Soft handover issues in radio resource management for 3G WCDMA networks

Transcription

1 Soft handover ssues n rado resource management for 3G WCDMA networks Chen, Yue For addtonal nformaton about ths publcaton clck ths lnk. nformaton about ths research obect was correct at the tme of download; we occasonally make correctons to records, please therefore check the publshed record when ctng. For more nformaton contact scholarlycommuncatons@qmul.ac.uk

2 Soft Handover ssues n Rado Resource Management for 3G WCDMA Networks Yue Chen Submtted for the degree of Doctor of hlosophy Department of Electronc Engneerng Queen Mary, Unversty of London September 3

3 To my famly

4 Abstract Moble termnals allow users to access servces whle on the move. Ths unque feature has drven the rapd growth n the moble network ndustry, changng t from a new technology nto a massve ndustry wthn less than two decades. Handover s the essental functonalty for dealng wth the moblty of the moble users. Compared wth the conventonal hard handover employed n the GSM moble networks, the soft handover used n S-95 and beng proposed for 3G has better performance on both lnk and system level. revous work on soft handover has led to several algorthms beng proposed and extensve research has been conducted on the performance analyss and parameters optmsaton of these algorthms. Most of the prevous analyss focused on the uplnk drecton. However, n future moble networks, the downlnk s more lkely to be the bottleneck of the system capacty because of the asymmetrc nature of new servces, such as nternet traffc. n ths thess, an n-depth study of the soft handover effects on the downlnk drecton of WCDMA networks s carred out, leadng to a new method of optmsng soft handover for maxmsng the downlnk capacty and a new power control approach. 3

5 Acknowledgement would lke to thank rof. Laure Cuthbert for hs supervson, knowledge, support and persstent encouragement durng my hd at Queen Mary, Unversty of London. Many other staff ncludng John Bgham, John Schormans and Mark Rayner, have gven me help and valuable advce durng ths perod, they have my thanks. My studes would not have been complete wthout the help and the frendshp of others, Felca, Ene, Elane, Laurssa, Rob and Veseln gave me so much help and support when frst came to UK. They wll always have a place n my fond memores at QMUL. Also, want to thank many other frends, Jb, Yong, Nng, Xaoyan, Hua, Shaowen and Bo for the good tmes and the frendshp. would also lke to extend my thanks to the numerous members of support staff who have made the techncal and fnancal part of my work possble. Fnally, wth my love and grattude, want to dedcate ths thess to my famly who have supported me throughout, especally to my mother and my late father. 4

6 Table of Contents Abstract...3 Acknowledgement...4 Table of Contents...5 Lst of Fgures...9 Lst of Tables... Glossary...3 Chapter ntroducton...6. ntroducton...6. Contrbuton Organsaton of the Thess...7 Chapter Moble Communcaton Networks...9. Evolvng Moble Networks Frst-generaton analogue moble systems Second-generaton & phase moble systems Thrd-generaton moble systems and beyond Obectves and requrement Ar nterface and spectrum allocaton G systems and beyond...5. Overvew of CDMA Technology rncples of spectrum spreadng (CDMA) Spreadng and de-spreadng Multple access Features of WCDMA Rado Resource Management RRM n moble networks Functonaltes of RRM ower control Handover control Admsson control Load control (congeston control)

7 .4 Summary...4 Chapter 3 Handover Overvew of Handover n Moble Networks Types of handover n 3G WCDMA systems Obectves of handover Handover measurements and procedures Soft Handover (SHO) rncples of soft handover Algorthm of soft handover Features of soft handover Motvaton for the Work n ths Thess revous work on soft handover Exstng problems (motvaton of the work n ths thess) The work n ths thess...55 Chapter 4 Lnk Level erformance Analyss ntroducton System Models Rado envronment of moble networks Rado channel model System scenaro Downlnk nterference Analyss Overvew ntra-cell & nter-cell nterference Soft handover effects on downlnk nterference Downlnk ower Allocaton ower allocaton wthout SHO ower allocaton wth SHO Conclusons...75 Chapter 5 System Level erformance Analyss ntroducton Downlnk Soft Handover Gan ntroducton Soft handover gan mpacts for soft handover gan...8 6

8 5.3 Cell Selecton/Reselecton Schemes ntroducton Basc prncples of dfferent CS schemes Effects of dfferent CS schemes on SHO gan Soft Handover Algorthms ntroducton Dfferent SHO algorthms SHO zone of dfferent SHO algorthms Downlnk ower Control ntroducton ower allocaton under three power control condtons ower control effects on SHO gan Mult-rate Servces Envronment ntroducton Mult-servce structure Results and Dscusson ntroducton SHO gan wth dfferent CS schemes SHO gan wth dfferent algorthms SHO gan under dfferent power control condtons SHO gan n mult-servce envronment Summary...7 Chapter 6 Soft Handover Optmsaton ntroducton rncples of Optmsaton Dervaton of Optmum Overhead and Thresholds Results and Conclusons...5 Chapter 7 Optmsed ower Control Strategy durng Soft Handover ntroducton Overvew of ower Control durng SHO rncples of New Approach Feasblty Evaluaton erformance Analyss Results and Dscussons...4 7

9 7.7 Conclusons...8 Chapter 8 Conclusons Conclusons Future Work...3 Appendx A Author s ublcatons...3 Appendx B Verfcaton and Valdaton...33 Appendx C nterference Calculaton of 37 cells...35 Appendx D Load Factor & Downlnk ole Equaton...37 References

10 Lst of Fgures Fgure. Evoluton of moble networks... Fgure. Bt rate requrements for some 3G applcatons [Sau]... Fgure.3 User bt rate versus coverage and moblty...4 Fgure.4 MT- ar nterfaces [Scr]...4 Fgure.5 Spectrum allocaton n dfferent countres...5 Fgure.6 Systems beyond 3G (Source: TU-R M. [FLMTS.REVAL])...6 Fgure.7 Heterogeneous network wth nterworkng access...7 Fgure.8 Spreadng and de-spreadng...9 Fgure.9 Multple access technologes...3 Fgure. rncple of spread-spectrum multple access...3 Fgure. Typcal locatons of RRM functonaltes wthn a WCDMA network...34 Fgure. Near-Far effects (power control n UL)...35 Fgure.3 Compensatng the nter-cell nterference (power control n DL)...35 Fgure.4 General outer-loop power control algorthm...37 Fgure.5 Load curve...38 Fgure 3. Scenaros of dfferent types of handover...4 Fgure 3. Handover procedures...44 Fgure 3.3 Comparson between hard and soft handover...46 Fgure 3.4 rncples of soft handover (-way case)...47 Fgure 3.5 S-95A soft handover algorthm...48 Fgure 3.6 WCDMA soft handover algorthm...49 Fgure 3.7 nterference-reducton by SHO n UL...5 Fgure 3.8 Asymmetrc applcatons for 3G systems...55 Fgure 4. Rado channel attenuaton...59 Fgure 4. System scenaro...6 Fgure 4.3 Uplnk nterference...6 9

