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The effective use of predetection (or IF) linear combining of diversity channels requires that all frequency components be added in = phase. In view of the time-variant phase characteristics of fading channels, some form of instantaneous phase correction must be provided prior to the diversity combining so that proper in-phase addition will result. If the instantaneous fading is not uniform over the width of the channel, the resulting channel incoherence will reduce the efficacy of the phase correction. One way to avoid this difficulty is to divide the selectively fading channel into "flatly" fading subchannels and perform the necessary phasing and combining operations per subchannel. The question arises as to how much frequency selective fading can be tolerated within a channel, or a subchannel, before serious deterioration in performance results. This paper considers this question with reference to FDM-SSB binary data transmission and matched filter reception, wherein a pilot tone is used both for deriving proper phase correction and for providing the weighting necessary for maximal ratio combining. Analytical expressions are derived for error probabilities which show the deleterious effects of frequency selectivity and pilot tone additive noise.