Pulse or digital communications – Systems using alternating or pulsating current – Plural channels for transmission of a single pulse train
Reexamination Certificate
1996-12-18
2001-02-27
Tsang, Fan (Department: 2683)
Pulse or digital communications
Systems using alternating or pulsating current
Plural channels for transmission of a single pulse train
C329S304000, C332S103000, C375S298000
Reexamination Certificate
active
06195396
ABSTRACT:
The invention relates to encoding and decoding digital signals, and it relates more precisely to an encoding/decoding system using 16-QAM modulation encoded in multi-level blocks, which system is transparent to phase shifts of ±&pgr;/2 and of &pgr;, has a high coding rate, and is simple to implement.
BACKGROUND OF THE INVENTION
16-state quadrature amplitude modulation (16-QAM) is widely used in the field of radio communications because of its high spectral effectiveness (compared with the commonly used techniques of phase modulation and of frequency modulation), and because it is relatively simple to implement (implementing the modulator/demodulator, processing the signal, etc.).
Furthermore, in the context of radio transmission, the cost of leasing transmission channels is directly related to the transmitted spectrum width. That is why the channel encoding technique chosen for this type of link must have a high rate.
Finally, error-correcting encoding must not only have performance levels that are acceptable from the system point of view, but must also be as simple as possible to implement, in particular as regards the decoder.
In the context of the present invention, the data transmitted from a transmitter to a receiver is integrated into a framed structure having fixed redundancy. In which case, block coded modulation is preferable to trellis coded modulation (TCM).
It is known that, for such transmission, it is possible to use modulation and high-rate convolutional encoding that are separated from each other. In the case of a framed structure having a fixed rate (N redundancy bits being added to every K message information bits to be transmitted), 16-QAM modulation associated with a block code like the Reed-Solomon code with firm decision is a natural choice.
Unfortunately, the choice of convolutional encoding either separate from or associated with modulation (TCM encoding) implies a rate that is necessarily limited. In addition, such a choice is not compatible with the choice of a framed structure having a fixed rate. Furthermore, the corresponding decoder associated with a convolutional code (Viterbi algorithm) is relatively complex.
For modulation associating 16-QAM mapping with Reed-Solomon coding, it is necessary to use a complex decoder. Furthermore, that solution does not enable decoding to take place under flexible-decision conditions.
Moreover, when coherent demodulation is performed by a carrier-recovery system enabling the phase and the frequency of the signal to be synchronized on reception, there still remain phase ambiguities that must be removed after such synchronization. For example, all QPSK and n-QAM constellations (two-dimensional constellations) have three phase ambiguities (+&pgr;/2, −&pgr;/2, and &pgr;) which must be removed. The encoding scheme is said to be “transparent to phase ambiguities” if it enables the sequence to be retrieved on reception as transmitted regardless of the phase ambiguity imparted on reception by carrier-recovery.
OBJECTS AND SUMMARY OF THE INVENTION
The present invention proposes a block-encoded modulation scheme using multi-level partitioning techniques of the multi-level coding (MLC) type. This scheme is made transparent to phase ambiguities of +&pgr;/2 and of &pgr; by means of differential encoding and appropriate mapping, it is applicable to 16-QAM modulation, and it has theoretical encoding gain that is optimal for the rate of the code. The encoding is of the systematic type (the message information bits to be transmitted are not modified, only parity information is added), and the decoder associated with this scheme uses the Wagner algorithm which is much less complicated to implement than the Viterbi algorithm or than the Reed-Solomon algorithm, and it operates under flexible-decision conditions.
More precisely, the present invention provides an encoding/decoding system using 16-QAM modulation encoded in multi-level blocks, said system including a transmitter for transmitting message information bits and a receiver for receiving message information bits;
said transmitter including:
two transition encoders, each of which has two dimensions, a first of said encoders receiving two message information bits to be transmitted referred to as more significant bits, the second of said encoders receiving two other message information bits to be transmitted referred to as less significant bits;
two parity encoders having a coding rate N/N−1, where N is even, receiving the bits output from said first transition encoder, said parity encoders supplying, every N symbols, said bits output from said first transition encoder together with parity information;
an allocation unit for allocating a symbol from the complex plane of the 16-QAM constellation to each group of four bits output by said parity encoders and by said second transition encoders, said allocation unit supplying the respective amplitude levels of said symbols on respective transmission paths, while satisfying the following rules:
all of the symbols in the same quadrant of said complex plane have the same two less significant bits; and
the two more significant bits of the symbols in the same quadrant and close to the same axis of said complex plane differ from symbol to symbol by a single bit, the symbols in said complex plane being split up into subsets, each subset having a pair of more significant bits that is different from the pair of more significant bits of another subset; and
quadrature modulation means for modulating said amplitude levels in quadrature, and supplying a signal to be transmitted; and
said receiver including:
quadrature demodulation means for demodulating the received signal in quadrature, and supplying two received amplitude levels;
a pre-decision unit which takes a firm decision on the received amplitude levels, and which, for each of said received amplitude levels, firstly calculates a reliability coefficient equal to the difference in absolute terms between the squared Euclidean distances of each received amplitude level and the amplitude level corresponding to the closest ideal symbol in the 16-QAM constellation, and secondly supplies sector information making it possible to define which one of the three sectors delimited by decision thresholds corresponding to expected ideal amplitude levels for the four symbols closest to the two axes of said 16-QAM constellation is the sector in which the received symbol lies;
two Wagner decoders taking flexible decisions, each of which decoders receiving firstly the firm decisions corresponding to said more significant bits and secondly one of said reliability coefficients, said Wagner decoders supplying, every N information symbols, said firm decisions together with correction information indicating that said parity information checks;
a decision unit receiving said firm decisions, said correction information and said sector information, said decision unit correcting the two more significant bits output by said Wagner decoders as a function of said correction information, and correcting the two decided less significant bits as a function of said sector information, of the corrected more significant bits, and of said correction information; and
two transition decoders, each of which has two dimensions, a first one of said decoders receiving the two corrected more significant bits, the second one of said decoders receiving the two corrected less significant bits, said decoders supplying received and decoded bits.
For example, N may be equal to 16.
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R. A. Wagner et al, “The string-to-string correctin problem”,Journal of The Association for Computing Machinery, Jan. 1974, USA, vol. 21, No. 1, ISSN 0
Fang Juing
Houplain Jean-Francois
Roux Pierre
Alcatel Telspace
Sughrue Mion Zinn Macpeak & Seas, PLLC
Tran Congvan
Tsang Fan
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