11 Fgure 4.4 Downlnk nterference...63 Fgure 4.5 χ, relatve downlnk nter-cell nterference...65 Fgure 4.6 η, nter-cell to ntra-cell nterference rato...66 Fgure 4.7 Senstvty of relatve downlnk nterference to rado parameters...66 Fgure 4.8 Soft handover effects on the downlnk nterference...67 Fgure 4.9 Mean β, β, β 3 vs. moble locaton...7 Fgure 4. Cumulatve dstrbuton functon of β, β, β Fgure 4. Mean total power vs. moble locaton...74 Fgure 4. Downlnk traffc channel power...74 Fgure 5. Soft handover zone and effectve cell coverage...78 Fgure 5. Cell layout...79 Fgure 5.3 Cell layout for cell selecton...83 Fgure 5.4 Flowchart of perfect cell selecton...84 Fgure 5.5 Flowchart of normal cell selecton...85 Fgure 5.6 Flowchart of S-95A soft handover algorthm...87 Fgure 5.7 Flowchart of UTRA soft handover algorthm...88 Fgure 5.8 Comparson of soft handover zone of dfferent algorthms...89 Fgure 5.9 Soft handover gan wth dfferent cell selecton schemes...99 Fgure 5. Soft handover gan under normal cell selecton wth dfferent CS_th... Fgure 5. Soft handover gan wth dfferent sze of actve set... Fgure 5. Comparson of S-95A and UTRA soft handover algorthms... Fgure 5.3 Senstvtes of soft handover gan...3 Fgure 5.4 E b / vs. moble locaton...4 Fgure 5.5 Soft handover gan under dfferent power control condtons...5 Fgure 5.6 Capacty gan wth dfferent mult-servce structures...5 Fgure 6. rncple of the soft handover optmsaton... Fgure 6. Flowchart of optmsed soft handover...3 Fgure 6.3 Soft handover optmsaton for maxmsng downlnk capacty...5 Fgure 7. Downlnk power control durng soft handover...8

12 Fgure 7. Relatve total transmt power for mobles n soft handover...3 Fgure 7.3 DL capacty gan wth dfferent power control schemes...5 Fgure 7.4 Capacty gan wth dfferent actve set sze...6 Fgure 7.5 Senstvtes of the capacty gan due to the optmsed power control...7 Fgure 7.6 Optmum overhead wth dfferent power control schemes...7

13 Lst of Tables Table. Techncal parameters of second-generaton dgtal systems... Table. Expected spectrums and ar nterfaces for provdng 3G servces...5 Table.3 Man WCDMA parameters...33 Table 5. Assocaton table...8 Table 5. arameters of dfferent servces...95 Table 5.3 System parameters...98 Table 5.4 Mult-servce assocaton table...6

14 Glossary G 3G 3G 3G 4G AC AGC AMS AMR B(T)S B3G systems BER BoD BSK CDF CDMA CN CCH DAB DECT DL DCCH DDCH DQSK DS-CDMA nd Generaton 3 rd Generaton 3 rd Generaton artnershp roect (produces WCDMA standard) 3 rd Generaton artnershp roect (produces cdma standard) 4 th Generaton Admsson Control Automatc Gan Control Advanced Moble hone Servce Adaptve Multrate (speech codec) Base (Transcever) Staton systems Beyond 3G Bt Error Rate Bandwdth on Demand Bnary hase Shft Keyng Cumulatve Dstrbuton Functon Code Dvson Multple Access Core Network Common lot Channel Dgtal Audo Broadcastng Dgtal Enhanced Cordless Telecommuncaton Downlnk Dedcated hyscal Control Channel Dedcated hyscal Data Channel Dfferental Quadrature hase Shft Keyng Drect-Sequence Code Dvson Multple Access 3

15 DVB EDGE ETS FCC FDD FDMA FLMTS GMSK GRS GS GSM HHO HO HSCSD Dgtal Vdeo Broadcastng Enhanced Data Rates for GSM Evoluton European Telecommuncaton Standard nsttute Federal Communcaton Commsson (US) Frequency Dvson Duplex Frequency Dvson Multple Access Future ublc Land Moble Telecommuncatons System Gaussan Mnmum Shft Keyng General acket Rado Servce Global ostonng System Global System for Moble Communcatons Hard Handover Handover Hgh Speed Crcut Swtched Data MT- nternatonal Moble Telecommuncatons - S-36 S-95 SDN TU JDC LOS D-AMS, US-TDMA system cdmaone, US-CDMA system ntegrated Servces Dgtal Network nternatonal Telecommuncatons Unon Japanese Dgtal Cellular Lne-of-sght MBWA Moble Broadband Wreless Access (EEE 8.) MS MUD NMT NTT O&M O-QSK OVSF Moble Staton Multuser Detecton Nordc Moble Telephones Nppon Telephone and Telegraph Operaton and Mantenance Offset Quadrature hase Shft Keyng Orthogonal Varable Spreadng Factor 4

16 C DC SK QoS RAM RAT RF RNC RRC RRM SCH SHO SR SSMA TACS TDD TDMA TC UE UL UMTS UTRA UTRAN UWC WARC WCDMA WLAN WWRF ower Control ersonal Dgtal Cellular hase Shft Keyng Qualty of Servce Rado Access Mode Rado Access Technology Rado Frequency Rado Network Controller Rado Resource Control Rado Resource Management Synchronzaton Channel Soft Handover Sgnal to nterference Rato Spread-Spectrum Multple Access Total Access Communcaton Systems Tme Dvson Duplex Tme Dvson Multple Access Transmt ower Control User Equpment Uplnk Unversal Moble Telecommuncaton Servces UMTS Terrestral Rado Access UMTS Terrestral Rado Access Network Unversal Wreless Communcatons World Admnstratve Rado Conference Wdeband Code Dvson Multple Access Wreless Local Access Network Wreless World Research Forum 5

