DFT-Spreading in OFDM-FDMA Uplink

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DFT-Spreading in OFDM-FDMA Uplink Powered By Docstoc
					                  OFDM
based Systems and related Multiple
        Access Schemes

              Antalya, July 2005


                Hermann Rohling
         Technical University of Hamburg-Harburg
           Department of Telecommunications
                  Eißendorfer Straße 40
                    D-21073 Hamburg
Department for Telecommunications   2
                                         Ultrasound
                                     Broadband Mobile
                                     Communications

    Ultrasound
 Self-Organizing                                                            Ultrasound
                                                                        CellularNetworks
                                                                         Cellular Networks
Wireless Networks
            1   01
          11      01
        01          01
      01              01
    01




                                  TUHH
  01                    11
                          1




                              Technical University of Hamburg-Harburg

                         Department of Telecommunications
                            Prof. Dr. rer. nat. H. Rohling
  Ultrasound
  Multi Sensor                                                             Ultrasound
   Systems
                                     Automotive Radar
  Overview
      •    Requirements of 4G Systems

      •    The Broadband Radio Channel

      •    OFDM Basics

      •    OFDM System Building Blocks
            • Modulation: Coherent, Incoherent, Adaptive
            • Channel Estimation: Pilot-based, Blind
            • Channel Coding
            • Synchronisation

      •    OFDM for Multi-User Communications

      •    OFDM System Design and Performance

      •    Advanced OFDM Techniques
            • Joint Layer Optimization
            • MIMO
            • Cellular Environment: Synchronisation, Radio Resource Management




Department for Telecommunications                                                4
  Evolution of Mobile Communications


                                  4G
                                                                                     Today

                             3G
                                                                    1991          2002

                       2G                         1982                     1990
                             Research                 Deployment
                  1G           1969                       1981



                      1970         1980         1990         2000          2010

                       Technique            Data Rate                  Systems
                 1G     Analogue          < 300 bps        AMPS, NMT, …
                 2G      Digital          9.6k – 64kbps    GSM, PDC, IS-95, …
                 3G      CDMA             64kbps -         W-CDMA, TD-CDMA, …
                                          2Mbps
                 4G          ???          2M - 20Mbps                       ?


Department for Telecommunications                                                            5
  Requirements for Future Systems


                Mobility



         vehicular


                                                4th Generation


        pedestrian         3rd Generation
                             (IMT-2000)


                                                    Wireless
                                                     LAN
         stationary

                                    0.1     1          10        100   Data rate [Mbps]




Department for Telecommunications                                                         6
  Packet-based Data Streams

                circuit data        packet data




Department for Telecommunications                 7
  General Requirements on Future Systems


     •    High spectral efficiency

     •    Support of high user mobility

     •    High flexibility to deal with a broad range of user and traffic
          scenarios

     •    React to changing transmission environments by a high adaptivity




Department for Telecommunications                                            8
       The Broadband Radio Channel




Department for Telecommunications    9
  Multipath Propagation (Power Delay Profile)




                                               Propagation paths

            a3,t3       a2,t2




                                               h(t) [dB]
                      a1,t1
                                    Receiver
                                                           tmax   t


    Transmitter




Department for Telecommunications                                     10
  The Linear Time-Invariant (LTI) Radio Channel

  •   Behaviour of multipath propagation with no movement is characterized
      by a LTI-system in the equivalent lowpass domain:
                                                            

      sT (t )         Radio Channel
                            hT (t )       rT (t ) rT (t )   sT t hT t  t  dt
                                                            

  •   The channel impulse response is given by
                            L
            hT (t )   hT ,l t  t l 
                           l 1


                            L
           H T ( f )   hT ,l e  j 2ft l
                           l 1
  •   HT(f) denotes the channel transfer function


Department for Telecommunications                                                  11
  Narrowband Channel

   •    Symbol duration TS is much larger than the                         hT t 
        maximum channel tap delay:


                           TS  t m ax
                                                                                              t
                                                                       0             t m ax


          HT  f   const
                                                      B
                                          for   f      with B  1
                                                      2            TS

   •    Channel transfer function is assumed to be                           HT  f 
        constant over the signal bandwidth B


        No frequency-selective fading!
        No Inter Symbol Interference (ISI)!                                                  f
                                                              B              B
                                                                   2              2



Department for Telecommunications                                                                 12
  Broadband Channel


   •   Symbol duration TS is much smaller than the maximum channel
       tap delay

                                    TS  t m ax
                                                                     HT  f 

   •   Channel transfer function HT(f)
       fluctuates over signal bandwidth B

                                                                                f
                                                            B         B
    Frequency-selective fading, ISI occurs!                     2         2



   •   Quasi time-invariance during a small time interval
       ( → coherence time TC)




Department for Telecommunications                                                   13
  Frequency Selectivity  ISI

     •          Frequency Selectivity                           •     Inter Symbol Interference
                           t m ax  TS  1s
                                                                     h t 
       10

            0
  |H(f)| / dB




                                                                      0dB
       -10

       -20

       -30
                                                                     30dB
                                                                               0   TS   2TS 3TS 4TS 5TS   t
          0       1   2     3   4    5     6   7   8   9   10
                                 f / MHz
                          t m ax  5  TS  5s
                                                                     h t 
       10
  |H(f)| / dB




            0
                                                                      0dB
       -10



                                                                                                          t
       -20
                                                                     30dB
                                                                               0   TS   2TS 3TS 4TS 5TS
       -30
          0       1   2     3    4   5     6   7   8   9   10
                                 f / MHz



Department for Telecommunications                                                                             14
  User Mobility (Doppler Profile)

                                                                        Line-of-sight path
            Independent
         propagation paths


                                                                    f

                                                             v
                                                                                  Angle of incident


         Mobile with omni-
        directional antenna



                                        Doppler profile


                                                                           “Jakes Doppler Profile”


                              -fD,max     Frequency       +fD,max



Department for Telecommunications                                                                     15
  The Time-Variant Radio Channel

   •    Behaviour of multipath propagation with movement is characterized by a
        linear time-variant system in the equivalent lowpass domain:
                                                                  
       sT (t )         Radio Channel
                           hT (t , t )        rT (t ) rT (t )     s t t  h t , t   dt
                                                                  
                                                                       T        T



   •    The time-variant channel impulse response is given by
                          L
        hT (t , t )   hT ,l  t    t  t l 
                         l 1


                          L
       H T ( f , t )   hT ,l  t   e  j 2 f t l
                         l 1
   •    HT(f,t) denotes the time-variant channel transfer function


Department for Telecommunications                                                          16
  Time-Variant Transfer Function




                                    Channel Parameters
                                    B = 20 MHz
                                    Non Line-of-Sight (NLOS)
                                    Exp. Power Delay Profile
                                    tmax = 0.8 ms
                                    Jakes Doppler Profile
                                    fD,max = 15 Hz
                                    3 km/h @ 5.5 GHz




Department for Telecommunications                              17
  How Broad is Broadband ?
                                                  |H(f)|




                                    -B/2     Frequency     B/2



     The sampling time of a broadband system T=1/B is much smaller than
     the maximum multipath delay of the channel tmax


                                           1/B << tmax

           The channel transfer function |H(f)| fluctuates over the system
           bandwidth B
           Frequency selective fading  Inter symbol interferences (ISI)


Department for Telecommunications                                            18
                                OFDM Basics




Department for Telecommunications             19
  Single-Carrier Transmission with increasing Data Rates

       Example 1:                                        tmax = 10 s
          Data Rate = 90 kbps
                                                  h(t)
          BPSK, Bandwidth = 90 kHz

       Data 1      Data 2     Data 3   Data 4
                                                         Symbol Duration
            ISI effects 90% of a single Symbol                            tmax
            Easy to equalize!


       Example 2:                                h(t)
          Data Rate = 1 Mbps
          BPSK, Bandwidth = 1 MHz


                D1 D2 D3 D4
                                                                           tmax

            ISI effects 10 adjacent Symbols!
            Equalization becomes very complex!


Department for Telecommunications                                                 20
  Multi-Carrier Transmission: Basic Idea
         Bandwidth is splitted in N narrowband subchannels

                       Example: Splitting of a broadband channel into N=32 subchannels


        H f                                 f
                                                       „narrowband“
     10dB
                                                        subchannel

         0dB

    10dB

     20dB

         30dB
                                                                                         f



   Each subcarrier is flat faded. Channel influence can be described by a
    complex valued factor for each subcarrier


Department for Telecommunications                                                            21
  Multi-Carrier Transmission: Advantage
                  T                              h(t)
      s(t)


                                        t

                                                                    tmax   t

        Serial Transmission (Single Carrier):
        • Maximum multipath delay tmax >> Symbol duration TSC
                                Inter-Symbol Interferences (ISI)
                                Complex time domain equalizer


        Parallel Transmission (Multi-Carrier):
        • Maximum multipath delay tmax << Symbol duration TOFDM
                              No Inter-Symbol Interferences (ISI)
                              Simple frequency domain equalizer


Department for Telecommunications                                              22
  Multi-Carrier Transmission: Comparison
      • Data Rate: 10 MBit/s
      • BPSK transmission  Bandwidth B=10 MHz
      • Multipath channel with maximum delay tmax = 10 s

      Single-Carrier
          Symbol duration depends directly on system bandwidth:
                     TSC = 0.1 s = tmax /100

                              ISI extends over 100 symbols!

