Coordinated M ulti-point Transmission Aided Cell Switch O ff Schemes. for Energy Efficient M obile Cellular Networks

Transcription

1 Coordinated M ulti-point Transmission Aided Cell Switch O ff Schemes for Energy Efficient M obile Cellular Networks by G encer Cili A thesis submitted to the Faculty o f Graduate Studies and Research in partial fulfillment o f the requirements for the degree o f M aster of Applied Science in Electrical and Com puter Engineering Ottawa-Carleton Institute for Electrical and Computer Engineering Department o f Systems and Computer Engineering Carleton University. Ottawa, Ontario, Canada 2012 Gencer Cili

2 1+1 Library and Archives Canada Published Heritage Branch 395 Wellington Street Ottawa ON K1A0N4 Canada Bibliotheque et Archives Canada Direction du Patrimoine de I'edition 395, rue Wellington Ottawa ON K1A 0N4 Canada Your file Votre reference ISBN: Our file Notre reference ISBN: NOTICE: The author has granted a nonexclusive license allowing Library and Archives Canada to reproduce, publish, archive, preserve, conserve, communicate to the public by telecommunication or on the Internet, loan, distrbute and sell theses worldwide, for commercial or noncommercial purposes, in microform, paper, electronic and/or any other formats. AVIS: L'auteur a accorde une licence non exclusive permettant a la Bibliotheque et Archives Canada de reproduire, publier, archiver, sauvegarder, conserver, transmettre au public par telecommunication ou par I'lnternet, preter, distribuer et vendre des theses partout dans le monde, a des fins commerciales ou autres, sur support microforme, papier, electronique et/ou autres formats. The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. L'auteur conserve la propriete du droit d'auteur et des droits moraux qui protege cette these. Ni la these ni des extraits substantiels de celle-ci ne doivent etre imprimes ou autrement reproduits sans son autorisation. In compliance with the Canadian Privacy Act some supporting forms may have been removed from this thesis. While these forms may be included in the document page count, their removal does not represent any loss of content from the thesis. Conformement a la loi canadienne sur la protection de la vie privee, quelques formulaires secondaires ont ete enleves de cette these. Bien que ces formulaires aient inclus dans la pagination, il n'y aura aucun contenu manquant. Canada

3 Abstract D ue to the increased energy consum ption o f cellular access netw orks, energy efficiency o f cellular system s should be considered jo in tly w ith spectral efficiency to obtain the overall perform ance m etrics and trade-offs. D ow nlink coordinated m ultipoint (C om P ) jo in t transm ission aided cell sw itch o ff schem es m itigate inter-cell interference and create an energy efficient system by elim inating the need to increase the transm it antenna pow ers o f the active cells serving the users in the sw itched o ff cell. H ow ever, the perform ance o f this new ly proposed schem e is heavily dependent on the accuracy o f the selected C om P jo in t transm ission set. W e m odel the m ulti-point channel estim ation enabled via channel state inform ation reference sym bols (C S I-R S ) introduced in 3G P P release 10 system s and sim ulate possible scenarios that w ould lead to inaccurate transm ission set clustering: m ulti-point channel estim ation errors and possible C om P system delays due to CSI transfers, node processing delays and netw ork topology lim itations. Individual and jo in t im pacts o f system delays and estim ation errors on energy efficiency and capacity perform ance degradations for various m obility conditions are dem onstrated. T hese technical challenges and the clustering accuracy bottleneck are overcom e by users perform ing m ulti-point channel estim ation and serving en B perform ing channel prediction procedures. S im ulations are perform ed according to realistic large and sm all scale fading m odels for m ultipoint radio links. It is dem onstrated that the users being served by larger jo in t transm ission clusters are m ore vulnerable to perform ance degradation. N ovel m ulti-point channel estim ation schem es are proposed and discussed, w here the users w ith higher clustering degrees enlarge the channel estim ation filter lengths for a certain subset o f the C om P m easurem ent set w hich has high chance o f being included in the jo in t transm ission cluster. M ulti-point channel estim ation accuracy can be m axim ized w hen users track each m ultipath com ponent o f channel im pulse responses (C IR s) separately, how ever com putation com plexity o f such schem es are significantly high com pared to the m ethods w hich ju st track the superim posed C IR s. Serving en B can avoid clustering decisions based on outdated CSI feedbacks by predicting the channel conditions on upcom ing transm ission tim e intervals (T T Is). To reduce the C om P clustering accuracy responsibility o f the serving en B s, users can take initiative to influence the clustering decisions by sending selective CSI feedbacks. T his novel C SI feedback reporting schem e not only reduces the possible C om P access netw ork system delays but also reduces the serving en B processing tim e for clustering decisions.

