Downhole telemetry system having discrete multi-tone modulation with QAM fallback

A method for use in a downhole communications system, the method comprising:

• using a downhole transmitter to transmit uplink signals having a fixed modulation method to a surface transceiver via a first transmission mode, wherein the fixed modulation method is discrete multi-tone (DMT) modulation;
• measuring channel characteristics including line noise measured during a quiet period, and a power spectral density (PSD) that is determined while receiving a wideband training signal from the surface transceiver via a second transmission mode orthogonal to the first transmission mode;
• receiving information from the surface transceiver that specifies a proposed data rate;
• selecting a downlink modulation technique from a predetermined set of modulation techniques based at least in part on said information and channel characteristics, wherein the predetermined set of modulation techniques includes at least DMT modulation and single-channel QAM;
• and
• transmitting the selected downlink modulation technique to the surface transceiver for use in communicating a downlink signal from the surface transceiver to the downhole transceiver.

A downhole transceiver that comprises:

• a transmitter chain configured to transmit an uplink signal having a fixed modulation method to a surface transceiver via a first of multiple orthogonal transmission modes, wherein the fixed modulation method is DMT modulation;
• a receiver chain configured to receive from the surface transceiver via a second of said orthogonal transmission modes a selectable one of a) a DMT-modulated signal, and b) a single-channel QAM signal;
• and
• a microcontroller coupled to the transmitter and receiver chains, wherein the microcontroller is configured to determine channel characteristics including channel noise and power spectral density, and is further configured to respond to a proposed downlink data rate from the surface transceiver by transmitting a downlink modulation selection based at least in part on the channel characteristics and the proposed downlink data rate.