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PublicatienummerWO2006070211 A1
PublicatietypeAanvraag
AanvraagnummerPCT/GR2005/000036
Publicatiedatum6 juli 2006
Aanvraagdatum5 dec 2005
Prioriteitsdatum27 dec 2004
Ook gepubliceerd alsEP1832138A1
PublicatienummerPCT/2005/36, PCT/GR/2005/000036, PCT/GR/2005/00036, PCT/GR/5/000036, PCT/GR/5/00036, PCT/GR2005/000036, PCT/GR2005/00036, PCT/GR2005000036, PCT/GR200500036, PCT/GR5/000036, PCT/GR5/00036, PCT/GR5000036, PCT/GR500036, WO 2006/070211 A1, WO 2006070211 A1, WO 2006070211A1, WO-A1-2006070211, WO2006/070211A1, WO2006070211 A1, WO2006070211A1
UitvindersA. Stylianos Mytilinaios
AanvragerMytilinaios A Stylianos
Citatie exporterenBiBTeX, EndNote, RefMan
Externe links:  Patentscope, Espacenet
Position location via geometric loci construction
WO 2006070211 A1
Samenvatting
The invention falls into the field of wireless communications and electromagnetic waves propagation. More specifically, part of the invention falls into the field of position location, or estimation of the location of a wireless device. The invention solves the problem of categorizing transmitters with contiguous propagation attenuation factors to a wireless device. The invention estimates the values of these factors, and also estimates the position of the wireless device - or 'locates' the device. This problem is affronted by proper geometric loci construction. At the same time, an internal criterion is calculated in order to evaluate the results of the location procedure. The invention comes at the desiring results with minimum input: the positions of the transmitters and the Received Signal Strength (R.S.S.) from each transmitter at the position of the wireless device (and also supplemental information, such as the gain of the transmitters' and receiver's antenna etc.). The invention may be employed to any wireless network, since it may be incorporated into existing and inexpensive network equipment. Therefore, the invention may be employed to either new or already existing and running networks. It is especially suited for operation in unknown environments, since topographical maps are not needed. The costly and sometimes impracticable procedure of on-the-field pre-measurements is avoided. Additionally, expensive equipment, such as directional or switched-beam or adaptive antenna arrays, is not required. The invention overcomes most of these problems, which are characteristic of existing location methods, as presented in the 'Description' part.
Claims  (OCR-tekst kan fouten bevatten)
1.
A method for the determination and categorization of transmitters, with the same or contiguous propagation attenuation factors with respect to a wireless device, estimation of these attenuation factors and estimation of the wireless device's position.
The position of the z-th transmitter is considered to be known, and is denoted by (xBS , , yBS , ) , while the position of the wireless device is unknown and is denoted by (xm,yMS ) ■ The unknown distance between the z-th transmitter and the device is denoted by dt and is given by
d-i = V(X BS i ~ X MS )2 ^ (yεs i ~ yMs)2 ■ ^he attenuation factor between the i-th transmitter and the device is unknown and is assumed to lie within [«min , W010x] • The theoretical signal strength (in Watts) that the device receives at a distance of Im from the z-th transmitter is denoted by P0 , and is considered to be known. The antennas of the transmitters and the device are assumed to be omnidirectional with known gain(s).
Hereinafter, a reference to a step signifies a step of the specific claim. A reference to a sub-step signifies a sub-step of the specific step of the specific claim. A reference to a step or sub-step of another claim will be explicitly stated.
The method is characterized by the steps of:
I. The device, placed at the unknown location (x^ , y^ ) , measures the local mean signal strength received from all transmitters (let there be a total of NBS ^4x transmitters). The local mean signal strength is calculated as the average of a sample of measures. The size of the sample is arbitrarily determined by the network administrator (or the user of the invention). The local mean signal strength is denoted by P1. π. A number NBS ≤ Ng3J114x of transmitters are selected. The number NBS and the selected transmitters are determined by the network administrator (or the user of the invention). Definitely, the number of selected transmitters, N33 , shall be greater than or equal to 4, i.e.
NBS > 4. 1<1
IK. Given the NBS transmitters, four of them are selected in all possible ways (i.e. all combinations of NBS transmitters taken four at a time - the order of selection is immaterial). Then, or each one of these quadruplets, the following sub-steps are executed:
(i) It is assumed that the transmitters of the quadruplet are characterized by a common -and unknown - attenuation factor to the device, denoted by nx .
(ii) One transmitter is randomly selected, and is named "central".
(iii) All three pairs between the central and the other transmitters are formed.
(iv) The estimator nA of the common attenuation factor nx is generated. The estimator nA scans \n - ,nmavλ with step n.,m . The value of n,,an is determined by the network administrator (or the user of the invention).
(v) For each value of nA e [«minmax], and for each pair of transmitters i and j constructed in (iii), (indexes i and j correspond to transmitters): The estimates of the distances between the
transmitters i , j and the device, denoted by dt (nA ) , d} (nA ) respectively, are calculated using:
(vi) For each value of nA e [nmin , rømax], and for each pair of transmitters / and j constructed in (iii), (indexes i and j correspond to transmitters): The intersection points of the circles, with
centers (xBS t , yBS 1 ) and (xBS , yBS ) , and radii the estimates dt (JIA ), dj (riA) respectively, are calculated. The intersection points will be at most two. However, there may be one, or even none intersection point between the two circles. (vϋ) As nA scans [77minmax] , and for each pair of transmitters i and j constructed in (iii),
(indexes / and j correspond to transmitters), the corresponding geometric locus of the intersection points defined in (vi) is constructed. Thus, a total of three geometric loci are constructed, corresponding to the three transmitters pairs (the wireless device must he on these loci). 30
(viii) For each value of nA e [nmin ,nmax], the sum of the corresponding distances between the
geometric loci is calculated, as a function of nA . The distance between two geometric loci is
calculated using the following definition: If for a value nvάn e [nmin ,nmax] the corresponding
