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

TL;DR

This paper analyzes the ergodic sum secrecy rate of downlink NOMA in underlay spectrum sharing, considering imperfect CSI, and demonstrates NOMA's advantages over OMA in secrecy performance through derived closed-form expressions and simulations.

Contribution

It provides exact and asymptotic formulas for the ESSR of downlink NOMA with imperfect CSI under interference and power constraints, highlighting NOMA's superiority over OMA.

Findings

NOMA outperforms OMA in secrecy rate.

NOMA maintains strong user secrecy and enhances weak user secrecy.

Performance is limited by either interference or power constraints.

Abstract

In this paper, we present the ergodic sum secrecy rate (ESSR) analysis of an underlay spectrum sharing non-orthogonal 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 closed-form 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.