LTE Cell Reselection.doc

T-Mobile – Small Cell Requirements

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T-Mobile – Intra LTE Mobility Solution CONTENTS 1.

INTRODUCTION.... INTRODUCTION.............. ..................... ..................... ..................... ..................... .................... ..................... ..................... ..................... ..................... .................... ..................... .........................2 ..............2

2.

MOBILITY CHALLENGES CHALLENGES AND TOP LEVEL REQUIREMENTS...... REQUIREMENTS................. ..................... .................... ..................... ......................2 ...........2

3.

MOBILITY SOLUTIONS........... SOLUTIONS..................... ..................... ..................... .................... ..................... ..................... ..................... ..................... .................... ..................... ..........................3 ...............3

3.1. 3.2. 3.3. 4.

INTRA-LTE INTRA-LTE MOBILITY METHODS............ METHODS....................... ..................... .................... ..................... ..................... ..................... ..................... .................... .........................8 ...............8

4.1. 4.2. 4.3. 5.

IDLE MODE MOBILITY CONTROL.................................................................................................................3 I NTRA-FREQUENCY CONNECTED MODE MOBILITY.................... .............................. ..................... ..................... .................... ..................................4 ........................4 I NTER -FREQUENCY CONNECTED MODE MOBILITY......................................................................................6

I NTERFERENCE MITIGATION..........................................................................................................................8 PCI PLANNING AND TIME SYNCHRONIZED CELLS..................... ................................ ..................... .................... ..................... ....................................8 .........................8 I NTER -FREQUENCY TARGET MEASUREMENTS AND EVALUATION..................................................................

SUMMARY......... SUMMARY.................... ..................... .................... ..................... ..................... ..................... ..................... .................... ..................... ..................... ..................... ..................... ............................11 ..................11

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1

Introduction This document describes intra-LTE mobility challenges and solutions with ocus on LTE !mall Cell"!mall Cell# LTE $acro"!mall Cell scenarios% The description co&ers both idle and connected mode mobility% This document ser&es only as basis or discussion and hence no commercial commitments are intended by the inormation gi&en in this document%

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Mobility challenges and top level requirements The main challenge or mobility solutions are To be easy to use and be robust against &ariations in 'E capabilities# 'E mo&ements and propagation conditions% •

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Pro&ide non-interrupted and consistent ser&ices to subscribers that eperience &arying radio conditions and at the same time minimi*e the radio resource usage and signaling load% +e consistent with capacity"energy management unctionality and support operators di,erent strategy or trac distribution# networ. migration and /ntersystem"0T interaction% Control 'E cell reselection in idle mode to minimi*e setup delays and be consistent with mobility and the intended trac distribution and camping strategies% 0egulatory reuirements also needs to be addressed with due consideration to the subscriber de&ice capabilities%

The specifc challenges or LTE $acro"!mall Cell and LTE !mall Cell"!mall Cell are3 4icult to achie&e a good compromise between number o hando&ers and retainability (0adio +earer drop# 56 ailures) and integrity (+itrates# 4elay) Pro&ide means to control mobility decisions to allow ull utili*ation o LTE !mall cells but at the same time a&oid using !mall cells or 'Es that will stay in !mall cell co&erage only or a &ery short time% •

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E&en i some specifc networ. scenarios are used to describe the solutions in this document# one challenge is that there should be no inherent restriction on networ. scenarios supported% !ome o the networ. scenario &ariations3 !ingle carrier or multi carrier sites /ntra-band or /nter-band carriers LTE carriers only or /0T carriers 4i,erent LTE carrier +7 4i,erent LTE carriers in di,erent sectors 5igh power and low power cells using same or di,erent LTE carrier • • • • • •

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5igh power and low power cells using same or di,erent 0T Co-located or non co-located cells 6pen access or closed access cells /ndoor and outdoor cells