points on locus 1 are A and B, and for a value nval j2 e [nmia ,nmΛX] the corresponding points on locus 2 are C and D, the distance between these two loci, for the specific values of attenuation factor estimators, is the minimum among the distances A-C, A-D, B-C and B-D, i.e. the distance given by:
Distance{n ~ aLl,n;al 2) = min{d(A,C),d(A,O),d(B,C),d(B,D)} (2) where d(K, Y) is the distance between the points X and Y. (ix) The minimum value of the function of the sum of distances between the geometric loci is calculated. The value nA t of nA which minimizes the sum of distances is also found. The inverse of the minimum value of the function of sum, is the measure of applicability of the specific quadriψlet of transmitters. Furthermore, the three points on the geometric loci, which correspond to the minimum value of the function of sum and the measure of applicability, are determined. IV. All significant data are stored, i.e.: All quadruplets (as combinations of transmitters), the measure of applicability of each quadruplet, the value nA which corresponds to the measure of applicability of each quadruplet, and the points of the geometric loci which correspond to the measure of applicability of each quadruplet.
2.
A method for the determination and categorization of transmitters, with the same or contiguous propagation attenuation factors with respect to a wireless device, estimation of these attenuation factors and estimation of the wireless device's position, according to claim 1, where, after the step IV of claim 1, and when the number NBS is greater than or equal to 5 (i.e. NBS ≥ 5 ), the following steps are also executed: Si
I. Given the NBS transmitters, five of them are selected in all possible ways (i.e. all combinations of NBS transmitters taken five at a time - the order of selection is immaterial). π. For each one pentad, three transmitters are selected in all possible ways (i.e. all combinations of five transmitters taken three at a time - the order of selection is immaterial). Thus, all possible combinations of a triplet and a pair are configured. For each triplet-pair combination, the following sub-steps are executed:
(i) For the triplet, it is assumed that the corresponding transmitters are characterized by a common -and unknown - attenuation factor to the device, denoted by nx .
(ii) From the triplet, one transmitter is randomly selected, and is named "central". (iii) From the triplet, the two pairs between the central and the other two transmitters are formed.
(iv) For the triplet, the estimator nA of nx is generated. The estimator nΛ scans [nmin ,nm!(X] with step nstep .
(v) For each value of nA e [W111Jn , nmax ] , and for each pair of transmitters / and J constructed in (iii), (indexes / and j correspond to transmitters): The estimates of the distances between the
transmitters i , j and the device, denoted by J1 (nA ) , d} (nΛ ) respectively, are calculated using:
(vi) For each value of nA e [nmin , «max ] , and for each pair of transmitters i and j constructed in (iii), (indexes / and j correspond to transmitters): The intersection points of the circles, with
centers (xBS t , yBS t ) and (xBS , yBS ) , and radii the estimates dt (n A) ,d} (n A) respectively, are calculated. The intersection points will be at most two. However, there may be one, or even none intersection point between the two circles.
(vii) As nA scans [wminmax] , and for each pair / and j constructed in (iii), (indexes / and j correspond to transmitters), the corresponding geometric locus of the intersection points defined in (vi) is constructed. Thus, a total of two geometric loci are constructed, corresponding to the two transmitters pairs (the wireless device must lie on these loci). n
(viii) For the pair of the pentad (rest two transmitters), it is assumed that the corresponding transmitters are characterized by a common —and unknown - attenuation factor to the device, denoted by n2 . The transmitters of this pair are denoted by k and m, for clarity.
(ix) For this pair, the estimator nB of n% is generated. The estimator nB scans [nmia ,nm!Bi] with step nstep .
(x) For each value of nB e [nmia,nm∞i], and for the transmitters k and m, the estimates of the
distances between the transmitters k,m and the device, denoted by dk {nB *), dm (nB) respectively, are calculated using:
(xi) For each value of nB e [nmia,nm!Bi], and for the transmitters k and m, the intersection points of the circles, with centers (xB$ t^ss k) ^d (xBs m>yBS *»)> an(i rac^u ^e estimates
^k O7 B ) > ^m (n B ) respectively, are calculated. The intersection points will be at most two. However, there may be one, or even none intersection point between the two circles.
(xii) As nB scans [nmin ,nm∞c] , and for the transmitters k and m, the corresponding geometric locus of the intersection points defined in (xi) is constructed. Thus, one geometric locus is constructed, corresponding to the transmitters pair k and m (the wireless device must lie on this locus).
(xiii) For each value of nA and for each value of nB , the sum of the corresponding distances between the geometric loci of sub-steps (vii) and (xii), is calculated. The sum of distances is a function of nA and nB .
(xiv) The minimum value of the function of the sum of distances between the geometric loci is calculated. The values nA opt ,nB opl of nA and nB respectively, which minimize the sum of distances are also found. The inverse of the minimum value of the function of sum, is the measure of applicability of the specific triplet-pair combination of transmitters. Furthermore, the three points on the geometric loci, which correspond to the minimum value of the function of sum and the measure of applicability, are determined. B<
HI. All significant data are stored, i.e.: All pentads, all triplet-pair combinations (as combinations of transmitters), the measure of applicability of each triplet-pair combination, the values
n A,oPt ' n B,oPt which correspond to the measure of applicability of each triplet-pair combination, and the points of the geometric loci which correspond to the measure of applicability of each triplet-pair combination.
3.
A method for the determination and categorization of transmitters, with the same or contiguous propagation attenuation factors with respect to a wireless device, estimation of these attenuation factors and estimation of the wireless device's position, according to claim 2, where, after the step
III of claim 2, and when the number NBS is greater than or equal to 6 (i.e. NBS ≥ 6 ), the following steps are also executed:
L Given the NBS transmitters, six of them are selected in all possible ways (i.e. all combinations of NBS transmitters taken six at a time - the order of selection is immaterial).
