Joint Beamforming-Power-Bandwidth Allocation in Terahertz NOMA Networks

A downlink Terahertz Non-Orthogonal Multiple Access (THz-NOMA) system with M beams is proposed in this paper, where each beam serves four users as a user cluster. Furthermore, two NOMA groups are grouped in each cluster. The Beamforming-Power-Bandwidth (B-P-B) problem is formulated aiming to maximize the network throughput while satisfying the QoS requirement of each user. Based on the THz-NOMA architecture and user distribution information, the beamforming design is first performed at base station (BS), in terms of the beam direction and the beamwidth. Then the B-P-B problem is evolved to the Power-Bandwidth (P-B) resource allocation problem, which is of importance to exploit the full benefit of THz-NOMA system. The mixed integer P-B problem can be solved with the support of decomposition theory, which allows sub-band assignment to be conducted in each cluster, while allowing power allocation to be carried out in the cell. The Long-User-Central-Window (LUCW) peculiarity of THz is captured by the Hungarian algorithm to adaptively assign sub-band between NOMA groups of a user cluster. According to the LUCW principle, the central sub-band of a THz window is assigned to the long NOMA group, while the side sub-band unavailable at long NOMA group is allocated to the short NOMA group. Then an iterative algorithm is proposed to acquire the optimal power allocation. Simulation results show an improved sum rate performance of THz-NOMA, 928.4 Gbps can be achieved, compared with the OMA system. Furthermore, the effects of sub-band assignment coefficient and NOMA grouping scheme are investigated. The results indicate that the sub-band assignment coefficient is preferred to be set between 0.47 and 0.7. Moreover, the favored NOMA grouping scheme requires a larger distance difference.