Patent Publication Number: US-2005143091-A1

Title: Indoor location identification system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      The present invention claims priority from U.S. Provisional Patent Application No. 60/499,009 filed 2 Sep., 2003, the content of which is incorporated herein by reference. 
    
    
     FIELD AND BACKGROUND OF THE INVENTION  
      The present invention relates to emergency location services, and in particular to methods and systems for locating individuals inside buildings during emergencies in real time, using cellular technology.  
      It is expected that within 3-5 years, 50% of cellular calls will be conducted inside buildings. The cellular coverage level in many buildings is not adequate to guarantee a successful call session. Reception of cellular signals in buildings is disturbed because:  
      1) Building obstructions (exterior and interior walls) attenuate the signal below the level required for maintaining a link.  
      2) In high floors (usually above the clutter), a mobile cellular telephone is exposed to multiple base station signals that create “co-channel interference” or “pilot pollution”.  
      An internal coverage system is often deployed to overcome coverage problems inside buildings. The most common internal coverage system has a Distributed Antenna System (DAS) architecture in which the antenna output of a base station or a repeater is connected through a coaxial network or fiber network (or combined coaxial and fiber network) to a set of indoor antennas.  FIG. 1  shows an exemplary prior art DAS architecture  100  in which the system is built in a “star” configuration, where a single base station  101  feeds two buildings  102  and  104  through two respective base units (BU)  106  and  108  and through (exemplarily) eight remote hub units (RHU)  110   a - d  and  112   a - d . Base units are sometimes called central hub units or base hub units, and are characterized by being a central distribution point for multiple remote units. RHUs are sometimes called remote access units (RAUs) or by other names. A RHU is a local distribution point for at least one antenna. Typically, one RHU covers a floor of a building. Exemplarily, in  FIG. 1  RHU  110   a  covers a floor  114   a , RHU  110   b  covers a floor  114   b , RHU  110   c  covers a floor  114   c , and RHU  110   d  covers a floor  114   d  of building  102 . Typical installations of such systems are in airports, commercial centers, shopping malls, corporate and high education campuses, hospitals, and other major sites.  
      In many cases, a single base station feeds several buildings located hundreds of yards or sometimes even more than a mile apart. Sometimes the buildings themselves are large horizontal constructions, with at least one dimension larger than 100 meters. In other cases, a DAS is fed by an “over the air repeater” that uses a “donor” base station which may be 1-3 miles remote from the covered building. It is obvious that location identification based on any kind of triangulation is not possible in such cases since the subscriber unit is received only by one base station by definition.  
      Multi-floor buildings pose additional obstacles for indoor location identification, as they require three-dimensional positioning. Even if the longitude and latitude of an individual cellular telephone in a fifty floor high building were known with great accuracy, that knowledge would be insufficient because an emergency team would have to search every floor. For an accuracy of 200 meters, the location fix may cover many multi-floor buildings. Under these conditions, a rescue team could spend hours just searching for the caller.  
      Several technologies have been proposed for outdoor location identification, including network-based technologies, e.g. Time Difference of Arrival (TDOA) and others, and handset-based technologies such as the Assisted Global Positioning System (AGPS). All these technologies are flawed in terms of their ability to locate indoor cellular callers. Large steel and concrete buildings, subways and large malls may be difficult or even impossible to cover using traditional wide area location technologies such as AGPS and TDOA. Low signal levels and signal multipath effects in these environments decrease the location identification accuracy or totally prevent signal acquisition. None of these technologies can locate a cellular telephone indoors, due to the low signal levels and the fact that these technologies provide in effect only a two-dimensional location identification, where a three-dimensional one is needed.  
      There is therefore a need for, and it would be advantageous to have a simple and reliable solution for real-time accurate indoor location of a mobile cellular telephone in case of an emergency.  
