Access Network Evolution Beyond Third Generation Mobile Communications

Transcription

1 BEYOND THIRD GENERATION SYSTEMS Access Network Evolution Beyond Third Generation Mobile Communications Werner Mohr and Walter Konhäuser, Siemens Editorial liaison: R. Prasad. ABSTRACT Second-generation mobile radio systems have been deployed successfully worldwide. These systems have evolved to higher data rates and packet transmission. Third-generation mobile radio systems are currently being standardized worldwide to be initially deployed in 2001 and 2002 in different regions of the world. New advanced multimedia services are under development, and first services are already being offered in second-generation systems, which will provide new business opportunities. Already today discussion is starting on the development of systems beyond third-generation mobile radio systems due to the long timeframe for system specification and international standardization. However, today there is no clear vision available on such systems. This discussion takes into account the new deregulated and liberalized communication environment. This article presents a concept for a system beyond third-generation mobile radio systems, which comprises a combination of several optimized access systems in a common IP-based medium access and core network platform. These different access systems will interwork through horizontal and vertical handover, service negotiation, and global roaming. The different access systems are allocated to different mutually complementing cell layers in the sense of hierarchical cells with respect to cell size, coverage, and mobility to provide globally optimized seamless services to users. This vision requires extensive international research and standardization activities to solve many technical challenges. Key issues are the global interworking of different access systems on a common platform, advanced antenna concepts, and the implementation of multimode and multiband terminals and base stations through software-defined radio concepts. THE DEVELOPMENT OF MOBILE RADIO SYSTEMS Mobile radio communication systems have been successfully deployed since about 1980 in different regions of the world to extend telephone services to mobile users. In the first generation, analog systems were developed to support mainly voice telephony. These narrowband systems are based on frequency-division duplex (FDD) and frequency modulation. In the beginning of mobile communications there was only home network roaming; as a first step forward, national roaming was required by the national monopoly network operators. The main representatives of first-generation systems are Advanced Mobile Phone Service (AMPS) at 800 MHz, Total Access Communication Standard (TACS) at 900 MHz, NMT-450 and NMT-900, and C-Net in the 450 MHz frequency range [1; 2, pp ]. In different European countries different analog systems have been deployed without international roaming. Due to smaller markets for the different analog systems in Europe than the big national AMPS market in the United States and the limitation of national roaming, the Conférence Européenne des Administrations des Postes et des Télécommunications (CEPT) decided in 1982 to develop a pan-european mobile radio system for international roaming. This resulted in the second-generation Global System for Mobile Communications (GSM), which has been deployed since The radio interface is based on time-division multiple access (TDMA), digital modulation, forward error correction and digital signal. Its main objective was to support voice telephony and international roaming. Therefore, GSM was designed as a narrowband FDD system. The main step forward was the introduction of digital technology for similar services as in analog systems. In addition to GSM, other digital systems were developed, such as the code-division multiple access (CDMA)- based IS-95 system and TDMA-based IS-54, ANSI-136, and Personal Digital Cellular (PDC) systems. The deployment of these systems started later than GSM. Today, GSM is the dominant second-generation standard in the world with significantly more than 250 million subscribers in about 120 countries. Due to the introduction of these second-generation systems, the penetration of analog systems is decreasing. Based on the big worldwide footprint of GSM, this system is being further developed to offer more advanced data services (Fig. 1) /00/$ IEEE IEEE Communications Magazine December 2000

2 Fax/data SMS 9.6 Kb/s 1993 Evolution of mobile data services Data compression (2.5 x 9.6 Kb/s) 1996 Mobile Internet access 1998 Figure 1. Evolution of the GSM system toward the third generation. GSM phase 2+ supports, in addition to voice, fax and enhanced data rate services; compression techniques and half rate voice coding enable the transmission of higher source data rate services and increases system capacity. The first simple mobile Internet applications were introduced in More advanced techniques such as high-speed circuit-switched data (HSCSD) and General Packet Radio Service (GPRS) are increasing the user data rate by pooling time slots and changed coding schemes. A further step in the GSM evolution toward higher data rates is the enhanced data rates for GSM evolution (EDGE) system with a new offset 8-phase shift keying (PSK) modulation. The main objectives in the beginning of GSM deployment were coverage, increasing system capacity with increasing numbers of subscribers, and network optimization. Basic supported services are voice and circuit-switched data. In GSM phase 2, advanced network features such as micro base stations, dual band operation, and half-rate voice coding increased network capacity, and enhanced full-rate voice quality is supported. In GSM phase 2+ features like GPRS and HSCSD enable higher data rates. CAMEL introduced intelligent network (IN) functionality. Similar developments can also be observed in the evolution of other second-generation systems like IS-95 to higher data rates and ANSI-136 with EDGE. The evolution of both ANSI-136 and GSM leads to EDGE. Despite these steps in the evolution of second-generation systems, third-generation mobile radio systems International Mobile Telecommunications in 2000 (IMT-2000) in the International Telecommunication Union (ITU) and Universal Mobile Telecommunications Service () in Europe are currently being standardized and developed worldwide. These activities started around Their main goals are to support broadband data services up to 2 Mb/s with a wideband radio interface, and international roaming for circuit-switched and packet-oriented services. These systems aim to HSCSD 28.8 Kb/s (+ data compression) GPRS