UMTS Optimization Question & Answer _ Wireless Exam

3/15/2011

UMTS Optimization Question & Answer …

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UMTS Optimization Question & Answer February 1, 2011 Radio Network 1. What are the optimization tools you use? Drive test, analysis, others? 2. Are System Information Blocks (SIB) transmitted all the time? No, system information block is multiplexed with synchronization channel. Synchronization channel occupies the first time slot (TS) and SIB occupies the other 9 time slots. 3. How does UE camp (synchronize) to a NodeB? 1.

UE uses the primary synchronization channel (P-SCH) for slot alignment (TS synchronization).

2. After aligning to NodeB time slot, UE then uses secondary synchronization channel (S-SCH) to obtain frame synchronization and scrambling code group identification. 3.

UE then uses scrambling code ID to obtain CPICH, thus camping to a NodeB.

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4. What could be the cause of soft handover failure? UE issue. Resource unavailable at target NodeB. Inadequate SHO threshold defined. Etc. 5. What are the three sets in handover? The 3 sets in handover are: Active set – the list of cells which are in soft handover with UE. Monitored set – the list of cells not in active set but RNC has told UE to monitor. Detected set – list of cells detected by the UE but not configured in the neighbor list. 6. What are the major differences between GSM and UMTS handover decision? 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. 7. What are the events 1a, 1b, 1c, etc.? 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. 8. What are event 2a-2d and 3a-3d? 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. 9. What may happen when there’s a missing neighbor or an incorrect neighbor? wirelessexam.com/umts-optimization-q…

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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. 10. What can we try to improve when access failure is high? When access failure is high we can try the following to improve RACH performance: Increase maximum UE transmit power allowed: Max_allowed_UL_TX_Power. Increase power quickly: power_Offset_P0. Increase number of preambles sent in a given preamble cycle: preamble_Retrans_Max. Increase the number of preamble cycles: max_Preamble_Cycle. Increase number of RRC Connection Request retries: N300. 11. What are the conditions you typically set to trigger IRAT handover? RSCP and Ec/Io are used to trigger IRAT handover: RSCP ≤ -100dBm. Ec/Io ≤ -16dBm. 12. What are the typical KPIs you use to measure a network and what criteria? Access failure rate (≤ 2%). Call setup time (CS: over 95% of the time < 6-second for mobile-to-PSTN, 9-second for mobile-mobile. PS: over 95% of the time < 5-second). Dropped call rate (≤ 2%). BLER: over 95% of the blocks ≤ 2%. Average DL/UL throughput for PSD: 210kbps for loaded, 240kbps for unloaded. 13. What is the typical UE transmit power? Varies – most of the time below 0dBm. 14. Have your used Ericsson TEMS? If so: Do you know how to create command sequence? What are the call sequences you typically have? CS long call, CS short call, PSD call, etc. What are the typical commands you have for CS and PS call? Do you regularly stop and restart a new log file? Why and when to stop and start a new file? How do you stop a log file? Stop command sequence first, wait and make sure all equipment are in idle mode before stop logging. 15. Did you work on neighbor prioritization?


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16. What is the typical event sequence of IRAT Handover from 3G to 2G Event 2d – entering into compressed mode – measurement of 2G candidates – Event 3a – Verification of 2G resources – Handover from UTRAN Command from 3G RNC to UE 17. What are the possible causes for an IRAT Failure? Missing 2G relations Non availability of 2G Resources Poor 2G Coverage Missing 3G Relations 18. What is Paging Success Ratio? What is the typical PSR that you have seen in a UMTS network? PSR – Paging Responses to the Paging Attempts About 90% 19. What are the possible causes for a lower PSR? Non-continuous RF Coverage – UE going in and out of coverage area frequently Very High ‘Periodic Location Update Timer’ – Keeping UEs in VLR long time after it moved out of coverage Lower Paging Channel Power Access Channel Parameter Issues Delayed Location Update when crossing the LA / CN Boundaries 20. What are the possible causes for a Drop Call on a UMTS network? Poor Coverage (DL / UL) Pilot Pollution / Pilot Spillover Missing Neighbor SC Collisions Delayed Handovers No resource availability (Congestion) for Hand in Loss of Synchronization Fast Fading Delayed IRAT Triggers Hardware Issues External Interference 21. A UE is served by 2 or 3 SC in AS. It is identifying a SC from 3rd tier, Stronger and meets the criteria for Event1a or Event1c. But SHO did not happen because of missing neighbor relations? How do you optimize this issue? Study the Pilot spillover from the 3rd Tier SC and control its coverage Even after controlling the coverage, if the spillover is there, Add the neighbor. 22. A UE is served by 2 SC in AS, a SC is coming in to Monitored Set and Event1a is triggered. But wirelessexam.com/umts-optimization-q…

