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					           Overview of
Direct Sequence Spread Spectrum

 Code Division Multiple Access

      Prof. Dr. Essam Sourour


                                  1
        Overview of Presentation

•   Spread Spectrum techniques
•   Direct Sequence Spread Spectrum
•   DS-SS CDMA
•   CDMA in Cellular Systems
•   CDMA versus TDMA
•   Spreading Codes
•   Multipath fading
•   Rake receiver


                                      2
    Overview of Presentation (cont.)

•   Voice Activity and variable rate Vocoder
•   Interference cancellation
•   Near Far Problem & Power Control
•   Soft handoff
•   Cell breathing
•   Multi-Carrier CDMA
•   Advantages and disadvantages
•   Summary


                                               3
       Spread Spectrum Systems
• Invented for military communications
• Two main types:
   – Direct Sequence: Wide band all the time
   – Frequency Hopping: Narrow band hopping signal
          frequency                     frequency


                                        f10
                                         f9
                                         f8
                                         f7
                                         f6
           fc
                                         f5
                                         f4
                                         f3
                                         f2
                                         f1
                                 time                           time
                Ts     Direct                 Ts    Frequency
                      Sequence                       Fopping



                                                                       4
      Spread Spectrum Systems
• Frequency Hopping
   – Narrow band signal hopping pseudo-randomly
   – Mitigates narrow band jamming
   – Different users use different hopping patterns
• Direct Sequence
   – Spreading narrow band signal over wide
     bandwidth using a high rate spreading code
   – Receiver de-spreads signal and spreads
     jammer signal
   – Most jamming signal power filtered out
   – Different users use different spreading codes
     having small cross-correlations

                                                      5
                  DS-SS System

• At transmitter: low rate data is spread by a high chip
  rate pseudo-noise (PN) code
• Jamming or interference present
• At receiver: data de-spread by a synchronized PN code
• Processing Gain=Bs/Bd = Tb/Tc = # of code chips per bit
  data

 PN Code
                 data                                                           data
                                                                       Filter
    Tb     Tc
                                          Jammer
                        PN code Carrier            Carrier   PN code
                                                                       LPF
                                                    Jammer

                           f
                                                     f                             f
                  Bd                      Bs

                                                                                       6
                                  DS-SS CDMA

• Unique code per user (typically, 64 or 128 chips)
• Synchronized Walsh codes = Zero cross-correlation
• Example: 2 synchronized users
    Tb
                           T b   i j
     C i t C j t dt   0
    0                            i j

         d1(t)                                                                     Z1(t)              d1(t)
                                                                                            . dt
                                    d1(t)
                                                                                           Tb
         C1(t)
                                             C1(t)     Carrier   Carrier   C1(t)

         d2(t)
                                     d2(t)

         C2(t)
                                               C2(t)   Carrier

                 Tb        Tc



                                                                                                              7
           Principle of CDMA
• Assume codes take values ±1
• Received signal = d1(t) C1(t) + d2(t) C2(t)
• Receiver 1 multiplies by own code C1(t)
• Receiver 1 gets:
     Z1(t)= C1(t) ( d1(t) C1(t) + d2(t) C2(t) )
     Z1(t)= d1(t) + d2(t) C1(t) C2(t)
• After integration over code period, output is:
     User 1 Output = Tb d1(t) + Zero
• Same applies for more users

                                                   8
          CDMA Quick Exercise

•   For the previous signal:
•   Multiply: s1(t) = d1(t)  C1(t)
•   Multiply: s2(t) = d2(t)  C2(t)
•   Add Z(t) = s1(t) + s2(t) , chip by chip
•   Multiply the sum Z(t)  C1(t)
•   Accumulate the first 8 chips and the
    second 8 chips



                                              9
           CDMA Quick Exercise:

    d1       1    1    1    1    1    1    1    1    -1   -1   -1   -1   -1   -1   -1   -1