      OFDM
         Large Number of Subcarriers:         N = 1000
         OFDM symbol duration:       TOFDM = TSC N = 10  tmax
         Required guard interval:    TGuard  tmax = 0.1 TOFDM

                              ISI free transmission!




Department for Telecommunications                                 23
                          OFDM
   Orthogonal Frequency Division    Multiplexing




Department for Telecommunications              24
  OFDM Transmission Technique

              S n (f)




                                    f




Department for Telecommunications   25
  OFDM Transmission Technique - Transmitter

     •   Time-continuous signal of the ith OFDM block

                                  N 1                                   t  iTs ,mc 
              si t  
                        1
                                  S     i ,k   e   j 2 πkft
                                                                  rect 
                                                                         T
                                                                                      
                                                                                      
                        N         k 0                                       s , mc  

     •   Time-discrete signal of the ith OFDM block

                                                     N 1
             si ,n  si n  t  
                                    1
                                    N
                                                     S
                                                     k 0
                                                                i ,k   e   j 2 πnkft
                                                                                         with f  t 
                                                                                                            1 Ts ,mc 1
                                                                                                          Ts ,mc N
                                                                                                                    
                                                                                                                      N




                                                    N 1                          nk
                                    1                                      j 2π
                          si ,n   
                                    N
                                                    S
                                                    k 0
                                                                i ,k   e          N
                                                                                         (IDFT)


Department for Telecommunications                                                                                         26
    OFDM Transmission Technique - Channel
        Influence of the linear time-(in)variant radio channel

    Transmitted signal:
                 N 1                      nk
          1                         j 2π
si ,n   
          N
                 S
                  k 0
                         i ,k   e          N




                                                Linear Channel

                                                                                        orthogonal

                                                |H(
        orthogonal

                                                 f)|
        subcarriers                                                                     subcarriers
                                                       Frequency




                                                                        Received signal:
        linear operations                                                      N 1                            nk
                                                                           1                            j 2π
        Eigenfunctions of the channel                              ri ,n 
                                                                           N
                                                                               S
                                                                               k 0
                                                                                      i ,k   H i ,k e          N

        Subcarrier-wise Channel Transfer Factors


 Department for Telecommunications                                                                         27
  OFDM Transmission Technique - Receiver
   •       Received time-continuous signal of the ith OFDM block

       •     Time-Domain:                      ri t   si (t )  hi (t )  ni (t )

       •     Frequency-Domain:                 Ri ( f )  Si ( f )  H i ( f )  N i ( f )


                                    Ri ( f )                    ri t 
                 Demodulator                        FFT                   Synchronisation




   •       Received time-discrete signal of the ith OFDM block

       •     Time-Domain:                         ri ,n  si ,n  hi ,n  ni ,n

       •     Frequency-Domain:                     Ri ,n  Si ,n  H i ,n  N i ,n


Department for Telecommunications                                                            28
  Why Guard Interval?

                                                                                                    Time


                                Data 1                   Data 2                   Data 3                            Path 1

                                    Data 1                  Data 2                  Data 3                          Path 2
     Without
                                         Data 1                  Data 2               Data 3                        Path 3
Guard Interval
                                                       FFT window

                                             Data 1               Data 2                   Data 3                   Path P


                          t1 tP
                                                                                                    Time


                              GI               Data 1             GI              Data 2            GI                   Path 1

                                   GI            Data 1                GI           Data 2               GI              Path 2
       With                                                                                                              Path 3
                                        GI            Data 1                GI        Data 2                  GI
Guard Interval                               FFT window

                                         GI             Data 1               GI            Data 2              GI        Path P


                          t1 tP

Department for Telecommunications                                                                                                 29
  OFDM Spectrum


            OFDM Spectrum           Subcarrier spacing f




                                                                 Frequency

                                     k-2 k-1    k   k+1 k+2



                                           sinT  f  kf 
                            Gk ( f )  T 
                                             T  f  kf 

Department for Telecommunications                                            30
  OFDM Spectrum


                                             10
               Spectral Power Density [dB]

                                              0


                                             -10


                                             -20


                                             -30


                                             -40
                                               -100   -50   0   50     100   150   200
                                                                 f/ f




Department for Telecommunications                                                        31
  Single-Carrier vs. Multi-Carrier Systems



                                                        Digital Radio Mondial

                                                        xDSL
                       Max. Multi-Path Delay
                                                                 Multi-Carrier
                                                                    DVB-T

                                                                                 4G

                                                                         WLAN


                                               Single-Carrier



                                                Data Rate (System Bandwidth)




Department for Telecommunications                                                     32
  OFDM Based Systems

                    Wireless                                Wireline
     Broadcast:
      Digital Audio Broadcasting (DAB)
      Digital Video Broadcasting (DVB-T)
      Digital Radio Mondial (DRM)
      Terrestrial repeaters for U.S. satellite
       Digital Audio Radio Service (SDARS)
     …                                           Communications:
                                                   Digital Subscriber Line (xDSL)
     Communications:
                                                   Power Line Communications (PLC)
      HIPERLAN/2                                  Cable TV Network (CATV / MMDS)
      IEEE 802.11a                               …
      IEEE 802.16
      Home RF
     …




Department for Telecommunications                                                     33
                                    OFDM
                           System Structure




Department for Telecommunications             34
  OFDM System Structure


         Transmitter                                     Sn.0

                                                                               P

        Bitstream Channel                   Modulation               inverse           sn,m
                              Puncturing                                                                  D
                                                                       FFT                    Guardint.
                  Coding     Interleaving                                                                     A
                                              Mapping
                                                                                                                      en(t)
                                                                                   S
                                                            Sn.K-1
                                            Pilot symbols
                                                                                                              Channel

                                               Channel Estimation
          Receiver                             Equalisation
                                                         Rn.0                                          AWGN       +

                                                                               P                                  rn(t)
                                               De-
       Bitstream Viterbi                     mapping                                   rn,m
                              Depunct.                                FFT                                     A
                                                                                              Window
                 decoding     Deinterl.                                                                   D
                                             Demodu-
                                              lation
                                                                                   S          Synchronization
                                                         Rn.K-1




Department for Telecommunications                                                                                             35
  Digital Modulation Schemes


         Transmitter                                     Sn.0

                                                                               P

        Bitstream Channel                   Modulation               inverse           sn,m
                              Puncturing                                                                  D
                                                                       FFT                    Guardint.
                  Coding     Interleaving                                                                     A
                                              Mapping
                                                                                                                      en(t)
                                                                                   S
                                                            Sn.K-1
                                            Pilot symbols
                                                                                                              Channel

                                               Channel Estimation
          Receiver                             Equalisation
                                                         Rn.0                                          AWGN       +

                                                                               P                                  rn(t)
                                              De-
       Bitstream Viterbi                     mapping                                   rn,m
                              Depunct.                                FFT                                     A
                                                                                              Window
                 decoding     Deinterl.                                                                   D
                                             Demodu-
                                              lation
                                                                                   S          Synchronization
                                                         Rn.K-1




Department for Telecommunications                                                                                             36
  Representation in Signal Space


            Constellation diagram                    •   Real part („inphase component“)
                                                         and imaginary part („quadrature
                           ImsT 
                                                         component“) of the baseband
                                                         signal can be depicted in one
             1011 1010 0010 0011
                                                         two-dimensional diagram


             1001 1000 0000 0001                            Constellation diagram
                                           ResT 
             1101   1100 0100 0101
                                                     •   Modulation symbols usually
                                                         plotted as fixed points in
             1111   1110   0110     0111
                                                         diagram
                                                     •   Constellation diagram is very
                                                         convenient for representation
                                                         of linear modulation schemes



Department for Telecommunications                                                        37
  Amplitude Shift Keying (ASK)

                       Bandpass signal                       Example: 4-ASK

    s t      10       11          01   00   11           sT t    I n p t  nTS 
                                                                     n
                                                   t                       t 1
         0                                                    pt   rect  
                                                                          T 2
                                                                           S  
                       TS
                       Baseband signal                     Constellation diagram
    sTr t                                                              sTi
         0
                                                   t

                                                       t      00   01     11     s
                                                                               10 Tr


    sTi t 
         0
                                                   t



Department for Telecommunications                                                         38
  Phase Shift Keying (PSK)

                       Bandpass signal                        Example: QPSK

    s t      10       11          01   00   11           sT t    I n p t  nTS 
                                                                      n
                                                   t                       t 1
         0                                                    pt   rect  
                                                                          T 2
                                                                           S  
                       TS
                       Baseband signal                     Constellation diagram
    sTr t                                                                sTi
           1
         0
                                                   t                      10

         -1
                                                       t         00              11   sTr
    sTi t                                                               01
           1
         0
                                                   t
                                                                 -1              1
         -1



Department for Telecommunications                                                           39
  Amplitude and Phase Shift Keying (APSK)

                       Bandpass signal                          Example: 8-APSK

    s t      101      011         111   001   100
                                                              sT t    I n p t  nTS 
                                                                        n
                                                                              t 1
         0
                                                      t          pt   rect  
                                                                             T 2
                                                                              S  
                       TS
                       Baseband signal                        Constellation diagram
    sTr t                                                                 sTi
         0
                                                      t                 110

                                                                        010

                                                          t      100 000            s
                                                                           011 111 Tr
    sTi t                                                             001


         0
                                                      t                 101




Department for Telecommunications                                                            40
  Coherent Modulation

    Received symbol on subcarrier k :

                                     Rn ,k  H n ,k S n ,k  N n ,k


    The equalization requires only a simple complex multiplication with the
    inverse channel transfer factor:

                                                             Sn ,k  decDn ,k 
                                Rn ,k             N n ,k      ˆ
                    D  c
                                       S n ,k                           c
                       n ,k
                                ˆ n ,k
                                H                 ˆ
                                                  H n ,k



                     Channel transfer factors have to be estimated !




