A Novel Cooperative NOMA in Wireless Backhaul Heterogeneous Networks

This paper proposes a novel cooperative transmission scheme based on non-orthogonal multiple access (NOMA) to improve the performance of wireless backhaul two-tier heterogeneous networks. Our work's novelty lies in the formulation to solve for the NOMA decoding order along with the cooperation rule between downlink small cell transmissions. We first employ the cochannel time division duplexing (CoTDD) combined with spectrum partitioning to manage interference. Then, we propose an optimization problem which maximizes the total achievable rate by jointly designing the NOMA decoding order and small cell cooperation rule along with the beamformer at the macro base station and small cells. The formulated problem is a mixed integer non-convex one which is generally NP-hard. To attain the solution, we exploit the structure of difference of convex functions to rewrite the formulated binary variables and then to equivalently transform the optimization problem into more amenable form. Finally, we develop an iterative lowcomplexity algorithm based on successive convex approximation (SCA) principle, which is provable to eventually converge at a sub-optimal solution. Numerical results show that our proposed strategy outperforms the conventional designs in terms of total achievable rate.