Optimal User Pairing in Cache-Based NOMA Systems with Index Coding

The user pairing problem for cache-based time-slotted non-orthogonal multiple access (NOMA) system with index coding is investigated. During each time slot, the packets of two users are scheduled at the base station. In accordance with different cache information of the scheduled users, either superposition coding or index coding is applied for base station transmission. For some specific case, the superior performance on the aspect of power consumption of index coding compared to that of superposition coding is analyzed. Besides, the power saving of our design system when compared to that of the NOMA system with pure superposition coding is also demonstrated in a mathematical way. Subsequently, we show that the original user scheduling problem can be transformed in quadratic time into a minimum weight perfect matching problem of an undirected graph, which can be solved with time complexity O(K³), where K is the number of users. Based on this transformation, the feasibility of any given system is analyzed. Furthermore, we formulate the minimum weight perfect matching problem as an integer linear problem and solve it by integer linear programming. Numerical results validate the performance gains of our proposed system from the aspects of total transmit power and outage probability.