Joint Subcarrier Assignment and Power Allocation in Downlink SCMA Systems

Sparse code multiple access (SCMA) represents a paradigm shift from conventional multiple access concept and is considered as a promising candidate multiple access technique for the fifth generation (5G) wireless communication systems. In this paper, we investigates the downlink resource allocation problem in SCMA systems. Taking user fairness into account, we formulate a problem to maximize the sum of logarithmic user data rate by jointly optimizing the subcarrier assignment and power allocation. The formulated problem is combinatorial and non-convex. To tackle this problem, we first convert the power allocation problem into a standard convex problem by assuming the subcarrier assignment is fixed. Based on this, then the algorithm ``remove and reallocate'' for joint power allocation and subcarrier assignment is proposed with low complexity. Simulation results demonstrate that our proposal is near-optimal and can achieve good performance over both traditional OFDM systems and random subcarrier assignment in SCMA systems.