me slots European GSM 4.615 msec for eight time slots One problem with TDMA, often not appreciated by many, is delay. In particular, this is delay on the uplink. Consider Figure 18.9, where we set up a scenario. A base station receives mobile time slots in a circular pattern and the radius of the circle of responsibility of that base station is 10 km. Let the velocity of a radio wave be 3 X 108 m/sec. The time for the wave to traverse 1 km is 1000 m/(3 X 108) or 3.333 usec. In the uplink frame we have a mobile station right on top of the base station with essentially no delay and another mobile right at 10 km with 10 X 3.33 usec or 33.3 usec delay. A GSM time slot is about 576 usec in duration. The terminal at the 10-km range will have its time slot arriving 33.3 usec late compared to the terminal with no delay. A GSM bit period is about 3.69 usec SO that the late arrival mutilates about 10 bits, and unless something is done, the last bit of the burst will overlap the next burst (Refs. 2 and 11). Mobile station 10 km Mobile station Base station Figure 18.9 A TDMA delay scenario. 18.6 NETWORK ACCESS TECHNIQUES TDMA frame = 8 time slots 1 time slot = 156.25 bit durations Normal Burst Tail bits Encrypted bits Training sequence Encrypted bits Tail bits Guard period Frequency Correction Burst Tail bits Fixed bits Tail bits Guard period Synchronization Burst Tail bits Encrypted sync bits Extended training sequence Encrypted sync bits Tail bits Guard period Access Burst Tail bits Synchro sequence Encrypted bits Tail bits Guard period 68.25 Figure 18.10 GSM frame and burst structures. (From Figure 8.7, Ref. 2. Reprinted with permission.) Refer now to Figure 18.10, which illustrates GSM burst structures. Note that the access burst has a guard period of 68.25 bit durations or a slop of 3.69 X 68.25 usec, which will well accommodate the later arrival of the 10-km mobile terminal of only 33.3 usec. To provide the same long guard period in the other bursts is a waste of valuable "spec- trum." The GSM system overcomes this pro