The solutions or idle mode and connected mode /ntra reuency mobility# /nter reuency mobility and /0T mobility# need to harmoni*e (no ping pong beha&ior# no dead loc. beha&ior) in real time # regardless o scenario and interaction with other unctions(e%g% Load balancing)% $ulti-&endor support or mobility is co&ered in terms o signaling by 89PP% There is also some support in :2 control signaling or hando&er tuning% 5owe&er since mobility perormance depends on harmoni*ed solutions and obser&ables or hando&er tuning# a multi-&endor scenario will add yet another dimension o challenges% $ulti-&endor harmoni*ation or the basic co&erage triggered hando&ers can typically be managed by agreements on parameters settings% The parameters used or tuning are typically common between &endors since 89PP e&ents and associated parameters in /dle and Connected mode are defned specifcally or support o co&erage triggered hando&er% ;or more elaborate networ. associated unctions where 89PP ha&e less detailed specifc support# multi-&endor harmoni*ation is more challenging and probably need ocused e,orts rom in&ol&ed &endors to reach ull unctionality and optimal perormance% Eamples o such areas are3 • • •

 3.1

/ntra LTE and /0T 5ando&er 6ptimi*ation utomatic
Mobility Solutions Idle Mode Mobility Control

The options or idle mode control are gi&en by the confguration options in 89PP T! 8=%881 and 'E beha&ior reuirements in 89PP T! 8=%8>?% The fgure below illustrates the typical use o the idle mode parameters%

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Figure 1: Typical Idle Mode Parameters 6ne important aspect or idle mode"connected mode interaction is that the 'E should preerably stay in the same cell when doing reuent transitions between idle and connected mode% This can be achie&ed by using the ollowing concept% /dle mode +roadcast a higher cell0eselectionPriority or the ser&ing reuency"cell in !/+ 8 than what the neighbors broadcast or the same reuency"cell in !/+ @% A6nce the cell is selected by the 'E the 'E will stay on the cell according to priority since the priority o the chosen reuency"cell Awent upB ater cell reselectionB •

Connected mode 6perator parameters or neighbour reuency"0T priority should typically be set eual to the cell0eselectionPriority or the neighbor reuency"0T used or idle mode% AConnected mode will in this way inherit the Ahigh priorityB or the reuency"cell 'E rom /dle modeB •

;urthermore# hando&er or co&erage reasons to other reuency"0T is only done when the decision to lea&e the current reuency"cell has been made and thus ser&ing cell reuency priority is o less concern or mobility% /t is possible to a&oid reuent Trac.ing area updates rom 'Es by utili*ing the T list concept rom 89PP that allow hysteresis by using o&erlapping Trac.ing reas% /n a $acro"!mall cell scenario the !mall cells should preerably belong to the same trac.ing area list as the co&erage o&erlapping macro cells as long as the paging capacity is sucient%

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3.2

Intra-frequency Connected Mode Mobility

;or /ntra reuency mobility in a reuency re-use 1 scenario it is &ery essential to .eep 'E connected to the best cell at all times% This is achie&ed by using /ntra reuency 8 e&aluation to trigger /ntra reuency hando&er any time needed% / not connected to the best cell the 'E will &ery soon su,er rom high /nter cell intererence le&els that e&entually will cause high P0+ usage# pea. rate degradations and e&entually 'E drops due to radio Lin. ailures# signaling ailures or hando&er ailures% The 'E ability to detect and measure on neighboring /ntra reuency cells will be impaired by high intererence le&els and to some etent also the time relation between ser&ing cell and measured neighbor% 'es using 40: cycles  ?> ms will also degrade the ability to detect and measure on neighboring cells% Deeping 'E connected to best cell at all times imply high hando&er reuencies and higher ris.s or oscillating hando&ers% /t also implies that hando&er preparation times should preerably be .ept low since the 'E is already in a Anon best cellB when hando&er preparation is initiated% t high load situations where admission denials start to occurred# e<+ will ha&e a possibility to attempt hando&er to Asecond bestB intra reuency cells ulflling the e&ent criteria% This allows the 'E to be connected to a better cell e&en i admission was not granted to the best cell% This also imply that there is much less reedom to use e%g% cell indi&idual o,set other than rather small o,sets or the purpose o tuning the ability to .eep the 'E connected to the best cell# balance 'L and 4L or to get an earlier indication that hando&er is needed% +elow is fgure illustrating simulation results showing the typical compromises when using di,erent hando&er o,set (8 o,set) and TTT in a 5et%> where confguration ?b is specifcally defned or 5otspot capacity enhancements% The absolute perormance in the results is &ery 'E and e<+ model and <7 scenario dependent and may or may not represent absolute perormance in feld% The results can though be used or understanding the relati&e e,ects rom changing parameters and the typical relation between ailure causes%

