ORG010202 CDMA 1x Handoff Algorithm

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What is Handoff in mobile network?
What is the Search Window?
How to optimize handoff parameters?
Upon completion of this course, you will be able to:
Know handoff principle and classification.
Know search window and key parameters.
Master handoff tuning in network.
Chapter 1 Basic Concept
Chapter 2 Handoff in CDMA
Chapter 3 Optimization Command
Chapter 4 Summary
Soft handoff
It is a process of establishing a link with a target sector before breaking the link with the serving sector.
Softer handoff
Similar to the soft handoff, but the softer handoff is occurred among multi-sectors in the same base station.
Hard handoff
Hard handoff occurs when the two sectors are not synchronized or are not on the same frequency. Interruption in voice or data communication occurs but this interruption does not effect the user communication.
CDMA soft handoff is driven by the handset
Handset continuously checks available pilots
Handset tells system pilots it currently sees
System assigns sectors (up to 6 max.), tells handset
Handset assigns its fingers accordingly
All messages sent by dim-and-burst, no muting!
Each end of the link chooses what works best, on a frame-by-frame basis!
Users are totally unaware of handoff

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ORG010202 CDMA 1x Handoff Algorithm

  1. 1. ORG010202 CDMA 1xHandoff AlgorithmISSUE 1.2
  2. 2. Page 2Learning Guide What is Handoff in mobile network? What is the Search Window? How to optimize handoff parameters?
  3. 3. Page 3Upon completion of this course, you will be able to: Know handoff principle and classification. Know search window and key parameters. Master handoff tuning in network.
  4. 4. Page 4Chapter 1 Basic ConceptChapter 1 Basic ConceptChapter 2 Handoff in CDMAChapter 2 Handoff in CDMAChapter 3 Optimization CommandChapter 3 Optimization CommandChapter 4 SummaryChapter 4 Summary
  5. 5. Page 5What is handoffBTS_ABTS_B
  6. 6. Page 6Why do we need handoffTo provide the continuous service for subscribersIn wireless communications, during HO process, there is ahigh probability of call drop, which directly effects thesystem performance ( 90% of radio call drop occursduring handoff). Proper handoff algorithm can reducesystem call drop and increase network capacity.Notice
  7. 7. Page 7How many kinds of handoff in CDMA Soft handoffIt is a process of establishing a link with a target sector beforebreaking the link with the serving sector. Softer handoffSimilar to the soft handoff, but the softer handoff is occurredamong multi-sectors in the same base station. Hard handoffHard handoff occurs when the two sectors are notsynchronized or are not on the same frequency. Interruption invoice or data communication occurs but this interruption doesnot effect the user communication.
  8. 8. Page 8Chapter 1 Basic ConceptChapter 1 Basic ConceptChapter 2 Handoff in CDMAChapter 2 Handoff in CDMAChapter 3 Optimization CommandChapter 3 Optimization CommandChapter 4 SummaryChapter 4 Summary
  9. 9. Page 9Soft Handoff It is a process of establishing a link with a target sector beforebreaking the link with the serving sector. Soft handoff branches are selected by BSC.
  10. 10. Page 10Soft Handoff CDMA soft handoff is driven by the handset Handset continuously checks available pilots Handset tells system pilots it currently sees System assigns sectors (up to 6 max.), tells handset Handset assigns its fingers accordingly All messages sent by dim-and-burst, no muting! Each end of the link chooses what works best, on a frame-by-frame basis! Users are totally unaware of handoff
  11. 11. Page 11Softer Handoff Similar to the soft handoff, but thesofter handoff is occurred amongmulti-sectors in the same basestation. Softer handoff: Signal of differentbranches are combined in BTS.
  12. 12. Page 12Softer Handoff Each BTS sector has unique PN offset & pilot Handset will ask for whatever pilots it wants If multiple sectors of one BTS simultaneously serve a handset,this is called Softer Handoff Handset can’t tell the difference, but softer handoff occurs inBTS in a single channel element Handset can even use combination soft-softer handoff onmultiple BTS & sectors
  13. 13. Page 13Questions Which channel signal strength from BTS should be compared byMS during HO process? How does MS identify which signal is better than others?
