As I understand it, the frequency associated with 5GHz band channels are their central frequency, just like on the 2.4GHz band. Thus adjacent selectable channels, e.g. 36 (5180 MHz), 40 (5200 MHz), 44 (5220 MHz), etc., don't overlap when their channel width are set to be only 20 MHz. Since frequencies of channel 36 spans 5170-5190MHz, channel 40 spans 5190-5210MHz, and so on.
But when channel width of 40 or 80MHz are used, how exactly does the work in those cases? Would channel 36 with a width of 40MHz span the frequency range 5160-5200MHz, and 40 span 5180-5220MHz, and so on? I know this is how it works on 2.4GHz, but of course people tend to restrict themselves to 20MHz wide channels.
I'd like to use 80MHz wide channels for the 5GHz wifi on both of my access points, and just want to understand how I should be selecting the second channel.
How you "name" the channels when you're using widths above 20mhz is probably a contentious subject. I don't know if there is a standard. I've seen:
First channel
OR
Middle Channel
I want to point this out because in your original post you say:
40 when used in 40MHz configuration is 36+40 by definition.
The correct ways to name this channel are either 36 or 38. Calling it 40 seems wrong to me.
Note that choosing channel 40 may affect which channel becomes your "primary" channel, but you still always occupy 36+40.
When using 80MHz, the channel would cover from 36-48, and the naming would be 36 or 42.
Anyway, i think the above graphic should help answer your original question of how to pick your 80MHz channels properly.