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Figure 2: HetNet Scenario
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The manual tuning e,orts to get optimal hando&er perormance or each scenario will be reduced by !6< eatures li.e e%g% 'E or Cell le&el oscillating hando&er minimi*ation# which detects that indi&idual 'Es or a specifc cell relation ha&e an oscillating tendency and increase the hando&er o,set temporarily or that 'E"cell relation% $obility 0obustness 6ptimi*ation unctionality with signaling support defned by 89PP is also studied and may be incorporated in the uture% 5ando&er optimi*ations on 'E or Cell le&el typically allows the initial deault hando&er o,set settings to be set lower and still .eep the
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be the same as beore acti&ation o the eature% Lower hando&er o,set settings ma.es 'Es to be connected to the best cell a larger portion o time at cell edge areas% Possible other enhancements or optimal hando&er perormance under study are to include 'E speed estimates and a&erage cell si*e estimates% 'E speed estimates are assumed to be possible to get rom 'L 4oppler shit estimates ta.en rom e<+% To support !mall cell range epansion the use o 'E perormance enhancements (/0C)# reuency domain /C/C and time domain /C/C (e%g% +!) schemes are under in&estigation% lso schemes where e<+ use coordinated cells that are using the same PC/ on the same reuency are in&estigated% /n this case e<+ need to Ata.e o&er rom the 'EB and pro&ide most o mobility support by itsel with no e&ent 8 triggered 'E PC/ measurement support in connected mode% This implies that e<+ need to decide which antennas points and which resources to use or 'L and 4L% 6ne challenge is that e<+ does not ha&e 'L transmission to measure on rom each 'E at all times unless e<+ periodically reuest Channel Guality /ndicators and Power 5eadroom 0eports rom the 'E% This e<+ ability will typically also be de&eloped or 'L comp and 4L comp schemes%

3.3

Inter-frequency Connected Mode Mobility /nter-reuency networ. scenarios with source and target cell on di,erent carriers are typically less challenging in terms o delay impact since there is no inter-cell intererence rom target cell that increase when 'E mo&es towards the target% 6n the other hand the 'E measurements on inter reuency target cells are more delayed than intra-reuency measurements and typically reuires the use o measurement gaps%

The use o measurement gaps may reduce 4L and 'L pea. rates in low loaded scenarios% The gap causes a 1> ms interruption (? ms to a&oid 50G in gap and = ms gap) once e&ery ?> ms or > ms which will gi&e 2@H and 12%@ H reduction in theoretical maimum pea. rates respecti&ely% To reduce the usage o gaps e<+ will use 2 and 1 e&ents to control when 'E is confgured to use gaps and search or inter reuency cells%
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2 - 'E ha&e entered +ad co&erage 1 - 'E ha&e let +ad co&erage
Figure : Typical use o! t"e di#erent $1%$2 t"res"olds 7here3 Close enough to ser&ing cell antenna indication with di,erent thresholds are used by e<+ to /ndicate that it is possible or a 'E to participate in load balancing (see +2-@) between co-located cells% /ndicate that it is possible to use co-located cells or carrier aggregation% /ndicate that there is no need in general or +! usage or other /C/C schemes% !tart using blind hando&er or hando&er between co-located cells% •

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!tart"!top inter-reuency cell search is used by e<+ or3 !earching or /ntra LTE reuencies"cells and"or /0T reuencies"cells that ha&e more optimal propagation conditions or the connection% !earching or /ntra LTE reuencies"cells and"or /0T reuencies"cells that it is possible or a 'E to participate in load balancing with% •

