URLLC Over Slow Fading Channels: K-Repetition Versus Multi-Connectivity

In this work, we analyze the performance of the uplink ultra-reliable and low-latency communications (URLLC) for the low-mobility users (UEs) in a factory environment. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$K$</tex-math></inline-formula>-repetition scheme is a promising approach for improving URLLC reliability, but most studies so far have been carried out assuming fast fading. A UE with low mobility typically experiences a time-correlated slow fading channel. We therefore consider the effects of time correlation in analyzing the performance of a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$K$</tex-math></inline-formula>-repetition scheme. On the other hand, multi-connectivity (MC) schemes can overcome such effects and improve reliability through space diversity. We then apply differential modulation to both schemes to support URLLC services, and compare their performance. The simulation results show that such slow fading channels severely decreases the time diversity of a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$K$</tex-math></inline-formula>-repetition scheme, and the MC scheme outperforms the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$K$</tex-math></inline-formula>-repetition scheme in terms of reliability and transmission latency.