With high power towers for cell network and plenty of devices that emit radio, you can have a wireless power harvester that can be powered forever if you lower

With high power towers for cell network and plenty of devices that emit radio, you can have a wireless power harvester that can be powered forever if you lower you energy consumption below a given threshold. Also was an experiment a while ago where photons were both feeding the camera sensor and used to get the image itself. Granted resolution and clarity was bad but the images still could be used to recognize stuff. Combine that with a smart AI at receiver site and you can have full surveillance, at least for counting bodies (humans + cars) for your desired place (think gathering places like Tienanmen).