Patent Publication Number: US-2021173037-A1

Title: Apparatus and method for locating a mobile device in a network system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/EP2018/069173, filed on Jul. 13, 2018, the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The disclosure relates to an apparatus and a method for locating a mobile device in a network system. Furthermore, the disclosure also relates to a corresponding network system, a computer program product and a computer readable storage medium. 
     BACKGROUND 
     An Indoor Positioning System (IPS) is a network system used to wirelessly locate objects, such as a mobile device, or people inside a building or in dense industrial areas. A special solution is needed since global positioning systems (GPS) are typically not suitable to establish indoor locations as they need an unobstructed line of sight (LOS) to four or more GPS satellites. Microwaves will be attenuated and scattered by roofs, walls and other objects and multiple reflections at surfaces cause multipath propagation serving for uncontrollable errors. 
     Time of flight, ToF, is the amount of time a signal takes to propagate from a transmitter to a receiver. Because the signal propagation rate is constant and known, the travel time of a signal can be used directly to calculate the distance between the transmitter and the receiver. Multiple (in GPS at least four satellites) measurements or multiple anchor stations can be combined with trilateration to find the location of a mobile device. 
     A trilateration method based on Time Difference of Arrival, TDOA, is a common scheme for locating a mobile device in a network system. In the network system, three or more anchor stations are used. The position of the mobile device is estimated according to the time difference of arrivals from the mobile device to each anchor station respectively. However, in commercial systems, receiver channel delays are different for different anchor stations because of manufacture process of these devices. Different receiver channel delays (non-synchronization) leads to inaccurate localization when using a TDOA-based method to locate the mobile device. 
     SUMMARY 
     An objective of the disclosure is to provide a solution which mitigates the drawbacks of conventional device location techniques. 
     The above and further objectives are solved by the subject matter of the independent claims. Further advantageous embodiments of the disclosure can be found in the dependent claims. 
     The disclosure aims at improving the accuracy for locating the mobile device by reducing different receiver channel delays among different anchors, or base stations, in the network system. 
     The term “RF” refers to radio frequency of any appropriate wavelength. 
     The term “anchor station” refers to a base transmitter whose location is known and is used as a reference location in determining the location of the mobile device, e.g. a base station, BS, or an access point, AP. 
     The term “mobile device” refers to a device, such as a mobile station, whose location is being identified. 
     The term “first radio frequency signal” refers to a radio frequency signal transmitted (broadcasted) from one anchor station (e.g., a base station or an access point), and received by anchor stations located in the vicinity of the transmitting anchor station. 
     The term “second radio frequency signal” refers to a radio frequency signal transmitted from a mobile device (e.g., a terminal device), which is being received at anchor stations in the vicinity of the mobile device. 
     According to a first aspect of the disclosure, the above mentioned and other objectives are achieved with a method for locating a mobile device in a network system. The network system comprises a plurality of anchor stations. The method comprises the steps: for each pair of anchor stations A i  and A j : determining a receiver channel delay difference, ΔT rx(A     i     ,A     j     )  of receiving times of a first signal transmitted by a different anchor station A k  and received at both anchor stations A i  and A j , wherein i, j, k are integers, i, j, k≥1, and i≠j≠k; determining a time difference of arrival, ΔT (MD,A     i     ,A     j     ) , of receiving times of a second signal transmitted by the mobile device to the pair of anchor stations A i  and A j ; obtaining a compensated time difference of arrivals Comp_ΔT (MD,A     i     ,A     j     )  based on the time difference of arrival, ΔT (MD,A     i     ,A     j     )  and the receiver channel delay difference ΔT rx(A     i     ,A     j     ) ; determining the location of the mobile device based on the compensated time difference of arrivals Comp_ΔT (MD,A     i     ,A     j     ) . 
     It should be noted that the determination of the receiver channel delay difference ΔT rx(A     i     ,A     j     )  and the determination of the time difference of arrival ΔT (MD,A     i     ,A     j     )  can be processed one after another, or processed concurrently. 
     An advantage of the method according to the first aspect is that by compensating the time difference of arrival, of a RF signal propagated from the mobile device to the pair of anchor stations, by the receiver channel delay difference between the two anchor stations, influences of different receiver channel delay at different anchor stations are reduced, improving thus the accuracy of estimation of the position of the mobile device. 
     In an implementation form of the method according to the first aspect, the receiver channel delay difference ΔT rx(A     i     ,A     j     )  is determined as follows: a pair of time of arrivals T (A     k     A     i     )  and T (A     k     A     j     )  are received, wherein T (A     k     A     i     )  and T (A     k     A     j     )  specify receiving times of the first signal transmitted by anchor station A k  and received at anchor stations A i  and A j  respectively. The difference of receiver channel delays ΔT rx(A     i     ,A     j     )  from the pair of received time of arrivals T (A     k     A     i     )  and T (A     k     A     j     )  is then determined. 
     In an implementation form of the method according to the first aspect, the time difference of arrival, ΔT (MD,A     i     ,A     j     )  is determined as follows: a pair of time of arrivals T (MD,A     i     )  and T (MD,A     j     )  are respectively received from anchor stations A i  and A j . The time of arrivals T (MD,A     i     )  and T (MD,A     j     )  specify receiving times of the second signal transmitted by the mobile device and received at the pair of anchor stations A i  and A j  respectively. The time difference of arrival ΔT (MD,A     i     ,A     j     )  is determined from the pair of time of arrivals T (MD,A     i     )  and T (MD,A     j     ) . 
     In an implementation form of the method according to the first aspect, a compensated time difference of arrivals Comp_ΔT (MD,A     i     ,A     j     )  is obtained by subtracting the receiver channel delay difference ΔT rx(A     i     ,A     j     )  from the determined time difference of arrival ΔT (MD,A     i     ,A     j     ) . 
     In an implementation form of the method according to the first aspect, the location of the mobile device is determined based on the compensated time difference of arrivals Comp_ΔT (MD,A     i     ,A     j     )  as follows: N different pairs of anchor stations are chosen from the plurality of anchor stations, wherein N is an integer and N≥2. N compensated time difference of arrivals are obtained which correspond to the N different pairs of anchor stations, respectively. The location of the mobile device is determined according to the N compensated time difference of arrivals. In particular, the location of the mobile device is determined by multiplication of the compensated time difference of arrivals and the speed of light. 
     An advantage with this implementation form is that multiple compensated time difference of arrivals are obtained, and these can be used in locating of the mobile device, further improving the accuracy of device localization. 
     The time of arrivals T (A     k     ,A     i     )  and T (A     k     ,A     j     ) , respectively, comprises a transmitting time T A     k    for the first signal transmitted by anchor station A k ; propagation times T AIR(A     k     ,A     i     )  and T AIR(A     k     ,A     j     ) , respectively, for the first signal being propagated from anchor station A k  to the pair of anchor stations A i  and A j , respectively; and receiving channel delays T rx(A     i     )  and T rx(A     j     ) , respectively, of receiving time of the first signal transmitted by anchor station A k  and received at anchor station A i  and A j , respectively; in particular, 
     
