Patent Publication Number: US-2011050501-A1

Title: Location system and method with a fiber optic link

Description:
RELATED PATENT APPLICATIONS  
     The present application claims the benefit of U.S. Provisional Patent Application entitled “LOCATION SYSTEM AND METHOD WITH FIBER OPTIC LINK”, filed Oct. 27, 2008, having Ser. No. 61/108, in the name of the same inventor. 
     The present application is further related to U.S. Patent Application entitled “METHOD AND SYSTEM FOR LOCATION FINDING IN A WIRELESS LOCAL AREA NETWORK”, filed on Aug. 20, 2002, having a Ser. No. 10/225,267; U.S. Pat. No. 6,968,194, entitled “METHOD AND SYSTEM FOR SYNCHRONIZING LOCATION FINDING MEASUREMENTS IN A WIRELESS LOCAL AREA NETWORK”, issued on Nov. 22, 2005; and United States Patent Application, entitled “METHOD AND SYSTEM FOR SYNCHRONIZATION OFFSET REDUCTION IN A TDOA LOCATION SYSTEM”, filed on Oct. 24, 2006, having a Ser. No. 11/552,211; the specifications of which are herein incorporated by reference. 
    
    
     FIELD OF THE INVENTION  
     The present invention relates generally to communications networks, and more specifically, to the use of fiber optic links in a radio location system as a replacement to RF coaxial cable links. 
     BACKGROUND OF THE INVENTION 
     A multitude of wireless communications systems are in common use today. Mobile telephones, pagers and wireless-connected computing devices such as personal digital assistants (PDAs) and laptop computers provide portable communications at virtually any locality. Wireless local area networks (WLANs) and wireless personal area networks (WPANs) according to the Institute of Electrical and Electronic Engineers (IEEE) specifications 802.11 (WLAN) (including 802.11a, 802.11b, 802.11g, 802.11n, etc.), 802.15.1 (WPAN) and 802.15.4 (WPAN-LR) also provide wireless interconnection of computing devices and personal communications devices, as well as other devices such as home automation devices. 
     Within the above-listed networks and wireless networks in general, in many commercial and industrial applications it is desirable to know the location of wireless devices and RFID tags. The above-incorporated patent applications describe a system for location finding in a wireless area network. 
     Techniques that may be used to determine location are disclosed in the above-incorporated patent applications. The techniques include loop delay measurement for distance determination or received signal strength measurement (RSSI), time-difference-of-arrival techniques (TDOA), and angle-of-arrival techniques (AOA) for location finding. 
     A typical deployment of such a location system includes a plurality of WLAN transceivers and/or access points, each unit connected to one or two antennas to receive and transmit wireless signals. According to different deployment alternatives, the antennas of those transceivers and/or access points can be directly connected to the unit or through a suitable RF coaxial cable. 
     Typical uses of RF coaxial cables include many cases where the antenna shall be mounted on a mast or pole to ensure proper coverage while the transceiver or access point unit needs to be mounted on the ground or in a covered area far away from its antenna. In those cases, the length of the coaxial cable is a critical factor since it directly affects the system performance. A long RF coaxial cable (e.g. &gt;10-15 m) may have a significant attenuation (e.g. &gt;3 dB) that will degrade the overall system performance. 
     This problem has already been identified and fiber optic solutions have been proposed and commercially implemented. This solution consists of replacing the RF coaxial cable with a fiber optic link and two transponders which convert the RF signal to a light signal and vice versa. Since the fiber optic cable has very low signal attenuation over distance, a very long link between the antenna and the transceiver or access point can be deployed while still maintaining a good system performance. 
     In many communication systems, those fiber optic links operating as a replacement to RF links are already available from several vendors. In those cases, knowing the overall delay of the link with a high precision (e.g. ˜1 nsec or less) is not important since this delay does not affect the received or transmitted signal. 
     However, when using those links in a TDOA location system, knowing those link delays is critical to ensure proper operation of the system. In addition, those fiber optic links enable several advantages which are specifically beneficial to location systems. 
     For example, US20080194226 discloses a method and system for providing. E911 services for a distributed antenna system uses a lookup table including round trip delay (RTD) ranges for a number of nodes of the distributed antenna system. The system has a lookup table based on the values of the fiber delays and air delays for each node on the distributed antenna system to determine the exact location of the wireless unit generating the E911 call. 
     U.S. Pat. No. 5,457,557 discloses a fiber optic RF signal distribution system which has a plurality of antenna stations, each station including an RF antenna. A central RF signal distribution hub receives and transmits signals external to the system. A pair of optical fibers connects each antenna station directly to the distribution hub with the connections being in a star configuration. 
     Similar systems are disclosed in U.S. Pat. No. 6,812,905, U.S. Pat. No. 5,936,754, U.S. Pat. No. 6,801,767, U.S. Pat. No. 6,597,325, U.S. Pat. No. 7,469,105 and U.S. Pat. No. 6,826,164. 
     Other implementations including optical fibers include conversion of RF signals to digital signals and their transmission over fibers. U.S. Pat. No. 7,366,150 discloses an indoor local area network (LAN) system using an ultra wide-band (UWB) communication system. The system comprises access point adapted to receive the analog signal of the ultra wide-bandwidth transmitted from the remote terminal and convert the received analog signal into an optical signal. 
     A common problem of TDOA location systems is the receiver time synchronization which is essential to allow a correct TDOA calculation when a wireless signal is time stamped by two or more receivers. This synchronization can be achieved by providing a common clock to all the receivers through cables connected between them and the common clock source or by using wireless methods. Both techniques are well known and widely used in the industry. 
     Although clock distribution solves the problem of the continuous drifts between the clocks in the different receivers, the initial offset of the time counters is a problem that requires special solutions. 
