Rotatable Antenna-Enhanced Beamforming: Signal Enhancement and Interference Suppression

Conventional beamforming with fixed-orientation antenna (FOA) arrays often struggles to effectively enhance signal and/or suppress interference due to significant variations in antenna directive gains across different steering angles. To overcome this limitation, this paper investigates the rotatable antenna (RA)-enhanced beamforming by leveraging the new spatial degrees of freedom (DoFs) via antenna orientation optimization. Specifically, the antenna rotation angle vector (ARAV) and antenna weight vector (AWV) are jointly optimized to maximize the minimum array gain over signal directions, while constraining the maximum array gain over interference directions. For the special single-/double-beam forming case, the optimal ARAV is derived in closed-form, which reveals the relationship between the antenna rotation angles and the antenna directive gain. For the special case of single-beam forming without interference, the optimal ARAV is derived in closed-form with the maximum ratio combining (MRC) beamformer applied to the AWV. For the general multi-beam forming case, we propose an efficient alternating optimization (AO) algorithm to obtain a high-quality suboptimal solution by alternatively optimizing the ARAV and AWV in an iterative manner. Simulation results demonstrate that the proposed RA-based scheme significantly outperforms the traditional FOA-based and isotropic antenna (IA)-based schemes in terms of array gain.