   C1

 d1  C1

    d2       -1   -1   -1   -1   -1   -1   -1   -1   1    1    1    1    1    1    1    1

   C2

 d2  C2

d1C1+ d2C2

C1(d1C1+
 d2C2)

 Add 8’s


                                                                                             10
               Orthogonal Codes
• Codes of previous example are orthogonal Walsh
  codes
• Orthogonal codes have cross-correlation=0
• Orthogonality preserved only if the codes are
  synchronized, i.e., start and end together
• If one code is delayed compared to the other,
  orthogonalty is lost
• System still works, with some interference among
  users
                                Tb
                                                      T b   i j
         cross  correlation   C i t C j t dt  
                               0                      0     i j




                                                                    11
           Non-Orthogonal Users




• Receiver 1 gets:
      Z1(t)= C1(t) ( d1(t) C1(t) + d2(t-t) C2(t-t) )
      Z1(t)= d1(t) + d2(t-t) C1(t) C2(t-t)
• After integration over code period, output is:
      User 1 Output = Tb d1(t) + Interference
• More non-orthogonal users = more interference

                                                       12
         CDMA Quick Exercise 2:

    d1       1   1   1   1    1    1    1    1    -1   -1   -1   -1   -1   -1   -1   -1

   C1

 d1  C1

Delayed d2   1   1   1   -1   -1   -1   -1   -1   -1   -1   -1   1    1    1    1    1

Delayed C2

 Delayed
 d2  C2

  Get Z1

 C1  Z1

 Add 8’s


                                                                                          13
        Non-orthogonal CDMA

• User orthogonal only in a downlink without
  large multipath spread
• With multipath, delayed paths cause
  interference to each other
• In an uplink, users are not synchronized,
  they cause interference to each other
• CDMA capacity is a soft capacity, number of
  users limited by allowed interference level
• You can always squeeze in one more person


                                                14
Orthogonal Versus Non-Orthogonal


           Orthogonal if orthogonal
               codes are used

                                                                         Orthogonal
                                                                          Signals




        Uplink

                                      Building 1



                                                   Path 1

                                                                      Path 2

                     Downlink
                                                     Non-Orthogonal
                                                        Signals




                                                                                      15
                 Spreading Detailed
                                                               Code
  Example: 8 chips spreading code                          I
                                                                                time

                                                                       Filter      D/A
            I                   Encoded bits
                                               Modulator                                 RF
                                                                       Filter      D/A
                                                           Q
                                      time                             Pulse
                                                               Code   Shaping
  Spreading      Tc
    code
                       time
  After
Spreading
                                                           time



After D/A
                                                               time




                                                                                              16
             De-Spreading Details
                                                               Code
                                                   time

                         I
                             A/D          Filter                                          Encoded
                                                                        code   Demodulat     bits
                    RF
                                                                        
                                                                                  or
                             A/D          Filter
                                                                        code
                         Q
 After A/D                             Pulse              Tc
  & Filter                            Shaping                  Code



                                                                 time




  After
Sampling
                                                                time

  After
  Sum

                                   time



                                                                                                     17
                  Complex Spreading Codes
                            Complex
                           Multiplication
                                            Filter      D/A
                            I
Encoded bits
               Modulator                                               RF
                           Q                Filter      D/A

                          CI CQ      Pulse
                       Complex Code Shaping

                                                      Pulse          Complex
                                                     Shaping        Multiplication
                                            A/D       Filter
                                                                             I       S               Encoded bits
                                                                                           De-
                                RF
                                                                                         Modulator
                                            A/D       Filter                 Q       S

                                                               Tc     CI -CQ
                                                                    Complex Conjugate
                                                                         of Code
         • CI and CQ are two different orthogonal codes
         • Chips of CI and CQ take values ±1/SQRT(2)
         • Despreading with complex conjugate of the code

