I. Introduction
With the development of communication and radar, the millimeter-wave technology has increasingly drawn attention. In recent years, radar systems have been used widely. Many applications such as cancer detection as well as crash-avoidance system enormously benefit from the use of millimeter imaging systems. There are many kinds of imaging modalities, including binocular camera imaging, ultrasonic imaging, and near-field imaging. These imaging modalities have their own limitations such as sensitivity to environmental factors. Thus, millimeter-wave imaging has become a hotspot because of its anti-interference ability, its low-cost characteristic and its high resolution. It has not only small size but also light weight, and, as such, it can be conveniently integrated. Millimeter-wave imaging is less affected by the weather than the microwave counterparts [1]–[3].