Joint Transceiver Orientation Optimization for Rotatable-Antenna MIMO Capacity Maximization

TL;DR

This paper proposes a joint optimization framework for transceiver and rotatable antenna orientations in MIMO systems, significantly enhancing channel capacity by exploiting orientation degrees of freedom.

Contribution

It introduces an orientation-dependent MIMO channel model and develops an alternating optimization algorithm for capacity maximization with practical constraints.

Findings

The proposed method outperforms fixed-orientation benchmarks in capacity.

The algorithm effectively optimizes antenna orientations and transmit covariance.

Simplified designs are derived for low-SNR MISO and SIMO cases.

Abstract

Conventional multiple-input multiple-output (MIMO) systems mainly rely on fixed antenna arrays, which limits their capability to adapt the effective channel matrix to the propagation environment. Rotatable antennas (RAs), which enable mechanical or electronic adjustment of antenna boresight directions, introduce a new orientation-domain degree of freedom for channel reconfiguration. In this paper, we investigate an RA-aided MIMO communication system for channel capacity enhancement. We first establish an orientation-dependent MIMO channel model. Then, we formulate a capacity maximization problem by jointly optimizing the transmit covariance matrix and the transmit/receive RA orientations under practical spherical-cap constraints. To solve this non-convex problem, we develop an alternating optimization algorithm, where the transmit covariance matrix is updated via eigenmode transmission and water-filling, while each RA orientation is optimized through a Riemannian Frank-Wolfe method. We further investigate the low-SNR regime and derive simplified designs for multiple-input single-output (MISO) and single-input multiple-output (SIMO) special cases. Numerical results show that the proposed RA-aided MIMO design significantly improves the channel capacity compared with the fixed-orientation benchmark, demonstrating the benefits of orientation-domain channel reconfiguration.