Handover (2)

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UMTS Handover

description

Handover (2)

Transcript of Handover (2)

Principles of the WCDMA System

UMTS Handover2UE Working Modes and statesIdle mode Connected modeCell_DCHCell_FACHCell_PCHURA_PCH23UE Working Modes and statesIdle ModeThe UE has no relation to UTRAN, only to CN. For data transfer, a signalling connection has to be established. UE camps on a cellIt enables the UE to receive system information from the PLMN When registered and if the UE wishes to establish an RRC connection, it can do this by initially accessing the network on the control channel of the cell on which it is camped UE can receive "paging" message from control channels of the cell.It enables the UE to receive cell broadcast services. The idle mode tasks can be subdivided into three processes:PLMN selection and reselection;Cell selection and reselection;Location registration. When a UE is switched on, a public land mobile network (PLMN) is selected and the UE searches for a suitable cell of this PLMN to camp on. The NAS shall provide a list of equivalent PLMNs, if available, that the AS shall use for cell selection and cell reselection.The UE searches for a suitable cell of the chosen PLMN and chooses that cell to provide available services, and tunes to its control channel. This choosing is known as "camping on the cell". The UE will, if necessary, then register its presence, by means of a NAS registration procedure, in the registration area of the chosen cell.If the UE finds a more suitable cell, it reselects onto that cell and camps on it. If the new cell is in a different registration area, location registration is performed .34UE Working Modes and statesConnected Mode (Cell-DCH, Cell-FACH, Cell-PCH, URA-PCH)When at least one signalling connection exists, the UE is in connected mode and there is normally an RRC connection between UE and UTRAN. The UE position can be known on different levels: UTRAN Registration Area (URA) levelThe UE position is known on URA level. The URA is a set of cellsCell levelThe UE position is known on cell level. Different transport channel types can be used for data transfer:Common transport channels (RACH / FACH, DSCH, CPCH)Dedicated transport Channels (DCH) The connected mode is entered when the RRC connection is established. The UE is assigned a Radio Network Temporary Identity (RNTI) to be used as UE identity on common transport channels. Two types of RNTI exist. The Serving RNC allocates an s-RNTI for all UEs having an RRC connection. The combination of s-RNTI and an RNC-ID is unique within a PLMN. c-RNTI is allocated by each Controlling RNC through which UE is able to communicate on DCCH. cRNTI is always allocated by UTRAN when a new UE context is created to an RNC, but the UE needs its c-RNTI only for communicating on common transport channels.The UE leaves the connected mode and returns to idle mode when the RRC connection is released or at RRC connection failure.Within connected mode the level of UE connection to UTRAN is determined by the quality of service requirements of the active radio bearers and the characteristics of the traffic on those bearers.The UE-UTRAN interface is designed to support a large number of UEs using packet data services by providing flexible means to utilize statistical multiplexing. Due to limitations, such as air interface capacity, UE power consumption and network h/w availability, the dedicated resources cannot be allocated to all of the packet service users at all times.Variable rate transmission provides the means that for services of variable rate the data rate is adapted according to the maximum allowable output power.The UE state in the connected mode defines the level of activity associated to the UE. The key parameters of each state are the required activity and resources within the state and the required signalling prior to the data transmission. The state of the UE shall at least be dependent on the application requirement and the period of inactivity.Common Packet Channel (CPCH) uplink resources are available to UEs with an access protocol similar to the RACH. The CPCH resources support uplink packet communication for numerous UEs with a set of shared, contention-based CPCH channels allocated to the cell.The different levels of UE connection to UTRAN are listed below:-No signalling connection existsThe UE is in idle mode and has no relation to UTRAN, only to CN. For data transfer, a signalling connection has to be established.-Signalling connection existsWhen at least one signalling connection exists, the UE is in connected mode and there is normally an RRC connection between UE and UTRAN. The UE position can be known on different levels:-UTRAN Registration Area (URA) levelThe UE position is known on URA level. The URA is a set of cells-Cell levelThe UE position is known on cell level. Different transport channel types can be used for data transfer:-Common transport channels (RACH / FACH, DSCH, CPCH)-Dedicated transport CHannels (DCH)Assuming that there exists an RRC connection, there are two basic families of RRC connection mobility procedures, URA updating and handover. Different families of RRC connection mobility procedures are used in different levels of UE connection (cell level and URA level):-URA updating is a family of procedures that updates the UTRAN registration area of a UE when an RRC connection exists and the position of the UE is known on URA level in the UTRAN;-handover is a family of procedures that adds or removes one or several radio links between one UE and UTRAN when an RRC connection exists and the position of the UE is known on cell level in the UTRAN. 45UE Working Modes and statesCell-DCHIn active stateCommunicating via its dedicated channels UTRAN knows which cell UE is in. 56UE Working Modes and statesCell-FACHIn active stateFew data to be transmitted both in uplink and in downlink. There is no need to allocate dedicated channel for this UE. Downlink uses FACH and uplink uses RACH. UE need to monitor the FACH for its relative information. UTRAN knows which cell UE is in. Cell-PCHNo data to be transmitted or received. Monitor PICH, to receive its paging. lower the power consumption of UE.UTRAN knows which cell UE is in.UTRAN have to update cell information of UE when UE roams to another cellIf there is only few data to be transmitted, there is no need to allocate dedicated channel. Thus UE will be in Cell-FACH. UE in Cell-FACH state is communicating via FACH (downlink) and RACH (uplink) with UTRAN. UE need to monitor the FACH for its relative information because FACHs is shared for all users in the cell.If UE has no data to be transmitted or received, UE will be in Cell-PCH or URA-PCH. In these two states, UE needs to monitor PICH,to receive its paging. UTRAN knows which cell or URA UE is now in. The difference between Cell-PCH and URA-PCH is that UTRAN update UE information only after UE which is in URA-PCH state has roamed to other URA. UTRAN have to update cell information of UE when UE roams to another cell. UE migrates to cell-FACH state to complete the cell update. If there is also no data to be transmitted or received, UE is back to CELL-PCH state after cell update. If the cell update times in a fixed time reach a preset value, UTRAN will let UE migrate to URA-PCH. URA is an area of several cells.

