Cognitive Radio Based NOMA for The Next Generations of Wireless Communications

In this paper, cognitive radio (CR) based on non-orthogonal multiple access (NOMA) is considered over frequency non-selective Rayleigh fading channels. The two techniques are merged for the sake of improving the performance of multiuser wireless communications. Moreover, CR and NOMA can improve significantly the spectrum utilization and spectrum efficiency, respectively, for the next generations of wireless communications. Furthermore, NOMA can enable CR to work without the need to implement spectrum sensing as a traditional CR works. This can be achieved by sharing the available channels among different users by utilizing simultaneously the same frequency band, but with different power allocation factors (PAF) in the power domain. Additionally, CR-NOMA is proposed in this paper by giving a primary user the priority to communicate with the base station (BS) with its required target rate without any effect on its performance, while a secondary user can use the service with its minimum required rate in the presence of the primary user. This can be achieved by arranging the PAF to control this process. Different scenarios have been considered for two users, and simulation results demonstrate that the primary user has the ability to keep its target achievable rate with acceptable outage probability, while the secondary user can be served simultaneously with satisfactory performance in the presence of the primary user.

I. Introduction

Non-orthogonal multiple access (NOMA) can play a vital role in satisfying massive connectivity by sharing the available power among several users in the cell via exploiting simultaneously the same frequency band. In the transmission with a base station (BS) in the uplink and downlink phases, NOMA can satisfy user fairness, secure connectivity, and spectral efficiency by applying the power allocation method. The latter technique can be achieved by giving the furthest users from the BS, i.e. the users with low channel gain, more portion of the available power than the users near to BS, i.e. the users that have high channel gain. The successive-interference cancellation (SIC) model is applied to each user in the NOMA system, except the furthest user from the source, by detecting the signal of the higher-order user followed by implanting a subtraction from the entire incoming NOMA signal to obtain the signal of interest for that user. It is interesting to mention that the furthest user, which has the highest portion of the power can detect its signal without the need for SIC [1] - [2].