Time-varying trellis-coded modulation formats which are robust in channels with phase variations
First Claim
1. A method for the digitally-coded transmission of information, said method being of the type including the steps ofproviding a carrier wave in the transmitter,coding the information according to a type of convolutional digital coding, andmodulating the carrier wave with the convolutionally-coded information according to a nonbinary digital modulation scheme involving a mapping of convolutionally-coded information values to nonbinary modulation values,said method being characterized bythe step of varying periodically the number of modulation values in the nonbinary digital modulation scheme to change correspondingly the mappingfrom the convolutionally coded information to the nonbinary modulation values to reduce the effect of phase noise or phase-offset upon the transmission.
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Abstract
There are described apparatus and methods for producing time-varying of trellis-coded modulations, specifically, time-varying trellis-coded phase-shift-keying, to facilitate carrier phase recovery when the signal has been exposed to extraneous phase variations and noise.
Typically, eight-phase-shift-keying (8-PSK), with rate 2/3 convolutional coding, is followed on a repetitive basis by one or two formats of four-phase-shift-keying (4-PSK), which formats are chosen to ease phase synchronization and to conserve the use of bandwidth and not to worsen the bit error probability while reducing the probability of synchronization loss and the length of error bursts or "random walks".
The variation of the modulation formats could be:
(1) 8-PSK (rate 2/3) followed by 4-PSK,
(2) 8-PSK (rate 2/3) followed by 4-PSK (preferred format) and then 4-PSK (secondary format) or,
(3) any of many other periodic formats of 8-PSK and 4-PSK sections.
The use of this improved type of modulation is contemplated, for example, mobile radio links, particularly to counteract fading, or for optical fiber links to counteract other causes of signal degradation.
116 Citations
9 Claims
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1. A method for the digitally-coded transmission of information, said method being of the type including the steps of
providing a carrier wave in the transmitter, coding the information according to a type of convolutional digital coding, and modulating the carrier wave with the convolutionally-coded information according to a nonbinary digital modulation scheme involving a mapping of convolutionally-coded information values to nonbinary modulation values, said method being characterized by the step of varying periodically the number of modulation values in the nonbinary digital modulation scheme to change correspondingly the mapping from the convolutionally coded information to the nonbinary modulation values to reduce the effect of phase noise or phase-offset upon the transmission.
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6. Apparatus for the digitally-coded transmission of information, including a transmitter comprising
means for generating a carrier wave, means for preparing an information signal for combined convolutional coding and modulation of the carrier wave, means for so modulating the carrier wave including logic apparatus for providing the convolutional coding, and means for mapping the convolutional code to a set of multiple discrete modulation values, said apparatus being characterized in the transmitter by a time-varying mapping means for generating a selected number of modulation values in response to the convolutional coding, and for reducing the number of modulation values to strengthen carrier recovery in the receiver, and for thereafter restoring the selected number of modulation values.
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8. Apparatus for transmitting phase-shift-keyed signals, comprising
means for generating a carrier wave, means for converting every two input information bits to 3 coded bits derived from a rate 2/3 convolutional code, and means for modulating the carrier wave as a phase-shift-keyed signal in response to said coded bits, said apparatus being characterized by first means between the converting means and the modulating means for mapping from three coded bits to an 8-PSK signal in which the modulation values have eight different relative phases which are equally spaced, said first mapping means controlling the modulating means during first signal periods, second means between the converting means and the modulating means for mapping from the three signal bits to a 4-PSK signal in which alternate modulation values starting with the second one from the zero phase reference are changed in phase by +45° - to have the phase angle of the next following modulation values, in terms of increasing phase angle, said second mapping means controlling the modulating means during second signal periods immediately following the first signal periods.
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Specification