Diversity Combining for Signals of Different Medians

Over-the-horizon communication systems normally use two identical antennas at each end. It is shown that an asymmetrical arrangement, for example, 6o'( )60' 30'( )30' is better than a symmetrical arrangement when both antennas at one end are used for diversity reception and the larger antenna at the other end is used for transmitting. This arrangement is important because for a given performance it costs less than the symmetrical arrangement. Depending on the exact relationship between cost and antenna gain, the cost is a minimum when the difference in antenna gains lies between 3.2 and 4.8 db. Statistical performance for various combining methods and degrees of diversity are presented. As for the case of equal medians derived previously, selector diversity combining is decidedly inferior to equal gain combining, which in turn is about one db worse than maximal ratio combining. The "instantaneous" SNR's (applicable to data transmission) are derived and compared to the usual short-term SNR's applicable to telephony.