       
      
       T 
       (A 
       
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       ,A 
       
         i 
       
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       =T 
       A 
       
         k 
       
       +T 
       AIR(A 
       
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         i 
       
       ) 
       +T 
       rx(A 
       
         i 
       
       )  
      
     
         T   (A     k     ,A     j     )   =T   A     k     +T   AIR(A     k     ,A     j     )   +T   rx(A     j     ) . 
     These times of arrivals are determined by each pair of receiving anchor stations according to the above formula. 
     In an implementation form of the method according to the first aspect, if the to-be-determined position of the mobile device is in two dimension, a minimum number for the pairs of anchor stations is 2; and if the to-be-determined position of the mobile device is in three dimension, a minimum number for the pairs of anchor stations is 3. 
     In an implementation form of the method according to the first aspect, if N is greater than the minimum number, the position of the mobile device can be determined based on the N compensated time difference of arrivals, the positions of the N different pairs of anchor stations according to a linear least square algorithm. 
     An advantage with this implementation form is that by using the linear least square algorithm, the determination of the position of the mobile device can be more accurate. 
     In an implementation form of the method according to the first aspect, the first signal and the second signal are two different radio frequency signals. 
     According to a second aspect of the disclosure, the above mentioned and other objectives are achieved with an apparatus for locating a mobile device in a network system. The network system comprises a plurality of anchor stations. The apparatus can be a proceeding module which can be deployed in one of the plurality of anchor stations. The apparatus can be also realized with a separate device, for example an application server. For the skilled person in the art, it is to be understood that there are a plurality of modules to implement the functions. In particular, the apparatus is configured to: 
     for each pair of anchor stations A i  and A j :
 
