Abstract:
A location sensing system and method for a mobile communication system is provided for maintaining location sensing service with the support of mobile nodes when there is an insufficient number of anchor nodes for reference points. A location sensing method for a mobile communication system includes discovering, at a source mobile node, anchor nodes in a vicinity; discovering, when a number of discovered anchor nodes is less than a predetermined number of reference nodes required for location sensing, neighboring mobile nodes; selecting at least one of neighboring mobile nodes as a reference mobile node; and computing a location of the source mobile node in cooperation with the predetermined number of reference nodes including the discovered anchor nodes and the reference mobile node.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims priority under 35 U.S.C. §119 from an application entitled “LOCATION SENSING SYSTEM AND METHOD FOR MOBILE COMMUNICATION SYSTEM” filed in the Korean Intellectual Property Office on Jan. 29, 2008 and assigned Serial No. 2008-0009077, the contents of which are incorporated herein by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a mobile communication system. More particularly, the present invention relates to a location sensing system and method for a mobile communication system that is capable of providing location sensing service with the support of mobile nodes. 
         [0004]    2. Description of the Related Art 
         [0005]    Location sensing is one of the more important techniques in which recent mobile communication systems operate. Triangulation, scene analysis, and proximity are the three principal techniques for automatic location sensing. 
         [0006]    Of the three techniques, triangulation location sensing technique computes the position of a mobile node by measuring its distance from three anchor nodes. In more detail, the mobile node measures the time-of-arrival to the three anchor nodes and determines the point at which three circles drawn with the radii calculated on the basis of the time-of-arrival as the location of the mobile node. 
         [0007]    With regard to another of the three techniques, the scene analysis location sensing technique utilizes features of a scene sensed from a particular vantage point to draw conclusions about the location of an object in the scene. The scene analysis is categorized into a static scene analysis, which compares the observed image with reference images stored in a database, and a differential scene analysis, which tracks the difference between successive scenes to estimate location. 
         [0008]    Finally, the third technique, a proximity location sensing technique, entails determining when the mobile node is “near” another node. There are three general approaches to sensing proximity: detecting physical contact with an object, monitoring wireless cellular access points which monitoring when a mobile device is in range of one or more access points in a cellular network, and observing automatic ID systems through the operation of various identification systems. 
         [0009]    We refer now to  FIG. 1 , which is a schematic diagram illustrating a conventional triangular location sensing technique. 
         [0010]    The triangular location sensing is known as the most accurate location sensing technique. Referring to the example in  FIG. 1 , a mobile node  100  communicates with three anchor nodes  110 ,  130 , and  150 . 
         [0011]    In order to locate the mobile node  100  with the triangular location sensing technique, at least three anchor nodes have to be within the transmission range of the mobile node  100 . Also, in view of the anchor nodes, the mobile node  100  should be within the transmission ranges of the respective anchor nodes  110 ,  130 , and  150 . 
         [0012]    In other words, if the mobile node  100  is out of the range of any one of three anchor nodes  110 ,  130 , and  150 , it is impossible to locate the mobile node using the triangular location sensing technique. Also, in a case where at least one of three anchor nodes  110 ,  130 , and  150  involved in an ongoing triangular location sensing is temporarily erroneous, the mobile node  100  fails computing its location. 
         [0013]    Accordingly, there is a need for an improved triangular location sensing method that is capable of securing stability and reliability of location service even when the mobile node does not find three anchor nodes or an anchor node involved in the ongoing triangular location sensing process is erroneous. 
       SUMMARY OF THE INVENTION 
       [0014]    The present invention provides a location sensing system and method for a mobile communication system that improves location information reliability. 
         [0015]    Also, the present invention provides a location sensing system and method for a mobile communication system that provides stable location sensing service without support of an anchor node. 
         [0016]    In addition, the present invention provides a location sensing system and method for a mobile communication system that provides location sensing service in cooperation with other mobile nodes without engagement of anchor node. 
         [0017]    Also, the present invention provides a location sensing system and method for a mobile communication system that operates a location sensing process in cooperation with another mobile node which is calculating the location of the mobile node, especially when a number of reference anchor nodes are not enough. 
