UMTS Question and answer part 4

61. What could be the cause of soft handover failure?


Answer –

• Undefined neighbors

• One way Neighbor definition

• UE issue.

• Resource unavailable at target NodeB.

• Inadequate SHO threshold defined.

62. What are the three sets in handover?

Answer –

Active Set

Monitored Set

Detected Set

63. What are the major differences between GSM and UMTS handover decision?

Answer –

GSM:

• Time-based mobile measures of RxLev and RxQual – mobile sends measurement report every SACH period (480ms).

• BSC instructs mobile to handover based on these reports.

UMTS:

• Event-triggered reporting – UE sends a measurement report only on certain event “triggers”.

• UE plays more part in the handover decision.



64. What are the events 1a, 1b, 1c, etc.?

Answer –

e1a – a Primary CPICH enters the reporting range, i.e. add a cell to active set.

e1b – a primary CPICH leaves the reporting range, i.e. removed a cell from active set.

e1c – a non-active primary CPICH becomes better than an active primary CPICH, i.e. replace a cell.

e1d: change of best cell.

e1e: a Primary CPICH becomes better than an absolute threshold.

e1f: a Primary CPICH becomes worse than an absolute threshold.



65. What are event 2a-2d and 3a-3d?

Answer –

Events 2a-2d are for inter-frequency handover measurements and events 3a-3d are for IRAT handover measurements.

e3a: the UMTS cell quality has moved below a threshold and a GSM cell quality had moved above a threshold.

e3b: the GSM cell quality has moved below a threshold.

e3c: the GSM cell quality has moved above a threshold.

e3d: there was a change in the order of best GSM cell list.



66. What may happen when there’s a missing neighbor or an incorrect neighbor?

Answer –

• Access failure and handover failure: may attempt to access to a wrong scrambling code.

• Dropped call: UE not aware of a strong scrambling code, strong interference.

• Poor data throughput.

• Poor voice quality.

• Etc.



67. How is inter-frequency Handover triggered?

Answer –

The network decides that inter frequency measurements need to be performed and sends the MEASUREMENT CONTROL MESSAGE with Measurement type set to Inter-Frequency measurements. Generally it will set an Event as well along with the measurements. The following are list of Events that can trigger Measurement Report.

• Event 2a: Change of Best Frequency

• Event 2b: The estimated quality of the currently used frequency is below a certain threshold and the estimated quality of a non-used frequency is above a certain threshold

• Event 2c: The estimated quality of a non-used frequency is above a certain threshold

• Event 2d: The estimated quality of the currently used frequency is below a certain threshold

• Event 2e: The estimated quality of a non-used frequency is below a certain threshold

• Event 2f: The estimated quality of the currently used frequency is above a certain threshold

The Inter-Frequency Handover Evaluation bases its decision on P-CPICH quality measures on the currently used frequency and on one or more non-used frequencies. If the evaluation result is positive, one cell on a non-used frequency is proposed to Inter-Frequency handover Execution.

Inter-Frequency Handover is a hard handover where the UE is ordered by the network to tune to another frequency. This means that there will be small interruptions in the data flow to and from the UE.

68. What kind of Handover takes place in Load Sharing?

Answer –

It’s a blind handover to the co-located cell. IFHO i.e.



69. What do you understand by IFHO?

Answer –

IFHO – Inter Frequency Handover

70. What do you understand by Directed Retry?

Answer –

When there is a co-existing GSM RAN, excess traffic in a WCDMA cell may be off-loaded to GSM

If a call is chosen for Directed Retry to GSM, the request for the speech RAB will be rejected with cause "Directed retry" and then a request is made to the core network to relocate the UE to a specific GSM cell, using the Inter-RAT handover procedure. This handover is a blind one since the target cell is chosen not based on UE measurements. Therefore, the target cell must be co-located with the WCDMA cell. Co-located GSM cells are assumed to have similar coverage and accessibility as their respective WCDMA cells.