  Wireless data aggregation (WDA), referring to aggregating data distributed at
devices (e.g., sensors and smartphone), is a common operation in 5G-and-beyond
machine-type communications to support Internet-of-Things (IoT), which lays the
foundation for diversified applications such as distributed sensing, learning,
and control. Conventional WDA techniques that are designed based on a
separated-communication-and-computation principle encounter difficulty in
accommodating the massive access under the limited radio resource and stringent
latency constraints imposed by emerging applications (e.g, auto-driving). To
address this issue, over-the-air computation (AirComp) is being developed as a
new WDA solution by seamlessly integrating computation and communication. By
exploiting the waveform superposition property of a multiple-access channel,
AirComp turns the air into a computer for computing and communicating functions
of distributed data at many devices, thereby allowing low-latency WDA over
massive devices. In view of growing interests on AirComp, this article provides
a timely overview of the technology by introducing basic principles, discussing
advanced techniques and applications, and identifying promising research
opportunities.
