256 QAM Demodulation problems

[Jdjames]

Hi again!

I am using Labview Modulation 3.0 Toolkit to generate and demodulate a 256 QAM signal.  This setup works perfectly until I add in the physical channel.  At lower QAM levels, everything works fine, at 256 QAM, all I am getting is circular constellation plots and noise and every once in a great while, I will get a decent constellation graph.  There isn't enough noise being introduced into the system to cause this behavior.  Any ideas?

Thanks!

Jennifer

[Travis_W]

Jennifer,

There are a couple reasons that your constellation plot could be circling.

1)  The noise introduced by the physical channel causes the demodulator to lose carrier locking.  If this is the case, switching to a lower M-ary should fix the problem.  This sounds a lot like what you are describing.  How have you verified that there is not enough noise in the channel for this to be the case?

2)  If you're only hitting a small number of the symbols on the map, the receiver can have a hard time locking to the actual generated phase, and the same result can occur.  This is called a false-lock, and is quite common in higher M-ary modulation schemes.  The solution to this would be ensure generation of all symbols by padding your data with a PN Sequence of a high order (N>13).  To test this out, try sending just a PN Sequence instead of user-defined data.  If the problem goes away, then try reintroducing your data padded with a PN Sequence to hit all the symbols.

Best of luck, Jennifer!  Let me know how this goes.

Travis W

[Jdjames]

Travis-

I think you are going to get tired of me by the end of the summer!

1) The CNR on the signal is high ~ 35dB.   In addition, for my highest CNR level, the problem disappears when I take the carrier frequency above 1GHz.

2)  I will give this a shot when I get to the lab.

How is the carrier tracking being implemented?  I tried to go into the VI to look at this more closely, but it appears this is being done internally??

Thanks again!

Jennifer

[Travis_W]

Jennifer,

Getting tired of QAM is impossible.

It sounds like you might not be hitting enough of your symbols for a lock.  Let me know how the PN sequence goes.

Travis

[Jdjames]

I tried increasing the PN sequence number to over 13 and there was no difference.

[Travis_W]

Jennifer,

Have you tried decreasing your symbol rate? 
Also, are you sharing clocks between your RFSA and RFSG?

Thanks,
Travis

[Jdjames]

1) Not yet

2) Yes, I am sharing them across the backplane as instructed.

Thanks!

Jenn

One of the things that might be contributing, we did a quick frequency response test of the PXI and noted that the frequencies up to 1GHz experienced a drop of about 3 dB then it goes back up at that point.  This may be causing the problem.  I will try your first suggestion and report back.