determine a receiver channel delay difference, ΔT rx(A     i     ,A     j     )  of receiving times of a first signal transmitted by a different anchor station A k  and received at both anchor stations A i  and A j , wherein i, j, k are integers, i, j, k≥1, and i≠j≠k;
 
determine a time difference of arrival, ΔT (MD,A     i     ,A     j     ) , of receiving times of a second signal transmitted by the mobile device to the pair of anchor stations A i  and A j ;
 
obtain a compensated time difference of arrivals Comp_ΔT (MD,A     i     ,A     j     )  based on the time difference of arrival. ΔT (MD,A     i     ,A     j     )  and the receiver channel delay difference ΔT rx(A     i     ,A     j     ) ;
 
determine the location of the mobile device based on the compensated time difference of arrivals Comp_ΔT (MD,A     i     ,A     j     ) .
 
     The apparatus according to the second aspect can be extended into implementation forms corresponding to the implementation forms of the method according to the first aspect. Hence, an implementation form of the apparatus comprises the feature(s) of the corresponding implementation form of the method. 
     The advantages of the methods according to the second aspect are the same as those for the corresponding implementation forms of the first apparatus according to the first aspect. 
     The disclosure also relates to a network system, comprises a mobile device, an apparatus according to any of second aspect of the disclosure, and a plurality of anchor stations. 
     The disclosure also relates to a computer program, characterized in program code, which when run by at least one processor causes said at least one processor to execute a method according to any of first aspect of the disclosure. 
     Further, the disclosure also relates to a computer program product comprising a computer readable medium and said mentioned computer program, wherein said computer program is included in the computer readable medium, and comprises of one or more from the group: ROM (Read-Only Memory), PROM (Programmable ROM), EPROM (Erasable PROM), Flash memory, EEPROM (Electrically EPROM) and hard disk drive. 
     Further, the disclosure also relates to a computer readable storage medium comprising computer program code instructions, being executable by a computer, for performing a method according to any of first aspect of the disclosure when the computer program code instructions runs on a computer. 
     Further applications and advantages of the embodiments of the disclosure will be apparent from the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The appended drawings are intended to clarify and explain different embodiments of the disclosure, in which: 
         FIG. 1  shows a network system according to an embodiment of the disclosure; 
         FIG. 2  shows a timeline flowchart for a method according to an embodiment of the disclosure; 
         FIG. 3  shows a server according to an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Illustrative embodiments of method, apparatus, and program product for efficient packet transmission in a communication system are described with reference to the figures. Although this description provides a detailed example of possible implementations, it should be noted that the details are intended to be exemplary and in no way limit the scope of the application. 
     Moreover, an embodiment/example may refer to other embodiments/examples. For example, any description including but not limited to terminology, element, process, explanation and/or technical advantage mentioned in one embodiment/example is applicative to the other embodiments/examples. 
     In order to reduce the influence of different receiver channel delays corresponding to different anchor stations in the process of locating a mobile device, an embodiment for an optimized triangulation method based on TDOA is provided. 
       FIG. 1  shows a network system  100  according to an embodiment of the disclosure. The network system  100  comprises a mobile device  110  (e.g. a terminal device, user equipment) and three anchor stations  120 A- 120 C (e.g. base stations or access points), and a server  130 . For simplicity, the network system  100  shown in  FIG. 1  only comprises one mobile device  110  and three anchor station  120 A- 120 C. However, the network system  100  may comprise any number of mobile devices  110  and any number of anchor stations  120  without deviating from the scope of the disclosure. The server  130  can be implemented with a separate apparatus (e.g., an application server device), or one or a plurality of modules integrated in one of the three anchor stations  120 A- 120 C or integrated in another anchor station (now shown in  FIG. 1 ) except the three anchor stations  120 A- 120 C. 
