Phase ambiguity resolution for offset QPSK modulation systems
First Claim
1. In an offset quaternary phase-shift-keyed modulation system for digital communications, wherein unique inphase and quadrature words are separately modulated onto respective inphase and quadrature channels of a preamble before combining the channels to form a quadraphase modulated carrier for transmission of data bits of a quaternary phase-shift-keyed modulated message, and in which a receiver includes means for detecting said inphase and quadrature data channels of the received data from said quadraphase modulated carrier, an improvement in said receiver comprisingmeans for tracking the phase of said modulated carrier and deriving a synchronization signal, said tracking means comprising a digital Costas loop and an integrated symbol synchronizer, said Costas loop comprising inphase and quadrature accumulators connected to receive data from said means of said receiver for detecting said inphase and quadrature data channels, respectively, said integrated symbol synchronizer being connected to receive the output of said inphase accumulator and produce said synchronizing signal, said integrated symbol synchronizer being connected to provide said synchronizing signal as a feedback to said inphase accumulator and with a delay of one half a bit symbol duration time Ts to a quadrature accumulator for correcting synchronization timing, thereby resolving four-phase ambiguity conditions using the inphase and quadrature channel reversal correcting property of said Costas loop with said integrated symbol synchronizer to correct errors associated with 90°
- and 270°
carrier phase errors, anda phase ambiguity resolver connected to receive said inphase and quadrature data channels of said tracking means in which 90° and
270°
carrier phase errors have been corrected, said phase ambiguity resolver includingunique inphase word detector means and unique quadrature word detector means for separately determining when said unique inphase and quadrature words are received with bits 180°
out of phase,means connected to said unique inphase word detector means and said unique quadrature word detector means for resolving phase ambiguity by separately inverting inphase and quadrature data bits thereby to so complement said bits as to correspond to bits of said unique words, whereby remaining phase ambiguity conditions associated with 0° and
180°
carrier phase error are resolved, wherein each of said unique word detector means is comprised ofmeans for separately correlating said unique inphase and quadrature words received with corresponding unique inphase and quadrature words transmitted, andmeans responsive to said means for correlating said unique inphase and quadrature words received for inverting data bits of a corresponding one of said inphase and quadrature data channels received thereafter, andmeans responsive to said means for separately correlating said inphase and said quadrature phase unique words for generating synchronizing pulses for demodulated data bits received thereafter.
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Abstract
A demodulator for OQPSK signals modulated with two unique words resolves eight possible cominbations of phase ambiguity which may product data error by first processing received IR and QR data in an integrated carrier loop/symbol synchronizer using a digital Costas loop with matched filters for correcting four of eight possible phase lock errors, and then the remaining four using a phase ambiguity resolver which detects the unique words to not only reverse the received IR and QR data channels, but to also invert (complement) the IR and/or QR data, or to at least complement the IR and QR data for systems using nontransparent codes that do not have rotation direction ambiguity.
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Citations
4 Claims
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1. In an offset quaternary phase-shift-keyed modulation system for digital communications, wherein unique inphase and quadrature words are separately modulated onto respective inphase and quadrature channels of a preamble before combining the channels to form a quadraphase modulated carrier for transmission of data bits of a quaternary phase-shift-keyed modulated message, and in which a receiver includes means for detecting said inphase and quadrature data channels of the received data from said quadraphase modulated carrier, an improvement in said receiver comprising
means for tracking the phase of said modulated carrier and deriving a synchronization signal, said tracking means comprising a digital Costas loop and an integrated symbol synchronizer, said Costas loop comprising inphase and quadrature accumulators connected to receive data from said means of said receiver for detecting said inphase and quadrature data channels, respectively, said integrated symbol synchronizer being connected to receive the output of said inphase accumulator and produce said synchronizing signal, said integrated symbol synchronizer being connected to provide said synchronizing signal as a feedback to said inphase accumulator and with a delay of one half a bit symbol duration time Ts to a quadrature accumulator for correcting synchronization timing, thereby resolving four-phase ambiguity conditions using the inphase and quadrature channel reversal correcting property of said Costas loop with said integrated symbol synchronizer to correct errors associated with 90° - and 270°
carrier phase errors, anda phase ambiguity resolver connected to receive said inphase and quadrature data channels of said tracking means in which 90° and
270°
carrier phase errors have been corrected, said phase ambiguity resolver includingunique inphase word detector means and unique quadrature word detector means for separately determining when said unique inphase and quadrature words are received with bits 180°
out of phase,means connected to said unique inphase word detector means and said unique quadrature word detector means for resolving phase ambiguity by separately inverting inphase and quadrature data bits thereby to so complement said bits as to correspond to bits of said unique words, whereby remaining phase ambiguity conditions associated with 0° and
180°
carrier phase error are resolved, wherein each of said unique word detector means is comprised ofmeans for separately correlating said unique inphase and quadrature words received with corresponding unique inphase and quadrature words transmitted, and means responsive to said means for correlating said unique inphase and quadrature words received for inverting data bits of a corresponding one of said inphase and quadrature data channels received thereafter, and means responsive to said means for separately correlating said inphase and said quadrature phase unique words for generating synchronizing pulses for demodulated data bits received thereafter. - View Dependent Claims (2, 3, 4)
- and 270°
Specification