Wideband Code Division Multiple Access (WCDMA) Interview Questions & Answers

  1. Question 1. What Are The Rrc States?

    Answer :

    • Cell DCH
    • Cell FACH
    • Cell PCH
    • URA PCH
  2. Question 2. Describe Different Rrc States?

    Answer :

    a. Cell DCH:

    • When UE is switched ON, it assumes the CELL_DCH State by default.
    • It exists when UE really requires UL/DL dedicated resource.
    • UE is characterized at cell level according to its current Active set.
    • Logical channels DCCH+DTCH are mapped to transport channel DCH.
    • E.g. Voice call(CS), Video telephony(CS+PS)

    b. Cell FACH:

    • This happens when,
    1. UE doesn’t require continuous circuit connection with n/w, but DL packets will work.
    2. UE is forced to release the n/w resources to decrease the congestion problems (flip-flop between DCH and FACH).
    3. Eventual location updates to support CELL_PCH and URA_PCH states (flip-flop between FACH and PCH).
    • E.g. Call on hold(CS), File downloading(PS)
    • No dedicated UL/DL is provided. However, UE will keep monitoring to DL common transport channel FACH ‘continuously’ and receives packet data meant for UE itself.
    • When UL transmission is required, it uses RACH for 1-2 frames or CPCH for few more frames, depending on common access procedures; still UL transmission will be ‘discontinuous’.
    • UE is known on cell level, but this time it’s based on the last cell update it made.

    c. Cell PCH:

    • This happens when UE doesn’t require an active connection(common/dedicated). But still needs to be connected, as it’s expecting a n/w initiated or terminated call(CS or PS) in future.
    • So, no UL transmission is allowed. Selects a PCH(based on algo.) and keep monitoring (receiving) it in ‘discontinuous’ mode(DRX).
    • Can commute only into CELL_FACH state.
    • E.g. No outgoing call or data receive for long time
    • UE is still identified on cell level, but now in the cell where UE made its last update, when it was in CELL_FACH state.
    • At regular time interval, UE would have to move in CELL_FACH state to update the location area. If UE is traveling, this update will become frequent because of changing cells.
    • In some cases UE can have the transition from CELL_DCH to CELL_PCH, but not vice versa; it has to go via CELL_FACH.

    d. URA PCH:

    • This can be considered as a general case of CELL_PCH state. Because all the characteristics are same except one.
    • That is, UE is known on URA(UTRAN Registration Area) level rather than cell level. It listens to PCH(and updates URA when required).
    • If UE is traveling continuously from one cell to another. It would have to transit multiple time to CELL_FACH state for location update. By assigning URA_PCH state, this condition is avoided, since 1 URA contains one or more cells and vice versa.
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  4. Question 3. In Which Conditions Ue Will Be In Cell Fach State?

    Answer :

    This happens when,

    • UE doesn’t require continuous circuit connection with n/w, but DL packets will work.
    • UE is forced to release the n/w resources to decrease the congestion problems (flip-flop between DCH and FACH).
    • Eventual location updates to support CELL_PCH and URA_PCH states (flip-flop between FACH and PCH).
      E.g. Call on hold(CS), File downloading(PS)
  5. Question 4. What Is The Difference Between Cell Pch And Ura Pch State?

    Answer :

    • UE is known on URA(UTRAN Registration Area) level rather than cell level
    • If UE is traveling continuously from one cell to another. It would have to transit multiple time to CELL_FACH state for location update. By assigning URA_PCH state, this condition is avoided, since 1 URA contains one or more cells and vice versa
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  7. Question 5. What Is U-rnti?

    Answer :

    URNTI is a 32bit identity.
    SRNC identifier + SRNTI makes URNTI

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  9. Question 6. Describe The Cell Search Procedure?

