Various aspects are related to a time-of-flight sensor and methods thereof (e.g., a method of carrying out a time-of-flight measurement).

Light-based time-of-flight measurements provide a simple, yet powerful approach to collect information about a scene. In general, a time-of-flight sensor operates by emitting light and calculating the time it takes for the emitted light to arrive back at the sensor after hitting an object in the scene. This measurement allows estimating the distance between the object and the sensor, thus allowing to create a map of the scene and to collect information about the object. Time-of-flight sensors have a wide range of applications, for example for assisting automated driving in a vehicle, for indoor monitoring systems, for smart farm systems, for movement tracking, and the like.

A common issue with time-of-flight sensors is their sensitivity to external light (such as ambient light, sun light, or direct light from other light sources), which may cause noise in the measurement, making it difficult to distinguish the reflection of the emitted light. Improvements in time-of-flight detection, for example in terms of robustness to noise, may thus be of particular relevance for advancing a variety of technological applications.

The present disclosure is directed to a time-of-flight sensor that makes use of the properties of a cross-correlation operation to enhance its distance measurement capabilities. The present disclosure is based on the realization that a cross-correlation allows finding a known pattern even in case the pattern is buried in noise. Such peculiar property of cross-correlation may be advantageously used in the time-of-flight context by emitting a light signal having a known pattern, and carrying out a cross-correlation to find the pattern in the light detected at the sensor, thus enabling an accurate and reliable measurement even in presence of noise.

The present disclosure is thus related to a time-of-flight sensor configured to measure a distance between the time-of-flight sensor and an object in the field of view via emitting a light signal having a predefined pulse pattern, and via carrying out a cross-correlation operation at the receiver side to find the pulse pattern in the detected light and thus determine the time-of-flight of the emitted multi-pulse light signal. The present disclosure is thus related to a new mechanism to detect the distance based on direct time-of-flight, aimed at improving signal-to-noise ratio, which subsequently improves power consumption, integration time, and overall performance of the sensor. According to various aspects, a time-of-flight sensor may include: a processing circuit configured to: receive a detection signal representative of light detected at the time-of-flight sensor during a detection period; carry out a cross-correlation of the detection signal with a reference signal, wherein the reference signal is representative of a predefined pulse pattern; and carry out a measurement of a time-of-flight of an emitted light signal based on a result of the cross-correlation, wherein the emitted light signal comprises a plurality of light pulses distributed in time according to the predefined pulse pattern.

According to various aspects, a method of carrying out a time-of-flight measurement may include: carrying out a cross-correlation between a detection signal and a reference signal, wherein the detection signal is representative of light detected during a detection period, wherein the reference signal is representative of a predefined pulse pattern; and carrying out a measurement of a time-of-flight of an emitted light signal based on a result of the cross-correlation, wherein the emitted light signal comprises a plurality of light pulses distributed in time according to the predefined pulse pattern.

The approach described herein may be readily integrated in existing time-of-flight sensors, with a minimal modification of the underlying hardware and/or software components, or may be implemented in dedicated time-of-flight sensors configured to operate according to the principles outlined in the following. A time-of-flight sensor configured as described herein may be implemented, for example, in a LIDAR system, in a movement tracker (such as an eye tracker), in an indoor monitoring system, and the like.

According to various aspects, a time-of-flight sensor described herein may be understood as a time-to-digital-converter (TDC)-based architecture that uses several pulses (a so-called multi-pulse) for ranging. A TDC-based solution may ensure low system-complexity (e.g., compared to an ADC-based solution), and may be suitable for high speed implementations, while ensuring a low data rate. Furthermore, a TDC-based solution may work with “binary” detector signals (e.g., with SPAD detector outputs), and may be suitable for multi-hit detection.

The expression “signal level” may be used herein to describe a parameter associated with a signal (e.g., with a light signal, a cross-correlation signal, a current signal, a voltage signal, etc.) or with a portion of a signal (e.g., with a pulse). As examples, a “signal level” as used herein may include at least one of a power level, a current level, a voltage level, an intensity level (e.g., a light intensity), or an amplitude level (also referred to herein as amplitude). It is however understood that the quantity represented by the term “signal level” may be suitably selected depending on the type of signal. The term “amplitude” may be used herein to describe the height of a pulse. The term “amplitude” may describe the signal level of the signal at the pulse (illustratively, at the peak of the pulse) with respect to a reference value for the signal level. The term “amplitude” may be used herein also in relation to a signal that is not a symmetric periodic wave, e.g. also in relation to an asymmetric wave (for example in relation to a signal including periodic pulses in one direction). In this regard, the term “amplitude” may be understood to describe the amplitude of the signal as measured from the reference value of the signal level.

In the following, various graphs associated with a signal may be illustrated and described, in which a signal level (e.g., a power or an amplitude) associated with the signal is plotted versus the time. It is understood that the values illustrated in the graphs and described in relation to the graphs are exemplary values which may be adapted in accordance with desired properties of the signal (e.g., a power may be increased or decreased, for example).

In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles disclosed herein. In the following description, various aspects disclosed herein are described with reference to the following drawings, in which:

FIG. 1 A shows a time-of-flight sensor in a schematic view, according to various aspects;

FIG. IB shows a graph representative of light detected at the time-of-flight sensor, according to various aspects;

FIG. 2A shows a time-of-flight sensor in a schematic view, according to various aspects;

FIG. 2B and FIG. 2C each shows an exemplary cross-correlation operation between a reference signal and a detection signal, according to various aspects;

FIG. 2D shows an emitted light signal in a schematic view, according to various aspects;

FIG. 2E and FIG. 2F each shows an exemplary cross-correlation operation between a reference signal and a detection signal, according to various aspects;

FIG. 3 A and FIG. 3B each shows an exemplary pulse pattern of a multi-pulse light signal in a schematic view, according to various aspects; FIG. 4A shows a processing circuit for use in a time-of-flight sensor in a schematic view, according to various aspects;

FIG. 4B shows a light detection circuit for use in a time-of-flight sensor in a schematic view, according to various aspects;

FIG. 4C shows a light emission circuit for use in a time-of-flight sensor in a schematic view, according to various aspects;

FIG. 4D shows an exemplary operation of the light emission circuit in a schematic view, according to various aspects;

FIG. 5 shows a time-of-flight sensor in a schematic view, according to various aspects; and

FIG. 6 shows a schematic flow diagram of a method of carrying out a time-of-flight measurement, according to various aspects.

The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and aspects disclosed herein may be practiced. These aspects are described in sufficient detail to enable those skilled in the art to practice the disclosed implementations. Other aspects may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the disclosed implementations. The various aspects are not necessarily mutually exclusive, as some aspects may be combined with one or more other aspects to form new aspects. Various aspects are described in connection with methods and various aspects are described in connection with devices (e.g., a time-of-flight sensor, a processing circuit, etc.). However, it is understood that aspects described in connection with methods may similarly apply to the devices, and vice versa.

FIG. 1A shows a time-of-flight sensor 100 in a schematic view, according to various aspects. In general, the basic principles of time-of-flight measurements, as well as basic hardware/ software components of a time-of-flight sensor, are known in the art. A brief description is provided herein to introduce aspects relevant for the present disclosure. FIG. 1 A illustrates the general operation of a time-of-flight sensor 100. A detailed description of the adapted configuration proposed herein will be provided in relation to FIG. 2A.

The time-of-flight sensor 100 may include, in general, a light emission circuit 102, a light detection circuit 104, and a processing circuit 106. In general, light is not the only option as a signal for carrying out time-of-flight sensing, e.g. sound waves may also be used, as another example. However, light may be a preferred choice since it ensures high traveling speed, safety of operation, and allows having a compact and portable system.

