GSM EDGE Tutorial

- GSM EDGE, Enhanced Data rates for GSM Evolution, was the evolution of GSM, & GPRS which used 8PSK modulation to achieve data transfer rates up to 384 kbps.

EDGE is an evolution to the GSM mobile cellular phone system. The name EDGE stands for Enhanced Data rates for GSM Evolution and it enables data to be sent over a GSM TDMA system at speeds up to 384 kbps. In some instances GSM EDGE evolution systems may also be known as EGPRS, or Enhanced General Packet Radio Service systems. Although strictly speaking a "2.5G" system, the GSM EDGE cellular technology is capable of providing data rates that are a distinct increase on those that could be supported by GPRS.

EDGE evolution 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 evolution changes the modulation to 8PSK and thereby enabling a significant increase in data rate to be achieved.

What is EDGE? - the basics

GSM EDGE cellular technology is an upgrade to the existing GSM / GPRS networks, and can often be implemented as a software upgrade to existing GSM / GPRS networks. This makes it a particularly attractive option proving virtually 3G data rates for a small upgrade to an existing GPRS network.

GSM EDGE evolution can provide data rates of up to 384 kbps, and this means that it offers a significantly higher data rate than GPRS.

There are a number of key elements in the upgrade from GSM or GPRS to EDGE. The GSM EDGE technology requires a number of new elements to be added to the system:

  • Use of 8PSK modulation:   In order to achieve the higher data rates within GSM EDGE, the modulation format can be changed from GMSK to 8PSK. This provides a significant advantage in being able to convey 3 bits per symbol, thereby increasing the maximum data rate. This upgrade requires a change to the base station. Sometimes hardware upgrades may be required, although it is often simply a software change.
  • Base station:   Apart from the upgrade to incorporate the 8PSK modulation capability, other small changes are required to the base station. These are normally relatively small and can often be accomplished by software upgrades.
  • Upgrade to network architecture:   GSM EDGE provides the capability for IP based data transfer. As a result, additional network elements are required. These are the same as those needed for GPRS and later for UMTS. In this way the introduction of EDGE technology is part of the overall migration path from GSM to UMTS.

    The two main additional nodes required for the network are the Gateway GPRS Service Node (GGSN) and the Serving GPRS Service Node (SGSN). 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.
  • Mobile stations:   It is necessary to have a GSM EDGE handset that is EDGE compatible. As it is not possible to upgrade handsets, this means that the user needs to buy a new GSM EDGE handset.

Despite the number of changes that need to be made, the cost of the upgrade to move to GSM EDGE cellular technology is normally relatively small. The elements in the core network are required for GPRS which may already be available on the network, and hence these elements will already be present. The new network entities are also needed for UMTS and therefore they are on the overall upgrade and migration path. Other changes to the base stations are comparatively small and can often be achieved very easily.

GSM EDGE evolution specification overview

It is worth summarizing the key parameters of GSM EDGE cellular technology.

Parameter Details
Multiple Access Technology FDMA / TDMA
Duplex Technique FDD
Channel Spacing 200 kHz
Modulation GMSK, 8PSK
Slots per channel 8
Frame duration 4.615 ms
Latency Below 100 ms
Overall symbol rate 270 k symbols / s
Overall modulation bit rate 810 kbps
Radio data rate per time slot 69.2 kbps
Max user data rate per time slot 59.2 kbps (MCS-9)
Max user data rate when using 8 time slots 473.6 kbps **

GSM EDGE specification highlights

  **   A maximum user data rate of 384 kbps is often seen quoted as the data rate for GSM EDGE. This data rate corresponds to the International Telecommunications Union (ITU) definition of the data rate limit required for a service to fulfill the International Mobile Telecommunications-2000 (IMT-2000) standard(i.e. 3G) in a pedestrian environment.

By Ian Poole

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