  Ultra-reliable and low-latency communications (URLLC) have stringent
requirements on quality-of-service and network availability. Due to path loss
and shadowing, it is very challenging to guarantee the stringent requirements
of URLLC with satisfactory communication range. In this paper, we first provide
a quantitative definition of network availability in the short blocklength
regime: the probability that the reliability and latency requirements can be
satisfied when the blocklength of channel codes is short. Then, we establish a
framework to maximize available range, defined as the maximal communication
distance subject to the network availability requirement, by exploiting
multi-connectivity. The basic idea is using both device-to-device (D2D) and
cellular links to transmit each packet. The practical setup with correlated
shadowing between D2D and cellular links is considered. Besides, since
processing delay for decoding packets cannot be ignored in URLLC, its impacts
on the available range are studied. By comparing the available ranges of
different transmission modes, we obtained some useful insights on how to choose
transmission modes. Simulation and numerical results validate our analysis, and
show that multi-connectivity can improve the available ranges of D2D and
cellular links remarkably.
