Simple digital data transmitter

david · May 12, 2010, 6:12pm

Hi,
Â
I have set up the following simple digital data transmitter:
Â
Â
File source -Ã Â Â throttleÂ Â Â -Ã Â Â Â packet encoder — >Â DBPSK modulator
-Ã USRP2Â Â Â Â
File sizeÂ Â Â Â Â Â Â Â Â Â Â Â sample rateÂ Â Â Â Â Â samples/sym - 2Â
Â Â Â Â Â Â samples/sym â€“ 2Â Â Â Â Â Â Â Â Â Â Inter rate 32Â Â
-variableÂ Â Â Â Â Â Â Â Â Â Â Â 3.125M Â Â Â Â Â Â Â Â Â Â Â Â bits/sym - 1Â Â Â Â Â Â Â Â Â Â Â Â Â
excess BW â€“ 0.35Â Â Â Â Â Â Â Â Freq 400M
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â access code â€“
n/aÂ Â Â Â Â grey code - yesÂ Â Â Â Â Â Â Â Â Â Â Â Gain 0
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â pad for USRP â€“ yes
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â payload len â€“ 0
Â
Â
What is the transmit (over the air) data rate of this line up ?
Â
Please explain how the rate is derived.
Â
Thanks,
Dave

david · May 12, 2010, 8:06pm

On Wed, May 12, 2010 at 09:06:13AM -0700, David B. wrote:

Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â payload len â€“ 0
Â
Â
What is the transmit (over the air) data rate of this line up ?
Â
Please explain how the rate is derived.
Â
Thanks,
Dave

Remove the “throttle”, it’s not needed.

Your baseband sample rate is 100M/32 -> 3.125MS/s

You’re using 2 samples/symbol, thus your symbol rate is
3.125MS/s / 2 -> 1.5625Msymbols/s
And since you’re using DBPSK, you’re getting 1 bit/symbol, thus your
raw over-the-air bit rate is 1.5625Mbit/s

Eric

david · May 12, 2010, 8:56pm

Thanks Eric.

So the packet encoder samples /symbol does not affect the bit rate but
just needs to match the dpsk samples/ symbol rate?

Dave

From: Eric B. [email protected]
To: David B. [email protected]
Cc: [email protected]
Sent: Wed, May 12, 2010 2:02:42 PM
Subject: Re: [Discuss-gnuradio] Simple digital data transmitter

On Wed, May 12, 2010 at 09:06:13AM -0700, David B. wrote:

Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â payload len â€“ 0
Â
Â
What is the transmit (over the air) data rate of this line up ?
Â
Please explain how the rate is derived.
Â
Thanks,
Dave

Remove the “throttle”, it’s not needed.

Your baseband sample rate is 100M/32 → 3.125MS/s

You’re using 2 samples/symbol, thus your symbol rate is
Â 3.125MS/s / 2 → 1.5625Msymbols/s
And since you’re using DBPSK, you’re getting 1 bit/symbol, thus your
raw over-the-air bit rate is 1.5625Mbit/s

Eric

david · May 12, 2010, 8:56pm

On Wed, May 12, 2010 at 11:42:55AM -0700, David B. wrote:

Thanks Eric.

So the packet encoder samples /symbol does not affect the bit rate but just needs to match the dpsk samples/ symbol rate?

Dave

Right.

Eric

david · May 13, 2010, 7:31am

Eric B. wrote:

Your baseband sample rate is 100M/32 → 3.125MS/s

You’re using 2 samples/symbol, thus your symbol rate is
3.125MS/s / 2 → 1.5625Msymbols/s
And since you’re using DBPSK, you’re getting 1 bit/symbol, thus your
raw over-the-air bit rate is 1.5625Mbit/s

Hi Eric, a quick question about this.

According to:
_pick_bitrate(bitrate, bits_per_symbol, samples_per_symbol, xrate,
converter_rate, xrates, gen_info) in pick_bitrate.py

when the samples per symbol and interpolation are given, then our
bitrate
can be determined straight away.
i.e Bitrate = converter_rate / xrate / samples_per_symbol

Is this correct though, in determining the bitrate for both dbpsk &
dqpsk?
For instance if we passed the_pick_bitrate() function an interpolation
of
128 and Samples/symbol of 2, then according to the code, our bit rate
for
both dqpsk and dbpsk will be 500kbs.

But according to what your saying:

for dbpsk

if DAC rate = 128e6 Interpolation = 128 Samples/symbol = 2
Bitrate = 128e6 / 128 / 2 = 500k

for dqpsk

Using the same parameters
Bitrate = 128e6 / 128 / 2 / 2bits per symbol = 256K

Have i missed something here, or in the code?

Regards,

Marcin

–
View this message in context:
http://old.nabble.com/Simple-digital-data-transmitter-tp28538027p28543866.html
Sent from the GnuRadio mailing list archive at Nabble.com.

david · May 13, 2010, 2:07pm

On Thu, May 13, 2010 at 1:18 AM, marcin_w [email protected]
wrote:

for dqpsk

Using the same parameters
Bitrate = 128e6 / 128 / 2 / 2bits per symbol = 256K

Have i missed something here, or in the code?

Regards,

Marcin

This does appear to be a bug. That calculation in pick_bitrate.py
should also be divided by bits_per_symbol.

Tom