  With various time-sensitive tasks to be served, ultra-reliable and
low-latency communications (URLLC) has become one of the most important
scenarios for the fifth generation (5G) wireless communications. The end-to-end
delay from the sub-millisecond-level to the second-level is first put forward
for a wide range of delay-sensitive tasks in the future sixth generation (6G)
communication networks, which imposes a strict requirement on satisfying
various real-time services and applications with different stringent quality of
service (QoS) demands. Thus, we need to find out new delay reduction framework
to satisfy the more stringent delay requirements. In this article, a
state-of-the-art overview of end-to-end delay composition and delay analysis
combined with access protocols are elaborated. We reveal the tradeoff
relationship among transmission delay, queuing delay, and retransmission times
with the change of blocklength in the finite blocklength (FBL) regime. Based on
the tradeoff and combining the grant-free (GF) random access (RA) scheme, we
propose the adaptive blocklength framework and investigate several effective
algorithms for efficiently reducing the over-the-air delay. Numerical results
show that our proposed framework and schemes can significantly reduce the
over-the-air delay for URLLC.
