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1 © Nokia Siemens Networks NSN DC Wrocław
For internal use
Lecture #9 Handovers and mobility
19th April 2011
2 © Nokia Siemens Networks NSN DC Wrocław
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
3 © Nokia Siemens Networks NSN DC Wrocław
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
4 © Nokia Siemens Networks NSN DC Wrocław
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 ).
5 © Nokia Siemens Networks NSN DC Wrocław
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
6 © Nokia Siemens Networks NSN DC Wrocław
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
Hierarchical Cell Structure (HCS)
#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
#7, HCS_PRIO=3#5, HCS_PRIO=3
High Mobility State OFF
Considered priorities:
• • •0 1 2 3 4 7
7 © Nokia Siemens Networks NSN DC Wrocław
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
Hierarchical Cell Structure (HCS)
#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
#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
#7, HCS_PRIO=3#5, HCS_PRIO=3
High Mobility State ON
Considered priorities:
• • •0 1 2 3 4 7
8 © Nokia Siemens Networks NSN DC Wrocław
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
9 © Nokia Siemens Networks NSN DC Wrocław
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 )
10 © Nokia Siemens Networks NSN DC Wrocław
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
11 © Nokia Siemens Networks NSN DC Wrocław
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
12 © Nokia Siemens Networks NSN DC Wrocław
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 ).
13 © Nokia Siemens Networks NSN DC Wrocław
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
14 © Nokia Siemens Networks NSN DC Wrocław
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
15 © Nokia Siemens Networks NSN DC Wrocław
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
16 © Nokia Siemens Networks NSN DC Wrocław
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
17 © Nokia Siemens Networks NSN DC Wrocław
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
18 © Nokia Siemens Networks NSN DC Wrocław
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
19 © Nokia Siemens Networks NSN DC Wrocław
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
20 © Nokia Siemens Networks NSN DC Wrocław
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
21 © Nokia Siemens Networks NSN DC Wrocław
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)
22 © Nokia Siemens Networks NSN DC Wrocław
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)
23 © Nokia Siemens Networks NSN DC Wrocław
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
24 © Nokia Siemens Networks NSN DC Wrocław
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
25 © Nokia Siemens Networks NSN DC Wrocław
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
26 © Nokia Siemens Networks NSN DC Wrocław
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/
27 © Nokia Siemens Networks NSN DC Wrocław
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
28 © Nokia Siemens Networks NSN DC Wrocław
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
29 © Nokia Siemens Networks NSN DC Wrocław
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
establishment/release
Connection
establishment/release
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
30 © Nokia Siemens Networks NSN DC Wrocław
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)
31 © Nokia Siemens Networks NSN DC Wrocław
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
32 © Nokia Siemens Networks NSN DC Wrocław
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
33 © Nokia Siemens Networks NSN DC Wrocław
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)
34 © Nokia Siemens Networks NSN DC Wrocław
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
35 © Nokia Siemens Networks NSN DC Wrocław
Late path switching – general idea
• User connected to eNB1 moving to eNB2
eNB1 eNB2
S-GW
DL
UL
DL UL
36 © Nokia Siemens Networks NSN DC Wrocław
Late path switching – general idea
• 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
37 © Nokia Siemens Networks NSN DC Wrocław
Late path switching – general idea
• 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
38 © Nokia Siemens Networks NSN DC Wrocław
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
39 © Nokia Siemens Networks NSN DC Wrocław
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
40 © Nokia Siemens Networks NSN DC Wrocław
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
41 © Nokia Siemens Networks NSN DC Wrocław
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
42 © Nokia Siemens Networks NSN DC Wrocław
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,
43 © Nokia Siemens Networks NSN DC Wrocław
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
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