Short Block-length Codes for Ultra-Reliable Low-Latency Communications

Abstract: This paper reviews the state of the art channel coding techniques for ultra-reliable low latency communication (URLLC). The stringent requirements of URLLC services, such as ultra-high reliability and low latency, have made it the most challenging feature of the fifth generation (5G) mobile systems. The problem is even more challenging for the services beyond the 5G promise, such as tele-surgery and factory automation, which require latencies less than 1ms and failure rate as low as $10^{-9}$. The very low latency requirements of URLLC do not allow traditional approaches such as re-transmission to be used to increase the reliability. On the other hand, to guarantee the delay requirements, the block length needs to be small, so conventional channel codes, originally designed and optimised for moderate-to-long block-lengths, show notable deficiencies for short blocks. This paper provides an overview on channel coding techniques for short block lengths and compares them in terms of performance and complexity. Several important research directions are identified and discussed in more detail with several possible solutions.

• The paper compares various state-of-the-art channel codes against finite block-length normal approximations to meet URLLC performance standards.
• It demonstrates that BCH codes with order-statistics decoding achieve near-optimal error performance at high reliability, despite high computational costs.
• The study highlights trade-offs between decoding complexity and performance, suggesting future research into low-complexity and adaptive coding schemes.

Short Block-Length Codes for Ultra-Reliable Low Latency Communications: An Overview

The paper "Short Block-length Codes for Ultra-Reliable Low Latency Communications" presents a detailed comparative study of various channel coding techniques aimed at meeting the stringent requirements of Ultra-Reliable Low Latency Communications (URLLC). Advanced channel coding is essential in the context of 5G and beyond, where services such as tele-surgery and factory automation demand ultra-low latency (below 1ms) and extremely high reliability (packet error rates as low as $10^{-9}$).

The authors provide a critical assessment of several state-of-the-art channel codes, focusing on their suitability for short block lengths—a key parameter for achieving low latency. These include classical codes like BCH and convolutional codes, as well as more contemporary approaches such as LDPC, Turbo, and Polar codes, which are discussed in terms of both performance and complexity under practical decoding scenarios.

Key Findings

1. Distinctive Reliability Requirements: The paper outlines the particular requirements for various URLLC applications, highlighting the trade-offs between latency and reliability. It underscores that standard error correction techniques, traditionally tailored for long block lengths, are inadequate when applied to URLLC scenarios without modifications.
2. Performance Benchmarks: Through simulations, the paper benchmarks the performance of these codes against the normal approximation, which accounts for finite blocklengths, providing a more realistic comparison than asymptotic limits like Shannon's capacity.
3. Error Performance Analysis: Among the codes evaluated, BCH codes with order-statistics decoding (OSD) achieved performance close to the normal approximation even at high reliability requirements, showing their potential albeit at a high computational cost. TB-CC, LDPC, and CA-Polar codes displayed promising results as well, with trade-offs between performance and complexity.
4. Complexity Considerations: The study notes the significant computational complexities associated with some advanced decoders, such as OSD for BCH codes. Polar codes with successive cancellation list (SCL) decoding also demonstrate high performance with reasonable computational demands, particularly as list size increases, though complexity remains a challenge at larger scales.

Implications and Future Directions

The findings of the paper suggest several implications for both theoretical research and practical application. The notable promise of BCH codes, when decoding complexity can be managed, indicates potential areas for optimization in decoder design to mitigate computational demands while maintaining high reliability. The analysis of BCH and TB-CC codes reveals the importance of decoding algorithm enhancements, such as OSD, to unlock their full capabilities in URLLC contexts.

Potential future work highlighted by the authors includes:

• Development of Low Complexity OSD: Continued research into simplifying OSD while preserving its near-optimal performance for BCH codes represents a viable pathway for enhancing URLLC systems.
• Self-Adaptive Joint Coding and Modulation: Exploring self-adaptive coding frameworks that eliminate the need for CQI feedback could significantly reduce latency, pointing towards rateless coding approaches integrated with intelligent modulation schemes.
• Space-Frequency Diversity: Expanding coding strategies to leverage spatial and frequency diversity can further enhance reliability without incurring additional latency, a valuable trait for mission-critical applications.

Ultimately, the paper elucidates that while substantial strides have been made, the pursuit of truly efficient and ultra-reliable low latency communications remains an active and critical domain of research, inviting further innovation in channel coding techniques tailored for short block-length regimes.