# An All-Digital Optical Phase-Locked Loop Suitable for Satellite Downlinks +
## 1 Introduction  
Due to the enormous increase in data traffic, multimedia service demands, and radio-frequency (RF) spectrum congestion, FSO has recently emerged as a promising communications technology [1]. As a result, switching to optical carriers has appeared as a solution to comply with the current and future demands. Furthermore, FSO has numerous advantages over RF communications, including large modulation bandwidth, narrow beam divergence, high security, high directivity, and no licensing requirement. However, optical signal propagation is impacted by absorption, scattering, and changes in the atmospheric refractive index or turbulence, which causes optical signal attenuation and degrades link quality [2].  
Coherent optical communication is advantageous for long-distance FSO, such as satellite-to-ground links, as shown in Figure 1. It can achieve higher-order modulation formats, and better signal sensitivity [3]. Although coherent detection is widely used in fiber-optic networks, its use in satellite links is still in the research stage. For FSO communication, the two techniques used to recover the data in a coherent optical system are optical phase-locked loop (OPLL) and digital signal processing (DSP). OPLL is typically used in inter-satellite communication, while satellite-to-ground links can use adaptive optics (AO) in combination with DSP techniques [4].  
For example, an FSO using BPSK modulation was demonstrated over a range of 10.45 km to emulate the atmospheric channel conditions for a geostationary feeder link and achieved a bit rate of 30 Gbps [5]. A digital synchronization system was evaluated inside a DSP, where a digital PLL and two open-loop techniques were analyzed [6]. Both systems were evaluated considering the effects of atmospheric turbulence and using the AO correction system.  
Regarding the OPLL technique for FSO, an optical injection phase-locked loop (OIPLL) was demonstrated where a pilot carrier was transmitted to synchronize with the local oscillator [7]. The OIPLL was evaluated over the fiber optic transmission channel, and the atmospheric turbulence effects were not considered. For an inter-satellite link, an OPLL for a bit rate of 1 Gbps was demonstrated where a sensitivity of \(-\)59.2 dBm was achieved for a BER below \(1\times 10^{-3}\)[8]. The analog OPLLs' structure is fixed to a specific modulation format due to the phase detector. Hence, the complexity of the OPLL phase detector increases with the modulation order. In addition, the loop time constants are fixed due to the components used, such as resistors and capacitors.  
Therefore, the use of DSP and OPLLs appears in different scenarios and applications, each with advantages and disadvantages. As a result, using OPLL and DSP techniques such as real-time data recovery and parameter reconfiguration flexibility, a fully digital OPLL to demodulate a digitally modulated optical signal is proposed. In this way, an all-digital OPLL will be evaluated using the DSP technique for the phase detector to handle a fading signal. This signal results from the effects of atmospheric turbulence of an optical signal propagating a low Earth orbit (LEO) slant path shown in Figure 1. The system will be evaluated under three scenarios of atmospheric turbulence and several wind speed values, aided by simulations using VPIphotonics in a co-simulation technique. Hence, we extended our previous work, explaining the project decisions from OPLL behavior, the proposed hybrid approach, and the modification in the phase estimator used in DSPs [9]. Additionally, the OPLL is now analyzed using two sample rates: 625 MSa/s and 10 GSa/s.