Abstract:
A system for estimating range to an object comprising a system for estimating range to an object comprising a transmitter to transmit at least one UWB signal, a receiver to receive at least one UWB signal, a sampler to sample the received UWB signal depending on a plurality of clock and/or sample pulses having a first frequency, and a circuit and/or processor configured to generate a fractional signal having a second frequency that is lower that the first frequency and a phase that is dependant on the delay between when the UWB signal is actually received and when the received UWB signal is first sampled, and determine the range based on at least a first number of clock or sample pulses between transmitting and receiving the UWB signal and the phase of the fractional signal. Also a method of estimating range to an object.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
       [0001]    The present invention contains subject matter related to Singapore Patent Application No. 200703957-1, filed in the Singapore Patent Office on Jun. 4, 2007, the entire contents of which being incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to estimation of time of arrival and/or range, particularly though not solely to a method and system for estimating range based on the time of arrival of an Ultra Wide Band (UWB) signal. 
         [0004]    2. Description of Related Art 
         [0005]    Methods for estimating position or range are widely used in many wireless devices. For example it is known to use a pulse-based UWB transceiver system to estimate the range to an object or between devices by determining the delay between transmission and receipt of each UWB pulse, which is also known as the Time of Arrival (TOA). Such systems may be used in applications such as radar, object location or tracking, and military applications. 
         [0006]    In  FIG. 1  shows prior art system for range estimation using TOA, as disclosed by Sinan Gezici, Zhi Tian, Georgios B. Giannakis, Hisashi Kobayashi, Andreas F. Molisch, H. Vincent Poor, and Zafer Sahinoglu, in “Localization via ultra-wideband radios.”  IEEE Signal Processing Magazine,  vol. 22, 2005. While a number of methods of range estimation exist, the TOA method for range estimation has particular advantages when used with UWB signals. The TOA method includes sending a coded UWB pulse  100  from the transmitter. The receiver receives the reflected pulse  101 , as shown in  FIG. 1(   a ), which is smoothed using an envelope detector or a low pass filter. Then the envelope is sampled and becomes time discrete samples  102 , as shown in  FIG. 1(   b ). The TOA of each pulse is estimated based on these discrete samples, for example the number of clock pulses between transmission and receipt. 
         [0007]    A sampling error may occur as the estimated TOA  104  may be different from the actual TOA  106 , as shown in  FIG. 1(   c ). The discrete time sampling step and low time resolution of the sampler, means a TOA between two sample points will not be identified. That is to say, that the accuracy is limited by the sampling rate. Therefore the lower the sampling rate, the bigger the possible sampling error. 
         [0008]    Various systems have been proposed in the prior art. For example a positioning system is disclosed in U.S. Pat. No. 7,171,303 that uses a common clock unit to form the sampling basis in both the local receiver and the relative sensor. Both the local receiver and the relative sensor have the same reference during the ranging processing. In U.S. Pat. No. 7,057,553 a method for processing the positioning signals in the global positioning system (GPS) is disclosed. The method collects pseudo-range samples from positioning signals received at the ranging receiver from several satellites. U.S. Pat. No. 7,042,868 discloses a ranging method between two packet signal transceivers. The time delay or distance between two transceivers can be determined through sending and received packet signals to each other. U.S. Pat. No. 6,239,741 describes a tunnel diode detector for UWB object detection. 
         [0009]    The present inventors have also filed Singapore Patent Application No. 200604221-2 on Jun. 23, 2006, relating to a method using a time to voltage converter to estimate the TOA. 
       SUMMARY OF THE INVENTION 
       [0010]    In general terms the invention relates to determining the delay between when a UWB signal is actually received and when the received UWB signal is first sampled. This may have the advantage that a timing difference smaller than the sampling period can be detected without undue hardware complexity and/or that a high sampling rate is not necessary. 
         [0011]    In one specific expression of the invention there is provided a method of estimating range to an object comprising: 
         [0012]    transmitting at least one UWB signal, 
         [0013]    receiving at least one UWB signal, 
         [0014]    sampling the received UWB signal depending on a plurality of clock and/or sample pulses having a first frequency, 
         [0015]    providing a fractional signal having a second frequency that is lower that the first frequency and a phase that is dependent on the delay between when the UWB signal is actually received and when the received UWB signal is first sampled, and 
         [0016]    determining the range based on at least a first number of clock or sample pulses between transmitting and receiving the UWB signal and the phase of the fractional signal. 
         [0017]    In a second independent specific expression of the invention there is provided a system for estimating range to an object comprising: 
         [0018]    a transmitter to transmit at least one UWB signal, 
         [0019]    a receiver to receive at least one UWB signal, 
         [0020]    a sampler to sample the received UWB signal depending on a plurality of clock and/or sample pulses having a first frequency, and 
