Time-of-flight rising edge adaptive cross-talk correction

1 . A method of operating a time-of-flight (ToF) ranging system, the method comprising:

receiving a histogram generated by a ToF imager of the ToF ranging system, wherein the histogram comprises a cross-talk signal and a reflected light signal from a target, wherein the cross-talk signal corresponds to an interference in the histogram due to reflected light pulses from a cover glass of the ToF ranging system;

finding, in a first region of the histogram, a first rising edge having a first gradient, wherein the first gradient is larger than a first threshold or is a maximum gradient in the first region, wherein the first rising edge is in a first histogram bin having a first value;

determining a second value of a second histogram bin in the first region, wherein the first histogram bin precedes the second histogram bin by a pre-determined distance;

estimating a ratio between the first region of the histogram and a pre-stored light pulse shape based on the first value and the second value, comprising:

comparing the second value with the first value; and

in response to determining that the second value is larger than the first value:

checking if the second value is between an upper bound and a lower bound; and

in response to determining that the second value is between the upper bound and the lower bound, generating the estimate of the ratio by multiplying the second value with a first pre-determined scaling factor;

scaling the pre-stored light pulse shape with the estimated ratio;

subtracting the scaled pre-stored light pulse shape from the histogram, wherein the subtracting reduces the interference in the histogram due to the reflected light pulses from the cover glass; and

after the subtracting, estimating a distance of the target using the histogram.

2 . The method of claim 1 , wherein estimating the distance of the target comprises:

finding, in a second region of the histogram, a second rising edge having a second gradient, wherein the second gradient is larger than a second threshold different from the first threshold, or is a maximum gradient in the second region; and

computing an estimate of the distance of the target by adding a pre-determined offset to a distance of the second rising edge.

3 . The method of claim 1 , further comprising, before finding the first rising edge:

computing differences between adjacent histogram bins in the first region; and

assigning the computed differences as gradients of corresponding histogram bins in the first region.

4 . The method of claim 1 , wherein the first region comprises a plurality of histogram bins around a reference zero-point histogram bin, wherein a distance of the reference zero-point histogram bin corresponds to a distance between an emitter of the ToF ranging system and the cover glass of the ToF ranging system.

5 . The method of claim 1 , further comprising, in response to determining that the second value is above the upper bound or below the lower bound, assigning a pre-determined default value as the estimate of the ratio.

6 . The method of claim 5 , wherein estimating the ratio further comprises:

in response to determining that the second value is smaller than the first value:

checking if the first value is between the upper bound and another lower bound; and

in response to determining that the first value is between the upper bound and the another lower bound, generating the estimate of the ratio by multiplying the first value with a second pre-determined scaling factor.

7 . The method of claim 6 , further comprising, in response to determining that the first value is above the upper bound or below the another lower bound, assigning the pre-determined default value as the estimate of the ratio.

8 . The method of claim 1 , further comprising, after finding the first rising edge and before determining the second value:

fine-tuning a location of the first rising edge by computing an adjustment term and adding the adjustment term to the location of the first rising edge.

9 . The method of claim 8 , wherein the adjustment term is a fractional number, wherein determining the second value of the second histogram bin comprises:

performing an interpolation process using values of the histogram bins around the second histogram bin to generate an interpolated histogram value; and

assigning the interpolated histogram value as the second value.

10 . A method of operating a time-of-flight (ToF) ranging system, the method comprising:

generating, by a ToF sensor of the ToF ranging system, a histogram, wherein the histogram comprises a cross-talk signal and a target signal, wherein the target signal is generated by a reflected light signal from a target of the ToF ranging system, wherein the cross-talk signal is generated by a reflected light signal from a cover glass of the ToF ranging system;

removing the cross-talk signal from the histogram, comprising:

estimating gradients of histogram bins in a first region of the histogram by computing differences between adjacent histogram bins;

finding, in a first histogram bin in the first region of the histogram, a first rising edge, wherein the first histogram bin has a first value, and a gradient of the first histogram bin is larger than a threshold or is a maximum gradient in the first region;

