Evolution from GSM to the 3rd Generation Review

GSM has been designed, above all, for speech communication. Although the standard also
offers data services, their possibilities are limited by a data rate of 9.6 kbps. To meet the
growing demands of increasingly complex data applications, e.g. in the multimedia or
Internet sector, higher data rates are necessary. Therefore, the system's capacity is
constantly being enhanced.

HSCSD

This innovation gives the bit rates an enormous boost. HSCSD is circuit-switched and
optimizes the existing transmission rates in two respects:
Firstly, a new channel coding method, available in GSM Phase 2+ in single circuit-switched
data, increases the data rate from 9.6 kbps to 14.4.
Secondly, the bundling of up to 4 timeslots can increase the user data rate to 57.6 kbps. This
allows, for example,
• Faster e-mail transfer
• Faster file transfer, and
• Speedier and thus cheaper webbrowsing and data download from the Internet.
As most of the services used require higher data rates in the downlink than in the uplink,
HSCSD really shows an asymmetrical implementation, e.g. 3 timeslots in the downlink and 1
in the uplink. This also facilitates mobile station design and avoids battery capacity problems.

GPRS

In contrast to HSCSD, the General Packet Radio Service GPRS is packet-switched instead
of circuit-switched. The radio network resources are only used if data is actually being
transmitted.
Thus, billing is no longer based on the duration of the call, but on the amount of transmitted
data. Additionally, the type of data service can be charged, for example web browsing or
WAP access.
By bundling up to 8 channels, a data rate of up to 171.2 kbps can be achieved, with up to 8
subscribers per channel. The more subscribers transmit data within a cell, the more the
datarate available to each individual user is reduced. GPRS requires some modifications to
the existing GSM infrastructure. Together with HSCSD, GPRS is a further step towards
mobile multimedia.

EDGE

Enhanced Data Rates for Global Evolution (EDGE) is a technology concentrating on the air
interface between the mobile and the base station. Based on a new modulation process, 8
Phase Shift Keying, or 8-PSK, EDGE achieves three times the data rates of HSCSD and
GPRS at the air interface. Furthermore, with EDGE, the subscriber can use all 8 timeslots at
the air interface. In contrast to GSM, a digital data unit at the air interface, a symbol, is not
just one bit, but three bits.
This technology allows data rates of almost 474 kbps per user. On the other hand, EDGE is
very bit error sensitive and requires careful planning and a sufficient number of base stations.

UMTS

The Universal Mobile Telecommunications System (UMTS), represents the 3rd generation of
mobile communication. It achieves bit rates of up to 2 Mbit/s if the mobile is static and no
other user is transferring any data. Realistic situations will allow a maximum of 144 or 384
kbps. The introduction of UMTS won't make GSM unnecessary. Simple services like speech
will be dealt with by GSM in the future, whereas high data rate multimedia services and
mobile office applications will be handled by UMTS. UMTS mainly affects the air interface,
where a broadband transmission method achieves high transmission speeds. This requires
considerable changes to the network architecture. Thus, especially at the beginning, a close
cooperation with the existing GSM network structure is necessary to guarantee that services
are supplied area-wide.