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HARQ(ACK/NACK)

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Hybrid Automatic Repeat reQuest (HARQ) is a communication protocol used in wireless communication systems to improve the reliability of data transmission. HARQ combines automatic repeat request (ARQ) and forward error correction (FEC) techniques to enhance the robustness of the communication link.

Here's a technical explanation of HARQ with a focus on the acknowledgment (ACK) and negative acknowledgment (NACK) mechanisms:

  1. Transmission Process:
    • When a sender transmits a data packet, it awaits an acknowledgment from the receiver.
    • The acknowledgment indicates whether the transmitted packet was successfully received or if there were errors.
  2. ACK and NACK Signals:
    • If the receiver successfully receives the packet without errors, it sends an ACK signal back to the sender.
    • If the receiver detects errors in the received packet, it sends a NACK signal to the sender.
  3. Soft Combining:
    • HARQ uses a technique called soft combining. If the sender receives a NACK for a transmitted packet, instead of discarding the entire packet, it stores the received bits in a buffer.
    • In subsequent retransmissions, the sender combines the new transmission with the stored bits from the previous attempts. This process continues until either an ACK is received, or a maximum number of retransmission attempts is reached.
  4. Adaptive Modulation and Coding (AMC):
    • HARQ often works in conjunction with AMC, where the sender adapts the modulation and coding scheme based on the channel conditions.
    • If the channel quality is good, the sender may use a higher order modulation and less error correction. If the channel quality degrades, it may switch to a lower modulation and higher error correction.
  5. Round-Trip Time (RTT) Considerations:
    • The sender adjusts the retransmission timeout based on the round-trip time (RTT) of the communication link. A longer RTT may require a longer timeout to allow for potential delays in the acknowledgment.
  6. Incremental Redundancy:
    • HARQ uses incremental redundancy, meaning that each retransmission contains additional redundant information. This allows for progressive improvement in the likelihood of correct reception with each retransmission.
  7. Fast Retransmission:
    • In some HARQ implementations, if the sender receives multiple NACKs for the same packet, it may trigger a fast retransmission without waiting for the regular timeout.
  8. Performance Optimization:
    • HARQ parameters and strategies are often optimized based on the characteristics of the communication channel, the type of data being transmitted, and the quality of service requirements.

HARQ is a sophisticated protocol that leverages a combination of ARQ, FEC, soft combining, and other techniques to enhance the reliability of wireless communication by managing retransmissions based on feedback from the receiver.