4 Acknowledgements I w ould like to express m y sincerest gratitude and respect for m y supervisors Prof. H alim Y anikom eroglu and Prof. Fei R ichard Y u for th eir d edicated support, guidance, supervision, and m otivation throughout the thesis research. W ithout their contributions, this w ork w ould not have been possible. Prof. Y anikom eroglu has been an inspiration for m e during the overall research process. H is expertise in the field o f w ireless com m unications, know ledge o f the latest radio technologies and the ability to relate academ ic research w ork to industrial im plem entation proposals helped m e significantly extend by telecom m unications know ledge. H is continuous m otivation m ade it possible to use th e thesis research w ork to create international conference papers and patent filings. Prof. Y u 's attention to detail, invaluable suggestions throughout the research, and thorough know ledge o f the latest industry trends and academ ic research topics enhanced the quality o f the thesis. H is advice about the choice o f research topic, suggestions about literatures to review, and technical recom m endations for focusing on interesting problem s helped m ajorly expedite the w ork. B oth o f my supervisors not only helped me for the research, but also provided invaluable com m ents about my professional career choices. I am extrem ely thankful to have the opportunity to w ork w ith them throughout this enjoyable and unforgettable journey. I w ould also like to thank m y form er m anager from R esearch in M otion, H ongchang T ian, for providing all the possible support to create an ideal environm ent for me to w ork for the RIM R adio SW team and pursue m y M aster s degree concurrently. W ithout his understanding, this w ork w ould not have been possible. I m ust also express my appreciation to m y form er colleague, H ikm et A sm er, for alw ays m otivating m e to continue m y career in the w ireless industry and pursue a graduate degree at the sam e tim e. Sincere thanks to m y undergraduate supervisor from M cg ill U niversity, Prof. Jan B ajcsy, for helping m e shape m y career choices. He will alw ays be my role m odel due to his intelligence, sm art w ay o f thinking, and passion for field o f w ireless com m unications. Last but not the least, I w ould like to express m y special thanks and respect for m y parents. T here are not enough w ords to describe m y appreciation for all the effort they spent on m e. I am extrem ely grateful for their continuous m oral support, patience and love. IV

5 Table of Contents A bstract A cknow ledgem ents Table o f Contents List o f Tables List of Figures N om enclature iii iv v viii ix xii 1 Introduction Thesis Problem Statement and M o tiv a tio n Thesis C ontributions Publications. Patent Filings and Thesis O rg an izatio n O verview on Cell Switch O ff M ethods and CoM P Enhancem ents Analysis o f Existing Energy Efficient Cellular S c h e m e s Enabling Methods for Green R a d io Energy Efficient Resource Utilization and Performance Trade-offs Analysis and Discussion o f Existing Cell Sw itch-o ff Techniques LTE - Advanced CoMP System F ram ew o rk Introduction to Downlink LTE T ransm issio n CoMP Definitions and S tandardization Joint Transmission P ro ced u res Downlink Channel Estimation for Beyond LTE S y ste m s Technical Challenges and Discussion o f Existing L ite ra tu re S u m m a ry v

6 3 Coordinated M ulti-point Aided Cell Switch O ff Schem es Cellular System M o d e l Cellular Layout and Uniform User D istrib u tio n Large Scale Propagation and Pathloss M o d e l Downlink CoMP Performance Metrics F o rm u latio n Capacity Calculation for CoMP S y s te m s Power Consumption M o d e l Energy Efficiency M e tric Simulation Results and D iscu ssio n Traditional Cell Switch O ff versus CoMP Aided S c h e m e s Threshold-Based Joint Transm ission Clustering CoM P Schemes S u m m a ry Performance Analysis o f Joint Transm ission Scheme Subject to Im perfect CSI Feedback ^6 4.1 Small Scale Fading M o d e l Rayleigh Channel M o d e l Winner SCME M o d e l Formulation o f CoMP Performance Metrics for Time-varying Channels Simulation Results and D iscu ssio n Impact o f Channel Estimation E rro rs Impact o f CoMP System D e la y Joint Impact o f Channel Estimation Errors and D e la y s S u m m a ry vi

7 5 M ulti-point Statistical Channel Estim ation and Prediction Schem es Stochastic Characteristics o f CIR and C T F Time Dispersive C haracteristics Time Varying C haracteristics Channel Estimation T echniq u es Frequency Domain E stim atio n Time Domain Channel Estimation and P red ictio n CoMP Performance Gains due to Channel Estimation and P rediction CoMP Adaptive Channel Estimation Filter D e s ig n User Driven Instantaneous Received Power T h resh o ld in g Moving Mean o f Joint Transmission Cluster D e g re e Independent Tracking o f CoMP Measurement Set P o in ts Adaptive Filter Lookup Table F o rm atio n UE Anchored Down-Selection for CoMP Joint Transmission C lu s te r S u m m a ry C onclusion and Future W ork Thesis C onclusions Possible Enhancements and Future W o r k Bibliography 84 vii

8 List of Tables 2.1 LTE D ow nlink O FD M P aram eters standardized in [ 3 1] assum ing 15 kh z subcarrier s p a c in g Sim ulation param eters for U M A pathloss m o d e l Pow er consum ption param eters for e-n B s using C om P according to [47] and [ 4 8 ] M ean system energy efficiency and user perceived DL capacity for cell sw itch o f s c h e m e s Sim ulation param eters for sm all scale fading m o d e l T im e-invariant C T F interpolation filter coefficients for various estim ation m ethods show n in [29] M ulti-point adaptive estim ation filter length lookup table using C IR autocorrelation v a lu e s 77 viii

