  In this paper, we propose a novel antenna architecture called movable antenna
(MA) to improve the performance of wireless communication systems. Different
from conventional fixed-position antennas (FPAs) that undergo random wireless
channel variation, the MAs with the capability of flexible movement can be
deployed at positions with more favorable channel conditions to achieve higher
spatial diversity gains. To characterize the general multi-path channel in a
given region or field where the MAs are deployed, a field-response model is
developed by leveraging the amplitude, phase, and angle of arrival/angle of
departure (AoA/AoD) information on each of the multiple channel paths under the
far-field condition. Based on this model, we then analyze the maximum channel
gain achieved by a single receive MA as compared to its FPA counterpart in both
deterministic and stochastic channels. First, in the deterministic channel
case, we show the periodic behavior of the multi-path channel gain in a given
spatial field, which can be exploited for analyzing the maximum channel gain of
the MA. Next, in the case of stochastic channels, the expected value of an
upper bound on the maximum channel gain of the MA in an infinitely large
receive region is derived for different numbers of channel paths. The
approximate cumulative distribution function (CDF) for the maximum channel gain
is also obtained in closed form, which is useful to evaluate the outage
probability of the MA system. Moreover, our results reveal that higher
performance gains by the MA over the FPA can be acquired when the number of
channel paths increases due to more pronounced small-scale fading effects in
the spatial domain. Numerical examples are presented which validate our
analytical results and demonstrate that the MA system can reap considerable
performance gains over the conventional FPA systems with/without antenna
selection (AS).
