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Antenna Spacing Requirement for a Mobile Radio Base-Station Diversity

01 July 1971

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It has been shown previously that diversity reception techniques at the mobile unit often help to reduce the fading rate of a mobile radio signal. 1-3 Here we try to determine the limitations on using space diversity at the base station in order to reduce the signal fading. In particular, we would like to know how large the antenna spacing should be between two base-station antennas in order to take advantage of the diversity technique. If this turns out to be practical, then we might prefer to build a diversity system, even a complicated one, from the economical point of view at the base station, and let the transmitter and receiver in the mobile unit be as simple as pos1859 1860 THE BELL SYSTEM TECHNICAL JOURNAL, JULY-AUGUST 1971 sible. 4,5 In the experiments we varied the antenna spacing between two base-station antennas and calculated the cross-correlation of the envelopes of the signals received from these antennas for a number of different antenna spacings. We also determined the theoretical relationship between the cross-correlation and the cumulative distribution curve of the combined signal. If we select a particular cumulative distribution curve as acceptable for diversity operation, the corresponding correlation then indicates the required antenna spacing. Using the experimental correlation data of both the in-line propagation case and the broadside propagation case as a guide, we will try to reduce the required antenna spacings by using an array of three base-station antennas.