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IR HARQ (incremental redundancy HARQ)

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In wireless communications, Hybrid Automatic Repeat Request (HARQ) is a technique that is used to improve the reliability and efficiency of data transmission. It is essentially a feedback mechanism that enables the receiver to request retransmissions of lost or erroneous data packets. HARQ can be implemented in a number of different ways, one of which is Incremental Redundancy (IR) HARQ.

In this article, we will provide a detailed explanation of IR HARQ, including its benefits, operation, and implementation in wireless communication systems.

Introduction to HARQ

Before diving into IR HARQ, it is important to understand the basic concept of HARQ. HARQ is a mechanism that is used to improve the reliability of data transmission over a wireless channel. It is particularly useful in scenarios where the wireless channel is prone to errors, such as in mobile networks, where the signal quality can be affected by factors such as distance, interference, and multipath propagation.

In HARQ, the transmitter sends a packet of data to the receiver, which then checks the packet for errors. If the packet is received correctly, the receiver sends an acknowledgement (ACK) back to the transmitter to indicate that the data has been successfully received. However, if the packet contains errors, the receiver sends a negative acknowledgement (NACK) back to the transmitter, indicating that the packet needs to be retransmitted.

The key advantage of HARQ is that it enables the receiver to request retransmissions of only the data that was lost or corrupted, rather than requiring the entire packet to be resent. This reduces the amount of retransmitted data and improves the overall efficiency of the system.

Introduction to IR HARQ

IR HARQ is a specific type of HARQ that is used in wireless communication systems. It is based on the idea of sending data in multiple, incremental transmissions, each of which contains additional information that can be used to improve the chances of successful reception.

In IR HARQ, the transmitter sends a series of data packets, each of which contains a subset of the original data. The receiver checks each packet for errors, and sends a feedback signal (either an ACK or a NACK) back to the transmitter. If the packet is received correctly, the receiver can use the information in the packet to decode a portion of the original data. If the packet contains errors, the transmitter can send additional packets that contain additional information to help the receiver decode the original data.

The basic idea behind IR HARQ is that the incremental transmissions allow the receiver to gradually improve its estimate of the original data, even if some of the transmissions are lost or corrupted. This makes it possible to achieve high reliability even in the presence of a noisy or unreliable wireless channel.

Operation of IR HARQ

The operation of IR HARQ can be divided into two main phases: transmission and retransmission.

Transmission Phase

During the transmission phase, the transmitter sends a series of data packets, each of which contains a subset of the original data. The packets are sent in a specific order, with each packet containing additional information that can be used to decode the original data.

For example, suppose that the original data consists of four bits: 0101. The transmitter could send the data in two packets, as follows:

Packet 1: 01 Packet 2: 01

In this example, each packet contains two bits of the original data. The receiver checks each packet for errors and sends a feedback signal back to the transmitter.

Retransmission Phase

During the retransmission phase, the transmitter sends additional packets that contain additional information to help the receiver decode the original data. The number of retransmissions depends on the specific implementation of the system and the channel conditions.

For example, suppose that the first packet is lost or corrupted during transmission. In this case, the receiver would send a NACK back to the transmitter, indicating that the first packet needs to be retransmitted. The transmitter would then send a new packet that contains additional information that can be used to help decode the first packet. For example, it might send the following packet:

Packet 3: 00

In this example, the third packet contains two additional bits of information that can be used to help decode the first packet. The receiver would then use the information in packets 2 and 3 to decode the first packet.

If the second packet is also lost or corrupted during transmission, the receiver would send another NACK back to the transmitter. The transmitter would then send another packet that contains additional information. For example:

Packet 4: 11

In this example, the fourth packet contains two additional bits of information that can be used to help decode the second packet. The receiver would then use the information in packets 3 and 4 to decode the second packet.

The process continues until all of the packets have been successfully received and decoded, or until a maximum number of retransmissions has been reached.

Benefits of IR HARQ

IR HARQ has several benefits over other HARQ schemes, including:

  1. Improved reliability: The incremental transmissions allow the receiver to gradually improve its estimate of the original data, even if some of the transmissions are lost or corrupted. This makes it possible to achieve high reliability even in the presence of a noisy or unreliable wireless channel.
  2. Reduced latency: IR HARQ can reduce the overall latency of the system, since it allows the receiver to decode and use some of the data before all of the packets have been received.
  3. Efficient use of resources: IR HARQ can be more efficient than other HARQ schemes, since it only requires the retransmission of the data that was lost or corrupted, rather than requiring the entire packet to be resent.
  4. Compatibility with other schemes: IR HARQ can be combined with other HARQ schemes, such as Chase combining or Soft combining, to further improve the reliability and efficiency of the system.

Implementation of IR HARQ

IR HARQ can be implemented in a number of different ways, depending on the specific requirements of the system. Some common techniques include:

  1. Repetition coding: In repetition coding, the transmitter sends multiple copies of each bit in the original data. The receiver uses the information in each copy to gradually improve its estimate of the original data.
  2. Incremental redundancy: In incremental redundancy, the transmitter sends a series of data packets, each of which contains additional information that can be used to improve the chances of successful reception.
  3. Hybrid ARQ: Hybrid ARQ combines the benefits of repetition coding and incremental redundancy, by sending a series of data packets that each contain multiple copies of the data, as well as additional information that can be used to improve the chances of successful reception.
  4. Chase combining: Chase combining is a technique that combines multiple copies of the data, in order to improve the chances of successful reception. The receiver selects the best copy of the data and discards the others.
  5. Soft combining: Soft combining is a technique that uses the probabilities of the received symbols to combine multiple copies of the data. It can be used to improve the reliability of the system, particularly in scenarios where the wireless channel is highly variable.

Conclusion

In conclusion, Incremental Redundancy Hybrid Automatic Repeat Request (IR HARQ) is a feedback mechanism that is used to improve the reliability and efficiency of data transmission in wireless communication systems. It uses incremental transmissions to gradually improve the estimate of the original data, and it can be combined with other HARQ schemes to further improve the reliability and efficiency of the system.