US20070292136A1 - Transponder for a radio-over-fiber optical fiber cable - Google Patents

Abstract

The invention is a transponder for a radio-over-fiber (RoF) optical fiber cable. The transponder includes a converter unit made up of an electrical-to-optical (E/O) converter and an optical-to-electrical (O/E) converter. The optical fiber cable optically couples the converter unit to a head-end unit that sends and receives optical RF signals. A dipole antenna system is operably coupled to the converter unit and is arranged so as to create elongate picocell in a direction perpendicular to the optical fiber cable when the transponder is in communication with the head-end unit. The asymmetric picocell shape allows for creating a picocellular coverage area using fewer optical fiber cables than is possible with prior art transponders.

Description

FIELD OF THE INVENTION
• The present invention relates generally to radio-over-fiber (RoF) systems, and in particular relates to transponders for a RoF optical fiber cable used in RoF systems.
BACKGROUND OF THE INVENTION
• 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 (coffee shops, airports, hospitals, libraries, etc.). The typical wireless communication system has a head-end station connected to an access point device via a wire cable. The access point device includes a RF transmitter/receiver operably connected to an antenna, and digital information processing electronics. The access point device communicates with wireless devices called “clients,” which must reside within the wireless range or a “cell coverage area” in order to communicate with the access point device.
• The size of a given cell is determined by the amount of RF power transmitted by the access point device, the receiver sensitivity, antenna gain and the RF environment, as well as by the RF transmitter/receiver sensitivity of the wireless client device. Client devices usually have a fixed RF receive sensitivity, so that the above-mentioned properties of the access point device largely determine the cell size. Connecting a number of access point devices to the head-end controller creates an array of cells that cover an area called a “cellular coverage area.”
• One approach to deploying a wireless communication system involves creating “picocells,” which are wireless cells having a radius in the range from about a few meters up to about 20 meters. Because a picocell covers a small area, there are typically only a few users (clients) per picocell. A closely packed picocellular array provides high per-user data-throughput over the picocellular coverage area. Picocells also allow for selective wireless coverage in small regions that otherwise would have poor signal strength when covered by larger cells created by conventional base stations.
• One type of wireless system for creating picocells utilizes radio-frequency (RF) signals sent over optical fibers—called “radio over fiber” or “RoF” for short. Such systems include a head-end unit optically coupled to a transponder via an optical fiber link. Unlike a conventional access point device, the transponder has no digital information processing capability. Rather, the digital processing capability resides in the head-end unit. The transponder is transparent to the RF signals and simply converts incoming optical signals from the optical fiber link to electrical signals, which are then converted to electromagnetic signals via an antenna. The antenna also receives electromagnetic signals and converts them to electrical signals. The transponder then converts the electrical signals to optical signals, which are then sent to the head-end unit via the optical fiber link.
• The transponders are typically included in an optical fiber cable that includes the optical fiber links for each transponder. The picocells associated with the distributed transponders form a picocell coverage area. To reduce picocell cross-talk, high-directivity transponder antennas can be used. Their use, however, requires additional efforts at the manufacturing and installation stages because proper adjustment and orientation of each antenna is necessary. Installing multiple directive antennas per transponder (e.g., to support both data and voice services in different frequency bands) further complicates installation and imposes tight requirements for integration of antennas with the transponder. In addition, the size and orientation of the picocells requires direct adjustment of the antennas, which is difficult to do once the antennas are incorporated into the optical fiber cable.
SUMMARY OF THE INVENTION
• One aspect of the invention is a transponder for a radio-over-fiber (RoF) optical fiber cable. The transponder includes an electrical-to-optical (E/O) converter and an optical-to-electrical (O/E) converter. The system also includes a dipole antenna system operably coupled to the E/O converter and the O/E converter. The antenna system is arranged relative to the optical fiber cable so as to create an elongate picocell in a direction locally perpendicular to the optical fiber cable when the transponder is addressed.
• Another aspect of the invention is a RoF picocellular wireless system. The system includes a head-end unit adapted to send and receive optical RF signals. The system also includes one or more transponders of the type described immediately above. The system further includes one or more optical fiber cables that include the one or more transponders and that optically couple the head-end unit to each transponder.
