MISSED PACKETS IN AN ASTRO SIMULCAST SYSTEM
Original Publication Date: 1999-Jun-01
Included in the Prior Art Database: 2002-Aug-16
In order to provide contiguous talk-out coverage tribution network is required. Digital system design, area that is larger than can be accommodated by a as embodied in the ASVO system, use a different single base station transmitter, systems are com'ig- method of synchronization. Since the audio is now ured to provide simulcast transmission from multi- digitized, it is distributed from a central source, ple transmitters. The use of multiple transmitters much like the analog counterpart. The digitized will create areas in the coverage zone that will be audio is delivered to each transmitter over a much reached by two or more transmitters. These areas are less expensive link, typically a leased phone line. called simulcast overlap areas. In order to minimize The synchronization of the audio signal is accom- interference between the transmitters, the modulat- plished by using a 1 Pulse Per Second signal from a ing signal is synchronized so that it is transmitted at GPS receiver. The digital audio packets sent from the same time from each station. Analog systems the central source also include a launch time para- have typically handled this synchronization by using meter which is correlated to the 1 PPS signal from a single audio source and delivering it to each sta- the GPS receiver. A typical system configuration is tion at the same time for transmission. To imple- shown in Figure 1. ment this requirement a fairly expensive audio dis- - RccnWd audio - rim BFnqlxncy Rcfcmccs .---- voted audio or sonrole audio 0 PP.9 and *MHz) w launch tines, .-------- Co"ml *ignalrig dy 10 s*llits Rsssi"ar,dieilal Only, V Fig. 1 Typical ASTRO GPS Simulcast System Infrastructure Diagram ! 0 MOtanla.lnC. 1999 208 June 1999 MOTOROLA Technical Developments @ The connecting links between the station, receiver and comparator are typically 4 wire leased phone lines. These phone lines will at times exhibit noise characteristics. Under noise conditions, bit errors will occur which will cause voice packets to be dropped. When a voice packet is dropped, the station does not have any data to send out. Since a basic requirement of a simulcast system is that the same information must be transmitted by all the sta- tions at exactly the same, a condition now exists which prevents that requirement from being met.