Uncoupled antenna matching for performance optimization in compact MIMO systems using unbalanced load impedance

Some MIMO applications require antennas to be closely spaced, which result in mutual coupling among antennas and high spatial correlation for signals. In order to compensate for the performance degradation due to correlation and coupling, impedance matching networks may be used. Recently, it was shown that uncoupled matching networks could be optimized against a given performance metric with the constraint of similar matching impedance for all antennas, i.e., balanced matching. In this paper, we investigate the use of uncoupled matching networks with both balanced and unbalanced load impedances, where either the received power or the channel capacity is optimized. For two- and three-element dipole arrays, we show numerically that a significant performance improvement can be achieved by introducing unbalanced matching. Observations suggest that the achieved improvement varies with array geometry and propagation environment. For example, a large capacity gain of up to 23% is realized when matching a uniform linear array to propagation environments that are asymmetrical about the array broadside, whereas the symmetrical environments do not benefit as much from unbalanced matching

Tracking of characteristic modes through far-field pattern correlation

Recent developments in characteristic mode (CM) analysis enable a more systematic approach to designing multi-antennas for mobile terminals. However, some of the advantages of developing antennas through CMs are based on accurate mode tracking over frequency. Existing tracking methods are primarily based on the tracking of eigencurrents over frequency, by which mode swapping and degenerate modes can occur. To solve these problems, a completely different method of tracking CM by means of far-field pattern correlation was recently developed and shown to work well for perfect electric conductors (PECs). This paper reveals that the same method can also substantially reduce tracking errors for structures with both PEC and dielectric materials, relative to state-of-the-art methods

Design of low profile MIMO antennas for mobile handset using characteristic mode theory

Designing highly integrated and efficient MIMO antennas for mobile handset is challenging, especially for low frequency bands below 1 GHz. In this work, by analyzing and manipulating the characteristic modes of a mobile handset, we propose a low profile dual-band MIMO antenna with high integration ability. Both antennas cover a bandwidth of 100 MHz at the center frequency of 0.9 GHz. The isolation between the antennas is over 10 dB, and the envelope correlation is below 0.1, which ensures high efficiency of the antenna system and good MIMO performance

Degree-of-freedom evaluation of six-port antenna arrays in a rich scattering environment

It has been proposed that six co-located antennas, namely three electric and three magnetic dipoles, can offer up to a six-fold capacity increase in wireless channels, relative to that of single antennas. In other words, six degrees of freedom (DOFs) can be supported by co-located six-port transmit and receive antenna arrays. However, due to the complexity in designing and measuring such a six-port antenna, to our knowledge, no experimental verification has yet been successfully performed. In this paper, the six DOFs hypothesis is experimentally verified at the 300 MHz band. The experiment involved the design and fabrication of two six-port arrays, and MIMO channel measurements in a rich scattering environment with these arrays

Antenna design using characteristic modes for arbitrary materials

Characteristic mode analysis has traditionally been constrained to problems which utilize only perfect electric conductors (PEC). Through forced symmetry of a method of moments surface integral equation and newly proposed post-processing, characteristic modes can be solved for any material in a computationally efficient manner. As an example, the characteristic modes are solved for a mobile terminal consisting of both PEC and dielectric materials

Preliminary study on differences between full-and sub-structure characteristic modes

In this paper, full-and sub-structure characteristic modes (CMs) are explored through the effect of coupling between the target and background structures. The eigenvalues and eigen-current distributions of the first few modes are compared for different coupling levels in the numerical example. Similarities and differences between the two types of CMs in specific conditions are revealed, paving the way for further feasibility studies of the sub-structure method for antenna design

Antenna matching for performance optimization in compact MIMO systems

The implementation of MIMO technology on compact mobile terminal devices poses a unique challenge for system designers. This is because it requires that multiple antennas be closely separated in a confined volume, which results in strong mutual coupling among the antennas and high spatial correlation for the signals. In this paper, we present a review on the latest developments of using uncoupled impedance matching networks to counteract performance degradation due to the aforesaid effects. Then, we extend our previous study of utilizing identical uncoupled matching networks to optimize performance by allowing them to be different across the antennas. The numerical examples reveal that the enlarged optimization search space is effective in improving the received power and correlation, whereas only a modest gain in channel capacity is observed

Wide band characteristic mode tracking utilizing far-field patterns

The Theory of Characteristic Modes provides a convenient tool for designing multi-antennas for MIMO applications, as it enables orthogonal radiation patterns to be excited in a given antenna structure. Moreover, the frequency behavior of the modes reveals interesting wideband properties of the structure. However, the tracking of characteristic modes over frequency remains a challenge, especially when differences between modes are limited to high currents in small regions of the structure. The common approach of tracking characteristic modes is through correlating the modal currents over frequency, this leads to multiple eigenvalues being mapped to the same eigencurrent. In this work, we propose a new approach to track characteristic modes by means of cross correlating far-field patterns, which effectively eliminates the mode mapping ambiguity

On user effects in MIMO handset antennas designed using characteristic modes

The Theory of Characteristic Modes (TCM) has been applied to design high-performance MIMO antennas for mobile terminals. However, existing studies focus on free-space (FS) performance, which is mostly irrelevant in real usage. This paper investigates the performances of two TCM-based MIMO terminal antenna designs in 7 realistic user scenarios for frequencies below 1 GHz. Full-wave simulation results indicate that the TCM designs can significantly outperform conventional designs in user scenarios that require good MIMO performance. Higher multiplexing efficiency (ME), by up to 3 dB, was recorded for a TCM design relative to a conventional terminal in a two-hand scenario. Performance advantages of the TCM designs were mainly due to lower correlation as well as higher impedance matching and coupling efficiency. Moreover, a combined usage study based on weighted ME over different user cases established that on average TCM designs outperform conventional designs by up to 1.6 dB. This suggests that the TCM designs not only give superior performance in FS, but also in realistic user scenarios