Development of 38GHz-band wireless communication system using high altitude platform station (HAPS) for 5G network--study on delay calibration of full digital beamforming for HAPS with base station on-board--

<p>HAPS (High-Altitude Platform Station) is expected to extend the communication coverage in the era of beyond 5G. This paper treats a HAPS backhaul system using 38GHz 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 38GHz band, and full digital beamforming is desired to connect multiple ground stations for feeder and service links at the same time. Real-time phase coherency calibration to directly measure in-band characteristics in 38GHz band is desired for every array antenna element in very low temperature and very low atmospheric pressure at stratosphere to achieve good beamforming performance. It is technically challenging because of huge number of array antenna element. In our previous work, we proposed real-time phase coherency calibration to directly measure in-band characteristics in 38GHz band without interrupting to transmit main signal by superimposing known spread spectrum signal with low power level. However, in addition, delay differences within one sample among antenna elements cause in-band phase slope within wide bandwidth and lead to bad beamforming performance. This paper proposes delay calibration utilizing many common parts of real-time phase coherency calibration in our previous work. Computer simulation shows that “early - late” correlation, whose sample timings are +/-1.0 sample from the center respectively, has precise linearity for calibration signal plus 5G NR signal. Based on the linearity, the amount of delay estimation within one sample is possible by using a look up table.</p>