Because the fifth-generation (5G) mobile wireless networks show promise in meeting the service requirements of Internet of Things, 5G systems must be equipped with the ca-pacity to interconnect all existing and emerging technologies. To keep up with this chal-lenge, heterogeneous networks (HetNets) can be deployed; in HetNets, the coverage area of a macro base station is embedded with many smaller base stations. This paper presents the architecture of a HetNet with overlapping small cells (HNOSC) for 5G IoT networks. The proposed HNOSC architecture considers the movement of user equipment (UE) and the overlapping area between any two small cells. We develop an analytical model by using a Markov chain of three states to derive a closed-form formula for the total downlink transmission power (TDTP) in terms of the radius of each small cell, size of the overlap-ping area, and UE mobility. Given the path loss factor and associated parameters of UE movement, the relationship between the coverage area of overlapping small cells and the TDTP consumption can be derived. Thus, the deployment of small base stations can be optimised to obtain the best trade-off between UE service quality and operator cost.