A multiple APs cooperation access scheme for energy efficiency in UUDN with NOMA

With the continuous increase of mobile Internet traffic, the user-centric ultra-dense network (UUDN) is proposed as one of the promising solutions to provide very high area throughput density and flexible access service for users. The limited radio resources cause serious competition among numerous APs. Thus, Non-orthogonal multiple access (NOMA), which supports multiple signals transmission on the same frequency resource, is introduced into UUDN. Then, it is an interesting and still open problem on how to organize multiple APs to serve one user in NOMA-based UUDN. In this paper, we propose an access scheme by grouping multiple APs cooperatively to provide a user-centric access service. Firstly, a group evaluation model is set up for organizing multiple APs into AP Group (APG) to provide access service for each user. Then, the resource allocation problem is formulated as a mix-integer non-linear programming (MINP) problem to maximize the system energy efficiency (EE), which is hard to tackle. For tractability purpose, we transform this problem as a differ convex problem(DCP) and propose iterative algorithms to obtain a feasible solution. Extensive simulation results are analyzed to demonstrate the significant performance improvement compared to the existing schemes.