A communications system includes at least two communications units, each communication unit including a transmitter capable of transmitting to the other unit at different power levels and on different frequencies, the power levels and frequencies of transmission being controlled by a mode control unit...http://www.google.de/patents/US5528623?utm_source=gb-gplus-sharePatent US5528623 - Cordless telephone system having automatic control of transmitter power and frequency in response to changing conditions
