On the Secrecy Rate of Downlink NOMA in Underlay Spectrum Sharing with Imperfect CSI

Abstract

In this paper, we present the ergodic sum secrecy rate (ESSR) analysis of an underlay spectrum sharing nonorthogonal multiple access (NOMA) system. We consider the scenario where the power transmitted by the secondary transmitter (ST) is constrained by the peak tolerable interference at multiple primary receivers (PRs) as well as the maximum transmit power of the ST. The effect of channel estimation error is also taken into account in our analysis. We derive exact and asymptotic closedform expressions for the ESSR of the downlink NOMA system, and show that the performance can be classified into two distinct regimes, i.e., it is dictated either by the interference constraint or by the power constraint. Our results confirm the superiority of the NOMA-based system over its orthogonal multiple access (OMA) based counterpart. More interestingly, our results show that NOMA helps in maintaining the secrecy rate of the strong user while significantly enhancing the secrecy performance of the weak user as compared to OMA. The correctness of the proposed investigation is corroborated through Monte Carlo simulation.

On the Effective Rate of NOMA in Underlay Spectrum Sharing

“…The superiority of NOMA in an underlay spectrum sharing scenario over its OMA-based counterpart, in terms of the average achievable rate, outage probability and ergodic sum secrecy rate, has been well-established in [9]- [11]. However, to the best of the authors' knowledge, the delay-constrained analysis of a spectrum sharing NOMA system has not yet been presented in the open literature.…”