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The question of how many receiver antennas to employ in a diversity system operating in cochannel interference is examined. The long-term signal-power-to-interference-power ratio, the long-term signal amplitude to the square root of the interference power ratio, the average instantaneous signal-to-interference ratio, and the average bit-error rate of a maximal ratio combining diversity system in the presence of an arbitrary number of cochannel interferers are evaluated when the desired user signal and the interfering user signals are independent, and each of them experiences correlated Ricean fading at the receiver antennas. The best number of antennas required for a fixed-size antenna array to achieve a good compromise between system performance and system cost is investigated. The results show that, in general, the performance of a fixed-size antenna array containing the maximum number of independent antennas cannot be significantly improved by adding more than one additional antenna. Some special cases where particular gains can be achieved by adding additional correlated antennas are also discussed.