QAM encoding

Details QAM encoding QAM encoding

1993-06-25

1996-03-19

O'Shea, Sandra L.

Pulse or digital communications

Systems using alternating or pulsating current

Plural channels for transmission of a single pulse train

375341, H03D 100, H04L 512

Patent

active

055008755

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

This invention relates to the coding of constellation points in a QAM (Quadrature Amplitude Modulation) communications system.


BACKGROUND

QAM modulation techniques are increasingly used to obtain maximum bandwidth efficiency in many communications systems. Such systems encode data by assigning data strings to one of a plurality of points in a signal constellation map in the complex plane.
A problem encountered in all such systems relates to the resolution of quadrant ambiguity resulting from carrier recovery. The receiver has no way of determining which quadrant of the signal constellation the transmitter has used to send a received point. Traditional modem techniques employ differential encoding, by assigning two bits of each data string to define the absolute quadrant. The remaining bits, for example 8 in a 1024 QAM system, are used to define the point within the quadrant. This is illustrated in FIG. 1.
However, there are still difficulties in determining the absolute bit pattern for the received quadrant. In order to overcome ambiguity due to rotation, the points are generally coded such that they incorporate rotational symmetry.
U.S. Pat. Nos. 4.601,044 to Kromer etal, and 4,416,016 to Iapicco disclose techniques for extracting phase information from trellis or convolutional encoded modulations. U.S. Pat. No. 4,866,395 to Hostetter discloses a system for carrier recovery. None of these disclosures addresses the issue of avoiding the requirement for differential encoding.
Conventional design methodologies dictate that differential encoding is essential to solve the phase ambiguity problem.
However, differential encoding in turn creates problems which become particularly evident in a large, for example 1024 point, QAM system. Gray coding on quadrant boundaries is very poor, and can lead to unacceptable error bursts when coupled with differential decoding errors.
It is an object of the present invention to provide a modem design and encoding system which ameliorates the deficiencies of the prior art.


SUMMARY OF INVENTION

According to one aspect, the present invention comprises an improved system for encoding and decoding QAM signals, wherein prior to transmission the signal constellation is directly encoded such that successive rotations about the axes of symmetry of the constellation do not create identical signals, and
during reception said signal is decoded by a method comprising successively comparing a received signal with a set of stored signals corresponding to the possible received rotations of a known signal, until a match is found, and thereby resolving the rotational orientation of the received signal.
According to a further aspect, the present invention comprises a method for encoding digital signals for transmission on analog media, comprising the steps of:
converting a digital signal into a set of digital words, each having a predefined length;
encoding each of said words as a point in a signal constellation, each point being defined by a quadrature component and an in-phase component;
wherein said constellation is chosen such that successive rotations with respect to the axes of symmetry will not map the constellation onto itself.
Contrary to prior art modem design, the present invention entirely discards differential encoding, and as a result rotational symmetry is unnecessary. The full QAM constellation may be completely, for example, gray coded.
This creates a difficulty at the receiver, as there is now no explicit specification of the rotational orientation of the constellation. This is overcome by rotating the constellation by predetermined steps, illustratively 90.degree., until the decoded information matches a known transmitted signal. This is preferably performed as part of the normal synchronisation procedure.


BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described with reference to the accompanying figures, in which:
FIG. 1 illustrates prior art differential encoding;
FIG. 2 illustrates a transmitted constell

REFERENCES:
patent: 3925611 (1975-12-01), Dennis
patent: 4271527 (1981-01-01), Armstrong
patent: 4483012 (1984-11-01), Wei
patent: 4873701 (1989-10-01), Tretter
patent: 4941154 (1990-07-01), Wei
patent: 4987569 (1991-01-01), Ling
patent: 5195107 (1993-03-01), Wei

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