Buffer-aided relay selection for cooperative NOMA in the internet of things

The nonorthogonal multiple access (NOMA) well improves the spectrum efficiency which is particularly essential in the Internet of Things (IoT) system involving massive number of connections. It has been shown that applying buffers at relays can further increase the throughput in the NOMA relay network. This is however valid only when the channel signal-to-noise ratios (SNRs) are large enough to support the NOMA transmission. While it would be straightforward for the cooperative network to switch between the NOMA and the traditional orthogonal multiple access (OMA) transmission modes based on the channel SNR-s, the best potential throughput would not be achieved. In this paper, we propose a novel prioritization-based buffer-aided relay selection scheme which is able to seamlessly combine the NOMA and OMA transmission in the relay network. The analytical expression of average throughput of the proposed scheme is successfully derived. The proposed scheme significantly improves the data throughput at both low and high SNR ranges, making it an attractive scheme for cooperative NOMA in the IoT

Performance analysis of reconfigurable intelligent surface assisted wireless communications

The advent of the smart radio environment has challenged traditional wireless communication systems, and reconfigurable intelligent surfaces (RIS) have emerged as a promising solution to enhance the performance limits of wireless communications. This thesis aims to study RIS technology in future networks by examining the performance of four typical RIS-assisted communication systems, each featuring unique models and practical RIS characteristics. The research begins with the performance analysis of RIS-assisted wireless communication systems with phase errors in both line-of-sight (LoS) and Rayleigh fading scenarios. It investigates the impact of different types of phase errors and other system parameters on system performance. Next, the impact of channel correlations on RIS-assisted communication systems is examined by deriving closed-form expressions of outage probability and average achievable rates for both correlated Rayleigh fading and correlated Nakagami-m fading models. The relationship between RIS size and performance degradation caused by channel correlations is explored. The study further investigates the application of RIS technology in more intricate wireless communication systems, specifically focusing on two spectrum efficiency techniques: non-orthogonal multiple access (NOMA) and full-duplex (FD) communications. For RIS-assisted NOMA, the research derives closed-form expressions for the outage probability of users with strong and weak channel conditions, while for RIS-assisted FD systems, the research proposes a setup where an FD transceiver serves uplink and downlink users simultaneously with two dedicated RISs and examines the impact of residual self-interference (SI). Finally, the performance of a RIS-assisted largescale network is examined through stochastic geometry, assessing coverage probability and average achievable rate. The research discusses two association strategies: nearest and fixed association, and investigates the effects of increasing transmitter (TX) density and RIS association probability on system performance. This thesis provides valuable analytical resources on RIS-assisted wireless communications performance, laying a foundation for future research and application of RIS technology