     Using fiber optic links enables one to concentrate in a single place all the transceivers used to locate in a specific area thus significantly simplifying the clock distribution and also providing several solutions to the initial offsets of the TOA counters used to time stamp the received signals. 
     Therefore, it would be desirable to provide a method and system for using fiber optic links in a TDOA location system, said fiber optic link having the properties and functionality required to solve common problems found in TDOA location systems and to ensure their proper system operation. 
     SUMMARY OF THE INVENTION 
     The above objectives of using fiber optic links in a location system are achieved in a method, system and related elements. 
     The method is embodied in a system that determines the physical location of a first mobile wireless device coupled to a wireless network by processing the measured characteristics of signals received from the first wireless device by one or more other wireless transceiver devices deployed in the location area. 
     More specifically, this invention applies to a TDOA (time difference of arrival) location system, in which mobile wireless devices broadcast wireless signals which are received by two or more transceivers deployed in the vicinity of said mobile wireless device. Each transceiver measures the TOA (time of arrival) of the received broadcasted signal and reports the TOA to a central server. Said server then calculates the mobile device position using multi-lateration of TDOA values. 
     This patent further refers to the use of fiber optic links between the antennas and the transceivers deployed in the location area to provide unique advantages that could not be achieved using RF coaxial cables. Those advantages are specifically useful in TDOA location systems. 
     In one preferred embodiment of this invention, the wireless transceivers used to receive wireless signals from the mobile wireless device are installed in one central place and the antennas of each of said transceivers are mounted on poles, walls, buildings, etc in the located area. 
     Each of said transceivers is connected to its antenna(s) using a fiber optic link including at least three main components: A local transponder connected to the transceiver which converts RF signals from the transceiver to light signals and vice versa, a fiber optic cable to transmit those light signals to long distances (e.g. from tens of meters to few kilometers) and a remote transponder which converts the light signals from the fiber optic cable to RF signals and vice versa. This remote transponder is also connected to the antenna(s). 
     In such a system, the transceivers are timed synchronized using wireless synchronization. The delay of the fiber optic link is automatically cancelled during the process of offset correction of the TOA counters used for time stamping. 
     In another preferred embodiment, part or all of the transceivers are replaced by receivers (without transmitter) thus simplifying the fiber optic link and the transponders. 
     Since all the transceivers associated to a specific location area can now be concentrated in one single place, there are several advantages as follows:
         All the transceivers can be connected with short cables (e.g. CAT5 or CAT6) to an Ethernet switch or hub located in the same place, thus saving the cost of those cables.   Since the fiber optic is inherently immune from lightning there is no need to protect the system against it This is a significant problem when an RF coaxial cable is connected between the antenna and the transceiver.   The transceivers can be installed in an indoor place although the location area is outdoors. This allows using transceivers rated to indoors environmental conditions which are cheaper than units that must withstand severe outdoors environmental conditions.   The maintenance of the transceivers is simpler since all the equipment is installed in one place.   It is possible to provide to all the location transceivers a common timing signal and provide a more stable synchronization since there is no drift between the clocks in the transceivers. Although this architecture is also possible when the transceivers are deployed in different places (several commercial systems work in this way), this requires sending those timing signals over long cables thus imposing deployment limitations and making the deployment more complicate and expensive.   The common timing signal can also be used to provide a common synchronization (sync) marker to align the offset of all the TOA counters in the transceivers. This technique is widely used and also implemented when the transceivers are not installed in one place. According to the present invention, this marker can be provided to all the transceivers at almost the same time thus providing full synchronization of all the TOA counters.       

     Other preferred embodiments include the integration of multiple transceivers in one single enclosure sharing a common data bus and a common TOA counter. 
     Other embodiments include means for self calibration of the RF and fiber optic link delay, integration of the remote transponder into the antenna and providing power to the remote transponder using a copper cable which is bundled in the fiber optic cable. 
     The foregoing and other objectives, features, and advantages of the invention will be apparent from the following and more particular, descriptions of the preferred embodiments of the invention, as illustrated in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a pictorial diagram depicting a wireless network with a location system in which preferred embodiments of the invention may be practiced. 
         FIG. 2  is a pictorial diagram depicting a wireless network with a location system in which a preferred embodiment of the invention is shown. This embodiment includes the provision of a common timing signal to some of the transceivers. 
         FIG. 3  is a pictorial diagram depicting a block diagram of a transceiver according to a preferred embodiment and its connection to the fiber optic link. 
         FIG. 4  is a pictorial diagram depicting a block diagram of a location transceiver according to another preferred embodiment and its connection to the fiber optic link. 
         FIG. 5  is a pictorial diagram depicting a block diagram of a location transceiver according to another preferred embodiment supporting antenna diversity architecture. The diagram shows the transceiver connection to the transponder. 
         FIG. 6  is a pictorial diagram depicting a detailed block diagram of a location transceiver according to a preferred embodiment of this invention supporting a mechanism that allows integrated measurement of the RF and fiber optic link delay. 
         FIG. 7  is a pictorial diagram depicting a detailed block diagram of a section of the remote transponder according to a preferred embodiment of this invention supporting a mechanism that allows integrated measurement of the RF and fiber optic link delay with antenna diversity. 
         FIG. 8  is a pictorial diagram depicting a detailed block diagram of a section of the remote transponder according to another preferred embodiment of this invention supporting a mechanism that allows integrated measurement of the RF and fiber optic link delay and antenna diversity whether all the remote transponder circuitry is embedded in the diversity antenna case. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention provides TDOA location of a mobile wireless device (e.g. tag, laptop, VoIP phone, bar code reader. etc.) within a wireless network such as a WLAN (e.g. IEEE 802.11a/b/g/a/n) or other any other suitable wireless network for TDOA location such as networks using UWB technology. 
     As described in the above-incorporated patent applications, the TDOA location system may include wireless synchronization as well as other improvements to reduce the synchronization offsets that may be caused by such synchronization method. 
