Abstract:
Ultrasonic locationing of a tag with an emitter transducer operable to emit a single frequency ultrasonic burst. A receiver with at least two microphones is operable to receive the ultrasonic burst. Each microphone receiver contains two narrowband filters to extract frequencies above and below the ultrasonic burst frequency. A processor derives a low frequency waveform indicative of the phase difference between the dual frequency pulses coming from each microphone receiver. A correlator then determines the time difference of arrival (TDOA) between each microphone receiver by correlating the phase difference waveforms. This invention enables the creation of an ultrasonic locationing system requiring microsecond accuracy on TDOA data, as is necessary with microphone spacing of less than one foot, while using a simple single high frequency emitter source.

Description:
FIELD OF THE DISCLOSURE 
       [0001]    The present disclosure relates generally to an ultrasonic locationing system, and more particularly to locating ultrasonic tags having a single frequency emitter. 
       BACKGROUND 
       [0002]    An ultrasonic receiver can be used to determine the location of items that contain ultrasonic emitters, such as a mobile device present within a retail, factory, or warehouse environment, for example. The ultrasonic emitter can transmit ultrasonic energy in a short burst which can be received by an ultrasonic transducer (microphone) in the ultrasonic receiver, thereby establishing the presence of the device within the environment. 
         [0003]    Further, the use of several ultrasonic microphones distributed within the environment can also be used to provide a specific location of a particular device using techniques known in the art such as triangulation, trilateration, and the like. However, unlike radio frequency locationing systems, ultrasonic locationing systems suffer from particular problems related to the characteristics of ultrasonic sound waves and their environment of use. Firstly, ultrasonic waves are typically subject to acoustic reverberation due to multipath reflections. Therefore, a signal sent by an emitter can be interfered with by its own reverberations. Secondly, ultrasonic signals are easily subject to noise. 
         [0004]    A solution to this problem is to provide a very short burst width of an ultrasonic signal from the emitter such that a signal sent by any emitter in the system will be properly received by the receiver before any multipath energy is included in the signal. In addition, a narrow bandwidth (high Q) receiver can be used to reject nearby environmental acoustic noise while being able to receive the very short burst. However, a high Q receiver implies a slow amplifier response time resulting in poor correlation margin between the peak output (indicating the correct TDOA) and adjacent cycles. This poor correlation margin results in a high likelihood of incorrect TDOA information which would make a receiver design with microphone spacing of less than one foot infeasible. 
         [0005]    Accordingly, there is a need for an improved technique to resolve the above issues with ultrasonic locationing. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0006]    The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views, together with the detailed description below, are incorporated in and form part of the specification, and serve to further illustrate embodiments of concepts that include the claimed invention, and explain various principles and advantages of those embodiments. 
           [0007]      FIG. 1  is a simplified block diagram of an ultrasonic locationing system, in accordance with some embodiments of the present invention. 
           [0008]      FIG. 2  is a simplified block diagram of a two microphone receiver, in accordance with some embodiments of the present invention. 
           [0009]      FIG. 3  is a simplified graphical representation of the operation of the correlator of  FIG. 2 . 
           [0010]      FIG. 4  is a graphical representation of an ideal single frequency ultrasonic burst received by two different microphones with high Q amplifier chains. 
           [0011]      FIG. 5  is a graphical representation of an ideal correlation result from the two pulses of  FIG. 4 . 
           [0012]      FIG. 6  is a graphical representation of an ultrasonic burst in the frequency domain. 
           [0013]      FIG. 7  is a graphical representation of two band pass filters in the receiver overlaid on the spectrum of the burst in  FIG. 6 . 
           [0014]      FIG. 8  is a graphical representation of two different frequency outputs of a single microphone receiver resulting from the filtering of  FIG. 7 . 
           [0015]      FIG. 9  is a simplified block diagram of a receiver, in accordance with some embodiments of the present invention. 
           [0016]      FIG. 10  is a graphical representation of separated different frequency components received in each microphone path of the receiver of  FIG. 9 . 
