  Ultra reliable and low latency communications (URLLC) is a new service
category in 5G to accommodate emerging services and applications having
stringent latency and reliability requirements. In order to support URLLC,
there should be both evolutionary and revolutionary changes in the air
interface named 5G new radio (NR). In this article, we provide an up-to-date
overview of URLLC with an emphasis on the physical layer challenges and
solutions in 5G NR downlink. We highlight key requirements of URLLC and then
elaborate the physical layer issues and enabling technologies including packet
and frame structure, scheduling schemes, and reliability improvement
techniques, which have been discussed in the 3GPP Release 15 standardization.