9 List of Figures 2. 1 P ow er consum ption distribution o f radio access netw orks, adapted from [ 8 ] D eploym ent and spectrum efficiency versus energy efficiency trade-off, adapted from [ 1 4 ] E nergy efficient w ireless resource utilization, taken from [ 1 7 ]... II 2.4 L ow traffic cells sw itching o ff fully o r zoom ing in w hile neig h b o r cells zoom out by antenna tilts, C om P, o r relaying approaches to serve the users located in the sw itched o ff cells as show n in [ 19] D aily traffic distribution and energy saving regions d uring night zones w ith low traffic periods show n in [ 2 1 ] C ell sw itch o ff schem e enabled via cooperation am ong m ultiple netw ork service providers show n in [ 2 2 ] LTE cell sw itch off/on solution suggested in 3G P P release 8 w orkshops by [ 2 4 ] T im e-f requency dom ain representation o f an O FD M signal show n in [ 2 8 ] C yclic prefix utilization in LTE system s to reduce ISI due to m ultipath reception, taken from [27] LTE dow nlink O FD M A transm itter and receiver a rc h ite c tu re Fram e structure and resource blocks in LTE FD D system s dem onstrated in [33], assum ing norm al C P u s e D ow nlink C om P schem es dem onstrated in [ 3 4 ] U ser plane data flow for dow nlink inter-en B C om P jo in t transm ission s c h e m e DL C om P procedures for inter-en B jo in t transm ission s c h e m e s R eference sym bol m apping in LTE-A D L C om P system s, adapted from [ 3 0 ] U niform user distribution and hexagonal cellular la y o u t Large scale urban m acro spatial pathloss m odel including both LoS and N L os p ro b a b ilitie s R eceived S1NR C D F com parison betw een traditional and C om P aided cell sw itch o ff schem es E nergy efficiency and DL capacity com parison betw een traditional and C om P aided cell sw itch o ff s c h e m e s PD F o f C om P jo in t transm ission cluster degrees for cell sw itch aiding versus regular C om P schem es, sim ulated in stationary channels according to 3 db clustering th re s h o ld ix

10 4.1 D em onstration o f tim e dispersive and varying nature o f the channel due to m ultipath propagation and m o b ility Sm all scale m ultipath fading m odel under different U E receiver m obility c o n d itio n s Joint transm ission cluster degree changes due to channel estim ation errors in fading channels D ow nlink capacity and energy efficiency perform ance o f C om P schem es subject to channel estim ation e r r o r s P erform ance degradation o f C om P schem es subject to system delays under various m obility sc e n a rio s Perform ance degradation o f C om P schem es subject to both system delays and m ulti-point channel estim ation errors under various m obility s c e n a rio s D ow nlink capacity and energy efficiency perform ance o f C om P schem es subject to both system delays and m ulti-point channel estim ation errors under low m obility conditions, v = 6 k m /h D ow nlink capacity and energy efficiency perform ance o f C om P schem es subject to both system delays and m ulti-point channel estim ation errors under high m obility conditions, v = 120 k m /h D ow nlink capacity and energy efficiency increases due to m ulti-point C IR estim ation by tracking each delay tap, /rn ; ( t, r (), individually using the delay-cross pow er density functions form ulated in ( ) C om parison o f m ulti-point channel estim ation done by tracking C IR at each delay tap separately as show n in (5.2 3 ) versus tracking the superim posed C IR sam ples as show n in ( ) D ow nlink capacity and energy efficiency gains o f th e C om P system due to U Es perform ing superim posed C IR estim ation using (5.2 8 ) and serving en B perform ing C IR prediction using ( ) P erform ance im provem ent o f the C om P system by utilizing m ulti-point channel estim ation and prediction s c h e m e s Instantaneous received pow er thresholding to predict the m em bers o f the jo in t transm ission cluster and adapt the m ulti-point channel estim ation filter le n g th s x

11 5.6 T racking tim e-varying m ean o f the C om P jo in t PD SC H transm ission clustering degrees to adapt m ulti-point channel estim ation filter le n g th s Independent tracking o f tim e-varying C om P M easurem ent set points to dynam ically adapt the filter lengths separately for each n e Nmeas C o M P jo in t transm ission cluster dow n-selection anchored by the U E after perform ing m ulti-point channel estim ation and thresholding the received pow er estim ates for each m easured p o in t 78 xi

12 Nomenclature AIPN A M C All IP N etw ork A daptive M odulation and C oding A W G N BC BS C 2PO W E R C A PE X C D F C D M A CIR C LT CN C O M P C P CQI C R C C RS CS C S/C B A dditive W hite G aussian N oise B illing C enter B ase Station C ognitive R adio and C ooperative Strategies for PO W E R Saving C apital E xpenditure C um ulative D istribution Function C ode D ivision M ultiple A ccess C hannel Im pulse R esponse C entral Lim it T heorem C ore N etw ork C oordinated M ulti-point C yclic Prefix C hannel Q uality Indicator C yclic R edundancy C heck C ell Specific R eference Sym bol C ircuit Sw itched C oordinated Scheduling C oordinated B eam form ing CSI C TF DC I DE D FT DL DPS C hannel S tate Inform ation C hannel T ransfer Function D ow nlink C ontrol Inform ation D eploym ent E fficiency D iscrete Fourier T ransform D ow nlink D ynam ic Point Selection

13 DRX D iscontinuous R eception D TX D iscontinuous T ransm ission E A R TH E nergy A w are R adio and N etw ork T echnologies E D G E EE E-N B E -PD C C H E nhanced D ata R ates for G SM E volution E nergy E fficiency E nhanced N ode B E nhanced Physical D ow nlink C ontrol C hannel ES E-U TR A N FDD FDM FFT G PR S H A R Q H etn et H LR H SPA ICI ICT IFFT IP I/o ISI ITU JT KPI LOS LTE E nergy Saving Evolved U niversal T errestrial R adio A ccess N etw ork Frequency D ivision D uplex Frequency D ivision M ultiplexing Fast Fourier T ransform G eneral P acket R adio Service H ybrid A utom atic R epeat R equest H eterogonous N etw ork H om e L ocation R egister H igh S peed Packet A ccess Inter C arrier Interference Inform ation and C om m unications T echnology Inverse Fast Fourier T ransform Internet Protocol In-phase and Q uadrature Inter Sym bol Interference International T elecom m unication U nion Joint T ransm ission K ey P erform ance Indicator L ine-of-sight Long T erm E volution

