Cell Reselection, Redirection and Handover (2G/3G/4G)

Idle Mode Procedure (Reselection)

Reselection and its trigger conditions

Reselection procedure is triggered by UE when UE is in idle mode and finds the neighbor and serving cell received signal power/quality levels are satisfying the below conditions.

1. Neighbor cell is of high priority and received signal power/quality level is better than thresholdHigh limit or
2. Neighbor cell is of lower priority and serving received signal power/quality level is below the thresholdLow limit and the neighbor received signal power/quality is above the thresholdLow and
3. More than 1 second has elapsed since the UE camped on the current serving cell.

Cell properties like ThresholdLow, ThresholdHigh and Cell Priority values are obtained from the below System Information Blocks 

•         LTE SIB-3
•        LTE SIB-4                         (for Intra LTE) 
•         LTE SIB-5                         (for Inter LTE)
•         LTE SIB-6                         (for UMTS)
•         LTE SIB-7                         (for GSM)
•         LTE SIB-8                         (for CDMA)
•         WCDMA SIB19                 (for LTE)
•         GSM SI2quater                  (for 3G/LTE)

Lower the value of priority in SIB, lower is cell priority.

Cell Selection/Reselection Criteria

For cell camp

Srxlev> 0 AND Squal> 0

In case of reselection from a lower priority cell to higher priority cell the below condition should hold good for atleast the t-reselection timer

If threshServingLowQ is provided in SIB3, then

Squal > ThresxHighQ (FDD) or SrxlevTarget > ThresxHigh (TDD/GERAN/CDMA)

else 

            SrxlevTarget > ThresxHigh  

In case of reselection from a high priority cell to lower priority cell the below condition should hold good for atleast the t-reselection timer

If threshServingLowQ is provided in SIB3, then

SqualServing < ThresxServingLowQ &

SqualTarget > ThresxLowQ (FDD) or SrxlevTarget > ThresxLow (TDD/GERAN/CDMA)

else

SrxlevServing < ThresServingLow

SrxlevTarget  > ThresxLow

So when will UE start measuring the neighbor cells?

1. UE will always measure all the high priority cells (Inter & Intra RAT).
2. UE will start measuring Intra Freq low priority cells if serving cell satisfies the below condition 
   SrxlevServing < sIntraSearch(SIB3)
3. UE will start measuring Inter Freq and Inter RAT low priority cells if serving cell satisfies the below condition
   SrxlevServing < sNonIntraSearch(SIB3)

Where

Srxlev= Qrxlevmeas–(Qrxlevmin+ Qrxlevminoffset) –Pcompensation

Squal= Qqualmeas–(Qqualmin+ Qqualminoffset)

Srxlev	Cell selection RX level value (dB)
Squal	Cell selection quality value (dB)
Qrxlevmeas	Measured cell RX level value (RSRP)
Qqualmeas	Measured cell quality value (RSRQ)
Qrxlevmin	Minimum required RX level in the cell (dBm), obtained in SIB1
Qqualmin	Minimum required quality level in the cell (dB), obtained in SIB1
Qrxlevminoffset	Offset to the signalled Qrxlevmintaken into account in the Srxlevevaluation as a result of a periodic search for a higher priority PLMN while camped normally in a VPLMN,obtained in SIB1
Qqualminoffset	Offset to the signalled Qqualmintaken into account in the Squalevaluation as a result of a periodic search for a higher priority PLMN while camped normally in a VPLMN,obtained in SIB1
Pcompensation	max(PEMAX_H–PPowerClass, 0) (dB)
PEMAX_H	Maximum TX power level an UE may use when transmitting on the uplink in the cell (dBm) defined as PEMAX_H in [TS 36.101],obtained in SIB1
PPowerClass	Maximum RF output power of the UE (dBm) according to the UE power class as defined in [TS 36.101]

Connected Mode Procedure (Redirection & Handover)

Connected mode procedures are driven by eNB. eNB uses the measurement reports sent from UE to decide if redirection/handover is required.

When does UE get chance to measure neighbor cells?

eNB configures UE with specific measurement gap. During this time, UE can stop monitoring PDCCH channel and start measuring neighbor cells.  Measurement Gap duration is fixed to 6msec. Measurement gap config has a periodicity of either 40ms (gp0) or 80ms (gp1) and gapoffset value. Measurement gap will start from the SFN and sub-frame which satisfies the below condition  

           SFN mod T = FLOOR(gapOffset/10);

           subframe = gapOffset mod 10;

           with T = MGRP/10; MGRP = 40 for gp0 and 80 for gp1

Measurement gap configuration is received through RRC reconfiguration message. UE can also use the DRX cycles if configured for measurements.

Which Neighbor cells to measure?

