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HARQ-ACK (Hybrid ARQ acknowledgement)

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Introduction:

Hybrid Automatic Repeat Request (HARQ) is a technique used in wireless communication systems to improve the reliability of data transmission. It combines both forward error correction (FEC) and automatic repeat request (ARQ) methods to provide a more efficient way of retransmitting lost or corrupted data. In HARQ, the receiver sends an acknowledgment message (ACK) to the transmitter indicating that the received data is error-free. If errors are detected, the receiver sends a negative acknowledgment message (NACK) requesting the transmitter to retransmit the data. HARQ-ACK is a term used to refer to the acknowledgment message sent by the receiver in HARQ.

How HARQ-ACK Works:

The HARQ process involves multiple rounds of transmission and retransmission until the data is successfully received at the receiver end. In each round, the transmitter sends a packet of data, and the receiver sends an acknowledgment message indicating whether the data was received correctly or not.

There are two types of acknowledgment messages in HARQ, which are:

  1. Positive Acknowledgment (ACK): This message is sent by the receiver to the transmitter to indicate that the received data is error-free. When the transmitter receives the ACK message, it assumes that the data has been successfully transmitted and proceeds to send the next packet of data.
  2. Negative Acknowledgment (NACK): This message is sent by the receiver to the transmitter to indicate that the received data is corrupted or lost. When the transmitter receives the NACK message, it retransmits the same packet of data again, hoping that it will be received correctly this time.

In a typical HARQ process, the transmitter sends a packet of data to the receiver. If the receiver detects errors in the received data, it sends a NACK message to the transmitter requesting it to retransmit the data. The transmitter then retransmits the same packet of data, and the process repeats until the data is successfully received without any errors. If the receiver receives the data without any errors, it sends an ACK message to the transmitter indicating that the data has been successfully received.

HARQ-ACK Feedback Mechanisms:

There are two types of feedback mechanisms used in HARQ-ACK, which are:

  1. Chase Combining: This mechanism is used when the receiver sends a NACK message to the transmitter requesting it to retransmit the data. In chase combining, the receiver combines the retransmitted data with the previously received data to obtain a better version of the original data. This process continues until the receiver receives the data without any errors, or the maximum number of retransmissions is reached.
  2. Incremental Redundancy: This mechanism is used when the receiver sends a NACK message to the transmitter requesting it to retransmit the data. In incremental redundancy, the transmitter adds more redundancy to the data in each retransmission to improve the chances of successful reception. This process continues until the receiver receives the data without any errors, or the maximum number of retransmissions is reached.

Advantages of HARQ-ACK:

HARQ-ACK has several advantages over traditional ARQ methods. Some of the advantages are:

  1. Improved Reliability: HARQ-ACK provides improved reliability by combining both FEC and ARQ methods. The receiver can use the FEC method to correct errors in the received data, and if errors still persist, it can use the ARQ method to request the transmitter to retransmit the data.
  2. Reduced Latency: HARQ-ACK reduces latency by reducing the time taken to retransmit lost or corrupted data. Instead of waiting for the entire packet to be retransmitted, the receiver can request the transmitter to retransmit only the corrupted portion of the data.
  3. Increased Throughput: HARQ-ACK can improve the throughput of a communication system by reducing the number of retransmissions required to successfully transmit data. By combining both FEC and ARQ methods, the system can achieve higher reliability with fewer retransmissions, resulting in increased throughput.
  4. Better Spectral Efficiency: HARQ-ACK can improve the spectral efficiency of a communication system by reducing the number of retransmissions required. This means that more data can be transmitted over the same bandwidth, resulting in better spectral efficiency.
  5. Adaptive Modulation and Coding: HARQ-ACK can be used with adaptive modulation and coding (AMC) techniques to further improve the reliability and throughput of a communication system. AMC techniques adapt the modulation and coding scheme based on the channel conditions, and HARQ-ACK can be used to further improve the reliability of the transmission.

Applications of HARQ-ACK:

HARQ-ACK is used in various wireless communication systems, including cellular networks, satellite communications, and Wi-Fi networks. In cellular networks, HARQ-ACK is used to improve the reliability of data transmission and reduce latency. In satellite communications, HARQ-ACK is used to improve the throughput and spectral efficiency of the system. In Wi-Fi networks, HARQ-ACK is used to improve the reliability and throughput of data transmission.

HARQ-ACK in LTE Networks:

HARQ-ACK is used in Long-Term Evolution (LTE) networks to improve the reliability and throughput of data transmission. LTE uses the Hybrid Automatic Repeat Request Protocol (HARQ) to provide ARQ functionality with incremental redundancy. In LTE, the HARQ-ACK feedback mechanism is used to provide feedback to the transmitter regarding the success or failure of the transmission.

In LTE, the HARQ-ACK feedback mechanism is based on a two-bit ACK/NACK scheme. The receiver sends either an ACK or a NACK message to the transmitter, depending on whether the received data is error-free or not. The two-bit ACK/NACK scheme is used to reduce the overhead of the feedback mechanism and improve the throughput of the system.

Conclusion:

HARQ-ACK is a technique used in wireless communication systems to improve the reliability and throughput of data transmission. It combines both FEC and ARQ methods to provide a more efficient way of retransmitting lost or corrupted data. HARQ-ACK provides improved reliability, reduced latency, increased throughput, better spectral efficiency, and can be used with adaptive modulation and coding techniques. It is used in various wireless communication systems, including cellular networks, satellite communications, and Wi-Fi networks. In LTE networks, HARQ-ACK is used to provide feedback to the transmitter regarding the success or failure of the transmission.