                                                                                                            18
                  Cellular DS-SS CDMA

• In a CDMA Cellular system:
        – Code(s) per user (assigned per call) c(t)
        – Code per base station, p(t)

                                                                                      di(t)
di(t)
                                                                        Tb
                                                                              dt

                                           carrier   p(t)       Ci(t)
          Ci(t)
                  S


                          p(t)   carrier                                              dj(t)
                                                                          dt
dj(t)
                                                                        Tb


          Cj(t)                            carrier   p(t)      Cj(t)

                      Base Station                          Cell Phones


                                                                                              19
        DS-SS CDMA Frequency Reuse

                                          Frequency re-use
                                              factor =1
            B
        G       C
            A
        F       D
    B       E
G       C       B
    A       G       C
F       D       A
    E       F       D
                E
TDMA Systems, frequency
re-use, example 7

                          CDMA Systems, frequency
                          re-use = 1


                                                             20
         CDMA versus TDMA

• TDMA: Users separated by time slots
• CDMA: Users separated by codes
            user 1    user 2   user 3



                                   Time
                     TDMA


                                   Time

                                   Time

                                   Time
                     CDMA
                                          21
            CDMA versus TDMA

• TDMA:
  –   Users separated by time slot and frequency
  –   Burst transmission & reception
  –   Narrower bandwidth
  –   Frequency reuse > 1 (4 or 7 usually)
• CDMA:
  –   Users separated by code and frequency
  –   Continuous transmission & reception
  –   Wider bandwidth
  –   Frequency reuse = 1

                                                   22
               Spreading Codes

• Codes with low or zero cross-correlation required to
  reduce multi-user interference
• WCDMA & CDMA-2000: Walsh code assigned to
  user/call
• Synchronous Walsh codes  zero cross-correlation
   – Synchronous Forward link without multipath fading
      no intra-cell multi-user interference
• Asynchronous Walsh codes  non-zero cross-
  correlation
   – Forward link inter-cell interference
   – Multipath in Forward link intra-cell interference
   – Reverse link  Inter/Intra-cell interference



                                                          23
       Spreading Codes (Cont.)

• Several types of SS codes are used in both
  3G CDMA Systems:
• Walsh codes: assigned per call in CDMA-
  2000 & WCDMA
• Maximal length codes (m-sequences): used
  in pilot code for CDMA-2000
• Gold codes: used as scrambling codes in
  WCDMA
• Kasami codes: Optional additional
  scrambling code in WCDMA

                                               24
                 Walsh Codes Generation

Length N = 2n              +1   +1
                           +1   -1

                      
                           +1   +1   +1   +1
             Hn H
H 2n    
         
                     n 
                       
                           +1   -1   +1   -1
         
         
         
             H n H   n
                       
                       
                           +1   +1   -1   -1
                           +1   -1   -1   +1

                           +1   +1   +1   +1   +1   +1   +1   +1
                
  H2  1 1   
             
                 
                 
                           +1   -1   +1   -1   +1   -1   +1   -1
       1 1  
             
                 
                 
                           +1   +1   -1   -1   +1   +1   -1   -1
                           +1   -1   -1   +1   +1   -1   -1   +1
                           +1   +1   +1   +1   -1   -1   -1   -1
                           +1   -1   +1   -1   -1   +1   -1   +1
                           +1   +1   -1   -1   -1   -1   +1   +1
                           +1   -1   -1   +1   -1   +1   +1   -1


                                                                   25
                   OVSF Codes
OVSF=Orthogonal Variable Spreading Factor
• Walsh codes of different lengths
• When code used, branching codes not used
• Shorter code for higher data rate, but lower
  processing gain                     11111111
                                   1111
                        11                    1 1 1 1 -1 -1 -1 -1
                                              1 1 -1 -1 1 1 -1 -1
                                  1 1 -1 -1
                                              1 1 -1 -1 -1 -1 1 1
                    1
                                              1 -1 1 -1 1 -1 1 -1
                                  1 -1 1 -1
                                              1 -1 1 -1 -1 1 -1 1
                        1 -1                  1 -1 -1 1 1 -1 -1 1
                                  1 -1 -1 1
                                              1 -1 -1 1 -1 1 1 -1

