(A) design of digital beam-steering system with signal calibration structure for millimeter-wave beamforming implementation

Abstract

As interest in 5G($5^{th}$ generation) mobile communication has increased recently, research regarding new mobile communication technologies has been actively conducted. Among the various communication technologies, the system using beamforming technique is one of the leading technologies in the 5G. The beamforming technique is addressed in several topics as well as in radar as a way of transmitting or receiving the beam of the array antenna in one direction. South Korea’ 5G mobile telecommunication frequency band is 3.5GHz or millimeter-wave band, which is higher than the 3G and 4G. Because the higher the frequency band, the higher the linearity of the propagation, and the higher the loss in air, beamforming technique can be used to solve this and implement mobile communication. It is the key objective of the topic to precisely control the phase differences between the antennas, because the beamforming technique uses multiple antennas. This thesis discusses the proposed new signal calibration method and signal phase control system for the topic in the millimeter-wave band. The higher the frequency, the shorter the wavelength, so the phase of signal can be changed by several factors. For example, phase-stabilized systems may have different phases of the output signal, even though they have the same structure. In this case, even if the beamforming is applied, the beamforming system cannot implement a beam in a desired direction. Thus, the structure of a phase comparator and calibration system is necessary in the beamforming system. For signal phase detection, the phase difference between RF(Radio Frequency) signals can be measured up to 360° and the phase comparator with an error of less than 1° was implemented. The phase differences between millimeter-wave signals obtained through the phase comparator is synchronized with the DDS(Direct Digital Synthesizer), and a digital beam-steering system for controlling the phase of each system is proposed. Finally, the beamforming radiation pattern of the phase calibrated system is measured and compared to the simulation results using the proposed method. First, after stabilizing the system, the phase difference before the phase calibration is detected using the designed phase comparator and the radiation pattern is measured. As a result of synchronizing the phase using the signal calibration system and implementing beamforming with the digital beam-steering system, the measured radiation patterns have been verified that it is similar the simulated pattern. The digital beamforming system also showed that the main beam of radiation pattern can be implemented in the desired direction and the beam can be controlled at 1° intervals.