Adaptive modulation schemes for frequency selective fading channels Mª Carmen Aguayo Torres, José Paris Angel, José Tomás Entrambasaguas Muñoz Universidad de Málaga Escuela Técnica Superior de Ingeniería de Telecomunicación

Universidad de Málaga 2 Conclusions AOFDM, ADFE, ATHP Contents Adaptive QAM (AQAM) OFDM, DFE and THP Introduction

Universidad de Málaga 3 Introduction Time spread –Frequency selectivity Multipath –Time selectivity or fading Time and frequency selective channel At an instant of time, distinct frequencies have different gains and phases At a single frequency: the channel change with time Mobile environment

Universidad de Málaga 4 Time selectivity Adaptive transmission techniques Simple way to face fading: use enough power or transmit slowly enough Inefficient Possibility: Modify the transmitted signal depending on the instantaneous channel conditions Any signal parameter can be modified: power, symbol period, modulation scheme...

Universidad de Málaga 5 Adaptive QAM High bit rates when channel is suitable Slow down the transmission when the channel gets worse Time selectivity Modify constellation size Power and symbol period are kept Adaptive modulation level Received signal time Used constellation: BPSK Used constellation: 16QAM

Universidad de Málaga 6 Frequency selectivity Several techniques against frequency selectivity Equalization Decision feedback equalization Problem: error propagation Tomlinson-Harashima pre-equalization Linear equalization enhances noise Disadvantage: transmitter needs knowledge about the channel TDD

Universidad de Málaga 7 Other possibility: splitting up the whole band width in narrow flat subbands Frequency selectivity Overlapped but orthogonal spectra Multicarrier modulation or OFDM Problem: time selective channels destroy the orthogonality

Universidad de Málaga 8 Time and frequency selective channel Adaptive DFE, THP or OFDM Adaptive QAM Fading channel Adaptive OFDM, DFE and THP DFE, THP or OFDM Frequency selective channel Adaptive modulation can be used over each subcarrier in OFDM or over the equivalent channel after equalization

Universidad de Málaga 9 Time and frequency selective channel Conclusions AOFDM, ADFE, ATH Fixed constellation Frequency selective Flat fading channel Contents OFDM, DFE and THP Adaptive QAM (AQAM) Introduction

Universidad de Málaga 10 Channel model System model Efficiency and BER Conclusions AOFDM, ADFE, ATH Contents OFDM, DFE and THP Adaptive QAM (AQAM) Introduction

Universidad de Málaga 11 Flat fading channel model Propagation model The signal arrives through a multitude of rays of different gains and phases but similar lengths Channel model System model Efficiency and BER Flat fading channel

Universidad de Málaga 12 Flat fading channel model x(t) h(t) y(t) FLAT FADING CHANNEL Flat fading channel Propagation model The signal arrives through a multitude of rays of different gains and phases but similar lengths Complex Gaussian distribution Time variations: f D

Universidad de Málaga 13 AQAM system model Modify the bit rate to get closer the variable channel capacity Channel model System model Efficiency and BER

Universidad de Málaga 14 AQAM system model Return channel R[n] bits/seg Symbol period: T b[n] x[n] h[n]n AWGN [n] Adaptive transmitter Adaptive receiver Channel y[n] Modify the bit rate to get closer the variable channel capacity

Universidad de Málaga 15 AQAM transmitter model m[n+1] 1 U 2 Adaptive modulator x[n] m[n] b[n] To the direct channel From return channel S/P m[n] is variable and under receiver control U constellations and no-transmission U+1 modulation regions

Universidad de Málaga 16 AQAM receiver model m[n] 1 S/P y[n] U 2 Adaptive detector m[n] m[n+1] To the return channel From the direct channel  [n] Grid adjust Channel estimation Contellation selector

Universidad de Málaga 17 Adaptation algorithm 5 modulation regions –No transmission –QPSK –16QAM –64QAM –256QAM ber(m,  )  (dB) m = 2 QPSK Average BER PoPo

