1. Field of the Invention
The present invention relates to modulation systems for the transmission of information in digital form.
2. Description With Regard to Related Art
In particular, the invention is concerned with multi-frequency systems, where, rather than using all the available bandwidth by wideband modulation of a single carrier, a number of carriers each occupy a portion of the bandwidth, the total number of bits to be transmitted being distributed among the carriers. If the properties of the channel to be used are uniform across all portions of the available bandwidth, then there is no advantage in this; but if, on the other hand, channel characteristics such as noise level or distortion vary with frequency then one can obtain an improvement in bit rate for a given bit error rate (or vice versa) by tailoring the bit rate on each carrier according to the characteristics of the channel at the relevant frequency. Preferably the allocation of bits is performed dynamically--i.e., by continually readjusting the allocation to take account of variations in the channel.
It is proposed that transmission on each channel takes place using quadrature amplitude modulation, where a carrier is set, during each of successive symbol periods, to a phase and amplitude corresponding to a symbol to be transmitted (normally by controlling the amplitudes of two phase-quadrature carriers). The set of allowable discrete combinations can conveniently be plotted as a set of points on a phase diagram and is commonly referred to as a constellation. In many systems the number of points in the constellation is a power of two; so that, for example, a symbol selected from a sixteen-point constellation carries four bits.