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UMTS/IMT-2000 Based on Wideband CDMA

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 IEE E Communi cations Magazi ne • September 19 98 70 UMTS/IMT-2000 B ased on Wid eb and C DM A 0163-6804/ 98/$10.00 © 1998 IE E E or more than a decade, research has been ongo- ing to find ena bling techniques to introduce multimedia capabilities into mobile communications. Research efforts have been aligned with efforts in the Inter- nationa l T elecommunication U nion (ITU ) and other bodies to find standards and recommendations which ensure that mobile communications of the future have access to multi- media ca pabilities and service quality similar to the fixed network. In Europe the E uropean C ommission has spon- sored research programs such as Research and D evelop- ment of Advanced C ommunication Technologies i n E urope, R ACE -1, and R ACE -2, and Advanced Communications Technology a nd Services (ACTS) in order to stimulate research on future mobile communication. In pa rticular, the projects COD IT (evaluation of code-division multiple access, CD MA) [1] and ATDMA (evaluation of time-divi- sion multiple access, TD MA) [2] within R ACE -2 and FR AME S [3] within ACTS have been very important fo r U niversal Mobile Telecommunications System (U MTS)/ Interna tional Mo bile Telecommunications in the year 2000 (IMT-2000). These efforts have been denoted third-generation (3G ) mobile communications” in pursuit of global recommenda - tions and standards for multimedia-capable mobile communi- cations. The q uestion of ma ss-market a cceptance of multimedia and discussion on finding market drivers for the enabling technologies have always been imperative. Many doubts have been raised on mass-market acceptance of nonvoice services. The “ silver bullet” in terms of data communication for the mass market has always been subject to questions. However, with the strong emergence of the Internet a nd Internet-based techniques to provide multimedia services to the mass market, this “silver bullet” has been clearly identi- fied. With the strong growth of the Internet, parallel with the growth of mobile telephony, provi ding multimedia capa bili ties to mo bile communications is equivalent to providing good I nternet access to mobile users. First-generation mobile communica- tions provided analog voice communi- cations and other telephony services to mobile users. The ma in first-generation standards are AMPS, TACS, and NMT. Second-generation mobile communi- cations provided digital voice communi- cations, and with that also data services, mainly circuit-switched low- to medium- rate data communications (e.g., 9.6 kb/ s). In a dditio n, the digita l systems facilitated potential service enhance- ments such as ma ny supplementary ser- vices and intelligent network capabilities. These second-generation systems are now well on the wa y to penetrat ing a globa l mass market. The second- gen- eration standards are G lobal System for Mobile Communica- tions (G SM), D igital AMP S (DAMPS)/IS-136, Personal D igital Cellular (PD C), a nd cdma One/IS-95. Wi th the intro duction of the third gen erat ion (U MTS/ IMT- 20 00 ), second- genera tion capa bilities (voice a nd low-/ medium- rate d ata) are extended, adding multimedia capabilities to second- generation plat forms such as support for high bit rates and intro duction of packet da ta/ IP a ccess . In line with the efforts of ITU to provide global recom- menda tions for IMT-20 00 , a spectrum identification ha s been mad e, identifying parts of the 2 G H z band fo r IMT-2000 usage (Fig. 1). From a sta ndard ization perspective, ITU has developed recommendations over a long period of time (since the late 1980s) for I MT-2000. In line with the ITU work a nd a lso with regulatory effo rts to ensure spectrum availability of the I TU identified IMT-2000 spectrum, various standards bodies are in the process of making standards for IMT-2000: the European Telecommunications Standards Institute (ETSI) in Europe, Association of R adio I ndustries and B usiness (ARIB ) in Ja pan, Telecommunications Industry Association (TIA) a nd T1P1 in the U nited St ates, a nd Telecommunications Technol- ogy Association (TTA) in South Korea. ETSI/ Special Mobile G roup (SMG ) has been responsible for U MTS stan da rdiza tion since the early 19 90s. A historic milestone was reached in January 1998, when the basic tech- nology for the U MTS terrestrial rad io access ( U TR A) system was selected. This decision conta ined the following key ele- ments: For th e paired ba nds 1920–1980 and 2110–2170 MH z wideband C D MA (W- CD MA) shall be used in frequen- cy- divisi on duplex (FD D ) operation For the unpaired bands of total 35 MHz time- division code-division multiple access (T D -CD MA) shall be used E rik Dahlman, Björn Gudm und s on, Mat s Nils son, and J ohan S köl d Ericsson Radio Systems AB F The UMTS terrestrial radio acces s is based on w ideband 4.096 Mchip/s DS-CDM A technolo gy. UTRA w ill be connected to an evolved GSM core netw ork fo r bot h circuit and packet services. A m erge betw een ETS I/E urop e and ARIB/J apan b ased on W-CDM A, a GSM core netw ork, and a com mo n fre- quency allocation according to the ITU Recommendation of 2 GHz makes a global IMT- 2000 standard feasible. UTRA based on W-CDMA fully support s the UM TS /I MT-2000 requirements (e.g., supp ort o f 38 4 kb/s for w ide-area coverage and 2 M b/s for local cover- age). Furthermore, the air interface has flexible support of mixed services, variable-rate ser- vices, and an eff icient packet mode. Key W-CDMA features also include imp roved basic capacity/coverage perform ance com pared to second-generation systems, full supp ort o f adaptive antenna arrays, support o f hierarchical cell structures with interfrequency han- dover, and suppo rt of asynchrono us inter-base-s tation operation. There have been no con- straints due to strong requirements for backward compatibility w ith second-generation systems. This has facilitated a h igh d egree of flexibility and a fut ure-proof air interface. Extensive evaluations by means of simulations and field trials have been carried out by a number of companies, and full system tests are ongoing. Consequently, W-CDMA technol- ogy can now be regarded as a matu re technology, ready to pro vide the basis for UMTS/IMT-2000. ABSTRACT
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IEE E Communications Magazine • September 199870

UMTS/IMT-2000Based on Wideband CDMA

0163-6804/98/$10.00 © 1998 IE EE

or more than a d ecade, research has been ongo-i n g t o f i n d e n a b l i n g t e c h ni q u e s t o i n t r o d u c e

m u l t i m e d i a c a p a b i l i t i e s i n t o m o b i l e c o m m u n i c a t i o n s .Research efforts have been aligned with efforts in the Inter-nationa l Telecommunication U nion (ITU ) and other bodiesto find standards and recommendations which ensure thatmobile communications of the future ha ve access to multi-media ca pabi l i t ies and service qual i ty s imilar to the f ixed

network. In Europe the E uropean C ommission has spon-sored research programs such as Research and D evelop-ment of Advanced C ommunication Technologies in E urope,R ACE -1 , and R ACE -2 , and Advanced Communica t ionsTechnology a nd Serv ices (ACTS) in o rder to s t imula teresearch on future mobile communication. In pa rticular, thep r o j e c t s C O D I T ( e va l u a t i o n o f c o d e - d i vi s io n m u l t i p l eaccess, CD MA) [1] and ATDMA (evaluation of time-divi-s i o n m u l t i p l e a c c e s s , TD M A ) [ 2 ] w i t h i n R A C E -2 a n dFR AME S [3] within ACTS have been very important fo rU n i v e r s a l M o b i l e Te l e c o m m u n i c a t i o n s S y s t e m(U MTS)/Interna tiona l Mo bile Telecommunications in theyear 2000 (IMT-2000).

These efforts have been denoted “ third-generation (3G )mobile communications” in pursuit of global recommenda -

tions and standards for multimedia-capable mobile communi-cations.The q uestion of ma ss-market a cceptance of multimedia

and discussion on f inding market dr ivers for the enabl ingtechnologies have always been imperative. Many doubts havebeen raised on mass-market acceptance of nonvoice services.The “ silver bullet” in terms of data communication for themass market has always been subject to questions.

Ho wever, with the strong emergence of the Internet a ndInternet-based techniques to provide multimedia services tothe mass market, this “silver bullet” has been clearly identi-fied. With the strong growth of the Internet, parallel with thegrowth of mobile telephony, providing multimedia capa bilities

to mo bile communications is equivalentto p rov id ing good I n te rne t access tomobile users.

First-generation mobile communica-tions provided analog voice communi-cations and other telephony services tomobile users. The ma in first-generatio nstandards are AMPS, TACS, and NMT.

