Page 93 - From GMS to LTE

General Packet Radio Service (GPRS) and EDGE 79

GMSK 8PSK
Q 010 Q
000 011
1

0
001 111
I I

110
101
100

Figure 2.7 GMSK (GPRS) and 8‐PSK (EDGE) modulation.

capability bit in the GPRS cell options of the System Information 13 message, which is
broadcast on the Broadcast Common Control Channel (BCCH). From the mobile
device side, the network is informed of the mobile device’s EDGE capability during the
establishment of a new connection. Hence, EDGE is fully backward compatible to GPRS
and allows the mixed use of GPRS and EDGE mobile devices in the same cell. EDGE
mobile devices are also able to use the standard GMSK modulation for GPRS and can
thus be used also in networks that do not offer EDGE functionality.
Another advantage of the new modulation and the nine different coding schemes
(MCS) compared to the four coding schemes of GPRS is the precise use of the best
modulation and coding for the current radio conditions. This is done in the mobile
device by continuously calculating the current bit error probability (BEP) and reporting
the values to the network. The network in turn can then adapt its current downlink
modulation and coding to the appropriate value. For the uplink direction, the network
can measure the error rate of data that was recently received and instruct the mobile
device to change its MCS accordingly. As both network and mobile device can report
the BEP very quickly, it is also possible to quickly adapt to changing signal conditions,
especially when the mobile device is in a moving car or train. This reduces the error rate
and ensures the highest transmission speed in every radio condition. In practice, it can
be observed that this control mechanism allows the use of MCS‐8 and MCS‐9 if reception
conditions are good, and a quick fallback to other MCS if radio conditions deteriorate.
In practice, transmission speeds of over 270 kbit/s can be reached with a class 32 EDGE
mobile device. Table 2.3 gives an overview of the possible modulation and coding
schemes and the datarates that can be achieved per timeslot.
Despite the ability to react quickly to changing transmission conditions, it is of course
still possible that a block contains too many errors and thus the data cannot be recon-
structed correctly. This is even desired to some extent because retransmitting a few
faulty blocks is preferred over switching to a slower coding scheme.
To preserve the continuity of the data flow on higher layers, EDGE introduces a
number of enhancements in this area as well. To correct transmission errors a
method called ‘incremental redundancy’ has been introduced. As is already the case