can be resolved and the system is then capable of credible data detection. Obviously, because of the lack of perfect phase information per symbol, some degradation in system performance over coherent detection of the same modulation will exist. In the spread-spectrum (SS) application, the need for differentially coher- ent detection comes about as follows [1]-[4]. Conventional frequency hop- ping (FH) as used to protect communication systems from radio frequency interference (RFI) or jamming has non-continuous carrier phase from hop to hop. For the fast hop rates needed in most RFI and jamming environ- ments, the receiver dehopper does not have a chance to acquire the phase of each new carrier in the hop sequence. Differentially coherent detection provides a possible solution to the effect of phase discontinuities introduced by frequency hopping since the absolute phase of each carrier in the hop sequence is irrelevant insofar as the data detection process is concerned. More explicitly, the case of interest in FH differentially coherent modula- tion systems is where there are many (at least two) data symbols per hop, i.e., the so-called slow frequency hopping (SFH) case. Thus, for all the data symbols on a given hop, the carrier phase is constant and, regardless of its value, the combination of differential phase encoding and phase difference detection is sufficient to produce reliable communication. Of course, as in any differentially coherent system, the first data symbol in the sequence Differentially Coherent Modulation Techniques (here the first data symbol in each hop) is lost since a phase reference has not yet been established for its detection. This again emphasizes the neces- sity of having many symbols per hop SO as to reduce this effective loss in information rate to a minimum. As in the previous chapter, we shall again examine the performance of the various modulations considered in the presence of partial-band noise and partial-band multitone jamming. In each case, the worst case jamming strat- egy 