Abstract
The nonorthogonal multiple access (NOMA) well improves the spectrum efficiency which is particularly essential in the Internet of Things (IoT) system involving massive number of connections. It has been shown that applying buffers at relays can further increase the throughput in the NOMA relay network. This is however valid only when the channel signal-to-noise ratios (SNRs) are large enough to support the NOMA transmission. While it would be straightforward for the cooperative network to switch between the NOMA and the traditional orthogonal multiple access (OMA) transmission modes based on the channel SNR-s, the best potential throughput would not be achieved. In this paper, we propose a novel prioritization-based buffer-aided relay selection scheme which is able to seamlessly combine the NOMA and OMA transmission in the relay network. The analytical expression of average throughput of the proposed scheme is successfully derived. The proposed scheme significantly improves the data throughput at both low and high SNR ranges, making it an attractive scheme for cooperative NOMA in the IoT.
Highlights
The Internet of Things (IoT) aims at connecting massive number of devices, imposing great challenges in mobile network design [1], [2]
In [11], a two-stage relay selection scheme is described to maximize the throughput for one non-orthogonal multiple access (NOMA) user upon satisfying the target transmission for the other
This becomes more serious in the NOMA cooperative network: TABLE I LIST OF NOTATIONS
Summary
The Internet of Things (IoT) aims at connecting massive number of devices, imposing great challenges in mobile network design [1], [2]. The proposed scheme only has higher throughput than its OMA counterpart in the high SNR range This is because the network throughput can be regarded as η(1 − Pout) in the delay-limited scenario [24], where η is the data rate (without considering the outage) and Pout is the outage probability. The early proposed buffer-aided max-link relay selection [25] may achieve full diversity order (i.e. twice the number of relay nodes) when the buffers have infinite size and balanced input/output data rates which is not always the case in practice. The state-based relay selection scheme achieves better outage performance than the max-link scheme, but the improvement becomes less significant for unbalanced channels. This becomes more serious in the NOMA cooperative network:
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