Sorry for being so late to answer this one - I’ve been on a “no-TTN January” for my new years resolution.
There’s a lot to read in the thread above, so I’m going to jump back to the question from 28days ago - are these 8-channel gateways violating the FCC regs. The answer is no, they are fine.
FCC describes two types of spectrum ‘fair use’. One is hopping over 50+ channels and the other is DTS: Digital Transmission System. Both are really trying to ensure that a user doesn’t hog and block a channel, either by hopping off it or by keeping the spectral density low.
There is also an option for ‘hybrid’ mode which as the name suggests, is a hybrid of the two schemes. Easiest to link to here rather than re-write it all: https://iotblog.org/lora-hybrid-mode-and-fcc/
If you search Google for FCC DTS Hybrid there is plenty of material.
When LoRaWAN spec was originally written, the USA channel plan was based on 64 channels just because it could be. It satisfied the 50+ restriction, gave good capacity. All sounds great but the gateways were damn expensive. I remember LinkLabs was promising to make them but didn’t, and also remember having to beg Senet for one of their early 64ch gateways to test against.
So some smart dude noticed the ‘hybrid’ rule and I guess plenty of 8ch EU gateways were available to be converted to US frequencies. So an 8ch system is a capacity restriction but still good enough for most cases, and the price is right.
The problem is that the LoRaWAN spec was written for 64ch gateways at that time and so these 8ch hybrid sub-bands were an unofficial de-facto standard that was deviating from the official spec. Some companies labelled the sub-bands as 0-7, some as 1-8, I think Semtech was suggesting A-H. Interoperability nightmare ensued and as written above, devices needed to be aligned to whatever random choice the network operator decided.
I guess what I’m saying is that there was no centralised design by the LoRa Alliance here, just practical implementations by its members.
All a real pain for devices/modules/stacks trying to match the 64ch spec but knowing they would not find service on 7/8 of the channels. And its not just during join, also needs to be considered in the ADR back-off strategy if you fall off your network.
As correctly stated above, the solution is now that the LoRaWAN spec is including ways to steer devices to the right sub-band using a channel mask. Happy days
Maybe 64ch gateways will come back if the cost of SX1301 comes down or a future version gets more demodulators integrated. 16ch seems like a reasonable compromise