     In said TDOA location system, multiple (two or more) receivers or transceivers are used to calculate the time-difference-of-arrival (TDOA) of wireless signals received from a transmitting source. Some or all the transceivers and/or receivers are connected to their respective antennas using a fiber optic link thus providing the capability to install those units far from their antennas without degrading the system performance as typically caused by long RF coaxial cables. 
     The location of the transmitting source (e.g. RFID tag or mobile station) can be determined by triangulation, based on the difference between the signal arrivals at the multiple receivers. Angle of arrival methods (AOA) may also be used to locate a unit by intersecting the line of position from each of the receivers. Those and other techniques for providing wireless device location information are well known to those skilled in the art and may be used within the method and system of the present invention taking advantage of the special architecture and benefits provided by this invention. 
     Unless otherwise defined, all technical and/or scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention pertains. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of embodiments of the invention, exemplary methods and/or materials are described below. In case of conflict, the patent specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and are not intended to be necessarily limiting. 
     Some embodiments of the invention are herein described, by way of example only, with reference to the associated drawings. With specific reference now to the drawings in detail, it is stressed that the details shown are by way of example and for purposes of illustrative discussion of embodiments of the invention. In this regard, the description taken with the drawings makes apparent to those skilled in the art how embodiments of the invention may be practiced. 
     The terms “comprises”, “comprising”, “includes”, “including”, “having” and their conjugates mean “including but not limited to”. The term “consisting of” means “including and limited to”. 
     The term “location transceiver” means any wireless communication unit which is part of a location system, and used to communicate with tags or any other wireless mobile devices being located by the location system. The term “location transceiver” includes WLAN Access Points (e.g. APs compliant to IEEE 802.11a/b/g/n), location receivers (in those cases where there is no need for 2-way communication), location transceivers, combinations of the above and other wireless location devices operating in the unlicensed frequency bands (e.g. ISM bands), UWB bands or any other radio frequency band (licensed or unlicensed) applicable to a location system. 
     The term “tag” means any portable wireless device being located by the location system, including unidirectional or bidirectional communication means, stand alone or integrated into other devices, battery powered or externally powered by any other source, passive, semi-passive or active RFID tags and also portable wireless devices including other communication means in addition to the one used to communicate with the location transceivers (e.g. ultrasound, infrared, low frequency magnetic interface, wired serial interface, etc.). 
     The term “transponder” means an electronic unit able to convert RF signals (e.g. signals in the 2.4 GHz, 5.7 GHz ISM bands or UWB signals) to light signals and vice versa. The transponder may be adapted to different types of fiber optic cables used to transmit and receive the light signals as well as to different kinds of RF signals. RF-Fiber optic transponders are commercially used for many applications. According to this invention, the transponder unit may also comprise other functions which enhance its functionality in accordance to some preferred embodiments. 
     The term “antenna” means any RF antenna used to transmit and/or receive RF signals. It includes both omni-directional and directional antennas of any type and gain. 
     The term “fiber optic” means a fiber optic cable used for communication (RF and/or digital) including single mode and multi-mode fibers. 
     Referring now to the figures and in particular to  FIG. 1 , a location system comprising four locations transceivers  3 - 6  are connected to a server  2  through a Ethernet  7  network. The location transceivers  3 - 6  are all connected to an Ethernet switch, hub or router  9  but any other configuration including several switches, hubs or routers is also possible and within the scope of this embodiment. The server  2  is also connected to the Ethernet switch  9  using a commonly used CAT5 cable  8  or any other suitable replacement. 
     In this location system as depicted in  FIG. 1 , location transceiver # 1   3  and location transceiver # 4   6  have their antennas  10 - 11  connected with RF coaxial cables  15 - 16 . Location transceiver # 2   4  is connected to its antenna  12  through a fiber optic link comprising a local RF-fiber transponder  18  connected to the location transceiver  4  and a fiber optic cable  21 , a fiber optic cable  21  and a remote RF-fiber transponder  17  connected to the antenna  12  and the fiber optic cable  21 . In a very similar way, location transceiver # 3   5  is connected to its antenna  13  through a fiber optic link comprising a local RF-fiber transponder  20  connected to the location transceiver  5  and the fiber optic cable  22 , a fiber optic cable  22  and a remote RF-fiber transponder  19  connected to the antenna  13  and the fiber optic cable  22 . The power to the remote transponder for both location transceivers can be supplied either from a power source close to the antenna or via a copper cable bundled in the fiber optic cable (not shown). 
     The location system further comprises a wireless sync source  27  transmitting beacons  29  which are received by the location transceivers  3 - 6 . By measuring the time of arrival (TOA) of those beacons  29  at each location transceiver  3 - 6  and reporting the TOA values to the server  2 , it is possible to estimate the continuous TOA counter offset between all the location transceivers  3 - 6  and synchronize the whole location system to perform TDOA location. As previously mentioned, this synchronization technique is described in U.S. Pat. No. 6,968,194, entitled “METHOD AND SYSTEM FOR SYNCHRONIZING LOCATION FINDING MEASUREMENTS IN A WIRELESS LOCAL AREA NETWORK”, issued on Nov. 22, 2005 and United States Patent Application, entitled “METHOD AND SYSTEM FOR SYNCHRONIZATION OFFSET REDUCTION IN A TDOA LOCATION SYSTEM”, filed on Oct. 24, 2006, having a Ser. No. 11/552,211. 
     Also according to the preferred embodiment depicted in  FIG. 1 , four tags  23 - 26  are located by the location system. Each tag  23 - 26  transmits messages  28 - 31  which are received by two or more location transceivers  3 - 6 . By measuring the TOA of those messages  28 - 31  when received by the location transceivers  3 - 6  and reporting them to the server  2 , the server  2  can calculate the position of the tags  23 - 26  using TDOA multi-lateration. TDOA location techniques are well known to the skilled in the art and beyond the scope of this patent. 
