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HARQ (Hybrid automatic repeat and request)

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Hybrid Automatic Repeat and Request (HARQ) is a mechanism used in communication systems to improve the reliability of data transmissions over wireless or wired networks. HARQ combines both automatic repeat request (ARQ) and error correction coding techniques to minimize the number of retransmissions needed to correct errors in data transmission. In this article, we will explain how HARQ works, its types, advantages, and disadvantages.

How does HARQ work?

HARQ operates by using a combination of error detection and correction techniques to minimize the number of retransmissions required. When a transmitter sends data to a receiver, the receiver checks the received data for errors using an error detection code such as cyclic redundancy check (CRC). If the data is error-free, the receiver sends an acknowledgement (ACK) message to the transmitter, indicating that the data was received correctly.

However, if the receiver detects errors in the data, it sends a negative acknowledgement (NACK) message to the transmitter, indicating that the data was received incorrectly. The transmitter then uses the error correction code to correct the errors and retransmits the data. This process continues until the data is successfully received by the receiver or a maximum number of retransmissions is reached.

There are two types of HARQ techniques: Type I and Type II.

Type I HARQ

Type I HARQ is also known as Chase combining or incremental redundancy. In this technique, the transmitter sends multiple copies of the same data, each with increasing redundancy. The receiver combines the redundant copies to improve the signal-to-noise ratio (SNR) and reduce the number of errors. If the receiver detects errors in the data, it sends a NACK message to the transmitter, indicating that the data was received incorrectly. The transmitter then sends additional redundant data to the receiver to correct the errors.

Type I HARQ is suitable for low-latency applications such as voice and video communication, where the delay caused by retransmissions can be disruptive to the user experience.

Type II HARQ

Type II HARQ is also known as parallel concatenation or Chase-2 combining. In this technique, the transmitter sends multiple independent copies of the same data, each with different encoding. The receiver combines the copies using a decoder to improve the SNR and reduce the number of errors. If the receiver detects errors in the data, it sends a NACK message to the transmitter, indicating that the data was received incorrectly. The transmitter then sends additional independent copies of the data with different encoding to correct the errors.

Type II HARQ is suitable for high-throughput applications such as data transfer, where the retransmission delay is less critical compared to the overall throughput.

Advantages of HARQ

HARQ has several advantages over traditional ARQ techniques:

  1. Improved reliability: HARQ combines both error detection and correction techniques, which improves the reliability of data transmission over wireless or wired networks.
  2. Reduced latency: HARQ reduces the number of retransmissions required to correct errors, which reduces the latency of data transmission.
  3. Higher throughput: HARQ uses parallel encoding and decoding techniques, which increases the overall throughput of data transmission.

Disadvantages of HARQ

HARQ also has some disadvantages:

  1. Increased complexity: HARQ requires additional hardware and software to support the error correction and detection techniques, which increases the complexity of the communication system.
  2. Increased overhead: HARQ requires additional bandwidth and processing power to transmit and decode redundant data, which increases the overhead of the communication system.
  3. Limited scalability: HARQ may not scale well for large networks with many nodes, as the retransmission delay may increase significantly, leading to higher latency and reduced throughput.

Conclusion

HARQ is a powerful technique for improving the reliability of data transmission over wireless or wired networks. It combines both error detection and correction techniques to minimize the number of retransmissions required to correct errors in data transmission, which reduces the latency and increases the overall throughput of the communication system.