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

Non-orthogonal multiple access (NOMA) aims to increase the spectral efficiency of fifth generation networks by relaxing the orthogonal use of radio-resources. In this paper, a network with multiple half-duplex buffer-aided (BA) relays is considered, where the source transmits with a fixed rate toward 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 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 NOMA and hybrid NOMA/OMA, namely BA-NOMA and BA-NOMA/OMA, respectively. BA-NOMA can improve the performance in terms of the outage probability when the power allocation factor α is selected such that robustness against channel uncertainties due to, e.g., outdated channel state information is provided.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.

I. Introduction

The expected success of 5G networks relies, not only on traditional communication practices, but also on novel communication paradigms, thus deviating from the current trends. Among those paradigms, non-orthogonal multiple access (NOMA) differentiates from traditional orthogonal multiple access (OMA), allowing users to share the same resource (time/frequency/code) and exploit different channel power levels [1]. Under NOMA, at the transmitter, superposition coding is deployed [2], while at the receivers, successive interference cancellation (SIC) is performed [3], [4]. The surveys in [5], [6] presented various challenges of NOMA, including the need for efficient user pairing, based on different channel quality for optimal NOMA, while the potential of NOMA for increasing the resource efficiency was underlined. Regarding resource allocation in NOMA, various works have provided efficient solutions. The authors in [7] studied joint sub-channel assignment and power allocation in the downlink of a NOMA network, aiming to maximize the weighted sum-rate and maintaining user fairness. In addition, spectral and power resource allocation optimization, towards increasing the energy efficiency of the network, was presented in [8]. As power allocation to multiple users is a key issue in NOMA networks, several surveys studied various power-domain (PD) NOMA scenarios, emphasizing the need for the development of novel cooperation techniques, facilitating NOMA when relays are available in multi-user networks [9]–[11]. Also, an important practical issue raised by the aforementioned works is the deteriorated performance of NOMA under imperfect/outdated channel state information (CSI) [9]. Furthermore, when only statistical CSI is available, NOMA was shown to outperform OMA when power allocation and decoding order are optimized under the max-min fairness criterion [12]. Finally, when multiple antennas are available at the source, precoding and the minorization-maximization algorithm were used to maximize the sum-rate of single-antenna destination [13], while for multi-antenna destinations, multiple-input multiple output (MIMO) NOMA was proved to provide improved performance compared to MIMO OMA [14].