Ruiyuan Tian

Multiplexing Efficiency of MIMO Antennas

A simple and intuitive metric of multiplexing efficiency is proposed for evaluating the performance of multiple-input-multiple-output (MIMO) antennas in the spatial multiplexing mode of operation. Apart from gaining valuable insights into the impact of antenna efficiency, efficiency imbalance, and correlation on multiplexing performance, the metric is particularly useful for antenna engineers whose goal is to achieve the optimum antenna system design. Experimental results involving prototype mobile terminals highlight the effectiveness of our proposal.

Tracking Time-Variant Cluster Parameters in MIMO Channel Measurements

This paper presents a joint clustering-and-tracking framework to identify time-variant cluster parameters for geometry-based stochastic MIMO channel models. The method uses a Kalman filter for tracking and predicting cluster positions, a novel consistent initial guess procedure that accounts for predicted cluster centroids, and the well-known KPowerMeans algorithm for cluster identification. We tested the framework by applying it to two different sets of MIMO channel measurement data, indoor measurements conducted at 2.55 GHz and outdoor measurements at 300 MHz. The results from our joint clustering-and-tracking algorithm provide a good match with the physical propagation mechanisms observed in the measured scenarios.

A Compact Six-Port Dielectric Resonator Antenna Array: MIMO Channel Measurements and Performance Analysis

MIMO systems ideally achieve linear capacity gain proportional to the number of antennas. However, the compactness of terminal devices limits the number of spatial degrees of freedom (DOFs) in such systems, which motivates efficient antenna design techniques to exploit all available DOFs. In this contribution, we present a compact six-port dielectric resonator antenna (DRA) array which utilizes spatial, polarization and angle diversities. To evaluate the proposed DRA array, a measurement campaign was conducted at 2.65 GHz in indoor office scenarios for four 6 × 6 multiple antenna systems. Compared to the reference system of monopole arrays which only exploit spatial diversity, the use of dual-polarized patch antennas at the transmitter enriches the channels DOF in the non-line-of-sight scenario. Replacing the monopole array at the receiver with the DRA array that has a 95% smaller ground plane, the 10% outage capacity evaluated at 10 dB reference signal-to-noise ratio becomes equivalent to that of the reference system, due to the DRAs rich diversity characteristics. In the line-of-sight scenario, the DRA array gives a higher DOF than the monopole array as the receive counterpart to the transmit patch array. However, the outage capacity is 1.5 bits/s/Hz lower, due to the DRA arrays lower channel gain.

Experimental Verification of Degrees of Freedom for Colocated Antennas in Wireless Channels

It has been postulated that six colocated orthogonally polarized electrical and magnetic dipoles can offer up to six degrees of freedom (DOFs) in a multipath scattering environment. In other words, a sixfold increase in channel capacity can be achieved in the context of multiple-input-multiple-output (MIMO) systems. 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 77 MHz. The experiment involves the design and fabrication of two six-port arrays. In particular, one array is made compact with colocated antenna elements enclosed in a small cubic volume of (0.24λ)3. Using the two arrays, MIMO channel measurements are performed in an RF shielded laboratory in order to measure a multipath scattering environment. Analysis on eigenvalues of the measured channel shows that six DOFs are attained due to the orthogonality in radiation patterns of the two arrays.

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.

On MIMO Performance Enhancement With Multisector Cooperation in a Measured Urban Environment

Capacity improvement of intrasite cooperative multiple-input-multiple-output (MIMO) using all three 120° sectors of a base station (BS) site is studied for the first time in a measured urban environment at 2.65 GHz. The results show that the mean uplink capacity gain exceeds 40% in as much as 1/4 of the coverage area, relative to the best single-sector link with no cooperation. In addition, a simple simulation model is developed for predicting the capacity gain from intrasite multisector cooperation. The model is used to unravel the respective roles of the BS antenna patterns and propagation mechanisms in determining cooperative performance .

Multiplexing efficiency of MIMO antennas with user effects

Recently, the multiplexing efficiency metric is proposed for characterizing the MIMO performance of terminal antennas. Here, we show that it can also provide useful insights into the impact of user on MIMO terminal performance. In particular, the impact of user hand and head on the efficiency and correlation of a MIMO antenna can be conveniently quantified using the multiplexing efficiency metric. For example, we find that the hand introduces a 4 dB loss in total efficiency relative to the free space case, for a penta-band two-element MIMO terminal antenna operating at 750MHz. However, the multiplexing efficiency drops by only 2.4 dB, due to the de-correlation effect of the hand partly compensating the loss in total efficiency.

Assessment and Modelling of Terminal Antenna Systems

The pervasive use and insatiable demand for high-performance wireless devices globally are driving the development of new communication technologies and standards. In this chapter, we present advancements in the area of terminal antenna systems and describe how they play a major role in enabling strict performance requirements to be met in future systems. As opposed to conventional systems that do not require explicit considerations of the terminal antenna beyond the satisfaction of some predefined design criteria in impedance bandwidth, radiation pattern and efficiency, multiple antenna systems in upcoming and future wireless networks must deliver significantly higher performance goals.

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.

Effective degree-of-freedom of a compact six-port MIMO antenna

In this paper, we confirm that angle and polarization diversities are indeed the dominant diversity mechanisms of a recently proposed compact six-port MIMO antenna array, as it exhibits similar effective degree-of-freedom (EDOF) performance as that of the ideal array with co-located antenna elements evaluated in propagation scenarios with different angular spreads. Furthermore, the same approach can also be applied to determine the relative contributions of different diversity mechanisms to the EDOF performance of a given array.