Channel Customization for Joint Tx-RISs-Rx Design in Hybrid mmWave Systems

TL;DR

This paper proposes a novel joint design of transmitter, RISs, and receiver in hybrid mmWave systems to customize the wireless channel, overcoming rank deficiency and enhancing spectral efficiency through RIS placement and reflection matrix optimization.

Contribution

It introduces a method to tailor the channel rank and condition number using multiple RISs, and develops a joint Tx-RISs-Rx design that improves spectral efficiency without complex matrix decompositions.

Findings

Achieves a customizable channel with arbitrary rank and minimized condition number.

Demonstrates substantial spectral efficiency gains over non-RIS systems.

Proves RISs can independently control sub-channel modes in water-filling.

Abstract

In strong line-of-sight millimeter-wave (mmWave) wireless systems, the rank-deficient channel severely hampers spatial multiplexing. To address this inherent deficiency, multiple reconfigurable-intelligent-surfaces (RISs) are introduced in this study to customize the wireless channel. Utilizing the RIS to reshape electromagnetic waves, we theoretically show that a favorable channel with an arbitrary tunable rank and a minimized truncated condition number can be established by elaborately designing the placement and reflection matrix of RISs. Different from existing works on multi-RISs, the number of elements needed for each RIS to combat the path loss and the limited phase control is also considered. On the basis of the proposed channel customization, a joint transmitter-RISs-receiver (Tx-RISs-Rx) design under a hybrid mmWave system is investigated to maximize the spectral efficiency. Using the proposed scheme, the optimal singular value decomposition-based hybrid beamforming at the Tx and Rx can be obtained without matrix decomposition for the digital and analog beamforming. The bottoms of the sub-channel mode in the water-filling algorithm, which are conventionally uncontrollable, are proven to be independently adjustable by RISs. Moreover, the transmit power required for realizing multi-stream transmission is derived. Numerical results are presented to verify our theoretical analysis and exhibit substantial gains over systems without RISs.