Relay selection for amplify-and-forward systems with differential and double-differential modulation

Most of existing research on cooperative diversity systems have assumed that the destination node and the relay node have perfect channel state information. However, it is difficult to obtain all the channel state information in the systems, especially in a multi-relay node environment. To greatly reduce the cost required for those channel estimations, a differential modulation technique can be applied to multi-node cooperative communications. However, the performance of this system can be severely degraded if the transmission among the relays is not synchronized. Therefore we first combine relay selection schemes with differential modulation. Even if some existing literatures considered how to select a relay, the selection methods in those papers still require the channel state information. In this paper, we propose a simple relay selection scheme which does not require any channel state information. Our method is based only on the received signal power at the destination. This selection method can also be applied to double-differential modulation and show that the proposed selection scheme can work well for both differential modulation schemes. Our relay selection scheme combined with these differential modulation can significantly reduce the receiver complexity and also improve data throughput by eliminating pilot symbol period needed for channel estimation.