EL. For each one hexad, two transmitters are selected in all possible ways (i.e. all combinations of six transmitters taken two at a time - the order of selection is immaterial).
DI. For each pair of π, there is a corresponding quadruplet, which is the rest of the transmitters of the hexad. For this quadruplet, two transmitters are selected in all possible ways (i.e. all combinations of four transmitters taken two at a time - the order of selection is immaterial). Thus, for each hexad, all possible combinations of the type pair-pair-pair are constructed. For each pair- pair-pair combination, the following sub-steps are executed:
(i) For the first pair of transmitters, denoted by i and/, it is assumed that they are characterized by a common -and unknown - attenuation factor to the device, denoted by nx .
(ii) For the first pair, the estimator nA of W1 is generated. The estimator nA scans [nmin ,nmaκ J with step nslep .
(iii) For each value of nA e [nmia , nm∞i ] , and for the transmitters / and j , the estimates of the
distances between the transmitters i,j and the device, denoted by dt(nA) , d ,(W4) respectively, are calculated using: 14 ■
(iv) For each value of n A e [nmin , nmsκ ] , and for the transmitters i and/, the intersection points of the circles, with centers {xBSji , yBSti ) and (XESJ ^BSJ ) ^ an(i rame estimates
d.QiA) ,dj (nA) respectively, are calculated. The intersection points will be at most two. However, there may be one, or even none intersection point between the two circles.
(v) As nA scans !)?„,!„ ,rømax] , and for the transmitters i and/, the corresponding geometric locus of the intersection points defined in (iv) is constructed. Thus, one geometric locus is constructed, corresponding to the transmitters pair i and/ (the wireless device must lie on this locus). (vi) For the second pair of transmitters, denoted by k and m, it is assumed that they are characterized by a common -and unknown - attenuation factor to the device, denoted by n2.
(vϋ) For the second pair, the estimator nB of n2 is generated. The estimator nB scans
IXnin with Step Ustep .
(viii) For each value of nB e [nmja , nmaχ] , and for the transmitters k and m, the estimates of the
distances between the transmitters k, m and the device, denoted by dk (fiB), dm (nB) respectively, are calculated using:
(ix) For each value of nB e [nmia ,nmaκ], and for the transmitters k and m, the intersection points of the circles, with centers {.^BS k^BS k) an<^ (X BS m^BS m)> an(^ radϋ the estimates
dk(nB) , dm (nB) respectively, are calculated. The intersection points will be at most two. However, there may be one, or even none intersection point between the two circles.
(x) As nB scans [«minmax] , and for the transmitters k and m, the corresponding geometric locus of the intersection points defined in (ix) is constructed. Thus, one geometric locus is 15
constructed, corresponding to the transmitters pair k and m (the wireless device must lie on this locus).
(xi) For the third pair of transmitters, denoted by p and q, it is assumed that they are characterized by a common -and unknown - attenuation factor to the device, denoted by n3.
(xii) For the third pair, the estimator nc of M3 is generated. The estimator nc scans
Kin XaJ with step nslep
(xiii) For each value of nc e [nmia , «max ] , and for the transmitters p and q, the estimates of the
distances between the transmitters p , q and the device, denoted by d (nc) , d (nc) respectively, are calculated using:
(xiv) For each value of nc e [fimia ,nmwi], and for the transmitters p and q, the intersection points of the circles, with centers (xBS p,yBS ) and (xBS q,yBS q)> an<^ radii the estimates
dp tøc) > dq (nc) respectively, are calculated. The intersection points will be at most two. However, there may be one, or even none intersection point between the two circles. (xv) As nc scans [nmin , nmax] , and for the transmitters p and q, the corresponding geometric locus of the intersection points defined in (xiv) is constructed. Thus, one geometric locus is constructed, corresponding to the transmitters pair p and q (the wireless device must lie on this locus).
(xvi) For each value of nA , nB , and nc , the sum of the corresponding distances between the geometric loci of sub-steps (v), (x), and (xv), is calculated. The sum of distances is a function of nA , nB and nc .
(xvii) The minimum value of the function of the sum of distances between the geometric loci is calculated. The values nA opt , nB opt , nCfipt of nA , nB and nc respectively, which minimize the sum of distances, are also found. The inverse of the minimum value of the function of sum, is the measure of applicability of the specific pair-pair-pair combination of transmitters . Furthermore, %
the three points on the geometric loci, which correspond to the τniniτnnm value of the function of sum and the measure of applicability, are determined.
IV. All significant data are stored, i.e.: All hexads, all pair-pair-pair combinations (as combinations of transmitters), the measure of applicability of each pair-pair-pair combination, the values
n A opt ' n B opt ' nc opt which correspond to the measure of applicability of each pair-pair-pair combination, and the points of the geometric loci which correspond to the measure of applicability of each pair-pair-pair combination.
4.
A method for the determination and categorization of transmitters, with the same or contiguous propagation attenuation factors with respect to a wireless device, estimation of these attenuation factors and estimation of the wireless device's position, according to claim 1 or 2 or 3, where, after the act of storage (step TV of claim 1 , step III of claim 2, or step IV of claim 3), the following steps are also executed:
I. From all examined combinations (quadruplets, triplet-pair combinations, and pair-pair-pair combinations) the one with the optimum {maximum) measure of applicability is selected. This combination is named "dominant" combination. π. The position of the wireless device is estimated as the average of the three geometric loci points which correspond to the measure of applicability of the dominant combination. The average (x,y) of three points, (JC1, yx) , (x2 ,y2) , (x3,y3) respectively, is calculated using: χ = X1 + χ 2 + χ 3 y! + y2 + y3 (8)
HI. Those attenuation factors, from transmitters to the wireless device, which are not determined
during the described procedure, are calculated using n. = — , nt being the logfø) attenuation factor of the /-th transmitter, dt being the distance between the z-th transmitter and the wireless device, and d( being estimated using the location produced by step II, equation (8).
Beschrijving  (OCR-tekst kan fouten bevatten)

