Enhanced data rates for GSM evolution (EDGE) is a major
enhancement to the GSM data rates.
Available data rates (GSM) can be increased from 9.6 kbps up to
64 kbps (HSCSD) and 160 kbps (GPRS).
EDGE is specified in a way that will enhance the throughput per
timeslot for both HSCSD and GPRS. The enhancement of HSCSD is
called ECSD (enhanced circuit switched data), whereas the
enhancement of GPRS is called EGPRS (enhanced general packet
radio service). In ECSD, the maximum data rate will not increase
from 64 kbps because of the restrictions in the A-interface, but
the data rate per timeslot will triple. Similarly, in EGPRS, the data
rate per timeslot will triple and the peak throughput, with all
eight timeslots in the radio interface, will reach 473 kbps.
8-PSK Modulation in GSM/EDGE
The ‘enhancement’ of tripling the data rates is the introduction
of the 8-PSK (octagonal phase shift keying) modulation in
addition to the existing Gaussian minimum shift keying.
An 8-PSK signal is able to carry 3 bits per modulated symbol over
the radio path, while a GMSK signal carries only 1 bit per symbol
The increase in data throughput does not come for free, the
price being paid in the decreased sensitivity of the 8-PSK signal.
This affects, e.g. the radio network planning, and the highest
data rates can only be provided with limited Coverage.
8-PSK Modulation in GSM/EDGE
The bit rate in the table is the gross bit rate.
Part of the bits are used for error correction.
EDGE is specified in a way that will
enhance the throughput per
timeslot for both HSCSD and GPRS.
The enhancement of HSCSD is called
ECSD (enhanced circuit switched
data), whereas the enhancement of
GPRS is called EGPRS (enhanced
general packet radio service).
EGPRS (enhanced general packet radio service)
EGPRS is built on top of GPRS, which is the packetswitched data service of GSM.
It includes nine different modulation and coding
schemes (MCS-1–MCS-9) as well as related signalling
and other procedures for link adaptation (switching
between different MCSs).
link adaptation also known as type 1 hybrid ARQ
Link adaptation refers to a set of techniques where
modulation, coding rate and/or other signal transmission
parameters are changed on the fly to better adjust to the
changing channel conditions.
For example, EDGE uses a rate adaptation algorithm that
adapts the modulation and coding scheme (MCS)
according to the quality of the radio channel, and thus the
bit rate and robustness of data transmission.
Higher data rates can be offered with limited coverage
and, therefore, the link adaptation mechanism becomes
essential. Switching between 8-PSK- and GMSK-based
channels is done with the standard intra-cell handover
Table shows the EGPRS modulation and coding
schemes (MCS) and their data throughputs. New
GMSK coding schemes (MCS-1–MCS-4), different
from GPRS GMSK coding schemes (CS-1–CS-4) are
needed for the incremental redundancy.
In incremental redundancy, also known as Type II
Hybrid Automatic Repeat Request, retransmission of
the data block is different to the initial transmission.
Upon reception failure, additional redundancy is
included which is combined with the earlier
(re)transmission(s) and can thus be used to correct
EGPRS (enhanced general packet radio service)
in EGPRS, the data rate per timeslot will triple and the peak throughput,
with all eight timeslots in the radio interface, will reach 473 kbps (59.2 x 3).
HSCSD data rates
HSCSD provides different levels of possible error
correction which can be used according to the quality
of the radio link. This means that in the best
conditions 14.4 kbit/s can be put through a single
The other innovation in HSCSD is the ability to use
multiple time slots at the same time. Using the
maximum of four time slots, this can provide an
increase in maximum transfer rate of up to 57.6 kbit/s
(i.e., 4 × 14.4 kbit/s).
ECSD (enhanced circuit switched data)
ECSD uses current HSCSD as a basis.
In ECSD, the maximum data rate will not increase
from 64 kbps because of the restrictions in the
A- interface, but the data rate per timeslot will triple.
Data rates to be provided with ECSD, although limited
to 64 kbps, are still sufficient for providing various
transparent and non-transparent services.
Transparent bearer services use only the functions of the physical
layer (layer 1 ISO/OSI reference model) to transmit data; data
transmission consequently has a constant delay and throughput,
that is if no errors occur. The only mechanism of any use to try and
increase the quality of the transmission is forward error correction
(FEC). This mechanism codes redundancy into the data-stream and
Depending on the FEC, data rate of 2.4, 4.8, or 9.6 kbit/s are
Non-Transparent bearer services use protocols of the layers two and
three to implement error correction and flow control. Nontransparent bearer services use the transparent bearer services,
while adding a radio link protocol (RLP). This protocol uses
mechanisms of high-level data link control (HDLC), and special
selective-reject mechanisms to trigger retransmission of erroneous
data. The achieved bit error rate is less than 10-7, but now
throughput may vary, this depending on the transmission quality.
ECSD enables inter-working with audio modems at
higher data rates than in current GSM networks,
inter-working with ISDN at various data rates and
various video-based services ranging from still image
transfer to videoconferencing services.
EDGE vs GPRS
General Packet Radio Service (GPRS)
Extends GPRS data with adaptive modulation
Less spectral efficiency
2x spect. Eff. of GPRS for best effort data
Same GMSK modulation as GSM
4 channel coding modes
Adaptive coding techniques
Packet-mode supports up to 115 kbps
Supports peak rates over 384 kbps
Flexible time slot allocation (1-8)
Smart antennas & adaptive antennas
Radio resources shared dynamically
between speech and data services
N/w based intelligent resource assignment
Independent uplink and downlink
Downlink speeds at several Mbps based on
wideband OFDM and/or multiple virtual
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
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
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 packetswitched networks such as the Internet and other GPRS
networks. The SGSN provides the packet-switched link to
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.