How does 5G manage control signaling for PDSCH retransmissions?

Last updated on Oct 4, 2023

In 5G, managing control signaling for Physical Downlink Shared Channel (PDSCH) retransmissions is essential to ensure the reliable delivery of data to user equipment (UE). Retransmissions are a crucial part of the communication process to correct errors that may occur during the initial transmission. Here's a technical explanation of how 5G manages control signaling for PDSCH retransmissions:

Initial Data Transmission:

When the base station (gNB - gNodeB) transmits data on the PDSCH to a UE, it divides the data into packets and sends them.
Each packet is assigned a unique identifier, typically referred to as a Hybrid Automatic Repeat Request (HARQ) process.

HARQ Process Numbering:

The gNB assigns a specific HARQ process number to each packet it sends to the UE.
The UE uses this HARQ process number to keep track of the status of each received packet.

Error Detection and HARQ Feedback:

Upon receiving a packet on the PDSCH, the UE performs error detection and checks whether the packet was received correctly.
If the UE successfully decodes the packet without errors, it sends a positive acknowledgment (ACK) to the gNB.

Negative Acknowledgment (NACK):

If the UE detects errors in the received packet, it sends a negative acknowledgment (NACK) to the gNB.
The NACK informs the gNB that the packet was received with errors and requests retransmission.

ACK/NACK Bundling:

To reduce overhead, 5G allows the bundling of multiple ACK/NACK signals.
The UE can combine acknowledgments or negative acknowledgments for several HARQ processes into a single message.

HARQ Round Trip Time (RTT):

HARQ typically operates within a specific round-trip time (RTT) window.
If the gNB receives a NACK within this RTT window for a particular HARQ process, it will initiate retransmission for the corresponding packet.

Soft Combining and Incremental Redundancy:

To improve reliability, 5G may use soft combining and incremental redundancy techniques.
Soft combining combines multiple transmissions of the same packet, weighting them by their likelihood of being correct.
Incremental redundancy involves sending additional redundancy bits in retransmissions to recover lost data more effectively.

Maximum Retransmission Limit:

5G defines a maximum number of retransmissions for each packet.
If a packet exceeds the maximum retransmission limit without successful decoding, it is considered lost, and higher-layer protocols may handle error recovery.

Fast Retransmission and HARQ Feedback:

In some cases, 5G may employ fast retransmission mechanisms that allow the UE to request retransmission immediately after detecting errors.
The gNB may use HARQ feedback channels to respond quickly to fast retransmission requests.

Retransmission Resource Allocation:

When retransmissions are necessary, the gNB allocates resources for retransmitting the specific packet.
These resources may include time-frequency resources, modulation and coding schemes, and transmit power settings.

Retransmission Scheduling and Control Signaling:

The gNB generates Downlink Control Information (DCI) messages to schedule retransmissions.
The DCI messages convey information about which packet is being retransmitted, the resource allocation for the retransmission, and other transmission parameters.

Reassembly at UE:

Upon receiving a retransmission, the UE reassembles the received packets to recover the original data.
Soft combining and incremental redundancy techniques may be used to improve the chances of successful recovery.

In summary, 5G manages control signaling for PDSCH retransmissions through a combination of error detection, HARQ feedback (ACK/NACK), ACK/NACK bundling, HARQ RTT timing, soft combining, incremental redundancy, maximum retransmission limits, fast retransmissions, resource allocation, scheduling, DCI messages, and reassembly at the UE. These mechanisms collectively ensure the reliable delivery of data and efficient use of radio resources in a 5G network.