800 MHz for public safety — Design considerations for conventional systems

Major questions arising in the design of a conventional (i.e., not trunked) public safety 800 MHz land-mobile communication system are discussed. Public safety agencies consider implementing systems in the 800 MHz band because of the difficulty of obtaining channels in the VHF high band and UHF 450-512 MHz bands, because of the excellent coverage available in urban and suburban environments, or for both reasons. Somewhat higher costs in this band are often offset by the advantages cited. Two very different types of systems are described. One is a complex, multi-channel tollroad system that is primarily mobile radio oriented, and is unified through an extensive microwave backbone system. This system serves both police and tollroad maintenance, engineering and administrative functions. The other is a municipal police and fire system with channels to serve both dispatch operations of the two departments and other functions such as fire ground, detective and administrative users. Both the fire ground and police dispatch systems are designed to provide highly reliable portable radio coverage over the entire jurisdiction. For a ribbon type system, such as the tollroad system, the use of the 800 MHz band allows special requirements to be imposed on antenna pattern specifications and on individual channel assignments. The restrictions placed on transmitter power and satellite receiver placement add to the design constraints normally found in such a system. The municipal system design poses the challenge of meeting the diverse requirements of dispatch and tactical communications (such as fire ground and detective message traffic) and portable coverage on both police and fire channels, while providing mobile coverage in a compatible system. Location of base stations and receivers, in light of available transmitter powers and known system losses also requires careful attention. The aspects of 800 MHz system design for conventional, public safety systems that are unique to these situations are presented and discussed. They include ERP limitations, antenna patterns available, and licensing requirements, as well as special consideration needed to produce competitive bid specifications in a market in which not every equipment feature or type is available from every supplier. Many of the design considerations discussed for public safety systems are equally applicable to business and industrial users of 800 MHz systems.