JOURNAL OF SCIENCE & TECHNOLOGY • No. 95 - 2013
INDOOR POSITIONING SYSTEM BASED ON PASSFVE RFID H$ T H 6 N G D I N H VI RFID THU B O N G 0 M(5l TRlTt^NG TRONG NHA
Dao Thi Mao, Doan Thi Ngoc Hien, Tran Duy Khanh, Nguyen Quoc Cuong Hanoi Universily of Science and Technology
Received February 20, 2013; accepted April 22, 2013 ABSTRACT
Nowadays, with the advance of communication & information technology, the need for gtoballzat'ion and localization on subjects is paid more attention. The localization depends much on the propagation environment. Esfiecially, In the indoor environment - complex environment, the diiedm of radio wave is hardly predictable. If we know effects of propagation environment, we are able to calculate and estimate the position of objects in the effective average area of RF signal. Hence, (ftis paper Is going to present an overview of some lectors that limit the performance range of indm passive RFID positioning systems. Received Information of passive reference tags are used to piedia approximately transmission tosses along tha transmission distance. Then, the two ~ dimensionai localization of objects in the indoor environment is presented into details. Our system Is built wilh (he improved Landmark model: using the pessive RFID tags, using tmly one reader antenna, desigtnng system In a small expenmenlal room with many moving people and electronic devices around.
Keywords: Passive RFID system, interfacial phenomenon of indoor spaces. Path Loss Model TOM TAT
Swphit triin cua cdng ngh$ thdng tin vi truyin thdng lam ting nhu ciu vi khoanh vCing, (J/i/i n dii tuwig. Nhung vi^c dinh vi phu thu^ nhiiu vio mil tnri/ng truyin sdng diin tCr. B$c bi$t ii ma tnidng trong nhi cd nhiiu v^t cin sS khO xie dinh dugc phuong truyin cija sdng di$n t(t. Niu bk ducrc nhung inh hu-dng cua mil tnrdng truyin sdng thi cd thi tlnh toin. u<rc lugng din^c vl tri cQa ck dii tugng. Bit bio trinh biy mdt s6 yiu ti inh hudng tdl hi$u suit cua h$ thing dinh vj RFID Um ding d mdi tnidng trong nhi Cic thi tham chiiu thu ding giup uOc Itigng dugc lirgng tin hao duim tmyin theo khoing cich tniyin sdng. Tiip theo. qui trinh dinh vj 2 chiiu 6 mdi trutmg tiong nhi dm trinh biy cu Ihi. Hi thing dinh vi dugc xiy dung (/(ra theo md hlnh Landmark vdi nhiiu dtim cii tik:
si dung cic thi RFID thu ding, 1 angten ctia diu dgc: dugc lip dit trong phdng thi nghiem nM nhiiu thiit bi diin tCr, ngudi di chuyin d xung quanh.
1. INTRODUCTION The structure of this paper is arranged in The RFID (Radio Frequency ' ™ ™^'" sections: theories and experiments.
Identiflcation) localization is a technique to Theories are presented in three following parts locate the position of objects by using the RFID ^^ " ' released some basics and an approach system. In today's short-range RF scene, many ' ° analyze the collected database. The second applications are operated in an indoor section is the experimental part that shows the environment, such as residential homes or ''^Ps ' ° ' ' collecting and processing the collected office buildings. The indoor RF signal ^^^ ' ° '"'^^'^ * c position of target tags. The propagation is effected by many factors such as ' ^ ' " ° ' ' ' ' °^ * ' ^ P^P^'' ' ' some goals and reflection, diffraction and scattering conclusions,
phenomenon. Thus, the RF signal at a given 2. LOCALIZATION USING RFID point can be a combination of reflected signals
diffracted signal components from various 2.1. Basics of RFID positioDing system soiirces[l]. This causes difficulties in A RFID positioning system basically estimating the radio wave propagation. In this consists of two componentsp]: RFID reader paper, we propose a localization approach using and RFID tag. The reader normally consists ofa passive RFID tags. transmitter, a receiver, a microprocessor and antennas that transmit the RF wave from the
JOURNAL OF SCIENCE & TECHNOLOGY * No. 95 - 2013 reader to Interrogate for tags and then receive
responses from tags.
A RFID tag basically consists ofa small antenna and an IC {Integrated Circuit). This IC stores some information regarding the tag. The tag antenna is used to receive and transmit the RF signal. There are two types of RFID tags:
passive tags and active tags. Passive tags are supplied power while active tags have their own transmitter battery. Therefore, passive tags cannot work outside the detection range ofthe reader. However, It makes inexpensive system setup and longer lifetime of system.
2.2. Database processing approach a. Data Collection
Reader collects data of all tags. This informadon is stored in profile including number of collected tags, ID, collection time and RSSI value of collected tags. Next step, database is constructed based on collected data and known position of reference tags.
