Fading-Resilient Coherent Optical Transceivers
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Solution Overview
Problem
Optical signals in free-space optical links are vulnerable to degradation by air turbulence, leading to random fluctuations in received optical power and signal fading, which can compromise data transmission quality.
Innovation Solution
A modified transmitter and receiver system that duplicates the data stream, delays one copy relative to the other, and transmits both copies on orthogonal polarization channels. The receiver selects the copy with higher signal quality for transmission, thereby enhancing the resilience of the optical link to fading.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If optical signals are transmitted through free-space optical links, then high-capacity data transmission is achieved, but signal degradation due to air turbulence causes power fluctuations and fading
Solution Approach 1:
The patent transmits multiple copies of the same data stream through different polarization channels (e.g., horizontal and vertical polarizations). Each copy experiences independent fading, and the receiver combines them to achieve diversity gain, ensuring that not all copies fade simultaneously.
Solution Approach 2:
The patent exploits the polarization dimension of optical signals to create independent transmission channels. By encoding data on orthogonal polarization states, the system adds a dimensional degree of freedom that allows simultaneous transmission of identical or complementary data streams, which are then combined at the receiver to mitigate fading effects.
2Reliability
If powerful error correction techniques are implemented, then transmission reliability is improved, but the system still cannot prevent signal power from falling below threshold due to severe fading
Solution Approach 1:
The patent prepares multiple redundant data copies in advance, each encoded with error correction capabilities. When fading occurs, the system has pre-prepared alternative copies that can be activated, providing a cushion against severe power drops before they become critical.
Solution Approach 2:
The patent changes the temporal parameter of signal transmission by introducing controlled time delays between copies transmitted on different polarization channels. This temporal separation ensures that copies experience fading at different times, and the receiver can select or combine copies based on their instantaneous quality, effectively combating severe fading.
3Reliability
If data streams are transmitted on orthogonal polarization channels, then fading resilience is enhanced, but device complexity increases due to additional modulation and demodulation requirements
Solution Approach 1:
The patent uses a single optical transmitter and receiver system that simultaneously handles multiple polarization channels. The same hardware components perform dual functions: transmitting/receiving both horizontally and vertically polarized signals, and the digital signal processor combines both channels to achieve fading mitigation, reducing the need for separate dedicated systems.
Solution Approach 2:
The patent replaces complex optical switching or mechanical beam steering mechanisms with digital signal processing. Instead of physically switching between polarization channels or using complex optical path modulation, the system uses digital algorithms to separate, process, and combine the polarization-diverse signals, simplifying the hardware while maintaining fading resilience.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach significantly reduces power fluctuations, maintaining the received power above a threshold, and thereby reduces the required optical launch power, making the system more practical for a wider range of applications.
Implementation Method 1
the respective copies of the data stream are recovered using an optical coherent receiver
Implementation Method 2
an optical front end is configured to modulate the first output digital data sequence onto a first polarization component of an optical signal and to modulate the second output digital data sequence onto a different second polarization component of the optical signal
Implementation Method 3
first and second data-modulated optical signals are transmitted over a free-space optical link
Data Source
AI summary
Apparatus and methods are provided for transmitting and/or for receiving data over a free-space optical link. An optical data transmitter generates duplicate digital data streams carrying a same digital data sequence with a relative temporal offset and mixes them onto two different polarization channels according to a preselected linear mixing rule. An optical data receiver demodulates first and second digital streams from the two polarization channels and unmixes the demodulated data, to produce first and second output digital data streams according to a preselected linear unmixing rule. The receiver applies a quality indicator to select between segments of the respective output digital data streams that correspond to the same transmitted data sequence.