A Comprehensive Survey of 5G URLLC and Challenges in the 6G Era

As the wireless communication paradigm is being transformed from human centered communication services towards machine centered communication services, the requirements of rate, latency and reliability for these services have also been transformed drastically. Thus the concept of Ultra Reliable and Low Latency Communication (URLLC) has emerged as a dominant theme for 5G and 6G systems. Though the latency and reliability requirement varies from one use case to another, URLLC services generally aim to achieve very high reliability in the range of 99.999\% while ensuring the latency of up to 1 ms. These two targets are however inherently opposed to one another. Significant amounts of work have been carried out to meet these ambitious but conflicting targets. In this article a comprehensive survey of the URLLC approaches in 5G systems are analysed in detail. Effort has been made to trace the history and evolution of latency and reliability issues in wireless communication. A layered approach is taken where physical layer, Medium Access Control (MAC) layer as well as cross layer techniques are discussed in detail. It also covers the design consideration for various 5G and beyond verticals. Finally the article concludes by providing a detailed discussion on challenges and future outlook with particular focus on the emerging 6G paradigm.