Method and system to avoid inter-system interference for phase-based time-of-flight systems

• US 7,405,812 B1
• Filed: 05/14/2007
• Issued: 07/29/2008
• Est. Priority Date: 05/18/2006
• Status: Active Grant

• Current Assignee: Microsoft Technology Licensing LLC (Microsoft Corporation)
• Original Assignee: Canesta Incorporated (Microsoft Corporation)
• Inventors: Bamji, Cyrus
• Primary Examiner(s): Tarcza; Thomas H.
• Assistant Examiner(s): Ratcliffe; Luke D
• Application Number:
  US11/803,660
• Time in Patent Office: 442 Days
• Field of Search: 245/5.1
• US Class Current: 356/5.1
• CPC Class Codes: G01S 13/325 using transmission of coded signals, e.g. P.S.K. signals
• CPC Class Codes: G01S 13/931 of land vehicles
• CPC Class Codes: G01S 17/36 with phase comparison between the received signal and the contemporaneously transmitted signal
• CPC Class Codes: G01S 17/894 3D imaging with simultaneous measurement of time-of-flight at a 2D array of receiver pixels, e.g. time-of-flight cameras or flash lidar
• CPC Class Codes: G01S 17/931 of land vehicles
• CPC Class Codes: G01S 2013/93272 in the back of the vehicles
• CPC Class Codes: G01S 2013/93274 on the side of the vehicles
• CPC Class Codes: G01S 7/0235 Avoidance by time multiplex
• CPC Class Codes: G01S 7/4911 Transmitters

Abstract

Inter-system interference cross-produce P₁₂ is reduced in a phase-based TOF system by randomizing instantaneous phase and/or frequency within a capture interval (or detection integration period). In one aspect, the TOF clock system frequency preserves long-term clock frequency stability but intentionally includes random or pseudo-random clock noise. The noise ensures the generated clock signals are temporally imperfect and lack substantial perfect periodicity. A second aspect causes the TOF clock system to hop frequency, preferably pseudo-randomly. TOF system homodyning favors detection of optical energy whose frequency correlates to the time-varying frequency of the emitted optical energy. Thus, the varying spectral spacing of the emitted optical energy reduces likelihood that an adjacent TOF system at any given time will emit optical energy of an interfering frequency. At least one aspect is employed, both aspects being mutually complementary to reduce the cross-correlation product P₁₂ without substantially affecting TOF system performance.

Claims

1. In a first time-of-flight (TOF) detection system that emits time modulated optical energy and examines phase shift in a fraction of said optical energy reflected by a target object to determine distance Z therebetween, a method of reducing potential interference from an adjacent second TOF system also emitting optical energy, the method comprising:

generating for said first TOF system a clock signal having long term frequency stability while exhibiting at least one characteristic selected from a group consisting of (a) a signal having a temporally imperfect waveform, (b) a signal having a substantially imperfect periodicity, (c) a signal including at least pseudo-random clock noise, (d) a signal including random clock noise, (d) a signal whose frequency hops between at least two discrete frequencies, (e) a signal whose frequency hops randomly between at least two discrete frequencies, and (f) a clock whose frequencies are characterized by a spread spectrum;

wherein a cross-correlation product P₁₂ between detected signals from said first TOF and said second TOF system is reduced while substantially maintaining first TOF detection accuracy.

2. The method of claim 1, wherein said first TOF system employs homodyne detection.

3. The method of claim 1, wherein over a time interval approximating a detection integration time period T, said clock signal has at least one characteristic selected from a group consisting of (i) average clock signal frequency accuracy is better than about 1%, (ii) frequency of said clock signal changes at least ten times, (iii) frequency of said clock signal changes at least one hundred times, and (iv) frequency of said clock signal changes at least one thousand times.

4. The method of claim 1, wherein said cross-correlation product P₁₂ is less than about 1% of a self-correlation product P₁₁.

5. The method of claim 1, wherein at least one of characteristics (a), (b), (c), and (d) is achieved carrying out at least one procedure selected from a group consisting of (i) promoting clock noise in said clock signal, (ii) actively coupling a source of noise to said clock signal, and (iii) promoting phase noise in said clock signal.