pilot system 1999 GPRS (8-92 Kb/s per subscriber + data compression) 2000 EDGE (8-384 Kb/s per subscriber + data compression) 2001 ( Kb/s per subscriber + data compression) 2002 integrate different second-generation cellular and cordless services. IMT-2000 supports timedivision duplex (TDD) and FDD to enable asymmetric and symmetric data services in a spectrally efficient way. The transport in the radio access network is based on asynchronous transfer mode (ATM) and IP [3 5]. IMT-2000, which is optimized for data services, will open new business opportunities such as seamless and bandwidth-on-demand services. Third-generation systems are addressing a mass market for mobile multimedia communications and enhanced multimedia services with roaming through different networks. At the end of 1999 the ITU Radiocommunication Standardization Sector (ITU-R) approved the specifications of the IMT-2000 radio interfaces, which are part of the IMT-2000 family [6]. The terrestrial component comprises: IMT-2000 direct spread CDMA based on the Third Generation Partnership Project (3GPP) concept [4, 6] IMT-2000 multicarrier CDMA based on 3GPP2 [6] IMT-2000 TDD CDMA based on the 3GPP concept and the Chinese concept from CWTS [4, 6] IMT-2000 single-carrier TDMA based on the evolution of ANSI-136 with EDGE and a high-speed mode [6] IMT-2000 multicarrier TDMA based on the DECT concept [6] Third-generation mobile radio systems will be deployed starting in Based on the successful development of mobile communications and the long-term development of new systems with a time frame on the order of about 10 years, discussions have already started on the development beyond third-generation mobile radio systems. This article is mainly focused on the European view from the GSM and perspective and its terrestrial components. Developments in other regions based on other second- and third-generation systems are similar. Despite these steps in the evolution of secondgeneration systems, thirdgeneration mobile radio systems (IMT-2000 in ITU and in Europe) are currently being standardized and developed worldwide. IEEE Communications Magazine December

3 Payment User Subscriber Access network operator Service providers Core transport network operator Value-added service providers Content providers Subscriber user Billing Communications Service broker Home environment Service management (e.g., ISP or corporate network) Access network operator Core network operator Value-added service providers Content providers Figure 2. New role model and service delivery for. TRENDS IN MOBILE COMMUNICATIONS Mobile communications are determined by economic and technical trends and, in the future, mainly by application requirements. With the evolution of second-generation systems and the emerging third-generation systems, more advanced data and multimedia services are becoming available in addition to mobile telephony. These trends and requirements are affecting the vision of future systems beyond the third generation. ECONOMIC TRENDS AND SERVICE DEMANDS In the last 10 years many countries introduced deregulation of telecommunications services through liberalization and privatization to support competition in the telecommunications markets. Due to this competition the rates for telecommunication services decreased and the number of subscribers, in particular for mobile radio systems, increased much faster than expected. The annual growth rates in important markets increased from 1998 with about 60 % to expected 100 percent per year in In 2000 the number of mobile subscribers is higher than 400 million worldwide, and for 2010 more than 1700 million mobile subscribers are expected worldwide [4, 7, 8]. This new environment is changing the role model for service delivery and the value chain [9, 10] for mobile service provision. Compared to a telephone service provider and network operator more players are active now as shown in Fig. 2. The roles of access network operators, core transport network operators, service providers, and content providers have to be distinguished. There are interrelations between these different possible players. In addition, users and subscribers are not necessarily the same. Service and content providers play an increasing role in the value chain. The dominant part of the revenues moves from the network operator to the content provider. Both are using the services of the access network and core network operators. It is expected that value-added data services and content provisioning will create the main growth. The combination and convergence of the different worlds should involve: Photo Report Video clip GSM 2+ Web Photo Report Video clip ISDN Web Photo Report Video clip PSTN Web Photo Report Video clip GSM Ph1 Web Photo Report Video clip 10 s 1 min Transmission time 10 min 1 hr Figure 3. Transmission time versus service and mobile radio system. 124 IEEE Communications Magazine December 2000

4 Petabits/day Million subscribers Fixed access Mobile access Mobile Internet Fixed and mobile Internet Internet access Voice Figure 4. Growth in traffic for different access systems and voice and data services. IT; for example, Internet access, , real-time image transfer, multimedia document transfer, browsing, broadcasting, publishing, and mobile computing Media; for example, audio-video content, video on demand, interactive video services, infotainment, value-added Internet services, and TV and radio contribution Telecommunications; for example, mobility, video telephony, wideband data services and global connectivity, access security, and QoS Integration of communication with information technology is a prerequisite for the information society. Advanced services for the information society are being and will be developed in many areas information, Internet browsing, communication, videoconferencing, education, financial services, e-commerce, telemetric services, location-based services, personal navigation, personal health, security, remote monitoring and controlling, social networks, and leisure. It is expected by the Forum that in 2010 in Europe more than 90 million mobile subscribers will use mobile multimedia services and will generate about 60 percent of the traffic in terms of transmitted bits. These different types of services are subdivided into individual services such as multimedia, , file transfer, and so on, symmetrical and asymmetrical services, real-time and non-real-time services, and distribution services such as radio, TV, and software provision. In addition, services might be distinguished in: Wide area services like mobile telephony, messaging services as always on (e.g., GPRS), mobile multimedia Local area