Abstract:
An optical medium, such as fiber, is tapped to provide an antenna port wherever radio service coverage is desired. Each antenna port is a bi-directional remote unit that receives a digital optical signal from a host unit and transforms the signal to a radio frequency signal for transmission by the remote unit. The remote unit receives radio frequency signals that are converted to digital signals and summed with signals from other remote units and converted to an optical signal for transmission to the host unit.

Description:
RELATED APPLICATION  
       [0001]    This application claims priority to U.S. Provisional Patent Application Serial No. 60/430,434 filed Dec. 3, 2002, and titled “Distributed Digital Antenna System,” which is commonly assigned and incorporated by reference herein. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention relates generally to communications and particularly to communications through a distributed antenna system.  
         BACKGROUND  
         [0003]    Various types of wireless communication systems have become prevalent around the world. For example, cellular communication systems cover most major metropolitan areas as well as major highways through remote areas. Cellular systems permit individuals with cellular handsets to communicate with base stations that are connected to the public switched telephone network (PSTN) or some other communication network.  
           [0004]    As with any communication system, cellular systems can leave coverage “holes” where the signal from the base stations cannot reach. The holes can be in tunnels, valleys, city streets between tall buildings, or any other location where a radio frequency (RF) signal is blocked.  
           [0005]    Placing additional base stations where these coverage holes are located is not always an option. Base stations tend to be very expensive due not only to the cost of the equipment but also because of land acquisition costs. Additionally, large base station antennas may not fit within an area either physically or aesthetically.  
           [0006]    One solution to hole coverage is to use smaller remote antennas where coverage is needed but a base station is not warranted or desired. One problem with remote antennas, however, is that coaxial cable cannot be run long distances due to attenuation. Remote antennas are difficult to install along a highway or through a tunnel due to this attenuation problem. Using repeaters may not be an option since this only adds to the expense and complexity of the system. There is a resulting need in the art for a distributed antenna system that does not suffer from attenuation problems.  
         SUMMARY OF THE INVENTION  
         [0007]    The embodiments of the present invention encompass a distributed digital antenna system that has a host unit for converting radio frequency signals to digital optical signals and digital optical signals to radio frequency signals. The digital optical signals are transmitted over an optical medium to a plurality of remote units that are daisy-chained along the optical medium. Each remote unit transmits an analog representation of the digital optical signals from the host unit and receives radio frequency signals that are converted by the remote unit to digital optical signals for use by the host unit. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 shows a block diagram of one embodiment of a distributed digital antenna system of the present invention.  
         [0009]    [0009]FIG. 2 shows a block diagram of another embodiment of a distributed digital antenna system of the present invention.  
         [0010]    [0010]FIG. 3 shows a block diagram of one embodiment of a remote unit in accordance with the system of FIG. 1.  
         [0011]    [0011]FIG. 4 shows a block diagram of one embodiment of a remote unit in accordance with the system of FIG. 2.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0012]    The embodiments of the present invention provide a digital distributed antenna system that enables a communication system to fill coverage holes without the expense of additional base stations. This is accomplished by distributing a fiber optic cable through the area in which coverage is desired and tapping into the fiber at desired antenna locations.  
         [0013]    The embodiments of the present invention refer to fiber optics as a means of communication between remote units and the host unit. However, any optical medium, such as a laser through the air, can be substituted for the optical fiber.  
         [0014]    [0014]FIG. 1 illustrates a block diagram of one embodiment of a distributed digital antenna system of the present invention. The system has a base station ( 100 ) that communicates over an RF link using an antenna ( 110 ). The base station communicates over the RF link using any appropriate air interface standard. For example, the air interface standard comprises one of Advanced Mobile Phone System (AMPS), code division multiple access (CDMA), time division multiple access (TDMA), or Global System for Mobile communications (GSM) or any other appropriate air interface standard.  
         [0015]    The RF link is made up of a forward link over which the base station ( 100 ) transmits to a subscriber unit wireless terminal ( 150 ). The subscriber unit ( 150 ) transmits back to the base station ( 100 ) over a reverse link. The subscriber unit ( 150 ) is either a mobile station or a fixed station such as in a wireless local loop system.  
         [0016]    The base station ( 100 ) has the transmitters and receivers that enable the subscriber unit ( 150 ) to communicate with the public switched telephone network (PSTN) ( 130 ). In one embodiment, the base station also links the subscriber unit ( 150 ) to other subscriber units that are communicating with other base stations. In one embodiment, the base station ( 100 ) is connected to the PSTN through a mobile switching center that handles the switching of calls with multiple base stations.  
         [0017]    A host unit ( 101 ) is connected to the base station ( 100 ) through an RF link ( 115 ). In one embodiment, this link ( 115 ) is a coaxial cable. Other embodiments use other types of connections such as an air interface or an optical fiber carrying digital RF signals. U.S. patent application Ser. No. 09/619,431, assigned to ADC Telecommunications, Inc. and incorporated herein by reference, discusses digital RF signals.  
