Discrete multitone transmission systems

Apparatus for generating a discrete multitone signal, the apparatus comprising:

• a modulator for modulating N carriers, corresponding to respective subchannels, in each sample period of the signal by performing an inverse discrete Fourier transform of N modulation symbols, each of which symbols represents a signal point in a QAM constellation for a respective subchannel, said QAM constellation comprising a basic constellation of M≧2m signal points where m is the number of data bits to be communicated over the subchannel in a said sample period;
• means defining, for each of Nc≦N of said subchannels, an expanded QAM constellation comprising pM signal points including said basic constellation of points, where p is an integer greater than 1, and further defining, in each said expanded constellation, p−1 equivalent signal points for each signal point of the basic constellation, where each of the p−1 equivalent points is selected from redundant points in the opposite quadrant of the constellation to the corresponding point of the basic constellation;
• and
• control logic for selecting, for each of the Nc subchannels, the modulation symbol representing either a basic constellation point or a corresponding equivalent point for generation of the discrete multitone signal in a said sample period such that the peak value of the signal does not exceed a predetermined threshold value.

A communication system comprising:

• a transmitter for transmitting a discrete multitone signal, the transmitter comprising:
• a modulator for modulating N carriers, corresponding to respective subchannels, in each sample period of the signal by performing an inverse discrete Fourier transform of N modulation symbols, each of which symbols represents a signal point in a QAM constellation for a respective subchannel, said QAM constellation comprising a basic constellation of M≧2m signal points where m is the number of data bits to be communicated over the subchannel in a said sample period;
• means defining, for each of Nc≦N of said subchannels, an expanded QAM constellation comprising pM signal points including said basic constellation of points, where p is an integer greater than 1, and further defining, in each said expanded constellation, p−1 equivalent signal points for each signal point of the basic constellation, where each of the p−1 equivalent points is selected from redundant points in the opposite quadrant of the constellation to the corresponding point of the basic constellation;
• and control logic for selecting, for each of the Nc subchannels, the modulation symbol representing either a basic constellation point or a corresponding equivalent point for generation of the discrete multitone signal for transmission in said sample period such that the peak value of the signal does not exceed a predetermined threshold value;
• and a receiver for receiving and demodulating such a discrete multitone signal to generate the data represented thereby.