Newbie DSP Questions

Avner Ezra avner.ezra at yahoo.com
Sat Dec 31 22:16:14 UTC 2011


Hi

I'm newbie in DSP and SDR, but I'm really interested in this matter and I want to learn. I have bunch of questions which has short answers, if you can please answer them in easy terms and words, so I can understand them well. I hope this thread will help a lot of other newbies to understand DSP stuff better.

I have a Linux with every needed tools like osmocomm, gnuradio, openbts, kalibrate, uhd drivers, ... and USRP N210 hardware ready. Everything works normal and well. 

A) Sometimes when I scan a network bandwidth like GSM1800 using kalibrate, I see some channels like 820, 538, etc. When I re-scan, I cannot find them. Does this mean that kalibrate finds a channel when a mobile handset has a live conversation or sms send or receive in progress? or what?

B) I want to wideband capture for example 125 ARFCN. It needs 25MHz bandwidth which USRP N210 can handle and stream it easily. Even AFAIK I can double this number and capture 250 ARFCN using single N210 (50MHz, in USRPN210 data sheet it says it's capable of streaming 50MHz wide signals). 

How can I wideband capture 125 ARFCNs? I tried to do it using:
./uhd_rx_cfile.py -f `arfcncalc -a 512 -b 1800 -d` --samp-rate=25000000 -N 200000000 -g 70 b1.cfile

What I understood and decided to write such command above:

B-1) arfcncalc calculates frequency of first GSM1800 channel (512 ARFCN) which is start point (in above command)
B-2) Sampling Rate is the bandwidth I want to capture, in our case it's 25MHz means 125 ARFCN which each ARFCN has 200kHz bandwidth
B-3) 200M samples will be received (-N parameter)
B-4) Gain value is 70, means it will boost antennas to maximum power to receive signals, I think USRPN210 max. gain is 80
B-5) My decimation rate here using 25M sampling rate and USRP N210 which has 100MHz ADC, will be 4. So if I decided to read cfile I have to use 4 as decimation rate.

Please explain and tell me if I'm correct or wrong about above statements.

Does this will work and I'll have a 125 ARFCN from 512 to 637 ARFCNs captures into b1.cfile?
If so why in http://gnuradio.org/doc/doxygen-3.4/classusrp__source__base.html#adf8529d74696b7476e78bb0ef7b630af documentation, it says set_rx_freq should set center of frequency, so for example if you want to capture from 512 to 637, you should set frequency to frequency of ARFCN 574?


C) How can I seperate and process 200khz by 200khz channels in wideband captured file?


Thank you so much for your time
Your help and answers will solve problem of a lot of newbies like me.
Best Wishes
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