Hybrid Automatic Repeat reQuest (HARQ) is a crucial mechanism in 5G that plays a fundamental role in ensuring reliable and error-free data transmission over the radio interface. HARQ is designed to improve the efficiency and robustness of communication by combining both automatic repeat reQuest (ARQ) and error correction coding. Here’s a detailed explanation of the HARQ process in 5G:
- Purpose of HARQ:
- The primary purpose of HARQ is to address errors that may occur during the transmission of data over the wireless channel. The radio channel is subject to various impairments, such as fading, interference, and noise, leading to the occasional corruption of transmitted data. HARQ aims to recover these errors and enhance the overall reliability of the communication link.
- Hybrid Approach – Combining ARQ and FEC:
- HARQ adopts a hybrid approach by combining the principles of ARQ and Forward Error Correction (FEC). This allows for the correction of errors at both the sender (transmitter) and receiver sides, increasing the likelihood of successful data transmission.
- Transmission of Data Blocks:
- The data to be transmitted is divided into smaller blocks, known as transport blocks. These transport blocks are then encoded using a coding scheme to add redundancy, enabling error detection and correction.
- Initial Transmission (New Transmission):
- The initial transmission involves sending the transport blocks from the transmitter to the receiver. If the receiver successfully receives and decodes the blocks without errors, the process ends, and the data is considered delivered.
- Acknowledgment (ACK) and Negative Acknowledgment (NACK):
- Upon receiving the data, the receiver sends an acknowledgment (ACK) to the transmitter if the decoding is successful. If errors are detected, the receiver sends a negative acknowledgment (NACK), indicating that some or all of the data blocks are corrupted.
- Retransmission (ARQ Process):
- In the case of a NACK, the transmitter retransmits the same data blocks, providing the receiver with another opportunity to successfully receive and decode the information. This retransmission is a classic ARQ process.
- Combining FEC and ARQ:
- The retransmitted data may still contain errors, or new errors may be introduced. To enhance error recovery, FEC is applied to the retransmitted data, adding an additional layer of protection beyond the original encoding.
- Soft Combining and Incremental Redundancy:
- HARQ introduces concepts like soft combining and incremental redundancy to further improve the chances of successful error recovery. Soft combining involves combining the information from multiple transmission attempts, and incremental redundancy allows for sending additional parity information during retransmissions.
- Round Trip Time (RTT) Considerations:
- The efficiency of HARQ is influenced by the round-trip time (RTT) between the transmitter and the receiver. A shorter RTT allows for quicker feedback, enabling faster retransmissions and reducing the impact of channel variations.
- Adaptive Modulation and Coding (AMC):
- HARQ is often integrated with Adaptive Modulation and Coding (AMC), where the modulation and coding scheme can be adjusted dynamically based on the channel conditions. This adaptability improves overall system efficiency.
- Combining with Multiple Antennas (MIMO):
- HARQ can be further enhanced when combined with Multiple Input Multiple Output (MIMO) techniques, leveraging spatial diversity to improve the chances of successful data transmission.
In summary, the HARQ process in 5G is a dynamic and adaptive mechanism that combines ARQ and FEC to address errors in the wireless communication channel. It plays a crucial role in achieving reliable and robust data transmission, contributing to the overall performance and quality of service in 5G networks.