Thus far in our discussions, we have considered only those frequency-hopped (FH) systems in which the carrier phases of the individual transmitted hop frequency pulses bear no relation to one another. Such a spread-spectrum (SS) technique was referred to as non-coherent FH and the method for recovering the data modulation appropriately employed some form of energy detection. Clearly, another possibility exists for implementing the FH modulator, wherein phase continuity is maintained from one hop pulse to another thereby resulting in so-called coherent FH. Assuming that the coherent frequency synthesizer in the receiver is capable of estimating and correcting for the phase errors associated with the electrical path length over the transmission channel and the Doppler shift, then following the dehopping operation one could employ coherent detection techniques for recovery of 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 quadrature partial response (QPR). ^[1] We shall consider binary phase-shift-keying (BPSK) as a degenerate case of QPSK. The performance of these modulation 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...