           [0017]      FIG. 11  is a graphical representation of multiplication products of the different frequency components of  FIG. 10 . 
           [0018]      FIG. 12  is a graphical representation of the multiplication products of  FIG. 11  after low pass filtering. This represents the difference in phase between the dual frequency outputs. 
           [0019]      FIG. 13  is a graphical representation of a correlation result from the two low pass multiplication products of  FIG. 12 . 
           [0020]      FIG. 14  is a flow diagram illustrating a method for ultrasonic locationing, in accordance with some embodiments of the present invention. 
       
    
    
       [0021]    Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention. 
         [0022]    The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. 
       DETAILED DESCRIPTION 
       [0023]    According to some embodiments of the present invention, an improved technique is described to resolve the issues with ultrasonic locationing of a device with an ultrasonic emitter within an environment. In particular, the present invention utilizes a cross correlation of ultrasonic signals impinging on multiple microphones for more accurate locationing of a mobile device. Specifically, the present invention derives two frequencies from the ultrasonic burst to be used for cross correlation. The present invention recognizes that an ideal reception of a burst, using perfectly matched ideal amplifiers yields very little cross correlation strength. In addition, while it is commonly known that noise does not correlate, noise does reduce the cross correlation strength and can shift the point of maximum cross correlation. Moreover, due to the relatively low speed of ultrasonic signals it proves difficult to be sure exactly which ultrasonic cycle is the correct cycle to be correlated to. As a result, picking the correct cross correlation cycle when the signal-to-noise ratio drops below ideal conditions is not possible with any known techniques. The present invention resolves this difficulty by deriving two frequencies from the single frequency emitter ultrasonic burst and correlating the phase difference between those frequencies, as will be described below. 
         [0024]    The device to be locationed and incorporating the emitter can include a wide variety of business and consumer electronic platforms such as cellular radio telephones, mobile stations, mobile units, mobile nodes, user equipment, subscriber equipment, subscriber stations, mobile computers, access terminals, remote terminals, terminal equipment, cordless handsets, gaming devices, personal computers, and personal digital assistants, and the like, all referred to herein as a device. Each device comprises a processor that can be further coupled to a keypad, a speaker, a microphone, a display, signal processors, and other features, as are known in the art and therefore not shown. 
         [0025]    Various entities are adapted to support the inventive concepts of the embodiments of the present invention. Those skilled in the art will recognize that the drawings herein do not depict all of the equipment necessary for system to operate but only those system components and logical entities particularly relevant to the description of embodiments herein. For example, routers, controllers, switches, access points/ports, and wireless clients can all includes separate communication interfaces, transceivers, memories, and the like, all under control of a processor. In general, components such as processors, transceivers, memories, and interfaces are well-known. For example, processing units are known to comprise basic components such as, but not limited to, microprocessors, microcontrollers, memory cache, application-specific integrated circuits, and/or logic circuitry. Such components are typically adapted to implement algorithms and/or protocols that have been expressed using high-level design languages or descriptions, expressed using computer instructions, expressed using messaging logic flow diagrams. 
         [0026]    Thus, given an algorithm, a logic flow, a messaging/signaling flow, and/or a protocol specification, those skilled in the art are aware of the many design and development techniques available to implement one or more processors that perform the given logic. Therefore, the entities shown represent a system that has been adapted, in accordance with the description herein, to implement various embodiments of the present invention. Furthermore, those skilled in the art will recognize that aspects of the present invention may be implemented in and across various physical components and none are necessarily limited to single platform implementations. For example, the memory and control aspects of the present invention may be implemented in any of the devices listed above or distributed across such components. 
         [0027]      FIG. 1  is a block diagram of a system for ultrasonic locationing of a single emitter, in accordance with the present invention. A single transponder such as a piezoelectric speaker or emitter  106  can be implemented within a mobile device  100 . The emitter can send a short burst of ultrasonic sound (e.g.  140 ) to indicate the presence of the mobile device  100  within the environment. The mobile device can include a controller  102  that can be coupled to a wireless local area network interface  104  for wireless communication with other devices in the communication network  120 . 
