ABSTRACT Service Evolution The approaching 4G (fourthgeneration) mobile communication systems The evolution from 3G to 4G will be drivenare projected to solve still-remaining by services that offer better quality (e.g.problems of 3G (third generation) systems video and sound) thanks to greaterand to provide a wide variety of new services, bandwidth, more sophistication in thefrom high-quality voice to high-definition association of a large quantity of information,video to high-data-rate wireless channels. The and improved personalization. Convergenceterm 4G is used broadly to include several with other network (enterprise, fixed) servicestypes of broadband wireless access will come about through the high session datacommunication systems, not only cellular rate. Machine-to-machine transmission willtelephone systems. One of the terms used to involve two basic equipment types: sensorsdescribe 4G is MAGIC—Mobile multimedia, (which measure arameters) and tags (whichanytime anywhere, Global mobility support, are generally read/write equipment). It isintegrated wireless solution, and customized expected that users will require high datapersonal service. As a promise for the future, rates, similar to those on fixed networks, for4G systems, that is, cellular broadband data and streaming applications. Mobilewireless access systems have been attracting terminal usage (laptops, Personal digitalmuch interest in the mobile communication assistants, handhelds) is expected toarena. The 4G systems not only will supportthe next generation of mobile service, but alsowill support the fixed wireless networks. Thispaper presents an overall vision of the 4Gfeatures, framework, and integration ofmobile communication. The features of 4Gsystems might be summarized with one word—integration. The 4G systems are aboutseamlessly integrating terminals, networks,and applications to satisfy increasing user Figure 1:Service evolution visiondemands. The continuous expansion of Grow rapidly as they become more usersmobile communication and wireless networks friendly. Fluid high quality video andshows evidence of exceptional growth in the network creativity are important userareas of mobile subscriber, wireless network requirements. Key infrastructure designaccess, mobile services, and applications.
requirements include: fast response, high in which Code Division Multiple Accesssession rate, high capacity, low user charges, (CDMA) will be progressively pushed to therapid return on investment for operators, point at which terminal manufacturers willinvestment that is in line with the growth in give up. When this point is reached, anotherdemand, and simple autonomous terminals. technology will be needed to realize the required increases in capacity and data Figure 3: Multiple overlay architectureDimensioning targets rates. The second path is the radio LAN one. WidespreadFigure 2: Dimensioning examplesA simple calculation illustrates the order ofmagnitude. The design target in terms ofradio performance is to achieve a scalablecapacity from 50 to 500 bit/s/Hz/km2 deployment of WiFi is expected to start in 2005 for PCs,(including capacity for indoor use), as shown laptops and PDAs. In enterprises, voice may start to be carriedin Figure 2.Gebit/s/km2)0000 by Voice over Wireless LAN (VoWLAN).As a comparison, the expected best However, it is not clear what the nextperformance of 3G is around 10 bit/s/Hz/km2 successful technology will be. Reaching ausing High Speed Downlink Packet Access consensus on a 200 Mbit/s (and more)(HSDPA), Multiple-Input Multiple-Output technology will be a lengthy task, with too(MIMO), etc. No current technology is many proprietary solutions on offer. A thirdcapable of such performance. path is IEEE 802.16e and 802.20, which are simpler than 3G for the equivalentMulti-technology Approach performance. A core network evolutionMany technologies are competing on the road towards a broadband Next Generationto 4G, as can be seen in Figure 3. Three paths Network (NGN) will facilitate theare possible, even if they are more or less introduction of new access networkspecialized. The first is the 3G-centric path, technologies through standard access
gateways, based on ETSI-TISPAN, ITU-T, Key 4G Technologies 3GPP, China Communication Standards Some of the key technologies required for 4G Association (CCSA) and other standards. are briefly described below: How can an operator provide a large number of users with high session data rates using its OFDMA existing infrastructure? At least two Orthogonal Frequency Division Multiplexing technologies are needed. The first (called (OFDM) not only provides clear advantages “parent coverage”) is dedicated to large for physical layer performance, but also a coverage and real-time services. Legacy framework for improving layer 2 technologies, such as 2G/3G and their performance by proposing an additional evolutions will be complemented by Wi-Fi degree of free-dom. Using ODFM, it is and WiMAX. A second set of technologies is possible to exploit the time domain, the space needed to increase capacity, and can be domain, the frequency domain and even the designed without any constraints on coverage code domain to optimize radio channel usage. continuity. This is known as Pico-cell It ensures very robust transmission in multi- coverage. Only the use of both technologies path environments with reduced receiver can achieve both targets (Figure 4). complexity. As shown in Figure 5, the signal Handover between parent coverage and Pico is split into orthogonal subcarriers, on each of cell coverage is different from a classical which the signal is “narrowband” (a few kHz) roaming process, but similar to classical and therefore immune to multi-path effects, handover. Parent coverage can also be used as provided a guard interval is inserted between a back-up when service delivery in the Pico each OFDM symbol. cell becomes too difficult. Figure 5: OFDM principles OFDM also provides a frequency diversity gain, improving the physical layerFig 4: Coverage performance trends performance.It is also compatible with other
