Ergodic Capacity Analysis of Alamouti Coded Cooperative Communication in Downlink NOMA

The demand for massive connectivity and low latency in future wireless networks has outlined the non-orthogonal multiple access (NOMA) scheme a key technology due to its effectiveness to meet the massive capacity requirement of the beyond fifth-generation (B5G) and the sixth-generation (6G) systems. However, with the increase in the total number of users in NOMA, the number of successive interference cancellations (SICs) at each user is also increased, which ultimately incurs high computational complexity and hardware cost of the overall system. Space-time block code (STBC)-based cooperative NOMA (STBC-NOMA) requires a smaller number of SICs as compared to that of conventional cooperative NOMA. However, prior work on STBC-NOMA assumes perfect SIC, which is challenging in practice. Motivated by this fact, in this paper, we study the impact of imperfect SICs on the performance of STBC-NOMA. Further, we derive the closed-form expression for the ergodic capacity of STBC-NOMA for imperfect SICs and use that to draw a comparative analysis between STBC-NOMA, conventional cooperative NOMA, and non-cooperative NOMA. We find the maximum number of users accommodated by STBCNOMA for different levels of imperfect SICs.