Application: USE YOUR BROWSER "BACK" BUTTON TO RETURN © 1999 DR. ERNEST SIMO - SPACE2000 
Example: Suppose the end of the superframe within which the synch message ended occurs at time X. The base station knows this time since it transmitted the message. The mobile, by virtue of being synchronizing to a pilot, can detect the end of the superframe, (which equals three pilot-code periods) by recognizing the presence in its short-code pilot generator of the first binary one following 15 consecutive binary zeros. The mobile can also mark the time instant 4 superframes beyond this event by simply counting 12 periods of the pilot code (4 superframes). That is, the mobile can accurately start its clock 320 msec after it identifies the end of the proper synch superframe. However, the beginning of the short pilot code, as marked by the binary one which follows 15 binary zeros, shifts in increments of 64 chips depending on the PILOT_PN offset. To provide this vital piece of time information, the synch message also conveys to the mobile, the PILOT_PN offset of the pilot it is synchronized to. Hence, the mobile, can start a timing signal at exactly 320 msec after the time X minus the time shift due to the PILOT_PN offset. This time shift equals PILOT_PNx52.083µsec, where 52.083 µsec is the time interval of 64 chips. The base station knowing time X also knows the time at X plus 320 msec minus PILOT_PNx52.083 µsec. The synch message ends in the third of the three pilot-code periods which make-up a superframe. Therefore, the first binary one following 15 zeros after the end of the synch message marks the end of the superframe in which the message ended. This will be the case for messages of reasonable length, since the synch message plus padding bits is required by IS-95 to equal an integer multiple of 93 bits. In the 80 msec superframe interval, 96 bits oc-cur at the 1200 bit/sec rate on the synch channel. Therefore, each group of message-plus-padding is .9688 of a superframe. The base station sends to the mobile in the synch message what the system time, SYS_TIME, will be at time X plus 320 msec minus PILOT_PNx52.083 µsec. Telling the mobile via the synch message the time into the future gives the mobile time to receive, demodulate, and set its clock to the communicated 36-bit state. With the clock in the mobile equipment set to SYS_TIME, i.e., X plus 320 msec minus PILOT_PNx52.083 µsec, the mobile starts its clock at exactly X plus 320 msec minus PILOT_PNx52.083 µsec. See IS-95, Fig. 6.6.1.4-1 for a diagram showing the time adjustment during the Timing Change Substate. Hence, the clock in the mobile and the system time are almost exactly synchronized, except for one issue. The mobile never knows the propagation delay between the base and itself and the base station does not try to compensate for this delay in setting SYS_TIME. Therefore, the time at the mobile lags behind the time at the base station by the propagation delay at the time of synchronization. USE YOUR BROWSER "BACK" BUTTON TO RETURN © 1999 DR. ERNEST SIMO - SPACE2000 |
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