     In the embodiment shown in  FIG. 1 , the mobile device  110  is in connected mode with the three anchor stations  120 A- 120 C and three radio links (RL) are configured between the mobile device  110  and each of the three anchor stations  120 A- 120 C. The radio links (RL) may be configured to work in an uplink (UL) mode, or in a downlink (DL) mode. 
     In the embodiment shown in  FIG. 1 , the server  130  is connected with the three anchor stations  120 A- 120 C via wireless connection, wired connection or both of wireless and wired connections. 
     To determine position of the mobile device  110  (e.g., denoted as (x,y)), a plurality of anchor stations are selected as reference positions. The positions of the anchor stations is known in advance. Just as an example, positions of the three anchor stations  120 A- 120 C are given as (x0, y0) for anchor station_ 0   120 A, (x1, y1) for anchor station_ 1   120 B, and (x2, y2) for anchor station_ 2   120 C. 
       FIG. 2  shows a timeline flow chart of a method  200  for locating a mobile device  110  in a network system  100  according to an embodiment of the disclosure. 
     In the embodiment of the disclosure, N different pairs of anchor stations A i  and A j  from the plurality of anchor stations are selected as receivers, wherein i, j are integers, i, j≥1, and i≠j. For each pair of anchor stations A i  and A j , another different anchor station A k  is selected as a transmitter, wherein k are integers, k≥1, and i≠j≠k. 
     It may be known that, by using a set of three anchor stations (e.g., anchor_ 1 , anchor_ 2 , anchor_ 3 ), the position of the mobile device  110  can be also determined. In the three anchor stations, at most three different pairs of anchor stations can be chosen as receivers (e.g., a first pair is anchor_ 1  and anchor_ 2 , a second pair is anchor_ 1  and anchor_ 3 , and a third pair is anchor_ 2  and anchor_ 3 ), and for each pair of anchor stations, another different anchor station in the set may be chosen as a transmitter. 
     For simplicity,  FIG. 2  shows an embodiment with a set of three anchor stations, i.e. anchor station_ 0   120 A, anchor station_ 1   120 B, anchor station_ 2   120 C. Among them, anchor station_ 0   120 A is chosen as a transmitter, and the other two anchor stations (i.e., anchor station_ 1   120 B, anchor station_ 2   120 C) are chosen as receivers. 
     Steps  201  (S 201 ) and  202  (S 202 ): a first signal (e.g. a radio frequency signal, RF 1 ) is transmitted from anchor station_ 0  (i.e. a transmitter)  120 A to anchor station_ 1   120 B (i.e. a first receiver) and anchor station_ 2   120 C (i.e. a second receiver), respectively. 
     In the implementation, anchor station_ 0   120 A broadcasts the first signal (e.g. a radio frequency signal, RF 1 ) in an omnidirectional form. 
     Steps  203  (S 203 ) and  204  (S 204 ): after receiving the first signal RF 1 , the first and the second receiver anchor stations anchor station_ 1   120 B, and anchor station_ 2   120 C, record the respective time of arrivals T TOA(AS0, AS1)  and T TOA(AS0, AS2) . The time of arrivals T TOA(AS0, AS1)  and T TOA(AS0, AS2)  specify receiving time of the first signal (e.g., the radio frequency signal RF 1 ) transmitted by anchor station_ 0   120 A and received at anchor station_ 1   120 B and anchor station_ 2   120 C respectively. 
     Each receiving anchor station AS i  (i=1, 2 . . . ,) determines the corresponding receiving time of arrival T TOA(AS0,AS1)  of the first signal (e.g. the first radio frequency signal RF 1 ) sent by AS 0  based on three components.
         (1) a transmitting time T Tx(AS0) : refers to a time delay, for anchor station_ 0   120 A, of transmitting the first signal (e.g., the radio frequency signal RF 1 ).   (2) a propagation time T air(AS0,ASi) : refers to a time for the first signal (e.g., the radio frequency signal RF 1 ), being propagated from anchor station_ 0   120 A to the receiving anchor station AS i  (in  FIG. 1 , anchor station_ 1   120 B and anchor station_ 2   120 C).       