    Answer :

    The Cell Search procedure can be summarised into the following steps:

    1. Cell Selection: Determine the cell whose received carrier power is the strongest
    2. Slot Synchronisation: the UE reads the P-SCH of the strongest carrier and feeds the received signal through a matched filter (that is matched to the Primary Synchronisation Code). The slot timing is detected via detection of peaks in the output of the matched filter.
    3. Frame Synchronisation and Scrambling Code Group Identification: the UE reads the S-SCH of the strongest carrier, and correlates the received signal with all the possible 64 Secondary Synchronisation Codes. The output that gives maximum correlation gives the group of the Primary Scrambling Code used in the cell. This process also gives the frame timing since the Secondary Synchronisation Code is different for each slot and the group tells the mapping between slots and SSCs.
    4. Scrambling Code Identification: The UE attempts then to read the P-CPICH so as to extract the Primary Scrambling Code used in the cell. The P-CPICH is always scrambled by the Primary Scrambling Code of the cell and is spread a factor 256 and an OVSF whose index is zero. Since the Primary Scrambling Codes are grouped into 64 groups of 8 Scrambling Codes and the S-SCH has given the Scrambling Code group only 8 possibilities remain. So the P-CPICH is unscrambled separately withevery possible Scrambling Code (8) in the group and only one output will produce correct results and the Primary Scrambling Code of the cell is determined.
    5. Read Broadcast Information: Since the P-CCPCH is always scrambled by the Primary Scrambling Code of the cell (which has now been determined) and is always spread using SF=256 with OVSF index=1, The UE can read the P-CCPCH without any problem. The P-CCPCH carries the BCH where system information blocks are broadcast throughout the entire cell. The UE can acquire at this stage various information (e.g. Spreading Factors, OVSF indexes and Scrambling Codes used in other downlink channels, important

    parameters for PRACH transmission, Measurement Parameters, Transport Formats etc.) and can then decodeother downlink channels, initiate a PRACH procedure, performs some specific measurements etc.

  10. Question 7. Radio Bearer Configuration Mappings?

    Answer :

    • Signalling radio bearer RB0 shall be used for all messages sent on the CCCH (UL: RLC-TM, DL: RLC-UM).
    • Signalling radio bearer RB1 shall be used for all messages sent on the DCCH, when using RLC unacknowledged mode (RLC-UM).
    • Signalling radio bearer RB2 shall be used for all messages sent on the DCCH, when using RLC acknowledged mode (RLC-AM), except for the RRC messages carrying higher layer (NAS) signalling.
    • Signalling radio bearer RB3 and optionally Signalling radio bearer RB4 shall be used for the RRC messages carrying higher layer (NAS) signalling and sent on the DCCH in RLC acknowledged mode (RLC-AM).
    • Additionally, RBs whose identities shall be set between 5 and 32 may be used as signalling radio bearer for the RRC messages on the DCCH sent in RLC transparent mode (RLC-TM).
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  12. Question 8. What Are The Types Of Hand Over?

    Answer :

    Here are following categories of handover (also referred to as handoff):

    • Hard Handover: Hard handover means that all the old radio links in the UE are removed before the new radio links are established. Hard handover can be seamless or non-seamless. Seamless hard handover means that the handover is not perceptible to the user. In practice a handover that requires a change of the carrier frequency (inter-frequency handover) is always performed as hard handover.
    • Soft Handover: Soft handover means that the radio links are added and removed in a way that the UE always keeps at least one radio link to the UTRAN. Soft handover is performed by means of macro diversity, which refers to the condition that several radio links are active at the same time. Normally soft handover can be used when cells operated on the same frequency are changed.
    • Softer handover: Softer handover is a special case of soft handover where the radio links that are added and removed belong to the same Node B (i.e. the site of co-located base stations from which several sector-cells are served. In softer handover, macro diversity with maximum ratio combining can be performed in the Node B, whereas generally in soft handover on the downlink, macro diversity with selection combining is applied.
    • Generally we can distinguish between intra-cell handover and inter-cell handover. For UMTS the following types of handover are specified:
    1. Handover 3G -3G (i.e. between UMTS and other 3G systems)
    2. FDD soft/softer handover
    3. FDD inter-frequency hard handover
    4. FDD/TDD handover (change of cell)
    5. TDD/FDD handover (change of cell)
    6. TDD/TDD handover
    7. Handover 3G – 2G (e.g. handover to GSM)
    8. Handover 2G – 3G (e.g. handover from GSM)

    The most obvious cause for performing a handover is that due to its movement a user can be served in another cell more efficiently (like less power emission, less interference). It may however also be performed for other reasons such as system load control.

