ystem with potential diversity factor K, shown in Figure 1.1. In an SS system, the K transmitter-receiver pairs of Figure 1.1 are replaced by a single wideband communication link having the capability to synthetize and detect all of the waveforms poten- tially generated by the orthogonal communication system complex. The pure SS strategy of randomly selecting a link for communication is replaced with an equivalent approach, namely, selecting a D-dimensional subspace for waveform synthesis out of the system's 2T,W--dimensional signal space. This random selection process must be independently repeated each time a sym- bol is transmitted. Independent selections are necessary to avoid exposing the communication link to the threat that the jammer will predict the sig- nal set to be used, will confine his jamming energy to that set, and hence, will reduce the apparent multiplicity and energy gain to unity. Three system configurations are shown in Figure 1.2, which illustrate basic techniques that the designer may use to insure that transmitter and receiver operate synchronously with the same apparently random set of signals. The portions of the SS system which are charged with the respon- sibility of maintaining the unpredictable nature of the transmission are double-boxed in Figure 1.2. The modus operandi of these systems is as follows: 1. Transmitted reference (TR) systems accomplish SS operation by trans- mitting two versions of a wideband, unpredictable carrier, one (x(t)) modulated by data and the other (r(t)) unmodulated (Figure 1.2(a)). These signals, being separately recovered by the receiver (e.g., one may be displaced in frequency from the other), are the inputs to a correla- tion detector which recovers the data modulation. The wideband carrier in a TR-SS system may be a truly random, wideband noise source, unknown by transmitter and receiver until the instant it is generated for use in communication. SOURCE NOISE DEMOD SOURCE (a) Transmitted Reference System SOURCE DEMOD (b) Stored Reference System MATCHED 