ertain advantages and disadvantages, depending on the system design objec- tives (AJ protection is just one facet of the design problem). Potentially, a blend of modulation techniques may provide better performance at the cost of some complexity. For example, the choice (1.23) may capture the advantages of the individual wideband waveforms c(i)(t) and mitigate their individual disadvantages. Three schemes seem to be prevalent for combining the data signal d(t) with the SS modulation waveform c(t) to produce the transmitted SS signal x(t). 1. Multiplicative modulation: Used in many modern systems, the transmit- ted signal for multiplicative modulation is of the form (1.24) Mechanization simplicity usually suggests certain combinations of data and SS formats, e.g., binary phase-shift-keyed (BPSK) data on a DS sig- Spread-Spectrum System Configurations and Components nal, or multiple frequency-shift-keyed (MFSK) data on a FH signal. These modulation schemes are the ones of primary interest in this book. 2. Delay modulation: Suggested for use in several early systems, and a nat- ural for mechanization with TH-SS modulation, this technique transmits the signal (1.25) Independent (switching) modulation: Techniques (1) and (2) are sus- ceptible to a jamming strategy in which the jammer forwards the trans- mitted signal to the receiver with no significant additional delay (a severe geometric constraint on the location of the jammer with respect to the transmitter and receiver), but with modified modulation. This repeater strategy, which if implementable, clearly reduces the multiplicity factor K of the SS system to unity, can be nullified by using a transmitted sig- nal of the form x(t) Re{(d()(t)ei(). (1.26) Here the data signal, quantized to M levels, determines which of M distinct SS modulations c(d)(t), d = 1, , 2, M, is transmitted. The key assumption here is that even though the jammer can observe the above waveform, it cannot reliably produce an alternate waveform c6()(t),j(t) # d(t), acceptable to the receiver as