  13. Question 9. What Are Types Of Measurements?

    Answer :

    • Intra-frequency measurements: measurements on downlink physical channels at the same frequency as the active set. A measurement object corresponds to one cell.
    • Inter-frequency measurements: measurements on downlink physical channels at frequencies that differ from the frequency of the active set and on downlink physical channels in the active set. A measurement object corresponds to one cell.
    • Inter-RAT measurements: measurements on downlink physical channels belonging to another radio access technology than UTRAN, e.g. GSM. A measurement object corresponds to one cell.
    • Traffic volume measurements: measurements on uplink traffic volume. A measurement object corresponds to one cell.
    • Quality measurements: Measurements of downlink quality parameters, e.g. downlink transport block error rate. A measurement object corresponds to one transport channel in case of BLER. A measurement object corresponds to one timeslot in case of SIR (TDD only).
    • UE-internal measurements: Measurements of UE transmission power and UE received signal level.
    • UE positioning measurements: Measurements of UE position.
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  15. Question 10. What Is Paging? Why Paging Is Required?

    Answer :

    Paging: This procedure is used to transmit paging information to selected UEs in idle mode, CELL_PCH or URA_PCH state using the paging control channel (PCCH).

    Paging can be initiated :

    • Upper layers in the network may request paging, to e.g. establish a signaling connection.
    • UTRAN may initiate paging for UEs in CELL_PCH or URA_PCH state to trigger a cell update procedure.
    • UTRAN may initiate paging for UEs in idle mode, CELL_PCH and URA_PCH state to trigger reading of updated system information.
    • UTRAN may also initiate paging for UEs in CELL_PCH and URA_PCH state to release the RRC connection.
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  17. Question 11. Type Of Paging?

    Answer :

    • Paging Type 1
    • Paging Type 2(UE dedicated paging)
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  19. Question 12. When Dedicated Paging Is Used?

    Answer :

    1. This procedure is used to transmit dedicated paging information to one UE in connected mode in CELL_DCH , CELL_FACH or CELL_PCH (FDD only) state. Upper layers in the network may request initiation of paging.
    2. For a UE in CELL_DCH, CELL_FACH or CELL_PCH (FDD only) state, UTRAN initiates the procedure by transmitting a PAGING TYPE 2 message on the DCCH using AM RLC. When not stated otherwise elsewhere, the UTRAN may initiate the UE dedicated paging procedure also when another RRC procedure is ongoing, and in that case the state of the latter procedure shall not be affected.
    3. UTRAN should set the IE “Paging cause” to the cause for paging received from upper layers. If no cause for paging is received from upper layers, UTRAN should set the value “Terminating – cause unknown”.
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  21. Question 13. In What Situation Ue Goes To Cell Pch And Ura Pch States?

    Answer :

    • Cell PCH: This happens when UE doesn’t require an active Connection(common/dedicated). But still needs to be connected, as it’s expecting a n/w initiated or terminated call(CS or PS) in future
    • URA PCH: In Cell PCH UE is still identified on cell level, but now in the cell where UE made its last update, when it was in CELL_FACH state.At regular time interval, UE would have to move in CELL_FACH state to update the location area. If UE is traveling, this update will become frequent because of changing cells. If UE is traveling continuously from one cell to another. It would have to transit multiple time to CELL_FACH state for location update. By assigning URA_PCH state, this condition is avoided, since 1 URA contains one or more cells and vice versa
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  23. Question 14. What Is Cell Update Procedure? What Are Various Cell Update Causes?

