End-to-End Delay Analysis of K-Repetition Grant-Free Access in Cell-Free Massive MIMO Systems

In order to meet the massive ultra-reliable and low-latency communication (mURLLC) demands of vertical industries in the Internet of Things (IoT), the research for end-to-end (E2E) delay has become a key direction of the sixth generation (6G) mobile communication. Cell-free massive multiple-input-multiple-output (CF-mMIMO) systems can provide enhanced anti-interference performance and reliability. Existing research is not comprehensive enough to analyze the E2E delay and are mostly limited to traditional cellular systems. In this paper, we propose the E2E delay model based on K-repetition grant-free (GF) access scheme in CF-mMIMO systems. Taking the finite blocklength regime into consideration, we present the process of packet queuing, random access, collisions and transmission errors. In particular, we analyze three significant components of the E2E delay, namely access delay, queuing delay and transmission delay. On this basis, we formulate the average E2E delay. Simulation results reveal the tradeoff relationship among access delay, queuing delay and transmission delay with the change of finite blocklength.