# Time-Varying Communication Channel High Altitude Platform Station Link Budget and Channel Modeling
## 1 Introduction  
As an airspace that humans have not systematically developed and utilized, near space is an important operational space for the future air and space weaponry system, and it is also a new \"strategic place\" for the great powers [1; 2]. Near space often refers to an airspace with a height of 20 to 100 km above the flying altitude and below the satellite's orbit. The high-altitude platform station (HAPS) is a platform that is stuck in a space of 20 to 50 km in height and is stationary relative to a specific location on the earth [3; 4]. HAPS can be seen as a communication system between a terrestrial communication system and a satellite communication system. The aim is to develop the potential benefits of a high space between land and space, improve communication capacity and spectrum utilization, and reduce system equipment cost and complexity. HAPS is smaller than the path fading of terrestrial communication systems. And has a smaller delay than the satellite communication system [5; 6]. To improve observation and coverage, the construction of surveillance and communication systems on near space platform has received further attention. HAPS has a similar development history to satellite communications in the 1960s [7]. The international telecommunication union (ITU) describes HAPS as \"a new technology with a promising application prospect that can subvert the development of the telecommunications industry.\" It is the foundation of next-generation wireless communications and has the ability to take full advantage of wireless spectrum resources, the system has large user capacity, good communication quality, and low operational risk. And you can upgrade the communication load operation [8; 9; 10] at any time. [PERSON] et al. [11] studied the inter-frequency forwarding of space satellite communications. The signal-to-noise ratio of the signal quality of the spatial communication link is quantitativelyanalyzed under the conditions of multipath fading and rain attenuation. [PERSON] et al. [12] analyzed the HATS IMT-2000 system. Mathematical modeling and simulation analysis of ultra-high frequency (49 GHz) link characteristics were carried out. [PERSON] et al. [13] analyzed the uplink capacity limitation of near space and the method of expanding the uplink and downlink capacity. [PERSON] [14] analyzed the relationship between Adaptive Modulation and Coding (AMC) and Automatic Repeat Request (ARQ) and Quality of Service (QoS) in the time domain. The time domain parameters such as modulation mode and coding mode can be dynamically adjusted according to the channel state. [PERSON] et al. [15] combines spatial channel transmission characteristics. The AMC and the joint physical layer are studied based on the cross-layer optimization design of spatial information based on link adaptation. ARQ technology at the data link layer [16; 17; 18]. The authors of [19; 20; 21; 22; 23], for high-altitude platform CDMA communication systems, established a wireless link model in quasi-stationary state. The main impact of the swing on the communication system is analyzed. Literatures [24; 25; 26] studied the effect of horizontal movement of the platform on HAPS. The probability of handover between terminals in the cell caused by the horizontal movement of the platform is analyzed. The fixed beam-width coverage cell model is given in [27; 28; 29; 30]. And analyzes the impact of platform rotation on the probability of switching between terminal cells. The rotation has the greatest influence on the outer honeycomb. The authors of [31; 32; 33] studied the state of platform motion (horizontal and vertical motion), changes in the ground coverage area. [PERSON] et al. [34] proposed a dynamic downlink transmission power of femtocells. Each femtocell adjusts its transmission power autonomously based on measured cost function unit. The transmission power level of femtocell is constrained by the rate of interference that femtocell produced to adjacent femtocells. [PERSON] et al.