Therefore, in a database, each reference position has RSSI value and distance to reader antenna (coordinates) at a collection time.
b. Data processing
The most common method of predicting indoor propagation loss is by using empirical models - Radio Wave Propagation (RWP). The regression analysis method is used to construct it. TTie most basic model is a free space RWP model that is released following the Ferris propagation equation:
' * ( — ) '
In open environments, this equation is converted into a popular equation (the Log - Distance path loss equation) that predicts the Path Loss (PL) along the distance (the effective coverage area ofa reader):
PLu, = PL^^ + 20logd(dBm) (2.2) In Indoor environments, the walls, ceilings and floors can form a structure. This structure forces the RF signals to propagate in particular directions, consequently, changes the PL pattems of the signal propagation.
According to practical experiments of
researchers, the Log - Distance Path Loss form is changed to an almost linear fonn[3],[4]:
PL = PL^+20n\ogd±d(dBm) (2.3) In the above formulas, P, is the transmitting power from reader, P^. Is the receiving power from tag, g, and g^ are transmitting and receiving antenna gains, A is the radio wave length, i is the distance between transmitting and receiving antenna;
Pl^ and Pl^i are the reference losses at the reference distance in each case; n is the path toss exponent; d is the regression standard error. The most common technique for determining the coefficients i^/^, n and d is Ordinary Least Squares (OLS). The values of PL^ and n are chosen to minimize the sum of the squared residuals.
The stability of the database is evaluated by following two parameters[5]: the regression standard error ( 5 ) and the "goodness of fit" ( R^). There are two equadons for calculadng these values;
a = , f ^ , i i ^ = l - ^ (2.4)
\ n - 2 SST Where,. SSR is the Sum of Squared Residuals, SST is Total Variation - Total Sum of Squares.
The Standard Error d shows that the sample data level is fitted with the established equation. R^ is a more standardized statistic, which also gives a measure ofthe "goodness of fit" of the estimated equation with value ranging 0<R^<1. The value ^ ^ =1 is a
"perfect score" that is obtained only if the data points lie exactly along a straight line.
2.3. Localization Approach
Actually, our localization process is described in following cycle:
dinance bewt tbe reads IDTFI
& Ibe lass
It & lelerted referi
Fig. 1. Process of our localization approach
JOURNAL OF SCIENCE & TECHNOLOGY * No. 95 - 2013 3. EXPERIMENTS
3.L How to setup the RFID experimental Model
Our system is based on the Landmark model. However, some characteristics are modified into match actual conditions. The distance between two reference positions Is 30 centimeters (larger than distance A./2) to reduce the radio signal interface between tags in experimental table[6],[7]. The size of table Is 90cmX180cm. Hence, it can only contain 28 reference positions and some target positions on an experimental table. The system uses only one reader antenna and one passive tag, which is put through all reference and target positions.
The experiments are performed by using the RFID reader ThingMagic 6 which works In the UHF frequency band (the EU3 frequency region)[7]. The coaxial cable 1.85m long Is used to connect between the antenna and the reader.
An Antenna 865 - 956 MHz MT - 242025/TRH/A is applied to distribute uniformly the radio wave following a radiated symmetrical pattern over the interrogation area.
The best antenna orientation is the one in which the detection of the reference landmark tags is more and the database has the highest stability level.
The experiments are implemented in a closed room with a lot of electrical equipment around. The tag is horizontally attached to the wood bar that stands on the experimental table roughly facing the antenna. Fig.2 shows the experimental setup.
Fig. 2. Experiment setup Twelve databases are collected at ten different reader antenna positions and different data collecting times.
3.2. Data process and localization steps a. The data process
An approach similar to the approach of RADAR system is used to locate target tags[9]:
choosing the most suitable reference position.
The following table shows some quali^
coefficients of 12 databases:
Table 1- Coefficients of databases
fil l
0 10 10 10 13 IS IS 15 15 15 20 20
ThtluJiht efrtidtr inttnni
104 170 150 160 US 157 147 137 137 137 156 147
Thi coffRchnt
0.32 0.25 0.S2 0.37 0J5 0J4 0.37 0.72 0.7 OSS 0.2S 0.66
T h t rifrtuisn itindird trrarfdBni 2.7 236 2.5 2.66 27 3.45 2.36 2.1S 2i3 2.48 2.9 2.57
T h t ttlrmittt
dirtince trr«(K) S7.3B 159.06 35J4
«M 44 S6 46.7 47J3 2 2 i 9 23J9 28.43 48.64 24.43
D i t i e f coHtction
04J«iuaivaH]
QJImwylOU 03Ja[iuaiv»13 (Ulanirim 03 January 2013 03Janurvllll3 03 January 2(113 03 January 2013 OdJanua(vllll3 09 January 2013 03 January 2(113 a3Jangarr2(ll3
In all cases, we have chosen the database having the best quality (the antenna 137cm height and the 15-degree angle). The PL equation is drawn with the corresponding coefficients:
Path L o u & Oinanc* Equation
Fig. 3 The Best Quality Path loss equation m our experimentation
Distance Error of Reference Positions 3S
3 —-— •desttmited distance Idactuilditunce
1 2 3 4 6 910111213141516171920212225272!
