Relay Selection for Energy Harvesting Cooperative NOMA

We consider the downlink transmission of a energy harvesting (EH) cooperative non-orthogonal multiple access (NOMA) system in which decode and forward (DF) EH relays convey the source's message to the two downlink users. Transmission occurs in two phases. In the first phase, the source transmits the message to the relays through fading channels. In the next phase, one of the relays, which has decoded the message, forwards it to the users via wireless links. Superposition coding is used for transmission and successive interference cancellation (SIC) is applied to decode the message. The system experiences an outage when the transmitted message is not received by the users. We employ an optimal relay selection algorithm that minimizes the time-averaged outage of EH cooperative NOMA system for any i.i.d. fading distributions and any i.i.d. energy harvesting processes. We compare the performance of this algorithm with a low-complexity random selection scheme. Our results show the effect of the quanta of energy harvested, data rate requirements of the users, and the transmit power on the system outage.