3.
From the above table, we can see the theoretical maximum data rate is 59.2 kbps times 8 time
slots 473.6 kbps using 8-PSK. These schemes allow the network and MS to trade off data rate,
depending on the channel conditions. The data rate changes occur by changing the modulation
scheme and the amount of parity bits inserted (code rate) in the transmitted bit stream. These are captured
in the modulation and coding scheme (MCS) values in the above table.
As discussed above, GSM and GPRS use the same modulation scheme, which is GMSK. The
EDGE modulation scheme is 8-PSK where 3 bits are grouped together to form a symbol. The
complex-valued symbol is given as
(1.3) s(k) expQj
2p
8
d(k)R
The mapping of the bits to symbols are governed by the Gray coding rule, which is shown in the signal
constellation diagram in Fig. 1.12.
Prior to performing the pulse-shaping operation, the symbols are rotated by 3/8 radians every
symbol time. The pulse shaping for EDGE is based on a linearized GMSK pulse and is described as:
(1.4)
(1.5) S(t) g sin ap 3
t
0
g(x) dxb , 0 t 4T
sin ap
2 p 3
t4T
0
g(x) dxb , 4T t 8T
0 , otherwise
h(t) ?µq3
i0
S(t iT ) , 0 t 5T
0 , otherwise
WIRELESS TOPICS 9
TABLE 1.
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