determining a second value of a second histogram bin in the first region, wherein the second histogram bin is behind the first histogram bin by a pre-determined number;

computing a scaling factor for a pre-stored pulse shape based on the first value and the second value, comprising:

determining if the second value is larger than the first value; and

in response to determining that the second value is larger than the first value:

checking if the second value is between an upper bound and a lower bound; and

in response to determining that the second value is between the upper bound and the lower bound, computing the scaling factor by multiplying the second value with a pre-determined value;

scaling the pre-stored pulse shape with the scaling factor; and

subtracting the scaled pre-stored pulse shape from the histogram; and

after removing the cross-talk signal, estimating a distance of the target using the histogram.

11 . The method of claim 10 , wherein the first region comprises a plurality of histogram bins around a reference zero-point histogram bin, wherein a distance of the reference zero-point histogram bin corresponds to a distance between a ToF emitter of the ToF ranging system and the cover glass.

12 . The method of claim 10 , wherein the ToF sensor is configured to emit a light signal comprising light pulses, wherein the pre-stored pulse shape corresponds to a normalized histogram of the light pulses emitted by the ToF sensor.

13 . The method of claim 12 , wherein the scaling factor is a ratio between a third value in a third histogram bin in the first region of the histogram and a corresponding fourth value of the normalized histogram.

14 . The method of claim 10 , further comprising, in response to determining that the second value is outside of a range between the upper bound and the lower bound, assigning a default value as the scaling factor.

15 . A time-of-flight (ToF) ranging system comprising:

a ToF sensor configured to emit light pulses and to generate a histogram based on reflected light pulses, wherein the histogram comprises a cross-talk signal and a target signal, wherein the target signal is generated by reflected light pulses from a target of the ToF ranging system, wherein the cross-talk signal is generated by reflected light pulses from a cover glass of the ToF ranging system; and

a processor coupled to the ToF sensor, wherein the processor is configured to:

find, in a first region of the histogram, a rising edge having a gradient, wherein the gradient is larger than a threshold or is a maximum gradient in the first region, wherein the rising edge is in a first histogram bin having a first value;

determine a second value of a second histogram bin in the first region, wherein the first histogram bin precedes the second histogram bin by a pre-determined number;

estimate, based on the first value and the second value, a ratio between the first region of the histogram and a pre-stored light pulse shape, comprising:

comparing the second value with the first value; and

in response to determining that the second value is smaller than the first value:

checking if the first value is between an upper bound and a first lower bound; and

in response to determining that the first value is between the upper bound and the first lower bound, generating the estimate of the ratio by multiplying the first value with a first pre-determined scaling factor;

scale the pre-stored light pulse shape with the estimated ratio;

subtract the scaled pre-stored light pulse shape from the histogram; and

estimate a distance of the target using the histogram after subtracting the scaled pre-stored light pulse shape from the histogram.

16 . The ToF ranging system of claim 15 , wherein the first region includes a plurality of histogram bins around a reference zero-point histogram bin, wherein a distance of the reference zero-point histogram bin corresponds to a distance between a ToF emitter of the ToF ranging system and the cover glass of the ToF ranging system.

17 . The ToF ranging system of claim 16 , wherein the pre-stored light pulse shape is a normalized histogram of light pulses emitted by the ToF sensor.

18 . The ToF ranging system of claim 15 , further comprising, in response to determining that the first value is above the upper bound or below the first lower bound, assigning a pre-determined default value as the estimate of the ratio.

19 . The ToF ranging system of claim 15 , wherein estimating the ratio further comprises:

in response to determining that the second value is larger than the first value:

checking if the second value is between the upper bound and a second lower bound; and

in response to determining that the second value is between the upper bound and the second lower bound, generating the estimate of the ratio by multiplying the second value with a second pre-determined scaling factor.

20 . The ToF ranging system of claim 19 , further comprising, in response to determining that the second value is above the upper bound or below the second lower bound, assigning a pre-determined default value as the estimate of the ratio.