14 LTE-A L ong T erm E volution A dvanced M A C M IB M edia A ccess C ontrol M aster Inform ation B lock M IM O M M SE M ultiple Input M ultiple O utput M inim um M ean S quare E rror N A S N on A ccess Stratum N LO S OAM O FD M O FD M A O PEX PA PBCH PD CCH PD F PD N -G W PD P PD SCH PMI PRB PS PU CCH PU SCH Q A M Q os Q PSK R A T RB N on-l ine-of-s ight O perations A dm inistration and M anagem ent O rthogonal Frequency D ivision M ultiplexing O rthogonal Frequency D ivision M ultiple A ccess O perational E xpenditure Pow er A m plifier Physical B roadcast C hannel Physical D ow nlink C ontrol C hannel Probability D istribution Function Packet D ata N etw ork G atew ay Pow er D elay Profile Physical D ow nlink Shared C hannel P recoding M atrix Indicator Physical R esource Block Packet Sw itched Physical U plink C ontrol C hannel Physical U plink Shared C hannel Q uadrature A m plitude M odulation Q uality o f Service Q uadrature Phase Shift K eying R adio A ccess T echnology R esource B lock

15 RE RI R esource E lem ent R ank Indicator RM S R oot M ean Square RRC R adio R esource C ontrol RRH R em ote R adio H ead RRM RS R adio R esource M anagem ent R eference Signal R SRP R SR Q SC M E R eference Signal R eceived Pow er R eference Signal R eceived Q uality Spatial C hannel M odel E xtended SE SIB SIN R SON T D M TM TTI UE UL UM A U M T S U TR A N V LR W C D M A W IN N E R Spectral E fficiency System Inform ation B lock Signal-to-Interference-plus-N oise R atio S elf-o rganizing N etw ork T im e D ivision M ultiplexing T ransm ission M ode T ransm ission T im e Interval U ser E quipm ent U plink U rban M acro U niversal M obile T elecom m unications System U niversal T errestrial R adio A ccess N etw ork V isiting L ocation R egister W ideband C ode D ivision M ultiple A ccess W ireless W orld Initiative N ew R adio XV

16 Chapter 1 Introduction 1.1 Thesis Problem Statem ent and M otivation E xponential rise in cellular device usage in the recent years along w ith the increase in th e m inim um required received quality o f service lead the innovation in th e direction o f cellular enhancem ents enabling spectrally efficient system s. Future w ireless standards like L TE-A (L ong T erm E volution A dvanced) and beyond are m aking significant changes to the overall system architecture, air interface and quality o f service offered to the users. Increased com plexity o f these cellular features and the rising m obile usage rates create a m ajor pow er consum ption burden on the overall system s. A s a result, all the key features considered by future w ireless technologies should be jo in tly review ed u nder the green radio um brella to check for possibilities o f energy saving im plem entations and increased total system capacity concurrently. R ecent developm ent and enhancem ents in w ireless com m unications do not only focus on increasing the data rates, capacity o r spectral efficiency, but also on im plem enting energy saving m ethods. T his is m ostly due to th e observation o f high energy consum ption resulting from inform ation and com m unications technologies and m ainly w ireless access netw orks. The inform ation and com m unications technology (IC T ) is responsible for 2-10% o f the global energy consum ption and the access netw orks (G E R A N for G PR S, U TR A N for U M T S and e-u T R A N for L TE) are responsible for 60-80% o f the w hole cellular netw ork energy consum ption as m entioned in [ 1] - [3], A s a consequence, optim izing the w ireless access stratum plays a m ore im portant role in overall energy savings in cellular architectures com pared to the core netw ork energy efficiency. Fourth generation w ireless standards pioneered by LTE (L ong T erm E volution) technology utilize adaptive m odulation, O F D M A (O rthogonal Frequency D ivision M ultiple A ccess), and H A R Q (H ybrid A utom atic R epeat R equest) schem es to m axim ize the observed capacity lim its. H ow ever, C oordinated M ulti- Point (C om P ) transm ission technology, w hich is listed as one o f the key features for L T E -A dvanced, aim s on decreasing the inter-cell interference via coordination am ong different transm ission points to further increase the achieved cell edge data rates. U sers w ill receive data transm ission from m ultiple cells w ith better SIN R (S ignal-to-lnterference-plus-n o ise R atio) values. T his m akes C om P a potential candidate to deliver green radio solutions by decreasing the energy cost per bit, if used correctly. 1

17 The thesis aim s on perform ing thorough analysis o f C om P perform ance in term s o f energy efficiency and capacity, investigating jo in t use o f C om P w ith existing energy efficient cell sw itch o ff schem e, identifying technical challenges and perform ance bottlenecks for C om P schem es, creating a fram ew ork for tim e-varying C om P jo in t transm ission schem e, and developing C om P adaptive channel estim ation and prediction schem es to tackle possible system delays and m ulti-point channel estim ation errors. 1.2 T hesis C ontributions N ovel contributions o f the thesis w hich have not been analyzed o r proposed in any ex istin g literature, to the best o f our know ledge, are listed as follow s: Individual and jo in t im pacts o f channel estim ation errors and system delays on D L C om P perform ance m etrics are evaluated. Im pacts o f inaccurate C om P active set clustering due to th e m ulti-point faulty tim e-varying channel feedbacks on overall system bits/joule energy efficiency and dow nlink capacity rates are analyzed. It is show n that the accuracy o f the tim e-varying jo in t transm ission set clustering decisions, in term s o f both the m em ber choices and the clustering degrees, is the m ajor perform ance determ ining factor for C om P system s. P erform ance degradation sensitivities o f various user locations in the cellular layout are characterized both for low and high m obility conditions. It is dem onstrated that the users being served w ith higher C om P clustering degrees get affected m ore severely due to inaccurate jo in t transm ission sets. N ovel C om P adaptive m ulti-point channel estim ation filter designs are proposed, w here the user equipm ents (U E s) dynam ically change the estim ation filter lengths according to the observed C om P clustering degrees and the probability o f each m easured point being an active m em ber o f the jo in t transm ission set on the upcom ing T TI, unlike the single point channel estim ation techniques w hich adapt the filter lengths solely based on the coherence tim e o f the channel and the U E receiver velocity. T his m ethod both im proves the C om P clustering accuracy and reduces the channel estim ation com putation com plexity by m aking sure only the U Es observing higher clustering degrees utilize enlarged estim ation filters for points that are m ore likely to take active role in jo in t PD SCH transm ission. A novel m ulti-point channel feedback reporting m ethod is proposed, w here the UE perform s dow n- selection on the jo in t transm ission set for the upcom ing TTI to reduce the clustering accuracy burden on the serving en B. UE perform s received pow er thresholding technique using the estim ated m ulti