         [0028]    The wireless communication network  120  can include local and wide-area wireless networks, wired networks, or other IEEE 802.11 wireless communication systems, including virtual and extended virtual networks. However, it should be recognized that the present invention can also be applied to other wireless communication systems. For example, the description that follows can apply to one or more communication networks that are IEEE 802.xx-based, employing wireless technologies such as IEEE&#39;s 802.11, 802.16, or 802.20, modified to implement embodiments of the present invention. The protocols and messaging needed to establish such networks are known in the art and will not be presented here for the sake of brevity. 
         [0029]    An ultrasonic receiver  110  includes a transducer such as one or more ultrasonic microphones  116  that can respond to an ultrasonic sound pressure wave (e.g.  140 ) transmitted from the ultrasonic emitter  106  of the mobile device. Each microphone  116  provides electrical signals  118  to a receiver circuitry including signal processors (not shown) and a controller  112 , such that the receiver controller will be aware of the presence of a device incorporating that ultrasonic emitter within the environment. The receiver controller  112  can also be coupled to a wireless local area network interface  114  for wireless communication with other devices in the communication network  120 . Alternatively, the controller  112  could be connected to the communication network  120  through a wired interface connection (not shown), such as an Ethernet interface connection. 
         [0030]    In order to provide positioning ability, using a multilateration technique for example, the transducers of the present invention includes a plurality of microphones  116  able to discriminate between different arrival times of a particular ultrasonic signal. In one embodiment, there are four microphones  116  integrated within a single receiver  110  housing. In this embodiment, three of the microphones can be disposed at each apex of a substantially triangular configuration, such as in a substantially flat triangular housing, and are configured in an array having a maximum dimension of not more than twelve inches, and where the fourth microphone is disposed in the middle of the triangular configuration, substantially coplanar with the other microphones. For unobtrusiveness and clear signaling, the housing can be affixed to a ceiling of the environment, where the position of each microphone is known and fixed. Of course, it should be recognized than many different housing and microphone configurations could be utilized with any number of microphones. However, the embodiment described herein utilizes relatively closely-spaced microphones within a singular housing, where the closeness of the microphones is accommodated by the present invention as described below. 
         [0031]    As the location and position of these microphones  116  is known and fixed, a signal received by these microphones can be used to locate and track the position of an emitter device using time difference of arrival (TDOA) at each microphone and employing multilateration, trilateration, or other suitable locationing technique. 
         [0032]    In the embodiment described herein, the mobile device emits an ultrasonic frequency burst at about 43.5 kHz, although it should be realized that other frequencies could be used. The emission may be accomplished by the controller driving the emitter at its resonant frequency. Also, it may be that more than one mobile device within the environment carries an emitter. In this case, the same frequencies can be used for all devices or different frequencies can be used for each device to better distinguish the devices by the receiver. In addition, the emitter frequency of one device could be changed during operation. Choosing which frequency to use can be accomplished by a backend controller  130  of the locationing system, which can communicate over the communication network  120  in order to direct different mobile devices  100  to emit the same or different frequencies in its an ultrasonic signal burst. Upon receiving the burst, the receiver  110  can communicate with the backend controller over the communication network that it has received the burst, and the backend controller will then know that the burst came from a particular mobile device. 
         [0033]    The backend controller also knows when the burst was sent, and can then determine the flight time of the burst by subtracting the emitting time from the acknowledgment of the reception time from the receiver. Alternatively, the backend controller could also radio the time of the originating burst to the receiver which would allow the receiver to convert TDOA values into flight times allowing trilateration, which has accuracy advantages over multilateration is some cases. Flight time can also be calculated once the position is determined by multilateration by simply taking the square root of the sum of the squares of the emitter&#39;s relative position in three dimensional space of the environment and then dividing by the speed of sound. It should be noted that the radio frequency communications are relatively instantaneous next to the flight time of the ultrasonic signal and could be ignored. Using a locationing technique such as multilateration, along with the flight time, the receiver  110  could determine and inform the backend controller of the location of the mobile device, which the backend controller can use to track its location during subsequent bursts. 
