Cyclically Optimal PPM Waveform Generation for Lidar Range Ambiguity
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Solution Overview
Problem
Current methods for generating Pulse Position Modulated (PPM) lidar waveforms are computationally intensive and limited in applicability, particularly in scenarios requiring long codes for unambiguous ranging, especially under low signal-to-noise ratios, where high pulse-to-photon-return ratios necessitate transmitting many pulses, and high PRFs exacerbate range ambiguity.
Innovation Solution
A method for generating cyclically optimal PPM waveforms involves creating a modulation pool, eliminating bad modulation levels, selecting and concatenating modulation levels to form an N-element modulation sequence, and applying it to a PRF less than the maximum nominal PRF, ensuring the waveform remains optimal and repeatable, even when concatenated, thus maintaining peak-to-side-lobe ratio independent of total code length.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If high PRF is used to increase integration time and reduce dwell time, then productivity is improved, but range ambiguity increases
Solution Approach 1:
The waveform is segmented into multiple pulses with positions modulated according to a code sequence. This segmentation allows the use of high PRF for improved productivity while the coded structure resolves range ambiguity by providing unique pulse position patterns that can be decoded to determine unambiguous range.
Solution Approach 2:
The patent employs periodic pulse trains with pulse positions modulated according to cyclically optimal codes. The periodic structure enables high PRF operation while the cyclic optimality ensures that autocorrelation properties remain favorable even when pulses are repeated, resolving the range ambiguity problem.
2Adaptability or versatility
If conventional PPM waveform generation methods are used, then adaptability is improved, but device complexity increases due to computational intensity
Solution Approach 1:
Cyclically optimal codes are pre-calculated and stored, eliminating the need for complex real-time computations during waveform generation. The preliminary computation of codes with optimal autocorrelation properties allows for simple, efficient waveform generation while maintaining adaptability through code selection.
Solution Approach 2:
The patent uses repeated instances of the same cyclically optimal code sequence to generate waveforms of different lengths. Instead of computing new complex codes each time, simple copying and concatenation of the base cyclically optimal code achieves waveform generation with minimal computational resources.
3Measurement precision
If PPM codes are made longer to improve measurement precision for low signal-to-noise ratio applications, then measurement precision is improved, but device complexity increases
Solution Approach 1:
By using cyclically optimal codes that can be repeatedly concatenated, the system achieves long effective code lengths for improved measurement precision in low SNR conditions. The periodic, cyclic structure allows simple repetition rather than complex generation of long unique sequences, avoiding increased device complexity.
Solution Approach 2:
Long PPM codes are constructed by copying and concatenating shorter cyclically optimal code sequences. This approach achieves the necessary code length for high precision range determination in low SNR applications while keeping the generation process simple and computationally efficient.
Data Source
AI summary
Lidar and method for generating repeatable PPM waveforms to determine a range to a target include: a processor for a) creating a modulation pool, based on a maximum nominal PRF and a specified final PPM code length of N; b) obtaining a seed code; c) eliminating bad modulation levels from the modulation pool to generate a good modulation pool, d) selecting a modulation level from the good modulation pool; e) concatenating the selected modulation level to the seed code to generate an i-element modulation sequence; f) repeating steps c to e N times to generate an N-element modulation sequence; g) selecting a PRF less than the maximum nominal PRF; and h) generating a repeatable PPM waveform by applying the N-element modulation sequence to the selected PRF.