Keyoor Gosalia

Increasing wireless channel capacity through MIMO systems employing co-located antennas

Wireless networks consisting of compact antennas find applications in diverse areas such as communication systems, direction of arrival estimation, sensor networks, and imaging. The effectiveness of many of these systems depend on maximizing the reception of RF power and extracting maximum information from the incident electromagnetic (EM) wave. Traditionally, this has been achieved through multiple-input multiple-output (MIMO) systems employing a spatial array of antennas that enhance the channel capacity. In this paper, we report similar increases in channel capacity obtained through the use of vector antennas consisting of co-located loops and dipoles, which can respond to more than one component of the EM field. It is shown that systems with three- and four-element vector antennas at both the transmitter and receiver operating around the frequency of 2.25 GHz support three and four times more information, respectively, as compared to conventional systems consisting of sensors with single antennas. Comparison with a simplified theoretical model of a MIMO system with co-located antennas in a rich multipath environment shows good agreement.

Investigation of a microwave data telemetry link for a retinal prosthesis

In this paper, we investigate a novel approach of establishing a data telemetry link for a dual-unit retinal prosthesis at microwave frequencies (1.45 and 2.45 GHz) using a pair of microstrip patch antennas. Appropriately sized extraocular (25/spl times/25 mm) and intraocular (6/spl times/6 mm) antennas are designed to operate at both the frequencies using the finite-difference time-domain method, and the coupling between them is examined computationally in the presence of a 0.25-mm resolution human-head model. Good agreement between numerical and experimental coupling results is shown and it is observed that the eyeball acts as a dielectric lens for the implanted antenna, thus improving the coupling between the extraocular and intraocular antennas. Specific absorption rate (SAR) computations are also performed at both the frequencies, and the peak 1-g SAR value is calculated. Detailed analysis of the design issues of the antennas, results of the numerical and experimental coupling measurements, and SAR calculations are presented.

Impedance matching and implementation of planar space-filling dipoles as intraocular implanted antennas in a retinal prosthesis

In this work, an extremely compact planar meander line dipole is designed and implemented for use as an intraocular element in a retinal prosthesis. This planar meander dipole antenna exhibits a high degree of current vector alignment and is impedance matched by inducing a current phase reversal along its length. This current phase reversal is induced by a minor offset in feed location which yields a highly directive broadside radiation pattern on this particular planar antenna geometry. This concept is applied in designing and implementing a 6/spl times/6 mm planar compact wire dipole at 1.4 GHz as the intraocular element for the data telemetry link of a retinal prosthesis. Coupling measurements between an external microstrip patch antenna and the intraocular wire dipole are presented and compared with those obtained with intraocular microstrip patch antennas in place of the wire dipole. It is demonstrated that such compact meander dipoles can perform better than previously reported microstrip patch antennas as intraocular elements for a retinal prosthesis.

Thermal elevation in the human eye and head due to the operation of a retinal prosthesis

An explicit finite-difference time-domain formulation of the bio-heat equation is employed with a three-dimensional head eye model to evaluate the temperature increase in the eye and surrounding head tissues due to the operation of the implanted stimulator IC chip of a retinal prosthesis designed to restore partial vision to the blind. As a first step, a validation of the thermal model and method used is carried out by comparison with in vivo measurements of intraocular heating performed in the eyes of dogs. Induced temperature increase in the eye and surrounding tissues is then estimated for several different operational conditions of the implanted chip. In the vitreous cavity, temperature elevation of 0.26/spl deg/C is observed after 26 min for a chip dissipating 12.4 mW when positioned in the mid-vitreous cavity while it is 0.16/spl deg/C when the chip is positioned in the anterior portion between the eyes ciliary muscles. Corresponding temperature rises observed on chip are 0.82/spl deg/C for both the positions of the chip. A comprehensive account of temperature elevations in different tissues under different operational conditions is presented.

