EDGE Modulation

As mentioned, EDGE uses the same 200-kHz channels
and eight-timeslot structure as used for GSM and GPRS.With EDGE,
however, 8-PSK modulation is introduced in addition to the 0.3 Gaussian
Minimum Shift Keying (GMSK) used in GSM.

0.3 GMSK means that the modulator has a bandpass filter with a 3dB
bandwidth of 81.25 kHz. In GSM, the symbol rate is a 270.833
ksymbols/second, with each symbol representing one bit, leading to
270.833 Kbps. The value of 81.25 is 0.3 times 270.833, which is why it is
called 0.3 GMSK. The 270.833 Kbps is carried on a 200-kHz carrier, so that
GSM provides a bandwidth efficiency of 270.833/200, which equals approximately
1.35 bits/s/Hz.

The objective with EDGE is to offer higher bandwidth efficiency, so that
we can squeeze more user data from the same 200-kHz channel. This
higher bandwidth efficiency is achieved through 8-PSK. In general, PSK
involves a phase change of the carrier signal according to the incoming bit
stream. The simplest form of PSK involves a 180° phase change at every
transition from 0 to 1, or vice versa, in the incoming bit stream.With 8-PSK,
we treat the incoming bit stream in groups of three bits at a time and allow
phase changes of 45°, 90°, 135°, 180°, 225°, 270°, or 315°. The specific phase
change of the signal represents the change from one set of three bits to the
next, as shown in Figure 5-9.With EDGE, the symbol rate is still 270.833
ksymbols/second, as it is in GSM. Each symbol, however, is three bits, such
that we have a bit rate of 812.5 Kbps.

Of course, we do not get this great increase in bandwidth efficiency for
free. In addition to any extra cost associated with producing devices that
can support 8-PSK modulation, we must also contend with the fact that
8-PSK is more sensitive to noise than GMSK. Noise in a signal can make it
more difficult for a receiver to determine the exact phase change when the
signal changes from one state to another. Because of the fact that the states
in 8-PSK are quite close together, the amount of noise required for errors to
occur can be relatively small—certainly smaller than the amount of noise that GMSK can handle. The direct result of this is that if a BTS supports
both GMSK and 8-PSK modulation and has the same output power for
both, then the cell footprint is smaller for 8-PSK than for GMSK. Recognizing
this limitation, however, the specifications for EDGE are such that both
the coding scheme and modulation scheme can be changed in response to
RF conditions. Thus, as a user moves towards the edge of a cell, the effect of
lower signal to noise will mean that the network can reduce the user’s
throughput, either by changing the modulation scheme to GMSK or by
changing the coding scheme to include greater error detection. All that the
user will notice is somewhat slower throughput.