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Trends and Challenges in Broadband Wireless AccessHikmet Sari

Pacific Broadband Communications, 4-14 rue Ferms, 75683 Paris Cedex 14 [email protected]

Abstract This paper gives an overview of broadbandwireless access systems, discusses the current trends,and highlights the major technical challenges fo r futuresystems. The study covers local multipoint distributionservice @MDS networh that operate at millimeter-wave rad io frequencies above 20 GHz, as well asbroadband w ireless access at microwa ve frequenciesbelow I I GHz. We outline the current standardizationwork in this fie ld by the ETSI in Eu rope and the IEEE802.16 Group in the US. We point o ut that one of themajor problems is the intercell interference which limitsthe frequen cy reuse fac tor an d the cell capacity. Weaddress the issue of increasing cell capaciv usingadaptive modulation, code-division multiple accessCDMA), and adaptive antennas. We also discuss thebasic transmission technologies fo r lower frequencybands @elow 11 GHz where a significant amount ofmultipath propagation must be compen sated.

I. INTRODUCTIONWith the introduction of multi-gigabit routers andoptical transmission lines, the core telecommunications

network has become a very high-speed network that canoffer a large variety of services to the users. The speedbottleneck is the access network that connects the endusers to the edge and core networks, typically to thenearest central officeor add-drop multiplexer. The mostwell-known access network is the twisted-pair coppercable, which serves virtually all homes and businesses.These cables were traditionally used to carry voiceservices and low-speed data communications usingvoiceband modems. They are now used to offer digitalsubscriber line @SL) services which come in differentforms. High-speed DSL HDSL) uses two or threetwisted pairs to offer symmetric 2 Mbitfs data services,while the more recently developed asymmetric DSLADSL) technology offers a 6-8 Mbitfs data ratedownstream and several hundreds of kbit/s upstream.Similarly, coaxial cable networks were traditionally usedfor broadcast TV services, and they have recentlyevolved to bidirectional networks that offer high-speeddata and telephony services to the subscribers.In countries with a well-developed telecommuni-cations infrastructure, there has been little need in thepast for fixed wireless access. This type of systems wereessentially deployed in developing countries with a large

population that is not served by the twisted pairtelephone cable. Those wireless access systems,however, are narrowband, and can only carry telephonyand low bit-rate data services.The emergence of broadband wireless access is veryclosely related to the recent deregulation of the worldtelecommunications market. This deregulation hascreated a new environment in which new operators cancompete with incumbent operators that often are formerstate-owned monopolies. Wireless access networks arevery appealing to new operators without an existingwired infrastructure, because with respect to building awired network; they have the advantage of rapiddeployment and low initial investment. The initialinvestment is determined by the initial customer baserather than the complete target network, because once inplace, wireless networks are easily upgraded toaccommodate additional subscribers as the customerbase grows. This is a very attractive feature with respectto wired networks where most of the investment needs tobe made during the initial deployment phase.Most frequencies available for broadband wirelessaccess are at millimeter-wave frequencies between 20and 5 GHz. Dedicated frequency bands for theseapplications have recently become available in Europe,North America, Asia-Pacific, and other regions. After anextensive field trial period, broadband wireless accesssystems operating at millimeter-wave frequencies arecurrently in an initial field introduction phase beforeundergoing massive commercial deployment in theseveral years to come. These cellular radio networks,which are commonly referred to as local multipointdistribution service LMDS) networks, are intended tooffer integrated broadband services to residential andbusiness customers. LMDS networks are particularlysuited for urban or suburban areas with high userdensity, because the cell capacity is typically in the rangeof the STM-1 data rate (155 Mbit/s) and the cellcoverage is only 2 to 5km

Although less than in the millimeter-wave frequencyrange, there are also some frequency bands availablebelow 11 GHz. These include the microwave multipointdistribution service MMDS) and in the US, the 3.5GHz band in Europe, and the 10 Hz band in a numberof countries. Below 11 GHz, there are also some unlicensed frequency bands that may be used for wirelessaccess, but these will not be covered in the present paper.