As an abridged overview, the time-of-flight sensor 100 may be configured to determine the distance d at which an object 110 is located with respect to the sensor 100 by emitting light 108 and measuring the time it takes for the emitted light 108 to hit the object 110 and be reflected back (as reflected light 112) to the sensor 100. The light emission circuit 102 may thus be configured to emit a light signal 108 towards the field of view of the time-of-flight sensor 100. The emitted light signal 108 may hit an object 110 in the field of view, and at least part of the emitted light may be reflected back towards the time-of-flight sensor 100. The light detection system 104 may receive and detect a reflected light signal 112 corresponding to the back-reflection of the emitted light signal 108, and the processing circuit 106 may be configured to calculate the time-of-flight and the distance d based on the reflected light signal 112. The object 110 may be any type of target in the field of view of the sensor 100, e.g. any type of inanimate object or animate object, such as a car, a tree, a traffic sign, an animal, a person, etc.

In general, a time-of-flight sensor 100 may be configured as a direct time-of-flight (DTOF) sensor, or as an indirect time-of-flight (ITOF) sensor.

In an ITOF-configuration, the emitted light signal 108 may be a continuous light signal having a predefined modulation, such as an amplitude modulation. At the receiver side, the processing circuit 106 may determine a phase shift between the modulation of the emitted light signal 108 and a modulation of the received light signal 112. The processing circuit 106 may then convert the phase shift into a corresponding distance d using the formula d = (c*tp)/(2*27t*f), where (p is the phase shift, c is the speed of light in a given medium (e.g., air in a common use case, or water, as examples) and f is the modulation frequency. An ITOF-configuration may be preferred in the context of three-dimensional imaging as it allows capturing more data points compared to a DTOF-approach. However it suffers from some limitations in view of the long integration time, possible ambiguities due to phase wrapping, and the difficulty in distinguishing multiple reflected signals.

In a DTOF-configuration, the light signal 108 may include a single light pulse, and the processing circuit 106 may be configured to carry out a timing measurement to measure a time difference between the emission of the light pulse and the reception of the reflected light pulse back at the sensor 100. From the time difference At, the processing circuit 106 may be configured to calculate the distance d at which the object 110 is located via the formula d = At*c/2, where c is the speed of light in the medium (e.g., air), and the factor 1/2 is to take into account the round-trip from the sensor 100 to the object 110 and back to the sensor 100. The processing circuit 106 may be configured to adjust the calculated distance by using one or more correction factors, e.g. to take into account effects of the optics of the time-of-flight sensor 100 as an example. A DTOF-sensor may thus have a simple architecture, e.g. using a time-to-digital converter to measure the time difference between a start signal representing the emission of the light signal 108, and a stop signal representing the arrival time of the reflected light signal 112.

In an exemplary configuration, a DTOF-sensor may be configured to carry out the distance calculation via histogram analysis. The time-of-flight measurement may include emitting short single light pulses and detecting their time of arrival via a highly sensitive photo diode, such as a single-photon avalanche diode (SP D), and a TDC. The times of arrival of the light pulses may be accumulated in a histogram, and the distance to the object 110 may be calculated using the peak value of the histogram to determine the time-of-flight of the emitted light 108 with higher accuracy with respect to a single measurement.

Such approach is illustrated in FIG. IB, which shows a graph 120 representing a direct time-of-flight measurement, according to various aspects.

In a conventional DTOF-scheme, the light emission circuit 102 may emit a laser pulse (e.g., a VCSEL pulse) at a fixed frequency that matches the maximum distance (illustratively, the maximum detectable range). During a given integration time, the SPAD may accumulate photon counts in a histogram with a fixed number of bins. After the integration is completed, the histogram may have a peak 122, as shown in FIG. IB. The peak may mark the position of the object.

In the exemplary scenario in FIG. IB, the signal is visibly larger than the noise threshold (caused for example by the ambient light). For example, the processing circuit 106 may be configured to detect a peak 122 in the histogram according to the following formula: where Npeak is the number of photon counts in the bin with the highest photon count (illustratively, the peak), <N> is the average photon count in the histogram, and SNRth is a predefined minimum SNR threshold. Illustratively, once the integration is completed, the formula above may be used to determine whether the signal may be distinguished from the noise to find the round-trip time of the signal. This time is translated into distance by knowing the speed of light in the medium (e.g., in air), as discussed above. Such DTOF-approach provides thus a straightforward and readily implemented distance measurement. However, the integration process takes time, which translates into power while the laser (e.g., the VCSEL) is on. Furthermore, in scenarios with a high level of noise (e.g., in an outdoor environment with sun light), additional integration time may be needed to accumulate sufficient signal and distinguish it from the noise with a sufficient level of confidence.

The present disclosure may be based on the realization that a cross-correlation operation may be introduced in the process flow of a time-of-flight sensor, by suitably adapting the emitted light signal, to detect an object with potentially less integration cycles (less integration time and therefore power) while maintaining the same level of confidence.

The adapted time-of-flight sensing scheme proposed herein provides thus a power-efficient approach that may operate even in conditions with a relatively high level of noise. The cross-correlation allows identifying a pattern in the detected light with a high level of confidence, and then carrying out the time-of-flight and distance measurement/calculation possibly in less time compared to a conventional approach (e.g., a conventional DTOF-approach). Illustratively, the cross-correlation may provide finding the signal in the noise without having to accumulate a large number of single shots, as for a DTOF approach, but rather relying on the properties of the cross-correlation to find a matching pattern in the detected light.

FIG. 2A shows a time-of-flight sensor 200 in a schematic view, according to various aspects. The time-of-flight sensor 200 may be configured to operate according to the cross-correlation-based measurement described herein. According to various aspects, the time-of-flight sensor 200 may be a direct time-of-flight sensor, adapted to carry out a cross-correlation operation at the receiver side. In general, the time-of-flight sensor 200 may be dedicated to the cross-correlation-based approach. Alternatively, the time-of-flight sensor 200 may combine the DTOF-approach or ITOF-approach described in FIG. 1 A with the cross-correlation-based approach, e.g. to enhance the overall capabilities of the sensor and adapt its operation to difference scenarios. A time-of-flight sensor may also be referred to herein as time-of-flight detector, or time-of-flight measurement device.

The time-of-flight sensor 200 may include a light emission circuit 202, a light detection circuit 204, and a processing circuit 206. A light emission circuit may be referred to herein also as emitter. A light detection circuit may also be referred to herein as light detector. FIG. 2A provides a schematic illustration to introduce the principles of the cross-correlation-based approach. A more detailed description of the various components of the time-of-flight sensor 200 will be provided in relation to FIG. 4A to FIG. 4D. It is also understood that the representation in FIG. 2A may be simplified for the purpose of illustration, and that the time-of-flight sensor 200 may include additional components with respect to the components shown in FIG. 2A. As an example, the time-of-flight sensor 200 may include emitter optics to direct emitted light towards the field of view, e.g. a diffuser to diffuse the emitted light to cover the field of view. As another example, the time-of-flight sensor 200 may include receiver optics (e.g., one or more lenses, a liquid crystal, etc.) to collect light from the field of view.

The processing circuit 206 may be configured to receive a detection signal 214 representative of light detected at the time-of-flight sensor 200 during a detection period. The detection signal 214 may be or include a representation of the detected light in a format that allows processing by the processing circuit 206. In general, the detection signal 214 may thus be an analog signal representing the detected light in an analog manner, or a digital signal representing the detected light in a digital manner. In a preferred configuration, the detection signal 214 may provide a digital representation of the detected light, as discussed in further detail below.

The processing circuit 206 may be further configured to carry out a cross-correlation 216 of the detection signal 214 with a reference signal 218 representative of a predefined pulse pattern. The reference signal 218 may encode a pattern of light pulses (illustratively a sequence of light pulses) to be matched with the detection signal 214. The processing circuit 206 may be configured to carry out a measurement 220 of a time-of-flight of an emitted light signal 208 based on a result of the cross-correlation 216. The emitted light signal 208 may include a plurality of light pulses distributed in time according to the predefined pulse pattern.

Illustratively, the operation of the time-of-flight sensor 200 may be based on emitting a multi-pulse light signal 208 and finding a match for the pulse pattern of the multi-pulse light signal 208 within the light detected at the time-of-flight sensor 200. The light detected during the detection period may illustratively include a reflection 212 of the emitted multi-pulse light signal 208, e.g. a reflection from an object 210 in the field of view of the time-of-flight sensor 200, and a noise component superimposed to the signal component. The cross-correlation 216 may provide finding the signal component (illustratively, the reflected light signal 212 corresponding to the reflection of the emitted multi-pulse light signal 208) within the noise component.

