Method and system for estimating the location of a wireless terminal from a received signal strength

A first estimated location of a target terminal to be estimated in its location is obtained based on information derived from radio signals for location estimation and also locations of anchor terminals. Then, on the basis of the first estimated location thus obtained, estimated distances are calculated while expected values are calculated based on information required for location estimation to compare the estimated distances with the expected-values. Depending on the comparison result, one or more anchor terminals are selected as nonuse terminals. On the basis of information required for location estimation derived from radio signals sent by the anchor terminals other than the selected nonuse terminals and also on the locations of the anchor terminals except the nonuse terminals, a second estimated location of the target terminal is obtained.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and a system for estimating the location of a wireless terminal from radio signals received, and a wireless terminal therefor.

2. Description of the Background Art

Conventionally, as disclosed by Japanese patent laid-open publication No. 2004-112482, in a system for estimating the location of a radio communication terminal, base stations receive a radio signal sent from a radio communication terminal and measures the signal strength of the received signal to transmit a measurement of the strength level to an information server, which in turn uses the measurement of signal strength received to estimate areas where the radio communication terminal associated with the measurement of signal strength received can exist to determine the location of the radio communication terminal from how the estimated areas overlap with each other.

U.S. Pat. No. 6,473,038 to Patwari et al., discloses a system for locating a number of devices by measuring signals transmitted between known location devices and unknown location devices and signals transmitted between pairs of unknown location devices, entering signal measurements into a graph function that includes a number of first sub-expressions, a number of which include signal measurement prediction sub-expressions and have extreme when a predicted signal measurement is equal to an actual signal measurement, and optimizing the graph function.

Furthermore, there are some examples of location estimation of a target terminal, in which a target terminal sends signals to anchor terminals, which in turn transmit received signal strength measurements of the received signals to a location estimation server, which uses the received signal strength measurements from the anchor terminals to estimate the most probable location of the target terminal. These examples are disclosed in D. Zhao et al., “A Maximum Likelihood Estimation Method of Localization using RSSI in Wireless Sensor Networks”, Technical Report, The Institute of Electronics, Information and Communication Engineers (IECE), IN2004-327, pp. 409-414; M. Takashima et al., “An Experiment of Indoor Location Estimation using IEEE 802.15.4”, Technical Report, IECE, IN2005-7, pp. 27-32; D. Zhao et al., “A Method for Reducing Location Estimation Data Traffic in Sensor Networks”, The Society Conference Transactions in 2005, IECE, A-21-20; M. Takashima et al., “An Experiment on Indoor Location Estimation Using IEEE 802.15.4—Effect by Pedestrians and RF Sensitivities—”, The Society Conference Transactions in 2005, IECE, A-21-22; and K. Iwamoto et al., “An Iterative Location Estimation Method for Wireless Sensor Networks”, Technical Report, IECE, IN2006-39, pp. 7-14.

In the conventional location estimation systems, however, the accuracy of location estimation depends on the placing distances between anchor terminals, so that the anchor terminals must be placed densely to increase the estimation accuracy. Placement of a number of anchor terminals leads to the increase of cost of the system. Furthermore, it is conceivable that the placement of a lot of terminals is physically impossible depending on the placement circumstances. In addition, the more the anchor terminals, the higher the communications traffic for the location estimation.

In some instances, the reliability of information derived from an anchor terminal located in a certain direction is lower than that of information derived from other anchor terminals. For example, when location estimation is carried out on a person who is moving about, his/her body may be an obstacle to the estimation unless he/she carries his/her terminal overhead. Also, if there are, for instance, partitions, shelves or walls between a target terminal and anchor terminals located in a target space for the estimation, they can be obstructions to the estimation. As described, information derived from a specific anchor terminal can be less reliable, and therefore the use of such information for the location estimation may cause a decrease in estimation accuracy.

Thus, there has been a demand for a method and a system for location estimation, and a radio terminal therefor to overcome the problem of increase of communications traffic arising from the expansion of area for the location estimation and the increase of anchor terminals to be placed, thereby enabling high-accuracy estimation even if the reliability of information derived from a specific anchor terminal is lower.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a method and a system for location estimation, and a radio terminal therefor specifically improved in accuracy of estimating the location of a radio terminal.

