It would be interesting to know how often the system can actually operate in 1024 QAM mode in the real world. In a pure AWGN channel, 1024 QAM requires "only" four times as much power as 256 QAM at the same BER, but the situation quickly gets much worse in the presence of interference.
The 25% capacity gain over 256 QAM is probably well worth the higher electric bill, but what is the effective capacity gain on a typical long-haul system? Can an operator expect to operate in 1024 QAM mode 25% of the time, 75% of the time, or what?
Well, for sure, you can only run 1024 QAM on "bluebird days," but that's the point of adaptive modulation. Although having this sort capability that can only be used under the right conditions would seem like a huge waste in the consumer electronics space, infrastructure is a completely different space.
It seems to me a waste of hardware upgrade. Performance requirements such as SNR for 1024-QAM is so high that it will appear to be defensless against channel fading, not to mention interferers. With adaptive modulation, it turns out to be running as 256-QAM at best as before. But it can be a good fit to wired channels such as cable, etc.
I guess it's important to understand that we are talking about carrier grade long haul (transport) links here. These links are dimensioned/engineered for very high availability, typically in the range of 99.995 or 99.999 % availability. This means less than half hour of outage per year! In order to achieve these figures the links have ample fading margin; typically in the range of 30 to 40 dB. Increasing modulation to 1024QAM reduces fading margin with 6dB compared to 256QAM. The effect on availability is just a factor of about 2 (doubling the outage), considering flat fading and interference free conditions. It has very limited effect on distance, as these links are not operating at the maximum possible distance anyway (due to needed fading margin). Interference can be an issue, again the sensitivity is increased by 6dB, so it needs to be factored into the availability calculations. Higher lever modulation does not change power consumption or other factors. So bottom line; as it uses adaptive modulation it offers 25% more capacity for 99.99% of the time. For the 0.01% of the time the links will scale down to lower modulation.
We are running a trial link over a 65 km path in Western Norway, using the 6.7 GHz frequency band. This has been running so far for about 2 months without even once changing to lower modulation. I hope this has helped clarify the technology.
Thanks for the interesting article. I'd like to know more about the additional complexity and cost needed to increase the throughput. It seems like you will definitely need more linear components, lower phase noise and maybe more DSP. Does the extra cost scale with the extra throughput? Or does it cost more than 1.25x to get 1.25x data throughput?
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