Re: [Discuss-gnuradio] GMSK minimum bitrate

[Brian Padalino]

On 5/8/07, Håvard Espeland <address@hidden> wrote:
> We're writing a gmsk radio module with low bitrate (<5 kbps) and
> continuous constant bitrate transmission using Gnuradio 3.0.3 and USRP.
> The code is based on the digital/tx_voice.py example, and I have two
> unanswered questions.
>
> By hacking on the example, pick_bitrate fails to return a lower bitrate
> than 35.714kb/sec. Is it possible to support a precisely defined lower
> bitrate, or is there a technical limitation?

You should be able to achieve that, but you have to do more than 1 or
2 samples per symbol.

All the TX samples are sent out of the AD9862 at a rate of 128MHz.
The FR_INTERP_RATE register within the FPGA has a maximum value of
1024 which is the maximum amount of interpolation between samples that
are given to the FPGA.

See here:
   
http://gnuradio.org/trac/browser/gnuradio/trunk/usrp/firmware/include/fpga_regs_standard.h

Just doing some off the cuff calculations, I believe that if you set
the interpolation rate to the maximum (1024), and the number of
samples per symbol to 25, you should be able to achieve 5kbps.

Hopefully this will work out for you?

> Another issue is underflow handling. Is there a way to gracefully
> detect and handle underflow if the producer (codec) lacks behind or
> stops? If unhandled, we would loose syncronization with the receiver.

As for this, I have absolutely no idea - so hopefully someone else
might be able to help you out.

Brian