Beamforming for Combating Inter-cluster and Intra-cluster Interference in Hybrid NOMA Systems

In this paper, downlink beamforming (BF) for hybrid non-orthogonal multiple access (NOMA) systems is considered, in order to combat inter and intra cluster interference. First, to minimize the inter- and intra-cluster interference, the projection hybrid NOMA (PH-NOMA) BF algorithm is introduced, by combining conventional zero-forcing BF (ZFBF) and hybrid NOMA (H-NOMA) precoding. Second, to further reduce the overall interference, two user pairing algorithms, termed projection-based pairing algorithm (PBPA) and inversion-based pairing algorithm (IBPA), are also proposed, by adopting the properties of quasi-degradation developed previously. Consequently, the proposed BF algorithm is obtained by combining PH-NOMA and PBPA/IBPA. Moreover, the system performance in terms of outage probability and diversity is analyzed for both the proposed BF algorithms and conventional ZFBF. Finally, computer simulations are conducted to demonstrate the efficiency of the proposed BF algorithms and to validate the correctness of the performance analysis.