INCREASING THE RELIABILITY OF DIGITAL COMMUNICATIONS SYSTEMS UNDER POOR SIGNAL CONDITIONS

摘要

The present digital communications equipment for the AN/TSQ-1 3 sys¬tem uses amplitude modulation to transmit binary digital data between sites. Theoretical studies and field experience have shown that a minimum signal-to-noise ratio of 22 db is essential for complete reliability.nTheoretical studies indicated that this minimum could be lowered by several db with other modulation and/or detection techniques. Two specific approaches were investigated:n1.Double sideband phase modulation and phase coherent detection.n2.Amplitude modulation and autocorrelation detection.nThe equipment was built and tested in the laboratory. Then tests were made over telephone lines and multiplex equipment using both techniques.nThe phase modulation technique showed a 7-db improvement over con¬ventional amplitude modulation. A variation in amplitude of 20 db at the receiver input could be tolerated with no deterioration of reliability. This feature is very desirable when scatter communication techniques are employed.nThe autocorrelation method showed a 4-db improvement over conven¬tional amplitude modulation. While this is not as great as the improvement realized from phase modulation, the autocorrelation method has the added advantage that no change in the transmitting or modulating equipment, or in the method of transmission, is required.