LTE Cell Reselection

T-Mobile – Small Cell Requirements

Confidential Information: Do Not Disclose

T-Mobile – Intra LTE Mobility Solution

CONTENTS

1. INTRODUCTION...............................................................................................................................................2

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

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

3.1. IDLE MODE MOBILITY CONTROL.................................................................................................................33.2. INTRA-FREQUENCY CONNECTED MODE MOBILITY.....................................................................................43.3. INTER-FREQUENCY CONNECTED MODE MOBILITY......................................................................................6

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

4.1. INTERFERENCE MITIGATION.........................................................................................................................84.2. PCI PLANNING AND TIME SYNCHRONIZED CELLS........................................................................................84.3. INTER-FREQUENCY TARGET MEASUREMENTS AND EVALUATION.................................................................9

5. SUMMARY........................................................................................................................................................11

2012-03-26 Proprietary and Confidential 1(12)



1 Introduction This document describes intra-LTE mobility challenges and solutions with focus on LTE Small Cell/Small Cell, LTE Macro/Small Cell scenarios. The description covers both idle and connected mode mobility.

This document serves only as basis for discussion and hence no commercial commitments are intended by the information given in this document.

2 Mobility challenges and top level requirementsThe main challenge for mobility solutions are

To be easy to use and be robust against variations in UE capabilities, UE movements and propagation conditions.

Provide non-interrupted and consistent service’s to subscribers that experience varying radio conditions and at the same time minimize the radio resource usage and signaling load.

Be consistent with capacity/energy management functionality and support operator’s different strategy for traffic distribution, network migration and Inter-system/RAT interaction.

Control UE cell reselection in idle mode to minimize setup delays and be consistent with mobility and the intended traffic distribution and camping strategies.

Regulatory requirements also needs to be addressed with due consideration to the subscriber device capabilities.

The specific challenges for LTE Macro/Small Cell and LTE Small Cell/Small Cell are: Difficult to achieve a good compromise between number of handover’s and

retainability (Radio Bearer drop, HO failures) and integrity (Bitrates, Delay) Provide means to control mobility decisions to allow full utilization of LTE Small

cells but at the same time avoid using Small cells for UE’s that will stay in Small cell coverage only for a very short time.

Even if some specific network scenarios are used to describe the solutions in this document, one challenge is that there should be no inherent restriction on network scenario’s supported.

Some of the network scenario variations: Single carrier or multi carrier sites Intra-band or Inter-band carriers LTE carriers only or IRAT carriers Different LTE carrier BW Different LTE carriers in different sectors High power and low power cells using same or different LTE carrier High power and low power cells using same or different RAT Co-located or non co-located cells




Open access or closed access cells Indoor and outdoor cells

The solutions for idle mode and connected mode Intra frequency mobility, Inter frequency mobility and IRAT mobility, need to harmonize (no ping pong behavior, no dead lock behavior) in real time , regardless of scenario and interaction with other functions(e.g. Load balancing).

Multi-vendor support for mobility is covered in terms of signaling by 3GPP. There is also some support in X2 control signaling for handover tuning. However since mobility performance depends on harmonized solutions and observables for handover tuning, a multi-vendor scenario will add yet another dimension of challenges.

Multi-vendor harmonization for the basic coverage triggered handovers can typically be managed by agreements on parameters settings. The parameters used for tuning are typically common between vendors since 3GPP events and associated parameters in Idle and Connected mode are defined specifically for support of coverage triggered handover.

For more elaborate network associated functions where 3GPP have less detailed specific support, multi-vendor harmonization is more challenging and probably need focused efforts from involved vendors to reach full functionality and optimal performance. Examples of such areas are:

Intra LTE and IRAT Handover Optimization Automatic Neighbor Relation Detection Load and service triggered usage of handover

3 Mobility Solutions

3.1 Idle Mode Mobility ControlThe options for idle mode control are given by the configuration options in 3GPP TS 36.331 and UE behavior requirements in 3GPP TS 36.304. The figure below illustrates the typical use of the idle mode parameters.




