Antenna arrays can be employed in mobile communications to increase channel capacity as well as communication quality via spatially selective reception/transmission at base stations. In most wireless communications systems, directions of arrival of multipath signals need to be found for spatial selective transmission. Unfortunately, due to the coherent nature of multipath signals, it is quite difficult to find their directions of arrival. In this paper, we will present a subspace smoothing algorithm for finding the directions of arrival of multipath signals based on the mobile terminal signals received at different time instances. More importantly, we will present our experimental results to demonstrate that the spatial diversity is present for slight movements of a mobile terminal and that the subspace smoothing approach is effective in real wireless scenarios. All of the experiments were performed using the smart antenna testbed at the University of Texas at Austin.

The National Aeronautics and Space Administration Advanced Communications Technology Satellite (ACTS) propagation experiment was designed to obtain slant-path attenuation statistics for locations within the United States and Canada for use in the design of low-margin Ka-band satellite communication systems. Experimenters at seven different locations have collected propagation data for move than two years. The propagation terminals used for the experiment were identical. A single preprocessing program was used by the experimenters to provide for automatic calibration, generation of attenuation histograms, and data archival. In this paper, the calibration procedures are described mid estimates given for measurement accuracy. ACTS provided beacons at 20.2 and 27.5 GHz for use in making attenuation measurements. In addition to the beacon receivers, each ACTS propagation terminal has two total power radiometers with center frequencies at the beacon frequencies. The radiometers are used to establish the beacon signal reference levels needed for calculating beacon attenuation values. For the combined radiometer and beacon measurement system, the attenuation measurement error was less than a maximum of 1.0 dB and was generally less than 0.3 dB. The dynamic range for attenuation measurement varied from site to site depending on location relative to the peak of the satellite beacon antenna pattern. For locations within the continental United States, the dynamic range was better than 20 dB.

Cumulative fade distributions were derived from measured transmissions at 20 GHz emanating from the geostationary Advanced Communications Technology Satellite (ACTS) and at 12 GHz from the television broadcasting digital satellite system (DSS) over the one-year period September 1, 1995-August 31, 1996. The transmissions were acquired at two collocated receivers at the Applied Physics Laboratory of The Johns Hopkins University (central Maryland). Since both geostationary satellites are positioned within 1/spl deg/ from one another, the geometric pointing parameters at the receiver locations are approximately coincident (e.g., 38/spl deg/ elevation angle). The 20-GHz fades were noted to be two-four times larger than those measured at 12 GHz. Two frequency scaling techniques were employed for estimating the distribution at one frequency given a measurement at the other. The methods pertained to the frequency scaling formulation of the radiocommunications sector of the International Telecommunications Union and the ratio of attenuations in terms of an equal probability rain rate along an effective path. The latter method gave agreement to within 1 dB when adjusted for antenna-wetting signal degradation.

This paper presents some preliminary results of experimental studies of space-division-multiple-access (SDMA) systems for wireless communications to expand capacity, increase coverage, and improve quality. Although SDMA schemes have been studied by a number of researchers, most of these studies are based on theoretical analyses and computer simulations. Very few real RF or microwave experiments have been conducted to validate the feasibility of various signal processing algorithms, such as direction finding and signal copy techniques. Also, no extensive experiments have been conducted to study the channel propagation associated with multiple antennas. The purpose of this paper is to present our preliminary experimental results using our recently developed antenna array testbed. We will also discuss the implications of these results on various array signal processing algorithms.

A spatial signature is a response vector of an antenna array to a mobile unit at a certain location. The mobile subscribers at different locations exhibit different spatial signatures. The exploitation of spatial diversity (or difference of spatial signatures) is the basic idea of the so-called space-division-multiple-access (SDMA) scheme, which can be used to significantly increase the channel capacity and quality of a wireless communication system. Although the SDMA schemes have been studied by a number of researchers, most of these studies are based on theoretical analysis and computer simulations with ideal assumptions. Not much experimental study has been conducted on spatial signature variation due to nonideal perturbations in a real wireless communication environment. The purpose of this paper is to present our preliminary experimental results of spatial signature variation using our recently developed antenna array testbed. The results to be presented include the spatial signature variation with time and frequency and in several typical scenarios: (1) the mobile unit is stationary, so are its surroundings; (2) the mobile unit is stationary, but the surroundings vary; (3) the mobile unit moves in several different directions.

We characterize multipath fading at low elevation angles for unshadowed line-of-sight propagation over a land-mobile satellite link. Equipment aboard a van recorded a CW signal from INMARSAT's geostationary satellite MARECS-B2 while driving in the states of Utah, Nevada, Washington, and Oregon. Elevation angles ranged from 7/spl deg/ to 14/spl deg/. The van carried a tracking helix antenna with beamwidths in the principal planes of approximately /spl plusmn/18/spl deg/. Multipath fading was observed in flat and hilly terrain and is compared with an example of fading due to tree shadowing. The measurements demonstrate that multipath effects at low elevation angles may result in fades exceeding 7 dB for approximately 1% of the driving distance for a worst case scenario corresponding to driving in rolling, hilly terrain where the satellite is in front or behind of the vehicle. Fading in such a case is typically dominated by a single multipath reflection from a terrain feature. On the other hand, tree shadowing at low elevation (7/spl deg/) may result in fades which exceed 25 dB at the 1% level.>

Personal communication systems within office buildings are designed on the premise of slow (>

Only a few years of rain attenuation data for Earth-space paths are available for low elevation angle paths, and these show a consistent tendency by the CCIR fade prediction model to underpredict rain fades. The authors contribute to the database four years of 11.2 GHz measurements, taken on a 5.8 degrees elevation path in Austin, Texas, USA and assess higher than predicted fade results with respect to earlier measurements and the CCIR model. At the fiducial 0.01 percent of time, the rain fall rate was 73 mm/h and the attenuation exceeded the 25 dB measurement fade margin. It is found that the monthly variability of rain fall rates and decibel fades follows a normal distribution. Durations of rainfall and fades are classified into events, episodes, interevent gaps (longer than 4.2 h), and interepisode gaps (shorter than 4.2 h). Rainfall interevent gaps and event durations, as well as fade event, episode, and gap durations are log-normally distributed.>

Transmissions from the Japanese ETS-V geostationary satellite were measured at L-band (1.5 GHz) in a vehicle driving on roads of south-eastern Australia. The measurements were part of a program designed to characterize propagation effects due to roadside trees and terrain for mobile-satellite-service (MSS). It is shown that the cumulative distributions of fade and nonfade durations follow a lognormal and power law, respectively. At 1% probability, fades last 2-8 m, and nonfades 10-100 m, depending on the degree of shadowing. Phase fluctuations are generally small, allowing the channel characteristics to be estimated from levels only.>

Depolarization caused by rain at 11.7 GHz is due to both differential phase shift and attenuation. In dual polarized communications systems it gives rise to interference which may be severe enough to cause an outage even though the allotted fade margin has not been exceeded. Described is a depolarization compensation experiment over a 10 km path with near circular polarization at 11.7 GHz. It made use of a turnstile junction feed system under computer control. Data presented demonstrate the stable performance of the method used during several rain events. At fades below about 15 dB significant isolation improvements were achieved, compared to a fixed cross-polarization channel. Finally, a method using the turnstile is suggested to receive two nonorthogonal signals without interference.