Kapil R. Dandekar

Design and Evaluation of a Reconfigurable Antenna Array for MIMO Systems

New reconfigurable antenna array is demonstrated for multiple input multiple output (MIMO) communication systems that improves link capacity in closely spaced antenna arrays. The antenna system consists of an array of two printed dipoles separated by a distance of a quarter wavelength. Each of the dipoles can be reconfigured in length using PIN diode switches. The switch configuration can be modified in a manner adaptive to changes in the environment. The configuration of switches effects the mutual coupling between the array elements, and subsequently, the radiation pattern of each antenna, leading to different degrees of pattern diversity which can be used to improve link capacity. The PIN diode-based reconfigurable antenna solution is first motivated through a capacity analysis of the antenna in a clustered MIMO channel model. A new definition of spatial correlation coefficient is introduced to include the effects of antenna mismatch and radiation efficiency when quantifying the benefit of pattern diversity. Next, the widespread applicability of the proposed technique is demonstrated, relative to conventional half wavelength printed dipoles, using computational electromagnetic simulation in an outdoor and indoor environment and field measurements in an indoor laboratory environment. It is shown for the 2 times 2 system considered in this paper, that an average improvement of 10% and 8% is achieved in link capacity for a signal to noise ratio (SNR) respectively of 10 dB and 20 dB in an indoor environment compared to a system employing non reconfigurable antenna arrays.

Experimental study of mutual coupling compensation in smart antenna applications

This paper investigates the benefit of mutual coupling compensation via a method of moments (MoM) approach in a uniform circular antenna array operating at 1.8 GHz. This mutual coupling compensation technique is applied to a direction of arrival (DOA) study of up to two cochannel mobile users. Field measurements and computer simulations are examined to explore the assumptions of the technique and verify its effect when using the Bartlett and MUSIC DOA algorithms. Computer simulations considering the application of the technique to down-link beamforming are also included. Experimental results show that the mutual coupling compensation technique improves up-link DOA algorithm performance primarily by reducing unwanted sidelobe levels. This reduction in sidelobe levels aids in down-link beamforming weight design. Specifically, simulation results show that use of the compensation technique allows DOA-based down-link beamforming algorithms to perform similarly to spatial signature-based algorithms. All field measurements were made using the smart antenna testbed at the University of Texas at Austin.

Power Management in MIMO Ad Hoc Networks: A Game-Theoretic Approach

This paper considers interference characterization and management in wireless ad hoc networks using MIMO techniques. The power allocation in each link is built into a non-cooperative game where a utility function is identified and maximized. Due to poor channel conditions, some links have very low data transmission rates even though their transmit powers are high. Therefore, a mechanism for shutting down links is proposed in order to reduce cochannel interference and improve energy efficiency. The multiuser water-filling and the gradient projection methods are compared with the proposed game theoretic approach in terms of system capacity and energy efficiency. It is shown that using the proposed method with the link shut-down mechanism allows the MIMO ad hoc network to achieve the highest energy efficiency and the highest system capacity

Smart antenna array calibration procedure including amplitude and phase mismatch and mutual coupling effects

This paper presents a procedure used to calibrate a smart antenna array to compensate for channel amplitude and phase mismatch as well as mutual coupling effects between antenna array elements. A theoretical basis for this calibration procedure in terms of the smart antenna mathematical model is given. Direction of arrival (DOA) analysis of data collected using the Smart Antenna Testbed at the University of Texas at Austin is presented to illustrate the effectiveness of this calibration procedure and quantify the improvement in system performance. It is shown that amplitude and phase mismatch effects have a much stronger impact on overall system performance compared to mutual coupling between array elements.

Experimental Analysis of Pattern and Polarization Reconfigurable Circular Patch Antennas for MIMO Systems

In this paper, we investigate the performance achievable with pattern and polarization multielement reconfigurable antennas in narrowband and broadband multiple-input-multiple-output (MIMO) systems by means of two-port reconfigurable circular patch antennas (RCPAs). We use field measurements collected in an indoor environment to determine the channel capacity achievable with the RCPAs when used at one or at both ends of the communication link. To conduct this analysis, we use two types of RCPAs: 1) an RCPA with two different antenna configurations, exploiting only pattern diversity and 2) an RCPA with three different antenna configurations, exploiting both pattern and polarization diversity. The benefits offered by each state (i.e., excited radiation pattern and polarization) of the RCPAs are investigated in both line-of-sight (LOS) and nonline-of-sight (NLOS) scenarios. The analysis shows the effectiveness of each array configuration in increasing the diversity level of the system and the amount of signal power received in LOS and NLOS communication links. The radiation efficiency and the level of pattern and polarization diversity of each configuration are investigated to explain the performance offered by this class of antennas in MIMO systems. Finally, we show the difference in performance achievable with these antennas when employed in narrowband or broadband communication systems.

