Fwd: BPSK transmitter - time domain waveform

GNU Radio
1

Hi Tom,

Yes, I am seeing sinusoids with low freq BPSK envelope → But, Not
clean
shift of 180deg as shown in figures of standard text books. Am i rite??

Also I didn’t get good idea of sps in root-raised cosine. I learnt that
taps of filter = 11*sps → smoothing freq domain. When googling mail
list,
i learnt that tx_original bw = 128e6 / interp / sps and hence i thought
that for each symb, two samples (out of root-coisned wave from for each
sym) and everything went fine.

After seeing baseband waveform, I lost all my understanding. My
questions;’

  1. For vector source and psk mod, there is no option of sample rate.
    What
    is the input symbol rate to psk mod and output sample rate of psk mod
    block? For each symbol, I am getting 23 samples (for sps : 2 ). If that
    is
    the case, how come equation of tx_original bw valid?

  2. As you said, I upsampled baseband signal before multiplying with
    signal
    source (for freq xlation simulation). Still I am not attaining clean
    shift
    of 180deg ? If you are able to attain that in GRC simulation, could you
    send that for me?

Thanks
Senthil

Thanks
Senthil

On Thu, Apr 5, 2012 at 7:15 PM, Tom R. [email protected] wrote:

of constant 1 for some samples (for 0) , inverted root-raised cosine
signal with 180deg phase shift as we all seen in stand. communication
Senthil
frequency offset between them.

Tom

–
http://amritasenthil.wordpress.com/

2

I am saying this because in theory, ppl assume that sin(2pifc*t +
Pshi)
where Pshi = 1(for 0) and -1 (for 1). Where as in Gnu Radio, we have
root-raised cosine impulses and freq.xlating it is more like amplitude
modulation --> so, no possibility of having waveform

3

On Thu, Apr 5, 2012 at 10:23 AM, senthil murugan
[email protected] wrote:

Hi Tom,

Yes, I am seeing sinusoids with low freq BPSK envelope → But, Not clean
shift of 180deg as shown in figures of standard text books. Am i rite??

I’m not sure what you mean by no clean shift. The BPSK sends symbols
at +1 and -1. When you modulate it onto a carrier, you can only see
the envelope of the signal, but there will be a phase change when
there’s a bit transition. I just mocked up a demo here and everything
looks as expected.

For each symbol, I am getting 23 samples (for sps : 2 ). If that is the
case, how come equation of tx_original bw valid?

Sample “rate” isn’t a concept an SDR has in software; it’s all
relative. You set the number of samples per symbol. When you send that
to the USRP, you now have a clocked rate that means something.

In these examples, you go into the modulator with 1 sample per bit.
The RRC filter inside is an interpolating filter, so the output has
some number of samples per symbol. The length of the filter has
nothing to do with this (it has an effect on the ISI memory). Most of
the time, we think in terms of samples per symbol and a normalized
sample rate of 1.

  1. As you said, I upsampled baseband signal beforemultiplyingwith signal
    source (for freq xlation simulation). Still I am not attaining clean shift
    of 180deg ? If you are able to attain that in GRC simulation, could you send
    that for me?

Thanks
Senthil

I’m sure it’s working correctly. My guess is you’re looking at it
wrong. I’m really not trying to sound snarky here. You are asking some
fundamental SDR questions that are understandable. These principles
are not well taught anywhere, and I don’t think I’m doing a much
better job here.

Tom

4

On Thu, Apr 5, 2012 at 8:02 PM, senthil murugan
[email protected]wrote: