In this paper, we investigate the performance of an overlay spectrum sharing system by employing an opportunistic multiuser two-way relaying with time division broadcast (TDBC) protocol over Nakagami-m fading channels. Herein, one-end primary user (PU) exchanges information with one of the best selected other end-PUs with the assistance of a secondary user (SU) using TDBC protocol. With an overlay approach, the SU allocates its partial power to relay the PUs' signals and uses the remaining power to transmit its own signal. The underlying PU selection strategy is designed to minimize the primary system outage probability. For such a set-up, we derive a closed-form expression for the overall outage probability. Based on its asymptotic representation at high signal-to-noise ratio (SNR), we illustrate that the primary system can achieve maximum diversity order and allows spectrum sharing as long as its outage constraints are met. Numerical and simulation results are provided to validate our theoretical analysis.
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