         [0034]    In the above scenario, the receiver is subject to reverberations of the ultrasonic signal due to multipath and reflections. Therefore, the ultrasonic burst must have a very short burst width such that the capture window is closed before any multipath energy is included in the cross correlation. If not, ultrasonic signal collisions could occur, and emitter signal would not be received properly. In addition, the receiver is configured to have a narrow bandwidth (high Q) receiver to reject as much environmental acoustic noise as possible. In this example, it is assumed that a  2500  ultrasonic burst is emitted by the emitter. 
         [0035]    Typically, for a single frequency burst, as each microphone of the receiver receives the ultrasonic burst the receiver can use cross-correlation of the burst to determine a direction of an emitter along with distances between microphones to determine flight time distance. For example, existing ultrasonic locationing systems have used a microphone spacing that is typically ten feet. Such system can simply correlate on burst envelopes. However, a microphone spacing of one foot or less, as in the present invention, requires correlation to an exact sine wave cycle of a burst. In this case, correlation accuracy drops since correlation must identify the correct correlation peak of the cycle. In particular, the locationing accuracy for closely-spaced microphone decreases with distance. Specifically, the sine wave correlations could shift an entire wavelength at the slow speeds of ultrasonic frequencies, thereby providing an incorrect distance calculation. At 40 kHz, for example, the period is 25 μs. Therefore, locking correlation to the wrong sine wave cycle can introduce integer multiple errors of ±25 μs, 50 μs or even 75 μs, which can be many feet of location error. 
         [0036]      FIG. 2  shows a single frequency correlation using two closely-spaced microphones. A correlator  20  is provided under control of a controller  112 . A mobile device with an emitter  21  emits a single frequency ultrasonic pulse which impinges on the microphones, reaching the first microphone  26  before the second microphone  28 . Although only two microphones are shown and the pulse is applied collinearly with respect to the microphone positions, it should be recognized that the number and position of the microphones can be expanded into a three dimensional environment. As shown, the correlator  20  obtains the pulse  22  from the first microphone and the pulse  24  from the second microphone as two time shifted signals. 
         [0037]      FIG. 3  shows the correlation function in the correlator. Specifically, the correlator takes the two obtained signals and correlates them, as is known in the art, by time shifting one signal over the other to find a point with the closest match of the waveforms. The amount if time shifting needed to correctly align the waveforms defines the time difference of arrival (TDOA) of the pulse between the microphones, which can then be used to locate the emitter. In the first correlation  30 , the correlator finds the proper correlation of pulse  1  and  2 , which the receiver can use to properly locate the emitter at location A (see  FIG. 1 ). However, if the correlator aligns to an improper waveform match, such as matching a sine wave cycle of one pulse to an adjacent cycle  32  of the other pulse, then the calculation of the location of the emitter with be off by 25 μs, which will improperly locate the emitter at location B, for example. 
         [0038]    Even if using a separate flight time calculation using RF synchronization, correlation to the wrong sine wave cycle cannot be tolerated. The key to implementing a precision ultrasonic locationing system with small receiver geometry (approximately one foot microphone spacing) is the accurate measurement of the TDOA of the ultrasonic burst between the microphones. However, accurate TDOA measurements have proven to be problematic in single frequency systems with narrow receiver bandwidths (i.e. high Q factor). An ultrasonic receiver system necessitates a high system Q factor to maximize signal-to-noise ratio in an environment where acoustic noise is abundant. This high Q factor inhibits the designing of a pulse shape that will facilitate accurate correlation results. 
         [0039]    As shown in  FIG. 4 , the pulse shape that results from a high Q system consists of sine waves having an amplitude that slowly rises and then decays. Frequency pulses of 40 kHz from two microphones in a high Q receiver are shown in this ideal representation. In this example, pulse  1  is the ultrasonic pulse received by a first microphone and pulse  2  is that same ultrasonic pulse received by a second microphone, but later in time. These two pulses are cross correlated with each other to yield the correlation buffer shown in  FIG. 5 . 
