Cooperative Hybrid ARQ in Wireless Decode-and-Forward Relay Networks

Wireless decode-and-forward (DF) relay networks suffer from severe latency due to multi-hop propagation. When hybrid ARQ (HARQ) is employed, the latency leads to the throughout degradation of individual HARQ processes because of the decreased number of retransmissions of unsuccessful packets in a given time. We propose two new distributed cooperative HARQ protocols to not only reduce latency but also optimize throughput in both forward and reverse DF links. Additional selection diversity from multiple packets is exploited to compensate for the throughput loss stemming from the less powerful source in the reverse DF link. Based on the 1(st) hop reception quality, each relay station (RS) independently forwards the packet in such a manner that, from the perspective of the destination, one of the three cooperative relaying modes is effectively formed with the highest instantaneous throughput: spatial multiplexing (SM), space-time transmit diversity (STTD) and unicast (UC). The maximal throughput can be achieved with minimal latency. The superiority of the proposed approaches has been demonstrated in terms of the optimal spectral efficiency and significant reduction in latency. Compared to the centralized approaches, the reduction is up to 40% at high signal-to-noise ratio (SNR) and the throughput of individual HARQ processes increases as a result.