Dr. Stefan Brück. Qualcomm Corporate R&D Center Germany. 3G/4G Mobile Communications Systems. Page 2. Chapter IX: M
3G/4G Mobile Communications Systems Dr. Stefan Brück Qualcomm Corporate R&D Center Germany
Chapter IX:
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Mobility Control
Slide 2
Mobility Control Handover Types Mobility Measurements in UMTS Mobility Procedures for HSDPA and HSUPA (E-DCH) Mobility Measurements in LTE X2/S1 based Mobility Procedures in LTE
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Slide 3
Cell (Re-)Selection and Handover Cell (Re-)Selection Procedure that allows the UE to change the cell it is camped on (E)-UTRAN provides parameters to control (re-)selection
Handover (Hand-off) Procedure that allows the UE to change from one cell to another, while the UE has radio resources allocated to it
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Slide 4
Types of Cell (Re-)Selection and Handover Intra-Frequency Within the same carrier frequency Inter-Frequency Between different carrier frequencies Inter-RAT Between different Radio Access technologies (RAT), e.g. from UMTS FDD to GSM Soft handover Multiple radio links exist to cells of different Node Bs Softer handover Multiple radio links exist to cells of the same Node B Hard handover Existing radio links are dropped before a new link is established
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Slide 5
Handover Control: Basics General: Mechanism of changing a cell or base station during a call or session UE may have active radio links to more than one Node B Mobile-assisted & network-based handover in UMTS: UE reports measurements to UTRAN if reporting criteria (which are set by the UTRAN) are met UTRAN then decides to dynamically add or delete radio links depending on the measurement results
Types of Handover: Soft/Softer Handover (dedicated channels) Hard Handover (shared channels) Inter Frequency (Hard) Handover Inter System Handover (e.g. UMTS-GSM) Cell selection/re-selection (inactive or idle)
All handover types require heavy support from the UMTS network infrastructure! 6
Slide 6
Soft/Softer Handover In soft/softer handover the UE maintains active radio links to more than one Node B Combination of the signals from multiple active radio links is necessary Soft Handover The mobile is connected to (at least) two cells belonging to different Node Bs In uplink, the signals are combined in the RNC, e.g. by means of selection combining using CRC
Softer Handover The mobile is connected to two sectors within one Node B More efficient combining in the uplink is possible like maximum ratio combining (MRC) in the Node B instead of RNC
Note: In uplink no additional signal is transmitted, while in downlink each new link causes interference to other users, therefore: Uplink: HO general increase performance Downlink: Trade-off
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Slide 7
Soft Handover – Example: UMTS
Multiple Node Bs are involved
Combining and Selecting • UE combines symbols received from each Node B • RNC selects the best radio frame from each Node B
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Slide 8
Softer Handover – Example: UMTS
Only one Node B, but multiple Cells are involved
Combining and Selecting UE combines symbols received from each cell Node B combines symbols received from each cell
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Slide 9
Hard Handover – Example: UMTS
Reasons for Hard Handover Inter-frequency handover Inter-RAT handover Shared transport channels
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Slide 10
Example: Soft Handover Control – UMTS
soft handover area NodeB 2
NodeB 1 UE
Measurement quantity, e.g. EC/I0 on CPICH Relative thresholds δadd & δdrop for adding & dropping Preservation time Tlink to avoid “ping-pong” effects Event triggered measurement reporting to decrease signalling load
Measurement Quantity CPICH 1
δdrop
δadd
Tlink
CPICH 2
Link to 1
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Link to 1 & 2
Link to 2
time
Slide 11
UMTS Soft/Softer Handover in Practice
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Slide 12
Soft Handover – Simulation Results 25%
Outage Probability (Blocking and Dropping)
20% 1 link
15%
max 2 SHO links max 4 SHO links
10%
max 6 SHO links
5%
0% 5
15
25
35
45
55
Offered Traffic [Erlang per site]
Soft handover significantly improves the performance, but … 13
Slide 13
Soft Handover – Simulation Results II
Mean Number of Active Links
2
1,5
1
0,5
0 1
2
4
6
Max. Active Set Size
… the overhead due to simultaneous connections becomes higher! 14
Slide 14
Inter-Frequency Handover in UMTS Hot-spot
Hierarchical cell structure (HCS)
Macro f1
Micro f2
Macro
Hot spot
f1
f2 f1
Handover f1 ⇔ f2 always needed between layers
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f1
f1
Handover f1 ⇔ f2 needed sometimes at hot spot
Hard handover
Inter-frequency measurements of target cell needed in both scenarios
Slide 15
Measurement Control and Reporting in UMTS Categories of Cells Active Set Cells for which a radio links is established between UE and UTRAN UE is in soft/softer handover with all cells in the active set
Monitored (Neighbor) Set UTRAN instructs UE to perform measurements on a list of cell in the geographic neighborhood All such cells that are not in the active set are in the monitored set Most likely candidates for soft/softer handover
