Average interference-constrained cognitive two-way relaying with efficient link utilization

In this paper, we investigate the outage performance of a decode-and-forward (DF) underlay cognitive two-way relay system with direct link using time division broadcast protocol in the presence of primary user's interference. Herein, relying on the practicability and low-complexity of statistical channel state information (CSI), the secondary transmissions satisfy the fading-averaged interference constraints at the primary receiver. We study an efficient link utilization scheme that can exploit both direct and relay links with appropriate combining methods to improve the performance of secondary users. Considering Nakagami-m fading channels, we derive a tight closed-form expression of outage probability for the secondary communications. We further perform the asymptotic high signal-to-interference noise ratio (SINR) analysis to gain more insight. We quantify the implications of primary user's interference on the secondary system performance and present several useful insights. With the proposed scheme, secondary system is shown to harness full diversity as long as the primary interference remains limited otherwise outage floor phenomenon occurs. Numerical and simulation results corroborate our theoretical developments.