Coherent Lidar Signal Processing for Extended Range
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Coherent lidars with frequency modulation face limitations in processing beat signals at long distances, leading to the disappearance of frequency plateaus and variable instantaneous frequencies, which complicates distance and speed measurement, especially when the time of flight exceeds the modulation waveform duration.
Innovation Solution
A method involving complex modulation and demodulation of the beat signal using multiple frequency slopes, followed by spectral density determination and characteristic frequency extraction, allows for the identification of target distance and speed information even at extended ranges.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of stationary object
If the time of flight exceeds the modulation waveform duration, then the lidar can measure longer distances, but the frequency plateaus disappear and instantaneous frequencies become variable making measurement complex
Solution Approach 1:
The patent segments the continuous modulation waveform into multiple linear frequency slopes (at least two). Each slope is processed separately through dedicated demodulation channels that apply appropriate time shifts and frequency adjustments. This segmentation transforms the complex continuous signal into discrete manageable segments, allowing frequency plateaus to be recovered even when total measurement time exceeds waveform duration.
Solution Approach 2:
The patent employs periodic modulation waveforms with repeating frequency slopes. By structure the modulation as periodic sequences of linear frequency ramps, the system can handle multiple waveform periods within the measurement interval. This periodic structure enables the recovery of characteristic frequency plateaus through synchronous detection, maintaining measurement accuracy regardless of whether the time of flight exceeds a single waveform duration.
2Measurement precision
If frequency modulation is used for long-range detection, then detection capability is improved, but the beat signal processing becomes limited at extended distances
Solution Approach 1:
The patent introduces intermediary processing steps between signal reception and final measurement extraction. Multiple demodulation channels act as intermediaries, each tailored to specific frequency slopes. These channels perform intermediate frequency transformations and time-shift operations that preserve signal integrity. This intermediary processing layer ensures reliable extraction of distance and velocity information even when the beat signal becomes complex at long ranges.
Solution Approach 2:
The patent dynamically adjusts processing parameters based on the detected frequency slopes. Each demodulation channel is configured with specific time shifts and frequency adjustments matched to its assigned slope. This parameter adaptation allows the system to maintain optimal processing conditions across varying distance ranges, ensuring reliable measurement extraction regardless of signal characteristics at extended distances.
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 enables the detection of characteristic frequency plateaus at longer distances, overcoming previous limitations and maintaining resolution, thus enhancing the lidar's range and accuracy without being limited by signal processing.
Implementation Method 1
The interference of these two waves is detected by a photodetector D, and the electric signal at the output of the detector has an oscillating term called beat signal Sb
Implementation Method 2
a fraction of the light wave backscattered by a target T
Implementation Method 3
The Doppler frequency shift v Dop of the backscattered wave is a function of the radial velocity v of the target T
Data Source
Figure 1~2
Figure 3a~3b
Figure 4a~4c
AI summary
The invention relates to a method for processing (50) a signal arising from coherent lidar comprising a coherent source (L) that is periodically frequency-modulated; - a beat signal (Sb) being generated by photodetector (D) on the basis of the interference between an optical signal that is referred to as the local oscillator having a local oscillator frequency (fOL(t)) and an optical signal that is backscattered by a target (T) illuminated by the lidar, said beat signal being digitized; - the local oscillator frequency (fOL(t)) being made up of the sum of a mean value (f0) and of a modulation frequency (fmod(t)) arising from the modulation of the source, the modulation frequency being periodic according to a modulation period (TFO), each period comprising n linear portions having n frequency slopes (αi), respectively, where n is greater than or equal to 2, the method comprising the steps consisting in: - complexly modulating (501) the beat signal (Sb); - complexly demodulating (502) the modulated signal (Smod) by n demodulation frequencies (fdemod(i)) each having a single slope that is equal to the respective frequency slope (αi) of the modulation frequency (fmod), in order to obtain n demodulated signals (Sdemod(i)); - determining (503) n spectral densities (SP(i)) of the n demodulated signals (SdemodO)); - determining (504) n characteristic frequencies (ναi) - determining (505) information on the velocity (v) and information on the distance (d) of the target (T) on the basis of said n characteristic frequencies (ναi).