Interference mitigation for non-orthogonal multiple access in heterogeneous networks

Non-orthogonal multiple access (NOMA) technique is a promising alternative for enhancing network sum-rate while increasing the interference experienced by the users. In heterogeneous networks, the signal of a NOMA user is affected by three types of interference; intra-cell, inter-cell, and inter-tier. In an environment with high interference, using orthogonal multiple access (OMA) along with NOMA is beneficial to improve the edge user performance. This work proposes a hybrid model, where NOMA and OMA are deployed together in the same spectrum band. The performance of hybrid model is compared to NOMA and OMA for heterogeneous networks by stochastic geometry analysis. The analysis includes pico- and femto-cell alternatives to elaborate varying interference conditions. Using the capacity analysis formulation of NOMA, a power level optimization framework is presented for maximum sum-rate and fairness objectives. Besides, two pairing methods, near-to-near and near-to-far pairing, are included from the literature to reveal pairing effect on user-specific and system-level performance. The numerical results indicate NOMA trade-offs for increasing edge user capacity by power optimization and resistance of hybrid model to interference under different conditions.