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Re: [Discuss-gnuradio] Re g : spectrum sensing code

From: sreeraj r
Subject: Re: [Discuss-gnuradio] Re g : spectrum sensing code
Date: Wed, 22 Aug 2012 15:17:22 +0800 (SGT)

>Question - 1
>I was working with the UHD version of spectrum sensing code.
>I have USRP2 with me. I learnt that USRP2 can achieve 50MHz of RF bandwidth
>with 8 bit samples and 25MHz of RF bandwidth with 16 bit samples.
>I wanted to know how does this information translates while passing min_freq
>ans max_freq parameters to the spectrum sensing code. Does it mean (max_freq
>- min_freq) = 25MHz.
>Please correct me if I am wrong :teeth:

As far as I understand spectrum sensing code is used for wideband spectrum analysis, that is
to scan across wide RF bands which USRP is not able to analyze at a time. So obviously
max_freq - min_freq > 25 MHz .

e.g Conider you have to analyze the spectrum from 400 MHz to 500 MHz. You can keep
min_freq = 400 MHz and max_freq = 500MHz. If you have configured USRP sample rate
to get 25 MHz bandwidth, ideally one round of scan will be complete by 4 tuning steps.
(practically 6 steps as the code is using 25% overlap to reduce non-linearity in DDC response).

Firas posted a detailed explanation of the code in the mailing list long back. You can find it
here http://www.ruby-forum.com/topic/174437.

>Question - 2

>Why the minimum center frequency is set as follows :
>self.min_center_freq = self.min_freq + self.freq_step/2

>Why it simply doesn't take the minimum frequency as we provided. Setting the
>max_center_freq I understood somehow.

e.g. Scan range 10MHz to 40 MHz
     USRP1 B/W = 8MHz
     First tuning step is with centre freq 14Mhz (10+8/2 ideally) which will cover the bandwidth from 10 to 18 Mhz

>Question - 3

>In non UHD version of spectrum sensing code the adc_rate was fixed fro
>USRP-1 i.e. 64M and that was used to calculate usrp_rate while in UHD
>version of the same code usrp_rate is calculated by the sampling rate we

>I was wondering how does the code manages to work with devices of different
>sampling rates i.e. USRP-1 with 64M USRP2 with 100M

sampling rate ----> Bandwidth  ----> feq_step

>Context : I am trying to scan WLAN channels 1, 6, 11 continuously. So I have
>to keep working with a total bandwidth of 76 MHz

Check the tuning delay of the device you are using and make sure that you are not losing any data during the sweeping process.I may be wrong somewhere. Please go through Firas's explanation which will make things clear.

Sreeraj Rajendran

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