INNOVATIVE TECHNIQUES FOR OVERCOMING COCHANNEL INTERFERENCE IN HF MODEMS

CO-CHANNEL INTERFERENCE REMAINS A PERSISTENT AND PERVASIVE ANNOYANCE FOR MOST HF BAND COMMUNICATIONS. THE MULTIRATE ORTHOGONAL SAMPLING (MOS) TECHNIQUE PRESENTED IS AN INNOVATIVE, YET INTUITIVELY STRAIGHTFORWARD APPROACH TO THE PROBLEM OF CO-CHANNEL INTERFERENCE. THE PREMISE OF THE MOS TECHNIQUE RELIES ON TWO FUNDAMENTAL PROPERTIES OF COMMUNICATION SIGNALS: (1) THE SIGNALS ARE CARRIER-BORNE I.E. THE INFORMATION SIGNAL HAS BEEN MODULATED ON TO A HIGH FREQUENCY CARRIER, AND (2) THE BANDWIDTH OF THE INFORMATION SIGNAL IS SIGNIFICANTLY LESS THAN CARRIER FREQUENCY. MOS DIVERGES FROM CONVENTIONAL DEMODULATION/ SUPPRESSION METHODS BY RECASTING THE PROBLEM TO ONE INVOLVING MULTIPLE WAVEFORM ESTIMATION. MOS DERIVES A SOLUTION BY CONSIDERING THE DEMODULATION/SUPPRESSION SIMULTANEOUSLY AS ONE OPTIMAL PARAMETER ESTIMATION PROBLEM. FOR TWO INTERFERING SINUSOIDS, THE BEST ESTIMATE OF ONE WAVEFORM IS OBTAINED WHEN THE COMPOSITE WAVEFORM IS SAMPLED AT A ZERO-CROSSING OF THE OTHER. MOS IS CONVENIENTLY IMPLEMENTED AS A SET OF MULTI-RATE SAMPLERS THAT COOPERATIVELY DEVELOP OPTIMAL SAMPLE CLOCKS TO PROVIDE INTERFERENCE SUPPRESSED ESTIMATES. MOS IS ESILY IMPLEMENTED USING INEXPENSIVE COMPONENTS, EASILY INTEGRATED INTO EXISTING HF MODEMS, AND DOESN'T REQUIRE COMPLEX ANTENNAS OR DEMODULATION CIRCUITRY.