Compact MIMO Terminals with Matching Networks
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Date
2008Author
Fei, Yuanyuan
Metadata
Abstract
The development of wireless communications has significantly changed people’s lifestyles in
the last century. It can be shown that the use of multiple antennas, so called multiple-input
multiple-output (MIMO) at both link ends can offer further improvement of spectral efficiency
and link reliability of wireless systems, which has been studied in the past decade. With the
maturity of theory, the practice ofMIMO technology has become feasible, though several problems
still need to be solved especially at the receive end. The inadequate antenna spacing limited
by the small dimension of the receive terminal causes the mutual coupling (MC) effect,
which may degrade MIMO system performance. This thesis focuses on the performance improvement
of compact MIMO terminals by introducing matching networks between the receive
antenna array and the load terminations. The investigation spans from practical implementation
to data analysis, and theoretical derivation to system performance optimisation.
The contributions of this thesis are threefold. Firstly, three performance metrics signal correlation
(SC), received power and capacity are well studied for compact MIMO terminals with
matching networks. An overview of the existing methods to examine these metrics is presented.
Especially for the received power, one proper approach is found and two methods are compared
and unified analytically. The simulation results reveal that adding matching networks into compact
MIMO terminals can significantly improve the system performance. However, SC and
received power should be well balanced to achieve a good capacity performance.
The second contribution describes the experimental measurement of the analytical SC and received
power study of the first contribution. A set-up of two quarter-wavelength monopoles
and a ground plane with various matching networks are measured and the system design is
aided by two simulation tools SEMCAD and FEKO. The measured results agree well with the
analytical prediction though discrepancies exist. The implementation confirms that relatively
high total receive power and low SC of the compact array can be achieved by choosing proper
load impedance in practice. It also indicates that the load impedance to optimise the received
power is different from the one for MIMO capacity maximisation.
Finally, inspired by the last finding of the second contribution, the optimal single-port matching
(SPM) impedance for capacity maximisation in a two by two compact MIMO system is derived
using an upper bound of the ergodic capacity for simplicity. A complete framework for MIMO
systems with compact arrays at both link ends is deduced using Z-parameters for the analysis. A
closed-form result for the optimal SPMimpedance in high signal-to-noise ratio (SNR) regime is
given and proved to be the input impedance of the receive antennas. The optimal SPMnetworks
outperform other matching networks for small antenna spacings with any SNR. Moreover, the
system performance sensitivity of a two by two MIMO system with coupled half-wavelength
dipoles and SPMnetworks is evaluated versus antenna spacing and dipole length. It shows that
MIMO capacity is not sensitive to the optimal impedance mismatch with fixed antenna spacing
and dipole lengths. However, the MIMO system is relatively sensitive to antenna structure
mismatch with a precise optimal matching network. Overall, the optimal single-port match is a
feasible technique to improve the performance of the compact MIMO systems.