DESCRIPTION

Position Location via Geometric Loci Construction.

The scientific field of the invention is wireless communications and electromagnetic waves propagation. More specifically, part of the invention falls into the field of position location, or estimation of the location of a wireless device.

The calculation of the propagation attenuation factor, in relative research, is usually performed as part of a larger procedure, related to propagation modeling. In the literature, there are a large number of relative papers, such as in [l]-[8]. In these papers, the positions of both transmitter and receiver are known, and the object of research is the extraction of a law of propagation, and/or the calculation of the propagation attenuation factor, which determines the loss of signal strength with respect to the transmitter-receiver distance.

Location of a wireless unit is also subject of extended research during the last years. As an example, consider the development of Global Positioning System (GPS). In GPS, an extra component, known as GPS receiver, is needed at the user's side, in order for the system to work. This means extra cost and size for the wireless unit. Furthermore, GPS suffers in indoor environments, due to large loss of signal strength during penetration of concrete walls, which means that GPS can not be used in indoor LANs.

There are a number of advanced position location (PL) techniques, alternative to GPS, for wireless device positioning. These techniques are known as Radio-Frequency (RF) or network- based techniques, and are categorized as Direction Finding systems (DF systems) and Range Based systems (RB systems) [9]. DF techniques estimate the position location of a mobile station (MS) by measuring the Angle-Of-Arrival (AOA) of the incoming signal, from the MS to a sufficient number of Base Stations (BSs). (Hereinafter, the term BS will generally refer to a fixed transceiver, with known position, and not only to cellular systems BSs.) On the other hand, RB techniques estimate the position of a MS by measuring the distance between the MS and a sufficient number of BSs. RB techniques may be further categorized as ranging, range- sum or range difference, if range estimates are based on Time-Of-Arrival (TOA), Time-Sum- Of-Arrival (TSOA) or Time-Difference-Of-Arrival (TDOA) measurements of the incoming signal, respectively. Furthermore, distance measurements may be implemented based on the signal strength of the incoming signal. DF techniques are used in macrocell networks, such as cellular telephony networks, where

BSs and the MS are located at large distances, and there is also a large difference between the heights of the antennas of the BSs and that of the MS. Advanced AOA measurement techniques, such as MUSIC or ESPRIT are used [9J-[Il]. A major disadvantage of these techniques is the need for expensive equipment and hardware for the AOA estimation [12], such as accurately calibrated antenna arrays etc. Furthermore, in microcells, BSs may be placed at heights below building rooftops. This case also occurs in indoor networks (such as WLANs). In this and other cases, multipath propagation strongly affects AOA techniques, because multiple replicas of the signal arrive at the receiver from different directions [12]. Therefore, DF systems may not perform accurately in such cases.

RB techniques based on TOA measurements are also widespread. These techniques also suffer from multipath propagation, because multiple replicas of the signal arrive at the receiver at different times of arrival. Furthermore, in Non-Line-Of-Sight (NLOS) conditions, only reflected and scattered replicas of the signal arrive at the receiver, making TOA measurements and position location even more inaccurate. Finally, these techniques also need expensive and accurately synchronized hardware (clocks) at the BSs.

RB techniques, which utilize signal strength measurements, have the advantage that they do not need expensive hardware, because signal strength measurement is implemented using inexpensive off-the-shelf components. These systems use either models of propagation and topographical maps of the area of coverage, or in-situ signal strength measurements in selected locations. In the literature, these systems are used in outdoor [13] and indoor environments [14]-[18]. The disadvantage of these techniques is exactly the need for topographical maps or in-situ measurements. Using a statistical propagation model only, yields large inaccuracies, due to the large-scale fading phenomenon. Large-scale fading is caused by shadowing effects of large buildings or natural features [19]-[21]. Even when topographical maps are available, the achieved accuracy is usually not adequate. Furthermore, in-situ measurements are an extremely expensive procedure, and are not always applicable, such as in emergency situations.

The invention, with minimum input the positions of a sufficient number of BSs, and the signal strength from each BS atthe MS's position, determines which of the BSs have the same (or contiguous) attenuation factor to the MS, calculates these factors and estimates the MS's position. This is accomplished by using propagation laws and constructing proper geometric loci. The core of the invention is the construction of the geometric loci, as well as a method for revealing the values of the attenuation factors and the position of the MS. The invention may be incorporated into any wireless network (cellular networks, wireless local area networks, sensor networks etc.), since it may be embodied into existing and inexpensive network equipment (e.g. 802.11 standard mandates on received signal strength indicators [RSSI]). Furthermore, the invention may be incorporated to already existing and functional networks. It is especially suited for operation in unknown environments (e.g. sensor networks), since topographical maps are not needed. The costly and sometimes impracticable procedure of on-the-field pre-measurements is avoided. Additionally, expensive equipment, such as directional, or switched-bearn, or adaptive antenna arrays, is no longer required. The invention overcomes most of these problems, which are characteristic of existing location methods, as aforementioned. For these reasons, the invention offers the possibility of locating a wireless unit with minimum prerequisites and inexpensive equipment.

Attached figures present the principles of the invention, and offer an intuitive understanding.

In figure 1, an example of two Base Stations (BSs), namely BSl, BS2, with the same attenuation factor to a Mobile Station (MS) is presented.

The example of figure 1 is further analyzed in figure 2. If an arbitrary attenuation factor is presumed, then, by measuring the received signal strength by the MS from each BS, two circles will be created. The MS must lie on these circles. Therefore, for this arbitrary attenuation factor, the MS must lie on either of the two intersection points, Pl, P2, of these circles (see figure 2). If no intersection point exists, then the presumed attenuation factor is a priori rejected. In figure 3, the evolution of the case study presented in figure 2 is illustrated. If the arbitrary attenuation factor scans an (arbitrary) interval, then, two circle groups, Cl , Cl , are constructed. The circles of these groups are centered at the fixed positions of BSl and BS2 respectively, and their radii are calculated using the instantaneous hypothetical value of the attenuation factor (similarly to figure 2 description). The geometric locus of the intersection points of the groups Cl, Cl is denoted by Ta_C2 in figure 2. The MS must lie on this locus, and the locus represents all possible locations of the MS.

In figure 4, the case study becomes a little more complicated. Let there be a third BS, namely BS3, with the same attenuation factor to the MS as BSl and BS2. Then, similarly as aforementioned, a third circle group, namely C3 , is constructed. In figure 4, the geometric loci T1 Ci-Ci ' ^ci-c3 of the intersection points of the groups C1-C2 and Cl -C3 respectively are illustrated. The locus TC2_C3 of the intersection points of the groups C2 -C3 is not illustrated for clarity, since it bears no further information on the location of the MS. The MS must lie on the intersection points between the loci T_C2 , ^1.C3, TC1_C3, which are illustrated in figure 4 as Pl and P2. The reason why TC2_C3 bears no further information, is because the intersection points between the loci are not further restricted if TC2_C3 is used instead of TC1_C2 or TC1_C3. The case of a fourth BS, namely BS4, with the same attenuation factor to the MS as BSl,

BS2 and BS3, is presented in figure 5. Similarly, the circle group C4 is constructed. Figure 5 presents the geometric loci T_C2 , T_C3 , T_C4. The locus TCX_CA corresponds to the intersection points of the groups Cl-CA . The MS possible location is restricted to an unambiguous location, namely the joint intersection point of the loci T_C2 , T_C3, T_C4 .

