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12 LTE interview questions and answers

LTE and LTE advanced technology is fast evolving in cellular arena and demand in the industries have been increased for LTE skilled engineers. These top 12 LTE interview questions and answers help engineers seeking LTE technology job to crack the interview with ease. One can refer page links mentioned on left side panel to learn more about LTE. These questions are very useful as viva questions also.

Question-1: What is the difference between LTE FDD and LTE TDD?
Answer-1:The difference lies in the LTE frame structure in both the FDD and TDD versions of the LTE. In FDD there will be pair of frequencies assigned in the downlink and uplink directions and hence transmissions from multiple subscribes can happen at the same time but on different frequencies as mentioned. In TDD, one single frequency will be used at different time instants by multiple subscriber terminals (UEs). Both frame versions of LTE will have 1 ms sub-frame duration and 0.5 ms slot duration. Read more.

Question-2: What is resource block in LTE?
Answer-2:LTE frame is divided based on time slots on time axis and frequency subcarrier on frequency axis. Resource block is the smallest unit of resource allocation in LTE system. It is of about 0.5ms duration and composed of 12 subcarriers in 1 OFDM symbol. One time slot is equal to 7 OFDM symbols in normal cyclic prefix and 6 OFDM symbols in extended cyclic prefix. One full resource block is equal to 12 subcarriers by 7 symbols in normal CP. Hence it consists of total 84 time/frequency elements referred as resource elements in LTE network. Refer LTE Terminology

Question-3: What are the LTE logical, transport and physical channels? Answer-3:All these channels help LTE UE establish the connection with the eNodeB, maintain the connection and terminate the same. Logical channels are characterized by the information that is transferred. Transport channels are characterized by how the data are transferred over the radio interface. Physical channel corresponds to a set of resource elements used by the physical layer. Channels are further divided into control channel and traffic channel at logical channel stage. Read more.

Question-4: Explain the difference between Reference signal (RS) and synchronization signal (SS) in the LTE? Also mention types of RS and SS.
Answer-4:Reference signal (RS) is used as pilot subcarrier in LTE similar to other broadband wireless technologies such as WLAN, WIMAX etc. Synchronization signal is used as preamble sequence in LTE for synchronization purpose. RS is used for channel estimation and tracking. SS are of two types viz. P-SS and S-SS. P-SS is used for initial synchronization. S-SS is used for frame boundary determination. RS are of two types viz. Demodulation RS (DRS) and Sounding RS (SRS). DRS is used for sync and channel estimation purpose. SRS is used for channel quality estimation purpose. DRS is used in both the uplink and downlink, while SRS is used only in the uplink. Refer LTE PSS SSS
pages to know insight concepts of synchronization signal and reference signal.

LTE PSS vs SSS or primary SCH secondary SCH

Question-5: Explain LTE cell search procedure followed by UE.
Answer-5:LTE cell search procedure is used by UE to camp onto the LTE cell i.e. eNodeB. Refer LTE UE cell search procedure and network entry procedure.

Question-6: What is the function of LTE physical broadcast channel i.e. PBCH?
Answer-6:After initial cell synchronization is completed, UE reads MIB (Master information block) on PBCH (Physical channel). Broadcast channel is referred as BCH at transport level and BCCH at logical level. MIB composed of downlink channel bandwidth in units of RBs, PHICH duration, PHICH resource and system frame number. Read more.

Question-7: What is the advantage of using SC-FDMA in the LTE uplink?
Answer-7:The main advantage of SC-FDMA is low PAPR compare to OFDMA used in LTE downlink. This increases the efficiency of power amplifier and hence increases the battery life. Read more.

Question-8: What is RSSI?
Answer-8:RSSI stands for Received Signal Strength Indication. It is used almost in all the RATs to identify power received from the cell in idle as well as connected/dedicated modes. This helps UE always camped on to the best cell all the time. In case of drop in power measured using RSSI, either UE or network initiates the handover or cell re-selection is carried out. Read more.

Question-9: Explain Circuit Switch Fall Back i.e. CSFB with respect to LTE and GSM.
Answer-9:Framework allowing the provisioning of voice services by reuse of legacy GSM served CS infrastructure when the UE is served by E-UTRAN (LTE).To provide voice call support, Circuit Switch Fall Back is carried out to GSM RAT from LTE RAT to facilitate the voice over LTE (VoLTE) feature. Read more.

LTE-interview questions and answers

Question-10: Explain LTE network architecture and various interfaces.
Answer-10:There are various entities forming the LTE network architecture, the main interfaces are Uu between UE and eNB, X2 interface between eNBs and S1 interface between eNB and EPC(Evolved Packet Core). Read more.

Question-11: What is SRVCC?
Answer-11:SRVCC is the short form of Single-Radio Voice Call Continuity. SRVCC handover is supported from E-UTRAN (i.e. LTE) to UTRAN/GERAN (WCDMA/GSM). SRVCC procedure is used for transferring an on-going PS voice call (IMS) in LTE to a CS voice call via Handover from LTE to GERAN/UTRAN. Read more.

Question-12:What is the difference between LTE and LTE Advanced?
Answer-12:LTE is specified in 3GPP release 8 and release 9. LTE advanced is specified in 3GPP release 10. The main difference between them is carrier aggregation is introduced in LTE advanced. Number of antennas supported by MIMO has been increased to 8 in LTE advanced,Read more.

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