     SUMMARY OF THE INVENTION  
      The present invention is of a system and method for accurate indoor location of a mobile cellular telephone (cellular user) (or simply “indoor location identification”) in case of an emergency in real time. The invention uses an existing DAS-like infrastructure to extract the location of the cellular user from the energy of radio frequency (RF) signals gathered through the antennas of the infrastructure. The DAS-like infrastructure may also serve (but does not have to) as a regular DAS. The RF signals are tapped into and processed by a location identification unit (LIU) coupled to the DAS. Preferably, the LIU taps into the DAS at a most convenient tapping point, most preferably near a base unit that concentrates the RF signals received from a plurality of remote hub units. Preferably, a separate LIU is coupled to each BU. Alternatively, a single LIU may be coupled to a plurality of BUs. The energy of RF signals is processed by a processor in the LIU to provide the location of the mobile cellular telephone, by identifying the RHU with the highest received energy at the channel frequency and time slot (or code) of the emergency call.  
      According to the present invention there is provided a system for accurate location of a mobile cellular telephone in an indoor environment, comprising: a DAS that includes a plurality of antennas located in known areas of the indoor environment; and at least one location identification unit coupled to the DAS and operative to extract a location of the mobile cellular telephone from the energy of RF signals obtained from the mobile cellular telephone.  
      According to the present invention there is provided a method for locating a mobile cellular user inside an indoor environment comprising the steps of obtaining RF signals from a plurality of antennas of a distributed antenna system located inside the indoor environment, and extracting the location of the mobile cellular user from the energy of the RF signals.  
      According to one feature in the method for locating a mobile cellular user inside an indoor environment, the step of obtaining RF signals includes using at least one LIU to tap into the DAS to receive the RF signals, and the step of extracting the location includes obtaining parameters of the emergency call, and identifying a RHU with the highest signal energy that matches the call parameters.  
      According to the present invention there is provided a method for identifying the indoor location of a mobile cellular telephone in case of an emergency, comprising the steps of acquiring an RF signal originated by the mobile cellular telephone through an indoor DAS that includes at least one RHU connected to at least one antenna, and identifying a specific RHU through which the RF signal was acquired.  
      According to one feature in the method for identifying the indoor location of a mobile cellular telephone in case of an emergency, the step of acquiring includes using a location identification unit to tap into the DAS to obtain the RF signal, and the step of identifying includes identifying the RF signal as belonging to an emergency call, providing the parameters of the emergency call to a processor inside a location identification unit, and identifying a RHU with highest signal energy at a tuning window defined by the call parameters. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:  
       FIG. 1  shows a prior art distributed antenna system architecture;  
       FIG. 2  shows a preferred embodiment of an indoor locating system of the present invention;  
       FIG. 3  shows details of the internal structure of a base unit and a location identification unit. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The present invention is of a system and method for accurate location, in real time, of a mobile cellular telephone in an indoor environment during emergencies (e.g. upon a “911” emergency call). The invention is based advantageously on the fact that a distributed antenna system can be used to monitor the RF signals received in each floor or area of each building covered by the DAS, specifically by the RHU covering that floor or area. The RHU identified with a particular emergency call indicates the location from which the call is made. Such a use of a DAS or DAS-like infrastructure is unknown in prior art, as well as unexpected. The indoor location identification system of the present invention can interface with existing distributed antenna systems, and can, upon request, provide information on the location of an indoor subscriber.  
       FIG. 2  shows a preferred embodiment of an indoor location identification system  200  of the present invention. System  200  is shown exemplarily in conjunction with DAS  100 . System  200  comprises, in addition to the elements of DAS  100 , at least one location identification unit (LIU) that is operative to extract the location of a mobile cellular telephone located indoors, by processing the energy of RF signals obtained from the DAS. The energy of the RF signals is obtained by the LIU by tapping into the DAS. The tapping may be done at different point of the DAS. However, preferably, the tapping is done close to a BU, which concentrates all RF signals from a plurality of RHUs connected thereto. In  FIG. 2 , a LIU  202  is connected to BU  106 , and a LIU  204  connected to BU  108  of DAS system  100 . Each LIU monitors the signals received from a group of floors (or areas) in a building served by the respective BU. Alternatively, a single LIU can be configured to be connected to a plurality of BUs and to monitor RF signals received from a group of floors (or areas) in the building served by a respective plurality of BUs. An emergency call is identified by data that includes the channel frequency and either a time slot or a code. These data are referred to herein also as “tuning parameters” which defines a “tuning window”. The tuning parameters are provided to the LIU by an external system (see  FIG. 3 ). The LIU then searches all RHUs to identify the RHU with the highest signal energy at the tuning window (channel frequency and time slot or code). This RHU is then assumed as the RHU receiving the emergency call, and the location of the caller is identified as lying in the area covered by this RHU.  
