3G/4G Mobile Communications Systems. Dr. Stefan Brück Qualcomm Corporate R&D Center Germany

Transcription

1 3G/4G Mobile Communications Systems Dr. Stefan Brück Qualcomm Corporate R&D Center Germany

2 Chapter IX: Mobility Control 2 Slide 2

3 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 3 Slide 3

4 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 4 Slide 4

5 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 5 Slide 5

6 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

7 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 7 Slide 7

8 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 8 Slide 8

9 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 9 Slide 9

10 Hard Handover Example: UMTS Reasons for Hard Handover Inter-frequency handover Inter-RAT handover Shared transport channels 10 Slide 10

11 Example: Soft Handover Control UMTS soft handover area NodeB 1 UE NodeB 2 Measurement Quantity CPICH 1 CPICH 2 δ add δ drop T link Measurement quantity, e.g. E C /I 0 on CPICH Relative thresholds δ add & δ drop for adding & dropping Preservation time T link to avoid ping-pong effects Event triggered measurement reporting to decrease signalling load Link to 1 Link to 1 & 2 Link to 2 time 11 Slide 11

12 UMTS Soft/Softer Handover in Practice 12 Slide 12

13 Soft Handover Simulation Results 25% ge Probability g and Dropping) Outag (Blocking 20% 15% 10% 5% 1 link max 2 SHO links max 4 SHO links max 6 SHO links 0% Offered Traffic [Erlang per site] Soft handover significantly improves the performance, but 13 Slide 13

14 Soft Handover Simulation Results II 2 er of Active Links Mean Numb 1,5 1 0, Max. Active Set Size the overhead due to simultaneous connections becomes higher! 14 Slide 14

15 Inter-Frequency Handover in UMTS Hierarchical cell structure (HCS) Hot-spot Macro Micro Macro f 2 f 1 f 1 Handover f 1 f 2 always needed between layers Hot spot f 1 f 1 f 1 f 2 Handover f 1 f 2 needed sometimes at hot spot Hard handover Inter-frequency measurements of target cell needed in both scenarios 15 Slide 15

16 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 16 Slide 16

17 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 17 Slide 17

18 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 18 Slide 18

19 Event 1a: A P-CPICH enters the Reporting Range 19 Slide 19

20 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

21 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 21 Slide 21

22 HSDPA Mobility Procedures II CRNC CRNC Source HS- DSCH Node B Target HS- DSCH Node B MAC-hs MAC-hs NodeB NodeB NodeB NodeB s t Serving HS-DSCH radio link 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

23 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) 23 Slide 23

24 HSDPA Handover Procedure UE Target HS-DSCH cell Source HS-DSCH cell SRNC = DRNC RL Reconfiguration Prepare RL Reconfiguration Ready ALCAP Iub HS-DSCH Data Transport Bearer Setup Serving HS-DSCH cell change decision i.e. event 1D If new NodeB RL Reconfiguration Prepare Radio Bearer Reconfiguration RL Reconfiguration Commit RL Reconfiguration Ready RL Reconfiguration Commit Synchronous Reconfiguration with Tactivation Radio Bearer Reconfiguration Complete Reset MAChs entity DATA ALCAP Iub HS-DSCH Data Transport Bearer Release 24 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 After the transport channel configuration is completed the UE sends the RRC message RADIO BEARER RECONFIGURATION COMPLETE Slide 24

25 E-DCH Operation in Soft Handover scheduling grant HARQ ACK/ NACK UE scheduling grant HARQ ACK/ NACK 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 25 Slide 25

26 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 26 Slide 26

27 EDCH Mobility Procedures SRNC MAC-es SRNC MAC-es MAC-e MAC-e MAC-e MAC-e NodeB NodeB NodeB NodeB s t Serving E-DCH radio link 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

28 Serving E-DCH Cell Change UE Target serving E-DCH cell Source serving E-DCH cell SRNC = DRNC RL Reconfiguration Prepare RL Reconfiguration Ready Serving E-DCH cell change decision i.e. event 1D RL Reconfiguration Prepare If new NodeB RL Reconfiguration Ready Radio Bearer Reconfiguration RL Reconfiguration Commit RL Reconfiguration Commit Synchronous Reconfiguration with T activation 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 Handover of HS-DSCH serving cell DL transport bearer setup MAC-hs RESET 28 Slide 28

29 Mobility Measurement Reporting in LTE LTE mobility measurements are similar as in UMTS Event triggered Event triggered with periodic reporting Periodic reporting Event A1 A2 A3 A4 A5 B1 B2 Purpose Serving cell becomes better than an absolute threshold Serving cell becomes worse than an absolute threshold Neighbor E-UTRA cell becomes an offset better than the serving cell Neighbor E-UTRA cell becomes better than an absolute threshold Serving cell becomes worse than an absolute threshold AND neighbor E-UTRA cell becomes better than another absolute threshold Inter-RAT neighbor cell becomes better than an absolute threshold Serving cell becomes worse than an absolute threshold AND inter-rat neighbor cell becomes better than an absolute threshold 29 Slide 29

30 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 30 Slide 30

31 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 31 Slide 31

32 LTE Handover: Preparation Phase UEUE Source enb enb Target enb enb MME sgw Measurement Control Packet Data UL allocation Measurement Reports HO decision HO Request Packet Data L1/L2 signaling L3 signaling User data Admission Control DL allocation RRC Connection Reconfig. HO Request Ack 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

33 LTE Handover: Execution Phase UEUE Source enb enb Target enb enb MME sgw Packet Data Detach from old cell, sync with new cell Deliver buffered packets and forward new packets to target enb Synchronisation DL data forwarding via X2 Buffer packets from source enb L1/L2 signaling L3 signaling User data 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 33 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

34 LTE Handover: Completion Phase UEUE Source enb enb Target enb enb MME sgw DL Packet Data DL data forwarding Packet Data Path switch req User plane update req End Marker Release resources Path switch req ACK Switch DL path User plane update response Flush DL buffer, continue delivering in-transit packets L1/L2 signaling End Marker L3 signaling Release resources User data Packet Data Packet Data 34 Slide 34

35 LTE Handover: Illustration of Interruption Period UEs stops Rx/Tx on the old cell UEUE Source enb enb Target enb enb UL U- plane active Measurement Report HO Command HO Request HO Confirm Handover Preparation Handover Interruption (approx 35 ms) approx 20 ms DL DL synchronisation + RACH (no contention) + + Timing advance Adv + UL Resource + Req and Grant UL resource request/grant Handover Latency (approx 55 ms) HO Complete ACK U- plane active 35 Slide 35