Benchmark_tx/rx does not work below 500k bitrate

hi
I tried setting the bit rate of benchmark_tx/rx below 500k and the
receiver
senses no messages below this rate. Is there any specific reason why is
it
so? I use xcvr2450 boards. and i transmit at 5.1G.

Thanks
Udaya

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udadidd wrote:

hi
I tried setting the bit rate of benchmark_tx/rx below 500k and the receiver
senses no messages below this rate. Is there any specific reason why is it
so? I use xcvr2450 boards. and i transmit at 5.1G.

The lower your bitrate, the narrower the signal gets. The narrower the
signal, the narrower the filters. With narrow filters, the frequency
offsets become a problem. The best way to handle this would be to add
some sort of coarse frequency acquisition and tracking, but that is not
in the code now, only fine tracking.

Matt

I faced the same issues and my findings on this was:

•Empirically:
for Reliable transmission 120 Kbps <= Br (data rate) <= 2 Mbps (For
GMSK, Pkt. Size = 1500)

•Theoretically:

  • Maximum Bit rate corresponds to Minimum Decimation Rate and min.
    samples/symbol value
  • ADC Sampling Rate = 100 MSamples/Sec
  • Min. Decimation Factor (D) =4, Min. Samples/Symbol (S) = 2
  • Maximum bitrate = ADC rate / (D * S) = 12.5 Mbps

However the host computer is not fast enough to generate samples at
this rate and therefore the practical max. is limited to 2 - 5 Mbps

•Minimum Bit rate corresponds to Max. Interpolation and Max Samples/Sym

  • DAC Sampling Rate = 400 Msamples/Sec
  • Max. Interpolation Factor (I) = 512, Max Samples/Symbol (S) = 7
  • Minimum bit rate = DAC rate /(I * S) = 111.6 Kbps

which more or less confirms with the empirical minimum

Srinivas
Graduate Student
WINLAB, Rutgers University

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