18 point channel im pulse responses and reports CSI feedback only for the points that are likely to be a part o f the jo in t transm ission cluster. T his schem e reduces the uplink payload required to report m ultipoint feedbacks, avoids the unnecessary feedbacks to be transferred w ithin the C om P access netw ork nodes, m inim izes C SI processing tim e at the serving en B, and yields m ore up-to-date CSI feedbacks to be used for C om P clustering decisions. A dditional contributions o f the thesis, w hich can be used to enhance the existing literature, are listed as follow s: Joint use o f the L TE-A feature, C om P, w ith the traditional cell sw itch o ff schem e is analyzed. Increasing the transm it pow er m ethod in the rem aining active cells d uring the energy saving period is replaced by the rem aining active cells using C om P jo in t transm ission technique in the dow nlink to jo in tly serve the users in the sw itched o ff cell. R ealistic w ireless channel and propagation m odels are used in accordance to 3G PP specifications and the sim ulation results d em onstrate that the C om P aided cell sw itch o ff schem es not only im prove the energy efficiency o f the radio access netw orks, but also increase the user perceived quality o f service in term s o f received dow nlink capacity rates w ith respect to the traditional cell sw itch o ff schem es. A study on optim al C om P jo in t transm ission clustering degree choices have been conducted, and it is dem onstrated that the serving en B has to perform threshold-based clustering decisions based on the received DL pow er values every TTI to balance the trade-o ff betw een the capacity and the energy efficiency o f the access netw orks. T his technique prevents energy efficiency degradations in the C om P access netw ork by avoiding inclusion o f unnecessary points in the jo in t transm ission cluster, w hich w ould lead to additional back hauling and signal processing pow er consum ption that cannot be com pensated by enough capacity gains. V arious m ulti-point channel estim ation/prediction schem es are analyzed to im prove the jo in t transm ission set clustering accuracy. M ulti-point channel im pulse responses can be tracked either by superim posed o r decom posed form at. L atter m entioned schem e tracks each m ultipath com ponent o f every C om P m easurem ent set m em ber and yields m ore accurate estim ates, how ever leads to significantly higher com putation tim es as opposed to the superim posed tracking. It is also dem onstrated that the serving en B can m axim ize the perform ance gains by setting the channel prediction range equal to observed system delay betw een the CSI reports and the PD SC H transm ission. 3

19 1.3 Publications, Patent Filings and Thesis O rganization T he research w ork presented in this thesis has been published, accepted, subm itted, o r is in progress for subm ission. Produced articles are referenced across the chapters according to the follow ing organization: C hapter 2 serves as a literature survey and tutorial. Introduction to green radio is presented and existing energy efficient cell sw itch o ff schem es are analyzed according to possible trade-offs. A rchitecture o f dow nlink L TE transm ission is explained thoroughly and C om P jo in t transm ission standardization process is described w ith respect to the new ly introduced radio procedures. E xisting C om P literature is discussed and the technical challenges are identified. C h ap ter 3 describes the cellular system m odel used in our study, form ulates the dow nlink C om P perform ance m etrics and presents a perform ance com parison o f the traditional versus C om P aided cell sw itch o ff schem es in term s o f energy efficiency and capacity m etrics assu m in g stationary channel conditions. P ro o f o f concept is provided for the serving en B to form the C om P jo in t transm ission clusters by thresholding the dow nlink received pow ers. C ontents o f the chapter are published in the follow ing conference paper: G. C ili, H. Y anikom eroglu, and F. R. Y u, C ell sw itch o ff technique com bined w ith coordinated m ulti-point (C om P ) transm ission for energy efficiency in beyond-l T E cellu lar netw orks," in Proc. IEEE ICC'12 Workshops, O ttaw a, O N, C anada, June C hapter 4 describes the sm all scale fading m odel used in o u r study, form ulates the tim e-varying C om P perform ance m etrics, and analyzes the perform ance sensitivity o f the proposed schem e for various users under different channel conditions by taking m ulti-point channel estim ation errors and system delays into consideration. Im pacts o f the jo in t transm ission clustering accuracy on the achieved C om P perform ance gains are discussed. C ontents o f the chapter are presented in the follow ing conference pap er subm ission: G. C ili. H. Y anikom eroglu, and F. R. Yu, E nergy efficiency and capacity evaluation o f LTE- A dvanced dow nlink C om P schem es subject to channel estim ation errors and system d elay, subm itted to IEEE IC C 13, B udapest, H ungary, June

