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Mobile
users continue to demand higher data rates. With the continued growth in
cellular services, laptop computer use and the Internet, wireless network
providers are beginning to pay an increasing amount of attention to packet data
networks. Enhanced Global Packet Radio Service (EGPRS) offers a substantial
improvement in performance and capacity over existing GPRS services, in return
for a relatively minimal additional investment. EGPRS, commonly called EDGE,
achieves these enhancements to the GPRS system primarily by implementing
changes to the Physical layer and to the Medium Access Control/Radio Link
Control (MAC/RLC) layer. The significant improvements are a new modulation
technique, additional modulation coding schemes, a combined Link Adaptation and
Incremental Redundancy technique, re-segmentation of erroneously received
packets, and a larger transmission window size.
Technical Differences
Between GPRS And EGPRS
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.
Measurement Accuracy
As
in the GSM environment, GPRS measures the radio environment by analyzing the
channel for carrier strength, bit error rate, etc. Performing these
measurements takes time for a mobile station, which is of no concern in the
speech world as the same coding is used all the time. In a packet-switched
environment, it is essential to analyze the radio link quickly in order to
adapt the coding toward the new environment. The channel analysis procedure
that is used for GPRS makes the selection of the right coding scheme difficult
since measurements for interference are performed only during idle bursts. As a
result, measurements can only be performed twice during a 240-millisecond
period.
For
EGPRS, the standard does not rely on the same "slow" measurement
mechanism. Measurements are taken on each and every burst within the equalizer
of the terminal, resulting in an estimate of the bit error probability (BEP).
Estimated for every burst, the BEP is a reflection of the current C/I, the time
dispersion of the signal and the velocity of the terminal. The variation of the
BEP value over several bursts will also provide additional information
regarding velocity and frequency hopping. A very accurate estimation of the BEP
is then possible to achieve.
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 shows
both GPRS and EGPRS coding schemes, along with their maximum throughputs.
EDGE Standard And
References
The
EDGE base station system work item provides a platform to employ new modulation
techniques, whereas the EDGE network support subsystem work item defines the
network changes to facilitate the physical layer. According to the work item
descriptions, EDGE will provide two phases: Phase 1: Single- and multislot
packet-switched services and single and multislot circuit switched services
Phase 2: Real-time services employing the new modulation
techniques that
are not included in Phase 1.
Phase
1 has been completed with 3GPP Release 99. Phase 2 is ongoing in the 3GPP
standardization, and its scope has been extended to cover the alignment with
WCDMA and the provisioning of Internet protocol (IP) multimedia. This concept,
currently standardized in 3GPP, is known as GERAN.
Conclusion
This
paper has presented an overview of EDGE with particular focus on the physical
layer and the data link layer. The goal of EDGE is to provide a packet data network that provides operating rates that are of adequate speed for most
applications. EDGE achieves this increase in throughput rate mainly through
enhancements to the physical layer and the RLC/MAC layer of the GPRS system.
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