Understanding HARQ and ARQ: Error Correction in 4G & 5G Networks.

HARQ (Hybrid Automatic Repeat Request):- Works at the MAC (Medium Access Control) layer to provide error correction and retransmission at the physical layer level.

ARQ (Automatic Repeat Request):- Works at the RLC (Radio Link Control) layer for error detection and retransmission when HARQ fails.

2. Which Layer Controls HARQ and ARQ?

Mechanism	Layer	Purpose	Control
HARQ	MAC Layer	Fast retransmissions with soft combining	eNB (gNB in 5G)
ARQ	RLC Layer	Ensures error-free packet delivery	UE & eNB (gNB in 5G)

 4. Failure Reasons in HARQ & ARQ

Failure Scenario	Reason	Solution
HARQ failure	Poor SNR, interference, packet corruption	Retransmission via ARQ
ARQ failure	Network congestion, excessive retransmissions	RLC reassembly failure, packet drop
HARQ Timeout	UE does not send ACK/NACK due to link loss	Packet dropped, retransmission at RLC
Excessive HARQ retransmissions	Signal degradation, mobility issues	Switch to ARQ
ARQ Buffer Overflow	Too many unacknowledged packets	Packet discard, throughput reduction