Top 5G Interview Question – 5G RACH Process

In this article, we will discuss the top 5G Interview Question – 5G RACH Process.

In 5G, The RACH Process (Random Access) is very important especially in case of NSA since a 5G RACH failure in NSA results in a 5G Call Drop. In this session, we go through UE logs to understand various RACH Information Elements (IE) while explaining how UE decodes those IEs to find the RACH configuration and location. The session also explains the basic RACH Issues that are observed in the networks along with potential troubleshooting and optimization actions.

Reference: 3GPP 38.211

Top 5G Interview Question – 5G RACH Process

The term “RACH” stands for “Random Access Channel” and is a concept used in wireless communication systems, including 5G. The RACH is a channel through which a user equipment (UE) device can initiate a connection with a base station or access point.

The 5G Random Access Channel (RACH) process involves several steps for a user equipment (UE) device to establish a connection with a base station. Here’s a detailed overview of the 5G RACH process:

1. RACH Preamble Transmission
   
   – The UE selects a random access preamble from a predefined set of preambles.
   – The preamble carries synchronization and identification information.
   – The UE transmits the selected preamble on the Physical Random Access Channel (PRACH).
   
2. RACH Detection and Timing Alignment:
   
   – The base station receives the PRACH transmissions from multiple UEs.
   – It detects the received preambles and estimates the timing offset of each UE.
   – The base station sends timing alignment commands (Timing Advance) to the UEs to align their transmission timing.
   
3. Random Access Response (RAR) Transmission
   
   – The base station selects one UE for contention resolution based on the received preambles.
   – It sends a RAR message on the Physical Downlink Control Channel (PDCCH) to the selected UE.
   – The RAR message contains information such as a temporary identifier (Temporary C-RNTI) and the uplink resources assigned to the UE.
   
4. RACH Contention Resolution
   
   – If multiple UEs transmit the same preamble, a contention occurs.
   – The UEs that experience contention follow a backoff and retransmission procedure.
   – They randomly select backoff durations and retransmit the preambles during contention resolution slots on the PRACH.
   
5. Random Access Response (RAR) Reception
   
   – The UE that successfully transmitted the preamble and received the RAR decodes the RAR message.
   – It obtains the temporary identifier (Temporary C-RNTI) and the assigned uplink resources.
   – The UE uses the received information to establish a connection with the base station.
   
6. Connection Establishment
   
   – The UE uses the assigned uplink resources to send a Connection Request message on the Physical Uplink Control Channel (PUCCH).
   – The base station receives the Connection Request and sends a Connection Setup message to the UE.
   – The UE and the base station exchange further control messages to establish the necessary resources for data transmission.

It’s important to note that the 5G RACH process can vary based on network configurations, deployment scenarios, and specific parameters set by network operators.

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Ali Khalid

Senior Solution Architect at Ericsson SCU, Australia | 5G NR | VoLTE | LTE-A | Massive MIMO | NB-IoT

Ali Khalid is a Senior LTE/VoLTE RNPO, NB-IoT and 5G Solution Architect who has successfully led and delivered a number of projects in different regions across the globe including Pakistan, Bahrain, UAE, Qatar, Oman, KSA, Nigeria, Turkey, Poland and Japan. He is currently working in Strategic Competence Unit (SCU), a highly experienced global team at Ericsson, Australia. In case of any questions or feedback, please feel free to drop a comment below or connect with him on LinkedIn.

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