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The purpose of the present paper is to give a state-of-the-art review of broadband wireless access systems,discuss the current trends and on-going standardizationwork, and indicate the technical challenges for futuresystems as well as some potential solutions. First, in thenext section, we give a brief introduction to LMDSnetworks. In Section 111, we present an analysis of theintercell interference in LMDS systems based on time-division multiplexing (TDM) on the downstream channel(from base station to users) and time-division multipleaccess (TDMA) on the upstream channel (from users tobase station). Next, in Section IV, we outline the currentstandardization work by the IEEE 802.16 and the ETSIBRAN Groups for next generation systems. Next, inSection V, we discuss several potential solutions toreduce intercell interference and increase cell capacity.Finally, In Section VI, we discuss broadband wirelessaccess at microwave frequencies below 11 GHz whichis essentially focused on residential applications. Ourconclusions are given in Section VII.11. AN OVERVIEW OF LMDS NETWORKS

LMDS was originally used to designate the 28 GHzband in the US, but it is used today to designate allbroadband wireless access systems operating atmillimeter-wave frequencies above 20 GHz. LMDSnetworks are cellular, each cell serving a number offixed subscribers located in its coverage area which has aradius of 2-5 km. The base station (BS) is connected tothe backbone network through a backhaul point-to-pointlink, which can be a radio link or a fiber.The network topology resembles that of mobilecellular radio systems, but fixed wireless access systemshave several distinctive features. The main of those isthat since users are at fixed locations, each user isassigned to a predetermined BS (typically the nearestBS). Furthermore, fixed wireless access systems employnarrowbeam directional subscriber antennas pointed tothe servingBS during installation. The increased gain inthe direction of the BS reduces network interference andincreases cell coverage.Another major difference concerns propagation.While mobile radio systems are subjected to shadowingand severe multipath propagation, LMDS systems arebased on clear line-of-sight between the BS and the fixedusers, and are virtually free of multipath propagation.Signal attenuation during normal propagation conditionsis proportional to the square of the distance, and whattruly limits the cell coverage is rain fading which furtherattenuates the transmitted signal by several dB or severaltens of dB per km. Due to this phenomenon and to thelimited power that can be generated at low cost atmillimeter-wave frequencies, the cell radius in LMDSnetworks is in the range of 2 to km depending on theclimatic zone, the available transmit power, and the.required availability objectives.Although LMDS systems can be based on hexagonalcell patterns which are commonly used in mobile radiosystems [ IJ rectangular cell patterns with 90 cell

sectoring have become very popular in LMDS networkdesign ([2], [3]) and will be considered throughout thispaper. Each sector is served by a 90 sector antenna, anddifferent frequency channels are assigned to the differentsectors. The channel bandwidth differs from region toregion, and for Europe and other countries which followthe CEPT channeling, the channel spacing is of the form112/2 MHz, where is an integer. The typical channelspacing for broadband wireless access can be expected tobe 28 MHz, unless the operator does not have sufficientbandwidth allocation.With a simple quatemary phase-shift keying (QPSK)modulation, a 28-MHz channel is sufficient to transmit auseful data rate of 16x2 Mbit/s. The total bit rate per cellis then 64x2 Mbit/s and can be used to serve for example64 business customers with a 2 Mbith leased line each.This example is only to give an idea of the cell capacity.In practice, the number of subscribers per cell may beseveral hundreds or several thousands, and such a largenumber of users are accommodated by dynamicallysharing the available resources between them. Assuming1000 subscribers, the 64x2 Mbit/s cell capacity gives abit rate of 128 kbit/s if all users are simultaneously activeand the total capacity is evenly shared between them.But different users have different types of traffic, andwhile some users may be requesting high instantaneousdata rates for high-speed intemet access, some otherusers may be idle. Therefore, the broadband wirelessaccess system example at hand can accommodate manyusers and guarantee very high peak data rates provided ithas an efficient medium access control (MAC) protocol.

111. ANALYSIS OF TDMA-BASED SYSTEMSDue to the lack of industry standards, first-generationbroadband wireless access systems are based onproprietary solutions. In fact, technical specifications for

LMDS systems were developed by the Digital VideoBroadcasting (DVB) Project [4] and the DigitalAudioVisual Council (DAVIC) [ 5 ] several years ago, butthese specifications were primarily intended for digitalbroadcasting applications and two-way communicationswith low interactivity. Virtually all proprietary LMDSsystems today as well s the DVB and DAVICspecifications are based on TDM on the downstreamchannel, TDMA on the upstream channel, andfrequency-division duplexing (FDD).Since bandwidth is a limited and costly resource, thefrequency reuse factor and the a'chievable cell capacityare crucial to the deployment of LMDS networks. Thesefactors are strongly impacted by intercell interference. Inthis section, we discuss intercell interference assuming arectangular cell pattern with 90 sectoring as mentionedpreviously. Throughout the paper, it is assumed that aseparate channel is assigned to each sector, which meansthat four channels are used to cover each cell. But thesame channels are reused in all cells as shown in Fig. 1.Note that channel assignment between neighboring cellsfollows a mirror-image rule in the horizontal, vertical,and diagonal directions.

