Patent Publication Number: US-11653175-B2

Title: Apparatuses, systems, and methods for determining location of a mobile device(s) in a distributed antenna system(s)

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/539,927, filed Aug. 13, 2019, which is a continuation of U.S. patent application Ser. No. 15/876,754, filed Jan. 22, 2018, now U.S. Pat. No. 10,448,205, which is a continuation of U.S. patent application Ser. No. 15/356,723, filed on Nov. 21, 2016, now U.S. Pat. No. 9,913,094, which is a continuation of U.S. patent application Ser. No. 14/873,483, filed Oct. 2, 2015, now U.S. Pat. No. 9,532,329, which is continuation of U.S. patent application Ser. No. 14/034,948, filed Sep. 24, 2013, now U.S. Pat. No. 9,185,674, which is a continuation of U.S. patent application Ser. No. 13/365,843, filed on Feb. 3, 2012, now U.S. Pat. No. 8,570,914, which is a continuation of International App. No. PCT/US2010/044884, filed Aug. 9, 2010, the contents of which are relied upon and incorporated herein by reference in their entireties, and the benefit of priority under 35 U.S.C. § 120 is hereby claimed. 
    
    
     BACKGROUND 
     Field of the Disclosure 
     The technology of the disclosure relates to distributed antenna and communications systems, including mobile distributed telecommunication systems and networks, for distributing communications signals to remote antenna units. The distributed antenna and communications systems can include any type of media, including but not limited to optical fiber to provide an optical fiber-based distributed antenna system. 
     Technical Background 
     Wireless communication is rapidly growing, with ever-increasing demands for high-speed mobile data communication. As an example, so-called “wireless fidelity” or “WiFi” systems and wireless local area networks (WLANs) are being deployed in many different types of areas (e.g., coffee shops, airports, libraries, etc.). Distributed antenna systems communicate with wireless devices called “clients” or “client devices,” which must reside within the wireless range or “cell coverage area” in order to communicate with an access point device. A distributed antenna system (DAS) comprises multiple antennas connected to a common cellular base station and can provide cellular coverage over the same area as a single antenna. 
     One approach to deploying a distributed antenna system involves the use of radio frequency (RF) antenna coverage areas, also referred to as “antenna coverage areas.” Antenna coverage areas can have a radius in the range from a few meters up to twenty meters as examples. Combining a number of access point devices creates an array of antenna coverage areas. Because the antenna coverage areas each cover a small area, there are typically only a few users (clients) per antenna coverage area. This allows for minimizing the amount of RF bandwidth shared among the wireless system users. 
     A distributed antenna system can be implemented to provide adequate cellular telephone and internet coverage within an area where the propagation of an RF signal is disturbed. For example, transmission and reception of RF signals are often blocked inside high buildings due to thick steel, concrete floors and walls. Similar problems can be found in other areas such as airports, shopping malls or tunnels, etc. To overcome this coverage problem, a distributed antenna system may comprise components that receive an input RF signal and convert it to a wired signal, for example, an optical signal. The distributed antenna system may include fiber optic cables to transmit optical signals in an area where RF signals are blocked, e.g., inside the buildings. The antennas can be placed close to the possible locations of mobile or portable terminals, originated from a utility or service room and then arranged to form a star-like topology. The distributed antenna system may also comprise components that re-convert the wired signals back to the RF signals. 
     As discussed above, it may be desired to provide such distributed antenna systems indoors, such as inside a building or other facility, to provide indoor wireless communication for clients. Otherwise, wireless reception may be poor or not possible for wireless communication clients located inside the building. In this regard, the remote antenna units can be distributed throughout locations inside a building to extend wireless communication coverage throughout the building. While extending the remote antenna units to locations in the building can provide seamless wireless coverage to wireless clients, other services may be negatively affected or not possible due to the indoor environment. For example, it may be desired or required to determine the location of client devices or provide localization services for client devices, such as emergency 911 (E911) services as an example. If the client device is located indoors, techniques such as global positioning services (GPSs) may not be possible to determine the location of the client device. Further, triangulation techniques may not be able to determine the location of the client device due to the remote antenna units typically being arranged to avoid overlapping regions between antenna coverage areas. 
     SUMMARY OF THE DETAILED DESCRIPTION 
     Embodiments disclosed in the detailed description include distributed antenna apparatuses, systems, methods, and computer-readable mediums to provide location information regarding client devices communicating with remote antenna units in a distributed antenna system. The location information can be used to determine the location of the client devices relative to the remote antenna unit(s) in which the client devices are communicating. In this scenario, the client devices would be known to be within communication range of the remote antenna units. This information can be used to determine or provide a more precise area of location of the client devices. The distributed antenna components and systems, and related methods disclosed herein may be well suited for indoor environments where other methods of providing and/or determining the location of client devices may be obstructed or not possible due to the indoor environment. 
     In this regard, in certain embodiments disclosed herein, a location processing unit (LPU) configured to provide location information for at least one client device wirelessly communicating in a distributed antenna system can be provided. The LPU includes a control system configured to receive uplink radio frequency (RF) signals communicated by at least one client device wirelessly communicating to a plurality of antenna units. The control system is further configured to determine the signal strengths of the uplink RF signals. The control system is further configured to determine which antenna unit among the plurality of antenna units is receiving uplink RF signals from the at least one client device having the greatest signal strength. The control system is further configured to determine location information for the at least one client device based on identification of the antenna unit receiving the uplink RF signals from the at least one client device having the greatest signal strength. 
     In another embodiment, a method of determining location information for at least one client device wirelessly communicating in a distributed antenna system is provided. The method includes receiving uplink RF signals communicated by at least one client device wirelessly communicating to a plurality of antenna units. The method further includes determining the signal strengths of the uplink RF signals. The method further includes determining which antenna unit among the plurality of antenna units is receiving uplink RF signals from the at least one client device having the greatest signal strength. The method further includes determining the location of the at least one client device based on identification of the antenna unit receiving the uplink RF signals from the at least one client device having the greatest signal strength. 
     In another embodiment, a computer-readable medium having stored thereon computer-executable instructions to cause an LPU configured to determine the location of at least one client device wirelessly communicating in a distributed antenna system is provided. The computer-executable instructions cause the LPU to receive uplink RF signals communicated by at least one client device wirelessly communicating to a plurality of antenna units. The computer-executable instructions cause the LPU to determine the signal strengths of the uplink RF signals. The computer-executable instructions cause the LPU to determine which antenna unit among the plurality of antenna units is receiving uplink RF signals from the at least one client device having the greatest signal strength. The computer-executable instructions cause the LPU to determine location information for the at least one client device based on identification of the antenna unit receiving the uplink RF signals from the at least one client device having the greatest signal strength. 
     In another embodiment, a head-end unit configured to determine the location of at least one client device wirelessly communicating in a distributed antenna system is provided. The head-end unit comprises an uplink receiver (URX) configured to receive uplink RF signals communicated by at least one client device wirelessly communicating to a plurality of antenna units. The URX is further configured to determine the signal strengths of the uplink RF signals. The URX is further configured to provide the signal strengths of the uplink RF signals to an LPU. The LPU is configured to determine which antenna unit among the plurality of antenna units is receiving uplink RF signals from the at least one client device having the greatest signal strength. The LPU is further configured to determine location information for the at least one client device based on identification of the antenna unit receiving the uplink RF signals from the at least one client device having the greatest signal strength. 
     Embodiments disclosed herein also include apparatuses and methods for determining the location of a mobile terminal in a distributed antenna system (DAS). An additional LPU is coupled to a typical DAS and preferably integrated in the head-end unit. Each RF uplink signal is transmitted to the LPU before being combined together and all of the split downlink signals are sent to the LPU as well. The LPU is communicatively linked to the base station and sends the location information of all distributed antennas to the base station. In order to extract the location information of a mobile terminal, the LPU monitors the usage of the frequency band which follows the long term evolution (LTE) standard. 
     In accordance with another embodiment, apparatuses for determining the location of a mobile terminal are provided and comprise a distributed antenna system that includes multiple antennas located in an indoor region where each of the antennas is located in a known area and provides a respective coverage area for communicating with a mobile terminal; a head-end unit that distributes the downlink signals and combines the uplink signals; and an LPU that is integrated in the head-end unit and is communicatively linked to the base station. The RF transmission signals in the system are modulated according to the LTE standard. 
     In accordance with another embodiment, apparatuses for determining the location of a mobile terminal, the location processing unit (LPU), are provided and comprise a plurality of signal monitoring devices that receive each of the uplink signals transmitted by the multiple antennas located in the known areas and acquire the time slots of the downlink signals sent by the base station and split by the head-end unit; and a location server that identifies a transmitting mobile terminal by monitoring the usage of the frequency band and sends the location information to the base station. 
     In accordance with another embodiment, methods for determining the location of a mobile terminal are provided and comprise selecting a specific time slot from the downlink signals; calculating the received signal strength indication (RSSI) values for each of the resource blocks at the specific time slot from the uplink signals; delivering the RSSI values of all the antennas to the location server of the LPU; and identifying which of the antennas is closest to the transmitting mobile terminal by monitoring RSSI values. 
