Technology for send/receive data on power lines has been around for years. Mostly used in building automation systems sending DC signals on AC lines. As for spying on citizens the concept would have been thought impossible before the super-computers. What kind of power is needed to monitor our phone calls? I know I won't be spied on though because I'm just not that interesting and I don't know any secrets. Except of course where the Finn treasure is.
DC in presence of AC is used, but data is not sent. It may be used to actuate a valve for instrumentation systems etc.
Data communication lines must be kept at distance from power cabling due to the EMF interference.
Extra twists are even required in Data lines to help combat this problem.
Most DC in controls is not generated, but is AC that has gone through a full bridge (Diode) rectifier with a capacitor. It is actually pulsating DC
Removing the Bottom half cycle in the sine wave, then the capacitor charges and fires to fill in the missing half cycle.
It is most useful due to being able to control movements and speed in motors by just adjusting voltage up and down, where AC will cause start winding to drop out. The way around that is with a Frequency drive which raises and lowers the Hz without changing the voltage, but they are expensive.
Woodworkers like DC Lathes for this reason, no need for different belts to change the ratio.
Current data is sent up to about 340 MHz max for now, even though giga speed cabling is sold, nothing currently can transmit at that level. The only thing with almost unlimited bandwidth would be Optical Fiber. For now I'm aware of no way to transmit data on a power line.
T. Reid Marcum
IBEW LU 1925
Union Journeyman Wireman/Nuclear Test Tech