20 C hapter 5 describes the stochastic m odeling o f the channel im pulse response and channel transfer function along w ith possible tim e-varying channel estim ation/prediction schem es. P erform ance gains due to m ulti-point channel estim ation/prediction procedures are analyzed w ith respect to energy efficiency and capacity perform ance m etrics. N ovel C om P adaptive channel estim ation filter designs and UE aided jo in t transm ission set clustering m ethods are discussed. C ontents o f th e chap ter are presented in the follow ing invention disclosures: G. C ili, H. Y anikom eroglu, and F. R. Yu, U E anchored dow n-selection for C om P jo in t transm ission cluster, Filed by A pple Inc.. U.S. Patent A pplication N o: 61/674,854 (filing date: July 2 4, ). G. C ili, H. Y anikom eroglu, and F. R. Y u, C om P adaptive channel estim ation prediction filter design, Filed by A pple Inc., U.S. Patent A pplication N o: 61/674,852 (filing date: July 23, 2012). A jo u rn al p ap er w ith the follow ing title is being prepared for subm ission: G. C ili. H. Y anikom eroglu. and F. R. Yu, C oordinated m ulti-point adaptive channel estim ation and prediction schem es for accurate jo in t transm ission clustering," to be subm itted to an IEEE jo u rn al, Sept C hapter 6 highlights the conclusions o f the thesis and explains the possible enhancem ents for future w ork. 5

21 Chapter 2 Overview on Cell Switch Off Methods and CoMP Enhancements 2.1 A nalysis o f Existing Energy Efficient C ellular Schem es R ecent m arket innovations for green radio" are pioneered by EA R TH (E nergy A w are R adio and N etw ork T echnologies) and C 2P O W E R (C ognitive R adio and C ooperative S trategies for PO W E R saving in m ultistandard w ireless devices) projects w hich have strong partners in both academ ia and the industry. T he goal o f both projects explained in [4] and [5] can be listed as follow s: D evelop energy saving m ethods for w ireless m obile devices and energy efficient m obile com m unication system s using cognitive and cooperative radio to decrease the C02 em ission by the IC T (Inform ation and C om m unications T echnology) industry. Propose energy efficient netw ork deploym ent and resource m anagem ent schem es w ithout sacrificing the quality o f service perceived by the users and the total system capacity. T hese innovations lead to new technology requirem ents for upcom ing w ireless technologies. E nergy saving and low C A P E X /O P E X for the netw orks are now considered as future technology requirem ents for 3G P P release 11 and beyond standards by D ocom o as m entioned in [6], As a result, all the key features considered by future w ireless technologies should be review ed under the "green radio" um brella to check for possibilities o f energy saving im plem entations and design Enabling Methods for Green Radio V arious system level approaches are possible to obtain energy efficiency in cellular netw orks. A detailed survey on m otivations for green cellular netw orks and different m ethods for energy savings are presented in [7] and categorized under energy savings via cooperative netw orks, renew able energy resources, heterogonous netw orks and cognitive radio. C ore netw ork operation, sustaining backhaul data traffic, access netw orks p roviding radio interface both for user and control planes, and m obile handsets contribute to the overall pow er consum ption o f the cellular system s. E nergy efficiency approaches w ere categorized in [8] as: com ponent level, radio interface, and netw ork deploym ent m ethodologies. 6

22 Total Conventional Advanced 1800W 1400W Power amplif met feeder 50-80% (-1200W) Air conditioning 50-80% (-300W) 700W Steep modes Power supply 5-10% (-100W ) r Signal processing (analogue and digital) 5-15% (-200W ) (120W ) Figure 2.1: Power consum ption distribution o f radio access networks, adapted from [8] and [9], C om ponent level energy savings are obtained by focusing on the various pow er consum ing parts o f a base station. Figure 2.1 show s that the pow er am plifiers, air conditioners, pow er supply and signal processing all consum e energy to operate a base station. C om ponent level energy saving focuses on the PA (pow er am plifier) efficiency to draw less current during operation since the PA consum es betw een 50-80% o f the overall base station pow er [9], T his is because the pow er am plification at the base station is essential w hile serving users facing m ajor path losses. M ethods like peak-to-average pow er ratio reduction, linearity increase, dynam ic frequency and voltage scaling according to traffic load, com ponent deactivation for digital/analog inactive base station com ponents, digitally flexible drivers and PA s to control the output pow er, DC pow er consum ption and linearity are som e o f the suggested m ethods. Pow er am plifier efficiency, the ratio o f the input AC pow er to the dow nlink transm itted output pow er, degrades from 50% to 5% as the served num ber o f users in the netw ork decreases [10]. H ence, sw itching o ff the base station d uring low traffic loads can be favorable for overall energy consum ption. It is m entioned in [11] that 80-90% o f the energy consum ed by the pow er am plifiers are w asted as heat w hich increases the pow er consum ption o f the air conditioning com ponents as w ell. P ow er consum ption o f the signal processors are directly related to the used transm ission and m odulation schem es. A dvanced spectrally efficient m odulation schem es lead to higher signal processing pow ers. It should be noted that even w hen there is no load in the netw ork, access netw ork still consum es 50% o f the peak DC pow er due to the pow er supply operating the base station and active air cooling as m entioned in [12] w hich is show n in Figure 2.1 w here the base station still consum es an input pow er even w hen the dow nlink transm itted output antenna pow er is zero. As a result, m ain energy saving potentials at the com ponent level are deactivating inactive parts o f the digital/analogue circuits, im plem enting sleep m odes during dow nlink transm ission or sw itching o ff the base stations com pletely during the low traffic periods. 7