67UE Working Modes and statesURA-PCHNo data to be transmitted or received. Monitor PICH. UTRAN only knows which URA (UTRAN Registration Area, which consists of multiple cells) that UE is in.UTRAN update UE information only after UE has roamed to other URA. A better way to lower the resource occupancy and signaling transmission It is the same as the CELL-PCH state. UE should migrate to CELL-FACH state to complete the URA update.

78CELL_DCHCELL_FACHCELL_PCHURA_PCHIDLEDEAD- Scanning networks (PLMN)- Camp on cell- Monitor paging channel- cell re-selection- Dedicated Channel- Radio bearers Transmission Services- upper layer Signalingtrigger (CN)- Reduce actionDTXand save powerRRC connectionUE statesIt is the same as the CELL-PCH state. UE should migrate to CELL-FACH state to complete the URA update.

89Handover MethodsChanging cells in the status of cell_DCH: -Handover-Direct retry(special case of handover)

Changing cells in the status of IDLE, Cell-PCH, Cell-FACH, URA-PCH -Cell reselection (forward handover)

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WCDMA RAKE Receiver10RAKE Receiver

Transmissions arriving causes deep multipath fading at the receiver that have followed different propagation paths. CDMA is less prone to this effect. In fact, one approach in common use with CDMA system, the Rake receiver, takes advantages of multipath, normally a major source of interference and signal degradation in other systems. In a Rake receiver, the signals of several correlation receivers belonging to the strongest multipath components are combined to provide an enhanced signal with better quality.The users in a CDMA cellular environment simultaneously share the same radio frequency band and can be separated at the receiver end with the knowledge of their unique code using a Rake receiver.

An optimum receiver contains several detection channels with different code delays, which are adjusted to match the major components of the impulse response. The timing accuracy to obtain full processing gain is approximately one chip time, i. e. the inverse of the channel bandwidth. The fingers in the rake collect together the contributions of the total signal energy from several multipath components. The impulse response is measured continuously in order to set the delay and phase of the different rake fingers. Thus the output from the channels can be added coherently giving diversity combining.Both the right coding and the right timing must be done to be able to dispreads the wanted signal in the receiver. An optimum receiver contains several detection channels with different code delays, which are adjusted to match the major components of the impulse response.In the Rake receiver the contribution from several multipath components are combined. It is necessary to measure continuously the impulse response of the propagation channel in order to set the delay and phase on the different rake branches. The output from the channels can then be added giving diversity combining.13Handover Overview3.1 Handover Types

