Synchronization Message
Definition: The message transmitted on the forward-link base-to-mobile Synchronization channel.
Synchronization Message
Application: The Synchronization message contains the first information bits actually demodulated by the mobiles. Prior to demodulating the Synchronization message, the mobile has only acquired a pilot PN code at a suitable power level. The Synchronization message contains critical information needed by the mobile receiver to progress have knowing the pilot to having the information needed to demodulate the other forward-link channels. This information includes the pilot offset of the pilot the mobile has acquired. This information allows the mobile to know where to search for the pilots in the neighbor list. System time, the time of day in the IS-95 system, is based on Global Positioning Satellite (GPS) time. The system time is used to synchronize system functions. For instance, the PN generators on the reverse link use zero offset relative to the even numbered seconds in GPS time. However, the mobiles only know system time at the base stations plus an uncertainty due to the propagation delay from its base station to the mobile's location. The state of the long code generator at system time is also sent to the mobile in the Synchronization message. This allows the mobile to initialize and run its long code generator very nearly in time synchronism with the long code generators in the base stations. The Synchronization message also notifies the mobile of the paging channel data rate, which may be either 4800 or 9600 bits/sec.
The transmission of the Synchronization message begins at the same time that a period of the PN pilot begins and ends within three PN pilot code periods. Three PN pilot code periods equal exactly 80 msec and this 80 msec interval is called a Synchronization super frame. The system time sent in the synch message is a time with respect to the time at the end of the super frame (end of the third pilot period) in which the synch message ends. To account for propagation and signal processing delays, the system time sent to the mobile is the GPS time of day exactly 320 msec after the end of the super frame in which the synch message would end on the zero offset pilot.
That is, assuming that the pilot is the zero offset pilot, the system time is 4 synch super frames or 12 PN pilot-code periods into the future with respect to the end of the super frame in which the synch message ends. The mobile can recognize the end of the message and the end of the PN pilot-code which first occurs after the end of the message. It then counts 12 pilot-code periods and adjusts for the actual PN code offset knowing the PN code offset which is contained in the Synchronization message.
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