PASS: power allocation and SIC order selection of cache-aided NOMA in vehicular networks

Non-orthogonal multiple access (NOMA) and caching are prominent technologies to support massive content delivery in wireless networks. By the advantage of superior data storage capability of the vehicles, a novel cache-aided NOMA paradigms has been introduced to vehicular networks for multimedia services, in which the cache utility and NOMA efficiency are both improved. However, the high mobility of vehicles causes a fast variation of channel state, which brings challenges in utilizing NOMA technology to guarantee the reliability of communication. In this context, we explore the optimization of power allocation and successive interference cancellation (SIC) ordering. Specially, we investigate the optimal Power Allocation and SIC ordering Selection (PASS) with imperfect channel estimation. To improve the quality of service and guarantee user fairness, we try to maximize the minimum achieved average outage data rate for all the vehicles under service. The PASS problem is non-convex because of the power and outage probability constraints. By Markov inequality, the probability constraint problem is firstly transformed into the non-probability problem for simplicity. Although the transformed problem is still non-convex, it is shown that the power allocation problem is quasi-concave when given the SIC ordering. Hence, we first solve the power allocation problem by the bisection method. Then, a greedy-meta schedule algorithm is proposed to determine the SIC ordering with low complexity. Numerical results show our proposed cache-aided scheme achieves a significant performance improvement.