  In this paper, we consider an uplink cellular Internet-of-Things (IoT)
network, where a cellular user (CU) can serve as the mobile data aggregator for
a cluster of IoT devices. To be specific, the IoT devices can either transmit
the sensory data to the base station (BS) directly by cellular communications,
or first aggregate the data to a CU through Machine-to-Machine (M2M)
communications before the CU uploads the aggregated data to the BS. To support
massive connections, the IoT devices can leverage the unlicensed spectrum for
M2M communications, referred to as IoT Unlicensed (IoT-U). Aiming to maximize
the number of scheduled IoT devices and meanwhile associate each IoT devices
with the right CU or BS with the minimum transmit power, we first introduce a
single-stage formulation that captures these objectives simultaneously. To
tackle the NP-hard problem efficiently, we decouple the problem into two
subproblems, which are solved by successive linear programming and convex
optimization techniques, respectively. Simulation results show that the
proposed IoT-U scheme can support more IoT devices than that only using the
licensed spectrum.
