Adaptive transmission-reception-sensing strategy for cognitive radios with full-duplex capabilities

In this paper, we exploit recent advances in full-duplex (FD) communications and self-interference suppression (SIS) to improve the performance of an opportunistic spectrum access (OSA) system. Specifically, we consider secondary users (SUs) that are equipped with SIS-capable radios. These radios can operate in a simultaneous transmission-and-sensing (TS) mode to improve the detection probability of primary users (PUs), or in a simultaneous transmission-and-reception (TR) mode to enhance the SU throughput. The radios can also revert to the standard sensing-only (SO) mode or perform channel switching (CS). The competing goals of the full-duplex TS and TR modes give rise to a spectrum-awareness/efficiency tradeoff, which can be optimized by allowing the SU link to adaptively switch between various modes, depending on the forecasted PU dynamics. In practice, SIS is imperfect, resulting in residual self-interference that degrades the sensing performance in the TS mode. Accordingly, we adopt a waveform-based sensing approach, which allows an SU to detect (with high accuracy) the PU signal in the presence of self-interference (and noise). In such a context, we analyze the sensing performance in the TS mode by deriving the false-alarm and detection probabilities. We also derive the throughput and the PU-SU collision probability for the TS and TR modes, which we then use to establish an optimal mode-selection strategy that maximizes an SU utility function subject to a constraint on the PU collision probability. This utility rewards the SU instantly for successful communication (throughput), but also includes a long-term component that depends on the outcomes of the action taken by the SU (the selected mode from the set {TR, TS, SO, CS}). Our results show that the proposed adaptive strategy results in about 50% reduction in the collision probability and twice the throughput of the half-duplex case. The results also indicate that the SU should operate in the TR mode if it has a high belief regarding the PU idleness over a given channel. As this belief decreases, the SU should switch to the TS mode to monitor any change in the PU activity while transmitting. At very low belief values, where the PU is highly likely to be active, the SU should switch to another channel.