     In accordance with one feature in the method for determining the location of a mobile terminal, the signal processing steps include converting the RF signals acquired from both downlink and uplink to baseband by transceivers (TRXs); digitizing the downlink and uplink signals by a pair of analog-to-digital converters (ADCs); selecting the specific window of data samples from the sample streams by time synchronization; and calculating the RSSI values for each of the resource blocks by a fast Fourier transform (FFT). 
     In according with a modification of embodiments disclosed herein, the location information comprising of the maximum RSSI values with the respective antenna locations where those maximum values have been received are provided to the base station, which then combines this location information with the prior user allocation to provide a location estimate to the network. 
     In a further modification of the method, the downlink and the uplink RF signals are temporal synchronized by means of standard techniques used in mobile terminal devices. 
     In another embodiment, the RSSI values for each of the resource blocks (RB) are calculated by an FFT. 
     In another modification, the location information of the transmitting mobile terminal is sent to the base station. An alternative embodiment of the method is to instruct the mobile device to modulate its output power, to identify a received signal from the mobile device having modulated output power; and to identify a particular antenna unit having a highest received power level from the mobile device. 
     Another embodiment of the method is provided by using time division multiple access (TDMA) protocol to identify a received signal from the mobile device in a frequency channel and time slot of the mobile device; and to determine which of the antennas is closest to the mobile device to be located by monitoring received signal strength of the identified signal. 
     Additional features and advantages will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the embodiments as described herein, including the detailed description that follows, the claims, as well as the appended drawings. 
     It is to be understood that both the foregoing general description and the following detailed description present embodiments, and are intended to provide an overview or framework for understanding the nature and character of the disclosure. The accompanying drawings are included to provide a further understanding, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments, and together with the description serve to explain the principles and operation of the concepts disclosed. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG.  1    is a schematic diagram of an exemplary distributed antenna system; 
         FIG.  2    is a partially schematic cut-away diagram of an exemplary building infrastructure in which a distributed antenna system can be employed; 
         FIG.  3    is an exemplary schematic diagram of an exemplary head-end unit (HEU) deployed in an distributed antenna system; 
         FIG.  4    shows an example of resource allocation in the frequency-time grid, received from a particular remote antenna unit(s) (RAUs) in a distributed antenna system; 
         FIG.  5    is a schematic diagram of an exemplary distributed antenna system illustrating location of client devices in relation to their communication with one or more RAUs in the distributed antenna system; 
         FIG.  6    is a schematic diagram of a distributed antenna system integrated with a location processing unit (LPU) in accordance with one embodiment; 
         FIG.  7    is a schematic diagram illustrating more detail of the internal components of an exemplary LPU, which may include the LPU of  FIG.  6   ; 
         FIG.  8    is a schematic diagram illustrating exemplary signal processing steps that can be performed by an LPU, including the LPU in  FIGS.  6  and  7   , to provide location processing and location services; 
         FIG.  9 A  is a schematic diagram of the HEU in  FIG.  3    that includes an LPU and other components to determine location of client devices in a distributed antenna system; 
         FIG.  9 B  is a schematic diagram of an alternative HEU that includes a co-located LPU and downlink receiver (DRX); 
         FIG.  10    is a schematic diagram illustrating components that may be included in an LPU, including the LPU in  FIGS.  6 ,  9 A, and  9 B ; 
         FIG.  11    is a schematic diagram of an exemplary downlink base station interface card (BIC) that can be provided in the exemplary HEU in  FIG.  9 A ; 
         FIG.  12    is a schematic diagram of an exemplary DRX that can be provided in the exemplary HEU in  FIGS.  9 A and  9 B ; 
         FIG.  13    is a schematic diagram of an exemplary uplink BIC that can be provided in the exemplary HEU in  FIGS.  9 A and  9 B ; 
         FIG.  14    is a schematic diagram of an exemplary uplink receiver (URX) that can be provided in the exemplary HEU in  FIGS.  9 A and  9 B ; 
         FIG.  15    is a schematic diagram of an exemplary uplink spectrum analyzer provided in the URX in  FIG.  14   ; 
         FIG.  16    is an exemplary URX message communicated from a URX to an LPU to provide energy levels associated with RAUs assigned to the URX for client device communications to the RAUs; 
         FIG.  17    is an exemplary LPU message communicated from an LPU to a base station to provide RAUs associated with the maximum energy level for client device communications; 
         FIG.  18    is a schematic diagram of an exemplary HEU board configuration; 
         FIG.  19    is a schematic diagram of another exemplary HEU board configuration; 
         FIG.  20    is a schematic diagram of a master HEU configured to provide location information for client devices communicating with a plurality of slave HEUs communicatively coupled to the master HEU; 
         FIG.  21    is a graph illustrating exemplary time-frequency separation of client devices; 
         FIG.  22    is a graph illustrating exemplary SC-FDMA spectrum of a 0 dB SC-FDMA signal compared to noise level; 
         FIG.  23    is a graph illustrating exemplary false detection probability for one client device and one resource block (RB); 
         FIG.  24    is a graph illustrating exemplary probability of not having 10 RBs pointing to the same RAU; 
         FIG.  25    is a graph illustrating exemplary probability of not having 100 RBs pointing to the same RAU; 
         FIG.  26    is a graph illustrating exemplary energy leakage caused by frequency offset; and 
         FIG.  27    is a graph illustrating exemplary energy leakage caused by time offset. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the embodiments, examples of which are illustrated in the accompanying drawings, in which some, but not all embodiments are shown. Indeed, the concepts may be embodied in many different forms and should not be construed as limiting herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Whenever possible, like reference numbers will be used to refer to like components or parts. 
     Embodiments disclosed in the detailed description include distributed antenna apparatuses, systems, methods, and computer-readable mediums to provide location information regarding client devices communicating with remote antenna units in a distributed antenna system. Providing location information is also providing “location services.” The location information can be used to determine the location of the client devices relative to the remote antenna unit(s) in which the client devices are communicating. In this scenario, the client devices would be known to be within communication range of the remote antenna units. This information can be used to determine or provide a more precise area of location of the client devices. The distributed antenna components and systems, and related methods disclosed herein may be well suited for indoor environments where other methods of providing and/or determining the location of client devices may be obstructed or not possible due to the indoor environment. 
     In this regard, in certain embodiments disclosed herein, a location processing unit (LPU) configured to provide location information for at least one client device wirelessly communicating in a distributed antenna system can be provided. The LPU includes a control system configured to receive uplink radio frequency (RF) signals communicated by at least one client device wirelessly communicating to a plurality of antenna units. The control system is further configured to determine the signal strengths of the uplink RF signals. The control system is further configured to determine which antenna unit among the plurality of antenna units is receiving uplink RF signals from the at least one client device having the greatest signal strength. The control system is further configured to determine location information for the at least one client device based on identification of the antenna unit receiving the uplink RF signals from the at least one client device having the greatest signal strength. 
     Before discussing the exemplary apparatuses, systems, methods, and computer-readable mediums that are configured to determine location information of a client device(s) in a distributed antenna system starting at  FIG.  5   , exemplary distributed antenna systems that do not include location processing according to embodiments disclosed herein are first described with regard to  FIGS.  1 - 4   . 
     Distributed antenna systems can employ different transmission mediums, including for example, conductive wire and optical fiber. A possible configuration of a distributed antenna system using fiber optic cables is shown in  FIG.  1   . In this regard,  FIG.  1    is a schematic diagram of a generalized embodiment of an antenna system. In this embodiment, the antenna system is a distributed antenna system  10 . The distributed antenna system  10  is configured to create one or more antenna coverage areas for establishing communications with wireless client devices located in the radio frequency (RF) range of the antenna coverage areas. In this regard, the distributed antenna system  10  includes a head-end unit (HEU)  12 , one or more remote antenna units (RAUs)  14  and an optical fiber link  16  that optically couples the HEU  12  to the RAU  14 . The HEU  12  is configured to receive communications over downlink electrical RF signals  18 D from a source or sources, such as a network or carrier as examples, and provide such communications to the RAU  14 . The HEU  12  is also configured to return communications received from the RAU  14 , via uplink electrical RF signals  18 U, back to the source or sources. In this regard, in this embodiment, the optical fiber link  16  includes at least one downlink optical fiber  16 D to carry signals communicated from the HEU  12  to the RAU  14  and at least one uplink optical fiber  16 U to carry signals communicated from the RAU  14  back to the HEU  12 . 
