On Sat, Dec 31, 2011 at 5:16 PM, Avner Ezra <avner.ezra(a)yahoo.com> wrote:
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?
No, it most cases it means a weak signal. Kalibrate performs
per-channel energy detection prior to offset calculation against the
FCCH, which is always active. The variance is probably just noise.
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).
To capture 50MHz you need to 8-bit samples to avoid saturating the
Ethernet bus. So yes it's possible, though I'm not sure about the
current state of device or API support for the feature.
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.
You want the centre frequency of the spectrum you want to capture -
not the start point. Also, the maximum gain value will also vary
depending on the daughterboard. Otherwise, you are correct.
C) How can I seperate and process 200khz by 200khz
channels in wideband
captured file?
The brute force approach is to individually frequency translate each
channel down to baseband, low-pass filter, and perform sample rate
conversion. The more efficient method it to construct a multirate
channelizer. GNU Radio has blocks for either approach, though
construction may not be straightforward.
Thomas