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

Nowlan, Sean Sean.Nowlan at gtri.gatech.edu
Wed Jul 22 15:21:30 EDT 2015


FYI - copying the GNU Radio mailing list, which would be a more appropriate place to post a question such as this one.

Some comments on your GNU Radio setup:
* You're using a "PSK Mod" block, which applies a root-raised cosine pulse-shaping filter to  the +1/-1 stream, so you wouldn't exactly see +1/-1 at the output, but instead the sum of symbol-period delayed filter responses.
* If you look at the documentation for the "PSK Mod" block in GRC, it will tell you that the input expects unsigned chars, which equates to values in the range [0,256). Instead of using a vector source, use a "Random Source" with Minimum=0 and Maximum=256 to feed the PSK Mod block.
* Your throttle block is what's setting the sample rate in this example, and the modulator is using 4 sps (samples-per-symbol), so your symbol rate is 32k/4 = 8 ksym/s. Since this is BPSK, the data rate is also 8 kbps.
* If you were to use a USRP to transmit, you would set its sample rate and then change the SPS to achieve the desired symbol rate: sym_rate = usrp_samp_rate / SPS.
* If you're using the QT GUI Constellation Sink, it will display all the complex points, including those interpolated by the filter in the PSK Mod block. Therefore you won't see symbol-clock sampled constellation points - you will also see intermediate points as the signal transitions between constellation points.

From: USRP-users [mailto:usrp-users-bounces at lists.ettus.com] On Behalf Of Moon, Seung - Exelis via USRP-users
Sent: Wednesday, July 22, 2015 2:55 PM
To: usrp-users at lists.ettus.com
Subject: [USRP-users] Simple BPSK modulation with alternating 1's and 0's

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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