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  1. 1. TOPICS IN WIRELESS COMMUNICATIONSChallenges in the Migration to4G Mobile SystemsSuk Yu Hui and Kai Hau Yeung, City University of Hong Kong ABSTRACT from the users’ point of view) of 4G networks are stated as follows: With the rapid development of wireless com- • High usability: anytime, anywhere, and with munication networks, it is expected that fourth- any technology generation mobile systems will be launched • Support for multimedia services at low within decades. 4G mobile systems focus on transmission cost seamlessly integrating the existing wireless tech- • Personalization nologies including GSM, wireless LAN, and • Integrated services Bluetooth. This contrasts with 3G, which merely First, 4G networks are all-IP based heteroge- focuses on developing new standards and hard- neous networks that allow users to use any sys- ware. 4G systems will support comprehensive tem at any time and anywhere. Users carrying an and personalized services, providing stable sys- integrated terminal can use a wide range of tem performance and quality service. However, applications provided by multiple wireless net- migrating current systems to 4G presents enor- works. Second, 4G systems provide not only mous challenges. In this article these challenges telecommunications services, but also data and are discussed under the headings of mobile sta- multimedia services. To support multimedia ser- tion, system, and service aspects. Proposed solu- vices, high-data-rate services with good system tions to the research problems in each aspect reliability will be provided. At the same time, a will also be examined. low per-bit transmission cost will be maintained. Third, personalized service will be provided by INTRODUCTION this new-generation network. It is expected that when 4G services are launched, users in widely Second-generation (2G) mobile systems were different locations, occupations, and economic very successful in the previous decade. Their classes will use the services. In order to meet the success prompted the development of third-gen- demands of these diverse users, service providers eration (3G) mobile systems. While 2G systems should design personal and customized services such as GSM, IS-95, and cdmaOne were for them. Finally, 4G systems also provide facili- designed to carry speech and low-bit-rate data, ties for integrated services. Users can use multi- 3G systems were designed to provide higher- ple services from any service provider at the data-rate services. During the evolution from 2G same time. Just imagine a 4G mobile user, Mary, to 3G, a range of wireless systems, including who is looking for information on movies shown GPRS, IMT-2000, Bluetooth, WLAN, and in nearby cinemas. Her mobile may simultane- HiperLAN, have been developed. All these sys- ously connect to different wireless systems. These tems were designed independently, targeting dif- wireless systems may include a Global Position- ferent service types, data rates, and users. As ing System (GPS) (for tracking her current loca- these systems all have their own merits and tion), a wireless LAN (for receiving previews of shortcomings, there is no single system that is the movies in nearby cinemas), and a code-divi- good enough to replace all the other technolo- sion multiple access (CDMA) (for making a tele- gies. Instead of putting efforts into developing phone call to one of the cinemas). In this new radio interfaces and technologies for 4G example, Mary is actually using multiple wireless systems, which some researchers are doing, we services that differ in quality of service (QoS) believe establishing 4G systems that integrate levels, security policies, device settings, charging existing and newly developed wireless systems is methods, and applications. It will be a significant a more feasible option. revolution if such highly integrated services are Researchers are currently developing frame- made possible in 4G mobile applications. works for future 4G networks. Different research To migrate current systems to 4G with the programs, such as Mobile VCE, MIRAI, and features mentioned above, we have to face a DoCoMo, have their own visions on 4G features number of challenges. In this article these chal- and implementations. Some key features (mainly lenges are highlighted and grouped into various54 0163-6804/03/$17.00 © 2003 IEEE IEEE Communications Magazine • December 2003