  14. 14. Page 14SearcherCorrelatorPilot SetsPilots of sectors actually in usePilots mobile requested, but not yet setup & transmitting by systemPilots told to mobile by system, as nearbysectors to checkAny pilots used by system but not already inthe other setsActive SetCandidateSetNeighborSetRemainingSet
  15. 15. Page 15Handoff Parameters Handset sends PPilot SStrength MMeasurement MMessage to thesystem whenever It notices a pilot in neighbor or remaining set exceeds T_ADDT_ADD An active set pilot drops below T_DROPT_DROP for T_TDROPT_TDROP time A candidate pilot exceeds an active by T_COMPT_COMP
  16. 16. Page 16How to implement Soft HandoffT_ADDT_ADDT_DROPT_DROPPilot 1PilotStrengthPilot 2T_TDROPT_TDROPNeighborSetCandidateSetActiveSetNeighborSetTIME1 2 3 4 5 6ActiveSetT_COMPT_COMP
  17. 17. Page 17Hard HandoffMS assistedHard handoffPseudo pilotHard handoffHand downHard handoffDirectHard handoffHard Handoff
  18. 18. Page 18MS Assisted Hard Handoff CFSRQM: Candidate Frequency Search Request Message CFSRSM: Candidate Frequency Search Response Message CFSCNM: Candidate Frequency Search Order Message CFSRPM: Candidate Frequency Search Report MessageCFSRPM(Report)CFSCNM(Search Order)F1 (serving)F2 (candidate)GHDM(HHO)CFSRQM(Search Parameters)CFSRSM(Acceptance)
  19. 19. Page 19Pseudo Pilot Hard HandoffF0F0F3F3F2F2F1F1F0F0F1F1F2F2F0F0Sector 1Sector 1 Sector 2Sector 2 Sector 3Sector 3Pilot BeaconF3Pilot BeaconF3Pilot BeaconF1Pilot BeaconF1Pilot BeaconF2Pilot BeaconF2
  20. 20. Page 20F0F0Hand Down Hard HandoffF3F3F2F2F1F1F0F0F1F1F2F2F0F0Sector 1Sector 1 Sector 2Sector 2 Sector 3Sector 3
  21. 21. Page 21Direct Hard HandoffF2F2 F1F1
  22. 22. Page 22Chapter 1 Basic ConceptChapter 1 Basic ConceptChapter 2 Handoff in CDMAChapter 2 Handoff in CDMAChapter 3 Optimization CommandChapter 3 Optimization CommandChapter 4 SummaryChapter 4 Summary
  23. 23. Page 23How to search pilot The purpose of SRCH-WIN set by MS is to capture the multi-pathcomponents, which arrived before or after the current signal. Based onexperience, the SRCH-WIN should be set wide enough so that it cancapture the signal, which has the maximum time delay due to multi-path.Search Window
  24. 24. Page 24SRCH-WIN Classification Currently MS uses three kinds of search windows, these are: SRCH-WIN-ASRCH-WIN-A , used for searching the pilots in the active set andthe candidate set SRCH-WIN-NSRCH-WIN-N , used for searching the pilots in the neighbor set SRCH-WIN-RSRCH-WIN-R , used for searching the pilots in the remain set The MS fixes the width of the search window according to the systemmessage. An MS can search the multi-path components of the availablepilot signals within the setting.
  25. 25. Page 25The Complete Picture of Handoff & Pilot Sets
  26. 26. Page 26Pilot Searching SRCH-WIN-ASRCH-WIN-A The MS can utilize the search window to search the pilots inthe active set and the candidate set based on the PN code.For pilots in the active set and candidate set, the MS fixesthe center of the search window at the moment, when themulti-path signal of the pilot reaches earliest.SRCH-WIN-ASRCH-WIN-AThe earliest arrived pilotPILOT PHASE
  27. 27. Page 27Pilot Searching SRCH-WIN-NSRCH-WIN-N Uses to monitor neighbor set , the size of this window islarger than SRCH-WIN-A. The distance is measured in termof chips (1 chips approximately covers 244m).The distance is measured in term of chips (1 chips approximately covers244m).PN4PN4{{n chip offset with delayed signal necessaryn chip offset with delayed signal necessaryPN16PN16{n chip offset because of the search center of active setn chip offset because of the search center of active set{Expected windowExpected window
  28. 28. Page 28Pilot Searching SRCH-WIN-RSRCH-WIN-R Uses to track the remaining pilots. A typical requirement forthe size of windows is, it should be equal or larger thanSRCH-WIN-N.
  29. 29. Page 29The Pilot Searcher’s Measurement Process
  30. 30. Page 30Delay budget and size of search windowSRCH_WIN_AWidth of SRCH_WIN_A(Chips)Delay Budget(ms)Available PN0 4 T < 1.64 5121 6 1.64 < T < 2.45 5122 8 2.45 < T < 3.27 5123 10 3.27 < T < 4.09 5124 14 4.09 < T < 5.72 5125 20 5.72 < T < 8.17 5126 28 8.17 < T < 11.44 5127 40 11.44 < T < 16.34 5128 60 16.34 < T < 24.51 5129 80 24.51 < T < 32.68 51210 100 32.68 < T < 40.85 51211 130 40.85 < T < 53.11 51212 160 53.11 < T < 65.36 25613 226 65.36 < T < 92.32 25614 320 92.32 < T < 130.72 17015 452 130.72 < T < 184.42 128
  31. 31. Page 31Questions Whether data service supports soft handoff? Considering the fact, high power may affect the system, whethersoft handoff can be performed after reducing the rate?
  32. 32. Page 32How To Optimize Soft Handoff Reasonable coverage planning and BTS layout Reasonable setting of T_Add, T_Drop, T_TDROP Optimize the maximum branches in active handoff Reasonable planning of handoff region Reasonable setting of neighbor and neighbor priority
  33. 33. Page 33How To Optimize Soft Handoff MOD HOMOD HO (Modify Handoff Parameters ) This command is executed to modify handoff parameters. Index: CN -- Cell number, SCTID -- Sector ID, CRRID -- Carrier ID.
