  We consider the design of a uniform circular array (UCA) based multiple-input
multiple-output (MIMO) system over line-of-sight (LoS) environments in which
array misalignment exists. In particular, optimal antenna placement in UCAs and
transceiver architectures to achieve the maximum channel capacity without the
knowledge of misalignment components are presented. To this end, we first
derive a generic channel model of UCA-based LoS MIMO systems in which three
misalignment factors including relative array rotation, tilting and
center-shift are reflected concurrently. By factorizing the channel matrix into
the singular value decomposition (SVD) form, we demonstrate that the singular
values of UCA-based LoS MIMO systems are \textit{independent} of tilting and
center-shift. Rather, they can be expressed as a function of the \textit{radii
product-to-distance ratio} (RPDR) and the angle of relative array rotation.
Numerical analyses of singular values show that the RPDR is a key design
parameter of UCA systems. Based on this result, we propose an optimal design
method for UCA systems which performs a one-dimensional search of RPDR to
maximize channel capacity. It is observed that the channel matrix of the
optimally designed UCA system is close to an orthogonal matrix; this fact
allows channel capacity to be achieved by a simple zero-forcing (ZF) receiver.
Additionally, we propose a low-complexity precoding scheme for UCA systems in
which the optimal design criteria cannot be fulfilled because of limits on
array size. The simulation results demonstrate the validity of the proposed
design method and transceiver architectures.