23 Link level energy saving m ethods contains all the air interface strategies and protocols that can be im plem ented to save pow er. R educed control plane RRC and N A S signaling. M IB (M aster Inform ation B lock) and SIB s (System Inform ation B locks) transm ission, and synchronization signals w ill help the netw ork save pow er during dow nlink operation. H ow ever, this m ethod com es w ith the risk o f bringing the overall netw ork perform ance dow n for cell selection, cell acquisition and R RM procedures. A nother link level solution to shift the focus from spectral efficiency to energy efficiency is to create m icro-sleep m odes for discontinuous transm ission (D T X ) on the dow nlinks. T hese deep/m icro sleep m odes for the base stations are enabled for 3G P P release 8 and further since LTE does not need to transm it the reference sym bols continuously unlike U M T S /H SPA protocols w here pilot sym bols had to be continuously transm itted on the dow nlink. D iscontinuous reception (D R X ) operation is already enabled in 3G P P release 8 both for U Es in RRC idle and connected m odes. M obile units m onitor paging m essages and U L (U plink)/d L (D ow nlink) scheduling grants on pre-configured cycles to save pow er. Sam e logic can be applied to the base stations w here control and user plane data are transm itted on specific periods enabling the transm it circuits to be pow ered o ff during sleep m odes. N etw ork level energy savings m ethods can be applied by deploying heterogonous netw orks, enabling coordination am ong the nodes, cooperative relaying and cognitive radio to dynam ically adapt to changing traffic loads. H ierarchical deploym ent o f netw orks via m acro, m icro, pico and fem to cells decrease the propagation distance betw een the transm it and receive antennas, hence, reduce the required transm it pow er. W ireless relays, w hich are cheap er to deploy com pared to base stations, do not require any backhaul links or com plex routing schem es, create an in-direct m ulti-hop transm ission environm ent w ith shorter propagation distances and decrease the burden on the pow er am plifier com ponents. T he challenge for H etn et and w ireless relaying schem es is the optim al deploym ent strategy to balance the trad e-o ff betw een increasing the outdoor coverage areas via sm all cells and still keeping the traffic load o f the m acro-cells at a certain level. As m entioned earlier, if m ajority o f the coverage is sustained by the sm all cells, the traffic loads and the pow er am plifier efficiencies o f the m acro cells reduce and the overall deploym ent becom es m ore expensive due to the short range coverages o f the sm all cells and relays. C ooperation am ong base stations can also help save energy in the cellular system either by load balancing options to d ecrease the transm ission pow er in the cells w ith high traffic loads and increasing the transm ission pow er o f low traffic cells o r by com pletely shutting o f base stations under low traffic conditions and increasing the coverage area o f the neighbor cells not to create any coverage 8

24 A) Theoretical Trade-Offs B) Practical Trade-Offs Realistic enb Power Consumption Modeling Increased Transmit Power SE figure 2.2: Deployment and spectrum efficiency versus energy efficiency trade-off. adapted from [ 14], DE SE holes. A m ore environm entalist solution is proposed in [7] to save energy resources and decrease carbon em ission is achieved by using alternative green resources such as biofuels, solar and w ind energy in cellular netw orks. C ell sw itch o ff schem e is the m ost prom ising candidate for energy savings in cellular netw orks since it provides a holistic approach by com bining link, com ponent and netw ork level pow er saving m ethodologies. Cell w ith the low traffic conditions is sw itched o f f com pletely to m axim ize the com ponent level pow er savings, and rem aining active cells in the netw ork coordinate to serve the users located in the sw itched o ff region. V arious existing cell zoom ing strategies are analyzed and discussed in Section C ell sw itch o ff schem es aided w ith dow nlink C om P transm ission w ill be discussed in C hapter 3, w hich enable the users in the sw itched o ff cell to be served sim ultaneously by m ultiple radio links Energy Efficient Resource Utilization and Performance Trade-offs Energy efficient cellular m ethodologies are being standardized by m ajor standardization bodies like 3G P P and ITU and existing key perform ance indicators like spectral efficiency and deploym ent efficiency should be jo in tly considered w ith the energy efficiency m etrics. S tandalone m ethods that aim on im proving a certain perform ance m etric and ignoring the rem aining m etrics are not o f interest. G reen radio im plem ented in any aspect o f the w ireless com m unication including access netw ork enhancem ents, core netw ork im provem ents, protocol stack changes, scheduling im plem entations or cooperating netw orks have to be re-considered according to deploym ent efficiency (DE) versus energy efficiency (EE) and spectral efficiency (SE) versus energy efficiency (EE) trade-offs. D eploym ent efficiency is considered as system throughput per unit deploym ent cost w hich can be sim ply im proved by enlarging the cellular coverage o f the existing base stations. 9