     The distributed antenna system  10  has an antenna coverage area  20  that can be substantially centered about the RAU  14 . The antenna coverage area  20  of the RAU  14  forms an RF coverage area  21 . The distributed antenna system  10  in this example is an optical fiber-based distributed antenna system. In this regard, the HEU  12  is adapted to perform or to facilitate any one of a number of Radio-over-Fiber (RoF) applications, such as radio-frequency (RF) identification (RFID), wireless local-area network (WLAN) communication, or cellular phone service. “Radio-over-Fiber,” or “RoF,” utilizes RF signals sent over optical fibers. Shown within the antenna coverage area  20  is a client device  24  in the form of a mobile device as an example, which may be a cellular telephone as an example. The client device  24  can be any device that is capable of receiving RF communication signals. The client device  24  includes an antenna  26  (e.g., a wireless card) adapted to receive and/or send electromagnetic RF signals. 
     As discussed above, the distributed antenna system  10  may, but is not required to, employ RoF. RoF is a technology whereby light is modulated by a radio signal and transmitted over an optical fiber link to facilitate wireless access. In an RoF architecture, a data-carrying RF signal with a high frequency (e.g. only, greater than 10 GHz) is imposed on a lightwave signal before being transported over the optical link. Therefore, wireless signals are optically distributed to base stations directly at high frequencies and converted to from optical to electrical domain at the base stations before being amplified and radiated by an antenna. As a result, no frequency up/down conversion is required at the various base station, thereby resulting in simple and rather cost-effective implementation is enabled at the base stations. 
     With continuing reference to  FIG.  1   , to communicate the electrical RF signals over the downlink optical fiber  16 D to the RAU  14 , to in turn be communicated to the client device  24  in the antenna coverage area  20  formed by the RAU  14 , the HEU  12  includes an electrical-to-optical (E/O) converter  28 . The E/O converter  28  converts the downlink electrical RF signals  18 D to downlink optical RF signals  22 D to be communicated over the downlink optical fiber  16 D. The RAU  14  includes an optical-to-electrical (O/E) converter  30  to convert received downlink optical RF signals  22 D back to electrical RF signals to be communicated wirelessly through an antenna  32  of the RAU  14  to client devices  24  located in the antenna coverage area  20 . The antenna  32  may be referred to as a “remote antenna unit  32 ” herein, but such only means that the antenna  32  is located a desired distance from the HEU  12 . 
     Similarly, the antenna  32  is also configured to receive wireless RF communications from client devices  24  in the antenna coverage area  20 . In this regard, the antenna  32  receives wireless RF communications from client devices  24  and communicates electrical RF signals representing the wireless RF communications to an E/O converter  34  in the RAU  14 . The E/O converter  34  converts the electrical RF signals into uplink optical RF signals  22 U to be communicated over the uplink optical fiber  16 U. An O/E converter  36  provided in the HEU  12  converts the uplink optical RF signals  22 U into uplink electrical RF signals, which can then be communicated as uplink electrical RF signals  18 U back to a network or other source. The HEU  12  in this embodiment is not able to distinguish the location of the client devices  24  in this embodiment. The client device  24  could be in the range of any antenna coverage area  20  formed by an RAU  14 . 
     To provide further exemplary illustration of how a distributed antenna system, such as distributed antenna system  10  in  FIG.  1   , can be deployed indoors,  FIG.  2    is a partially schematic cut-away diagram of a building infrastructure  40  employing the distributed antenna system  10  of  FIG.  2   . The building infrastructure  40  generally represents any type of building in which the distributed antenna system  10  can be deployed. As previously discussed with regard to  FIG.  1   , the distributed antenna system  10  incorporates the HEU  12  to provide various types of communication services to coverage areas within the building infrastructure  40 , as an example. For example, as discussed in more detail below, the distributed antenna system  10  in this embodiment is configured to receive wireless RF signals and convert the RF signals into RoF signals to be communicated over the optical fiber link  16  to the RAUs  14 . The distributed antenna system  10  in this embodiment can be, for example, an indoor distributed antenna system (IDAS) to provide wireless service inside the building infrastructure  40 . These wireless signals can include cellular service, wireless services such as radio frequency identification (RFID) tracking, Wireless Fidelity (WiFi), local area network (LAN), and combinations thereof, as examples. 
     With continuing reference to  FIG.  2   , the building infrastructure  40  includes a first (ground) floor  42 , a second floor  44 , and a third floor  46 . The floors  42 ,  44 ,  46  are serviced by the HEU  12  through a main distribution frame  48  to provide antenna coverage areas  50  in the building infrastructure  40 . Only the ceilings of the floors  42 ,  44 ,  46  are shown in  FIG.  2    for simplicity of illustration. In the example embodiment, a main cable  52  has a number of different sections that facilitate the placement of a large number of RAUs  14  in the building infrastructure  40 . Each RAU  14  in turn services its own coverage area in the antenna coverage areas  50 . The main cable  52  can include, for example, a riser section  54  that carries all of the downlink and uplink optical fibers  16 D,  16 U to and from the HEU  12 . The main cable  52  can include one or more multi-cable (MC) connectors adapted to connect select downlink and uplink optical fibers  16 D,  16 U, along with an electrical power line, to a number of optical fiber cables  56 . 
     The main cable  52  enables multiple optical fiber cables  56  to be distributed throughout the building infrastructure  40  (e.g., fixed to the ceilings or other support surfaces of each floor  42 ,  44 ,  46 ) to provide the antenna coverage areas  50  for the first, second and third floors  42 ,  44  and  46 . In an example embodiment, the HEU  12  is located within the building infrastructure  40  (e.g., in a closet or control room), while in another example embodiment the HEU  12  may be located outside of the building infrastructure  40  at a remote location. A base station  58 , which may be provided by a second party such as a cellular service provider, is connected to the HEU  12 , and can be co-located or located remotely from the HEU  12 . A base station is any station or source that provides an input signal to the HEU  12  and can receive a return signal from the HEU  12 . In a typical cellular system, for example, a plurality of base stations are deployed at a plurality of remote locations to provide wireless telephone coverage. Each base station serves a corresponding cell and when a mobile station enters the cell, the base station communicates with the mobile station. Each base station can include at least one radio transceiver for enabling communication with one or more subscriber units operating within the associated cell. 
     To provide further detail on components that can be provided in a HEU, including the HEU  12  provided in the distributed antenna system  10  of  FIGS.  1  and  2   ,  FIG.  3    is provided. As illustrated therein, the HEU  12  in this embodiment includes a head-end controller (HEC)  60  that manages the functions of the HEU  12  components and communicates with external devices via interfaces, such as a RS-232 port  62 , a Universal Serial Bus (USB) port  64 , and an Ethernet port  66 , as examples. The HEU  12  can be connected to a plurality of base stations (BTSs)  69 ( 1 )- 69 (N), transceivers, and the like via base station inputs  70  and base station outputs  72 . The base station inputs  70  are downlink connections and the base station outputs  72  are uplink connections. Each base station input  70  is connected to a downlink base station interface card (BIC)  74  located in the HEU  12 , and each base station output  72  is connected to an uplink BIC  76  also located in the HEU  12 . The downlink BIC  74  is configured to receive incoming or downlink RF signals from the base station inputs  70  and split the downlink RF signals into copies to be communicated to the RAUs  14 , as illustrated in  FIG.  4   . The uplink BIC  76  is configured to receive the combined outgoing or uplink RF signals from the RAUs  14  and split the uplink RF signals into individual base station outputs  72  as a return communication path. 
     The downlink BIC  74  is connected to a midplane interface card  78  panel in this embodiment. The uplink BIC  76  is also connected to the midplane interface card  78 . The downlink BIC  74  and uplink BIC  76  can be provided in printed circuit boards (PCBs) that include connectors that can plug directly into the midplane interface card  78 . The midplane interface card  78  is in electrical communication with a plurality of optical interface cards (OICs)  80 , which provide an optical to electrical communication interface and vice versa between the RAUs  14  via the downlink and uplink optical fibers  16 D,  16 U and the downlink BIC  74  and uplink BIC  76 . The OICs  80  include the E/O converter  28  in  FIG.  2    that converts electrical RF signals from the downlink BIC  74  to optical RF signals, which are then communicated over the downlink optical fibers  16 D to the RAUs  14  and then to client devices. The OICs  80  also include the O/E converter  36  in  FIG.  1    that converts optical RF signals communicated from the RAUs  14  over the uplink optical fibers  16 U to the HEU  12  and then to the base station outputs  72 . 
     The OICs  80  in this embodiment support up to three (3) RAUs  14  each. The OICs  80  can also be provided in a PCB that includes a connector that can plug directly into the midplane interface card  78  to couple the links in the OICs  80  to the midplane interface card  78 . The OICs  80  may consist of one or multiple optical interface cards (OICs). In this manner, the HEU  12  is scalable to support up to thirty-six (36) RAUs  14  in this embodiment since the HEU  12  can support up to twelve (12) OICs  80 . If less than thirty-six (36) RAUs  14  are to be supported by the HEU  12 , less than twelve OICs  80  can be included in the HEU  12  and plugged into the midplane interface card  78 . One OIC  80  is provided for every three (3) RAUs  14  supported by the HEU  12  in this embodiment. OICs  80  can also be added to the HEU  12  and connected to the midplane interface card  78  if additional RAUs  14  are desired to be supported beyond an initial configuration. The HEC  60  can also be provided that is configured to be able to communicate with the downlink BIC  74 , the uplink BIC  76 , and the OICs  80  to provide various functions, including configurations of amplifiers and attenuators provided therein. Note that although  FIG.  3    illustrates specific exemplary components for the HEU  12 , the HEU  12  is not limited to such components. 