                               Set of Walsh   Set of Walsh
                                codes N=4      codes N=8

                                                                    26
           M-Sequence Generation

• Generating polynomials set feedback nodes
• Length N = 2n-1
• Longest sequence that can be generated with shift
  register of length n (maximal-length sequence)
• Generating polynomial g(x)
        g(x) = 1 + g1x + g2x2 +….. + gn-1xn-1 + xn

                                         0/1    0→1     +1/-1
       D         D        D          D
                                                1 →-1

g0=1        g1       g2       gn-1       gn=1




                                                                27
           M-Sequence properties
• Code period is N=2n-1
• If the PN code is represented in 0 and 1
   – There 2n-1 ones and 2n-1-1 zeros
   – Shift and add: any modulo 2 sum of the code with a circular
     shifted version of itself generates the same code but shifted
• If the PN code is represented in +1 and -1
   – The autocorrelation of the code is given by

                               N    t 0
                      C t   
                               1 otherwise




                                                                     28
Autocorrelation of m-sequence
                   35
                          Auto correlation of M-sequence of length 31 (n=5)
                   30


                   25


                   20
Auto Correlation




                   15


                   10


                    5


                    0


                   -5
                    -30   -20        -10          0          10         20    30
                                                  t


                                                                                   29
           Gold Codes Generation

•   Two preferred m-sequence polynomials added
•   Can generate N+2 codes of length N= 2n-1
                            
                            
                                          n 2 
                                          2     
                                                  
                                                         n 2 
                                                         2 
•   Low cross-correlations=  1,  1  2
                                          , 1 2
                                                
                                                  
                                                              

                            
                                                 
                                                  
•   Example: n=5, cross correlation= {-1,-9,7}
                   D        D        D    D

               1       g1       g2                  1     0/1
                                                                   0→ 1   -1/+1
                                                                   1→-1


                   D        D        D    D

               1       g1       g2                  1




                                                                                  30
                 Codes Examples:
m-sequence             D          D         D           D   D     D
with n=6
                                          g(x)=1+x+x6




     D       D     D         D        D


                   g(x)=1+x2+x5




     D       D     D         D        D
                                                        Gold sequence
                                                        with n=5

                 g(x)=1+x+x2+x4+x5



                                                                        31
       Gold codes example (n=7)
• Gold code example with n=7:
             g1  x   1  x 3  x 7
             g 2 x   1 x  x 2  x 3  x 7

• This generates 129 Gold codes of period 127




                                                 32
               Multipath Fading
•   Multipath causes Rayleigh fading
•   When Delay Spread > Tc  resolvable multipath
•   WCDMA  small Tc  more resolvable paths
•   Each path adds Interference & also Diversity
•   Example WCDMA: Tc= 0.26 ms
•   Delay spread 2.5 ms  2.5/0.26  9 paths




                                 chip

                                                     33
                    Rake Receiver
•   Multipath diversity = multipath is advantageous
•   One finger (correlator) per path
•   Each finger synchronized to one path
•   Finger outputs combined (MRC)
         t
    g2
    g1
                                                   . dt
                                                  Tb
             chip
                               p(t)     C1(t)                 
                                                             g1   d(t)


                     Carrier
                                                   . dt
                                                  Tb


                               p(t-t)   C1(t-t)               
                                                             g2

                                                                         34
         Rake Receiver Fingers

• One rake finger for each channel path
• Codes on each finger delayed to desired path
• Gain in each finger is complex conjugate of
  path gain (Maximal Ratio Combining)
• Additional subsystems needed:
  – Delay tracking: to track delay changes
  – Channel gain estimation: to find paths gains {gi}
  – Searcher: to find new stronger paths (and new
                                   t
    base stations)            g2
                              g1
                                                                            . dt
                                                                           Tb
                                       chip
                                                        p(t)     C1(t)                 
                                                                                      g1   d(t)


