🔬 optics

Wavelength-diverse transmission for turbulence-resistant free-space optical communication

This paper demonstrates that a wavelength-diverse transmission system using two C-band carriers and maximal ratio combining can reduce outage probability by nearly 20-fold and achieve an 86% reduction in fading-induced outages for free-space optical communication links affected by atmospheric turbulence.

Original authors: Nicolas Couture, Brandon Buscaino, Douglas Charlton, Mohammad E. Mousa-Pasandi, Kim B. Roberts

Published 2026-07-28

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CC BY 4.0

Original authors: Nicolas Couture, Brandon Buscaino, Douglas Charlton, Mohammad E. Mousa-Pasandi, Kim B. Roberts

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine trying to shout a secret across a crowded, windy stadium. Sometimes, a sudden gust of wind or a shifting crowd creates a "dead zone" where your voice gets swallowed up, and the person on the other side hears nothing. This is the daily struggle of sending data through the air using light instead of radio waves. While light can carry vastly more information than radio, it is a bit of a diva; it gets easily confused by the atmosphere. Tiny pockets of hot and cold air, known as turbulence, act like invisible lenses that bend and scramble the light, causing the signal to fade or disappear entirely. Scientists call this "fading," and it's the main reason why high-speed light-based internet links in the sky aren't everywhere yet. To fix this, engineers have tried many tricks, like using giant, wobbly mirrors to straighten the light (adaptive optics) or sending the same message at different times. But there's another clever idea: what if you sent the message on two different "colors" of light at the same time? If the wind messes up one color, the other might stay clear, giving the receiver a backup plan.

This is exactly what a team of researchers from Ciena Corporation set out to test. They wanted to see if sending a single, super-fast data stream across two slightly different colors of light (wavelengths) could save the connection when the air gets turbulent. They didn't just simulate this on a computer; they built a real-life experiment on a lab bench. They created a "turbulence machine"—a spinning plate that mimics the chaotic air of a stormy day—and shot a 200 Gbps data stream (that's incredibly fast, like downloading a whole movie in a blink) across it using two laser colors. The results were a huge success. By combining the signals from both colors, they found that the chance of the connection dropping out (an "outage") became nearly 20 times smaller than if they had used just one color. Even better, they discovered that when the signal gets really weak, the two colors don't always fade together; sometimes one is struggling while the other is still strong. By using a smart digital trick called "maximal ratio combining" to weigh the stronger signal more heavily, they managed to cut the failure rate by an additional 86%. This proves that using a little bit of color diversity is a practical, powerful way to make free-space optical communication much more reliable, paving the way for faster, more robust connections for satellites and cities alike.

Technical Summary: Wavelength-Diverse Transmission for Turbulence-Resistant Free-Space Optical Communication

Problem Statement
Free-space optical communication (FSOC) links offer high bandwidth and data rates essential for modern connectivity, particularly for remote areas and low-earth orbit (LEO) satellite networks. However, their widespread adoption is hindered by environmental constraints, specifically clear-air turbulence. Turbulence, caused by air movement, temperature gradients, and pressure fluctuations, induces power fading through multi-path interference. This fading can reduce received power below the receiver's sensitivity threshold, leading to link outages. While adaptive optics offer a solution, they require complex hardware and calibration. Diversity techniques, which transmit redundant information across uncorrelated channels, present a promising alternative, but existing wavelength diversity schemes often require large spectral separations (hundreds of nanometers) that may not be compatible with standard infrastructure.

Methodology
The authors present an experimental demonstration of a wavelength diversity system designed to mitigate turbulence-induced fading within the C-band using standard optical components.

System Architecture: A modified polarization-multiplexed coherent optical transceiver was used to transmit a 56.8 GBaud signal (200 Gbps client rate) across two carrier frequencies (191.5 THz and 195.95 THz, separated by ~35 nm) within the C-band. Both wavelengths shared the same electro-optics at the transmitter and the same opto-electronics at the receiver.
Experimental Setup: The transmission occurred over a bench-top free-space link utilizing fiber-to-free-space terminals. Atmospheric turbulence was emulated using a rotating phase plate (Lexitek PRP-100) with a coherence length ( $r_0$ ) of 0.5 mm, designed to mimic pseudo-Kolmogorov phase variations.
Reception and Processing: Two synchronized single-wavelength coherent receivers captured the waveforms at each carrier frequency. The system did not combine signals optically; instead, the received waveforms were processed offline. The researchers applied receiver calibration, polarization alignment, and time synchronization.
Combining Strategy: The core of the methodology involved Maximal Ratio Combining (MRC) in the digital domain. The waveforms were weighted based on their respective Error Vector Magnitudes (EVM) to simulate the coherent addition of signals. The weighting parameter $\alpha$ was defined as the ratio of the EVMs of the two receivers, ensuring that lower-power (higher EVM) channels contributed less to the combined signal, mimicking an optical domain combination.

Key Results

Turbulence Characterization: The received power histograms for both wavelengths fit the Gamma-Gamma turbulence model. The overall power correlation coefficient between the two channels was measured at 0.9, consistent with the moderate spectral separation at the C-band edges.
Fade-Dependent Correlation: A critical finding was that while the overall correlation was high, the correlation between channels decreased significantly as fade depth increased. When one or both channels experienced deep fades (several dB below the mean), the correlation dropped, meaning the channels did not fade simultaneously.
Outage Probability Reduction:
- Single-Wavelength Performance: Individual receivers (Rx(1) and Rx(2)) exhibited outage probabilities of 3.50% and 3.34%, respectively, defined by the signal falling below the required SNR (RSNR).
- Combined Performance: After digital MRC, the outage probability dropped to 0.17%. This represents a reduction by a factor of nearly 20 compared to the single-carrier scenario.
- Impact of Correlation Modeling: The authors compared their experimental results against a theoretical model assuming a constant fade-independent correlation of 0.9. The theoretical model predicted an outage probability of 1.22%. The experimental result (0.17%) was 86% lower than this theoretical expectation, demonstrating that the fade-dependent decorrelation observed in the experiment significantly enhances system resilience.

Significance and Claims
The paper claims that wavelength diversity utilizing C-band edges offers a practical and efficient solution for turbulence mitigation without requiring the large spectral separations or complex adaptive optics of other methods. The primary contribution is the experimental demonstration that even with moderate spectral decorrelation (0.9 correlation), the system achieves a substantial reduction in outage probability (factor of ~20) by exploiting the fact that deep fades are not perfectly correlated between wavelengths.

The authors emphasize that the observed "fade-dependent correlation" is a key factor in the system's success, allowing the MRC algorithm to effectively utilize the redundant information when one channel is severely attenuated. They conclude that this technique improves the reliability of FSOC links in turbulent conditions, paving the way for applications in satellite communications, urban connectivity, and disaster recovery, provided the system can leverage the inherent robustness of redundant information across multiple wavelengths.

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