LTE RF – Technical Interview Questions

Let’s Crack the Interviews … Technically.

Here is the list of questions, you may be asked during the technical interview of LTE RF planning and optimization job.

  1. Tell us about LTE Frame Structure?
  2. What are the Bandwidths used for LTE deployment?
  3. How Many subcarriers/RBs are there in 5/10/20 MHz channel
  4. What is Subcarrier Bandwidth in LTE?
  5. How can we calculate LTE DL/UL throughput?
  6. What Maximum LTE throughput can be achieved in the field?
  7. How many states a UE can have ?
  8. What is difference between Idle & Connected Mode
  9. What is difference between HO , Redirection, Cell Selection / Re-Selection?
  10. What is RRC Reconfiguration?
  11. What is Difference between MIB and SIB?
  12. Message flow during LTE Call?
  13. Can you tell us about Prach/PCI/Pusch/Pucch Planning?
  14. Why does RRC Connection Reestablishment trigger?
  15. Handover types in LTE
  16. What is difference between X2 and S1 HO?
  17. Explain Handover signalling messages?
  18. Explain Events measurements in LTE
  19. What are main reasons of HO failures? how do you solve them?
  20. What is CIO? hysteresis?Time to trigger?
  21. What is MRO?
  22. Which event trigger during the reselection from LTE to 3G and vice versa
  23. What is ANR?
  24. What are the types of ANR? Which one is better?
  25. What is QCI?
  26. What is LTE modulation?AMC?
  27. What is MIMO?
  28. What is TM? TM1? TM2? TM3? TM4?
  29. What is close loop MIMO?
  30. Mapping of MIB and SIB messages?
  31. Explain SIB messages?
  32. What is purpose of SIB19?
  33. What information PDCCH carries?
  34. What are DCI formats? Which DCI format used for Uplink allocation?
  35. Explain Resource allocation to PDCCH? What is CCE ?
  36. What does Aggregation Level mean ?
  37. Why do we need multiple aggregation levels?
  38. How EnodeB decides the Aggregation Level to use?
  39. What Message will be observed on layer-3 During handover stage?
  40. Which channel carries CQI information to enodB?
  41. What other info PUCCH carries?
  42. What is DRX?
  43. What is  UE disconnection Timer?
  44. What is the Location of PUCCH in the UL Spectrum?
  45. CS Fall back criteria and event?
  46. Explain CSFallback signalling?
  47. What basic parameters are required from Planning to configure a site?
  48. Explain PCI (Physical Cell id) Planning?
  49. How do you deal with Interference issues during PCI planning
  50. How do we do optimization of site?
  51. What are the Features available to improve accessibility, retainability?
  52. How does TA work in LTE?
  53. What is BLER? What’s Target value?
  54. How to investigate low throughput?
  55. What major KPIs have you been analyzing most?
  56. What do you know about LTE-A?
  57. What is carrier aggregation? Types of CA?
  58. What CA configuration? CA bandwidth class?
  59. What are PCC and SCC?
  60. what is difference between CSFB and VoLTE?
  61. Can you explain VoLTE call flow?
  62. How do you take care of VoLTE DCR?
  63. What QCIs are used for VoLTE?
  64. What is IMS? IMS Signalling?
  65. Does QCI1 and QCI5 setup simulateniously in VoLTE?
  66. Why QCI 9 and QCI 5 are always established and deactivated together?
  67. what is Semi-Persistent Scheduling (SPS) ?
  68. what is TTI Bundling?
  69. What is ROHC?
  70. What is SRVCC?
  71. what are the main issues you have faced in VoLTE? how do you solvethem?

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34 thoughts on “LTE RF – Technical Interview Questions

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    1. System information is transmitted via broadcast ch and that System information is in two parts 1.Static and 2. Daynamic ….ist one is called MIB and 2nd one is SIB…..MIB carries information like no of anteena,bandwith etc it is fixed thats why called static and send in every 40 ms….in daynamic there are total 13 SIB are available with diffrent no of information

    2. question 21:LTE provides different QoS for a given application by putting it into the different bearer . each bearer has its own QCI (QoS Class Identifier) which introduce a particular service for a particular application. as far as I know 9 different QCIs have been defined in LTE based on priority,packet loss and delay.

      QoS class identifier (QCI) is a scalar that is used as a reference to a specific packet forwarding behaviour (e.g. packet loss rate, packet delay budget) to be provided to a SDF. This may be implemented in the access network by the QCI referencing node specific parameters that control packet forwarding treatment (e.g. scheduling weights, admission thresholds, queue management thresholds, link layer protocol configuration, etc.), that have been pre-configured by the operator at a specific node(s) (e.g. eNodeB).

      The QoS architecture in EPC enables a number of important capabilities for both operators and users:

      •VoIP support with IMS. QoS is a crucial element for providing LTE/IMS voice service.
      •Enhanced application performance. Applications such as gaming or video can operate more reliably.
      •More flexible business models. With flexible, policy-based charging control, operators and third-parties will be able to offer content in creative new ways. For example, an enhanced video stream to a user could be paid for by an advertiser.
      •Congestion control. In congestion situations, certain traffic flows (e.g., bulk transfers, abusive users) can be throttled down to provide a better user experience for others.