                                              Carrier
                                                                            . dt
                                                                           Tb


                                                        p(t-t)   C1(t-t)               
                                                                                      g2

                                                                                                  35
   Multipath Interference on Rake Rx.

• Assume 3 CDMA users
• User A Channel with 2 paths User C                  User B
• Phone A sets 2 Rake fingers,
  one for each path
• Each path carries the signal of
  the 3 users
• Within a path, user codes are  Building 1



  orthogonal                      Path 1

                                                  Path 2
• However, path 1 causes
  interference to finger 2, and
  vice versa                             User A


                                                           36
       Performance of Rake receiver
• Assume K CDMA users, with random codes Ck (may be complex)
• Code is N chips per bit (or symbols), random ±1, with chip time is Tc
• Channel of user k is multipath with impulse response
                                     L 1

                                    g
                                     l 0
                                             l ,k    t  l T c 
• The L gains gl,k are all independent complex Gaussian with zero mean
                                   L 1
  and variance v l2
                                    v  2

                                     l 0
                                            l     1
• Assume BPSK modulation (bk=±1). We find the BER of user k=0
                                        desiredsignal
• Received signal is given by
                                    L 1
                           r t    g l ,0 b 0 t  lT c  C 0 t  lT c 
                                    l 0
                                    K 1 L 1
                                    g l ,k b k t  lT c  C k t  lT c   n t 
                                     k 1 l  0
                                                                                     AWGN
                                                      multiple access interference
                                                                                        37
        Performance of Rake receiver
• Rake receiver includes L fingers, each is performing:
    –   Multiples the received signal by delayed versions of the code of user 0
    –   Integrates over the bit duration (or symbol if not BPSK)
    –   Multiplies the sum by the conjugate of the path gain
    –   Results of all fingers are added (call it z)
    –   The output z is compared to zero (or sent to demapper if not BPSK)

                L 1        mT c  NT c

          z   g m ,0
                  *
                                         r t C 0 t  mT c  dt
                                                  *

                m 0           mT c

              P1             P2                           P3                  P4
               desired   self interference       multiple access interference    AWGN

• Interference terms will be approximated as Gaussian.
• To find the BER we need to find the 4 terms above



                                                                                        38
      Illustration of Rake receiver
• K users, L paths




                                      39
         Performance of Rake receiver
                                 L 1                                                    L 1
                       P1   g m ,0 b0 N T c  b0 N T c                                 g
                                                      2                                                2
                                                                                                m ,0
                                 m 0                                                    m 0

         L 1             L 1            mTc  NTc

 P2   g m ,0  g l ,0
          *
                                                         b0 t  lT c C 0 t  lT c C 0 t  mT c  dt
                                                                                         *

        m 0              l 0                  mTc
                          l m
• Assume the code are random ±1 and random bits ±1
                                                            L 1          L 1
                                           P2  T c         g g
                                                            m 0
                                                                   *
                                                                   m ,0
                                                                          l 0
                                                                                 l ,0   Al
                                                                          l m
• Al is a random variable which is the sum of N random variables ±1
• All random variable are independent. P2 is zero mean with variance:
                       L 1              L 1                                                              L 1
var  P2   T c       g                 var  g  var  A                     var  P2   T c N       g
                   2                 2                                                             2                     2
                              m ,0                        l ,0            l                                       m ,0
                       m 0              l 0                                                              m 0
                                         l m


                                                                                                                             40
      Performance of Rake receiver
     L 1        L 1          K 1 mTc  NTc
P3   g m ,0  g l ,k
         *
                                                         bk t  lT c C k t  lT c C 0 t  mT c  dt
                                                                                          *

     m 0        l 0          k 1           mTc
          L 1          L 1          K 1
    Tc    g g  B
          m 0
                 *
                 m ,0
                        l 0
                               l ,k
                                       k 1
                                                    l ,k



