  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.
