—Sheer number of devices in Internet of Things (IoTs) fundamentally challenge the ubiquitous information transmissions through the backbone networks, such as cellular systems. The heterogeneity of IoT devices and the hardware-based, inflexible cellular architectures impose even greater challenges to enable efficient communication. To address these challenges, this paper introduces the so-called SoftAir architecture on wireless software-defined networking and proposes software-defined gateways (SD-GWs) that jointly optimize cross-layer communication function-alities between heterogeneous IoT devices and cellular systems. First, the SoftAir architecture is proposed to support a unified software-defined platform for quality-of-service aware IoT systems and software-defined radio access networks (SD-RANs) with millimeter-wave transmissions. Next, the SD-GWs are designed in SoftAir to explore the interactions between two-types of networks (i.e., IoTs and SD-RANs) and enable cross-layer solutions that simultaneously achieve optimal energy savings and throughput gain in IoTs and maximum sum-rates in SD-RANs. Simulation results validate that our SoftAir solutions surpass classical IoT schemes by jointly optimizing communication functionalities for both IoTs and SD-RANs and bring significant system synergies for reliable 5G IoT communication.