         [0040]      FIG. 5  shows the correlation buffer results from two ideal 40 kHz pulses in a high Q system. The key point to recognize here is how close in amplitude the adjacent correlation peaks are at ±one cycle away from the maximum correlation point. In particular, the margin between the maximum correlation point  50  and an adjacent correlation point  52  in this ideal case is only 1.1%. In actuality, these pulse shapes are slightly different from each other and can be affected by a number of different external factors that cannot be controlled (i.e. noise level, microphone response, electronic component tolerances, etc.). Since the correlation margins are so weak to begin with, slight differences in the external factors will most likely result in incorrect correlation results and therefore incorrect TDOA measurements. 
         [0041]    Therefore, the present invention derives two frequencies from an ultrasonic burst received by each microphone, wherein these two frequencies are then used for correlation. The present invention allows the ultrasonic locationing system to more consistently produce accurate TDOA measurements with the use of dual frequencies obtained from the ultrasonic burst emitted from the emitter without any loss in system Q factor or signal-to-noise ratio. The key piece of information that is makes this possible is the difference in phase between the two frequencies. 
         [0042]    Referring to  FIG. 6 , a representation of the ultrasonic burst received from the emitter is shown in the frequency domain. As shown the ultrasonic burst is centered on the emitter frequency, f t . As can be seen, the frequency spectrum of the ultrasonic burst is not an impulse at f t , but rather a broad pulse incorporating other frequencies. The (3 dB) frequency bandwidth of the burst will be determined by the duration of the drive signal and the quality factor (Q) of the emitter transducer. Assuming the Q of the transducer is relatively low, the bandwidth will be determined by the pulse duration as follows, BW=2/T, where T is the duration of the pulsed drive signal. If the Q of the transducer is relatively high, the bandwidth will be determined more from the transducer itself rather than from the burst duration. 
         [0043]    Referring to  FIG. 7 , two frequency pulses can be derived from the one ultrasonic burst. In particular, the receiver of the present invention consists of a microphone connected to two high Q amplifier chains (see  FIG. 9 ) with respective band pass filters tuned to different frequencies, i.e. center frequency f L  and f H , where f L &lt;f t &lt;f H . The required separation of the two receiver band pass filter center frequencies is a function of the bandwidth of the ultrasonic burst. A wider bandwidth burst will allow the filters to be spaced further apart while a narrower bandwidth pulse will require the filters to be spaced closer. For example, the filters can be configured such that the 3 dB points of the filter and the burst align, as shown. If properly designed, a portion of the energy contained in the ultrasonic burst (less than f t ) will be located within the pass band of the first receive band pass filter centered at f L  and will therefore derive an output pulse of frequency f 1 , somewhere between f L  and f t . Similarly, a portion of energy contained in the ultrasonic burst (greater than f t ) will be located within the pass band of the second receive band pass filter centered at f H  and will therefore derive an output pulse of frequency f 2 , somewhere between f t  and f H . In one example, f t  can be 43.5 kHz, f 1  can be 40 kHz, and f 2  can be 47 kHz. 
         [0044]    Referring to  FIG. 8 , since the two output pulse frequencies, f 1  and f 2 , from the band pass filters are derived from the same burst, they will maintain a fixed phase relationship across multiple microphone amplifier chains assuming the frequency and phase response of all the filters are matched. In this example, the two output pulse frequencies have about 6 dB less amplitude than the burst itself, and have frequencies corresponding to the 3 dB bandwidth of the burst (of  FIG. 6 ). 