services such as wireless secure high-speed access, fast Internet and intranets, and shared databases and applications The mobile radio systems for telephony and low-data-rate services have been driven by technology as the change from analog to digital systems from the first to the second generation. The major step from the second to the third generation was not directly driven by technology since both are based on digital techniques; the major step was the ability to support advanced and wideband services. Therefore, the user is much more in focus today. Users do not care about the technology. Their expectations are increasing for a large variety of services and applications with high quality of service (QoS), which is related to delay, data rate, and bit error requirements. IMT-2000 will be an important step to enable such services and will improve the grade of service for data services (Fig. 3). Therefore, seamless services and applications via different access systems will be the driving forces for future developments. Today s communications model is still governed by communication between people. However, in the future this paradigm will be shifted to communication from people to people, people to machines, machines to people and machines to machines. It is expected that due to the dominating role of mobile radio access the number of portable handsets will exceed the number of PCs connected to the Internet around Therefore, mobile terminals will be the major man machine interface in the future instead of PCs. These new roles of different players and the increasing user expectations require more advanced multimedia services, flexibility, and interworking in the network between different access technologies, including service negotiation between the different access technologies. Due to the dominant role of IP-based data traffic, in the future networks and systems have to be designed for economic packet data transfer. The expected new data services consume high amounts of bandwidth. This results in increasing data rate requirements for future systems. Major areas of growth will be terminals (e.g., palm-sized PCs, net appliances) for data applications, Internet, smartcards for software download, and security. Decreasing costs per transmitted megabyte will drive new bandwidth-consuming applications. TECHNOLOGICAL TRENDS The overwhelming growth of Internet usage is one of the major technical trends in future communications. The growth of subscriber numbers for fixed access will reach saturation around 2004 (for voice usage). Mobile radio access is increasing very fast, and the number of mobile subscribers is expected to exceed the number of fixed subscribers also around The penetration of fixed Internet is growing parallel to mobile radio access penetration. In Japan a penetration of IEEE Communications Magazine December

5 Terminal Radio Core Application 1996 Single GSM, DECT Source: Forum Multi GPRS, EDGE Figure 5. Technological trends on the way to IMT-2000 /. User mobility Fast mobile Slow mobile Movable Fixed PSTN, ISDN, GSM, IP-based Voice, data information, World Wide Web G S M Adaptive IP-based, B-ISDN, ATM High speed Service portablility and roaming between systems ISDN Source: ACTS project the magic WAND 2M WAND SAMBA B-ISDN 20M 34M Supported data rate MEDIAN Figure 6. New frequency ranges and related research projects. A W AC S 155M 60 GHz 40 GHz 19 GHz 5 GHz 2 GHz Frequency band about 40 percent will be reached in 2002 [11], which is similar to other countries. According to the ITU the number of Internet hosts increased from a few hundred thousand to 30 million between 1991 and In the year 2002 nearly 350 million Internet users are expected. This growth is increasing. About 80 percent of Internet users connected to fixed access networks are also using mobile communications. Therefore, these users are interested in getting the same services on mobile terminals. This results in a big market potential for mobile multimedia, which will start with the availability of mobile data systems like GPRS, HSCSD, EDGE, and IMT-2000/ (Fig. 4). The first mobile Internet applications for low data rates of 9.6 kb/s in GSM are based on Wireless Application Protocol (WAP) to adapt the content to the available mobile data rates and display capabilities. The success of the i-mode Internet service in Japan with more than 11 million subscribers by May 2000 about one and a half years after service launch shows the potential of data applications. With increasing data rates and improved displays, more advanced content will be provided. Mobile telephone traffic will overtake fixed telephony in terms of subscribers [11]. Overall data traffic in terms of bits per second is already exceeding voice traffic with much higher growth rates than voice (Fig. 4). The growing data and Internet traffic results in packet-oriented traffic dominating over circuit-switched traffic in the access systems [11; 12]. These data services require a high degree of asymmetry between uplink and downlink, especially for Internet-type services with much higher expected capacity on the downlink [4], which has already been taken into account by IMT-2000/ due to the combination of FDD and TDD. In addition to the fast growth of Internet usage, other technical trends are observed, which have been presented by the Forum as shown in Fig. 5 [9]. The first mobile radio terminals were singlemode and single-band terminals. Today, secondgeneration terminals are implemented for several bands (e.g., combinations of 900, 1800, and 1900 MHz). The implementation of dual-mode terminals is proceeding as GSM plus ANSI-136 and, for example, IS-95 and (or combined with) AMPS. In the third generation terminals will be combined, say, with GSM. Due to the multiplicity of available standards, multimode terminals and finally adaptive terminals based on software-defined radio concepts will be introduced. This trend is supported by increasing available signal power, which is expected to grow by a factor of 10 according to a conservative estimate up to a factor of 100 according to Moore s law within the next 10 years [13]. The software-defined radio concept will also be applied to base station equipment. In addition, source coding technologies using suitable compression algorithms reduce the necessary data rate for efficient multimedia transport. Several access technologies are evolving and emerging. The second-generation GSM is evolving via GPRS, HSCSD, and EDGE toward (Fig. 1). In addition, wireless local area network (WLAN) type systems like HIPERLAN 2, IEEE , digital audio broadcast (DAB), and digital video broadcasting (DVB-T) are becoming available. For short-range