Abstract:
A technique for improving the capability of measuring the distance between nodes of a wireless communication network is provided. Technique ( 800 ) includes receiving a measured signal, correlating the measured signal with a reference signal to output a measured correlated function, comparing the correlation function to a predetermined reference correlation function, the reference correlation function being based on a predetermined direct path sequence and an estimation of the phase delay of the measured correlated function. A score is assigned to the measured correlation function based on how close the measured correlation function resembles the predetermined reference correlation function. Technique ( 800 ) provides improved location accuracy, even in multipath environments, by indicating the quality of the TOA measurement and enabling the selection of a correction mechanism.

Description:
TECHNICAL FIELD  
       [0001]     This invention relates in general to wireless networks and more particularly to improving Time of Arrival (TOA) measurements between nodes of a wireless communications network.  
       BACKGROUND  
       [0002]     Wireless communication networks have become increasingly prevalent over the past decade. In recent years, a type of mobile communication network known as an “ad-hoc” network has been developed. In this type of network, a mobile node is capable of operating as a base station or router for other mobile nodes without using fixed infrastructure base stations. More sophisticated ad-hoc networks are also being developed which, in addition to enabling mobile nodes to communicate with each other as in conventional ad-hoc networks, further enable the mobile nodes to access a fixed network and thus communicate with other mobile nodes, such as those of a switched telephone network (PSTN), and on other networks such as the Internet. The mobile nodes of such networks may assume any number of random positions within the network, making exact node location determinations difficult when needed. For computing node geographical coordinates in such ad-hoc wireless networks, algorithms in use at individual nodes in typical networks use a “Time of Arrival” (TOA) measurement technique.  
         [0003]     A Time of Arrival (TOA) measurement provides the distance between mobile nodes for computing a mobile node position. The measurements are based upon signal propagation times, specifically the time a signal needs for traveling between transceivers of a target node and a reference node. Historically, TOA measurements provide an estimate of the distance between two transceivers, or nodes, using approaches that assume that any information received is via a direct path channel. Existing TOA measurement methods detect a peak of a correlation function of a received signal. This peak, however, could be the manifestation of the direct path only or the direct path “tainted” with delay spread. TOA measurements can thus be inaccurate due to delay spread and multipath in the communication channel. Furthermore, existing methods typically average TOA measurements to determine distance which can lead to a trade off between convergence and accuracy.  
         [0004]     Accordingly, it would be beneficial to have an improved method of determining the distance between nodes of a wireless communications network that would address the delay spread and multipath problem. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]     The features of the present invention, which are believed to be novel, are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in conjunction with the accompanying drawings, in the several figures of which like reference numerals identify like elements, and in which:  
         [0006]      FIG. 1  is a block diagram of an example ad-hoc wireless communications network including a plurality of nodes employing a system and method in accordance with an embodiment of the present invention;  
         [0007]      FIG. 2  is a block diagram illustrating an example of a mobile node employed in the network shown in  FIG. 1 ;  
         [0008]      FIG. 3  is an example of a graph of a quadratic approximation for estimating phase delay for use in the method and apparatus of the present invention;  
         [0009]      FIG. 4  is an example of a graph representing a measured correlation function having a high score in accordance with the present invention;  
         [0010]      FIG. 5  is an example of a graph representing a measured correlation function having a low score in accordance with the present invention;  
         [0011]      FIG. 6  is an example of a reference correlation function for use in the method and apparatus of the present invention;  
         [0012]      FIG. 7  is a graph of an empirical reference correlation function in accordance with an embodiment of the present invention; and  
         [0013]      FIG. 8  is a flowchart summarizing the steps for estimating time of arrival (TOA) in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0014]     While the specification concludes with claims defining the features of the invention that are regarded as novel, it is believed that the invention will be better understood from a consideration of the following description in conjunction with the drawing figures, in which like reference numerals are carried forward.  
         [0015]     The present invention may be embodied in several forms and manners. The description provided below and the drawings show exemplary embodiments of the invention. Those of skill in the art will appreciate that the invention may be embodied in other forms and manners not shown below. The invention shall have the full scope of the claims and shall not be limited by the embodiments shown below. It is further understood that the use of relational terms, if any, such as first, second, top and bottom, front and rear and the like are used solely for distinguishing one entity or action from another, without necessarily requiring or implying any such actual relationship or order between such entities or actions.  
