An Experimental Study on Automatic Gain Control in HAPS Wireless Repeater System

A high-altitude platform station (HAPS) is a new mobile communication platform that directly provides mobile communication services such as fourth-generation long-term evolution (4G LTE) or fifth-generation new radio (5G NR) from the stratosphere to terrestrial user equipment (UE) by utilizing aircraft such as solar planes flying in the stratosphere. In this paper, we study on an automatic gain control (AGC) system for HAPS wireless repeater systems. We assume a repeater system, which uses different frequencies for the feeder and service links to avoid loop-back interference between the links. Measuring the receive power in the feeder link according to a known reference signal instead of the total receive power avoids fluctuations in the transmit power depending on the existence of traffic channels. We propose an AGC system for a HAPS system compatible with 5G NR, which enables the repeater to transmit at maximum power to maximize the coverage area regardless of changes in the propagation loss or polarization in the feeder link due to the flight of the HAPS aircraft. We conducted experiments to demonstrate the performance of the proposed AGC system.