Phased array antenna is an antenna system consisting of a large number of elements, each of which can control its phase and amplitude independently. It can provide high-precision beam control and realize rapid beam direction switching. Therefore, phased array antennas are widely used in aerospace, navigation, radar and other fields. Typically, phased arrays use transmit/receive (T/R) technology to control the elements. By adjusting the amplitude and phase of the elements using digital phase shifters and digital attenuators, the direction of beam can be determined. However, this conventional approach can be costly, especially for achieving high-resolution beam scanning, as it requires complex and expensive phase shifters. In addition, the control accuracy and bandwidth of traditional digital phase shifters are also constrained by the performance of hardware circuit. To address the need for high accuracy, cost effectiveness, and ultra-wideband capability in phased array antenna systems, time-modulated array (TMA) has received much attention in recent years, which has many advantages, such as high accuracy, low cost, wide bandwidth and reconfigurability. In the 1960s, researchers introduced the concept of TMA, which uses the time dimension as an additional variable to control the radiation characteristics of antennas. By periodically adjusting antenna parameters such as excitation, aperture shape, frequency, phase distribution, and aperture size, a periodic radiation pattern that varies with time can be achieved. Based on the periodic fluctuation of the radiation pattern, an infinite number of independent channels can be obtained, each corresponding to a harmonic frequency component of the time modulation. Each harmonic has a distinct spatial factor, allowing for simultaneous operation of multiple modes in a single antenna, synthesizing radiation characteristics that conventional antennas do not possess. It has great potential for applications in military and civilian fields, such as precision guidance and mobile communications. This paper provides a comprehensive review of the development process of time-modulated phased array (TMPA) technology, introduces the latest research progress in efficiency improvement, sideband suppression, and sidelobe suppression. Firstly, we introduce the basic principles and relevant concepts of TMA, and provide the fundamental formulas of time sequence, spectrum, and radiation pattern. Then, based on the analysis of TMA loss mechanisms, several implementation methods for improving the efficiency of TMA are discussed, including array energy allocation and element efficiency enhancement. Then, this paper analyzes a series of performance enhancement schemes for low sideband level TMPA and low sidelobe TMPA, such as "-1/+1" rectangular pulse modulation, in-phase/quadrature (I/Q) modulation, stepped waveform modulation, phase modulation, double modulation, and pseudorandom modulation. Finally, the paper presents research on time-modulated phased array chips based on CMOS (complementary metal-oxide semiconductor) technology. Although TMPA technology has made significant progress in recent years, there is still a need for further development in terms of establishing a comprehensive theory and addressing practical details. For example, there is a need to explore the integration between amplitude modulation and phase modulation, the calibration of the time modulation system, and breakthroughs in the design of new array structures beyond traditional linear and planar arrays. The above introduction can serve as a reference for further research on time-modulated phased arrays, and the relevant results are expected to be promoted and applied in the frontiers of military, aerospace and next-generation wireless communications.
