Channel Estimation Schemes for OFDM Relay-Assisted System Darlene Maciel, C. Ribeiro, A. Silva e Atílio Gameiro darlene@av.it.pt Workshop 2009 2 nd Dec, FEUP, Porto, Portugal
Outline • Introduction • Motivation • PACE Schemes • Simulation Scenario • Results • Conclusion • Future Works 2 nd Dec, FEUP, Porto, Portugal MAP-Tele Workshop 2009
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) 2 nd Dec, FEUP, Porto, Portugal MAP-Tele Workshop 2009
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 AF , DF , SDF or CF S Half dupplex AF: 2 phases RN Received D S Transmitted signal signal Simple forms of cooperation involves 3 links 2 nd Dec, FEUP, Porto, Portugal MAP-Tele Workshop 2009
Introduction: Amplify-and-Forward Protocol RN D S • 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 2 nd Dec, FEUP, Porto, Portugal MAP-Tele Workshop 2009
Motivation: Equivalent Channel S RN D (1) ( ) (2) ( ) h t h t • Compound Channel (1) (2) (1) (2) h h h h h h ( ) ( ) (t) ( ) ( ) ( ) t A t m A m m Eq Eq • The Power Delay Profile - PDP (1) (2) (1) (2) 2 ( ) ( ) | } PDP = E{| h t h t = PDP PDP Maximum delay = Delay channel 1 + Delay channel 2 # Taps of the compound channel will depend on the both channels PDP 2 nd Dec, FEUP, Porto, Portugal MAP-Tele Workshop 2009
Motivation: Equivalent Channel S RN D (1) ( ) (2) ( ) (1) (2) , : 0, 1 ES h t h t h h • Two sources of Noise. The total noise at the D: 2 2 2 2 (2) (1) (2) {| | } 2 ( ) ( ) ( ) ( ) ( ) E ( ) A w m Ah m w m w m m w m t n n t t • Conditioned to a specific channel realization the noise variance: 2 2 2 2 2 (2) | ( )| ( ) A h m m t n n The conventional channel estimation schemes should be adapted to this scenario LS , MMSE 2 nd Dec, FEUP, Porto, Portugal MAP-Tele Workshop 2009
Motivation: Questions • Questions to be Solved: ▫ How does the statistics of the compound channel affect the performance of classical PACE in OFDM signalling? ▫ How much can be gained through the knowledge of ? (2) h 2 nd Dec, FEUP, Porto, Portugal MAP-Tele Workshop 2009
Classical Pilot Aided Channel Estimation Schemes • FD – LS (Least Square): ˆ h Channel ˆ P LS ˆ 1 h X Y AWGN h Estimator LS FD MMSE LS P P MMSE ˆ ˆ P Estimate Filter h Wh + DFT LS LS Equalisation W : Interpolator • FD – MMSE (Mean Minimum Square Error) : ˆ ˆ P h W h LS MMSE MMSE Autocorrelation; R 1 W R R P MMSE HP P Cross-correlation; R HP 2 nd Dec, FEUP, Porto, Portugal MAP-Tele Workshop 2009
Classical Pilot Aided Channel Estimation Schemes • TD-MMSE ˆ ˆ h Channel h MMSE LS TD MMSE CIR AWGN Estimator CIR Estimate Filter Group + Equalisation Example CIR estimate [ ] Channel PDP R n Auto-correlation Function [ ] R n hh hh W n PDP PDP PDP 2 [ ] 1 2 R n n hh 2 2 n t 2 nd Dec, FEUP, Porto, Portugal MAP-Tele Workshop 2009