Department for Telecommunications                                                   41
  Differential Modulation




    Differential modulation in time direction:         S n , k  S n 1 , k Qn , k

    M-DPSK signal constellation:         Bn,k    {e j 2i / M | i  0,, M 1}

                                            Rn ,k    S         B  H n ,k  N n ,k
    Incoherent demodulation: Dn ,k 
                              nc
                                                     n 1,k n ,k
                                            Rn 1,k    S n 1,k  H n 1,k  N n 1,k
                                    Bn ,k  decDn ,k 
                                    ˆ            nc




Department for Telecommunications                                                       42
  Differential Modulation: Example 8-DPSK




Department for Telecommunications           43
  Differential Modulation in OFDM Systems

                       Time direction                                    Frequency direction




                                                             Frequency
           Frequency




                           Time                                                 Time

                                           Time and frequency direction
                                        Frequency




                                                     Time

Department for Telecommunications                                                              44
  Differential Modulation in OFDM Systems

 Performance of differential modulation depends on the correlation of symbols

                                                                                                                  BER
                          1.01
   Correlation Function




                            1                                                                       0.5

                          0.99                                                                      0.4
                                                                                                            TIME




                                                                                          f tmax
                          0.98                                                                      0.3

                          0.97                                                                      0.2               FREQ
                          0.96                 Frequency correlation function                       0.1
                                               Time correlation function
                          0.95                                                                      0.0
                             -0.1 -0.08 -0.06 -0.04 -0.02   0   0.02 0.04 0.06 0.08 0.1                0.0 0.2 0.4 0.6 0.8 1.0 1.2
                                                     f/BC, t/TC
                                                                                                                TS fD,max

                             When is differential modulation in frequency direction better ?

                                                                 TOFDM  f D ,m ax  2  f  t m ax

Department for Telecommunications                                                                                                    45
  Higher Order Differential Modulation M-DAPSK

                                                                                                DAPSK signal constellation:

                                                                                                                                              
                                                                                           A  e jP | A  0, N a  1 P   , N p  1
                                                                                             a                               ,     0



                                                                                                             Amplitude    Phase    Amplitude
                                                                                                               Bits        Bits     Factor a
                                                                                                    M < 16       1          M          -
                                                   Amplitude   Qn
                         (b1 , ... , b a )
                                     m n            Mapping
                                                                    Differential   Sn
                      (bma +1 , ... , b a +mp n)     Phase           Encoding
                                      m             Mapping




                                                                           -1
                                                                           z
                                                                                                    M = 16       2          8         2.0
                                                   Modified
                         (b1 , ... , b a )n
                                     m             Amplitude   Bn
                                                   Mapping
                                                                                   Sn



                                                                                                    M = 32       2         16         1.6
                      (bma +1 , ... , b a +mp n)
                                      m              Phase
                                                    Mapping




                                                                                                    M = 64       4         16         1.4
                                                                                                   M = 128       4         32         1.3




Department for Telecommunications                                                                                                              46
  Performance of Differential Modulation

                                    0
                               10
                                                        64-DAPSK, quasi-coherent
                                                         64-DAPSK, non-coherent
                                                               64-QAM, coherent
                                -1
                               10
              Bit Error Rate




                                -2
                               10



                                -3
                               10



                                -4
                               10
                                        12   14   16   18   20    22    24   26    28   30
                                                            S/N [dB]




Department for Telecommunications                                                            47
  Adaptive Modulation

 OFDM gives the opportunity to use:                                |H(k)|
                                         10


 •    different modulation schemes for
      each subchannel                      1



 •    different power for each
                                         0.1
      subchannel
                                                                        b(k)
                                           6


                                           4


                                           2


                                           0
                                               10   30   50   70   90     110   130   150   170   190
     Adaptive Modulation                                                 k
     Adaptation to the channel transfer function using subchannel
     specific modulation schemes and power


Department for Telecommunications                                                                   48
  Adaptive Modulation


                                                                         256QAM
                                                                         64QAM
                 Average SNR                                             16QAM
                                                                         QPSK
                                                                         BPSK

                                                                    Not used due to
                                                                       low SNR

                                           Subcarriers


          Algorithms: Chow, Cioffi and Bingham: capacity maximization
                               Fischer: Error probability minimization
                               Grünheid: simple blockwise loading algorithm
                               Hughes-Hartogs: sets target rate R, intensive searching




Department for Telecommunications                                                        49
  Adaptive Modulation

    • Adaptive modulation (average 2 bits per subcarrier)
                        0
                   10
                                                              adapt modulation
                                                              fixed modulation
                        -1
                   10




                        -2
                   10
             BER




                        -3
                   10
                                                                        1.5 dB

                        -4
                   10




                        -5
                   10
                            -6   -4   -2   0        2     4         6        8   10
                                               SNR (dB)

 Bit loading by Fischer Algorithm

Department for Telecommunications                                                     50
  Channel Estimation


         Transmitter                                     Sn.0

                                                                               P

        Bitstream Channel                   Modulation               inverse           sn,m
                              Puncturing                                                                  D
                                                                       FFT                    Guardint.
                  Coding     Interleaving                                                                     A
                                              Mapping
                                                                                                                      en(t)
                                                                                   S
                                                            Sn.K-1
                                            Pilot symbols
                                                                                                              Channel

                                               Channel Estimation
          Receiver                             Equalisation
                                                         Rn.0                                          AWGN       +

                                                                               P                                  rn(t)
                                              De-
       Bitstream Viterbi                     mapping                                   rn,m
                              Depunct.                                FFT                                     A
                                                                                              Window
                 decoding     Deinterl.                                                                   D
                                             Demodu-
                                              lation
                                                                                   S          Synchronization
                                                         Rn.K-1




Department for Telecommunications                                                                                             51
  Pilot-Based Channel Estimation

                                              Nt                         Least-Squares Estimation:

                                                                                               Rn , k
                                                                                    H n ,k 
                                                                                               S n,k
                                Frequency Interpolation
                  Subcarriers




                                                           Two-Dimensional Interpolation


                                                                               Nf

                                           Time Interpolation




                                             OFDM symbols
        Pilots
        Virtual pilots by time interpolation
        Desired transfer factor



Department for Telecommunications                                                                       52
  Interpolation Methods


         Linear interpolation

         Second order interpolation

         Low pass interpolation

         Spline cubic interpolation

         Time domain interpolation




Department for Telecommunications      53
  Pilot-Based / Blind Channel Estimation

                                                      Current OFDM symbol                                 nfT S
    Frequency




                                                                     Frequency
                                                                                                                         ntf



                                          Time                                                          Time
                 „Decision Directed“                                                    „Pilot Based“
                                                 Rk,i =Sk,i Hk,i +Nk,i
                                                                                        Soft/Hard-
                                                                                                                                Sk,i
                FFT                                                                                               Decoding
                                                                                         Decision
                            Rl,j

 A priori known                                                                  Hk,i
 pilot symbols                      Remove             Interpolation /
                                   Modulation            Estimation




Department for Telecommunications                                                                                                      54
  Decision-Directed Channel Estimation
                      Subcarrierwise channel estimation with a,  filtering


                                            ˆ
     Hn,k  (1  a )( Hn1,k  n1,k )  a Hn,k
                                                                                      ˆ  Rn,k
                                                                                     Yeqn ,k            ˆ  Rn , k
                                                                                                        H n ,k
                                 ˆ
     n,k  (1   )n1,k   ( Hn,k  Hn1,k )                                             H n1,k           Yeqn , k



                               FFT                                 Yeqn,k                     Decoder
                                                       Div                  Demod
                                                           ~
                                                           H n,k
                                              a,  filter
                                                                                        Hard Decision

                                                   ˆ
                                                   H n,k
                                                                            Yeqn,k
                                                                   Div                Mod

                                ~
                                              Rn , k
                                H1,k


Department for Telecommunications                                                                                         55
  Decision-Directed Channel Estimation
                     Subcarrierwise channel estimation with a,  filtering


