Two-way SWIPT-aided hybrid NOMA relaying for out-of-coverage devices

Relaying is a tool to solve basic problem of poor coverage and low capacity at the cell border due to low signal to interference and noise ratio. In this paper, we propose a novel non-orthogonal multiple access (NOMA) based relaying mechanism to extend the coverage of a source for bi-directional communication with some users in an out-of-coverage area by exploiting two users from a NOMA pair to act as relays for remote devices. A combination of multiple access schemes is used, source-relay communication is established with NOMA while relay-device communication is orthogonal multiple access (OMA) based. To avoid battery consumption of the relay nodes, power-switching based simultaneous wireless information and power transfer protocol is used for relaying uplink and downlink data between source and the users. Potentially, the scheme provides parallel coverage to multiple disconnected users with the help of wireless-powered NOMA relays (WPNRs). To characterise the performance gains of proposed system, two important metrics outage probability and ergodic rate are discussed. Specifically, analytical expressions for outage probabilities and delay-limited throughput in downlink (DL) and uplink (UL) communications are derived, which are also verified with simulations. Additionally, we also obtain closed-form expressions for the DL and UL asymptotic ergodic rates. Our results demonstrate that: (1) WPNRs establish bi-directional communication link between the source and remotely located users solely on the basis of scavenged energy; (2) WPNR based relaying network is superior in performance than OMA based relaying network in high SNR region; (3) Orientation of WPNRs contributes in improved system throughput.