GNU Radio QPSK 4 Symbols

Question

I'd like to generate a simple QPSK signal in GNU radio, but so far I haven't been able to get it to perform the way I'd like and I'm not sure if this has to do with some misunderstanding I have about how QPSK works or if I just am not familiar enough with GNU Radio.

Essentially, my conception of QPSK modulation is this - 2 bits are fed into the modulator (4 possible symbols - 00, 01, 10, 11) and, based on which combination of those 2 bits was received, one of 4 possible waveforms is generated (corresponding to +/-.707 +/- .707j). Thus, when using a QPSK modulator in GNU Radio I would expect to see, in the constellation diagram, a square with 4 points at each corner, but instead I see diamonds with thousands of points, even in the absence of any introduced noise. When I print the output of the QPSK modulator to a file sink, I also see more than 4 distinct points.

Is there a way to take a random input stream of 0's and 1's and produce a modulated signal from it, like QPSK, that has only one of four values? It is important for the simulation that I'm running that there are only 4 possible modulation values before noise is added to the system. Also, I will be sending these signals between USRP's, so if there is some way to generate a QPSK signal and transmit it on a carrier frequency between USRP's that doesn't rely on the PSK Mod block (I'm not too familiar with how to implement a raised cosine filter) that would be appreciated.

Answer 1

It is important for the simulation that I'm running that there are only 4 possible modulation values before noise is added to the system.

So, you made a mistake when building your simulation, or your simulation already includes a frequency offset. The way you ask this, you probably did the former, but since we have zero knowledge of what exactly you've built: Can't help you with that, make sure your receiver simulation does exactly the same as what your transmitter simulation does. ("fix your program!")

I'll answer as if this was a system that's generally correctly designed:

Thus, when using a QPSK modulator in GNU Radio I would expect to see, in the constellation diagram, a square with 4 points at each corner, but instead I see diamonds with thousands of points, even in the absence of any introduced noise.

That's not surprising! No two oscillators are exactly the same, and so your receiver and transmitter have a frequency offset.

A frequency is the linear change of phase over time. That means that your symbol phase at the receiver has a phase offset that grows constantly every time you look. Hence, the "diamond".

You need what you've been taught in your digital communications course: a frequency synchronization!

You can do that with a second order costas loop.

Answer 2

Hi Zman147 this is John BG [email protected]
1. what board are you using? 2. If you haven't chosen yet and are on budget have a look at the following link

https://osmocom.org/projects/rtl-sdr/wiki/Rtl-sdr

Note that there is already a Simulink package available, so you can easily play with IQ constellations.

If you have more budget, they you should consider something like:

http://www.ni.com/en-gb/shop/select/usrp-software-defined-radio-device?storeId=11212&userLocale=en-GB&requestLanguage=en&localeGroup=en-gb&requestLangId=-1&langId=-1

hope it helps.