Lecture 09 2011 Big

1 © Nokia Siemens Networks NSN DC Wrocław

For internal use

Lecture #9 Handovers and mobility

19th April 2011


Mobility – clear advantage

Freedom of making and receiving calls or browsing internet anywhere and

anytime is the biggest advantage of wireless cellular communication over fixed line

networks


How to provide mobility? Lecture Agenda

1. UMTS cell selection in idle mode

2. RRC states in UMTS

a) Paging

b) Cell / URA Update procedure

3. Handovers – general classification

a) Handover measurements

4. Soft Handovers

5. Softer Handovers

6. Hard Handovers

a) Compressed Mode

b) Iur Interface

7. LTE Handovers

a) RRC States

b) Mobility in RRC idle

c) Mobility in RRC connected

d) X2 interface

8. UMTS vs LTE mobility comparison


Cell selection in idle mode ( UMTS )

What happens if you switch on your mobile phone?

Cell selection procedure:

1. UE looks for Synchronization Channel.

2. Search for pilot signals looking for a suitable cell to camp on

3. Decode broadcast information from selected cell looking for information about

best neighbor cell

4. Reselect cell based on information about:

• cell belonging to HPLMN ( Home Public Land Mobile Network ) supporting desired

communication technology e.g. HSPA or

• cells belonging to HPLMN supporting other technologies UE is capable of e.g.

WCDMA, EDGE, GPRS, GSM

• cells belonging to PPLMN ( Prefered PLMN )

• cell belonging to other operators with best pilot signal

• HCS priorities

5. Since there is no signaling connection yet, UE tries to establish one via selected

cell using RRC Connection Request message ( UE registration at the network ).


HCS HSPA Freq 1 Freq 1macro

cells

pico

cells

Hierarchical Cell Structure (HCS)

• Hierarchical Cell Structure (HCS) is an optional 3G feature to prioritize layers in cell reselection process

• HCS enables:

– prioritizing layers based on RAT (3G, 2G), frequency, nominal Tx power...

– setting individual priorities for each layer (8 different priorities available)

– controlling transitions between layers by means of measurements rules

and thresholds settings

HCS_PRIO range: 0 – lowest priority … 7 – highest priority

HCS 2G HSPA . . .Freq 1 Freq 2Freq 1


Criterion H

to identify target cell on

different layers

Criterion R

target cell ranking based

on quality

S criterion fulfilled

start looking for other

cells


#2 HCS_PRIO=1

#1, HCS_PRIO=2

#10, HCS_PRIO=4

#3, HCS_PRIO=4

New serving

cell

Current

serving cell#10, HCS_PRIO=4

#2 HCS_PRIO=1

#3, HCS_PRIO=4

#1, HCS_PRIO=2

#7, HCS_PRIO=3#5, HCS_PRIO=3

Cells fullfilling the H>=0 criterion

#1, HCS_PRIO=2

#3, HCS_PRIO=4

#10, HCS_PRIO=4

#2 HCS_PRIO=1

Cells not fullfilling the H-criterion


High Mobility State OFF

Considered priorities:

• • •0 1 2 3 4 7


Criterion H

to identify target cell on

different layers

Criterion R

target cell ranking based

on quality

S criterion fulfilled

start looking for other

cells


#2 HCS_PRIO=1

#10, HCS_PRIO=4

#3, HCS_PRIO=4

#1, HCS_PRIO=2New serving

cell

Current

serving cell#10, HCS_PRIO=4

#2 HCS_PRIO=1

#3, HCS_PRIO=4

#1, HCS_PRIO=2


Cells fullfilling the H>=0 criterion

#1, HCS_PRIO=2

#3, HCS_PRIO=4

#10, HCS_PRIO=4

#2 HCS_PRIO=1

Cells not fullfilling the H-criterion


High Mobility State ON

Considered priorities:

• • •0 1 2 3 4 7


Radio Resource Control states ( UMTS )

• RRC protocol is responsible for mobility aspects in UMTS Terrestial Radio Access

Network ( UTRAN ) and LTE evolved UTRAN ( eUTRAN )

• RRC states are trade offs between User Equipment ( UE ) power consumption,

location information precision and instant available data rate.

• In each state mobility is provided by either paging, location update or handovers.

