Discrete multitone modulation with reduced peak-to-average ratio using unloaded subchannels

A transceiver for transmitting Discrete Multitone (DMT) modulated signals, comprising:

• an analog front end function, coupled on one side to a communications facility;
• an interface, for interfacing the transceiver to a data source;
• and
• programmable logic circuitry, coupled to the interface and to the analog front end function, for modulating a digital payload bitstream from the data source and applying the modulated signal to the analog front end function, by executing an instruction routine to perform a sequence of operations comprising:
• bit loading a plurality of subchannels;
• identifying at least one of the plurality of subchannels that is unloaded;
• receiving the payload bitstream;
• encoding the received payload bitstream into a plurality of symbols, the symbols each associated with one of the plurality of subchannels, the plurality of symbols arranged as a digital payload signal;
• for each of a plurality of phases corresponding to an upsampling function, forming a time-domain sum signal of the payload signal and a trial clip prevention signal, the trial clip prevention signal being orthogonal to the payload signal and associated with the at least one unloaded subchannel, and the trial clip prevention and payload signals having been filtered by a filter associated with the current phase;
• applying a function to the sum signal to derive a clipped sum signal;
• responsive to the clipped sum signal differing from the sum signal, modifying the trial clip prevention signal for the current phase;
• repeating the forming, applying, and modifying steps for each of the plurality of phases;
• responsive to the clipped sum signal having differed from the sum signal for each of the plurality of phases, repeating the forming, applying, and modifying steps for the plurality of phases;
• and
• responsive to the clipped sum signal not differing from the sum signal over all of the plurality of phases, processing a sum of the payload signal and the current trial clip prevention signal and forwarding the processed signal to the analog front end function.

A method of processing a payload signal to avoid clipping, comprising the steps of:

• encoding the payload signal into a plurality of symbols, the symbols each associated with one of a plurality of subchannels, the plurality of subchannels including at least one unloaded subchannel for which none of the encoded symbols of the payload signal are associated;
• for each of a plurality of phases corresponding to an upsampling function, forming a time-domain sum signal of the payload signal and a trial clip prevention signal, the trial clip prevention signal being orthogonal to the payload signal and associated with the at least one unloaded subchannel, and the trial clip prevention and payload signals having been filtered by a filter associated with the current phase;
• applying a function to the sum signal to derive a clipped sum signal;
• responsive to the clipped sum signal differing from the sum signal, modifying the trial clip prevention signal for the current phase;
• repeating the forming, applying, and modifying steps for each of the plurality of phases;
• responsive to the clipped sum signal having differed from the sum signal for each of the plurality of phases, repeating the forming, applying, and modifying steps for the plurality of phases;
• and
• responsive to the clipped sum signal not differing from the sum signal over all of the plurality of phases, processing a sum of the payload signal and the current trial clip prevention signal.