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+ad 4L co&erage indication is used by e<+ or3 !earching or /ntra LTE reuencies"cells and"or /0T reuencies"cells that is li.ely to sa&e the connection rom dropping% +4 'L co&erage indication is e&aluated by e<+ by estimating long term 'L 9/<0 (path 9ain /ntererence and
Target e&aluations are done by using3

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! 4.1

@ (+2 or /0T) !er&ing cell worse than threshold 1 and neighbor cell better than threshold 2% 8
Intra-LTE Mobility Methods Interference mitigation There se&eral techniues to mitigate intererence% The techniues can be di&ided into these general categories% /ntererence reIection techniues in 'E and"or e<+ recei&er Coordinated short term use o radio resources or transmission and reception in time# reuency# power and direction"area Coordinated long term planning o radio resources e%g% reuency re-use planning
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$obility perormance will beneft rom any techniues that reduce intererence when the 'E is in a hando&er situation either by impro&ing the connection to source and"or target cell but also impro&ing the ability and perormance to measure on neighboring cells accurate and with low delays% The impro&ed ability can be utili*ed to epand cell co&erage areas to impro&e the ability or an /ntra reuency low power cell to oKoad a macro cell% +esides reducing the hando&er ailure rate the techniues are also epected to reduce the hando&er reuency needed to .eep connections optimal rom a radio resource perspecti&e% dditional inormation describing the /ntererence mitigation solution or the cochannel deployment o the LTE layer !mall cell"LTE $acro and LTE !mall cell"!mall cell is pro&ided# please refer to our !ite paper B2-3 Interference Mitigation Solution"

4.2

PCI planning and time synchronied cells The prime purpose o PC/ planning in relation to mobility is to assure that each 'E can identiy a&ailable cells uniuely in an area and select or report the ound PC/s and associated measurements to the e<+ or urther hando&er e&aluation% 89PP ha&e also chosen to re-use PC/ &alues or coordinated control o C0! reuency shit% The intention is that it should be possible to a&oid o&erlapping C0! 0E in time and reuency between adIacent cells or a constellation o 8 intra reuency cells using 2 antenna ports per cell% The typical usage is to plan or shited C0! transmitted on the same reuency rom the same e<+ site% / the system is low loaded and small cell si*es are used it ha&e been obser&ed in feld tests that it may be better rom a pea. rate perspecti&e to allow C0! rom adIacent cells rom the same site to hit each other all the time rather than hitting data parts o each other transmission%

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Cell border areas where hando&er is done in poor co&erage conditions will typically beneft rom shited C0! since shited C0! will impro&e perormance or channel estimation% !o i the scenario is dominated by many but wea. recei&ed cells# a shited C0! approach is epected to be benefcial%
4.3

Inter-frequency target measurements and e!aluation The e&ents defned in 89PP or inter reuency target e&aluation is essentially o 2 .ind% 0elati&e comparison# supported by e&ent 8 (
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;or mobility purposes both conditions are applicable%
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Figure &: ' (ltered )S)P *e"a+ior in a typical multi,carrier net-or. scenario 2 will be possible to confgure or 0!0P e&aluation and 0!0G e&aluation in parallel# both or search and bad co&erage reasons% s a complement 'L 9/<0 will also be possible to monitor and e&aluated by using a Ae<+ 2 e&entB% d+ a /nterreuency hando&er could still be moti&ated e&en i @ condition Aser&ing cell worse than threshold 1B is not ulflled%

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This means that it is not mobility that triggers hando&er when the connection is good enoughO it is rather other unctions li.e load balancing% This also imply that it is possible to a&oid doing intra reuency hando&er ollowed immediately by a /nter reuency hando&er i load balancing is chosen by e<+ to be done on 'Es that ha&e been connected to the cell or a while%

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Summary This document presents a set o tools to balance capacity# load and ser&ice eperience or the end-user when it comes to intra-LTE mobility scenarios% lso# it is important to ta.e into account what networ. operators should be considering rom planning perspecti&e that maimi*es LTE eperience% The mobility scenarios must consider idle mode and connected mode beha&ior as well as the interaction o both under changing radio en&ironment and load conditions% lso# it is important to minimi*e radio resource usage# signaling load and setup delays"latency%

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LTE Cell Reselection.doc

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