Pulse or digital communications – Repeaters – Testing
Patent
1989-05-31
1991-10-01
Safourek, Benedict V.
Pulse or digital communications
Repeaters
Testing
375 39, 371 43, H04L 2700, H03M 1312
Patent
active
050540360
DESCRIPTION:
BRIEF SUMMARY
The present invention concerns modulation and demodulation of digital signals.
In phase amplitude modulation, each group of input bits selects one of a number of points in the phase diagram. In a trellis code, more points are allowed than are necessary for a one to one assignment of bit combinations and points; the selection is made with the aid of a convolutional encoder.
A convolutional encoder is at any time, in one of a number of states. It has an input which changes at symbol clock rate. The coder moves from its present state to a follower state in dependence on the input. The follower state may in some circumstances (i.e. some combinations of present state and present input) be the same as the present state. The output depends on the state, and may also depend on the input. This arrangement permits certain sequences of points whilst other sequences are inadmissible. A Viterbi decoder can then carry out `soft decision` decoding, with an improvement in noise performance compared with coding using a non-redundant constellation of points. Trellis coding is discussed in some detail in "Channel Coding with Multilevel/Phase Signals" by G.Ungerboeck, IEEE Trans on Information Theory, Vol. IT-28 No. 1, January 1982.
If differential coding is employed, the coding can be chosen such that a 90.degree. phase jump causes errors only temporarily after a jump occurs, and errors do not persist. This is referred to below as 90.degree. phase jump immunity. Hitherto it has been thought that this requires differential coding on at least two bits per symbol.
It is now shown however, that trellis coding can be used with one bit per symbol and allow 90.degree. phase jump immunity. This requires four points in the q.a.m. signal constellation.
According to the present invention there is provided a coding apparatus comprising
(i) a convolutional coder having a plurality n of defined states and operable at regular intervals to assume a state which, for any preexisting state, is one of two mutually different states according to the binary value of an input thereto, such that if the behaviour of the coder is represented by a diagram in which each such state is represented by a point and each transition, and its direction, between two states or, as the case may be, transition from a state to the same state, is represented by a line whose ends are joined to the point(s) representing those states or that state, then that diagram can be so drawn that it is unchanged when rotated by any multiple of 90 degrees;
(ii) means responsive to the coder states to generate in respect of each possible pair of consecutive states an assigned one of four signals having relative phases 0, 90, 180 and 270 degrees, the assignment being such that each of the 4n possible sequences of three states gives rise to a pair of signals having the same signed phase difference as the pair generated by a corresponding sequence which, in the diagram, occupies a position rotated by 90.degree. from that of the sequence in question; and
(iii) input means responsive to a single bit input stream supplied thereto to produce the said binary input to the convolutional coder to so control the state transitions that one bit value gives rise to a singal of phase difference a or b from the preceding signal and the other bit value gives rise to a signal of phase difference c or d from the preceding signal, where a, b, c and d are different ones of the values 0, 90, 180 and 270 degrees.
Some embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:
FIG. 1 is an outline diagram of a coding apparatus according to the invention, with a decoder;
FIG. 2a,b,c, illustrate signal point mappings, and a block diagram, of a CCITT standard V32 coder;
FIG. 3a,b illustrate state transitions and signal subsets of the V32 coder;
FIG. 4a,b illustrates the state transition and subsets for a first embodiment of coding apparatus according to the invention;
FIG. 5 is a block diagram of the apparatus illustrated by FIG. 4;
FIG. 6 ill
REFERENCES:
patent: 4077021 (1978-02-01), Csajka et al.
patent: 4601044 (1986-07-01), Kromer, III et al.
Br Telecom Technol J, vol. 2, No. 4, Sep. 1984, pp. 64-73, "Duplex Transmission at 4800 and 9600 Bit/s on the PSTN and The Use of Channel Coding with a Partitioned Signal Constellation", J. D. Brownie et al.
IEEE Transactions on Information Theory, vol. IT-28, No. 1, Jan. 1982, pp. 55-67, "Channel Coding with Multilevel/Phase Signals", G. Ungerboeck.
IEEE Transactions on Communications, vol. COM-33 (1985), Jan. No. 1, New York, USA, pp. 20-29, "Convolutional Coding Combined with Continuous Phase Modulation", S. Pizzi et al.
IEEE International Conference on Communications, 1985, vol. 2, Jun. 23-26, 1985, pp. 18.1.1-18.1.5, "Rotationally Invariant Trellis Codes for mPSK Modulation", M. Oerder.
Advances in Instrumentations, 1976, pp. 678-1-678-4, "Sequential vs. Viterbi Decoding for RF Data Transmission Links", Batson et al.
Brownlie John D.
Lloyd Barry G.
British Telecommunications public limited company
Safourek Benedict V.
Smith Ralph
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