6. The method of claim 1, wherein said first TOF system is disposed within a motor vehicle.

7. For use with a time-of-flight (TOF) homodyning detection system of the type that emits time modulated optical energy and examines phase shift in a fraction of said optical energy reflected by a target object to determine distance Z therebetween, a clock system that reduces potential interference from an adjacent second TOF system also emitting optical energy, the clock system comprising:

a clock generator outputting a clock signal having long term frequency stability while exhibiting at least one characteristic selected from a group consisting of (a) a signal having a temporally imperfect waveform, (b) a signal having a substantially imperfect periodicity, (c) a signal including at least pseudo-random clock noise, (d) a signal including random clock noise, (d) a signal whose frequency hops between at least two discrete frequencies, and (e) a signal whose frequency hops randomly between at least two discrete frequencies;

wherein a cross-correlation product P₁₂ between detected signals from said first TOF and said second TOF system is reduced while substantially maintaining first TOF detection accuracy.

8. The clock system of claim 7, wherein said first TOF system employs homodyne detection.

9. The clock system of claim 7, wherein over a time interval approximating a detection integration time period T, said clock signal has at least one characteristic selected from a group consisting of (i) average clock signal frequency accuracy is better than about 1%, (ii) frequency of said clock signal changes at least ten times, (iii) frequency of said clock signal changes at least one hundred times, and (iv) frequency of said clock signal changes at least one thousand times.

10. The clock system of claim 7, wherein absolute magnitude of said cross-correlation product P₁₂ is less than about 1% of absolute magnitude of a self-correlation product P₁₁.

11. The clock system of claim 7, further including at least one of (i) means for promoting clock noise in said clock signal, (ii) a source of noise coupled to said clock signal, and (iii) means for promoting phase noise in said clock signal;

wherein at least one of characteristics (a), (b), (c), and (d) is achieved.

12. The clock system of claim 7, wherein said first TOF system is disposed within a motor vehicle.

13. For use with a first time-of-flight (TOF) detection system of the type that emits modulated optical energy and examines phase shift in a fraction of said optical energy reflected by a target object to determine distance Z therebetween, a clock system that reduces potential interference from an adjacent second TOF system also emitting optical energy, the clock system comprising:

a clock generator outputting a clock signal having long term frequency stability;

means, coupled to said clock generator, for reducing a cross-correlation product P₁₂ between detected signals from said first TOF and said second TOF system, while substantially maintaining first TOF detection accuracy.

14. The clock system of claim 13, wherein said clock generator outputs a clock signal having long term frequency stability while exhibiting at least one characteristic selected from a group consisting of (a) a signal having a temporally imperfect waveform, (b) a signal having a substantially imperfect periodicity, (c) a signal including at least pseudo-random clock noise, (d) a signal including random clock noise, (d) a signal whose frequency hops between at least two discrete frequencies, and (e) a signal whose frequency hops randomly between at least two discrete frequencies.

15. The clock system of claim 13, wherein said first TOF system employs homodyne detection.

16. The clock system of claim 13, wherein over an time interval approximating a detection integration time period T, said clock signal has at least one characteristic selected from a group consisting of (i) average clock signal frequency accuracy is better than about 1%, (ii) frequency of said clock signal changes at least ten times, (iii) frequency of said clock signal changes at least one hundred times, and (iv) frequency of said clock signal changes at least one thousand times.

17. The clock system of claim 13, wherein absolute magnitude of said cross-correlation product P₁₂ is less than about 1% of absolute magnitude of a self-correlation product P₁₁.

18. The clock system of claim 14, further including at least one of (i) means for promoting clock noise in said clock signal, (ii) a source of noise coupled to said clock signal, and (iii) means for promoting phase noise in said clock signal;

wherein at least one of characteristics (a), (b), (c), and (d) is achieved.

19. The clock system of claim 13, wherein said first TOF system is disposed within a motor vehicle.

20. The clock system of claim 13, wherein said clock system is implemented on a CMOS IC containing at least part of said first TOF system.