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Non-orthogonal multiple access (NOMA) has been considered as an essential enabling technology for fifth generation (5G) and beyond 5G (B5G) cellular networks to meet the increasing demands on low latency, high reliability, massive connectivity, improved fairness, and high throughput. Cooperative relay networks, on the other hand, have been shown to significantly improve the throughput, coverage, and achievable rates of wireless networks, in which, the relay nodes assist the communication between the source node and the destination node when the direct channel is poor. There has been extensive research on the subject of NOMA and cooperative relay networks. However, most of the existing research relies on two main assumptions: at fading channels and perfect channel state information (CSI) at the transmitting nodes. Instead of those idealistic assumptions, we study NOMA in cooperative relay networks under practical assumptions: frequency selective multipath fading channels and statistical CSI at the transmitting nodes. This dissertation begins with an overview of the basic concept of NOMA. Then, it focuses on downlink NOMA transmissions in a cooperative relay network. Specifcally, a base station communicates with two paired mobile users simultaneously via superposition coded orthogonal frequency division multiplexing (OFDM) transmissions with the help of a half-duplex relay under either the decode-and-forward (DF) or amplify and-forward (AF) scheme, where the power splitting between two mobile users depends on the statistical CSI. We derive the ergodic rates under Rayleigh fading channels and present an ad-hoc approach to determine the power splitting parameters when the target data rates are given. After that, this dissertation presents uplink NOMA transmissions in a cooperative relay network. Specifcally, two users, a near user and a far user, communicate with a base station at the same time with existence of a half-duplex relay employing the DF scheme to assist the far user. At the transmitting nodes, the power allocation factors are decided based on the statistical CSI while, at the receiving nodes, the effect of imperfect successive interference cancellation (SIC) implementation is taken into account. Under Rayleigh fading channels, we derive the exact ergodic rate for the near user and the upper bound of the ergodic rate for the far user, then present an ad-hoc approach to determine the power allocation factors when the target data rates are given, and we study the outage probability of the two users at the base station. The last part of this dissertation extends the study into an energy harvesting cooperative wireless sensor network. We consider a two-hop network consisting of a source, two parallel half-duplex relay nodes, and two destinations. While the destinations have adequate power supply, the source and relay nodes rely on harvested energy for data transmission. Different from all existing works, the two relay nodes can also transfer their harvested energy to each other via energy conferencing channels. For such a system, an optimization problem is formulated with the objective of maximizing the total data rate and conserving the source and relay transmission energy.
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