    Answer :

    Uplink data transmission:

    For FDD, if the variable H_RNTI is not set, and for TDD:

    • if the UE is in URA_PCH or CELL_PCH state; and
    • if the UE has uplink RLC data PDU or uplink RLC control PDU on RB1 or upwards to transmit: perform cell update using the cause “uplink data transmission“.

    Paging response:

    • if the criteria for performing cell update with the cause specified above. are not met; and
    • if the UE in URA_PCH or CELL_PCH state, receives a PAGING TYPE 1 .message fulfilling the conditions for initiating a cell update procedure. perform cell update using the cause “paging response“.

    Radio link failure:

    • if none of the criteria for performing cell update with the causes specified above in the current subclause is met:
    • if the UE is in CELL_DCH state and the criteria for radio link failure are ,or
    • if the transmission of the UE CAPABILITY INFORMATION message fails. perform cell update using the cause “radio link failure“.

    MBMS ptp RB request:

    • if none of the criteria for performing cell update with the causes specified above in the current subclause is met; and
    • if the UE is in URA_PCH, Cell_PCH or Cell_FACH state; and
    • if the UE should perform cell update for MBMS ptp radio bearer request, perform cell update using the cause “MBMS ptp RB request“.

    Re-entering service area:

    • if none of the criteria for performing cell update with the causes specified above in the current subclause is met; and
    • if the UE is in CELL_FACH or CELL_PCH state; and
    • 2> if the UE has been out of service area and re-enters service area before T307 or T317 expires:
    • 3> perform cell update using the cause “re-entering service area“.

    RLC unrecoverable error:

    • if none of the criteria for performing cell update with the causes specified above in the current subclause is met; and
    • if the UE detects RLC unrecoverable error [16] in an AM RLC entity: perform cell update using the cause “RLC unrecoverable error“.

    Cell reselection:

    • if none of the criteria for performing cell update with the causes specified above in the current subclause is met:
    • if the UE is in CELL_FACH or CELL_PCH state and the UE performs cell re-selection; or
    • if the UE is in CELL_FACH state and the variable C_RNTI is empty: perform cell update using the cause “cell reselection“.

    Periodical cell update:

    • if none of the criteria for performing cell update with the causes specified above in the current subclause is met; and
    • if the UE is in CELL_FACH or CELL_PCH state; and
    • if the timer T305 expires; and
    • if the criteria for “in service area” and
    • if periodic updating has been configured by T305 in the IE “UE Timers and constants in connected mode” set to any other value than “infinity”: perform cell update using the cause “periodical cell update“.

    MBMS reception:

    • if none of the criteria for performing cell update with the causes specified above in the current subclause is met; and
    • if the UE is in URA_PCH, Cell_PCH or Cell_FACH state; and
    • if the UE should perform cell update for MBMS counting , perform cell update using the cause “MBMS reception“.
  24. Question 15. What Is Active Set?

    Answer :

    Active Set is defined as the set of Node-Bs the UE is simultaneously connected to (i.e., the UTRA cells currently assigning a downlink DPCH to the UE constitute the active set).

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  26. Question 16. What Is Monitor Set And Detected Set Cells?

    Answer :

    • Cells, which are not included in the active set, but are included in the CELL_INFO_LIST belong to the Monitored Set.
    • Cells detected by the UE, which are neither in the CELL_INFO_LIST nor in the active set belong to the Detected Set. Reporting of measurements of the detected set is only applicable to intra-frequency measurements made by UEs in CELL_DCH state.
  27. Question 17. What Are The Various Types Of Rnc?

    Answer :

    • Serving RNC
    • Drift RNC
    • Controlling RNC
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  29. Question 18. What Is The Use And Difference Between Crnc, Drift Rnc And Srnc?

    Answer :

    RNC from which the UE is currently getting Served or connected to is called SRNC or Serving RNC.

