 the data modulation. Of all the modulation techniques which lend themselves to coherent detection, those that are most commonly found in present-day applications are the so-called quadrature modulations which include quadriphsae-shift- keying (QPSK), quadrature amplitude-shift-keying (QASK), and quadra- ture partial response (QPR). 1 We shall consider binary phase-shift-keying (BPSK) as a degenerate case of QPSK. The performance of these modula- tion techniques over the additive white Gaussian noise (AWGN) channel is well documented in the literature [1], [2], [3]. This chapter presents the comparable results when an FH/SS modulation is superimposed on these conventional techniques in order to combat the intentional interference introduced by a jammer [4], [5]. The receiver structures that will be analyzed will be those normally employed for coherent detection of these modula- tions in an AWGN background. As such, our intent here is not to consider 1The QPR Class I modulation, which is the only case we shall consider, is also referred to in the literature as duobinary-encoded QPSK [1]. Coherent Modulation Techniques optimum receiver structures based upon jamming state information, but rather to demonstrate the effect of the despread jammer on the conven- tional structures. The scenario under which the jammer is assumed to operate here is the same as that proposed in the previous chapters for the design of non-coher- ent FH/SS systems. In particular, we assume an intelligent jammer that has knowledge of the form of data and SS modulations, including such items as data rate, spreading bandwidth and hop rate, but no knowledge of the code selected for determining the spectrum-spreading hop frequencies. Once again, the strategy employed is to design for the worst case jammer in the sense that, given the modulation form of the communicator, the jammer is assumed to employ the type of jamming which is most deleterious to the communication receiver. The two most common jammer types are still the partial-band noise and p