     It may be desired to provide location information/localization services in the distributed antenna system  10  illustrated in  FIGS.  1  and  2   , as an example. For example, it may be desired determine the location of client devices  24  communicating with antennas  32  in the distributed antenna system  10 . Localization services may be desired or required to provide certain services, such as, for example, emergency 911 (E911) services in the case of a cellular client device. Localization services may require a certain percentage of client devices  24  to be locatable within a given distance to comply with communication requirements. As an example, it may be desired or required by E911 services to be able to locate a given percentage of all client device users within one hundred (100) feet (ft.) as an example. Localization services may be desired or required for other types of wireless clients other than cellular clients as well. If client devices  24  are located inside the building infrastructure  40  and establishes communication with the HEU  12 , it can be determined that the client devices  24  are located within at least the distance between the farthest RAU  14  located from the HEU  12 . However, it may not be possible to determine the location of client devices  24  with greater specificity and resolution. For example, in indoor environments, global positioning services (GPSs) provided in the client devices  24  may be inoperable to report a location. Further, triangulation techniques as a method of determining location of client devices  24  may not be possible due to separation of the antenna coverage areas in the distributed antenna system  10 . 
     If it could be determined to which RAU(s)  14  in the distributed antenna system  10  a client device  24  establishes communications, this information could be used to provide location information for a client device  24 . The client device  24  would be known to be within communication range of such RAU(s)  14 . This information coupled with knowing the location of the HEU  12  can be used to determine or provide a more precise area of location of the client device  24 . In essence, linking communication of client devices  24  with a particular RAU(s)  14  provides another layer of location determination in addition to knowing the location of the HEU  12 . Cellular networks, for example, provide methods of determining location. 
     For example, Global System for Mobile Communications (GSM) network compatible client devices are configured to automatically initiate providing client device identification information over the network that can be exploited to provide location services for a distributed antenna system. The locations of the RAUs in the system are also configured and known in the HEU. By knowing and correlating the particular RAU(s) in which the client device established communication, the HEU is able to determine and/or provide the location of the client device as being within the antenna coverage area formed by the particular RAU. The correlation of client device identification information from the client device with the location of the RAU is retained when communicated to the HEU and is not lost by being combined, such as by splitters or containers, with communications from other RAUs. 
     As another example, in a code division multiple access (CDMA) network, a specific notification channel is provided to carry a tracking signal that can be exploited to provide location services in a distributed antenna system. In this manner, the tracking signal is radiated through the RAU to be communicated to client devices within range of the antenna coverage area formed by the RAU. When the client device wirelessly receives the tracking signal, the client device communicates its identification information and identification of the tracking signal to an RAU to be communicated back to the HEU. The HEU can provide this information to a network or carrier. In this manner, the client device identification information and identification of the tracking signal can be associated with the location of a particular RAU(s) that received and transmitted the tracking signal in the distributed antenna system to provide or determine a location of the client device. 
     As another example, the long term evolution (LTE) standard supports both frequency division duplexing (FDD) and time division duplexing (TDD) modes that can be exploited to provide location services in a distributed antenna system. LTE uses orthogonal frequency-division multiplexing (OFDM) for the downlink and a pre-coded version of OFDM called single carrier-frequency division multiple access (SC-FDMA) for the uplink. Furthermore, LTE employs a multiple input/multiple output (MIMO) antenna scheme to achieve the requirements of throughput and spectral efficiency. The LTE standard supports both FDD and TDD modes. In the time domain, the time slot is fixed to 0.5 milliseconds (ms) long which is half of a subframe. A radio frame is ten (10) ms long and it contains ten (10) subframes. In the frequency domain, the smallest resource unit is denoted as a resource element and twelve of these elements together (per slot) are called a resource block (RB) that is 180 kiloHertz (kHz). Uplink and downlink transmissions are separated in the frequency domain. For TDD mode, a subframe is either allocated to downlink or uplink transmission. Uplink and downlink transmissions alternate in the time domain using the same frequency bands. 
     In this regard,  FIG.  4    illustrates that in an uplink, data is allocated in multiples of one resource block. In FDD applications, the uplink resource block size in the frequency domain contains twelve (12) sub-carriers and the transmission time interval is one (1) ms long. The uplink resource blocks are assigned to the user equipment (UE) by the base station scheduler, which is called evolved Node B (eNB). Since the base station assigns certain time (t) and frequency (f) blocks to the UEs and informs UEs about the transmission format to use, the base station has complete knowledge of which user has used a specific frequency bin at a specific time slot. The UEs may hop resource blocks RB from subframe to subframe. In LTE PUSCH hopping mode, a UE may even use different frequencies from one slot to another for added frequency diversity.  FIG.  4    shows an exemplary diagram of resource allocation in the frequency-time grid, received from a particular antenna in the distributed antenna system. As shown in  FIG.  4   , the UE hops to another frequency allocation from one slot to another within one subframe. 
     Since there is a growing demand for increasing the capacity and speed of mobile telecommunication networks, mobile communication technology is currently being developed toward the 4th generation (4G), which is mainly based on the LTE standard. Therefore, it is desired to provide a method for determining the location of a mobile terminal in a distributed antenna system that can meet the LTE standard. 
     In each of these technologies and any others that may be selected for employment in a distributed antenna system, if communications between client devices and particular RAU(s) can be determined and recognized, the location of the client devices in the distributed antenna system can be determined. Depending on the communication technologies employed or supported in a distributed antenna system, how a particular RAU is linked to a particular client device can vary, but the concept of linking particular RAU(s) to client devices to determine location can be employed. 
     In this regard,  FIG.  5    illustrates a schematic diagram of an exemplary distributed antenna system  90  that is configured to provide localization services for locating particular client devices  92  communicating with RAUs  94 A- 94 D within the distributed antenna system  90 . In this example, the RAUs  94 A- 94 D are strategically located within different tracking zones  96 A- 96 D in a building  98  or other infrastructure. For example,  FIG.  5    illustrates four tracking zones  96 A- 96 D, which may each represent a given floor within the building  98 . Note that although four (4) tracking zones  96 A- 96 D are shown, the disclosure herein is not limited to providing a particular number of tracking zones. Thus, when the client devices  92  are located within range of a particular RAU  94 A- 94 D, the client device  92  will communicate with a particular RAU(s)  94 A- 94 D in range. 
     With continuing reference to  FIG.  5   , an HEU  102  provided in the distributed antenna system  90  and communicatively coupled to the RAUs  94 A- 94 D can receive communications from the client devices  92  and determine from which RAU(s)  94 A- 94 D communications from the client devices  92  are being received. Thus, location information regarding the client devices  92  can be determined based on linking communications of the client devices  92  to known locations of the RAUs  94 A- 94 D in the distributed antenna system  90 . The location information can be provided by the HEU  102  over a wired and/or wireless network  104  to a base station  106 , if desired. The base station  106  may contain information that allows the client devices  92  to be specifically identified by user or subscriber to then know the location of such user or subscriber. 
     Embodiments disclosed herein include modified HEUs that provide exemplary solutions to locate client devices based on their communications with a particular RAU(s) in a distributed antenna system. In this regard,  FIG.  6    provides one embodiment of determining the location of a client device in a distributed antenna system. As illustrated therein, a distributed antenna system  110  is provided, which in this example is an optical fiber-based distributed antenna system. The distributed antenna system  110  contains multiple antennas  32  provided in remote antenna units (RAU)  14  that provide respective coverage areas for communicating with client devices  24 , which may be for example cellular devices and/or terminals. A main antenna  32  and an auxiliary antenna  32 ′ may be provided for antenna diversity. A HEU  116  is provided that is communicatively coupled to a base station  118 , which may be a cellular base station, to receive input electrical RF signals  120  from the base station  118  and provide output electrical RF signals  122  to the base station  118 . 
     The HEU  116  includes a combiner/splitter  124  that splits the input electrical RF signals  120  into downlink electrical RF signals  126 . A plurality of RF-to-FO (RF2FO) converters  130  are provided to convert the downlink electrical RF signals  126  to downlink optical RF signals  132 . The downlink optical RF signals  132  are transmitted in an indoor region via fiber optic cables  134  and converted back to downlink electrical RF signals  136  by a plurality of FO-to-RF (FO2RF) converters  138 . The converted downlink electrical RF signals  136  are further transmitted to the multiple antennas  32  for communicating with the client devices  24 . A plurality of RF2FO converters  140  are also provided to convert uplink electrical RF signals  142  from the client devices  24  to uplink optical RF signals  144 . The uplink optical RF signals  144  are communicated over fiber optic cables  146  to FO2RF converters  148  at the HEU  116  to be converted into uplink electrical RF signals  128 . The combiner/splitter  124  combines the uplink electrical RF signals  128  into the output electrical RF signals  122  communicated to the base station  118 . 
