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Re: [Discuss-gnuradio] Fwd: BPSK transmitter - time domain waveform

From: Tom Rondeau
Subject: Re: [Discuss-gnuradio] Fwd: BPSK transmitter - time domain waveform
Date: Thu, 5 Apr 2012 10:33:22 -0400

On Thu, Apr 5, 2012 at 10:23 AM, senthil murugan
<address@hidden> 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.

> 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?

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.

> 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

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.


> On Thu, Apr 5, 2012 at 7:15 PM, Tom Rondeau <address@hidden> wrote:
>> On Thu, Apr 5, 2012 at 9:33 AM, senthil murugan
>> <address@hidden> wrote:
>> > Hi all,
>> >
>> > I have some doubt on the time domain view of bpsk signal generated in
>> > Gnuradio and usrp. I used psk mod block in GRC (sps =2) and created bpsk
>> > (baseband )waveform with vector source producing 0,1,1 continuously.
>> >
>> > When I plotted the output of psk mod block using scope sink, I seen
>> > waveform
>> > of  constant 1 for some samples (for 0) , inverted root-raised cosine
>> > waveform (for 1) and another inverted root-raised cosine waveform (for
>> > 1).
>> > This is repeated continuously and i understand that it because of
>> > transmit
>> > pulse shaping.
>> >
>> > Now, the question is when I give this signal to USRP for transmission,
>> > how
>> > the time-domain view of that waveform look like?  Will it look like
>> > sinusoid
>> > signal with 180deg phase shift as we all seen in stand. communication
>> > text
>> > books? I guess it will not the case. I tried  multiplying  the baseband
>> > waveform with signal source (complex) -- mathematical equivalent of what
>> > happening in usrp -- and seen waveform. No smooth shift of 180deg.
>> >
>> > If anyone having usrp and digital oscilloscope, could you please send me
>> > the
>> > pic of time domain view of output of usrp?
>> >
>> > Thanks
>> > Senthil
>> If you want to see this in simulation, make sure that you are
>> upsampling your signal before doing the frequency conversion so your
>> sample rate can support the new frequency range you are interested in
>> seeing.
>> The answer is yes, the resulting waveform will be a sinusoid at the
>> carrier frequency with a lower frequency BPSK envelope. If you are
>> doing this over the air, remember that if the oscillators in your
>> transmitter and receiver are not locked together, there will be a
>> frequency offset between them.
>> Tom
> --
> http://amritasenthil.wordpress.com/
> --
> http://amritasenthil.wordpress.com/
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