In accordance with the present invention, a method Of estimating the location of a target terminal by using a radio signal and the location of a plurality of anchor terminals, the radio signal being transmitted at least in either of directions from the target terminal to the plurality of anchor terminals and from the plurality of anchor terminals to the target terminal, comprises the steps of: using information required for location estimation derived from the radio signal and the location of at least one of the plurality of anchor terminals to obtain a first estimated location of the target terminal; calculating an estimated distance between the target terminal and the at least one anchor terminal based on the first estimated location, and calculating an expected value of a distance between the target terminal and the at least one anchor terminal based on the information required for the location estimation; comparing the estimated distance with the expected value to thereby select at least one of the plurality of anchor terminals as a nonuse terminal; and using the information required for the location estimation derived from the radio signal transmitted at least either of the directions except for the at least one nonuse terminal, and the location of ones of the plurality of anchor terminals other than the at least one nonuse terminal to obtain a second estimated location of the target terminal.

In accordance with the present invention, a system for estimating a location of a target terminal comprises: a target terminal for at least transmitting a radio signal; a plurality of anchor terminals each arranged at a known location for at least receiving the radio signal; and a location estimator for estimating the location of the target terminal. The location estimator uses information required for location estimation derived from the radio signal and the location of the plurality of anchor terminals to obtain a first estimated location of the target terminal, calculates an estimated distance between the target terminal and at least one of the anchor terminals based on the first estimated location, calculates an expected value of a distance between the target terminal and the at least one anchor terminal based on the information required for the location estimation, compares the estimated distance with the expected value to thereby select at least one of the plurality of anchor terminals as a nonuse terminal, and uses the information required for the location estimation derived from the radio signal transmitted to or from ones of the plurality of anchor terminals other than the at least one nonuse terminal and the location of the anchor terminals other than the at least one nonuse terminal to obtain a second estimated location of the target terminal.

Also, in accordance with the present invention, a radio terminal consists of the location estimator, the target terminal or the anchor terminal of the aforementioned location estimation system.

The present invention can improve the accuracy of location estimation for a target terminal by selecting at least one of the plurality of anchor terminals as at least one nonuse terminal and obtaining the location of the target terminal on the basis of information required for the location estimation, which is derived from radio signals transmitted between the plurality of anchor terminals other than the selected nonuse terminal and the target terminal and also based on the locations of the plurality of anchor terminals other than the selected nonuse terminal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1schematically shows a preferred embodiment of the location estimation system. InFIG. 1, the location estimation system of the embodiment includes two target terminals10and a number of anchor terminals20. This figure shows dotted arrows22indicating that signals are transmitted to all neighboring terminals without specifying recipients, i.e. broadcast transmission. Signals are designated with reference numerals of connections on which they are conveyed. The anchor terminals20are at known locations and transmit location-estimation request signals. The target terminals10are the objects of location estimation. Each target terminal10receives the location-estimation request signals from the anchor terminals20and derives information required for location estimation, such as received signal strength (received power value), thereby estimating its location.

FIG. 2schematically shows the configuration of one of the anchor terminals20of the embodiment shown inFIG. 1, which shows the six anchor terminals20that may be of the same structure as each other. InFIG. 2, the anchor terminal20comprises an antenna201, a transmitter202and a transmission data generator203, which are interconnected as illustrated. The antenna201serves as transmitting a radio signal22. The transmission data generator203has an identification code set specific to the anchor terminal20, i.e. different from anchor terminal to terminal, and is adapted to generate a location-estimation request signal24which includes the identification code. The transmitter202functions as converting the location-estimation request signal24generated by the generator203to a corresponding radio signal26to output the signal to the antenna201.

Now,FIG. 3schematically shows the configuration of the target terminal10in the illustrative embodiment. InFIG. 3, the target terminal10comprises an antenna101, a receiver104interconnected to the antenna101, a data collector105and a location estimation processor106serving as a location estimator, which are interconnected as illustrated. The receiver104includes a receiving circuit102and a received data processor103, interconnected as shown.

The antenna101is used to receive the radio signal22and input a received signal12to the receiving circuit102. The receiving circuit102is adapted to convert the radio signal12to corresponding digital data14to output the data14as received data to the received data processor103. In addition, the receiving circuit102has the function of measuring the received power value S1of the radio signal22and then outputs the measured value S1on its output14to the received data processor103as information necessary to the location estimation. The received data processor103is adapted for processing the received data14input by the receiving circuit102to thereby deliver, if the received data14is the location-estimation request signal transmitted from the anchor terminal20, the information contained in the request signal as well as the information on the received power value S1measured by the received circuit102on its output16to the data collector105.

