High-Performance Cooperative Demodulation With Decode-and-Forward Relays

Cooperative communication systems using various relay strategies can achieve spatial diversity gains, enhance coverage, and potentially increase capacity. For the practically attractive decode-and-forward (DF) relay strategy, we derive a high-performance low-complexity coherent demodulator at the destination in the form of a weighted combiner. The weights are selected adaptively to account for the quality of both source-relay-destination and source-destination links. Analysis proves that the novel coherent demodulator can achieve the maximum possible diversity, regardless of the underlying constellation. Its error performance tightly bounds that of maximum-likelihood (ML) demodulation, which provably quantifies the diversity gain of ML detection with DF relaying. Simulations corroborate the analysis and compare the performance of the novel decoder with existing diversity-achieving strategies including analog amplify-and-forward and selective-relaying.

I. Introduction

The proliferation of wireless terminals has naturally led to cooperative (also known as relay) links whereby communicators benefit from their neighbors [4], [8], [9], [11], [13], [14]. As with multi-input multi-output (MIMO) systems, where multiple collocated antennas are deployed at the transmit- or receive-ends, a main objective with single-antenna cooperating terminals is also to enable spatial diversity. Beyond MIMO with collocated antennas, relay transmissions offer resilience against shadowing and enhanced coverage. In such cooperative links, the message sent by the source arrives at the destination through diverse paths: one directly from the source node and others through relay nodes. Performance will, thus, depend on the number of cooperating relay nodes as well as the processing operations at both relays and destination. If properly designed, cooperative networks can achieve diversity order up to the number of diverse paths (what we, henceforth, refer to as full diversity). To put our contribution in context, we next review the existing relay strategies, detectors, and their error performance in terms of diversity.