         [0018]    Moreover, the present invention provides a location sensing system and method for a mobile communication system that improves location sensing accuracy, when performing the location sensing in cooperation with another mobile node, by selecting an optimal mobile node on the basis of sensing information (parameters). 
         [0019]    Additionally, the present invention provides a location sensing system and method for a mobile communication system that selects an optimal mobile node on the basis of standard deviation of location sensing, Received Signal Strength Indication (RSSI) of an anchor node, and acceleration of mobile node. 
         [0020]    In accordance with an exemplary embodiment of the present invention, a location sensing method for a mobile communication system includes discovering, at a source mobile node, anchor nodes in vicinity; discovering, when a number of discovered anchor nodes is less than a predetermined number of reference nodes required for location sensing, neighbor mobile nodes; selecting at least one of neighbor mobile nodes as a reference mobile node; and computing location of the source mobile node in cooperation with the predetermined number of reference nodes including the discovered anchor nodes and the reference mobile node. 
         [0021]    In accordance with another exemplary embodiment of the present invention, a location sensing method for a mobile communication system includes broadcasting, when a source mobile node detects lack of number of anchor nodes, a device discovery request message; receiving one or more device discovery response messages transmitted by at least one neighbor mobile node positioned in a transmission range of the source mobile node; selecting one of the at least one neighbor mobile node as a reference mobile node on the basis of measurement information contained in the device discovery response messages; and computing a location of the source mobile node with support of the reference mobile node. 
         [0022]    In accordance with yet another exemplary embodiment of the present invention, a location sensing method for a mobile communication system includes broadcasting, when a source mobile node detects lack of number of anchor nodes, a device discovery request message; transmitting, at neighbor mobile nodes received the device discovery request message, device discovery response message containing location information and measurement information to the source mobile node; analyzing, at the source mobile node, the location and measurement information contained in the received device discovery response messages; and selecting at least one of the neighbor mobile nodes as a reference mobile node on the basis of analysis result. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The above features and advantages of the present invention will become more apparent from the following detailed description in conjunction with the accompanying drawings, in which: 
           [0024]      FIG. 1  is a schematic diagram illustrating a conventional triangular location sensing technique; 
           [0025]      FIG. 2  is a schematic diagram illustrating a location sensing system according to an exemplary embodiment of the present invention; 
           [0026]      FIG. 3  is a flowchart illustrating a location sensing method according to an exemplary embodiment of the present invention; 
           [0027]      FIG. 4  is a diagram illustrating a triangular calculation process of the location sensing method of  FIG. 3 ; and 
           [0028]      FIG. 5  is a diagram illustrating an expansion of sensing area in a location sensing method according to an exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    Exemplary embodiments of the present invention are described herein below with reference to the accompanying drawings in detail. The same reference numbers are used throughout the drawings to refer to the same or like parts. Detailed description of well-known functions and structures incorporated herein may be omitted to avoid obscuring appreciation of the subject matter of the present invention by a person of ordinary skill in the art with such well-known functions and structures. 
         [0030]    The terms and phrases used in the specification and appended claims are provided to enable a clear and consistent understanding of the detailed description and the claims. Unless otherwise noted, the terms and phrases are to be understood according to conventional usage by those skilled in the relevant art. While the invention is shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. 
         [0031]    The present invention proposes a location sensing system and method for locating a position of a mobile node in a mobile communication system. More particularly, the location sensing system and method enables the mobile node to proceed with an ongoing location sensing process with the support of another mobile node, even when the anchor node involved in the ongoing location sensing process has erroneous results. 
         [0032]    That is, the location sensing system and method of the present invention allows a mobile node to locate its position in cooperation with other mobile nodes. Also, the present invention proposes an algorithm for selecting a reference mobile node representing the erroneous anchor node. The location sensing system and method of the present invention is applied to a mobile communication system comprising a plurality of mobile nodes and anchor nodes. 
         [0033]      FIG. 2  is a schematic diagram illustrating a location sensing system according to an exemplary embodiment of the present invention. In  FIG. 2 , four mobile nodes  205 ,  215 ,  225 , and  235  are distributed in an available sensing range, and a mobile node  245  is out of the sensing area. Here, the sensing area is defined by the overlapping transmission ranges of three anchor nodes  210 ,  230 , and  250 . 
