Cross-Tier Interference Mitigation for RIS-Assisted Heterogeneous Networks

Abstract

With the development of the next generation of mobile networks, new research challenges have emerged, and new technologies have been proposed to address them. On the other hand, reconfigurable intelligent surface (RIS) technology is being investigated for partially controlling wireless channels. RIS is a promising technology for improving signal quality by controlling the scattering of electromagnetic waves in a nearly passive manner. Heterogeneous networks (HetNets) are another promising technology that is designed to meet the capacity requirements of the network. RIS technology can be used to improve system performance in the context of HetNets. This study investigates the applications of reconfigurable intelligent surfaces (RISs) in heterogeneous downlink networks (HetNets). Due to the network densification, the small cell base station (SBS) interferes with the macrocell users (MUEs). In this paper, we utilise RIS to mitigate cross-tier interference in a HetNet via directional beamforming by adjusting the phase shift of the RIS. We consider RIS-assisted heterogeneous networks consisting of multiple SBS nodes and MUEs that utilise both direct paths and reflected paths. Therefore, the aim of this study is to maximise the sum rate of all MUEs by jointly optimising the transmit beamforming of the macrocell base station (MBS) and the phase shift of the RIS. An efficient RIS reflecting coefficient-based optimisation (RCO) is proposed based on a successive convex approximation approach. Simulation results are provided to show the effectiveness of the proposed scheme in terms of its sum rate in comparison with the scheme HetNet without RIS and the scheme HetNet with RIS but with random phase shifts.

A hybrid PSO-GWO-based phase shift design for a hybrid-RIS-aided heterogeneous network system

“…The efficiency of the proposed approach is evaluated by comparing it to the baseline methods, which are specified as Fully passive RIS (SDR (RIS)) [1] : In this method, the RIS phase shift matrix for a RIS-assisted HetNet is created using a semidefinite relaxation (SDR) method, but all the reflecting elements are passive. Fully active RIS (LM (Active RIS)) [30] : In this approach, the RIS phase shift matrix for an RIS-assisted HetNet is tailored using the Lagrange multiplier (LM) method, with all the reflecting elements being active.…”

“…Fully passive RIS (SDR (RIS)) [1] : In this method, the RIS phase shift matrix for a RIS-assisted HetNet is created using a semidefinite relaxation (SDR) method, but all the reflecting elements are passive.…”

“…Reconfigurable intelligent surfaces (RISs) are already widely recognized as a new technology that can offer excellent spectrum efficiency and reasonably priced coverage [1] , [2] . An RIS, in particular, is a metasurface composed of multiple reflective parts capable of reflecting the input signal with dynamically adjustable amplitude and/or phase [3] , [4] , [5] , [6] .…”

Hybrid RIS-assisted interference mitigation for heterogeneous networks

“…The H-RIS element is hence passive when n r = 0 and active when n r ∼ CN 0, σ 2 r . From (4), the signal-to-interference-plus-noise ratio at the u th user is expressed as (5). Where h i = h iu + f u T Ag i is the channel from the i th SBS to the u th user, which includes both direct link and reflected link, and h n = a u + d T u Ac n ∈ C 1×M .…”