17 Chapter ntroducton. ntroducton Handover deals wth the moblty of the end users n a moble network: t guarantees the contnuty of the wreless servces when the moble user moves across the cellular boundares. n frst and second generaton moble networks, hard handover s employed; n thrd generaton networks, whch are predomnantly based on CDMA technology, the soft handover concept s ntroduced. Compared wth the conventonal hard handover, soft handover has the advantages of smoother transmsson and less png-pong effects. As well as leadng to contnuty of the wreless servces, t also brngs macrodversty gan to the system. However, soft handover has the dsadvantages of complexty and extra resource consumpton. Therefore, optmsaton s crucal for guaranteeng the performance of soft handover. Untl now, several algorthms have been proposed amng at maxmsng the macrodversty gan and mnmsng the handover falure rate and extensve research has been conducted on the optmsaton of the parameters for these soft handover algorthms. t has been proved that the ndvdual lnk qualty can be mproved by soft handover and n the uplnk soft handover can ncrease the capacty and expand the coverage. However, n the downlnk, there are no qualfed results about the trade-off between the macrodversty gan and the extra resource consumpton caused by soft handover. Because the downlnk s more lkely to be the bottleneck of the system capacty n the future moble networks, the system level performance of soft handover n the downlnk needs to be further nvestgated. n ths thess, an n-depth study of the soft handover effects on the downlnk drecton of WCDMA networks s carred out, leadng to a new method for optmsng soft handover and a new power control approach. Fast downlnk power control and ntal cell selecton schemes that had been gnored by most of the prevous lterature are both ncluded n the analyss. Ths work quantfes the trade-off between the macrodversty gan and the extra resource 6

18 consumpton, proposes a new method for optmsng soft handover n order to maxmse the downlnk capacty and a new power control approach for ncreasng the soft handover gan by mtgatng the nterference. The optmsaton of soft handover s valuable for rado network dmensonng and the new power control scheme s shown to have better performance when compared wth the balanced power control scheme adopted by 3G.. Contrbuton The maor contrbutons of the work n ths thess are: An n-depth study of soft handover effects on the downlnk drecton of the WCDMA networks, whch quantfes the trade-off between the macrodversty gan and the extra resource consumpton. Senstvty analyss of the soft handover gan to dfferent cell selecton schemes, dfferent power control condtons and dfferent rado parameters gves a better understandng of soft handover effects on the downlnk capacty. A new method for optmsng soft handover s proposed n order to maxmse the downlnk capacty. The optmum soft handover overhead and threshold ranges are obtaned. The results are valuable for the rado resource management n future moble networks where the downlnk s more lkely to be the bottleneck of the whole system. A new power dvson approach s proposed for controllng the power dvson between the Base Statons (BSs) n the actve set durng the soft handover. The new approach has a better performance than the balanced power dvson scheme adopted by 3G, mnmsng the nterference and mantanng the benefts from the macrodversty at the same tme. The author s papers are lst n Appendx A..3 Organsaton of the Thess Chapter gves a bref overvew of the evoluton of moble networks. The key technologes and landmark features of dfferent generaton moble networks are 7

19 descrbed. Ths chapter also ntroduces the basc concepts of CDMA technology and functonaltes of rado resource management that are used throughout ths thess. Chapter 3 manly ntroduces handover. The dfference between hard and soft handover, prncples, algorthms, prevous work and exstng problems about soft handover are presented. Ths chapter reveals the motvaton of the work n ths thess. Chapter 4 begns wth the downlnk nterference analyss, followed by the lnk level performance analyss of soft handover n terms of total power consumpton. The work s new and t sets the foundaton for the system level performance analyss n later chapters and provdes a theoretcal bass for the soft handover optmsaton and the new power control approach. n Chapter 5, the system level performance of soft handover s analysed. The tradeoff between the macrodversty gan and the extra resource consumpton s quantfed. The effects of dfferent cell selecton schemes and dfferent power control condtons on the soft handover gan are evaluated and the senstvty of the soft handover gan to rado parameters and mult-servce envronments s also estmated. Chapter 6 proposes a new method for optmsng the soft handover n order to maxmsng the downlnk capacty. The optmum soft handover overhead and threshold ranges are obtaned. The results are valuable for rado resource management n future moble networks where the downlnk s more lkely to be the bottleneck of the whole system. Chapter 7 presents the new approach for optmsng power dvson durng the soft handover. The performance s verfed by comparng to the balanced power dvson strategy adopted by 3G. Chapter 8 summarses the work n ths thess, draws the conclusons and also dscusses the future work. Verfcaton and valdaton are very mportant n any research work: detals of that used for ths work s gven n Appendx B. 8

20 Chapter Moble Communcaton Networks. Evolvng Moble Networks.. Frst-generaton analogue moble systems n 98 the moble cellular era had started, and snce then moble communcatons have undergone sgnfcant changes and experenced enormous growth. Fgure. shows the evoluton of the moble networks. Frst-generaton moble systems used analogue transmsson for speech servces. n 979, the frst cellular system n the world became operatonal by Nppon Telephone and Telegraph (NTT) n Tokyo, Japan. The system utlsed 6 duplex channels over a spectrum of 3 MHz n the 8 MHz band, wth a channel separaton of 5 khz. Two years later, the cellular epoch reached Europe. The two most popular analogue systems were Nordc Moble Telephones (NMT) and Total Access Communcaton Systems (TACS). n 98, the NMT-45 system was commercalsed by NMT n Scandnava. The system operated n the 45 MHz and 9 MHz band wth a total bandwdth of MHz. TACS, launched n the Unted Kngdom n 98, operated at 9 MHz wth a band of 5 MHz for each path and a channel bandwdth of 5 khz. Extended TACS was deployed n 985. Other than NMT and TACS, some other analogue systems were also ntroduced n 98s across the Europe. For example, n Germany, the C-45 cellular system, operatng at 45 MHz and 9 MHz (later), was deployed n September n 985. All of these systems offered handover and roamng capabltes but the cellular networks were unable to nteroperate between countres. Ths was one of the nevtable dsadvantages of frst-generaton moble networks. n the Unted States, the Advanced Moble hone System (AMS) was launched n 98. The system was allocated a 4-MHz bandwdth wthn the 8 to 9 MHz frequency range. n 988, an addtonal MHz bandwdth, called Expanded Spectrum (ES) was allocated to AMS... Second-generaton & phase moble systems Second-generaton (G) moble systems were ntroduced n the end of 98s. Low bt rate data servces were supported as well as the tradtonal speech servce. Dgtal 9

