Sign in Subscribe

How does 5G manage physical layer procedures for retransmissions?

Last updated on 


5G, like previous cellular generations, manages physical layer procedures for retransmissions to ensure reliable and error-free communication between the base station (gNB - gNodeB) and the user equipment (UE). Retransmissions are essential for handling data that may be lost or corrupted during transmission. Let's delve into the technical details of how 5G manages physical layer procedures for retransmissions:

Error Detection and Acknowledgment:

  • When the gNB sends data to the UE, it includes a sequence number or other identifying information for each data packet.
  • The UE receives the data and checks for errors using error-detection mechanisms such as cyclic redundancy checks (CRC).
  • If the UE detects an error in a received packet, it sends a negative acknowledgment (NACK) to the gNB for that specific packet.

Harq (Hybrid Automatic Repeat reQuest):

  • 5G employs a retransmission scheme called HARQ, which is a combination of automatic repeat request (ARQ) and forward error correction (FEC).
  • When the gNB receives a NACK from the UE, it retransmits the erroneous packet.
  • The gNB may also use FEC to correct errors if the NACK indicates that the errors can be corrected through FEC.

HARQ Processes:

  • 5G typically employs multiple HARQ processes in parallel for each data connection, allowing for simultaneous retransmissions of different packets.
  • Each HARQ process has a designated set of resources (e.g., time-frequency resources) for retransmissions.

Adaptive Modulation and Coding (AMC):

  • To improve the efficiency of retransmissions, 5G uses adaptive modulation and coding (AMC).
  • When a retransmission is required, the gNB may adapt the modulation and coding scheme to make the transmission more robust, especially in poor radio conditions.

Scheduling and Resource Allocation:

  • The gNB schedules retransmissions based on the priority and importance of data packets.
  • It allocates suitable radio resources (time and frequency slots) for retransmissions, ensuring that they do not interfere with new data transmissions or other UEs' communications.

Timing and Retransmission Limits:

  • 5G defines specific timing constraints for retransmissions to minimize latency.
  • There are limits on the number of retransmission attempts for a packet to avoid excessive use of radio resources.

Incremental Redundancy:

  • In some cases, instead of retransmitting the entire packet, 5G uses incremental redundancy.
  • Incremental redundancy involves transmitting only the redundant information needed to correct errors in the original packet, reducing overhead.

Combining and Decoding:

  • At the UE, multiple received copies of the same packet (original and retransmissions) are combined using techniques like soft combining.
  • Decoding algorithms are used to recover the original packet from the combined information.

Feedback Mechanisms:

  • The UE provides feedback to the gNB regarding the successful reception of retransmitted packets.
  • If a retransmission is successful, the UE sends an acknowledgment (ACK) to the gNB.

In summary, 5G manages physical layer procedures for retransmissions through a combination of error detection, HARQ, adaptive modulation and coding, resource allocation, and feedback mechanisms. These techniques ensure that data is reliably delivered even in challenging radio conditions, contributing to the high reliability and low-latency communication capabilities of 5G networks.