Second-generation
(2G) mobile systems were very successful in the previous decade.
Their success prompted the development of third generation (3G)
mobile systems. While 2G systems such as GSM, IS-95, and cdmaOne
were designed to carry speech and low-bit-rate data, 3G systems
were designed to provide higher-data-rate services. During the
evolution from 2G to 3G, a range of wireless systems, including
GPRS, IMT-2000, Bluetooth, WLAN, and HiperLAN, have been developed.
All these systems were designed independently, targeting different
service types, data rates, and users. As all these systems have
their own merits and shortcomings, there is no single system that
is good enough to replace all the other technologies. Instead
of putting efforts into developing new radio interfaces and technologies
for 4G systems, which some researchers are doing, we believe establishing
4G systems that integrate existing and newly developed wireless
systems is a more feasible option.
Researchers
are currently developing frameworks for future 4G networks. Different
research programs, such as Mobile VCE, MIRAI, and DoCoMo, have
their own visions on 4G features and implementations. Some key
features (mainly from user's point of view) of 4G networks are
stated as follows:
" High usability: anytime, anywhere, and with any technology
" Support for multimedia services at low transmission cost
" Personalization
" Integrated services
First, 4G networks are all IP based heterogeneous networks that
allow users to use any system at any time and anywhere. Users
carrying an integrated terminal can use a wide range of applications
provided by multiple wireless networks.
Second, 4G
systems provide not only telecommunications services, but also
data and multimedia services. To support multimedia services,
high-data-rate services with good system reliability will be provided.
At the same time, a low per-bit transmission cost will be maintained.
Third, personalized
service will be provided by this new-generation network. It is
expected that when 4G services are launched, users in widely different
locations, occupations, and economic classes will use the services.
In order to meet the demands of these diverse users, service providers
should design personal and customized services for them.
Finally,
4G systems also provide facilities for integrated services. Users
can use multiple services from any service provider at the same
time. Just imagine a 4G mobile user, Mary, who is looking for
information on movies shown in nearby cinemas. Her mobile may
simultaneously connect to different wireless systems. These wireless
systems may include a Global Positioning System (GPS) (for tracking
her current location), a wireless LAN (for receiving previews
of the movies in nearby cinemas), and a code-division multiple
access (CDMA) (for making a telephone call to one of the cinemas).
In this example Mary is actually using multiple wireless services
that differ in quality of service (QoS) levels, security policies,
device settings, charging methods and applications. It will be
a significant revolution if such highly integrated services are
made possible in 4G mobile applications.
To migrate
current systems to 4G with the features mentioned above, we have
to face a number of challenges. In this article these challenges
are highlighted and grouped into various research areas. An overview
of the challenges in future heterogeneous systems will be provided.
Each area of challenges will be examined in detail. The article
is then concluded.