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  3G / WCDMA / UMTS  Thursday, 12 January 2012 Interview Question & Answer For 3G / WCDMA / UMTS UMTS interview questions:   1)   What are the RRC states?   a.   Cell DCH b.   Cell FACH c.   Cell PCH   d.   URA PCH   2)   Describe different RRC States?   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 wi ll 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.   3)   In which conditions UE will be in Cell FACH state?   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)   4)   What is the difference between Cell PCH and URA PCH state?   ·   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   5)   What is U-RNTI?   URNTI is a 32bit identity.   SRNC identifier + SRNTI makes URNTI   6)   Describe the CELL search procedure?   The Cell Search procedure can be summarised into the following steps:   a.   Cell Selection : Determine the cell whose received carrier power is the strongest   b.   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.   c.   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.   d.   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.   e.   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.   7)   Radio bearer configuration mappings?    ·   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).   8)   What are the types of hand over?   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: ·   Handover 3G -3G (i.e. between UMTS and other 3G systems) ·   FDD soft/softer handover ·   FDD inter-frequency hard handover ·   FDD/TDD handover (change of cell) ·   TDD/FDD handover (change of cell) ·   TDD/TDD handover ·   Handover 3G - 2G (e.g. handover to GSM) ·   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.   9)   What are types of measurements?   ·   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. 10)   What is paging? Why paging is required?   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.   11)   Type of Paging?   ·   Paging Type 1 ·   Paging Type 2(UE dedicated paging)   12)   When dedicated paging is used?   a.   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.   b.   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.   c.   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 .   13)   In what situation UE goes to CELL PCH and URA PCH states?   a.   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   b.   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   14)   What is Cell Update Procedure? What are various Cell Update causes?  ·   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  
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