(Meta: I do not know an appropriate place for this on Stack Exchange. There does not appear to be any groups related to autonomous driving technology and computer 3D vision / perception systems.)

For self driving vehicles using 3D depth perception LIDAR on a highway with hundreds of other vehicles also using various other LIDAR sweep beam or spot-field (kinect style) emission scanners, how is it able to distinguish its own signal returns, from the scanning being done by the other systems?

For an extremely large multilane highway, or complex multi-way intersections such emissions can be seen in all directions, covering all surfaces, and there is no way to avoid detecting the beam emissions from other scanners.

This seems to be the main technical hurdle for implementing LIDAR for autonomous driving vehicles. It does not matter if it works perfectly if it’s the only vehicle on the road using LIDAR.

The real question is how it deals with being inundated with spurious signals from similar systems in a future scenario where LIDAR is present on every vehicle, potentially with multiple scanners per vehicle and scanning in all directions around each vehicle.

Is it capable of functioning normally, can it somehow distinguish its own scanning and reject others, or in the worst case can it fail completely and just report garbage data that is useless, and it doesn’t know that it’s reporting garbage data?

This at least seems to be a strong case for having passive 3D computer vision that’s just based on natural light and stereo camera depth integration, as is done in the human brain.

  • 1
    \$\begingroup\$ Or you know...the eye hazard. As for distinguishing your own return signal from others, you could do some modulation or auto-correlation. I'm not sure how compatible that is with time-of-flight schemes but it would increase processing in something that already needs to differentiate extremely small time differences. \$\endgroup\$
    – DKNguyen
    Commented Aug 5, 2019 at 19:16
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    \$\begingroup\$ @DKNguyen, you can modulate your laser with an RF subcarrier, and do any kind of RF or phase modulation you like on that subcarrier. \$\endgroup\$
    – The Photon
    Commented Aug 5, 2019 at 20:27
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    \$\begingroup\$ @VoltageSpike Well, TDMA (Time Division Multiple Access) is one way, but CDMA (Code Division Multiple Access) is far more sophisticated. Alas, I studied those things more than 20 years ago and I hadn't a refresher since then. I remember when I studied them I thought "TDMA or FDMA? Meh! Easy concepts.", whereas CDMA was mind-boggling! Essentially you mix each stream (from each user) with a different numeric orthogonal code sequence. Extraction of the right stream is done by math operations involving the specific code sequence. The rest of the streams appears just as background noise. \$\endgroup\$ Commented Aug 5, 2019 at 21:25
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    \$\begingroup\$ @ThePhoton When you say "phase modulation" are you actually referring to modulating the phase of the photon/electromagnetic wave as would normally be meant if we were talking about radio? Or are you referring to modulating the phase of the amplitude variations of (brightness) the laser? Because my understanding is that as long as lasers are not being produced by optical antenna, we have little control over the actual phase of individual photons. \$\endgroup\$
    – DKNguyen
    Commented Aug 5, 2019 at 21:26
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    \$\begingroup\$ @DKNguyen, I'm talking about modulating the subcarrier, AM, PM, FM, whatever. \$\endgroup\$
    – The Photon
    Commented Aug 5, 2019 at 21:34

2 Answers 2


Commercial LIDAR units modulate the light with a very long pseudo-random sequence.

The modulation is primarily to (1) have a modulation for determining distance and (2) to avoid interference with ambient sources of DC and AC light.

The long sequence makes it unlikely that any other source, even a modulated one like another LIDAR, will line up and interfere.

  • \$\begingroup\$ Exactly. The interference issues exist at least as significantly from ambient noise, so the "competing" noise is relatively small, by comparison. \$\endgroup\$ Commented Aug 6, 2019 at 21:08
  • \$\begingroup\$ Is there a reason you don't refer to it as FHSS? Is it not actually FHSS? \$\endgroup\$
    – forest
    Commented Aug 7, 2019 at 15:48
  • \$\begingroup\$ The “FH” part doesn’t feel quite right to me. The base color of the laser doesn’t change, so the hopping is in time more than frequency. Yes, the modulation sequence means the laser is SS, but the side bands are really tiny. \$\endgroup\$ Commented Aug 7, 2019 at 15:59

Depends on the LIDAR. My experience with it is out of date (over 10 years), but I cannot imagine that the basics have changed that much.

Most will be using a form of lock in to discriminate their signals. They treat other LIDARs as noise just as they do anything that is not locked to their signal. While you don't have the same frequency agility that a radar does, you do have the ability to modulate your carrier using the many forms of modulation. They can definitely change modulation schema as required to find the least noise.

A modern DSP version of a Lock in Amplifier or the equivalent would be used at minimum.

After more "looking" around, it also looks like some research versions have the ability to combine transmitters into a single "beam"(wrong term I know, but easy to understand) and this would make it much harder to confuse and increase imaging definitional all at one go. Way beyond what I worked on.


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