SPREAD SPECTRUM SIGNAL SETS WITH REDUCED MEAN-SQUARE CROSSCORRELATION (FREQUENCY HOPPING, TIME PHASE)
Abstract
In a spread spectrum multiple-access communication system, we wish to support not only a large population of potential users, but also a large number of active users at any one time. For a given time-bandwidth product and signal-to-noise ratio, the population of users M is approximately proportional to the mean-square crosscorrelation function envelope, V(,C)('2), and the number of active users U is inversely proportional to the V(,C)('2). Therefore, when designing a signal set for this application, a trade off must be made between these two numbers in order to fulfill the requirements of the system. Most previously known design procedures generate sets of signals which provide a large number of potential users but a small number of active users. If we want to increase U, a signal set with smaller V(,C)('2) must be found. This implies that the total number of signals in the set is also reduced. In the optimum case, when U is equal to M, every user can be active all of the time. This is optimum because it is meaningless to have U larger than M. The question, therefore, is whether a set of signals that can accomplish this optimum condition really exists. The design procedures presented in this research show that signal sets with optimum V(,C)('2) do exist for all three basic spread spectrum signal types: frequency-hopping, time-hopping, and phase-hopping. Similar signal sets can also be found for all different types of hybrid signals: frequency + time hopping, frequency + phase hopping, time + phase hopping, and frequency + time + phase hopping. A general signal representation method using complete orthonormal basis functions is also investigated. This method increases the flexibility of the simulation program such that different signals can be calculated with the same program by merely changing in the coefficients of the basis functions. The peak values of the correlation function envelope at the integer shifts of the hop duration can be controlled when a special set of coefficients generated from a PN sequence is used. However, the V(,C)('2) of this signal set increases and moves away from the optimum situation.
Degree
Ph.D.
Subject Area
Electrical engineering
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