Improved communication systems and methods are disclosed. In one embodiment, a downhole communications system comprises a downhole transceiver and a surface transceiver. The surface transceiver receives a discrete multi-tone (DMT) modulated uplink signal from the downhole transceiver, and transmits a...http://www.google.ca/patents/US7443312?utm_source=gb-gplus-sharePatent US7443312 - Downhole telemetry system having discrete multi-tone modulation with QAM fallback
