APPLICATION OF DISTRIBUTED ANTENNA SYSTEMS FOR DETERMINATION OF RADIATION SOURCE COORDINATES WITH USE OF PHASE METHOD IN FRESNEL ZONE
Keywords:source of radiation, phase direction-finder, curvature, phase front, antenna array, bearing angle, distance.
Background. Talking about evolution of passive radar systems and multifunction antenna systems, it is necessary to acknowledge that distributed antenna systems (DAS) are supposed to be widely spread in the very near future in many areas of human activity, such as organization of high-quality mobile communication in large offices, shopping centers, fitness buildings and so on. Structurally, DAS consist of different groups of antennas and base stations with active and passive repeaters of radio signals. Generally, mutual location of such antennas and base stations can be chosen experimentally according to the criterion of service area maximization. For the further quality of service improvement, it is necessary to use smart antennas in DAS as basic structure elements. The use of smart antennas will allow tracking the subscriber`s (or SOR – source of radiation) location by means of direction-finding (DF) and to increase radiation power of radio signal towards SOR thus decreasing interference to other subscribers. It is important to note that amplitude methods of DF are unacceptable in this case because base stations have antenna beam patterns which are rather wide. Use of phase methods for direction finding is actual but constrained because of electromagnetic waves (EMW) multipath propagation and large DF errors caused by the following reason – it happens that SOR is located in Fresnel zone of direction-finder antenna system. So, consideration of the phenomenon of EMW front curvature in DAS is very important in order to construct direction-finders.
Objective. Consideration of EMW phase front sphericity (curvature) and its influence on the accuracy of SOR coordinates determination measured by DAS phase methods. Scheme technical decisions of phase direction-finders (PDF) based on this process are also represented.
Methods. To study the effect of the EMW phase front curvature on the accuracy of determining the coordinates of the SOR analytical dependencies are synthesized. Calculations of the direction characteristics, errors in determining the phase shifts between the antenna elements of the phase direction-finder, as well as the error in determining the distance to SOR in the Fresnel zone as a function of the range to SOR and its bearing were made based on MathCad 14 program.
Results. During the process of SOR angular coordinates determination in Fresnel zone with use of EMW phase front curvature оn the basis of planar antenna array, it is necessary to execute consistently two operations: 1) to define the fact of presence of EMW phase front curvature; 2) to switch PDF from the mode of bearing and tilt angles estimation (when it is in plane phase front) to the mode of bearing and tilt angles estimation (and also distance to SOR if needed) with phase front curvature of EMW. Taking into account the analysis relating to errors of bearing angle measurements in Fresnel zone (depending on the location of rough, but exact bases, for three-element sparse LAA) it is possible to conclude that their position doesn’t impact on exactness of bearing angle measuring in PDF. But to support more exact determination of SOR distance they must be placed in the center of exact, but ambiguous bases of PDF.
Conclusions. Failure to take into account EMW phase front curvature from SOR harmonic signal leads to errors of SOR coordinate calculation under conditions of its location in Fresnel zone and absence of other SORs. Knowledge of distance
to SOR can be used in passive radar systems, in radio monitoring systems of radio interference sources which have bearing angles equal with useful SOR ones, but located on different distances from useful SOR. This knowledge can also be used in mobile communication networks to strengthen signal level on certain distance with EMW energy focusing, in vehicle control systems on the basis of the satellite navigation systems etc.
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