The enhanced data rates for GSM evolution (EDGE) provides an evolutionary path from existing 2G systems (GSM, IS-136, PDC) to deliver some 3G services in existing spectrum bands. The advantages of EDGE include fast availability, reuse of existing GSM and other technologies like IS-136 and PDC infrastructure, as well as support for the gradual introduction of 3G capabilities. Hence EDGE is considered to be a technology of 2.75 G. EDGE is primarily a radio interface improvement, but it can also be viewed as a system concept that allows GSM, IS-136, and PDC networks to offer a set of new services.

1) Air interface Specifications

GSM EDGE evolution was able to provide data rates of up to 384 kbps, and this meant that it offered a significantly higher data rate than GPRS.

There were a number of key elements in the evolution from GSM or GPRS to EDGE. The GSM EDGE technology required 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 was changed from GMSK to 8PSK. This provided a significant advantage in being able to convey 3 bits per symbol as compared to 1 bit/symbol in GMSK, thereby increasing the maximum data rate. This upgrade required a change to the base station.

• Channel coding schemes: Apart from the upgrade to incorporate the 8-PSK modulation capability, other small changes were required to the Base Station. These were normally relatively small and could often be accomplished by software upgrades. GPRS used 9 modulation coding schemes named as MCS -1 to MCS -9 (as illustrated in Table 2 ). MCS1 would be used in cases where in the signal was prone to maximum fading. More number of error correction bits and channel coding bits had to be added which decreased the data rate. MCS-9 was used in cases wherein fading effects were minimum.

• Link quality variations: A link adaptation scheme regularly estimates the link quality and subsequently selects the most appropriate modulation and coding scheme for the transmission to maximize the user bit rate. The link adaptation scheme offers mechanisms for choosing the best modulation and coding scheme for the radio link. In GPRS only the coding schemes can be changed between two consecutive link layer control (LLC) frames. In the EGPRS even the modulation can be changed. Different coding and modulation schemes enable adjustment for the robustness of the transmission according to the environment.

  EDGE has been designed to improve S/I by using link quality control. Link quality control adapts the protection of the data to the channel quality so that for all channel qualities an optimal bit rate is achieved. Thus, it is capable of giving a variable data rate depending on the quality of radio link using strong link adaptation algorithms.

• Incremental Redundancy: Another way to handle link quality variations is incremental redundancy. In this scheme, information is first sent with very little coding, yielding a high bit rate if decoding is immediately successful. If decoding is not successful, additional coded bits (redundancy) are sent until decoding succeeds. The more coding that has to be sent, the lower the resulting bit rate and the higher the delay. EDGE supports combined link adaptation and incremental redundancy schemes. In this case, the initial code rate of the incremental redundancy scheme is based on measurements of the link quality. Benefits of this approach are the robustness and high throughput of the incremental redundancy operation in combination with lower delays and lower memory requirements enabled by the adaptive initial code rate.

  Actual performance of modulation and coding scheme together with channel characteristics form the basis for link adaptation. Channel characteristics are needed to estimate the effects of a switch to another modulation and coding combination and include an estimated S/I ratio, but also time dispersion and fading characteristics (that affect the efficiency of interleaving).

• Retransmission: One of the main difference between GPRS and EDGE is that in former retransmissions with another Coding scheme is not possible while in latter it is possible. Below table summarizes all the specifications defined for GSM EDGE.

EDGE can be seen as a generic air interface for efficiently providing high bit rates, facilitating an evolution of existing 2G systems toward 3G systems. The modulation scheme based on 8- PSK is used to increase the gross bit rate. GMSK modulation as defined in GSM is also part of the EDGE system. The symbol rate is 271 ksps for both GMSK and 8-PSK, leading to gross bit rates per time slot of 22.8 kbps and 69.2 kbps, respectively. The 8-PSK pulse shape is linearized by GMSK to allow 8-PSK to fit into the GSM spectrum mask. The 8-PSK burst format is similar to GSM as shown in Figure 6. EDGE reuses the GSM carrier bandwidth and time slot structure.

2) Network architecture

Upgrade to network architecture: GSM EDGE provided the capability for IP based data transfer. As a result, additional network elements were required. Since EDGE is packet switched technology, these were 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 were the Gateway GPRS Service Node (GGSN) and the Serving GPRS Service Node (SGSN). The GGSN connected to packet switched networks such as the Internet and other GPRS networks. The SGSN provided the packet-switched link to mobile stations.

Mobile stations: It was necessary to have a GSM EDGE handset that is EDGE compatible. As it was not possible to upgrade handsets, this meant that the user had 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 was normally relatively small. The elements in the network required for GPRS may already have been in place and therefore not needed fort he EGE upgrade.

The new network entities were also needed for 3G UMTS and therefore they were on the overall evolution path. Other changes to the base stations were comparatively small and could often be achieved very easily.