450 SM modulation

I'm seeing that on our 3.65 PMP450 SM's we cannot obtain over a 25% 256-QAM score no matter how close or clean the signal.  Link efficiency is near 100% too.  Is this the highest this rating can go on an SM in the "Receive Fragments  Modulation" section under the Alignment Tool? 

Currently both A and B paths show: QPSK: 25% , 16 QAM: 25%, 64 -QAM: 25%, 256- QAM: 25%  (does this mean it is 100% 256-QAM?)

But...Shouldn't a link like I've described be doing something like 90% 256-QAM, 9% 64-QAM, 1% 16 -QAM??



You have a good link. I can understand the confusion. If you have a good 256 QAM link all of the data is being sent at 256 qam and the percentages will be 25 fo all modulations.

Yes Nathan, I will agree with Rylan... that's about the healthiest link you can have.

Unfortunately, the way the modulation breakdown is reported is a bit confusing. When a fragment is received in 256-QAM modulation, it is also counted in the lower modulations. This has to do with how OFDM technology works, and the constellation diagram of the OFDM signal.

To give a few examples, a perfect 8x link (i.e. a link that is able to achieve and maintain 256-QAM modulation) will report:

256-QAM: 25%

64-QAM: 25%

16-QAM: 25%

QPSK: 25% 

just as you describe.

A link that cannot achieve 8x, but is a solid 6x link will show:

256-QAM: <1%

64-QAM: 33%

16-QAM: 33%

QPSK: 33%

In real world deployments, it would be common to see something like:

256-QAM: <1%

64-QAM: 14%

16-QAM: 42%

QPSK: 44%

which would indicate that the majority of the time, this link transmits at 4X (16-QAM), but sometimes is able to maintain 6X modulation... hope this explanation helps.

1 Like