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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 2ft 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 1s 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 5s 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 πkft 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 πnkft 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 sinT f kf Gk ( f ) T T f kf 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 ImsT component“) of the baseband signal can be depicted in one 1011 1010 0010 0011 two-dimensional diagram 1001 1000 0000 0001 Constellation diagram ResT 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 pt 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 pt 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 pt 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 decDn ,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 2i / 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 decDn ,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 jP | 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 ntf 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 )( Hn1,k n1,k ) a Hn,k ˆ Rn,k Yeqn ,k ˆ Rn , k H n ,k ˆ n,k (1 )n1,k ( Hn,k Hn1,k ) H n1,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 )( Hn1,k n1,k ) a Hn,k -1.0 -0.1 0.1 1.0 -0.1 ˆ n,k (1 )n1,k ( Hn,k Hn1,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 Hn1,k n1,k Optimal values a=0.1, =0.008 n,k n1,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 bf 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 8.5s TS 23 .5s 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 ,L1 si ,0 si ,1 si ,L1 si ,0 si ,1 si ,L1 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 , L1 i , L1 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 ,L1 Di ,0 Di ,1 Di ,L1 Di ,0 Di ,1 Di ,L1 identical si ,0 si ,1 si ,L1 si ,0 si ,1 si ,L1 si ,0 si ,1 si ,L1 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 ,L1 Si , L 1 Di ,L1 Di ,0 IDFT Di ,L1 Di ,0 Di ,1 Mth period Di ,L1 Department for Telecommunications 92 Multiple Users Other users allocate a shifted, but also equidistant subset of subcarriers: Si(,0 ) m Si(,1 ) m ) Si(,m1 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(,m1 L M-times repetition m m Di(,0 ) Di(,1 ) Di(,m1 Di(,0 ) Di(,1 ) L ) m m Di(,m1 L ) m Di(,0 ) Di(,1 ) m Di(,m1 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(,m1 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 ,L1 (0) (0) Sn,L1 (0) Dn , L1 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,L1 ( M 1) ( 1) S n ,M 1 L ( 1) Dn,M1 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 Department for Telecommunications 149 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 Department for Telecommunications 151

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