  In this paper, we propose a new multiple-input multiple-output (MIMO)
communication system with movable antennas (MAs) to exploit the antenna
position optimization for enhancing the capacity. Different from conventional
MIMO systems with fixed-position antennas (FPAs), the proposed system can
flexibly change the positions of transmit/receive MAs, such that the MIMO
channel between them is reconfigured to achieve higher capacity. We aim to
characterize the capacity of MA-enabled point-to-point MIMO communication
systems, by jointly optimizing the positions of transmit and receive MAs as
well as the covariance of transmit signals. First, we develop an efficient
alternating optimization algorithm to find a locally optimal solution by
iteratively optimizing the transmit covariance matrix and the position of each
transmit/receive MA with the other variables being fixed. Next, we propose
alternative algorithms of lower complexity for capacity maximization in the
low-SNR regime and for the multiple-input single-output (MISO) and single-input
multiple-output (SIMO) cases. Numerical results show that our proposed MA
systems significantly improve the MIMO channel capacity compared to traditional
FPA systems as well as various benchmark schemes, and useful insights are drawn
into the capacity gains of MA systems.
