1074 IEEE TRANSACTIONS ON SYSTEMS.MAN.AND CYBERNETICS-PART C:APPLICATIONS AND REVIEWS,VOL.37.NO.6.NOVEMBER 2007 a sensor measurement for estimating inter-tag distance.They interference because of the differences in signal types and radio exploit the density of tags and correlation of multiple measure- spectrum used.UWB short duration pulses are easy to filter in ments to improve both accuracy and precision.Another system order to determine which signals are correct and which are gen- is called LANDMARC (indoor location sensing using active erated from multipath.At the same time,the signal passes easily RFID)[29].Its prototype uses the RFID reader's operating fre- through walls,equipment and clothing.However metallic and quency with 308 MHz.In order to increase accuracy without liquid materials cause UWB signal interference.Use of more placing more readers,the system employs the idea of having UWB readers and strategic placement of UWB readers could extra fixed location reference tags to help location calibration. overcome this disadvantage.Short-pulse waveforms permit an These reference tags serve as reference points in the system. accurate determination of the precise TOA and,namely,the pre- The LANDMARC approach requires signal strength informa- cise TOF of a burst transmission from a short-pulse transmitter tion from each tag to readers.The kNN method is adopted to to a corresponding receiver [33],[32].UWB location exploits calculate the location of the RFID tags.It is reported that the 50 the characteristics of time synchronization of UWB communi- percentile has an error distance of around 1 m while the maxi-cation to achieve very high indoor location accuracy (20 cm). mum error distances are less than 2 m for LANDMARC system. So it is suitable for high-precision real-time 2-D and 3-D loca- tion.3-D location positioning can be performed by using two C.Cellular-Based different measuring means:TDOA,which is measuring the time difference between a UWB pulse arriving at multiple sensors, A number of systems have used global system of mobile/code division multiple access(GSM/CDMA)mobile cellular network and AOA.The advantage of using both means in conjunction to estimate the location of outdoor mobile clients.However,the is that a location can be determined from just two sensors de- accuracy of the method using cell-ID or enhanced observed time creasing the required sensor density over systems that just use difference(E-OTD)is generally low (in the range of 50-200 m), TDOA.More UWB knowledge and products are given in'and their related references. depending on the cell size.Generally speaking,the accuracy is To date,several UWB precision localization systems have higher in densely covered areas (e.g,urban places)and much been fielded [34].8.9,10 The Ubisense systems is a unidirectional lower in rural environments [30]. Indoor positioning based on mobile cellular network is pos- UWB location platform with a conventional bidirectional time sible if the building is covered by several base stations or one division multiple access (TDMA)control channel.The tags base station with strong RSS received by indoor mobile clients. transmit UWB signals to networked receivers and are located Otsasen et al.presented a GSM-based indoor localization sys- using AOA and TDOA.Ubisense works by creating sensor cells. Each cell requires at least four sensors or readers.Throughout tem in [31].Their key idea that makes accurate GSM-based in- buildings or collections of buildings,an unlimited number of door localization possible is the use of wide signal-strength fin- readers can be networked together in a manner similar to cellular gerprints.The wide fingerprint includes the six strongest GSM phone networks.The readers receive data from the tags,from cells and readings of up to 29 additional GSM channels,most of as far as 150 ft,and send it through the Ubisense Smart Space which are strong enough to be detected but too weak to be used software platform. for efficient communication.The higher dimensionality intro- duced by the additional channel dramatically increases localiza- Microwave frequency,covered by the UWB frequency band, tion accuracy.They present results for experiments conducted is used in Siemens local position radar(LPR)[24].Siemens LPR on signal-strength fingerprints collected from three multifloor is an RTOF system,in which the RTOF between a transponder unit and measuring units/base stations is measured via the fre- buildings using weighted kNN technique.The results show that their indoor localization system can differentiate between floors quency modulated continuous wave (FMCW)radar principle.It was launched for industrial applications like crane and forklift and achieve median within-floor accuracy as low as 2.5 m.The same method could be applied in IS-95 CDMA and 3G mobile positioning.It is applicable only for LOS environment. network. E.WLAN (IEEE 802.11) D.UWB This midrange wireless local area network (WLAN)stan- dard,operating in the 2.4-GHz Industrial,Scientific and Med- UWB is based on sending ultrashort pulses (typically <1 ns),with a low duty cycle(typically 1:1000).On the spec- ical (ISM)band,has become very popular in public hotspots and enterprise locations during the last few years.With a typical tral domain,the system,thus,uses an UWB (even >500 MHz wide).UWB location has the following advantages [32].Un- gross bit rate of 11,54,or 108 Mbps and a range of 50-100 m, like conventional RFID systems,which operate on single bands IEEE 802.11 is currently the dominant local wireless network- of the radio spectrum,UWB transmits a signal over multiple ing standard.It is,therefore,appealing to use an existing WLAN infrastructure for indoor location as well,by adding a location bands of frequencies simultaneously,from 3.1 to 10.6 GHz. UWB signals are also transmitted for a much shorter duration than those used in conventional RFID.UWB tags consume less 7Intel Corporation.http://www.intel.com/technology/comms/uwb/.And power than conventional RF tags and can operate across a broad Ultrawideband planet:http://www.ultrawidebandplanet.com UbiSense Company.http://www.ubisense.net area of the radio spectrum.UWB can be used in close prox- 9Aether Wire Location,Inc.http://www.aetherwire.com imity to other RF signals without causing or suffering from 10Time Domain Company.http://www.timedomain.com Authorized licensed use limited to:University of Pittsburgh.Downloaded on January 27.2009 at 17:04 from IEEE Xplore.Restrictions apply.1074 IEEE TRANSACTIONS ON SYSTEMS, MAN, AND