The explosive growth of the Internet of Things (IoT) is putting higher requirements for the performance of co-time co-frequency full-duplex relaying (CCFD-R) due to small-size devices appearing in abundance. In this paper, we present a novel CCFD-R approach with very low complexity to deal with the issue above. Based on the orthogonality of its two components, a distributed usage mode of quadrature amplitude modulation (QAM) constellation is proposed to perform self-interference cancellation (SIC) without the complicated processing modules, as used widely in traditional full-duplex techniques. Instead of the conventional practice of assigning the entire QAM constellation to only one communication node, our scheme (termed as QAM-R) splits the constellation into two parts and further assigns them to different communication nodes, respectively. With the principle above, the mapping pattern and the frame format are carefully designed. Then, the corresponding signal model is established and the approximation of end-to-end bit error probability (BEP) is derived. Finally, numerical simulations validate the theoretical analysis and show that QAM-R can balance spectrum effectiveness and transmission reliability very well and, especially, performs even better in the low range of signal noise ratio (SNR) when adopting higher-order QAM constellations.
Selection-and-stay combining for multi-branch (SSTC-MB) systems is investigated. The mathematical model is first established. Based on the Markovian property of the received signal-to-noise ratio, the outage performance is analysed and the closed-form expression is obtained. Numerical results validate the theoretical analysis and show that the SSTC-MB system not only enhances outage performance but also expands the applied scope of a diversity system. Introduction: Diversity, as a key technique, has always been a hot research direction in wireless communication systems. Recently, a variety of hybrid diversity strategies have been designed to meet the trade-off between performance and complexity (see, e.g. [1-3] and references therein). In particular, a selection-and-stay combining (SSTC) scheme was presented in [4], in which a useful attempt for the integration of threshold-switched combining (TSC) and selection combining (SC), with the advantage of both low complexity of TSC and high performance of SC, is made. Unfortunately, the scheme is essentially equivalent to TSC, except for the case that both diversity paths are poor. In this case, although SSTC picks out the stronger branch, the performance improvement, compared with TSC, is limited owing to the poor channel conditions. Moreover, SSTC merely applies to dual-branch systems, restricting its practicability. In this Letter, to overcome the above drawbacks, we extend the SSTC scheme to multi-branch systems, termed the SSTC-MB system. After designing and modelling the combining strategy of the SSTC-MB system, we investigate the outage performance, and obtain the closed-form expression, which indicates that the scheme is able to reach its highest potential, i.e. full diversity gain. Numerical results confirm the theoretical analysis and show that the SSTC-MB system can improve outage performance enormously and effectively enlarge the applied range of a diversity system.
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