21 transmsson rather than analogue transmsson was used by these systems. Consequently, compared wth frst-generaton systems, hgher spectrum effcency, better data servces, and more advanced roamng were offered by G systems. n Europe, the Global System for Moble Communcatons (GSM) was deployed to provde a sngle unfed standard. Ths enabled seamless servces through out Europe by means of nternatonal roamng. The earlest GSM system operated n the 9 MHz frequency band wth a total bandwdth of 5 MHz. Durng development over more than years, GSM technology has been contnuously mproved to offer better servces n the market. New technologes have been developed based on the orgnal GSM system, leadng to some more advanced systems known as.5 Generaton (.5G) systems. So far, as the largest moble system worldwde, GSM s the technology of choce n over 9 countres wth about 787 mllon subscrbers [GSMweb]. NTT TACS NMT AMS G GSM S-36 S-95 DC G GRS HSCSD EDGE S-95B.5G Speech & low rate data servce Dgtal transmsson Speech servce Analogue transmsson MT- UMTS cdma mc-cdma 3G 4G? Hgher bt rate? New applcatons? Speech, data, multmeda servces Bt rate up to Mbt/s Dgtal transmsson Fgure. Evoluton of moble networks n the Unted States, there were three lnes of development n second-generaton dgtal cellular systems. The frst dgtal system, ntroduced n 99, was the S-54 (North Amerca TDMA Dgtal Cellular), of whch a new verson supportng addtonal servces (S-36) was ntroduced n 996. Meanwhle, S-95 (cdmaone) was deployed n 993. The US Federal Communcatons Commsson (FCC) also auctoned a new block of spectrum n the 9 MHz band (CS), allowng GSM9 to enter the US market. n Japan, the ersonal Dgtal Cellular (DC) system, orgnally known as JDC (Japanese Dgtal Cellular) was ntally defned n 99.

22 Commercal servce was started by NTT n 993 n the 8 MHz band and n 994 n the.5 GHz band. Table. shows the techncal parameters of four typcal secondgeneraton dgtal moble systems. GSM S-36 S-95 DC Multple access TDMA TDMA CDMA TDMA Modulaton GMSK π/4-dqsk Coherent π/4-dqsk Coherent 8-SK QSK/O-QSK π/4-dqsk Carrer spacng khz 3 khz.5 khz 5 khz Carrer bt rate kbt/s 48.6 kbt/s (π/4-dqsk) 7.9 kbt/s (8-SK).88 Mchp/s 4 kbt/s Frame Length 4.65 ms 4 ms ms ms Slots per frame 8/6 6 3/6 Frequency band (uplnk/downlnk) (MHz) Maxmum possble data rate (kbt/s) 88-95/ / /85-9 HSCSD: 57.6 GRS: / /85-9 S-36: / /85-9 S-95A: 4.4 S-95B: 5. Handover Hard Hard Soft Hard 8-86/ / Table. Techncal parameters of second-generaton dgtal systems Nowadays, second-generaton dgtal cellular systems stll domnate the moble ndustry throughout the whole world. However, they are evolvng towards thrdgeneraton (3G) systems because of the demands mposed by ncreasng moble traffc and the emergence of new type of servces. The new systems, such as HSCSD (Hgh Speed Crcut Swtched Data), GRS (General acket Rado Servce), and S-95B, are commonly referred as generaton.5 (.5G). HSCSD, GRS and EDGE are all based on the orgnal GSM system. HSCSD s the frst enhancement of the GSM ar nterface: t bundles GSM tmeslots to gve a theoretcal maxmum data rate of 57.6 kbt/s (bundlng kbt/s full rate tmeslots). HSCSD provdes both symmetrc and asymmetrc servces and t s relatvely easy to deploy. However, HSCSD s not easy to prce compettvely snce each tmeslot s effectvely a GSM channel. Followng HSCSD, GRS s the next step of the evoluton of the GSM ar nterface. Other than bundlng tmeslots, 4 new channel codng schemes are proposed. GRS

23 provdes always on packet swtched servces wth bandwdth only beng used when needed. Therefore, GRS enables GSM wth nternet access at hgh spectrum effcency by sharng tme slots between dfferent users. Theoretcally, GRS can support data rate up to 6 kbt/s (current commercal GRS provdes 4 kbt/s). Deployng GRS s not as smple as HSCSD because the core network needs to be upgraded as well. EDGE uses the GSM rado structure and TDMA framng but wth a new modulaton scheme, 8QSK, nstead of GMSK, thereby ncreasng by three tmes the GSM throughput usng the same bandwdth. EDGE n combnaton wth GRS wll delver sngle user data rates of up to 384 kbt/s. For more detals on GSM phase and on GSM s evoluton towards 3G systems, refer to [ZJK98]...3 Thrd-generaton moble systems and beyond The massve success of G technologes s pushng moble networks to grow extremely fast as ever-growng moble traffc puts a lot of pressure on network capacty. n addton, the current strong drve towards new applcatons, such as wreless nternet access and vdeo telephony, has generated a need for a unversal standard at hgher user btrates: 3G. Fgure. shows the bt rate requrements for some of the applcatons that are predcated for 3G networks. Most of the new servces requre btrates up Mbt/s. data rate M 384 K 64 K 3 K 6 K N T E R N E T vdeo conference audo conference e vdeo streamng MMS vdeo on demand SDN TV moble rado moble 9.6 K.4 K voce Emal fax. K Fgure. Bt rate requrements for some 3G applcatons [Sau]

