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.