CYBERNETICS—PART C: APPLICATIONS AND REVIEWS, VOL. 37, NO. 6, NOVEMBER 2007 a sensor measurement for estimating inter-tag distance. They exploit the density of tags and correlation of multiple measure￾ments to improve both accuracy and precision. Another system is called LANDMARC (indoor location sensing using active RFID) [29]. Its prototype uses the RFID reader’s operating fre￾quency with 308 MHz. In order to increase accuracy without placing more readers, the system employs the idea of having extra fixed location reference tags to help location calibration. These reference tags serve as reference points in the system. The LANDMARC approach requires signal strength informa￾tion from each tag to readers. The kNN method is adopted to calculate the location of the RFID tags. It is reported that the 50 percentile has an error distance of around 1 m while the maxi￾mum error distances are less than 2 m for LANDMARC system. C. Cellular-Based A number of systems have used global system of mobile/code division multiple access (GSM/CDMA) mobile cellular network to estimate the location of outdoor mobile clients. However, the accuracy of the method using cell-ID or enhanced observed time difference (E-OTD) is generally low (in the range of 50–200 m), depending on the cell size. Generally speaking, the accuracy is higher in densely covered areas (e.g, urban places) and much lower in rural environments [30]. Indoor positioning based on mobile cellular network is pos￾sible if the building is covered by several base stations or one base station with strong RSS received by indoor mobile clients. Otsasen et al. presented a GSM-based indoor localization sys￾tem in [31]. Their key idea that makes accurate GSM-based in￾door localization possible is the use of wide signal-strength fin￾gerprints. The wide fingerprint includes the six strongest GSM cells and readings of up to 29 additional GSM channels, most of which are strong enough to be detected but too weak to be used for efficient communication. The higher dimensionality intro￾duced by the additional channel dramatically increases localiza￾tion accuracy. They present results for experiments conducted on signal-strength fingerprints collected from three multifloor buildings using weighted kNN technique. The results show that their indoor localization system can differentiate between floors and achieve median within-floor accuracy as low as 2.5 m. The same method could be applied in IS-95 CDMA and 3G mobile network. D. UWB UWB is based on sending ultrashort pulses (typically <1 ns), with a low duty cycle (typically 1 : 1000). On the spec￾tral domain, the system, thus, uses an UWB (even >500 MHz wide). UWB location has the following advantages [32]. Un￾like conventional RFID systems, which operate on single bands of the radio spectrum, UWB transmits a signal over multiple bands of frequencies simultaneously, from 3.1 to 10.6 GHz. UWB signals are also transmitted for a much shorter duration than those used in conventional RFID. UWB tags consume less power than conventional RF tags and can operate across a broad area of the radio spectrum. UWB can be used in close prox￾imity to other RF signals without causing or suffering from interference because of the differences in signal types and radio spectrum used. UWB short duration pulses are easy to filter in order to determine which signals are correct and which are gen￾erated from multipath. At the same time, the signal passes easily through walls, equipment and clothing. However metallic and liquid materials cause UWB signal interference. Use of more UWB readers and strategic placement of UWB readers could overcome this disadvantage. Short-pulse waveforms permit an accurate determination of the precise TOA and, namely, the pre￾cise TOF of a burst transmission from a short-pulse transmitter to a corresponding receiver [33], [32]. UWB location exploits the characteristics of time synchronization of UWB communi￾cation to achieve very high indoor location accuracy (20 cm). So it is suitable for high-precision real-time 2-D and 3-D loca￾tion. 3-D location positioning can be performed by using two different measuring means: TDOA, which is measuring the time difference between a UWB pulse arriving at multiple sensors, and AOA. The advantage of using both means in conjunction is that a location can be determined from just two sensors de￾creasing the required sensor density over systems that just use TDOA. More UWB knowledge and products are given in7 and their related references. To date, several UWB precision localization systems have been fielded [34].8,9,10 The Ubisense system8 is a unidirectional UWB location platform with a conventional bidirectional time division multiple access (TDMA) control channel. The tags transmit UWB signals to networked receivers and are located using AOA and TDOA. Ubisense works by creating sensor cells. Each cell requires at least four sensors or readers. Throughout buildings or collections of buildings, an unlimited number of readers can be networked together in a manner similar to cellular phone networks. The readers receive data from the tags, from as far as 150 ft, and send it through the Ubisense Smart Space software platform. Microwave frequency, covered by the UWB frequency band, is used in Siemens local position radar (LPR) [24]. Siemens LPR is an RTOF system, in which the RTOF between a transponder unit and measuring units/base stations is measured via the fre￾quency modulated continuous wave (FMCW) radar principle. It was launched for industrial applications like crane and forklift positioning. It is applicable only for LOS environment. E. WLAN (IEEE 802.11) This midrange wireless local area network (WLAN) stan￾dard, operating in the 2.4-GHz Industrial, Scientific and Med￾ical (ISM) band, has become very popular in public hotspots and enterprise locations during the last few years. With a typical gross bit rate of 11, 54, or 108 Mbps and a range of 50–100 m, IEEE 802.11 is currently the dominant local wireless network￾ing standard. It is, therefore, appealing to use an existing WLAN infrastructure for indoor location as well, by adding a location 7Intel Corporation. http://www.intel.com/technology/comms/uwb/. And Ultrawideband planet: http://www.ultrawidebandplanet.com 8UbiSense Company. http://www.ubisense.net 9Aether Wire & Location, Inc. http://www.aetherwire.com 10Time Domain Company. http://www.timedomain.com Authorized licensed use limited to: University of Pittsburgh. Downloaded on January 27, 2009 at 17:04 from IEEE Xplore. Restrictions apply