Detected Set All other cells which UE has detected and measured The UE may report such cells to the UTRAN to be added to the monitored set
Reporting is either event-triggered or periodic Periodic reports generate a high load on the uplink Event triggered reporting is therefore usually preferred
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Slide 16
Parameters for Event Triggered Reporting (UMTS) Events as a function of the Measurement Types Intra-frequency:
Events 1a to 1f
Inter-frequency:
Events 2a to 2f
Inter-RAT:
Events 3a to 3d
Each event is associated with a set of parameters What cells can trigger the events? Absolute and relative threshold Time-to-Trigger
Time-to-Trigger Interval between event detection and report sent Time-to-Trigger interval ranges between 0 and 5 seconds
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Slide 17
Intra-Frequency Reporting Events (UMTS) Event 1a: A P-CPICH enters the reporting range Used to indicate to UTRAN when a new cell should be added to the active set
Event 1b: A P-CPICH leaves the reporting range Used to indicate to UTRAN when a new cell should be removed from the active set
Event 1c: A non-active P-CPICH becomes better than an active P-CPICH Used to indicate to UTRAN to replace a cell in the active set with a different cell (active set is full)
Event 1d: Change of best cell Used for changing cells in HSDPA
Event 1e: A P-CPICH becomes better than an absolute threshold Used to indicate to UTRAN when a new cell should be added to the active set
Event 1f: A P-CPICH becomes worse than an absolute threshold Used to indicate to UTRAN when a new cell should be removed from the active set
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Slide 18
Event 1a: A P-CPICH enters the Reporting Range
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Slide 19
Event 1c: Active Set is full
If Event 1c is received the UTRAN should replace the weakest active set with the new rising cell Event 1c may be configured such that the UE begins using periodic reporting if the UTRAN does not send active set update message 20
Slide 20
HSDPA Mobility Procedures I
HS-DSCH for a given UE belongs to only one of the radio links assigned to the UE (serving HS-DSCH cell) The UE uses soft handover for the uplink, the downlink DCCH and any simultaneous CS voice or data Using existing triggers and procedures for the active set update (events 1A, 1B, 1C) Hard handover for the HS-DSCH, i.e. Change of Serving HS-DSCH Cell within active set Using RRC procedures, which are triggered by event 1D
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Slide 21
HSDPA Mobility Procedures II
CRNC
CRNC
Source HSDSCH Node B
Target HSDSCH Node B
MAC-hs NodeB
MAC-hs NodeB
NodeB
s Serving HS-DSCH radio link
NodeB
t Serving HS-DSCH radio link
Inter-Node B serving HS-DSCH cell change Note: MAC-hs needs to be transferred to new Node B ! 22
Slide 22
HS-DSCH Serving Cell Change
Measurement quantity CPICH 1
Hysteresis
CPICH 2
CPICH3
Time to trigger
Reporting event 1D
Time
Event 1D: change of best cell within the active set Hysteresis and time to trigger to avoid ping-pong (HS-DSCH: 1…2 dB, 0.5 sec)
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Slide 23
HSDPA Handover Procedure Target HS-DSCH cell
UE
Source HS-DSCH cell
SRNC = DRNC Serving HS-DSCH cell change decision i.e. event 1D
RL Reconfiguration Prepare RL Reconfiguration Ready ALCAP Iub HS-DSCH Data Transport Bearer Setup
If new NodeB
RL Reconfiguration Prepare RL Reconfiguration Ready
Radio Bearer Reconfiguration
RL Reconfiguration Commit
RL Reconfiguration Commit
Synchronous Reconfiguration with Tactivation Reset MAChs entity
Radio Bearer Reconfiguration Complete
DATA ALCAP Iub HS-DSCH Data Transport Bearer Release
The RNC determines the activation time for the serving HS-DSCH cell change Time is populated by the RNC In the RRC message TRANSPORT CHANNEL RECONFIGURATION to the UE In the NBAP message RADIO LINK RECONFIGURATION COMMIT to the involved Node Bs
At this time the Node B commits and the UE activates a new transport channel configuration for HS-DSCH serving cell change
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After the transport channel configuration is completed the UE sends the RRC message RADIO BEARER RECONFIGURATION COMPLETE
Slide 24
E-DCH Operation in Soft Handover
scheduling grant HARQ ACK/ NACK
scheduling grant HARQ ACK/ NACK
UE NodeB 1
NodeB 2
Macro-diversity operation on multiple Node Bs Softer handover combining in the same Node B Soft handover combining in RNC (part of MAC-es)
Independent MAC-e processing in both Node Bs HARQ handling rule: if at least one Node B tells ACK, then ACK Scheduling rule: relative grants “DOWN” from any Node B have precedence
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Slide 25
EDCH Mobility Handling The UE uses soft handover for associated DCH as well as for E-DCH Using existing triggers and procedures for the active set update (events 1A, 1B, 1C) E-DCH active set is equal or smaller than DCH active set New event 1J: non-active E-DCH link becomes better than active one
The UE receives AG on E-AGCH from only one cell out of the E-DCH active set (serving E-DCH cell) E-DCH and HSDPA serving cell must be the same Hard Handover, i.e. change of serving E-DCH cell Using RRC procedures, which maybe triggered by event 1D Could be also combined with Active Set Update
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Slide 26