     If the client device  24  sends an RF signal to any of the antennas  32  in this embodiment, the base station  118  cannot identify the location of the client device  24 . This is because the uplink electrical RF signals  128  from the various client devices  24  are combined by the combiner/splitter  124 . Thus, in this embodiment, a location processing unit (LPU)  150  is provided and integrated into the HEU  116 . As will be described in more detail below, the LPU  150  can determine the location of the client devices  24 . In certain embodiments, the LPU  150  can determine the location of the client devices  24  by monitoring the signal strength of the uplink electrical RF signals  142  received from the client devices  24 . By monitoring the signal strength of the uplink electrical RF signals  142  (either by direct measurement or indirectly such as measuring the signal strength of the uplink optical RF signals  144 ) the LPU  150  can determine with which antenna  32  in the distributed antenna system  110  the client device  24  is communicating. If the client device  24  is communicating with multiple antennas  32 , the LPU  150  can distinguish which antenna  32  is closest to the client device  24  by comparing the signal strengths of the uplink electrical RF signals  142  received by the multiple antennas  32 . The LPU  150  can then provide this location information regarding the client device  24  to the base station  118  via a communication link  152 , which may be a wired or wireless link. 
       FIG.  7    is a schematic diagram of one possible embodiment of the LPU  150  in  FIG.  6   . In this regard, a plurality of signal monitoring devices  154 ( 1 )- 154 (N) receive the uplink electrical RF signals  128 ( 1 )- 128 (N) from each of the distributed antennas  32  located in the known areas before being combined together by the HEU  116  and acquire the time slots of the downlink electrical RF signals  126  sent by the base station  118  after being split by the HEU  116 . The task of the signal monitoring devices  154 ( 1 )- 154 (N) is to provide the usage of the frequency band from each of the multiple antennas  32  (see  FIG.  6   ). For each of the uplink electrical RF signals  128 ( 1 )- 128 (N), the received signal strength indication (RSSI) value is determined by the signal monitoring devices  154 ( 1 )- 154 (N) for given time/frequency blocks. A location server  156  receives RSSI values of all of the antennas sent by the signal monitoring devices  154 ( 1 )- 154 (N) and identifies which of the antennas  32  is closest to the transmitting client device  24  to be located. The location information is then sent over the communication link  152  to the base station  118 . Since the base station  118  controls the assignment of certain time slot/frequency blocks to the client devices  24  in this embodiment, the base station  118  can uniquely identify which of the client devices  24  has used a specific frequency bin at a specific time slot. 
     In case of an emergency or a service request sent by the client device  24 , the base station  118  is asked to deliver the location information and it sends the request to the LPU  150 . Then, the LPU  150  acquires RSSI values for all particular time slots/frequency blocks from all the antennas and identifies the location of the transmitting client device  24  by identifying the antenna  32  for which the resource block (RB) energy is maximized. The location information is then sent from the LPU  150  to the base station  118  over the communication link  152 . An assessment of these RSSI values (e.g., triangulation) provides a good estimation of the location in which the client device  24  is sending the service request by monitoring the usage of the frequency band, and it is communicatively linked to the base station  118 . 
       FIG.  8    is a schematic diagram illustrating exemplary signal processing steps that can be performed by an LPU, including the LPU  150  in  FIGS.  6  and  7   , to provide location processing and location services for locating client devices. The signal processing is performed in the LPU  150  for antenna diversity, for example, when two receiving antennas  32 ,  32 ′ per antenna location are employed for communications to the client device  24 . The downlink electrical RF signal  126  is first down-converted to baseband by means of a transceiver (TRX)  158  that includes at least mixers and appropriate filters. The downlink electrical RF signal  126  is then digitized by a pair of analog-to-digital converters (ADCs)  160  to produce downlink data  162 . 
     With continuing reference to  FIG.  8   , the uplink electrical RF signal  128  received from the main antenna  32  at a specific location is converted to digital baseband by a TRX  164  and ADCs  166  to produce uplink data  168 . Time synchronization  170  of the downlink data  162  and the uplink data  168  is processed by means of standard techniques that are also employed in client devices  24 . For a given time slot (as illustrated in  FIG.  4   ), the signal from the time synchronization  170  is used in a window selection  172  to select a specific window of data samples from the sample streams to process a fast Fourier transform (FFT)  174 . The squared absolute value of each FFT output is computed in step  176  and the relevant outputs are combined to form an RSSI value for the given time slot/frequency block in step  178 . 
     Optionally, a second received uplink electrical RF signal  128 ′ coming from an auxiliary antenna  32 ′ at the same antenna location can be processed in the same manner. Uplink data  180  of this second path consisting of a TRX  182  and ADCs  183  are then combined together with the RSSI outputs of the main receiving antenna  32  in step  178  and this combined RSSI value can provide a better location estimation. 
     In an alternative embodiment applicable to TDD mode, in which uplink and downlink transmissions alternate in the time domain using the same frequency bands, a switching mechanism can be used to alternate the downlink and uplink transmissions on the same frequency. However, the downlink time synchronization block must additionally assess the control information about the downlink and uplink periods. In LTE, this control information can be retrieved from one of the control channels from the downlink. An additional signal needs to be generated and conveyed to the uplink signal processing paths to exclude downlink signals from being processed. Alternatively, a signal provided by the base station that is used to control a power amplifier in a TDD system can be used instead. 
     Now that generalized embodiments of providing location services have been described, more specific exemplary embodiments are discussed. In this regard,  FIG.  9 A  is a schematic diagram of the HEU  12  in  FIG.  3    that includes another example of an LPU and other components to determine location of client devices in a distributed antenna system. Components in  FIG.  7    that are common with components in  FIG.  3    are illustrated with common element numbers and thus will not be re-described here. To provide location information, an LPU  184  is provided in the HEU  12  and is interfaced with other additional components provided in the HEU  12 . In  FIG.  9 A , the LPU  184  is provided as a separately component from a digital receiver (DRX)  186 , which is discussed in more detail below. Alternatively, as illustrated in  FIG.  9 B , the LPU  184  and DRX  186  may be co-located in the same component, for example on the same PCB. The LPU  184  is the main interface to the base stations  69 ( 1 )- 69 (N) via communication links  192 . The base stations  69 ( 1 )- 69 (N) can request location processing services over the communication links  192  to the LPU  184 . In response, the LPU  184  can configure a downlink receiver (DRX)  186  and uplink receivers (URXs)  190 , which are described in more detail below. The URXs  190  provide a distributed configuration to provide information regarding energy levels of the uplink optical RF signals  22 U resulting from client device  24  communications to antennas at RAUs  14  coupled to the HEU  12  to the LPU  184 . The LPU  184  uses the energy levels to determine which antenna  32  (i.e., RAU) is closest to the client device  24  to perform location services. More detail regarding internal exemplary components of the LPU  184  is provided in  FIG.  10    described below. 
     With continuing reference to  FIG.  9 A , the DRX  186  is provided to retrieve specific settings from a downlink control channel sent by a base station  69 ( 1 )- 69 (N) over downlink electrical RF signals  188 ( 1 )- 188 (N) from the base station  69 ( 1 )- 69 (N). These settings are sent to the LPU  184  for analysis and control generation for analyzer functions performed by the LPU  184  for determining location of the client devices  24 . The DRX  186  uses the downlink electrical RF signals  188 ( 1 )- 188 (N) to synchronize a local oscillator. The DRX  186  also provides a reference frequency and reference timing to the LPU  184  and the uplink receivers (URXs)  190  (which are discussed below) to synchronize these components to the base station  69 ( 1 )- 69 (N). One DRX  186  can be provided in the HEU  12  providing settings to the LPU  184  and all URXs  190 . Alternatively, a DRX  186  can be provided for each OIC  80  if desired. More detail regarding an exemplary DRX will be discussed in more detail below with regard to  FIGS.  11  and  12   . 
     With continuing reference to  FIG.  9 A , the URXs  190  are provided to perform signal analysis on uplink optical RF signals  22 U received from antennas in RAUs  14  to provide the energy levels of these signals to the LPU  184  for processing. In essence, the URXs  190  listen on the uplink optical fiber  16 U to monitor the uplink optical RF signals  22 U to determine the energy level of these signals. In this embodiment, each URX  190  has three (3) uplink signal analyzing paths to support three (3) uplink optical RF signals  22 U coming from up to three (3) RAUs  14 . Implementing URX  190  functionality on the OIC  80  automatically takes into account the scalability of the HEU  12  so that sufficient resources are provided to timely analyze incoming uplink optical RF signals  22 U. Each analyzing path converts a specific channel that matches the channel of a base station  69 ( 1 )- 69 (N) to baseband and then performs spectral analysis and energy detection for each RAU  14 , respectively. The signal analysis performed in the URXs  190  is made according to the reference timing provided by the DRX  186 . The maximum energy values of each channel are provided to the LPU  184  to determine the locations of client devices  24  and provide this information to the base stations  69 ( 1 )- 69 (N). More detail regarding an exemplary URX will be discussed in more detail below with regard to  FIGS.  13 - 15   . 
