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Saturday, 9 November 2013

LTE Interview Questions?

1)     What happens when a LTE UE is powered on? From PHY Layer Point of view & NAS Point of view?
2)     Explain attach procedure in LTE?
3)     Why there is two types of security in LTE?
4)     What are the measurement events in LTE?
Ans: 
Intra/Inter Frequency Events:

Event A1 (Serving becomes better than threshold) 
Event A2 (Serving becomes worse than threshold) 
Event A3 (Neighbour becomes offset better than PCell) 
Event A4 (Neighbour becomes better than threshold) 
Event A5 (PCell becomes worse than threshold1 and neighbour becomes better than threshold2) 
Event A6 (Neighbour becomes offset better than SCell) 

Inter RAT Events:

Event B1 (Inter RAT neighbour becomes better than threshold) 
Event B2 (PCell becomes worse than threshold1 and inter RAT neighbour becomes better than
threshold2)
5)     What is DCI?
6)     What are the contents of DCI?
7)     What are the main difference between DCI0 and DCI1a?
8)     What is contention resolution?
9)     When Radio Link Failure is detected?
Ans:
 Radio link failure to be detected:
  1) upon T310 expiry
  2) upon random access problem indication from MAC while neither T300, T301, T304 nor T311 is running
  3) upon indication from RLC that the maximum number of re-transmissions has been reached

10)   What is SRS used for?
Ans: UL reference signal used to measure the channel quality over a section of the bandwidth.
Node B use this information for frequency selective scheduling and link adaptation decisions.
11)   What is DMRS/DRS?
Ans: DMRS/DRS is uplink reference signal.
     Used for : 1)Channel Estimation and synchronization in UL
                2)EnodeB can use DMRS for calculating TA command for each UE.
     Two Types: 1) PUSCH DMRS.
                2) PUCCH DMRS.
     PUSCH DMRS
                1) Included in every resource block allocated to UE for PUSCH transmission.
                2) Distributed only in Frequency domain to preserve the PAPR characteristic of SC-FDMA.
                3) 12 Resource element per resource block allocated to PUSCH DMRS.
     PUCCH DMRS
                1) Included in every resource block allocated to UE for PUCCH transmission(if transmitted).PUCCH occupies 2 resource block per 1 ms subframe when transmitted.
                2) No of REs used for PUCCH DMRS depends on a) PUCCH format to be transmitted and whether b) normal or extended cyclic prefix used.
                3) PUCCH DRMS used more no of bits in case of format 1,1a,1b and less no of bits in caseof format 2, 2a, 2b.
13)   What is DRX?
14)   Explain Connected mode DRX and Idle mode DRX?
15)   Why PHICH configuration is mentioned in MIB?
16)   In what are the scenario RACH is triggered?
17)   What is RACH Procedure?
18)   How UE come to know which RACH Preamble to USE?
19)   Why there is no SOFT HO in LTE?
20)   What PLMN Selection Order UE follows during  Automatic PLMN selection and Manual PLMN Selection?
21)   What is Timing Advance? What happens if Timing Advance Timer Expires?
Ans: The timing of UL radio frame is relative to DL radio frame. EnB provides timing advance command to each UE such that all UL transmissions arrive at the eNodeB in synchronous manner.

   If TA timer expires UE goes of reestablishment procedure or move to idle. 
22)   What is SR? What is the use of SR? 
23)   What is MAC CE? 
24)   What is BackOff Indicator? What is the use of Backoff indicator?
Ans : 
     Backoff Indicator is a special MAC subheader that carries the parameter indicating the time delay between a PRACH and the next PRACH.

if the Random Access Response contains a Backoff Indicator subheader
   set the backoff parameter value in the UE as indicated by the BI field of the Backoff Indicator subheader
else, 
   set the backoff parameter value in the UE to 0 ms.


25)   What is BSR?
      Ans: The Buffer Status reporting procedure is used to provide the serving eNB with information about the amount of data available for transmission in the UL buffers of the UE.