     The positions of anchor stations are pre-determined. In the implementation, this propagation time is determined by dividing the distance between anchor stations by the speed of the light. 
     (3) a receiver channel delay T Rx(ASi) : refer to a time delay for the receiving anchor station AS i  (in  FIG. 1 , anchor station_ 1   120 B or anchor station_ 2   120 C), to receive the first signal (e.g. the radio frequency signal RF 1 ). 
     That is, the time of arrival T TOA(AS0,ASi)  of the signal broadcasted by the anchor station AS 0  and received at AS i  can be determined as: 
         T   TOA(AS0,ASi)   =T   Tx(AS0)   +T   air(AS0,ASi)   +T   Rx(ASi)   ,i= 1, 2, . . . . 
     For two receiving anchor stations(AS 0 , and AS 1 ), the time of arrivals are determined as: 
     
       
      
       T 
       TOA(AS0,AS1) 
       =T 
       Tx(AS0) 
       +T 
       air(AS0,AS1) 
       +T 
       Rx(AS1)  
      
     
         T   TOA(AS0,AS2)   =T   Tx(AS0)   +T   air(AS0,AS2)   +T   Rx(AS2)   {circle around (1)}
 
     Steps  205  (S 205 ) and  206  (S 206 ): the time of arrivals T TOA(AS0, AS1)  and T TOA(AS0, AS2)  are transmitted from anchor station_ 1   120 B and anchor station_ 2   120 C to the server  130  respectively. 
     Step  207  (S 207 ): a receiver channel delay difference for the two receivers (i.e. anchor station_ 1   120 B and anchor station_ 2   120 C) ΔT RX(AS1, AS2)  is determined according to the time of arrivals T TOA(AS0, AS1)  and T TOA(AS0, AS2) . 
     In the implementation, the receiver channel delay difference ΔT TX(AS1, AS2)  can be specifically determined based on the equation {circle around (1)}: 
       Δ T   RX(AS1,AS2)   =T   RX(AS1)   −T   RX(AS2)   =T   TOA(AS0,AS1)   −T   TOA(AS0,AS2)   +T   air(AS0,AS2)   −T   air(AS0,AS1)   {circle around (2)}
 
     Steps  208  (S 208 ) and  209  (S 209 ), a second signal (e.g. a radio frequency signal, RF 2 ) is transmitted from the mobile device  110  to anchor station_ 1   120 B (i.e. the first receiver) and anchor station_ 2   120 C (i.e. the second receiver) simultaneously. 
     In the implementation, the mobile device  110  transmits the second signal (e.g. a radio frequency signal, RF 2 ) in an omnidirectional form. 
     Steps  210  (S 210 ) and  211  (S 211 ): the second signal (e.g. the radio frequency signal RF 2 ) is received by anchor station_ 1   120 B and anchor station_ 2   120 C respectively. The time of arrivals T TOA(MD, AS1)  and T TOA(MD, AS2)  are recorded. The time of arrivals T TOA(MD, AS1)  and T TOA(MD, AS2)  specify transmitting time for the second signal (e.g. the radio frequency signal RF 2 ) transmitted from the mobile device  110  to anchor station_ 1   120 B and anchor station_ 2   120 C respectively. 
     Each receiving anchor station AS i  (i=1, 2 . . . ,) determines the corresponding time of arrival T TOA(MD, ASi)  for the second signal (e.g. the radio frequency signal. RF 2 ) based on the three components as follows:
         (1) a transmitting time T Tx(MD)  for the mobile device  110  to transmit the second signal (e.g. the radio frequency signal RF 2 ).   (2) a propagation time T air(MD,ASi) : for the second signal (e.g. the radio frequency signal RF 2 ) being propagated from the mobile device  110  to the receiving anchor station AS i  (in  FIG. 1 , anchor station_ 1   120 B or anchor station_ 2   120 C).   (3) a receiver channel delay T Rx(ASi) : refer to a time delay, for the receiving anchor station AS i  (in  FIG. 1 , anchor station_ 1   120 B or anchor station_ 2   120 C), to receive the second signal (e.g. the radio frequency signal RF 2 ).       

