On the Outage Probability of Cognitive Two-Way Relaying Based on Superposition Coding

Cognitive two-way relaying is studied in this paper. A secondary relay is used to assist bidirectional communication between a pair of primary users. As a return, the secondary relay is allowed to transmit its own secondary signal on the licensed primary spectrum. For this purpose, superposition coding is employed at the relay to superimpose the secondary signal on the primary signals. Three typical cognitive two-way relaying strategies are considered in this paper, namely, amplify-andforward (AF), XOR- based decode-and-forward (DF-XOR) and superposition based DF (DF-SUP). Outage probabilities of the two primary users under these three strategies are derived for Rayleigh fading channels. Their performances are compared with direct transmission and another recently proposed cognitive two-way relaying strategy which enables interference cancelation of the secondary signal at the primary users. Our results show that a major portion of relay power has to be allocated for relaying primary signals because the superimposed secondary signal always plays as an interference to the primary signals. Finally, we propose several potential solutions to suppress this interference to enhance the performance of cognitive two-way relaying.