3.2 Intra-Frequency Handover

3.3 Inter-Frequency Handover

3.4 Inter-RAT Handover (3G to 2G)1314The Categories of HandoverAccording to the signaling characters:Soft handover (softer handover)Hard handoverAccording to the properties of source cell and target cellIntra-frequency handoverInter-frequency handoverInter-system handover (UMTS GSM)According to the purpose of handoverBased on CoverageBased on Load (Optional)Based on mobility of UE (Optional)Based on Service (Optional)1415

1516The Basic Concepts of HandoverActive SetMonitored SetDetected set Event reportingEvent reporting to Periodic reportingPeriodic reportingRadio Link (RL)Radio Link Set (RLS)Combination way:maximum ratio combinationselection combinationThe soft handover gainCPICHSoft handover, softer handover, hard handover1617The Characters of Different HandoversComparison between soft handover and hard handover:ItemSoft HandoverHard HandoverThe numbers of RL in active set after handoverSeveral OneInterruption during handoverNoYesThe frequencies of cellsOnly happened in Intra-frequency cellsCan be happened in Intra-frequency cells or Inter-frequency cells1718

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1920Inter-Frequency Handover

Inter-frequency handover provides supplementary coverage for inter-frequency cells to share load with each other and to ensure service continuity.

From the UE point of view, inter-frequency handover is the same as intra-frequency hardhandover, because for both cases, the old connection is released before a new connection isset up.

2021The types of inter-frequency handover are as follows:

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2223Rules for Enabling 3G-to-2G Handover

2324Measurement of Handover

Measurement Control, normal caseUEUTRANMEASUREMENT CONTROL

If the UE setup a RRC connection or trigger the 1D event, the UTRAN will send the measurement control to UE. The measurement control include the algorithm of measurement and handover, for example, the intra-frequency handover criteria, and how to report.The latter message includes the following measurement control information:1. Measurement identity: A reference number that should be used by the UTRAN when setting up, modifying or releasing the measurement and by the UE in the measurement report.2. Measurement command: One out of three different measurement commands.-Setup: Setup a new measurement.-Modify: Modify a previously defined measurement, e.g. to change the reporting criteria.-Release: Stop a measurement and clear all information in the UE that are related to that measurement.3. Measurement type: One of the types listed above describing what the UE shall measure. Presence or absence of the following control information depends on the measurement type.2425Measurement of Handover

Measurement Control, normal caseUEUTRANMEASUREMENT CONTROL

. Measurement objects: The objects the UE shall measure, and corresponding object information. 5. Measurement quantity: The quantity the UE shall measure. This also includes the filtering of the measurements.6. Reporting quantities: The quantities the UE shall include in the report in addition to the quantities that are mandatory to report for the specific event.7. Measurement reporting criteria: The triggering of the measurement report, e.g. periodical or event-triggered reporting.8. Measurement Validity: Defines in which UE states the measurement is valid.9. Measurement reporting mode: This specifies whether the UE shall transmit the measurement report using AM or UM RLC.10. Additional measurement identities: A list of references to other measurements. When this measurement triggers a measurement report, the UE shall also include the reporting quantities for the measurements referenced by the additional measurement identities.

2526Measurement of Handover

Measurement Control, normal caseUEUTRANMEASUREMENT REPORTThe purpose of the measurement reporting procedure is to transfer measurement results from the UE to UTRAN. Based on the algorithm in measurement control, the UE will measure the signal strength and check if it meet the requirement of all event. If it meet the requirement of any event, UE will send the measurement report to UTRAN to trigger the handover. The most important information in the measurement are the PSC and the CPICH Ec/No of the target cell, and the triggered event.

2627The Basic Concepts of MeasurementThe measurement values of HandoverSoft handover and Intra-frequency hard handover : CPICH RSCP, CPICH Ec/N0Inter-frequencyCPICH RSCP, CPICH Ec/N0Inter-systemGSM Carrier RSSI, BSIC Identification, BSIC ReconfirmationThe reporting methods of measurementPeriodic reporting Event reportingThe events of reportingIntra-frequency events1A,1B,1C,1D,1FInter-frequency events 2D,2F,2B,2CInter-system events 2D,2F,3A,3CIn the actual network, we usually use the CPICH Ec/No for measurement value of intra-frequency handover.