         [0016]      FIG. 1  is a block diagram illustrating an example of an ad-hoc wireless communications network  100  employing an embodiment of the present invention. Specifically, the network  100  includes a plurality of mobile wireless user terminals  102 - 1  through  102 - n  (referred to generally as nodes  102  or mobile nodes  102 ), and can, but is not required to, include a fixed network  104  having a plurality of access points  106 - 1 ,  106 - 2 , . . .  106 - n  (referred to generally as nodes  106 , access points (APs)  106  or intelligent access points (IAPs)  106 ), for providing nodes  102  with access to the fixed network  104 . The fixed network  104  can include, for example, a core local area network (LAN), and a plurality of servers and gateway routers to provide network nodes with access to other networks, such as other ad-hoc networks, the public switched telephone network (PSTN) and the Internet. The network  100  further can include a plurality of fixed routers  107 - 1  through  107 - n  (referred to generally as nodes  107 , wireless routers (WRs)  107  or fixed routers  107 ) for routing data packets between other nodes  102 ,  106  or  107 . It is noted that for purposes of this discussion, the nodes discussed above can be collectively referred to as “nodes 102, 106 and 107”, or simply “nodes”.  
         [0017]     As can be appreciated by one skilled in the art, the nodes  102 ,  106  and  107  are capable of communicating with each other directly, or via one or more other nodes  102 ,  106  or  107  operating as a router or routers for packets being sent between nodes, as described in U.S. patent application Ser. No. 09/897,790, and U.S. Pat. Nos. 6,807,165 and 6,873,839.  
         [0018]     As shown in  FIG. 2 , each node  102 ,  106  and  107  includes at least one transceiver or modem  108 , which is coupled to an antenna  110  and is capable of receiving and transmitting signals, such as packetized signals, to and from the node  102 ,  106  or  107 , under the control of a controller  112 . The packetized data signals can include, for example, voice, data or multimedia information, and packetized control signals, including node update information.  
         [0019]     Each node  102 ,  106  and  107  further includes a memory  114 , such as a random access memory (RAM) that is capable of storing, among other things, routing information pertaining to itself and other nodes in the network  100 . As further shown in  FIG. 2 , certain nodes, especially mobile nodes  102 , can include a host  116  which may consist of any number of devices, such as a notebook computer terminal, mobile telephone unit, mobile data unit, or any other suitable device. Each node  102 ,  106  and  107  also includes the appropriate hardware and software to perform Internet Protocol (IP) and Address Resolution Protocol (ARP), the purposes of which can be readily appreciated by one skilled in the art. The appropriate hardware and software to perform transmission control protocol (TCP) and user datagram protocol (UDP) may also be included. Additionally, each node includes the appropriate hardware and software to perform Time of Arrival (TOA) measurements, as set forth in greater detail below.  
         [0020]     As stated earlier, mobile nodes  102  of such networks may assume any number of random positions within the network, making exact node location determinations difficult when needed. In order for nodes  102 ,  106  and  107  to ascertain each others locations, a Time of Arrival (TOA) measurement can be used to provide an estimate of the distance between the two transceivers of a first and a second node. In order to perform high precision computations for mobile node location services, it is necessary to measure the distance between the two transceivers with a high degree of precision. Determining distance measurements in a multipath channel is particularly challenging, because it is sometimes impossible to extract the direct path information out of a received signal. If a direct path is weaker than the secondary paths, it is possible that the receiver will detect some of the secondary paths, but not the direct path. If the secondary paths are close in time to the direct path, the mechanism that determines the time of the direct path will be confused by the presence of secondary paths and consequently may not have enough resolution to distinguish between both. A small error in time calculation results in large errors in the determination of position.  
         [0021]     Briefly, in accordance with the present invention, direct path data is extracted from available channel information in order to determine whether a strong direct path is present. To accomplish this task, correlation functions of a received signal are measured and reference correlation functions are generated. To address the multi-path issue, a score is assigned based on similarities between the measured correlation function and the reference correlation function. The assigned score provides an indication of how close the measured correlation function is to a direct path function. The measured correlation function with the highest score is then used to improve the TOA measurement of the received signal.  
         [0022]     Referring back to  FIG. 1 , node  102  can be said to be operating as a ranger and node  104  can be said to be operating as a pinger. The ranger  102  is the node that sends a first ranging packet and the pinger  104  is the node that replies to the ranging packet with another ranging packet. In operation, ranger node  102  sends a ranging message to pinger node  104  to instigate time of arrival (TOA) measurements of received signals. Thus, when pinger node  104  receives a signal from node  102  over a communication channel, a TOA measurement of the received signal is taken.  
         [0023]     In accordance with the present invention, a received signal is correlated with a reference signal to obtain a measured correlation function. Examples of graphical representations of measured correlation functions are shown in  FIGS. 4 and 5 . Graph  400  includes measured correlation function  402  and reference correlation function  404 . The estimated phase delay is represented by designator  406 . Graph  500  includes measured correlation function  502  and reference correlation function  404 . The estimated phase delay is represented by designator  506 . Phase delay is calculated with using the following formula:  
       Δ   =       1   2     ·         Z   -     -     Z   +           2   ⁢     Z   0       -     Z   +     -     Z   -               
 
         [0024]     The phase delay (Δ) is the correction applied to the measured TOA of the received signal. The values of Z 0 , Z +  and Z −  are measured as shown in the quadratic approximation of  FIG. 3 . Graph  300  shows amplitude versus phase delay, expressed in multiples of the sampling interval.  
