Flow optimization in parallel relay networks with cooperative relaying

We investigate the relaying problem in parallel relay networks consisting of a source sending information to a destination via multiple relays. To exploit the broadcasting nature of the wireless channel, the source broadcasts to the relays and the relays forward the decoded information to the destination. The flow rates on all links in the network are selected to minimize the average transmit power required. We consider two optimal protocols, respectively in which the relays forward the decoded information phase-synchronously and phase-asynchronously. It turns out that not all relays forward information in the optimal phase-asynchronous protocol. Using this fact, we propose a sub-optimal protocol employing at most two relays. In phase-synchronous relaying, the optimal protocol gives around 1dB and over 3 dB improvements compared at the outage probability of 10^-4 with respect to the single-relay optimal protocol at low-rate and high-rate requirements, respectively. Again, we propose a two-relay sub-optimal protocol for phase-synchronous transmission. Simulation results indicate that these sub-optimal protocols give performances that are close to the performances of the optimal protocols and superior to that of the single-relay optimal protocol.