         [0034]    Still referring to  FIG. 2 , each of the mobile nodes  205 ,  215 ,  225 , and  235  positioned in the sensing area discovers the anchor nodes  210 ,  230 , and  250 , measures the distances from the respective anchor nodes  210 ,  230 , and  250 , and computes their respective location on the basis of the distances. The location is computed using the above-mentioned triangular location sensing technique. 
         [0035]    With continued reference to  FIG. 2 , one of the mobile nodes  205 ,  215 ,  225 , and  235  may move out of the sensing area, or at least one of the three anchor nodes  210 ,  230 , and  250  happens to be in an erroneous state of operation, resulting in an absence of enough number of anchor nodes. In this particular case, the mobile node detects the unreachable or erroneous anchor node and removes the anchor node from an anchor list with which each mobile node manages the reachable anchor nodes. In this exemplary embodiment, the mobile node-supportive location sensing method is described with an example in that the mobile node  245  moves out of the sensing area. 
         [0036]    The mobile node  245  moved out of the sensing area detects that the anchor node  230  is unreachable, and removes the anchor node  230  from its anchor list. Since at least three anchor nodes are required for the mobile node  245  to locate itself, the mobile node  245  searches for another anchor node. 
         [0037]    If no other anchor node has been found, the mobile node  245  broadcasts a Device Discovery Request message such that the neighbor mobile nodes  205 ,  215 ,  225 , and  235  receive the Device Discovery Request message. That is, the mobile node  245  can check the existence of the neighbor mobile nodes  205 ,  215 ,  225 , and  235  by broadcasting the Device Discovery, Request message. 
         [0038]    If the Device Discovery Request message has been received, each of the mobile nodes  205 ,  215 ,  225 , and  235  transmits a Device Discovery Response message to the mobile node  245 . The Device Discovery Response message contains information including position of the neighbor mobile node and the following parameters: 
         [0039]    (1) Standard deviation of position measurements 
         [0040]    (2) Standard deviation of RSSIs from the anchor node 
         [0041]    (3) Average speed of the mobile node 
         [0042]    (4) Acceleration sensed by acceleration sensor of the mobile node The parameters can be selectively contained in the Device Discovery Response message. In order to improve the location sensing accuracy, further parameters can be added. 
         [0043]    After receiving the Device Discovery Response messages from the neighbor mobile nodes  205 ,  215 ,  225 , and  235 , the mobile node  245  selects the neighbor mobile node of which parameters have the least difference in values of the parameters (1)-(4), as an optimal reference mobile node. Since the neighbor mobile node of which standard deviation of positions measurements, average speed, and acceleration are smaller than those of other neighbor mobile nodes is considered as the node which moves least, it is possible to reduce the location computation error caused by movement. In this exemplary embodiment, the mobile node  215  is selected as the reference mobile node. 
         [0044]    Once the reference mobile node is selected, the mobile node  245  can compute its location with the support of the two anchor node  210  and  250  and the reference mobile node  215 . That is, the location sensing system according to this exemplary embodiment of the present invention enables providing the location sensing service even when the number of available anchor nodes is not enough. 
         [0045]    The operation of the above-structured location sensing system is described hereinafter in more detail. However, the present invention is not limited to the following exemplary embodiments, but can be applied to various exemplary embodiments of triangular location sensing methods, for example. 
         [0046]      FIG. 3  is a flowchart illustrating a location sensing method according to an exemplary embodiment of the present invention. The location sensing method according to this exemplary embodiment is accomplished on the basis of a triangular location sensing technique in consideration of a lack of anchor nodes. 
         [0047]    Referring now to  FIG. 3 , a mobile node searches for anchor nodes around its vicinity (S 301 ) by broadcasting an Anchor Discovery Request message. The mobile node recognizes the existence of the anchor nodes in its transmission range (neighbor anchor nodes) with by receiving Anchor Discovery Response messages transmitted to the mobile node in response to broadcast of the Anchor Discovery Request message. 
         [0048]    After checking the existence of the neighbor anchor nodes, the mobile node determines whether a number of neighbor anchor nodes is equal to or greater than 3 (S 303 ). This is because the triangular location sensing requires three reference points. 