URA PCH

paging, dormant

mode

Cell PCH

paging,

dormant mode

Cell FACH

location update, common always

on low data rate channels

CELL DCH

set up „on demand” dedicated

channels, handovers

RRC states in Connected Mode

Idle Mode


RRC States – comparison ( UMTS )

Power consumption

Location information

Available data rates

URA/Cell PCH

< 5 mA URA / cell None

Cell FACH ~ 100 mA Cell Low

Cell DCH > 200 mA CellMedium /

High

State transitions examples:

• cell PCH -> cell URA - performed if UE traverse through several cells within

specific time

• cell PCH -> cell FACH – performed if low date rate transmission is done

between UE and NodeB e.g. cell update signalling

• cell FACH –> cell DCH – performed if higher data rate transmission is done

between UE and NodeB

• cell DCH -> cell PCH – performed after some inactivity time ( ~2-5s )


Paging ( UMTS )

• There are three situations when UE is paged:

1. In case of Mobile Terminating Call establishment procedure.

2. If the system information on Broadcast Channel ( BCH ) change.

3. When UE needs to perform Cell Update procedure.

• In cell / URA PCH UE utilize discontinuous reception ( DRX ) in order to provide

power savings. Transmitter is always off.

• When in PCH state UE switch on it’s receiver in so cold DRX cycles to check,

if there is Paging Indication ( PI ) on Paging Indication Channel ( PICH ).

If yes, UE reads information from Paging Channel ( PCH ).

Paging Channel

Paging Indication Channel

zzzzzz

zzzzzz

? PI

x

paging

message

7680 chipsDRX cycle


Cell / URA Update procedure ( UMTS )

• If UE is in URA / cell PCH it needs to move to cell FACH state in order to inform UTRAN about it’s position change.

• No Dedicated Channel ( DCH ) is assigned to UE. Common FACH/RACH channels are used instead.

• Based on periodic measurements UE performs cell reselection and upon selecting a new UTRAN cell, initiates a cell update procedure.

• Similar procedure apply for UtranRegistration Area ( URA ) Update.

Cell Update ( RACH )

When in URA / Cell PCH or FACH, UE has to monitor periodicaly CPICH level of

surrounding cells

Cell Update Confirm ( FACH )

NodeBUE RNC

If any monitored cell match criteria for cell reselection UE will update it’s position to

UTRAN

UE continue to monitor periodicaly available cells

UE position is updated


Handover ( Handoff ) types

If active UE due to its movement can be served in more efficient manner in another cell handover is performed.

Handover is transfer of running user connection from one radio channel to another.

There are following categories of handovers:

• Soft Handovers – UE has always at least one link to NodeB and links are added or removed between different NodeBs

• Softer Handovers - UE has always at least one link to NodeB and links are added/removed between sectors of same NodeB. There is no soft and softer hadover in HSDPA and LTE, they are common in WCDMA and HSUPA.

• Hard handovers – radio links between UE and NodeB are removed before adding new ones ( further division will be given later ).


How does the UE know when to perform a handover? ( UMTS )

NodeB X

UE - Handover

measurements

NodeB Y

Handover

decision - RNC• UE monitors Common Pilot Channel (CPICH)

level ( SINR, Eb/No, Es/No ) of neighbouring

cells.

• Results of those measurements are sent in so

called measurement events to Radio Network

Controller which is a handover decision unit.

• Surrounding monitored cells are categorized

into:

active set – list of cells where UE has

connection to UTRAN ( NodeBs X and Y )

monitored set – list of cells that UE

measure but their pilot level is to low to add

to active set ( NodeB Z )NodeB Z


Handover decision and measurements ( UMTS )

Time

Cell X

Cell Y

Cell Z

Pil

ot

Ec

/Io

∆t

∆t

∆t

1 23

1. Event 1A - radio link addition, adding radio link from Cell Y

Pilot Ec/No > Best Pilot Ec/No – Reporting Range + Hysteresis event 1A for period ∆t

2. Event 1C - radio link replacement, replacing radio link from Cell X with radio link from Cell Z

Best New Pilot Ec/No > Worst Old Pilot Ec/No – Reporting Range + Hysteresis event 1C for ∆t

3. Event 1B - radio link removal, remove radio link from Cell Z

Pilot Ec/Io < Best Pilot Ec/Io – Reporting Range – Hysteresis event 1B for a period of ∆t

Reporting range – threshold for certain event

Hysteresis event and ∆t preventing ping pong effect.

add cell Y replace cell

X with cell Z

remove cell Z


Soft Handover ( UMTS )

• Soft and softer handovers were not present in GSM. They are also not used in UMTS TDD, and HSDPA.

