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 an 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.