# Bidirectional Atmospheric Channel Reciprocity-Based Adaptive Power Transmission
## 1 Introduction  
Free-space optical communication has received a lot of interest recently due to the prevalence of data-traffic issues and its benefits of a small antenna size, no requirements of authorization, and high bandwidth [1]. The turbulence effect and background noise are the two primary features of the noise impact brought by the atmospheric channel in actual FSO communication system implementations. Due to atmospheric turbulence brought on by temperature, humidity, and other factors, a wireless optical signal sent over the atmosphere would have undesirable consequences such as beam expansion, beam drift, light-intensity flicker, and more [2]. The optical signal will be distorted by the background noise that solar radiation produces [3; 4]. Due to the turbulence effect and background radiation noise, the received signal has a high bit error rate [5], and the FSO communication system's performance is below optimal [6]. Researchers have investigated a variety of effective communication techniques to deal with the adverse impacts brought on by atmospheric turbulence. Multibeam light was proposed to be used for information transmission in FSO links in the literature [7; 8]. However, multibeam means multichannel, and the pointing inaccuracy is not insignificant. Multimode aperture diversity reception was studied in the literature [9; 10] to minimize turbulence damage. This method was more effective, but the system was less stable. According to the literature [11], optical phase conjugation (OPC) was used in FSO communication to mitigate the turbulence effect and correct for signal distortion. This technique could effectively improve the performance of FSO communication, but it might cause inter-channelcrosstalk and channel power imbalance. To decrease the system's emissive power and the likelihood of a system failure, ref. [12] proposed using statistical estimation of the approximate misalignment angle of multiple incident light beams. However, this method was less frequently used in weak turbulence scenarios. Ref. [13] came with a technique for high-dimensional coded communication employing polarization differential phase shift keying with the use of turbulence-resistant vector beams, but this method had not yet been used in practice. Refs. [14; 15] provided an in-depth study of compensation schemes using orbital angular momentum (OAM) in atmospheric turbulence, but OAM links are less effective in long-range communication. Additionally, the adaptive transmission method works well as a turbulence effect channel compensating technique. The rate adaptive transmission strategy, which had excellent results in communication transmission but was more challenging to implement, had been proposed in the literature [16]. The authors of ref. [17] replaced channel state information with the adaptive threshold decision (ATD) by using the low-pass characteristics of atmospheric turbulence to extract the instantaneous intensity. The quantization voltage of the judgment signal was used in ref. [18] as the judgment threshold for an adaptive threshold decision system. Ref. [19] utilized an adaptive linear prediction filter for CSI prediction and ATD using predicted CSI. Although FSO systems of refs. [17; 18; 19] based on adaptive approaches had higher performance and reduced loss, they all disregard the impact of background noise. Most studies have concentrated on unidirectional atmospheric channel transmission, although bidirectional atmospheric channel transmission has also drawn more interest since the introduction of [PERSON]'s original concept of channel reciprocity [20]. Bidirectional atmospheric channel transmission consists of two transmitters placed at the ends of the link with the same axial symmetry emitting beams. This method can send, receive, and send information in a very short period of time, which can be approximated by the same channel turbulence state.