ARQ vs HARQ: Understanding the Differences

This page discusses ARQ and HARQ techniques, highlighting the key differences between them.

ARQ (Automatic Repeat Request)

ARQ is a protocol used at the data link layer. It relies on Cyclic Redundancy Check (CRC) to determine the integrity of received packets.

• Successful Reception: If a packet is received correctly, the receiver sends an ACK (Acknowledgement) to the transmitter.
• Unsuccessful Reception: If a packet is received with errors, the receiver sends a NACK (Negative Acknowledgement) to the transmitter. Upon receiving a NACK, the transmitter re-transmits the same packet.

This retransmission process occurs for a pre-defined number of attempts. If the packet cannot be successfully transmitted within those attempts, the system exits the retransmission loop. This basic form is known as stop-and-wait ARQ.

A more common and efficient variation is the Go-Back-N ARQ technique:

Go-Back-N ARQ

This technique leverages the sliding window protocol. Here’s how it works:

1. The source/transmitting station transmits Protocol Data Units (PDUs) numbered sequentially (PDU-1, PDU-2, … PDU-n).
2. In the absence of errors, the receiver sends positive acknowledgements (ACKs) for each successfully received PDU.
3. If the receiver detects an error in a particular PDU, it sends a negative acknowledgement (NACK) specifically for that PDU.
4. Upon receiving a NACK, the transmitter suspends the transmission of any current and subsequent PDUs. It then re-transmits the erroneous PDU and all PDUs that were sent after it.

HARQ (Hybrid Automatic Repeat Request)

HARQ differs from ARQ in its retransmission strategy. Instead of retransmitting the exact same packet, HARQ modifies certain physical layer parameters before retransmission.

Several HARQ techniques exist:

• Chase Combining HARQ: In this type, the retransmitted packet is identical to the original. The receiver combines the multiple received copies of the packet, effectively increasing the signal-to-noise ratio (Eb/No) due to the Maximum Ratio Combining (MRC) technique. Each retransmission contributes to increasing the energy of the received packet.

• Incremental Redundancy (IR) HARQ (Type II and Type III): Instead of retransmitting the entire packet, these IR-HARQ types retransmit different redundant information. The specific redundant information to be retransmitted can be adjusted based on channel conditions by varying the puncturing configuration used at the physical layer of the transmitter.

Performance Considerations:

• In poor channel conditions, HARQ generally performs better than ARQ due to the gains from combining or incremental redundancy.
• In good channel conditions, ARQ can sometimes outperform HARQ, as the overhead of HARQ’s more complex retransmission schemes might outweigh its benefits.