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UE is not receiving Active Set Update from NodeB and the call drops. What could be possible causes for this drop? Delayed Handover Loss of Synchronization Fast Fading Pilot Pollution / Spillover issues 23. What is Hard Handover in UMTS? When will it happen? Hard Handover in UMTS is a break before make type Handover It can happen in the inter RNC boundaries where there is no Iur link. 24. What is the typical Call Setup Time for a 3G UE to 3G UE Call? What are the possible RF related causes for a delayed CST in this type of call? 6 to 9 seconds Multiple RRC Attempts (UE is on poor coverage – need more than Access Attempt) Delayed Page Responses High Load on Paging and/or Access Channel Paging / Access Parameters 25. What is Soft Handover Overhead? What is the typical value in UMTS network? Soft Handover Overhead is calculated in two ways. 1) Average Active Set Size – Total Traffic / Primary Traffic. 2) Secondary / Total Traffic Typical Values are like 1.7 (Avg Active Set Size) or 35% (Secondary / Total ) 26. What will happen to the Soft Handover Overhead when you apply OCNS on the network? And Why? With OCNS, the interference (load) increases. This leads to reduction in Ec/Io of a Pilot, which reduces the pilot spillovers. Reduction in Pilot Spillover will reduce the Soft Handover Overhead. 27. What are the possible causes for an Access Failure in UMTS? Missing Neighbors Poor Coverage Pilot Pollution / Spillover Poor Cell Reselection Core Network Issues Non – availability of resources. Admission Control denies Hardware Issues Improper RACH Parameters External Interference 28. (FOR ERICSSON EXPERIENCED) What is RTWP? What is the significance of it? wirelessexam.com/umts-optimization-q…

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Received Total Wide-band Power It gives the Total Uplink Power (Interference) level received at NodeB 29. (FOR ERICSSON EXPERIENCED) What is the System Reference Point at which all the Power Levels are measured in Ericsson NodeB? System Ref Point for E/// NodeB is at the output of TMA (Between TMA and Antenna) 30. What are the typical values for ‘reportingrange1a’ and ‘reportingrange1b’? 3 dB and 5 dB respectively. 31. What will be the impact when you change ‘reportingrange1a’ from 3 to 4 dB and ‘timetotrigger1a’ 100 to 320 ms, without changing any other parameters? Reduction in number of Event1a Delayed Event1a trigger Reduction in Average Active Set Size Delay in Event1a could increase DL interference, which could lead to a drop call or increase in Average Power Per User (reduction in cell capacity) 32. What is Admission Control? Admission Control is an algorithm which controls the Resource Allocation for a new call and additional resource allocation for an existing call. Incase, if a cell is heavily a loaded and enough resources in terms of power, codes or CEs are not available, admission control denies permission for the additional resource requirement. 33. What is Congestion Control? Congestion Control monitors the dynamic utilization of specific cell resources and insures that overload conditions do not occur. If overload conditions do occur, Congestion Control will immediately restrict Admission Control from granting additional resources. In addition, Congestion Control will attempt to resolve the congestion by either down switching, or terminating existing users. Once the congestion is corrected, the congestion resolution actions will cease, and Admission Control will be enabled.

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UMTS Optimization Question & Answer _ Wireless Exam

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