A RAKE type receiver structure for narrow band wireless systems can be designed using multiple antennas at the receiver

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Abstract
A new procedure is proposed for resolving multipaths for wireless channels. This method uses multiple antennas at the receiver to estimate the direction of arrival (DOA) of the multipaths. Assuming finite support, a finite matrix description of the channel is derived. It is shown that the rank of this matrix is equal to the minimum of the number of antennas, the number of multipaths, and the channel length. A sufficient condition for computing the DOAs is that the rank of this matrix be at least equal to the number of multipaths. This condition need not hold at all times, especially when the channel length in number of symbols is smaller than the number of multipaths. Usually this problem is solved via a so called spatial smoothing approach requiring a large array of antennas. We develop a so called fractionally spaced temporal averaging technique that finds the DOAs can be resolved as the path delays are distinct. Beams are formed in the specific DOAs. Thus the significant paths can be separated from one another and then fed into the fingers of a RAKE-type receiver. Each finger is adaptively adjusted for delay to achieve optimum combining. In absence of DOA estimation errors the 'matched filter bound' for a 'flat' channel is achieved with perfect beamforming.

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