24 Because of these drvers, the nternatonal Telecommuncatons Unon (TU) has been developng 3G snce G networks are referred as MT- (nternatonal Moble Telephony) wthn TU and UMTS (Unversal Moble Telecommuncatons Servces) n Europe. n ETS (European Telecommuncatons Standards nsttute), UMTS standardsaton started n Obectves and requrement Thrd generaton systems are desgned for multmeda communcatons: person-toperson communcaton can be enhanced wth hgh qualty mages and vdeo. Also, access to nformaton and servces on publc and prvate networks wll be enhanced by the hgher data rates and new flexble communcaton capabltes of these systems. Thrd generaton systems can offer smultaneous multple servces for one user and servces wth dfferent Qualty of Servce (QoS) classes. The man obectves for the MT- systems can be summarsed as: Full coverage and moblty for 44 kbt/s, preferably 384 kbt/s; Lmted coverage and moblty for Mbt/s; rovdes both symmetrc and asymmetrc data transmsson; rovdes both crcut swtched and packet swtched connectons; Capable of carryng nternet rotocol () traffc; Global roamng capabltes; Hgh spectrum effcency compared to exstng systems; Hgh flexblty to ntroduce new servces; The bt-rate targets have been specfed accordng to the ntegrated servces dgtal network (SDN). The 44 kbt/s data rate provdes the SDN BD channel confguraton, 384 kbt/s provdes the SDN H channel, and.9 Mbt/s provdes the SDN H channel. Fgure.3 shows the relaton between bt rates and moblty for the dfferent systems. 3

25 User bt rate Mbps 384 Kbps MT- 44 Kbps Kbps GSM EDGE.5 G (GSM HSCSD and GRS, S-95B) G (GSM, S-95, S-36, DC) Fxed / Low Moblty Wde Area / Hgh Moblty Fgure.3 User bt rate versus coverage and moblty..3. Ar nterface and spectrum allocaton Wthn the MT- framework, several dfferent ar nterfaces are defned for 3G systems, based on ether CDMA or TDMA technology. Currently, there are fve nterfaces agreed by TU and beng under standardsaton as shown n Fgure.4. drect sequence tme code sngle carrer multcarrer frequency tme DS TC SC MC FT C UTRA FDD (WCDMA) UTRA TDD hgh & low chp rates UWC 36 Cdma DECT 3G UWCC TR 45 3G ETS Fgure.4 MT- ar nterfaces [Scr] Among these nterfaces, WCDMA has been adopted as the rado access technology of UMTS; t s also to be used n Asa, ncludng Japan and Korea. More detals about WCDMA technology wll be presented n secton.. Multcarrer CDMA (cdma) can be used as an upgrade soluton for the exstng S-95. 4

26 n 99, the World Admnstratve Rado Conference (WARC) allocated spectrum bands MHz and MHz for MT- systems, and n WRC, two further spectrum bands MHz and 5 69 MHz were added. However, dfferent countres have ther own usage because of the dfferent choces of 3G ar nterface and the dfferent exstng G systems. Fgure.5 shows the spectrum allocatons for 3G systems n dfferent countres and Table. shows the expected frequency bands and geographcal areas where dfferent ar nterfaces are lkely to be appled [HT]. GSM8 MT- Downlnk DECT TDD MT- Uplnk MSS MT- Uplnk TDD MT- Uplnk MSS Downlnk EUROE JAAN HS MT- Uplnk MT- Uplnk KOREA S-95/Downlnk MT- Uplnk MT- Uplnk USA CS/Uplnk CS/Downlnk MHz Fgure.5 Spectrum allocaton n dfferent countres Area Frequency band Ar nterface Europe Amercas Asa MT- band GSM 8 band n the exstng bands that are already used by second generaton systems. MT- band GSM 8 band WCDMA EDGE EDGE, WCDMA, and multcarrer CDMA (cdma) WCDMA EDGE Japan MT- band WCDMA Korea MT- band WCDMA Table. Expected spectrums and ar nterfaces for provdng 3G servces G systems and beyond Currently 3G technologes are startng to be launched commercally; for example, Hutchson launched 3G n the UK n. 5

27 Recently, systems beyond 3G (B3G systems) are attractng more and more attenton. Many nternatonal Forums such as TU-R W8F Vson Group and the EU ntated Wreless World Research Forum (WWRF) are undertakng research on B3G systems. n, TU-R W8F held the 9 th meetng, subttled System Capabltes for system beyond 3G. Fgure.6 shows the expected capabltes for B3G systems from that. Hgher bt rate wth hgher moblty s expected to be requred for future new applcatons. Fgure.6 Systems beyond 3G (Source: TU-R M. [FLMTS.REVAL]) Meanwhle, fourth-generaton moble networks branded 4G have already been proposed. Although there s no unform defnton about what 4G s and what exactly marks the generaton, the vson for the 4G moble networks s developng, wth dfferent vews beng taken n dfferent areas around the world. n Asa, 4G s beng foreseen as an ar nterface that could support up to Mbt/s for hgh moblty and up to Gbt/s for low moblty. 6

28 n the US, 4G s expected to be the combnaton of Wreless Local Access Network (WLAN) and EEE 8.. n Europe, the understandng of 4G s a network of networks, whch ncludes multple nterworkng networks and devces. n ths knd of network, the followng dfferent technologes mght coexst wth seamless nterworkng beng supported between them. Cellular Moble (/.5/3G) Wreless LAN (EEE8.x) ersonal Area Networkng (Bluetooth) Dgtal Broadcastng (vdeo, audo, DVB, DAB) Home Entertanment Wreless Networkng Mult-Modal Servces Fgure.7 shows the concept of the heterogeneous network. Fgure.7 Heterogeneous network wth nterworkng access systems for next generaton [W] On December, the EEE Standards Board approved the establshment of EEE 8., the Moble Broadband Wreless Access (MBWA) Workng Group. EEE 8. specfes an effcent packet based ar nterface that s optmzed for the transport of based servces. The goal s to enable worldwde deployment of affordable, ubqutous, always-on and nteroperable mult-vendor moble broadband wreless access networks that meet the needs of busness and resdental end user markets. 7