The data collector105functions as collecting the information16input from the receiver104and outputting the information16to the location estimation processor106. The location estimation processor106operates as described below to estimate the location of each target terminal10based on the information16input by the data collector105. It is to be noted that the data collector105and the location estimation processor106may be designed in the form of separate unit interconnected to the received data processor103over telecommunications, rather than those illustrated as incorporated in the target terminal10. In this case, the received data processor103is adapted to transmit necessary data16in the form including the identification data of the target terminal10in which the processor103is included.

The operation of the illustrative embodiment of the location estimation system thus configured will be described with reference toFIG. 4. In operation, the system is specifically applied to a location estimation method utilizing the received power values of the radio signals22transmitted from the anchor terminals20to the target terminals10. However, since the system, comprising the plurality of anchor terminals20and the target terminals10, is operable to estimate the location of a target terminal10in some measures, the system is adaptable to any types of location estimation system capable of estimating the distance between each anchor terminal20and the target terminals10in some measures.

FIG. 4is a flowchart for use in describing the operation of the location estimation in the illustrative embodiment. Each anchor terminal20transmits a location-estimation request signal22including an identification code unique to the anchor terminal20, or transmitting terminal code. On the target terminals10, the receiver104receives the location-estimation request signal22from the anchor terminals20and measures the received power value S1of the signal22to output the data16of power value S1along with the transmitting terminal code to the data collector105(S101).

The data collector105, in turn, collects the data16from the receiver104to deliver the latter to the location estimation processor106. The location estimation processor106stores the data18provided from the collector105, and executes, based on the stored data, a first estimation step S102for obtaining a first estimated location of the target terminal10in question.

In the first estimation step S102, use is made of information about the received power value S1and the transmitting terminal code contained in the location-estimation request signal22received by the target terminal10as well as the locations of the anchor terminals20to obtain the first estimated location of the target terminal10in question.

In the location estimation in the step S102, the information transmitted from the anchor terminals20to the target terminal10in question and the locations of the anchor terminals20, for example, are used for calculating an existence probability at every possible position of the target terminal10in question. Then, a method, such as the maximum likelihood estimate method, is used to thereby locate a position where the existence probability of the antenna101is highest and define the position thus obtained as the estimated location of the target terminal10in question.

Alternatively, another method can be employed which uses the distances between each anchor terminal10and the target terminal in question10. That is, several pieces of distance information can be used to estimate the location of the target terminal10. In such a method, for instance, estimated distances R between each anchor terminal20and the target terminal10in question are calculated according to the information transmitted between each anchor terminal20and the target terminal10in question, and also distances D between the target terminal10at an assumed position (x, y) and each anchor terminal20are calculated. The absolute value of difference |R−D| is then calculated in relation to each anchor terminal20, and the sum of the absolute values of difference is calculated for all the anchor terminals10. The obtained sum is then minimized to thereby determine the estimated location (x, y) of the target terminal10. Note that the method for obtaining the estimated location is not limited to the specific one described above, but other suitable methods can be employed.

On the basis of the first estimated location, in the step S103, the location estimation processor106calculates estimated distances between the target terminals10and the anchor terminals20while using the information required for the location estimation to calculate expected values of the distances between the target terminals10and the anchor terminals20. The processor106then compares the estimated locations with the expected values so as to select at least one of the anchor terminals20as a nonuse or disused terminal/terminals, as not used for the location estimation.

Detailed description about selecting nonuse terminals will now be described. It is assumed that the first estimated location of the target terminal10obtained in step S102is positioned at the coordinates (x, y). Furthermore, the anchor terminals20are numbered such that the i-th anchor terminal20is positioned at the coordinates (xi, yi), where i is a natural number.

Now, assuming that the distance between the first estimated location of the target terminal10of interest and the i-th anchor terminal20, that is, the estimated distance Di, is given by an expression [(xi−x)^2+(yi−y)^2]1/2. In addition, the distance between the target terminal10in question and the i-th anchor terminal20estimated from the information transmitted between each anchor terminal20and the target terminal10in question is the expected value Ri.

