Ivaylo Vasilev

Impact of Antenna Design on MIMO Performance for Compact Terminals With Adaptive Impedance Matching

Using the metrics of channel capacity and multiplexing efficiency, the adaptive impedance matching (AIM) performances of two multiple-input multiple-output (MIMO) terminals with different antenna designs were evaluated and compared. The evaluation was performed in LTE Band 18 Downlink (860-875 MHz) under realistic usage conditions of two measured user handgrips and simulated propagation channels with different angular spreads (ASs). The results provide potential performance gains from AIM based on realistic MIMO terminal prototypes, and the underlying mechanisms by which the gains were achieved, which can serve as antenna and AIM circuit design guidelines. In particular, the evaluation revealed that ideal uncoupled AIM networks can increase the capacity by up to 52% relative to 50-Ω terminations. However, the observed gains depend heavily on the antenna design, the user scenario, and the channels AS. For example, the wideband design in different user cases experienced capacity gain of 4%-9% from AIM in uniform 3-D channels, in contrast to the 1.3%-44% gain seen in a conventional narrowband design. In nonuniform channels with small ASs, the AIM gain for different mean incident angles depends on the absolute mean effective gain (MEG) and the change in correlation due to AIM; in cases where AIM has little impact on correlation, the mean incident angles with high AIM gains were close to those with high MEGs.

On user effects in MIMO handset antennas designed using characteristic modes

The Theory of Characteristic Modes (TCM) has been applied to design high-performance multiple-input-multiple-output (MIMO) antennas for mobile terminals. However, existing studies focus on free-space (FS) performance, which is mostly irrelevant in real usage. This letter investigates the performances of two TCM-based MIMO terminal antenna designs in seven 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.

A low band cellular terminal antenna impedance tuner in 130nm CMOS-SOI technology

This paper presents a low band antenna impedance tuner in 130nm CMOS-SOI technology. It consists of three digitally controlled switched capacitor banks and two off-chip inductors and is intended for use in terminals supporting modern cellular standards like WCDMA and LTE. By using a negative gate bias in the off state, linearity can be improved and maintained. Measurements show an OIP3 exceeding +55dBm for all measured impedance states, which cover a VSWR of up to 5.4. The measured minimum loss is 1dB or lower in the frequency range from 700-900MHz with spurious emissions below -30dBm at +33dBm input power. The switched capacitors are implemented with eight stacked transistors to yield a voltage handling of at least 20V, and in order to handle the large voltages custom designed capacitors are used.

Experimental Investigation of Adaptive Impedance Matching for a MIMO Terminal With CMOS-SOI Tuners

It is well known that user proximity introduces absorption and impedance mismatch losses (MLs) that severely degrade multiple-input multiple-output (MIMO) performance of handset antennas. In this work, we experimentally verified the potential of adaptive impedance matching (AIM) to mitigate user interaction effects and identified the main AIM gain mechanism in realistic systems. A practical setup including custom-designed CMOS silicon-on-insulator (SOI) impedance tuners implemented on a MIMO handset was measured in three propagation environments and ten real user scenarios. The results indicate that AIM can improve MIMO capacity by up to 42% equivalent to 3.5 dB of multiplexing efficiency (ME) gain. Taking into account the measured losses of 1 dB in the integrated tuners, the maximum net ME gain is 2.5 dB suggesting applicability in practical systems. Variations in ME gains of up to 1.5 dB for different hand-grip styles were mainly due to differences in impedance mismatch and tuner loss distribution. The study also confirmed earlier results on the significant differences in mismatch and absorption between phantoms and real users in which the phantoms underestimated user effects and, therefore, AIM gains. Finally, propagation environments of different angular spreads were found to give only minor ME gain variations.

Experimental study of adaptive impedance matching in an indoor environment

In recent years, adaptive impedance matching (AIM) has been used to compensate for severe performance degradation in terminal antennas due to user proximity. In particular, user effect compensation is critical for multiple-input multiple-output (MIMO) terminals, to achieve high data rates. In this study, the AIM performance of a MIMO terminal is measured for three user scenarios at two locations in an indoor office environment, using real impedance tuners. It was established that AIM leads to MIMO capacity gains of up to 25%, corresponding to 2.2 dB of power gain. On the other hand, the insertion loss of the tuners was found to be about 0.3 dB for free-space conditions. These results suggest that AIM can offer significant net performance gains in practice. Moreover, we provide physical insight into the similar AIM results for the two measured locations, representing geometrical line-of-sight (LOS) and non-LOS links, respectively.