29 n the future, wreless communcaton s gong to move towards unversal communcaton that uses a very flexble networkng nfrastructure and dynamcally adapts to the changng requrements.. Overvew of CDMA Technology.. rncples of spectrum spreadng (CDMA) Dgtal communcatons systems are desgned to maxmse capacty utlsaton. From Shannon s channel capacty prncple expressed as (.), t s obvous that the channel capacty can be ncreased by ncreasng the channel bandwdth. ( S ) C B log (.) N Where B s the bandwdth (Hz), C s the channel capacty (bt/s), S s the sgnal power and N s the nose power. Thus, for a partcular S/N rato (Sgnal to Nose Rato: SNR), the capacty s ncreased f the bandwdth used to transfer nformaton s ncreased. CDMA s a technology that spreads the orgnal sgnal to a wdeband sgnal before transmsson. CDMA s often called as Spread-Spectrum Multple Access (SSMA). The rato of transmtted bandwdth to nformaton bandwdth s called the processng gan G p (also called spreadng factor). G Bt B or G p (.) B R p Where B t s the transmsson bandwdth, B s the bandwdth of the nformaton bearng sgnal, B s the RF bandwdth and R s the nformaton rate. Relatng the S/N rato to the E b / rato, where E b s the energy per bt, and s the nose power spectral densty, leadng to: S E R E N B G b b (.3) p 8

30 Therefore, for certan E b / requrement, the hgher the processng gan, the lower the S/N rato requred. n the frst CDMA system, S-95, the transmsson bandwdth s.5 MHz. n WCDMA system, the transmsson bandwdth s about 5 MHz. n CDMA, each user s assgned a unque code sequence (spreadng code) that s used to spread the nformaton sgnal to a wdeband sgnal before beng transmtted. The recever knows the code sequence for that user, and can hence decode t and recover the orgnal data... Spreadng and de-spreadng Spreadng and de-spreadng are the most basc operatons n DS-CDMA systems, shown as Fgure.8. User data here s assumed to be a BSK-modulated bt sequence of rate R. The spread operaton s the multplcaton of each user data bt wth a sequence of n code bts, called chps. Here, n8, and hence the spreadng factor s 8. Ths s also assumed for the BSK spreadng modulaton. The resultng spread data s at a rate of 8 R and has the same random (pseudo-nose-lke) appearance as the spreadng code. The ncrease of data rate by a factor 8 corresponds to a wdenng (by a factor 8) of the occuped spectrum of the spread user data sgnal. Ths wdeband sgnal would then be transmtted across a rado channel to the recevng end. symbol Spreadng data spreadng code chp De-spreadng spreadng code data Fgure.8 Spreadng and de-spreadng Durng de-spreadng, the spread user data/chp sequence s multpled bt by bt wth the same 8 code chps as used durng the spreadng process. As shown, the orgnal users data s recovered perfectly. 9

31 ..3 Multple access A moble communcaton network s a mult-user system, n whch a large number of users share a common physcal resource to transmt and receve nformaton. Multple access capablty s one of the fundamental components. The spectral spreadng of the transmtted sgnal gves the feasblty of multple access to CDMA systems. Fgure.9 shows three dfferent multple access technologes: TDMA, FDMA and CDMA. FDMA TDMA CDMA p p p t m f f f M f t t f t Fgure.9 Multple access technologes n FDMA, (Frequency Dvson Multple Access), sgnals for dfferent users are transmtted n dfferent channels each wth a dfferent modulatng frequency; n TDMA, (Tme Dvson Multple Access), sgnals for dfferent users are transmtted n dfferent tme slots. Wth these two technologes, the maxmum number of users who can share the physcal channels smultaneously s fxed. However, n CDMA, sgnals for dfferent users are transmtted n the same frequency band at the same tme. Each user s sgnal acts as nterference to other user s sgnals and hence the capacty of the CDMA system s related closely to the nterference level: there s no fxed maxmum number, so the term soft capacty s used. Fgure. shows an example of how 3 users can have smultaneous access n a CDMA system. At the recever, user de-spreads ts nformaton sgnal back to the narrow band sgnal, but nobody else s. Ths s because that the cross-correlatons between the code of the desred user and the codes of other users are small: coherent detecton wll only put the power of the desred sgnal and a small part of the sgnal from other users nto the nformaton bandwdth. The processng gan, together wth the wdeband nature of the process, gves benefts to CDMA systems, such as hgh spectral effcency and soft capacty. 3

32 However, all these benefts requre the use of tght power control and soft handover to avod one user s sgnal cloakng the communcaton of others. ower control and soft handover wll be explaned n more detaled level n secton.3.. and Chapter 3 respectvely. user user & & 3 & 3 user 3 3 Users transmt ther spread-spectrum sgnals smultaneously output of user s recever narrow-band data sgnals spread spectrum sgnals Fgure. rncple of spread-spectrum multple access..4 Features of WCDMA Wdeband-CDMA (WCDMA) has been adopted by UMTS as the multple access technology and t s also referred to as UMTS terrestral rado access (UTRA). Ths secton ntroduces the prncples of the WCDMA ar nterface. Specal attenton s drawn to those features by whch WCDMA dffers from GSM and S-95. Table.3 summarses the man parameters related to the WCDMA ar nterface. Some of the tems that characterse WCDMA are: WCDMA s a wdeband CDMA system. User nformaton bts are spread over a wde bandwdth (5 MHz) by multplyng wth spreadng codes before transmsson and are recovered by decodng n the recever [O98]. The chp rate of 3.84 Mchp/s used leads to a carrer bandwdth of approxmately 5 MHz. n GSM, carrer bandwdth s only khz. Even n narrowband CDMA systems, such as S-95, the carrer bandwdth s only.5 MHz. The nherently wde carrer bandwdth of WCDMA supports hgh user data 3

33 rates and also has certan performance benefts, such as ncreased multpath dversty [TR]. WCDMA supports hghly varable user data rates; n other words the concept of obtanng Bandwdth on Demand (BoD) s well supported. Each user s allocated frames of ms duraton, durng whch the user data rate s kept constant. However, the data capacty among the users can change from frame to frame. WCDMA supports two basc modes of operaton: Frequency Dvson Duplex (FDD) and Tme Dvson Duplex (TDD). n FDD mode, separate 5MHz carrers are used for the uplnk and downlnk respectvely, whereas n TDD only one 5 MHz s tme-shared between uplnk and downlnk. WCDMA supports the operaton of asynchronous base statons. Unlke the synchronous S-95 system, there s no need for a global tme reference, such as a GS, so makng deployment of ndoor and mcro base statons easer. WCDMA employs coherent detecton on uplnk and downlnk based on the use of plot symbols or common plot. n S-95 coherent detecton s only used on the downlnk. The use of coherent detecton on uplnk wll result n an overall ncrease of coverage and capacty on the uplnk. Ths makes the downlnk more lkely to be the bottleneck of the whole system. The WCDMA ar nterface has been crafted n such a way that advanced CDMA recever concepts, such as multuser detecton (MUD) [Ver86][OH98][JL] and smart adaptve antennas [VMB] [LA], can be deployed by the network operator as a system opton to ncrease capacty and/or coverage. n most second generaton systems no provson has been made for such concepts. WCDMA s desgned to be deployed n conuncton wth GSM. Therefore, handovers between GSM and WCDMA are supported. 3

