If you plot the fade statistics for HF signals, however, you find many times that the result looks much more like Rician fading than Rayleigh.
Jack K8ZOA On 9/16/2010 2:11 PM, Kok Chen wrote: > On Sep 16, 2010, at 9/16 9:59 AM, Jim Brown wrote: > >> A VERY large component of fading is due to multipath -- that is, >> the arrival of more two or more wavefronts that travel different >> paths, and >> thus have slightly different travel times. > Selective fading does not require multipath. The CCIR 520-2 profiles > for Raleigh fading are all single path models. Raleigh fading causes > selective fading. > > http://www.itu.int/rec/R-REC-F.520-2-199203-W/en > > Whenever you hear distorted AM signals on HF, chances are it is caused > by selective fading. You can see selective fading in a waterfall by > watching the fading occur as moving holes that sweep across the > spectrum (very visible when you tune in broadband signal such as a > Coast Guard weather FAX station on HF). You can also see it take away > individual tones in an Olivia signal in a waterfall. Selective fading > was also one of the primary impetus to switch from on-off keying to > FSK in the early days of RTTY. > > The Watterson model for ionospheric propagation breaks up a path into > a complex signal with in-phase and quadrature components: > > http://ieeexplore.ieee.org:80/xpl/freeabs_all.jsp?reload=true&arnumber=1090438 > > Each component passes through a independent scattering function that > have Gaussian random processes, both a Gaussian Doppler spreading term > and also a Gaussian amplitude term. > > The modulus (i.e. "amplitude", or square root of power of the I and Q > components) of a bivariate Gaussian random process happens to have > Rayleigh distribution. See references here (the Rician distribution > is more general in that the mean of the components need not be zero): > > http://en.wikipedia.org/wiki/Rayleigh_distribution > http://en.wikipedia.org/wiki/Rice_distribution > > I.e., the amplitude of a signal whose I and Q components each have > independent Gaussian statistics, has a Rayleigh distribution. > > As a consequence, a Rayleigh signal can cause selective fading without > the need for a second path. Since the Rayleigh probability density > function has finite probability of being infinitesimally small, the > fade has a chance of being very deep. > > If you run a signal through an HF Channel Simulator (such as AE4JY's > PathSim or cocoaPath) set to rayleigh fading parameters, you will see > selective fading. > > Watterson's paper also considers both the cases of multi- paths and > multi- magneto-ionic rays that are scattered by the ionosphere. > > Multipath signals have a time delay, multiray signals do not have a > time delay between the rays. > > It is a fascinating paper that hams interested in HF propagation > should read. Unfortunately, I have not found a free version on the > web that I can reference, even though the research was done using tax > payer's money at what is today NIST. But if you are an IEEE member, > you can download the paper for free. > > 73 > Chen, W7AY > > > > > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
