Polarization Diversity Fading Mitigation in Coherent Optical Receivers
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Solution Overview
Problem
Free-space optical communication systems face challenges in maintaining optical links due to air turbulence-induced power fades, which existing diversity schemes struggle to mitigate effectively without increasing hardware requirements or power consumption.
Innovation Solution
The method employs polarization diversity by transmitting optical signals on orthogonal polarization portions delayed in time, allowing the communication terminal to select the signal with the highest receiving power to maintain the link, thereby mitigating power fades without redundant hardware or increased power usage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If spatial diversity is used to transmit replicas from two different physical locations, then fading mitigation is improved, but device complexity and hardware requirements increase
Solution Approach 1:
The patent combines spatial diversity and time diversity into a unified system where multiple replicas are transmitted from the same physical location but at different time instances. This merging approach achieves fading mitigation through temporal separation while avoiding the need for multiple spatial locations, thereby reducing hardware complexity while maintaining reliability.
Solution Approach 2:
The patent transitions from spatial diversity (different locations) to time diversity (different time instances) as the primary dimension for replica transmission. By introducing temporal separation as the key dimension, the system achieves diversity gain without requiring redundant spatial hardware, thus resolving the contradiction between reliability and device complexity.
2Device complexity
If time/wavelength diversity is used to transmit replicas from the same physical location, then device complexity is reduced, but use of energy increases due to requiring twice as much transmit optical power
Solution Approach 1:
The patent makes a single optical channel perform multiple functions by transmitting multiple replicas with different time delays through the same physical location. This multi-functionality allows the system to achieve diversity mitigation without requiring additional spatial hardware or increased transmit power, as the same channel is reused efficiently across different time instances.
Solution Approach 2:
The patent changes the temporal parameter (time delay) of the transmitted replicas rather than increasing power or using additional spatial channels. By adjusting the time delay parameter between replicas, the system achieves diversity gain while maintaining constant transmit power levels, thus resolving the energy consumption issue.
3Reliability
If multiple replicas are transmitted to mitigate fading, then reliability is improved, but loss of time increases due to delayed reception
Solution Approach 1:
The patent transmits replicas with incremental time delays rather than simultaneously, allowing the receiver to potentially obtain the first successful replica before all delayed replicas arrive. This partial action approach means that while multiple replicas are transmitted, the system can stop processing once the first successful reception occurs, thus limiting the actual time loss while maintaining reliability through the availability of multiple delayed copies.
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 effectively maintains optical links by selecting the signal with the highest power, reducing the impact of turbulence-induced fades without the need for additional hardware or increased power consumption, optimizing latency based on environmental conditions.
Implementation Method 1
Air scintillation may result in optical power fades over optical signals communicated between the communication terminals
Implementation Method 2
transmitting two wavelengths carrying the same data from two different physical locations. However, spatial diversity requires redundant physical hardware
Data Source
AI summary
A method includes receiving a first data packet on a first polarization portion of an optical signal from a second communication terminal through a free space optical link during a first time period and receiving a first data packet replica on the first polarization portion of the optical signal during a second time period. The second time period is delayed in time relative to the first time period. The method also includes determining receiving powers for the optical link during both the first time period and the second time period based on at least one of the received first data packet and the received first data packet replica. The method also includes selecting the one of the first data packet or the first data packet replica that is associated with the highest receiving power for the optical link as surviving data for maintaining the optical link.