         [0049]    If the number of neighbor anchor nodes is equal to or greater than 3, the mobile node measures its distances from three reference nodes, i.e. the neighbor anchor nodes (S 309 ), and computes its location on the basis of the distances (S 311 ). 
         [0050]    Referring back to (S 303 ), if the number of neighbor anchor nodes is not equal to or greater than 3, at step (S 305 ) the mobile node broadcasts a Device Discovery Request message to discover neighbor mobile nodes (S 305 ). The mobile node can check the neighbor nodes on the basis of Device Discovery Response messages received in response to the Device Discovery Request message. 
         [0051]    Next, the mobile node selects from the response to the Device Discovery Request Message at least one of the discovered neighbor mobile nodes to become a reference mobile node on the basis of parameters extracted from the Device Discovery Response messages (S 307 ). The number of reference mobile nodes can be changed according to the number of neighbor anchor nodes discovered at step S 303 . For example, two reference mobile nodes are required when one anchor node is discovered, and three reference mobile nodes are required when no neighbor anchor node is discovered. 
         [0052]    As described above, the mobile node can select the reference mobile node on the basis of at least one of the parameters contained in the Device Discovery Response messages. 
         [0053]    Still referring to  FIG. 3 , the mobile node computes its location in cooperation with three reference nodes including the discovered anchor nodes and the reference mobile nodes. That is, the mobile node calculates the distances from the three reference nodes (S 309 ) and computes its location using the calculated distances (S 311 ). 
         [0054]      FIG. 4  is a diagram illustrating a triangular calculation process of the location sensing method of  FIG. 3 . In  FIG. 4 , a mobile node  445  moved out of a sensing area, which is defined by overlapping of the transmission ranges of three anchor node  410 ,  430 , and  450 , and the mobile node  445  calculates its location in cooperation with three reference nodes including two anchor nodes  410  and  450  and another mobile node  405 . The mobile node  405  was typically selected in accordance with the Device Discovery Request as discussed above. 
         [0055]    Referring to  FIGS. 3 and 4 , the mobile node  445  moved out of the sensing area broadcasts an Anchor Recovery Request message such that the two anchor nodes  410  and  450  located in the transmission range of the mobile node  445  can receive the Anchor Recovery Request message and transmit the Anchor Recovery Response messages to the mobile node  445 . At this time, since the anchor node  430  is out of the transmission range of the mobile node  445 , it cannot receive the Anchor Recovery Request message transmitted by the mobile node  445 . 
         [0056]    Since the two anchor nodes  410  and  450  are recovered, the mobile node  445  needs one more reference node for performing triangular calculation. In this case, the mobile node  445  broadcasts a Device Discovery Request message to discover another mobile node as a reference node. In response to the Device Discovery Request message, neighbor mobile nodes  405 ,  415 , and  425  located in the transmission range of the mobile node  445  transmit Device Discovery Response messages to the mobile node  445  such that the mobile node  445  recognizes the existence of the neighbor mobile nodes  405 ,  415 , and  425 . Since the mobile node  435  is located outside of the transmission range of the mobile node  445 , mobile node  435  does not receive the Device Discovery Request message from mobile node  445 . 
         [0057]    The mobile node  445  analyzes the Device Discovery Response messages transmitted by the neighbor mobile nodes  405 ,  415 , and  425 , and selects on the neighbor mobile nodes  405 ,  415 , and  425  as the reference mobile node on the basis of the analysis result. In more detail, the mobile node  445  compares the values of at least one of parameters contained in the Device Discovery response messages and selects the neighbor mobile node of which parameter value is least as the reference mobile node. In  FIG. 4 , the neighbor mobile node  405  is selected as the reference mobile node. 
         [0058]    After selecting the reference mobile node, the mobile node  445  then computes its location in cooperation with the three reference nodes, i.e. the two anchor nodes  410  and  450  and the reference mobile node  405  selected in the preceding paragraph. There can be additional advantages by simultaneous transmission in terms of the time for the entire operation being reduced. 
         [0059]    In the above description, the anchor node discovery process and neighbor mobile node discovery process are performed separately. That is, the mobile node discovers the anchor nodes by broadcasting the Anchor Discovery Request message and discovers the neighbor mobile nodes by broadcasting the Device Discover Request message. 
         [0060]    However, contrary to the above, it is also within the spirit and scope of the claimed invention that the anchor node discovery and mobile node discovery can be performed at the same time by broadcasting a combined device discovery request message. In this case, the mobile node computes, if at least three anchor nodes have been discovered, its location with the support of the three discovered anchor nodes. Otherwise, if less than three anchor nodes have been discovered, the mobile node selects at least one reference mobile node through the neighbor mobile node discovery process and computes its location with the support of the discovered anchor node and the at least one reference mobile node. 
         [0061]    In the particular case of the use of the combined device discovery request message, a threshold of a specific parameter can be configured such that the mobile node receives the device discovery response messages of which parameter values are greater than the threshold. For example, the specific parameter may comprise, for example, the distance between the mobile node and the neighbor mobile node, and the threshold distance can be set to 3 m such that the neighbor mobile nodes in the range of 3 m responses to the device discovery request message transmitted by the mobile node. In this manner, it is possible to reduce contention for transmitting the device discovery response message between the neighbor mobile nodes and message processing load of the mobile node. 
         [0062]    In another exemplary embodiment of the present invention, neighbor nodes (including anchor nodes and mobile nodes) overhear the device discovery messages to acquire the information about its neighbor nodes. In this case, a specific mobile node requests distance measurement using a first packet. At this time, the mobile node sends the standard deviation of its location measured until then together with the distance measurement request message, and other neighbor nodes overhear the distance measurement request message to acquire use the information from the distance measurement request message. That is, when a specific mobile node is required to send a device discovery request message, the mobile node generates the device discovery message with reference to the information acquired by overhearing the device discovery messages exchanged between other mobile nodes. 
         [0063]      FIG. 5  is a diagram illustrating an expansion of sensing area in a location sensing method according to an exemplary embodiment of the present invention. In  FIG. 5 , the sensing area is defined, for this example, by the transmission ranges of 6 anchor nodes. 
         [0064]    Referring to  FIG. 5 , if location sensing is available in the sensing area  500  depicted as a rectangular region and the transmission range of each anchor node is 50 m, the size of the rectangular region is 3872 m 2  (88 m×44 m), and sensing area including the area surrounding the rectangular region becomes 5401 m 2 . 
         [0065]    In such an exemplary environment, adding 1 mobile node to the sensing area increases the available sensing area to 8118 m 2  in average, which is 1.5 times larger than before adding the mobile node. In a case of adding 2 mobile nodes to the sensing area, the available sensing area increases to 10388 m 2  in average, which is 1.9 times larger than before adding the mobile nodes. 
         [0066]    As described above, the location sensing system and method of the present invention expands the sensing area by temporarily using the mobile node as a reference point for triangular location sensing when there are less than three anchor nodes within range. The location sensing method of the present invention can be used for expanding the location sensing service area by installing a mobile node having no mobility in an area requiring a location sensing service with the least number of anchor nodes. 
         [0067]    Also, the location sensing system and method of the present invention enables a mobile node to compute its location through the triangular location sensing technique with three reference points, including at least one mobile node providing one of the reference points even when the anchor nodes are erroneous, resulting in improvement of location sensing reliability. 
         [0068]    As described above, the location sensing system and method for a mobile communication system according to the present invention expands available sensing area in which a mobile node can compute its location by using neighbor mobile nodes, as reference points for triangular location sensing technique, as well as fixed anchor nodes. The present invention can be implemented with the fewest number of anchor nodes and still provide support of mobile nodes installed in shadow areas, resulting in reduction of system implementation and maintenance costs. The present invention allows mobile nodes to act as reference points for triangular location sensing especially when at least one anchor node has erroneous operations, resulting in improvement of location sensing service reliability. 
         [0069]    Although exemplary embodiments of the present invention have been described in detail hereinabove, it should be clearly understood that many variations and/or modifications of the basic inventive concepts herein taught which may appear to those skilled in the present art will still fall within the spirit and scope of the present invention, as defined in the appended claims.