  2. 2. Key challenges Proposed solutions Mobile station Multimode user To design a single user terminal that can operate in A software radio approach can be used: the user terminals different wireless networks, and overcome the design terminal adapts itself to the wireless interfaces of the problems such as limitations in device size, cost, power networks [1]. consumption, and backward compatibilities to systems. Wireless system To discover available wireless systems by processing the User- or system-initiated discoveries, with automatic discovery signals sent from different wireless systems (with different download of software modules for different wireless access protocols and incompatible with each other). systems [2]. Wireless system Every wireless system has its unique characteristic and role. The wireless system can be selected according to the selection The proliferation of wireless technologies complicates the best possible fit of user QoS requirements, available selection of the most suitable technology for a particular network resources, or user preferences [3, 4]. service at a particular time and place. System Terminal mobility To locate and update the locations of the terminals in Signaling schemes and fast handoff mechanisms are various systems. Also, to perform horizontal and vertical proposed in [5]. handoff as required with minimum handover latency and packet loss. Network To integrate the existing non-IP-based and IP-based A clear and comprehensive QoS scheme for UMTS system infrastructure systems, and to provide QoS guarantee for end-to-end has been proposed [6]. This scheme also supports and QoS support services that involves different systems. interworking with other common QoS technologies. Security The heterogeneity of wireless networks complicates the Modifications in existing security schemes may be security issue. Dynamic reconfigurable, adaptive, and applicable to heterogeneous systems. Security handoff lightweight security mechanisms should be developed. support for application sessions is also proposed [4]. Fault tolerance To minimize the failures and their potential impacts in any Fault-tolerant architectures for heterogeneous networks and survivability level of tree-like topology in wireless networks. and failure recovery protocols are proposed in [7]. Service Multi-operators To collect, manage, and store the customers’ accounting Various billing and accounting frameworks are and billing information from multiple service providers. Also, to bill proposed in [8, 9]. system the customers with simple but detailed information. Personal To provide seamless personal mobility to users without Personal mobility frameworks are proposed. Most of mobility modifying the existing servers in heterogeneous systems. them use mobile agents, but some do not [10, 11].I Table 1. A summary of key challenges and their proposed solutions.research areas. An overview of the challenges in are essential as they can adapt to different wire-future heterogeneous systems will be provided. less networks by reconfiguring themselves. ThisEach area of challenges will be examined in eliminates the need to use multiple terminalsdetail. The article is then concluded. (or multiple hardware components in a termi- nal). The most promising way of implementing multimode user terminals is to adopt the soft- AN OVERVIEW OF THE ware radio approach [1]. Figure 1 shows the design of an ideal software radio. The analog CHALLENGES IN INTEGRATING part of the receiver consists of an antenna, a HETEROGENEOUS SYSTEMS bandpass filter (BPF), and a low noise amplifier (LNA). The received analog signal is digitizedIt is convenient to discuss the challenges (and by the analog/digital converter (ADC) immedi-their proposed solutions) by grouping them into ately after the analog processing. The process-three different aspects: mobile station, system, ing in the next stage (usually still analogand service. Each of these aspects leads to sever- processing in conventional terminals) is thenal important research areas. performed by a reprogrammable baseband digi- tal signal processor (DSP). The DSP will pro- cess the digitized signal in accordance with the RESEARCH CHALLENGES wireless environment. Unfortunately, the cur-In this section each of the key research areas rent software radio technology is not completelylisted in Table 1 will be described in detail. feasible for all the different wireless networks due to the following technological problems. MOBILE STATION First, it is impossible to have just one antennaMultimode User Terminals — In order to use and one LNA to serve the wide range of fre-the large variety of services and wireless net- quency bands (i.e., to cover the bands of all 4Gworks in 4G systems, multimode user terminals wireless networks). The only solution is to useIEEE Communications Magazine • December 2003 55