Reduced size, dual-polarized microstrip patch antenna for wireless communications

A novel, compact, probe-fed microstrip patch antenna for operation in dual-polarization mode is proposed. The novel design is achieved by etching out a symmetric pattern of crossed slots from the surface of a square probe-fed patch. Reduction in patch size of up to 51% with respect to a traditional dual-polarized square patch operating at the same frequency is obtained. Results show linear polarizations in the +45 and -45/spl deg/ with a high isolation of 38 dB between the two ports. Moreover, the 50-/spl Omega/ feed position can be achieved by moving the feed point along the diagonal of the square patch, leading to ease in fabrication.

A Comparison of Two and Three Dimensional Dipole Antennas for an Implantable Retinal Prosthesis

The feasibility is investigated using three dimensional folded dipole antennas as a data-telemetry implantable receiving antenna in a dual-unit retinal prosthesis to restore partial vision to the blind. Three dimensional designs are explored in an effort to enhance certain antenna characteristics such as bandwidth and maximum gain while reducing the planar footprint size in comparison to its two dimensional equivalent. The current vector alignment between the three dimensional layers are examined through folding and rotating the dipole arms with respect to each other to fully optimize the antennas characteristics. The performance of the 2D and 3D antennas were compared in simulations and further examined by fabricating and characterizing the performance in a transmit/ receive system in air and inside eye phantoms. Results show that three-dimensional antennas can provide larger bandwidth while being physically smaller than the correspondent two-dimensional ones, thus providing larger channel capacity that could lead to a system with an increased number of stimulating electrodes.

Investigation of the performance of copolarized, colocated electric and magnetic dipoles for increasing channel capacity

An antenna system comprising of colocated and copolarized magnetic dipole (loop) and electric dipoles has been implemented in a planar stacked configuration. All the three elements of the antenna system show good return loss and coupling between copolarized elements is observed to be less than -10 dB. The feasibility of employing such a system to realize more information channels in the same frequency band has been demonstrated. It is expected that future work on the design aspect of the antenna elements will lead to improvement in the coupling performance of the copolarized elements. Also, means of implementing another orthogonal loop will be explored with the eventual aim of realizing the six element structure using printed loops and dipoles.

SAR distribution and thermal elevation in a human head model due to the operation of the data telemetry link and implanted chip in a retinal prosthesis

In this work, we use the 3-D finite difference time domain method to investigate the microwave data telemetry link at two widely different frequencies and examine the power deposition in the head-eye tissues quantified as peak 1-g SARs for an estimated maximum transmitted power. A 3-D thermal analysis is also performed to determine the increase in the steady state temperature distribution in the head-eye tissues due to the operation of the implanted chip.

FDTD investigation of a microwave link for data telemetry in retinal prosthesis applications

Retinitis pigmentosa (RP) or age-related macular degeneration (AMD) are leading causes of blindness worldwide. It has been shown that patients suffering from such diseases can recover partial vision by means of controlled electrical stimulation of the retina. Thus, a retinal prosthesis can be developed where the functionality of permanently damaged retinal photoreceptors is replaced by a 2D array of electrodes, which can provide electrical stimulation on the surface of the retina. A high bandwidth (of up to 30 MHz) microwave data telemetry link for a retinal prosthesis is computationally investigated. A pair of appropriately sized microstrip patch antennas (extra- and intraocular) has been designed. A high spatial resolution (0.25 mm) head-eye model has been developed and the coupling between the extraocular and intraocular units computed in free space and with the receiving antenna embedded in the model. A power coupling of -30 dB is obtained with the receiving antenna embedded in the model and -26 dB of coupling is obtained in free space. Modification of the current antenna designs to improve their gains is expected to lead to an enhancement in the coupling efficiency. Such high bandwidth data telemetry links can contribute to other potentially data intensive biomedical applications requiring chronically implanted electronic prosthetic devices.

A dual band (C/Ku) feed system with transmit and receive capability

In satellite earth station antenna systems there is an increasing demand for complex single aperture, multi-function and multi-frequency band capable feed systems. In this work, a multi band feed system (6/12 GHz) is described which employs quadrature junctions (QJ) and supports transmit and receive functionality in the C and Ku bands respectively. This feed system is designed for a 16.4 m diameter shaped cassegrain antenna. It is a single aperture, 4 port system with transmit capability in circular polarization (CP) mode over the 6.625-6.69 GHz band and receive in the linear polarization (LP) mode over the 12.1-12.3 GHz band