34 Multple access method Duplexng method Base staton synchronsaton Chp rate Frame length Servce multplexng Multrate concept Detecton Multuser detecton, Smart antennas DS-CDMA FDD/TDD Asynchronous operaton 3.84 Mcps ms Multple servces wth dfferent qualty of servce requrements multplexed on one connecton Varable spreadng factor and multcode Coherent usng plot symbols or common plot Supported by the standard, optonal n the mplementaton Table.3 Man WCDMA parameters.3 Rado Resource Management.3. RRM n moble networks Rado Resource Management (RRM) n 3G networks s responsble for mprovng the utlsaton of the ar nterface resources. The obectves of usng RRM can be summarsed as follows: Guarantee the QoS for dfferent applcatons Mantan the planned coverage Optmse the system capacty n 3G networks, pre-allocatng resource and over-dmensonng the network are not feasble any more because of the unpredctable need and the varable requrements of dfferent servces. Therefore, rado resource management s composed of two parts: rado resource confguraton and re-confguraton. Rado resource confguraton s responsble for allocatng the resource properly to new requests comng nto the system so that the network s not overloaded and remans stable. But, as congeston mght occur n 3G networks because of the moblty of users, rado resource re-confguraton s responsble for re-allocatng the resource wthn the network when load s buldng up or congeston starts to appear. t s responsble for returnng the overloaded system quckly and controllably back to the targeted load. 33

35 .3. Functonaltes of RRM Rado resource management can be dvded nto power control, handover, admsson control and load control functonaltes. Fgure. shows the typcal locatons of RRM functonaltes wthn a WCDMA network. Base Staton Moble Termnal Rado Network Controller ower Control ower Control Handover Control Load Control ower Control Handover Control Admsson Control Load Control Fgure. Typcal locatons of RRM functonaltes wthn a WCDMA network.3.. ower control ower control s a necessary element n all moble systems because of the battery lfe problem and safety reasons, but n CDMA systems, power control s essental because of the nterference-lmted nature of CDMA. n GSM slow (frequency approxmately Hz) power control s employed. n S-95 fast power control wth 8 Hz s supported n the uplnk, but n the downlnk, a relatvely slow (approxmately 5 Hz) power control loop controls the transmsson power. n WCDMA fast power control wth.5khz frequency s supported n both uplnk and downlnk [O98]. Tght and fast power control s one of the most mportant aspects of WCDMA systems. The reasons for usng power control are dfferent n the uplnk and downlnk. The overall obectves of power control can be summarsed as follows: Overcomng the near-far effect n the uplnk Optmsng system capacty by controllng nterference Maxmsng the battery lfe of moble termnals (not consdered further) 34

36 Fgure. shows near-far problem [SL-SWJ99] n the uplnk. Sgnals from dfferent MSs are transmtted n the same frequency band smultaneously n WCDMA systems. Wthout power control, the sgnal comng from the MS that s nearest to the BS may block sgnals from other MSs that are much farer away from the BS. n the worst stuaton one over-powered MS could block a whole cell. The soluton s to apply power control to guarantee that sgnals comng from dfferent termnals have the same power or the same SR (Sgnal-to-nterference Rato) when they arrve at the BS. Fgure. Near-Far effects (power control n UL) n the downlnk drecton, there s no near-far problem due to the one-to-many scenaro. ower control s responsble for compensatng the nter-cell nterference suffered by the mobles, especally those near cell boundares as shown n Fgure.3. Moreover, power control n the downlnk s responsble for mnmsng the total nterference by keepng the QoS at ts target value. Moble Moble Fgure.3 Compensatng the nter-cell nterference (power control n DL) n Fgure.3, moble suffers more nter-cell nterference than moble. Therefore, to meet the same qualty target, more power needs to be allocated to the downlnk channel between the BS and moble. 35

37 There are three types of power control n WCDMA systems: open-loop power control, closed-loop power control, and outer-loop power control.. Open-loop power control Open-loop power control s used n the UMTS FDD mode for the moble ntal power settng. The moble estmates the path loss between the base staton and the moble by measurng the receved sgnal strength usng an automatc gan control (AGC) crcut. Accordng to ths estmate of path loss, the moble can decde ts uplnk transmt power. Open-loop power control s effectve n a TDD system because the uplnk and downlnk are recprocal, but t s not very effectve wth FDD system because the uplnk and downlnk channels operate on dfferent frequency bands and the Raylegh fadng n the uplnk and downlnk s ndependent. So open-loop power control can only roughly compensate dstance attenuaton. That s why t s only used as an ntal power settng n FDD systems.. Closed-loop power control Closed-loop power control, also called fast power control n WCDMA systems, s responsble for controllng the transmtted power of the MS (uplnk) or of the base staton (downlnk) n order to counteract the fadng of the rado channel and meet the SR (sgnal-to-nterference rato) target set by the outer-loop. For example, n the uplnk, the base staton compares the receved SR from the MS wth the target SR once every tme slot (.666ms). f the receved SR s greater than the target, the BS transmts a TC command to the MS va the downlnk dedcated control channel. f the receved SR s lower than the target, the BS transmts a TC command to the MS. Because the frequency of closed-loop power control s very fast t can compensate fast fadng as well as slow fadng. 3. Outer-loop power control Outer-loop power control s needed to keep the qualty of communcaton at the requred level by settng the target for the fast closed-loop power control. t ams at provdng the requred qualty: no worse, no better. The frequency of outer-loop power control s typcally -Hz. Fgure.4 shows the general algorthm of outer-loop power control. 36