     With continuing reference to  FIG.  9 A , the communication link  192  may be an Ethernet communication link, which is well supported. Different network protocols, such as User Datagram Protocol (UDP) and Transmission Control Protocol (TCP)/Internet Protocol (IP) (TCP/IP), are also well supported. IP packets communicated from the LPU  184  to the base stations  69 ( 1 )- 69 (N) can also be routed via a wide area network (WAN) or via a cellular modem (e.g., LTE), as examples, to remote locations. In this manner, location processing provided by the LPU  184  can be supported even if the HEU  12  is remotely located from the base stations  69 ( 1 )- 69 (N), for example, when the HEU  12  is connected to a cellular network. 
     The base stations  69 ( 1 )- 69 (N) can request location processing services to the HEU  12  by sending a request message over the communication link  192  to the HEU  12 . In this instance, the LPU  184  wakes up the DRX  186  and the URXs  190 . Control messages from the LPU  184  to the DRX  186  request the DRX  186  to tune to the same channel as the base station  69 ( 1 )- 69 (N) requesting location services/information. The DRX  186  then acquires the base station  69 ( 1 )- 69 (N) downlink signal and decodes the control channel to get frame timing and cell-site specific configuration. These parameters are communicated from the DRX  186  to the LPU  184 , which in turn configures the URXs  190  based on this parameter information. The URXs  190  can then monitor the uplink optical RF signals  22 U on the configured channel for providing energy levels of uplink optical RF signals  22 U on the channel to the LPU  184 . If a common DRX  186  is provided, location services can be provided for one channel requested by the base station  69 ( 1 )- 69 (N) at one time. However, if multiple DRXs  186  are provided in the OICs  80 , location services for more than one base station channel can be performed at the same time. 
       FIG.  10    is a schematic diagram illustrating components that may be included in an LPU, which can include the LPU  184  in  FIGS.  6 ,  9 A, and  9 B . The LPU  184  in this embodiment includes one or more BTS ports  194 ( 1 )- 194 (N) that allow communication between the LPU  184  and the base stations  69 ( 1 )- 69 (N) over the communication link  192 . The BTS ports  194 ( 1 )- 194 (N) are connected to a BTS interface  196  provided in a control system  197  in the LPU  184  that is configured to receive requests to determine locations of client devices  24  for given channels of the base stations  69 ( 1 )- 69 (N). The BTS interface  196  is also configured to report to the base stations  69 ( 1 )- 69 (N) through the appropriate BTS port  194 ( 1 )- 194 (N) which antenna  32  is receiving maximum energy from client devices  24  for determining the location of the client devices  24 . 
     With continuing reference to  FIG.  10   , the LPU  184  also includes a DRX control  198  that is configured to power up and reset the DRX  186  when location services are requested or desired. The DRX control  198  is also configured to set the channel in the DRX  186  to distinguish downlink RF signals from the base station  69 ( 1 )- 69 (N) requesting location services to the LPU  184 . The DRX control  198  communicates to the DRX  186  in this regard through a DRX port  200  provided in the LPU  184 . Timing information from the DRX  186  received over a downlink RF signal from a base station  69 ( 1 )- 69 (N) requesting location services is provided to the LPU  184  through a timing port  202 . 
     With continuing reference to  FIG.  10   , the LPU  184  also includes a URX control  204  that is configured to power up and reset the URXs  190  when location services are requested or desired. The URX control  204  is also configured to set the channel in the URXs  190  to distinguish uplink RF signals from the client devices  24  destined for the base stations  69 ( 1 )- 69 (N) requesting location services to the LPU  184 . The URX control  204  can also relay timing information, such as frame number and frame timing, to the URXs  190 . The URX control  204  can also relay cell-site specific configuration data, such as cyclic prefix mode and bandwidth as examples, to the URXs  190 . The URX control  204  communicates to the URXs  190  in this regard through URX ports  206 ( 1 )- 206 (N) provided in the LPU  184 . 
     With continuing reference to  FIG.  10   , the LPU  184  also includes a location module  208  that is configured to collect data regarding energy levels of uplink RF signals from the URXs  190  over the URX ports  206 ( 1 )- 206 (N). Thus, the LPU  184  can receive energy levels of uplink RF signals from client devices  24  per URX  190  and per client device  24  since a URX  190  is provided per OIC  80  in one embodiment. The location module  208  identifies the antenna  32  (i.e., RAU  14 ) that has the maximum energy signal for each client device  24 . By selecting the URX  190  that has reported the maximum energy level for a given client device  24 , the client device  24  can be associated with a specific antenna  32  in a RAU  14  and thus the location of the client device  24  relative to the location of such antenna  32  can be determined. The location information determined by the location module  208  can be provided to the base stations  69 ( 1 )- 69 (N) via the BTS ports  194 ( 1 )- 194 (N). The LPU  184  includes a switch  210 , which may be an Ethernet switch, that concentrates traffic between the components of the LPU  184  and the ports  194 ( 1 )- 194 (N),  200 ,  206 ( 1 )- 206 (N). 
     The control system  197 , and any of the components provided therein as illustrated in  FIG.  10   , may be exclusively provided in circuitry, software instructions executing on a processor, or a combination of both. As examples, the control system  197  may include a circuit, which may be provided in a field-programmable gate array (FPGA), a microprocessor, a microcontroller, or any combination thereof. Memory  207  may be provided in the control system  197  that contains computer-executable instructions to perform some or all of the functionalities provided in the control system  197 . 
       FIG.  11    is a schematic diagram of an exemplary downlink BIC  74  in  FIGS.  9 A and  9 B , which can comprise a single printed circuit board. The downlink BIC  74  receives the downlink electrical RF signals  188 ( 1 )- 188 (N) from the base stations  69 ( 1 )- 69 (N), combines the downlink electrical RF signals  188 ( 1 )- 188 (N) via a combiner  212 , and then splits the combined signal into twelve (12) output signals to be communicated to the OICs  80  to be converted into downlink optical RF signals to be communicated to RAUs  14 . In this embodiment, the DRX  186  is coupled to an output  214  of the combiner  212 . As an example, the expected power level of the output  214  may be in the range of 8 dBm. However, as an example, the DRX  186  may be configured to receive signal levels from the output  214  from −10 to −90 dBm. Thus, the DRX  186  can receive downlink electrical RF signals  188 ( 1 )- 188 (N) for all base stations  69 ( 1 )- 69 (N) and thus communicate requests from the base stations  69 ( 1 )- 69 (N) requesting location services to the LPU  184 . Alternatively, multiple DRXs  186  could be provided to individually receive downlink electrical RF signals  188 ( 1 )- 188 (N) from the base stations  69 ( 1 )- 69 (N) before the downlink electrical RF signals  188 ( 1 )- 188 (N) are combined. In this instance, each DRX would communicate to the LPU  184  to provide requests for location services from the base stations  69 ( 1 )- 69 (N). 
       FIG.  12    is a schematic diagram of the DRX  186  in  FIGS.  9 A and  9 B  illustrating exemplary components that can be provided in the DRX  186 . In this example, the DRX  186  contains an RF transceiver  216 , a clock generation unit  218 , and a control module  220  which provides the logic for performing time synchronization via generation of a TIMING signal  219 . For example, the time synchronization performed may be LTE time synchronization. The RF transceiver  216  receives the downlink electrical RF signal  188 ( 1 )- 188 (N) through a BTS downlink RF signal port  217 . The control module  220  may be provided exclusively in circuitry, such as in an FPGA as an example, or software executed on a processor, or a combination of both. A DRX control  222  provided in the control module  220  is configured to interpret commands from the LPU  184  and send the detected cell-site specific parameters to the LPU  184 . For LTE processing as an example, an LTE cell searcher  224  and downlink receiver  226  are included. Automatic frequency control (AFC)  228  is also included. 
     Using LTE processing as a specific example, the downlink receiver  226  is set up and calibrated. A control interface  230  to set up and calibrate the RF transceiver  216  is provided by a downlink receiver control  232 . The LTE cell searcher  224  finds the frame timing using an LTE primary synchronization sequence (PSS) and secondary synchronization sequence (SSS). The downlink receiver  226  is responsible for retrieving further control parameters from the broadcast channel in the downlink electrical RF signals  188 ( 1 )- 188 (N). Frequency synchronization can be achieved by tuning a local voltage controlled oscillator (VCO)  234 . An external digital-to-analog converter (DAC)  236  is provided and used for generating the control voltage for the VCO  234 . The URXs  190  are synchronized in frequency to the uplink electrical RF signals received from the client devices  24 . Thus, the VCO&#39;s  234  reference frequency is buffered and distributed to the URXs  190  as the CLOCK signal  237  in this embodiment. The VCO&#39;s  234  reference frequency can also be provided to the LPU  184  for synchronization if the LPU  184  is not hosted on the same PCB as the DRX  184 . 
