A STUDY OF ADAPTIVE BEAMFORMING AND EQUALIZATION TECHNIQUES TO COMBAT MULTIPATH FADING

Abstract

The demand for wireless communication systems is experiencing an unprecedented growth, as these systems are able to support a large number of users at high data rates, under difficult propagation conditions. Half a billion subscribers to mobile networks are predicted by the year 2005. Also, market studies show that mobile multimedia services will attain in 2010- a penetration of more than 60 percent. in most industrialized countries [1]. Therefore there is a need to increase the spectrum efficiency. While a great effort in current (second generation) wireless communication systems has been directed towards modulation, coding etc antenna related technology has received less attention. Adaptive or self- configured antennas consist of an antenna array with signal processing in space. The spatial processing introduces a new degree of freedom in the system design with enormous potential to improve the performance including range, capacity enhancement, higher data rate and better bit-error-rate performance [21[3]. Beamformingg was developed for radar systems several decades ago. In a primitive type of beamforming, the various sensor- outputs are appropriately delayed and then summed, to steer the pencil beam towards the direction of desired signal. Thus the received power is maximized when the beam is steered towards the target. However this scheme has no provision for dealing with sources of interference.. Adaptive beamforming'(ABF) can respond to an unknown interference :environment, such that it can place 'nulls in the direction of interference. Adaptive antennas, are currently the subject of extensive investigation, as a means of reducing the vulnerability of reception of desired signals in the presence of interference signals in radar, sonar, seismic and communication systems. The principle reason behind their. widespread interest, lies in their ability to sense automatically, the presence of interference noise sources and to suppress them, while simultaneously enhancing the desired signal reception, without . prior knowledge of signal/interference environment. The U interference signals may not only consist of deliberate electronic counter measures, non-hostile radio-frequency interference, clutter-scatter returns and natural noise sources, but also coherent interferences. Coherent interferences can result, when multipath propagation is present or when `smart' jammer deliberately introduce coherent janitning, by retro-directing the signal energy towards the receiver. If the correlated interferers are spatially close to the received direction of target signal, then large number of sub-arrays are necessary to reduce correlation. Therefore this approach is not feasible for a portable unit (which has a limited physical dimension) and also not for mobile vehicular application devices, which are low weight, low power, hand held devices. Hence, the solution provided by space-time-processing will find application first at the base stations [4].