Through RRC reconfiguration messages, UE will receive a measurement object, which contains the neighbor cells information and event condition to report.
Below are the lists of events reported by UE to eNB

A1	Serving becomes better than threshold
A2	Serving becomes worse than threshold
A3	Neighbor becomes offset better than serving cell + extra margin
A4	Neighbor becomes better than threshold
A5	Serving becomes worse than threshold1 AND neighbor becomes better than threshold2
A6	Neighbour becomes offset better than SCell
B1	Inter RAT neighbor becomes better than threshold
B2	Serving becomes worse than threshold1 AND inter RAT neighbor becomes better than threshold2

Why different procedures - Redirection & Handover?

In Handover procedure, target cell allocates all the required radio resources (Radio bearers and dedicated PRACH resource) upfront for the UE to move seamlessly. Target Cell radio resource information is provided to UE by serving cell in RRC reconfiguration message. UE stays in connected mode during this procedure.

In Redirection procedure, target cell will not be allocating any radio resources upfront. So UE has to release the radio connection (in serving cell) and use the target cell info provided in the RRC Reconfiguration (if any, else UE needs to do full scan) to camp and further request for the radio resources in target cells. UE goes to idle mode in serving cell and once it moves to target cell, UE moves to connected mode through RRC connection request.

Conversion between RSRP<->RSCP<->RSSI/ ECNO<->RSRQ

4G->2G/3G

• RSRP

The reporting range of RSRP is defined from -140 dBm to -44 dBm with 1 dB resolution.

Reported value	Measured quantity value	Unit
RSRP_00	RSRP < -140	dBm
RSRP_01	-140 < RSRP < -139	dBm
RSRP_02	-139 < RSRP < -138	dBm
…	…	…
RSRP_95	-46 < RSRP < -45	dBm
RSRP_96	-45 < RSRP < -44	dBm
RSRP_97	-44 < RSRP	dBm

• RSRP

The reporting range of RSRQ is defined from -19.5 dB to -3 with 0.5 dB resolution.

Reported value	Measured quantity value	Unit
RSRQ_00	RSRQ < -19.5	dB
RSRQ_01	-19.5 < RSRQ < -19	dB
RSRQ_02	-19 < RSRQ < -18.5	dB
…	…	…
RSRQ_32	-4 < RSRQ < -3.5	dB
RSRQ_33	-3.5 < RSRQ < -3	dB
RSRQ_34	-3 < RSRQ	dB

3G->4G 

• ECNO

The reporting range is for CPICH Ec/Io is from -24dB ...0 dB.

Reported value	Measured quantity value	Unit
CPICH_Ec/No _00	CPICH Ec / Io < ‑24	dB
CPICH_Ec/No _01	-24 < CPICH Ec / Io < ‑23.5	dB
CPICH_Ec/No _02	-23.5 < CPICH Ec / Io < ‑23	dB
…	…	…
CPICH_Ec/No _47	-1 < CPICH Ec / Io < -0.5	dB
CPICH_Ec/No _48	-0.5 < CPICH Ec / Io < 0	dB
CPICH_Ec/No _49	0 < CPICH Ec / Io	dB

• UTRA RSSI

The reporting range for UTRA carrier RSSI is from -100dBm ...-25 dBm.

Reported value	Measured quantity value	Unit
UTRA_carrier_RSSI_LEV _00	UTRA carrier RSSI < ‑100	dBm
UTRA_carrier_RSSI_LEV _01	-100 < UTRA carrier RSSI < ‑99	dBm
UTRA_carrier_RSSI_LEV _02	-99 < UTRA carrier RSSI < ‑98	dBm
…	…	…
UTRA_carrier_RSSI_LEV _74	-27 < UTRA carrier RSSI < -26	dBm
UTRA_carrier_RSSI_LEV _75	-26 < UTRA carrier RSSI < -25	dBm
UTRA_carrier_RSSI_LEV _76	-25 < UTRA carrier RSSI	dBm

• RSCP

The reporting range for CPICH RSCP is from -120 dBm ...-25 dBm.

Reported value	Measured quantity value	Unit
CPICH_RSCP_LEV _-05	CPICH RSCP < ‑120	dBm
CPICH_RSCP_LEV _-04	-120 < CPICH RSCP < ‑119	dBm
CPICH_RSCP_LEV _-03	-119 < CPICH RSCP < ‑118	dBm
…	…	…
CPICH_RSCP_LEV _89	-27 < CPICH RSCP < -26	dBm
CPICH_RSCP_LEV _90	-26 < CPICH RSCP < -25	dBm
CPICH_RSCP_LEV _91	-25 < CPICH RSCP	dBm

References:

1. 3GPP 36.133
2. 3GPP 36.304
3. 3GPP 36.331
4. 3GPP 25.133
5. 3GPP 44.018
6. 3GPP 45.008