         [0021]    a circuit and/or processor configured to generate a fractional signal having a second frequency that is lower that the first frequency and a phase that is dependant on the delay between when the UWB signal is actually received and when the received UWB signal is first sampled, and determine the range based on at least a first number of clock or sample pulses between transmitting and receiving the UWB signal and the phase of the fractional signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    In order that one or more aspects of invention may be more readily understood and put into practical effect, there shall now be described, by way of non-limitative example only, one or more embodiments of the inventions, with reference to and as illustrated by the accompanying drawings; in which: 
           [0023]      FIG. 1  is a graph of a prior art method of TOA estimation 
           [0024]    (a) The UWB pulse sequence 
           [0025]    (b) The envelope detection and sampling 
           [0026]    (c) The sampling output and TOA estimation error 
           [0027]      FIG. 2  is a block diagram of the ranging system according to an exemplary embodiment. 
           [0028]      FIG. 3  is a graph showing the relationship between the signals in the system of  FIG. 2 . 
           [0029]      FIG. 4  is a graph of signal A and B triggered by the arrival pulse edge and the sampler clock in the system of  FIG. 2 . 
           [0030]      FIGS. 5(   a )-( c ) are graphs of the phase change of the signal F along with the pulse arrival time change. 
           [0031]      FIGS. 6(   a )-( c ) are circuit diagrams of the hardware for generating signals A, B and F(ω 1 −ω 0 ). 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0032]    An exemplary embodiment is shown in  FIG. 2  for estimating the TOA of a UWB signal. The system  200  includes a transmitter  202  for generating the UWB signal and a receiver  204  for receiving the reflected UWB signal. Typically the transmitter  202  sends a reference signal out and the receiver  204  receives the echo reference signal from the target object. By detecting the delay between the transmitting the signal and receiving the signal the distance to the object can be found. 
         [0033]    A clock  206  provides timing pulses to the transmitter  202 , which generates the coded pulses according to a preset pattern. Where repeated pulse coding is used, the maximum delay should not be longer in the time domain than the UWB pulse code length. The receiver  204  is connected to an integer delay detection  208  and a fractional delay detection  210 . The integer delay detection  208  determines the number of integer clock pulses between transmitting the UWB pulse, and receiving the reflected UWB pulse. The fractional delay detection  210  determines the delay between the first clock pulse after the reflected UWB signal is received and when the reflected UWB signal is actually received. 
         [0034]    The relationship between integer delay detection  208  and fractional delay detection is  210  shown in  FIG. 3 . The integer delay  306  is the time (measured in the number of clock pulses or samples) from when the UWB pulse  302  is transmitted  300  to the first sample  310  after the UWB pulse  304  is received. As explained in the background section the UWB pulse  304  may be received in between samples. Consequently the fractional delay  308  is the time from when the UWB pulse  304  is actually received  312  to the first sample  310  after the UWB pulse  304  is received. The actual time when the UWB pulse is received  312  may be determined as when the envelope of the signal from the receiver  204  is above a threshold. 
         [0035]    Therefore the total delay or actual TOA may be calculated by subtracting the fractional delay  308  from the integer delay  306 . A ranging calculation ( 212   FIG. 2 ) is based on the total delay. 
         [0036]    In order to determine the fractional delay  308 , a fractional signal F is generated. The fractional signal F has a lower frequency than the clock frequency and a phase  314  which is dependant on the delay between the first clock or sample pulse after the UWB signal is received and actually receiving the UWB signal. Because F has a large period relative to the sampling period, a small relative change in timing between the when the reflected UWB pulse is actually received and the first sample after it is received will result in a bigger change in the phase  314  of F. The phase of signal F represents a temporal expansion of the fractional delay  308 , so that the fractional delay  308  can be measured using the standard sampling frequency. 
         [0037]    An example method of fractional delay detection  210  is shown in  FIG. 4 . Two signals A and B are generated. Signal A is in phase with the received UWB pulse  400  with frequency ω 0 , which is slightly slower than the sampling frequency ω 1 . Signal B is in phase with the first clock or sample pulse  402  after the UWB signal is received and has a frequency the same as the sampling frequency ω 1 . 
         [0038]    A further signal C is generated by the multiplication of signals A and B as shown in Equation (1): 
         [0000]        C=A (ω 0 )· B (ω 1 )   (1) 
         [0039]    Signal C can be separated in the frequency domain into two parts. A first part signal D is the high frequency part centered at a frequency of ω 0 +ω 1  and a second part signal F is the low frequency part centered at a frequency of ω 0 −ω 0 . Thus signal C can also be represented as shown in Equation (2): 
         [0000]        C=D (ω 0 +ω 1 )+ F (ω 1 −ω 0 )   (2) 
         [0040]    Because signals D and F have significantly different frequencies, a low pass filter may be used to extract the low frequency part F(ω 1 −ω 0 ). The high frequency part D(ω 1 −ω 0 ) may be discarded. 
         [0041]    The signal F(ω 1 −ω 0 ) has a long period relative to the sampling period, because ω 1  is close to ω 0 . The phase of signal F will vary in accordance with the delay between the first clock or sample pulse after the UWB signal is received and when the UWB signal is actually received. Therefore the fractional delay  308  can be determined by sampling the phase  314  of F. 
         [0042]    A factor or multiplier K by which the fractional delay  308  is expanded in the time domain into the phase  314  of F(ω 1 −ω 0 ) can be calculated as shown in Equation (3): 
         [0000]    
       