       FIG.  13    is a schematic diagram of an exemplary OIC  80  provided in  FIGS.  9 A and  9 B . In this embodiment, the OIC  80  supports N number of RAUs  14  on a single PCB. The OIC  80  comprises an N-way downlink splitter  238  electrically coupled to a downlink coaxial connection  240 , an N-way uplink combiner  242  electrically coupled to an uplink coaxial connection  244 , N downlinks  246 ( 1 )- 246 (N), N uplinks  248 ( 1 )- 248 (N), N E/O converters  250 ( 1 )- 250 (N), N O/E converters  252 , and connectors  254 . Note that the number of RAUs  14  supported by the OIC  80  can be varied, however, depending upon the particular application. In the illustrated embodiment, the connectors  254  are dual SC/APC interfaces. A URX  190  is provided in the OIC  80  to tap off uplink optical RF signals  256 ( 1 )- 256 (N) that are the output of the N-way uplink combiner  242  to further process such signals and provide energy levels to the LPU  184  for location processing. 
       FIG.  14    is a schematic diagram of the URX  190  in  FIGS.  9 A and  9 B  illustrates exemplary components provided in the URX  190 . In this embodiment, the URX  190  has transceivers  258 ( 1 )- 258 (N), one for each OIC input  259 ( 1 )- 259 (N) supported by the URX  190 , which down-converts an uplink electrical RF signal from a client device  24  to baseband. A control module  260  is provided that contains uplink spectrum analyzers  262 ( 1 )- 262 (N) for each OIC input  259 ( 1 )- 259 (N). The uplink spectrum analyzers  262 ( 1 )- 262 (N) perform signal analysis on a digital baseband input  263  to determine the energy level on the uplink electrical RF signals on the baseband. A control interface  264  is provided in the control module  260  to provide energy level information regarding uplink electrical RF signals received from the OIC inputs  259 ( 1 )- 259 (N) to the LPU  184  via an LPU port  265 . The uplink spectrum analyzers  262 ( 1 )- 262 (N) can be configured via control signals  270 ( 1 )- 270 (N) provided by the control interface  264  to the uplink spectrum analyzers  262 ( 1 )- 262 (N). The URX  190  receives the clock signal  237  from the DRX  186  through a clock port  266  to use to synchronize control logic in the control module  260 . For accurate timing, the uplink spectrum analyzers  262 ( 1 )- 262 (N) receive the timing signal  219  through a timing port  268 . 
       FIG.  15    is a schematic diagram of an exemplary uplink spectrum analyzer  262  provided in the URX  190  in  FIG.  14   . The uplink spectrum analyzer  262  performs signal analysis on one digital baseband input  263  to determine signal strength based on a digital representation of an uplink electrical RF signal. The uplink spectrum analyzer  262  in this embodiment multiplies the digital baseband input  263  with a complex sinusoid signal using a multiplier  272 , and the half sub-carrier frequency shift of the uplink electrical RF signal is undone. In order to determine the energy or signal strength level in the uplink electrical RF signal, windowing is performed by a window selector  274 . On this sample vector, the FFT is computed. Then, for all used frequencies, the squared absolute value is computed and all squared values that belong to a client device  24  are added. The results are further averaged over a number of symbols  276  that belong to one slot in the example of LTE processing. The results are then serialized and provided as output  278  to the control interface  264 . 
       FIG.  16    illustrates exemplary URX communication messages  280 ( 1 )- 280 (N) communicated from the URX  190  to the LPU  184  to provide energy/signal strength levels associated with RAUs  14  assigned to the URX  190 . In this manner, as previously described, the LPU  184  can determine for which RAU the energy level of communications of a client device  24  is strongest. This information can indicate the location of the client device  24 , since the location of the RAUs  14  in the distributed antenna system are known. The URX communication messages  280 ( 1 )- 280 (N) are created by the control interface  264  in the URX  190  in the example of  FIG.  14    based on the output of the uplink spectrum analyzers  262 ( 1 )- 262 (N). 
     As illustrated in  FIG.  16   , each URX  190  provides a URX communication message  280  to the LPU  184 . The URX communication message  280  is provided over the LPU port  265  in  FIG.  14    to the LPU  184  in one embodiment. For each RAU  14  receiving communications with a client device  24 , a URX communication message  280  is provided to the LPU  184 . The URX communication message  280  contains a URX ADDRESS  282 , FRAME NUMBER  284 , and SLOT # 286 . In one embodiment of LTE processing, this is known as a resource block (RB). An RB  288  contains the energy level for a client device  24  communicating with an RAU. In a LTE processing example, RBs  288  are provided for all LTE resources blocks. 
       FIG.  17    is an exemplary LPU communication message  290  communicated from an LPU  184  to a base station  69 ( 1 )- 69 (N) to provide RAUs associated with the maximum energy level for client device  24  communications. In this example, the location module  208  in the LPU  184  in  FIG.  10    creates the LPU communication message  290  to send to a base station  69 ( 1 )- 69 (N) through BTS ports  194  over the communication link  192 . The LPU communication message  290  provide condensed information from the URX communication messages  280 ( 1 )- 280 (N) that provide the RB  288  containing the RAU that received the maximum energy level of communications from client devices  24 , or RBs. Thus, when this information is provided the base station  69 ( 1 )- 69 (N), the base station  69 ( 1 )- 69 (N) can determine to which RAU the client devices  24  are closest, and thus the location of the client devices  24 . 
       FIG.  18    is a schematic diagram of an exemplary HEU board configuration that can be provided in the HEU  12 . In this embodiment, one URX  190  is provided per OIC  80  as illustrated in  FIG.  18   . This configuration has the advantage of modularity, but also requires more URXs  190  as OICs  80  are added, thereby increasing expense and the space requirements. Thus, in this example, if the URX  190  consumes the same amount of space in the HEU  12  as the OIC  80 , providing a URX  190  per OIC  80  reduces the number of OICs  80  that can be provided in the HEU  12  by one half. 
       FIG.  19    is a schematic diagram of another exemplary HEU board configuration. In this example, a URX  190  is provided per optical interface module (OIM)  300 . An OIM  300  consists of two or more OICs  80 . Thus, in this example, less URXs  190  are provided for a given number of OICs  80  than the configuration in  FIG.  18   . This has the advantage of saving space when a large number of OICs  80  are included in the HEU  12 . However, if a small number of OICs  80  are included in the HEU  12 , the URX  190  may be more expensive since it provides resources in the URX  190  to support a plurality of OICs  80  in the OIM  300  instead of just one OIC  80  like provided in  FIG.  18   . 
     If it is desired to support providing location services for more client devices than a single HEU  12  can handle, multiple HEUs  12  can be provided in a master/slave arrangement. In this regard,  FIG.  20    is a schematic diagram of a master HEU  12 (M) configured to provide location information for client devices communicating with a plurality of slave HEUs  12 ( 1 )- 12 (N) communicatively coupled to the master HEU  12 (M). The components in the HEUs  12 (M),  12 ( 1 )- 12 (N) have been previously described and are not re-described here. Each slave HEU  12 ( 1 )- 12 (N) can provide location information as previously described above to the master HEU  12 (M), and more particularly to a master LPU  184 (M), which can in turn provide such location information to the base stations  69 ( 1 )- 69 (N). Location services can be requested over a master communication link  192 (M) to the master HEU  12 (M), which in turn may pass the location services request to the appropriate slave HEU  12 ( 1 )- 12 (N). 
     Some base stations support a transmission method using more than one antenna to receive or transmit RF signals along different propagation paths, for example, using antenna diversity or a multiple input/multiple output (MIMO) antenna scheme. In this case, more than one antenna can be used to receive the downlink signal at the head-end unit. The signals are individually transmitted to the head-end unit and then combined with the respective received signals. This method can provide better signal quality and increase reliability. 
     As previously discussed, the RF signals in the distributed antenna systems disclosed herein can be, but are not required, to be modulated according to the LTE standard. LTE employs OFDM for downlink data transmission and SC-FDMA for uplink transmission and furthermore, uses a MIMO antenna scheme for data transmission. In OFDM, a large number of sub-carrier frequencies are used to carry the data. The sub-carriers are orthogonal to each other so that the cross-talk between the sub-channels is eliminated. Each sub-carrier is independently modulated. Based on the orthogonality, a discrete Fourier transform (DFT) algorithm can be simply implemented on the receiver side and inverse DFT (IDFT) on the transmitter side. Similarly in SC-FDMA, both DFT and IDFT are applied on the transmitter side and also on the receiver side. 
