Beyond diagonal reconfigurable intelligent surfaces: mode analysis, signal processing, and hardware impairments

Reconfigurable intelligent surface (RIS) has been envisioned as a promising and cost-effective technique to enhance future wireless communications. Extensive research attention has been drawn to the use of conventional diagonal RIS with diagonal phase shift matrices, where each RIS element is connected to its own load to ground but not connected to other elements. However, the simple architecture of conventional diagonal RIS limits its flexibility of manipulating passive beamforming. To fully exploit the benefits of RIS, a new technical advance, namely, beyond diagonal (BD) RIS, has emerged by introducing inter-element connections. This thesis aims to develop novel architectures and modes of BD-RIS with performance enhancements, study corresponding signal processing solutions, and characterize hardware impairments of BD-RIS. First, we propose two new branches of BD-RIS, namely BD-RIS with hybrid transmitting and reflecting mode, and BD-RIS with multi-sector mode, which enable enhanced channel gain and enlarged coverage compared to conventional diagonal RIS. Specifically, the modeling, architecture design, discussions, and beamforming design for the proposed two modes are provided. Second, we study the signal processing techniques for BD-RIS aided multi-antenna systems. Specifically, we start by synergizing BD-RIS with rate-splitting multiple access, which is a non-orthogonal technique providing additional flexibility for the interference management. Under imperfect channel state information conditions at the transmitter, a stochastic optimization problem is formulated and efficiently solved. Then, we propose an efficient pilot sequence and BD-RIS design for channel estimation, which theoretically guarantees to achieve the performance lower-bound. Finally, we characterize two important hardware impairments at BD-RIS, namely, mutual coupling and frequency dependence in wideband systems. To evaluate the impact of mutual coupling at BD-RIS, we adopt dipole antennas and model the mutual coupling as a function of inter-antenna spacing. To evaluate the impact of frequency dependence at BD-RIS, we adopt lumped circuit models in wideband communication systems.