         
           
             
               
                 
                   K 
                   = 
                   
                     
                       ω 
                       1 
                     
                     
                       
                         ω 
                         1 
                       
                       - 
                       
                         ω 
                         0 
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0043]    For example, if the sampling frequency ω 1  is 100 MHz, and ω 0  is 99 MHz then F(ω 1 −ω 0 ) has a frequency of 1% of ω 1  or 1 MHz. This gives a resolution of 100 samples over the possible range of fractional delay of 10 ns (one sample period). This makes it easy to use a relatively low speed sampling frequency to obtain higher time resolution by sampling F(ω 1 −ω 0 ) instead of only sampling the received signal. In the above example, the time resolution of the pulse arrival time detection is improved by two orders of magnitude from 10 ns to 100 ps (10 ns/100). 
         [0044]    The period of F(ω 1 −ω 0 ), namely 2π/(ω 1 −ω 0 ), must be smaller than the period of coded pulse sequence. Otherwise, F(ω 1 −ω 0 ) can&#39;t be processed in time because it will overlay with the next coming pulse signal. 
         [0045]    For example as shown in  FIGS. 5(   a )-( c ) the phase of F varies depending on the fractional delay. In  FIG. 5(   a ) the signal is received at the same time as a sampling pulse (factional delay is zero), and according the phase of F is shown as 0°. In  FIG. 5(   b ) a small factional delay exists and accordingly the phase of F is 90°. In  FIG. 5(   c ) a large fractional delay exists and accordingly the phase of F is 180°. 
         [0046]    Integer delay detection  208  may be implemented by starting a Counter 1  when the UWB signal is transmitted and stopping the Counter 1  when the sampled received signal is higher than a threshold, (i.e. the first clock pulse after the UWB pulse is received). The integer delay τ 1  is calculated according to Equation (4): 
         [0000]      τ 1   =N   1   ·T    (4) 
         [0000]    where N 1  is value of Counter 1 , T is the sample period. 
         [0047]    Fractional delay detection  210  may be implemented by starting a Counter 2  when the envelope of the signal from the receiver is above a threshold, and stopping the Counter 2  when F(ω 1 −ω 0 ) changes state, (i.e. the number of clock pulses represent the phase of F). The fractional delay τ f  is calculated according to Equation (5): 
         [0000]      τ f   =N   f   ·T/K    (5) 
         [0000]    where N f  is the value of Counter 2 . Therefore total delay or actual TOA Δ can be calculated according to Equation (6): 
         [0000]      Δ=τ 1 −τ f    (6) 
         [0048]    The total delay or actual TOA Δ may be used to calculate range or position. For example in an UWB ranging system, the distance d between the transceiver and the object is proportional to the delay or actual TOA of the received signal as compared with the emitted signal, as shown in Equation (7): 
         [0000]        d =(Δ− X )·ζ/2   (7) 
         [0000]    where X is a known constant circuit delay and ζ is the speed of propagation for example the speed of light. 
         [0049]      FIG. 6  shows a hardware implementation for fractional delay detection  210 . Signals A(ω 0 ) and B(ω 1 ) are generated by two feedback oscillation circuits  600 ,  608 . 
         [0050]    In  FIG. 6(   a ), the first oscillator circuit  600  receives output of the envelope detector ( 214   FIG. 2)  which is supplied to a clock input of a latch register A. When the envelope detector  214  output rises above a threshold (detects a UWB pulse arrival), the latch register A output  606  will be high, which causes a feed back oscillator  602  to oscillate. The frequency of the oscillation is determined by the feedback line  604  length, which is set at D 0  which results in a frequency of ω 0 . The latch register A will be cleared to zero when F(ω 1 −ω 0 ) changes state stopping the oscillation or the next pulse arrives. 
         [0051]    In  FIG. 6(   b ), the second oscillator circuit  608  is used to generate signal B(ω 1 ). The latch register A&#39;s output  606  is combined at an AND gate  610  with the clock pulses. The output of the AND gate  610  is supplied to a clock input of latch register B. The latch register B output  612  will be high after the first clock pulse after a UWB pulse arrival. This causes a feed back oscillator  614  to oscillate. The frequency of the oscillation is determined by the feedback line  616  length, which is set at D 1  which results in a frequency of ω 1 , which is also stopped once F(ω 1 −ω 0 ) changes state or the next pulse arrives. 
         [0052]    In  FIG. 6(   c ), a further circuit  616  is shown. The signals A(ω 0 ) and B(ω 1 ) from circuits  600   608  are sent to an XOR gate  618  which is equivalent to a mixer or multiplication which results in signal C. Signal C from the XOR gate  618  is provided to the clock input of a latch register D to sample and hold the XOR results. A(ω 0 ) is supplied to the clock input of a latch register D so that signal C is sampled and registered at ω 0 . This is equivalent to a low-pass filter which results in signal F(ω 1 −ω 0 ) at the output. 
         [0053]    Whilst the preferred embodiments of the invention have been described in detail, many variations are possible within the scope of the invention as will be clear to a skilled reader.