MC-CDMA and SCMA Performance and Complexity Comparison in Overloaded Scenarios

In 5G wireless networks, the perspective of very large number of simultaneous connections is challenging. In this sense, an overloaded system is a basic scenario of 5G multiple access techniques, that is, the number of active users is greater than the number of system subcarriers. Some schemes of non-orthogonal multiple access (NOMA) have been proposed in the literature and, two of them are compared in this paper. These NOMA techniques are the multi-carrier code division multiple access (MC-CDMA) and sparse code multiple access (SCMA). The performance, in terms of the mean bit error rate, and the decoding complexity, in terms of number of mathematical operations are evaluated. In MC-CDMA, the multi-user maximum likelihood detector (MU-MLD) implemented via sphere decoding (SD) algorithm is considered, which is the optimum receiver, but with polynomially complexity. In SCMA, a near optimal receiver is used, named as message passing algorithm (MPA), which is based on the sum-product algorithm, whose complexity does not depend, directly, on the number of users. Monte Carlo simulations and theoretical expressions are used to compare both multiple access techniques in the uplink of a cellular scenario. Results show that, for an equivalent spectral efficiencies, MC-CDMA performs better than SCMA and SD algorithm is less complex than MPA.