Spectrum Sharing for Cognitive Two-Way Relaying: Superposition Coding Versus Time Division

Spectrum sharing for cognitive two-way relaying is considered in this paper. A secondary relay is used to assist two-way communications between two primary users, and as a return, the secondary relay is allowed to transmit its own secondary signal by using some of the licensed primary spectrum. Three different spectrum sharing strategies are studied in this paper. In the baseline superposition coding (SC) strategy, the relay superimposes its secondary signal on the relayed primary signals in the same phase, and in the SC strategy with secondary interference cancellation (SC-SIC), the interference from the superimposed secondary signal on the primary signals can be removed; whereas in the time division (TD) strategy, the relay uses two distinct phases to transmit the relayed primary signals and its own secondary signal. Therefore, relay power or transmission time is treated as the shared resource and needs to be allocated between primary and secondary transmissions in the three strategies. We develop resource allocation results which can minimize outage probability of the secondary signal, and meanwhile, can satisfy data rate targets of the two primary users. Our results show that, although the SC-SIC strategy requires the least shared resource to achieve the data rate target for primary users, the TD strategy can provide the best performance for secondary transmission because it can completely avoid mutual interference between primary and secondary signals.