Joint Resource Allocation and Passive Beamforming in RIS-Aided HetNets With Wireless Backhaul

Heterogeneous networks (HetNets) have been widely used in the development of 5G because a large number of small base stations (SBSs) can be deployed in hot spots to alleviate uneven traffic distribution. However, the performance of HetNet with wireless backhaul is limited by its backhaul capacity and the severe wireless interference. To solve this problem, we introduce reconfigurable intelligent surface (RIS) into the wireless backhaul HetNets and employ reversed time division duplex (TDD) in the time domain and dynamic soft frequency reuse (SFR) in the frequency domain. In the proposed RIS-aided dual-layer HetNets system, we formulate a joint optimization problem of bandwidth allocation, RIS passive beamforming, and power allocation to maximize the overall data rate. Two schemes are considered to adapt to different situations, i.e., unified dynamic SFR (U-SFR) and individual dynamic SFR (I-SFR). We solve the different sub-problems in U-SFR and I-SFR modes with the alternate optimization method, and obtain closed-form expressions for bandwidth allocation and power allocation. The convergence, feasibility, and complexity of the proposed schemes are also analyzed. In numerical results, the overall data rate is shown to be significantly increased with the help of RIS.