Free Space Optical Receiver Turbulence Mitigation
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Solution Overview
Problem
Optical satellite communications via free space are affected by air turbulence, which causes random fluctuations in optical beam profiles, leading to variations in received optical power and potential errors in data demodulation, even with sophisticated error correction engines, making it challenging to maintain error-free transmission.
Innovation Solution
An apparatus that uses spectral and time diversity by duplicating and delaying digital data streams in different wavelength bands, allowing selection of the best-quality stream based on quality indicators such as optical power and bit error rate, to ensure reliable data demodulation and reduce the impact of turbulence.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If optical signals are transmitted through the atmosphere via free space, then high-capacity data links can be established in low-density populated areas, but air turbulence causes random fluctuations in received optical power and beam profile
Solution Approach 1:
The patent segments the optical transmission system into multiple independent wavelength channels (first wavelength band and second wavelength band). Each channel transmits data separately, allowing the system to segment the data stream into multiple parallel transmission paths that can be independently processed and selected based on received power quality.
Solution Approach 2:
The patent introduces wavelength diversity as an additional dimension to the transmission system. Instead of relying on a single wavelength channel, the system transmits data across multiple wavelength bands, adding a spectral dimension that provides alternative transmission paths when one wavelength experiences turbulence-induced power fluctuations.
2Reliability
If spectral diversity is used by transmitting data in different wavelength bands, then link availability increases, but device complexity increases due to need for multiple optical carriers and selection mechanisms
Solution Approach 1:
The patent implements dynamic operation where the optical data receiver dynamically switches between different wavelength bands based on real-time received power measurements. The system dynamically adjusts which wavelength channel is actively used for data transmission, transitioning from static single-channel operation to dynamic multi-channel selection to maintain optimal link performance.
Solution Approach 2:
The patent creates copies of the data stream across multiple wavelength channels. The same data is transmitted simultaneously on different wavelengths, creating redundant copies that can be independently received and processed. This copying approach provides backup transmission paths without requiring separate data sources, managing complexity through replication rather than duplication.
3Reliability
If time diversity is applied by delaying data streams, then turbulence impact is reduced, but transmission delay increases
Solution Approach 1:
The patent applies periodic time diversity by introducing controlled delays between the transmission of data streams on different wavelength bands. The delay is applied periodically to create temporal separation that exploits the time-varying nature of atmospheric turbulence, allowing one wavelength to experience better conditions when another is delayed. The periodic application of delay mechanisms balances reliability improvement with acceptable latency.
Data Source
AI summary
An apparatus comprises an optical data receiver to receive first and second data-modulated optical carriers in different wavelength bands via a free space optical link and to generate an output digital data stream from demodulated segments of the first and second data-modulated optical carriers. The optical data receiver is configured to make selections between temporally corresponding portions of the first and second data-modulated optical carriers for generating successive portions of the output digital data stream. The optical data receiver is configured to make one of the selections between two of the temporally corresponding portions of the first and second data-modulated optical carriers based on at least one value of a quality indicator obtained for at least one of two temporally corresponding portions.