CubeSat-Enabled Free-Space Optics: Joint Data Communication and Fine Beam Tracking

Abstract

The integration of CubeSats with Free Space Optical (FSO) links accelerates a major advancement in high-throughput, low-Earth orbit communication systems. However, CubeSats face challenges such as size, weight, and power (SWaP) limitations, as well as vibrations that cause fluctuations in the angle-of-arrival (AoA) of the optical beam at the receiver. These practical challenges make establishing CubeSat-assisted FSO links complicated. To mitigate AoA fluctuations, we expand the receiver's field of view and track the location of the focused beam spot using an array of avalanche photodiodes at the receiver. Initially, we model the optical channel between the transmitter and the detector array. Furthermore, to reduce the computational load of maximum likelihood sequence detection, which is infeasible for CubeSats due to SWaP constraints, we propose a sub-optimal blind sequence data detection approach that relies on the generalized likelihood ratio test (GLRT) criterion. We also utilize combining methods such as equal gain combining (EGC) and maximal ratio combining (MRC) for data detection, benchmarking their performance against the GLRT-based method. Numerical results demonstrate that the proposed low-complexity GLRT-based method outperforms the combining methods, achieving performance close to that of the ideal receiver.

Figures (7)

• Figure 1: Graphical illustration of a ground-to-CubeSat FSO link for 6G and beyond wireless networks.
• Figure 2: A schematic of the deviated received beam due to the AoA fluctuations on a $2\times 2$ APD detector.
• Figure 3: The Graphical illustration of a $4\times4$ photodetector array where each photodetector is specified by $i$ and $j$ indices. Moreover, $w_f$ denotes the junction width (dead space) and $w_a$ denotes the active width of each photodetector.
• Figure 4: BER of the proposed GLRT-based detection method versus $P_t$ for two different angular instabilities. The case of employing the EGC method and the ideal Rx are considered as benchmarks.
• Figure 5: BER of the proposed GLRT-based detection method versus $L$ for two different values of $P_t$. The case of employing the EGC method and the ideal Rx are considered as benchmarks.