         [0045]      FIG. 9  shows a simplified block diagram of a dual frequency receiver, in accordance with the present invention. It is envisioned that this receiver is implemented in the digital domain, in a digital signal processor  650  for example. It should be recognized that other components, including a controller, amplifiers, analog-to-digital converters (i.e. digitizers), digital filters, and the like, are not shown for the sake of simplicity of the drawings. Dual frequencies, f 1  and f 2 , are derived from each microphone signal of the emitted single ultrasonic burst, as previously explained. Each microphone signal has been amplified in a high Q amplifier chain and digitized before being provided to the receiver. Preferably, matched components are used for the first and second high Q microphone signal chains of the receiver. Optionally, each receiver chain could be equipped with its own digitizer. 
         [0046]    The receiver splits each microphone signal into its two frequency components, f 1  and f 2 , by passing each microphone signal through two different band pass filters, as detailed above. One band pass filter  602  has a center frequency at f L , and passes the first frequency component, f 1 , of the first microphone signal of the ultrasonic burst (i.e. pulse  1 ). Another band pass filter  604  has a center frequency at f H  and passes the second frequency component, f 2 , of the first microphone signal of the ultrasonic burst (i.e. pulse  1 ). Similarly, another band pass filter  606  has a center frequency at f L , and passes the first frequency component, f 1 , of the second microphone signal of the ultrasonic burst (i.e. pulse  2 ). Another band pass filter  608  has a center frequency at f H  and passes the second frequency component, f 2 , of the second microphone signal of the ultrasonic burst (i.e. pulse  2 ). 
         [0047]      FIG. 10  shows the individual frequency components output from the band pass filters. In the example described herein, the lower first frequency components, f 1 ,  610 ,  614  are at 40 kHz and the higher second frequency components, f 2 ,  612 ,  616  are at 47 kHz. 
         [0048]    Referring back to  FIG. 9 , the output from each band pass filter is combined in a digital multiplier which provides sum and difference products of the first and second frequency components, f 1  and f 2 .  FIG. 11  shows the multiplied product of the two frequency components of each receiver after being multiplied in the digital domain. In particular, the pulse  1  frequency components  610  and  612  produce the product  618  in multiplier  640 , and the pulse  2  frequency components  614  and  616  produce the product  620  in multiplier  642 . Inasmuch as f 1  is 40 kHz and f 2  is 47 kHz, the combined waveform includes 87 kHz products (47 kHz+40 kHz) and 7 kHz products (47 kHz-40 kHz). 
         [0049]    Referring back to  FIG. 9 , the output from each multiplier is digitally filtered in a low pass filter to pass only the low frequency products.  FIG. 12  shows the pulse  1  low frequency product  618  and the pulse  2  low frequency product  620  of  FIG. 11  after being low pass filtered to provide the respective pulse  1  signal and pulse  2  signals  622  and  624  for correlation. The correlator  20  obtains these signals and correlates the phase difference between the two different frequency pulses as previously described to obtain a TDOA between the microphones for the emitted ultrasonic burst. 
         [0050]      FIG. 13  shows the correlation buffer results from two signals  622  and  624  of  FIG. 12 . The key point to recognize here is that the margin between the maximum correlation point  90  and an adjacent correlation point  92  resulting from the use of the dual frequencies in the present invention is about 21.4%, which is far better than the 1.1% margin of the previous single frequency embodiment of  FIG. 5 . Accordingly, the present invention provides much improved TDOA accuracy and locationing. 
         [0051]    However, if the system is subject to more than one cycle error, i.e. incorrect correlation to farther cycles than just the adjacent cycle, a less aggressive frequency selection must be made to reduce the correlation strength of those farther cycle points. The optimal frequency selection becomes difficult to calculate when the amplitudes vary during the burst. Therefore, frequency selection was computer modeled considering ±four cycles. Real world amplitude shaping was simulated to be 24% reduction in correlation strength of all cycles. However, 15% was measured on the actual system using the finally chosen modeled frequencies of 40 kHz and 47 kHz. The choice of these two frequencies results in a correlation strength of the correct cycle being considerably larger than that of the adjacent cycles, as shown in  FIG. 13 , thereby providing increased immunity to noise and non-ideal components. 