Optically transparent conductive polymer RFID meandering dipole antenna

In this paper, we present optically transparent flexible conductive polymer antennas for radio frequency identification systems. The designs for these antennas are presented along with simulated and measured results of antenna radiating properties. These conductive polymer antennas are compared to antennas with the same design fabricated out of copper. Finally, we include an analysis of the optical transparency of the conductive polymer antennas.

Vector channel modeling and prediction for the improvement of downlink received power

Many researchers have done significant work to reduce fast fading in single channel wireless systems using prediction. We introduce a novel synthesis and prediction filter at the smart antenna base station to predict the vector channel in time-division duplex systems. We show the advantage of modeling each component of the vector channel with the same coefficients over modeling each channel separately. Experimental results from measurements taken in downtown Austin show that prediction of the vector channel is feasible, even as far as ten steps ahead. Ray tracing simulations of downtown Austin, assuming noiseless line-of-sight and nonline-of-sight channels, show that these predictions enhance downlink beamforming resulting in improvements in downlink received power in excess of 10 dB in nonline-of-sight scenarios compared to beamforming without predictions. Furthermore, this improvement allows an increase in the duplex interval by more than three times, assuming constant mobile velocity.

Widely-linear beamforming and RF impairment suppression in massive antenna arrays

In this paper, the sensitivity of massive antenna arrays and digital beamforming to radio frequency (RF) chain in-phase quadrature-phase (I/Q) imbalance is studied and analyzed. The analysis shows that massive antenna arrays are increasingly sensitive to such RF chain imperfections, corrupting heavily the radiation pattern and beamforming capabilities. Motivated by this, novel RF-aware digital beamforming methods are then developed for automatically suppressing the unwanted effects of the RF I/Q imbalance without separate calibration loops in all individual receiver branches. More specifically, the paper covers closed-form analysis for signal processing properties as well as the associated radiation and beamforming properties of massive antenna arrays under both systematic and random RF I/Q imbalances. All analysis and derivations in this paper assume ideal signals to be circular. The well-known minimum variance distortionless response (MVDR) beamformer and a widely-linear (WL) extension of it, called WL-MVDR, are analyzed in detail from the RF imperfection perspective, in terms of interference attenuation and beamsteering. The optimum RF-aware WL-MVDR beamforming solution is formulated and shown to efficiently suppress the RF imperfections. Based on the obtained results, the developed solutions and in particular the RF-aware WL-MVDR method can provide efficient beamsteering and interference suppressing characteristics, despite of the imperfections in the RF circuits. This is seen critical especially in the massive antenna array context where the cost-efficiency of individual RF chains is emphasized.

Metamaterial-Substrate Antenna Array for MIMO Communication System

We demonstrate how a magnetic permeability enhanced metamaterial can enhance the antenna array of a multiple-input multiple-output(MIMO) communication system. The performance of a rectangular patch antenna array on a metamaterial substrate was studied relative to a similar array constructed on a conventional FR4 substrate. Differently spaced arrays were analytically compared using array correlation coefficients and mean effective gain as performance metrics. Achievable channel capacity were obtained through channel measurements made on a MIMO testbed. While results show that arrays on conventional FR4 substrates have higher capacity due to gain and efficiency factors, arrays can be made smaller, and have less mutual coupling and correlation coefficients, when using a metamaterial substrate, but the antenna built on the metamaterial substrate can be made more efficient through the use of better host materials. This was reflected in the analysis of both antenna arrays normalized to remove efficiency and gain differences where they showed similar performances. Hence, metamaterial substrates are a cost-effective solution when antenna miniaturization is a key design criteria compared to conventional substrates that achieve the same miniaturization factor without significantly sacrificing performance.

A technique for antenna configuration selection for reconfigurable circular patch arrays

This paper demonstrates a method that allows reconfigurable multi element antennas to select the antenna configuration at the receiver of a Multiple Input Multiple Output (MIMO) communication system. This antenna configuration selection scheme consists of using spatial correlation and average Signal to Noise Ratio (SNR) information to select the antenna radiation pattern at the receiver. We show that using this approach it is possible to achieve capacity gains in a multi element reconfigurable antenna system without modifying the data frame of a conventional MIMO system. We demonstrate our configuration selection algorithm through an analysis of reconfigurable circular patch antennas in realistic MIMO clustered channel models. Using this approach we also show that the functionality of the proposed selection scheme is connected to antenna parameters such as radiation efficiency, input impedance, the level of diversity between the radiation patterns of different antenna configurations, and the average system SNR. The capacity gain achievable with this configuration selection approach is calculated through numerical simulations using reconfigurable circular patch antennas at the receiver of a MIMO system that employs minimum mean square error receivers for channel estimation. The performance of the proposed method is compared to that of a MIMO system that estimates the channel transfer matrix for each antenna configuration in order to select the optimal receiver antenna radiation pattern. Channel capacity and Bit Error Rate (BER) results show the improvement offered by the proposed selection scheme relative to a conventional antenna selection technique for reconfigurable MIMO systems; in particular we show that the improvement increases with the number of configurations in the multi element reconfigurable antenna.