     That is, the time of arrival T TOA(MD,ASi)  of the signal broadcasted by the anchor station MD and received at AS i  can be determined as: 
         T   TOA(MD,ASi)   =T   Tx(MD)   +T   air(MD,ASi)   +T   Rx(ASi)   ,i= 1,2, . . . . 
     For two receiving anchor stations(AS 1 , and AS 2 ), the time of arrivals are determined as: 
     
       
      
       T 
       TOA(MD,AS1) 
       =T 
       Tx(MD) 
       +T 
       air(MD,AS1) 
       +T 
       Rx(AS1)  
      
     
         T   TOA(MD,AS2)   =T   Tx(MD)   +T   air(MD,AS2)   +T   Rx(AS2)   {circle around (3)}
 
     Steps  212  (S 212 ) and  213  (S 213 ): the time of arrivals T TOA(MD, AS1)  and T TOA(MD, AS2)  are transmitted from anchor station_ 1   120 B and anchor station_ 2   120 C to the server  130  respectively. 
     Step  214  (S 214 ): a time difference of arrival ΔT TOA(MD,AS1,AS2)  is obtained. The time difference of arrival ΔT TOA(MD,AS1,AS2)  specifies a difference of time of arrival, TDOA for the second signal (e.g. the radio frequency signal RF 2 ) transmitted from the mobile device  110  to anchor station_ 1   120 B and anchor station_ 2   120 C respectively. 
     In the implementation, the difference of time of arrivals ΔT TOA(MD,AS1,AS2)  can be determined based on the equation {circle around (3)}: 
       Δ T   TOA(MD,AS1,AS2)   =T   TOA(MD,AS1)   −T   TOA(MD,AS2) =( T   air(MD,AS1)   −T   air(MD,AS2) )+( T   Rx(MD,AS1)   −T   Rx(MD,AS2) )=Δ T   air(MD,AS1,AS2)   +T   Rx(AS1,AS2)   {circle around (4)}
 
     In the equation {circle around (4)}, the first component denoted as ΔT air(MD,AS1,AS2)  refers to a time difference for the second signal (e.g. the radio frequency signal RF 2 ) being propagated over the air from the mobile device  110  to anchor station_ 1   120 B and anchor station_ 2   120 C separately. The second component denoted as ΔT Rx(AS1,AS2)  refers to a receiver channel delay difference of receiving times for anchor station_ 1   120 B and anchor station_ 2   120 C, which has been obtained in equation {circle around (2)}. 
     Step  215  (S 215 ): a compensated time difference of arrival ΔT TOA_C(MD, AS1,AS2)  is determined based on the corresponding difference of receiving times ΔT RX(AS1,AS2)  and the estimated time difference of arrival ΔT TOA(MD,AS1,AS2) . 
     In the implementation, the compensated time difference of arrivals ΔT TOA_C(MD, AS1,AS2)  can be determined based on the equation {circle around (5)}: 
       Δ T   TOA_C(MD,AS1,AS2)   =ΔT   air(MD,AS1,AS2)   =ΔT   TOA(MD,AS1,AS2)   −ΔT   Rx(AS1,AS2)   {circle around (5)}
 
     In equation {circle around (5)}, the receiver channel delay difference ΔT Rx(AS1,AS2)  of receiving times for anchor station_ 1   120 B and anchor station_ 2   120 C can be obtained in equation {circle around (2)}. The time difference of arrival ΔT TOA(MD,AS1,AS2)  can be determined based on different known algorithms. Just as an example, in orthogonal frequency-division multiplexing, OFDM systems, assuming h k  and h k+1  are the received channel of sub-carrier k and k+1 respectively, and h* k  specifies conjugate of h k . Δf is the sub-carrier space between two adjacent sub-carriers k and k+1, τ is the time of arrival for a sub-carrier (e.g. sub-carrier k or k+1) being propagated from the mobile device  110  to an anchor station (e.g. anchor station_ 1   120 B or anchor station_ 2   120 C). Then the time of arrival τ can be determined as, wherein arg(A) denotes the phase difference of A: 
     