26)   At what scenario UE triggers BSR?
Ans: 
  • UL data, for a logical channel which belongs to a LCG, becomes available for transmission in the RLC entity or in the PDCP entity and either the data belongs to a logical channel with higher priority than the priorities of the logical channels which belong to any LCG and for which data is already available for transmission, or there is no data available for transmission for any of the logical channels which belong to a LCG, in which case the BSR is referred below to as "Regular BSR";
  • UL resources are allocated and number of padding bits is equal to or larger than the size of the Buffer Status Report MAC control element plus its subheader, in which case the BSR is referred below to as "Padding BSR"
  •  retxBSR-Timer expires and the UE has data available for transmission for any of the logical channels which belong to a LCG, in which case the BSR is referred below to as "Regular BSR"
  • periodicBSR-Timer expires, in which case the BSR is referred below to as "Periodic BSR".

27)   When different types of BSR are Triggered?
Ans:
For Regular and Periodic BSR:

 if more than one LCG has data available for transmission in the TTI where the BSR is transmitted
      report Long BSR
 else,
      report Short BSR.

For Padding BSR:

if the number of padding bits is equal to or larger than the size of the Short BSR plus its subheader but smaller than the size of the Long BSR plus its subheader:
       if more than one LCG has data available for transmission in the TTI where the BSR is transmitted: report Truncated BSR of the LCG with the highest priority logical channel with data available for transmission;
      else
      report Short BSR.
 else if the number of padding bits is equal to or larger than the size of the Long BSR plus its subheader,         
      report Long BSR.

28)   What is the use of system info modification period?
29)   What is the content of RAR?
Ans: 
A MAC RAR consists of the four fields
  •          R
  •          Timing Advance Command
  •          UL Grant
  •          Temporary C-RNTI


30) What is the USE of UE specific Reference signal?
31) What is Cell Specific Reference Signal?
32) In what are the scenario UE Triggers RRC Connection Reestablishment?
Ans: 
      UE Triggers RRC Connection Reestablishment procedure on following condition: 
  • Upon detecting Radio Link Failure
  • Handover Failure
  • Mobility From E-UTRA Failure 
  • Integrity Failure Indication Received From Lower Layers
  • RRC Connection Reconfiguration Failure
      33) What is GUTI?
      34) What is the significance of S-TMSI? 
      35) What is the content of Paging Message?
      36) When UE activates integrity and ciphering?
Ans:
  • The SECURITY MODE COMMAND message is used to command the UE for the activation of AS security. E-UTRAN always initiates this procedure prior to the establishment of Signalling Radio Bearer2 (SRB2) and Data Radio Bearers (DRBs).
  • AS security comprises of the integrity protection of RRC signalling (SRBs) as well as the ciphering of RRC signalling (SRBs) and user plane data (DRBs). The integrity protection algorithm is common for signalling radio bearers SRB1 and SRB2. The ciphering algorithm is common for all radio bearers (i.e. SRB1, SRB2 andDRBs). Neither integrity protection nor ciphering applies for SRB0.
  • The eNodeB sends integrity protected SECURITY MODE COMMAND message to the UE. The UE shall derive KeNB and KRRCint which is associated with integrity protection algorithm indicated in the SECURITY MODE COMMAND. Then, UE verifies the Integrity of the received SECURITY MODE COMMAND by checking the Message Authentication Code (MAC) in the SECURITY MODE COMMAND message. If the SECURITY MODE COMMANDmessage fails the integrity protection check, then the UE sends SECURITY MODE FAILURE to the eNodeB.
  • If the SECURITY MODE COMMAND passes the integrity protection check, then the UE shall derive the encryption keys KRRCenc key and the KUPenc keys associated with the ciphering algorithm indicated in theSECURITY MODE COMMAND.
  • The UE shall apply integrity protection using the indicated algorithm (EIA) and the integrity key, KRRCintimmediately, i.e. integrity protection shall be applied to all subsequent messages received and sent by the UE, including the SECURITY MODE COMPLETE message.
  •  The UE shall apply ciphering using the indicated algorithm (EEA), KRRCenc key and the KUPenc key after completing the procedure, i.e. ciphering shall be applied to all subsequent messages received and sent by the UE, except for the SECURITY MODE COMPLETE message which is sent un-ciphered.
36) How many default and dedicated bearer possible in lte?
37) Can there be multiple default bearer to same PDN?
38) How the position of each SIB is calculated in LTE?
39) How measurement GAP calculation happens in LTE?



      

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