         [0052]    Looking at  FIG. 13 , the sharpness of the correlation peak  90  is not as sharp as the correlation peak  50  of  FIG. 5 . Therefore, even though  FIG. 10  provides the correct cycle, the actual measurement of TDOA is less accurate than it would be if choosing the correct cycle from  FIG. 5 . In other words it can be difficult to find the exact peak of a sine wave, and therefore having a sine wave of a higher frequency will provide a more accurate time offset measurement. Therefore, the present invention can optionally combine the results from  FIG. 13  with  FIG. 5  to provide a more exact time offset measurement. 
         [0053]    Referring back to  FIG. 9 , the present invention can provide a second correlator  600  that can take a single frequency correlation (as was done for  FIGS. 4 and 5 ) for use to improve the time offset measurement. For example, the phase difference between the same frequency component,  610  and  614 , (although  612  and  616  could have been used) of each pulse chain is correlated in correlator  600  to provide the combined result of  FIG. 5 . The sine waves of  FIG. 5  are on approximately 40 kHz cycles, whereas the sine waves of  FIG. 10  are on approximately 7 kHz cycles. Therefore,  FIG. 5  can provide almost a 20× (40/2) improvement in TDOA measurement assuming the correct cycle is chosen. In practice, the ±time offset measurement error at 7 kHz is sufficient to bracket the correct cycle in  FIG. 5  and then the measured time offset of the correct cycle in  FIG. 5  is used to provide the more accurate TDOA result. In effect, the correlation result of  FIG. 5  is used to fine tune the correlation result of  FIG. 13 . 
         [0054]    In effect, the present invention provides a novel technique to determine if a correlation is wrong and how the correct cycle can be found with a high degree of confidence. If after correlating the f 1  outputs,  610  and  614 , from each microphone chain in correlator  600 , it is determined that the TDOA between the two pulses is T TDOA  microseconds, then it can be confirmed that the difference in phase between f 1  and f 2  at any time t microseconds on one microphone will be identical to the difference in phase between f 1  and f 2  on the other microphone at time t+T TDOA  microseconds. Therefore, the qualifying test for a correct TDOA calculation is: 
         [0000]      Δφ mic1 ( t )=Δφ mic2 ( t+T   TDOA )
 
         [0055]    One way to measure the changes in relative phase between the two frequencies is to multiply them together (see  FIG. 11 ). The trigonometric identity for the product of two different frequencies yields terms with both the sum and difference frequency: 
         [0000]      sin( f   1   t )*sin( f   2   t )=½*[cos(( f   1   −f   2 ) t )−cos(( f   1   +f   2 ) t )]
 
         [0056]    Filtering out the higher frequency sum component leaves the difference (beat) frequency (see  FIG. 12 ) which represents the relative change in phase between the two sine waves. Cross correlating the two low pass filter outputs (see  FIG. 13 ) give a good measurement of the TDOA between the channels accurate to well under half a cycle of the original f 1  and f 2  input frequencies. This TDOA value can then be used to choose the correct correlation cycle on the original single frequency correlation of  FIG. 5  to obtain the true TDOA. The correlation strength of the output will be relatively strong with little chance for an incorrect TDOA result, due primarily to the fact that the pulse is of short duration at low frequency (i.e. it will only contain as little as one or two complete sine wave cycles.) 
         [0057]      FIG. 14  is a diagram illustrating a method for ultrasonic locationing of a single emitter, according to some embodiments of the present invention. 
         [0058]    A first step  200  includes emitting a single frequency ultrasonic burst from one emitter of a mobile device. 
         [0059]    A next step  202  includes receiving the ultrasonic burst from the single frequency emitter in each of at least two microphones to provide respective microphone signals. 
         [0060]    A next step  204  includes deriving two different ultrasonic frequency pulses from the ultrasonic burst in each microphone signal. This can include band pass filtering each microphone, a first band pass filter passing a first frequency component from the ultrasonic burst and a second band pass filter passing a second frequency component from the ultrasonic burst different from the first frequency component. This step can also include multiplying the first and second frequency components from its respective microphone in the digital domain to provide the obtained signals for the correlator. This step can also include low pass filtering that passes a difference frequency of a multiplied combination of the two different ultrasonic frequency components from each microphone to the correlator. 
