Performance and Energy Efficiency of Cognitive Full-Duplex under Spectrum Sharing Constraints

In a cognitive radio context under spectrum sharing constraints, a secondary network may transmit concurrently with the primary one as long as the communication of this latter is not compromised. For such an operation, a maximum allowable interference level at the primary receiver is defined, and secondary users (SUs) should take into account this threshold during the transmission in order to adjust their transmit powers to not damage the reception of the primary receiver [1], [2]. This will allow a more efficient use of the frequency spectrum. On the other hand, cooperative communications [3], [4] have emerged as an alternative technique to boost the performance of communication systems. The idea behind this strategy is to make use of one or more nodes (called relays) in order to emulate a physical antenna array. Thus, the same benefits obtained in multiple-input multiple-output systems can also be achieved with the use of single-antenna nodes through the distributed transmission and processing of the information. In cooperative systems, the relay behavior is governed by the so-called cooperative protocols and it can operate on either half-duplex or full-duplex modes [3], [5]. Specifically, in half-duplex mode, the relay transmits and receives in orthogonal channels, whereas in full-duplex mode the transmission and reception are performed at the same time and at the same frequency band. Owing to this fact, halfduplex relays require the use of additional system resources, while full-duplex relays arise as a viable option to alleviate this problem. However, although ideal full-duplex relaying can achieve higher capacity than half-duplex relaying [5], its use introduces self-interference that is inherent to the fullduplex approach (please, see [6]–[8] and references therein). Nevertheless, the works in [6]–[8] showed that full-duplex relays can still achieve high performance, even in the presence of strong interference levels. Motivated by the important benefits acquired with cognitive radio and cooperative diversity techniques, several recent works have analyzed the performance of cooperative cognitive networks under spectrum sharing constraints [9].Common to these works is that they assumed that all nodes operate on a half-duplex mode. However, in [14] the authors considered a scenario with a full-duplex relay subject to self-interference. In that work, through the use of a whitening filter, the interference from the primary network was assumed to be approximately Gaussian. With this assumption in mind, [14] performed a closed-form outage analysis for a full-duplex dual-hop (DH) relaying scheme, in which the self-interference at the relay was taken into account and the direct link was seen as interference at the secondary destination. Recently, energy efficient schemes have become the focus of academia and industry. The objective of power consumption analysis is to propose alternatives to prolongate the battery lifetime of mobile devices as well as to reduce carbon emissions, and reduce energy consumption of the network as a whole [10]. In [11] power consumption was analyzed for non-cooperative and cooperative networks. Moreover, by defining an end-to-end throughput requirement, it is shown that incremental cooperation is more energy efficient than direct transmission and than multi-hop transmission, even at small transmission ranges. Moreover, in [12] an energy efficient analysis is carried out considering multi-hop HD and FD schemes in the AWGN relay channel. The results show that the multi-hop HD relay requires more bandwidth than multi-hop FD relay with the same rate and power constraints. Differently from all previous works, in this paper we consider a cooperative cognitive network operating on a spectrum sharing scenario with a full-duplex relay subject to selfinterference. In particular, this paper differs from [14] because the secondary destination applies joint decoding with the signals received from the relay and from the secondary source such that the direct link can be seen as useful information rather than interference. Closed-form expressions for the outage probability and throughput are derived and insightful discussions are provided. The proposed scheme, termed as full-duplex joint-decoding (JD) relaying, is compared with the full-duplex DH scheme presented in [14] as well as with the standard half-duplex (HD) relaying scheme. Our results demonstrate that the proposed cognitive cooperative full-duplex relaying scheme can considerably outperform the full-duplex relaying method proposed in [14] for the whole signal-to-noise ratio (SNR) range. Moreover, our results show that the proposed JD method performs better than the halfduplex HD scheme in terms of throughput even in the presence of self-interference.