• Soft and Softer Handovers are supported in HSUPA and LTE.

• The idea behind soft handover is to benefit from overlapping cell areas covered by different NodeBs

• Soft handovers occur in about 20 – 40% of connections.

NodeB

NodeB

NodeB

overlapping areas

RNCIub

Iub

UE


Soft Handover ( UMTS )

• In case of soft handover

between two NodeBs

connected to two different

RNCs signaling between

RNCs on Iur interface is

needed.

• Iur will be further explained

in Hard Handovers.

NodeB

NodeB

NodeB

overlapping areas

RNCIur

IubIub

RNC

UE


Softer Handover ( UMTS )

• Softer Handover occurs in about 5% -15% connections.

• Procedure is similar for softer and soft handover.

• Soft and softer handovers can occur together.

NodeB

UE

Radio link addition

Measurement Report 1a

Active Set Update

Active Set Update Complete

Measurement Report 1b

Active Set Update

Active Set Update Complete

RNC

Radio link deletion

Iub


Soft Handover vs. Softer Handover ( UMTS )

Softer Handover

• Occur between sectors of the

same NodeB

• In uplink received signal is

combined in NodeB like different

multipath signals using macro

diversity soft combining

• One power control loop

• No extra transmission on Iub

interface

Soft Handover

• Occur between different NodeBs

• In uplink received frames are

routed to RNC where based on

CRC check best candidate is

selected after each interleaving

period e.g 10, 20, 40 ms ( selection

combining )

• In fast power control loop if one

NodeB sends „power down”

command UE decreases it’s

transmission power

• Consumes more resources


Soft and Softer Handover gains and pains ( UMTS )

• Soft(er) handover can lead to reduction of interference and radio link performance

improvement due to macro diversity principle ( one of two radio links is always better

than another one )

• Soft and softer handovers prevent near far effect in uplink

• They also improve cell coverage

• On the other hand we need more resources like channelization codes (

transmission links), Iub capacity, additional rake fingers to cover handover demands

etc.

• In case of bigger path loss difference ( > 5 dB ) between radio links involved in soft

and softer handovers there are losses due to signalling errors in uplink power control

transmitted in DL and increased interferences in DL.

Soft and soft handovers parameters needs to be

adjusted so gains were higher than pains


Handover classification

Intra – without changing the specified element, ex.:

Intra-frequency – same frequency

Intra-NodeB – same NodeB

Intra-RNC – same RNC

Inter – with changing the specified element, ex.:

Inter-system – different system

Inter-mode – different duplex modes

Inter-frequency – different frequencies


Hard Handoverbreak-before-make

Used when it is impossible for a UE to communicate on two links (pre-HO &

post-HO) simultaneously, ex.:

Inter-RAT – UMTS↔GSM

Inter-mode – FDD ↔ TDD

Inter-frequency – between different frequencies

within operator’s band

Features:

+ every handover can be performed as a hard handover

+ only one connection active at a time

– there is a short brake in communication

– complicated measurements (compressed mode)


Compressed Mode

f1

f2

?

Most UMTS terminals are equiped with a single radio receiver

tuned to a single frequency. How to perform measurements on different

frequency and/or RAT during continous transmission/reception?

• To deal with this gaps are introduced within normal trasmission of the serving frequency. During those gaps the measurement on the other frequency/RAT is done

during this time the measurement on different frequency/RAT can

be performed!

time

Power

10 ms 10 ms 10 ms

Normal frame

Compressed frame

Gap

The time devoted for the gap has be be compensated by compressing the information which could have been sent during that time.