• Bl,k are independent random variables which are sums of N random
  variables ±1
• P3 is modeled as Gaussian. The mean is zero. Variance is:
                                                  L 1                 L 1           K 1
                   var  P3   T c               g                    var  g   var  B 
                                              2                    2
                                                            m ,0               l ,k          l ,k
                                                  m 0                 l 0           k 1
                                                                       L 1
                                       T c N  K  1  g m ,0
                                              2                               2

                                                                       m 0




                                                                                                             41
      Performance of Rake receiver
• Finally P4 is the AWGN
                                  L 1       mT c  NT c

                          P4   g m ,0
                                   *
                                                          n t C 0 t  mT c  dt
                                                                   *

                                  m 0            mT c

                                  L 1       NT c

                                  g m ,0
                                      *
                                               n t  dt
                                  m 0        0



• Noise n(t) is AWGN with two-sided power spectral density No/2
                              L 1                  NTc         
                 var  P4    var  g m ,0  var   n t  dt 
                                        *
                                                                
                              m 0                  0           
                                                         L 1
                                     N T c N o  g m ,0
                                                                  2

                                                         m 0
                L 1

                      g m ,0  Gaussian  0, N T c N o T c2 N T c2 N  K  1 
                            2
 z  b0 N T c
                m 0
                L 1

                      g m ,0  Gaussian  0, N T c N o T c2 N K              
                            2
 z  b0 N T c
                m 0
                                                                                      42
       Performance of Rake receiver
• The complete output z is:
                 L 1
                                                                          L 1                     L 1                         L 1
                                                                                                                                          2
                                                                                                        g m ,0 T c N  K  1  g m ,0 
                                 2                                                       2                         2
  z  b0 N T c          g m ,0        Gaussian  0, N T c N o                    g m ,0 T c N
                                                                                              2                        2

                 m 0                                                     m 0                     m 0                          m 0     
                 L 1
                                                                                            L 1
                                                                                                               
                                     Gaussian  0,  N T c N o T c2 N K                    g
                                 2                                                                         2
  z  b0 N T c          g m ,0                                                                      m ,0       
                 m 0                                                                       m 0              
• With the normalized bits and normalized channel, the combined
  average energy per bit is Eb=NTc and the Eb/No =b= NTc/No
• Overall SNR is given by:
                                                                       2
                                           L 1
                                                                   
                                            g
                                                               2
                                  N Tc                 m ,0       
      SNR                                 m 0                                                                  1          2
                                                               L 1                                     m   1         g
                    N T c N o T c N K   g m ,0                                                              b  K N  m ,0
                                                                              2
                                             2
                                                                                                             
                                                               m 0
                                                                                                                   1    
                        1                      L 1                        L 1                       m   1        vm
                                                 g                                                         b  K N 
                                                                   2
        total     1       
                    b  K N                   m 0
                                                          m ,0
                                                                             m 0
                                                                                     m



                                                                                                                                         43
      Performance of Rake receiver
• To get a closed form we assume equal power for each path
                                                  1     1
                         m                          
                                            b1  K N  L

• Similar to MRC, the pdf of the total SNR is given by

                                              L 1 e  
                          p total       L
                                              L  1!

