What I Wish I’d Known
For most of my career, signal was a number. You measured the level on a single-carrier QAM, you compared it to spec, and you knew. If the level was good, the carrier was good. If the carrier was bad, the level usually told you why. It was beautiful in the same way an arithmetic problem is beautiful: there was an answer, you could find the answer, the answer didn’t depend on how you felt that morning.
That’s not how OFDM works. I had to learn that the hard way. I learned several things about OFDM the hard way, actually. We’ll get there.
Got a ticket on a customer with intermittent service. The modem was within spec on every level you could measure. Forward signal levels: fine. Return levels: fine. SNR on the legacy carriers: fine. By every meter I trusted, this customer should have had the best service on the block.
I replaced the drop. Replaced the modem. Replaced the F-connector at the tap. Levels stayed perfect. Customer kept calling. By the third visit, the customer was no longer asking when the problem would be fixed. He was asking me, with what I believe was genuine curiosity, whether I had ever considered a different career. I told him I had not. He said it was something to think about.
A coworker showed me the OFDM sub-carrier MER plot on the modem’s PNM dashboard. Thousands of narrow sub-carriers, each one with its own quality measurement. Some were fine. Some weren’t. There was a clear shape to where the bad ones lived, a notch sitting in a specific frequency range.
That notch was the story. The aggregate level didn’t see it because the aggregate level doesn’t have the resolution. The legacy SNR measurement didn’t see it because the legacy carriers were in a different part of the spectrum. The OFDM sub-carrier plot saw it immediately, because it was looking at exactly the part of the spectrum where the problem lived.
My coworker said, calmly, “Have you tried looking at this before?” I said no. He said, “Maybe try looking at this.” I said I would. We left it there. I went home and thought about it for a while.
The notch lined up with an LTE downlink band. Some compromised termination somewhere on that customer’s line was acting as a receive antenna for off-air LTE energy, and that energy was landing right under the OFDM carrier, degrading the sub-carriers in that band, leaving the carriers above and below it untouched.
The level was fine because the level wasn’t being damaged. The MER was bad because the MER measures something the level can’t see. There’s a metaphor in there about hearing what people aren’t saying, but I’m a cable guy and I’ll let somebody else write that one.
I went out with a DRV3, drove the plant from the customer’s location, and found the source in 15 minutes. Bad shield on a span clamp 200 feet up the line. Repaired it. Customer’s service came back to spec while I was still on-site. He did not call me again. I do not know whether he is happy or whether he has accepted defeat. Either is fine with me.
What I wish I’d known years earlier: my meter was measuring the wrong thing for the modulation that was about to take over the plant. Single-carrier QAM lives or dies by the level. OFDM lives or dies by the per-sub-carrier MER, and the patterns that show up in the MER plot tell you not just that there’s a problem, but where in the spectrum the problem is, which often tells you what kind of fault it is and where to drive.
The meter still has a place. I use it at the end of a repair to validate. But the dashboard is where the diagnosis happens now, and pretending otherwise just costs you truck rolls. And, occasionally, your dignity in front of customers who have started asking you about career changes.
The single-carrier intuition was good for its time. The time changed. The intuition has had to make some adjustments.
