Pulse or digital communications – Systems using alternating or pulsating current
Patent
1993-09-28
1997-04-22
Chin, Stephen
Pulse or digital communications
Systems using alternating or pulsating current
375330, 375340, 370207, 370509, 329304, H04L 2700
Patent
active
056235160
DESCRIPTION:
BRIEF SUMMARY
RELATED APPLICATIONS
This application is related to commonly assigned U.S. patent application Ser. No. 08/030,232 filed Mar. 22, 1993 entitled CODED QAM SYSTEM and naming Richard G. Williams and John D. Brownlie as inventors. Priority rights and benefits under 35 U.S.C. .sctn.120 and 35 U.S.C. .sctn.365 are hereby claimed.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is concerned with modulation schemes for the transmission of digital data, more specifically those using quadrature amplitude modulation (QAM) in which the transmitted signal consists of a series of discrete signal portions ("symbols") in each of which a carrier assumes a selected phase and amplitude. More specifically, the phase and amplitude of a symbol are constrained to be equal to a selected one of a set of phase/amplitude combinations. This set is usually referred to as a "constellation" and may be depicted graphically as a set of points on a plot in which cartesian coordinates represent in-phase and quadrature components (or polar coordinates represent phase and amplitude).
2. Related Art
In the simplest of such systems, the number of points in the (single) constellation is a power of two (e.g. 2.sup.n) and each group of n bits to be transmitted selects the point for each symbol (where each symbol thus carries n bits). In more sophisticated systems additional bits may be generated by redundant-coding process; as a result (or otherwise) the number of bits to be carried per symbol may be non-integer, and bits to be carried by a number of symbols need to be processed together. Such an arrangement results in a framing structure for the transmitted signal and necessitates provision for frame synchronisation in order that the symbols can be correctly interpreted at a receiver. Alternatively (or in addition) the data to be sent may have an inherent framing structure so that synchronisation is required for the data to be correctly processed at a receiver.
BRIEF SUMMARY OF THE INVENTION
According to the present invention there is provided an apparatus for transmission of data using quadrature amplitude modulation comprising:
(a) modulation means controllable to produce consecutive output symbols in each of which a carrier has selected phase and amplitude;
(b) mapping means for receiving the data and controlling the modulation in accordance with a first signal point constellation having a plurality of signal points some of which, on a polar plot lie outside a predetermined boundary and others of which lie within it, and one more further constellation(s) which has or have points fewer in number than those of the first constellation and which lie within the said predetermined boundary;
(c) frame synchronisation means operable in accordance with a predetermined framing structure of the transmission so to control the mapping means that, for each symbol to be transmitted in one predetermined position of a frame, the symbol is chosen from the first constellation, other symbols of the frame being chosen from the further constellation(s).
The other symbols may be chosen from one such further (second) constellation, however, one (or more) of them may be chosen from a third constellation having a smaller number of points than the second constellation. In this case preferably a symbol chosen from the third constellation occupies a position in the frame adjacent to that of the symbol chosen from the first constellation.
Other aspects of the invention are set out in the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
Some embodiments of the invention will now be described, by way of example with reference to the accompanying drawings, in which:
FIG. 1 is a block diagram of a first embodiment of coding apparatus according to the invention;
FIG. 2 is a phase diagram showing constellations for use with the apparatus of FIG. 1;
FIG. 3 is a block diagram of a second embodiment of coding apparatus according to the invention;
FIG. 4 is a phase diagram showing constellations for use with the apparatus of FIG. 3;
FIG. 5 is a logic ci
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Brownlie John D.
Williams Richard G.
British Telecommunications public limited company
Chin Stephen
Webster Bryan E.
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