EDCH Mobility Procedures SRNC
SRNC
MAC-es
MAC-es
MAC-e
MAC-e
MAC-e
MAC-e
NodeB
NodeB
NodeB
NodeB
s Serving E-DCH radio link
t Serving E-DCH radio link
Inter-Node B serving E-DCH cell change within E-DCH active set Note: MAC-e still established in both Node Bs ! 27
Slide 27
Serving E-DCH Cell Change Target serving E-DCH cell
UE
Source serving E-DCH cell
SRNC = DRNC Serving E-DCH cell change decision i.e. event 1D
RL Reconfiguration Prepare RL Reconfiguration Ready RL Reconfiguration Prepare
If new NodeB
RL Reconfiguration Ready
Radio Bearer Reconfiguration
RL Reconfiguration Commit
RL Reconfiguration Commit
Synchronous Reconfiguration with Tactivation
Radio Bearer Reconfiguration Complete
UE receives now AG & dedicated RG from target cell
Handover of E-DCH scheduler control No changes in UL transport bearer No MAC-es RESET 28
Handover of HS-DSCH serving cell DL transport bearer setup MAC-hs RESET Slide 28
Mobility Measurement Reporting in LTE LTE mobility measurements are similar as in UMTS Event triggered Event triggered with periodic reporting Periodic reporting
Event
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Purpose
A1
Serving cell becomes better than an absolute threshold
A2
Serving cell becomes worse than an absolute threshold
A3
Neighbor E-UTRA cell becomes an offset better than the serving cell
A4
Neighbor E-UTRA cell becomes better than an absolute threshold
A5
Serving cell becomes worse than an absolute threshold AND neighbor E-UTRA cell becomes better than another absolute threshold
B1
Inter-RAT neighbor cell becomes better than an absolute threshold
B2
Serving cell becomes worse than an absolute threshold AND inter-RAT neighbor cell becomes better than an absolute threshold
Slide 29
Intra-LTE Handover Types LTE supports two types of handover signaling X2 based handover S1 based handover
From the air interface perspective Handovers are hard Procedure is identical for intra- and inter-frequency Random access is required for synchronization
From the core network perspective, handover type depends on the network topology Intra- or Inter-MME/S-GW handover
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Slide 30
LTE Handover LTE uses UE-assisted network controlled handover UE reports measurements; network decides when handover and to which cell Relies on UE to detect neighbor cells → no need to maintain and broadcast neighbor lists Allows "plug-and-play" capability; saves BCH resources
For search and measurement of inter-frequency neighboring cells only carrier frequency need to be indicated
X2 interface used for handover preparation and forwarding of user data Target eNB prepares handover by sending required information to UE transparently through source eNB as part of the Handover Request Acknowledge message New configuration information needed from system broadcast Accelerates handover as UE does not need to read BCH on target cell
Buffered and new data is transferred from source to target eNB until path switch → prevents data loss UE uses contention-free random access to accelerate handover
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Slide 31
LTE Handover: Preparation Phase Target Target eNB eNB
Source Source eNB eNB
UEUE
MME MME
sGW sGW
Measurement Control Packet Data
Packet Data L1/L2 signaling
UL allocation Measurement Reports
L3 signaling User data
HO decision HO Request Admission Control HO Request Ack DL allocation RRC Connection Reconfig.
SN Status Transfer
HO decision is made by source eNB based on UE measurement report Target eNB prepares HO by sending relevant info to UE through source eNB as part of HO request ACK command, so that UE does not need to read target cell BCH 32
Slide 32
LTE Handover: Execution Phase Target Target eNB eNB
Source Source eNB eNB
UEUE
MME MME
sGW sGW
Packet Data Detach from old cell, sync with new cell
Deliver buffered packets and forward new packets to target eNB
L1/L2 signaling
DL data forwarding via X2
L3 signaling
Buffer packets from source eNB
User data
Synchronisation UL allocation and Timing Advance RRC Connection Reconfig. Complete Packet Data
UL Packet Data
RACH is used here only so target eNB can estimate UE timing and provide timing advance for synchronization
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RACH timing agreements ensure UE does not need to read target cell P-BCH to obtain SFN (radio frame timing from SCH is sufficient to know PRACH locations)
Slide 33
LTE Handover: Completion Phase UEUE
Target Target eNB eNB
Source Source eNB eNB
MME MME
sGW sGW
DL Packet Data DL data forwarding Packet Data Path switch req User plane update req End Marker
Switch DL path Path switch req ACK
Release resources Flush DL buffer, continue delivering in-transit packets
L1/L2 signaling End Marker
L3 signaling User data
Release resources
Packet Data
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User plane update response
Packet Data
Slide 34
LTE Handover: Illustration of Interruption Period Source Source eNBeNB
UEUE
UEs stops Rx/Tx on the old cell UL
Measurement Report U- plane active
Handover Interruption (approx 35 ms)
Target Target eNB eNB
HO Request HO Confirm
Handover Preparation
HO Command
approx 20 ms
sync DL DL synchronisation + RACH (no contention) + + Timing Timingadvance Adv + + UL Resource Req and UL resource Grantrequest/grant
Handover Latency (approx 55 ms)
HO Complete ACK
U- plane active
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