Data Collected Reference Positions Fig. 4. Comparing estimated & actual distance 98
JOURNAL OF SCIENCE & TECHNOLOGY • No. 95 - 2013 The total equation is written In the form:
? i = 54.036 +1.2144*20*Log(/±2.148(rfSm) (3.1) After that, the estimated distance is calculated by using the derived equation:
/>X-54.036
^ 20*1.2144
Fig.4 shows the difference between the estimated distance and the actual distance that is relatively small.
b. Localization steps
The localization process is divided Into several steps. The first step is to install the coordinates of objects following the Fig.5.
.(,;,
- >7.Vj" "
P I =
. P i :
P i 1
P26 p ; 6 - - - ; 1 •
---r
r^ J'
--
' p j 1 p ; 7 P3S(0
.__
-—
f,..
^ - 1
ViU
Fig. 5. Theposition of all points In which, the green position Is reference position; the red position is the target position.
These target positions are installed In any positions in experimental table.
Table 2. Estimated distance range
The p i t h t e n v . r u . r > r c . { d B m )
PLmin - 55.85 PL = 56 PLmai = 6015
d , . t > r c . cne-(>n)
1-19 1.32 1.78
The second step is to calculate the estimated distance range of target tag. For example, we need to locate the target tag at the position 29 (located at the 3"" row). At dils point, the estimated distance range Is shown in Table 2. We can choose some appropriate
reference values with the estimated distance in estimated distance range of target position.
The third step is to compare the histograms between target and appropriate reference positions (four in this case: positions 1, 4, 11, 14). Histogram charts release RSSI value range and frequency of each RSSI value.
If reference position has similar histogram with target position, it has same RSSI value frequencies and small estimated distance error.
Finally, the most similar histogram to target point histogram is chosen:
Fig 6. The most similar histograms As seen in Fig.6, the target position has only RSSI value -58dBm (10 times) and the reference position 11 has RSSI value -58dBm (7 times) and -57dBm (3 times). This position 11 has highest RSSI value -58dBm frequency in four appropriate reference positions. Therefore, the coordinates at the target position 29 have values ofthe reference position 11.
Table 3. The estimated coordinate error of target points
Number
1 2 3 4 S
e
7 S
T i r g c t I M i i l i o n
2 1 3 4 3 3 1 3 3 5 3 2 2 5 2.6
Calibrated PL iinounts(dBm) 1S4 1 5 0 1 4 9 I S B 1 5 8 1 4 0 1 5 5 1 7 7
TIlB e i i l m M e d
poirtlon
e
11 11 15 15 4 19 25
arrorlm) 0 1 4 0 3G 0.20 0 51 0 28 0 53 0 22 0,50
3.3. Calibration process
As mentioned above, the indoor RF propagation depends much on the environmental characteristics. Therefore, the collected databases are different for different environments. Even in the same room, the RF propagation parameters (PI^ and n) may change over time. In practice, only the best-
JOURNAL OF SCIENCE & TECHNOLOGY * No. 95 - 2013 collected database is used to locate all target
positions. Hence, the main problem is how to adapt the received target tags data for use in the best quality collected database. It means that the PL value ofthe target tag in other measurement time is added a calibrated PL amount (APL )[9].
If the number of reference tags is small, the PL equation will be calibrated. In this case, we have two different quality databases that are collected in two different days at the same position of reader antenna;
P^= 54.036+ l.2\44* 20* Logd±2.\4^(dBm) PL2 = 53.537 +1.1623 * 20 * Logd ± 2.MS6(dBm) We must calibrate the data from the first equation because the first equation has better quality and higher stability. The total difference of equadon elements is the calibrated RSSI value:
APL = \.1996+ \.042* LogdidBm) (3.3) After calibrating, target positions are located following good results:
4. GOALS AND CONCLUSIONS With the above-achieved results, we can localize the position of objects through
comparing and choosing histograms of reference points in the estimated distance range.
The average error is 30 centimeters (almost the distance from the tag to the tag reference). In the world, there are few Indoor passive RHD localization systems. These systems have some readers (3 or 4 readers) with 37 cm localization average error[10] or 30 cm[ll]. However, our system uses only one reader antenna and one passive tag to get the data, which is one of its advantages. In the future, we are going to design the system with some reference tags in the experimental table to acquire the database.
In the last case (28 reference tags), we can use the KNN approach in order to locate the target positions.
5. Acknowledgements
This work is done in the framework of the Intemational cooperation project 10/2011 /HD-NDT. Audiors would like to thank HaNol University of Science and Technology for providing necessary financial assistance of our project. The authors also would like to thank the anonymous reviewers who had valuable suggestion that has helped to Improve the quality ofthe manuscript.
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Author's address: Nguyen Quoc Cuong - Email: [email protected] Hanoi University of Science and Technology No.l Dai Co Viet Str., Ha Noi, Viet Nam
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