The Significance Of Rayleigh Fading In Coverage Measurements & 40 Lambda Criteria
•Technical Paper: 40 Lambda Conversion
There are two types of fading that are relevant to performing coverage measurements, fast fading, also know as Rayleigh fading, and terrain based fading due to obstructions and propagation loss. The goal of coverage measurement is to measure the local signal strength in presence of terrain based fading.
Rayleigh fading is due to close in reflections. It produces drops in signal strength that are only a fraction of a wavelength in size (often on the scale of inches). These fades always exist and are only of interest in the sense that coverage measurements must be made in a manner that rejects Rayleigh fading, so that actuate measurements of terrain based fading can be made.
William C. Y. Lee derived a well accepted criteria for removing Rayleigh fading and retaining slower terrain based fading, known as 40 Lambda averaging [W.C. Y. Lee and Y. S. Yeh, "On the Estimation of the Second-Order Statistics of Log Normal Fading in Mobile Radio Environment", IEEE Trans. Comm., vol. 22, June 1974, pp. 869-873] (You can find an easier to read version of this criteria in most mobile communications text books). In order to remove Rayleigh fading, you should average data for a time period equal to the time it takes to traverse 40 wavelengths in your measurement vehicle and you should have no less than 36 to 50 points in that time. For the remainder of this text we will assume 40 points to make the calculations easier.
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•Technical Paper: 40 Lambda Conversion
SOURCE: Berkeley Varitronics Systems, Inc.