This point is denoted as P in figure 5.

The case of four BSs, which have similar (contiguous) but not equal attenuation factors to the MS, is presented in figure 6. The geometric loci T_C2 , T_C3 , T_C4 do not have a joint intersection point, as they did in figure 5. But, as the invention scans the estimator of the contiguous attenuation factors, there is a value of the estimator for which the sum of distances between the loci is minimal. This value may be used as the estimate of the contiguous attenuation factors, and the position of the device may be estimated as the average of the points

Pl, P2 and P3 of figure 6. The points Pl, P2 and P3 are the points of the geometric loci which correspond to the minimum sum of distances between these loci. The real MS position is not displayed for clarity.

In figure 7, another case study is presented. In this study, there are five BSs, namely BSl,

BS2, BS3, BS4, BS5. Let BSl, BS2 and BS3 have contiguous attenuation factors to the MS.

Let BS4 and BS5 have contiguous attenuation factors to the MS, but not contiguous to the ones of BSl, BS2 and BS3. The invention scans all possible values of the two estimators of these factors. Thus, the circle groups Cl , Cl , C3 , CA , C5 and the geometric loci T_C2 , T_C3 ,

TCA_C5 are constructed, in a way similar to the one aforementioned. The locus TC4_C5 corresponds to the intersection points between the groups CA - C5 . The values of the estimators which correspond to the minimum sum of distances between the respective points of the loci, are used as the estimates of the real attenuation factors. The location of the device is estimated as the average of the point Pl, P2 and P3 of figure 7. These points correspond to the estimates of the real attenuation factors. The real location of the MS is not displayed for clarity.

The case of six BSs, namely BSl, BS2, BS3, BS4, BS5, BS6, is presented in figure 8. Let

BSl and BS2 have contiguous attenuation factors to the MS. Let BS3 and BS4 have contiguous attenuation factors to the MS, but not contiguous to the ones of BSl and BS2. Finally, let BS5 and BS6 have contiguous attenuation factors to the MS, but not contiguous to the ones of BSl, BS2, BS3 and BS4. Similarly as in the four and five BSs cases, the circle groups Cl, C2, C3 ,CA, C5 , C6 and the loci T_C2 , TC3_C4 , TC5_C6 are constructed. Again, the values of the attenuation factors estimators, which correspond to the minimum sum of distances between the loci, are used as the estimates of the real attenuation factors. The location of the device is estimated as the average of the points Pl, P2 and P3 of figure 8. These points correspond to the estimates of the real attenuation factors. The real location of the MS is not displayed for clarity.

Hereinafter, it is assumed that there are a number of fixed transmitters, referred to as BSs, and a mobile receiver or wireless device/unit, referred to as MS. The terms BS and MS will generally refer to a transmitter and receiver, respectively, and should not be confused with cellular networks terminology. A number of assumptions need to be made prior to the presentation of the invention.

The MS is assumed to be placed at the unknown position (xAiS , yMS ) , in an arbitrary, but known, coordinates system. The MS can sense signals from a maximum number of BSs, namely NBSMAX . Let NBSMAX e N and NBSMAX ≥ 4. The received signal strength measurement from each BS can be performed by the MS, since most current MSs have the capability of signal strength indication. However, this is not a prerequisite for the invention. Let i e {l,...,NBSjiWC}, and let the i-th transmitter be placed at the known position

(xBS j,yBS j), of the same coordinates system to the MS (the i-th transmitter's position can be provided by the network administrator). Consequently, the unknown distance dt between the

MS and the i-th transmitter will be given by d, =

The BSs transmit to a known output power (easily provided by the network administrator). The antennas of the BSs and the MS are considered to be omnidirectional and their respective gains are considered to be known. Therefore, the reference power level, that the MS theoretically receives at Im distance from each BS, is known and denoted by P01. The received signal strength at the MS's position, from each BS, is denoted by P1.

The invention selects a number of BSs, namely NBS , which will be used for the position location of the MS. The selection of the specific NBS transmitters is based on criteria that are irrelevant to the algorithm of the invention. The main reason for restricting the number of BSs to be taken into account is computational burden, which means more time needed for the algorithm to be executed. On the other hand, a more accurate result is expected as NBS increases. The user of the invention will determine the value of NBS . A possible criterion for selecting transmitters is to select those transmitters with the strongest signal strength received by the MS, up to a maximum of e.g. nine transmitters (the maximum depends on the computational power available to the user of the invention). Furthermore, it should be noted, that the attenuation factor between the MS and the i-th transmitter, namely n,, ranges vastly, depending on the propagation channel. Evidently, nι e [^min 'w raaχ] • Determining the bounds of nt is irrelevant to the invention, and should be performed by the user of the invention. In practical cases, wmin will be around 2, and wraax will be around 7 to 8, or more.

Based on the assumptions made above, consider the simple case of two BSs, namely BSl and BS2, placed at (xBSι ,yBsi) md (xas2 >>as2) respectively, and a MS placed at

(X MS >yMs) > as illustrated in figure 1. The positions of BSl and BS2 are considered known, while the position of the MS is considered unknown. The distance between the

MS and BSl, BS2, is denoted by dx , d2 respectively, and given by:

di = T](XBS2 - XMS f + iyBS2 - yMS Y (2)

Hereinafter, the theoretical received signal strength by the MS, at Im distance by any BS, will be assumed to be equal to P0 , without loss of generality. The received signal strength from

BSl and BS2, is denoted by P1 , P2 respectively, and given by [19]:

p> - % (3) p> =f (4)

From equations (3) and (4), it is shown that the received power is inversely proportional to the distance raised by an exponent. This exponent is the attenuation factor, and is denoted by W1, n2 for BS1,BS2 respectively. The received signal strength exhibits large scale and small scale fading [19], which will be discussed further below.