    During a soft handover procedure the RNC UE is moving to, is called Drift RNC.

    The RNC which controls the movement of UE from Serving RNC to the Drift RNC is called Controlling RNC.

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  31. Question 19. What Is Initial Direct Transfer Message? What Is The Use?

    Answer :

    The initial direct transfer procedure is used in the uplink to establish a signaling connection. It is also used to carry an initial upper layer (NAS) message over the radio interface.

  32. Question 20. What Is Security Mode Command? Why It Is Used?

    Answer :

    Security mode command is a RRC Message,

    The purpose of this procedure :

    • To trigger the start of ciphering or to command the restart of the ciphering with a new ciphering configuration, for the radio bearers of one CN domain and for all signalling radio bearers.
    • It is also used to start integrity protection or to modify the integrity protection configuration for all signalling radio bearers
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  34. Question 21. What Is Cctrch ?

    Answer :

    A Coded Composite Transport Channel is the result of combining (multiplexing) several Transport Channels of the same type; i.e.: we can have a CCTrCh made up of one or several DCHs, a CCTrCh made up of DSCHs. One exception is that we can have one or several FACHs multiplexed with a PCH on the same CCTrCh.

    A better notation would be for example:

    • CCTrCh4DCH: A CCTrCh consisting of 4 DCHs
    • CCTrCh3FACH: A CCTrCh consisting of 3 FACHs
    • CCTrChPCH,2FACH: A CCTrCh consisting of the PCH and 2 FACHs
  35. Question 22. What Is Compressed Mode?

    Answer :

    Higher layers can request a UE to monitor cells on other frequencies (FDD, TDD), or other systems (GSM). This implies that the UE has to perform measurements, on the other frequencies/systems and thus higher layers command the UE to enter Compressed Mode. The UTRAN need to send to the UE all the parameters for the Compressed Mode.

    Compression Methods:

    When in Compressed Mode the information normally transmitted during a 10 ms frame is compressed in time sothat gaps of adequate length are created. To achieve this one of three mechanisms

    • Puncturing: This method is NOT applicable to downlink.
    • Spread Factor Reduction: Actually halving the Spread Factor. So with this method if SF is used in non-compressed frames then SF/2 is used in the compressed frames.
    • Higher Layer Scheduling: Higher Layers set restrictions so that only a subset of allowed TFCs are used in Compressed Mode.

    Compressed Mode Parameter:

    TGSN (Transmission Gap Starting Slot Number): A transmission gap pattern begins in a radio frame, called firstradio frame of the transmission gap pattern, containing at least one transmission gap slot. TGSN is the slot numberof the first transmission gap slot within the first radio frame of the transmission gap pattern;

    • TGL1 (Transmission Gap Length 1): Duration of the first transmission gap within the transmission gap pattern, expressed in number of slots;
    • TGL2 (Transmission Gap Length 2): Duration of the second transmission gap within the transmission gap pattern, expressed in number of slots. If this parameter is not explicitly set by higher layers, then TGL2 =TGL1;
    • TGD (Transmission Gap Start Distance): Duration between the starting slots of two consecutive transmission gaps within a transmission gap pattern, expressed in number of slots. The resulting position of the second transmission gap within its radio frame(s) shall comply with the limitations of TS 25.101 (Ref [2]). If this parameter is not set by higher layers, then there is only one transmission gap in the transmission gap pattern;
    • TGPL1 (Transmission Gap Pattern Length): Duration of transmission gap pattern 1;
    • TGPL2 (Transmission Gap Pattern Length): Duration of transmission gap pattern 2. If this parameter is not explicitly set by higher layers, then TGPL2 = TGPL1.

    The following two parameters (integers) control the transmission gap pattern sequence start and repetition:

    1. TGPRC (Transmission Gap Pattern Repetition Count): Number of transmission gap patterns within the transmission gap pattern sequence; From Figure 10 it seems TGPRC is even, therefore the number of slots in TG Sequence is: 0.5*TGPRC (TGPL1+TGPL2)
    2. TGCFN (Transmission Gap Connection Frame Number): CFN of the first radio frame of the first pattern 1 within the transmission gap pattern sequence.
  36. Question 23. What Is Fach Measurement Occasion?