Universidad de Málaga 18 Adaptation algorithm ber(m,  )  (dB) ber(  ) m(  ) 11 22 44 33 5 modulation regions –No transmission –QPSK –16QAM –64QAM –256QAM

Universidad de Málaga 19 Average efficiency and BER Average efficiency Average of m(  ) Channel model System model Efficiency and BER

Universidad de Málaga 20 Average efficiency and BER  (dB) ber(  )m(  ) Average efficiency Average of m(  )

Universidad de Málaga 21 Average efficiency and BER P o = Average efficiency Average of m(  )

Universidad de Málaga 22 Average BER Average of the instantaneous BER ber(  ) Average efficiency and BER  (dB) ber(  )m(  )

Universidad de Málaga 23 Average efficiency and BER Average BER Average of the instantaneous BER ber(  )

Universidad de Málaga 24 Average efficiency and BER At 23 dB, QPSK has the same BER than AQAM, but half efficiency

Universidad de Málaga 25 Conclusions AOFDM, ADFE, ATH Contents OFDM, DFE and THP Adaptive QAM (AQAM) Introduction Channel model OFDM DFE and THP

Universidad de Málaga 26 Frequency selective fading channel model Small symbol period  propagation paths can be distinguished frequency selective fading channel Channel model OFDM DFE and THP

Universidad de Málaga 27 Frequency selective fading channel model x(t) y(t) FREQUENCY SELECTIVE FADING CHANNEL h 0 (t) Retardo  0 h L-1 (t) Retardo  L-1 Channel output comes from adding L different echos which passed through a flat fading channel Small symbol period  propagation paths can be distinguished frequency selective fading channel

Universidad de Málaga 28 Frequency selective fading channel model number, delay and average power of each ray Ray delay,  l (  s) Normalized power Typical Urban Channel Power profile

Universidad de Málaga 29 Frequency selective fading channel model Frequency response At each frequency, the response is different and variable in time In average, SNR is the same for all frequencies

Universidad de Málaga 30 OFDM system model IDFT a 1,s x[n] n W [n] h[n,i] y[n] P/S + Ext DFT Ext + S/P a N-1,s a 0,s Channel model OFDM DFE and THP

Universidad de Málaga 31 OFDM system model IDFT a 1,s x[n] n W [n] h[n,i] y[n] P/S + Ext DFT Ext + S/P a N-1,s a 0,s Cyclic extension Eliminates ISI Linear convolution with the channel appears as circular Total period T t = T + T g Useful period: N T s For a constant channel, OFDM can be considered as N parallel channels Fourier transform of the channel impulse response sampled at each subcarrier frequency

Universidad de Málaga 32 OFDM system model IDFT a 1,s x[n] n W [n] h[n,i] y[n] P/S + Ext DFT Ext + S/P a N-1,s a 0,s f Fourier transform of the pulse

Universidad de Málaga 33 Intercarrier Interference (ICI) Doppler spread effects f The transference function H m,m results from the average over a symbol period Received pulses over fading channels are not orthogonal

Universidad de Málaga 34 Interference analysis Signal to Interference Ratio, SIR SIR depends on f D T but not on the power profile Useful signal powerInterference of the k subcarrier on the m one

Universidad de Málaga 35 High SNR are limited by interference Interference analysis Intercarrier interference  added Gaussian noise Signal to Noise and Interference ratio, SNIR

Universidad de Málaga 36 Performance results BER QPSK Rayleigh channel

Universidad de Málaga 37 DFE system model Channel model OFDM DFE and THP x[n] b[n] DFE TX

Universidad de Málaga 38 n W [n] y[n] h[n,i] DFE system model Feedforward Filter Feedback Filter DFE RX x[n] b[n] DFE TX Error propagation: Depends on specific channel