A pattern with a plurality of light pulses may provide an encoding of the emitted light signal 208 that allows identifying the pattern within the noise with a high level of confidence (e.g., compared to emitting single light pulses), and may be readily implemented with minimal adaptation of the existing hardware, e.g. in a direct time-of-flight sensor. Illustratively, other types of encoding may in general be provided for the cross-correlation (for example, based on modulating the amplitude of a continuous light signal), which however require a more complex hardware and are not easily implemented. The reference signal 218 may be, for example, stored in a memory of the time-of-flight sensor 200. The processing circuit 206 may be communicatively coupled with the memory (not shown) and may be configured to retrieve the reference signal 218 for carrying out the cross-correlation 216. In an exemplary configuration, the processing circuit 206 may be configured to deliver the reference signal 218 as control signal to the light emission circuit 202 to control an emission of light according to the pulse pattern, and to further use the delivered reference signal 218 for the cross-correlation 216. In another exemplary, configuration, both the light emission circuit 202 and the processing circuit 206 may retrieve the (same) reference signal 218 from the memory. In a yet further exemplary configuration, the processing circuit 206 may be configured to receive the reference signal 218 from the light emission circuit 202, e.g. from a controller of the light emission circuit 202.

To carry out the measurement 220 of the time-of-flight of the emitted light signal 208, the processing circuit 216 may be configured to derive a value of the time-of-flight from the result of the cross-correlation, as discussed in further detail below. In this regard, the time-of-flight of the emitted light signal 208 may be understood as the time it takes for the emitted light signal 208 to be received back at the time-of-flight sensor 200 (illustratively, as reflected light signal 212). The time-of-flight of the emitted light signal 208 may thus be a round-trip time of the emitted light signal 208 between its emission and the arrival of its reflection 212 at the time-of-flight sensor 200.

According to various aspects, carrying out the measurement 220 of the time-of-flight may be or include carrying out a distance measurement to determine a distance d at which an object 210 is located with respect to the time-of-flight sensor 200. Illustratively, the processing circuit 206 may be configured to determine (e.g., calculate) a distance d between the time-of-flight sensor 200 and the object 210 based on the result of the cross-correlation 216 (and, accordingly, based on the result of the time-of-flight measurement 220). The distance d may be calculated using the formula discussed above taking as At the determined time-of-flight of the emitted light signal 208, optionally including corrections and/or adjustments to take into account effects of the optics of the time-of-flight sensor 200.

An exemplary cross-correlation operation 216 is illustrated in FIG. 2B, according to various aspects.

To carry out the cross-correlation 216 of the detection signal 214 with the reference signal 218, the processing circuit 206 may be configured to determine a match between the predefined pulse pattern and a waveform of the detection signal 214. Illustratively, the processing circuit 206 may be configured to determine a similarity between the reference signal 218 and the detection signal 214, to find if and where there is an occurrence of the reference signal 218 within the detection signal 214. The reference signal 218 may thus be understood as a template pattern that the processing circuit 216 shifts across the detection signal 214 to measure a degree of correspondence of the reference signal 218 with the detection signal 214. In general, the processing circuit 206 may be configured to carry out the cross-correlation 216 in the time domain or in the frequency domain (e.g., via a Fourier transform of the signals 218, 214). In a preferred configuration, the processing circuit 206 may be configured to perform the crosscorrelation 216 in the time domain, as this may simplify the overall processing and hardware architecture.

An exemplary result of a cross-correlation 216 is shown in the graph 222. In general, the result of the cross-correlation 216 may be or include a cross-correlation signal 224. The cross-correlation signal 224 may vary over time according to a degree of similarity (illustratively, a degree of matching) between the pattern of the reference signal 218 and the waveform of the detection signal 214. A signal level of the cross-correlation signal 224 over time may thus represent a correspondence between the reference signal 218 and the detection signal 214.

In FIG. 2B, the result of the cross-correlation 216 is represented in terms of cross-correlation coefficient. In the graph 222 the cross-correlation coefficient is normalized, so that it may vary between 1 (indicating a perfect match) and -1. Illustratively, the closer to 1 the signal level of the cross-correlation signal 222 is, the more matching occurs between the reference signal 218 and the detection signal 214. It is however understood that in general the result of the crosscorrelation 216 may be expressed or visualized in various forms to represent the occurrence of a match between the reference signal 218 and the detection signal 214. As another example, the cross-correlation signal 222 may be represented in terms of a non-normalized cross-correlation coefficient.

The processing circuit 206 may be configured to identify a maximum of the result of the cross-correlation 216 (illustratively, a peak of the cross-correlation signal 224) to determine the time-of-flight of the emitted light signal 208. The cross-correlation coefficient may be maximum at the time-delay where the reference pattern has a correspondence to the detection signal 214. As an exemplary configuration, to carry out the measurement of the time-of-flight, the processing circuit 206 may be configured to determine a time location within the crosscorrelation signal 224 of an absolute maximum of the cross-correlation signal 224. As illustrated in the graph 222, the result of the cross-correlation 216 may be representative of a time-delay 226 between the predefined pulse pattern and a matching pattern in the detection signal 214. By way of illustration, considering an initial time point (e.g., time zero), the maximum of the cross-correlation signal 224 may be at a time offset with respect to the initial time point obtained by shifting in time the emitted pattern and finding the maximum correspondence within the detection signal 214. In the context of time-of-flight measurement, such time-delay (also referred to herein as time-offset, or time-shift) may be indicative of the time it took for the pattern to be emitted (at the initial time point) and be detected back at the time-of-flight sensor 200. The time delay 226 may thus be or represent the time-of-flight of the emitted light signal 208. The time-of-flight measurement may thus be carried out without the usual start signal and stop signal of a TDC-measurement of a conventional DTOF-approach, but rather relies on determining a time-delay between the emitted pattern and the detected signal.

According to various aspects, the processing circuit 206 may be configured to discard the result of the cross-correlation 216 in the case that the signal level at the maximum of the cross-correlation signal 224 is outside of a predefined range, e.g. in the case that the maximum of the cross-correlation coefficient is less than a predefined threshold (illustratively, a minimum confidence threshold). As a numerical example, considering a normalized cross-correlation coefficient, the predefined threshold may be 0.7, or 0.5. The predefined threshold may represent a desired level of confidence for the detection of the pattern within the detection signal 214, and may thus be selected or adapted to define a desired accuracy of the measurement.

The detection signal 214 may represent the light detected at the time-of-flight sensor 200 during the detection period in any suitable form to enable the cross-correlation 216. As an example, the detection signal 214 may represent the detected light in an analog manner, e.g. as a continuous analog signal with varying signal level according to the intensity of the received light over time. In a preferred configuration, which may be readily integrated in existing DTOF -hardware, the detection signal 214 may include a plurality of photon counts accumulated during the detection period. In this configuration, the detection signal 214 may represent the detected light as a number of photon counts per time point (or per time interval), and may thus be provided as a digital representation. Each photon count may be associated with a respective time of arrival of light at the time-of-flight sensor 200 with respect to a starting time of an emission of the emitted light signal 208. Illustratively, each photon count may be assigned to a respective time-delay with respect to the starting time at which the multi-pulse light signal 208 has been emitted, and the time-delay may represent the time at which that photon (or those photons) arrived at the time-of-flight sensor 200 measured from the starting time of the light emission. As discussed in further detail in relation to FIG. 4B, such type of representation may be generated with hardware commonly used in DTOF-measurements, so that the approach described herein may be integrated in DTOF-sensors with minimal adaptations.

