Top LTE Interview Questions and Answers

LTE and LTE Advanced technology are rapidly evolving in the cellular domain, leading to increased industry demand for skilled LTE engineers. This compilation of top 12 LTE interview questions and answers aims to assist engineers seeking LTE technology jobs in acing their interviews.

Question 1: What is the difference between LTE FDD and LTE TDD?

Answer: The key difference lies in the LTE frame structure used by each technology.

• LTE FDD (Frequency Division Duplex): Employs paired frequencies for downlink and uplink transmissions. This allows simultaneous transmissions from multiple subscribers, each using a different frequency.

• LTE TDD (Time Division Duplex): Utilizes a single frequency, with different time slots allocated for uplink and downlink transmissions by multiple user equipments (UEs).

Both LTE frame versions have a 1 ms sub-frame duration and a 0.5 ms slot duration.

Question 2: What is a resource block in LTE?

Answer: An LTE frame is divided based on time slots (time axis) and frequency subcarriers (frequency axis). A resource block is the smallest unit of resource allocation in the LTE system. It has a duration of 0.5 ms and consists of 12 subcarriers within 1 OFDM symbol.

• One time slot equals 7 OFDM symbols in a normal cyclic prefix and 6 OFDM symbols in an extended cyclic prefix.
• A full resource block is comprised of 12 subcarriers by 7 symbols (normal CP).

Therefore, it consists of a total of 84 time/frequency elements, known as resource elements in the LTE network.

Question 3: What are the LTE logical, transport, and physical channels?

Answer: These channels facilitate the establishment, maintenance, and termination of a connection between an LTE UE and the eNodeB.

• Logical Channels: Characterized by the type of information being transferred.
• Transport Channels: Characterized by how the data is transferred over the radio interface.
• Physical Channels: Correspond to a set of resource elements used by the physical layer.

At the logical channel stage, channels are further categorized into control channels and traffic channels.

Question 4: Explain the difference between Reference Signal (RS) and Synchronization Signal (SS) in LTE. Also, mention the types of RS and SS.

Answer:

• Reference Signal (RS): Functions as a pilot subcarrier in LTE, similar to other broadband wireless technologies like WLAN and WiMAX. It is used for channel estimation and tracking.

• Synchronization Signal (SS): Serves as a preamble sequence in LTE for synchronization purposes.

Types of SS:

• P-SS (Primary Synchronization Signal): Used for initial synchronization.
• S-SS (Secondary Synchronization Signal): Used for determining frame boundaries.

Types of RS:

• Demodulation RS (DRS): Used for synchronization and channel estimation purposes in both uplink and downlink.
• Sounding RS (SRS): Used for channel quality estimation purposes, exclusively in the uplink.

Question 5: Explain the LTE cell search procedure followed by a UE.

Answer: The LTE cell search procedure is used by the UE to connect to (camp onto) an LTE cell, i.e., an eNodeB.

Question 6: What is the function of the LTE physical broadcast channel (PBCH)?

Answer: After initial cell synchronization is complete, the UE reads the MIB (Master Information Block) on the PBCH (Physical Broadcast Channel).

• At the transport level, the broadcast channel is referred to as BCH, and at the logical level, it’s referred to as BCCH.
• The MIB contains information such as the downlink channel bandwidth (in units of RBs), PHICH duration, PHICH resource, and the system frame number.

Question 7: What is the advantage of using SC-FDMA in the LTE uplink?

Answer: The primary advantage of SC-FDMA (Single-Carrier Frequency-Division Multiple Access) is its lower PAPR (Peak-to-Average Power Ratio) compared to OFDMA (Orthogonal Frequency-Division Multiple Access) used in the LTE downlink. This improves the efficiency of the power amplifier, extending battery life.

Question 8: What is RSSI?

Answer: RSSI stands for Received Signal Strength Indication. It is used in almost all Radio Access Technologies (RATs) to measure the power received from a cell in both idle and connected/dedicated modes. This helps the UE stay connected to the best cell at all times. If the power measured via RSSI drops, either the UE or the network initiates a handover, or cell re-selection is performed.

Question 9: Explain Circuit Switched Fallback (CSFB) with respect to LTE and GSM.

Answer: CSFB provides a framework for delivering voice services by leveraging the existing GSM Circuit Switched infrastructure when the UE is being served by E-UTRAN (LTE). To provide voice call support, a Circuit Switch Fall Back is initiated from the LTE RAT to the GSM RAT, facilitating voice over LTE (VoLTE).

Question 10: Explain the LTE network architecture and various interfaces.

Answer: The LTE network architecture consists of various entities. The main interfaces are:

• Uu: Between the UE and the eNB.
• X2: Between eNBs.
• S1: Between the eNB and the EPC (Evolved Packet Core).

Question 11: What is SRVCC?

Answer: SRVCC is the abbreviation for Single-Radio Voice Call Continuity. SRVCC handover is supported from E-UTRAN (i.e., LTE) to UTRAN/GERAN (WCDMA/GSM). The SRVCC procedure is used to transfer an ongoing PS (Packet Switched) voice call (IMS) in LTE to a CS (Circuit Switched) voice call via Handover from LTE to GERAN/UTRAN.

Question 12: What is the difference between LTE and LTE Advanced?

Answer: LTE is specified in 3GPP Release 8 and Release 9. LTE Advanced is specified in 3GPP Release 10.

The main difference is that carrier aggregation is introduced in LTE Advanced. Additionally, the number of antennas supported by MIMO (Multiple-Input Multiple-Output) has been increased to 8 in LTE Advanced.