Phase-Coherent Optical Carrier Generation for Distributed Units
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Solution Overview
Problem
Current microwave backhaul systems face challenges in achieving optimal antenna deployments for high data-rate demands, leading to suboptimal system gain, throughput, and availability due to unsynchronized local oscillators and high phase noise in MIMO links, especially at larger distances from the central unit.
Innovation Solution
A method using a single light source to generate phase-coherent optical carriers, which are then used to create synchronized RF carriers for distributed units, enabling correlated phase noise and reduced attenuation over fiber, allowing for precoding and flexible frequency operation from 6 to 72 GHz.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If multiple light sources are used to generate RF carriers for different distributed units, then each DU can operate independently, but the RF carrier frequencies become unsynchronized and phase noises are uncorrelated, reducing system capacity
Solution Approach 1:
The patent merges multiple light sources into a single light source that generates multiple optical carriers through frequency division. This single light source provides phase-coherent optical carriers to multiple distributed units, ensuring correlated phase noises while maintaining independent operation. The optical carriers are derived from the same laser, creating a unified phase reference across all DUs.
Solution Approach 2:
The single light source performs multiple functions by generating multiple optical carriers simultaneously through frequency division. Each optical carrier serves a different distributed unit, but all carriers originate from the same light source, providing universal phase coherence across the entire system while maintaining individual unit independence.
2Reliability
If digital signals are transmitted alongside reference signals through fiber links, then synchronization can be achieved, but the fiber link spectral efficiency is low and DU complexity increases due to digital-to-analog conversion requirements
Solution Approach 1:
The patent replaces the mechanical/electrical digital-to-analog conversion process with an optical generation process. Instead of transmitting digital signals and converting them at the DU, the system directly generates analog optical carriers at the CU using a single light source. This substitution eliminates the need for digital-to-analog converters at DUs, reducing complexity while maintaining synchronization through inherent optical phase coherence.
3Reliability
If subharmonic LO is distributed electrically along the baseband signal, then phase coherence can be maintained, but the achievable distance from CU to DUs is limited due to attenuation
Solution Approach 1:
The patent replaces electrical distribution of subharmonic LO with optical distribution of carriers. Optical signals have much lower attenuation than electrical signals over long distances. By generating optical carriers from a single light source and distributing them through fiber, the system maintains phase coherence while extending the achievable distance from CU to DUs far beyond what electrical distribution can achieve.
4Productivity
If optimal antenna separation of 13 meters is deployed for 4x4 MIMO at 18 GHz, then system capacity is maximized, but the deployment becomes problematic to accommodate in practical scenarios
Solution Approach 1:
The patent changes the system parameters by introducing phase-coherent optical carriers and precoding techniques. This allows the system to achieve optimal capacity performance without requiring strict adherence to optimal antenna separation distances. The phase coherence enables signal processing compensation that makes the system less sensitive to suboptimal antenna deployments, allowing flexible installation while maintaining high capacity.
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 enhances system capacity, reduces phase noise requirements, and allows for longer distances between the central unit and distributed units, while simplifying transmitter complexity and enabling flexible frequency operation, improving overall system performance and scalability.
Implementation Method 1
The CU uses a single light source to generate two or more optical carriers, wherein the generated optical carriers are all phase coherent with one another
Implementation Method 2
When the analog RF signal for each DU is generated using different LSs and LOs at the CU, the RF carrier frequencies of the DUs are not synchronized, and its phase noises are uncorrelated... only an optical-to-electrical conversion using a photodetector (PD) is needed at the DUs to generate the analog RF signals
Data Source
AI summary
A method performed by a CU (202, 302) for enabling at least two DUs, to generate an RF carrier. In one embodiment the method includes the CU using a single light source (212) to generate two or more optical carriers, wherein the generated optical carriers are all phase coherent with one another. The method also includes the CU generating a first single sideband (SSB) signal for a first DU using two of the generated optical carriers and generating a second SSB signal for a second DU using two of the generated optical carriers. The method also includes the CU transmitting the first SSB to the first DU and transmitting the second SSB to the second DU.