The factors W1 , W2 and the distances dx , d2 are unknown. Let W1 , W2 be equal to each other, Le. W1 = W2 = w . By using the equations (3) and (4), estimates for dx , d2 can be

obtained, for any arbitrary value of the estimator w of w . The estimates of dx , d2, for an

arbitrary value of w , are denoted by dx (w) , d2 (w) and given by:

dM) (5)

As the estimator n scans the interval [«„,;„ ,«max] with step nstep , the estimates

dx in) , d2 (ή) are calculated by equations (5) and (6).

(The value of nstep is irrelevant to the algorithms of the invention. Evidently, a larger value of nstep implies faster execution of the algorithm and a less accurate result. On the other hand, a smaller value of nstep implies more accurate results with the cost of increased execution time. Determining the value of nstep should take into account the computational power available to the user of the invention.)

The estimates dx in) , d2 (n) define circle groups, centered at the BSs locations. These

groups are denoted by C1 (xBSl , yBSl , dx («)) , C2 (xBS2 , yBS2 , d2 O)) , i.e. by their respective centers and radii. The MS must he on the intersection points of these circles. This is illustrated

in figure 2, where an arbitrary value of n is selected (different to the real value of n ). The

intersection points Pl and P2 constrain the possible MS locations, for the specific value of n .

If for a value of n the corresponding circles do not intersect each other, then this value is discarded. If the circles are adjacent, there is only one "intersection" point and one possible MS location.

The set of all intersection points, so far defined, for all values of n e [nmin,nmax] , determines a geometric locus. This locus, displayed in figure 3, is denoted by TC1_C2 . The geometric locus is a closed curve, definitely passes by the MS location, and encloses one BS. The BS enclosed is the one closest to the MS. Note that figure 3 displays a geometric locus and not a circle centered at BS2. Evidently, the MS location lies on the locus TCX_C2 .

Let there be a third BS, namely BS3, placed at (xBSi , yBSz ) , at a distance d3 from the MS, transmitting at the same power level as BSl and BS2. Let the attenuation factor of BS3 to the MS be n , equal to the ones of BSl and BS2. The power P3 that the MS receives from BS3 is given by:

"3 while the estimate of the distance d3 for an arbitrary value of the estimator n e [nmin , nmaκ] , is

denoted by di{ri) and given by:

Similarly to the aforementioned analysis, a circle group C3(xBS3,yBS3,d3(n)) is constructed. Furthermore, the geometric locus TC1_C3 of the intersection points between

Q (X BSI j yεsi > di («)) and C3 (xBS3 , J^53 , ds (/?)) , as well as the geometric locus TC2_C3 of the

intersection points between C2(xBS2,yBS2,d2(n)) and C3{xBS3,yBS3,dτ,{nγ) are constructed. The loci TC1_C2 and TC1_C3 are displayed in figure 4. The locus TC2_C3 is not displayed for clarity. The locus TC1_C3 is a closed curve, enclosing BSl, since BSl is closer to the MS than BS3. The intersection points of the two loci, denoted by Pl and P2, are also displayed in figure 4. The points Pl and P2 of figure 4, are the only possible locations of the

MS. Thus, the location of the MS and the value of n are significantly restricted. It should be noted that the locus TC2_C3 is also a closed curve, enclosing BS2. However, the locus TC2_C3 will have the same intersection points Pl and P2 with the other two loci. Therefore, the locus Zc2-C3 does not b∞e any further information on the MS location or the attenuation factor value, and is not displayed in figure 4 for clarity.

Let there be a fourth BS, namely BS4, placed at (xBS4, JF^4), at a distance d4 from the MS, and let the attenuation factor between BS4 and the MS be also n . The received power by the MS, from BS4, is denoted by P4 and given by:

P4 = ^- (9) dA while the estimate of the distance between the MS and BS4, for a value of the estimator

« Kin '"max ] ' isVen by :

d*{n) = ( MpM Y" (10) Similarly, the circle group C4(xBS4,yBS4,d4(n)) is created, and the geometric locus

!TC1_C4 of the intersection points between the groups C1 (X381, yBSι,dι(n)) and

C 4 (X BS45 ^545 ^ 4 (n)) is constructed. The geometric loci T02^3 , TC2_C4, TC3_C4 may also be constructed. Figure 5 displays the loci TC1_C2 , TC1_C3 , T_C4. Evidently, these loci have a joint intersection point, which is denoted by P in figure 5. The loci TC2_C3 , TC2_CA, TC3_C4 do not bear additional information on the MS's location, and are not displayed in figure 5, for clarity. Consequently, the only possible location of the MS is the joint intersection point P, and the only possible attenuation factor value, is the estimate that corresponds to this point. Thus, an estimate of the attenuation factor n , and also an estimate of the MS's position, are specified. The existence of more than four BSs with the same attenuation factor to the MS5 will not improve the estimation of the MS's location.

The invention can also estimate the attenuation factors between four BSs and the MS, in the case where these factors are contiguous to one another instead of equal. In this case, the received signal strength from each BS is given by:

P p1, = ^ dx r (H)

P0

P2 (12) d α2"2

Po

Ps (13) d3

where n1,n2,n3,nA are the attenuation factors between the MS and BSl, BS2, BS3 and

BS4 respectively. In this case, a common estimator n ≡ [nmin,nmax] of W15W25W35W4 is

generated. For each value of « e [wmin,wmax] , the estimates

di(n) , di(ή) , d4(n) of the distances dx , d2 , d3 , d4 respectively are calculated:

In this case, the constructed loci T_C2 , T_C3 , T_C4 do not present a joint intersection

point. However, as n scans [nmia ,nmaκ] with step nstep , there are up to two corresponding

points on each locus. Therefore, the distance between two loci, for a value of n , is defined as

the minimum distance between the points of these loci, for this value of n . If for a value

72rø/ 1 & [nmin ,nmax] the corresponding points on locus 1 are A and B, and for a value

n i,2 e [w min 'ra maχ] ^e corresponding points on locus 2 are C and D, the distance between these two loci, for the specific values of attenuation factor estimators, is the minimum among the distances A-C, A-D, B-C and B-D, i.e. the distance given by:

Distαnce{nv ~ αU,n ~ αh2) = min{d(A, C)5 J(A5 D)5J(B5 C)5 J(B3D)) (19) where J(X5 Y) denotes the distance between the points X and Y. Hereinafter, the term distance between two loci will refer to the distance defined by equation (19).

