H-ARQ Mechanism in DL

1. The Transport block is transmitted to the UE on the PDSCH.
2. The UE receives it but it is erroneous. The TB is stored in a buffer.
3. The UE transmits directly a NACK concerning the erroneous block on the PUCCH.
One HARQ entity in the UE and the e-UTRANMade of 8 HARQ processes in each direction (extended to 16 in DL MIMO)
Each process handles STOP and WAIT HARQ protocolEach process is responsible for generating ACK or NACK indicating delivery status of PDSCH/PUSCH

1. On reception of the NACK, the eNodeB retransmits the TB.
2. The UE receives it. Even if the retransmitted TB (Transport Block) is erroneous, the UE can try to recombine the 2 TB to have a correct one.
3. The UE send a ACK on the PUCCH.

In UL, it is the same principle, but the eNodeB send the ACK/NACK on a channel dedicated to the H-ARQ called the PHICH.

How H-ARQ Works in Downlink (DL) in LTE

H-ARQ (Hybrid Automatic Repeat Request) in the downlink helps make sure the data you receive from the network is correct. If there’s an error, the system quickly sends the data again without starting over completely.

Step-by-Step DL H-ARQ Process

Initial Transmission: The network sends data to your device over PDSCH.
Check for Errors: Your device checks the data using CRC (Cyclic Redundancy Check).
Send ACK/NACK: If the data is fine, your device sends an ACK. If not, it sends a NACK back using PUCCH.
Retransmission: The network resends only the parts with errors, not the whole packet. This saves time and resources.

Soft Combining in H-ARQ

Your device doesn’t throw away the bad packet—it combines the old and new data to rebuild the correct version. This is called soft combining, and it improves the chance of successful decoding.

Why H-ARQ Matters

H-ARQ keeps your downloads, streams, and calls clear and stable, even when the signal isn’t perfect. It reacts fast and fixes issues without you noticing delays or glitches.