Offset Between Transmit and Receive

 IS-54 is a frequency
duplex TDMA system. In other words, the mobile transmits on one frequency
and receives on another frequency. In the uplink, the mobile transmits
on a given pair of time slots, and on the downlink, it receives on the
corresponding pair of time slots. If, for example, a given mobile transmits
on time slots 1 and 4 on the uplink, then it receives on time slot 1 and 4 on
the downlink. Time slots 1 and 4 on the downlink do not, however, correspond
to the same instants in time as time slots 1 and 4 on the uplink. A
time offset between the downlink and the uplink corresponds to one time
slot plus 45 symbol periods (207 symbol periods total or 8.5185 ms), with
the downlink lagging the uplink. Therefore, the mobile does not transmit
and receive simultaneously. Rather, during a conversation, it receives a
time slot on the downlink shortly after sending a time slot on the uplink.
Figure 3-16 depicts this offset, showing the transmission and reception by
a given mobile on time slots 1 and 4.

As can be seen from Figure 3-16, times will occur when the mobile is neither
transmitting on a given time slot nor listening to the base station on
the corresponding downlink time slot. So what does it do during these
times? Rather than do nothing, the mobile tunes briefly to other base stations
to measure the signal from those base stations. As described later in this chapter, those measurements can be provided to the network to assist
the network in determining when a handoff should take place.