Two analytical models for the parameter of rms delay spread are presented for a three-path radio environment. The reasons that led to the development of these models are : (i) the fact that the assumption of the existence of two paths in the radio channel did not seem to be enough for the description of all cases, (ii) simulation results showed that in most cases there are significant differences between the Cumulative Distribution Functions (CDFs) of rms-delay spread obtained for a two-path and a three-path radio environment. On the contrary, the CDFs obtained for L = 3, 4, 5 and 10 (L is the number of paths) are similar, and (iii) experimental results in indoor environment showed that in most cases there are 3 dominant paths in the radio channel. Probability Density Functions of the resulting rms delay spread are plotted for different values of the parameters of the models and compared with simulation results. In order to examine the validity of the models, measurements have been conducted in National Technical University of Athens while others have been obtained from the literature and compared with theoretical results. In all cases, the X-square criterion showed that the match between experimental and theoretical results is very good. These models, therefore, can be used for the design of high bit-rate systems that will operate without problems resulting from intersymbol interference.
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