2728Intra-frequency Measurement EventsIntra-frequency measurement report events are identified with 1X1A : A primary pilot channel enters the reporting range. When the active set of UE is full, UE stops reporting 1A event;1B : A primary pilot channel leaves the reporting range; 1C : The primary pilot channel in a non active set is better than the primary pilot channel in an active set;1D : The best cell changes;1f : The measurement value of a primary pilot channel exceeds the absolute thresholdAbsolute threshold: These parameters correspond to the signal strength that satisfies the basic QoS assurance. The soft handover absolute threshold parameters include IntraAblThdFor1E (absolute threshold for event 1E) and IntraAblThdFor1F (absolute threshold for event 1F). Intra-frequency Measurement Events are for soft handover and Intra-frequency hard handover.

2829Key Parameters

The strength of CPICH 1 is decreasing and the strength of CPICH 2 is increasing. If the strength difference between CPICH 1 and CPICH 2 is less than window (AS_TH - AS_TH_Hyst) for T, the 1A event will be triggered. The basic principle of 1C and 1B is same with 1A event.If the best serving cell changed, window will be changed and follow the current best serving cell.

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35Inter-frequency Measurement EventsInter-frequency measurement events are identified with 2X2B : The estimated quality value of the used frequency is lower than a certain threshold, and that of the non-used frequency is higher than a certain threshold; 2C : The estimated quality value of the non-used frequency is higher than a certain threshold; 2D : The estimated quality value of the used frequency is lower than a certain threshold 2F : The estimated quality value of the used frequency is higher than a certain threshold2D and 2F are to trigger compressed mode.2B is for Inter-frequency hard handover based on coverage.2C is for Inter-frequency hard handover based on non-coverage, that is load and traffic.

3536Inter-system Measurement EventsInter-system measurement events are identified with 3X3A : The estimated quality value of the used UTRAN frequency is lower than a certain threshold, and that of the other system is higher than a certain threshold3C : The estimated quality value of the other system is higher than a certain thresholdInter-system hard handover also use 2D and 2F to initiate and stop compressed mode. 3A is for Inter-system hard handover based on coverage.3C is for Inter-system hard handover based on non-coverage, that is load and traffic.

3637The Basic Handovers3.1 Soft Handover3.2 Intra-frequency Hard Handover3.3 Inter-frequency Hard Handover3.4 Inter-system Hard Handover3738Brief Introduction of Soft Handover Characters of Soft HandoverDuring handover, UE has several RLs with different cells - active setThe handover among different cells which are in same RLS can be softer handoverSoft handover:Selection combination in uplinkMaximum combination in downlinkSofter handoverMaximum combination in uplink and downlink3839Brief Introduction of Soft HandoverAdvantagesSoft handover gainDecrease the possibility of call drop caused by ping-pong handoverDisadvantagesMore resource needed in downlink, especially for the code resource of BE serviceUsually, the gain of downlink power is negativeWhen the downlink power from different cells is not balanced, it will bring side-effect3940Measurement of Soft Handover The measurement of soft handover/softer handover Measurement valueCPICH RSCPCPICH Ec/NoPathlossProcess of MeasurementLayer 1 filter, Layer 3 filterReporting wayPeriodic reporting Event reportingEvent type1A1B1C1D,1FUE measures the time difference between CFN and SFNReporting rules: trigger condition, Relative threshold, Absolute threshold, Hysteresis, Time to triggerEvent reporting to periodic reporting4041The Decision Algorithm of Soft HandoverSoft/softer handover decision 1A : Radio Link Addition1B : Radio Link Removal1C : Combined Radio Link Addition and Removal1D : change best cellIf Pilot_Ec /I0 > Best_ Pilot_Ec /I0 Reporting_range t Hysteresis_event1A for a period of T and the active set is not full, the cell is added to the active set. This event is called Event 1A or Radio Link Addition.If Pilot_Ec /I0 < Best_ Pilot_Ec /I0 Reporting_range Hysteresis_event1B for a period of T, then the cell is removed from the active set. This event is called Event 1B or Radio Link Removal. If the active set is full and Best_candidate_Pilot_Ec /I0 > Worst_Old_Pilot_Ec /I0 t Hysteresis_event1C for a period of T, then the weakest cell in the active set is replaced by the strongest candidate cell (i.e. strongest cell in the monitored set). This event is called Event 1C or Combined Radio Link Addition and Removal.

