Acquisition and tracking of independent quadrature modulated bitstreams

Communication apparatus for communicating dual bitstream quadrature modulated digital data for receipt by a receive apparatus, said communication apparatus comprising:

• a first encoder for encoding a first stream of digital information data;
• a second encoder for encoding a second stream of digital information data;
• means for applying an invertible alteration function to said first stream of data to alter said first stream in a manner that will produce a high error rate if said first stream is decoded without first applying the inverse of said alteration function;
• means for quadrature modulating said altered first stream of data and said second stream of data to provide separate in-phase (I) and quadrature (Q) data streams for transmission;
• said receive apparatus being capable of recovering a desired one of said first and second information streams from quadrature modulated data transmitted by said communication apparatus and including:
• a quadrature demodulator coupled to receive said quadrature modulated data, said demodulator outputting a demodulated in-phase (I) bitstream and a demodulated quadrature (Q) bitstream;
• first switch means coupled to said demodulator for selecting one of said demodulated I and Q bitstreams;
• an inverter coupled to invert the bitstream selected by said first switch means;
• second switch means for selectively outputting either the bitstream selected by said first switch means or the inverted bitstream from said inverter;
• means for applying the inverse of said alteration function to the bitstream output from said second switch means;
• a decoder for receiving either the bitstream output from said second switch means or an inverse altered version thereof from said inverse alteration function applying means;
• and
• third switch means for selecting the desired one of said first and second information streams by selectively outputting to said decoder either the bitstream output from said second switch means or the inverse altered version thereof.

A receiver for recovering a desired information stream from a pair of independent bitstreams transmitted using quadrature modulation, one of said independent bitstreams having been altered by an invertible alteration function prior to transmission, said receiver comprising:

• a quadrature demodulator coupled to receive quadrature modulated data, said demodulator outputting a demodulated in-phase (I) bitstream and a demodulated quadrature (Q) bitstream;
• first switch means coupled to said demodulator for selecting one of said demodulated I and Q bitstreams;
• an inverter coupled to invert the bitstream selected by said first switch means;
• second switch means for selectively outputting either the inverted bitstream from said inverter or the bitstream selected by said first switch means;
• means for applying the inverse of said alteration function to the bitstream output from said second switch means;
• a decoder for receiving either the bitstream output from said second switch means or an inverse altered version thereof from said inverse alteration function applying means;
• and
• third switch means for selecting the desired information stream by selectively outputting to said decoder either the bitstream output from said second switch means or the inverse altered version thereof.

A method for recovering a desired information stream from a pair of independent bitstreams transmitted using quadrature modulation, one of said independent bitstreams having been altered by an invertible alteration function prior to transmission, comprising the steps of:

• demodulating said quadrature modulated bitstreams to provide a demodulated in-phase (I) bitstream and a demodulated quadrature (Q) bitstream;
• tentatively providing one of said demodulated I and Q bitstreams for decoding;
• optionally inverting the tentatively provided bitstream;
• decoding one of the tentatively provided bitstream and the optionally inverted bitstream if it is desired to recover the bitstream that was not altered by said alteration function prior to transmission;
• decoding one of the tentatively provided bitstream and the optionally inverted bitstream after applying the inverse of said alteration function thereto if it is desired to recover the bitstream that was altered by said alteration function prior to transmission;
• determining an error rate of data decoded during the decoding step;
• and
• if the error rate is not acceptable, trying different choices of said demodulated I and Q bitstreams with and without inversion to attempt to obtain an acceptable error rate.