Interference is the major bottleneck for the performance of the two-way K-user interference channel. In this paper, we demonstrate that by employing a reconfigurable intelligent surface (RIS), we can indeed achieve the interference-free degree of freedom (DoF). Mathematically, achieving interference-free DoF amounts to finding the solutions of a linear system with the modulus-1 constraint. To address this challenging problem, we take a geometric perspective and leverage the tools of high-dimensional probability. As a result, we are able to provide the nearly exact characterizations of both the maximal and the minimal number of RIS elements required for the interference-free DoF. Simulation results match our theoretical prediction quite well.

Analog self-interference cancellation (A-SIC) plays a crucial role in the implementation of in-band full-duplex (IBFD) radios, due to the fact that the inherent transmit (Tx) noise can only be addressed in the analog domain. It is thus natural to ask what the performance limit of A-SIC is in practical systems, which is still quite underexplored so far. In this paper, we aim to close this gap by characterizing the fundamental performance of A-SIC which employs the common multi-tap delay (MTD) architecture, by accounting for the following practical issues: 1) Nonstationarity of the Tx signal; 2) Nonlinear distortions on the Tx signal; 3) Multipath channel corresponding to the self-interference (SI); 4) Maximum amplitude constraint on the MTD tap weights. Our findings include: 1) The average approximation error for the cyclostationary Tx signals isequal tothat for thestationarywhiteGaussianprocess, thus greatly simplifying the performance analysis and the optimization procedure. 2) The approximation error for the multipath SI channel can bedecomposedas the sum of the approximation error for the single-path scenario. By leveraging these structural results, the optimization framework and algorithms which characterize the fundamental limit of A-SIC, by taking into account all the aforementioned practical factors, are provided.

The traditional fully‐digital beamforming is realised by the full radio‐frequency (RF) chain configuration, which will be impracticable in massive multiple‐input multiple‐output (MIMO) systems because of the overburden energy consumption of RF chains at millimetre wave frequencies. To address this issue, a series of hybrid beamforming schemes have been proposed to reduce the number of RF chains. Although the hybrid beamforming schemes for single‐user MIMO (SU‐MIMO) have been studied extensively, the performance of hybrid beamforming for multi‐user MIMO (MU‐MIMO), especially for partially‐connected hybrid architecture, still has room for improvement. In this study, the authors propose a hybrid beamforming scheme for MU‐MIMO systems. Specifically, they focus on the design of analogue beamforming and optimise the analogue RF precoders and combiners jointly. The simulation results show that the proposed hybrid beamforming in fully‐connected structure can achieve a better performance compared with the existing hybrid beamforming schemes in both SU‐MIMO and MU‐MIMO systems. It is also observed that the proposed hybrid beamforming can achieve significant performance advantages compared with the existing hybrid beamforming schemes in partially‐connected structures.