Feed-Forward Equalization for LiDAR Inter-Symbol Interference
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Solution Overview
Problem
LiDAR systems face inter symbol interference (ISI) due to the fast rise time and slow RC decay tail of silicon photomultiplier (SiPM) detectors, which can overlap with subsequent pulses, making it difficult to detect reflections from multiple close objects.
Innovation Solution
A feed-forward equalization circuit is implemented, which includes a delay path circuit with a delay line and gain control circuit to produce a delayed, reduced amplitude voltage signal that is subtracted from the original signal, effectively truncating the trailing edge of the SiPM voltage signal and eliminating ISI by ensuring it falls below the noise threshold before the next pulse arrives.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a SiPM photodetector is used with fast rise time, then the response speed is improved, but inter symbol interference increases due to slow RC decay tail
Solution Approach 1:
The patent applies preliminary action by generating a delayed replica of the photodetector output signal before the actual interference occurs. The delay path circuit creates a time-shifted version of the signal that is used to cancel the trailing edge, preventing the interference from affecting subsequent pulse detection.
Solution Approach 2:
The patent converts the harmful slow RC decay tail into a beneficial signal by using it to generate the cancellation signal. The same exponential decay that causes interference is replicated and inverted to create the equalization signal that removes the interference, turning the harmful characteristic into the solution.
2Object-generated harmful factors
If the trailing edge of the voltage signal is truncated, then inter symbol interference is reduced, but signal processing complexity increases
Solution Approach 1:
The patent uses copying by creating a replica of the photodetector output signal in the delay path circuit. This copied signal is then processed with gain control and subtracted from the original signal to achieve the truncation effect, which is a simpler approach than direct digital signal processing.
Solution Approach 2:
The patent replaces complex digital signal processing with an analog circuit implementation using operational amplifiers, resistors, and capacitors. This mechanical/electronic substitution reduces computational complexity while achieving the same signal truncation effect.
3Measurement precision
If gain control is applied to the delayed signal, then the trailing edge cancellation is optimized, but circuit complexity increases
Solution Approach 1:
The patent applies parameter changes by dynamically adjusting the gain of the delayed signal through the gain control circuit. This allows optimization of the cancellation precision by adapting the gain parameter to match the specific characteristics of the photodetector output and the desired truncation point.
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 solution improves the accuracy of detecting objects by eliminating ISI, allowing for the detection of closely spaced reflections and enhancing the overall performance of LiDAR systems in autonomous vehicles by ensuring that subsequent pulses can be accurately identified.
Implementation Method 1
A SiPM is a solid-state photomultiplier comprised of a high density matrix of Geiger-mode-operated avalanche photodiodes also known as SPAD (single-photon avalanche photodiode). These SPADs have high internal gain which enable single photon detection.
Implementation Method 2
A SiPM is a solid-state photomultiplier comprised of a high density matrix of Geiger-mode-operated avalanche photodiodes
Implementation Method 3
The voltage signal is provided to a delay path circuit having a delay line and a gain control circuit to provide a delayed, reduced amplitude voltage signal.
Implementation Method 4
The delayed, reduced amplitude voltage signal is subtracted from the voltage signal in a subtraction circuit. The smaller, delayed signal cancels out the slowly decaying trailing edge of the SiPM voltage signal.
Data Source
AI summary
Apparatus and methods for reducing inter symbol interference from reflected laser pulses that are received close in time. A laser is provided to emit a laser beam pulse. A photodetector is mounted to receive a reflected laser beam pulse after reflecting off an object in an external environment, and produce a voltage signal corresponding to the reflected laser beam pulse. The voltage signal is provided to a delay path circuit having a delay line and a gain control circuit to provide a delayed, reduced amplitude voltage signal. The delayed, reduced amplitude voltage signal is subtracted from the voltage signal in a subtraction circuit to produce a truncated pulse. The output of the subtraction circuit is provided to a pulse detector circuit to detect the arrival time of the leading edge of the truncated pulse.