25 Increasing the dow nlink transm it pow ers decreases the total num ber o f base stations deployed in the netw ork and save unit cost for both capital expenditure (C ape x) and operational expenditure (O pe x). H ow ever, the im plem entation should be the exact opposite for system s aim ing energy efficiency since denser base station deploym ents correspond to low er transm it pow ers due to shorter radio links [13], A t first, energy efficiency o f the cellular netw ork seem s to d ecrease w ith low er deploym ent costs as show n in Figure 2.2a: h o w ever the relation betw een the access netw ork pow er consum ption and the deploym ent cost should be m odeled realistically. E nergy consum ption and the deploym ent cost o f the netw orks do not only depend on the transm it pow er o f the base station. PTX, but also on the operational pow er consum ption, P0P, such as the air cooling, signal processing and the pow er supply as explained in Section As a result, increased cell sizes do not alw ays yield energy inefficient solutions. E valuating EE - DE trad e-o ff according to the realistic pow er consum ption m odels show s that there exists an optim um solution w hich im proves both th e perform ance m etrics as show n in Figure 2.2b. b its/sec. A nother perform ance indicator for cellular system s is the spectral efficiency m easured in -, w hich Hz has alw ays been the m ain focus o f im provem ent during the w ireless evolution. E xpressing energy efficiency o f the system in b its /jo u le, the theoretical relation betw een spectral and energy efficiency is derived by authors o f [14] using Shannon s capacity form ula for additive w hite G aussian noise (A W G N ) channels explained in [15] as: SE = logz( l + ^ ) ; (2.1) W-N0J ' EE = Wlog2{l + P^ f ) / P TXg ; (2.2) E E = 1~,. (2.3) (2 l)*v0 T otal bandw idth assigned to the user is expressed as W, channel gain is denoted as g, and noise spectral density is represented by N0. T heoretical derivation for EE SE trad e-o ff for an A W G N channel show n in (2.3), w hich ignored the transm ission independent contributors to the overall pow er consum ption in (2.1) and (2.2), suggests m onotonically decreasing relation betw een spectral and energy efficiency perform ance m etrics. In fact, under practical considerations for realistic access netw ork pow er consum ption m odel in [16], pathloss and propagation schem es, m odulation/coding schem es, resource m anagem ent algorithm s, p o w er am p lifier linearity/efficiency and m ulti-user/m ulti-cell scenarios, EE SE trad e-o ff leads to a non-m onotonic relation show n in Figure 2.2.b. 10

26 A) Generic Resource Utilization B) Green Resource Utilization Frequency Frequency Time Energy Time Energy Space Code Space Code f igure 2.3: Energy efficient wireless resource utilization, taken from [ I7. T his is the fundam ental m otivation for developing energy efficient fram ew orks w ithout sacrificing the user perceived quality o f service (Q os ) in term s o f spectral efficiency and achieved data rates. B ottleneck for the perform ance m etrics m entioned above is the lim itation o f w ireless resources like tim e, frequency, space, energy and code. U tilization ratio o f these w ireless resources can be optim ized in favor o f certain perform ance m etrics, w here Figure 2.3 show s an exam ple green resource trading schem e that im proves the energy efficiency by o ver utilizing the rem aining w ireless resources. A prelim inary fram ew ork have been proposed by authors o f [17] to find the optim al bandw idth and tim e resource utilization w hich results in m inim al energy consum ption to transm it one bit, Ejoule/bit. C apacity o f the A W G N channel, C, m easured in bits/sec is expressed as the inverse o f the tim e it takes to transm it one bit, tbit, as C = = W lo g,( l + ^ 2 ). (2.4) tbit * W No Energy consum ption o f the access netw ork due to the dow nlink tran sm it pow er consum ption o f the base station is derived using (2.4) as 1 F - P t - V W' tbu-f)'w >N0.tbit t TX r TXt bit (2.3 } and the overall energy consum ption o f the access netw ork is represented as the sum o f energy spent for transm ission and the energy spent due to the circuitry o f the base station, Ecir, as 3 r... r. p Ejoule/bit - E-tx + fcd r - + w ^circtbu + ^ b ^b U - w here Pcirc is the transm ission independent operational pow er consum ption w hich scales proportionally w ith the used frequency bandw idth and Psb denotes the operational pow er consum ption w hich is independent o f the transm ission and the utilized bandw idth. It is clear from (2.6) that the operational energy consum ption o f the 11

27 system increases w ith increasing bandw idth and tim e resources w hile the transm itting energy consum ption o f the system m onotonically decreases. A s a result there exists an optim um bandw idth and tim e resource allocation in the system w hich m inim izes the energy consum ption d epending on the observed channel gain, g. T he system im plem ented in [17] finds the optim um W for a fixed t and vice versa by using the convex energy consum ption functions resulting from (2.6) for different channel gains by m aking sure the user still receives the m inim um Q os required in term s o f assigned frequency range and the delay observed during transm ission. Proposed schem e can be further im proved by sim ulating the large scale pathloss w ith realistic propagation m odels instead o f the sim plistic free space propagation m odel, including sm all scale fading in the system and arranging the m edia access control (M A C ) scheduling decisions every TTI according to the green resource trading function Analysis and Discussion of Existing Cell Switch-Off Techniques V arious green techniques have been proposed by academ ia and industry recently for the w ireless access netw orks w ith different optim ization m ethods using various energy efficiency and tra d e -o ff m etrics. A lthough there are m ethods to sustain long term energy savings by reducing peak user dem and as proposed by authors o f [18], access netw ork energy savings are m ostly im plem ented by cell size adjustm ents according to traffic load fluctuations. A n exam ple o f energy saving w ith cooperated base stations schem e is dem onstrated in [19] that aim s to find w hich cells in the netw ork should be sw itched o ff so that the traffic load is concentrated around the base stations providing highest spectral efficiencies to the served users. T he cell sizes in the netw ork are dynam ically adapted according to th e traffic load fluctuations in the netw ork. A load concentration approach is show n w here the cells w ith the low traffic zoom into zero and the neighbor cells zoom out by using C om P, relaying approaches o r physical adjustm ent m ethods including antenna tilts and increased transm it pow ers to sustain the traffic as show n in Figure 2.4. T his is the exact opposite approach com pared to the load balancing approach w here the high traffic cells used to zoom in to disperse the total traffic in the netw ork. The proposed system com es alo n g w ith m any challenges such as short term traffic fluctuations, risk o f coverage holes in the netw ork, and special control channels needed for cooperated signaling and the com patibility lim itations due to cells w hich are incapable o f cell zoom ing. A centralized approach is proposed w here the virtual cell zoom ing server collects all the channel conditions and rate requirem ents from the users assum ing each user can only be served by one base station. T he algorithm in the central server loops through all the m obile users, i G /, and assigns 12