What is HARQ (Hybrid automatic repeat and request)

Hybrid Automatic Repeat and Request (HARQ) Explained Technically

HARQ (Hybrid Automatic Repeat and Request) is a powerful error correction technique used in wireless communication systems to ensure reliable data transmission over error-prone channels. It combines the strengths of two fundamental error control mechanisms:

1. Forward Error Correction (FEC): Adds redundant bits to the transmitted data using error-correcting codes. The receiver can use these redundant bits to detect and sometimes even correct errors without requiring a retransmission.
2. Automatic Repeat Request (ARQ): Detects errors at the receiver and requests retransmission of the corrupted data packets from the transmitter.

HARQ offers a more efficient and reliable approach compared to using either FEC or ARQ alone. Here's a breakdown of its key aspects:

Functionality of HARQ:

1. Transmission with FEC: The transmitter encodes the data with an error-correcting code and transmits it along with the encoded redundant bits.
2. Error Detection at Receiver: The receiver attempts to decode the received data.
   • If the decoding is successful and no errors are detected, the receiver acknowledges (ACK) the reception of the correct data.
   • If the decoding fails due to excessive errors, the receiver cannot recover the original data.
3. Automatic Repeat Request (Hybrid Approach):
   • HARQ with Chase Combining: This approach utilizes the information from both the received corrupted packet and any previously received (potentially partially correct) versions of the same packet for decoding. This can improve the chances of successful decoding even with a high number of errors.
   • HARQ with Incremental Redundancy: Instead of retransmitting the entire packet upon error detection, the transmitter sends only the erroneous portion of the data along with additional redundancy in a retransmission packet. This reduces the amount of data retransmitted and improves efficiency.
4. Positive Acknowledgment (ACK) or Negative Acknowledgment (NACK): Based on the decoding outcome, the receiver sends an ACK or NACK to the transmitter.
   • ACK: Indicates successful reception of the data.
   • NACK: Informs the transmitter about the need for retransmission.
5. Retransmission (if NACK received): Upon receiving a NACK, the transmitter retransmits the data (potentially using the incremental redundancy approach) until an ACK is received, confirming successful delivery.

Benefits of HARQ:

• Improved Reliability: Compared to FEC alone, HARQ provides higher data reliability by leveraging retransmissions when necessary.
• Reduced Overhead: Compared to pure ARQ, HARQ avoids unnecessary retransmissions of entire packets by utilizing FEC and potentially retransmitting only the erroneous portions.
• Flexibility: Different HARQ schemes (Chase Combining, Incremental Redundancy) can be chosen based on channel conditions and desired trade-offs between throughput and complexity.

HARQ Applications:

HARQ is widely used in various wireless communication systems, including:

• Cellular networks (e.g., LTE, 5G)
• Wireless LAN (Wi-Fi) standards (e.g., 802.11ax - Wi-Fi 6)
• Satellite communication systems

In conclusion, HARQ is a crucial error correction technique that plays a vital role in ensuring reliable data transmission in modern wireless communication systems. By combining FEC with selective retransmissions, HARQ offers a balance between error correction capability, efficiency, and overall communication performance.