Non-orthogonal Multiple Access for next Generation Wireless Communications
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Non-orthogonal multiple access (NOMA) is considered as a strong candidate for en- hancing the spectrum efficiency of future fifth generation (5G) wireless systems. A key feature of NOMA is the superposition of multiple users’ messages in a single resource by allocating different power levels to each user and applies successive interference cancellation (SIC) at receiver to suppress intra-user interference. The current literature is sparse in exploring the application of NOMA to multi-tier cellular network, group device-to-device (D2D) communication and wireless sensor networks (WSN). Previous studies have also shown that NOMA has no significant gain over OMA in low signal-to-noise ratio (SNR) regime, and there are some potential drawbacks of SIC that can limit NOMA performance. In order to address these gaps in knowledge, extensive research is conducted in this thesis. The first three chapters provide introduction and research motivations, background concepts relevant to this thesis, and literature review to identify the research gaps. In particular, research work conducted in this thesis can be divided into two parts. The first part considers the application and performance analysis of NOMA for multi- tier cellular networks, group device-to-device (D2D) communications and ubiquitous wireless sensor networks (UWSNs). Based on the work done in first part, three problems are identified to be addressed: a) to enhance the performance of NOMA in low SNR regime; b) to enhance the performance of NOMA under similar channel conditions; c) to resolve the issues related to the use of SIC with NOMA. As a result, the second part of the thesis proposes solutions to solve the aforementioned problems. The specific novel contributions of this thesis can be summarised as follows: 1) developed a novel analytical framework to investigate multi-tier NOMA networks with underlay D2D communications; 2) proposed a quality of service (QoS) based NOMA group D2D communication scheme where unlike existing works, the D2D users (DRs) are ordered in NOMA according to their QoS requirements; 3) investigated and analysed NOMA for ubiquitous wireless sensor networks (UWSNs) in the presence of cross- technology (CT) nodes; 4) proposed a hybrid multiple access (HMA) scheme where users are scheduled either for NOMA and OMA in order to enhance the performance in low SNR regime; and 5) designed an alternate receiver structure based on parallel interference cancellation (PIC) to alleviate the SIC issues as well as an equivalent transmission model for downlink NOMA. Moreover, in order to analyse the performance of all the considered NOMA systems, closed-form expressions for outage probability are also derived.