Time Alignment

Since three mobiles use a given RF channel on
a time-sharing basis, it is necessary that they each time their transmissions
exactly. Otherwise, their signals would overlap and cause interference at
the base station receiver. Furthermore, a given cell may be many miles in
diameter, and the time for transmission from one mobile to the base station
may be different than the time taken by the transmission from another
mobile. Therefore, if one mobile begins transmission immediately after
another mobile stops transmission, it is possible that the two signals could
collide at the base station.
For example, consider a situation where Mobile A is far away from the
base station and Mobile B is close to the base station. It takes longer for
Mobile B’s transmission to reach the base station than that of Mobile A.
Therefore, if Mobile A starts transmitting immediately after Mobile B stops
transmitting, the transmission from Mobile B could still be arriving at the
base station when Mobile A’s transmission starts to arrive. Consequently, it
is necessary not just to ensure that no two mobiles transmit at the same
time, but it is also necessary to time transmissions such that no two transmissions
arrive at the base station at the same time. The methodology for
this timing is called time alignment, which involves advancing or retarding
the transmission from a given mobile so that the transmission arrives at
the base station at the correct time relative to transmissions from other
mobiles using the same RF channel.
When a mobile first accesses the system, the network assigns it a traffic
channel, including a Digital Voice Color Code (DVCC). At this point, however,
the network has not provided any time alignment information. Given
that the mobile could be close to the base station or far away, it needs the
correct time alignment information before transmitting real user data,
which means that the base station must determine roughly how far away
the mobile happens to be and must send time alignment instructions. In
order to help the base station determine what the time alignment instructions
should be, the mobile sends a special sequence of 324-bit duration,
called a shortened burst, as shown in Figure 3-18. The structure of the
shortened burst is such that if the base station detects two or more sync
words of the burst it can determine the mobile’s distance from the base station.
The base station then sends a Time Alignment Message instructing
the mobile to adjust its transmission timing.