An example will be described which uses the received signal strengths of the signals transmitted from the anchor terminals20to the target terminals10. Received signal strength is generally in inverse relation to a constant multiplier of the distance. Accordingly, the expected values of the distances, i.e. Ri, between the target terminals10and the anchor terminals20can be calculated from the received signal strengths used for the location estimation.

Hereinafter, the strength of a signal received by one target terminal10from the i-th anchor terminal20, or information similar thereto, is represented by Pi. Furthermore, assume that represented by pi is the received signal strength, or similar information, between the target terminal10and the i-th anchor terminal20, which is expected from the estimated distance Di between these terminals resultant from the position estimation.

The location estimation processor106selects a certain number, described later, of anchor terminals20as nonuse or disused terminals by any of the following selection methods:

#1—select anchor terminals20in the order descending from the largest value of parameter |Di−Ri|;

#2—select anchor terminals20in the order descending from the largest value of parameter |Di−Ri|/Ri;

#3—select anchor terminals20in the order descending from the largest value of parameter |Di—Ri|/Di;

#4—select anchor terminals20in the order descending from the largest value of parameter |Pi−pi|;

#5—select anchor terminals20in the order descending from the largest value of parameter |Pi−pi|/Pi;

#6—select anchor terminals20in the order descending from the largest value of parameter |Pi−pi|/pi;

#7—select anchor terminals20in the order descending from the largest value of parameter Di;

#8—select anchor terminals20in the order descending from the largest value of parameter Ri;

#9—select anchor terminals20in the order descending from the largest or smallest value of parameter Pi; and

#10—select anchor terminals20in the order descending from the largest or smallest value of parameter pi.

The location estimation processor106then determines the certain number of nonuse terminals to be selected by any of the following determining methods 1 to 6. It means-that the estimation processor106determines the number based on at least one of the estimated distance Di and the expected value Ri.

Determining Method 1—always select one or a predetermined number of, anchor terminals20;

Determining Method 2—derive an average with respect to the parameter specified in any of the above selection methods #1 to #10 for all anchor terminals20, and select all anchor terminals20having the value above or below the average derived;

Determining Method 3—derive a variance and a standard deviation with respect to the parameter specified in any of the above selection methods #1 to #10 for all anchor terminals20, and select all anchor terminals20which are determined as having an outlier therefrom;

Determining Method 4—select all anchor terminals20which are above or below a predetermined value in respect of the parameter specified in any of the above selection methods #1 to #10;

Determining Method 5—implement Determining Methods #1 to 4 and, in addition, specify a minimum required number of anchor terminals20to modify the number of anchor terminals20to be selected such that the minimum number of terminals20or more will remain; and

Determining Method 6—implement Determining Methods #1 to #4 and, in addition, specify a minimum required ratio of anchor terminals20to modify the number of anchor terminals20to be selected such that if the number of the anchor terminals, which could derive the information, or all anchor terminals is normalized to be unity, the number of anchor terminals20corresponding to the minimum ratio or more will remain.

Next in the S104, the location estimation processor106deletes the information on the anchor terminals20selected as nonuse terminals from the stored information which is received from the data collector105in step S101. Note that, as an alternative to deleting the information, an identifier, e.g. a flag, may be set to define such a state that the information set with the identifier will not be used in the succeeding processes.

Based on the information on the anchor terminals20excluding one on the nonuse terminals, the location estimation processor106executes a second estimation S105for obtaining a second estimated location of the target terminal10in question.

In the second estimation stage, use is made of information that the target terminal10has received from the anchor terminals20other than the selected nonuse anchor terminals, that is, information about the received power value S1and the transmitting terminal code contained in the location-estimation request signal as well as the locations of the anchor terminals20to obtain the second estimated location of the target terminal10in question. The estimated location may be obtained by any of the methods employed in the first estimation step S102.

The location estimation processor106determines whether or not a prescribed condition, described later, is fulfilled (S106). If any condition is fulfilled, then the processor106finishes the location estimation processing, or otherwise iterates the processes in steps S103to S105. At this time, the processor106uses as the first estimated location the second estimated location of the target terminal10in question obtained in step S105to execute the step S103of selecting nonuse terminals to thereby obtain a second estimated location once again.