Simulation Scenario • Scenario and Parameter S RN D Modulation Path Delay(ns) QPSK Relative Power(dB) Sampling frequency (LTE) 1 0.0 15.36 MHz 0.0 # Subcarriers 2 65.1 (T) 1024 -0.7 Link Analized 3 260.4 (4T) Compound channel -0.8 For reference 4 586.0 (9T) Conv. SISO -6.0 Channels’ Noise statistics 5 1041.67 (16T) identical -10.0 Channel 6 1627.6 (25T) 7 Taps -14.0 7 2474.0 (38T) -19.0 2 nd Dec, FEUP, Porto, Portugal MAP-Tele Workshop 2009
Simulation Scenario • Simulation Parameters ▫ The pilots are multiplexing in the symbol: Frequency Data Pilot Nf= 32; 4 Time Nt= 1; 12 2 nd Dec, FEUP, Porto, Portugal MAP-Tele Workshop 2009
Results • FD – LS Estimator LS Channel Estimation LS Channel Estimation -2 -2 -3 -3 -4 -4 -5 -5 -6 -6 MSE (dB) MSE (dB) -7 -7 Nf=32, Nt=1,Relay on Nf=32, Nt=1,Relay on -8 -8 Nf=32, Nt=12,Relay on Nf=32, Nt=12,Relay on Nf=4, Nt=1,Relay on -9 -9 Nf=4, Nt=1,Relay on Nf=4, Nt=12,Relay on Nf=4, Nt=12,Relay on -10 -10 Nf=32, Nt=1,Conv. SISO Nf=32, Nt=1,Conv. SISO Nf=32, Nt=12,Conv. SISO Nf=32, Nt=12,Conv. SISO -11 -11 Nf=4, Nt=1,Conv. SISO Nf=4, Nt=1,Conv. SISO Nf=4, Nt=12,Conv. SISO Nf=4, Nt=12,Conv. SISO -12 -12 0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 E b /N0 (dB) E b /N0 (dB) 2 nd Dec, FEUP, Porto, Portugal MAP-Tele Workshop 2009
Results • TD – MMSE Estimator MMSE Channel Estimation MMSE Channel Estimation -8 -8 -10 -10 -12 -12 -14 -14 -16 -16 MSE (dB) MSE (dB) -18 -18 Nf=32, Nt=1,Relay on Nf=32, Nt=1,Relay on Nf=32, Nt=12,Relay on -20 -20 Nf=32, Nt=12,Relay on Nf=4, Nt=1,Relay on Nf=4, Nt=1,Relay on ≈ 5 dB -22 -22 Nf=4, Nt=12,Relay on Nf=4, Nt=12,Relay on Nf=32, Nt=1,Conv. SISO Nf=32, Nt=1,Conv. SISO -24 -24 Nf=32, Nt=12,Conv. SISO Nf=32, Nt=12,Conv. SISO Nf=4, Nt=1,Conv. SISO Nf=4, Nt=1,Conv. SISO -26 -26 Nf=4, Nt=12,Conv. SISO Nf=4, Nt=12,Conv. SISO -28 -28 0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 E b /N0 (dB) E b /N0 (dB) 2 nd Dec, FEUP, Porto, Portugal MAP-Tele Workshop 2009
Example Point-to-point Channel PDP Magnitude 0.4 P2P Channel N1 = 7 Taps 0.2 0 0 10 20 30 40 50 Compound Channel Compound Channel PDP 0.2 Magnitude conv (P2P Ch, P2P Ch) N2 = 27 Taps 0.1 0 1 1 1 N N 0 10 20 30 40 50 2 N 2 N2 can be quite larger than N1 Taps SNR per Tap MSE 2 nd Dec, FEUP, Porto, Portugal MAP-Tele Workshop 2009
How much can be gained through the Knowledge oh h2? Channel Estimation MSE vs. Eb/N0 2 2 2 2 2 2 A h m m 2 ChEst MSE (dB) n 2 mm -12 m t n n t -14 2 n 2 No noticeable improvement by n -16 the knowledge of h2; -18 Number of non-zero taps << -20 Nc/Nf 0 5 10 Eb/N0 (dB) Filter design is robust to errors in the estimate of the noise variance 2 nd Dec, FEUP, Porto, Portugal MAP-Tele Workshop 2009
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; 2 nd Dec, FEUP, Porto, Portugal MAP-Tele Workshop 2009
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. 2 nd Dec, FEUP, Porto, Portugal MAP-Tele Workshop 2009
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