         [0018]    The host unit ( 101 ) is responsible for converting the RF signal from the base station ( 100 ) to an optical signal for transmission over an optical medium. The host unit ( 101 ) also converts a received optical signal to an RF signal for transmission to the base station ( 100 ). In other embodiments, the host unit ( 101 ) performs additional functions.  
         [0019]    One or more remote units ( 105 - 108 ) are connected to the host unit ( 101 ) through an optical medium, such as fiber optic lines ( 120  and  125 ), in a daisy-chain arrangement. The remote units ( 105 - 108 ) are placed in locations that require additional signal coverage due to a lack of coverage by the base station ( 100 ). The remote units ( 105 - 108 ) communicate with subscriber units in a particular remote unit&#39;s coverage area over an RF link provided by the remote unit antennas ( 135 - 138 ).  
         [0020]    For purposes of illustration, four remote units ( 105 - 108 ) are shown. However, alternate embodiments use other quantities of remote units. If only a small geographic area requires coverage, as few as one remote unit ( 105 ) is used. If a highway in a remote area requires additional coverage, more than four remote units are typically used.  
         [0021]    The embodiment of FIG. 1 uses a separate fiber optic line for each direction of communication. Each fiber carries a different wavelength. For example, the fiber optic line ( 120 ) from the host unit ( 101 ) to the remote units ( 105 - 108 ) carries a wavelength of λ 1 . The fiber optic line ( 125 ) from the remote units ( 105 - 108 ) to the host unit ( 101 ) carries a wavelength of λ 2 . In alternate embodiments, each fiber carries the same wavelength.  
         [0022]    The fiber optic line ( 120 ) from the host unit ( 101 ) to the remote units ( 105 - 108 ) carries the digital optical signal for transmission by the ( 105 - 108 ). The fiber optic line ( 125 ) from the remote units ( 105 - 108 ) carries a digital optical signal comprising the sum of the received signals from each of the remote units ( 105 - 108 ). The generation of this summation signal from the remote units is discussed subsequently.  
         [0023]    [0023]FIG. 2 illustrates a block diagram of another embodiment of a distributed digital antenna system of the present invention. This system is similar to the embodiment of FIG. 1 except that the remote units ( 205 - 208 ) are connected to the host unit ( 201 ) over a single optical medium ( 220 ).  
         [0024]    The system of FIG. 2 has a base station ( 200 ) that communicates over an RF link using an antenna ( 210 ). The base station can communicate over the RF link using any air interface standard. For example, the air interface standard may be code division multiple access (CDMA), time division multiple access (TDMA), or Global System for Mobile communications (GSM).  
         [0025]    The RF link is made up of a forward link over which the base station ( 200 ) transmits to a subscriber unit ( 250 ). The subscriber unit ( 250 ) transmits back to the base station ( 200 ) over a reverse link. The subscriber unit ( 250 ) may be a mobile station or a fixed station such as in a wireless local loop system.  
         [0026]    The base station ( 200 ) has the transmitters and receivers that enable the subscriber unit ( 250 ) to communicate with the public switched telephone network (PSTN) ( 230 ). The base station may also link the subscriber unit ( 250 ) to other subscriber units that are communicating with other base stations. In one embodiment, the base station ( 200 ) is connected to the PSTN through a mobile switching center that handles the switching of calls with multiple base stations.  
         [0027]    A host unit ( 201 ) is connected to the base station ( 200 ) through an RF link ( 215 ). In one embodiment, this link ( 215 ) is a coaxial cable. Other embodiments use other types of connections such as an air interface or an optical fiber carrying digital RF signals.  
         [0028]    The host unit ( 201 ) is responsible for converting the RF signal from the base station ( 200 ) to a digital optical signal for transmission over an optical medium. The host unit ( 201 ) also converts a received optical signal to an RF signal for transmission to the base station ( 200 ). In other embodiments, the host unit ( 201 ) performs additional functions.  
         [0029]    One or more remote units ( 205 - 208 ) are connected to the host unit ( 201 ) through an optical medium, such as a fiber optic line ( 220 ), that is connected in a daisy-chain arrangement. The remote units ( 205 - 208 ) are placed in locations that require additional signal coverage due to a lack of coverage by the base station ( 200 ).  
         [0030]    For purposes of illustration, four remote units ( 205 - 208 ) are shown. However, alternate embodiments use other quantities of remote units.  
         [0031]    The embodiment of FIG. 2 uses a single fiber optic line ( 220 ) for communication both to and from the remote units ( 205 - 208 ). This is accomplished by the single fiber ( 220 ) carrying multiple wavelengths. For example, the fiber optic line ( 220 ) uses a wavelength of λ 1  for the digital signal from the host unit to the remote units ( 205 - 208 ). The fiber optic line ( 220 ) also carries a digital summation signal with a wavelength of λ 2 . This digital summation signal is the sum of the received signals from the remote units ( 205 - 208 ). The generation of this summation signal from the remote units is discussed subsequently.  
