This paper comprehensively reviews the recent 5G and future 6G Internet of Things (IoT) protocols/standards, applications, and access networks used. First, most of the IoT protocols/standards and application scenarios are summarized in the form of tables, pictures, and diagrams to facilitate readers to understand and compare current and future Internet of Things technologies more easily and quickly. Second, the terrestrial and aerial radio access networks are analyzed and discussed in detail. The evolution of 5G terrestrial access networks is briefly described and its performance limitations are quantitatively analyzed and discussed. When the operating frequency reaches the sub-millimeter wave band, the terrestrial radio access network will deal with high path loss caused by weather factors, such as oxygen and water vapor absorption in the atmosphere, rainfall, and cloud/fog attenuation. The development of aerial radio access networks is preparing for 6G IoT to solve the coverage and path loss issues. In this survey, the aerial radio access architectures and infrastructure are also surveyed. This survey aims to guide readers to better understand the technical status of 5G IoT and the milestones as well as key performance indicators that need to be reached for 6G IoT in the future.

One of the important studies in 6G aerial radio access networks is the propagation channel modeling. The high accurate propagation channel model will save cost and time, and is more effective in the design of the air radio access network system. However, existing channel models are limited to 1 THz, while 6G wireless technology is expected to operate up to 3 THz. In this paper, the propagation channel from 275 GHz to 3 THz is modeled by modifying the Friis equation, and each parameter in the model is described and analyzed analytically. The main factors that contribute to wireless signal attenuation at terahertz, such as atmospheric oxygen and water vapor, rainfall, and cloud factors, are also discussed in detail. Furthermore, the propagation channel calculation App for 6G low-altitude platform access networks application has been built using MATLAB GUI.