An overview of EDGE

- the 2.5 G development for GSM

EDGE is an enhancement to the GSM mobile cellular phone system. The name EDGE stands for Enhanced Data for GSM Evolution and it enables data to be sent over a GSM TDMA system at speeds up to 384 kbps. In some instances EDGE systems may also be known as EGPRS, or Enhanced General Packet Radio Service systems. Although strictly speaking a "2.5G" system, it is anticipated that it will be used to provide data services by operators who have not been able to secure the full 3G licences.

EDGE is intended to build on the enhancements provided by the addition of GPRS (General Packet Radio Service) where packet switching is applied to a network. It then enables a three fold increase in the speed at which data can be transferred by adopting a new form of modulation. GSM uses a form of modulation known as Gaussian Minimum Shift Keying (GMSK), but EDGE changes the modulation to 8PSK and thereby enabling a significant increase in data rate to be achieved.

Technical Overview
It is generally expected that EDGE will be applied to networks where the enhancements provided by GPRS have already been added. Under the original GSM system, a circuit would be allocated to a given user whether data was being transmitted or not. This was fine for voice communications because there would normally be some data present for most of the time. However this is not the case for data transmissions where high levels of data are transmitted in short bursts. TO make more efficient use of the available capability, packet switching is used. Here individual packets of data are routed to the user, enabling the channel or channels to be shared by several users.

To achieve this requires the addition of two additional nodes to the network, namely the Gateway GPRS Service Node (GGSN) and the Serving GPRS Service Node (SGSN). Here the GGSN connects to packet-switched networks such as the Internet and other GPRS networks. The SGSN provides the packet-switched link to mobile stations.

In terms of implementation EDGE systems require an EDGE transceiver unit to be added to each cell along with software upgrades to allow its use. This software upgrades may be implemented remotely. This change means that the inclusion of EDGE onto a network requires a significant investment in the infrastructure and as a result it is these upgrades will normally be implemented over a period of time. However GSM, GPRS and EDGE can all co-exist on the same network.

As both GPRS and EDGE represent significant upgrades to handsets and they are not just software upgrades, new mobile handsets are required.

Modulation
One of the key elements of EDGE is the form of modulation that is used. GPRS, being essentially a packet switched version of GSM uses GMSK, along with GSM itself. This form of modulation limits the data rate that can be transmitted over the air interface. EDGE uses a form of modulation known as 8 PSK. This is a form of phase shift keying where 8 phase states are used. The advantage is that it can transmit high data rates, although it is not as immune to interference and noise. The network therefore switches to 8PSK to allow the high data transfer rates when signal strengths are sufficient to permit the data transfer with a sufficiently low Bit Error Rate. By using 8PSK it is possible to transfer data at 48 kbps per channel rather than 9.6 kbps that is possible using GMSK. By allowing the use of multiple channels the technology allows the transfer of data at rates up to 384 kbps. However it should be remembered that these data transfer rates are only possible when the network is not highly loaded as access to all the channels would not be allowed.