Abstract
In this article, we study the performance of an uplink non-orthogonal multiple access (NOMA) network under statistical quality of service (QoS) delay constraints, captured through each user's effective capacity (EC). We first propose novel closed-form expressions for the EC in a two-user NOMA network and show that in the high signal-to-noise ratio (SNR) region, the “strong” NOMA user, referred to as U <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , has a limited EC, assuming the same delay constraint as the “weak” user, referred to as U1. We demonstrate that for the weak user U1, OMA and NOMA have comparable performance at low transmit SNRs, while NOMA outperforms OMA in terms of EC at high SNRs. On the other hand, for the strong user U <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , NOMA achieves higher EC than OMA at small SNRs, while OMA becomes more beneficial at high SNRs. Furthermore, we show that at high transmit SNRs, irrespective of whether the application is delay tolerant, or not, the performance gains of NOMA over OMA for U <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> , and OMA over NOMA for U <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> remain unchanged. When the delay QoS of one user is fixed, the performance gap between NOMA and OMA in terms of total EC increases with decreasing statistical delay QoS constraints for the other user. Next, by introducing pairing, we show that NOMA with user-pairing outperforms OMA, in terms of total uplink EC. The best pairing strategies are given in the cases of four and six users NOMA, raising once again the importance of power allocation in the optimization of NOMA's performance.
Highlights
N On-orthogonal multiple access (NOMA) schemes have attracted a lot of attention recently, allowing multiple users to be served simultaneously with enhanced spectral efficiency; it is known that the boundary of achievable rate pairs using non-orthogonal multiple access (NOMA) is outside the capacity region achievable with orthogonal multiple access (OMA) techniques [1]–[5]
The authors in [11] introduce an iterative interference cancellation (IIC) detection scheme for uplink NOMA, and proposed a new detection scheme based on IIC, which is called advanced IIC
A detailed comparison between NOMA and OMA is provided; through an extensive set of simulation results, we show that NOMA does not always perform better than OMA in the presence of delay constraints
Summary
N On-orthogonal multiple access (NOMA) schemes have attracted a lot of attention recently, allowing multiple users to be served simultaneously with enhanced spectral efficiency; it is known that the boundary of achievable rate pairs using NOMA is outside the capacity region achievable with orthogonal multiple access (OMA) techniques [1]–[5]. The present work extends our recent publication [34] in which novel closed form expressions for the effective rate in a two user uplink NOMA network were presented. Parameters Total number of users User i Allocated transmission power to Ui Total power Transmit SNR Delay QoS constraint of Ui Channel gain of Ui Received superimposed signal at the base station Achievable rate of Ui under NOMA Achievable rate of Ui under OMA Duration of each fading-block System bandwidth Normalized QoS exponent Effective capacity of i-th user NOMA Sum ECs under NOMA Sum ECs under OMA ically, to be the pairing of users with the maximum channel gains gaps, which is in agreement to previous results in systems without delay constraints [35].
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