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

From: senthil murugan
Subject: Re: [Discuss-gnuradio] BPSK transmitter - time domain waveform
Date: Thu, 5 Apr 2012 20:02:45 +0530

I am saying this because in theory, ppl assume that sin(2*pi*fc*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

On Thu, Apr 5, 2012 at 7:53 PM, 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?? 

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?



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

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.





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