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Fig. : Rectangular cell pattern with 90 sectorsThe BS's, which are designated by heavy dots in Fig.1, are located on a rectangular grid. The solid linesrepresent the sector borders, and the dotted lines indicatethe cell boundaries. The labels A, B, C, and D representthe channels used in different sectors. As it is indicatedin [2] and [3], the mirror-image assignment of thesechannels eliminates nterference between adjacent cells.But the second-nearest cells have the same channelassignment as the cell at hand and interfere with it. Let2 A designate the distance between two adjacent BS's inthe horizontal and vertical directions. Suppose now thata user is located on the border of two sectors at adistance a fiom the serving BS. This user's antenna willalso be pointed toward a second-nearest BS that is at adistance 4A+ a Assuming that all BS's transmit thesame signal power and that the signal attenuation is

proportional to the squared distance, which is a commonassumption in line-of-sight microwave and millimeter-wave radio systems, the downstream signal-to-interference ratio SIR) for this user isSIR dB)= 20.10{?)-

This expression, which is valid for 0 e a A , achieves itsminimum value for a A. This corresponds to an SIR of14 dB which represents the worst-case SIR for TDMA-based systems with a rectangular cell pattem and 90sectoring. In writing (l), we have assumed that BS'sfurther than the second-nearest BS are not in clear line-of-sight with the user of interest, i.e., their signals areblocked by buildings, trees, or other obstacles.As it is shown in [3], the worst-case SIR of 14dB salso valid for the upstream channel when automaticpower control is used. But the similarity of downstreamand upstream channels in terms of interference is limitedto the value of the worst-case SIR. On the downstreamchannel, the SIR is a function of the user position, andonly in a very small part of the cell, the users are

subjected to strong interference. Using a commonsubscriber antenna radiation diagram with a beamwidthof 5 , we have plotted in Fig. 2 the SIR distributionwithin a sector when the BS is located in the upper leftcomer. Specifically, the figure shows the boundaries ofthe regions corresponding to an SIR higher than a givenvalue. It clearly indicates that only in very small regionslocated at the other 3 comers, the SIR is lower than 15dB. t also shows that in virtually half of the cell size, theSIR is higher than 30 dB.

Fig.2: SIR distribution within a sector when the BS islocated in the upper left corner and the subscriberantenna beamwidth is 5 .This figure indicates that if the system design requires anSIR value higher than 15 dB, here will be three smallregions which will not be covered. Coverage will beeven smaller if the system design requires an SIR higherthan 20 or 25 dB. Also, a bandwidth-efficientmodulation scheme that requires a high SIR value willnot be usable if full cell coverage is required. But thefigure also suggests that while users at unfavorablepositions (regions of low SIR values) must use a low-level modulation scheme such as QPSK, users in morefavorable locations can use higher-level quadratureamplitude modulation (QAM) schemes such as 16-QAMor 64-QAM, at least during normal propagationconditions. This adaptive modulation (and coding)concept will be discussed in Section V.The situation is quite different on the upstreamchannel, because in this direction, all users get the sameamount of interference, i.e., the SIR is not a function ofthe user position. Consequently, it makes little sense touse different modulation schemes for different users, butthe adaptive modulation concept can still be used toadapt the modulation to propagation conditions.

IV. CURRENT STANDARDIZATIONWhile first-generation LMDS systems are currentlygoing into field deployment, standardization activities

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are now underway at both the IEEE and the ETSI todefine technical specifications for future systems. Thegroups that are carrying out this work within the IEEEand the ETSI are the IEEE 802.16.1 and the ETSI BRANGroups. Specification work by both groups covers thephysical-layer and the MAC-layer functions, and at thetime of this writing, the IEEE 802.16 Group has justfinalized and released its technical specifications [6] forbroadband wireless access systems at frequencies from11 to 60 GHz. As for the ETSI BRAN Group, it startedits standardization work later than the IEEE 802.16.1Group, and today, it can be expected that the ETSIBRAN specifications will not be completed before mid-2001. But this group has already made a number of basicchoices. These choices, which have a strong commona-lity with one of the physical-layer options in the IEEE802.16. specifications, include the following [7]:

The transmission technique is based on single-carriertransmission. The reason for this is that LMDSsystems suffer very little intersymbol interferenceISI), and this does not give much motivation forusing orthogonal frequency-division multiplexing(OFDM) which is attractive for strong Is1 channels[8]. In addition, the strong sensitivity of OFDM tooscillator phase noise and transmit power amplifiernonlinearity makes this technique rather undesirablefor systems operating at millimeter-wave frequen-cies, where high transmit power and low phase noiseincur high cost.

