5G NR Interview Questions and Answers

This document provides a collection of frequently asked questions (FAQs) related to 5G New Radio (NR) technology, along with their corresponding answers. These questions are designed to help candidates prepare for technical interviews for 5G engineer positions.

Question 1: What is 5G NR (New Radio)?

Answer: 5G stands for Fifth Generation, and NR stands for New Radio. This technology represents an evolution from LTE-Advanced and LTE-Advanced Pro. The standard is defined in 3GPP Release 15 and later versions.

Question 2: What is the difference between 5G NR and 4G (LTE)?

Answer: 4G LTE and LTE-Advanced adhere to 3GPP Releases 8 and 12, respectively. 5G follows Release 15 and later.

• 4G operates below 6 GHz, whereas 5G NR operates across a wider range of frequency bands, including:
  • Sub-1 GHz
  • 1 to 6 GHz
  • Above 6 GHz in mmWave bands (28 GHz, 40 GHz, etc.)
• 5G supports significantly higher data rates compared to 4G. 5G can deliver around 10 Gbps, while LTE-Advanced Pro offers up to 3 Gbps.
• 5G offers lower latency, typically less than 1 ms, compared to LTE-Advanced Pro, which provides latency under 2 ms.

Question 3: Explain the 5G NR network architecture, its elements, and its network interfaces.

Answer: The 5G NR architecture comprises three primary elements:

• UE (User Equipment): The mobile device.
• RAN (Radio Access Network): The network responsible for radio communication.
• Core Network: The central part of the network managing services and data.

The NG-RAN contains the gNB radio (base station), Control Unit, and Data Unit. AMF stands for Access and Mobility Management Function and UPF stands for User Plane Function.

Question 4: Explain the difference between TDD and FDD in 5G NR.

Answer:

• FDD (Frequency Division Duplex): Uplink and downlink transmissions occur simultaneously using different frequency bands.
• TDD (Time Division Duplex): Uplink and downlink transmissions share the same frequency band but occur at different times.

FDD is suitable for scenarios where uplink and downlink data rates are symmetrical, and it typically experiences less interference. TDD is better suited for asymmetrical data rate requirements.

Question 5: Define use cases of 5G NR: eMBB, mMTC, and URLLC.

Answer: These terms refer to the different Quality of Service (QoS) categories supported by 5G:

• eMBB (Enhanced Mobile Broadband): Supports high peak data rates (10-20 Gbps) for applications like high-definition video streaming and AR/VR.
• mMTC (Massive Machine Type Communications): Supports a large number of devices with low data rates (1-100 Kbps) for IoT applications.
• URLLC (Ultra-Reliable Low Latency Communications): Provides ultra-responsive connections with very low latency (less than 1 ms) for critical applications like industrial automation and autonomous vehicles.

Question 6: What are the different types of handover in 5G NR? Explain each in brief.

Answer: There are primarily two categories of handover in 5G NR:

• Network Controlled: The network makes the handover decision.
• UE Controlled: The UE assists in the handover process.

Other types mentioned in the standard include:

• 5G NR Inter gNB handover: Handover between two different gNBs.
• 5G NR intra RAN handover (C-Plane Handling): Handover within the same RAN, specifically managing the control plane.

Question 7: What are the functions of MIB and SIB in 5G?

Answer:

• MIB (Master Information Block): Contains essential system information transmitted on the xBCH transport channel and xBCCH logical channel.
• SIB (System Information Block): Contains RRC (Radio Resource Configuration) information that is common for all UEs. It’s transmitted on the xBCCH logical channel and xBCH transport channel. The IE (Information Element) XSystemInformationBlock is part of the SIB.

Question 8: Explain 5G NR deployment scenarios or modes: NSA (Non-Standalone), SA (Standalone), Homogeneous mode, and Heterogeneous mode.

Answer:

• SA (Standalone): The UE relies solely on the 5G RAT (Radio Access Technology). Both the Control Plane (C-Plane) and User Plane (U-Plane) are handled by the 5G cell, managing signaling and data transfer.
• NSA (Non-Standalone): Requires a connection to both an LTE cell and a 5G cell. LTE is used for C-Plane functions (e.g., call origination, termination, location registration), while 5G NR focuses on the U-Plane.
• Homogeneous mode: All the cells in the network use the same technology and frequency band
• Heterogeneous mode: The network consists of cells using different technologies (e.g. 4G, 5G) and/or different frequency bands.

Question 9: What is the range and speed in 5G NR technology?

Answer:

• Speed: 5G networks are benchmarked to achieve speeds of approximately 20 Gbps in the downlink and 10 Gbps in the uplink as the network matures. Many operators have already reached throughputs around 5 Gbps.
• Range: The range of 5G NR is generally shorter than that of 4G LTE or 3G networks, more comparable to Wi-Fi range.

Question 10: What are the functions of the RRC layer in 5G NR? Explain RRC states with a state diagram.

Answer: The RRC (Radio Resource Control) layer performs the following functions:

• Broadcasts System Information (SI) messages to the Access Stratum (AS) and Non-Access Stratum (NAS).
• Handles paging initiated by the 5G Core Network (5GC) or NG-RAN (Radio Access Network).
• Establishes, maintains, and releases RRC Connections between the 5G NR UE and NG-RAN, including the addition, modification, and release of Carrier Aggregation (CA) and Dual Connectivity in NR or between E-UTRA and NR.
• Manages security, including key management.
• Establishes, configures, maintains, and releases Signaling Radio Bearers (SRBs) and Data Radio Bearers (DRBs).
• Handles mobility functions such as handover, context transfer, UE cell selection/re-selection, control of cell selection/re-selection, and Inter-RAT mobility.
• Manages QoS.
• Handles UE measurement reporting and reporting control.
• Detects and recovers from radio link failures.
• Transfers NAS messages to/from the UE.

The RRC layer has three states:

• RRC_IDLE: The UE is not actively connected to the network.
• RRC_INACTIVE: The UE is known to the network but is not actively transmitting or receiving data. This state allows for faster transition back to connected mode compared to RRC_IDLE.
• RRC_CONNECTED: The UE is actively connected to the network and can transmit and receive data.

Question 11: Which are the KPIs used in 5G NR?

Answer: 5G KPIs (Key Performance Indicators) are categorized according to the use cases: eMBB, URLLC, and mMTC.

Common KPIs include:

• Peak data rate
• Peak spectral efficiency
• Latency
• Traffic capacity
• Connection density
• Energy efficiency
• Reliability

Question 12: What are the frequency bands of 5G NR used in various parts of the world such as USA, Europe, China, Japan, Korea, INDIA, and so on?

Answer: The specific frequency bands used for 5G NR vary by region.

Question 13: What is CORESET in 5G?

Answer: In 5G NR, the resource allocation unit for control channels is the CORESET (Control Resource Set). Time-domain and frequency-domain parameters of the CORESET are defined in the TS 38.211 document.

Question 14: What is a Mini-slot in 5G?

Answer: Mini-slots are a feature of 5G NR that allows for more flexible scheduling.

Mini-slots occupy 2, 4, or 7 OFDM symbols, enabling non-slot-based scheduling. It is the minimum scheduling unit used in 5G NR.

Question 15: Explain the 5G NR frame and subframe structure with possible combinations.

Answer: The 5G NR frame structure consists of frames, subframes, and slots. The flexibility in slot duration and the introduction of mini-slots allow for dynamic adaptation to different service requirements.

This collection of 5G NR interview questions and answers is intended to be helpful for both entry-level and experienced 5G engineers. In addition to these questions, exploring resources on 5G core technology is highly recommended. Good luck!