       
         
           
             
               
                 
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     The time difference of arrival TDOA ΔT TOA(MD,AS1,AS2)  can be determined as: 
     
       
         
           
             
               
                 
                   
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     From  FIG. 1 , assuming position of the mobile device denoted as(x, y), the positions of the anchor stations (i.e. anchor station_ 0 , anchor station_ 1 , and anchor station_ 2 ) can be pre-determined and they are denoted as respectively: (x 0 , y 0 ), (x 1 , y 1 ), (x 2 , y 2 ). 
     Based on Steps  201  to  215 , an equation can be obtained as follows, C denotes the speed of light: 
       √{square root over (( x−x   1 ) 2 +( y−y   1 ) 2 )}−√{square root over (( x−x   2 ) 2 +( y−y   2 ) 2 )}=Δ T   air(MD,AS1,AS2)   *C   {circle around (8)}
 
     Step  216  (S 216 ): other different pairs of anchor stations are chosen as receivers, and the steps  201  to  215  are performed repeatedly. 
     In the implementation, the pair of anchor stations (i.e. anchor station_ 1   120 B and anchor station_ 2   120 C) are selected as two receivers in Steps  201  and  202 , and Steps  208  and  209 . In this step, another N−1 different pairs of anchor stations are chosen as N−1 pairs of receivers, for example, anchor station_ 0   120  A and anchor station_ 1   120 B, or anchor station_ 0   120 A and anchor station_ 2   120 C, or other anchor stations which are not shown in  FIG. 1 . Just as an example, assuming anchor station_ 0   120  A and anchor station_ 1   120 B are another pair of receivers, so an equation correspondingly can be obtained after performing the steps  201  to  215 , which is shown as follows (C also denotes the speed of light): 
       √{square root over (( x−x   0 ) 2 +( y−y   0 ) 2 )}−√{square root over (( x−x   1 ) 2 +( y−y   1 ) 2 )}=Δ T   air(MD,AS1,AS2)   *C   {circle around (9)}
 
     Step  217  (S 217 ): The position of mobile device  110  is determined by the server  130  according to N compensated time difference of arrivals ΔT TOA_C(MD, ASi, ASj) . 
     In the implementation, the position of mobile device  110  can be determined based on the equations {circle around (8)} and {circle around (9)}). 
     In order to reduce inaccurate estimation of the position of the mobile device, N different pair of anchor stations are chosen and N compensated time difference of arrivals ΔT TOA_C(MD, ASi,ASj)  can be thus determined. For example, when N is 3, such equations are determined as: 
       √{square root over (( x−x   1 ) 2 +( y−y   1 ) 2 )}−√{square root over (( x−x   2 ) 2 +( y−y   2 ) 2 )}=Δ T   air(MD,AS1,AS2)   *C   {circle around (8)}
 
       √{square root over (( x−x   0 ) 2 +( y−y   0 ) 2 )}−√{square root over (( x−x   1 ) 2 +( y−y   1 ) 2 )}=Δ T   air(MD,AS0,AS1)   *C   {circle around (9)}
 
       √{square root over (( x−x   0 ) 2 +( y−y   0 ) 2 )}−√{square root over (( x−x   2 ) 2 +( y−y   2 ) 2 )}=Δ T   air(MD,AS0,AS2)   *C   {circle around (10)}
 
