Link-Layer Capacity of Downlink NOMA with Generalized Selection Combining Receivers

TL;DR

This paper evaluates the link-layer capacity of downlink NOMA systems with generalized selection combining, considering delay constraints, and provides approximate formulas for effective capacity at different SNR regimes.

Contribution

It introduces an analysis of effective capacity for downlink NOMA with GSC, bridging the gap between traditional diversity schemes and considering practical delay constraints.

Findings

GSC improves link-layer performance over traditional schemes.

Approximate EC expressions are accurate at low and high SNR.

Tradeoff identified between RF chains and system performance.

Abstract

Non-orthogonal multiple access (NOMA) has drawn tremendous attention, being a potential candidate for the spectrum access technology for the fifth-generation (5G) and beyond 5G (B5G) wireless communications standards. Most research related to NOMA focuses on the system performance from Shannon's capacity perspective, which, although a critical system design criterion, fails to quantify the effect of delay constraints imposed by future wireless applications. In this paper, we analyze the performance of a single-input multiple-output (SIMO) two-user downlink NOMA system, in terms of the link-layer achievable rate, known as effective capacity (EC), which captures the performance of the system under a delay-limited quality-of-service (QoS) constraint. For signal combining at the receiver side, we use generalized selection combining (GSC), which bridges the performance gap between the two conventional diversity combining schemes, namely selection combining (SC) and maximal-ratio combining (MRC). We also derive two approximate expressions for the EC of NOMA-GSC which are accurate at low-SNR and at high-SNR, respectively. The analysis reveals a tradeoff between the number of implemented receiver radio-frequency (RF) chains and the achieved performance, and can be used to determine the appropriate number of paths to combine in a practical receiver design.