  3. 3. becomes complicated in 4G heterogeneous sys- tems because of the differences in wireless tech- Analog Digital nologies and access protocols. One of the proposed solutions is to use software radio devices that can scan the available networks. After scanning, they will load the required soft- Baseband ware and reconfigure themselves for the selected BPF LNA ADC DSP network. There are a number of ways to facili- tate the downloading of software modules [2]. Figure 2 shows an example of how a multimode terminal attached to a WLAN is scanning the available wireless networks. Once the terminalI Figure 1. An ideal software radio receiver. discovers the available systems, it can download the suitable software to reconfigure the software radio. As shown, the software can be download- multiple analog parts to work in different fre- ed from media such as a PC server, smart card, quency bands. This certainly increases the design or memory card, or over the air (OTA). Each complexity and physical size of a terminal. The downloading method has its own advantages and second challenge is that existing ADCs are not disadvantages with respect to speed, accuracy, fast enough. For example, the GSM and Univer- resource usage, and convenience. OTA is the sal Mobile Telecommunications Service most challenging way to achieve wireless system (UMTS) waveforms require at least 17 bits reso- discovery, but its availability frees users from the lution with very high sampling rates (over 100 tedium of downloading. Operators will also Msamples/s). To provide such bit resolution, the enjoy simplified network management. Le and speed of the fastest current ADC is still two to Aghvami [2] proposed an OTA downloading three orders of magnitude slower than required. approach in which multimode user terminals Finally, in order to allow real-time execution of constantly monitor a predefined broadcasting software implemented radio interface functions channel (global pilot and download channel, such as frequency conversion, digital filtering, GPDCH) to check for available networks. Once spreading, and despreading, parallel DSPs have they detect a new available network, they can to be used. This also creates problems such as decide whether or not a change should be made. high circuit complexity, and high power con- As pointed out in [2], we still need to solve sumption and dissipation. problems such as the long downloading time and slow speed of the GPDCH. Wireless System Discovery — To use 4G ser- vices, multimode user terminals should be able Wireless System Selection — With the sup- to select the target wireless systems. In current port of 4G user terminals, we can choose any GSM systems, base stations periodically broad- available wireless network for each particular cast signaling messages for service subscription communication session. As every network has to mobile stations. However, this process unique features, using a suitable network for a specific service may optimize system perfor- mance and resource usage. Furthermore, the Via right network selection can ensure the QoS PC required by each session. However, it is compli- GSM server cated to select a suitable network for each com- munication session since network availability changes from time to time. Moreover, adequate GPRS knowledge of each network is required before a selection is made. This includes precise under- standing of the supported service types, system data rates, QoS requirements, communication Via costs, and user preferences. Eguchi et al. [3] pro- CDMA Scanning... smart posed a selection scheme in which Session Initia- card tion Protocol (SIP) messages, location information of the source mobile node, available networks of both mobile nodes, and user prefer- ences are all taken into account in the selection UMTS when a mobile node makes a call to another OTA Via mobile node. Other researchers also suggest that memory card network resources and minimum QoS require- WLAN ments should be considered in network selection [5]. Despite these research efforts, we believe that there are many issues to be resolved in selecting the appropriate wireless system. Available systems SYSTEM Ways to download software Terminal Mobility — In order to provide wireless services at any time and anywhere, ter-I Figure 2. A multimode terminal attaches to the WLAN and scans the avail- minal mobility is a must in 4G infrastructure. able systems. It can download suitable software manually or automatically. Terminal mobility allows mobile clients to roam56 IEEE Communications Magazine • December 2003
  4. 4. across geographic boundaries of wireless net-works. There are two main issues in terminalmobility: location management and handoffmanagement. With location management, the UMTSsystem tracks and locates a mobile terminal for coveragepossible connection. Location managementinvolves handling all the information about theroaming terminals, such as original and currentlocated cells, authentication information, andQoS capabilities. On the other hand, handoff GSMmanagement maintains ongoing communica- coveragetions when the terminal roams. Mobile IPv6(MIPv6) is a standardized IP-based mobilityprotocol for IPv6 wireless systems. In thisdesign, each terminal has an IPv6 homeaddress. Whenever the terminal moves outsidethe local network, the home address becomes WLANinvalid, and the terminal obtains a new IPv6 coverageaddress (called a care-of address) in the visitednetwork. A binding between the terminal’shome address and care-of address is updated to Vertical handoff Horizontal handoffits home agent in order to