Flow Optimization in Parallel Relay Networks: A Routing Perspective

We investigate the routing problem in parallel relay networks consisting of a source sending information to a destination via 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 synchronously and asynchronously. It turns out that not all relays forward information in the optimal asynchronous protocol. Using this fact, we propose a sub-optimal protocol employing at most two relays. In synchronous relaying, the optimal protocol gives around 1dB and over 3dB improvements compared at the outage probability of 10−4 with respect to traditional routing at low-rate and high-rate requirements, respectively. Again, we propose a two-relay sub-optimal protocol for synchronous transmission. These sub-optimal relaying protocols can be viewed as generalizations to traditional routing. 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 traditional routing.