Consequently, the loci TC1_C2 , TCX_C3 > ^ci-c4 me characterized by a distance between each

other, which is a function of n . The sum of these distances is also a function of n . The

minimum distant points of the loci T_C2 , TC1_C3 , T_C4 , as n scans [^min ,^max] , are used for the estimation of the MS's location. The term "minimum distant points" means that the sum of distances between the loci T_C2 , T01-03 , T_C4 is minimum. The corresponding value of

the estimator n e [nmin,nmaκ] is the estimate of the contiguous factors «l5«25κ35«4. Furthermore, the MS's location is estimated by the average of the corresponding "minimum distant" points of the loci T_C2 , TC\_C3 , T_C4 . This case is illustrated in figure 6, where the "minimum distant points" are denoted by Pl, P2 and P3. Thereby, the estimate of the MS's location is (x,y), where x and y are given by: χ = X1 + X2 +X3 y = *±ψ* (21) where (X1^1) , (x2 ,y2), C^, ^) are tlie coordinates of the points Pl, P2, P3 in figure 6, respectively.

It should be noted that in figure 6, the loci intersect. In another case study, the loci will possibly not intersect, but the attenuation factors and the MS's location are estimated using the minimum sum of distances between the geometric loci in any case. If one or more of the loci

^C2-C3 > ^C2-C4> ^C3-C4 ^ usec* alternatively, the estimation outcome may be slightly different.

The invention uses an internal criterion in order to estimate the accuracy of the attenuation factor and the MS's position estimation procedure. Evidently, this criterion cannot be the true accuracy of position location, since the position of the MS is unknown. Thereby, it should be noted that when the attenuation factors M1 , n2 , n3 , n4 are equal to one another, there is only one joint intersection point, while the same is not true when the factors nγ,n2,n^,n4 are contiguous. The minimum sum of distances between the "minimum distant points" will be larger for more dissimilar attenuation factors. Therefore, the Measure of Applicability (MA) is defined as:

where (X1^1) , (x2,y2), (x3,y3) are the coordinates of the "minimum distant points" (e.g. the points Pl, P2, P3 in figure 6, respectively). The Measure of Applicability is an alternative to the real accuracy and can be used in order to evaluate the result of the overall estimation procedure. Evidently, the MA in equation (22) is the inverse of the minimum sum of distances between the geometric loci. A larger MA corresponds to a more accurate estimation.

The invention, described so far, is able of revealing the MS's location, in the case where there are four BSs, as long as these BSs are characterized by similar attenuation factors to the

MS. In the case where more than four BSs are available, the invention is able to categorize BSs with contiguous attenuation factors, estimate these factors, and also estimate the MS's position.

The cases of 5, 6 or more BSs are now described.

Let there be five BSs, namely BSl, BS2, BS3, BS4, BS5, characterized by the attenuation factors nx, n2 , W3, n4 , n5 to the MS, at a distance of dx , d2 , d3 , dA , d5 away from the MS, placed at (xBSl , yBSl ) , (xBS2 , yBS2 ) , (Xj383 , yBS3 ) , (xBS4 , yBS4 ) , (xBS5 ,yBS5) respectively. The received power by the MS, from each BS, is given by:

P P P P P

P - ° P - ° P — 0 P _ 0 p _ rO (r)n\

M ~ U J1ill ' r2 * d ,2„2 ' r3 ~ d J3iH ' r4 ~ d JA»4 ' ^5 ~ d ,5„5 V^

Let the factors W1, W2 , W3 be contiguous to each other, and the factors W4 , W5 be contiguous to each other, but not contiguous to W1 , W2 , W3. The estimator nA e [wmin , wmax ] of

W1, W2 , W3, and the estimator nB e [wmin , wmax] of W4 , W5 are generated. Using the method that was described for the case of three BSs (refer to figure 4), the geometric loci

T_C2 , TC1_C3 are constructed. Then, the method that was used in the case of two BSs (refer to figure 3) is applied, and the locus TC4_C5 is constructed. Thus, three loci are constructed, which ideally should have a joint intersection point. Practically, as nA scans [wmin,wmax] with step n step ^ and n B scansfflin ^maχ] ™& steP n step ^ raere is & value for U A and a value for nB , for which the sum of distances between the loci is minimum. These values are the estimates of the attenuation factors. It should be noted, that the sum of distances is now a function of nA and nB . The case study is illustrated in figure 7, where the points Pl, P2 and P3, of the loci T_C2 , T_C3 , TC4_C5 , corresponding to the minimum sum of distances, are displayed. The

MS's position is estimated as the average of Pl, P2 and P3, as defined by equations (20) and (21). Finally, theM4 of the specific triplet-pair combination is calculated.

The invention is functional even in the case where it is not known which exactly of the five BSs are characterized by contiguous attenuation factors. In this case, all possible combinations of five BSs taken three at a time are configured (the order of selection is immaterial). Then, the MA is calculated for each triplet-pair combination. The combination corresponding to the optimum (maximum) MA, among the configured triplet-pair combinations, is the dominant combination, and is used in order to estimate the MS's position.