                          1.0
                                                   If received symbols are inside
                                                           the reliable area:

                          0.1
                                                                                       ˆ
                                                Hn,k  (1  a )( Hn1,k  n1,k )  a Hn,k
         -1.0      -0.1       0.1   1.0
                          -0.1
                                                                            ˆ
                                                n,k  (1   )n1,k   ( Hn,k  Hn1,k )

                           -
                          1.0

                                                   If received symbols are outside
                                                      the reliable area: (a=0, =0)
       Locating a reliability area in the
       constellation diagramm (QPSK)                        Hn,k  Hn1,k  n1,k

        Optimal values a=0.1, =0.008
                                                             n,k  n1,k


Department for Telecommunications                                                             56
  Channel Coding


         Transmitter                                     Sn.0

                                                                               P

        Bitstream Channel                   Modulation               inverse           sn,m
                              Puncturing                                                                  D
                                                                       FFT                    Guardint.
                  Coding     Interleaving                                                                     A
                                              Mapping
                                                                                                                      en(t)
                                                                                   S
                                                            Sn.K-1
                                            Pilot symbols
                                                                                                              Channel

                                               Channel Estimation
          Receiver                             Equalisation
                                                         Rn.0                                          AWGN       +

                                                                               P                                  rn(t)
                                              De-
       Bitstream Viterbi                     mapping                                   rn,m
                              Depunct.                                FFT                                     A
                                                                                              Window
                 decoding     Deinterl.                                                                   D
                                             Demodu-
                                              lation
                                                                                   S          Synchronization
                                                         Rn.K-1




Department for Telecommunications                                                                                             57
  Channel Coding

     Flat bit error rate curve in the Rayleigh channel
     due to faded subcarriers                1e-0
                                                         BPSK, Rayleigh-Kanal
                                                         BPSK, Rayleigh channel
                                                         BPSK, AWGN-Kanal
                                                         BPSK, AWGN channel
                                             1e-1


                                             1e-2




                                       BER
      Channel Coding
                                             1e-3


                                             1e-4


                                             1e-5
     Questions:                           0     5    10   15    20   25           30
     • choice of appropriate codes                       S/N
       (spreading codes, block codes, convolutional codes, turbo codes)
     • optimal modulation schemes (coded modulation)
     • metrics for soft-decision decoding
     • decoding techniques


Department for Telecommunications                                                 58
  Soft-Output Demodulation of Coherent Signals

    Received modulation symbol:                        Rn , k  H n , k S n , k  N n , k
                                                                                                                                    2
                                                                                                          Rn , k  H n , k Sn , k

    Posteriori PDF is Gaussian:                                
                                                       p Rn ,k S n ,k              2
                                                                                          1
                                                                                              2
                                                                                                  e
                                                                                                      
                                                                                                                   2 2




    Maximum Likelihood Sequence Estimation (MLSE):

                                    S n ,k  arg max P Rn ,k                             
                                                                                    ()
                                    ˆ                                 S n ,k             
                                                                                        

                                                  
                                                                   
                                          arg max  p Rn ,k S n ,k (  )                 
                                                           k

                                          arg min  Rn ,k  H n ,k S n ,k (  )
                                                                                              2

                                                  
                                                       k


    Metrik information fed to the Viterbi decoder:
                                                                                2
                                           ,k  Rn ,k  H n ,k S ,k
                                           n                       n




Department for Telecommunications                                                                                                       59
  Soft-Output Demodulation of Incoherent Signals

   Differential phase modulation                                               Differential amplitude modulation
   (DPSK):                                                                     (DASK):
                                                              2
                                            n , k  n , k                                                   Rn ,k                S
                                                                                             Wn ,k  ln               , Vn ,k  ln n ,k
               
                                       
                            1                   2   2
   p  n ,k  n ,k                e                                                                          Rn 1,k             S n 1,k
                        2 
                            2
                                                                                                                                              2
                                                                                                                             Wn , k Vn , k

                                                                                                  
                                                                                                                         
                                                                                                              1                   2 w
                                                                                      p Wn ,k Vn ,k 
                                                                                                                                     2
                                                                                                                     e
                                                                                                         2 w
                                                                                                              2
    n,k  arg min  H n ,k  n ,k   n,k (  )
                                       2                              2

                    
                        k                                                                                             2                                 2
                                                                                                 
                                                                                                   2
                                                                                                   w
                                                                                                          2
                                                                                                                                     2
                                                                                                                                                                     2
                                                                                                                  H n ,k S n ,k                   H n 1,k S n 1,k


                                Bn ,k  arg min  d n ,k (  ) RIn ,k
                                ˆ                   2            2
                                                              
                                                                  k
     DAPSK:
                         d n ,k (  )  Wn ,k  Vn ,k (  )    n ,k  n ,k (  )  , RIn ,k 
                                                                          2                  2 2                                1
                                                                                                                         2                           2
                                                                                                               1 Rn ,k  1 Rn 1,k

    Metrik information for DAPSK fed to the Viterbi decoder:

                                                                   ,k  d n ,k  RIn ,k
                                                                                 2   2
                                                                   n


Department for Telecommunications                                                                                                                                         60
  Concatenation of Coding and Differential Modulation



                Convolutional                                   Differential
                                           Interleaver
                  Coding                                        Modulation




          Convolutional                              Differential        Non-Diff.
                                    Interleaver
            Coding                                     Coding           Modulation




            Concatenated Code  Turbo-Decoding




Department for Telecommunications                                                    61
  Differential Modulation with Turbo Decoding



                                         AWGN channel
                                         8-DPSK (bzw. 8-PSK)
                                         Convolutional code:
                                                   [171]8 [133]8
                                         Block-Interleaver: 3066 Bits
                                         OFDM: 1024 subcarrier




Department for Telecommunications                                   62
  Synchronization


         Transmitter                                     Sn.0

                                                                               P

        Bitstream Channel                   Modulation               inverse           sn,m
                              Puncturing                                                                  D
                                                                       FFT                    Guardint.
                  Coding     Interleaving                                                                     A
                                              Mapping
                                                                                                                      en(t)
                                                                                   S
                                                            Sn.K-1
                                            Pilot symbols
                                                                                                              Channel

                                               Channel Estimation
          Receiver                             Equalisation
                                                         Rn.0                                          AWGN       +

                                                                               P                                  rn(t)
                                              De-
       Bitstream Viterbi                     mapping                                   rn,m
                              Depunct.                                FFT                                     A
                                                                                              Window
                 decoding     Deinterl.                                                                   D
                                             Demodu-
                                              lation
                                                                                   S          Synchronization
                                                         Rn.K-1




Department for Telecommunications                                                                                             63
  Synchronization




                       coding /                      cyclic
      10100011                      IFFT   P/S                         D/A
                     modulation                    extension



                                                                                     channel




      10100011      demodulation/   FFT    S/P     windowing           A/D
                      decoding



                                                    frame
                                                 synchronization

                                                                       clock           frequency
                                                                   synchronization   synchronization




Department for Telecommunications                                                                      64
  Guard Interval based Technique

          Exploit the correlation introduced by the guard interval:




                                    OFDM symbol


                      GI                DATA              GI GI

                                                  ()*                            t
                                                                Sliding window

                                      Moving                                Fractional
                                                        Phase
                                       sum
                                                                         frequency offset



                                        ||             argmax            Time offset



                                Maximum likelihood estimator !


Department for Telecommunications                                                           65
                                    OFDM
                                     for
            Multi-User Communications



Department for Telecommunications          66
  OFDM for Multi-Use Communications

      For a given OFDM system find a suitable multiple access scheme that
      maps the user data to a modulation block !

                                              L              NC
          User 1




                                    Mapping
              Coding
              +Interl.
                            S/P                   Dk,l


          User 2
              Coding                Mapping
                            S/P
              +Interl.

                                                         ?                       Add




                                                                         IFFT
                                                                  Sn,i          Guard




          User K
                                    Mapping




              Coding
                            S/P
              +Interl.




Department for Telecommunications                                                       67
  OFDM Multiple Access Schemes

                OFDM-FDMA                    OFDM-TDMA


           f                             f




                                    t                          t
                                        OFDM-CDMA

                           f



                                                     User / Code
                                                 t



Department for Telecommunications                                  68
  OFDM-TDMA

                Principle:
                Every user allocates all subcarriers in a certain number of time slots
                (OFDM symbols) in each OFDM modulation block


                                                           Advantages:
                          User                              No multiple access interferences (MAI)
                  1                  NS
                                                            Incoherent or coherent modulation
                                          NC
                                                            Adaptation to channel characteristics
                                                            High coding gain due to diversity
  Subcarriers




                                               User Data


                                                            Robust against estimation errors
                                                            No MAI in case of synchronisation errors
                                                            Easy implementation
                                          1

                      OFDM Symbols                         Disadvantages:
                                                            Performance of „normal“ OFDM system



Department for Telecommunications                                                                  69
  OFDM-FDMA

                Principle:
                Every user transmits on a certain number of OFDM subcarriers during
                all time slots of the OFDM modulation block


                                                      Advantages:
                        User Data                      No multiple access interference
                  1                  NS
                                                       Incoherent or coherent modulation
                                          NC
                                                       Adaptation to channel characteristics
                                                         • Select good subcarriers
  Subcarriers




                                                         • Bitloading on selected subcarriers
                                               User




                                                       Robust against estimation errors

                                          1
                                                      Disadvantages:
                      OFDM Symbols                     Stronger requirements on carrier
                                                         frequency synchronisation between
                                                         users in the uplink


Department for Telecommunications                                                               70
  FDMA Transmitter

              User 1




                                                         Interleaver
                                                         Frequency
                                    Mapping
                 Coding
                              S/P             Di,k
                 +Interl.



