Conventional beamforming with fixed-orientation antenna (FOA) arrays may struggle to effectively enhance signal and/or suppress interference due to significant variations in antenna directive gains over different steering angles. To break this limitation, we investigate in this paper the rotatable antenna (RA)-enhanced single/multi-beam forming by exploiting the new spatial degrees of freedom (DoFs) via antennas' rotation 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, subject to a given constraint on the maximum array gain over interference directions. 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 case of multi-beam forming, we propose an efficient alternating optimization (AO) algorithm to find a high-quality suboptimal solution by iteratively optimizing one of the ARAV and AWV with the other being fixed. Simulation results demonstrate that the proposed RA-based scheme can significantly outperform the traditional FOA-based and isotropic antenna (IA)-based schemes in terms of array gain.

翻译：传统采用固定方向天线（FOA）阵列的波束成形技术，由于天线方向性增益在不同波束指向角上存在显著变化，可能难以有效增强信号和/或抑制干扰。为突破此限制，本文通过天线旋转优化引入新的空间自由度（DoFs），研究可旋转天线（RA）增强的单/多波束成形技术。具体而言，我们联合优化天线旋转角矢量（ARAV）与天线权重矢量（AWV），在给定干扰方向最大阵列增益约束条件下，最大化信号方向的最小阵列增益。针对无干扰单波束成形的特殊情况，推导出闭式最优ARAV解，并对AWV采用最大比合并（MRC）波束成形器。针对多波束成形的一般情况，提出一种高效的交替优化（AO）算法，通过固定其中一个变量迭代优化另一个变量（ARAV或AWV），以获得高质量的次优解。仿真结果表明，所提出的基于RA的方案在阵列增益方面显著优于传统的基于FOA和各向同性天线（IA）的方案。