With Full CSI: the gain is small.

1. R g h2 D S h1 S2 R D S1 S D Relay Symmetrical Diversity at Low SNR h e : Outage Probability (Quality of Service) The impact of fading The impact of diversity (e=0.01) • The impact of diversity is more significant • Outage Capacity: Cooperative Strategies for the Relay Channel In the Low SNR Regime Salman Avestimehr and David Tse University of California at Berkeley The Relay Scenario Cooperative Communication • Transmit diversity: may not be practical • - most handsets (size) • - wireless sensor network (size, power) • Single-antenna mobiles share their antennas • - Virtual Multiple-Antenna system • Rayleigh fading channels + AWGN noise (variance 1) • Slow fading: Outage event • Half-Duplex constraint • Channel State Information available at the receiver only • Average power constraint (P) on both S and R • Two scenarios: 1 2 3 A Better Protocol? Upper Bound on the Performance Cooperative Schemes • Bursty Amplify-Forward: • To have less noisy observation at the Relay, source transmits rarely (a fraction of the time) but with high power: be large -In order to be power efficient, we should pick a such that the effective rate is small:be small - Can we pick such a ? Yes, because at low outage probability is small. - The achievable rate of this scheme: - Why optimal? R R • Decode-Forward: • -Not Optimal, Why? • Cut-set: • Amplify-Forward: • -Relay becomes useless by injecting more noise than signal. P g g h2 h2 Exploits Diversity bP D D S S h1 (1-b)P • Min-Cut Max-Flow Bound: • The corresponding bound on the outage capacity No Diversity 6 4 5 Full CSI Scenario The Gain From Full CSI Network with k Relays R R • With Full CSI: • the gain is small. • The reason is that in previous optimization only small amount of power will be allocated for beam forming (6%) • In the MISO case, the gain is 3dB as all the power is used for beam forming P g g h2 h2 bP D D S S h1 D D S S (1-b)P • Allocate some power for beam forming • For any choice of b, r it can be achieved by BAF + Beam forming • Min-Cut Max-Flow Bound: • The corresponding bound on the outage capacity • The cut-set is achieved by BAF protocol on each relay 7 9 8