connectivity, systems like Bluetooth and the DECT data system are being developed. In fixed access, systems like digital subscriber line (xdsl), in particular asymmetric DSL (ADSL), are increasing the user data rate significantly on the last mile. All these technologies might also be part of systems beyond the third generation in the sense of fixed mobile convergence. However, this list of access technologies is not exhaustive. The transport capacity of the core network increased within about 10 years by a factor of 10 6 with decreasing transmission costs through technology steps from plesiochronous digital hierarchy (PDH) to synchronous digital hierarchy (SDH) and optical communications with wavelength-division multiplexing (WDM) and dense WDM (DWDM) [14]. Therefore, transmission speed is growing faster than signal power. The core networks are moving to more transparent transport techniques without any distinction between circuit-switched and packet-oriented networks to support real-time and non-real-time services in the same network [15]. From today s perspective IP is the most promising solution. IP transport will also be used in the radio access network with routers close to the base stations. The progress in such areas as 126 IEEE Communications Magazine December 2000

6 software technology, Java Virtual Machines, and routers supports this trend. Frequency spectrum is a scarce resource. Therefore, on one hand spectrum-efficient radio access schemes are required, and on the other new frequency bands will be used. In addition to the already deployed GSM system and the standardized system, the European Commission partly funded several research projects within the framework of the Research of Advanced Communications in Europe (RACE) Program in the Mobile Broadband System (MBS) project and the Advanced Communication Technologies and Services (ACTS) Program for wideband radio access systems in the frequency ranges GHz (AWACS Project), GHz (SAMBA Project), and 60 GHz (MEDIAN Project) (Fig. 6) [16]. These projects significantly influenced the standardization in the European Telecommunications Standards Institute (ETSI) Broadband Radio Access Network (BRAN) Project for the WLAN-type systems HIPERLAN 2 and HIPERACCESS. With increasing carrier frequency, the path loss is increasing for constant antenna gain, which on one hand reduces the covered area and the mobility of an economically deployed system; on the other hand, it enables high frequency reuse by a more dense cluster size. Especially for the 60 GHz frequency range, the peak in atmospheric attenuation due to hydrogen resonance increases the isolation to adjacent cells significantly. Internationally, different frequency bands are already allocated to WLAN-type systems and future systems in the millimeter-wave frequency range (Fig. 7). There are several internationally harmonized frequency bands identified for this type of system, which will allow global roaming. Already today an increasing number of terminal equipment such as handhelds, notebooks, and desktops/workstations are supplied with modem cards like Personal Computer Memory Card International Association (PCMCIA) to enable connection to fixed and/or mobile networks [7]. All these developments are preparing the market for mobile data applications. EVOLVING AND EMERGING ACCESS TECHNOLOGIES From the user s perspective several access technologies are available and will become so in the near future. These technologies can be grouped in the following categories: Cellular mobile radio systems (second-generation, e.g., GSM; third-generation, e.g., IMT-2000/) Cordless systems (e.g., DECT) Systems for short-range connectivity such as Bluetooth and the DECT data system WLAN-type systems such as ETSI BRAN HIPERLAN 2 and HIPERACCESS, and IEEE a Fixed wireless access or wireless local loop systems Satellite systems Broadcasting systems such as DAB and DVB-T Cable systems such as xdsl over twisted pair and cable modems using transmission over coaxial cables (e.g., CATV systems) Europe USA Japan ISM UNII* *Unlicenced national information infrastructure band Figure 7. International frequency allocation for WLAN type systems. Mobility and range High speed vehicular rural Vehicular urban Pedestrian Indoor Fixed urban Personal area GSM These systems have been developed for special purposes. All of them are applicable to data communications. Figure 8 shows the basic application areas for wireless systems vs. supported data rate, range, and grade of mobility. The WLAN-type systems are designed in particular for high-data-rate access and low range, and in general for low mobility. They are applicable to corporate networks and public access as a complement to cellular mobile radio systems (e.g., GSM and ) for hot spot applications such as company campuses, conference centers, airports, hotels, and railway stations. The physical layer of the ETSI BRAN system HIPERLAN 2 is harmonized with IEEE a and MMAC in Japan, which would basically allow global roaming. In addition to these evolving and emerging radio access technologies, research on a new radio interface is proposed, especially in Japan, which should support high mobility and high data rates. Ad hoc or self-organizing networks will play a complementary role to extend coverage for low-power systems and unlicensed applications. In these systems mobile stations may act as relay stations in a multihop transmission environment from distant mobiles to base stations. Mobile stations will have the ability to support base station functionality. Direct mobile-tomobile calls will be possible. The network organization will be based on interference measurements by all mobiles and base stations for automatic and dynamic network organization according to the actual interference and channel assignment situation for the channel allocation of new connections and link optimization. Fixed wireless access or wireless local loop sys UNII* DECT BlueTooth Proposed new radio interface IEEE a BRAN Hiperian2 WLAN MBS WLAN MBS WLAN MBS 64 Frequency (GHz) BRAN Hiperaccess Mb/s Total data rate per call Figure 8. Wireless systems versus supported data rate, range and mobility. IEEE Communications Magazine December

