ributed random variables with values selected from a frequency set F accord- ing to a probability distribution function PF(), then (1.61) b. Periodic Hopping. if {fn} is sequence of frequencies with period N, gen- erated by a finite state machine, and if N denotes the number of occurrences of the frequency value f' in one sequence period, then (1.62) Notice that the PSD of the deterministically generated sequence will match that of the randomly generated sequence with PF(f) = NIN. Furthermore, the number of hops MD spanned by a data symbol is not a fac- tor in the spectral density of the resultant signal in either case. A rough measure of the hop pulse transform's width is 1/T Since integer multiples of 1/Th are required for orthogonal randomly phased tones of dura- The Advantages of Spectrum Spreading tion Th, the assumption that the radiated tones d + f, f E F and d E D, are orthogonal leads to the conclusion that the minimum required bandwidth is approximately = D = M. The PSDs which we have just calculated correspond to asymptotically long-term ensemble-averaged, time-normalized, energy spectra. Attempts at interception of these signals in reality will be made over short time inter- vals, usually much shorter than the periods of the pseudorandom number generators driving the SS waveform generator. Hence, signals with compa- rable PSDs may have differing short-term energy density characteristics, and hence, different LPI capabilities. 1.5.2 Independent Interference Rejection and Multiple-Access Operation We have already discussed the energy gain achievable against a jammer whose radiated signal is generated without knowledge of the key parame- ters used in generating the SS transmitter's modulation. We refer to this type of interference as independent, the connotation also applying to in-band interference from other friendly communication systems. The ability of a SS system to reject independent interference is the basis for the multiple-access capability of SS systems, SO called because several SS systems can