Study on Full Digital Beamforming for HAPS Backhaul System with Base Station On-board Using 38 GHz Band Frequency

Extreme coverage extension is one of its requirements and use cases in beyond 5G and 6G era. HAPS (high-altitude platform station) has drawn attention because of its characteristics to stay at a fixed location of an altitude of about 20 km, typically forming a coverage area with a cell radius of more than 50 km on the ground and having much lower latency than LEO with a one-way propagation time of about 0.1 ms. This paper treats a HAPS backhaul system using the millimeter wave band (38 GHz band) with base station on-board. In this target system, huge number of antenna array elements for beamforming is needed to compensate free space loss in 38 GHz band, and full digital beamforming is desired to connect multiple ground stations for feeder and service links and at the same time. Since digital beamforming needs analog circuit for each antenna array element, it leads to high power consumption. Phase coherency calibration is needed for every array antenna element to achieve good beamforming performance, and real-time calibration to directly measure in-band characteristics in 38 GHz band is desired in a circumstance of very low temperature and very low atmospheric pressure at stratosphere. It is technically challenging because of huge number of array antenna element. For these technical issues, we propose power reduction by low resolution ADC/DAC with dithering by utilizing beamforming gain and real-time phase coherency calibration to directly measure in-band characteristics in 38 GHz band without interrupting to transmit main signal by superimposing known spread spectrum signal with low power level. Computer simulation shows that when the target of phase error is $\sigma$ = 1 degree (i.e., SNR = 35 dB) or less with 5 bits of DAC input, calibration for 2 polarization (vertical and horizontal) and 1024 antenna elements can be done every 4 seconds.