     Therefore and according to this preferred embodiment, the location system includes two location transceivers  4 - 5  which can be deployed far from their antennas  12 - 13 . As can be easily understood, another preferred embodiment of the location system may include location transceivers which all of them are far from their antennas and connected to the antennas with fiber optic links. 
     In the depicted embodiment, the fiber optic links  21 ,  22  enables to concentrate two location transceivers  4 ,  5  in a single place (together or not with other equipment) thus providing several advantages to the user as easier maintenance, lightning protection (due to the fiber link), protection against hard environmental conditions, etc. 
     As can be easily understood, the location transceivers  3 - 6  can also be replaced by receivers only, since in some location system architectures they are not required to transmit any message. As already explained and for the sake of simplicity, in any case where a location transceiver is mentioned in this invention, it can be optionally replaced by a location receiver if that unit is not required to transmit messages as part of its normal operation. 
     Optionally, in another preferred embodiment, the sync source is one of the location transceivers  3 - 6  which transmit beacons  29  used for the location system synchronization. 
     Referring now to  FIG. 2 , another embodiment of the location system is depicted.  FIG. 2  depicts a location system comprising four locations transceivers  3 - 6  connected to a server  2  through an Ethernet  7  network. The location transceivers  3 - 6  are all connected to an Ethernet switch, hub or router  9  but any other configuration including several switches, hubs or routers is also possible and within the scope of this embodiment. The server  2  is also connected to the Ethernet switch  9  using a commonly used CAT5 cable  8  or any other suitable replacement. 
     In this location system as depicted in  FIG. 2 , location transceiver # 1   3  and location transceiver # 4   6 , have their antennas  10 - 11  connected with RF coaxial cables  15 - 16 . Location transceiver # 2   4  is connected to its antenna  12  through a fiber optic link comprising a local RF-fiber transponder  18  connected to the location transceiver  4  and the fiber optic cable  21 , a fiber optic cable  21  and a remote RF-fiber transponder  17  connected to the antenna  12  and the fiber optic cable  21 . In a very similar way, location transceiver # 3   5  is connected to its antenna  13  through a fiber optic link comprising a local RF-fiber transponder  20  connected to the location transceiver  5  and the fiber optic cable  22 , a fiber optic cable  22  and a remote RF-fiber transponder  19  connected to the antenna  13  and the fiber optic cable  22 . The power to the remote transponder in both location transceivers can be supplied either from a power source close to the antenna or via a copper cable bundled in the fiber optic cable. 
     Also according to this preferred embodiment as depicted in  FIG. 2 , four tags  23 - 26  are located by the location system. Each tag  23 - 26  transmits messages  28 - 31  which are received by two or more location transceivers  3 - 6 . By measuring the TOA of those messages  28 - 31  when received by the location transceivers  3 - 6  and reported to the server  2 , the server  2  can calculate the position of the tags using TDOA multi-lateration. 
     According to this preferred embodiment, all the four location transceivers  3 - 6  are installed in one single place, close each to other. Two location transceivers  4 - 5  are deployed far from their antennas  12 - 13  while the other two location transceivers  3 ,  6  are connected relatively close to their antennas  10 - 11  through RF coaxial cables  15 - 16  (e.g. antennas mounted on a roof or wall close to the location transceiver installation). As can be easily understood, another preferred embodiment of the location system may include location transceivers which all of them are far from their antennas and connected to the antennas with fiber optic links. 
     In the depicted embodiment, and taking advantage of the concentrated deployment of the location transceivers  3 - 6 , additional advantages can be provided in addition to the already mentioned advantages as easier maintenance, lightning protection (due to the fiber link), protection against hard environmental conditions, etc. 
     The location system further comprises a central timing source  40  which provides a wired timing signal  41  to all the location transceivers  3 - 6 . According to a preferred embodiment, this timing signal may be a clock at frequencies in the range of 10-100 MHz. This timing signal  41  or a synchronized derivative of it is used in each location transceiver to clock the TOA counter which is used to timestamp the received wireless signals. Since the location transceivers  3 - 6  are all deployed in a single place, providing this timing signal  41  to all the transceivers is very simple since it can be performed with very short cables and without having the limitations normally found when providing fast clocks over long lines. 
     Optionally and still taking advantage of the transceivers being close each to other, a common TOA counter reset signal  42  can be provided by one transceiver  3  to the other transceivers  4 - 6 . Since the distances between the units are short and can be fully controlled, it is possible to reset the TOA counters of all the transceivers  3 - 6  almost simultaneously thus providing a time synchronization between all the transceivers. 
     Since each of the transceivers  3 - 6  may have a different time delay of the received signals from the antenna to the time stamp section in the transceiver itself, it is possible to cancel this fixed offset by an initial calibration. 
     According to this preferred embodiment, a wireless sync source unit  27  transmits periodic beacons  29  which are received by each of the location transceivers  3 - 6 . By measuring the time of arrival (TOA) of those beacons  29  at each location transceiver  3 - 6  and reporting them to the server  2 , it is possible to estimate the time offset between all the location transceivers  3 - 6  and synchronize the whole location system to perform TDOA location. Since all the transceivers are clocked from a common timing signal  41  there is no drift between the TOA counters (one TOA counter in each location transceiver) and the TOA offsets due to different cable lengths remain fixed so far the cables connecting the antennas to the transceivers  15 - 16  and  21 - 22  are not modified. Even if one or more of the location transceivers are powered off and on, there is no need to recalibrate the system since the time offsets calculated during the calibration process are still valid and can be used. 