support continuouscommunications. However, this handoff processcauses an increase in system load, high han- I Figure 3. Vertical handoff and horizontal handoff of a mobile terminal.dover latency, and packet losses. Althoughsome enhanced Mobile IPv6 (MIPv6) schemeshave been proposed to solve these problems, particular wireless system in mind. For example,more needs to be done to satisfactorily over- the 3G Partnership Project (3GPP) has pro-come these problems [5]. It is even more diffi- posed a comprehensive QoS architecture forcult to solve these problems in 4G networks. UMTS. It realizes QoS in UMTS via the UMTSThe reason is that besides horizontal handoff, Bearer Service and its underlying bearer servicesvertical handoff is also needed. Figure 3 shows [6]. There are clear definitions of characteristicsan example of horizontal and vertical handoff. and functionalities of each bearer service on aHorizontal handoff is performed when the ter- specific layer. These enable the provision of aminal moves from one cell to another within contracted QoS in all aspects, including controlthe same wireless system. Vertical handoff, signaling, radio interface transport, and QoShowever, handles the terminal movement management functionality. Additionally, in orderbetween two different wireless systems (e.g., to support various services, the UMTS specifica-from WLAN to GSM). Moreover, 4G networks tion has defined QoS classes and their attributesare expected to support real-time multimedia for dealing with differentiated QoS require-services that are highly time-sensitive. It is ments. However, providing QoS only in UMTSunacceptable if the MIPv6 handoff process sig- cannot guarantee end-to-end QoS because sys-nificantly degrades system performance, espe- tems that are non-UMTS are involved. Tocially QoS performance. In addition, it is hard address this problem, internetworking with mostto decide the correct handoff time because common QoS architectures is studied in 3GPP.measuring handoffs among different wireless We believe that internetworking mechanismssystems is very complicated. Finally, the uncer- involving layer 3 (or above) operations may betain handoff completion time adds to the com- needed.plexity in designing good handoff mechanisms.To overcome these problems, researchers are Security and Privacy — Security requirementscurrently investigating new handoff decision of 2G and 3G networks have been widely stud-policies and new handoff algorithms for hetero- ied in the literature. Different standards imple-geneous networks. ment their security for their unique security requirements. For example, GSM provides high-Network Infrastructure and QoS Support — ly secured voice communications among users.Existing wireless systems can be classified into However, the existing security schemes for wire-two types: non-IP-based and IP-based. Many less systems are inadequate for 4G networks, asnon-IP-based systems are highly optimized for stated in [4]. The key concern in security designsvoice delivery (e.g., GSM, cdma2000, and for 4G networks is flexibility. As the existingUMTS). On the other hand, IP-based systems security schemes are mainly designed for specif-are usually optimized for data services (e.g., ic services, such as voice service, they may not802.11 WLAN and HiperLAN). In 4G wireless be applicable to 4G environments that will con-environments, the problem in integrating these sist of many heterogeneous systems. Moreover,two systems becomes apparent. Research chal- the key sizes and encryption and decryptionlenges such as QoS guarantee for end-to-end algorithms of existing schemes are also fixed.services need to be addressed, although they are They become inflexible when applied to differ-by no means easy to tackle, especially when ent technologies and devices (with varied capa-time-sensitive or multimedia applications are bilities, processing powers, and security needs).considered. To design flexible security systems, some Current QoS designs are usually made with a researchers are starting to consider reconfig-IEEE Communications Magazine • December 2003 57
  5. 5. There are two At 10:00 a.m., a video message At 6:00 p.m., another video At 8:30 p.m., a video message is sent to Mary. She reads message is sent to Mary. is sent to Mary again. She ways to achieve the message using her She reads the message reads the message using her desktop PC in her office. using her PDA when laptop PC at home. fault-tolerant driving her car. architectures to support QoS under failures. The first is to use a hierarchical cellular network system; the second is to use collocated or overlapping heterogeneous wireless networks. However, more work should be I Figure 4. An example of personal mobility. done in order to urable security mechanisms. As an example, cated or overlapping heterogeneous wireless build fault-tolerant Tiny SESAME is a lightweight reconfigurable networks. However, more work should be done 4G systems in security mechanism that provides security ser- in order to build fault-tolerant 4G systems in vices for multimedia or IP-based applications in both models. both models. 