A new hybrid CDMA–NOMA scheme with power allocation and user clustering for capacity improvement

Non-orthogonal multiple access (NOMA) is a prominent 5G wireless communication technique which ensures higher spectral efficiency, low latency and user fairness. As power-domain NOMA exploits different power levels assigned to each user for decoding, therefore, performance of this scheme degrades badly in a network having approximately equidistant users. To address this challenge, a novel hybrid CDMA–NOMA scheme for 5G downlink is proposed in this paper. Superposition coding is done at the transmitter side while successive interference cancellation (SIC) is performed at the receiver. The proposed scheme differentiates inter-cluster users on the basis of spreading codes, whereas intra-cluster users are differentiated on the basis of different power levels. Interference in the system is mitigated through two different techniques, inter-cluster interference through spreading codes (PN and Walsh codes) while intra-cluster interference through SIC technique. We implement an accurate power allocation strategy with users clustering best with poor model (users with nearest and farthest distances to BS) which is based on the distance of each user from base station. Considering user’s distance and channel coefficient, the proposed technique adapts intelligently among NOMA and hybrid CDMA–NOMA. BER performance of the hybrid CDMA–NOMA system is analyzed in NYUSIM channel model. The proposed approach improves the capacity without increasing the bandwidth and/or number of antennas, and results in reduced interference in congested NOMA systems.