  Compared with a single-input-single-output (SISO) wireless communication
system, the benefit of multiple-input-multiple-output (MIMO) technology
originates from its extra degree of freedom (DOF), also referred as scattering
channels or spatial electromagnetic (EM) modes, brought by spatial
multiplexing. When the physical sizes of transmitting and receiving arrays are
fixed, and there are sufficient antennas (typically with half-wavelength
spacings), the DOF limit is only dependent on the propagating environment.
Analytical methods can be used to estimate this limit in free space, and some
approximate models are adopted in stochastic environments, such as Clarke's
model and Ray-tracing methods. However, this DOF limit in an certain
inhomogeneous environment has not been well discussed with rigorous full-wave
numerical methods. In this work, volume integral equation (VIE) is implemented
for investigating the limit of MIMO effective degree of freedom (EDOF) in three
representative two-dimensional (2-D) inhomogeneous environments. Moreover, we
clarify the relation between the performance of a MIMO system and the
scattering characteristics of its propagating environment.
