A transmitting controller is connected to a powerline and on command places a reference signal and a series of signal pulses in the powerline at a series of signal timing positions related to zero voltage crossing points so that the signals pulses are substantially in the powerline temporal quiet zone...http://www.google.co.uk/patents/US7688183?utm_source=gb-gplus-sharePatent US7688183 - Powerline pulse position modulated transmitter apparatus and method