     Comparing this embodiment to the embodiment in  FIG. 1 , the sync source  27  in  FIG. 1  must be continuously used since there is a continuous drift between the clocks at each location transceivers and therefore the TOA counter offsets cannot be kept with a fixed offset. The embodiment in  FIG. 2  uses a common timing signal  41  and TOA counter reset  42  and therefore the sync source is only required for an initial calibration process and can be removed during the normal system operation. This is a significant advantage in many deployments since it provides a simpler and more stable synchronization. 
     Optionally, the TOA counter reset signal  42  can be combined with the timing signal itself thus providing both functions directly from the timing source  40  and using a single signal. This feature will be further described in other embodiments of this invention. 
     Although it is possible to use this synchronization technique when the location transceivers are installed close to their antennas and far from each other, the distribution of the timing signal is problematic and requires high quality shielded cables. 
     In another preferred embodiment, all (or part) the location transceivers installed in one place are enclosed in a single enclosure. For example, a motherboard including several slot connectors where each location transceiver is connected to the motherboard through one of said slot connectors. In this preferred embodiment a very easy and reliable distribution of the clock and TOA counter initialization can be implemented. The common clock may be generated internally on the motherboard and distributed through the motherboard to all the location transceiver boards. Optionally, all the location transceivers may share a single TOA counter thus eliminating the need for any distributed sync timing signal. In addition, it is also possible to have one central processor used to process the signals received from all the receivers enclosed in the same case. 
     In another embodiment the fiber cables  21 - 22  are bundled into a single cable which is chained from antenna to antenna. Since the location transceivers  4 - 5  are located in the same place, it is convenient to connect those units to a single cable with multiple fibers which is chained to both antennas  12 - 13 . Other preferred embodiments may include cable chaining to a plurality of antennas. 
     Referring now to  FIG. 3 , a block diagram of a location transceiver connected to an antenna  63  through a fiber optic link is depicted. According to this preferred embodiment, the location transceiver comprises a controller unit  51 , a WLAN transmitter  52 , a WLAN receiver  53 , and additional functions which will be further described. Typically, the location transceiver receives signals, measures and reports their Time of Arrival (TOA). It also reports other information which may comprise the received data and other generated data in the location transceiver. 
     According to this preferred embodiment as depicted in  FIG. 3 , when a wireless signal is received by the antenna  63 , the signal is sent to the RF-fiber remote transponder  62  through a short coaxial cable. Since the remote transponder  62  is located close to the antenna the attenuation losses of the coaxial cable are very low. The RF-fiber transponder  62  amplifies the received signal and converts it to a light signal which can be transmitted over a fiber optic cable  61 . This cable  61  may be very long since the attenuation of the fiber optic is very low compared to a typical coaxial cable. The light signal is converted back to an RF signal by a local RF-fiber transponder  60  and fed to the location transceiver RE section. The received signal is transferred through a transmit/receive (T/R) switch  59  to a low noise amplifier  55 . In many cases the received signal  58  after the T/R switch  59  is strong enough (due to the LNA in the remote transponder  62 ) so there is no need for the LNA  55  and it can be avoided. The signal  59  can be fed directly to the WLAN receiver  53 . 
     The received signal is demodulated by the WLAN receiver  53  and converted to baseband signals I and Q  67 . Those signals are decoded by the baseband controller  51  and in parallel sampled by two A/D converters included in the A/D, MF, RAM and TOA counter function  50 . The sampled signals (e.g. with a resolution of 8-10 bits) are passed through a matched filter (ME) of the A/D, MF, RAM and TOA counter function  50  and the results stored in a dedicated RAM of the A/D, MF, RAM and TOA counter function  50 . As an example, a typical IEEE 802.11b at 1 Mbps BPSK signal will be sampled at a rate of 22 MHz. The RAM of the A/D, MF, RAM and TOA counter function  50  stores the matched filter output of around 128 bits (2816 I and Q matched filter results). For the skilled in the art, it shall be obvious that a hardware implementation of the matched filter is just one preferred embodiment. Fast digital signal processors (DSP) can perform the same function by reading directly the I and Q samples  67 . In order to calculate the time of arrival of the received signal, a TOA counter of the A/D, MF, RAM and TOA counter function  50  is used to time stamp the samples. The timing of the TOA counter is controlled by a timing function  54  which provides the clock for this TOA counter of the A/D, MF, RAM and TOA counter function  50 . The master clock of this timing function  54  may be provided from an internal clock (e.g. TCXO or OCXO) or from an external signal  64  which can also be supplied to other location transceivers. The final TOA of the received signal is calculated by the controller  51  which reads the I&amp;Q matched filter results and the TOA counter data  66  from the A/D, MF, RAM and TOA counter function  50 . The final TOA calculation may optionally include fine interpolation and sophisticated algorithms to reduce the effects of noise, multipaths and other interference conditions which may cause an error in the TOA calculation. Many of those algorithms are well known and beyond the scope of this invention. 
     Optionally the TOA counter can be initialized from the controller  51  or from an external signal  65 . Initializing the TOA counter of the A/D, MF, RAM and TOA counter function  50  from an external signal enables a controlled and synchronized initialization of those TOA counters in multiple location transceivers. 
     According to the preferred embodiment as depicted in  FIG. 3 , the location transceiver can also transmit messages. Messages to be transmitted are prepared by the controller  51  and encoded by the baseband controller  51  which generates transmit I&amp;Q signals  68 . Those I&amp;Q signals  68  are modulated by the WLAN transmitter  52  and its output fed to a power amplifier  56 . The amplified signal  57  is conducted to the local RF-fiber optic transponder  60  through a T/R switch  59 . In many cases, it is preferable to avoid the use of the power amplifier  56  since the local transponder  60  does not require a high level input signal. Thus an input signal level of approximately 0 dBm can be directly provided by the WLAN transmitter  52  to the T/R switch  59  and then to the local transponder  60 . The transmitted signal is then converted by the local transponder  60  to a light signal which is sent through a fiber optic  61  to the remote RF-fiber optic transponder  62 . Note that the fiber optic cable  61  comprises two separate fibers, one used for the received signals and one for the transmitted signals. Although in principle it is possible to use a single fiber for both signals using well known techniques, using two fibers is in most of the cases (for relatively short distances of up to several hundred meters) a more cost effective solution. 
