# A Channel Compensation Technique Based on Frequency-Hopping Binary Offset Carrier Modulated Signal
## 1 Introduction  
The Space-Air-Ground Integrated Network (SAGIN), which has been acknowledged as promising future communication network, has gained considerable attention due to its high mobility and seamless global coverage. During the past years, the limited network capacity and resilience, depending only on terrestrial communications, is impractical for supporting progressive standards and technologies [1; 2; 3]. Additionally, what allows SAGIN to outperform others is the additional two segments, besides the terrestrial network. Satellites, as a space segment, are able to provide seamless connectivity across huge areas according to the spacious coverage, and such coverage is supported by maturing construction of satellite communication networks [4]. Meanwhile, unmanned aerial vehicles (UAVs) behave as air segments, can enhance communication networks capacity, and can support high throughput accesses for ground devices [5]. In the construction of SAGIN, UAVs are indispensable and behave as a mainstay in the integration of satellite-terrestrial communication. Satellites themselves are not capable of handily satisfying tremendous requests from the ground, but numerous applications can be realized, likely with the assistance of UAVs [6]. In fact, several advanced projects have been developed based on SAGIN, such as the OneWeb constellation [7], the Global Information Grid (GIG) [8], SpaceX [9], etc. Therefore, such potential and importance motivate us to study the channel characteristics of space-to-air communications. In the construction of SAGIN, current wireless communication systems for UAVs and satellites, respectively, are designed with distinct demands and limitations. The aerial communication platform comprised of UAVs possesses the dynamic deployment abilityto arrange a network corresponding to requirements in a moment, thus a problem raised on real-time capacity optimization in connection with UAV placement and trajectory to refine overall network performance [10; 11]. It also contributes to the solution or a relief of line-of-sight (LoS) problems, which refers to the obstructed transmission path in terrestrial channels having problems with providing reliable data flow or energy signals to terminal devices in occasions of numerous air-to-ground communications majorly. As for satellites communication, the global navigation satellite system (GNSS) has been developed for decades, from the initial military proposal of designing global positioning system (GPS) to various versatile satellite systems satisfying distinct demands, such as atmosphere monitoring, geophysical parameters observing [12] on the basis of navigation, and measuring functions. Additionally, quite a few limitations have been imposed, such as occasional dynamic environment and ionospheric scintillation as a common atmospheric interference [13]. In fact, Figure 1, below, suggests a practical application scenario for SAGIN, where either segment has inner connections and is responsible for coordinating each other. As a consequence, however, SAGIN inevitably adopts different communication protocols in accordance with separated segments for maintaining its superiority regarding high capacity and data throughput [14]. Hence, in order to refine the performance of integrated network, communication between UAVs and satellites as space-to-air propagation is acknowledged as a highly comparative factor. Focusing on space-to-air communications, a lot of related studies have been performed to deal with all kinds of problems and challenges. Signal attenuation can be easily noticed, since there exists quite a long distance between UAVs and satellites, and the directions of the antennas are required to be adjusted to retain a higher gain [15; 16], which is somehow beyond the issue discussed in our work. Meanwhile, such fast relative movements can cause a dynamic circumstance and lead to Doppler frequency shift.