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Multiple adaptive recursive array for multipath environment
In a wireless communication system, signals sent into the channel interact with the environment in a very complex way. Thereby transmitted signals may be subject to many forms of degradation among which there are causes of multipath propagation: • Reflections due to obstacles with the size greater than a wavelength; • Refractions due to the non-homogeneity of the medium; • Diffractions due to obstacles with the size smaller than a wavelength. The most relevant effect caused by the aforementioned phenomena is related to the simultaneous presence of several copies (multipath) of the original signal, differing in attenuation, phase, delay, and direction of arrival. In frequency selective slow varying fading channels these copies give rise to inter symbol interference (ISI). Apparently, a natural remedy for the multipath problem would be either to reject all delayed versions of the transmitted signal or to properly combine them. The first approach based on using an adaptive infinite impulse response (IIR) filter at the single element receiver has been investigated in [1]. The proposed processor with adaptive IIR filter is capable of rejecting multiple reflections, and, moreover, it may be used for the estimation of channel parameters. The computational complexity of suggested algorithm is kept at a very low level. The second approach used for dealing with the multipath propagation involves spatial-temporal processors whose purpose is to combine the multipath in order to take advantage of the inherent diversity factors of the channel. The main problem concerning the application of such techniques is that they are computationally intensive and that the complete source path must be known at the receiver. Therefore, suitable channel estimation algorithms able to estimate and track channel variations are critical and must be implemented at the receiver. In this case the computational complexity should be kept as low as possible, so that multi-user real-time implementation can be afforded at reasonable costs. In the present work we suggest to combine both approaches above-mentioned to design the optimal solution of the problem of signal detection by an antenna array with quadratic processor [2]. We propose to employ the all-poles adaptive IIR filter at the output of each sensor of antenna array with quadratic processing in order to easily estimate and track the required channel parameters [3]. Note, that signals processed by such filters are ISI-free.
[1] Sellone F., Serebryakov G. //Proc. of the European Personal Mobile Communications Conference (EPMCC 2001). 2001. Vienna. [2] Morgan D.R, Smith T.M. //Journal of the Acoust. Soc. of America. 1990. V.87, №2. P.737. Sellone F., Serebryakov G., Tiraspolsky S. //Proc. of the International Conference on Electromagnetics in Advanced Applications (ICEAA 01). 2001. Torino, Italy.