Author: Wang, Xinyu
Title: High-spectrum-efficiency transmission technique based on full-duplex cooperative non-orthogonal multiple access
Advisors: Lau, Chung-ming, Francis (EIE)
Ho, Wang Hei Ivan (EIE)
Degree: Ph.D.
Year: 2023
Subject: Wireless communication systems
Digital communications
Signal processing -- Digital techniques
Hong Kong Polytechnic University -- Dissertations
Department: Department of Electronic and Information Engineering
Pages: xxii, 156 pages : color illustrations
Language: English
Abstract: To meet the increasing demand and the requirements for low latency and massive connectivity, non-orthogonal multiple access (NOMA) has recently received significant attention due to its potential of achieving enhanced user fairness, enlarged connections, reduced access latency, and facilitated diverse quality of service. In the meanwhile, cooperative communication has obtained a lot of attention because of its ability to provide spatial diversity gain to mitigate fading, to extend coverage and to improve the communication reliability. Therefore, cooperative non-orthogonal multiple access (CNOMA) has attracted a great deal of attention since it is promising to further improve the system efficiency and to achieve an ideal win-win situation for NOMA users. This thesis investigates and proposes the CNOMA scheme from four perspectives for high spectrum efficiency.
Firstly, we propose a half-duplex (HD) CNOMA scheme based on spectrum sensing. In this scheme, a primary user (PU), i.e., a weak user, intends to communicate with the base station with the assistance of a secondary user (SU), i.e., a strong user, which works as a HD relay. In the meanwhile, the SU obtains an additional opportunity to share the same spectrum band originally belonging to the PU by NOMA. This scheme adopts spectrum sensing technique to identify spectrum holes, accordingly, to choose an appropriate form of transmitted signals, avoid useless transmission, and eventually obtain superior system capacity. We take the practical assumption of imperfect spectrum sensing into consideration and characterize the performance of our proposed scheme. Closed-form expressions for outage probability, system throughput in delay-limited transmission mode, ergodic rate, and system throughput in delay-tolerant transmission mode are derived, with or without the direct link between the base station and the PU. Simulation results are presented to further validate these closed-form derivation expressions, verify the effectiveness of employing the spectrum sensing technique, and illustrate the superior performance of the proposed HD CNOMA scheme compared with two cooperative benchmarks.
Secondly, we investigate a full-duplex (FD) CNOMA scheme based on spectrum sensing. In this scheme, PU communicates with the base station with the assistance of SU which works as a FD relay. The proposed collaboration scheme adopts spectrum sensing to identify spectrum holes and accordingly avoid the waste of spectrum hole resources. It enables PU and SU to share the licensed spectrum band of PU by NOMA, which further increases the spectrum efficiency. By adopting FD mode, the proposed FD CNOMA scheme not only achieves better system performance than HD mode by receiving and transmitting simultaneously, but also overcomes inherent issues of a conventional cognitive radio network. To characterize the performance of the proposed scheme, expressions of exact and asymptotic ergodic rates and system throughput are worked out. Accordingly, the high signal-to-noise ratio slopes for these ergodic rates are further derived. Simulation results are presented to verify the correctness of all these derivation results and to illustrate the performance superiority of FD CNOMA compared with other cooperative benchmarks in real-world scenarios in terms of both ergodic rates and system throughput.
Thirdly, we present a three-stage relay selection strategy with dynamic power allocation (TRSPA) for the above-proposed FD CNOMA scheme based on spectrum sensing. Considering the higher priority of PU than SU, the relay selection objective is to maximize the transmission data rate of the selected relay, i.e., SU, on the condition that PU's signal is successfully uploaded to the base station by precisely narrowing down relay candidates step-by-step and dynamically allocating optimal power coefficients. Exact and asymptotic outage probabilities and ergodic rates are worked out.
Accordingly, diversity orders and spatial multiplexing gains are derived. We further exploit the impact of self-interference (SI) on TRSPA for FD CNOMA and then compare its performance with TRSPA applied in other relaying modes, that is half-duplex and orthogonal multiple access. Finally, simulation results are presented to reveal that: (i) theoretical derivation results are correct; (ii) TRSPA always outperforms other relay selection strategies in terms of outage probability and ergodic rate; and (iii) TRSPA for FD CNOMA in a real-world scenario achieves better performance than other relaying modes in spite of the adverse effect of SI in FD mode.
Fourthly, we exploit a two-way relaying FD CNOMA scheme, where NOMA users intend to exchange messages via a two-way FD relay. To characterize the potential performance gain brought by this proposed FD CNOMA scheme, the outage probability and ergodic rate are analyzed. Specifically, the closed-form expressions for the outage probabilities, diversity orders, ergodic rates, and system throughputs in delay-limited and delay-tolerant transmission modes are derived under the realistic assumption of imperfect self-interference cancellation. Furthermore, to present the comprehensive performance evaluation, both perfect and imperfect successive interference cancellations (SICs) are taken into consideration. Simulations are performed to validate the accuracy of the derivation results and to illustrate the outstanding performance of the proposed scheme in low signal-to-noise ratio region compared with HD CNOMA system and cooperative orthogonal multiple access system. Our results show that under the conditions of both perfect and imperfect SICs, outage probability floors and ergodic rate ceilings exist for the proposed FD CNOMA scheme due to the inter-user interference among superimposed NOMA signals and the residual self-interference caused by the imperfect self-interference cancellation.
Rights: All rights reserved
Access: open access

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