What is ARQ and HARQ?

Both HARQ (Hybrid Automatic Repeat Request) and ARQ (Automatic Repeat Request) are error control mechanisms used in wireless communication to ensure data is received correctly. They mechanism helps in recovery of lost or corrupted packets during transmission.

ARQ (Automatic Repeat Request):

ARQ stands for Automatic Repeat Request. This is the protocol used at data link layer (RLC layer in 5G/4G) . it is an error-control mechanism that is being used in a two-way communication systems.  It is used to achieve reliable data transmission over an unreliable source or service.

 It uses CRC(cyclic redundancy check) to determine, whether the received packet is correct or not. If the packet is received correctly at receiver side, receiver sends ACK to the transmitter, but in case if the packet is not received correctly at receiver side, then receiver send NACK to the transmitter. And then after receiving NACK from receiver side, the transmitter re-transmits the same packet again and so on.

 Concept:

ARQ is a basic error correction method. If a receiver detects an error in a packet (using CRC), it asks the sender to retransmit the entire packet.

How It Works:

1. Sender transmits a data packet.
2. Receiver checks for errors using CRC.
3. If errors are found, receiver sends a NACK (Negative Acknowledgment).
4. Sender retransmits the same packet.

Types of ARQ:

• Stop-and-Wait ARQ: Waits for ACK/NACK before sending the next packet.
• Go-Back-N ARQ: Retransmits from the error point onward.
• Selective Repeat ARQ: Only retransmits erroneous packets.

Used In:

• Higher layers like RLC (Radio Link Control) in 5G.

HARQ (Hybrid Automatic Repeat Request)

Concept:

HARQ is a more advanced version of ARQ. It combines error detection with forward error correction (FEC). Instead of resending the same packet, it sends redundant bits to help the receiver decode the original message.

How It Works:

1. Sender transmits a packet with FEC.
2. Receiver checks for errors.
3. If errors are found, receiver sends a NACK.
4. Sender sends additional redundancy bits (not the same packet).
5. Receiver combines original and new bits to decode the message.

Key Feature:

• Uses soft combining (e.g., Chase Combining or Incremental Redundancy).
• Reduces retransmissions and improves efficiency.

Used In:

• MAC layer in 5G NR.
• Works with transport blocks and physical layer transmissions.

HARQ vs ARQ: Key Differences

Feature	ARQ	HARQ
Layer Used	RLC	MAC
Retransmission Type	Same packet	Redundant bits (soft combining)
Error Correction	No (only detection)	Yes (FEC + detection)
Efficiency	Lower	Higher
Latency	Higher	Lower
Complexity	Simple	Complex
Use in 5G	RLC layer	MAC layer