Universidad de Málaga 39 DFE system model ZF DFE For FIR channels, only feedback filter is necessary Feedback Filter ZF DFE RX n W [n] y[n] h[n,i] x[n] b[n] DFE TX Grid adjustment

Universidad de Málaga 40 THP system model Moving the feedback filter to the transmitter: no error propagation Preequalizer Filter s[n] b[n] THP TX x y[n] GkGk x[n] v[n] x Gain Control + u[n] Mod{·} G k -1

Universidad de Málaga 41 THP system model n W [n] r[n] h[n,i] y[n] ZF THP RX x Gain Control Mod{·} A[n] ^

Universidad de Málaga 42 THP system model Moving the feedback filter to the transmitter: no error propagation Preequalizer Filter s[n] b[n] THP TX x y[n] GkGk x[n] v[n] x Gain Control + u[n] Mod{·} G k -1 Transmitted signal is like-QAM but continuous Power penalty QPSK: Average: 1.0 dB Maximum: 5.6 dB

Universidad de Málaga 43 Performance results QPSK TU channel BER Power penalty Error propagation

Universidad de Málaga 44 Adaptive QAM (AQAM) AOFDM ADFE ATHP Conclusions AOFDM, ADFE, ATH Contents OFDM, DFE and THP Introduction

Universidad de Málaga 45 System model Return channel R[n] bits/sec Adaptive receiver Adaptive transmitter x[n] b[n] n AWGN [n] h[n,i] y[n] Channel Modifying the bits per second transmitted depending on the instantaneous channel conditions

Universidad de Málaga 46 Adaptive OFDM Signal properties can be selected depending on gain and noise at each subcarrier OFDM splits up the whole bandwidth in parallel flat channels AOFDM ADFE ATHP

Universidad de Málaga 47 Adaptive OFDM f SNR(f) f Bits per carrier Modifying the bit rate over each sub-band depending on the instantaneous channel shape Signal properties can be selected depending on gain and noise at each subcarrier OFDM splits up the whole bandwidth in parallel flat channels

Universidad de Málaga 48 AOFDM transmitter model P/S + Ext P/S + Ext Adaptive modulator S/P x[n] m m [n] N b[n] m[n+1] N From the return channel To the forward channel m o [n] m 1 [n] m N-1 [n] IDFT OFDM modulator

Universidad de Málaga 49 AOFDM receiver model Ext + S/P Ext + S/P Channel estimator and constellation selector Adaptive OFDM demodulator H[n] N N To return channel m[n+1] N m[n] N From the forward channel Constellation adaptation algorithm: AQAM

Universidad de Málaga 50 AOFDM receiver model To return channel Ext + S/P Ext + S/P m[n+1] Channel estimator and constellation selector Adaptive OFMD demodulator H[n] N N N m[n] N From the forward channel DFT Adaptive grid adjust and detector P/S N H o [n] m o [n] m[n] N H N-1 [n] m N-1 [n]

Universidad de Málaga 51 AOFDM receiver model To return channel Ext + S/P Ext + S/P m[n+1] Channel estimator and constellation selector Adaptive OFMD demodulator H[n] N N N m[n] N From the forward channel DFT Adaptive grid adjust and detector P/S N H o [n] m o [n] m[n] N H N-1 [n] m N-1 [n] Grid adjust m m [n] H m [n]

Universidad de Málaga 52 AOFDM efficiency Calculated from adaptive QAM efficiency Losses Intercarrier interference Cyclic extension Independent of power profile Independent of channel variations 1º Flat fading channel 2º Frequency selective fading channel

Universidad de Málaga 53 AOFDM efficiency Efficiency is a function of f D T Flat fading channel Slow channel Equivalent to AQAM Doppler spread SNIR instead SNR

Universidad de Málaga 54 AOFDM efficiency Flat fading channel Rayleigh channel Limited by ICI

