  As the number of real-time applications with ultra-low delay requirements
quickly grows, massive ultra-reliable and low-latency communication (mURLLC)
has been proposed to provide a wide range of delay-sensitive services for the
sixth generation (6G) wireless networks. However, it is difficult to meet the
stringent delay demand of massive connectivity with existing grant-based (GB)
random access and fixed frame structure in long-term evolution (LTE) and the
fifth generation (5G) new radio (NR) systems. To solve this problem, in this
paper we propose the new grant-free (GF) based adaptive blocklength scheme for
short packet transmission to reduce the access delay. We develop the adaptive
blocklength framework where the blocklength can be adaptively changed according
to the real-time load, to revise the traditional non-flexible frame structure
which impacts the delay performance. Taking the features of mURLLC into
consideration, we analyze the GF random access procedure, packet arrival
behavior, packet collision, and packet transmission error in the finite
blocklength (FB) regime. On this basis, we derive the closed-form expression of
successful access and transmission probability and give the GF-based status
update model. Then, we propose the access delay minimization problem that
jointly considers queuing delay and transmission delay to reduce the overall
access delay. With the alternating optimization algorithm, we obtain the
optimal blocklength of each packet, thus forming the corresponding adaptive
blocklength scheme for mURLLC. Simulation results verify the correctness of
theoretical results and show that our proposed adaptive blocklength scheme can
significantly reduce the access delay compared with that of LTE and 5G NR
systems.
