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Re: [Discuss-gnuradio] Cycle time detection for an unknown cyclic radio

From: kaleem ahmad
Subject: Re: [Discuss-gnuradio] Cycle time detection for an unknown cyclic radio system
Date: Tue, 26 May 2009 00:57:27 -0700 (PDT)

Can someone at least comment on first part of the problem, I mean where I am
getting this 'factor' from


kaleem ahmad wrote:
> Dear All, 
> A few weeks ago I was involved in a discussion about calculating the cycle
> time of an unknown system present in the area by sensing the spectrum with
> the help of RFX2400+USRP1 system. For details you may like to have a look
> on previous mails on this thread. 
> http://www.nabble.com/cycle-period-detection-of-a-cyclic-periodic-transmitter-td23171564.html#a23174531
> As Firas suggested to choose a decimation factor with respect to duty
> cycle of the cyclic system. I implemented it in this way but I am facing
> two problems: 
> 1- I am using following formula to calculate the cycle time: 
> if 
> ADC_rate = 64MHz, D=decimation rate, fft_size, count= total no. of
> fft_scans where signal is not present 
> Then following would give me the value of cycle time: 
> [1/(ADC_rate/D)]*fft_size*count*1e3 ---> where [1/(ADC_rate/D)] simply
> gives the time resolution or time gap between two points in fft bin. 1e3
> for conversion in ms. 
> with D=128, fft_size=512, I scanned the spectrum for more than one times
> and stored the results in an array, then I choose maximum occerence in
> that array as 'count' in above formula e.g. 
> for 200ms cycle time my array look like [27, 200, 27, 27, 27, 27, 276,
> 27], here count = 27, other values are actually some errors 
> for 100ms cycle time my array look like [13, 13, 87, 13, 13, 13, 13, 167],
> here count = 13, other values are actually some errors 
> for 50ms cycle time my array look like [6, 6, 6, 6, 6, 6, 6, 6], here
> count = 6. 
> From this array with multiple scans I was able to see that some cyclic
> behaviour is correctly detected. BUT when I put all values in above
> mentioned formula then I found that to get exact value of cycle time I
> must multiply my answer with a 'factor' ranging from 7-8 (some times 7.25,
> or 7.4, or 7.5 etc). But this 'factor' is not a fixed value and variation
> in its value depend on the cycle time. So final formula becomes 
> [1/(ADC_rate/D)]*fft_size*count * factor *1.e3  --->  The value of factor
> is changing on following pattern: 
> for 200ms cycle time factor ~ 7.25 would give me exact answer 
> for 150ms cycle time factor ~ 7.35 would give me exact answer 
> for 100ms cycle time factor ~ 7.5 would give me exact answer 
> for 50ms cycle time factor ~ 8 would give me exact answer 
> I cant understand what this factor is and where it is comming from. Is
> there any processing in USRP which I am forgetting? Can someone help me to
> figureout this factor. 
> 2-My second problem is that, I am able to detect cycle time for any duty
> cycle with the help of 128, and 256 decimation rates , but with any other
> decimation rate my answers are quite random and I cant get a cyclic answer
> in my above mentioned array. Or simply you can say as it was looking from
> theretical side that for any specific decimation rate I should get a
> cyclic answer but the only thing which would change was the no. of
> detected and undetected scans. But its not happening in this way. e.g with
> any other decimation rate except 128, 256, I am getting answer like
> follows: 
> for 200ms cycle time my array look like [20, 2, 187, 17, 87, 65, 13, 167],
> which dont provide me any clue. Similarly for any other cycle time I am
> getting random values. 
> Is there any thing wrong with selection of decimation rate? can some one
> help me to figure out where I am doing wrong. 
> Thanks and Best Regards 

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