     The remote transponder  62  converts the light signal back to an RF signal and amplifies it to get the required signal power (e.g. +20 dBm). The amplified RF signal is transmitted using the location transceiver antenna  63 . 
     Also according to this preferred embodiment, the transmitted I&amp;Q signals  68  are also sampled using the same function  50  used to sample the received signals. This sampling enables the controller  51  to calculate the time of transmission with the same level of accuracy as done with the received signals. Having this capability, the location transceiver can synchronize itself when transmitting beacons for wireless synchronization or when performing a distance measurement. This capability will be described in more detail when describing a method which allows self calibration of the fiber cable length. The controller  51  has an Ethernet interface  69  which allows communication to a server or to any other unit connected to the network. 
     In another preferred embodiment, the location transceiver operates as a receiver only. In that case, all the functions related to the transmission of signals can be saved including the relevant functions in the local and remote transponders. Also according to this preferred embodiment and referring to  FIG. 3 , only a single fiber is used to receive signals. 
     Referring now to  FIG. 4 , another preferred embodiment of the location transceiver is depicted. This embodiment comprises the same basic functions as described in the embodiment of  FIG. 3 . However, according to this preferred embodiment, the location transceiver has no power amplifier and no T/R switch. The transmitted signal  57  is directly coupled to the local RF-fiber transponder  70 . In the receive path, the received signal  58  is directly connected to the LNA  55  without passing a T/R switch as in  FIG. 3 . This embodiment has the advantage of having a simpler coupling to the local transponder  70  which optionally can be an integral part of the location transceiver. This option reduces equipment cost and simplifies the deployment since the fiber optic cable can be directly connected to the location transceiver. 
     In addition, and according to this preferred embodiment the initialization of the TOA counter of the A/D, MF, RAM and TOA counter function  50  used for time stamping of the received and/or transmitted signals is provided by the timing function  54 . The advantage of this approach is the fact that this initialization signal  71  can be directly derived from a periodic marker in the external timing signal  64 . A simple and common method to generate this marker in the timing signal is by masking one cycle of the timing signal (e.g. the timing signal amplitude will remain constant for one cycle) without changing the timing signal frequency. Preferably, this marker shall have a repetition period long enough to avoid TOA ambiguity of the time stamped signals. For example a repetition period of 1 every 10 6  cycles of a 50 MHz clock, will create a marker every 20 msec, time which is long enough to avoid any TOA ambiguity in typical WLAN location systems. 
     According to this preferred embodiment, a location system including location transceivers having the functionality as depicted in  FIG. 4  and installed in a single place can be easily synchronized by a common timing signal which also includes a marker. This marker is used to generate a synchronous reset signal to the TOA counters of all said location transceivers. 
     Referring now to  FIG. 5 , the block diagram of another preferred embodiment of the location transceiver is depicted. Similarly to the description of the block diagram in  FIG. 3 , the location transceiver includes a Controller and TOA function  51 , a WLAN transmitter  52 , a WLAN receiver  53  and LNA  55 , an A/D, MF, RAM and TOA counter function  50 , a timing function  54  and additional functions which will be further described. 
     According to this preferred embodiment as depicted in  FIG. 5 , the location transceiver is connected to two antennas  63  operating as diversity antennas. When a wireless signal is received by one or both antennas  63 , the signal received by each antenna is sent to the RF-fiber remote transponder  82  through a short coaxial cable. Since the transponder  82  is located close to the antennas the attenuation losses of the coaxial cables are very low. The RF-fiber transponder  82  amplifies each of the received signals and converts them to separate light signals which are transmitted over a fiber optic cable  81 . In this embodiment the fiber optic cable  81  includes a separate fiber optic for each received signal. The light signals are converted back to RF signals by a local RE-fiber transponder  80  and fed to the location transceiver RE section. 
     The received RF signals  84  and  85  are connected to a diversity switch  86  and then a selected signal is connected to a low noise amplifier  55  part of the receiver chain. In many cases the received signals  84  and  85  are strong enough (due to the LNA in the remote transponder  82 ) so there is no need for an LNA  55  and it can be avoided. The selected signal from the diversity switch  86  can be fed directly to the WLAN receiver  53 . Note that also this embodiment has no T/R switch as the embodiment described in  FIG. 4 . In this preferred embodiment, there is a direct and separate coupling of transmit and receive paths in the location transceiver and the RF-fiber transponder  80 . 
     The received signal selected by the diversity switch  86  is demodulated by the WLAN receiver  53  and converted to baseband signals I and Q  67 . Those signals are decoded by the baseband controller  51  and in parallel sampled by two A/D converters included in the A/D, MF, RAM and TOA counter function  50 . The sampled signals (e.g. with a resolution of 8-10 bits) are passed through a matched filter (MF) of the A/D, MF, RAM and TOA counter function  50  and the results stored in a dedicated RAM of the A/D, MF, RAM and TOA counter function  50 . The RAM of the A/D, MF, RAM and TOA counter function  50  stores the matched filter output of around 128 bits (2816 I and Q matched filter results). For the skilled in the art, it shall be obvious that a hardware implementation of the matched filter is just one preferred embodiment. Fast digital signal processors (DSP) can perform the same function by reading directly the I and Q samples  67 . In order to calculate the time of arrival of the received signal, a TOA counter of the A/D, MF, RAM and TOA counter function  50  is used to time stamp the samples. The timing of the TOA counter is controlled by a timing function  54  which provides the clock for this TOA counter of the A/D, MF, RAM and TOA counter function  50 . The master clock of this timing function  54  maybe provided from an internal clock (e.g. TCXO or OCKO) or from an external signal  64  which can also be supplied to other location transceivers. The final TOA of the received signal is calculated by the controller  51  which reads the I&amp;Q matched filter results and the TOA counter data  66  from the A/D, MF, RAM and TOA counter function  50 . As previously mentioned, the final TOA calculation may optionally include fine interpolation and sophisticated algorithms to reduce the effects of noise, multipaths and other interference conditions which may cause an error in the TOA calculation. 
