SCA (Subsidiary Communication Authority) Coverage Predictions
SCA service is relies upon an FM radio broadcast signal to deliver the information that is provided through the use of a transmitted sub-carrier. It was originally authorized as a 67 kHz sub-carrier, typically injected to the FM carrier at approximately 10%. It is a one-way wide area network. There is no provision for two-way communication. Information is supplied to the FM broadcast site and then can be delivered to very large geographic areas, roughly equivalent to that of the coverage of the FM station employed. Early applications were narrow band voice links (approx 5 KHz) for transmission of reading of books to the bind. It is also used for distribution of musical programs played in the background for offices and specialized audio networks common factors might include ethnic or professional backgrounds. Many other applications permit a wide variety of data transmission for medical and industrial applications. Audio up to 15kHz can now be received, with minimal distortion, but the application is primarily between fixed locations, rather than a mobile receiver.
For these types of service or coverage areas an engineer must identify the sensitivity of the receiver and the receiving antenna gain that will be used at the remote end of the link. There are often two sensitivity specifications, one for the FM broadcast carrier level and the second for the level of the SCA subcarrier. With these specifications identified the designer can then model the area coverage from the FM broadcast station and then plot the desired received threshold. A commonly used propagation model is the Longley-Rice propagation module in TAP.
(20070514) Copyright 2007 by SoftWright LLC, Aurora, Colorado USA