Modeling Multipath Fading Responses Using Multitone Probing Signals and Polynomial Approximation

Multipath fading (hereafter abbreviated MPF) on terrestrial microwave paths can be a major cause of outage in digital radio systems.1"4 Numerous efforts have been aimed at understanding, analyzing, and correcting this source of disruption, and some have led to new statistical models for MPF responses. 5 " 9 The particular model that inspires the present work approximates the MPF response by a low-order complex polynomial in frequency. 8 For a particular 26-mile path in Georgia, it was shown that a firstorder polynomial suffices to characterize the fading response in a 25193 MHz band centered near 6 GHz. The joint probability distribution for the polynomial coefficients was derived for that path, thus permitting a complete statistical description of the MPF response. Now another experiment is being instrumented, this time for a 23mile path in New Jersey operating in the 11-GHz band. The aim of the new experiment is to add to, and in several respects improve upon, the data base used to quantify the earlier polynomial model. The improvements include higher measurement signal to noise ratios (SNRS), higher sampling rates (20 measurements per second rather than 5), coherent processing to obtain phase information (previously absent), and a wider measurement bandwidth (40 MHz rather than 25 MHz). Given the highly variable nature of multipath fading, such improved measurements for a new path in a different frequency band and locale should add importantly to our knowledge of this phenomenon.