4 Time-slot structure.
I Channel
Q Channel
FIGURE 1.5 /4-DQPSK signal constellation diagram.
given in the form of a phase state table (see Table 1.1).
The input bit stream is converted to symbols or di-bit
pairs using the mapping rule in the phase state table.
1.2.2 Group Special Mobile (GSM)
In this section, we will provide the physical layer details
of the European-born TDMA system, GSM, which has
been adapted practically worldwide. This system uses
FDD for uplink and downlink communications [8??“11].
The time-slot structures for both uplink and downlink are given in Fig. 1.6. There are a total of
8 time slots per radio frame. The aggregate data rate is 270.8 kbps and the per user data rate is 33.85
kbps. The uplink and downlink frames have an offset of 3 time slots.
WIRELESS TOPICS 5
TABLE 1.1 /4-DQPSK Phase State Table
Di-bit pair Phase change ()
0??“0 /4
1??“0 /4
0??“1 3/4
1??“1 3/4
I-Axis
Q-Axis
Phase State
FIGURE 1.7 MSK signal constellation diagram.
1 2 3 4 5 6 7 8
Frame = 4.615 msec
FIGURE 1.6 Time-slot structure.
The modulation scheme used is GMSK with the signal constellation diagram shown in Fig. 1.7.
Some of the benefits of this modulation scheme are constant envelope, spectrally efficient, and excellent
Bit Error Rate (BER) performance.
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