Sign in Subscribe

HI (Hybrid ARQ indicator)

Last updated on 

Hybrid Automatic Repeat reQuest (HARQ) is a technology used in wireless communication systems to improve the reliability of data transmission by providing error correction and retransmission capabilities. The Hybrid ARQ Indicator (HI) is a signaling mechanism used in HARQ to inform the receiver about the type of retransmission that should be used in case of a transmission error. In this article, we will discuss the HI mechanism, its different types, and its applications in modern wireless communication systems.

HARQ Mechanism:

Before discussing the HI mechanism, it is important to understand the basic working of HARQ. In a typical wireless communication system, the transmitter sends data to the receiver, which then decodes the received data. However, due to the presence of noise and interference in the wireless channel, errors can occur during data transmission. To handle these errors, HARQ uses a combination of error correction coding and retransmission techniques.

In HARQ, the data is first encoded using an error correction code, which adds redundancy to the data to enable error detection and correction. The encoded data is then transmitted over the wireless channel. At the receiver side, the data is decoded, and if any errors are detected, the receiver sends a request for retransmission to the transmitter. The transmitter then sends the same data again, but this time, with a different encoding or modulation scheme, depending on the type of retransmission requested by the receiver.

HARQ can be classified into two main types: Chase Combining (CC) and Incremental Redundancy (IR). In CC, the receiver combines the data received from multiple transmissions to improve the decoding performance. In IR, the transmitter sends additional redundancy in each retransmission to improve the chances of successful decoding.

HI Mechanism:

The Hybrid ARQ Indicator (HI) is a signaling mechanism used in HARQ to inform the receiver about the type of retransmission that should be used in case of a transmission error. The HI is typically included in the header of the data packet and is sent along with the data.

The HI mechanism allows the transmitter to choose the appropriate type of retransmission for each data packet, depending on the channel conditions and the performance requirements. The HI can indicate to the receiver whether the retransmission should be performed using CC or IR, and also provide other information, such as the modulation and coding scheme to be used in the retransmission.

HI Types:

There are different types of HI mechanisms used in modern wireless communication systems, depending on the specific requirements and constraints of the system. Some of the commonly used HI types are discussed below:

  1. Channel Quality Indicator (CQI): In this type of HI mechanism, the receiver sends a CQI to the transmitter to indicate the current channel conditions. The CQI is used by the transmitter to choose the appropriate modulation and coding scheme for the initial transmission and any retransmissions.
  2. Acknowledgment/Negative Acknowledgment (ACK/NACK): In this type of HI mechanism, the receiver sends an ACK or NACK to the transmitter to indicate whether the received data was successfully decoded or not. If the data was not successfully decoded, the NACK includes information about the type of retransmission that should be used.
  3. Hybrid ARQ with Soft Combining (HARQ-SC): In HARQ-SC, the receiver sends a Soft Combining (SC) indicator to the transmitter to indicate whether soft combining should be used in the retransmission. Soft combining allows the receiver to combine the soft information (reliability metric) from multiple transmissions to improve the decoding performance.
  4. Hybrid ARQ with Incremental Redundancy and Chase Combining (HARQ-IRC): In HARQ-IRC, the HI includes information about whether IR or CC should be used for the retransmission. The transmitter uses this information to determine the appropriate retransmission strategy for the specific data packet.

Applications of HI:

The HI mechanism has several important applications in modern wireless communication systems, including:

  1. Adaptive Modulation and Coding (AMC): The HI mechanism is used in AMC to select the appropriate modulation and coding scheme for each data packet based on the channel conditions. By selecting the appropriate modulation and coding scheme, the system can achieve a higher data rate and better spectral efficiency.
  2. Multiple Input Multiple Output (MIMO): The HI mechanism is used in MIMO to select the appropriate spatial streams for each data packet based on the channel conditions. By selecting the appropriate spatial streams, the system can achieve a higher data rate and better spectral efficiency.
  3. Quality of Service (QoS): The HI mechanism is used in QoS to prioritize different types of data packets based on their importance and the required quality of service. By prioritizing the data packets, the system can ensure that the most important data is delivered with the required quality of service.
  4. Interference Mitigation: The HI mechanism is used in interference mitigation techniques to select the appropriate interference cancellation scheme for each data packet based on the interference conditions. By selecting the appropriate interference cancellation scheme, the system can mitigate the effects of interference and improve the overall system performance.

Conclusion:

In conclusion, the Hybrid ARQ Indicator (HI) is an important signaling mechanism used in HARQ to inform the receiver about the type of retransmission that should be used in case of a transmission error. The HI mechanism allows the transmitter to choose the appropriate type of retransmission for each data packet, depending on the channel conditions and the performance requirements. The HI mechanism has several important applications in modern wireless communication systems, including adaptive modulation and coding, multiple input multiple output, quality of service, and interference mitigation. By using the HI mechanism, wireless communication systems can achieve a higher data rate, better spectral efficiency, and improved system performance.