     Optionally the TOA counter can be initialized from the controller  51  or from an external signal  65 . Initializing the TOA counter of the A/D, MF, RAM and TOA counter function  50  from an external signal enables a controlled and synchronized initialization of those counters in multiple location transceivers. 
     According to the preferred embodiment as depicted in  FIG. 5 , the location transceiver can also transmit messages. Messages to be transmitted by the controller  51  are encoded by a baseband controller  51  which generates transmit I&amp;Q signals  68 . Those I&amp;Q signals  68  are modulated by the WLAN transmitter  52  and its output  57  is directly fed to the local RF-fiber optic transponder  80 . In this preferred embodiment there is no power amplifier in the location transceiver since the local transponder  80  does not require a high level input signal. Thus a signal level of approximately  0  dBm can be directly provided by the WLAN transmitter  52  to the local transponder  60 . The transmitted signal  57  is then converted by the local transponder  80  to a light signal which is sent through a fiber optic  81  to the remote RF-fiber optic transponder  82 . Note that the fiber optic cable  81  comprises three separate fibers, two fibers used for the received signals and one fiber used for the transmitted signal. 
     The remote transponder  82  converts the light signal back to an RF signal and amplifies it to generate the required signal power (e.g. +20 dBm). The amplified RF signal is then transmitted using one of the location transceiver antennas  63 . 
     Also according to this preferred embodiment, the transmitted I&amp;Q signals  68  are also sampled using the same function  50  used to sample the received signals. This sampling enables the controller  51  to calculate the time of transmission with the same level of accuracy as done with the received signals. Having this capability, the location transceiver can synchronize itself when transmitting beacons for wireless synchronization or perform a distance measurement. 
     The controller  51  has an Ethernet interface  69  which allows communication to a server or any other unit connected to the network. 
     Referring now to  FIG. 6 , the block diagram of another preferred embodiment of the location transceiver is depicted. In this preferred embodiment, the sampling of the transmitted and received I&amp;Q signals includes a special implementation. 
     The transmitted I&amp;Q signals  105 - 106  and the received I&amp;Q signals  107 - 108  are connected to two analog switches (multiplexers)  103 - 104  in a way that enables unique functionality. 
     In normal operation during signal reception, both I&amp;Q components  107 - 108  of the received signal are sampled by two parallel A/D converters  101 - 102 . Switch  103  is set to position 1 by the controller  51  through control lines  109 . The received 1-signal  107  is then sampled by A/D  101 . Switch  104  is set to position 1 by the controller  51  through control lines  109 . The received Q-signal  108  is then sampled by A/D  102 . 
     In normal operation during signal transmission, both I&amp;Q components  105 - 106  of the transmitted signal are sampled by the two A/D converters  101 - 102 . Switch  103  is set to position 2 by the controller  51  through control lines  109 . The transmitted I-signal  105  is then sampled by A/D  101 . Switch  104  is set to position 2 by the controller  51  through control lines  109 . The transmitted Q-signal  106  is then sampled by A/D  102 . 
     In addition to this normal operation, the location transceiver can perform a self measurement of the RF and fiber optic link delay thus allowing a self calibration process. 
     Referring now to  FIG. 7 , a fiber optic link with a remote transponder that supports this self calibration is depicted. In this preferred embodiment, the remote transponder  131  and additional functions are integrated in a remote antenna unit  132 . Those functions include T/R switches  135 ,  136  and  140 , LNAs  133 - 134  and a power amplifier  141 . The power  148  to this remote antenna unit is provided by a local power source close to the remote antenna unit  132 . 
     In another preferred embodiment this local power source is a solar power unit mounted in the same pole as the antenna and the remote antenna unit  132 . 
     A transmitted signal  142  is converted by the local transponder  130  to a light signal and sent to the remote transponder  131  through a fiber optic  145 . The remote transponder  131  converts the light signal back to an RF signal  139  which drives the power amplifier  141 . In addition, the transmitted signal  139  is connected to two T/R switches  135 - 136  which send this signal back to the remote transponder  131 . Therefore the transmitted signal  139  is received back by the local transponder  130  after it passed through two fibers  146 - 147 . 
     Note that the remote transponder automatically controls the T/R switches  135 - 136 . When a signal is transmitted, the remote transponder senses the presence of energy of signal  139  and automatically sets both T/R switches  135 - 136  to transmit mode. In this mode, the transmitted signal is sent back to the local transponder  130 . When there is no signal being transmitted (absence of energy), the remote transponder  131  sets the T/R switches  135 - 136  to their normal receive mode thus allowing the reception of signals from antennas  137 - 138 . 
     When a signal is being transmitted by the location transceiver, the remote transponder also sets T/R switch  140  to transmit mode thus allowing the transmission of the signal through antenna  137 . When there is no signal being transmitted, the remote transponder  131  sets T/R switch  140  to its normal receive mode thus allowing the reception of signals from antenna  137 . 
     According to a preferred embodiment of a location transceiver as depicted in  FIG. 6 , when the location transceiver desires to perform a self measurement of the RF and fiber optic link delay (including the local and remote transponders delays), it transmits a signal with an I-component  105  (e.g. a BPSK signal). This signal is transmitted through the transmitter  52  and then fed  142  to the local transponder  130  as depicted in  FIG. 7 . 