The determination in step S106is made on whether or not the prescribed condition is fulfilled according to any of the following Determinations:

#1—finish the procedure if the process in step S106is executed for a predetermined number of times;

#2—store the result of the location estimation carried out last time in step S106, and then finish the procedure if the difference between the previous and current results comes down to a certain value, that is, finish the procedure when the results of the location estimation are determined as having converged;

#3—in addition to the condition in the above Determination #1 or #2, record the time of the first execution of the process in step S106, and finish the procedure when a certain period of time elapses therefrom;

#4—finish the procedure if the existence probability of the target terminal10at the second estimated location becomes lower than the probability at the first estimated location or at the second estimated location obtained in the previous execution of process in step S106. At this time, the estimated location of the target terminal10ultimately determined is where the existence probability is highest, i.e. either the first estimated location or the second estimated location obtained in the previous execution of process in step S106. To this end, the results of calculating estimated location and the existence probability have to be recorded in the location estimation processor106each time the calculation of the estimated location of the target terminal10is implemented. Furthermore, when employing Determination #4, in the step S103of selecting a nonuse anchor terminal, Determining Method #1 is employed so as to select a single nonuse anchor terminal all the time;

#5—alternative to the existence probability used in Determination #4, use a value resultant from dividing the existence probability of the target terminal10at the estimated location by the number of anchor terminals20used for the location estimation; and

#6—determine by any combination of the conditions of Determinations #1 to #5.

In this way, the repetition of the steps S103to S106enables to exclude from the location-estimating calculation the anchor terminals20that have transmitted information including large errors for estimating the distances between the target terminal10and the anchor terminals20, so that high-accuracy estimation can be achieved. The following are examples of situations where such transmitted information includes large errors in an application in which information on a received signal power is transmitted.

Example #1—a situation where a location estimation object attaching the target terminal10is per se an obstacle;

Example 2—a situation where the target terminal10locates with respect to a specific anchor terminal20with an obstacle existing in between, the obstacle being, for instance, a column standing on a location estimation plane; and

Example 3—a situation where the transmission output power of a target terminal10of interest differs from an assumed output power for some cause.

As described, in the illustrative embodiment, one or more anchor terminals20are selected as nonuse terminals depending on a result of a comparison between the estimated distance Di and the expected value Ri, and then the estimation is iterated by using the information on the anchor terminals20other than the information on the nonuse anchor terminals, so that the accuracy of the estimated location can be improved.

In the illustrative embodiment, the selection of nonuse terminal is performed in step S103by using the estimated distance Di and the expected value Ri, but is not limited in scope of the invention in this respect. Alternatively, the system may be adapted such that a defect or malfunction of the anchor terminals20may be determined to thereby define the anchor terminals thus determined defective as nonuse terminals.

The operation of the alternative example will now be described. The location estimation processor106stores earlier results of selection of nonuse terminals obtained in the repetition of the step S103. On the basis of the stored data, if a specific anchor terminal20has been selected as a nonuse terminal continuously or with a predetermined probability or more, then the anchor terminal20in question is determined as a defective terminal. The defective anchor terminal20thus determined is subsequently selected as the nonuse terminal in step S103.

Furthermore, the location estimation processor106can use for the location estimation a plurality of location-estimation request signals derived from the same anchor terminal20. More specifically, information required for the estimation derived from radio signals22transmitted to the target terminal10from the anchor terminal20may be stored to use the stored information to calculate the estimated location of the target terminal10. Taking the method using the received power as an example, the following ways can be applied to the estimation:

Way #1—use the average of the stored received power values; and

Way #2—use the largest one of the stored received power values.

Moreover, a period of time can be used which has elapsed from the time of reception of location-estimation request signal22to the time of execution of location estimation. More specifically, according to the time period elapsing from-the reception of a radio signal22, the stored old information is selected to be used in the location estimation. Such information is used because, considering that the target terminal10moves around, the data with a time elapsing longer is very likely not to reflect the latest information on the location of the target terminal.

Now, reference will be made toFIG. 5schematically showing an alternative embodiment of the location estimation system of the invention. InFIG. 5, the system comprises a location estimator30in addition to the target terminals10, the anchor terminals20. Like components are designated with the same reference numerals, and a repetitive description thereon will be avoided.

Each target terminal10is an object of location estimation and is adapted to transmit a location-estimation request signal32to the anchor terminals20. The anchor terminals20are at known locations and adapted to receive the location-estimation request signal32, and derive information from the received signal32, which is required for estimating the location, such as received signal strength, or received power value. The derived information for the location estimation is transmitted to the location estimator30in the form of location estimation information signal34.

