Channel Estimation Schemes for OFDM Relay-Assisted System

Transcription

1 Channel Estimation Schemes for OFDM Relay-Assisted System Darlene Maciel, C. Ribeiro, A. Silva e Atílio Gameiro darlene@av.it.pt Workshop 2009

2 Outline Introduction Motivation PACE Schemes Simulation Scenario Results Conclusion Future Works

3 Introduction: Diversity Diversity is inherent in the physical layer: PHY diversity Time, frequency, space (antenna) and polarization diversity Combat the fading channel by trying to flatten the channel Diversity can also be achieved in the MAC or higher layer: Network diversity Multiuser diversity (by scheduling or routing) Cooperative diversity (by cooperative transmission)

4 Introduction: Cooperation Redundant transmission is realized via the cooperation of third party devices rather than solely from the originating device; D Node S cooperates with neighbors to send information to D S AF, DF, SDF or CF Half dupplex AF: 2 phases RN Transmitted signal S D Received signal Simple forms of cooperation involves 3 links

5 Introduction: Amplify-and-Forward Protocol RN S D In the half duplex AF protocol receiver at D needs First phase: Estimate channel S-D: Single link Conventional Channel Estimation Second Phase: Estimate channel S-RN-D: Compound Channel

6 Motivation: Equivalent Channel Compound Channel S h (1) () t RN h (2) () t D h h h h h h Eq (1) (2) (1) (2) Eq ( t) A ( t) (t) ( m) A ( m) ( m) The Power Delay Profile - PDP PDP = E{ h ( t) h ( t) } = PDP PDP (1) (2) 2 (1) (2) Maximum delay = Delay channel 1 + Delay channel 2 # Taps of the compound channel will depend on the both channels PDP

7 Motivation: Equivalent Channel S h (1) () t RN h (2) () t D (1) (2) h h ES, : 0, 1 Two sources of Noise. The total noise at the D: w m Ah m w m w m (2) (1) (2) ( ) ( ) ( ) ( ) t ( m) E{ w ( m) } A t t n n Conditioned to a specific channel realization the noise variance: 2 ( m) A h ( m) 2 2 (2) 2 2 t n n The conventional channel estimation schemes should be adapted to this scenario LS, MMSE

8 Motivation: Questions Questions to be Solved: How does the statistics of the compound channel affect the performance of classical PACE in OFDM signalling? (2) How much can be gained through the knowledge of h?

9 DFT Classical Pilot Aided Channel Estimation Schemes FD LS (Least Square): h ˆP LS hˆ LS X Y 1 P Wh ˆP LS P W: Interpolator AWGN + LS Estimate Channel Estimator ĥ LS FD MMSE Filter ĥ MMSE Equalisation FD MMSE (Mean Minimum Square Error) : ĥ W MMSE W MMSE R R 1 MMSE HP P hˆ P LS R R P HP Autocorrelation; Cross-correlation;

10 Classical Pilot Aided Channel Estimation Schemes TD-MMSE AWGN CIR Group + h ˆ LS Channel Estimator TD MMSE Filter h ˆ MMSE Equalisation Example CIR Estimate CIR estimate [ n] hh 2 W n R [ n] n hh R R [ n] Auto-correlation Function Channel PDP hh PDP PDP PDP n t

11 Simulation Scenario Scenario and Parameter S RN D Modulation Path Delay(ns) QPSK Relative Power(dB) Sampling frequency 1 (LTE) MHz 0.0 # Subcarriers (T) Link Analized (4T) Compound -0.8 channel For reference (9T) Conv. SISO -6.0 Channels 5Noise statistics (16T) identical Channel (25T) 7 Taps (38T) -19.0

12 Simulation Scenario Simulation Parameters The pilots are multiplexing in the symbol: Frequency Data Pilot Nf= 32; 4 Nt= 1; 12 Time

13 MSE (db) MSE (db) Results FD LS Estimator LS Channel Estimation Nf=32, Nf=32, Nt=1,Relay Nt=1,Relay on on Nf=32, Nt=12,Relay on on Nf=4, Nf=4, Nt=1,Relay on on Nf=4, Nt=12,Relay on on Nf=32, Nt=1,Conv. SISO Nf=32, Nt=12,Conv. SISO Nf=4, Nt=1,Conv. SISO Nf=4, Nt=12,Conv. SISO E b /N0 (db)

14 MSE (db) MSE (db) Results TD MMSE Estimator MMSE Channel Estimation Nf=32, Nt=1,Relay on Nf=32, Nt=1,Relay on Nf=32, Nt=12,Relay on Nf=32, Nt=12,Relay on Nf=4, Nt=1,Relay on Nf=4, Nt=1,Relay on Nf=4, Nf=4, Nt=12,Relay Nt=12,Relay on on Nf=32, Nt=1,Conv. SISO Nf=32, Nt=12,Conv. SISO Nf=4, Nt=1,Conv. SISO Nf=4, Nt=12,Conv. SISO E b /N0 (db) db

15 Magnitude Magnitude Example Point-to-point Channel PDP P2P Channel N1 = 7 Taps Compound Channel PDP N2 can be quite larger than N1 Compound Channel conv (P2P Ch, P2P Ch) N2 = 27 Taps N2 N1 1 N1 2 Taps SNR per Tap MSE

16 ChEst MSE (db) How much can be gained through the Knowledge oh h2? Channel Estimation MSE vs. Eb/N0 2 n t mm m n 2 n Eb/N0 (db) m A h m t n n No noticeable improvement by the knowledge of h2; Number of non-zero taps << Nc/Nf Filter design is robust to errors in the estimate of the noise variance

17 Conclusion In AF the equivalent channel S-RN-D has a larger delay than point-to-point Increases the minimum pilot density that can be used; Degrades the performance of the MMSE; The robustness of the TD-MMSE filter to errors in the estimate of noise variance The knowledge of individual P2P channels does not bring any noticeable improvement;

18 Future Works Consider a scenario which the channel statistics can bring improvements to the channel estimate: Antenna array at the BS; Equalize-and-Forward Protocol; Power constraints at the RN; Channels with different statistics.