To exchange information between two sources in a two-way relaying network with multiple potential relays, most researches focus on two-hop relay system with single-relay-selection (SRS) scheme. Comparing with SRS scheme, the authors first design a paired-relay-selection (PRS) scheme in which a pair of `best’ relays broadcast network-coded information to other nodes (source or relay). They propose an optimal selection algorithm and a suboptimal algorithm that selects the pair of `best’ relays in the PRS scheme and they describe how the nodes exchange information in a frame consisting of four timeslots.
Both the analytical and simulation results show that when the pathloss exponent is large and/or there is a sufficient number of relays to choose from, using two relay nodes can provide a lower outage compared with using only one relay node even under the same total transmit power in uniformly distributed relay networks. In addition, to reduce the overhead of the PRS scheme, they propose an iterative-PRS (I-PRS) scheme in which the paired relay is selected in an iterative and opportunistic way. Simulation results show that the I-PRS scheme has nearly the same outage performance as the PRS scheme under time-invariant channels and significantly outperforms the PRS scheme under time-varying channels.