As in the earlier DVBDAVIC specifications, TDMand TDMA have been adopted for the downstreamchannel and the upstream channel, respectively. Thischoice can be justified by the relative maturity ofTDMA with respect to code-division multiple access(CDMA) that has been adopted in third-generationdigital mobile radio standards 191To increase cell capacity with respect to pure QPSK,adaptive modulation and coding will be used. Thepurpose here is to use the most bandwidth-efficientmodulation and coding schemes that are compatiblewith the signal-to-noise ratio (SNR) and theinterference level affecting the user of interest. Thisis function of the user position on the one hand (onthe downstream channel), and the instantaneous fadelevel on the other hand. The candidate modulationschemes are 4-QAM (QPSK), 16-QAM7 and 64-QAM for the downstream channel, and 4-QAM and16-QAM for the upstream channel.V. TECHNIQUES TO INCREASE CAPACITYWith respect to QPSK-based LMDS systems, the cellcapacity can be potentially increased using adaptivemodulation and coding and/or adaptive antennas at theBS. In this section, we briefly discuss the potential ofthese techniques on one hand and investigate the use ofCDMA on the other hand.

A . Adaptive modulationFor simplicity, we will limit our discussion here toadaptive modulation, but of course, the same applies toadaptive coding and to adaptive combined modulationand coding. Assuming that the SIR required is 12 dB for4-QAM (QPSK), 19dB for 16-QAM1and 25 dB for 64-QAM and using a subscriber antenna beamwidth of 6 , itwas shown in [lo] that an adaptive modulation thatcombines these three signal formats on the downstreamchannel achieves an increase of cell capacity by a factorof 2.7 with respect to QPSK. It was also indicated thatadaptive modulation is not directly applicable on theupstream channel, because all users are subjected to thesame level of interference. To use adaptive modulationon the upstream channel, it was proposed in [ O] to splitthe channel in two parts and assign each subchannel to aspecific region of the sector. It is possible to make thisassignment in such a way that some subscribers arenever affected by a high level of interference. Usingsubchanneling and an adaptive modulation involving the

QPSK and the 16-QAM signal formats, a capacityimprovement by a factor of 1.4 was achieved on theupstream channel. These results indicate that adaptivemodulation substantially increases the cell capacity,although to a lesser extent on the upstream channel.B. Adaptive antennasAdaptive antennas are another potential technique toincrease capacity on the upstream channel. Indeed, if theBS employs a steered narrowbeam antenna, only theusers near the sector borders in the horizontal andvertical directions and those near the diagonal will besubjected to strong upstream interference, and thesituation becomes similar to that on the downstreamchannel. Users located outside these regions can use a16-QAM or a 64-QAM modulation depending on theirlocation and the SIR value affecting them. The upstreamcell capacity then becomes similar to that of thedownstream channel. One difficulty to apply this conceptis that adaptive antenna technology is not yet mature formicrowave and millimeter-wave frequencies, but it canbe expected to mature within the next few years at leastfor microwave frequencies below 11 GHZ.C. CDMAlthough very popular in new generations of digitalmobile radio systems, CDMA has not yet become astrong candidate for broadband wireless access. Butdespite this, it is of significant interest to study itspotential for LMDS applications. A comparison ofTDMA and CDMA in LMDS systems [I 13 showed thatwhile CDMA reduces worst-case interference on theupstream channel, it leads to higher interference thanTDMA on the downstream channel. The comparison wasmade using TDMA and CDMA schemes with the samebandwidth occupancy and maximum cell capacity. Amultimode CDMA concept was then introduced for the

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downstream channel. This technique, which consists ofusing spreading sequences whose length and number area function of the user position, was shown to signify-cantly improve the worst-case SIR with respect toconventional TDMA and CDMA.VI. LOWER FREQUENCY BANDS

