Compact MIMO Terminals with Matching Networks

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.