Optimal relaying strategy for UE relays

In cooperative cellular wireless networks, a user equipment (UE) relay can be a good alternative to rendering an end user by forwarding the signal overheard from the source to the end user. However, due to its practical limitation, it has no cell-specific reference signal. Therefore, different from conventional fixed relay station, the channel state information (CSI) is not available to the UE relay. Since the UE relay cannot estimate relay-to-destination (R-D) channel capacity, there arises the possibility of an outage event in cooperating phase. In this paper, we introduce an outage based rate control under multiple relay networks in which there are one source, multiple UE relays and one end user. To achieve maximal overall transmission rate, resource allocation during the cooperating phase is done at the source in advance by considering both the expected transmission rate and the resulting outage probability. Especially we consider three cases of relaying strategies in such open-loop R-D links and observe that they complement each other depending on the R-D link geometry and channel conditions, leading to optimal relaying strategy for UE relays.