The location estimator30is adapted for estimating the location of each target terminal10using the received information signals34. Note that the dotted arrows32shown in the figure indicate that signals are transmitted to all peripheral terminals without specifying recipients, i.e. broadcast, whereas the solid arrows34represent that signals are directed to a terminal or station, such as the location estimator30, pointed by the arrows, the transmission being of unicast.

FIG. 6schematically shows the configuration of one of the anchor terminals20of the alternative embodiment. The anchor terminals20may be of the same structure as each other, and the anchor terminal20depicted comprises an antenna211, a transmitter212, a transmission data generator213, a receiving circuit214and a received data processor215, which are interconnected as illustrated.

The antenna211is adapted to transmit and receive radio signals34and32, respectively. The antenna211is adapted to input a received radio signal217to the receiving circuit214, which is adapted to convert the signal217to corresponding digital information219to output the information219in the form of received data to the received data processor215. In addition, the receiving circuit214measures a received power value S1of the radio signal32to output information on the measured value S1on its output219to the received data processor215.

The received data processor215processes the received data219input by the receiving circuit214, and functions as instructing, if the received data219is the location-estimation request signal transmitted from the target terminal10, the transmission data generator213to generate transmission data221including the information about the received power value S1measured by the receiving circuit214and a transmitting terminal code contained in the location-estimation request signal.

The transmission data generator213is operative in response to the instruction221by the received data processor215, to produce a location estimation information signal223containing the information of the received power value S1input by the received data processor215and an identification code unique to the terminal20in question to output the thus produced signal223to the transmitter212.

The transmitter212serves to convert the signal223representative of a location-estimation request or location estimation information produced by the transmission data generator213to a corresponding radio signal225to output the radio signal225to the antenna211.

The configuration of the target terminal10of the alternative embodiment may be similar to that of the anchor terminal20of the illustrative embodiment shown inFIG. 2.

The configuration of the location estimator30of the alternative embodiment may be similar to that of the target terminal10of the illustrative embodiment shown inFIG. 3. It is to be noted that the receiver104, the data collector105and the location estimation processor106can be provided separately, and the received data processor103maybe connected to the data collector105over telecommunications.

The operation of the alternative embodiment of the location estimation system thus configured will now be described. As in the case of the illustrative embodiment describe earlier, the alternative embodiment will be described when applied to a location estimation method utilizing the received power values of the radio signals32and34transmitted between the anchor terminals20and the target terminals10. However, since the system of the alternative embodiment comprises the plurality of anchor terminals20and the target terminals10and is operable to estimate the location of a target terminal in some measures, the system is adaptable to any kinds of location estimation system which is capable of estimating the distance between each anchor terminal20and the target terminals10in some measures.

Each target terminal10sends toward all neighboring terminals a location-estimation request signal32including an identification code as a transmitting terminal code unique to the target terminal10.

Each anchor terminal20receives the location-estimation request signal32and measures the received power value S1of the signal. The anchor terminal20in turn outputs the information on the power value S1and the transmitting terminal code along with a location estimation information signal containing its own identification code to the location estimator30in the form of radio signal34.

The location estimator30, in turn, collects and stores the information contained in the location estimation information signals34delivered from the anchor terminals20in every identification code of the target terminals10, thereby estimating, based on the stored information, the location of each target terminal10.

The location estimator30estimates the location of target terminal10in a way similar to the operation carried by the location estimation processor106in the illustrative embodiment shown in and described with reference toFIG. 4. Note that, as the location estimator30estimates the location of the target terminals10separately from each other, the location estimator30may be adapted to perform a simple parallel processing.

As described, since the location estimation operation is not required in each target terminal10in the alternative embodiment, the system can achieve, in addition to the advantages attained in the illustrative embodiment described with reference toFIG. 1, the reduction in size and weight of the target terminal10and also achieve its longer mission time. Furthermore, if several target terminals10exist, the location estimator30can utilize information on these terminals10to thereby estimate the location of these terminals, whereby the accuracy of the estimation can be further improved than in the illustrative embodiment shown inFIG. 1.