Let there be 6 BSs, namely BSl, BS2, BS3, BS4, BS5, BS6, with attenuation factors W1,

W2 , w3, W4 , W5, W6 , at a distance dx, d2 , d3, d4 , d5 , d6 away from the MS, placed at

(XBSI > yβsi ) > (XBS2 > yΕs2 ) ' (XBS3 > Λ53 ) > (XBS4 > y>BS4 ) ' (xBss > -Xros ) > (XBS6 > ^6 ) respectively. The received power by the MS, from each BS, is given by:

P P P P P P

P - Q p - ° p - Q p - o p - Q p - Q OΛ\

1 1 ~ JBl ' r2 ~ ,«2 ' ^3 ~ ,„3 ' r4 - ,nA ' λ 5 ~ ,„5 ' T6 ~ ,n6 V") ai a2 U3 a4 U5 U6 Let the factors H1 , n2 be contiguous to each other, the factors n3, n4 be contiguous to each other but not contiguous to W1, n2 , and the factors n5, W6 be contiguous to each other, but not contiguous to W1 , W2 , W3 , W4. The estimator nA e [nmin , wmax ] of W1 , W2 , the estimator n B e Kπn > w maJ °f «3 > U A aad me estimator wc e [wmin,wmax] of n5, n6 are generated. Similarly to the case of two BSs (refer to figure 3), the geometric loci T_C2 , TC3_C4 , TC5_C6 are constructed. Thus, three loci are constructed, which ideally should have a joint intersection point. Practically, as nA scans [wmin ,wmax] with step nslep , nB scans [wmin ,wmaj with step nstep , and nc scans [wmin , wraax] with step nslep , there is a value for nA , a value for nB and a value for nc , for which the sum of distances between the loci is minimum. These values are the estimates of the attenuation factors. It should be noted, that the sum of distances is now a function of nA , nB and nc . This case is illustrated in figure 8, where the points Pl, P2 and P3 of the loci TC1_C2 , TC3_C4 , TC5_C6 correspond to the minimum sum of distances. The MS's position is estimated as the average of Pl, P2 and P3, as defined by equations (20) and (21). Finally, the MA of the specific pair-pair-pair combination is calculated. The invention is functional even in the case where it is not known which exactly of the six

BSs are characterized by contiguous attenuation factors. In this case, all possible combinations of six BSs taken two at a time are configured (the order of selection is immaterial). Thus, a pair and a remaining quadruplet are configured. Then, all possible combinations of the remaining four BSs, taken two at a time, are configured (the order of selection is immaterial). Thus, all possible combinations of the type pair-pair-pair are formed, and the MA is calculated for each one of them. The combination corresponding to the optimum (maximum) MA, among the configured pair-pair-pair combinations, is the dominant combination, and is used in order to estimate the MS's position.

Evidently, the invention may be developed, in order to categorize any number of BSs

(greater than six) with respect to the propagation attenuation factor to the MS. Based on the analysis of four, five and six BSs, all combinations of the BSs, taken four, five or six at a time are configured (the order of selection is immaterial). Each combination is scrutinized, properly analyzed, if needed, to triplet-pair and pair-pair-pair combinations (in the case of pentad or hexads, respectively). The invention measures the MA of each sub-combination, and determines the optimum group of BSs. Thus, the unknown attenuation factors are estimated, and the MS position is located. The residual attenuation factors, after the MS is located, are calculated, using the equation: log^hlogrø

logtø) where «, is Hie attenuation factor of the Mh BS to the MS, and d. is calculated considering the estimated MS position and the known position of the Mh BS.

Finally, a discussion follows, regarding technical issues that may arise during the employment of the invention to real-world problems, and ways of overcoming these issues.

If the number of BSs that the MS can sense is too large, the execution of the algorithm may significantly delay. A number of BSs needs to be selected, in order to provide proper input to the algorithm. A smaller number of BSs means less accuracy but smaller execution time, while a larger number of BSs improves accuracy but execution time delays significantly. A possible criterion for BS selecting, is to select those BSs whose received signal is strongest. Another criterion is to select those BSs, whose signal strength fluctuation, as measured by the MS, is minimum. Actually, the criterion that the user of the invention will adopt is irrelevant to the algorithm of the invention. Small-scale fading refers to the rapid fluctuations of the received signal in space, time and frequency, and is caused by the signal scattering off objects between the transmitter and receiver [19], [20], [21], [22]. The large signal fluctuation introduced by small-scale fading means that the instantaneous value of the received signal strength may be very different than the local mean, and that this value changes rapidly with time and with movement of the order of wavelength. Performance degradation of the proposed algorithm may appear under strong fading conditions. In order to mitigate the consequences of small-scale fading, antenna diversity may be utilized at the transmitter, receiver, or both. Furthermore, a large number of signal samples from all BS's may be collected by the MS, in order to affront time-varying fading. Also, a large number of signal samples may be collected while moving slightly the MS, at distances of the order of wavelength. The invention can be used together with any of the techniques of small-scale fading mitigation described herein.

Large-scale fading or shadowing is caused by buildings or natural features and is determined by the local mean of a small-scale fading signal [19], [20], [21]. Large-scale fading means that the mean signal strength value varies from the value predicted by path loss slope [23]. In other words, large-scale fading describes the effect which occurs over a large number of measurement locations, which have the same transmitter-receiver separation, but have different levels of clutter on the propagation path. Consequently, even after mitigating small- scale fading using the techniques described above, the local mean signal strength may be measured to be different than predicted by the slope of the path loss diagram. However, large- scale fading does not need to be resolved when using the invention method. Rather, the invention dynamically resolves any changes to the effective attenuation factors by anew estimating the MS location.

When the transmitted power by each BS is not constant, the invention uses the information of the theoretical received power at Im transmitter-receiver separation (provided by the administrator of the network), in order to categorize the BSs and locate the MS. The equations

(3) to (18), and (23) to (25) are used with no modification other than the power received at Im distance.

In some cases there is prior information on the position of the MS. As an example, consider the case of cellular systems, where the MS position is restricted into the cell of the associated BS. In a case like this, this information may be combined with the MS location provided by the invention, in order to ameliorate the accuracy of the location system.

If the orientation of the MS user affects the received power, the invention may sample the received power for different user orientations. This case is relevant to space-selective fading. In any case, the orientation with the optimum MA is selected.

The resolution of the estimator n may also be an issue. The computational burden of the algorithm increases as the value of nstep decreases. Consequently, there is a trade off between the precision of the estimated attenuation factors and execution time. The selection of nstep values should be performed by the invention user, and is irrelevant to the algorithm of the invention.

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Classificaties
Internationale classificatieG01S5/10, H04W64/00, G01S5/02, G01S5/14
Coöperatieve classificatieG01S11/06, H04W64/00, G01S5/02, G01S5/14
Europese classificatieG01S11/06, G01S5/02, H04W64/00, G01S5/14
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