    Answer :

    GAPs in S-CCPCH to perform measurement on Different Frequencies or Interrat resection.

  37. Question 24. Mib/sb/sib Details?

    Answer :

    MIB: The Scheduling of MIB is Static.

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  39. Question 25. Why We Perform Channelisation Or Spreading Before Scrambling?

    Answer :

    Before sending a data we need to identify the channel to send the data. A particular Channelisation code is the “identity” for the channel.

    Hence we use Channelisation code to select the channel and then scramble it so as to send the identity of the UE over the channel

  40. Question 26. When Tmsi , P- Tmsi , Imsi , Imei Used?

    Answer :

    • TMSI (GSM-MAP): The TMSI (GSM-MAP) shall be chosen if available. The IE “LAI” in the IE “Initial UE identity” shall also be present when TMSI (GSM-MAP) is used, for making it unique.
    • P-TMSI (GSM-MAP): The P-TMSI (GSM-MAP) shall be chosen if available and no TMSI (GSM-MAP) is available. The IE “RAI” in the IE “Initial UE identity” shall in this case also be present when P-TMSI (GSM-MAP) is used, for making it unique.
    • IMSI (GSM-MAP): The IMSI (GSM-MAP) shall be chosen if available and no TMSI (GSM-MAP) or P-TMSI is available.
    • IMEI: The IMEI shall be chosen when none of the above three conditions are fulfilled.
  41. Question 27. Why We Do A Combined Attach?

    Answer :

    If the network operates in Network Mode Of Operation I i.e. NMO I and GPRS MS that operates in Mode A or Mode B will do a simultaneous IMSI attach for GPRS and non GPRS services and shall use the Combined GPRS attach, combined and periodic routing area updating procedures instead of the corresponding MM specific procedures IMSI attach and normal and periodic Location area updating.

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  43. Question 28. When Ue Triggers Event 3a Measurement Report?

    Answer :

    Event 3A:The Estimated Quality of currently used UTRAN frequency is below a certain threshold and the estimated quality of the other system is above a certain threshold.

  44. Question 29. How Many Transport Channels Will Be Configured When A Csv And A Psd Call Is Simultaneously Going On?

    Answer :

    Five transport channels will be configured.

    1 for 4 SRBs
    3 for CSV
    1 for PSD

  45. Question 30. When Event 2a, 2b, 2c,2e Occurs?

    Answer :

    • 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
  46. Question 31. When Event 1a – 1f Event Occurs?

    Answer :

    • event 1A: A Primary CPICH enters the reporting range
    • event 1B: A primary CPICH leaves the reporting range
    • event 1C: A non-active primary CPICH becomes better than an active primary CPICH
    • event 1D: Change of best cell
    • event 1E: A Primary CPICH becomes better than an absolute threshold
    • event 1F: A Primary CPICH becomes worse than an absolute threshold
  47. Question 32. Where Ck And Ik Gets Generated?

    Answer :

    In RRC CK and IK gets generated.

  48. Question 33. Where Ck And Ik Gets Stored?

    Answer :

    In USIM CK and IK will get stored.

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  50. Question 34. Where The Security Procedure Will Start?

    Answer :

    Security Procedure will start in RRC but Ciphering will start for AM and UM data security procedure will start at RLC layer. And for TM mode security procedure start at MAC layer.

  51. Question 35. What Are The Mandatory Sib’s Which Ue Has To Read Before Campaign?

    Answer :

    SIB 1, 3,5,7,11

  52. Question 36. What Are The Important Rach Parameters And Which Sib Contains This Information?

    Answer :

    SIB 7/5

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  54. Question 37. Which Sib Contains S Criteria Information?

    Answer :

    SIB 3