DIGITAL-VOICE OVERLAY FOR LAND MOBILE COMMUNICATIONS

The technical feasibility of alternative means of multiplexing data and voice transmissions in a vehicle location and communications system is treated. Multiplexing refers to the sharing of a transmitter, a receiver, and a radio frequency channel by different users. Two multiplexing techniques are discussed: "time division" methods (tdm), in which each broadcaster is given a particular time slot during which he may transmit data and all other users remain silent; and frequency slot separated by guard bands to minimize cross-channel effects. Because tdm technology cannot easily or economically handle both voice and digital transmissions, the report focuses on fdm applications. Fdm systems necessarily employ equipment to modulate voice and data inputs in order to channel them into their appropriate slots. Considerations in applying channel modulation in location and communications systems are (1) frequency modulation, which is standard for land mobile services but requires potentially undesirable bandwidth occupancy requirements; (2) amplitude modulation, which suffers potential fading and drifting problems; (3) frequency shift keying; (4) phase-shift keying; and (5) on-off keying. Bandwidth requirements and data rate capability are discussed with reference to alternative fdm applications. Trade-offs in voice-data multiplex are considered in terms of power requirements for switching between voice and digital transmissions in the same frequency subchannel. Baseband spectrum occupancy, power requirements, and on operational considerations are also examined. The report concludes with a brief discussion of federal communications regulations that apply to implementation.

• Corporate Authors:

  IIT Research Institute

  10 West 35th Street
  Chicago, IL  United States  60616
• Authors:
  • Scales, W C
• Publication Date: 1970-11

Subject/Index Terms

• TRT Terms: Communications; Data communications; Rapid transit
• Old TRIS Terms: Land mobile communications
• Subject Areas: Highways; Operations and Traffic Management;

Filing Info

• Accession Number: 00044204
• Record Type: Publication
• Source Agency: Urban Mass Transportation Administration
• Files: TRIS, USDOT
• Created Date: Jun 15 1976 12:00AM