As wireless communication systems evolve
As wireless communication systems evolve, service quality and capacity are of primary importance. To ensure reliable communication over a mobile radio channel, a system must overcome multipath fading, polarization mismatch, and interference. The trend towards low power hand held transceivers increases all of these challenges. Even as more spectrum is allocated, demand for higher data rate services and steadily increasing numbers of users will motivate service providers to seek ways of increasing the capacity of their systems.
Antenna arrays can improve reliability and capacity in two ways. First, diversity combining or adaptive beamforming techniques can combine the signals from multiple antennas in a way that mitigates multipath fading. Second, adaptive beamforming using antenna arrays can provide capacity improvement through interference reduction. The use of adaptive arrays is an alternative to the expensive approach of cell splitting, which increases capacity by increasing the number of base station sites. Most adaptive arrays that have been considered for such applications are located at the base station and perform spatial filtering. They cancel or coherently combine multipath components of the desired signal and null interfering signals that have different directions of arrival from the desired signal.
Multi-polarized adaptive arrays, sometimes called polarization-sensitive adaptive arrays, can also match the polarization of a desired signal or null an interferer having the same direction of arrival as the desired signal, if the two signals have different polarization states. If base stations or mobile units in a peer-to-peer system can match the polarization states of hand-held transceivers, link quality and reliability will be enhanced, and power consumption in the hand-held units will be reduced, increasing battery life. It is possible that 100% or greater increase in system capacity can be achieved through a combination of spatial and polarization reuse. Because they offer large untapped potential performance gains, multi-polarized adaptive arrays should be studied extensively to determine what performance improvements are feasible. Currently, however, little is known about the performance of multi-polarized adaptive arrays in mobile communication systems.
Multi-polarized arrays have been considered as a means of rejecting jammers in military applications. The potential of multi-polarized arrays for interference rejection in wireless communication systems has been investigated in recent years. This research indicates that 20 to 35 dB of interference rejection is possible if interfering and desired signals differ in either polarization state or angle of arrival.
However, neither measurements nor simulations have been reported that show the performance of multi-polarized adaptive arrays in typical mobile multipath channels. Some researchers have proposed diversity combining at handheld radios and shown that significant performance gains can be achieved. The use of adaptive antennas on handheld radios is a new area of research. In 1988, Vaughn [1.7] concluded that with then-current technology, adaptive beamforming would work for units moving at pedestrian speeds but would be difficult for high-speed mobile units. In 1999, Braun, et al reported experiments in which data was recorded using a two-element handheld antenna array, and processed using diversity and optimum beamforming techniques.
While this was the first publication of its kind, some assumptions were made in the experiments and data processing that limit the applicability of the results.
This dissertation evaluates the performance improvement that can be achieved using co-polarized and dual-polarized antenna arrays at handheld receivers. This was done by measuring and modeling the performance of small handheld array configurations to determine the degree of diversity gain and interference rejection they can provide in typical mobile radio channels. Multi-polarized geometrically based multipath propagation models were developed for use in this study. A software package, the vector multipath propagation simulator (VMPS) was developed that can model the transmission, reflection, and reception of polarized waves and account for the effects of element and array pattern and orientation. Design guidelines for multi-polarized arrays and recommendations for their integration into new wireless communication systems are presented based on the results of this study. An overview of the dissertation follows.
Cesar Augusto Suarez
CI 17394384
CAF
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