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Data are transmitted using multiple carriers modulated by quadrature amplitude modulation. In general, the carriers carry different numbers of bits, to exploit the fact that the properties of a transmission path are more favorable to carriers at some frequencies than to those at others. The bit allo

Data are transmitted using multiple carriers modulated by quadrature amplitude modulation. In general, the carriers carry different numbers of bits, to exploit the fact that the properties of a transmission path are more favorable to carriers at some frequencies than to those at others. The bit allocation process is made closer to optimum by permitting allocation of a non-integer number of bits to a carrier. Input bits are grouped for several carriers and this binary representation is converted to a number representation in which the base of each digit corresponds to the number of points in the QAM constellation for a respective carrier (or to this number divided by a power of two).

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An apparatus for transmission of data using quadrature amplitude modulation, said apparatus comprising: (a) modulation means controllable to produce consecutive output symbols in which a plurality of carriers of different frequencies each have a selected phase and amplitude; (b) mapping means for co

An apparatus for transmission of data using quadrature amplitude modulation, said apparatus comprising: (a) modulation means controllable to produce consecutive output symbols in which a plurality of carriers of different frequencies each have a selected phase and amplitude; (b) mapping means for controlling modulation of a carrier in accordance with a signal point constellation having a plurality of signal points, wherein at least two carriers are controlled in accordance with signal point constellations having mutually different numbers of points and wherein at least two signal point constellations have a number of points not equal to a power of two; (c) conversion means operable to receive a group of bits to be transmitted by two or more carriers controlled in accordance with signal point constellations having a number of points not equal to a power of two and to generate from the group of bits, considered as a binary number, the digits of a number having the same numerical value, the base of each digit being the number of points in the signal point constellation employed for a respective carrier, the mapping means being responsive to a respective digit to select a point from the relevant signal point constellation.