Enhanced Data Rates for GSM Evolution (EDGE) – EGPRS

other of their capabilities it is possible to use the standard GMSK modulation with older

terminals and the new 8PSK modulation with new terminals at the same time in the same

cell. From the network side, the terminal is informed of the EGPRS capability of a cell by the

EGPRS 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 side, the

network is informed of the terminal’s EDGE capability during the establishment of a new

connection. Therefore, EGPRS is fully backward compatible to GPRS and allows the mixed

use of GPRS and EDGE terminals in the same cell. EDGE terminals are also able to use the

standard GMSK modulation for GPRS and can thus also be used in networks that do not offer

EDGE functionality.

Another advantage of the new modulation and the nine different coding schemes compared

to the four different coding schemes of GPRS is a precise use of the best modulation and

coding for the current radio conditions. This is done in the terminal by constantly 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 to change its MCS accordingly. As both network and

terminal can report the BEP very quickly it is possible to also quickly adapt to changing

signal conditions especially when the terminal is moving in a 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 the situation deteriorates.

Therefore, transmission speeds of over 200 kbit/s can be reached with a class 10 EDGE

terminal under real conditions. Table 2.3 gives an overview of the possible modulation and

coding schemes and the data rates 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 toomanyerrors and thus the data cannot be reconstructed correctly.

To some extent this is even desired as retransmitting a few faulty blocks is preferred over

switching to a slower coding scheme. In order to preserve the continuity of the data flow on

higher layers, EGPRS introduces a number of enhancements in this area as well. In order to

correct transmission errors a method called ‘incremental redundancy’ has been introduced. As