         [0061]    A next step  206  includes correlating the phase difference between the two different frequency pulses in each microphone signal to establish a time difference of arrival of the ultrasonic burst at each microphone. This step can also include a second correlation of a same frequency component from each microphone to produce a second frequency correlation, and wherein the (first) correlation providing its time difference of arrival measurement to the second correlation which is used to select the correct correlation cycle in the second frequency correlation and its respective time difference of arrival measurement. 
         [0062]    A next step  208  includes using the time difference of arrival of the ultrasonic burst from the emitter transducer impinging on each microphone to determine a location of the emitter and mobile device. 
         [0063]    Advantageously, the present invention provides an accurate ultrasonic locationing of a single frequency emitter using an ultrasonic receiver integrated with closely spaced microphones, which is not known in the prior art. In addition, the present invention provides reasonable immunity to close reflectors (multipath signals), and a reasonable minimum SNR ratio. The present invention has been demonstrated in the lab to show a large increase in correlation strength when subject to actual Center of Excellence in Wireless and Information Technology (CEWIT) noise environment. The present invention opens the door to phase measurements which have been shown to provide further immunity against picking the wrong cycle during correlation. 
         [0064]    In the foregoing specification, specific embodiments have been described. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present teachings. 
         [0065]    The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all the claims. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued. 
         [0066]    Moreover in this document, relational terms such as first and second, top and bottom, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “comprises,” “comprising,” “has”, “having,” “includes”, “including,” “contains”, “containing” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises, has, includes, contains a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by “comprises . . . a”, “has . . . a”, “includes . . . a”, “contains . . . a” does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises, has, includes, contains the element. The terms “a” and “an” are defined as one or more unless explicitly stated otherwise herein. The terms “substantially”, “essentially”, “approximately”, “about” or any other version thereof, are defined as being close to as understood by one of ordinary skill in the art, and in one non-limiting embodiment the term is defined to be within 10%, in another embodiment within 5%, in another embodiment within 1% and in another embodiment within 0.5%. The term “coupled” as used herein is defined as connected, although not necessarily directly and not necessarily mechanically. A device or structure that is “configured” in a certain way is configured in at least that way, but may also be configured in ways that are not listed. 
         [0067]    It will be appreciated that some embodiments may be comprised of one or more generic or specialized processors or processing devices such as microprocessors, digital signal processors, customized processors and field programmable gate arrays and unique stored program instructions (including both software and firmware) that control the one or more processors to implement, in conjunction with certain non-processor circuits, some, most, or all of the functions of the method and/or apparatus described herein. Alternatively, some or all functions could be implemented by a state machine that has no stored program instructions, or in one or more application specific integrated circuits, in which each function or some combinations of certain of the functions are implemented as custom logic. Of course, a combination of the two approaches could be used. 
         [0068]    Moreover, an embodiment can be implemented as a computer-readable storage medium having computer readable code stored thereon for programming a computer (e.g., comprising a processor) to perform a method as described and claimed herein. Examples of such computer-readable storage mediums include, but are not limited to, a hard disk, a compact disc Read Only Memory, an optical storage device, a magnetic storage device, a Read Only Memory, a Programmable Read Only Memory, an Erasable Programmable Read Only Memory, an Electrically Erasable Programmable Read Only Memory, and a Flash memory. Further, it is expected that one of ordinary skill, notwithstanding possibly significant effort and many design choices motivated by, for example, available time, current technology, and economic considerations, when guided by the concepts and principles disclosed herein will be readily capable of generating such software instructions and programs and integrated circuits with minimal experimentation. 
         [0069]    The Abstract is provided to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In addition, in the foregoing Detailed Description, it can be seen that various features are grouped together in various embodiments for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separately claimed subject matter.