On Preamble-based Grant-Free Transmission in Low Power Wide Area (LPWA) IoT Networks

Grant-free (GF) access is one of the prominent access strategies being considered for massive machine type communication (mMTC) facilitating the low power wide area (LPWA) Internet of things (IoT). GF access in the uplink typically uses a preamble followed by data transmissions. Building GF systems ensuring a high success probability for user access while utilizing a limited number of preambles is a challenge. In this work, a preamble-based GF IoT system is considered, where the total number of users in the system correspond to two categories - (i) fixed and (ii) random (modelled using a Poisson distribution). Analytical expressions are derived for the number of preambles required for GF access in each of the categories, such that the user transmissions are successful with high probability. It is shown that the analytical results match with the simulation results. Furthermore, an approach to choose the number of preambles based on the probability of success of at least all but two users is proposed, which results in a significant reduction in the number of preambles.