Common Path Distortion Detection in RFoG Networks
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Solution Overview
Problem
Conventional digital transmission systems, such as RFoG and HFC networks, face challenges with common path distortion (CPD) and common mode distortion, which affect signal quality and are difficult to detect and mitigate, especially in determining the location and number of corrosion diodes causing these distortions.
Innovation Solution
A modified SCTE-109 procedure is implemented to detect and locate CPD by adjusting downstream carrier frequencies, using a switchable common mode test signal to assess wiring integrity, and employing adaptive polarization for receiver antennas to enhance signal communication.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional SCTE-109 procedure is used to detect common path distortion, then downstream carriers can be monitored for distortion products, but the distance to corrosion diodes cannot be determined and multiple diodes cannot be distinguished
Solution Approach 1:
The patent segments the monitoring approach by dividing the downstream spectrum into multiple frequency regions and using separate test signals for different segments. This allows the system to distinguish between multiple corrosion diodes at different locations by analyzing which frequency segments contain distortion products from each diode's mixing action.
Solution Approach 2:
The patent adds a spatial dimension to the detection by mapping frequency offsets to physical distances. By measuring the frequency offset between the original downstream carrier and the distorted upstream signal, the system can calculate the round-trip delay and determine the distance to each corrosion diode, transforming a one-dimensional spectral analysis into a two-dimensional analysis that includes both frequency and distance information.
2Reliability
If AC powering is used at corrosion diodes, then the diodes can operate, but the distortion products are modulated at 120 Hz making detection difficult
Solution Approach 1:
The patent exploits the periodic nature of the 120 Hz modulation caused by AC powering. By using correlation techniques that specifically look for signals at this known frequency offset, the system can detect the modulation signature and distinguish it from other noise sources, thereby simplifying detection rather than complicating it.
Solution Approach 2:
The system uses feedback by continuously monitoring the upstream spectrum for distortion products and adjusting the test signal parameters based on detected corrosion diode locations. This closed-loop approach allows the system to adapt to changing conditions and maintain accurate detection despite the periodic modulation effects.
3Measurement precision
If common mode test signal is used to assess wiring integrity, then wiring issues can be detected, but the test signal may interfere with normal data transmission
Solution Approach 1:
The patent implements preliminary action by performing common mode distortion testing during initialization or maintenance windows before normal data transmission begins. The test signals are injected and measured in advance, allowing wiring integrity to be assessed without interrupting or degrading ongoing data communications.
Solution Approach 2:
The system uses an intermediary approach by introducing test signals at controlled levels and through specific pathways that minimize interference with data signals. The common mode test signals are designed to be distinguishable from data signals through frequency or temporal separation, allowing both testing and transmission to coexist without significant mutual interference.
Data Source
AI summary
A transmitter is configured to transmit data to a receiver over a communications network. The transmitter includes a memory for storing computer-executable instructions and a processor in operable communication with the memory. The processor is configured to (i) collect spectrum data of a spectral band over a first sampling interval, and (ii) store the collected spectrum data in the memory. The transmitter further includes a common path distortion (CPD) detection unit configured to (i) sample the collected spectrum data, over a second sampling interval less than the first sampling interval, for a plurality of sub-bands within the spectral band, (ii) analyze the spectral band and each sub-band for indications of common path distortion, and (iii) inject a test signal configured for detection at a headend of the communications network. The transmitter further includes a modem portion configured to operably communicate with the headend and the receiver.