As an example, the starting time of the emission of the emitted light signal 208 may correspond to a time point of an emission of an initial pulse of the plurality of pulses of the emitted light signal 208. Illustratively, the starting time may be considered, as an exemplary convention, as the time at which the temporally first pulse in the train of pulses is emitted. This convention enables a simple and straightforward association of the photon counts to the emission time and then arrival time. It is however understood that other time points may be considered as starting time for the emission of the multi-pulse light signal 208, with an adaptation of how the photon counts are recorded.

FIG. 2C shows an exemplary cross-correlation 216c considering a detection signal 214c including a plurality of photon counts.

In this scenario, the processing circuit 206 may be configured to determine a match between the predefined pulse pattern and a pattern defined by the photon counts accumulated during the detection period. Illustratively, the photon counts may be approximated as a continuous waveform over time, and the processing circuit 206 may be configured to determine a match between the predefined pulse pattern and such approximated waveform.

In an exemplary configuration, as shown in FIG. 2C (see also FIG. 2D and FIG. 2E), the detection signal 214c may be or include a histogram. The histogram may provide a compact representation of the detected light. The histogram may include a plurality of bins, e.g. histogram bins, each corresponding to a respective arrival time or a respective range for the arrival time of light at the time-of-flight sensor 200 with respect to the starting time of the emission of the emitted light signal. The histogram 214c may include, for each histogram bin, a respective photon count according to the arrival time of the photons during the detection period. According to the formula discussed above, each bin may correspondingly be associated with a distance between the sensor 200 and the object 210 on which the light 208 is reflected.

According to the multi-pulse approach proposed herein, the final histogram 214c presents multiple peaks instead of a single peak, as it is the case in a conventional DTOF-configuration. In general, the amplitude of the multiple peaks may be smaller than the mean noise value, but the cross-correlation between the known signal and the received noisy variation still enables detecting the peaks with a high level of confidence. In the exemplary scenario in FIG. 2C, the pulse pattern of the emitted light signal, and accordingly of the reference signal 218c, may include a plurality of pulses, e.g. six pulses (e.g., six VCSEL pulses) with a predefined amplitude (for example, an amplitude of ten counts). At the receiver side, the multi-pulse signal is mixed with noise, e.g. a noise with a mean of 100 counts as an example. The cross-correlation 216c between the signal 218c and the received signal including noise 214c results in a clear peak at one of the histogram bins (e.g., bin 32 in this exemplary case), as shown in the graph of the cross-correlation signal 222c. The bin at which the peak is located may correspond to the signal shift (in time), and accordingly to the distance at which the object is located.

Therefore, the cross-correlation-based approach allows to detect a signal even when the signal is clearly not immediately visible in the histogram 214c. As an exemplary configuration, the processing circuit 206 may be configured to find the absolute maximum of the cross-correlation signal by comparing the signal level of the cross-correlation signal to a predefined noise threshold. For example, the formula mentioned above may be used. Illustratively, considering a histogram representation, the processing circuit 206 may be configured to find the maximum photon count using the formula where Npeak is the number of photon counts in the bin with the highest photon count (illustratively, the peak), <N> is the average photon count in the histogram, and SNRth is a predefined minimum SNR threshold. Such configuration provides a simple, yet accurate manner to find the peak in the histogram (or, with a corresponding adaptation, the peak in a crosscorrelation signal with a different representation). It is however understood that other strategies for identifying the peak may be provided.

According to various aspects, as shown in FIG. 2D, the time-of-flight measurement may be carried out on the basis of a plurality of multi-pulse light signals. Illustratively, in various aspects, the emitted light signal 208d may include a plurality of partial light signals 232, each having a plurality of light pulses distributed in time according to the predefined pulse pattern. This configuration allows accumulating, at the receiver side, light from a plurality of signal components, rather than relying on just one multi-pulse signal, thus providing an improvement (illustratively, an increase) in the signal-to-noise ratio of the detection signal 214.

This configuration may be conceptually analogous to a DTOF -measurement, in which a plurality of shots of single pulses are shone onto a target, with the difference that according to the approach described herein each shot includes a multi-pulse light signal 232. The emission of a plurality of multi-pulse light signals 232 allows thus summing, at the receiver side, the reflection from each multi-pulse light signal 232, and facilitate the detection even in presence of strong noise.

In this configuration, the detection period may illustratively include a plurality of detection intervals 236, and each detection interval 236 may be associated with the emission of a corresponding partial light signal 232. Considering a configuration in which the detection signal 214 represents photon counts, the detection signal 214 may include a plurality of photon counts accumulated during each detection interval 236. Within each detection interval 236, each photon count may be associated with a respective time of arrival of light at the time-of-flight sensor 200 with respect to a starting time of an emission of the corresponding partial light signal 232. As mentioned for the case with one multi-pulse light signal, the starting time of the emission of a partial light signal 232 may correspond to a time point of an emission of an initial pulse of the plurality of pulses of the partial light signal 232, as an exemplary convention.

During each detection interval 236, the detection signal 214 may include noise and a signal component provided by the corresponding partial multi-pulse light signal 232. The contributions of the individual multi-pulse light signals 232 may be summed (or, in general, combined) to generate a detection signal 214 representative of the detection period 234 with a plurality of detection intervals 236. The processing circuit 206, or the light detection device 204, may be configured to carry out such combination.

By way of illustration, each of the partial light signals 232 may have substantially the same time-of-flight for hitting an object 210 and returning back to the time-of-flight sensor 200. By suitably choosing a period for the emission of the partial light signals 232, the object 210 may be considered substantially stationary during the emission of an emitted light signal 208 including a plurality of partial light signals 232. Thus, considering the accumulation of photon counts, the photon counts may be assigned to the same time of arrival (e.g., the same histogram bin) for each partial light signal 232, so that the photon counts of the signal component of the detection signal 214 increase (illustratively, accumulate) over time.

The period of the emission of the partial light signals 232 may be selected according to the maximum range of the time-of-flight sensor 200, e.g. the maximum distance at which an object 210 may be located and still be detected. Illustratively, the emission period may be selected to allow each partial light signal 232 to travel to the maximum distance before emitting the subsequent partial light signal 232, thus preventing an overlap that would make it difficult to distinguish the signals at the receiver side. Accordingly, each detection interval 236 may have a duration in time based on the emission period, e.g. a duration in time equal to the emission period of the partial light signals 232, so that during each detection interval 236 only the contribution of one of the multi-pulse light signals 232 is detected.

In general, the processing circuit 206 may be configured to receive the detection signal 214 continuously during the detection period, e.g. the processing circuit 206 may be configured to continuously carry out a cross-correlation. In an exemplary configuration, the processing circuit 206 may be configured to carry out the cross-correlation at periodic time intervals, for example at the end of each detection interval 236 considering an emission of a plurality of multi-pulse light signals 232.

FIG. 2E and FIG. 2F each shows an exemplary cross-correlation 216e, 216f considering the accumulation of a plurality of partial light signals.

As shown, the predefined pattern (represented by the reference signal 218e, 218f) may include a plurality of pulses (e.g., three pulses), each having a same photon count, e.g. 10 as a numerical example.

At the receiver side, as an example, ten partial light signals may be accumulated, so that the detection signal 214e, 214f may include peaks having a photon count of 100. FIG. 2E shows the ideal case in absence of noise, whereas FIG. 2F shows the signal component together with a noise component providing noise photon counts in the histogram. As shown in FIG. 2F, the accumulation of photon counts over a plurality of detection intervals ensures that the signal component may be more easily distinguished from the noise component.

The cross-correlation 216e, 216f may allow to identify a peak in a histogram representation of the cross-correlation signal 222e, 222f, and thus determine a shift 226e, 226f in time between the emitted pattern and a matching pattern in the detection signal 214e, 214f. The time-delay 226e, 226f may be used to determine a corresponding distance at which the object by which the light was reflected is located, as discussed above.