F1: Prio 0 Prio 0

F3: Prio 1

F4: Prio 2

Srxlevserving < 0

Or

Squalserving < 0

F2: Prio 0

Srxlevserving < ThreshServing, LowP

And

Srxlevneighbour > ThreshX, LowP

Alternative if RSRQ used

Squalserving < ThreshServing, LowQ

And

Squalneighbour > ThreshX, LowQ

TreselectionRATx used

Srxlevneighbour > ThreshX, HighP

Alternative if RSRQ used

Srxlevneighbour > ThreshX, HighP

Or

Squalneighbour > ThreshX, HighQ


RN > RS

where

RS = Qmeas,s +Qhyst

RN = Qmeas,n – Qoffset


Figure 1: Typical Idle Mode Parameters

One important aspect for idle mode/connected mode interaction is that the UE should preferably stay in the same cell when doing frequent transitions between idle and connected mode. This can be achieved by using the following concept.

Idle mode Broadcast a higher cellReselectionPriority for the serving frequency/cell in SIB 3 than

what the neighbors broadcast for the same frequency/cell in SIB 5.“Once the cell is selected by the UE the UE will stay on the cell according to priority since the priority of the chosen frequency/cell “went up” after cell reselection”

Connected mode Operator parameters for neighbour frequency/RAT priority should typically be set

equal to the cellReselectionPriority for the neighbor frequency/RAT used for idle mode.

“Connected mode will in this way inherit the “high priority” for the frequency/cell UE from Idle mode”

Furthermore, handover for coverage reasons to other frequency/RAT is only done when the decision to leave the current frequency/cell has been made and thus serving cell frequency priority is of less concern for mobility.

It is possible to avoid frequent Tracking area update’s from UE’s by utilizing the TA list concept from 3GPP that allow hysteresis by using overlapping Tracking Areas. In a Macro/Small cell scenario the Small cells should preferably belong to the same tracking area list as the coverage overlapping macro cells as long as the paging capacity is sufficient.




3.2 Intra-frequency Connected Mode MobilityFor Intra frequency mobility in a frequency re-use 1 scenario it is very essential to keep UE connected to the best cell at all times. This is achieved by using Intra frequency A3 evaluation to trigger Intra frequency handover any time needed. If not connected to the best cell the UE will very soon suffer from high Inter cell interference levels that eventually will cause high PRB usage, peak rate degradations and eventually UE drop’s due to radio Link failures, signaling failures or handover failures. The UE ability to detect and measure on neighboring Intra frequency cells will be impaired by high interference levels and to some extent also the time relation between serving cell and measured neighbor. Ue’s using DRX cycles > 40 ms will also degrade the ability to detect and measure on neighboring cells.

Keeping UE connected to best cell at all times imply high handover frequencies and higher risks for oscillating handovers. It also implies that handover preparation times should preferably be kept low since the UE is already in a “non best cell” when handover preparation is initiated.

At high load situations where admission denials start to occurred, eNB will have a possibility to attempt handover to “second best” intra frequency cells fulfilling the event criteria. This allows the UE to be connected to a better cell even if admission was not granted to the best cell.

This also imply that there is much less freedom to use e.g. cell individual offset other than rather small offsets for the purpose of tuning the ability to keep the UE connected to the best cell, balance UL and DL or to get an earlier indication that handover is needed.

Below is figure illustrating simulation results showing the typical compromises when using different handover offset (A3 offset) and TTT in a HetNet scenario, here the UE is moving 3km/h. The simulations are based on models and parameter settings defined in 3GPP TR 36.814 v9.0.0 where configuration 4b is specifically defined for Hotspot capacity enhancements. The absolute performance in the results is very UE and eNB model and NW scenario dependent and may or may not represent absolute performance in field. The results can though be used for understanding the relative effects from changing parameters and the typical relation between failure causes.




