  In this paper, we propose a new six-dimensional (6D) movable antenna (6DMA)
system for future wireless networks to improve the communication performance.
Unlike the traditional fixed-position antenna (FPA) and existing fluid
antenna/two-dimensional (2D) movable antenna (FA/2DMA) systems that adjust the
positions of antennas only, the proposed 6DMA system consists of distributed
antenna surfaces with independently adjustable three-dimensional (3D) positions
as well as 3D rotations within a given space. In particular, this paper applies
the 6DMA to the base station (BS) in wireless networks to provide full degrees
of freedom (DoFs) for the BS to adapt to the dynamic user spatial distribution
in the network. However, a challenging new problem arises on how to optimally
control the 6D positions and rotations of all 6DMA surfaces at the BS to
maximize the network capacity based on the user spatial distribution, subject
to the practical constraints on 6D antennas' movement. To tackle this problem,
we first model the 6DMA-enabled BS and the user channels with the BS in terms
of 6D positions and rotations of all 6DMA surfaces. Next, we propose an
efficient alternating optimization algorithm to search for the best 6D
positions and rotations of all 6DMA surfaces by leveraging the Monte Carlo
simulation technique. Specifically, we sequentially optimize the 3D position/3D
rotation of each 6DMA surface with those of the other surfaces fixed in an
iterative manner. Numerical results show that our proposed 6DMA-BS can
significantly improve the network capacity as compared to the benchmark BS
architectures with FPAs or 6DMAs with limited/partial movability, especially
when the user distribution is more spatially non-uniform.
