LTE Downlink HARQ Mechanism

based on scheduler decision the eNodeB send the data over Air interface on PDSCH, then, The UE receive the Data and verifies the checksum and send to eNodeB an ACK or NACK, on PUCCH and PUSCH channels.

The reported CQI is an estimation and errors can still occur on the transmission, it may be necessary to retransmit a packet that could not be decoded successfully by the UE.

LTE provides two retransmission mechanisms:
Hybrid ARQ(HARQ): implemented on the physical layer for fast recovery from errors.
Automatic Repeat request(ARQ): is a NACK based implemented on the RLC layer invoked only if the HARQ fails.
if neither HARQ or ARQ can recover a lost or corrupted packet then the application layer is responsible for retransmitting the packet.

Lte throughput Optimization course

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