• The BER is given by
                                  
                         Pb   Q
                                  0
                                             
                                          2 p  total    d 

                                                  L 1  m   1  
                                                                         m
                              1  
                                           L L 1

                            
                              2 
                                             0  m   2 
                                              m                  

                    1                          is given above


                                                                             44
       0
                           BER for BPSK with Rake receiver
      10
                                                                       numPaths=1
                                                                       numPaths=2
                       10 CDMA users, Code length 64                   numPaths=4
                                                                       numPaths=8

       -1
      10
BER




       -2
      10




       -3
      10




       -4
      10
           0   2   4   6         8       10       12         14   16    18      20
                                        b dB




                                                                                     45
    Voice Activity - Variable Vocoder
• CDMA capacity is multi-user interference
  limited
• Ideal: No voice  stop transmission  reduce
  interference  increase capacity
• Practical: No voice  lower Vocoder rate &                 Voice active ,higher data rate
  lower transmission power
• Energy per bit is same
• CDMA-2000: Vocoder rates: 1.5, 2.7, 4.8 & 9.6         Tb
                                                                 No Voice ,lower power &
  kbps or 1.5, 2.7, 4.8 &14.4 kbps                                   lower data rate

• WCDMA uses Adaptive Multirate (AMR)
                                                                Tb
  Vocoder
• AMR rates: 12.2, 10.2, 7.95, 7.4, 6.7, 5.9, 5.15 or
  4.75 kbps



                                                                                        46
         Interference Cancellation
• Base Station:
   – Knows all assigned PN codes of all users
   – One receiver synchronized to each user
   – Performs channel estimation for all users
   – Demodulates all users signals
   – Has all information to calculate interference
     from one user on an other
• Intra-cell interference cancellation possible
• Reduces Near-Far problem
• Implementation is complex



                                                     47
   Interference Cancellation (Cont.)
• Parallel IC (for user 1)
                                                          +                                 +
                                                                  -                               -

    received
     signal    Conventional                reconstruct                       reconstruct
               Demodulation      .        Interference                .     Interference              .
                                 .                                    .                               .
                for all users    .
                                         felt by user 1               .    felt by user 1             .


                     Conventional decision                First stage IC decision           Second stage IC decision
                      varaible for all users               varaible for all users             varaible for all users



• Sequential IC
                                                  +                                                       +
                                                              -                                                 -

 received      Conventional                                           Conventional
  signal                               Regenarte                                               Regenarte
               Demodulation                                           Demodulation
                                        Signal of                                            Signal of 2'nd
                for stongest                                             for 2'nd
                                     Strongest user                                          Strongest user
                    user                                              stongest user

                     Decision varaible for                                Decision varaible for 2'nd
                        stongest user                                          stongest user


                                                                                                                       48
            Near Far Problem

• All users transmit on the same frequency
• Signal from near users cause high
  interference to far users
• Reverse link power control is crucial
• Also saves phone battery




                                             49
     Reverse Link Power Control
• Goal: Equal received power from all users
   – Also saves mobile battery
• Open loop Power Control:
   – Slow, based on average power
   – Fixes Near-Far Problem                           po
                                                        we




                                                  e
                                              tim
                                                           r

   – Fixes Shadowing




                                          r
                                       we
   – Cannot fix Rayleigh fading




                                     po
                                                               tim
• Closed loop Power Control:                                         e


   – Fast, with instantaneous SNR
   – Fixes Rayleigh fading



                                                                         50
     Open Loop RL Power Control

• Mobile measures averaged forward link power
  – High  MS reduces its transmit power
  – Low  MS increases its transmit power
• Open Loop Example:
  Bt = BS transmit power (dB)    Mt = MS transmit power (dB)
  Br = BS received power (dB)    Mr = MS received power (dB)
  L = Path loss in dB
  C = correction factor (dB)

  Mr=Bt-L    &   Br = Mt-L
  Power Control Rule: Mt = -Mr + C
  Br = -Bt + C        Constant received power at BS


                                                               51
   Closed Loop RL Power Control

• Closed and open loop power control work
  simultaneously
• BS measures SNR for each MS
• BS sends PC bits to MS, up or down
• MS adjusts power immediately
• WCDMA: PC bits at 1500 Hz, step size is 1, 2
  or 3 dB
• IS-95 & CDMA-2000 1X: PC bits at 800 Hz,
  step size 0.25, 0.5 or 1 dB
• Effect of fading is partially removed