4142Soft Handover Flow (add RL)

4243Soft Handover Flow (del RL)

4344Brief Introduction of Intra-frequency Hard HandoverCharacters UE only has one RL with one cell in the process of handoverDisconnect UE with the original cell, then hand it over to target cellThe working frequency of source cell and target cell are sameAdvantagesEnhance the using efficiency of the OVSF code and hardware resourceDisadvantagesHigh call drop possibility because of the intra-frequency interferenceApplication scenarios Without Iur interface between two RNCs or the Iur interface jam (only happened in inter-RNC handover)The different strategies can be used for different conditions, such as code resource condition, the QoS condition and so no4445Intra-frequency Hard Handover Flow

4546The Brief Introduction of Inter-frequency Hard HandoverCharactersThe working frequency is different after handoverThe compressed mode needed if the UE only has one receiverUsually, the timing re-initiation hard handover is used for hard handoverApplication scenarios Disconnected coverage Handover based on loadAdvantagesCompare to intra-frequency hard handover, the success possibility is higherThe load balance among cells with different frequencies DisadvantagesCompressed mode results in extra radio resource occupiedThe timing re-initiation hard handover increase the time of handover and the risk of call drop

4647Measurement Values and EventsThe Inter-frequency hard handover measurement valuesMeasurement valuesCPICH RSCPCPICH Ec/N0Different handover purpose for different measurement typeIn the edge of carrier coverage: CPICH RSCPIn the center of carrier coverage: CPICH Ec/No4748Compressed Mode InitiationConditions for measurement initiated2D event : The estimated quality value of the used frequency is lower than a certain threshold Conditions for Measurement stopping 2F event : The estimated quality value of the used frequency is higher than a certain threshold48

Compressed mode by higher layer scheduling reducing the data rate used in the upper layers ,It is basically applicable only to non-real-timeservices, such as packet transmissions.

Compressed mode by reducing the spreading factor by 2 (SF/2) A way to temporarily increase the transmission speed byhalving SF so that the same number of bits can betransmitted as in the normal mode in slots other than thetransmission gap.4950Decision AlgorithmThe inter-frequency hard handover decisionThe coverage triggers the handover2B eventthe quality in the used frequency is lower than absolute threshold, but the quality in another non-used frequency is higher than another absolute threshold.The load triggers handover2C eventthe quality in another frequency is higher than an absolute threshold

5051Inter-frequency Hard Handover Signaling Flow

5152The Brief Introduction of Inter-system Hard HandoverApplication scenarios WCDMA FDD GSMCharactersDifferent radio access technology is used after handoverUsually, compressed mode should be used to help the measurementAdvantagesFor coverage, it can solve the problems from one system to another systemFor capacity, it can enhance the utilizing efficiency of old equipments(2G->3G)DisadvantagesThe flow is complicated, and it demands higher compatibility for equipmentsDemands more complicated UE5253The inter-system measurement (GSM measurement)Measurement type:GSM Carrier RSSIBSIC Identification BSIC ReconfirmationThe process of measurement: layer 1 filter, layer 3 filterMeasurement reportingEvent reporting2D Event: initiate GSM measurement2F Event: stop GSM measurementCompressed Mode Initiated5354Decision AlgorithmInter-system handover caused by coverageEvent reporting:3A eventThe estimated quality value of the used UTRAN frequency is lower than a certain threshold, and that of the other system is higher than a certain thresholdPeriodic reporting:EvaluationAccording to the periodic reported GSM RSSI measurement value and the BSIC confirming state of target cell of GSM system, and meanwhile the UE evaluates the GSM RSSI of target cell is greater than the absolute threshold, then consider the cell confirmed by BSIC5455Inter-system handover caused by non-coverage Event reporting3C event: The estimated quality value of the other system is higher than a absolute thresholdDecision Algorithm5556

Inter-system Handover Flow5657Key Parameters (I)Inter-system handover caused by coverage parametersInter-system measurement initiated and stopped threshold: Considering the different demands of CPICH Ec/No and CPICH RSCP for PS domain and CS domain, the different 2D and 2F parameters are configuredInter-system measurement values (2D, 2F)CPICH Ec/NoCPICH RSCPBSIC confirmed (Optional)Inter-system handover caused by coverage parametersconfigure the GSM RSSI threshold of CS domain and PS domain separatelyUsing inter-system frequency quality handover threshold Configure the delay trigger time, hysteresis for each event57Thank You

10. Handover

Complete

BSC

MSC

CN

SRNC

Node B

8. Handover

Detect

6. Relocation Command

7. DCCH : Handover from UTRAN Command

1. Relocation Required

5. Prepare Handover

Response

4. Handover

Request Ack

2. Prepare

Handover

UE

11. Send End

Signal

Request

12. Iu Release Command

13. Iu Release Complete

14. Send End Signal Response

3. Handover

Request

9. Handover Complete