In this paper, we propose to recover collided packets between Primary Users (PUs) and Secondary Users (SUs) in Cognitive Radio Networks (CRNs) for two scenarios. When a collision occurs between an SU and a PU transmitters, the SU's receiver considers the PU's transmitted packet's signals as an interference, and hence, cancels its effect in order to recover its corresponding received packet's signals. Recovering collided packets, instead of retransmitting them saves transmitters' energy. In the first scenario, we assume PUs and SUs employ the standard Binary Phase-Shift keying (BPSK) and a 90 degree phase shifted version, i.e., orthogonal to BPSK, respectively, as their modulation techniques. In the Second scenario, we assume PUs and SUs employ BPSK and QPSK as their modulation techniques, respectively, or vice versa. In both scenarios, we propose protocols to recover the SU collided packets, depending on the received phase shifts. We show through numerical analysis that a significant fraction of collided packets can be recovered. We also derive an energy saving performance metric for our proposed mechanisms, in order to assess the saved energy due to recovering the collided packets. Our numerical analysis also shows that a high percentage of energy can be saved over the traditional scheme, in which our packets recovery mechanisms are not employed.
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