Resource Allocation for Two-Tier RIS-Assisted Heterogeneous NOMA Networks

Reconfigurable intelligent surface(RIS)as a promising technology has been proposed to change weak communication environ-ments.However,most of the current resource allocation(RA)schemes have focused on RIS-assisted homogeneous networks,and there is still no open works about RA schemes of RIS-assisted heterogeneous networks(HetNets).In this paper,we design an RA scheme for a RIS-assisted HetNet with non-orthogonal multiple access to improve spectrum efficiency and transmission rates.In particular,we jointly optimize the transmit power of the small-cell base station and the phase-shift matrix of the RIS to maximize the sum rates of all small-cell users,subject to the unit modulus constraint,the minimum signal-to-interference-plus-noise ratio constraint,and the cross-tier interference constraint for protecting communication quality of microcell users.An efficient suboptimal RA scheme is proposed based on the alternating iteration ap-proach,and successive convex approximation and logarithmic transformation approach.Simulation results verify the effectiveness of the pro-posed scheme in terms of data rates.