Beamforming Techniques for MIMO-NOMA for 5G and Beyond 5G: Research Gaps and Future Directions

Effective sharing of the communication channel among many users, or multiple access (MA) techniques, can play a vital role in meeting the diverse demands of low latency, high reliability, massive connectivity, better fairness, and high throughput. In this context, non-orthogonal multiple access with multiple antennas, also known as multiple-input, multiple-output NOMA (MIMO-NOMA), is a promising enabling technology for fifth-generation (5G) and beyond (5G) wireless networks. The proper design of beamforming systems is one of the major difficulties in developing MIMO-NOMA. There are various ways to design beamforming for MIMO-NOMA in the literature. However, there is not much work dedicated to the survey focusing only on beamforming design in MIMO-NOMA systems. This work presents a comprehensive overview of beamforming methods in MIMO-NOMA for 5G and B5G. These strategies are classified in detail and have varied attributes, benefits, and drawbacks. As a result, future research gaps are also highlighted. Moreover, a simulation study is presented as a case study on the impact of random beamforming in various scenarios of heterogeneous environments with small and macro-cells. For this purpose, users' outage probability is simulated with various types of interference in the heterogeneous systems, including inter-cluster, cross-tier, and co-tier interferences. This analysis also helps to contrast the performance of small and macro-cells. Finally, future research directions are discussed for beamforming in MIMO-NOMA for 5G and B5G.