  Massive Machine-Type Communication (mMTC) is expected to be strongly
supported by future 5G wireless networks. Its deployment, however, is seriously
challenged by the high risk of random access (RA) collision. A popular concept
is to reduce RA collisions by clustering mMTC devices into groups, and to
connect group members with device-to-device (D2D) links. However, analytical
models of this method and discussions about the reliability of D2D links are
still absent. In this paper, existing grouping-based solutions are reviewed, an
analytical model of grouped RA collision is proposed. Based on the analytical
model, the impact of D2D reliability on the collision rate is also
investigated. Afterwards, an efficient grouped RA procedure is designed to
extend the state-of-the-art with an efficient local group update mechanism
against D2D link exceptions.
