Developing techniques are the perspective of advances .GSM ,the digital communication of Generation 2 ,GPRS i.e. General Packet Radio Service ,the Generation 2.5 data communication ,further extending ,EDGE viz., Enhanced Data for Global Evolution. EDGE introduces a new modulation technique and new channel coding that can be used to transmit both packet-switched and circuit-switched voice and data services . EDGE makes it easier for circuit-switched and packet-switched traffic to coexist while making more efficient use of the same radio resources. EDGE is specified to reuse the channel structure, channel width, channel coding and the existing mechanisms and functionality of GPRS and HSCSD. The modulation standard selected for EDGE, 8-phase shift keying (8PSK), fulfills all of those requirements. EGPRS is capable of offering data rates of 384 kbps and, theoretically, of up to 473.6 kbps.
Introduction
Regarded as a subsystem within the GSM standard,GPRS has introduced packet-switched data into GSM networks. Many new protocols and new nodes have been introduced to make this possible. EDGE is a method to increase the data rates on the radio link for GSM. Basically, EDGE only introduces a new modulation technique and new channel coding that can be used to transmit both packet-switched and circuit-switched voice and data services. EDGE is therefore an add-on to GPRS and cannot work alone.GPRS has a greater impact on the GSM system than EDGE has. By adding the new modulation and coding to GPRS and by making adjustments to the radio link protocols, EGPRS offers significantly higher throughput and capacity.
Figure 1. EGPRS introduces changes to GPRS only on the base
station system part of the network.
GPRS and EGPRS have different protocols and different behavior on the base station system side.However, on the core network side, GPRS and EGPRSshare the same packet-handling protocols and,therefore, behave in the same way. Reuse of the existing GPRS core infrastructure (serving GRPS support node/gateway GPRS support node) emphasizes the fact that EGPRS is only an “add-on” to the base station system and is therefore much easier to introduce than GPRS (Figure 1).
In addition to enhancing the throughput for each data user, EDGE also increases capacity. With EDGE, the same time slot can support more users. This decreases the number of radio resources required to support the same traffic, thus freeing up capacity for more data or voice services. EDGE makes it easier for circuit-switched and packet-switched traffic to coexist while making more efficient use of the same radio resources. Thus in tightly planned networks with limited spectrum, EDGE may also be seen as a
Capacity booster for the data traffic.
Coding schemes
For GPRS, four different coding schemes, designated CS1 through CS4, are defined. Each has different amounts of error-correcting coding that is optimized for different radio environments. For EGPRS, nine modulation coding schemes, designated MCS1 through MCS9, are introduced. These fulfill the same task as the GPRS coding schemes. The lower four EGPRS coding schemes (MSC1 to MSC4) use GMSK, whereas the upper five (MSC5 to MSC9) use 8PSK modulation. Figure 4 shows both GPRS and EGPRS
coding schemes, along with their maximum throughputs.GPRS user throughput reaches saturation at a maximum of 20 kbps with CS4, whereas the EGPRS bit rate continues to increase as the radio quality increases, until throughput reaches saturation at 59.2 kbps.
Both GPRS CS1 to CS4 and EGPRS MCS1 to MCS4 use GMSK modulation with slightly different throughput performances. This is due to differences in the header size (and payload size) of the EGPRS packets. This makes it possible to resegment EGPRS
packets. A packet sent with a higher coding scheme (less error correction) that is not properly received, can be retransmitted with a lower coding scheme (more error correction) if the new radio environment requires it. This resegmenting (retransmitting with another coding scheme) requires changes in the payload sizes of the radio blocks, which is why EGPRS and GPRS do not have the same performance for the MSKmodulated coding schemes. Resegmentation is not possible with GPRS.
8-phase shift keying; CS, Coding scheme; EGPRS, Enhanced GPRS; GMSK,
Gaussian minimum shift keying; MCS, Modulation coding scheme)
Packet handling
Another improvement that has been made to the EGPRS standard is the ability to retransmit a packetthat has not been decoded properly with a more robust coding scheme. For GPRS, resegmentation is not possible. Once packets have been sent, they must
be retransmitted using the original coding scheme even if the radio environment has changed. This has a significant impact on the throughput, as the algorithm decides the level of confidence with which the link adaptation (LA) must work.
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