Differential Modulation Schemes for Cooperative Diversity

Recent developments of wireless networks show that cooperative diversity can produce multi-input multi-output (MIMO) advantages by forming a virtual antenna array with distributed network nodes. Although differential schemes have long been appreciated in conventional MIMO communications since they provide the benefit of simplicity at the receiver by avoiding channel estimation, their performance and possible benefits in the distributed scenario have not been thoroughly investigated. In this paper, we examine two differential schemes for a cooperative diversity setup using a decode-and-forward relaying protocol. The first scheme (scheme I) is a direct extension of the conventional differential scheme for a single-input single-output (SISO) channel; whereas the second (scheme II) is a distributed cooperative counterpart of differential space-time coding (STC). We develop the maximum likelihood (ML) decision rules for two cases, in which the effect of the relay decision is captured by the transition probability. Simulations confirm that both schemes provide full diversity gain. Comparisons of the two schemes show that scheme I outperforms scheme II in terms of symbol error rate (SER); while scheme II can support a higher transmission rate, and this rate advantage increases as the number of relays increases