An important application of spread spectrum modulation is to provide interference immune communications. Generally, the approach to system performkce analysis will assume a host of various interference situations with the hope the results give wide applicability, while, based on the concept and properties of pseudo-random spreading waveforms, it is possible to rid the analysis of artificial or restricting interference assumptions and obtain completely general performance bounds.
Using a channel impulse responsehnction formalism, an overall channel model which combines ionospheric scintillation fading and terrestrial multipath efects has been developed for the Mobile User Objective System (W O S). For ionospheric scintillation fading we construct a channel impulse response function using a mutual coherence hnction. For terrestrial multipath we use a channel impulse response$nction based on a tapped-delay line model. We solve the combined model numerically using the Naval Research Laborato y Impulse Response Function Simulator code (NCIRFS) for several modulation waveforms for bothfiequencyselective andfiequency nonselective fading conditions. We compute the delay spread for a simulated ground terminal in an urban and maritime fading environment. For an urban or maritime scintillation environment we find that the delay spread is a multiplicative combination of both multipath and ionospheric scintillation fading and can extend over several modulation periods. We present results for several waveforms and for combinations of fading and multipath environments.
Statistical propagation models for the combined efect of ionospheric scintillation and terrestrial multipath are derived. These models are used to determine the satellite link performance of the Mobile User Objective System ( W O S ) under various operational conditions. The models are described byJirst order statistics in the form of probability density functions (PDFs) and cumulative distribution ficnctions (CDFs). The PDFs include combinations of Rayleigh, Rician and lognornial distribution functions. The corresponding CDFs are used to assess satellite link performance and establish link margins. The degrading effect of combined ionospheric scintillation and terrestrial multipath fading on link availability is illustrated with a number of examples.
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