In a conventional DTOF-approach, the time-of-flight measurement may be carried out for a predefined time, e.g. a predefined integration time of a SPAD, during which multiple single shots are emitted towards the target. The cross-correlation approach allows, however, to identify the reflection of the emitted light over an overall shorter period of time with respect to a conventional DTOF-approach. The cross-correlation allows identifying peaks even in the presence of strong noise, and the measurement may thus be carried out without the need of accumulating a large number of shots, as it is instead the case for DTOF-configurations. According to various aspects, the processing circuit 206 may be configured to stop the detection of light (e.g., an integration at a SP D, as discussed in further detail below) based on the result of the cross-correlation. Illustratively, the processing circuit 206 may be configured to generate a stop signal to terminate the detection period in the case that a signal level of the cross-correlation signal is in a predefined threshold range, e.g. in the case that the maximum of the cross-correlation coefficient is greater than a predefined threshold (e.g., greater than 0.7, or 0.8, or 0.9, as numerical examples considering a normalized representation). The predefined threshold range may be defined by a predefined signal-to-noise ratio, e.g. a lower boundary of the predefined threshold range may be or represent a threshold SNR above which it may be assumed that the signal has been detected, and not just random noise. This configuration may thus provide stopping the measurement as soon as it may be confidently assumed that the signal has been identified, without having to wait for a fixed detection time. This configuration may be particularly advantageous, without limitation, in the case that the light detection is carried out via a SPAD (see also FIG. 4B), as it allows having an overall shorter integration time for the SPAD. This approach allows thus saving power both at the receiver side and the emitter side.

In various aspects, additionally or alternatively, the processing circuit 206 may be configured to stop the emission of light based on the result of the cross-correlation. Illustratively, the processing circuit 206 may be configured to generate a further stop signal to terminate the light emission in the case that a signal level of the cross-correlation signal is in the predefined threshold range, e.g. in the case that the maximum of the cross-correlation coefficient is greater than the predefined threshold.

FIG. 3A and FIG. 3B each shows an exemplary pattern 300a, 300b of a multi-pulse light signal in a schematic view, according to various aspects. Illustratively, the pattern 300a, 300b may be an exemplary configuration of the reference signal 218 (and accordingly of the light signal 208, or partial light signal 232).

In general, the pattern 300a, 300b according to which light is emitted (e.g., the light signal 208, e.g. each partial light signal 232) may include a plurality of pulses 302a, 302b distributed over time. The properties of the predefined pattern 300a, 300b may be adapted according to a desired operation of the time-of-flight sensor 200, while ensuring the possibility of carrying out the cross-correlation.

The properties of the predefined pattern may include, as examples, a number of pulses, a duration of a pulse, a time-spacing between pulses, an overall duration of the signal, a photon count per pulse, and the like. In general, the approach described herein may be carried out without amplitude or frequency modulation of the emitted light, e.g. without emitting a continuous light signal having a modulated amplitude or frequency (as instead done in ITFO-architectures). Illustratively, the pulses 302a, 302b may all have the same amplitude, e.g. the same photon count.

Numerical examples, without limitation, are provided in the following. A predefined pattern 300a, 300b may include a number of pulses 302a, 302b in the range from 2 to 10, for example in the range from 3 to 5. A duration of a pulse 302a, 302b may be in the range from 1 ns to 100 ns, for example in the range from 10 ns to 50 ns. A time-spacing between pulses 302a, 302b (also referred to as pulse interval) may be in the range from 2 ns to 100 ns, for example in the range from 5 ns to 50 ns. An overall duration of the pattern 302a, 302b may be in the range from 100 ns to 1000 ns, for example in the range from 200 ns to 500 ns. A photon count per pulse may be in the range from 5 to 50, for example in the range from 10 to 30.

In general, the overall duration of the pattern 302a, 302b may be selected to allow for the emission of multiple multi-pulse light signals configured according to the pattern, e.g. the overall duration of the pattern 302a, 302b may be less than half of a detection interval 236, e.g. less than one third of a detection interval, as examples. The time-spacing between pulses may be, in general, less than the interval at which single shots are emitted in a conventional DTOF-configuration.

In a simple configuration, as shown in FIG. 3 A, the pulses may be regularly spaced in time within the pattern, e.g. the pattern 300a may include a plurality of light pulses distributed in time at regular time intervals 304a between consecutive pulses. In this scenario, a time interval 304a between a first pulse and a second pulse of the pattern 300a may be equal to the time interval 304a between the second pulse and a third pulse of the pattern 300a, or between the third pulse and a fourth pulse, etc.

In a preferred configuration, as shown in FIG. 3B, the predefined pulse pattern 300b may include a plurality of light pulses distributed in time with an irregular distribution, in particular according to a pseudo-random sequence. A distribution according to a pseudo-random sequence may provide an increased robustness against noise, in particular against external light coming from other time-of-flight sensors. In this configuration, a first time interval 304b-l between a first pulse and a second pulse of the pattern 300b may have a first value, a second time interval 304b-2 between the second pulse and a third pulse may have a second value, a third time interval 304b-3 between the third pulse and a fourth pulse may have a third value, etc. For example, the values of the time intervals may be all different from each other and have respective values selected (e.g., by the processing circuit 206, or by a controller of the light emission device 202, as examples) in a pseudo-random manner. As another example, some of the values may be equal to each other, overall according to a pseudo-random distribution.

FIG. 4A, FIG. 4B, and FIG. 4C illustrate exemplary configurations of a processing circuit 400, light detection circuit 420, and light emission circuit 440 for use in a time-of-flight sensor. Illustratively, the processing circuit 400, light detection circuit 420, and light emission circuit 440 may be exemplary configurations of the processing circuit 206, light detection circuit 204, and light emission circuit 202 of the time-of-flight sensor 200. In general, the operations of the processing circuit 400, light detection circuit 420, and light emission circuit 440 may be carried out in the digital domain and/or in the analog domain, so that the components may be adapted according to the selected approach, without loss of generality.

It is understood that, in general, there may be various options to implement the aspects of the cross-correlation-based time-of-flight detection described herein. FIG. 4A to FIG. 4C illustrate exemplary realizations which have been found particularly suitable for emitting and detecting multi-pulse light signals, and for carrying out the cross-correlation. However, the various circuits may include additional, less, or alternative components with respect to those shown, to provide overall the functionality described herein.

With reference to FIG. 4A, the processing circuit 400 may include a cross-correlator 402 configured to carry out a cross-correlation between a detection signal 404 and a reference signal 406 (e.g., configured as the detection signal 214 and reference signal 218 described in FIG. 2A). The cross-correlator 402 may receive, at a first input, the detection signal 404 and, at a second input, the reference signal 406. The cross-correlator 402 may be configured to cross-correlate the reference signal 406 with the detection signal 404 to generate, as output, a cross-correlation signal 408.

In general, the cross-correlator 402 may be configured to operate in the analog domain or in the digital domain. Depending on the selected implementation, the input signals 404, 406 may be converted to the appropriate format via digital-to-analog converters or analog-to-digital converters. As an example, considering an analog implementation, the cross-correlator 402 may include one or more amplifiers, one or more diodes, one or more flip-flops, and/or the like.

As an optional component, the processing circuit 400 may include a stop signal generation circuit 410 configured to generate a stop signal 412 to terminate the detection (and, in some aspects, the light emission) based on the cross-correlation signal 408 outputted by the cross-correlator 402. The stop signal generation circuit 410 may be configured to deliver the stop signal 412 to the light detection circuit and/or light emission circuit of the time-of-flight sensor. The stop signal generation circuit 410 may be configured to compare the signal level of the cross-correlation signal 408 with a signal representing a predefined threshold level, and generate the stop signal 412 in the case that the signal level of the cross-correlation signal 408 is or becomes greater than the predefined threshold level. As an exemplary configuration, the stop signal generation circuit 410 may include a comparator (e.g., including an operational amplifier) configured to receive, at a first input, the cross-correlation signal 408 and, at a second input, a signal representing the predefined threshold level.

The processing circuit 400 may further include a time-of-flight measurement circuit 414 configured to determine the time-of-flight of the emitted light signal based on the crosscorrelation signal 408 outputted by the cross-correlator 402. The time-of-flight measurement circuit 414 may be or include, for example, a digital signal processing circuit configured to receive a digital representation of the cross-correlation signal 408 and identify a peak in the cross-correlation signal 408.

The time-of-flight measurement circuit 414 may be configured to provide (e.g., deliver, or output) an output signal 416 representative of the time-of-flight of the emitted light signal and/or representative of the distance at which the object is located. Illustratively, the time-of-flight measurement circuit 414 may be also configured to calculate the distance at which the object is located based on the time-of-flight, as discussed above. The time-of-flight measurement circuit 414 may be configured to provide the output signal 416 to other circuits or devices. For example, the time-of-flight measurement circuit 414 may be configured to wirelessly transmit the output signal 416 to a central controller in a network, to a host processor, to a user device, and/or the like.

FIG. 4B illustrates a light detection circuit 420. In general, the light detection circuit 420 may be configured to detect light, generate a corresponding detection signal 430 representative of the light detected during a detection period, and deliver the detection signal 430 to a processing circuit of the time-of-flight sensor (e.g., the processing circuit 206, 400). The light detection circuit 420 may be configured to carry out light detection according to a histogram-based approach. In this configuration, the light detection circuit 420 may be configured to receive a start signal 422 representative of a starting time of an emission of an emitted light signal. The light detection circuit 420 may be further configured to generate the detection signal 430 as a plurality of photon counts, and to assign each photon count to a respective time of arrival of light at the light detection circuit 420 with respect to the starting time of the emission of the emitted light signal. In case the emitted light signal includes a plurality of partial light signals, the start signal 422 may correspondingly include a plurality of partial start signals, each representative of a starting time of an emission of the corresponding partial light signal. In this scenario, the light detection circuit 420 may be configured, for each partial start signal, to assign each photon count to a respective time of arrival of light with respect to the starting time of the emission of the corresponding partial light signal (illustratively, during a corresponding light detection interval).

In an exemplary configuration, the hardware components of the light detection circuit 420 may be adapted from components usually implemented in DTOF-sensors, so that the configuration and operation of these components is in general well-known in the art. A detailed description is thus dispensed with, and just aspects relevant to the present disclosure are described.

According to various aspects, the light detection circuit 420 may include a light receiver 424 configured to receive and detect light (e.g., configured to generate a signal upon light being received at the light receiver 424). For example, the light receiver 424 may include one or more photo diodes, for example a one-dimensional array of photo diodes 424 or a two-dimensional array of photo diodes 424. A two-dimensional array of photo diodes may provide determining from which direction in the x-y plane the reflection of the emitted light is coming, thus allowing to reconstruct a map of the scene. The light receiver 424 (e.g., the one or more photo diodes) may be configured to be sensitive for the emitted light, e.g. may be configured to be sensitive in a predefined wavelength range, for example in the visible range (e.g., from about 380 nm to about 700 nm), infra-red and/or near infra-red range (e.g., in the range from about 700 nm to about 5000 nm, for example in the range from about 860 nm to about 1600 nm, or for example at 905 nm or 1550 nm), or ultraviolet range (e.g., from about 100 nm to about 400 nm).

In a preferred configuration, the light receiver 424 may include one or more single-photon avalanche photo diodes. The single-photon avalanche photo diodes (SPADs) allow generating a strong (avalanche) signal upon reception of single photons impinging on the photo diodes, thus providing a high responsivity and a fast optical response. In addition, SPADs have a low noise characteristic that makes them well suited for TOF-applications. In such a configuration, the detection period for the light detection may be or include an integration time of the SPADs. As other examples, the light receiver 424 may include one or more avalanche photo diodes, or a silicon photomultiplier.

The light detection circuit 420 may further include a time-to-digital converter 426 configured to provide time-stamping of the photons detected by the light receiver 424. Illustratively, the time-to-digital converter 426 may be configured to receive the start signal 422, receive a stop signal upon detection of a photon at the light receiver 424; and generate a digital signal 432 representative of the time difference between the start signal and the stop signal. As an example, the time-to-digital converter 426 may include a plurality of time-to-digital converters, e.g. one for each photo diode of the light receiver 424 (illustratively, one for each pixel of a photo diode array). As another example, the photo diodes may be grouped into one or more subsets of photo diodes, and the time-to-digital converter 426 may include one or more time-to-digital converters, each corresponding to a respective subset of photo diodes.

The light detection circuit 420 may further include a histogram generation circuit 428 configured to receive the output of the time-to-digital converter 426 and accumulate the photon counts in a corresponding histogram. Illustratively, the histogram generation circuit 428 may be configured to increase a counter corresponding to a certain arrival time upon receiving a corresponding digital signal 432 representing that arrival time from the time-to-digital converter 426. The histogram generation circuit 428 may be configured to output the histogram as detection signal 430, e.g. at periodic time intervals (e.g., after each detection interval) or continuously during the detection. As an exemplary configuration, the histogram generation circuit 428 may be configured to store the photon counts in a memory (e.g., in a register, or a buffer), to accumulate multiple photon counts, and sum the photon counts over a plurality of detection intervals.

FIG. 4C shows a light emission circuit 440. In general, the light emission circuit 440 may include a light source 442 configured to emit light (e.g., a light signal 448, an example of light signal 208), and a controller 444 configured to control the light source to emit light.

The light source 442 may be configured to emit light having a predefined wavelength, for example in the visible range (e.g., from about 380 nm to about 700 nm), infra-red and/or near infra-red range (e.g., in the range from about 700 nm to about 5000 nm, for example in the range from about 860 nm to about 1600 nm, or for example at 905 nm or 1550 nm), or ultraviolet range (e.g., from about 100 nm to about 400 nm). Illustratively, the light source 442 may include at least one of an infrared light source, an ultraviolet light source, or a visible light source.

In a preferred configuration, particularly suitable for TOF -measurements to reduce the impact of ambient light, the light source 442 may be or include a laser source. Illustratively, the emitted light signal (e.g., the signal 208, 448) may be an emitted laser signal. As an exemplary configuration, the light source 442 may include a vertical cavity surface emitting laser (VCSEL), or a VCSEL array (e.g., a one-dimensional or two-dimensional array). As another example, the light source 442 may include one or more edge emitting laser diodes.

The controller 444 may be configured to encode an emission signal 446, and deliver the emission signal 446 to the light source 442 to drive the light emission. According to the cross-correlation-based approach, the controller 444 may be configured to encode the emission signal 446 to include a plurality of pulses according to the predefined pulse pattern, and may be configured to control the light source 442 to emit the light signal according to the encoded emission signal 446. For example, the emission signal 446 may drive the discharge of one or more capacitors to provide accordingly a driving current to the emitting diodes according to the emission signal. As another example, the emission signal 446 may open/close one or more access transistors to allow/prevent a current flow to the emitting diodes of the light source 442.

As an exemplary configuration, the light emission circuit may include a clock signal generator (not shown) configured to generate a clock signal to trigger a light emission by the light source 442. To control the emission of the predefined pattern, the controller 444 may be configured to impose a plurality of phase-delays onto the clock signal to encode the emission signal. Such approach is shown in FIG. 4D for the exemplary scenario of a VCSEL. Illustratively, based on a desired pulse pattern 450, the controller 444 may be configured to impose a plurality of phase-delays onto the clock signal such that a clock pulse reaches the light source 442 and triggers light emission in correspondence of the pulses within the pulse pattern 450. The phase-delays may correspond to the spacing between pulses in the pattern 450. A first clock pulse 452 may be delayed to provide emission of a first light pulse, a second clock pulse 454 may be delayed to provide emission of a second light pulse, a third clock pulse 456 may be delayed to provide emission of a third light pulse, etc. This configuration may provide a modulation of the phase of the VCSEL pulse in each integration cycle. By performing this modulation in a circular manner with a number of phase delays the final histogram will present multiple peaks instead of a single peak, as discussed above.

As an exemplary implementation, the controller 444 may include a field-programmable gate array (FPGA) configured to receive the clock signal and to output a phase-delayed version of the clock signal according to the predefined pulse pattern. The FPGA may cycle through all the phase delays of each pulse in a sequence, e.g. in a pseudo-random sequence. As another exemplary implementation, the controller 444 may be or include an Application Specific Integrated Circuit (ASIC) configured to shift the clock signal.

The controller 444 may encode the emission signal 446 according to any desired pattern for the light emission (e.g., according to the pattern 300a, 300b). In a preferred configuration, the controller 444 may be configured to encode the emission signal according to a pseudo-random sequence for distributing the plurality of pulses in time. Considering the configuration with the clock signal generator, the controller 444 may be configured to impose the plurality of phase-delays onto the clock signal according to a pseudo-random sequence. According to various aspects, the controller 444 may be configured to generate a start signal 422 to initiate the light emission by the light source 442 (e.g., as part of the emission signal 446), and may be configured to deliver the start signal 422 to a light detection circuit of the time-of-flight sensor (e.g., to the light detection circuit 204, 420).

FIG. 5 shows a time-of-flight sensor 500 in a schematic view, according to various aspects. The time-of-flight sensor 500 may be an exemplary implementation of the time-of-flight sensor 200, and of the various components.

At the emitter side, the time-of-flight sensor 500 may include an emitter 502 (an example of light emission circuit 202, 440). The emitter 502 may include a laser emitter 504 and a multi-pulse clock generator 506 configured to provide clock signals to trigger laser emission by the laser emitter 504 based on a predefined pulse pattern 508. According to various aspects, the multi-pulse clock generator 506 may receive the reference signal 508 (e.g., from an external circuit, or from a memory of the time-of-flight sensor 500) and may encode the clock pulses to control the emission of light by the laser emitter 504 according to the reference signal 508. In other aspects, the multi-pulse clock generator 506 may deliver, as output, the reference signal 508 for use in the cross-correlation operation at the receiver side. Illustratively, the multi-pulse clock generator 506 may control the laser source 504 according to an encoded emission signal representing the reference signal 508, and may output the encoded emission signal as input to the cross-correlation at the receiver side.

At the receiver side, the time-of-flight sensor 500 may include a photon detector array 510 (e.g., an array of SPADs), a time-to-digital converter 512, and a histogram generator 514 configured to store photon counts as histogram in corresponding bins (e.g., time of arrival bins, e.g. distance bins). The photon detector array 510, time-to-digital converter 512, and histogram generator 514 may be part of a light detection circuit of the time-of-flight sensor 500. It is however understood that the TDC 512 and histogram generator 514 may also be part of the processing circuit of the time-of-flight sensor 500.

Further, at the receiver side, the time-of-flight sensor 500 may include, as processing circuit, a cross-correlation circuit 516, a stop signal generation circuit 518 configured to stop the measurement based on a SNR-threshold, and a distance calculation circuit 520 configured to calculate a time-of-flight of the emitted light and thus a distance at which an object is located.

At the receiver side, a process start signal 522 may define a start of the processing of the detected light (e.g., according to the emission), and the processing may be executed until the end of a detection period (e.g., the end of an integration time of the detector array) or until the processing circuit generates a stop signal, as discussed above. According to various aspects, the multi-pulse clock generator 506 at the emitter side may receive a request signal 524 prompting the emission of a new multi-pulse light signal. The request signal may illustratively be a new multi-pulse request, e.g. sent by the receiver side of the time-of-flight sensor 500, for example after a detection period, or after each detection interval. As an exemplary configuration, the TDC 512 may be configured to deliver the request signal 524 to the multi-pulse clock generator 506 to prompt the emission of a multi-pulse signal, e.g. after an integration period of the detector 510.

FIG. 6 shows a schematic flow diagram of a method 600 of carrying out a time-of-flight measurement, e.g. a method of carrying out a distance measurement based on time-of-flight.

The method 600 may include, in 610, carrying out a cross-correlation between a detection signal and a reference signal. The detection signal may be representative of light detected during a detection period, and the reference signal may be representative of a predefined pulse pattern.

The method 600 may further include, in 620, carrying out a measurement of a time-of-flight of an emitted light signal based on a result of the cross-correlation. The emitted light signal may include a plurality of light pulses distributed in time according to the predefined pulse pattern.

In the following, various aspects of this disclosure will be illustrated. The aspects may refer to the time-of-flight sensor 200, 500, processing circuit 206, 400, light emission circuit 202, 440, light detection circuit 204, 420, and method 600.

Example l is a time-of-flight sensor including: a processing circuit configured to: receive a detection signal representative of light detected at the time-of-flight sensor during a detection period; carry out a cross-correlation of the detection signal with a reference signal, wherein the reference signal is representative of a predefined pulse pattern; and carry out a measurement of a time-of-flight of an emitted light signal based on a result of the cross-correlation, wherein the emitted light signal includes a plurality of light pulses distributed in time according to the predefined pulse pattern.

In Example 2, the time-of-flight sensor of example 1 may optionally further include that, to carry out the cross-correlation of the detection signal with the reference signal, the processing circuit is configured to determine a match between the predefined pulse pattern represented by the reference signal and a waveform of the detection signal, that the result of the cross-correlation is representative of a time delay between the predefined pulse pattern and a matching pattern included in the detection signal, and that the time delay is representative of the time-of-flight of the emitted light signal. In Example 3, the time-of-flight sensor of example 1 or 2 may optionally further include that the result of the cross-correlation includes a cross-correlation signal; and that, to carry out the measurement of the time-of-flight, the processing circuit is configured to determine a time location within the cross-correlation signal of an absolute maximum of the cross-correlation signal.

In Example 4, the time-of-flight sensor of any one of examples 1 to 3 may optionally further include that the processing circuit is configured to generate a stop signal to terminate the detection period in the case that a signal level of at least a portion of the cross-correlation signal is in a predefined threshold range.

In Example 5, the time-of-flight sensor of example 4 may optionally further include that the threshold range is defined by a predefined signal-to-noise ratio for the detection signal.

In Example 6, the time-of-flight sensor of any one of examples 1 to 5 may optionally further include that the emitted light signal includes a plurality of partial light signals, and that each partial light signal includes a plurality of light pulses distributed in time according to the predefined pulse pattern.

In Example 7, the time-of-flight sensor of example 6 may optionally further include that the detection period includes a plurality of detection intervals, and that each detection interval is associated with an emission of a corresponding partial light signal of the plurality of partial light signals.

In Example 8, the time-of-flight sensor of example 7 may optionally further include that a duration interval has a duration in time equal to a period of an emission of the plurality of partial light signals.

In Example 9, the time-of-flight sensor of any one of examples 1 to 8 may optionally further include that the detection signal includes a plurality of photon counts accumulated during the detection period, and that each photon count is associated with a respective time of arrival of light at the time-of-flight sensor with respect to a starting time of an emission of the emitted light signal.

In Example 10, the time-of-flight sensor of example 9 may optionally further include that the starting time of the emission of the emitted light signal corresponds to a time point of an emission of an initial pulse of the plurality of pulses of the emitted light signal. In Example 11, the time-of-flight sensor of example 9 or 10 may optionally further include that, to carry out the cross-correlation of the detection signal with the reference signal, the processing circuit is configured to determine a match between the predefined pulse pattern represented by the reference signal and a pattern defined by the photon counts accumulated during the detection period.

In Example 12, the time-of-flight sensor of any one of examples 9 to 11 may optionally further include that the detection signal includes a plurality of photon counts accumulated during each detection interval of the plurality of detection intervals, and that, within each detection interval, each photon count is associated with a respective time of arrival of light at the time-of-flight sensor with respect to a starting time of an emission of the corresponding partial light signal.

In Example 13, the time-of-flight sensor of example 12 may optionally further include that the starting time of the emission of the partial light signal corresponds to a time point of an emission of an initial pulse of the plurality of pulses of the partial light signal.

In Example 14, the time-of-flight sensor of any one of examples 9 to 13 may optionally further include that the detection signal is or includes a histogram, the histogram including a plurality of histogram bins, that each histogram bins corresponds to a respective range for the time of arrival of light at the light detector with respect to the starting time of the emission of the emitted light signal; and that each photon count is assigned to a respective histogram bin.

In Example 15, the time-of-flight sensor of any one of examples 1 to 14 may optionally further include that the processing circuit is configured to continuously receive the detection signal during the detection period. In an exemplary configuration, processing circuit may be configured to carry out the cross-correlation at the end of each detection interval.

In Example 16, the time-of-flight sensor of any one of examples 1 to 15 may optionally further include that the light detected during the detection period includes a reflection of the emitted multi-pulse light signal from an object in the field of view of the time-of-flight sensor.

In Example 17, the time-of-flight sensor of example 16 may optionally include that the processing circuit is configured to calculate a distance between the time-of-flight sensor and the object based on a result of the time-of-flight measurement.

In Example 18, the time-of-flight sensor of any one of examples 1 to 17 may optionally further include a light detection circuit configured to: detect light; generate the detection signal representative of the light detected during the detection period, and deliver the detection signal to the processing circuit.

In Example 19, the time-of-flight sensor of example 18 may optionally further include that the light detection circuit is configured to: receive a start signal representative of a starting time of an emission of the emitted light signal; and generate the detection signal as a plurality of photon counts, and that the light detector is configured to assign each photon count to a respective time of arrival of light at the light detector during the detection period with respect to the starting time of the emission of the emitted light signal.

In Example 20, the time-of-flight sensor of example 18 or 19 may optionally further include that the emitted light signal includes a plurality of partial light signals, each partial light signal including a plurality of light pulses distributed in time according to the predefined pulse pattern; that the start signal includes a plurality of partial start signals, each partial start signal representative of a starting time of an emission of the corresponding partial light signal; and that, for each partial start signal, the light detector is configured to assign each photon count to a respective time of arrival of light at the light detector with respect to the starting time of the emission of the corresponding partial light signal.

In Example 21, the time-of-flight sensor of any one of examples 18 to 20 may optionally further include that the light detection circuit includes a one-dimensional array of photo diodes or a two- dimensional array of photo diodes.

In Example 22, the time-of-flight sensor of any one of examples 18 to 21 may optionally further include that the light detection circuit includes one or more avalanche photo diodes or one or more single-photon avalanche photo diodes, or a silicon photomultiplier.

In Example 23, the time-of-flight sensor of any one of examples 19 to 22 may optionally further include that the light detection circuit includes a time-to-digital converter, and that the time-to-digital converter is configured to: receive the start signal; receive a stop signal upon generation of a photon at the light detection circuit (e.g., at the one or more photo diodes); and generate a digital signal representative of the time difference between the start signal and the stop signal.

In Example 24, the time-of-flight sensor of example 23 may optionally further include that the light detection circuit includes a histogram generation circuit configured to receive an output of the time-to-digital converter and generate a corresponding histogram representing the photon counts over the detection period. In Example 25, the time-of-flight sensor of any one of examples 1 to 24 may optionally further include that the predefined pulse pattern includes a plurality of light pulses distributed in time according to a pseudo-random sequence.

In Example 26, the time-of-flight sensor of any one of examples 1 to 25 may optionally further include that the emitted light signal is or includes an emitted laser signal.

In Example 27, the time-of-flight sensor of any one of examples 1 to 26 may optionally further include a light emission circuit configured to emit the light signal.

In Example 28, the time-of-flight sensor of example 27 may optionally further include that the light emission circuit includes: a light source configured to emit light; and a controller configured to: encode an emission signal including a plurality of pulses according to the predefined pulse pattern; and control the light source to emit the light signal according to the encoded emission signal.

In Example 29, the time-of-flight sensor of example 28 may optionally further include that the controller is configured to encode the emission signal according to a pseudo-random sequence for distributing the plurality of pulses in time.

In Example 30, the time-of-flight sensor of example 28 or 29 may optionally further include that the light emission circuit further includes: a clock generator configured to generate a clock signal to trigger a light emission by the light source; and that the controller is configured to impose a plurality of phase-delays onto the clock signal to encode the emission signal.

In Example 31, the time-of-flight sensor of example 30 may optionally further include that the controller is configured to impose the plurality of phase-delays onto the clock signal according to a pseudo-random sequence.

In Example 32, the time-of-flight sensor of any one of examples 28 to 31 may optionally further include that the light source is or includes a laser source.

In Example 33, the time-of-flight sensor of example 32 may optionally further include that the laser source is or includes a vertical cavity surface emitting laser.

Example 34 is a LIDAR system including a time-of-flight sensor according to any one of examples 1 to 33. Example 35 is a method of carrying out a time of flight measurement, the method including: carrying out a cross correlation between a detection signal and a reference signal, wherein the detection signal is representative of light detected during a detection period, wherein the reference signal is representative of a predefined pulse pattern; and carrying out a measurement of a time-of-flight of an emitted light signal based on a result of the cross correlation, wherein the emitted light signal includes a plurality of light pulses distributed in time according to the predefined pulse pattern.

The method of example 36 may optionally further include one, or more than one, or each of the features recited in the examples 1 to 34, where appropriate.

The term “processor” or “processing circuit” as used herein may be understood as any kind of technological entity that allows handling of data. The data may be handled according to one or more specific functions executed by the processor or processing circuit. Further, a processor or processing circuit as used herein may be understood as any kind of circuit, e.g., any kind of analog or digital circuit. A processor or processing circuit may thus be or include an analog circuit, digital circuit, mixed-signal circuit, logic circuit, processor, microprocessor, Central Processing Unit (CPU), Graphics Processing Unit (GPU), Digital Signal Processor (DSP), Field Programmable Gate Array (FPGA), integrated circuit, Application Specific Integrated Circuit (ASIC), etc., or any combination thereof. Any other kind of implementation of the respective functions, which will be described below in further detail, may also be understood as a processor or logic circuit. It is understood that any two (or more) of the processors or processing circuits detailed herein may be realized as a single entity with equivalent functionality or the like, and conversely that any single processor or processing circuit detailed herein may be realized as two (or more) separate entities with equivalent functionality or the like.

While various implementations have been particularly shown and described with reference to specific aspects, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope as defined by the appended claims. The scope is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced. LIST OF REFERENCE SIGNS

100 Time-of-flight sensor

102 Light emission circuit

104 Light detection circuit

106 Processing circuit

108 Emitted light

110 Object

112 Reflected light

120 Graph

122 Peak

200 Time-of-flight sensor

202 Light emission circuit

204 Light detection circuit

206 Processing circuit

208 Emitted light signal

208d Emitted light signal

210 Object

212 Reflected light

214 Detection signal

214c Detection signal

214e Detection signal

214f Detection signal

216 Cross-correlation operation

216c Cross-correlation operation

216e Cross-correlation operation

216f Cross-correlation operation

218 Reference signal

218c Reference signal

218e Reference signal

218f Reference signal

220 Time-of-flight measurement

222 Graph

222c Graph

222e Graph

222f Graph

224 Cross-correlation signal

226 Time-delay

226e Time-delay

226f Time-delay

232 Partial light signals

234 Detection period

236 Detection interval

300a Pulse pattern

300b Pulse pattern

302a Pulses

302b Pulses

304a Pulse interval

304b Pulse interval 400 Processing circuit

402 Cross-correlator

404 Detection signal

406 Reference signal

408 Cross-correlation signal

410 Stop signal generation circuit

412 Stop signal

414 Time-of-flight measurement circuit

416 Output signal

420 Light detection circuit

422 Start signal

424 Light receiver

426 Time-to-digital converter

428 Histogram generation circuit

430 Detection signal

432 Digital signal

440 Light emission circuit

442 Light source

444 Controller

446 Emission signal

448 Light signal

450 Pulse pattern

452 Clock pulse

454 Clock pulse

456 Clock pulse

500 Time-of-flight sensor

502 Emitter

504 Laser emitter

506 Multi-pulse clock generator

508 Reference signal / predefined pulse pattern

510 Photon detector array

512 Time-to-digital converter

514 Hi stogram generator

516 Cross-correlation circuit

518 Stop signal generation circuit

520 Distance calculation circuit

522 Process start signal

524 Request signal

600 Method

610 Method step

620 Method step