              User 2




                                                         Interleaver
                                                         Frequency
                                    Mapping




                                                                                   IFFT
                 Coding
                 +Interl.
                              S/P             Di,k                          Si,k




                                                     FDMA Multiple Access

                  How shall the subcarrier of each user be selected ?



Department for Telecommunications                                                         71
  Time-Frequency Block

     To allow the utilization of subcarrier by different users define a time-
     frequency modulation block consisting of b subcarriers in a OFDM
     symbols:




                  f
                                                         bf
                                                         aT




                   f
                         T                                    t


Department for Telecommunications                                               72
  OFDM-FDMA Resource Allocation


                                                       Independent multi-path channels




                                                           |H| [dB]
              User 1
                                                                      Subcarrier




                                                           |H| [dB]
                                    Frequency
              User 2

                                                ?                     Subcarrier



                                                Time
              User K




                                                           |H| [dB]
                                                                      Subcarrier




Department for Telecommunications                                                    73
  OFDM-(FH-)FDMA

     If no channel information is available the TDMA/FDMA concept can be
     used to implement a frequency hopping scheme.

                                    f




               |H(f)|                                            t



Department for Telecommunications                                          74
  OFDM-TDMA/FDMA

    With a OFDM-TDMA/FDMA multiple access scheme frequency bands can
    be assigned to users with highest SNR in that band

                                              f              aT
                                                                  b f




                                                                         t
                       |SNR 1(f)|    | SNR 2(f)|


                                     Multi-User diversity


Department for Telecommunications                                            75
  OFDM-CDMA

 Principle:
 Every user transmits on all OFDM subcarriers during all OFDM symbols of an
 OFDM modulation block using an orthogonal code (e.g. Walsh-Hadamard).

                      User Data
                                                    Advantages:
                1                  NS
                                        NC           Processing gain due to frequency
                                                      diversity
                                                     Robust against interferences
  Subcarriers




                                             User


                                                    Disadvantages:
                                                     Multiple access interferences
                                        1
                                                     Only coherent modulation possible
                    OFDM Symbols                     No adaptation to channel
                                                       characteristics




Department for Telecommunications                                                         76
  Performance Results for the Downlink
        BER performance comparison between OFDM multiple access
         techniques (QPSK, R=1/2)
                        0
                   10


                        -1                                    OFDM-CDMA
                   10


                        -2
                   10
                                                                   OFDM-TDMA
             BER




                        -3
                   10


                        -4                          5dB       2.5dB
                   10
                             OFDM-FDMA

                        -5
                   10
                        -6    -4    -2   0      2         4    6       8       10
                                             SNR (dB)

Department for Telecommunications                                                   77
                   OFDM System Design
                          and Performance




Department for Telecommunications           78
  OFDM System Design

    The overhead of the guard interval sets the lower limit on the OFDM
    symbol duration:
                                         t m ax  0.2  TS

    The maximum Doppler frequency sets the upper limit for the OFDM
    symbol duration:
                                             vfCarrier
                                f D ,max               0.03  f
                                                c
    Requirements for OFDM symbol duration:

                                                                     1
                                    5  t m ax  TS  0.03
                                                               f D ,m ax

    Example: Hiperlan/2 ETSI-E channel model and 250 km/h@5.5GHz
                                      8.5s  TS  23 .5s


Department for Telecommunications                                          79
  OFDM System Parameters
                            Parameter                         Value
                 System Bandwidth             B = 20 MHz
   Properties

                 Maximum Delay                tmax = 5 s
    Channel




                 Coherence Bandwidth          Bc = 200 kHz
                 Carrier Frequency            fC = 5.5 GHz
                 Maximum Speed                vmax = 200 km/h
                 Maximum Doppler Frequency    fDmax = 1 kHz                    1
                 OFDM Symbol Duration         TS = 25.6 s
   OFDM System




                 Guard Intervall Duration     TG = 6.4 s
    Parameters




                 Total OFDM Symbol Duration   TOFDM = 32 s
                 FFT Length                   NC = 512 (1024)
                 Guard Intervall Length       NG = 128 = NC /4 (NC /8)     2
                 Subcarrier Spacing           f = 39063 kHz (19531 kHz)
                 Modulation Technique         16-QAM, 16-DAPSK
                 Code Rate                    R=1/2
                 User Data Rate               32 MBit/s


Department for Telecommunications                                                  80
  Performance Results for the Downlink

                1
                                                                   TDMA
                                                      TDMA (Adapt. mod.)
                                                      CDMA (MMSE SUD)
             1e-01                                          Adapt. FDMA
                                               Adapt. FDMA (Adapt. Mod.)
                                                     Adapt. FDMA/CDMA


             1e-02
       BER




             1e-03



             1e-04



             1e-05 0                5     10                 15            20
                                        SNR [dB]


Department for Telecommunications                                               81
         Advanced OFDM Techniques




Department for Telecommunications   82
          OFDM-FDMA Scheme for the
              Uplink of a Mobile
            Communication System



Department for Telecommunications    83
  System Overview

                             OFDM-based Uplink Scheme

                           User                          User
                            0                             m



                           User                          User
                            1                            M 1
                           MT            BS

              Multiple Users
              Uplink from Mobile Terminal (MT) to Base Station (BS)
              Single Cell Environment
              Sharing of Bandwidth by a specific OFDM-FDMA-Scheme




Department for Telecommunications                                      84
  Mobile Terminal

           Two parts of the MT„s OFDM-Structure are considered:
                  Spreading matrix
                  Equidistant subcarrier allocation


 Mobile Terminal
                      Di ,n               Si ,k                       si ,n
       Encoder +                                  Subcarrier
                              Spreading                        IDFT           GI
      Modulation                                  Allocation



                Di ,n :   Modulation Symbols

                Si ,k :   Transmit Symbols (Freq. Domain)

                si ,n :   Transmit Symbols (Time Domain)

Department for Telecommunications                                                  85
  Subcarrier Allocation

    IDFT-Processing in an OFDM-system (ith OFDM-Block):
                                                    N 1
                                            1
                                    si ,n 
                                            N
                                                    Si ,k  e j 2 nk / N
                                                    k 0




    General Observation:

       Discrete Spectrum with equidistantly                                       Periodic
                                                                    IDFT
                spaced non-zero values                                           Time Signal


   This effect is used in the considered subcarrier allocation scheme:
                             Di ,n        Si , k                         si ,n
                                                   Subcarrier
                                                   Allocation
                                                                 IDFT




Department for Telecommunications                                                              86
  Subcarrier Allocation

    The subcarriers are allocated equidistantly:
                                   Di ,n          Si , k                             si ,n
                                                           Subcarrier
                                                           Allocation
                                                                            IDFT

                        Magn.    Si ,0         Si ,1                          Si , L 1
                                                                                   User 1

                                                           Subcarriers
                                         M

                                             si ,n  IDFT  Si ,k 


         si ,0 si ,1            si ,L1 si ,0 si ,1               si ,L1          si ,0 si ,1            si ,L1
                1st period                                                                   Mth period

                       → This leads to a periodic transmit time signal


Department for Telecommunications                                                                                   87
  Spreading

   Second design element:
    Spreading is applied to the user„s subcarriers
                               Di ,n            Si ,k                    si ,n

                                       Spread



  Multiplication of modulation symbols Di ,n with an orthogonal,
      unitary matrix

    Well known examples:
                        1 1 1 1                       1     1           1          1 
                        1 1 1 1                          j / 2                 j 3 
                                                        1 e            e  j     e 2 
                                  
                        1 1 1 1                     1 e  j       e  j 2   e  j 3 
                                                                                         
                        1 1 1 1                      1 e  j 3    e  j 3
                                                                                      j 9 
                                                                                  e 2 
                                                                  2


                         Walsh-Hadamard                        Discrete Fourier


Department for Telecommunications                                                               88
  Spreading Matrix

   In the considered OFDM-FDMA system,
   only DFT-matrices are applied for spreading:

                              Di ,n            Si ,k                    si ,n
                                      Spread



                            Si ,0                     Di ,0 
                            S                        D 
                            i ,1   
                                       
                                              DFT      i ,1 
                                                                  
                                                      D 
                            Si , L 1  
                                                        i , L 1 

   Joint application of DFT-spreading and equidistant subcarrier
   allocation leads to a greatly simplified system