1 2 3 4 5 6 7 8 90

10

20

30

40

50Number of Handovers per UE and Hour

macro-macromacro-picopico-macropico-pico

1 2 3 4 5 6 7 8 90

2

4

6

8

10

12

14pingPong Rate

1 2 3 4 5 6 7 8 90

0.1

0.2

0.3

0.4

0.5

0.6

0.7Number of Failures per UE and Hour

measurementReporthandoverCommandrandomAccesshandoverConfirmRLM

1 2 3 4 5 6 7 8 90

0.1

0.2

0.3

0.4

0.5

0.6

0.7Failure Scenarios

macro RLMmacro-macromacro-picopico RLMpico-macropico-pico

Scenario 1: HetNet Scenario, configuration 4b

48039

16038

4037

48026

16025

4024

48013

16012

4011

TTT (ms)Handover offset

Case

Figure 2: HetNet Scenario

Note that for this scenario the dominant reasons for failures where: Radio Link monitoring (RLM) failures which are based on the estimated ability for the

UE to detect PDCCH. Measurement report signaling failures which are based on maximum number of RLC

retransmission used by the UE to reach eNB acting as source.Other reasons modeled are: “Handover command” (RRC connection reconfiguration with mobilitycontrolinfo)

signaling failures which are based on maximum number of RLC retransmission used by the eNB acting as source cell.

Random access failures which are based on RACH quality in the eNB acting as target and allowed RA re-attempts used by the UE.

“Handover confirm” (RRC connection reconfiguration complete) signaling failures which are based on based on maximum number of RLC retransmission used by the UE to reach eNB acting as target.

The manual tuning efforts to get optimal handover performance for each scenario will be reduced by SON features like e.g. UE or Cell level oscillating handover minimization, which detects that individual UE’s or a specific cell relation have an oscillating tendency and increase the handover offset temporarily for that UE/cell relation.

Mobility Robustness Optimization functionality with signaling support defined by 3GPP is also studied and may be incorporated in the future.

Handover optimizations on UE or Cell level typically allows the initial default handover offset settings to be set lower and still keep the Number of handover per UE and hour to




be the same as before activation of the feature. Lower handover offset settings makes UE’s to be connected to the best cell a larger portion of time at cell edge areas.

Possible other enhancements for optimal handover performance under study are to include UE speed estimate’s and average cell size estimates. UE speed estimates are assumed to be possible to get from UL Doppler shift estimates taken from eNB.

To support Small cell range expansion the use of UE performance enhancements (IRC), frequency domain ICIC and time domain ICIC (e.g. ABS) schemes are under investigation.

Also schemes where eNB use coordinated cells that are using the same PCI on the same frequency are investigated. In this case eNB need to “take over from the UE” and provide most of mobility support by itself with no event A3 triggered UE PCI measurement support in connected mode. This implies that eNB need to decide which antenna’s points and which resources to use for UL and DL. One challenge is that eNB does not have UL transmission to measure on from each UE at all times unless eNB periodically request Channel Quality Indicators and Power Headroom Reports from the UE. This eNB ability will typically also be developed for UL comp and DL comp schemes.

3.3 Inter-frequency Connected Mode MobilityInter-frequency network scenarios with source and target cell on different carriers are typically less challenging in terms of delay impact since there is no inter-cell interference from target cell that increase when UE moves towards the target. On the other hand the UE measurements on inter frequency target cells are more delayed than intra-frequency measurements and typically requires the use of measurement gap’s.

The use of measurement gaps may reduce DL and UL peak rates in low loaded scenarios. The gap causes a 10 ms interruption (4 ms to avoid HARQ in gap and 6 ms gap) once every 40 ms or 80 ms which will give 25% and 12.5 % reduction in theoretical maximum peak rates respectively.

To reduce the usage of gap’s eNB will use A2 and A1 events to control when UE is configured to use gap’s and search for inter frequency cells. Note that for high speed UE’s, eNB may not initiate inter frequency cell search at all to avoid too short time in cell before handover is needed again. This will happen when UE speed is too high in relation target cell size. If UE speed estimates are not available, periodical search for valid targets can be used instead. By adjusting search periodicity and required number of consecutive target hits before Inter frequency handover is triggered the probability to connect high speed UE’s to small target cells will be reduced.

eNB use the A2/A1 event to get the following generalized information (Note that there could be several instances of the same event configured in the UE but with different thresholds):

A1 -> UE is “Close enough to antenna”A2 -> UE is not “Close enough to antenna”

A2 -> Start inter frequency cell searchA1 -> Stop inter frequency cells search




A2 -> UE have entered Bad coverageA1 -> UE have left Bad coverage

Next figure illustrates a typical use of the different A1/A2 thresholds:

