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 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.