7 System Data rates Technology Range Mobility Frequency range Original application area GSM (including 9.6 kb/s TDMA, FDD Up to 35 Km High 900, 1800, 1900 MHz Public and private GPRS, HSCSD up to 384 kb/s in GSM, environment and EDGE) lower for data IMT-2000, Max. 2 Mb/s IMT-2000 family, 30 m 20 Km High 2 GHz (ITU spectrum) Public and private (UTRA) WCDMA (FDD) + environment TD-CDMA (TDD) DECT/DECTlink Max. 2 Mb/s TDMA/TDD Up to 50 m Low MHz Office and residential environment Bluetooth Max. 721 kb/s Direct sequence or m Very low 2.4 GHz ISM band Cable replacement, frequency hopping SoHo environment HIPERLAN 2 25 Mb/s OFDM, TDD m Low 5 GHz Corporate environment, public hot spots IEEE a About 20 Mb/s OFDM, TDD m Low 5 GHz Corporate environment, public hot spots HIPERACCESS About 25 Mb/s Not yet specified 2 10 km No 5 40 GHz Business access, feeder DAB 1.5 Mb/s OFDM 100 km High E.g., MHz Audio broadcasting MHz DVB-T 5 31 Mb/s per 8 OFDM 100 km Medium TV bands below Video broadcasting MHz channel to high 860 MHz (mobile: 5 8, fixed 16 31) Cable modem Down < 40 Mb/s FDD 5 to ~20 km No Down ~60 to 860 MHz Residential environment Up < 10 Mb/s QAM/QPSK Up 10 to ~40 MHz ADSL Down DMT 2 6 Km No Baseband SoHo (small office- (8) Mb/s (carrierless home office), SME, Up (0.640 Mb/s) AM/PM CAP) residential environment Table 1. Main parameters of different access systems. tems are developed to replace or complement wired access systems. These systems do not support mobility. DAB and DVB-T can be applied to wideband broadcast data services in the downlink. These systems can be combined with cellular mobile radio systems like GSM and or the public switched telephone network (PSTN) and integrated services digital network (ISDN) for the uplink as a return channel for user requests and highly asymmetrical services. Fixed lines based on copper twisted pair or coaxial cables are widely distributed. Coaxial cables support high bandwidth and could be used for wideband data services. The potential of twisted pairs is used by ISDN and xdsl techniques (mainly ADSL) [1, 2, 17]. Table 1 summarizes the main parameters of important exemplary access technologies. These technologies represent a very flexible and powerful platform to support future requirements of services and applications. However, most of these systems have been designed in isolation without taking into account possible interworking with other access technologies. Their system design is mainly based on the traditional vertical approach to support a certain set of services with a particular technology. This situation is the starting point for the discussion of mobile radio systems beyond the third generation. Depending on future user requirements and economic demands, new radio access schemes such as wideband radio access for high mobility might be developed. A VISION FOR SYSTEMS BEYOND THIRD-GENERATION MOBILE RADIO COMMUNICATIONS Third-generation mobile radio systems for mobile multimedia applications will be deployed by 2001 and 2002 in different regions. Due to the long necessary timeframe for the definition, development, and standardization of new systems beyond third- generation mobile radio systems, discussions are already starting today. However, what are the motivations for further developments? Frequency spectrum is a very scarce resource, which requires more efficient use of spectrum and the exploration of new frequency bands for wireless applications. Increasing computing power at lower cost enables new possibilities for more sophisticated signal algorithms for, say, coding, decoding, detection, advanced antenna concepts, and software-defined radio implementations. Different flexible and broadband access technologies are evolving and emerging, which are optimized for special purposes. Their integration in a common flexible and expandable platform would provide a multiplicity of possibilities for current and future services and applications to users in a single terminal. User expectations are increasing to more sophisticated services with QoS comparable to wireline access based on the current development in the Internet. However, the major 128 IEEE Communications Magazine December 2000

8 The interworking between these different access systems in terms of horizontal and vertical handover and seamless services with service negotiation including mobility, security and QoS will be a key requirement. Services and applications Download channel DAB DVB Media access system IP-based core network New radio interface Wireline xdsl Return channel: (e.g., GSM) Cellular GSM IMT-2000 WLAN type Short-range connectivity Other entities Figure 9. A seamless future network including a variety of interworking access systems. driving forces for systems beyond the third generation will be economic success based on user demands for new and advanced services with high security and reliability, and economic advantages and business opportunities for the involved players (Fig. 2), competition, costs, size and weight of terminals, and battery lifetime. The ratio between cost and performance will improve beyond the third generation. The available, emerging, and evolving access technologies have basically been designed in the classical vertical communication model that a system has to provide a limited set of services to users in an optimized manner. In the original vision of the third generation around 1990, the capabilities of different wireless access systems such as cellular, cordless, and data services should be supported in all radio environments by a single radio interface. During the definition and standardization phase of, it turned out that there is no single radio technology which can be optimized for all applications. Therefore, the Terrestrial Radio Access (UTRA) concept combined the FDD and TDD components to support the different symmetrical and asymmetrical service needs in a spectrum efficient way [4, 5]. Therefore, systems beyond third generation will mainly be characterized by a horizontal communication model, where different access technologies like cellular, cordless, WLAN type systems, systems for short range connectivity and wired systems will be combined into a common platform to complement each other in an optimum way for different service requirements and radio environments. These access systems will be connected to a common, flexible, and seamless core network. The mobility management will be part of a new media access system as the interface between the core network and the particular access technology to connect a user via a single number for different access systems to the network. This will correspond to a generalized access network. Global roaming for all access technologies is required. The interworking between these different access systems in terms of horizontal and vertical handover and seamless services with service negotiation including mobility, security, and QoS will be a key requirement, which will be handled in the newly developed media access system and the core network. Further characteristics of such systems will be: A variety of supported data rates according to second-generation cellular systems to third-generation, broadband access (WLAN type systems), short-range connectivity systems, and wired systems such as xdsl with maximum data rates up to 10, 20, and 155 Mb/s for current and future services and applications Current and new frequency bands Sharing of frequencies for better use of resources between different systems and possibly different operators New network types and network management such as ad hoc and self-optimizing networks, automatic and dynamic network reconfiguration, and dynamic frequency allocation to support a variety of access systems on a common platform Support of symmetrical and asymmetrical services by FDD and TDD systems Core and radio access networks designed for efficient packet transmission by supporting improved QoS requirements for realtime services Optimization of transmission links for asymmetrical traffic Core and radio access network based from today s point of view on IP due to IEEE Communications Magazine December

