Flex-NOMA: Exploiting Buffer-Aided Relay Selection for Massive Connectivity in the 5G Uplink

The success of fifth generation (5G) mobile network and its long-term evolution (LTE) relies on enabling demanding services with massive connectivity requirements, including Internet of Things (IoT) nodes, mobile devices, or unmanned aerial vehicles (UAVs). Towards this end, non-orthogonal multiple access (NOMA) schemes allow multiple users and devices to communicate using the same temporal and spectral resources. In this context, buffer-aided (BA) relay selection can significantly enhance the quality and reliability of communication, through increased diversity. So, in this paper, we employ BA relay selection in the uplink of the NOMA networks where users and devices coexist and demand access to the wireless resources. The presented relay selection policy, namely flex - NOMA, facilitates simultaneous transmissions from multiple sources to multiple relays, exploiting channel state information at the reception, and dynamic decoding ordering by the relays performing successive interference cancellation (SIC). The theoretical analysis and performance evaluation results are provided and comparisons, in terms of outage probability, average sum-rate and average delay show that flex - NOMA offers improved performance without incurring high complexity and coordination overheads.