F1

F3

A1 close enough to serving cell antenna for blind HO

F2

F3 F3

A1 UE close enough to serving cell antenna for load balancing

A2 Inter Frequency cell search

A2 Bad coverage

A2 Inter Frequency cell search

F1

Figure 3: Typical use of the different A1/A2 thresholds

Where:Close enough to serving cell antenna indication with different thresholds are used by eNB to Indicate that it is possible for a UE to participate in load balancing (see B2-5)

between co-located cells. Indicate that it is possible to use co-located cells for carrier aggregation. Indicate that there is no need in general for ABS usage or other ICIC schemes. Start using blind handover for handover between co-located cells.

Start/Stop inter-frequency cell search is used by eNB for: Searching for Intra LTE frequencies/cells and/or IRAT frequencies/cells that have more

optimal propagation conditions for the connection. Searching for Intra LTE frequencies/cells and/or IRAT frequencies/cells that it is

possible for a UE to participate in load balancing with.

Bad DL coverage indication is used by eNB for: Searching for Intra LTE frequencies/cells and/or IRAT frequencies/cells that is likely to

save the connection from dropping.BAD UL coverage indication is evaluated by eNB by estimating long term UL GINR (path Gain Interference and Noise Ratio). Short term UL GINR is also used by link adaptation.

Target evaluations are done by using:




A5 (B2 for IRAT) Serving cell worse than threshold 1 and neighbor cell better than threshold 2.

A3 Neighbor cell is offset better than serving cell.

4 Intra-LTE Mobility Methods

4.1 Interference mitigationThere several techniques to mitigate interference. The techniques can be divided into these general categories. Interference rejection techniques in UE and/or eNB receiver Coordinated short term use of radio resources for transmission and reception in

time, frequency, power and direction/area Coordinated long term planning of radio resources e.g. frequency re-use planningNote that there is no clear distinction between the categories and features may utilize combinations from each.

Mobility performance will benefit from any techniques that reduce interference when the UE is in a handover situation either by improving the connection to source and/or target cell but also improving the ability and performance to measure on neighboring cells accurate and with low delays.The improved ability can be utilized to expand cell coverage areas to improve the ability for an Intra frequency low power cell to offload a macro cell.

Besides reducing the handover failure rate the techniques are also expected to reduce the handover frequency needed to keep connections optimal from a radio resource perspective.

Additional information describing the Interference mitigation solution for the co-channel deployment of the LTE layer Small cell/LTE Macro and LTE Small cell/Small cell is provided, please refer to our white paper B2-3 Interference Mitigation Solution.

4.2 PCI planning and time synchronized cellsThe prime purpose of PCI planning in relation to mobility is to assure that each UE can identify available cells uniquely in an area and select or report the found PCI’s and associated measurements to the eNB for further handover evaluation.3GPP have also chosen to re-use PCI value’s for coordinated control of CRS frequency shift. The intention is that it should be possible to avoid overlapping CRS RE in time and frequency between adjacent cells for a constellation of 3 intra frequency cells using 2 antenna ports per cell. The typical usage is to plan for shifted CRS transmitted on the same frequency from the same eNB site.

If the system is low loaded and small cell sizes are used it have been observed in field tests that it may be better from a peak rate perspective to allow CRS from adjacent cells from the same site to hit each other all the time rather than hitting data parts of each other transmission.

Cell border areas where handover is done in poor coverage conditions will typically benefit from shifted CRS since shifted CRS will improve performance for channel




estimation. So if the scenario is dominated by many but weak received cells, a shifted CRS approach is expected to be beneficial.

Note that having time synchronized inter eNB cells (but maybe not time aligned) provides full control of which parts of a cell transmission that hits adjacent cells transmission and vice versa and makes it possible to use non shifted CRS within the eNB and shifted CRS between eNB’s.