9 The combination of several available, evolving, and emerging access technologies into a common platform and their seamless interworking, combined with adaptive multi mode terminals, will be the key characteristics of systems beyond third generation mobile radio systems. Distribution layer Cellular layer Hot spot layer DAB and/or DVB Possible return channels Personal network layer 2G: e.g. GSM IMT-2000 WLAN type e.g. ETSI BRAN Fixed (wired) layer X X X X X X X X X X X X X Horizontal handover within a system Figure 10. Layered structure of seamless future network. Full coverage Global access Full mobility Individual links not necessary Full coverage and hot spots Global roaming Full mobility Individual links Local coverage Hot spots Global roaming Local mobility Individual links Short range communication (e.g., Bluetooth, DECT) Global roaming Individual links No mobility Global roaming Individual links Vertical handover between systems lower infrastructure costs, faster provisioning of new features, and easy integration of new network elements Separation of the physical layer and different access technologies from the applications by means such as Java Virtual Machines and Common Object Request Broker Architecture (CORBA) to decouple the applied software from the hardware Figure 9 shows this vision of a seamless network including a variety of interworking access systems, which are connected to a common IPbased core network. The media access system connects each access system to a common core network. A possible new radio interface is for further study, depending on economic requirements. Due to the different application areas, cell ranges, and radio environments, the different access systems are organized in a layered structure similar to hierarchical cell structures in cellular mobile radio systems according to Fig. 10. However, in addition to different cell layers, different access technologies also complement each other on a common platform: Distribution layer: Due to the big possible range and cell size of DAB and DVB-T, these systems are especially suited to distribution or broadcast services. Under the condition that the network is deployed to achieve coverage in these big cells, DAB and DVB-T cannot provide high system capacity in terms of users and/or data rate per area unit. For broadcast services individual links are not necessarily needed. However, in the case of using the DAB and DVB-T transmission technology as a wideband downlink channel, wideband Internet access can be provided with a limited amount of accessible Internet content. Basically, all other access systems can be used as a return channel for data request and acknowledgment signaling. Full coverage, full mobility, and global access can be supported. Therefore, these systems can be applied for distribution purposes and for wideband access in the downlink. Cellular layer: This layer provides high system capacity in terms of users and data rates per area unit. It will comprise second- (GSM and its evolution) and third-generation mobile radio systems (IMT- 2000/: UTRA FDD and TDD) for data rates up to 2 Mb/s. Full coverage and hot spot applications, full mobility, and global roaming are supported. These systems are designed especially for individual links. Broadcast transmission technology like DAB and DVB-T can also be applied in this layer in smaller cells than in the distribution layer to provide a wideband downlink for individual Internet access. The return channels are part of this system component as in the distribution layer. Hot spot layer: For very high-data-rate applications and individual links (e.g., in company campus areas, conference centers, airports), WLAN-type systems such as HIPERLAN 2 are best suited due to their flexibility in terms of asymmetrical data services, supported data rates, and adaptive modulation. However, the short range compared to cellular systems supports mainly local coverage with limited mobility for economic system deployment. Global roaming is possible and will be required. However, full coverage is not expected. 130 IEEE Communications Magazine December 2000

10 front-end BB signal BB signal BB signal front-end Diversity concepts Figure 11. Advanced antenna concepts. Multiple antenna concepts Adaptive antenna concepts Personal network layer: Personal networks will be applied for office and home (household) networking. Different equipment can be connected to each other via short-range connectivity systems like Bluetooth and DECT. These systems can also be connected as individual links to the other network layers or directly to the medium access system or via terminals equipped with shortrange connectivity equipment. This also allows direct connection from devices to the public network. These systems do not practically support mobility; however, global roaming should be ensured. Fixed (wired) layer: Fixed access systems are twisted pair systems (xdsl) and coaxial cable systems (e.g., CATV). Fixed wireless access or wireless local loops can be placed in this category. Fixed access systems do not support mobility. However, global roaming is feasible and will be required. These systems with high capacity support individual links as well as point-to-multipoint (PMP) links. The interworking between these systems on the common platform is ensured by horizontal handover within an access system, in particular by vertical handover between different access systems and in general between different layers of the common platform. Vertical handover is combined with service negotiation to ensure seamless service, because in general different access systems support different user data rates and other bearer and service parameters. Interworking, mobility