# Frequency response from fading simulator block not working as expected

**URL:** <https://www.ruby-forum.com/t/frequency-response-from-fading-simulator-block-not-working-as-expected/239480>\
**Category:** GNU Radio\
**Created:** [February 28, 2014, 2:14pm UTC](https://www.ruby-forum.com/t/frequency-response-from-fading-simulator-block-not-working-as-expected/239480 "2014-02-28T14:14:38Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![musicdenotation](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ea666f/32.png) [@musicdenotation](https://www.ruby-forum.com/u/musicdenotation)\
**Post date:** [February 28, 2014, 2:14pm UTC](https://www.ruby-forum.com/t/frequency-response-from-fading-simulator-block-not-working-as-expected/239480/1 "2014-02-28T14:14:38Z")

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Hello,

In order to simulate a multi-path scenario, Im trying to validate the  
“Frequency Selective Fading Model” block, but the results I found are  
not  
the ones I would expect.  
The set up consisted of 2 taps delayed 20 microseconds each other, so  
that  
the frequency response should have very deep fadings spaced regularly in  
frequency. The results I got showed a flat frequency response, as well  
as  
in other standardized power delay profile scenarios.  
The blocks I used were a random source -\> OFDM Mod -\> Throttle -\>  
Frequency  
Selective Fading Model -\> WX GUI FFT Sink. Sample rate was set as 1MHz,  
so  
that in Freq. Selective block I set PDP Delays as [0, 20] and PDP  
Magnitudes as [1, 1].  
Please tell me if I misunderstood any of these parameters or this block  
could be really improved.

Thanks and Regards,  
Ricardo

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**Author:** ![Ricardo\_Yoshimura](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e0b2c6/32.png) [@Ricardo\_Yoshimura](https://www.ruby-forum.com/u/Ricardo_Yoshimura)\
**Post date:** [March 1, 2014, 12:52pm UTC](https://www.ruby-forum.com/t/frequency-response-from-fading-simulator-block-not-working-as-expected/239480/2 "2014-03-01T12:52:51Z")

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On 02/28/2014 02:13 PM, Ricardo Yoshimura wrote:

> and PDP Magnitudes as [1, 1].  
> Please tell me if I misunderstood any of these parameters or this block  
> could be really improved.

Did you increase the number of taps?  
Also, note that this block provides a time-variant channel. If you  
average, you might not see fades. For a static channel, use the “Channel  
Model”.

If you use the default settings, you should see fades, right?

M

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**Author:** ![Ricardo\_Yoshimura](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e0b2c6/32.png) [@Ricardo\_Yoshimura](https://www.ruby-forum.com/u/Ricardo_Yoshimura)\
**Post date:** [March 7, 2014, 8:05pm UTC](https://www.ruby-forum.com/t/frequency-response-from-fading-simulator-block-not-working-as-expected/239480/3 "2014-03-07T20:05:49Z")

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Hi Martin,

Thats right! As the number of taps was increased, the deep fadings were  
visible, ranging until about 20dB.

Thanks and Regards,  
Ricardo