4G networks [6]. SERVICES Fault Tolerance and Survivability — In the Multiple Operators and Billing System — past, extensive work has been done to provide In today’s mobile market, an operator usually fault tolerance in wired networks and high- charges customers with a simple billing and speed data networks (e.g., public switched tele- accounting scheme. A flat rate based on sub- phone networks and asynchronous transfer scribed services, call durations, and transferred mode networks). These attempts have improved data volume is usually enough in many situa- the reliability, availability, and survivability of tions. However, with the increase of service the networks under study. However, there is varieties in 4G systems, more comprehensive inadequate study on the survivability of wireless billing and accounting systems are needed. Cus- access networks, even though they are more vul- tomers may no longer belong to only one oper- nerable than wired networks. A cellular wireless ator, but instead subscribe to many services network is typically designed as a tree-like from a number of service providers at the same topology that has several levels, including time. It may be very inconvenient for a cus- device, cell, switch, and network levels. One tomer to deal with multiple service providers. major weakness of this topology, though, is that Instead, a brokering service can be provided. when any level fails (in either hardware or soft- Customers do not have to waste time handling ware), all levels below will be affected. For all the financial transactions involved. To example, damage of a base station in a cell may achieve this, operators need to design new busi- cause partial or full service loss in that cell. The ness architecture, accounting processes, and situation becomes worse when multiple tree accounting data maintenance. Moreover, equal- topology networks work together in 4G systems. ization on different charging schemes is also Their fault-tolerant designs should consider needed. This is because different billing power consumption, user mobility, QoS man- schemes may be used for different types of ser- agement, security, system capacity, and link vices (e.g., charging can be based on data, time, error rates of many different wireless networks. or information). It is challenging to formulate To simplify the survivability design, Tipper et al. one single billing method that covers all the [7] propose three classes of strategies to improve billing schemes involved. Furthermore, 4G net- network survivability in different layers: preven- works support multimedia communications, tion, network design and capacity allocation, which consists of different media components and traffic management and restoration. But the with possibly different charging units. This adds work is not for 4G networks, so it remains to be difficulty to the task of designing a good charg- seen whether these strategies are applicable to ing scheme for all customers [8]. Besides, the 4G situations. There are two ways to achieve media components may have different QoS fault-tolerant architectures to support QoS in requirements. It is very complicated to decide a failures [6]. The first is to use a hierarchical cel- good tariff for all the possible components. In lular network system; the second is to use collo- order to build a structural billing system for 4G58 IEEE Communications Magazine • December 2003
  6. 6. networks, several frameworks have already include wireless system selection, security, fail-been studied. The requirements on these frame- ure, and survivability. Moreover, work on the In addition toworks include scalability, flexibility, stability, implementations of personal mobility, billing,accuracy, and usability [9]. and accounting systems are also needed in 4G terminal mobility, networks. The discussion in this article not personal mobilityPersonal Mobility — In addition to terminal only shows that there is much work to be donemobility, personal mobility is a concern in mobil- in the migration to 4G systems, but also high- is another concernity management. Personal mobility concentrates lights that current systems must be implement-on the movement of users instead of users’ ter- ed with a view to facilitate a seamless in mobilityminals, and involves the provision of personal integration into 4G infrastructure. Without management.communications and personalized operating these infrastructures, 4G services will not beenvironments. Figure 4 demonstrates the con- launched easily. Personal mobilitycept of personal communications using a person- ACKNOWLEDGMENT concentrates onalized video message application. As shown inthe figure, when there is a video message The authors are grateful to Professor David Hill, the movement ofaddressed to the mobile user, no matter where