Compressed mode methods:

• Lowering the data rate from higher levels (DL)

• Increasing data rate by lowering spreading factor and incresing transmission power (DL & UL)

• Reducing symbol rate by puncturing (DL, not implemented)


Intra-RNC Hard Handover

1. Radio link setup req.

2. Radio link setup ack.

3. Channel reconfiguration req.

4. Channel reconfiguration completed

5. Release radio resources req.

6. Release radio resources ack.

RNC

NodeB2NodeB1

UE

1

2

5

6

34


SRNC

Inter-RNC Handover

1. UE connected to SRNC through NodeB1

2. In soft handover

3. Right after handover

4. SRNC realocation

MSC

NodeB1

UE

NodeB2

SRNC RNC

Iu

Iur

Iub Iub

Iu

DRNCRNC


Iur Interface (RNC-to-RNC)

Functions of Iur interface:

• Support of basic Inter-RNC mobility

• Support of dedicated channel traffic

• Support of common channel traffic

• Support of global resource menagement

Core Network

Iub Iur Iub

Iu Iu

RNC RNCNodeBNodeB


Inter-RAT Handover

Reasons for Inter-RAT handovers:

• Lack of coverage

• load balancing

• QoS

Measurements:

• RNC/BSC triggered

• compressed modewhen in UMTS

Play Online: http://internet.playmobile.pl/maps/


Inter-RAT HO from UMTS to GSM

1. Reallocation(?)

required

2. Radio link setup req

3. Radio link setup ack.

4. Realocation command

5. Handover command

6. Access request

7. Handover complete

8. Release resources

MSC

UTRAN

UE

GSM BSS

1 2

34

56

78


LTE Mobility procedures

Mobilty procedures divided into idle state and connected state

RRC IDLE

RRC CONNECTED

•Cell reselections done autonomously by UE

•Based on UE measurements

•Controlled by broadcasted parameters

•Different priorities can be assigned to

frequency layers

•Network controlled

handovers

•Based on UE

measurements


Mapping of inter-RAT RRC states

Handover

CELL_PCH

URA_PCH

CELL_DCH

UTRA_Idle

E-UTRA

RRC_CONNECTED

E-UTRA

RRC_IDLE

GSM_Idle/GPRS

Packet_Idle

GPRS Packet

transfer mode

GSM_Connected

Handover

Reselection Reselection

Reselection

Connection

establishment/release

Connection


Connection


CCO,

Reselection

CCO with

optional

NACC

CELL_FACH

CCO, Reselection

• It’s not possible to move from CELL_FACH to LTE

• CELL_PCH and URA_PCH states match to RRC Idle in LTE


Mobility in RRC IDLE state

What happens when you turn on your LTE phone?

• (Initial) cell selection:

– UE scans all radio frequency(RF) channels in E-UTRAN bands to find a

suitable cell

▪ Cell ranking according to radio conditions

▪ The strongest cell is selected

– Once selected the UE reads broadcast channel (BCH) of that cell to find

out whether the cell is suitable for camping:

▪ Good radio quality

▪ Cell not barred / allowed PLMN / etc

– After cell selection the UE must register itself to the network (TAC update)

• Whenever UE has camped on a cell it will continue to look for better

cells according to reselection criteria received in the BCH channel

• The UE location in RRC idle is known by the MME with the accuracy

of a Tracking Area (corresponding to Routing Area in UMTS)


Absolute Priorities (AP)

• A mean to prioritize cells for re-selection

– Priorities were introduced to reduce the number of tracked cells, i.e. improve cell reselection speed and mobile batery life

• Applicable to Release 8 UEs and beyond

• Can be cell or UE specific

• Priorities are defined per layer

– Inter-frequency layer

– Inter-RAT layer

• Priority level: {0,1,…,7}, where

– 0 – low and

– 7 – high


Go for higher priority

cell N if SqualN > ThreshXhigh2

/

Thresholds for AP based reselection

Camped on (S)

NOT camped on (N)

time

ThreshServingLow2

ThreshXhigh2

ThreshXlow2

Squal

HSPA CELL #1

LTE CELL #2

LTE CELL #1

LTE: Higher

priority =2

HSPA: Lower

priority =1

Go for lower priority

cell N if SqualN > ThreshXlow2 AND

Squals <

ThreshServingLow2

I22

I22

I22


Mobility in RRC IDLE stateTracking Area optimization

• A Tracking Area(TA) is a group of

several eNBs. Each of those groups

has a specific number i.e. Tracking

Area Code

• The UE location is known by the MME

with the accuracy of a TA

• A large TA means less Tracking Area

Updates (TAUs), a small TA reduces

the paging signalling load for incoming

packet calls

• The size of TA can be optimized in the

network planning phase

• The corresponding concept in UMTS is

called Routing Area

Tracking Area 1 Tracking Area 2

MME

On moving from one

TA to another in

RRC IDLE mode the

UE has to perform a

Tracking Area

update (TAU)


