Re: [Discuss-gnuradio] Minimum and Maximum Bit rates for USRP2 benchmark_rx and tx

[Eric Blossom]

On Thu, Jul 16, 2009 at 03:30:26PM -0400, Srinivas wrote:
> Hi All:
> 
> I am new to GNU Radio and I am experiencing a few anomalies. My experimental
> setup is as follows:
> 
> - I am using USPR2 with GNU Radio ver. 3.2 installed on an Intel 64-bit Quad
> Core machine with GigEthernet.
> - I am using XCVR2450 daughter card and the latest firmware on the USRP2s.
> - I am running benchmark_rx.py and benchmark_tx.py using two USPR2s.
> - I have wired the two USRPs using a 60 dB attenuator.
> 
> My Observations:
> - The maximum bit rate I could achieve was 12.5 Mbps corresponding to
> samples/symbol = 2 and interpolation and decimation factor set to 4
> - The minimum bit rate I could see was 120 Kbps in the 2.4GHz band and 250
> Kbps on the 5GHz band.

The bit rates should not vary depending on the frequency.  The
appropriate gain required to achieve a given bit rate may be dependent on
frequency band on the XCVR2450 (2.4 vs 5 GHz).


> I have the following questions:
> 
> 1. What is the minimum and maximum bit rates that USPR2 can support ?

Not to be glib, but the correct answer is "it depends".
It depends on at least:

  * modulation
  * any additional s/w decimation/decimation that you add.
  * system h/w configuration (cpu, cache, power saving mode, memory
    bandwidth..., other things on the bus, other things on the
    ethernet, the particular peripherals in your pc, ...)
  * system o/s configuration
  * are you using gr.enable_realtime() ?


> My immediate guess for minimum bitrate is that it depends on the ADC/DAC
> sampling rates, samples/symbol and interpolation and decimation factor. So
> with least interpolation (4) and least Samples/Sym (2), the calculations
> give 12.5 Mbps (for GMSK - 1bit/sample/Hz).
> The maximum bit rate is limited by the host processor; whether it can churn
> out as many packets as the USPR2 wants in a second.
> 
> Is my analysis correct ?

Uhh, if you want the minimum bit rate, you'd want the maximum
interpolation (that is, the minimum bandwidth of the complex baseband).

> 2.  Why does the minimum bit rate depend on the carrier frequency ?

It shouldn't.

> I tried changing the Tx amplitude and Tx gain at 5G but it still didn't
> change anything.

By "didn't change anything" do you mean that when you measured the
output with a spectrum analyzer or o'scope, there was no change?  If
so, what values were you passing to the set_gain method?  Are they
within the values returned by gain_range() and/or gain_min(),
gain_max()?


> 3. How to perform benchmark_rx/tx in BERT mode ( from digital-bert folder) ?
> Looks like the scripts are written for USRP1. How to modify them to make it
> work on USRP2 ?
>
> Can anyone please help me resolve these issues ?

Did this help?  ;-)

> Thanks,
> Srinivas
> Graduate Student,
> WINLAB, Rutgers University, NJ

Eric