     The transmitted signal is looped back by the remote transponder  131  and fed back to the location transceiver by the local transponder  130 . One of the received signals  143 - 144  is selected using a diversity switch  86  (e.g. as depicted in  FIG. 5 ) and then converted back to I&amp;Q signals  107 - 108  by the WLAN receiver  53 . 
     The overall delay (receive +transmit) of the RF and fiber optic link including the transponders  130 - 131  can be measured by measuring the delay between the transmitted signal  105  and one or both of the received signals  107 - 108 . In this mode, switch  103  is set to position 1 and therefore A/D  101  samples the transmitted signal  105  while at the same time switch  104  is set to either position 1 or 3 to allow a simultaneous sampling of either one of the received signals  107 - 108  by A/D  102 . Since the delay of this link is unknown, also the phase of the received signal is unknown. Therefore the received signal energy may be either concentrated in one of its I&amp;Q components only or split in both I&amp;Q components  107 - 108 . 
     For this reason, the delay measurement may include two steps. In the first step the transmitted signal  105  is sampled together with the I-component  107  of the looped back signal and in the second step the same procedure is repeated with the Q component  108 . 
     Having the matched filter output of the sampled data of both transmitted  110  and received  111  signals, the controller  51  can calculate the overall delay using the TOA functions  50  which are normally used for the TOA measurement of the transmitted or received signals. 
     This delay measurement operation can be performed for each of the receive paths  143 - 144  thus providing a better accuracy of the overall delay since both receive fibers  146 - 147  are bundled in the same cable. 
     In principle, the overall signal delay (from the A/D′s  101 - 102  to the antennas  137 - 138 ) include a small additional delay consisting of the power amplifier, the LNAs and the short RF coaxial cable. Those delays are fixed and easily calculated or measured and can be taken in account in the overall self calibration process. Since the fiber optic delay is very stable, this self calibration process is not required very often. 
     Knowing the overall delay of the RF link and optical link is very useful for the following reasons:
         In a TDOA location system being synchronized by wireless beacons, a location transceiver operating also as synchronization source can synchronize itself. This is self synchronization is performed by measuring the TOA of the transmitted messages used for synchronization. The RF+fiber link delay is necessary to cancel the sync offset caused to the sync transceiver when performing self synchronization.   In a TDOA location system using distance measurement between the sync source transceiver and each of the other location transceiver, it is necessary to know the RF+fiber link delay to calculate the true distance between the units. Distance measurement is used to reduce wireless synchronization offsets caused by multipaths.   In a TDOA location system in which a group of location transceivers have a common TOA counter or their TOA counters are initialized simultaneously, knowing the RF+fiber link delay in each location transceiver avoids using a wireless signal to calculate the TOA offsets caused by this link.       

     In addition, the same mechanism can be used to provide additional advantages as follows:
         Detect link malfunctions: Periodic delay measurements can detect faults in the up or down links thus providing additional reliability to the system.   Calibrate the gain of the received signals paths. Transmitting a test signal and receiving it back from each of the receive paths used for diversity, it is possible to detect gain/attenuation differences between those two paths.       

     In another preferred embodiments, the measurement of the link delay can be done with other known techniques as transmission of very short pulses or phase delay variations using frequency hopping techniques. 
     Referring now to  FIG. 8 , the block diagram of another preferred embodiment of the fiber optic link is depicted showing additional advantages of the present invention. In this preferred embodiment, there is an integrated antenna unit  150  which includes the remote transponder  131  and additional functions all enclosed in the same case (radome) with the antennas  137 - 138 . The integrated antenna unit  150  is in practice, a diversity antenna with a fiber optic interface. This approach has several advantages as follows:
         The antennas  137 - 138  and all the remote transponder functions are all enclosed in a single case (radome) thus simplifying the installation and also reducing the overall cost.   Improved reliability and improved performance by avoiding the RF connectors used for the antenna connection. The transponder LNAs  133 - 134  can be located very close to the antennas elements  137 - 138  thus reducing the RF losses and improving the receiver sensitivity.       

     In this preferred embodiment the power to the integrated antenna unit is provided through the local transponder  130  (e.g. from the location transceiver). The fiber optic cable includes also a copper cable  151  used to provide the DC power to the integrated antenna unit  150 . Since the power consumption of this integrated antenna unit is relatively low (typically 1-2 watts), the requirements for the copper cable  151  are not severe. Typically the integrated antenna unit will also include a voltage regulator (not shown) to provide a stable and clean power to the integrated antenna unit  150 . 
     The fiber optic or the copper cables connecting between the location transceiver and the remote antenna unit can also be used to send digital commands to the remote antenna unit and receive digital messages from it. 
     In another preferred embodiment, the remote antenna unit includes a small micro controller able to receive commands from the location transceiver or a central unit. Those commands can be used to control the remote transponder and perform diagnostics. This microcontroller can also send back status messages. In chained configuration, it is possible to use also multicast and/or broadcast commands sent to multiple remote antenna units. 
     Other preferred embodiments comprise using the same antenna for a location transceiver and a WLAN Access Point located in the same place. This technique is well known and commercially available. 
     Large location systems may include several groups of location transceivers each group concentrated in a different place and synchronized by a different timing source. In a preferred embodiment of such location system it may be necessary to synchronize between two or more of said timing sources. This synchronization can be achieved by having a connection between those timing sources and defining one of them as a master timing source. This master timing source will provide the clock to each of the other slave timing sources connected to said master timing source. The connection between those synchronized timing sources may be implemented using a CAT5/CAT6 cable or a fiber optic. 
     The principles of this invention can also be applied to a TDOA location system also capable to locate using Angle of Arrival (AOA). 
     It may be appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination or as suitable in any other described embodiment of the invention. Certain features described in the context of various embodiments are not to be considered essential features of those embodiments, unless the embodiment is inoperative without those elements. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that many alternatives, modifications and variations and other changes in form, and details may be made therein without departing from the spirit and scope of the invention.