management, and roaming will be handled via the medium access system and the IP-based core network (Fig. 9). Multimode terminals and new appliances are key components. These terminals will comprise, say, a camera, a screen for video and high-resolution Internet applications, and systems for short-range connectivity for ad hoc networking with other devices. These terminals may be adaptive based on high signal power. The combination of several available, evolving, and emerging access technologies into a common platform and their seamless interworking combined with adaptive multimode terminals will be the key characteristics of systems beyond third-generation mobile radio systems. This could be supported by technologies like Java Virtual Machines and CORBA. These systems have to be flexible and reconfigurable by network management schemes, adaptive frequency allocation, and self-optimizing networks as far as possible to support the needs of the different players and a variety of terminals in future communication systems (Fig. 2). CHALLENGES Many technical challenges must be solved by extensive research to make the vision of systems beyond the third generation happen. The key points are the interworking of different access systems on a common platform and the necessary multimode or adaptive and multiband terminals for different access systems and a wide range of services. Challenges are in several areas, such as the radio interface, the radio access and core network, implementation issues, and services-related issues. Future systems have to use the frequency resources as efficiently as possible. Therefore, several physical-layer-related techniques have to be investigated: Optimization of evolving and emerging access systems by improved modulation and channel coding schemes for further enhancement of spectrum efficiency and system performance Advanced detection schemes such as multiuser detection and interference cancellation to gain from the a priori knowledge about intra- and intercell interference signals Signal algorithms to trade off between performance gain and computing complexity Compression techniques for source coding to reduce the needed user data rate Improved algorithms to support these mainly physical layer issues are: Link adaptation according to the channel conditions, traffic load, and services for better usage of the frequency resources and improved system performance Spectrum sharing between different systems and the investigation of coexistence conditions between different radio access systems Advanced antenna concepts to improve link quality and channel capacity (Fig. 11). These concepts are used to increase the channel capacity of the radio link. Diversity concepts basically reduce the impact of fading due to multipath transmission. Multiple antenna concepts are a further extension of diversity concepts gaining from uncorrelated multipath transmission IEEE Communications Magazine December

11 Mode 1 BB signal Mode 2 BB signal Mode n BB signal 1 2 n Memory for parameter sets BB signal Flexible and adaptive front-end Programmable high power BB signal Multimode terminal with parallel modes Multimode terminal with softwaredefined signal Fully adaptive softwaredefined terminal Figure 12. A concept for multimode and software-defined terminals. channels between the different antenna elements on the base station and terminal sides [18]. The basic idea is to reuse the same frequency band simultaneously for parallel transmission channels by space-time coding to increase the channel capacity. Adaptive antenna concepts improve link quality by reducing the co-channel interference from different directions and, in the more advanced space-division multiple access (SDMA) concept, by reusing the same frequency channels simultaneously for different users in distinct directions. System aspects like common control channels and the signaling concept are an essential part of advanced antenna concepts in order to achieve the same range extension for common control channels as for traffic channels. These are key concepts to use the scarce frequency spectrum as efficiently as possible without major impacts on evolving access systems. Concepts such as have already taken into account necessary prerequisites for adaptive antenna concepts. However, economic implementations of the different front-ends and baseband signal are technical challenges. System design and evaluation requires a realistic wideband channel characterization for new frequency bands up to about 60 GHz. Based on wideband propagation measurements, channel models are needed for the international standardization process, including models for direction of arrival. Higher-layer protocols in the access network (medium access system) are an additional area to improve system performance even further, for example: Self-optimizing networks and automatic network reconfiguration Resource allocation algorithms with respect to varying traffic load, services, bearer capabilities, radio environment, and channel conditions Dynamic frequency allocation Interworking of different access systems on higher layers via horizontal and vertical handover and service negotiation Network management The interworking of different access systems on an IP-based common platform via the medium access system with horizontal and vertical handover results in several challenges for the core and the medium access system. Major topics are related to the improvement and extension of IP for mobile applications and the optimization for radio transmission: Support of real-time and non-real-time services with respect to service requirements like QoS, and especially delay requirements for real-time services Service negotiation for seamless services vs. the available access systems and bearer capabilities Mobility management including handover and roaming Security mechanisms like authentication, authorization, and accounting (AAA) The seamless future network in Fig. 9 comprises several access systems with seamless interworking for different applications and radio environments. Users will only gain from this flexible concept when multimode and multiband terminals with low power consumption and reasonable size are feasible. A variety of terminal types such as PDAs, notebooks, and handsets support these applications. Other technical challenges are improved display techniques and battery technology. Several concepts for terminal implementation are under discussion (Fig. 12), which will also be applied to base station equipment. In the simplest concept several fixed modes are implemented in parallel. However, this straightforward concept is inflexible for future improvements. More advanced concepts apply a signal platform, where the parameter sets of different access systems are downloaded to the signal unit. In most advanced concepts a flexible and freely programmable signal unit can be adapted to the actual access system. The last two concepts are software-defined radios, which are the major technical challenge in terminal implementation [19]. Such concepts will become feasible with progress in semiconductor technology and increasing available signal power. From the user s point of view, the manmachine interface has to be easy to use and selfexplanatory to enable people even handicapped and elderly people to easily use advanced services. The content has to be adapted automatically to the actual bearer capability of the access system used. To meet these challenges, extensive international research activities are necessary to solve 132 IEEE Communications Magazine December 2000

