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Buffer-Aided (BA) relaying has shown tremendous performance improvements in terms of throughput and outage probability, although it has been criticized of suffering from long delays that are restrictive for applications, such as video streaming, Web browsing, and file sharing. In this paper, we propose novel relay selection policies aiming at reducing the average delay by incorporating the buffer size of the relays into the decision making of the relay selection process. More specifically, we first propose two new delay-aware policies. One is based on the hybrid relay selection algorithm, where the relay selection takes into account the queue sizes so that the delay is reduced and the diversity is maintained. The other approach is based on the max - link relay selection algorithm. For the max - link algorithm, a delay-aware only approach starves the buffers and increases the outage probability of the system. Thus, for max - link, we propose a delay- and diversity-aware BA relay selection policy targeting the reduction of the average delay, while maintaining the diversity of the transmission. The proposed policies are analyzed by means of Markov Chains and expressions for the outage, throughput, and delay are derived. The asymptotic performance of the policies is also discussed. The improved performance in terms of delays and the use of the proposed algorithms are demonstrated via extensive simulations and comparisons, signifying, at the same time, the need for adaptive mechanisms to handle the interplay between delay and diversity.

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. 7 88743 88755