(A) study on high resolution digital beamforming receiver and real-time calibration technique

Abstract

Currently, high resolution beamforming technology is required in various fields. High resolution beamforming technology is required in space astronomy to accurately track low-altitude space objects and to grasp three-dimensional information such as distance, altitude, azimuth, and speed from an observer through radio signals scattered from space objects. In addition, it is also required in 5G mobile communication, which requires accurate classification and identification of targets for data transmission and reception. Due to the necessity of such high resolution beamforming technology, this paper deals with high resolution beamforming technology in 28GHz. First, This paper presents a fully digital beamforming receiver (FDBR) using a method that calibrates the signals of all chains in real time. In the real-time calibration method, the received signals of all chains are adjusted to correct the errors of phase and amplitude using the in-band signal other than operating frequency for calibration. The proposed FDBR with real-time calibration is designed and fabricated. The FDBR consists of eight chains of the tapered slot antenna (TSA) element, low noise block (LNB), and software defined radios (SDRs). The 1 × 8 array TSA with the directional coupler and the 1:8 divider is designed to send eight uniform calibration signals along with the received signal of all the chains. In SDR, the digital phase shifter and the real-time calibration blocks are implemented to realize digital beamforming. The digital phase shifter has an extremely high resolution of 0.72°. After using the real-time calibration method, the average of measured phase and amplitude error between each chain is less than 0.9° and 0.5 dB, respectively. To verify the beamforming performance of the FDBR, the simulation radiation pattern and the measurement radiation pattern are compared for 0°, ±15°, ±30°, and ±45° beam angles. The simulation results are in good agreement with the measured results. An excellent beamforming performance is achieved in the 1 × 8 array FDBR using the real-time calibration. However, the Fully Digital Beamforming structure has a disadvantage in that it requires a very high specification of the signal processing unit to process the large amount of data coming through numerous chains. In order to improve these drawbacks, secondly, this paper presents a signal generator and a Digital Beamforming Receiver (DBR) with high phase control resolution and high beam control resolution, respectively. The signal generator is designed based on a Direct Digital Synthesizer (DDS) and a Phase Lock Loop (PLL). In the DDS-PLL signal generator, the conventional divider in PLL is replaced with a mixer, comb generator, and a doubler to utilize high phase control resolution of DDS (14bit, 0.022°) as it is and to synchronize the DDS-PLL signal generator with the entire system. In the DBR, the output signals of the DDS-PLL signal generators are used as the second Local Oscillator (LO) signal, and the beamforming technique is implemented through the change of the phase of the second LO signals. Also, for sophisticated beamforming, all signal generator components in the DBR are synchronized for generating same and fixed frequency and phase. Finally, the DBR has the same phase control resolution as the DDS, and the phase of each chain of the DBR can be adjusted up to 0.022° (14 bit). The DBR using the DDS-PLL signal generator is designed and fabricated. The DBR consists of a 1X8 array Tapered Slot Antenna (TSA) part, a 1st and 2nd frequency conversion part, a 1X8 data acquisition and combiner part, DDS-PLL signal generator part, and a reference signal generator part. The phase control performance of the DBR was verified by measuring the phase change of the chain in the DBR according to the DDS phase change. Also, to verify the beamforming performance of the DBR with high beam control resolution, the radiation pattern of the DBR when the angle of the main lobe was 0°, 0.2°, 0.4°, 15°, and 30° was measured in the 28GHz band. At the main lobe, the gain error is within 1dB and the beam angle error is within 0.2 degrees. The simulation results are in good agreement with the measured results. A high resolution beamforming performance is achieved in the DBR using the DDS-PLL signal generator.