Cognitive Spectrum Sharing With Bi-directional Secondary System

A cognitive spectrum sharing protocol based on the two-path relay channel is proposed in this paper, where a primary transmitter keeps transmitting new messages to a corresponding receiver and two secondary users alternately decode-and-forward (DF) the primary messages. Upon successful decoding of the primary message, each secondary user superimposes its own message on the relayed message through a certain power allocation. Otherwise, the secondary user simply stays silent. In view of the memorylessness that the DF process at a secondary user in the current time slot depends on the DF process at the other secondary user in the previous time slot, a Markov framework is proposed to analyze the state transitions between staying silent and accessing the spectrum of the secondary users. Based on this framework, the outage probability and throughput are characterized for the primary and secondary users respectively. Our results demonstrate that with a prudential power allocation, the performance of the primary users can be significantly improved and bi-directional communications between two secondary users can be facilitated simultaneously.