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

Josh Blum josh at ettus.com
Tue Jun 26 22:32:18 EDT 2012



On 06/26/2012 04:10 AM, Birhane Alemayoh wrote:
> Hello all
> 
> I understand the USB and gigiabit limitation when streaming data to a host
> PC. These  RF bandwidth of the FPGA  is decimated by a decimation factor to
> reduce the data rate to be streamed to the PC.
> 
> 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!!
> 

Decimation factors are integers such that host sample rate =
100e6/decimation. I recommend picking even decimations.

You can always perform re-sampling in the host to achieve the desired
rate. There are some nice rational re-samplers in gnuradio for example.

-josh

> Birhane
> 
> 
> 
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