UMTS CELL CELECTION AND RESELECTION: A DETAILED ANALYSIS

The purpose of cell reselection is to continuously re-select the most suitable cell in the network when UE is not operating on a dedicated channel.There are three types of cell reselection
1. Intra frequency cell reselection : occurs between cells on the same radio frequency.
2. Inter frequency cell reselection : occurs between cells on different radio frequenies with no interruption of its connectivity with current cell.
3. Inter-RAT cell reselection : : happens between cells on different radio access technologies.

UE performs location registration if the new cell belongs to a different registration area.
During cell reselection UE takes measurement of the serving cell as well as he neighbour cells provided in SIB-11 or 12.

The S-criteria (Suitable Criteria) and R-criteria (Ranking Criteria) are calculated for each measured cells. All suitable cells are ranked based on R criteria. If highest ranked cell is a cell other than current serving cell for Treselection second and UE has camped on current serving cell for at leas one second then that cell is chosen for cell reselection.

S-Criteria

It determines whether a cell is suitable. Cell must meet the following criteria to be suitable.

Srxlev > 0     and      Squal > 0

Where, Srxlev = Qrxlevmin - Pcompensation

             Squal = Qqualmeas - Qqualmin

refer: 3GPP TS 25.331

R-Criteria

It determine the ranking of the serving cell and other suitable neighbour cell. The highest ranked cell is chosen for cell reselection. The definition of R criteria for serving cell is given as

                  Rs = Qmeas,s + Qhyst

Qmeas,s = measured quality of serving cell

Qhyst,s = hysteresis applied to serving cell.

Qmeas,s stands for measured CPICH RSCP and CPICH Ec/I0 .

UE chooses the highest ranking cell to camp on if the cell ranks higher than the current serving cell for Treselection seconds and UE has camped on current serving cell for at least one second. The parameter Treselection is defined by UTRAN in SIB-3 or SIB-4.

UE MEASUREMENTS AND REPORTING IN UMTS

UE MEASUREMENTS AND REPORTING  IN UMTS 

UTRAN controls a measurement in the UE  : 

1. By using the measurement information broadcast in SIB -11 and SIB-12, or

2. By transmitting Measurement Control Message.

The information for UE measurements and reporting of measurement results includes:

1. Measurement Identity : is the refrence number used by UTRAN when setting up, modifying or releasing the measurement using Measurement Control message and used by UE in Measurement report.

2. Measurement Command : Setup, Modification or release Command. The setup command orders UE to setup a new measurement,Modification command instruct UE to modify previously defined measurement and the release command tells the UE to stop measurement and clear all information related to it.

3. Measurement Type :  UTRAN can configure the UE to take any of 5 types of measurement.

                                          a.  Intra Frequency Measurements

                                          b.  Inter Frequency Measurements

                                          c.  Inter RAT Measurements

                                          d.  Traffic Volume Measurements

                                          e.  UE Internal Measurements

                                          f.  Quality Measurements

                                          g.  UE Positioning Measurement

4. Measurement Objects List : The objects on which UE Measures

5. Measurement Quantity : Quantity that UE Measures (CPICH Ec/I0, CPICH RSCP) on measurement objects.

6 : Reporting Quantity : Quantity that UE must include in report. (Cell Id, CPICH Ec/I0, CPICH RSCP, Path Loss, Observed time difference, and cell synchronization)

7. Measurement reporting criteria : Triggering of Measurement Report (periodical or event-triggered). Periodic reporting may be used in intra-frequency, inter-frequency or inter RAT measurements.

8. Measurement Validity : defines the UE states in which measurements are to be performed (CELL-DCH, CELL_FACH, URA_PCH, CEL_PCH). Measurement that UE takes in CELL-PCH, URA-PCH or  IDLE states are used for cell reselection and measurement reported in RACH when UE transitions to CELL_DCH state are used for soft and softer handovers.

9. Measurement Reporting Mode : UE transfers Measurements in which RLC mode (UM/AM).

UE Measurement for Handover

When UE is at call setup it uses measurement parameters broadcasted by UTRAN in SIB-11. UE performs the measurements of the cells and report the results to the UTRAN on RACH with RRC connection request message. UTRAN then decides whether to setup single link or multiple links (for softer/ soft handover in active set) before call becomes active. Handover at call setup is considered as a network evaluated handover.

For CELL_DCH state (When call is setup) UE uses the parameters broadcasted in SIB-11 or SIB-12. UE starts to perform measurements upon entering to CELL_DCH state and also evaluates the measurements. If defined measurement criteria mathes, the UE sends an event measurement report to UTRAN.

When UE is in CELL_DCH state, UTRAN may request the UE to setup, modify or release a measurement by sending a measurement control message to UE using AM RLC on a downlink DCCH chanel.

When UTRAN recieves the UE measurement report, it will add or remove the radio link for UE. No evaluation process in UTRAN is needed on handover actions because this decision is already made by UE. Actual handover algorithm sits in UE.UTRAN only defines the handover rules by setting the parameters for the measurement and reporting process.

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