         [0032]    [0032]FIG. 3 illustrates a block diagram of one embodiment of a remote unit ( 105 ) of FIG. 1. Each of the remote units ( 105 - 108 ) of the embodiment of FIG. 1 are substantially identical in functional composition.  
         [0033]    The remote unit ( 105 ) transmits and receives RF signals over the antenna ( 135 ). Both the receive and transmit circuitry is connected to the antenna ( 135 ) through a diplexer ( 301 ).  
         [0034]    Alternate embodiments use other quantities of antennas. For example, one embodiment uses three antennas to cover three different sectors of an area.  
         [0035]    An analog signal that is received on the antenna ( 135 ) is split off by the diplexer ( 301 ) to an analog-to-digital converter ( 305 ). The analog-to-digital converter ( 305 ) digitizes the received analog signal by periodically sampling the signal. The sampling generates a digital representation of the received analog signal.  
         [0036]    The digitized received signal is input to a summer ( 315 ) to be added to the digitized signals from the preceding remote units in the daisy-chain. The input of the summer ( 315 ), therefore, is coupled to an output of a previous remote unit. The output of the summer ( 315 ) is a summation signal that is coupled to either the input of a subsequent remote unit or to the host unit. The host unit thus receives a summation signal that represents the sum of all the signals received by the remote units ( 105 - 108 ) of the system.  
         [0037]    A digital signal from the host unit is coupled to a digital-to-analog converter ( 310 ). The digital-to-analog converter ( 310 ) takes the digital representation of an analog signal and converts it to the analog signal for transmission by the antenna ( 135 ).  
         [0038]    Optical-to-Electrical converters ( 320 - 323 ) are located at the optical ports ( 330  and  335 ) of the remote unit ( 105 ). Each optical port ( 330  and  335 ) has an input and an output that are each coupled to an Optical-to-Electrical converter ( 320 - 323 ).  
         [0039]    Since the remote unit ( 105 ) operates with electrical signals that are represented by the optical signals coming in through the optical ports ( 330  and  335 ), the Optical-to-Electrical converters ( 320 - 323 ) are responsible for converting the optical signals to electrical signals for processing by the remote unit ( 105 ). The Optical-to-Electrical converters ( 320 - 323 ) are also responsible for converting received electrical signals from electrical to an optical representation for transmission over the optical fiber.  
         [0040]    [0040]FIG. 4 illustrates a block diagram of one embodiment of a remote unit ( 205 ) of FIG. 2. Each of the remote units ( 205 - 208 ) of the embodiment of FIG. 1 is substantially identical in functional composition.  
         [0041]    The remote unit ( 205 ) transmits and receives RF signals over the antenna ( 435 ). Both the receive and transmit circuitry are connected to the antenna ( 435 ) through a diplexer ( 401 ).  
         [0042]    Alternate embodiments use other quantities of antennas. For example, one embodiment uses three antennas to cover three sectors of an area.  
         [0043]    An analog signal that is received on the antenna ( 435 ) is split off by the diplexer ( 401 ) to an analog-to-digital converter ( 405 ). The analog-to-digital converter ( 405 ) digitizes the received analog signal by periodically sampling the signal. The sampling generates a digital representation of the received analog signal.  
         [0044]    The digitized received signal is input to a summer ( 415 ) to be added to the digitized signals from the preceding remote units in the daisy-chain. The host unit thus receives a summation signal that represents the sum of all the signals received by the remote units ( 205 - 208 ) of the system.  
         [0045]    A digital signal from the host unit is coupled to a digital-to-analog converter ( 410 ). The digital-to-analog converter ( 410 ) takes the digital representation of an analog signal and converts it to the analog signal for transmission by the antenna ( 435 ).  
         [0046]    Optical-to-Electrical converters ( 420 - 423 ) are located at the optical ports ( 440  and  445 ) of the remote unit ( 205 ). Each optical port ( 440  and  445 ) has an input and an output that are each coupled to an Optical-to-Electrical converter ( 420 - 423 ).  
         [0047]    Since the remote unit ( 205 ) operates with electrical signals that are represented by the optical signals coming in through the optical ports ( 440  and  445 ), the Optical-to-Electrical converters ( 420 - 423 ) are responsible for converting the optical signals to electrical signals for processing by the remote unit ( 205 ). The Optical-to-Electrical converters ( 420 - 423 ) are also responsible for converting received electrical signals from electrical to an optical representation for transmission over the optical fiber.  
         [0048]    A wavelength division multiplexer (WDM) ( 430  and  431 ) is located at each optical port ( 440  and  445 ). The WDMs ( 430  and  431 ) perform the optical processing necessary to combine several optical signals having several wavelengths. The WDMs ( 430  and  431 ) also perform the optical demultiplexing necessary to split the multiple wavelengths of a single fiber to their own signal paths.  
         [0049]    In summary, the distributed digital antenna system provides multiple daisy-chained antennas on a single medium such as optical fiber. The fiber can be tapped anywhere along its length multiple times to provide economical radio coverage in areas where a base station would be cost prohibitive.  
         [0050]    Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.