[USRP-users] Simple BPSK modulation with alternating 1's and 0's

Marcus Müller marcus.mueller at ettus.com
Wed Jul 22 15:24:36 EDT 2015


Hi Seung Moon,
> constellation point: 2, gray code: Y, differential encoding: Y, sps: 
> 4, excess BW: .35
>
> From my understanding, BPSK modulation modulates our data to +1 and -1.
>
> On the GUI constellation diagram, we see some points that are not 
> situated at +1 and -1.
>
That's the pulse shaping filter in action; depending on the sequence of 
symbols, the samples between the exact symbol times can (will) be at 
non-constellation points. That's why it's sps (samples per symbol) == 4: 
These are not 4 identical samples for every symbol, but are shaped to 
decrease the bandwidth of the symbol-to-symbol transition (which would 
have sinc spectrum, otherwise) and/or maximize SNR (matched filtering).

> We are also unsure on how to transmit our data at the desired data rate.
As mentioned before, the samples-per-symbol-number describes the 
relationship of symbol rate (which ultimately is data rate) to sample 
rate. You configure the sample rate in your USRP block.
That's fairly nicely described in the GNU Radio guided tutorials; 
digital modulation is a topic running through chapter 1 to 7, so I'd 
really like to ask you to read that tutorial:

https://gnuradio.org/redmine/projects/gnuradio/wiki/Guided_Tutorials

Also, please note that the throttle block is fine for simulation but 
doesn't actually do anything with the signal - it just slows down 
processing of the signal; that's a common pitfall.

Best regards,
Marcus

PS: I pointed your colleague at the GNU Radio mailing list yesterday -- 
if you've got any problems with the GNU Radio tutorials, that might be 
the optimal place to ask.


On 22.07.2015 20:54, Moon, Seung - Exelis via USRP-users wrote:
>
> Hello,
>
> I’m currently working an intern, and my teammate and I are trying to 
> generate a simple BPSK modulated signal.
>
> We both lack knowledge on GNURadio and communications for our project, 
> so we are hoping to get some help online.
>
> Currently, I have a repeating vector source of alternating 1 and 0 
> which feeds into PSK modulator block (constellation point: 2, gray 
> code: Y, differential encoding: Y, sps: 4, excess BW: .35) which feeds 
> into a throttle block (sample rate: 32k) which then feeds into a GUI sink.
>
> On the GUI constellation diagram, we see some points that are not 
> situated at +1 and -1.
>
> We are not sure on how to interpret what we are seeing on the 
> constellation diagram.
>
> From my understanding, BPSK modulation modulates our data to +1 and -1.
>
> Ultimately, we will be using USRP B200 to transmit our signal to lock 
> on to a receiver which expects a BPSK modulated signal at certain data 
> rate (let’s say 1kbps).
>
> We are also unsure on how to transmit our data at the desired data rate.
>
> Any guidance will be greatly appreciated.
>
> Thank you for your interest. Have a wonderful day.
>
>
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