Hybrid NOMA/OMA With Buffer-Aided Relay Selection in Cooperative Networks

Abstract

Non-orthogonal multiple access (NOMA) aims to increase the spectral efficiency of fifth generation (5G) networks by relaxing the orthogonal use of radio-resources. In this work, a network with multiple half-duplex (HD) buffer-aided (BA) relays is considered, where the source transmits with fixed rate towards two users. The users might demand the same rate by the source (e.g., two cellular users requiring the same service), or they could have different rate requirements (e.g., a cellular user coexisting with an Internet of Things (IoT) device). By deploying multiple BA relays, increased reliability and additional degrees of freedom are provided. Leveraging the spectral efficiency of NOMA and the increased diversity gain of BA relaying, two relay selection algorithms with broadcasting are proposed for power-domain (PD) NOMA and hybrid NOMA/OMA, namely BA-NOMA and BA-NOMA/OMA, respectively. BA-NOMA can improve the performance in terms of outage probability when the power allocation factor α is selected such that robustness against channel uncertainties due to, e.g., outdated channel state information (CSI), is provided. Moreover, BA-NOMA/OMA further improves the sum-rate by switching to OMA when the relays cannot serve the users through NOMA. For both cases, a theoretical analysis for the outage probability is conducted and the asymptotic performance is studied. Finally, numerical results and comparisons with other state-of-the-art algorithms are provided for the outage probability, average throughput and average delay.

Full-Duplex NOMA Transmission with Single-Antenna Buffer-Aided Relays

“…In downlink NOMA networks with a single source, multiple BA relays and two destinations, hybrid BA-NOMA/OMA was proposed in [31], integrating broadcasting in the {S→R} link and exploiting BSI in the {R→D} NOMA transmission. The steps of BA-NOMA/OMA are given in the following paragraphs:…”

“…There are different methods of determining the power allocation coefficient α k in NOMA networks. In this work, power allocation follows the method that was developed in [31] which considered a two-hop relay network with two destinations with possibly heterogeneous rate requirements, as it is the case of coexisting IoT devices and users. Since a similar {R→D} link setting is studied here, readers are referred to that work for more details on calculating α k .…”

“…Here, comparisons include the OMA schemes of [14] (LoCo − Link) and [39] (OMA RD), as well as equivalent NOMA-based versions (LoCo − Link-NOMA and NOMA RD). Finally, the hybrid BA-HD-NOMA/OMA of [31] is compared, where if NOMA fails, a single user D i is served with rate r i , i ∈ {1, 2}. However, BA-HD-NOMA/OMA is excluded from the outage results, as it cannot be directly compared to standalone NOMA and OMA [29].…”

“…In networks where multiple BA relays are present, two BA-ORS algorithms were given in [30], choosing the best relay, by considering the buffer state information (BSI) and adopting {R→D} prioritization for reduced latency. Then, the potential of BA-ORS in NOMA and hybrid NOMA/OMA networks was presented in [31,32], where hybrid NOMA/OMA selection policies were devised, further improving the sum-rate performance in the downlink and the uplink of two-hop networks, respectively.…”

Integrating Broadcasting and NOMA in Full-Duplex Buffer-Aided Opportunistic Relay Networks

“…The results showed that a diversity gain equal to two can be achieved for buffer sizes larger than two. Finally, Nomikos et al in [17] developed BA-ORS algorithms integrating broadcasting in the {S→R} link and combining NOMA with OMA for robustness against outages, increasing the average sum-rate and reducing the delay. Furthermore, Nomikos et al in [7] combined HD and FD BA relays with the objective of minimizing the power expenditure.…”