"For example, wouldn't all access points send the signals at exactly the same time? How can they insure that? How can they insure the interferences won't have changed between the test signals and when the real signals are sent?"
I think the key is to see the DIDO access points as part of a larger system and not as traditional access points. "So, you can think of the DIDO APs as a vast random array of antennas extending out from the DIDO Data Center for miles" as the whitepaper says. I assume that all the access points will be stationary and will talk to the DIDO centre very regularly. This would be required, at least in my mind, to ensure that their environment characterisation is accurate and up to date and to syncronise transmissions.
"For example, wouldn't all access points send the signals at exactly the same time?"
Yes, apparently there is a timing issue.
Maybe can resolve the timing issue by having the transmitters send timing signals. Then maybe one transmitter becomes the 'central timing' source.
So, maybe discretize time into, say, time windows where each window is, say, only a millisecond or so long. Maybe get a new window each, say, 3 milliseconds. Then at the beginning of each window, each transmitter sends. To get them all to send at the same time, use the central time source with each transmitter knowing its delay from the central source. So, for a window, the central time source says "SEND", and each transmitter knows just how long to wait after receiving the "SEND" message before starting to send.
Maybe some such.
"How can they insure the interferences won't have changed between the test signals and when the real signals are sent?"
Use the test signals every few milliseconds to recalculate all the transfer functions to all the receivers. If occasionally a receiver moves a little too fast, then depend on TCP to handle the error.
Maybe.
Do ask that mostly each receiver B is logically connected to nearly the nearest transmitter A. Then for transmitters a long way away from A and, thus, also B, they should be able mostly to f'get about the signal from A to B. That should help the timing issues.
So, in some area with a lot of users where want a lot of data rate at the one frequency available in that area, put in a lot of transmitters. Then expect that each user will get associated with a transmitter of their own that is nearly the nearest transmitter to them.
The location of each transmitter is essentially fixed; if the location varies a little day by day, no problem. The location of each receiver is essentially fixed over, say, a few milliseconds; that should be okay if redo the 'test signal' handshaking each few milliseconds.
Remember that are only sending digital packet data and looking for latencies only less than a few milliseconds. So, get a little time, a millisecond here and there, to send test signals, do the handshaking to get all the transmitters sending at the same time, buffer up at each transmitter the data to send during the next time window, etc.
For the communications between the central 'smart box' and each of the transmitters, have lots of options including, say, some form of multi-drop passive optical.
To make money, a key will be to keep the costs for installation down. So, need to be cheap at the central box, the communications from that box to the transmitters, the transmitters, and the card in each receiver.
Hmm, to 'wire' a suburb, go to a homeowner and rent a few square feet in their attic! Put in the central box and, say, 100 transmitters. To this house, run, say, 10 GbE over optical. So, that would be 100 Mbps per transmitter. Put a UPS in the attic. Hope to give really good service to, say, 25, maybe, 50 houses.
Cute solution to the 'last mile'!
If an attic gets busy, then light another 10 GbE wavelength in the optical fiber to the attic and put in another 100 transmitters.
I think the key is to see the DIDO access points as part of a larger system and not as traditional access points. "So, you can think of the DIDO APs as a vast random array of antennas extending out from the DIDO Data Center for miles" as the whitepaper says. I assume that all the access points will be stationary and will talk to the DIDO centre very regularly. This would be required, at least in my mind, to ensure that their environment characterisation is accurate and up to date and to syncronise transmissions.