enhancement technologies, such as smart multi-band equipment with reducedantennas and MIMO. OFDM modulation can development effort and costs throughalso be employed as a multiple access simultaneous multi-channel processing.technology (Orthogonal Frequency DivisionMultiple Access; OFDMA). In this case, each Multiple-input multiple-outputOFDM symbol can transmit information MIMO uses signal multiplexing betweento/from several users using a different set of multiple transmitting antennas (spacesubcarriers (subchannels). This not only multiplex) and time or frequency. It is wellprovides additional flexibility for resource suited to OFDM, as it is possible to processallocation (increasing the capacity), but also independent time symbols as soon as theenables cross-layer optimization of radio link OFDM waveform is correctly designed forusage. the channel. This aspect of OFDM greatly simplifies processing. The signal transmitted by m antennas is received by n antennas.Software defined radio Processing of the received signals may deliver several performance improvements:Software Defined Radio (SDR) benefits from range, quality of received signal and spectrumtoday’s high processing power to develop efficiency. In principle, MIMO is moremulti-band, multi-standard base stations and efficient when many multiple path signals areterminals. Although in future the terminals received. The performance in cellularwill adapt the air interface to the available deployments is still subject to research andradio access technology, at present this is simulations (see Figure 6). However, it isdone by the infrastructure. Several generally admitted that the gain in spectruminfrastructure gains are expected from SDR. efficiency is directly related to the minimumFor example, to increase network capacity at number of antennas in the link.a specific time (e.g. during a sports event), anoperator will reconfigure its network addingseveral modems at a given Base TransceiverStation (BTS). SDR makes thisreconfiguration easy. In the context of 4Gsystems, SDR will become an enabler for theaggregation of multi-standard pico/microcells. For a manufacturer, this can be apowerful aid to providing multi-standard,
Handover technologies based on mobile IP technology have been considered for data and voice. Mobile IP techniques are slow but can be accelerated with classical methods (hierarchical, fast mobile IP). These methods are applicable to data and probably also voice. In single-frequency networks, it is necessary to reconsider the handover methods. Several techniques can be used when the carrier to interference ratio is negative (e.g. VSFOFDM, bit repetition), but Figure 6: Alcatel test-bed performance roadmap the drawback of these techniques is capacity.Interlayer optimization In OFDM, the same alternative exists as in CDMA, which is to use macro-diversity. In The most obvious interaction is the one the case of OFDM, MIMO allows macro-between MIMO and the MAC layer. Other diversity processing with performance gains.interactions have been identified (see However, the implementation of macro-Figure7). diversity implies that MIMO processing is centralized and transmissions are synchronous. This is not as complex as in CDMA, but such a technique should only be used in situations where spectrum is very scarce. Caching and Pico Cells Memory in the network and terminals facilitates service delivery. In cellular systems, this extends the capabilities of the MAC scheduler, as it facilitates the delivery of real-time services. Resources can be assigned to data only when the radio conditions are favorable. This method canFigure 7: Layer interaction and associated optimization double the capacity of a classical cellularHandover and mobility system. In pico cellular coverage, high data
rate (non-real-time) services can be delivered shown in Figure 8. At the entrance of theeven when reception/transmission is access network, lines of cache at theinterrupted for a few seconds. Consequently, destination of a terminal are built and stored.the coverage zone within which data can be When a terminal enters an area in which areceived/transmitted can be designed with no transfer is possible, it simply asks for the lineconstraints other than limiting interference. of cache following the last received. betweenData delivery is preferred in places where the the terminal and the cache. A simple, robustbitrate is a maximum. Between these areas, and reliable protocol is used between thethe coverage is not used most of the time, terminal and the cache for every servicecreating an apparent discontinuity. In these delivered in this type of coverageareas, content is sent to the terminal cache at .Multimedia service delivery, servicethe high data rate and read at the service rate. adaptation and robust transmissionCoverages are “discontinuous”. The Audio and video coding