Department for Telecommunications                                               89
  Combination of Spreading and Subcarrier Allocation
   In effect, the DFT of the spreading matrix and the IDFT-processing
   in the OFDM-transmitter cancel out each other:


           si ,0          Si ,0               Di ,0    Di ,0 
                                                                  
           si ,1   IDFT  Si ,1   IDFT  DFT  Di ,1     Di ,1 
                                                                  
          s 
                         
                           S             
                                                  D   D 
                                                                       
           i , L 1       i , L 1             i , L1    i , L1 




   Consequence:
   The DFT-spreaded OFDM-FDMA system is equivalent to
   a single-carrier-system with guard intervall



Department for Telecommunications                                              90
  Combination of Spreading and Subcarrier Allocation

   Together with equidistant subcarrier allocation:


   Transmit signal si ,n is an M-times repetition of modulation
   symbol vector Di :

            1st period                                                      Mth period


   Di ,0 Di ,1            Di ,L1 Di ,0 Di ,1            Di ,L1    Di ,0 Di ,1          Di ,L1

                                                  identical

    si ,0   si ,1         si ,L1 si ,0   si ,1           si ,L1   si ,0   si ,1        si ,L1




Department for Telecommunications                                                                  91
  Combination of Spreading and Subcarrier Allocation
    As a result of combined spreading and subcarrier allocation,
     three components in the transmitter cancel out each other:
                       Di ,n             Si , k                            si ,n
                                                   Subcarrier
                               Spread                            IDFT
                                                   Allocation


                   Di ,0                   Si ,0                                    Di ,0
                   Di ,1                                                            Di ,1    1st
                                           Si ,1
                                DFT                                                        period
                  Di ,L1                 Si , L 1                                Di ,L1
                                                                                   Di ,0
                                                                 IDFT 
                                                                                   Di ,L1
                                                                                   Di ,0
                                                                                   Di ,1     Mth
                                                                                             period
                                                                                   Di ,L1


Department for Telecommunications                                                                     92
  Multiple Users

  Other users allocate a shifted, but also equidistant subset of subcarriers:


                                           Si(,0 )
                                               m
                                                          Si(,1 )
                                                              m                           )
                                                                                    Si(,m1 User m
                                                                                        L
                       Magnitude
                                                                                           User 1



                                                            Subcarriers
                                       m


            The frequency shift leads to a phase rotation of the transmit
            symbols in the time domain:


                                   Si(,m )   ( k  m)
                                       k                            si(,m )  exp( j 2 nm / N )
                                                                        n




Department for Telecommunications                                                                    93
  Resulting Multi-User System

    All this results in a very simple signal processing in the transmitter in a
                              multi-user uplink system

   L Modulation symbols
        m       m
    Di(,0 ) Di(,1 )             )
                          Di(,m1
                              L                     M-times repetition

        m       m
    Di(,0 ) Di(,1 )       Di(,m1 Di(,0 ) Di(,1 )
                              L
                                )     m       m
                                                            Di(,m1
                                                                L
                                                                  )       m
                                                                      Di(,0 ) Di(,1 )
                                                                                  m
                                                                                                Di(,m1
                                                                                                    L
                                                                                                      )




             1st period
                                                                             Mth period

                                                    j 2 nm / N                                               Phase
    e j 0... e j1...                           e                                         e j... e j...       Rotation

                                                
                                                                                                             (m)   (m)
        m       m
    si(,0 ) si(,1 )       si(,m1 si(,m )
                              L
                                )
                                      L                                                 si(,m )2 si(,m )1 si ,0 si ,1
                                                                                            N         N



                                       Transmit signal in time domain                                          GI



Department for Telecommunications                                                                                         94
  Receiver Structure

    Receiver is equivalent to conventional OFDM-FDMA receiver
      with additional despreading


                               (0)            One-Tap EQ                          Despread
                              Rn                                                               (0)
                                                                        (0)
                                                                       Sn ,0                  Dn ,0
                                        G (0)
                                           n ,0    0     0 
                                                               
                                                                                               (0)




                                                                                                        User 0
                                                                        (0)                   Dn ,1
                                         0              0 
                                                                       Sn ,1
                                                                                   IDFT 
                                         0        0    Gn ,L1 
                                                         (0)
                                                                     (0)
                                                                      Sn,L1
                                                                                              (0)
                                                                                             Dn , L1
     ri ,n
              DFT                                Detection for M users

                                                                     S n ,0 1)
                                                                       (M                    Dn,0 1)
                                                                                              (M
                                        G
                                         ( M 1)
                                                    0       0    




                                                                                                        User M-1
                                        
                                         n ,0
                                                                    S n ,1 1)
                                                                       (M                    Dn,1 1)
                                                                                              (M

                                         0                 0                     IDFT 
                                         0         0    ( M 1) 
                                                        Gn,L1 
                              ( M 1)                                 ( 1)
                                                                     S n ,M 1
                                                                          L
                                                                                              ( 1)
                                                                                             Dn,M1
                                                                                                L
                             Rn




Department for Telecommunications                                                                                  95
  Equalization

   The cyclic prefix in the transmit signal prevents ISI
   → Frequency domain equalization can be done by means
       of a one-tap equalizer

                              Snm )   Gnm )
                                  (
                                    ,0
                                            (
                                              ,0        0          0  Rnm )  (
                                                                                   ,0
                                                                                   
                                        0                      0                  
                              S (m)   0              0         ( m )  ( m ) 
                                                                 Gn , L 1  Rn , L 1 
                              n , L 1  

   In order to avoid high noise amplification in deep spectral fades,
                     (m)
   the coefficients Gn ,k are calculated from the MMSE-criterion:

                                                         (
                                                       H nm )*
                                    G (m)
                                      n ,k                ,k

                                                     (m) 2        1
                                                 H   n ,k    
                                                                 SNR
                (
              H nm ) are the channel coefficients of the k th subcarrier at time n
                  ,k



Department for Telecommunications                                                           96
  Bit Error Performance

   The receiver structure is equivalent to a spreaded OFDM-FDMA
   system and therefore, the same BER-Performance will be observed

   As an example, the performance of an uncoded system is evaluated
   System parameters:
          256 subcarriers             16 QAM Modulation
          20 MHz bandwidth            64 samples guard interval
          16 users

   Channel parameters: (WSSUS)
          Exponentially decreasing power delay (0 - 3.2µs)
          30 uncorrelated paths
          Rayleigh fading
          No Doppler-shift


Department for Telecommunications                                   97
  Bit Error Performance
   The performance figure gives a result for the introduced
   system in comparison with Zero-Forcing equalizaton.
                                    0
                               10
                                                                  ZF
                                    -1
                                                                  MMSE
                               10

                                    -2
                               10
                         BER




                                    -3
                               10

                                    -4
                               10

                                    -5
                               10
                                        0   5   10      15   20          25
                                                 SNR [dB]


   An arbitrary user is considered, because in general all users
   experience the same average performance

Department for Telecommunications                                             98
  PAR-Reduction
   Due to the duality to a single-carrier system, the Peak-to-Average
   ratio (PAR) is smaller compared to conventional OFDM-systems:

                   2    (a)                              2    (b)
                   1                                     1
            Imag




                                                  Imag
                   0                                     0


                   -1                                    -1


                   -2                                    -2

                         -2   -1    0     1   2                -2   -1    0     1   2
                                   Real                                  Real



   For comparison: Complex envelope of two DFT-spreaded
   transmission systems with (both QPSK)
         a) random subcarrier allocation
         b) equidistant subcarrier allocation


Department for Telecommunications                                                       99
  Benefits of this Uplink Scheme



            The OFDM-FDMA scheme reduces to a single-carrier
             system with guard interval


            Reduction of transmitter complexity

            Low PAR due to single-carrier equivalence


            Same BER-Performance as conventionally
             spread OFDM-FDMA




Department for Telecommunications                               100
  Channel Prediction Requirements




              Multiple Access Scheme   Channel Prediction Required
              OFDM-TDMA                            No
              OFDM-FDMA with
                                                  Yes
              Adaptive Modulation
              OFDM-FDMA with
                                                   No
              Spreading
              OFDM-CDMA                            No




Department for Telecommunications                                    101
            Joint Optimization of Layers




Department for Telecommunications          102
  Joint Optimization of PHY and DLC
                                    Higher Protocol Layers      Higher Protocol Layers


                                     DLC          Statistical
                                                  PER Data
                                                                  DLC
   Higher Protocol                                                             PHY Mode
                                                  PHY Mode                     Selection
       Layers                                     Selection

                                                                             PER / Goodput
                                                                               Prediction

 PHY                                 PHY                          PHY             SNR
                                                                            Transfer Function
                                                   SNR
                                                Measurement                    Soft Bits




       Channel                             Channel                      Channel

                                       Conventional                   Joint Link
      No Link
                                      Link Adaptation                Adaptation by
     Adaptation
                                          by DLC                       DLC/PHY



Department for Telecommunications                                                           103
  SNR Thresholds for HL/2 PHY Mode Selection

                       1
                                                                                    BPSK, R=1/2
                                                                                    BPSK, R=3/4
                                                                                    QPSK, R=1/2
                                                                                    QPSK, R=3/4
                                                                                 16-QAM, R=9/16
                                                                                  16-QAM, R=3/4
                                                                                  64-QAM, R=3/4
                      0.1
          LCH-PER




                                                                                                              PER threshold 10-2
                     0.01




                    0.001
                            -4   -2   0   2   4   6   8   10    12 14 16   18   20   22   24   26   28   30
                                                               SNR [dB]




     ETSI A Channel Model


Department for Telecommunications                                                                                         104
  16-QAM, R=3/4 PERs for an ETSI A Channel

    PER for individual ETSI A channel realizations vs. average PER
                        1

                                                                                      AWGN
                                                                               ETSI_A, av g.
                                                                            ETSI_A, samples


                       0.1
           LCH-PER




                      0.01




                     0.001
                             9   10   11   12   13   14   15   16 17 18      19   20   21      22   23   24   25
                                                                 SNR [dB]


        Use PHY layer information about channel to optimize link adaptation !