                                                 52
      Forward Link Power Control
• Goal: Reduce forward link interference
  – Reduce power to MS in favorable channels
  – Increase power to MS in unfavorable channels
• BS scales each user code in baseband
• All channels added for
  RF transmission



                                           User 2



                           User 1


                                                    53
              FL Power Control
• Slow FL Power Control:
   – Existed in IS-95 standard
   – MS measures FER and periodically reports to BS
   – BS adjusts power
   – Slow process
• Fast FL Closed loop Power Control:
   – In CDMA-2000 and WCDMA
   – MS measures SNR at a high rate
   – MS sends PC bits to BS
   – PC commands rate similar to RL power control
   – WCDMA step size is 0.5 and 1 dB

                                                      54
                  Soft Handoff
• Each BS transmits unmodulated pilot channel
• MS always measures all pilots strength
• MS requests handoff to BS with strong pilot
• Connection with old BS kept
• If granted, MS combines the two signals
• In the reverse link, MSC selects the best
  connection
• Old connection dropped if pilot strength gets <
  threshold
• IS-95, CDMA-2000 & WCDMA: maximum 6 BS in
  soft handoff. Typically 2 or 3.



                                                    55
                     Soft Handoff
MSC
                                    MSC   select




                                          combine



                  Step 1                           Step 2
 MSC     select
                              MSC




       combine




                   Step 3                     Step 4

 Make before brake (or even don’t brake)


                                                            56
              Cell Breathing
• Cell size controlled by      MSC     select


  pilot channel power
• Cell/sector overloaded?              combine


  reduce pilot channel
  power                                          Before
• Mobile stations at border
                                     MSC
  will handoff to neighbor
  Base stations and drop
  connection to loaded
  cell/sector
                                                   After



                                                           57
             Multi-Carrier CDMA

• CDMA-2000 allows 3X bandwidth Forward Link
• Multi-Carrier CDMA is used:
   – Bits encoded and interleaved normally
   – Encoded bits de-multiplexed to 3 branches
   – Each branch transmitted on a separate carrier
   – 3 receive RF chains at mobile station
   – MS receives separate CDMA carriers and de-
     multiplex bits
   – Leverage off current CDMA-2000 1X designs



                                                     58
                                Multi-Carrier CDMA
                                                             Walsh Complex
                                                             Code Pilot Code
After coding and
interleaving:                                           S1
                                                                               RF1   Carrier 1
b 1 , b 2 , b 3 , b4 , b 5 , b 6 , …

                                         De-multiplex
                                                        S2                           Carrier 2
  source bits     Coding and                                                   RF2
                  Interleaving                                                                   1.25 MHz

                                                        S3
                                                                               RF3   Carrier 3
                                                                   Complex
                                                                   Multiply                                           frequency
                                                                                            IS-95 and CDMA-2000 1X

                                       QPSK Symbols:
                                       S1=b1+jb2
                                       S2=b3+jb4                                                                       frequency
                                                                                         Multi-Carrier CDMA-2000 3X
                                       S3=b5+jb6



                                                                                                                            59
        Advantages of CDMA
•   Anti-interference and Jamming
•   Takes advantage of voice activity
•   No frequency cell planning
•   Soft capacity
•   Easy for variable data rate
•   Multipath diversity (Rake receiver)
•   Soft handoff (make before break)
•   Transmit diversity
•   Easier for packet data

                                          60
       Disadvantages of CDMA

• Sensitive to self interference
• Sensitive to power control
• Difficult handoff to other frequencies




                                           61
                   Summary

•   In CDMA user separation is by codes
•   Codes designed orthogonal
•   Codes: Walsh, m-sequences, Gold, Kasami
•   CDMA is cellular with frequency reuse = 1
•   Rake receiver benefits from multipath fading
•   Open & Closed loop Power Control necessary
•   Soft handoff possible in cellular CDMA




                                                   62

				
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