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1. Improving free-space optical communication with adaptive optics for higher order modulation

   https://doi.org/10.1117/12.2568713  

   Nafria, Vijay; Han, Xiao; Djordjevic, Ivan (August 2020, Proceedings Volume 11509, Optics and Photonics for Information Processing XIV)

   Iftekharuddin, Khan M.; Awwal, Abdul A.; Márquez, Andrés; Diaz-Ramirez, Victor H. (Ed.)

   One of the biggest challenges of free-space optical (FSO) communication is the wave-front aberration due to atmospheric turbulence. In FSO links the wave-front distortion manifests as a significant drop in received power, beam wander, information loss, and scintillation effects. The performance of FSO communication system is degraded significantly by the atmospheric turbulence effects. Fortunately, the adaptive optics system offers potential to mitigate the performance degradation, which is relevant for quantum communication applications as well. In our FSO experiment, we perform the transmission of 6.25 GBd QPSK signal over an FSO link without and with adaptive optics, operating at 1550nm. We emulate the atmospheric aberration in our indoor experimental setup by applying random Kolmogorov phase screens on spatial light modulators (SLMs). We demonstrate significant improvements in the power-collected, signal-to-noise-ratio (SNR), and bit-error-rate (BER) performance due to the application of adaptive optics. 

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2. Incoherent Light Sources-Based Low Probability of Detection and Covert Radars over Atmospheric Turbulence Channels

   https://doi.org/10.1109/TELSIKS57806.2023.10315967  

   Djordjevic, Ivan B; Nafria, Vijay (October 2023, IEEE)

   In this paper we are concerned with the low probability of detection (LPD) and covert radars employing optical incoherent sources. Key idea of our proposed LPD/covert radar concept is to hide the radar signal in solar radiation by employing the broadband (>30 nm) Erbium-doped fiber amplifier source, modulating such source output beam with a constant amplitude modulation format at high-speed, and detect the presence of the target by the cross-correlation method. To demonstrate the proposed concept we developed an outdoor free-space optical (FSO) testbed at the University of Arizona campus. To improve the tolerance to atmospheric turbulence effects the adaptive optics is used. We demonstrate that the LPD/covert radar concept over strong turbulent FSO channel is feasible in a desert environment. 

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3. Effect of Feedback Delay on Adaptive LDPC Coding in a Fading Free-Space Optical Channel

   https://doi.org/10.1109/ICC51166.2024.10622619  

   Galijasevic, Semira; Luo, Jingchao; Divsalar, Dariush; Wesel, Richard (June 2024, IEEE)

   Free-space optical (FSO) links are sensitive to channel fading caused by atmospheric turbulence, varying weather conditions, and changes in the distance between the transmitter and receiver. To mitigate FSO fading, this paper applies linear and quadratic prediction to estimate fading channel conditions and dynamically select the appropriate low-density parity check (LDPC) code rate. This adaptivity achieves reliable communication while efficiently utilizing the available channel mutual information. Protograph-based Raptor-like (PBRL) LDPC codes supporting a wide range of rates are designed, facilitating convenient rate switching. When channel state information (CSI) is known without delay, dynamically selecting LDPC code rate appropriately maximizes throughput. This work explores how such prediction behaves as the feedback delay is increased from no delay to a delay of 4 ms for a channel with a coherence time of 10 ms. 

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4. Channel-Prediction-Driven Rate Control for LDPC Coding in a Fading FSO Channel With Delayed Feedback

   https://doi.org/10.1109/OJCOMS.2025.3538839  

   Galijasevic, Semira; Luo, Jingchao; Divsalar, Dariush; Wesel, Richard (January 2025, IEEE Open Journal of the Communications Society)

   Free-space optical (FSO) links are sensitive to channel fading caused by atmospheric turbulence, varying weather conditions, and changes in the distance between the transmitter and receiver. To mitigate FSO fading, this paper applies linear and quadratic prediction to estimate fading channel conditions and dynamically select the appropriate low-density parity check (LDPC) code rate. This adaptivity achieves reliable communication while efficiently utilizing the available channel mutual information. Protograph-based Raptor-like (PBRL) LDPC codes supporting a wide range of rates are designed, facilitating convenient rate switching. When channel state information (CSI) is known without delay, dynamically selecting LDPC code rate appropriately maximizes throughput. This work explores how such prediction behaves as the feedback delay is increased from no delay to a delay of 4 ms for a channel with a coherence time of 10 ms. 

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5. Real-Time FPGA Demonstration of Hybrid Bi-directional MMW and FSO Fronthaul Architecture

   https://doi.org/10.1364/OFC.2019.W2A.39  

   Alfadhli, Yahya; Peng, Peng-Chun; Cho, Hyunwoo; Liu, Siming; Zhang, Rui; Chen, You-Wei; Chang, Gee-Kung (January 2019, 2019 Optical Fiber Communications Conference and Exhibition (OFC))

   We experimentally demonstrate a converged hybrid bi-directional mobile fronthaul by integrating MMW and FSO links with real-time FPGA processing. We achieve long-term stability under practical 5G operation scenarios with EVM variations of <0.7% for 16-QAM. 

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