Dr. K. T. Ko and Miss Judy Keung for theirthe user is located or what kind of terminal is helpful comments on the article. users instead ofbeing used, the message will be sent to the user users’ terminals,correctly. A personalized operating environment, REFERENCESon the other hand, is a service that enables and involves the [1] E. Buracchini, “The Software Radio Concept,” IEEE Com-adaptable service presentations (in order to fit mun. Mag., vol. 38, no. 9, 2000, pp. 138–43. provision ofthe capabilities of the terminals in use regardless [2] T. H. Le and A. H. Aghvami, “Performance of an Access-of network types). Currently, there are several ing and Allocation Scheme for the Download Channel personalframeworks on personal mobility found in the in Software Radio,” Proc. IEEE Wireless Commun. andliterature. Mobile-agent-based infrastructure is Net. Conf., vol. 2, 2000, pp. 517–21. communications [3] H. Eguchi, M. Nakajima, and G. Wu, “Signaling Schemesone widely studied solution [10, 11]. In this over a Dedicated Wireless Signaling System in the Hetero- and personalizedinfrastructure, each user is usually assigned a geneous Network,” Proc. IEEE VTC, Spring 2002, pp.unique identifier and served by some personal 464–67. operating [4] J. Al-Muhtadi, D. Mickunas, and R. Campbell, “Amobile agents (or specialized computer pro- Lightweight Reconfigurable Security Mechanism for environments.grams running on some servers). These agents 3G/4G Mobile Devices,” IEEE Wireless Commun., vol. 9,act as intermediaries between the user and the no. 2, Apr. 2002, pp. 60–65.Internet. A user also belongs to a home network [5] N. Montavont and T. Noel, “Handover Management for Mobile Nodes in IPv6 Networks,” IEEE Commun. Mag.,that has servers with the updated user profile vol. 40, no. 8, Aug. 2002, pp. 38–43.(including the current location of the user’s [6] 3GPP TS 23.107 v.5.9.0, “Quality of Service (QoS) Con-agents, user’s preferences, and currently used cept and Architecture,” June 2003.device descriptions). When the user moves from [7] D. Tipper et al., “Providing Fault Tolerance in Wireless Access Networks,” IEEE Commun. Mag., vol. 40, no. 1,his/her home network to a visiting network, Jan. 2002, pp. 58–64.his/her agents will migrate to the new network. [8] F. Ghys and A. Vaaraniemi, “Component-based Charg-Referring to the example shown in Fig. 4, when ing in a Next-generation Multimedia Network,” IEEEsomebody makes a call request to Mary, the Commun. Mag., vol. 41, no. 1, Jan. 2003, pp. 99–102. [9] J. Fleck, “A Distributed Near Real-time Billing Environment,”caller’s agent first locates Mary’s agent by mak- Telecommun. Info. Net. Architecture, 1999, pp. 142–48.ing a location request to her home network. By [10] A. D. Stefano and C. Santoro, “NetChaser: Agent Sup-looking up Mary’s profile, her home network port for Personal Mobility,” IEEE Internet Comp., vol. 4,sends back the location of Mary’s agent to the no. 2, Mar./Apr. 2000, pp. 74–79. [11] B. Thai and A. Seneviratne, “IPMoA: Integrated Person-caller’s agent. Once the caller’s agent identifies al Mobility Architecture,” Comp. and Commun., 2001,Mary’s location, the caller’s agent can directly pp. 485–90.communicate with her agent. Different agentsmay be used for different services. A mobile-agent-based infrastructure proposed in [10] uses BIOGRAPHIESfour assistants (user assistant, HTTP assistant, SUK YU HUI (syhui@ee.cityu.edu.hk) is an M.Phil. candidate in the Department of Electronic Engineering, City Universitymail assistant, and FTP assistant) to personalize of Hong Kong. Her research interests include mobile com-user operating environments. However, there are munication systems and wireless local area networks. Sheother personal mobility frameworks that do not received a B.Sc. in information technology from City Uni-rely on mobile agents. versity of Hong Kong. KAI HAU YEUNG [M] (eeayeung@cityu.edu.hk) is an associate CONCLUSION professor in the Department of Electronic Engineering, City University of Hong Kong. His research interests includeIn this article research challenges in the migra- Internet infrastructure security, mobile communication sys- tems, and Internet caching systems. He is also an activetion to 4G networks are studied and described. industry consultant in the areas of computer networkingThe challenges are grouped into three aspects: and communication systems. From 1996 to 1998, he wasmobile station, system, and service. Some of involved in a project to develop a 900 MHz GSM mobilethe challenges are well studied, such as multi- handset. The project team in City University successfully developed a handset prototype for a listed company inmode user terminals, wireless system discovery, Hong Kong. He is a member of the ACM, and BCS. He isterminal mobility, and QoS support. On the also a Cisco Certified Network Professional and Cisco Certi-other hand, others are less studied. These fied Academy Instructor.IEEE Communications Magazine • December 2003 59