     The position of mobile device  110  can be determined based on N (e.g. N=3) equations according to a known linear least square algorithm (e.g., a weighted least square, WLS algorithm). 
       FIG. 3  shows a server  130  according to an embodiment of the disclosure. In the embodiment shown in  FIG. 3 , the server  130  comprises a processor  131 , a transceiver  132  and a memory  133 . The processor  131  is coupled to the transceiver  132  and the memory  133  by communication means  134  known in the art. As an alternative, the server  130  further comprises an antenna or antenna array  135  coupled to the transceiver  132 , which means the server  130  is configured for wireless communications in a wireless communication system. As another alternative, the server  130  further comprises a wired interface  135  coupled to the transceiver  132 , which means that the server  130  is configured for wired communications in a wired communication system. 
     The server  130  is configured to perform certain actions in this disclosure can be understood to mean that the server  130  comprises suitable means, such as e.g. the processor  131  and the transceiver  132 , configured to perform said actions. 
     The mobile device  110  herein, may be denoted as a user device, a User Equipment (UE), an internet of things (IoT) device, a sensor device, a wireless terminal and/or a mobile terminal, is enabled to communicate wirelessly in a wireless communication system, sometimes also referred to as a cellular radio system. The UEs may further be referred to as mobile telephones, cellular telephones, computer tablets or laptops with wireless capability. The UEs in this context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another receiver or a server. The UE can be a Station (STA), which is any device that contains an IEEE 802.11-conformant Media Access Control (MAC) and Physical Layer (PHY) interface to the Wireless Medium (WM). The UE may also be configured for communication in 3GPP related LTE and LTE-Advanced, in WiMAX and its evolution, and in fifth generation wireless technologies, such as New Radio. 
     Anchor stations  120 A- 120 C herein may also be denoted as a radio client device, an access client device, an access point, or a base station, e.g. a Radio Base Station (RBS), which in some networks may be referred to as transmitter, “gNB,” “gNodeB,” “eNB,” eNodeB,” “NodeB” or “B node,” depending on the technology and terminology used. The radio client devices may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. The radio client device can be a Station (STA), which is any device that contains an IEEE 802.11-conformant Media Access Control (MAC) and Physical Layer (PHY) interface to the Wireless Medium (WM). The radio client device may also be a base station corresponding to the fifth generation (5G) wireless systems. 
     Furthermore, any method according to embodiments of the disclosure may be implemented in a computer program, having code means, which when run by processing means causes the processing means to execute the steps of the method. The computer program is included in a computer readable medium of a computer program product. The computer readable medium may comprise essentially any memory, such as a ROM (Read-Only Memory), a PROM (Programmable Read-Only Memory), an EPROM (Erasable PROM), a Flash memory, an EEPROM (Electrically Erasable PROM), or a hard disk drive. 
     Moreover, it is realized by the skilled person that embodiments of the mobile device  110  and anchor stations  120 A- 120 C comprises the necessary communication capabilities in the form of, e.g., functions, means, units, elements, etc., for performing the solution. Examples of other such means, units, elements and functions are: processors, memory, buffers, control logic, encoders, decoders, rate matchers, de-rate matchers, mapping units, multipliers, decision units, selecting units, switches, interleavers, de-interleavers, modulators, demodulators, inputs, outputs, antennas, amplifiers, receiver units, transmitter units, digital signal processors (DSPs), multi-stage decoding (MSDs), trellis-code modulation (TCM) encoder, TCM decoder, power supply units, power feeders, communication interfaces, communication protocols, etc. which are suitably arranged together for performing the solution. 
     Especially, the processor(s) of the mobile device  110  and anchor stations  120 A- 120 C may comprise, e.g., one or more instances of a Central Processing Unit (CPU), a processing unit, a processing circuit, a processor, an Application Specific Integrated Circuit (ASIC), a microprocessor, or other processing logic that may interpret and execute instructions. The expression “processor” may thus represent a processing circuitry comprising a plurality of processing circuits, such as, e.g., any, some or all of the ones mentioned above. The processing circuitry may further perform data processing functions for inputting, outputting, and processing of data comprising data buffering and device control functions, such as call processing control, user interface control, or the like. 
     Finally, it should be understood that the disclosure is not limited to the embodiments described above, but also relates to and incorporates all embodiments within the scope of the appended independent claims. 
     Although the exemplary embodiments of the present disclosure are disclosed herein, it should be noted that any various changes and modifications could be made in the embodiments of the present disclosure, without departing from the scope of legal protection which is defined by the appended claims. In the appended claims, the mention of elements in a singular form does not exclude the presence of the plurality of such elements, if not explicitly stated otherwise.