From: Tom R. [email protected]

To: Bill S. [email protected]

Cc: [email protected]

Sent: Friday, March 6, 2009 10:55:09 AM

Subject: Re: [Discuss-gnuradio] does the number of samples per symbol

for the dbpsk case really affect the PDR performance?

On Fri, Mar 6, 2009 at 9:38 AM, Bill S.

[email protected] wrote:

Bill S. wrote:

hand?

Tom

Bill

Yes, you are correct. There is a 2:1 difference in the interp to decim

rates. However, the bps rate of the digital system is based on the

number of samples per symbol and the interp/decim.

r_tx = r_dac / interp / sps

r_rx = r_adc / decim / sps

Where r_dac is the DAC rate, r_adc is the ADC rate, interp is the

interpolation rate, decim is the decimation rate, and sps is the

samples per symbol.

If you look in pick_bitrate.py, we make some calculations to determine

what the actual bit rate is depending on what parameters are supplied

based on these calculations above.

If you want 500 kbps, then r_dac = 128 MSps, interp = 128, and sps = 2

Sps

r_tx = 128e6 / 128 / 2 = 500e3

At the receiver, r_adc = 64 MSps, decim = 64, and sps = 2 Sps

r_rx = 64e6 / 64 / 2 = 500e3

If you had specified a different sps for each side, then the

pick_bitrate calculation would try to attain the specified rate of 500

kbps. It would find that with the receiver using 2, the interp rate

should be 128, but at the transmitter with 4 Sps, the decim rate would

be 32 instead.

Under these conditions, both sides would be able to hit the 500 kbps

rate exactly and you should have no problem.

On the other hand, if you specified BOTH the sps and the decim/interp

rates, then the resulting rate would be different by a factor of two

and therefore your tests would be screwed up. OR if you specified the

different sps and a rate that produces a different rate for both the

transmitter and receiver, then things could be screwed up.

I suggest rerunning both scripts quickly with your exact settings you

used in your experiments and see what the decim, interp, sps, and

resulting TX and RX rates were.

Tom

Hi, Tom

Thank you! I understand what you said! Actually, in our experiment, the

bitrates and number of samples per symbol were specified at both sides

and only these two parameters were specified for our experiment. The

decim and interp will be self-adjusted! I think that is the case!

Bill