As in the case of the illustrative embodiment shown inFIG. 1, defects or malfunctions of the anchor terminals20may be determined in the alternative embodiment to select the anchor terminals20, which are determined defective, as nonuse terminals. In this case, the location estimator30can store information on nonuse terminals each time nonuse terminals are determined so as to select nonuse terminals on the basis of the stored information to estimate the location of any of the target terminals10.

Defective terminals can be determined, for instance, with all target terminals10sharing information on defective terminals with each other, or separately from target terminal to terminal10. The latter way is implemented due to the possibility that the selection of defective terminals might be affected by the position or direction where the target terminals10are installed on targets, or objects.

In the above description, the location estimator30is in direct communication with each anchor terminal20as seen fromFIG. 5. Alternatively, the estimator30can be configured to collect data transmitted by multi-hop transmission via neighboring anchor terminals20or terminals for relaying data.

Furthermore, the location estimation processor106in the location estimator30of the alternative embodiment may be adapted to utilize, as with in the first embodiment shown inFIG. 1, the information derived from the same anchor terminal20in the past to issue a plurality of location-estimation request signals for location estimation.

Another alternative embodiment of the location estimation system is configured to utilize information in a location-estimation request signal sent from one target terminal10to neighboring target terminals10along with the information used in the alternative embodiment shown in and described with reference toFIG. 5.

FIG. 7schematically shows the other alternative embodiment of the location estimation system of the invention. As shown inFIG. 7, the system comprises the target terminals10, the anchor terminals20and the location estimator30. Each target terminal10is an object of location estimation and transmits a location-estimation request signal32. Furthermore, when a target terminal10receives a location-estimation request signal from another target terminal10, the target terminal10in question derives information from the received signal, which is required for location estimation, such as received signal strength, or received power value. The derived information for the location estimation is transmitted to the location estimator30in the form of location estimation information signal36.

The anchor terminals20are at known locations and receive the location-estimation request signal32from the target terminals10to derive information from the received signal32, which is required for estimating the location, such as received signal strength, or received power value. The derived information for the location estimation is transmitted to the location estimator30in the form of location estimation information signal34.

The location estimator30uses the received information signal34and36to estimate the location of each target terminal10. It is to be noted that the dotted arrows32and38shown in the figure indicate that signals are transmitted to all peripheral terminals without specifying recipients (broadcast), whereas the solid arrows34and36represent that signals are meant for a terminal or unit pointed by the arrows (unicast transmission).

The configurations of the target terminal10and the anchor terminal20of the third embodiment shown inFIG. 7may be similar to that of the anchor terminal20of the alternative embodiment shown inFIG. 6. The configuration of the location estimator30may be the same as the target terminal10of the first embodiment shown inFIG. 3. Alternatively, the receiver104, the data collector105and the location estimation processor106may be provided separately from each other, and the received data processor103and the data collector105can be connected to each other through telecommunications.

Next, the operation of the third embodiment with the above configuration will be described with further reference toFIG. 8. Exemplarily, the operation in this embodiment is directed to a location estimation method using received power values of radio signals transmitted between the anchor terminals20and the target terminals10. However, the present invention is directed to the system comprising a plurality of anchor terminals20and the target terminals10and adapted to estimate a location in any measures, the present invention is applicable to any kinds of location estimation system which can estimate a distance between each anchor terminal20and the target terminals10by any methods.

FIG. 8is a flowchart for use in understanding the operation of location estimation. Each target terminal10transmits to all neighboring terminals a location-estimation request signal32or38including a unique identification code of the target terminal10as a transmitting terminal code. The anchor terminals20or target terminals10which receives the location-estimation request signal32or38, in turn, measures the received power value S1of the received signal and transmits the information on the power value S1and the transmitting terminal code along with a location estimation information signal containing its own identification code to the location estimator30in the form of signals34and36, respectively.

The location estimator30receives the location estimation information signal34or36, respectively, from the anchor terminals20or target terminals10and collects information contained in the information signal to store the information in respect of the identification code of the respective target terminals10(S201).

The location estimator30then selects a target terminal to be estimated (S202) and performs, in the same manner as step S102in the first embodiment, a first estimation step S203on the target terminal thus selected for obtaining an estimated location of the target terminal10.

After that, corresponding to steps S103-S106in the first embodiment, the estimator30deletes information on nonuse terminals, obtaining a second estimated location of the target terminal10in question, executing a second estimation step S204to iterate the processing until a certain condition is fulfilled. The estimator30carries out the first and second estimation steps on all target terminals (S205). Consequently, the second estimated locations for all target terminals10are obtained.

