Non-orthogonal multiple access in wireless communication networks : transmission scheme design and performance analysis

• Main Author: Cheng, Yanyu
• Other Authors: Li Kwok Hung
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Electrical and electronic engineering::Wireless communication systems
• Online Access: https://hdl.handle.net/10356/146356
• Institution: Nanyang Technological University

Non-orthogonal multiple access (NOMA) has been regarded as a promising technique for next-generation wireless communication networks due to its potential capability on spectrum enhancement. It can meet the demand of high spectral efficiency and massive connectivity by allocating multiple users to a single resource block, which is different from the conventional orthogonal multiple access (OMA). This thesis provides a systematic discourse of this newly emerging technology including the transmission scheme design and the performance analysis. More specifically, it investigates multi-carrier NOMA, multi-carrier cooperative NOMA (C-NOMA), intelligent refecting surface (IRS)-aided NOMA, and NOMA assisted by multiple IRSs with discrete phase shifts. First, a problem related to user pairing and subcarrier allocation for multi-carrier NOMA systems is investigated. Correspondingly, a joint user pairing and subcarrier allocation scheme is proposed, and the closed-form expression for the outage probability of the worst-performance user in the worst case is derived. To gain further insight into the performance, the diversity order is obtained by asymptotic analysis. It is revealed that the proposed scheme can achieve the optimal diversity order. Second, multi-carrier C-NOMA systems are studied, where cell-edge users cannot communicate with the base station (BS) directly and need assistance from cell-center users. A two-step scheme pairing cell-center and cell-edge users is designed. To measure the performance of the proposed scheme, the closed-form expression for the upper bound of outage probability is derived. By asymptotic analysis, we find that our proposed scheme achieves the optimal performance in terms of diversity order. Third, multi-carrier C-NOMA systems are explored, in which all users are able to communicate the BS directly. Specifically, each NOMA pair can work in two modes: non-cooperative NOMA (NC-NOMA) and C-NOMA, where the mode se- lection is based on users’ channel conditions. Correspondingly, a scheme involving user pairing, subcarrier allocation, and mode selection is proposed. The closed- form expression for the upper bound of outage probability is derived. Following that, the achievable diversity order is obtained by asymptotic analysis, which is proved to be optimal. Fourth, since IRSs can enhance the channel quality, the combination of NOMA and IRS can further improve system performance, and IRS-aided NOMA systems are investigated, in which an IRS is deployed to intensify the coverage by assisting a cell-edge user device (UD) to communicate with the BS. To characterize system performance, new channel statistics of the BS-IRS-UD link with Nakagami-m fad-ing are investigated. For both downlink and uplink transmissions, the closed-form expressions for the outage probability and ergodic rate are derived. To gain fur-ther insight, the diversity order and the high signal-to-noise ratio (SNR) slope are obtained according to asymptotic approximations in the high-SNR regime. It is demonstrated that the diversity order is affected by the number of IRS reflecting el-ements and Nakagami-m fading parameters, but the high-SNR slope is not related to those parameters. It is also revealed that the IRS outperforms the full-duplex (FD) decode-and-forward (DF) relay in the high-SNR regime. Last, NOMA systems assisted by multiple IRSs with discrete phase shifts are explored, in which each UD is served by an IRS to enhance the received signal. Two scenarios are considered based on whether there is a direct link between the BS and each UD, and the outage performance is analyzed for each scenario. Specifically, the upper bound of outage probability is approximated in the high-SNR regime, and the achievable diversity order is obtained. OMA can be regarded as a special case of NOMA, and the outage performance of multi-IRS assisted OMA systems is also characterized. It is demonstrated that NOMA outperforms OMA in multi-IRS assisted systems. Furthermore, it is shown that discrete phase shifts do not degrade the performance in terms of diversity order as compared with continuous phase shifts. More importantly, simulation results further reveal that a 3-bit resolution for discrete phase shifts is sufficient to achieve near-optimal outage performance.