Both the IEEE 802.16 Group and ETSI BRAN firstput a priority on the definition of system specificationsfor broadband wireless access systems operating atfrequencies higher than 11 GHz, but they have nowtumed their attention toward licensed frequency bandsbelow 11 GHz. ETSI BRAN is still at the functionalrequirements phase, but the IEEE 802.16.3 Subgroup hasalready entered the technical specifications phase.In many aspects, broadband wireless access at lowerfrequencies is quite similar to LMDS, but it also has twobasic distinctive features: The first concerns the trafficmodel. Whereas LMDS systems are essentially intendedfor small business applications, frequencies below 11Hz are more likely to be used for residentialsubscribers where the major application is high-speedinternet access. The implication of this is that traffic willbe highly asymmetric, most of it being from the BS tosubscribers. This feature will have a strong impact onboth the physical layer and the DLC layer. The seconddistinctive feature is that due to larger cell sizes, smallersubscriber antenna directivity, and non-line-of-sightpropagation, lower-frequency bands are subjected to asignificant level of ISI, which must be compensated.One of the solutions for lower frequencies is to usethe same technical specifications as for millimeter-waves(LMDS). The only thing that needs to be added in thiscase is an adaptive equalizer that is capable of handlingthe multipath encountered in this kind of networks.Another solution consists of using the OFDM

technology which has been adopted in the IEEE 802.1and ETSI BRAN specifications for wireless local areanetworks (LANs) at 5 GHz. This technique is known tobe efficient against multipath fading when combinedwith appropriate channel coding and interleaving. This,together with the fact that it is relatively easier to designlow-cost radios than at millimeter-wave bands, mayfavor the use of OFDM at frequencies below 11 GHz.Both options may be viable, and it is not certain at thispoint which way standardization will go, but the recentadoption of OFDM in a number of ETSI and IEEEstandards puts this technique also in a strong position forbroadband wireless access at frequencies below GHz.VII. CONCLUSIONS

We have given an overview of broadband wirelessaccess networks which represent an attractive solutionfor new operators to reach the end users in high-densityurban or suburban areas. After briefly discussing the cellcapacity, frequency planning, and interference issues, wehave summarized the current status of standardizationwork by the ETSI BRAN and the IEEE 802.16 Groups.

We then highlighted the fact that the major problem inbroadband wireless access is intercell interference, anddiscussed a number of potential techniques to reduce itand increase cell capacity. These techniques includeadaptive modulation, adaptive antennas, and CDMA.Finally, broadband wireless access at frequencies below11 GHz is subjected to strong multipath propagation, andOFDM may be considered as an attractive technique forthis application.REFERENCES

T. S Rappaport, Wireless Communications:Principles and Practice, IEEE Press, New York,and Prentice Hall, New Jersey, 1996.H. Sari, Broadband Radio Access to Homes andBusinesses: MMDS and LMDS, ComputerNetworks, vol. 31, pp. 379 393, February 1999,Elsevier Science B.V., The Netherlands.G. LaBelle, LMDS: A Broadband Wireless Inter-active Access System at 28 GHz, in BroadbandWireless Comm., M. Luise and S Pupolin (Eds.),Springer-Verlag, Berlin, 1998, pp. 364-377.ETS 300 748, Digital Video Broadcasting (DVB):Framing Structure, Channel Coding, and Modula-tion for MVDS at 10 GHz and Above, ETSI,October 1996.DAVIC 1.1 Specifications art 8: Lower-LayerProtocols and Physical Interfaces, Revision 3.3,Geneva, September 1996.Air Interface for Fixed Broadband Wireless AccessSystems, IEEE 802.16.3Task Group, Sept. 2000.ETSI web site: www. etsi.orgH. Sari, G . Karam, and I. Jeanclaude, Transmis-sion Techniques for Digital Terrestrial V Broad-casting, IEEE Communications Magazine, vol. 33,pp. 100-109, February 1995.F.Adachi, M. Sawahashi, and H. Suda, WidebandDS-CDMA for Next-Generation Mobile Commu-nications Systems, IEEE Commun. Magazine, vol.36, no. 9, pp. 56-69, September 1998.J. P. Balech and H. Sari, Advanced ModulationTechniques for Broadband Wireless AccessSystems, Proc. the 7th European Conference onFixed Radio Systems and Networks (ECRR 2000),pp. 159-164, September 2000, Dresden, Germany.H. Sari, A Multimode CDMA Scheme withReduced Intercell Interference for LMDSNetworks, Proc. the 2 Intemational ZurichSeminar on Broadband Communications, pp. 307-312,February 2000,Zurich, Switzerland.

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