Cell detection performance and measurement accuracy for high interference scenarios is currently under investigation in 3GPP RAN 4. The work has been initiated for HetNet scenarios where cell range expansion is assumed to be used. In general current 3GPP R8/R9 UE requirements allow PCI’s to be detected by UE’s down to SINR of aprox. -6 dB. Also for cell detection it matters if PCH/SCH overlaps in time and frequency and the PCI combination that collides. More field evaluation is needed to understand when these properties are significant or not for mobility performance. The first results indicate that current UE’s overestimate RSRP at high interference levels. RSRP overestimation implies that cell individual offset should be used to fine tune the handover borders in high interference scenarios.

4.3 Inter-frequency target measurements and evaluationThe events defined in 3GPP for inter frequency target evaluation is essentially of 2 kind. Relative comparison, supported by event A3 (Neighbor becomes offset better

than serving) Absolute conditions, supported by event A5 (Serving becomes worse than

threshold 1 and neighbor becomes better than threshold)

For mobility purposes both conditions are applicable. Note however for co-located inter frequency cells there is a very high risk that a unnecessary high amount of inter frequency handovers are triggered if not a complemented with thresholds A2/A1 that prohibit inter frequency cell search and thus Inter frequency handovers.

The figure below illustrates the L3 filtered RSRP behavior in a typical multi carrier NW scenario.As can be seen in the magnified part the usage of A3 and A5 evaluation between co-located carriers all the time is not possible if handover frequency shall be kept low even if measurement gap usage is considered to be of no issue.

Inter frequency measurements and A3/A5 evaluation need to be motivated by: Bad serving frequency/cell coverage Significant long-term gains in radio resource usage Load balancing need when blind handover are not possible to use. For more

information regarding load balancing please refer to our white paper B2-5 LTE Load Balancing Solution.

Observables for NW tuning purposes




F1

F3

F2

A3 will be fulfilled very frequently even if large A3 offsets and long TTT are used while handover is really not motivated from a mobility perspective (Optimize Long term radio resource usage and avoid drops).

However Inter frequency handover to achieve load balancing between F1 and F2 and F3 can still be very motivated.

A2 triggered Inter Frequency cell search on F1 and F2 A2 Bad

coverage

A2 triggered Inter Frequency cell search on F3

A3 Intra frequency HO

Figure 4: L3 filtered RSRP behavior in a typical multi-carrier network scenario

A2 will be possible to configure for RSRP evaluation and RSRQ evaluation in parallel, both for search and bad coverage reasons. As a complement UL GINR will also be possible to monitor and evaluated by using a “eNB A2 event”.

Note that serving cell RSRQ values can fluctuate between -3 dB to -11 dB without any inter cell interference present due to the 3GPP definition of RSRQ. A Voip user will typically experience RSRQ= -3dB close to the antenna whereas full buffer user will experience RSRQ= -11 dB at the same position. This means that serving cell RSRQ thresholds above -11 dB should be avoided since it depends more on serving cell transmission than on inter cell interference. A full buffer single user in a 2 carrier co-located scenario in a position close to the antenna may actually start to oscillate between the carriers when serving cell RSRQ thresholds for coverage triggered handover above -11 dB is used.

RSRQ values for inter frequency neighbors will have a similar property since all RSRQ values above -11 dB could be due to the UE’s being scheduled in the neighbouring cell.

Intra frequency A3 will be used at all times.

Inter frequency A3 and A5 will be used either at the same time or one at a time.Example: Interfrequency handover is typically triggered by A5 but as a complement if the difference between F1 and F2 is really large e.g. >10 dB a Interfrequency handover could still be motivated even if A5 condition “serving cell worse than threshold 1” is not fulfilled.




This means that it is not mobility that triggers handover when the connection is good enough; it is rather other functions like load balancing. This also imply that it is possible to avoid doing intra frequency handover followed immediately by a Inter frequency handover if load balancing is chosen by eNB to be done on UE’s that have been connected to the cell for a while.

5 Summary

This document presents a set of tools to balance capacity, load and service experience for the end-user when it comes to intra-LTE mobility scenarios. Also, it is important to take into account what network operators should be considering from planning perspective that maximizes LTE experience.

The mobility scenarios must consider idle mode and connected mode behavior as well as the interaction of both under changing radio environment and load conditions. Also, it is important to minimize radio resource usage, signaling load and setup delays/latency.


LTE Cell Reselection

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