  This paper develops a linear minimum mean-square error (LMMSE) channel
estimator for single and multicarrier systems that takes advantage of the
mutual coupling in antenna arrays. We model the mutual coupling through
multiport networks and express the single-user multiple-input multiple-output
(MIMO) communication channel in terms of the impedance and scattering
parameters of the antenna arrays. We put forward a novel scattering description
of the communication channel which requires only the scattering parameters of
the arrays as well as the terminated far-field embedded antenna patterns. In
multi-antenna single-carrier systems under frequency-flat channels, we show
that neglecting the mutual coupling effects leads to inaccurate
characterization of the channel and noise correlations. We also extend the
analysis to frequency-selective multicarrier channels wherein we further
demonstrate that the coupling between the antenna elements within each array
increases the number of resolvable channel taps. Standard LMMSE estimators
based on existing inaccurate channel models become sub-optimal when applied to
the new physically consistent model. We hence develop a new LMMSE estimator
that calibrates the coupling and optimally estimates the MIMO channel. It is
shown that appropriately accounting for mutual coupling through the developed
physically consistent model leads to remarkable performance improvements both
in terms of channel estimation accuracy and achievable rate. We demonstrate
those gains in a rich-scattering environment using a connected array of slot
antennas both at the transmitter and receiver sides.
