[USRP-users] Regarding the 112 decimation and 174 decimation factors

Thomas Tsou ttsou at vt.edu
Wed Jun 27 15:10:53 EDT 2012


On Tue, Jun 26, 2012 at 7:10 AM, Birhane Alemayoh <birhanea at gmail.com> wrote:
> For GSM applications, many litratures recommended to use decimation factor
> of 112 for USRP1 and 174 for USRP2,  N2xxs to capture one GSM channle. But
> how can these decimation factors achieve a bandwidth of one  GSM channel.
> One GSM channel has a bandwidth of 200 kHz. But for a USRP N200  with 100MHz
> sampling rate for example;  if we use 174 decimation factor, we will end up
> with 100/174 =  574.7KHz. This will be the input for our PC.
> So what is the reason behaind selecting 112 and 174?
> Thank you!!

GSM channels are spaced 200 kHz apart, but the channels are
overlapping; the sample rate needed to capture a signal without
bandlimiting is somewhat higher. The GSM symbol rate is 270.833 ksps
and sampling at two samples-per-symbol will give a sample rate close
to the listed values.

  Thomas




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