     LTE users can be separated by the base station  69  in time and frequency domain. A media access controller (MAC) scheduler of a base station  69  is in control of assigning RBs to specific client devices  24  and has knowledge of which RB belongs to which client device  24 . For an outside observer, this knowledge is not readily obtainable. However, in order to locate a client device  24  within the proximity of an RAU  14 , as previously discussed, it can be sufficient to measure the RB energy from the client device  24  and send the maximum detected values together with the RAU  14  number to the base station. The base station  69  then can take the measurement results and relate it to the MAC scheduling information. 
     In this regard,  FIG.  21    shows a simple example how client devices  24  are separated by time and frequency in LTE. The base station  69  assigns different RBs to different client devices  24 . Due to the nature of a distributed antenna system, the base station  69  sees a superposition  310  of signals  312 ( 1 )- 312 (N) received from the individual antennas  32 ( 1 )- 32 (N). The base station  69  uses the scheduling information to demodulate and de-multiplex the received SC-FDMA multiplex. If the IDAS reports from which antenna  32 ( 1 )- 32 (N) RB is received with maximum energy, a client device  24  can be located. Thus, the location retrieval process can be summarized as follows for one embodiment. For each antenna  32  and channel, detect energy for every RB. For each RB, report max value together with antennas  32 ( 1 )- 32 (N) to the base station  69 . As the base station  69  knows each client device&#39;s  24  allocation in the superposition of signals, a user can be associated with an antenna. 
     In order to minimize interference to adjacent cells in this embodiment, LTE signals are sent typically close to the minimum required signal level necessary to demodulate the signal at the base station. It has been shown above that carrier to noise ratios can be as low as −3 dB.  FIG.  22    shows the spectrum of an SC-FMDA signal that is received with a CNR of 0 dB. Also shown is the spectrum of a noise signal. It can be seen that for this level, the signal is not possible to visually distinguish the signal from the noise signal (i.e., the presence of the uplink signal is hard to detect). It shall also be noted that in contrast to OFDM, an SC-FDMA signal does not have a flat spectrum. 
     For RB energy detection, at first, the time and frequency synchronized signal is shifted such by one half subcarrier (i.e., 7.5 kHz to remove the one half subcarrier frequency shift that is introduced at the uplink transmitter to avoid a possible DC notch). Then, the cyclic prefix is removed by selecting a window of FFT SIZE samples. The FFT size varies with the LTE channel bandwidth. On the selected samples, the FFT is computed and the squared absolute values of the FFT outputs are computed. These values are proportional to the energy received on one (1) subcarrier for one (1) SC-FDMA symbol. All squared outputs that belong to one (1) RB are now added to give the total RB energy. The addition takes place over twelve (12) adjacent FFT outputs and over six (6) or seven (7) SC-FDMA symbols depending on the LTE mode used. The sounding reference if present needs to be omitted. As the distributed antenna system may not know when the sounding reference symbol is sent, the last SC-FDMA symbol in a subframe shall always be omitted. In order to keep time slots symmetrical, omit the last SC-FDMA symbol in the first time slot of a subframe. 
     The robustness of the algorithm in Additive White Gaussian Noise (AWGN) channels has been analyzed. In this analysis, one client device  24  is added to one RAU  14 , the other antennas  32  are receiving white Gaussian noise only for that RB. Each RAU  14  represents a possible communication channel. The client device  24  just sends one (1) RB. Detection is positive if the received RB energy for the channel to which the client device  24  is connected is highest. The results are shown in  FIG.  23   . It can be seen that if the distributed antenna system has to choose between many channels (e.g., thirty-two (32)), and bases its decision solely on one RB&#39;s energy, the probability of a false decision is higher than if the distributed antenna system would have to choose between only two (2) channels. For low carrier-to-noise ratios of −3 dB, the probability of making a wrong decision is four (4) percent in this example, whereas it would be around 0.3 percent if the distributed antenna system would have to decide between just two channels. For higher CNRs of like 0 dB, the probability of a wrong decision is below 0.1 percent (i.e., the highest energy value reported by the IDAS would point to the right RAU with likelihood greater than 99.1 percent). 
     The detection probability has been further analyzed.  FIGS.  24  and  25    show the probability of a false detection as a function of RAU  14  channels if the location information is rejected if one or more maximum results point to different RAUs  14 , in this example. This is done for 10 or 100 RBs, respectively. As more observations are made, the probability for rejected location information increases with the number of observations. 
     For a CNR of −3 dB and thirty-two (32) RAU  14  channels, the probability of having at least one RB pointing at the wrong channel using the maximum energy criterion is close to 100 percent. An alternative for a base station  69  is to choose the most likely antenna after multiple observations (i.e., select the RAU  14  that is most often reported).  FIG.  25    also shows the probability that more than 50 out of 100 observations point to the correct RAU  14  for thirty-two (32) RAU  14  channels. This curve can be seen as an upper bound for making a wrong decision. Using this method, it can be seen from  FIG.  25    that an LTE user can be located with a probability of greater than 99.9 percent even if the received CNR is as low as −5 dB and fulfills all requirements on location processing with margin. 
     The impact of frequency offset has been analyzed. Frequency offset destroys the orthogonality of the SC-FDMA signal. In this regard,  FIG.  26    shows the energy leakage that is caused by frequency offset. It can be seen that one percent of the subcarrier spacing causes −37 dB leakages (i.e., an adjacent signal on a different RAU that is received at the base station 37 dB stronger than the signal for which the location needs to be determined can cause a wrong decision). One percent subcarrier spacing corresponds to 150 Hz. For three percent, i.e., 450 Hertz (Hz), the leakage already increases to −28 dB. It shall be noted that at a signal frequency of 2 GigaHertz (GHz), 150 Hz frequency offset can be caused by an oscillator inaccuracy of, i.e., 75 parts per billion which is a factor 500 less than the accuracy of an off-the-shelve crystal oscillator. Thus, frequency synchronization is performed. The frequency can be synchronized to the base station&#39;s downlink signal through standard techniques that are also used in mobile terminals. 
     Like frequency offset, time offset destroys the orthogonality of the SC-FDMA signal.  FIG.  27    shows the energy leakage as a function of time offset relative to the length of the cyclic prefix. A time offset causes intersymbol interference. At a time offset of twenty (20) percent of the cyclic prefix (approximately 1 pec), the leakage already has reached −15 dB. This would mean that a terminal that is received at the base station 15 dB stronger than the terminal whose location needs to be determined can significantly impact the location detection capabilities of the system. Therefore, time synchronization is performed. The symbol timing can be synchronized to the base station&#39;s downlink signal through standard techniques that are also used in mobile terminals. The accurate time shall be distributed over a dedicated wire. 
     Further, as used herein, it is intended that terms “fiber optic cables” and/or “optical fibers” include all types of single mode and multi-mode light waveguides, including one or more optical fibers that may be upcoated, colored, buffered, ribbonized and/or have other organizing or protective structure in a cable such as one or more tubes, strength members, jackets or the like. Likewise, other types of suitable optical fibers include bend-insensitive optical fibers, or any other expedient of a medium for transmitting light signals. An example of a bend-insensitive, or bend resistant, optical fiber is ClearCurve® Multimode fiber commercially available from Corning Incorporated. Suitable fibers of this type are disclosed, for example, in U.S. Patent Application Publication Nos. 2008/0166094 and 2009/0169163. 
     Those of skill in the art would further appreciate that the various illustrative logical blocks, modules, circuits, and algorithms described in connection with the embodiments disclosed herein may be implemented as electronic hardware, instructions stored in memory or in another computer-readable medium and executed by a processor or other processing device, or combinations of both. The memory controllers, arbiter, master units, and sub-master units described herein may be employed in any circuit, hardware component, IC, or IC chip, as examples. The memory may be any type and size of memory and may be configured to store any type of information desired. To clearly illustrate this interchangeability, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. How such functionality is implemented depends upon the particular application, design choices, and/or design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present invention. 
     The various illustrative logical blocks, modules, and circuits described in connection with the embodiments disclosed herein may be implemented or performed with a processor, a Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. 
     The embodiments disclosed herein may be embodied in hardware and in instructions that are stored in hardware, and may reside, for example, in Random Access Memory (RAM), flash memory, Read Only Memory (ROM), Electrically Programmable ROM (EPROM), Electrically Erasable Programmable ROM (EEPROM), registers, hard disk, a removable disk, a CD-ROM, or any other form of computer readable medium known in the art. An exemplary storage medium is coupled to the processor such that the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. The processor and the storage medium may reside in an ASIC. The ASIC may reside in a remote station. In the alternative, the processor and the storage medium may reside as discrete components in a remote station, base station, or server. 
     Many modifications and other embodiments of the embodiments set forth herein will come to mind to one skilled in the art to which the embodiments pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. These modifications include, but are not limited to, whether a tracking signal is provided, whether downlink and/or uplink BICs are included, whether tracking signal inputs are provided in the same distributed communications unit as downlink base station inputs, the number and type of OICs and RAUs provided in the distributed antenna system, etc. Therefore, it is to be understood that the description and claims are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. It is intended that the embodiments cover the modifications and variations of the embodiments provided they come within the scope of the appended claims and their equivalents. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.