Then, the location estimator30selects one target terminal10as an object of the location estimation (S206) and executes a third estimation step S207for obtaining a third estimated location of the selected terminal10. In the third estimation stage, each target terminal10is assumed as an anchor terminal,allocated at the second estimated locations obtained in step S204, thereby obtaining an estimated location of the selected terminal10in the same way as step S102in the first embodiment. More specifically, the location estimation in step S207is carried out by using information, inclusive of information on the anchor terminals20and neighboring target terminals10.

The location estimator30then calculates the estimated distances Di between the target terminal10in question and each anchor terminal20based on the third estimated location while calculating the expected values Ri of the distances between the target terminal10in question and each anchor terminal20based on the information required for the location estimation, and makes a comparison between the estimated distances Di and the expected values Ri to thereby select one or more anchor terminals20as nonuse terminal/terminals. The selection of nonuse terminals is performed in the same way as in step S103of the first embodiment.

After the selection of nonuse terminals, the location estimator30deletes the information on the selected-nonuse anchor terminals20in a manner similar to that described in connection with the step S104of the first embodiment, and based on the information on the anchor terminals20other than those of the nonuse terminal, executes a fourth estimation step S208for obtaining a fourth estimated location of the target terminal10in question. The estimated location herein is obtained in the similar way as the second estimation step S204. The third and fourth estimation steps S207and S208are carried out on all target terminals (S209).

The location estimator30then determines whether or not a prescribed condition is fulfilled (S210). If the condition is fulfilled, the estimator30finishes the location estimation processing, or otherwise iterates the steps S206to S210. At this time, the estimator30performs the fourth estimation step S208by using the fourth estimated location of the target terminal10in question obtained in step S208as third estimated location to thereby obtain the fourth estimated location once again. The determination in step S210on whether or not the prescribed condition is fulfilled is made by the similar determination step S106of the first embodiment.

Upon execution of the third and fourth estimation steps S207and208, the estimated location of the target terminal10is updated from the third estimated location to the fourth estimated location. Since the estimated location of the target terminal10in question is used to estimate the locations of other target terminals10, the update of the estimated location of the target terminal10has effect on the location estimation for the other target terminals10.

Therefore, when the estimated location of the target terminal10is changed due to the update, the location estimation is executed again on another target terminal10. More specifically, the fourth estimated location is used as the third estimated location, and the fourth estimation step S208is iterated on all target terminals10except the target terminal10in question through the steps S206to S210. The continuation of iteration of the fourth estimation step S208is determined either when the results of the location estimation of all target terminals10have converged or depending on whether or not a predetermined number of iteration has been done.

As described, in the third embodiment one of the target terminals10may send out information received from another of the target terminals10to the location estimator30. That enables the system to use the information transmitted between the target terminals10to accomplish the location estimation. Consequently, in addition to the advantages attained in the second embodiment, more accurate estimation can be implemented even where the number of anchor terminals20is small, so that the increase in communications traffic can be reduced. In addition, the location estimation is iterated by using the results of location estimation of neighboring target terminals10, thereby allowing location estimation with less error.

Furthermore, when selecting the anchor terminals20not to be used for the location estimation and the neighboring target terminals10, a target terminal10having a larger estimation error is selected on a priority basis, whereby the accuracy of the estimation can be significantly improved in comparison with a situation where anchor terminals to be used is not reduced.

In the other alternative embodiment described above, as well in the first embodiment, defects or malfunctions of the anchor terminals20may be determined to select the anchor terminals determined defective as nonuse terminals. In this case, the location estimator30stores information on a nonuse terminal each time of its selection. When the first to fourth estimated locations of another target terminal10are obtained and also when nonuse terminals are selected, the nonuse terminals can be selected on the basis of the stored information.

Furthermore, the location estimation processor106of the third embodiment can utilize, as in the case with the first embodiment, the information derived from the same anchor terminal20in the past to form a plurality of location-estimation request signals for the location estimation.

The entire disclosure of Japanese patent application Nos. 2007-45041 and 2007-209375 filed on Feb. 26 and Aug. 10, 2007, including the specifications, claims, accompanying drawings and abstracts of the disclosure, is incorporated herein by reference in its entirety.

While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by the embodiments. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention.