Method to resolve interfering targets jointly at multiple dimensions

A system and method for resolving a first target from a second target by radar is disclosed. The system includes a transmitter for transmitting a source signal, a receiver for receiving first and second echo signals from reflection of the source signal from at least a first target and a second target, respectively. A processor is used to subtract the first echo signal from the composite signal to obtain a second generation of the second echo signal, subtract the second generation of the second echo signal from the composite signal to obtain a second generation of the first echo signal, and estimate a parameter value for the first target from the second generation of the first echo signal and a parameter value for the second target from the second generation of the second echo signal.

FIELD OF THE INVENTION

The subject invention relates to a system and method for distinguishing target signals from one another using radar and, in particular, to a method and system for jointly improving the resolution of multiple targets in a composite radar signal.

BACKGROUND

Automobiles and other vehicles have come to employ safety systems which include radar technologies for detecting a location of an object or target with respect to the vehicle so that a driver or collision-avoidance device can react accordingly. A radar system includes a transmitter for sending out a source signal and a receiver for receiving an echo or reflection of the source signal from the target. The received signal (the echo signal) is sampled at a selected sampling frequency and the sampled data points of the received signal are entered into a Fast Fourier Transform (FFT) in order to determine a dominant frequency of the reflected signal. Various parameters and dimensions of the target, which are determined from the dominant frequency, are then used to provide an echo signal representative of the target in a data cube.

Due to the time-limited nature of digital sampling techniques, the echo signal in the data cube is not a centralized point but instead displays a central peak with multiple side lobes. The presence of side lobes produces complications when attempting to distinguish multiple echo signals from one another. For example, when a first target and a second target are in close proximity of each other, a side lobe of a first echo signal (representative of the first target) can overlap a peak of a second echo signal (representative of the second target). When the first echo signal is much stronger or more intense than the second echo signal, the side lobe of the first echo signal can mask the presence of the second echo signal, or alter the appearance of the second echo signal, thereby making accurate determination of parameters for the second echo signal difficult. Similarly, the presence of the second echo signal alters the appearance of the first echo signal and makes it difficult to accurately determinate parameters for the first echo signal. Accordingly, it is desirable to correct for the effect of echo signals on one another in order to distinguish multiple target signals from each other.

SUMMARY OF THE INVENTION

In one exemplary embodiment of the invention, a method of resolving a first target from a second target in a radar system is disclosed. A composite signal is obtained, the composite signal including a first echo signal obtained by reflection of a source signal of the radar system from the first target and a second echo signal obtained by reflection of the source signal from the second target. The first echo signal is subtracted from the composite signal to obtain a second generation of the second echo signal. The second generation of the second echo signal is subtracted from the composite signal to obtain a second generation of the first echo signal. A parameter value is estimated for the first target from the second generation of the first echo signal and a parameter value is estimated for the second target from the second generation of the second echo signal.

In another exemplary embodiment of the invention, a radar system is disclosed. The radar system includes a transmitter for transmitting a source signal, a receiver for receiving echo signals from reflection of the source signal from at least a first target and a second target, and a processor. The processor is configured to obtain a composite signal including a first echo signal representative of the first target and a second echo signal representative of the second target, subtract a signal representative of the first echo signal from the composite signal to generate of a second generation of the second echo signal, subtract a signal representative of the second generation of the second echo signal from the composite signal to obtain a second generation of the first echo signal, and estimate a parameter value for the first target from the second generation of the first echo signal and a parameter value for the second target from the second generation of the second echo signal.

DESCRIPTION OF THE EMBODIMENTS

In accordance with an exemplary embodiment of the invention,FIG. 1shows a vehicle100, such as an automobile, that includes a radar system102suitable for determining a location and/or a relative velocity of various objects or targets with respect to the vehicle100. In the embodiment shown inFIG. 1, the radar system102includes a transmitter106and a receiver108. In alternate embodiments, the radar system102may be a MIMO (multi-input, multi-output) radar system that includes an array of transmitters and an array of receivers. A control unit110on-board the vehicle100includes a processor which controls and operates the transmitter106to generate a radio frequency wave (a “source signal”120). In one embodiment, the source signal120includes a linear frequency-modulated continuous wave (LFM-CW), often referred to as a chirp signal. Alternately, the source signal120can be a pulsed signal or a combination of pulsed and chirp signals. A first target104and a second target114are shown in a possible pathway of the vehicle100. For illustrative purposes, the first target104is another vehicle that is at a first location with respect to the vehicle100and has a first relative velocity v1with respect to vehicle100. The second target114is a person who is at a second location (i.e., on a walk path) with respect to the vehicle100and has a second relative velocity v2with respect to vehicle100. A reflection of the source signal120from first target104provides a first echo signal122, and a reflection of the source signal120from the second target114provides a second echo signal124. The first echo signal122and second echo signal124are received at the receiver108, which generally includes circuitry for sampling the first echo signal122and the second echo signal124. The control unit110performs calculations on the first echo signal122and second echo signal124in order to determine locations and/or relative velocities of the first and second targets104,114with respect to the vehicle100. Knowledge of the location and/or the relative velocity of the first and second targets104,114with respect to the vehicle100can then be used to maneuver the vehicle100by, for example, accelerating or decelerating the vehicle100or steering the vehicle100in order to avoid the first and second targets104,114. In one embodiment, the control unit110determines the distance and/or velocity of the first and second targets104,114with respect to the vehicle100and may cooperate with a collision-avoidance system112to control steering and acceleration/deceleration components to perform necessary maneuvers at the vehicle100to avoid the first and second targets104,114. In another embodiment, the control unit110provides a signal to alert a driver of the vehicle100so that the driver can take a necessary action to avoid the first and second targets104,114.

While the radar system102is discussed herein as being on-board vehicle100, the radar system102may also be part of an immobile or stationary object in alternate embodiments. Similarly, each of the first target104and second target114can be a vehicle or any type of moving object or can be an immobile or stationary object.

FIG. 2shows an exemplary data space200for a single echo signal obtained via a radar system for a single target. The echo signal is shown for illustrative purposes as a signal in one dimension. This dimension can be one of range (distance to target), azimuth, and elevation of velocity (i.e., relative velocity) of the target. The radar system however is capable of obtaining values of parameters in multiple dimensions, including some or all of range, azimuth, elevation and velocity. When parameter values are obtained in all four dimensions, the radar system generates a four-dimensional data cube in which to represent the echo signal. In the four-dimensional data cube, the echo signal is represented as a four-dimensional point. For illustrative purposes,FIG. 2shows only a single dimension, that of range.

Data space200includes a time-delimited echo signal202, i.e., an echo signal that is obtained over an infinite sampling time. The time-delimited echo signal202is characterized by a single peak centered at a selected parameter value in parameter space. The illustrative echo signal202is located at a range of 12.98 m. The height of the peak is indicative of the strength or intensity of the signal from the target.FIG. 2also shows a time-limited echo signal204. The time-limited echo signal204is characterized by a peak204acentered at the location of the target (i.e., at 12.98 m) as well as various side lobes204b. The intensity of the side lobes204bis generally about 10 decibels less than the intensity of the peak204a. The shape of the time-limited target signal204is due to there being a finite time period for sampling.

FIG. 3shows an exemplary data space300including two time-limited echo signals obtained from multiple targets.FIG. 3shows a one-dimensional data space. However, as withFIG. 2, the radar system may present the echo signals ofFIG. 3as four-dimensional signals in a four-dimensional data cube when four parameters of the target have been measured. A first echo signal302is shown having a central peak302aand side lobe302b. A second echo signal304is shown having a central peak304aand side lobe304b. For illustrative purposes, the first echo signal302is caused by the first target104ofFIG. 1and the second echo signal304is caused by the second target114ofFIG. 1. The first echo signal302is stronger or more intense than the second echo signal304.

FIG. 3also shows a composite signal310that represents the sum of the first echo signal302and the second echo signal304. The composite signal310is the signal that is initially obtained at the radar system. The composite signal310exhibits two major composite peaks,310aand310b. First peak310ain the composite signal310is a summation of peak302aof the first echo signal302and side lobe304bof the second echo signal304. The value of the parameter (e.g. range) of peak310ais different from the value of the parameter for peak302a, generally due to the effects of the side lobe304b. Second peak310bin the composite signal310is a summation of peak304aof the second target signal304and side lobe302bof the first echo signal302. Peak310bis offset from peak304adue to the effects of the side lobe302b. For an example, peak304ais located at 25.1 sm and peak310bis located at 26.89 sm, for a difference of 1.79 sm. The method disclosed herein provides a method of determining the parameter values for peak302aof the first signal302and peak304aof the second signal304from the composite signal310. This method can be extended to four parameter dimensions as well as to the presence of multiple target signals within a four-dimensional data cube.

FIG. 4shows a data space400illustrating the results of applying the method disclosed herein in obtaining a parameter value representative of an original echo signal from the composite signal. The data space400shows only a single dimension, which is selected as range for illustrative purposes only. The composite signal408includes first echo signal402and a second echo signal404. The data space400shows a close-up in a region of a peak404aof the second echo signal404. The data space400shows a portion of a central lobe of the first echo signal402and a first side lobe402sof the first echo signal402. The data space400also shows peak408aof the component signal408. Due to the strength of the side lobe402s, an estimated value of range for the peak408ais significantly different that the estimated value of range for peak404a. In particular, peak408ais located at 26.76 sm, while peak404ais located at 25.13 sm for an error of about 6.4%

Curve410represents a remaining or residual signal that results from subtracting the first signal402from the composite signal408. It is clear that the peak410aof residual signal410has moved along the x-axis and is more closely aligned (along the x-axis) with peak404athan is the peak408a. In particular, peak410ais located at 25.39 sm while peak504ais located at 25.13 sm, for an error of about 0.95%. Therefore, the accuracy with which one is able to determine the peak404aof the second echo signal404, as well as its parameter value(s), is increased by subtracting out the first echo signal402and its side lobes from the composite signal408. This method similarly can be applied to subtract out the second echo signal404from the composite signal408in order to obtain an improved representation of the first echo signal402.

FIG. 5shows a flowchart500illustrating the method for peak resolution disclosed herein. The method is an iterative method for determining parameter values for the first echo signal and the second echo signal in a composite signal composed of the first and second echo signals to within a selected criterion. The method alternately subtracts signals representative of the first and second echo signals from the original composite signal in order to bring parameter values of the echo signals in line with the actual parameter values of their corresponding targets. Since the process is an iterative process, we used the terms “first generation,” “second generation,” etc. to described the results of each iteration, whereas the first generation of the first echo signal is the original first echo signal and the first generation of the second echo signal is the original second echo signal. However, it is to be understood that multiple echo signals can occur in a composite signal and that the methods disclosed herein can be extended to include these multiple echo signals. The method ofFIG. 5can be performed on a processor, such as the processor of control unit110.

In box502, the method begins with a composite signal that includes both the first generation of the first echo signal and the first generation of the second echo signal, the peaks (i.e., local maxima) are located for the composite signal.

In box504a peak for the first generation of the first echo signal is identified in the composite signal and in box506a parameter value (e.g., a value along the x-axis) is determined for the peak of the first generation of the first echo signal. In box508, a representative signal is constructed for the first generation of the first echo signal using the parameter value from box506. In one embodiment, constructing the representative signal includes constructing a sinc function having a central peak located at the parameter value. The sinc function can be constructed from a convolution of a delta function having the parameter value for the peak of the signal to be represented.

In box510, the representative signal from box508is subtracted from the composite signal to obtain a first residual signal. The first residual signal includes a peak for a second generation of the second echo signal.

In box512, the peak for the second generation of the second echo signal is identified and in box514a parameter value is determined for the peak from box512. Once the parameter value is determined at box514, the flowchart can perform a decision step at box516in which the parameter value for a current generation (e.g., the second generation) of the second echo signal is compared to the parameter value for the previous generation (e.g., the first generation) of the second echo signal. When the difference between the current and previous parameter values is less than a selected threshold value, the method comes to a stop (box526). Otherwise, the method continues to box518.

In box518, a representative signal is constructed for the second generation of the second echo signal using the determined parameter value from box514. Constructing the representative signal includes providing a sinc function having a central peak located at the parameter value. In box520, the representative signal from box518is subtracted from the composite signal (i.e., the original composite signal from box502) to obtain a second residual signal. The second residual signal includes a peak for a second generation of the first echo signal. In box522, a parameter for the second generation of the first echo signal is determined.

Once the parameter value is determined at box522, the flowchart performs a decision step at box524in which the parameter value for the current generation of the first echo signal is compared to the parameter value for the previous generation of the first echo signal. When the difference between the current and previous parameter values is less than a selected threshold, the method may come to a stop (box528). Otherwise, the method continues to determine a next generation of echo signal by returning to box504.

The second generations of the first echo signal and the second echo signals are considered to be more realistic representatives of their respective targets than the first generation of the respective signals. When the difference between the parameter values of successive generations is less than a selected criterion, the parameter value is considered to have converged and the method can stop. Otherwise, the method continues onto a next iteration. While the decision to stop or continue has been described either by comparing parameter values for consecutive generations of the first echo signal (box524) or by comparing parameter values for consecutive generations of the second echo signal (box516), the method may stop when the differences in parameter values for consecutive generations of both the first echo signal and the second echo signal are less than their respective threshold values.

FIGS. 6-9illustrate signals obtained using the method disclosed in the flowchart500.FIG. 6shows a first generation of a first echo signal601, a first generation of a second echo signal602and a composite signal605.FIG. 7shows the signals ofFIG. 6and a second generation of the second echo signal702obtained using the method disclosed herein, such as at box510ofFIG. 5.FIG. 8shows the signals ofFIG. 6and a second generation of a first echo signal801obtained using the method disclosed herein, such as at box520ofFIG. 5.

FIG. 9shows multiple generations of the first and second echo signals obtained via multiple iterations of the method disclosed herein. First generation of the first echo signal601, first generation of the second echo signal602and composite signal605are shown, as well as second (902), third (904) and fourth (906) generations of the first echo signal and second (912), third (914) and fourth (916) generations of the second echo signal.

The methods disclosed herein improve the ability of a radar system to distinguish multiple target signals from each other and to more accurately determine the values of parameters associated with the multiple target signals, such as their range, elevation, azimuth, relative velocity. These improved parameter values can be provided to the driver or the collision avoidance system (112,FIG. 1) in order for the driver or the collision avoidance system112to have improved reaction in avoiding targets, thus increasing a safety of the driver and vehicle.