Efficient Nonorthogonal Multiple Access: Cooperative Use of Distributed Space?Time Block Coding

The demand for higher data rates and seamless connectivity has caused researchers to investigate novel techniques for fifth-generation (5G) wireless systems. The scarcity and congestion of bandwidth in the current useable frequency spectrum have urged the use of bandwidth-efficient access techniques, with nonorthogonal multiple access (NOMA) being one of them. NOMA exploits the multiplexing gain in the power domain and provides large data rates at the expense of interference, which is catered through the use of interference cancellation techniques. Further enhancement in NOMA, such as bringing cooperation between users, has been a hot topic as it further improves the performance of the system. This article, besides giving an overview of various features of NOMA, investigates a new cooperative NOMA (C-NOMA) strategy that employs distributed space?time block coding (STBC) and is known as STBCNOMA. The proposed case study incorporates the use of orthogonal and nonorthogonal access simultaneously along with STBC and shows that the STBC-NOMA reduces the computational overhead at the user end while improving the system throughput and energy efficiency (EE). Finally, potential applications and future research directions in this area are provided.