  34. 34. Page 34How To Optimize Soft Handoff MOD BSCHOMOD BSCHO (Modify Inter-BSC Handoff Parameters ) This command is executed to modify handoff parameters at system level.
  35. 35. Page 35How To Optimize Hard Handoff In some cases, soft/softer handoff cannot be realized even thefrequencies of the pilots are the same. For example: Inter-BSC handoff without A3/A7 connection Handoff between different frame offsets on the same pilot In such cases, hard handoff is applied to ensure the uninterruptedcommunication.
  36. 36. Page 36Optimize Intra-frequency Hard Handoff MOD HHOSFMOD HHOSF (Modify Same-Frequency HHO Parameters ) This command is executed to modify same-frequency hardhandoff parameters. Index: CN -- Cell number, SCTID -- Sector ID, CRRID--Carrier ID.
  37. 37. Page 37Optimize MS Assistant Hard Handoff MOD HHOMAMOD HHOMA (Modify Mobile Assisted HHO Parameters ) This command is executed to modify the mobile assisted hardhandoff parameters. Index: CN -- Cell number, SCTID -- Sector ID, CRRID -- Carrier ID.
  38. 38. Page 38Optimize Pseudo Pilot Hard Handoff MOD HHOBPLTMOD HHOBPLT (Modify Pilot Beacon HHO Parameters ) This command is executed to modify pilot beacon hard handoffparameters. Index: CN -- Cell number, SCTID -- Sector ID, CRRID -- CarrierID.
  39. 39. Page 39Optimize Hand Down Hard Handoff MOD HNDDWNMOD HNDDWN (Modify Hand down HHO Parameters ) This command is executed to modify handdown hard handoff parameters. Index: CN -- Cell number, SCTID -- Sector ID, CRRID -- Carrier ID.
  40. 40. Page 40Optimize Direct Hard Handoff MOD DRCTMOD DRCT (Modify Direct HHO Parameters ) This command is executed to modify direct hard handoff parameters. Index: CN -- Cell number, SCTID -- Sector ID, CRRID -- Carrier ID.
  41. 41. Page 41Chapter 1 Basic ConceptChapter 1 Basic ConceptChapter 2 Handoff in CDMAChapter 2 Handoff in CDMAChapter 3 Optimization CommandChapter 3 Optimization CommandChapter 4 SummaryChapter 4 Summary
  42. 42. Page 42Handoff CaseEc/IoAll PN Offsets0PN0-20Rake FingersŒReference PNActive Pilot168 500220Mobile Rake RXSrch PN??? W0F1 PN168 W61F2 PN168 W61F3 PN168 W61
  43. 43. Page 43Handoff Case Just prior to this message, this particularmobile already was in handoff with PN168 and 220. This PSMM reports PN 500 has increasedabove T_Add, and the mobile wants touse it too.PILOT STRENGTH MEASUREMENT MESSAGE(PSMM) The base station acknowledges receivingthe PSMM.BASE STATION ACKNOWLEDGMENT
  44. 44. Page 44Handoff Case The base station sends a Handof Direction Messageauthorizing the mobile to begin soft handoff with allthree requested PNs. The pre-existing link on PN 168will continue to use Walsh code 61, the new link onPN220 will use Walsh Code 20, and the new link onPN500 will use Walsh code 50.HANDOFF DIRECTION MESSAGEMOBILE STATION ACKNOWLEDGMENT The mobile acknowledges it has receivedthe Handoff Direction Message.
  45. 45. Page 45Handoff CaseBASE STATION ACKNOWLEDGMENTHANDOFF COMPLETION MESSAGE The mobile searcher quickly re-checks allthree PNs. It still hears their pilots! The mobile sends a Handoff CompletionMessage, confirming it still wants to goahead with the handoff. The base station confirms it has receivedthe mobile’s Handoff Completionmessage, and will continue with all of thelinks active.
  46. 46. Page 46Handoff CaseMOBILE STATION ACKNOWLEDGMENTNEIGHBOR LIST UPDATE MESSAGE In response to the mobile’s Handoff Completion Message, thebase station assembles a new composite neighbor list includingall the neighbors of each of the three active pilots. This is necessary since the mobile could be traveling toward anyone of these pilots and may need to request soft handoff with anyof them soon. The mobile confirms receiving theNeighbor List Update Message. It isalready checking the neighbor list and willdo so continuously from now on. The handoff is fully established.
  47. 47. Page 47Handoff CaseEc/IoAll PN Offsets0 512PN0-20T_ADDRake FingersŒReference PNActive Set168 500220ŽT_DROPMobile Rake RXSrch PN??? W0F1 PN168 W61F2 PN500 W50F3 PN220 W20
  48. 48. Page 48Questions What is Handoff in mobile network? What is the Search Window? How to optimize handoff parameters?

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