Mobility in RRC CONNECTED stateLTE Handovers

• Network-controlled– Target cell is selected by the serving eNodeB

– UE always connected to only one eNodeB - no soft-handover

• UE-assisted– Measurements are made and reported by UE to eNodeB

• Lossless– Packets are forwarded from the source eNodeB to the taget

eNodeB (X2 interface)

• Late path switch– Only once the handover is successfull, the packet core is invloved


Late path switching – general idea

• User connected to eNB1 moving to eNB2

eNB1 eNB2

S-GW

DL

UL

DL UL



• eNB1 is forwarding DL packets to eNB2 over X2 interface!

eNB1 eNB2

S-GW

DL UL

DL

UL

DL

No packets are being lost!

X2 interface



• Late Path switch – the core network S1 connection is updated only once radio path is fully changed

eNB1 eNB2

S-GW

DL UL

DL UL

DL

X2 interface

DL

1

1

2


LTE Handovers - Preparation

1. Handover decision

2. Handover req.

(radio resources

reservation)

3. Handover Ack.

4. Handover command

5. Data forwarding

2

3

S-GWMME

4eNB1

UE

eNB2

Data link

Control link


79

LTE Handovers - Execution

6. Handover

7. Path switch request

8. Path switch execution

9. Path switch ack.

10.Release resources req.

11.Release resources

ack.

S-GW

eNB1eNB2

MME

1011

Data link

Control link

UE


X2 interface (eNB-to-eNB)

Functionalities of X2 interface:

• Data forwarding in Inter-eNodeB handover

• Information exchange between neighboring cells(ex. for load balancing)

• X2 is a logical interface i.e. direct connection isnot required, can be routed via core network

eNB

eNB

X2

eNB

X2

X2


Differences in UTRAN and E-UTRAN Mobility

UTRAN E-UTRAN

• Location Area (CS) /Routing Area (PS)

• Soft handover in uplink and downlink, also for HSUPA uplink fbdf

• CELL_FACH, CELL_PCH, URA_PCH

• Mobility mostly hidden by RNC in UTRAN

• Only Tracking Area (only PS connections)

• No soft handovers, UE always connected to only one eNB ( also for HSDPA DL )

• Only Connected state gggggggggggg

• Core network sees every handover

Tracking Area 2

RRC IDLE

RRC CONNECTED

MME

eNB1


Questions?

Thank you!References

[1] 3GPP TS 25.331v9.2.0, Radio Resource Control (RRC);

[2] 3GPP TS 25.304v9.0.0, User Equipment (UE) procedures in idle mode and procedures for cell reselection

in connected mode

[3] 3GPP TS 36.331v9.1.0, Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control

(RRC);

[4] 3GPP TS 36.304v9.0.0, User Equipment (UE) procedures in idle mode

[5] Harri Holma and Antii Toskala, LTE for UMTS – OFDMA and SC-FDMA Based Radio Access

[6] Harri Holma, Antii Toskala, WCDMA for UMTS – HSPA evolution and LTE, forth edition

[7] Jerzy Kołakowski, Jacek Cichocki, UMTS – System telefonii komórkowej trzeciej generacji, wydanie 2,


AbbreviationsBCH – Broadcast Channel

BSS – Base Station System

CPICH – Common Pilot Channel

DCH – Dedicated Channel

DL – Downlink

DRNC – Drifting RNC

DRX – Discontinous Reception

FACH – Forward Access Channel

FDD – Frequency Division Duplex

GW - Gateway

HARQ – Hybrid Automatic Repeat Request

HO – Handover

HSDPA - High Speed Downlink Packet Access

HSUPA – High Speed Uplink Packet Access

MME – Mobility Management Entity

PCH – Paging Channel

PDU – Protocol Data Units

PI – Paging Indicators

PICH – Paging Indication Channel

QoS – Quality of Service

RACH – Random Access Channel

RLC – Radio Link Control

RNC – Radio Network Controler

RRC – Radio Resource Control

SAE – System Architecture Evolution

SRNC – Serving RNC

TDD – Time Division Duplex

UE – User Equipment

UL - Uplink

URA – UTRAN Registration Area

UTRAN – UMTS Terrestrial Radio Access Network

Lecture 09 2011 Big

Documents

cell pch cell ura

cell reselection process

selected cell

suitable cell

nodeb cell dch cell

prio range

umts cell selection

cell ura pch ue