Performance Analysis and Optimization in Two-Way Cooperative NOMA Under Two-Phase Transmission Protocol

In this article, a novel two-phase transmission protocol is proposed to help the bidirectional transmission in a user-aid two-way cooperative nonorthogonal multiple access system, where superposition coding, bit-level Exclusive-Or operation, and successive interference cancellation (SIC) techniques are employed for improving the spectrum efficiency and throughput. Considering the residual interference generated in the practice process of decoding, both the perfect SIC and the imperfect SIC scenarios are discussed. Besides, the power allocation factor $\alpha _{1}$ and $\alpha _{2}$ are optimized under the perfect SIC to reduce the total outage probability, whose closed-form expression is derived and verified by simulations. Also, the approximated outage performance is derived in imperfect SIC scenario. Furthermore, two other conventional two-phase protocols and one three-phase protocol are provided as the benchmark to compare the outage probability and throughput with the proposed protocol, which shows the superiority of the proposed two-phase protocol and power allocation.