Universidad de Málaga 55 AOFDM efficiency Frequency selective fading channel Cyclic extension reduces efficiency Lenght of impulse response BER does not change T grows: smaller cyclic extesion effect T disminishes: smaller Doppler frequency effect

Universidad de Málaga 56 AOFDM efficiency  (dB)  AOFDM (bps/Hz) f D = 25 Hz f D = 900 Hz  max = 0.5  s  max = 5  s Maximum effiency

Universidad de Málaga 57 Adaptive DFE and THP AOFDM ADFE ATHP Signal properties can be selected depending on the first echo gain After removing ISI, equivalent channel can be considered flat fading

Universidad de Málaga 58 ADFE transmitter model m[n+1] 1 U 2 Adaptive modulator x[n] m[n] b[n] To the direct channel From return channel S/P

Universidad de Málaga 59 ADFE receiver model Thresholds are modified to include error propagation effects Channel estimator and constellation selector Adaptive DFE receiver G [n] To return channel m[n+1] m[n] f [n] From the forward channel

Universidad de Málaga 60 ADFE receiver model Channel estimator and constellation selector Adaptive DFE receiver G [n] To return channel m[n+1] m[n] f [n] From the forward channel f [n] Feedback filter G[n] m o [n] Adaptive grid adjust and detector Grid adjust m[n] G[n]

Universidad de Málaga 61 ADFE efficiency Calculated from adaptive QAM values Losses Only first echo power is used Error propagation Dependent on power profile Dependent on the power profile

Universidad de Málaga 62 ATHP transmitter model Preequalizer Filter x y[n] GkGk x[n] v[n] x Gain Control + u[n] Mod{·} G k -1 s[n] 1 U 2 m[n] AOFDM ADFE ATHP

Universidad de Málaga 63 From return channel To control channel ATHP transmitter model Preequalizer Filter x y[n] GkGk x[n] v[n] x Gain Control + u[n] Mod{·} G k -1 s[n] 1 U 2 m[n] Channel estimator

Universidad de Málaga 64 ADFE receiver model G [n] From control channel To return channel m[n+1] Constellation selector Adaptive THP receiver m[n] From the forward channel Thresholds are modified to include power loss

Universidad de Málaga 65 ATHP receiver model G [n] From control channel To return channel m[n+1] Constellation selector Adaptive THP receiver m[n] From the forward channel x Gain Control r[n] G[n] m[n] Adaptive modulo and detector Mod{·} m[n]

Universidad de Málaga 66 ATHP efficiency Calculated from adaptive QAM values Losses Only first echo power is used Power boost Dependent on power profile

Universidad de Málaga 67 ADFE and ATHP efficiency  (dB)  ADFE,ATHP (bps/Hz) DFE THP Efficiency for TU channel

Universidad de Málaga 68 Efficiency Comparison TU channel f D = 25 Hz  (dB)  (bps/Hz) ATHP ADFE AOFDM

Universidad de Málaga 69 Adaptive QAM (AQAM) Conclusions AOFDM, ADFE, ATH Contents OFDM, DFE and THP Introduction

Universidad de Málaga 70 Conclusions Adaptive modulation schemes Efficiency Function of target BER and SNR Average bit error rate Approximately the same for all SNR values

Universidad de Málaga 71 Conclusions Power penalty THP DFE Error propagation increases BER more than over AWGN channels OFDM over fading channels Orthogonality among subcarriers is lost

Universidad de Málaga 72 Cope with frequency selectivity Keep advantages of adaptive modulation Conclusions Adaptive OFDM Efficiency depends on T, f D and  max Adaptive DFE and THP Only use first echo power Adaptive DFE Error propagation reduces efficiency Adaptive schemes for frequency selective channels

Adaptive modulation schemes for frequency selective fading channels Mª Carmen Aguayo Torres, José Paris Angel, José Tomás Entrambasaguas Muñoz Universidad de Málaga Escuela Técnica Superior de Ingeniería de Telecomunicación