      In LTE there are 9 different services based on QCI characteristics which are as below.

      Click image for larger version. Name: 23_203_Table_6_1_7.jpg Views: 12 Size: 23.7 KB ID: 3182

    3. que 11
      well reselection happens from idle mode to idle mode (i.e. is an idle mode procedure). Handover, on the other hand, takes place between connected modes (e.g. LTE RRC_CONNECTED to 3G RRC_CONNECTED/CELL_DCH). Redirection moves the UE from connected mode to idle mode (e.g. in many cases for CS Fallback the UE is redirected from LTE RRC_CONNECTED mode to 2G/3G idle mode).

      For example in case of CS Fallback a handover is not required (nor possibly even supported by the network and/or the UE) and in such case redirection is de facto the only possible mechanism to use.

      Furthermore, in some cases the redirection can be faster than handover due to many reasons (e.g. inter-RAT measurement delay/cell detection, L3 filtering, time-to-trigger, handover preparation in the network etc.), especially in cases where MO/MT call (which requires the UE to move to RRC_CONNECTED state and perform CSFB) is triggered from LTE idle mode.

    4. Answer of Question No.10:-

      RRC CONNECTION RECONFIGURATION message is the command to modify an RRC connection. Its prime purpose is
      – To establish/modify/release Radio Bearers
      – To perform Handover
      – To setup/modify/release Measurements
      – To add/modify/release SCells
      – Dedicated NAS Information might also be transferred from eNodeB to UE
      Differences with UMTS:
      – In LTE, RRC CONNECTION RECONFIGURATION is the only message used to perform all logical, transport, and physical channel configurations where as in case UMTS, there exists Transport Channel, Physical Channel and Radio Bearer Reconfiguration messages
      – In LTE, RRC CONNECTION RECONFIGURATION can also be used to send NASdedicated signalling to the UE to reduce the latency whereas this option is not available in UMTS
      – LTE has only one RRC connected mode state (RRC_CONNECTED), so it has only one Temporary Radio Network Temporary Identity (RNTI) i.e. C-RNTI
      – In UMTS, MEASUREMENT CONTROL is used to setup/modify/release measurements where as in LTE, RRC CONNECTION RECONFIGURATION message is the only message for this purpose

  1. hi khalid…very good appraoch..I can understand that you have spent lot of time…link other webpages so that it becomes coplete

  2. Question no 17…In LTE two type of handover is defined that is Inter LTE and IRAT..Event A1 to A5 belongs to Inter LTE handover in which A5 state that neighbour is above the threshold and serving is below the threshold…

    and B1 and B2 are IRAT handover…

  3. Ans for Que-14 – HO type in LTE
    • Intra-LTE: Handover happens within the current
    LTE nodes (intra-MME and Intra-SGW)
    • Inter-LTE: Handover happens toward the other
    LTE nodes (inter-MME and Inter-SGW)
    • Inter-RAT: Handover between different radio
    technology networks, for example GSM/UMTS
    and UMTS

  4. Q:42 BLER is BLocked ERror Ratio, the ratio of the no of bits transmitted and received, It should not be more than 10%, if it is more than 10 it will affect the Downlink and Uplink speed

  5. Answer 10:-
    RRC CONNECTION RECONFIGURATION message is the command to modify an RRC connection. Its prime purpose is
    – To establish/modify/release Radio Bearers
    – To perform Handover
    – To setup/modify/release Measurements
    – To add/modify/release SCells
    – Dedicated NAS Information might also be transferred from eNodeB to UE
    Differences with UMTS:
    – In LTE, RRC CONNECTION RECONFIGURATION is the only message used to perform all logical, transport, and physical channel configurations where as in case UMTS, there exists Transport Channel, Physical Channel and Radio Bearer Reconfiguration messages
    – In LTE, RRC CONNECTION RECONFIGURATION can also be used to send NASdedicated signalling to the UE to reduce the latency whereas this option is not available in UMTS
    – LTE has only one RRC connected mode state (RRC_CONNECTED), so it has only one Temporary Radio Network Temporary Identity (RNTI) i.e. C-RNTI
    – In UMTS, MEASUREMENT CONTROL is used to setup/modify/release measurements where as in LTE, RRC CONNECTION RECONFIGURATION message is the only message for this purpose

  6. 15. X2 HO occurs when source and target eNBs are served within the same MME pool.The procedures relies on the presence of X2 interface between S and T eNB AND S1-MME interface between the MME and the source eNodeB as well as between the MME and the target eNodeB. Also as part of handover execution, downlink and optionally also uplink packets are forwarded from the source eNodeB to the target eNodeB
    Whereas, S1 handover is when the X2 procedure fails(due to unreachability/Error response etc).

  7. Here the answer of question 8

    Idle mode means there is no call is in processing and dedicated mode call is in processing. ..which means in idle mode we only transmit but in dedicated mode we rescieve and transmit .

  8. Answer 38: pci which carry the reference signal here the formula for calculating the pci
    3×physical group id+physical I’d after calculating we get 503 is the range of pci
    Range of physical I’d 0 to 2
    Range of physical group I’d 0 to 167

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