# 1. Re: Multipath Fading (Marcus M?ller) **URL:** \ **Category:** GNU Radio\ **Created:** [May 7, 2015, 7:26pm UTC](https://www.ruby-forum.com/t/1-re-multipath-fading-marcus-m-ller/241879 "2015-05-07T19:26:15Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![Detlef\_R](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/58f4c7/32.png) [@Detlef\_R](https://www.ruby-forum.com/u/Detlef_R)\ **Post date:** [May 7, 2015, 7:26pm UTC](https://www.ruby-forum.com/t/1-re-multipath-fading-marcus-m-ller/241879/1 "2015-05-07T19:26:15Z")
Hello, I working on a project wireless channel emulator. I have to implement multipath fading in gnu radio using signal source from usrp. I have tried using the fading blocks but i am not able to identify whether multipath fading is occuring in the output or not. Regarding the top blocks i have sent…it was just a screenshot before correcting the top block error(WX IN TOP BLOCK AND QT IN FLOWGRAPH) Also according to the theory power should attenuate after applying fading. In total i just need to implement multipath fading using usrp and gnu radio. ---
**Author:** ![Ritvik\_P](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/df705f/32.png) [@Ritvik\_P](https://www.ruby-forum.com/u/Ritvik_P)\ **Post date:** [May 8, 2015, 12:15pm UTC](https://www.ruby-forum.com/t/1-re-multipath-fading-marcus-m-ller/241879/2 "2015-05-08T12:15:09Z")
Hi Ritvik, to analyze whether you’re seeing attenuation, you should actually compare the power of a un-faded and the faded version of a signal with a _single_ QT sink. You can configure that sink to have two inputs. Your dB-scaled FFT display hardly lends itself to comparing powers. To test the fading model, you really should use noise from the GNU Radio noise source and not what you receive with a USRP, because in normal cases, what a USRP receives won’t be white, and hence is sub-optimal to test a channel model. Therefore, the testing setup I’d propose looks somewhat like [0]. > according to the theory power should attenuate after applying fading. > Talking of the gr::channels::fading\_model: It’s solely based on > flat\_fader\_impl[1], which, if I don’t misread the source, is really just > Clarke’s model of a Rayleigh channel[2], scaled. This will not give you > a strong attenuation on average. Rayleigh fading is based on the idea > that due to a large number of real-world scatterers, the channel impulse > response (which is a function) comes from a Gaussian process[3]; because > we consider complex channels, the amplitude (as the combination of > independent I and Q) is Rayleigh distributed. Now, you’re right, at no point there can be more power coming out of the channel than what went into it, so the maximum output of the channel is the input power, and on average, the power will be less. However, the statistical properties of the actual power of an actual channel depend on the statistical properties of the the absolute path lengths – something that a fading channel model itself doesn’t simulate, because it only cares about the path /differences/. Greetings, Marcus [0] [![](https://gist.githubusercontent.com/marcusmueller/1bbf7704afec20c4f00d/raw/6d5a0329e6fb8916be3e5eb4817e0c47b279a3c9/fading_test.grc.png) ](https://gist.githubusercontent.com/marcusmueller/1bbf7704afec20c4f00d/raw/6d5a0329e6fb8916be3e5eb4817e0c47b279a3c9/fading_test.grc.png) [0] GRC file also available: > [1] > [2] Since you’re after modeling RF channels a bit closer to the physical medium: Have a look at Appendix A /p.992 of Clarke, R.H., “A statistical theory of mobile-radio reception,” /Bell System Technical Journal/ , vol.47, no.6, pp.957,1000, July-Aug. 1968 URL: [http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6779222&isnumber=6779217](http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6779222&isnumber=6779217) [3] Central limit theorem: a sum of independent, identically distributed random variables follows a Gaussian distribution, no matter what the distribution of the individual variable is, as long as the number of variables is sufficiently large.