are scalable. For instance, a video flow can be split into three Flows which can be transported independently: one base layer (30 kbit/s), which is a robust flow but of limited quality (e.g. 5 images/s), and two enhancement flows (50 kbit/s and 200 kbit/s). The first flow provides availability, the other two quality and definition. In a streaming situation, the terminal will have three caches. In Pico cellular coverage, the parent coverage establishes the service dialog and service start-up (with the base layer). As soon as the terminal enters Pico cell coverage, the terminal caches are filled, starting with theFigure 8: Pico cell network design base cache. Video (and audio) transmissionsadvantage of coverage, especially when are currently transmitted without error anddesigned with caching technology, is high without packet loss. However, it is possible tospectrum efficiency, high scalability (from 50 allow error rates of about 10-5 /10-6 and ato 500 bit/s/Hz), high capacity and lower cost. packet loss around 10-2 /10-3. Coded imagesA specific architecture is needed to introduce still contain enough redundancy for errorcache memory in the network. An example is
correction. It is possible to gain about 10 dB coverage have yet been resolved. However,in transmission with a reasonable increase in indoor coverage can be obtained by:complexity. Using the described technologies, • Direct penetration; this is only possible inmultimedia transmission can provide a good low frequency bands (significantly Below 1quality user experience. GHz) and requires an excess of power, which may raise significant Interference issues.Coverage • Indoor short range radio connected to theCoverage is achieved by adding new fixed network.technologies (possibly in overlay mode) and • Connection via a relay to a Pico cellularprogressively enhancing density. Take a access point.WiMAX deployment, for example: first theparent coverage is deployed; it is then made Integration in a Broadband NGNdenser by adding discontinuous Pico cells, The focus is now on deploying anafter which the Pico cell is made denser but architecture realizing convergence betweenstill discontinuously. Finally the Pico cell the fixed and mobile networks (ITU-Tcoverage is made continuous either by using Broadband NGN and ETSI- TISPAN). ThisMIMO or by deploying another Pico cell generic architecture integrates all serviceCoverage in a different frequency band (see enablers (e.g. IMS, network selection,Figure 9). The ultimate performances of the middleware for applications providers), andvarious technologies are shown in Figure 10. offers a unique interface to applicationParent coverage performance may vary service providers.From 1 to 20 bit/s/Hz/km, while Pico cell Conclusiontechnology can achieve from 100 to 500 As the history of mobile communications shows, attempts have been made to reduce a number of technologies to a single global standard. Projected 4G systems offer this promise of a standard that can be embraced worldwide through its key concept ofFigure 9: example of deployment in dense traffic areas integration. Future wireless networks willBit/s/Hz/km?, depending on the complexity need to support diverse IP multimediaof the terminal hardware and software. These applications to allow sharing of resourcesperformances only refer to outdoor coverage; among multiple users. There must be a lownot all the issues associated with indoor
complexity of implementation and an which can reach between 100 and 500efficient means of negotiation between the bit/s/Hz/km2. The distributed, full IPend users and the wireless infrastructure. The architecture can deployed using two mainfourth generation promises to fulfill the goal products: base stations and the associatedof PCC (personal computing and controllers. Terminal complexity depends oncommunication)—a vision that affordably the number of technologies they can workprovides high data rates everywhere over a with. The minimum number of technologieswireless network. is two: one for the radio coverage and one forThe provision of megabit/s data rates to short range use (e.g. PANs). However, thethousands of radio and mobile terminals per presence of legacy networks will increase thissquare kilometer presents several challenges. to six or seven.Some key technologies permit the progressive REFERENCESintroduction of such networks without 1. B. G. Evans and K. Baughan, "Visions ofjeopardizing existing investment. Disruptive 4G," Electronics and Communicationtechnologies are needed to achieve high Engineering Journal, Dec. 2002.capacity at low cost, but it can still be done in 2. H. Huomo, Nokia, "Fourth Generationa progressive manner. The key enablers are: Mobile," presented at ACTS Mobile • Sufficient spectrum, with associated Summit99, Sorrento, Italy, June 1999.sharing mechanisms. 3. J. M. Pereira, "Fourth Generation: Now, It • Coverage with two technologies: parent Is Personal," Proceedings of the 11th IEEE(2G, 3G, and WiMAX) for real-time delivery, International Symposium on Personal, Indoorand discontinuous Pico cell for high data rate and Mobile Radio Communications, London,delivery. UK, September 2000.• Caching technology in the network andterminals.• OFDM and MIMO.• IP mobility.• Multi-technology distributed architecture. • Fixed-mobile convergence (for indoorservice).• Network selection mechanisms.Many other features, such as robusttransmission and cross-layer optimization,will contribute to optimizing the performance,