38 Decrease SR target ncrease SR target Yes Receved qualty better than requred qualty No Fgure.4 General outer-loop power control algorthm The outer-loop power control compares the receved qualty to the requred qualty. Usually the qualty s defned as a certan target Bt Error Rate (BER) or Frame Error Rate (FER). The relatonshp between the SR target and the qualty target depends on the moble speed and the multpath profle. f the receved qualty s better, t means the current SR target s hgh enough for guaranteeng the requred QoS. n order to mnmse the headroom, the SR target wll be reduced. However, f the receved qualty s worse than the requred qualty, the SR target needs to be ncreased for guaranteeng the requred QoS..3.. Handover control Handover s an essental component of moble cellular communcaton systems. Moblty causes dynamc varatons n lnk qualty and nterference levels n cellular systems, sometmes requrng that a partcular user change ts servng base staton. Ths change s known as handover. More detaled nformaton s presented n the next chapter (Chapter 3) Admsson control f the ar nterface loadng s allowed to ncrease excessvely, the coverage area of the cell s reduced below the planned values (so called cell breathng ), and the QoS of the exstng connectons cannot be guaranteed. The reason for the cell breathng phenomenon s because of the nterference-lmted feature of CDMA systems. Therefore, before admttng a new connecton, admsson control needs to check that 37

39 admttng the new connecton wll not sacrfce the planned coverage area or the QoS of exstng connectons. Admsson control accepts or reects a request to establsh a rado access bearer n the rado access network. The admsson control functonalty s located n RNC where the load nformaton of several cells can be obtaned. The admsson control algorthm estmates the load ncrease that the establshment of the bearer would cause n the rado access network. The load estmaton s appled for both uplnk and downlnk. The requestng bearer can be admtted only f the admsson controls n both drectons admt t, otherwse t s reected because of the excessve nterference that t adds to the network. Several admsson control schemes have been suggested. n [DKOR94] [HY96] and [KBY97], the use of the total power receved by the base staton s supported as the prmary uplnk admsson control decson crteron. n [DKOR94] and [KBY98] a downlnk admsson control algorthm based on the total downlnk transmsson power s presented. Generally, the admsson control strateges can be dvded nto two types: wdeband power-based admsson control strategy and throughput-based admsson control strategy. Fgure.5 shows the wdeband power-based admsson control. nterference level threshold total_old? Max nose rse?: Estmated ncrease of nterference?l: Estmated ncrease of load?l Load η Fgure.5 Load curve 38

40 The new user s not admtted f the new resultng total nterference level s hgher than the threshold value: reect : admt : total _ old total _ old > < threshold threshold (.4) The threshold value s the same as the maxmum uplnk nose ncrease and can be set by rado network plannng. n throughput-based admsson control strategy the new requestng user s not admtted nto the rado access network f the new resultng total load s hgher than the threshold value: reect : admt : η η total _ old total _ old L > η L < η threshold threshold (.5) t should be noted that as the admsson control s appled separately for uplnk and downlnk, dfferent admsson control strateges can be used n each drecton Load control (congeston control) One mportant task of the rado resource management functonalty s to ensure that the system s not overloaded and remans stable. f the system s properly planned, and the admsson control works well, overload stuatons should be exceptonal. However, n moble networks, overload somewhere s nevtable because the rado resource cannot be pre-allocated wthn the network. When overload s encountered, the load control, also called congeston control, returns the system quckly and controllably back to targeted load, whch s defned by the rado network plannng. The possble load control actons n order to reduce or balance load are lsted below: Deny downlnk power-up commands receved from the MS. Reduce the uplnk E b / target used by uplnk fast power control. Change the sze of soft handover zone to accommodate more users. Handover to another WCDMA carrer (nter-frequency handover). Handover to overlappng network (another UMTS network or GSM network). Decrease bt rates of real-tme users, e.g. AMR speech codec. Reduce the throughput of packet data traffc (non-real-tme servce). 39

41 Drop calls n a controlled way. The frst two n the lst are fast actons that are carred out wthn the BS. These actons can take place wthn one tmeslot,.e. wth.5khz frequency, and provde prortsaton of the dfferent servces. The thrd method, changng the sze of soft handover zone s especally useful to a downlnk-lmted network and ths s dscussed n more detal n the next chapter. The other load control actons are typcally slower. nter-frequency handover and nter-system handover can overcome overload by balancng the load. The fnal acton s to drop real-tme users (.e. speech or crcut swtched data users) n order to reduce the load. Ths acton s taken only f the load of the whole network remans very hgh even after other load control actons have been effected n order to reduce the overload. The WCDMA ar nterface and the expected ncrease of non-real-tme traffc n thrd generaton networks gve a large selecton of possble actons to handle overload stuatons, and therefore the need to drop real-tme users to reduce overload should be very rare..4 Summary Ths chapter gves a bref overvew of the evoluton of moble networks and ntroduces the basc concepts of CDMA technology and functonaltes of rado resource management. As one of the crucal components of rado resource management, handover (the focus of ths thess) s descrbed n the next chapter. The basc prncple, prevous research and exstng problems of soft handover are presented, whch brng out the motvaton of the work. 4

42 Chapter 3 Handover 3. Overvew of Handover n Moble Networks Moble networks allow users to access servces whle on the move so gvng end users freedom n terms of moblty. However, ths freedom does brng uncertantes to moble systems. The moblty of the end users causes dynamc varatons both n the lnk qualty and the nterference level, sometmes requrng that a partcular user change ts servng base staton. Ths process s known as handover (HO). Handover s the essental component for dealng wth the moblty of end users. t guarantees the contnuty of the wreless servces when the moble user moves across cellular boundares. n frst-generaton cellular systems lke AMS, handovers were relatvely smple. Second-generaton systems lke GSM and ACS are superor to G systems n many ways, ncludng the handover algorthms used. More sophstcated sgnal processng and handover decson procedures have been ncorporated n these systems and the handover decson delay has been substantally reduced. Snce the ntroducton of CDMA technology, another dea that has been proposed for mprovng the handover process s soft handover and ths s the focus of the work n ths thess. 3.. Types of handover n 3G WCDMA systems There are four dfferent types of handovers n WCDMA moble networks. They are: ntra-system HO ntra-system HO occurs wthn one system. t can be further dvded nto ntrafrequency HO and nter-frequency HO. ntra-frequency occurs between cells belongng to the same WCDMA carrer, whle nter-frequency occurs between cells operate on dfferent WCDMA carrers. nter-system HO nter-system HO takes places between cells belongng to two dfferent Rado Access Technologes (RAT) or dfferent Rado Access Modes (RAM). The most frequent case for the frst type s expected between WCDMA and GSM/EDGE 4