Rotatable Continuous Aperture Array (CAPA)-based Multi-user MIMO Communications

This paper proposes a rotatable continuous-aperture array (R-CAPA)-based multi-user multiple-input multiple-output (MIMO) architecture. Aiming at maximizing the system spectral efficiency (SE), an integral-based optimizing problem is formulated for the joint optimization of the CAPA beamformers at the base station (BS) and the boresight direction of each CAPA at each user. To address this problem, we develop an alternating optimization (AO)-based algorithm. For the BS-CAPA beamformer optimization subproblem, we employ the calculus of variation (CoV) theory to obtain the optimized beamformer. For the user CAPA boresight direction optimization subproblem, we leverage the gradient-ascent method to derive the optimized boresight direction of each R-CAPA with the current beamformers. Our simulation results show that, compared to conventional static CAPAs, R-CAPAs can achieve significantly higher system SE with enhanced degrees of freedom.