12 technical issues and prepare for the consensus building toward international standardization of such new ideas as the interworking of systems by vertical handover, global roaming, and the optimization of evolving and emerging access systems, as well as the radio access and core networks. CONCLUSIONS Mobile multimedia applications are already starting today with evolved second-generation mobile radio systems. Third-generation mobile radio systems provide more opportunities for mobile multimedia through improved wideband and more flexible radio interfaces. Systems beyond the third generation will not just be more wideband access systems to provide higher data rates to users. These systems will follow the general concept of many combined optimized access systems for special purposes on a common flexible network platform, which complement each other in an efficient and optimized way from the user s perspective. Depending on the selected services, available access systems, and bearer capabilities, this new system will select the most appropriate access. The different access systems are allocated to different cell layers with respect to cell size, coverage and mobility for globally optimized seamless service provision. The flexible platform of the nedium access system and the core network will be based, from today s point of view, on IP technology with transparent transmission to ensure flexibility for all involved players in the new deregulated and liberalized communications environment. In addition, the concept will use algorithms for reconfigurability, self-optimizing networks, and automatic network reconfiguration, so new network entities and access systems can easily be added. Key issues of the new concept are the interworking of a bundle of access systems on a common platform by horizontal and vertical handover, service negotiation for seamless service provisioning, and global roaming. The newly developed medium access system and the IPbased core network will handle interworking and mobility management. There are many technical challenges that must be solved to make this vision happen. This requires extensive international research activities in the areas of access system improvement, optimization of IP for radio transmission and mobility management, and implementation of multimode and multiband terminals. International standardization will play an important role in rendering the implementation of the concept on a global basis to ensure global roaming and seamless service provisioning. Finally, economic aspects will determine which access system elements are implemented. However, the concept should be open and flexible so that new access systems can be introduced later to improve service to users continuously. ACKNOWLEDGMENTS The authors would like to acknowledge the valuable contributions and comments of their colleagues Dr. R. R. Becher, Dr. M. Haardt, Dr. G. Klas, Dr. R. Lüder, Mr. K.-H. Möhrmann, Mr. H.-J. von der Neyen, and Mr. S. Schuster from Siemens ICM N and ICN M in Munich, Germany, for the preparation of this article. REFERENCES [1] J. D. Gibson, The Mobile Communications Handbook, IEEE Press, [2] V. Jung and H. J. Warnecke, Handbuch für die Telekommunikation, Berlin: Springer Verlag, [3] E. Berruto et al., Research Activities on Radio Interface, Network Architectures, and Planning, IEEE Commun. Mag., vol. 36, no. 2, Feb. 1998, pp [4] P. Chaudhury, W. Mohr, and S. Onoe, The 3GPP Proposal for IMT-2000, IEEE Commun. Mag., vol. 37. no. 12, Dec. 1999, pp [5] R. Prasad, W. Mohr, and W. 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Mag., vol. 36, no. 11, Nov. 1998, pp [15] A. Jajszczyk, What Is the Future of Telecommunications Networking? IEEE Commun. Mag., vol. 37, no. 6, June 1999, pp [16] M. Prögler, C. Evci, and M. Umehira, Air Interface Access Schemes for Broadband Mobile Systems, IEEE Commun. Mag., vol. 37, no. 9, Sept. 1999, pp [17] A. Dutta-Roy, A Second Wind for Wiring, IEEE Spectrum, vol. 36, no. 9, Sept. 1999, pp [18] IEEE Pers. Commun., feature topic on antenna concepts), vol. 5, no. 1, Feb. 1998, pp [19] IEEE Commun. Mag., feature topic on softwaredefined radio, vol. 37, no. 2, Feb. 1999, pp BIOGRAPHIES WERNER MOHR [SM] (werner.mohr@icn.siemens.de) graduated from the University of Hannover, Germany, with a Master s degree in electrical engineering in 1981, and a Ph.D. in He has been with Siemens AG, Germany, since He has been responsible for propagation measurements and channel modeling, and was involved in the European RACE-II Project ATDMA. From December 1996 he was project manager of the ACTS FRAMES Project through its conclusion in He is vice president, pre-engineering of Siemens ICM N. He is a member of VDE. In 1990 he received the Award for the ITG in VDE. WALTER KONHÄUSER (walter.konhaeuser@icn.siemens.de) graduated from the Technical University of Berlin, Germany, with a Master s degree in electrical engineering in 1976, and a Ph.D in He joined Siemens AG in 1982 and was involved in the development of security systems based on decentralized microcomputer networks, relay systems, and a plant manager. In 1992 he became responsible for software development of the mobile switching subsystem, including the implementation of data services. In 1995 he became responsible for product management of the mobile business networks group, and presently he is chief technical officer. He is active as a professor in industrial control systems at the Technical University of Berlin and is a member of VDE. More advanced concepts apply a signal platform, where the parameter sets of different access systems are downloaded to the signal unit. In most advanced concepts a flexible and freely programmable signal unit can be adapted to the actual access system. IEEE Communications Magazine December