         [0025]      FIG. 6  shows a reference correlation function  604 . The reference correlation function  604  is constructed based on the amplitude and the phase delay of the measured correlation function. The reference correlation function  604  can be determined analytically as shown in  FIG. 6  or empirically as graphically represented in  FIG. 7  by graph  700 . A reference correlation function is constructed for each measured correlation function, and thus the reference correlation function will vary from measurement to measurement thereby providing a dynamic reference correlation function. Hence, the reference correlation function  404  of  FIG. 4  and the reference correlation function  504  of  FIG. 5  do not have the same amplitude and are not sampled using the same phase delay.  
         [0026]     In accordance with the present invention, the score is calculated based on similarities between the reference correlation function and the measured correlation function. The calculated score is proportional to the inverted sum of the difference between the reference correlation function and the measured correlation function. The calculated score is used as an estimation of signal quality in a communication channel.  
         [0027]     As seen from the graphs  400 ,  500 , the estimated phase delay (Δ) is smaller in graph  400  and a higher score will be assigned and the estimated phase delay  506  will have a lower score assigned. Based on the assigned score, multipath is determined for the measured correlation function  402  and a TOA measurement is adjusted using the estimated phase delay  406 . The low score measured correlation function  502  will be considered multipath and alternate TOA techniques can be used.  
         [0028]     The scoring information may be used to: apply the Δ correction to the TOA to obtain an accurate measure; provide a weight to the location service calculation; apply specific smoothing filter to the TOA samples; and/or apply a specific elimination criteria for TOA samples. Thus, the position of a node in a network can now be determined utilizing a TOA having a higher degree of precision.  
         [0029]     Additionally, an error can be determined, the error being proportional to the difference between measurement and theory, as well as to the amplitude of the measurement, or the reference (whichever is greatest). The determined error represents mostly those errors that are measured at high amplitudes, which are most representative of what the input signal looks like, rather than low amplitudes, which are mostly driven by noise. The error can be biased by taking the difference between the reference correlation function and the measured correlation function and multiplying the difference by a power of the amplitude of the highest reference correlation function and the measured correlation function. The error is thus biased toward large amplitude discrepancies which do not have correlation noise.  
         [0030]     Referring now to  FIG. 8 , there is shown a flowchart  800  summarizing the steps for improving TOA measurements utilizing the scoring of correlation functions in accordance with the present invention. Beginning at step  802  a signal is received (either at the ranger or pinger) and correlated with a reference signal at step  804  to obtain a measured correlation function (such as  402  or  502  of graphs  FIG. 4  and  5 ). The phase delay of the received signal is estimated at step  806  utilizing the measured correlation function (such as  406 / 506  of graphs  FIG. 4  and  5 ). A reference correlation function is generated at step  808  (reference correlation function  404 ,  504 ,  604  and  704  are equivalent) based on the amplitude and the delay of the measured correlation function. The measured correlation function is compared to the reference correlation function at step  810  (graph of  FIG. 6 ). A score is calculated at step  812  and assigned based on how close the measured correlation function is to reference correlation function. The higher the score, the more likely a direct path was encountered. If the score indicates a direct path at step  814 , then the step of correcting the TOA by the estimated phase delay takes place at step  816 . If the score is low, indicating multipath, alternate TOA techniques can be used.  
         [0031]     Technique can further comprise the step of biasing an error of the measured correlation function by taking the difference between the reference correlation function and the measured correlation function and multiplying the difference by a power of the amplitude of the highest reference correlation function and the measured correlation function. The error is thus biased toward large amplitude discrepancies which do not have correlation noise.  
         [0032]     The technique of scoring a correlation function output in accordance with the present invention is thus able to determine whether a path is “tainted” or not. The technique of scoring a correlation function in accordance with the present invention provides the ability to detect single path correlation even if received signal strength (RSS) is low. The score represents the likelihood of the peak being a single path. Multiple paths will compress the peak or widen it, depending on how far apart the paths are. The score is used for various purposes including interpolating the samples to increase precision, eliminating imprecise data points and estimating the quality of the communication channel. The technique of the present invention interpolates the reference function based on the estimation of the phase of the input function. The method of scoring allows for fast convergence when the scores are high and allows for a fall-back on alternate distance estimation methods if the channel is poor.  
         [0033]     While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims.