Abstract:
A system and method selects secondary base stations to task that provide geo-location information used to geo-locate a mobile station. A network overlay location system may be co-located at base station sites of a wireless communications network. The location measurement units of the network overlay system provide location information of the mobile station, which may be determined via one or more geo-location techniques, for example: time of arrival (TOA), time difference of arrival (TDOA), uplink time difference of arrival (UTDOA), angle of arrival (AOA), signal power, radio fingerprinting etc. Selecting secondary base stations to task using various selection criteria may provide optimum results when geo-locating a mobile station.

Description:
BACKGROUND 
     The use of wireless communication devices such as telephones, pagers, personal digital assistants, laptop computers, etc., hereinafter referred to collectively as “mobile appliances” or “mobiles stations” has become prevalent in today&#39;s society. In recent years, at the urging of public safety groups, there has been increased interest in technology which can determine the geographic position or “geo-locate” a mobile station in certain circumstances. 
     In the United States, mobile station locating equipment is being deployed for the purpose of locating wireless callers who dial 911. Other services in addition to emergency call servicing are contemplated and are referred to as location based services (LBS). Wireless location equipment is typically employed as an overlay to wireless communication networks, thus forming a network overlay geo-location system. 
       FIG. 1  illustrates a wireless communications network ( 1 ), which typically includes a plurality of base stations (BS)  10   a ,  10   b ,  10   c  and  10   d  ( 10 ( a - d )) a mobile station (MS)  20 , and a mobile switching center (MSC)  30 , also known as a base station controller (BSC). The wireless communication network allows communication between the MS and the BS over an air interface as is well known in the art. 
     The network overlay geo-location system is designed to work with the wireless communications network, and is made up of a plurality of local measurement units (LMU)  15 , also known as wireless location sensors (WLS), a geo-location system controller (GCS)  35 , also known as a serving mobile location center (SMLC) and a mobile positioning center (MPC)  45 , also known as a gateway mobile location center (GMLC). 
     The LMUs  15  of the geo-location network overlay system are generally co-located at the wireless service provider BSs  10 ( a - d ). The LMUs  15  conduct measurements on radio signal emitted by the MS  20  under control of the GCS  35 . The GCS  35  is commanded by the MPC  45  to determine and report locations of specific MSs  20 . 
     The GCS  35  is responsible for processing sensor information gathered from the LMUs  15 . A database  55  stores the collected sensor information which is accessible by the GCS  35 . When a location request is received, the mobile network provider will notify the GCS  35  as to which BS  10 ( a - d ) and/or respective BS sector (not shown) is serving a current call of a MS  20 . 
     Most network overlay systems include a number of non-serving BS sites  10 ( b - d ) located around the vicinity of the serving BS site  10 ( a ), which are termed “secondary sites” and the corresponding sectors of the secondary sites are called “secondary sectors” (not shown). For this example, the secondary sites  10 ( b - d ) are the sites located furthest from the MS  20 , and the serving site  10 ( a ) is located closest to the MS  20 . The LMUs  15  co-located with the secondary sites  10 ( b - d ) return location measurement information that enables the GCS  35  to compute the location of a caller/MS  20 . 
     In operation, network overlay systems conduct measurements of radio frequency (RF) transmissions from the MS  20  and received by BSs  10 ( a - d ), and estimate the location of the MS  20 . The geographic location of the BSs  10 ( a - d ) are known, and the location of the MS  20  with respect to the BSs  10 ( a - d ) is represented by a reference position location which is an estimated location of the MS  20  that enables the geographic location of the MS  20  to be determined. 
     To geo-locate MS  20 , one or more of the BSs  10 ( a - d ) perform radio frequency measurements on the received signals transmitted from MS  20 . Some techniques used to perform geo-location from signal measurements can include, for example: time of arrival (TOA), time difference of arrival (TDOA), uplink time difference of arrival (UTDOA), the angle of arrival (AOA), the signal power, or the unique/repeatable radio propagation path (radio fingerprinting). In addition, the geo-location systems can also use collateral information (e.g., locations of roads, dead-reckoning, topography, map matching etc.) to assist in geo-locating the MS  20 . 
     UTDOA is a network-based geo-location algorithm that determines a MS&#39;s  20  position by calculating the difference in time required for a MS&#39;s  20  signal to arrive at different BSs  10 ( a - d ). The arrival time measurements are performed by the LMUs  15  installed at selected BSs  10 ( a - d ). UTDOA requires the target MS&#39;s  20  signal be measured by at least three LMUs  15 . These measurements are sent to the GCS  35 , which computes the location of the target MS  20  using the time difference of arrival of the RF signal transmitted from the MS  20  to the BSs  10 ( a - d ). 
     AOA is another network-based geo-location algorithm that determines the MS&#39;s  20  position by establishing lines of bearing from the BSs  10 ( a - d ) to the MS  20 . AOA requires the installation of a LMU  15  at each AOA-equipped BS, and also requires direction-sensing antennas. The AOA algorithm searches for the target MS  20  on a serving sector of a serving BS  10   a , and on secondary sectors of secondary sites  10 ( b - d ). The likelihood of obtaining an accurate location of the MS  20  using the AOA algorithm depends to a large extent on the correct choice of secondary sectors for the secondary sites  10 ( b - d ). AOA is normally deployed in especially challenging geographic circumstances. For example, AOA is sometimes used in mountainous areas with sparse BS installations and challenging BS site-to-site geometry. 
     In addition to optimum selection of LMUs  15  at secondary site locations, it is important to have optimum LMU  15  coverage of the MS  20 . Optimum coverage, or “good coverage”, illustrated in  FIG. 1 , would be realized by LMUs  15  surrounding the MS  20  from all sides and being physically close enough to receive accurate signal timing measurements. Geo-location accuracy can suffer from geometric dilution of precision, or “poor coverage” when the sensors are all located in roughly the same area and direction with respect to the MS  20 , as illustrated in  FIG. 2 . Selecting a reference position as an estimated location of the MS  20  prior to selecting a secondary base station to task may provide optimum secondary site selection. 
     In one embodiment of applicant&#39;s disclosure, it may be desirable to select a secondary BS site to task by determining a reference position within the geographical area of the serving sector before selecting a secondary BS site. In another embodiment of applicant&#39;s disclosure, it may be desirable to select a secondary BS site to task by determining a reference position to be the location of the serving BS site. In yet another embodiment of applicant&#39;s disclosure, it may be desirable to select a secondary BS site to task based on a function of the reference position. In still another embodiment of applicant&#39;s disclosure, it may be desirable to select a secondary BS site to task based on a preferred parameter that cannot be produced by the other secondary sites. 
     In another embodiment of applicant&#39;s disclosure, it may be desirable to select a secondary BS site to task by computing a distance range to one or more secondary sites, omitting the secondary sites outside the distance range as potential secondary site candidates, computing the bearing angle from at least one secondary site to the reference position, determining which secondary sites do not have at least one sector beam that covers the reference position, omitting the secondary sites that do not have at least one sector beam that covers the reference position as potential secondary site candidates, and flagging the secondary site that is closest to the mobile station reference position as a selected secondary site. 
     Accurate geo-location performance is predicated on optimum selection and tasking of LMUs  15  at secondary sites  10 ( a - d ) and their respective sectors. Optimum secondary site and/or secondary sector selection may provide superior geo-location performance results. Enabling the GCS  35  to select and task optimum secondary sites and/or secondary sectors may provide geo-location information necessary to estimate the location of the MS  20 . 
     These and other advantages of the disclosed subject matter over the prior art will be readily apparent to one skilled in the art to which the disclosure pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a wireless communications network and network overlay system. 
         FIG. 2  illustrates optimum LMU coverage of a MS in a wireless communications network environment. 
         FIG. 3  illustrates poor LMU coverage of a MS in a wireless communications network environment. 
         FIG. 4  illustrates a reference position selected as a function of inter-site distance. 
         FIG. 5  illustrates a wireless communications network where the reference position is selected to be the location of the serving site. 
         FIG. 6  illustrates a wireless communications network where the reference position is selected to be outside the location of the serving site. 
         FIG. 7  illustrates a flow diagram of a process to geo-locate a MS. 
         FIG. 8  illustrates a flow diagram of another process to geo-locate a MS. 
         FIG. 9  illustrates a flow diagram of a process to select secondary sites. 
         FIG. 10  illustrates a flow diagram of a process to calculate a cost function to select secondary sites. 
     
    
    
     DETAILED DESCRIPTION 
     A network overlay geo-location system is described herein that utilizes one or more algorithms to select secondary base station (BS) sites and/or secondary BS site sectors to task in order to obtain optimum geo-location information. The secondary sites are BS sites other than the serving BS site of a mobile station (MS). The location of the secondary sites is an important consideration when selecting secondary sites to task in order to geo-locate the MS. The geo-location information obtained by secondary sites may provide the network overlay geo-location system with the capability to geo-locate a MS operating within a mobile communications network. 
     Some examples of conventional geo-location methods include AOA, TDOA, UTDOA etc., and may include other methods not specifically described in this detailed description. The accuracy of the information used for geo-location is important to the success of locating a MS. Algorithms can be used to determine optimum secondary sites and/or sectors to task to provide geo-location measurement information to locate the MS. The secondary sites may be selected with respect to a mobile station reference position located generally within the communication area of the serving site. 
     Geo-location of a MS, in accordance with one or more embodiments described herein, can be divided into three parts. First, the MS reference position is selected as the estimated location of the MS within the communication area of the serving site. Second, the secondary site(s) and their respective sector(s) are selected to provide optimum geo-location information based on their relative locations and positions with respect to the MS reference position. Third, the secondary sectors of the selected secondary sites are tasked to obtain and provide the geo-location information of the MS to the GCS so the estimated location of the MS may be calculated. 
     The reference position of the MS may be selected a number of different ways. One option is to designate the location of the serving site as the MS reference position, as illustrated in  FIG. 5 . Once the serving site of the MS is known, it can be assumed that the MS is located within the communication range of the serving site. Therefore, the location of the serving site serves as an estimate of the MS&#39;s position regardless of the actual position of the MS. It is not necessary to know the serving sector of the serving site when designating the serving site as the MS reference position. 
     Another option is to designate the reference position a predetermined distance away from the serving site location.  FIG. 4  illustrates an example of a reference position located “X” meters on a heading of the serving sector of the serving site BS 1 . Referring to  FIG. 4 , a two-site configuration is illustrated having a reference position (denoted by the circle) located X meters from the serving site BS 1 . The serving sector should be known before the reference position can be designated on a heading of the serving site BS 1 . 
       FIG. 4  illustrates the reference position X located closer to BS 1  than BS 2 , and inside the serving sector of the serving site BS 1 . The reference position represents an estimated position for the MS, which is operating within the coverage area of its serving sector. In this example, the reference position is designated as twenty percent of the average inter-site distance on the heading of the serving sector. The average inter-site distance is defined as the average of the distances from the closest site to each of the sites in a given PSAP (Public Safety Access Position) boundary. 
     In the example shown in  FIG. 4 , the reference position is selected to be approximately twenty percent of the inter-site distance. Other percentages of the inter-site distance may be used to estimate the location of the MS. The dotted line along the center of BS 1 &#39;s sector coverage indicates its heading. The lines drawn on both sides of the heading of BS 1  show the edge of coverage of its serving sector. In  FIG. 4 , the mobile reference position is shown to be X meters on the heading of the serving sector, where X is twenty percent of the distance between BS 1  and BS 2 . 
     The reference position selected as a function of inter-site distance, as illustrated in  FIG. 4 , may prove to be more accurate when used in a secondary site and/or secondary sector selection algorithm as opposed to having BS 1  as the reference position. For example, if BS 1  and BS 2  were farther apart than what is shown in the  FIG. 4 , then the reference position selected as a function of twenty percent the inter-site distance, would move further into the serving sector of BS 1 . Having the reference position selected as a function of inter-site distance provides a dynamic approach to accurately selecting secondary sites and sectors to task. The MS reference position can be calculated using a variety of other different methods not described herein for convenience. 
     Implementing an adjustable MS reference position provides a way to estimate the location of the MS as a function of site and/or sector location. Both of the MS reference position examples discussed above have proven to be useful ways to estimate the location of the MS for geo-location purposes. Certain factors, such as, secondary site/sector geometry, serving site/sector location and other factors play a role in selecting which reference position determination technique to use. 
     Once the reference position is selected, the secondary site(s) and/or sector(s) can be selected and tasked to provide geo-location information to locate the MS. Secondary site selection can be accomplished a number of different ways. Regardless of what type of analysis is done to select secondary sites, the selected reference position provides a starting position to determine which secondary sites are in a position to provide optimum geo-location information. 
     One way to select secondary sites is to compute the distance range to the available LMUs co-located at BSs (i.e., potential secondary sites) from the MS reference position. Once the distances between the secondary sites and the reference position are known, the BSs whose distance from the reference position exceeds a predetermined distance threshold can be omitted. The predetermined distance threshold may be based on a known practical coverage distance capable of receiving accurate geo-location information. The predetermined distance may also be adjustable to accommodate different types of networks or geographical areas etc. 
     To further limit the list of potential secondary site candidates, further calculations may be necessary. For example, it may be useful to calculate the bearing angle from the remaining secondary BS candidates to the MS reference position (i.e., 180 degrees minus the bearing angle from the MS to the secondary BS) for each secondary site candidate, and check if the MS is within any sector beam of the secondary BS. If the MS is not within any sector beam of the secondary sites, then those secondary sites may be omitted as possible secondary site candidates. Calculating the bearing angle of the secondary sites is done to avoid tasking any secondary sites having sectorized antenna beams outside the location of the reference position. For example, when a site does not have 360 degree antenna coverage its antenna coverage may not include the reference position, in which case it may be omitted from the list of potential secondary site candidates. 
     An additional secondary site selection criteria may include selecting the secondary site that is closest to the MS reference position as a selected site. The selection of the closest site may be performed first, or after one or more of the distance range and the bearing angle position tests are performed, as described above. Once the secondary site that is closest to the MS reference position is determined, the site can be flagged so it is no longer considered to be an available secondary site candidate. 
     Once a potential secondary site candidate is selected, a new look direction B L  can be calculated based on the previously selected secondary site. The look direction B L  is calculated by determining the bearing angle from the MS reference position to the previously selected secondary site, and adding 180 degrees to the computed bearing angle. The new look direction B L  can act as a directional guide when selecting subsequent secondary sites. For example, once a given area is covered by a selected secondary site, it would be prudent to select a secondary site and/or sector located outside the general coverage area of the previously selected secondary site. 
     An additional secondary site selection criteria may include computing a cost function for one or more of the candidate secondary sites. The cost function may be calculated for those candidate sites that remain after one or more of the distance range, bearing angle position and/or the closest site tests are performed. The cost function may be a function of the distance from the MS reference position to the secondary site candidate, and the site&#39;s azimuth bearing angle difference from the look direction bearing angle. The cost function is the sum of the distance cost component and the bearing (angular) cost component:
 
COST=COST Distance +COST Bearing  
 
     The bearing cost is defined as the absolute value of the angular difference in degrees between the bearing angle, from the MS reference position to the secondary site candidate, and the look direction bearing angle:
 
COST Bearing   ==|B   L   −B   MS     —     BS |
 
     The distance cost may be computed different ways depending on the distance from the MS reference position to the secondary site candidate. Two different distance functions may be used so that candidate sites that are very close to the MS reference position are more likely to be chosen than would be the case with a single distance cost function. For sites that are inside a configurable distance cutoff (D cutoff ) radius from the MS reference position, the distance cost is:
 
COST Distance   ==W ( D   MS     —     BS   F )
 
     where D MS     —     BS  is the distance from the MS reference position to the candidate site in kilometers, F is an exponent that controls how steep the distance cost penalty increases inside the cutoff radius, and W is a weighting factor that controls the relative cost between distance and bearing angle costs. 
     For sites outside the distance cutoff radius, the distance cost function increases linearly with distance and is equal to:
 
COST Distance   ==WD   Ms     —     BS   +C  
 
     where the constant term (C) is equal to the distance cost evaluated at the distance cutoff radius. The constant term is necessary so that the overall distance cost is monotonically increasing with distance. 
     Once the cost function has been calculated, the secondary site candidate with the minimum total cost can be flagged so it is no longer considered to be an available secondary site candidate. 
     Once a new secondary site is selected a new look direction bearing angle (B L ) can be selected. The new look direction B L  can be calculated to bisect the largest uncovered sector (i.e. largest “pie slice” between secondary sites), as described below. First, the bearing angles from the MS reference position to the flagged secondary sites should be listed in ascending order. Next, the difference between adjacent bearing angles is calculated, and the largest difference is selected. The look direction B L  is measured in the direction that bisects the selected difference. Once the new look direction is calculated, the cost function calculation for the remaining secondary site candidates may be repeated. The process of calculating a new look direction and computing a cost function is recursive and may be repeated until the desired number of secondary sites have been selected, or no candidates remain. If for a particular look direction there are no secondary site candidates located in the uncovered sector, this look direction can be skipped and the next largest uncovered sector may be selected to obtain the new look direction. 
     As the secondary site selection algorithm proceeds, a situation may occur in which a secondary site candidate would be selected that is “geometrically redundant” with a previously chosen secondary site (i.e., it is in roughly the same direction as a previously chosen site but farther away). To prevent selection of geometrically redundant secondary sites, a bearing angle “buffer zone” may be defined around each previously selected secondary site such that no new secondary sites may be chosen if they are within +/−N degrees (typically 5 degrees) of the bearing angle from the secondary site to the MS reference position. 
     After the secondary sites and their respective sectors have been selected and identified as candidates for providing geo-location information, the optimum sectors of the selected sites can be tasked to obtain and provide the geo-location information of the MS to the GCS. 
       FIG. 5  illustrates a configuration of four BSs labeled BS 1 , BS 2 , BS 3  and BS 4 , where BS 1  is the serving site and BS 2 -BS 4  represent secondary sites. The three lines extending from BS 1  represent the edges of coverage for its respective alpha, beta and gamma sectors. The shorter line in the center of the gamma sector of BS 1  represents the heading of the gamma sector. The configuration illustrated in  FIG. 5  can be classified as “poor geometry” since the x-coordinates of BS 1 -BS 4  are close together. If either the x or the y coordinates of the BSs are close together, it is evident that the sites are located in a straight line, which indicates poor geometry. 
     As stated previously, an AOA geo-location configuration may be used to geo-locate a MS. An AOA geo-location configuration may include AOA antennas at each sector having approximately 120 degree coverage per sector. It is important that the correct sector of an AOA configuration is selected to be tasked or the AOA measurements may be inaccurate. 
     In TDOA geo-location systems, poor geometry can be characterized as horizontal dilution of precision (HDOP). If, however, the sensors of the LMUs co-located at the secondary sites surround the target MS from all sides, the coverage is considered good and dilution of precision is low. 
     The average inter-site distance is another factor that could directly affect the accuracy of geo-location information provided by secondary sites. For example, as the average inter-site distance increases, the GCS has to determine which sectors of secondary stations to task. 
     In one mode of operation, the GCS uses the serving site location as the MS reference position and selects one or more of the secondary sectors of the respective secondary sites enclosing the reference position as the sector(s)-to-task. For example, referring to  FIG. 5 , BS 1  is mostly enclosed by the alpha sector of BS 4 , so the GCS may decide to task the alpha sector of BS 4 . Following this approach to tasking secondary sectors, the GCS considers the available secondary sensor LMU sites co-located at the secondary sites and comes up with a list of sectors to task for the available secondary sites. Note that when the serving site acts as the reference position, the GCS is not required to know the serving site&#39;s serving sector when deciding which secondary sectors to task. 
     The above described method would most likely operate successfully if the mobile was indeed located geographically somewhere on the serving sector (alpha sector) of BS 1 . If, however, the MS was actually located geographically on the gamma sector of BS 1 , then the GCS would still select to task the alpha sector of BS 4  because the reference position is BS 1 , which is located in the alpha sector of BS 4 . 
     The AOA algorithm will search for the target within +/−60 degrees of the tasked sector heading. For the above example, the algorithm will search for the mobile on the alpha sector. Since that is the wrong sector, the directional antennas may receive the target MS&#39;s signal at a reduced level, if at all. As a result, the accuracy of the AOA algorithm will be degraded, which results in the final estimated location of the MS to be inaccurate. 
     In another mode of operation of one or more of the embodiments described herein, the fact that the GCS can obtain knowledge of the serving sector is exploited when it attempts to select optimum secondary sectors to task. Instead of using the serving site location as the location of the MS reference position, the GCS puts out the MS reference position “X” meters on the heading of the serving sector and uses this location to find the secondary sectors to task. 
     Consider the above-mentioned scenario where the MS is located geographically on and is served by the gamma sector of BS 1 , as illustrated in  FIG. 6 . The GCS in this example operates as though the MS reference position is located “X” meters on the heading of the serving sector. The network in  FIG. 6  illustrates the location of the MS reference position is part of the gamma sector of BS 4 , so the GCS may select the gamma sector as the sector-to-task for secondary BS 4 . Placing the MS reference position “X” meters on the heading of the serving sector provides the correct sector selection of the secondary sites. In the above example, the GCS was able to select a secondary sector better suited to provide accurate geo-location information. 
       FIG. 7  illustrates a flow diagram ( 1 A) of a process to geo-locate a MS. Initially, a location request to geo-locate a MS is received at one or more BSs and/or the GCS of the mobile communications network (operation  10 ). The GCS, for example, may then determine the serving site and/or the serving sector of the MS (operation  20 ). The secondary sites can then be tasked to provide location information location, which may be used to geo-locate the MS (operation  30 ). The location information is computed by the GCS to determine the location of the MS (operation  40 ). 
       FIG. 8  illustrates a flow diagram ( 1 B) of another process to geo-locate a MS. An initial geometry determination may be performed by the GCS to determine whether to use the serving site location or a heading position of the serving site as the MS reference position (operation  100 ). If, for example, the secondary sites are located around the serving site then the geometry may be “Good”. Conversely, if the secondary sites are not located around the serving site (i.e., the secondary sites share common X or Y-coordinates) then the geography may be “Poor”. Once the reference position is determined (operation  200  or  250 ), one or more secondary sites can be selected based on their relative positions with respect to the reference position (operation  300 ). The sector(s) of the selected secondary sites may be selected (operation  400 ) and tasked to obtain and provide location information received from the MS (operation  500 ). 
       FIG. 9  illustrates a flow diagram ( 1 C) of a process to select secondary sites. First, a reference position is calculated to provide a location to use as a reference to select secondary sites (operation  310 ). A range may be computed to the available LMS sites from the MS reference position (operation  320 ). A threshold distance (operation  330 ) may be used to omit sites that are too far from the reference position (operation  340 ), and consider those sites that are within the distance range as potential secondary sites. A bearing angle may be calculated (operation  350 ) to determine which sites have sector beams that cover the area of the reference position. The sites that do not include the MS reference position may be omitted (operation  360 ). A proximity test may be administered to flag the site that is closest to the MS reference position (operation  370 ) so it will not to be included in subsequent selection operations. A minimum total cost function may be applied to determine and/or flag the site with the minimum total cost, as described in detail above (operation  380 ). After a site is flagged, a new look direction may be calculated which may replace the previous look direction (operation  390 ). The process of calculating a new look direction and computing a cost function is recursive and may be repeated until the necessary number of secondary sites needed have been selected or no candidates remain (operation  395 ). Once the number of needed secondary sites have been obtained, the process may end (operation  396 ). 
       FIG. 10  illustrates a flow diagram ( 1 D) of a process to compute the cost function described in operation  390  of  FIG. 9 . A bearing cost is defined as the absolute value of the angular difference in degrees between the bearing angle from the MS reference position to the secondary site candidate and the look direction bearing angle (operation  391 ). The distance cost may be determined different ways depending on whether the candidate site is inside or outside a distance cutoff radius (operations  392  or  393 ). The cost function may be computed based on the determined bearing cost and the distance cost (operation  394 ). 
     The embodiments described above may be used in base station configurations that include, but are not limited to: predetermined data collected over varying terrain, different base station configurations, varying cellular communication protocols. 
     While preferred embodiments of the present invention have been described, it is to be understood that the embodiments described are illustrative only and the scope of the invention is to be defined solely by the appended claims when accorded a full range of equivalents, many variations and modifications naturally occurring to those skilled in the art from a perusal hereof.