Second-generation mobile communi-cations provided digital voice communi-cations, and with that also data services,mainly circuit-switched low- to medium-ra te da ta communica t ions (e .g . , 9 .6kb/s). In a dditio n, the digita l systemsfaci l i ta ted potent ial service enhance-ments such as ma ny supplementary ser-v ices and in te l l igen t ne twork

capabilities. These second-generation systems are now well onthe wa y to penetrat ing a globa l mass market. The second-gen-eration standards are G lobal System for Mobile Communica-t ions (G SM), D ig i t a l AMP S (D AMPS ) /IS-136, Persona lD igital Cellular (PD C), a nd cdma One/IS-95.

With the intro duction of the third gen erat ion (U MTS/IM T-2000), second -genera tion capa bilities (voice a nd low-/medium-rate d ata) are extended, adding mult imedia capabi li t ies to

second-generation plat forms such as support for high bit ratesand intro duction of packet da ta/IP a ccess.In line with the efforts of ITU to provide global recom-

menda tions for IMT-2000, a spectrum identification ha s beenmad e, ident ifying parts of the 2 G H z band fo r IMT-2000usage (Fig. 1).

From a sta ndard ization perspective, ITU has developedrecommendations over a long period of time (since the late1980s) for I MT-2000. In line w ith the ITU work a nd a lso withregulatory effo rts to ensure spectrum availability of the I TUidentified IMT-2000 spectrum, various standards bodies are inthe process of making standards for IMT-2000: the EuropeanTelecommunications Standards Institute (ETSI) in Europe,Assoc ia t ion o f R ad io I ndus t r ies and B usiness (ARIB ) inJa pan, Telecommunications Industry Association (TIA) a nd

T1P1 in the U nited St ates, a nd Telecommunications Technol-ogy Association (TTA) in South Korea.ETSI/Special Mobile G roup (SMG ) has been responsible

for U MTS stan da rdiza tion since the early 1990s. A historicmilestone was reached in January 1998, when the basic tech-nology for the U MTS terrestrial rad io access (U TR A) systemwas selected. This decision conta ined the following key ele-ments:• For th e paired ba nds 1920–1980 and 2110–2170 MH z

wideband C D MA (W-CD MA) shall be used in frequen-cy-division duplex (FD D ) operation

• For the unpaired bands of total 35 MH z t ime-divis ioncode-division multiple access (TD -CD MA) shall be used

Erik Dahlman, Björn Gudm und son, Mat s Nilsson, and Johan Sköl d

Ericsson Radio Systems AB

F

The UMTS terrestrial radio access is based on w ideband 4.096M chip/s DS-CDM A technolo gy. UTRA w il l be connected to an

e v o l ve d G SM c o r e n e t w o r k f o r b o t h c i r c u i t a n d p a c k et s e r vi c es. A m e rg e b e t w e e nETSI/Europ e and A RIB/Japan b ased on W-CDM A, a GSM core netw ork, and a com mo n fre-quency allocation according to the ITU Recommendation of 2 GHz makes a global IMT-2000 s t anda rd f eas ib l e . UTRA b ased on W-CDMA fu l ly suppor t s t he UM TS/ IM T-2000requirements (e.g., supp ort o f 38 4 kb/s for w ide-area coverage and 2 M b/s for local cover-age). Furthermore, the air interface has flexible support of mixed services, variable-rate ser-vices, and an eff icient packet m ode. Key W-CDM A featu res also includ e imp roved basiccapacity/coverage perform ance com pared to second-generation systems, full supp ort o fadaptive antenna arrays, supp ort o f hierarchical cell structures with interfrequency han-dover, and suppo rt o f asynchrono us inter-base-station operation. There have been no con-st ra in ts due to s t rong requirements for backward compat ib i l i ty w i th second-generat ion

systems. This has facil i tated a h igh d egree of flexibil i ty and a fut ure-proof air interface.Extensive evaluations by means of simulations and field trials have been carried out by anumber of companies, and full system tests are ongoing. Consequently, W-CDMA technol-o g y c a n n o w b e r e g a r d e d a s a m a t u r e t e c h n o l o g y, r e a d y t o p r o v i d e t h e b a s is f o rUMTS/IMT-2000.

ABSTRACT

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IEE E Communications Magazine • September 1998 71

in time-division duplex (TD D )operation

• P a r a m e t e r s sh a l l be c ho s en t ofacilitate easy implementation ofFD D /TD D dual-mode terminalsIn Ja pan, ARIB has been the focus

of IMT-2000 radio access activities.ARIB made an early decision for W-CD MA technology, and ARIB and E TSI have now harmo-nized their standards to the same W-CD MA technology.

In this article we will give the rationale behind the Euro-pean decision in fa vor of W-CD MA. In the next two sectionswe give some background information on the underlyingreq uirements o f U MTS/IM T-2000, and the e volutiona ry sce-narios from second-generation systems. We outline the mainelements from a network and radio access perspective to beintroduced into the second-generation platforms in their evo-lution t o U MTS/IM T-2000, thus providing t he fra mewo rk forthe introduction of W-CDMA technology. The following twosections are the ma in part of the article. The key features ofWC D M A a r e p o i n t e d o u t , a n d s o m e k e y a rg u m e n t s f o rchoosing W-CD MA a re given. We provide a detailed t echni-cal description of the W-CDMA air interface, with the mainfocus on the physical layer. Finally, a summary is given.

It should be mentioned that extensive evaluation of W-CD MA has been carried out, in both simulations and field tri-als. No results are given in this article; these can be found in[4, 5] and others.

UMTS/IMT-2000 R EQUIREMENTSTo provide end users with the necessary service quality formult imedia communicat ions, mainly Internet access andvideo/picture tra nsfer, high-bit-rate ca pabilities are req uired.

G ood -qua lity Internet access requires a couple of hundredkilobits per second peak rate (e.g., to download informationfrom the Web). Video, slow-scan video, and picture transferservices require bit rates ranging from a few tens of kilobitsper second to ro ughly 2 Mb/s, depending on q uality req uire-

ments.Thus, the bearer capability targets for the third generationhave been defined as:• 384 kb/s for full area coverage• 2 Mb/s for local area coverage

B ecause many multimedia applications are pa cket-orient-ed, it is essential to optimize third-generation techniques toeffectively cater fo r variable bit rate and packet capabilities.With this approach radio and network resources can be avail-able on a shared basis to many users, thus utilizing the natureof this type of communications in a resource-efficient manner.

Pro viding multimedia support also implies the need forflexibility. Being able to handle services with different bit ratesand E b /N 0 req uirements, and to multiplex such services in amultiservice environment is essential. Thus, third-generation

technology must be optimized fo r flexibility in a resource-effi-cient way.

GSM E VOLUTION TO UMTS/IMT-2000THE RADIO PERSPECTIVE

Adding third-generation capabilities from a radio access per-spective means mainly higher bit rate capabilities. Possiblescenarios depend on spectrum availability for the operato r.Depending on the spectrum situation, two different migrationscenarios must be supported, namely:• Refarming of existing spectrum bands• New or modified spectrum bands

To support the different spectrum scenarios, two third-gen-eration radio access building blocks have been defined:• ED G E uses high-level modulation for a 200 kHz TDMA

migration scenario, ba sed on plug-in transceiver equip-ment tha t can migra te existing bands in small spectrumchunks.

• U MTS is a new radio access network based on 5 MH zW-CDMA and optimized for efficient support of third-generat ion services. UM TS can be used in both new a ndexisting spectra .

THE NETWORK PERSPECTIVEAdding third-generation capabilities from a network perspec-tive implies the addition of packet switching, Internet access,and IP connectivity capabilities.

With this a pproach t he existing mobile net works will reusethe eleme nts of mo bility support, user authent icatio n/servicehand ling, and circuit switching. Pa cket switching/IP capa bili-ties will then be added to provide a mobile multimedia corenetwork by evolving existing mobile telephony networks.

THE GLOBAL APPROACHThe building blocks W-CD MA/ED G E for ra dio a ccess andpacket switching/IP n etwo rk capabilities have been acceptedin the standards selection process as the main technology ele-ments to e volve mobile communication s into multimedia /third-generat ion. The s tandardizat ion bodies represent ing theG SM, DAM PS, and PD C user communit ies have al l madethese selections. The standards selections are as follows:

ETSI/T1P1 (GSM)

• ED G E and W-CD MA radio access• Evolved GSM core network, including packet/IP capa bili-ties

ARIB/TTC • W-CD MA radio access• Evolved GSM core network, including packet/IP capa bili-

ties

TR45.3 (DAMPS/IS-136)• EDG E rad io access• Evolved I S-41 network with introduct ion of packet /IP

evolution

With these selections Japa n will join the global G SM com-

munity in the IM T-2000 perspect ive with W-CD MA a ndevolved G SM core networks. The D AMPS community will,with the ED G E select ion, obtain great synergies with theG SM community fo r IP -based services. Thus, the aspects of aglobal standard have been successfully reached.

From now o n , the focus o f th i s a r t i c le wi l l be on theETSI /U TR A W-CD MA pro posa l for IMT-2000.

KEY W-CDMA F EATURESIn this section, some of the key features of the W-CDMA airinterface will be discussed. A det ailed technical d escription,with emphasis on the physical layer, will be given later.

Figu re 1. Spectrum all ocation according to IT U.

M S S

r e g

. 2

M SSIMT-2000

Frequency in M Hz1800

M SS M S S

r e g

. 2

IMT-2000

1850 1900 1950 2000 2050 2100 2150 2200 2250

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IEE E Communications Magazine • September 199872

An air interface based on direct-sequence CDMA andoperation at a wide bandwidth gives the opportunity to designa sys tem wi th p roper t i e s fu l f i l l ing the th i rd -genera t ionreq uirements. The key properties emphasized in W-CD MAare:• Improved performance over second-generation systems,

including:– Improved capacity– Improved coverage, enabling migration from a second-generation d eployment

• A high degree of service flexibility, including:– Support of a wide range o f services with maximum bitrat es above 2 Mb/s and the po ssibility for mult iple para l-lel services on one connection– A fast and efficient packet-access scheme

• A high degree of operator flexibility, including:– Support of asynchronous inter-base-station operation– Efficient support of different deployment scenarios,including hierarchical cell structure (HCS) and hot-spotscenarios– Support of evolutionary technologies such as ada ptiveantenna arrays and multi-user detection

– A TD D mode designed for efficient operat ion in unco-ordinated environments

PERFORMANCE IMPROVEMENTSCapaci t y Imp rovement s — The wide ba ndwidth o f W-CD MA gives an inherent performance ga in over previous cel-lular systems, since it reduces the fading of the radio signal. Inadd i t ion , W-CD MA uses coheren t demod ula t ion in theuplink, a feature that has not previously been implemented ince l lu la r CD MA systems . Also , f a s t power con t ro l on thedownlink will give improved performance, especially in indoorand low-speed outdoor environments at low D oppler. In total,for a speech service, these improvements are expected toincrease the cell capacity of W-CD MA by at lea st a fa ctor oftwo (3 dB).

Also, for high-bit-rate packet services W-CDMA will givevery good capacity figures. In the ETSI W-CDMA evaluationrepor t, the simula tion s of a 384 kb/s packet da ta service show that fo r a low-mobility radio channel, a tota l bit rate of 1.9Mb/s is availa ble on the d ownlink of ea ch W-CD MA car rier.For a typical Web-browsing application this would mean that130 simultaneous active users could be supported with one W-CD MA ca rrier, ea ch user ha ving 384 kb/s packet a ccess withfast response. The tra ffic model a ssumes a user that activelylooks for information on the Web. The average page size is 40kbytes. Approximately every sixth page is read more carefully,giving an average time per page of 22 s.

In the above-mentioned capacity figures for W-CDMA,

improvements from techniques such a s ada ptiveantenna arrays, multi-user detection, or downlinka n t e n n a d i v e r s i t y h a v e n o t b e e n t a k e n i n t oaccount. W-CDMA has built in support for suchtechniques, which are likely to be introduced inthe future. Adaptive antenna arrays can be intro-duced efficiently since the downlink physical chan-nel carries dedicated pilot symbols. Furthermore,spread ing wi th shor t code s makes mul t i -use rdetection feasible. Transmit d iversity will also besupported.

Coverage and Link Budget Impro vements — The coverage o f W-CD MA is determined by thelink performance through the link budget as shownin [6]. The coverage d emonstrated for W-CD MAshows that it is possible to r euse G SM1800 cell

s i tes when migrat ing from G SM to W-CD MA support inghigh-rat e U MTS services. Assumptions for t he compa rison arethat the average mobile output power is equal in W-CDMAand G SM. W-CD MA receiver performance is based on theresul ts in [6], while G SM performance is based on a G SMimplementation.

The results show that a W-CDMA speech service will tol-erate a few dB higher path loss than a G SM speech service.This means that W-CD MA gives better speech coverage tha nG SM, reusing the same cell sites when being deployed in thesame or a ne arby freq uency band (e .g. , G SM1800 vs. theU MTS band). I n ad ditio n, a 144 kb/s circuit-switched d at aservice can operate with at lea st the same coverage as a G SMspeech service, thereby reusing the G SM cell sites.

SERVICE FLEXIBILITYOne of the most important characteristics of W-CDMA is thefact that power is the common shared resource for users. Inthe downlink, the total transmitted power of an RF carrier isshared between the users transmitting from the base stationby code-division multiplexing (CDM). In the uplink, there is amaximum tolerable interference level a t the base s tat ion

receiver. This maximum interference power is shared betweenthe transmitting mobile stations in the cell, each contributingto the interference.

Po wer as the common resource ma kes W-CD MA very flex-ible in handling mixed services and services with variable bitrate demands. Radio resource management is done by allocat-ing power to ea ch user (cal l ) to ensure that the maximuminterference is not exceeded. R eallocation of codes, time slots,and so on i s normal ly no t needed as the b i t r a te demandchanges, which means that the physical channel allocationremains unchanged even if the bit rate changes. Furthermore,W-CD MA req uires no frequency planning, since one cel lreuse is applied.

This flexibility is supported in W-CD MA w ith the use oforthogonal variable spreading factor (OVSF) codes for chan-

nelization of different users. The OVSF codes have the char-acter is t ic of maintaining downlink t ransmit or thogo nal i tybetween users (or different services allocat ed to o ne user)even i f they opera te a t d i ffe ren t b i t r a tes . One phys ica lresource can t hus carry multiple services with varia ble bitrates. As the bit rate demand changes, the power allocated tothis physical resource is adjusted so tha t q uality of service isguaranteed at any instant of the connection.

A typical scenario for a fully utilized W-CD MA systemincludes a mix of simultaneous high-speed packet da ta usersand low-rate voice user connections. Figure 2 shows an uplinkexample. This is done while mainta ining high capacity a ndcoverage fo r every se rvice . S ince power i s the common

Figu re 2. M ulti plexing variable bit-rate users.

Pow er levels from M S

Received po w er levels at BTSC A

C B

C C

C D

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IEE E Communications Magazine • September 1998 73

resource, multiplexing of services with very different char-acteristics can be achieved in an efficient way, utilizing thefull capacity of the W-CD MA carrier.

With the W-CDMA dual-mode packet access scheme,packet transfer can take place on both common and dedi-cated channels. In this way, the packet access can be opti-mized fo r fa st access/response as well a s for m aximumthroughput. The mode of o peration is ad aptively chosenbased on estimated packet traffic chara cteristics, and do esnot require any explicit user interaction.

OPERATOR FLEXIBILITYAn important flexibility aspect is the incorporation of linkimprovements. If a technique to improve the link-level perfor-mance is introd uced such as mult i -user ( joint) detect ion,downlink antenna diversity, or ada ptive antenna s, there is animmediate improvement for a ll users. The reason is tha t if thelink performance is improved even for only some of the links,the required power levels (and genera ted interference) forthese links is immediately reduced. With the common sharedpower resource, and since there is single-cell reuse, this has animmediate impa ct in reduced interfe rence for a ll users. Thereduced interference can be utilized as higher capacity, betterrange, or improved link quality.

Asynchronous Base Station Operation — In contrast tosecond-generation narro wband C D MA systems, W-CD MAdoes not require t ight inter-base-station synchronization. Thismeans, for example, there is no req uirement tha t ea ch basestation should be capable of reliable G lobal Po sitioning Sys-tem (G PS ) reception. This will significant ly reduce the deploy-ment efforts, especially in indoor environments.

Interfrequency Handover — The support of seamless inter-frequency handover through a downlink slotted mode is a keyfeature of W-CDMA, not previously implemented in cellularCDMA. Interfrequency handover is necessary for the supportof H CS with overlapping micro- and macrocells operating ondifferent ca rrier frequencies (Fig. 3). With the introduction o f

HCS, a cellular system can provide very high system capacitythrough the microcell layer, at t he same time o ffering full cov-erage and support of high mobility by the macrolayer. Inter-frequency handover is then needed fo r a ha ndover betweenthe different cell layers.

A second scenario where interfrequency handover is neces-sary is the hot-spot scenario, where a certain cell that serves ahigh traffic area uses add itional carriers to those used by theneighboring cells (Fig. 3). If the deployment of extra carriersis to be limited to the actual hot spot area, the possibility ofinterfrequency handover is essential.

Suppor t f o r Adap t i ve An t enna Ar r ays — As alreadymentioned, the W-CD MA system supports full utilization ofadaptive antennas through the use of dedicated pilot symbols

on both uplink and downlink.TDD Mode — Accord ing to the ETSI dec i s ion , theU TR A/TD D mode should be ba sed on TD /CD MA technolo -gy. During the subsequent E TSI process, the paramet ers ofthe TD D mode have been completely harmonized to the W-CD MA-based FD D mode. The main difference between theFD D and TD D modes is tha t the TD D mode inc ludes anadd i t iona l TD MA component , a l lowing fo r in te r fe renceavoidance by means of dynamic channel a l locat ion. Suchinterference avoidance capabilities are highly valuable in caseof uncoordinated operation of several systems within one geo-graphical area, a main appl icat ion for TDD mode. Conse-

quently, the U TR A/TD D mode no w provides a very goodcomplement to the U TR A/FD D mode.

A DETAILED DESCRIPTION OF THEW-CDMA A IR INTERFACE

This section d escribes the technical det ails of the W-CD MAair interface. The description is primarily focused on the phys-ical layer. Ho wever, a short discussion of the ra dio link con-trol (R LC) and media a ccess control (MAC) layers is a lsog iven . The desc r ipt ion focuses on the FD D mode of theU TR A concept. As already mentioned, the TDD mode is verysimilar to the FDD mode, but includes an additional TDMAcomponent to allow for operation in uncoordinated environ-ments. It should be noted tha t many of the technical deta ilsdescribed below are s t i l l under considerat ion a nd may bemodified during the ongoing refinement phase of the work onthe W-CD MA a ir interfa ce within E TSI. This is especiallytrue for the higher-layer protocols that were not part of theinitial U TR A evaluation and decision phase.

THE PHYSICAL LAYERBasic Radio Paramet ers — U TR A/FD D is based on 5 MH zW-CD MA wit h a basic chip ra te o f 4.096 Mchips/s, correspond -ing to a bandwidth o f approximately 5 MH z. Higher chip rates

(8.192 an d 16.384 Mchips/s) are a lso specif ied . These ch iprates are intended for the future evolution of the W-CDMAair interface to ward even higher da ta rates (> 2 Mb/s).

The basic radio frame length is 10 ms, allowing for low-delay speech and fast control messages.

W-CDMA carriers are located on a 200 kHz carrier gridwith typical carrier spacing in the range 4.2–5.0 MHz. Thisflexible carrier spacing allows for the optimization of ca rrierspacing for different deployment scenarios. As an example, alarger carrier spacing is typically needed between ca rriers indifferent cell layers, compared t o the ca rrier spacing neededbetween carriers in the same cell layer.

Transport Channels — Transport channels are the servicesoffe red by the W-CD MA phys ica l l ayer to h igher l ayers.

Transport channels are always unidirectional and either com-mon (i.e., shared amo ng several users) or dedicated (i.e., allo-cated to a specific user).

The following types of transport channels are defined inW-CD MA:• Broadcast control channel ( BCCH ) — A downlink com-

mon transport channel used to broadcast system- andcel l -specif ic control information. A BCCH is alwaystransmitted over the entire cell.

• Forwar d access channel (F AC H ) — A downlink commontransport channel used to carry control information andshort user packets to a mobile station, the location cell ofwhich is known to the system. An FACH may be trans-

Figu re 3. I nterfrequency handover.

Handover f 1 f 2 alwaysneeded betw een layers

Handover f 1 f 2 neededsometim es at Hot Spot

Hot-spot scenarioHCS-Scenario

f 1 f 1f 2

M acro M acroMicro

f 1 f 1

Hot spot

f 2f 1

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IEE E Communications Magazine • September 199874

mitted over the entire cell or over only a part of a cell(e.g., using adaptive antenna arrays).

• Pagin g channel (PCH ) — A do wnlink common tra nsportchannel used to carry control information to a mobilestation, the location cell of which is not known to the sys-tem. A PC H is always transmitted over the entire cell.

• Random access channel (RA CH ) — An uplink commontransport channel used to carry control information andshort user packets from a mobile stat ion. A R ACH isalways received from the entire cell.

• Dedicated channel (D CH ) — A downlink or uplink dedi-cated channel used to carry user data or control informa-t i o n t o /f r o m a m o b i l e st a t i o n . A D C H m a y b etra nsmitted /received over the ent ire cell or only part of acell (e.g., using adaptive antenna arrays).D ata arrives on the transport channel in the form of trans-

port blocks . A variable number of transport blocks arrive oneach transport channel at each transmission time instant. Thesize of the transport blocks is, in general, different betweendifferent transport channels and may also vary in time for aspecific transport channel. The time interval between consecu-tive transmission time instants, the transmission tim e interval ,is, in general, different for different transport channels but is

limit ed to the set { 10/20/40/80 ms} .The transmission time interval of atransport channel typically corre-sponds to the in te r l eaver spanapplied by the physical layer.

Table 1 illustrat es some exam-ples of t ransport channel formatsfor some typical service cases.

Physical Channel Structure Frame Struct ure — Figures 4 and 5 illustrate the basic W-CD MA fra me structure for downlink and uplink, respectively.Ea ch rad io fra me of lengt h 10 ms is spl i t into 16 slots oflength 0.625 ms, corresponding to one power-control period.

On the downlink, layer 2 dedicated data is t ime-mult i -plexed with layer 1 control information within each slot. Thelayer 1 control informa tion consists of known pilot bits fordownlink channel estimation, power contro l commands foruplink closed-loop power control, a nd a transport forma t indi-cator (TFI). As already mentioned, dedicated pilot bits areused instead of a common pilot in order to support, for exam-ple, the use of adaptive antenna arrays in the base station,also on the downlink. Dedicated pilot bits also allow for moreefficient downlink closed-loop power control. The TFI informsthe receiver about the instantaneous parameters (block sizeand number of blocks) of each transport channel multiplexedon the physical channel.

As shown in Fig. 4, the number of bits per downlink slot isnot fixed but may vary in the range 20–1280, corresponding toa ph ysical cha nnel bi t ra te in t he ra nge 32–2048 kb/s. Toachieve even higher bi t ra tes , mult iple downlink physicalchannels can be transmitted in parallel on one connection. Inthis case, the layer 1 control information is only transmitted

on o ne physical channel, while the corresponding fieldsof the other physical channels are empty.

In contrast to the downlink, there are two types ofdedicated physical channels defined for the W-CD MAupl ink: the up l ink ded ica ted phys ica l da ta channe l(uplink DPDCH) and the uplink dedicated physical con-

trol channel (uplink DPCCH). The DPDCH carries thelayer 2 dedicated da ta , while the D PC CH carr ies thelayer 1 control information. O n the uplink, layer 2 dataand layer 1 control information is thus transmitted i n parallel on different physical channels. The uplink layer 1control information is the same as for the do wnlink, (i.e.,pilot bits, power-control comma nds for downlink closed-loop power control, and a TFI).

On the uplink, the number of bits per slot may varyin the range 10–640, corresponding to a physical channelbit ra te in t he ra nge 16–1024 kb/s. To a chieve even high -er bit rates, multiple uplink DPDCHs can be transmittedin parallel on one connection.

Spreading and M odulation — Figure 6 illustrates the

spreading and modulat ion of the d ownlink dedicatedphysical channel. Data modulation is quaternary phaseshift keying (QPSK ), that is, a pair of bits is spread tothe chip rate using the same channelization code (binaryPSK, BPSK, spreading) and subsequently scrambled by acell-specific real scrambling code (BP SK scrambling).D ifferent physical channels in the same cell use differentchannelization codes. Several downlink physical channelscan be transmitted in parallel on one connection usingdifferent channelization codes in order to achieve higherchannel bit rates (multicode transmission).

The channelization codes are OVSF codes as definedin [7]. The OVSF codes preserve mutual transmit orthog-

Table 1. Transport-channel formats for some typical service cases.

Variable-rat e speech 10 or 20 m s Fixed (= 1) Variable

Packet data 10–80 m s Variab le Fixed (≈ 300 bits)

Circuit -sw itched data 10–80 m s Fixed (> = 1) Fixed

Service Transmission-t ime Number of t ransport Transport -blockint erval blocks per t ransmission- size

time interval

Figu re 4. Fram e structur e for the downli nk dedicated physical chan - nel.

Slot # 1 Slo t # m Slo t # 16

Layer 1 control(pilot + TPC + TFI)

0.625 m s, 20× 2k b i t s

One radio f rame = 10 ms

Layer 2 dat a

Figu re 5. Fram e structur e for the uplink dedicated physical chan nel.

Slot # 1 Slot # m Slo t # 16

DPDCH

0.625 m s, 10× 2k b i t sTPC:Transmit pow er control bitsTFI: Transpor t format indica tor ( ra te informat ion)

One radio f rame = 10 ms

Layer 2 dat a

DPCH Layer 1 con t rol (p ilo t + TPC + TFI)

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IEE E Communications Magazine • September 1998 75

onality between different downlinkphysical channels even if they usedifferent spreading factors and thusoffe r d i ffe ren t channe l b i t r a tes .The use of O VSF cod es is thus onekey fac t o r in the h igh degree o fservice flexibility of the W-CD MAair interface.

The d ownlink scrambling codeis a pseudo-noise code of len gth40,960 chips (10 ms). There are atota l of 512 different scramblingcodes available in the system, lead-ing to ve ry low requ i rements onexplicit scrambling code allocationbetween the cells. To support efficient cell search, the down-link scrambling codes are divided into 32 groups, each consist-ing of 16 codes.

Figure 7 illustrates the spreading and modulation for theuplink dedicated physical channels. D ata modulation is dual-channel QPSK, that is, the I and Q channels are used as twoindependent BPSK channels. For the case of a single DPDCH,the DP D CH and D PCC H a re spread by two different chan-ne l i za t ion codes and t ransmi t t ed on the I a nd Q bra nch ,respectively. If more than one D PD CH is to be transmitted,the additional DPDCHs can be transmitted on either the I orQ bra nch using additiona l channelization codes (multicodetransmission). The tota l spread signal I+ jQ is subsequentlycomplex scrambled by a connection-specific complex scram-bling code.

The upl ink channel iza t ion cod es are the same type ofO V S F c o d e s u se d f o r t h e d o w n l i nk i n o r d e r t o e n s ur eD PD CH /D PC CH transmit orthogonality. The uplink scram-bling code is norma lly a pseudo-noise code of lengt h 40,960chips (10 ms). However, as an option a short (256 chips) verylarge Ka sami code may be used as scrambling code. Short -code scrambling is used on system req uest to support low-complexity multi-user detection in the base station.

Downlink Common Physical Channels — The downlinkcommon physical channels have a structure very similar tothat o f the downlink dedicated physical channels; compareFigs. 4 and 6. The main difference is that the downlink com-mon physical channels are of fixed ra te (i.e., no TFI is need-ed). Furthermore, there is no corresponding power-controlleduplink (i.e., the down link common physical channels do notcarry any power-control commands. Consequently, the layer 1control information of the downlink common physical chan-nels consists of pilot bits only.

There are two types of downlink common physical channel:• The primary common control physical channel (primary

CC PC H) is of fixed predefined rate and is transmitted

on a predefined channel iza-tion code common to all cells.The primary CCPCH is usedto t ransmit the BC CH and isthe channel first acquired bythe mobile station (MS).

• The secondary common con-t ro l phys ica l channe l ( sec -o n d a r y C C P C H ) i s a l so o ffixed rate. H owever, the ratemay be different for differentsecondary CCPCHs within thece l l and be tween ce l l s. Thesecondary CCPCH is used totransmit the FACH and PC H.

Information about the channelization code of each sec-ondary CCPCH is broadcasted on the BCCH.

Transport-Channel Coding/Multiplexing — A key featureof t he W-CD MA a ir interface is the possibility to transportmultiple pa rallel services (transport channels) with d ifferentquality requirements on one connection.

The basic scheme for the channel cod ing and transport-channel multiplexing in W-CD MA is outlined in Fig. 8. Pa ral-lel transport cha nnels (TrCh -1 to TrCh -M) ar e separat elychannel-coded and interleaved. The coded transport channelsare then time-multiplexed into a coded composite transportchannel (C C-TrCh). F inal intraf rame (10 ms) interleaving iscarried out after transport-channel multiplexing.

Ch a n ne l C od in g — D i f f e r e n t c o d i n g a n d i n t e r le a v i n gschemes can be applied to a t ransport channel depending onthe specific requirements in terms of error rates, delay, a nd soforth. This includes the following:

Figu re 7. Upli nk spreading and modul ation

C c , C d Channelization codes (OVSF)S scramb Scrambling cod e (10 ms or 2 56 chips)p (t ) Ro o t -r ai sed co si n e, r o ll -o f f 0 .2 2

-sin(ω t )

C scramb

IQm ux

p (t )Im { }

I + jQ

cos(ω t )

p (t )

Re { }

C c

C d

Q

I

DPCCH

DPDCH

Figu re 8. Transport-channel coding/multipl exing in W-CD M A.

Rate-matching

Inter-frameinterleaving

CC-TrCh

Rate-matching

Inter-frameinterleaving

Channelcoding

Static rate-matching

M u l t i p l e x

Dynamic rate-mat ching

TrCh-1

Rate-matching

Inter-frameinterleaving

ChannelcodingTrCh-M

Figu re 6. Downl ink spreading and modulation.

S PDPCH

C ch Channelization code (OVSF)

S scramb Scram bling code (10 m s)p (t ) Ro o t -r ai sed co si ne , r o ll -o f f 0 .2 2

C ch C scram b

cos(ω t )

p (t )

-sin(ω t )

p (t )

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• R ate 1/3 convolutional coding is typically applied for low-delay services with modera te error rate requirements(BER ≈ 10–3).

• A concatenat ion of rate 1/3 convolut ional coding ando u t e r R e e d - S o l o m o n co d i n g + i n t e r le a v i n g c a n b eapplied for high-quality services (BE R ≈ 10–6).Turbo code s are a lso being considered a nd will most likely

be adopted for high-rate high-quality services.

Rate Mat ching — R ate ma tch ing i s app lied in o rder to

match the bit rate of the CC-TrCh to one of the limited set ofbit rates of the uplink or downlink physical channels; see theprevious section. As shown in Fig. 8, two different rate-match-ing steps are carried out:• Static rate matchin g — Stat ic rat e matching is carr ied

out at the add ition, removal, or redefinition of a trans-por t channe l ( i . e . , on a ve ry s low bas i s) . S ta t i c ra tematching is applied af ter channel coding and uses codepuncturing to a djust the channel-coding rat e of ea chtransport channel so that the ma ximum bit rate of theC C -Tr C h i s m a t c h e d t o t h e b i t r a t e o f t h e p h y si c a lchannel. Static rate matching is applied on both uplinkand d ownlink. On the downlink the static rate is usedto, if possible, reduce the CC -TrCh rate to the closestl o w e r p h y s i c a l c h a n n e l r a t e

(next higher spreading fa ctor),thus avoiding the over-al loca-tion of orthogonal codes on thedownlink and reducing the risko f a c o d e - l i m i t e d d o w n l i n kcapac i ty. S ta t i c ra te ma tch ingshould be dis tr ibuted betweenthe parallel transport channelsin such a way tha t the tra nsportc h a n n e l s f u l f i l l t h e i r q u a l i t yrequirements at a pproximatelyt h e s a m e c h a n n e l s i g n a l - t o -interference ratio (SIR ); that is,

s t a t i c r a t e m a t c h i n g a l so p e r -forms “SIR matching.”

• Dynamic ra te matching — D ynamic rate matching is carriedout once every 10 ms radio frame( i . e . , on a ve ry fas t bas i s ) .D y n a m i c r a t e m a t c h i n g i sapplied after tra nsport-channelmul t ip lexing a nd uses symbolrepet i t ion so that the instanta-neous bit rate of the CC-TrCh isexactly matched to the bit rate of

t h e p h y s i c a l c h a n n e l . D y n a m i c r a t ematching is only applied to the uplink.On the downlink, discontinuous transmis-sion within each s lot is used when theinstantaneous rate of the CC-TrCh doesnot exactly match the bit rate of the phys-ical channel.It should be noted that, although transport-

channel coding a nd multiplexing are carriedout by the physical layer, the process is fullycontrolled by the radio resource controller, forexample, in terms of choosing the appropriatecoding scheme, interleaving parameters, andrate-matching parameters.

Random Access — The W-CD MA ra ndomaccess i s based on a s lo t t ed Aloha scheme

where a random access burst can be transmitted in differentaccess slots , spaced 1.25 ms apart (Fig. 9).

Figure 10 illustrates the structure of a W-CD MA ra ndomaccess burst. It consists of two main parts, preamble and mes-sage. The pream ble consists of a length-16 complex symbolsequence, the rando m access signature , spread by a cell-specif-ic preamble co de o f length 256 chips. The message pa rt issplit into a data part and a control part similar to the uplinkDPDCH and DPCCH, respectively. The control part consistsof known pilot bits for channel estimation and a TFI which

indicates the bit rate of the data part of the random accessburst. The W-CD MA rando m a ccess scheme thus supportsvariable-rate random access messages. Between the preambleand message part s there is an idle time period of length 0.25ms (preliminary value). The idle t ime period allows for detec-tion of the preamble part and subsequent online processing ofthe message part.

Befo re a ra ndom access request can be carr ied out , themobile station must a cquire the following information fromthe BC CH of the target cell:• The cell-specific spreading codes available for the pream-

ble and message parts• The signatures and access slots available in the cell• The spreading factors allowed for the message part

Figu re 9. W-CD M A random-access structure.

Rando m -access burstAccess-slot # 1

1.25 ms

Rando m -access burstAccess-slot # 2

Rando m -access burstAccess-slot # 3

Rando m -access bu rstAccess-slot # 8

Figu re 11. Structure of W-CD M A synchron ization signal.

One radio frame (10 m s)

cp

cp : Pr imary synchronizat ion codecs

i,1:Secondary synchronization code

csi, 1

cp

csi, 2

cp

csi,16

One slot (0.625 m s)

Figu re 10. Structure of W-CD M A ran dom-access burst.

CRCM S ID

Preamble part

1 m s 0.25 m s

Rando m -access burst

10 ms

M essage part

Req. Ser.

Data part

Control part (pilot + TFI)

Option al packet

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IEE E Communications Magazine • September 1998 77

• Th e p ri ma r y C C P C H t r a nsm itpower level

• The uplink interference level at thebase stationThe s teps carr ied d uring a rand om

access request are as follows:1 Th e m o b i l e s t a t i o n s e l e c t s t h e

spreading codes to be used for thepreamble and message parts. Themobi le s t a t ion a l so se lec t s thespreading factor (i.e., the channelbit rate) for the message part.

2 The mobile station randomly selectsthe signature and a ccess slot to beused for the random access burst.

3 The mobi le s t a t ion es t imates thedownlink path loss and ca lculatesthe req uired uplink transmit powerto be used fo r the random a ccessburst.

4 The mobile station transmits the random access burst.5 The mobile stat ion waits for an a cknowledgment on a

corresponding downlink FACH. If no acknowledgment isreceived within a predefined timeout period, the randomaccess procedure of step 2 is repeated.

Asynchronous Base Station Operation — To a llow foreasy deployment in all types of environments, W-CD MA d oesnot require t ight inter-base-stat ion synchronizat ion. Thisaffects the way in which cell search as well as soft handoversynchronization are implemented in W-CD MA.

Cell Search w it h Asynchro nou s Base St atio ns— The W-CD MA cell search is a refinement of the scheme outlined in[8]. To support a n efficient cell search with a synchronousoperation, each W-CDMA base station transmits a specialsynchronization signal according to Fig. 11.

The synchronization signal consists of the following twosignals transmitted in parallel:

• A repeatedly t ransmitted unmodulated orthogonal G oldcode o f length 256 chips, the primary synchronizationcode (PSC ), with a period of one s lot . The PSC is thesame for every base station in the system and is transmit-ted time-aligned with the primary CCPCH (BCCH) slotboundary. By de tec t ing the PSC , the mobi le s t a t ionacquires slot synchronization to the target B S.

• A repeatedly transmitted length-16 sequence of unmodu- lated orthogo nal G old codes of length 256 chips, the sec-ondary synchronization code (SSC), with a period on oneframe. E ach SSC is chosen from a set of 17 differentorthogonal G old codes of length 256. There are a totalof 32 possible SSC sequences indicating to which of the32 d i ffe ren t code g roups the base s t a t io n downl inkscrambling code belong s. The sequences a re constructed

in such a way that their cyclic-shifts are unique, that is, anonzero cyc l i c sh i f t o f onesequence is not equal to any ofthe ot her 31 sequences. Co nse-quently, by detecting the SSCsequence the MS does not onlydetermine the scrambling codegroup, but also the frame tim-ing of the target BS.After the detection of the scram-

bling code group, the MS searchesall 16 dow nlink scrambling co des,typically using symbol-by-symbol

correlations over the f ixed-rate primaryCCPCH.

Figure 12 summarizes the three-stepcel l -search procedure ad opted for W-CDMA.

Soft Handover w ith AsynchronousBase Stations — Although W-CDMAbase stations are generally mutually asyn-chronous, inter-base-station synchroniza-tion on a connection level is needed incase o f so f t hand over. Sof t handoversynchronization is carried out as follows:• F ro m the cell sea rch, the mo bile

station can estimate the timing off-set between the downlink dedicatedchannel of the current base stationand the primary CCPCH of the tar-get base station.

• The e st ima te d t iming o ff set istransferred to the target ba se station using the currentlink with the old base station.

• The target base station uses the estimated timing offsetto adjust the timing of the new downlink dedicated chan-nel relative to that of the primary CC PC H. The adjust-ment i s do ne in s t eps o f 256 ch ips to p rese rve thedownlink transmit orthogonality of the ta rget base sta-tion.

• D u e t o a n (a p p r o xi m a t e l y) f i xe d o f f s e t be t w e e n t hedownlink and uplink frame timing, the target base stationcan , f rom the es t imated t iming o ff se t , e st imate theapproximate t iming of t he upl ink dedica ted physicalchannels to receive.

Slotted Mode — In order to support interfrequency hand-over measurements a W-CD MA connection can enter a slot-ted mode [9] . During the s lot ted mo de, the da ta norma llytransmitted during a 10 ms radio frame is instead transmittedin a shorter time, thereby creating an idle time period during

which the MS receiver is idle and t hus available for interfre-quency measurements (Fig. 13). The idle time period is creat-ed by either reducing the spreading factor or increasing thecoding rate, and thus does not lead to any loss of data. Notethat the slotted mod e is only needed fo r time-critical rea l-timeservices. In the case of non-real-time services, typically packetdata services, an idle time period for interfrequency measure-ments can ea sily be created by simply delaying packet trans-mission.

THE W-CDMA RLC/MAC L AYERIn ad di t ion to the d esign of the W-CD MA physical layer,there has also been significant effort devoted to the design ofthe higher layers of U TR A. Here we will focus on the RLCand MAC layers, which are responsible for efficiently transfer-

ring data of both real-time and non-real-time services. The

Figu re 13. Slotted-mode tran smi ssion .

M easurement p eriod

Synchronization signal o n different carrier

10 ms

Figu re 12. W-CD M A th ree-step cell search.

Search for PCSusing matched fil ter

DecodeSCS-sequence

Search all codes inlong-code group

Slot-tim ing acquired

Long-code acquired

Frame tim ing andlong-code groupacquired

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transfer of non-real-time data transfer includes the possibilityof low-level automatic repeat request (ARQ), offering higherprotocol layers reliable da ta transfer. In ad dition, the MAClayer controls but does not carry out the multiplexing of datastreams originating from different services.

Data Flow — In order to a chieve the requirements men-tioned above, the RLC layer segments the data streams intosmall packets, RLC protocol data units (RLC PDUs), suit-able for transmission over the ra dio interfa ce. In Fig. 14 thedata flow of the W-CD MA system is shown. Network-layerPD U s (N-PD U s) are f i rst segmented into smaller packetsand tra nsformed into link access control (LAC) PD U s. TheLAC overhead ( ≈ 3 octets) typical ly consists of a t least aservice access points identifier and sequence number forh i g he r -l e ve l A R Q a n d o t h e r f i e l d s . Th e L A C P D U s a r e

then segmented in to smal le r packet s , RLC PD U s, corre -sponding to the physical-layer transport blocks. Each R LCPD U conta ins a sequence number used fo r the low-leve lf a s t A R Q . A c y c li c r e d u n d a n c y c h e ck ( C R C ) f o r e r r o rdetection is calculated and appended to each R LC P D U bythe physical layer.

The da ta flow of the W-CD MA system is very similar tothe data flow of G eneral Packet Rad io Service (GP RS) [10].Ho wever, one important difference is that, in the G PR S, sys-tem a RLC PDU always consists of four bursts, while the coderate may vary.

O n t h e o t h e r h a n d , i n t h e W-C D M A s y st e m a l l R L CPD U s have the same size, regardless of t ra nsmission rate .This means that since the t ransmission rat e ma y change every10 ms , the number o f R LC PD U s t rans fe rred each 10 ms

varies.Model of Operation Packet Data Services — In this sect ion we describe themode l o f opera t ion when packe t s a re t r ansmi t t ed in theuplink. D ownlink packet tra nsmission is carried out in a verysimilar way.

In W-CD MA, packe t da ta can be t ransmi t t ed in th reeways. First, if a layer 3 packet is generated, the MS radio-resource control (RRC) may choose to transmit the packet onthe R ACH (i.e., included in the message part o f the accessburst) (Fig. 15). This type of common-channel packet trans-mission is typically chosen if there is only a small amo unt of

data to transmit (short or infrequentpackets) . In this case, no expl ici treservat ion is carr ied out , i .e . theoverhead is kept to a minimum. Fur-thermore, no explicit channel assign-ment is needed (i.e., the access delayis kept small). The ma in disadvan-tage is the risk of collisions on thec o m m o n R A C H a n d t h e f a c t t ha tthe R ACH is not power-controlled,l e a d i n g t o h i g h e r E b /N 0 r e q u i r e -ments.

The second a lternative for pa ckettransmission is illustra ted in Fig. 16.In th i s case , the MS f i r s t sends a

resource request message, indicating what type of tra f-fic is to be transmitted. The network then evaluatesw h e t h e r t h e M S c a n b e a s s i g n e d t h e n e c e s s a r yresources. If so, a resource allocation message is trans-mitted on the FACH. The resource allocation messageconsists of a set of transport formats and the specifica-tion of a dedicated channel to use for the packet trans-mission. Out of this set the MS will use one transport

format to tra nsmit the data on a D CH . Exactly which trans-port format the MS may use and at wha t t ime the MS mayinitiate its transmission is either transmitted t ogether with t heresource al locat ion message or is indicated in a separatecapaci ty-al locat ion message at a la ter t ime. In s i tuat ionswhere the traffic load is low, the first alternative is most like-ly to be used, while the second a lternat ive is used in caseswhere the load is high and the MS is not a llowed to immedi-ately tra nsmit the packet. In Fig. 16 the first alternative isillustrated.

This method of f irst requesting resources before tran s-mitting data is used in cases when the MS has large packetsto tra nsmit. The overhead caused by the reservation mecha-n i sm i s then neg lig ib le . D ue to the f ac t tha t the MS ge t sass igned a ded ica ted channe l , da ta t r ans fe r wi l l be morere l i ab le tha n wi th common-channe l t r ansmission in the

R ACH . The reason is that the dedicated channel is not as h a r e d c h a n n e l ( i . e . , n o c o l l i s i o n s w i ll o c c u r ) a n d t h a tclosed-loop power control is used on the ded icated cha nnel.The reason for assigning a set of transport formats and notonly one is that the t ra nsport forma t can t hen more easi lybe changed during transmission in order to allow for moreefficient interference control.

The third a lternative for packet transmission, illustrat ed inFig. 17, is used when there is already a dedicated channel avail-able. The MS ca n then either issue a capacity request on theD CH , when the MS has a large amount of data to tra nsmit, orsimply start. The MS can already have a DCH at its disposaldue to the fact that it uses it for another service. Another rea-son can be that the MS just finished transmitting packets on theD CH . It will then keep the D CH for a certain time. If in this

time new packets arrive, the MS may immediately start trans-mission, using the tra nsport format used during the la st datatransmission. Between packets on the DCH, link maintenanceis done by sending pilot bits and power control comma nds,ensuring tha t t he pa cket transmission is spectrally efficient.

If no new packets have been generated within a specifiedtimeout interval, the MS will release the D CH . H owever, theMS will keep the allocated transport forma t set. Thus, when ithas new packets to t ra nsmit , only a short capaci ty requestmessage need be transmitted on the RACH.

Real-Time Services — For real-time services the allocationprocedures are very similar. O nce an MS has data to tra ns-

Figu re 14. Segmentation and tran sform ation of network layer protocol data uni ts.

N-PDU

LAC-PDU LAC-overhead

Header

RLC-PDU

Transport block

Header

RLC-PDU

Transport block

Header

RLC-PDU

Transport block

Figu re 15. Packet transmission on a common channel ( RACH ).

Rando m -access burstincluding small packet

Rando m -access burstincluding small packet

Random-accesschannel (RACH)

Arbitrary time

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mit, it first issues a resource req uest mes-sage on the RAC H, o r, i f an MS a l readyhas dedicated channels assigned to it , onthe DC H a l loca ted fo r con t ro l in fo rma-t ion . As a resu l t the ne twork now a l lo -c a t e s t h e r e q u i r e d r e s o u r c e s, a g a i n b ym e a n s o f a s e t o f t r a n s p o r t f o r -mats. In contrast to the packet datacase, where the MS first waits for ac a p a c i t y a l l o c a t i o n m e s s a g e , t h eM S c a n n o w s t a r t t r a n s m i s si o nimmediately after it ha s received ar e s o u r c e a l l o c a t i o n m e s s a g e .A n o t h e r d i f f e r e n c e f r o m p a c k e tdata transmission is that the M S isnow a l lowed to use any t ranspor tf o r m a t a l l o c a t e d i n t h e r e s o ur c eallocation message. In this way the MS can support variablebit ra te services such as speech.

Mixed Services — The MAC should also be able to sup-port mult iple services. As mentioned previously, the physi-cal layer is capable of multiplexing bitstreams originatingfrom dif ferent services . The MAC protoco l controls thisp rocess by con t ro l l ing the da ta s t rea m de l ive red to thephysical layer over the transport channels. This control canbe particularly important when there is a lack of capacity inthe system.

If a n MS wants to transmit data of different services, forexample, a rea l-t ime service such as speech and a pa cketdata service, it is assigned two sets of transport fo rmats, onefor the real-time service and o ne for the packet da ta service.As mentioned in the single-service case, the MS may use anytransport format assigned for the real-time service, whereasit may only use one of the transport formats for the data ser-vice. In the multiservice case, the M S ma y use any tra nsportforma t assigned to it for t he speech service. In a ddition, theMS gets assigned a specific output power/rate threshold. Theaggregate rate of both services must be below this threshold.

The transport forma ts used for the da ta service are chosenout of the a llocated transport format set in such a wa y thatt h e a g g r e g a t e o u t p u t p o w e r /r a t e w i l l n e v e r e x ce e d t h ethreshold. Thus, the transport forma ts used for da ta servicef luctuates adapt ively to the used t ransport forma ts of thespeech service.

SUMMARYStarting in the early 1990s, extensive research in widebandCD MA has been carried out in Europe. EU -sponsored pro-grams such as CODIT and FRAMES have del ivered pro-posals fo r U MTS/IM T-2000. In J an uar y 1998 E TSI d ecide dto base the U MTS te r res t r i a l r ad io access (UTRA) on awideba nd 4.096 Mchip/s D S-CD MA techno logy. For t he

FD D mode (1920–1980 and 2110–2170 MH z) a pure W-CD MA technology wil l be used, and f or TD D (35 MHz intota l) a TD MA component is added . The radio netw ork willbe connected to an evolved G SM core network for both cir-cuit and packet connectio ns. A merge between ETSI/Euro peand AR IB /Ja pan based on W-CD MA, a G SM core network,and common freq uency al locat ions according to the ITUR ecommendat ion on 2 G Hz makes a global I MT-2000 stan-dard feasible.

U TR A based o n W-CD MA fully supports the U MTS/IM T-2000 req uireme nts (e.g. , support o f 384 kb/s with wid e ar eacoverage a nd 2 Mb/s with local co verage. H igh-bit-rate ser-vices will mainly be packet-oriented, with efficient a ccess to

the I nternet, and IP -based services. A very flexible air inter-face supporting a mix of services, variable-rate services, a nd avery efficient packet mode ha s been a strong driving forcewhen designing the proposal.

The key features of the U MTS W-CD MA proposal can besummarized as:• Fl exibl e suppor t of new mu lti media services: Mixed ser-

vices, var iable-rate services , and an ef f icient packetmode.

• I mpro vement o f basic capacity/coverage perform ance: Themain reason for the improvements is the extra frequencydiversi ty due to the high band width. With a coherentuplink and fast power control in both up- and downlinks,the basic performance is further improved.

• Support of in terfrequency hand over: To efficiently sup-por t h ie ra rch ica l ce l l s t ruc tu res and in te r f req uencyhandover, a slotted transmission with idle time periodsi s used . D ur ing the id le pe r iods , measurements onother freq uecies can be carr ied out . These measure-ments are then used by the radio netwo rk for handoverdecisions.

• Support of adapti ve antenna arr ays: Dedicated pilot sym-

bols in both up- and downlinks facilitate user-uniqueantenna patterns using adaptive antenna arrays.• Asynchrono us base stati ons: There is no requirement on

inter-base-station synchronization; therefore, there is norequirement on a ny external system such as G PS. New efficient cell search schemes have been designed to facili-tate this.

• The TDD mode: A TD MA component i s inc luded tosupport uncoordinated operation (e.g., in unlicensedbands).There have been no constraints due to backward compati-

bility requirements to second-generation systems, which hasfacilitated the high degree of flexibility and future-proofness.Extensive field trials have been made by a number of compa-nies , and f ul l system tests are ongoing . To conclude , W-

CD MA is now a mature t echnology, r eady to p rov ide thebasis for a true third-generation mobile communications sys-tem with full multimedia capabilities.

REFERENCES[1 ] A . Ba ie re t a l . , “Des ign S tudy f o r a CDMA-Based Th i rd Genera

Mobile Radio System,”IEEE JSA C , vol. 12, May 1994.[ 2 ] A . U r i e, M . S t r e et o n , a n d C. M o u r o t , “ A n A d v a n c e d TD M A

Access System for UMTS,”Proc. IEEE PIM RC ’94 , 1994.[3] P. O. And ressonet a l ., “FRAMES Multiple Access — A Concept fo

UMTS Radio Interface”Proc. ACTS Mobile Commun, Summit ’97 , vol. 1,Aalborg, Denmark, 1997.

[4] M . Ew erbring, J. Farjh, and W . Granzow, “ Performance Evaluation oband Testbed Based on CDM A,”Proc. VTC ’97 , Phoenix, AZ, May 199 7.

Figu re 16. Packet transmission on a dedicated channel (D CH ).

Random-accessburst

Packet

Random-access channel (RACH)

Dedicated channel (DCH)

Random-accessburst

Packet

Figu re 17. Packet tr ansmission on the dedicated channel.

Capacityrequest

Link m aintenance

Scheduledpacket

Dedicated channel (DCH)

Unscheduledpacket

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[5] M . Sund elin, W . Granzow, and H. Olofsson, “ A Test System for Evaluationof the W-CDMA technology,”Proc. VTC ’98 , Ottawa, Canada, May 1998.

[6] ETSI UM TS 30 .06, “ UM TS Terrestrial Radio Access (UTRA) Concep t Evalu-at ion,” 1998.

[7] F. Adachi, M. Sawahashi, and K. Okawa, “ Tree-structured generation ofor thog onal spreading codes with different lengths for forward l ink ofDS-CDMA mobile radio,”Elect. Lett., vol. 33, Jan. 1997.

[8] K. Higuchi, M . Sawah ashi, and F. Adachi, “ Fast cell search algorith m inDS-CDMA mo bi l e r ad io us ing long sp read ing codes, ”Proc. VTC ’97 Conf., Phoenix, AZ, May 1997.

[9] M. Gustafssonet al ., “Compressed Mode Techniques for Inter-FrequencyM e a su r e m e n t s i n a Wi d e - b a n d D S- CD M A Sy st e m , ”Proc . 8 t h IEEE PIMRC , Helsinki, Finland, Sept. 1997 .

[10] J. Cai, and D. J. Goodman, “General Packet Radio Service in GSM,”IEEE Commun. Mag., Oct. 1997.

ADDITIONALREADING[1] F. Ovesjoet al., “ FRAM ES Multiple Access Mod e 2 – Wideb and CDMA,”

Proc. IEEE PIM RC ’97 , Helsinki, Finland, Sept. 1997 .[ 2 ] H . H o l m ae t a l . , “ Ph y s i ca l L a ye r o f FRA M E S M o d e 2 – Wi d e b a n d

CDMA,”Proc. VTC ’98 , Ottawa, Canada, May 1998.[3] E. Dahlman, A. Toskala , and M . Latva-aho, “ FRAM ES FM A2, a W ide-

band-CDMA Air-Interface for UMTS,”Proc. CIC ’97 , Seoul, South Korea,Oct. 1997.

[4] A. Toskalaet al., “ FRAM ES M ultip le Access Mo de 2 Physical Transpo rt Con-trol Functions,”Proc. ACTS Summit 1997 , Aalborg, Denmark, Oct. 1997.

[5] E. Nikulaet al., “FRAM ES M ultip le Access for UM TS and IM T-2000 ,”IEEE Pers. Commun., Apr. 1998.

[6] S. Onoe, K. Ohno, K. Yamagata , and T. Nakamura, “W ideband-CDM A

Radio Control Techniques for Third-Generation Mobile CommunicationSystems,”Proc. VTC ’97 , Phoenix, AZ; May 1997.

BIOGRAPHIESERIKDAHLMAN(erik.dahlman @era-t.ericsson.se) received h is Master’s degreein electrical engineering from the Royal Institute of Technology, Stockholm,Sweden in 19 87, and his Doctorate in te lecomm unicat ion t heory f rom t hesame university in 199 2. In 1 993 he joined the Radio A ccess Research Labo-

ratory of Er icsson Radio Systems where he is current ly involveddevelopment and standardization of W-CDMA for UMTS/IMT-200ETSI and ARIB.

BJÖRNGUDMUNDSON[M ‘85] ([email protected]) recean M.S.Sc. degree in electrical engineering from the Royal Institute nology, Sweden, in 1981, an M.S.Sc. in electrical engineering from SUniversity, California, in 1985, and a Ph.D. degree in electrical engfrom the Royal Institut e of Technology, Sweden, in 1988. In 1 981 hEricsson Radio Systems, w orking primarily on development of privmobile radio systems. During 1984 to 1988 he studied adaptive eqt i o n t e c h n i q u e s f o r m o b i l e ra d i o c h a n n e l s , b o t h a s p a r t o f h i sresearch an d f or t he predevelopment of GSM signal p rocessing at ERadio Systems. In 1988 he joined the research depar tment a t Er iRadio System s, engag ed in the M obile RACE project , pr imari ly smicrocellular techniques. In 1990 he became manager for a researcon rad io access/radio transmission t echniques. From 1992 to 1995 hman ager of the Radio Access and Signal Processing Research departa l so i n c l u d i n g s p e ec h c o d i n g a n d e c h o c a n c el l a t i o n . I n 1 9 9 5 happointed research director for the Radio Access, Radio Network an& Visual Technology Research areas.

MA TSNILSSON([email protected]) joined Ericsson Radiotems AB in 1987 as a systems engineer and project manager. From 11992 h e was a technical manager for radio comm unications at Nipposson KK, Japan. His main respon sibility w as in developm ent o f PDC.s en t p o s i t i o n f r o m 1 9 9 2 i s d i r e ct o r o f t e c h n i c a l st r a t e g i e s f o r systems in Ericsson’s business area for mobile systems, responsible fur ther evolut ion of mob i le systems and t he int roduct ion of t h i rdt ion capabi l it ies (UMTS/IMT2000) into m obi le comm unicat ions . degrees in theoretical physics (B.Sc. and Licentiate of Philosophy) fUniversities of Uppsala and Stockholm.

JO H A NSKÖLD(Johan.Skold@era- t .er icsson.se) has been with the RaAccess Laborat ory o f Er icsson Radio System s since 198 9, w here hbeen actively involved with the standardization and evolution of Grecently also in the d evelopm ent and standardization of UMTS/IM TETSI. He has an M .Sc. in electrical engin eering fro m the Royal InstTechnology, Stockholm, Sweden, in 1987, and one from the UniveWashington, Seattle, in 1 989.


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