Department for Telecommunications                                                                                  105
  OFDM-TDMA - Downlink




Department for Telecommunications   106
  Delay Oriented Throughput

                                      5
                                               conv entional scheme
                                                  optimised scheme

                                      4
      Avg. Throughput [bits/modsym]




                                                                                                                         64-QAM
                                                                                                                          R=3/4

                                      3
                                                                                                              16-QAM
                                                                                                               R=2/4

                                      2                                                        16-QAM
                                                                                               R=9/16


                                      1
                                                                                   QPSK
                                                                                   R=1/2
                                                                  BPSK
                                                                  R=1/2
                                      0
                                          -4   -2    0   2    4       6   8   10    12 14 16   18   20   22    24   26    28   30
                                                                                    SNR [dB]




Department for Telecommunications                                                                                                   107
                                    MIMO
      Multiple Input - Multiple Output




Department for Telecommunications          108
  Higher Data Rates needed

     Mobility&
       Range
                      2G     2.5G         3G

                                                                Data rates greater
     High Speed
                                                                 than 100 MBit/s
       Vehicular



        Vehicular

                    GSM
      Pedestrian
                                    UMTS             WLAN/         ??
                            GPRS                      HL2
           Indoor           EDGE
            Fixed
                        0.01        0.1         1          10       100    1000
                                          Data Rate (Mbit/s)


Department for Telecommunications                                                    109
  How to increase the Data Rate?


         More Bandwidth ?

              April 2000: UMTS license auction in the UK: 5 licenses à
               10MHz pairs are auctioned off for 40.109 Euro

              August 2000: UMTS license auction in Germany: 6 licenses à
               10MHz pairs are auctioned off for 50.109 Euro, almost 900
               million Euro per MHz pair



         Bandwidth is limited and expensive




Department for Telecommunications                                           110
  How to increase the Data Rate?

         Higher bandwidth efficiency by using higher-order constellation
          diagrams

         But capacity cannot be larger than Shannon Limit

                                                              20
                    Bandwidth efficiency R/B [(bit/s) / Hz]



                                                                    Shannon limit for AWGN capacity
                                                              10
                                                                                               64-QAM
                                                                5                 16-QAM
                                                                                                        32-PSK
                                                                                              16-PSK
                                                                                      8-PSK
                                                                2
                                                                               QPSK

                                                                1
                                                                                      (p b 10 -5 )
                                                                                         =
                                                              0.5
                                                                    5     10          15        20       25      30
                                                                                  E b/ N0 in dB



Department for Telecommunications                                                                                     111
  MIMO - Multiple Input Multiple Output

                            s1                             r1

                            s2                             r2
        Transmitter                                               Receiver


                            sn                             rm

                        n Transmit antennas, m Receive antennas




Department for Telecommunications                                            112
  MIMO - Multiple Input Multiple Output

                            s1                             r1

                            s2                             r2
        Transmitter                                               Receiver


                            sn                             rm

                        n Transmit antennas, m Receive antennas


                   In flat fading channels:




Department for Telecommunications                                            113
  MIMO - Multiple Input Multiple Output

                            s1                            r1

                            s2                            r2
         Transmitter                                            Receiver


                            sn                             rm

         For flat fading channels, the MIMO radio channel is written as the
          Channel matrix




         The transmission is written as a matrix multiplication:



Department for Telecommunications                                              114
                      Cellular Environment




Department for Telecommunications            115
  OFDM Application Systems


         OFDM-based
                                    Single Cell       Cellular Networks
          Systems

                                       DAB         Multi-Frequency Network
                                                             (MFN)
         Broadcasting                 DVB-T
                                                   Single Frequency Network
                                       DRM                   (SFN)
                                                       MFN: HiperLAN/2
         Interactive                 HiperLAN/2
       Communication                IEEE 802.11a           TUHH



              TUHH has firstly proposed such a system!



Department for Telecommunications                                             116
  Conventional Cellular OFDMA Network

                                                               Modulation               Band
                                                                 block                  filter

                                                                    DL             UL

                                         BS




                                                   Frequency
              BS
                                                                            Time


                                               •        Different cells use different
                                          BS            resources
                                               •        Cells have to be separated by
                                    MT
                                                        filters
                                               •        Independent operation of cells



Department for Telecommunications                                                            117
   Conventional vs. Self-Organized Management


                 Conventional                Resources             SO-RRM
Resource are clustered with reuse factor 7          Resource are shared with reuse factor 1


                6       7       5

            2       4       3        6

        7       5       1       2        4

            3       6       7        5

                2       4       3


                Low flexibility                                   High flexibility
  Optimal for uniform user distribution                  Suitable for any user distribution


 Department for Telecommunications                                                       118
  OFDM-based Cellular Single Frequency Network
                                                                                Band
                                                                                filter
                                                               DL          UL


                                         BS




                                                   Frequency
              BS                                                    Time

                                               •   Different cells can access all
                                                   resources
                                               •   Cells need not to be separated
                                          BS
                                               •   Terminals have to be
                                                   synchronized
                                    MT         •   Propagation delay is
                                                   compensated by OFDM



Department for Telecommunications                                                        119
  Synchronisation Concept for a Selforganised SFN

   Main Task: Decentralised, self-organised synchronisation of the
   cellular network

                          Downlink                    Uplink
                                                                             Two dedicated Sync signals
    Resources
                MT SYNC
                MT SYNC




                                                                   BS SYNC
                           DL Data                 UL Data
                                                                             - preamble in downlink for
                                                                               Mobile Terminal synchronization
                                     Time
                                                                             - postamble in uplink for
                                                                               Base Station synchronization




    All MTs synchronize to the BS           All BS synchronize to MTs
             in “their” cell                     in “adjacent” cells




Department for Telecommunications                                                                             120
  Sync Signal Structure




                                                                       One pair of pilot subcarriers
                                                                       used by all MTs of single BS



                                               “0”           “0”      “0”             “0”

                                                                                                     Frequency
                                     Separate subsets of pilot     Distinct pairs of pilot subcarriers
                                    subcarriers by guard bands     used by MTs of different BS




Department for Telecommunications                                                                            121
  Sync Signal Properties

      Sync signals are transmitted with maximum transmit power
      Sync signals have much higher SNR compared to the data
       transmission




                               NC  2048  Gain  30dB


Department for Telecommunications                                 122
  Estimation Procedure

                            Evaluate each detected pair ofsubcarriers
                          separately to obtain a time and frequency offset
                                       estimate for each cell
           RX Pwr




                                                                             RX Pwr
                                                        FFT #1                                                   FFT #2

                                                            Subcarriers                                              Subcarriers



                         Time offset estimate                                             Frequency offset estimate

                                      Im                                                          Im           Phase difference
                                                         R1 (l )                                          between same subcarrier
                                                                                                          of consecutive symbols =
                                                                                                              frequency offset
                                                   Re                                                      Re
                                               I
                     R1 (l  1)             Phase difference between                  R2 (l )
                                           adjacent subcarriers of same
                                               symbol = time offset

                        TS                                                                   f
                tl        tan 1{R1 (l  1).R1 (l )*}                                fl       tan 1{R1 (l )* .R2 (l )}
                        2                                                                   2
                    - OFDM symbol duration              - OFDM subcarrierspacing




Department for Telecommunications                                                                                                    123
  System Overview

      All BSs and MTs share the whole resources and can access them at
       any time
      No BS controller – instead: radio resource management (RRM) using a
       self-organized dynamic channel allocation (SO-DCA)
      Each BS can observe MTs located in its own cell and in adjacent cells
      Challenges: Interference from adjacent cells




Department for Telecommunications                                              124
  Short Range Scenario

         Cell size: 30m (office) or 100m (outdoor)
         Low mobility (less than 10km/h)
         Proposal : OFDM-FDMA based
         Synchronization in time and frequency




                                               BS     Frequency

                                                                  DL   UL




                                    BS




                                                BS                      Time


                                          MT




Department for Telecommunications                                              125
  Wide Area Scenario

         Cell size: 400m till 2km
         High mobility (till 250km/h)
         Proposal : OFDM-TDMA based
         Synchronization only in time




                                              BS
                                                    Frequency
                                                                DL   UL