       FIG. 3  shows details of the internal structure of a base unit and a location identification unit. A base unit  302  comprises exemplarily two optics-to-RF transformer units  304  and  306  operative to receive optical signals from a RHU and convert them into RF signals for transmission to cellular base station  101 , and a RF combiner  308  that combines the RF signals before their transmission to the base station. A LIU  310  comprises a receiver  312  operative to receive the RF signals from BU  302 , and a processing unit  314  operative to process the these signals, and to communicate bi-directionally with at least one external system  301 . The external system provides the channel frequency and time slot of the emergency call in case of time division multiple access (TDMA), or the channel frequency and code of the emergency call in the case of code division multiple access (CDMA). An external system according to the present invention may be a location identification system for the macro-network. As well known, this is a system that enables locating emergency calls outside of buildings. It identifies the fact that an emergency call has been made, and provides information on the frequency and time slot (or code) to the relevant equipment (in this case the LIU). Alternatively, an external system may include other elements or combination of elements of the cellular network, referred to collectively herein as a “subsystem” of the cellular network.  
      The processing unit in the LIU provides the following functionality: 
          a) interfaces between the external system and the LIU. The external system provides a required channel frequency and time slot, or a required channel frequency and code related to the emergency call;     b) tunes the receiver to the required channel frequency and time slot, or channel frequency and code (“tuning window”);     c) measures, at the tuning window, the signal energy received from each RHU;     d) identifies, by comparison, which RHU has received the highest energy;     e) determines the location of the handset (cellular telephone) that conducts the emergency call in the coverage area of the RHU that received the highest energy at the given tuning window; and     f) provides to the external system the location of the mobile cellular telephone that conducts the emergency call.        

      Preferably, the processing unit is a microprocessor-based equipment with memory and other required peripherals.  
      In use, a call initiated in any of the buildings by a mobile cellular telephone is received by the DAS system, transferred to the cellular base station and further transferred to the cellular core network (not shown), where the dialed number is decoded, and a channel frequency and time slot (or code) are assigned to the call in well-known procedures. If the dialed number is an emergency number (e.g. 911) this information (of the call being an emergency call) plus the channel frequency and time slot (or code) of the call are provided to all the LIUs connected to the respective cellular base station through which the call was received. Each LIU, connected to a respective BU, is then tuned to the provided channel frequency and time slot (or code), and operated to scan through the inputs of all the RHUs connected to the respective BU. The LIU then determines which of the RHUs connected to the respective BU received the call, by measuring the energy at a tuning window defined by the provided channel frequency and time slot (or code). Since each RHU is assigned to a specific building/floor/area, the system can locate the building and/or floor and/or area from which the emergency call was originated.  
      An alternative technique may be used in order to shorten the time required for identification. According to this technique, the receiver (e.g.  312 ) in a LIU (e.g.  310 ) continuously monitors the output of the RHUs connected to its associated BU (e.g  302 ) and registers the characteristics (frequency/time slot/code) of the received signals. The receiver functions in this case like a well-known spectrum analyzer that searches a given frequency band in which the cellular transmissions are made, identifies received signals and records them. Whenever the information on an emergency call is provided to the LIU, the LIU searches its records for matching characteristics (i.e. same frequency/time slot/code) and responds immediately with the designation of the RHU that received the signal.  
      While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.