New wave of the technology revolution, often referred to as the fourthindustrial revolution, is changing the way we live, work, and communicate witheach other. These days, we are witnessing the emergence of unprecedentedservices and applications requiring lower latency, better reliability massiveconnection density, and improved energy efficiency. In accordance with thistrend and change, international telecommunication union (ITU) defined threerepresentative service categories, viz., enhanced mobile broadband (eMBB),massive machine-type communication (mMTC), and ultra-reliable and low latencycommunication (uRLLC). Among three service categories, physical-layer design ofthe uRLLC service is arguably the most challenging and problematic. This ismainly because uRLLC should satisfy two conflicting requirements: low latencyand ultra-high reliability. In this article, we provide the state-of-the-artoverview of uRLLC communications with an emphasis on technical challenges andsolutions. We highlight key requirements of uRLLC service and then discuss thephysical-layer issues and enabling technologies including packet and framestructure, multiplexing schemes, and reliability improvement techniques.
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