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In this paper, we study the achievable link-layer rate, namely, effective capacity (EC), under the per-user statistical delay quality-of-service (QoS) requirements, for a downlink nonorthogonal multiple access (NOMA) network with M users. Specifically, the M users are assumed to be divided into multiple NOMA pairs. Conventional orthogonal multiple access (OMA) then is applied for inter-NOMA-pairs multiple access. Focusing on the total link-layer rate for a downlink M-user network, we prove that OMA outperforms NOMA when the transmit signalto-noise ratio (SNR) is small. On the contrary, simulation results show that NOMA prevails over OMA at high values of SNR. Aware of the importance of a two-user NOMA network, we also theoretically investigate the impact of the transmit SNR and the delay QoS requirement on the individual EC performance and the total link-layer rate for a two-user network. Specifically, for delay-constrained and delay-unconstrained users, we prove that for the user with the stronger channel condition in a twouser network, NOMA prevails over OMA when the transmit SNR is large. On the other hand, for the user with the weaker channel condition in a two-user network, it is proved that NOMA outperforms OMA when the transmit SNR is small. Furthermore, for the user with the weaker channel condition, the individual EC in NOMA is limited to a maximum value, even if the transmit SNR goes to infinity. To confirm these insightful conclusions, the closed-form expressions for the individual EC in a two-user network, by applying NOMA or OMA, are derived for both users and then confirmed using Monte Carlo simulations.

This paper investigates delay constrained performance of a cognitive radio relay network when the cognitive (secondary) user transmission is subject to satisfying spectrumsharing restrictions imposed by a primary user. The primary user allows a secondary user to gain access to its allocated spectrum band as long as certain thresholds on the interference power, on the peak or average values, inflicted on the primary receiver are not exceeded by the transmission of the secondary users. In addition, we assume that the secondary transmitter benefits from an intermediate node, chosen from terminals, to relay its signal to the destination. Considering that the transmission of the secondary user is subject to satisfying a statistical delay quality-of-service (QoS) constraint, we study the maximum arrival rate of the secondary user's relay link while the interference limitations required by the primary user are satisfied. Particularly, we obtain the effective capacity of the secondary network and determine the power allocation policies that maximize the effective capacity of the secondary user's relaying channel. In addition, we derive closed-form expressions for the effective capacity of the channel in Rayleigh blockfading environment under peak or average interference-power constraints. Numerical simulations are provided to endorse our theoretical results.