                                    BS




                                                                          Time
                                               BS



                                         MT




Department for Telecommunications                                         126
  Simulation Parameters

  Parameters                                  Value

  System bandwidth                        B = 100 MHz
  Number of subcarriers                     N = 2048
  Subcarrier spacing                  F = B/N = 48.8 KHz
  Symbol duration                   Ts = 1/F = N/B = 20.48 s
  Guard interval length                      NG = 80
  Guard interval duration                     0.8 s
  Number of cells                           NBS = 19
  Cell radius                               R = 100 m
  Path-loss coefficient                        2.5
  Shadowing deviation                         4 dB
  SNR at propagation distance R               20 dB
  Average number of MTs per cell                7
  Channel model                              802.11n




Department for Telecommunications                                127
  Network Model
        Cellular network with identical cell radius
        MTs are uniformly located
        Quantitative results is counted only in central cell




Department for Telecommunications                               128
  Frequency Sync in a Cell
     Inside a cell, after 20 frames, frequency synchronization between all
      MTs and their BS is correctly achieved
     Frequency offset is about 0.5% of the subcarrier spacing

                                                             0.3
             Relative freq. offset to BS [ f/Sub.spacing]



                                                                        Convergence of MTs to their BS frequency
                                                             0.2


                                                             0.1


                                                               0


                                                             -0.1


                                                             -0.2


                                                                    0   20            40           60              80
                                                                                    Frame
Department for Telecommunications                                                                                       129
  Time Sync in a Cell
     Inside a cell, after 10 frames, time synchronization between all MTs and
      BS is achieved
     Time offset is about 8% of the guard interval
               Relative time offset to BS [ t/Symbol Duration]
                                                                  0.5

                                                                              Convergence of MTs to their BS timing




                                                                    0




                                                                  -0.5
                                                                         0   20          40           60              80
                                                                                       Frame
Department for Telecommunications                                                                                          130
  Frequency Sync in Cellular Network
     Frequency of all BSs within the network converge after 20 frames
     Frequency offset is about 1% of the subcarrier spacing
             Relative freq offset to sub. spacing [  f/  F]
                                                                0.3
                                                                           Convergence of other BSs to ref. BS frequency
                                                                0.2


                                                                0.1


                                                                  0


                                                                -0.1


                                                                -0.2



                                                                       0        20           40            60              80
                                                                                           Frame

Department for Telecommunications                                                                                               131
  Time Sync in Cellular Network
     Timing of all BSs within the network converge after 20 frames
     Time offset is about 10% of the guard interval
             Relative time offset to symbol duration [  t/Ts]

                                                                 0.5

                                                                            Convergence of other BSs to ref. BS timing




                                                                   0




                                                                 -0.5
                                                                        0     20           40            60              80
                                                                                         Frame

Department for Telecommunications                                                                                             132
  Result Animation




Department for Telecommunications   133
  Result Animation




Department for Telecommunications   134
  Data Transmission
                                         OFDM im Rayleigh Kanal




                       3 dB @        8 dB @       13 dB @         18 dB @      22 dB @
                     0.5Bit/s/Hz    1Bit/s/Hz     2 Bit/s/Hz      3 Bit/s/Hz   4 Bit/s/Hz


     If the target BER = 10-5, the highest PHY mode ½ 256-QAM can be used
      with SNR > 22dB

Department for Telecommunications                                                           135
  Frequency Offset Accuracy
     Frequency offset accuracy of 5% is sufficient for a data
      transmission (SINR > 22 dB with frequency offset of 5%)




Department for Telecommunications                                136
    Synchronisation Concept - Conclusion
    Synchronization concept is proposed, using Sync signals in preamble
     and postamble.

    A Sync signal is transmitted with the maximum power by two of
     subcarriers and three OFDM symbols.

    Time and frequency synchronization can be carried out simultaneously at
     the receiver.

    Simulation results shows synchronization in OFDM-based cellular
     networks is feasible




Department for Telecommunications                                          137
  Self-organized Radio Resource Management
     Characteristic system features:

        Resource allocation is done independently by each base station,
         without any information exchange.

        SINR is calculated from estimated signal power and co-channel
         interference.

        Resources with the highest SINR values are allocated.

        PHY mode are selected based on the SINR values.




Department for Telecommunications                                          138
  Interference Measurement for Resource Allocation


                                                                                    BS 2
                                                                                   MT3
                                                                                              MT5




                                                                 RX Power
                                                                                     MT4




                                                          BS
                                                                                   Resource


                                    BS 1                                                      BS
                             MT 1
                                           MT 2
                                                          BS 2
                          BS 1                    MT 3
                                                                            MT 5
                             MT2
        RX Power




                   MT1                             MT 4
                                     BS                                                       BS


                                                          BS
                         Resource




Department for Telecommunications                                                                   139
  Interference Measurement for Resource Allocation
        Interference are measured continuously and averaged over time.
        DL interference values are measured at MT.
         UL interference values are measured at BS.
        Co-channel interference is taken as the maximum value from
         UL/DL measurements.


                         k                DL             UL               k
                             NR                                     NR




         ÎDL                 1                                      1                 ÎUL
         MT measurements                         TMAC               BS measurements


                                                                              Frame
                                    ...    l-1    l     l+1   l+2   ...


Department for Telecommunications                                                           140
  Signal Power Measurements

     Signal power is measured exclusively in a reserved “Signal Measurement
      Slot”.
     An “SM-slot” is only for new users, containing all subcarriers in one
      OFDM symbol.
     The estimated value is the average received power over all subcarriers.




  f                                 MAC frame


                      Downlink                  Uplink



                                             Signal Measurement Slot t

Department for Telecommunications                                          141
  SINR Calculation and Resource Ranking

   1.    Balance between UL and DL       I k  max( I k ,UL , I k , DL )

                                                       N 1
                                             1
   2.    Signal power estimation         S 
                                             N
                                                       S
                                                       k 0
                                                                k




                                                    S
   3.    SINR calculation                SINRk 
                                                 Ik  N

   4.    Ranking of resources based on
         their SINR values



Department for Telecommunications                                          142
  Cellular Scenario
    19 cells, 100 users, uniform user distribution inside each cell
    OFDM-FDMA, 128 subcarriers, synchronized in time and frequency
    Hotspot fraction: the probability that a user is located inside central cell



          5 users in central cell                     Received power at the central AP
                                           60
                                                Signal
                                                Interference
                                           50

                                           40
                                    [dB]


                                           30

                                           20

                                           10

                                            0
                                                                Subcarrier



Department for Telecommunications                                                        143
  Hotspot Snapshots
                                25 users in central cell                         Received power at the central AP
                                                                      60
                                                                           Signal
                                                                      50   Interference


                                                                      40

Hotspot fraction:




                                                               [dB]
                                                                      30

        30%                                                           20

                                                                      10

                                                                      0
                                                                                             Subcarrier
                                    63 users in central cell                       Received power at the central AP
                                                                      60
                                                                           Signal
                                                                           Interference
                                                                      50


Hotspot fraction:                                                     40


                                                               [dB]   30
        60%
                                                                      20

                                                                      10

                                                                       0
                                                                                             Subcarrier



Department for Telecommunications                                                                                     144
  Hotspot Demonstration

      Two processes are included:
           User concentration towards the central cell
           User scatteration from the central cell




Department for Telecommunications                         145
  Dynamic Channel Allocation




                                         BS
                                                  Enable dynamic channel
                                                   allocation (DCA) technique
                                                  Measure co-channel
                                                   interference between
                                                   adjacent cells
                                                  Always assign resources
              BS                                   with minimum interference
                                                  Reuse all resources
                                                   dynamically in adjacent
                                                   cells
                                          BS


                                    MT




Department for Telecommunications                                            146
      Digital and Analog Hardware
         Aspects in OFDM Systems




Department for Telecommunications   147
  Digital and Analog Hardware Aspects in OFDM Systems

          Introduction Analog / Digital
          Analog Hardware Aspects
          Digital Hardware Aspects
          OFDM Demonstrator
          Performance Estimations




Department for Telecommunications                       148
  Introduction Analog / Digital




                        Analog                       Digital
              - IQ-Modulator, -Demodulator   - FPGA / ASIC
              - Amplifiers                   - D/A-, A/D-Converter
              - Antennas

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                                    Summary

     • Efficient mitigation of multipath propagation

     • Excellent performance in coded systems

     • Link adaptation techniques in OFDM makes the system
       very flexible and powerful

     • Choice of multiple access scheme allows adaptation to
       channel and user requirements

     • New aspect: cellular environment




Department for Telecommunications                              150
                Aspects of Future Systems

     • Modulation Techniques:         Coherent vs. Incoherent
                                      Adaptive Modulation

     • Channel Coding:                Coded Modulation
                                      Turbo Codes

     • Network Aspects:               Single Frequency Networks
                                      Ad-Hoc Networks

     • Multiple Access:               TDMA, FDMA, CDMA

     • Dynamic „Bandwidth“:           Dynamic Channel
                                      Dynamic Packet Allocation

     • Diversity


                                    Flexibility


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posted:9/3/2011
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