HARQ Enhancements

Hybrid Automatic Repeat Request (HARQ) is a technique used in wireless communication systems to improve the reliability of data transmission. HARQ combines forward error correction (FEC) and automatic repeat request (ARQ) to improve the reliability of data transmission. The basic idea behind HARQ is to detect and correct errors in the transmitted data at the receiver, and if necessary, request the retransmission of the lost data from the sender. In this way, HARQ can improve the throughput and quality of service of wireless communication systems.

Over the years, many enhancements have been proposed to HARQ to improve its efficiency, reliability, and performance. In this article, we will discuss some of the HARQ enhancements that have been proposed in the literature.

Incremental Redundancy

One of the main drawbacks of the conventional HARQ scheme is that it requires the retransmission of the entire data packet when an error is detected in the received packet. This can result in a significant delay and waste of resources, especially when the data packet is large. To overcome this problem, incremental redundancy (IR) has been proposed as an enhancement to HARQ.

In IR, the sender does not transmit the entire data packet in the first transmission, but only sends a part of it. If an error is detected in the received packet, the sender transmits additional redundant bits to correct the errors in the first transmission. This process continues until the entire data packet is successfully received at the receiver.

IR can significantly reduce the delay and resource usage in HARQ, as it avoids the retransmission of the entire data packet. However, IR requires the use of more sophisticated coding schemes, which can increase the complexity of the system.

Chase Combining

Chase combining (CC) is another enhancement to HARQ that can improve the reliability of data transmission. In CC, the receiver combines the information from multiple transmissions of the same data packet to improve the decoding performance. This is done by storing the received packets in a buffer and combining the information from them using a maximum likelihood decoder.

CC can improve the decoding performance of HARQ by exploiting the diversity in the received packets. However, CC requires additional storage and processing at the receiver, which can increase the complexity of the system.

Soft-Decision Decoding

Soft-decision decoding (SDD) is a technique that can improve the decoding performance of HARQ by using soft information about the received packets. In SDD, the receiver uses the likelihood values of the received symbols instead of the hard decisions to decode the received packet. This can significantly improve the decoding performance, especially in the presence of noise and interference.

SDD can improve the reliability and throughput of HARQ, as it can improve the decoding performance in noisy and interference-prone environments. However, SDD requires the use of more sophisticated decoding algorithms, which can increase the complexity of the system.

Joint Source-Channel Coding

Joint source-channel coding (JSCC) is an enhancement to HARQ that can improve the efficiency and reliability of data transmission by jointly optimizing the source and channel coding schemes. In JSCC, the coding schemes are optimized to minimize the total distortion of the transmitted data packet, taking into account the channel conditions and the constraints of the transmission system.

JSCC can improve the efficiency and reliability of data transmission by jointly optimizing the source and channel coding schemes. However, JSCC requires the use of more sophisticated coding schemes and optimization algorithms, which can increase the complexity of the system.

Rate-Compatible Punctured Codes

Rate-compatible punctured codes (RCPC) is an enhancement to HARQ that can improve the throughput and efficiency of data transmission. In RCPC, the coding scheme is designed to support multiple transmission rates, which can be used to adapt the transmission rate based on the channel conditions and the quality of service requirements. RCPC achieves this by puncturing the higher rate code to obtain lower rate codes, which can be used for lower quality channel conditions.

RCPC can improve the efficiency and throughput of data transmission by adapting the transmission rate to the channel conditions and quality of service requirements. However, RCPC requires the use of more sophisticated coding schemes and puncturing algorithms, which can increase the complexity of the system.

Adaptive Modulation and Coding

Adaptive modulation and coding (AMC) is an enhancement to HARQ that can improve the efficiency and reliability of data transmission by adapting the modulation and coding scheme to the channel conditions. In AMC, the transmission rate, modulation scheme, and coding scheme are adapted to the channel conditions to optimize the throughput and reliability of data transmission.

AMC can improve the efficiency and reliability of data transmission by adapting the modulation and coding scheme to the channel conditions. However, AMC requires the use of more sophisticated modulation and coding schemes and optimization algorithms, which can increase the complexity of the system.

Multi-User HARQ

Multi-user HARQ (MU-HARQ) is an enhancement to HARQ that can improve the efficiency and reliability of data transmission in multi-user environments. In MU-HARQ, the sender and receiver use different sets of HARQ processes for different users, which can reduce the interference and improve the throughput and reliability of data transmission.

MU-HARQ can improve the efficiency and reliability of data transmission in multi-user environments by reducing the interference and optimizing the use of resources. However, MU-HARQ requires the use of more sophisticated resource allocation algorithms, which can increase the complexity of the system.

Conclusion

In this article, we discussed some of the HARQ enhancements that have been proposed in the literature. These enhancements can improve the efficiency, reliability, and performance of HARQ in wireless communication systems. However, each enhancement has its own advantages and disadvantages, and the choice of enhancement depends on the specific requirements of the system. The selection of the appropriate HARQ enhancement can significantly improve the performance of wireless communication systems and provide a better quality of service for users.