• Another aspect of the invention is a method of forming an elongate picocell for a RoF system. The method includes transmitting optical RF signals to a transponder via a downlink optical fiber in the optical fiber cable, and converting the optical signals to electrical RF signals at the transponder. The method also includes converting the electrical signals to electromagnetic RF signals at the transponder using a dipole antenna system to create the elongate picocell in a direction locally perpendicular to the optical fiber cable.
• Additional features and advantages of the invention are set forth in the detailed description that follows, and will be readily apparent to those skilled in the art from that description or recognized by practicing the invention 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 of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention and, together with the description, serve to explain the principles and operations of the invention.
• Accordingly, various basic electronic circuit elements and signal-conditioning components, such as bias tees, RF filters, amplifiers, power dividers, etc., are not all shown in the Figures for ease of explanation and illustration. The application of such basic electronic circuit elements and components to the present invention will be apparent to one skilled in the art.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a schematic diagram of a generalized embodiment of a RoF picocellular wireless system according to the present invention;
• FIG. 2 is a schematic close-up view of an example embodiment of the RoF system ofFIG. 1, illustrating an example embodiment of the converter unit and dipole antenna system for the transponder of the present invention as arranged in the optical fiber cable;
• FIG. 3 is a close-up schematic diagram of an example embodiment of the configuration of the converter unit and dipole antenna system for the transponder of the present invention, wherein the dipole antenna system includes transmitting and receiving antennas;
• FIG. 4 is a schematic diagram of an example embodiment of the transponder of the present invention similar to that shown inFIG. 3, but wherein dipole antenna system includes a single antenna that both transmits and receives electromagnetic signals;
• FIG. 5 is a close-up schematic diagram of an example embodiment of an E/O converter in the converter unit that includes a number of amplifiers, with each amplifier adapted to amplify a different frequency in the electrical RF signal created by the photodetector;
• FIG. 6 is a schematic diagram of an example embodiment of the transponder of the present invention, wherein the dipole antenna system includes a power divider and three separate antennas;
• FIG. 7 is a schematic diagram of an example embodiment of the transponder of the present invention similar to that shown inFIG. 6, wherein the antenna system includes a plurality of power dividers each electrically coupled to an antenna;
• FIG. 8 is a schematic diagram of an example embodiment of the transponder according to the present invention similar to that shown inFIG. 7, wherein a portion of the antenna system lies outside of the optical fiber cable coating;
• FIG. 9 is a schematic diagram of an example embodiment of the transponder of the present invention similar to that shown inFIG. 8, wherein the converter unit and dipole antenna system both reside outside of the optical fiber cable coating;
• FIG. 10 is a schematic diagram of an example embodiment of the transponder of the present invention, wherein the antenna system includes two pairs of wire antennas, with each antenna connected to the converter unit via respective RF cable sections;
• FIG. 11 is a schematic diagram of an example embodiment of the RoF picocellular wireless system ofFIG. 1, showing details of an example embodiment of the head-end unit;
• FIG. 12 is a schematic diagram of a typical prior art picocellular coverage area formed by a conventional prior art picocellular wireless system that employs conventional omnidirectional transponders;
• FIG. 13 is a schematic diagram of an example picocellular coverage area formed by the RoF picocellular wireless system of the present invention that utilizes the transponders of the present invention; and
• FIG. 14 is a plot of RF power (dBm) emitted by the dipole antenna system of the transponder of the present invention vs. the distance (m) from the antenna system along both the x-direction (i.e., perpendicular to the optical fiber cable) and the y-direction (i.e., along the optical fiber cable).
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
• Reference is now made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Whenever possible, the same or analogous reference numbers are used throughout the drawings to refer to the same or like parts.
Generalized Picocellular Wireless System with Transponder
• FIG. 1 is a schematic diagram of a generalized embodiment of a RoFpicocellular wireless system10 according to the present invention.System10 includes a head-end unit20 adapted to transmit, receive and/or process RF optical signals. In an example embodiment, head-end unit20 is operably coupled to anoutside network24 via anetwork link25, and the head-end unit serves as a pass-through for RF signals sent to and from the outside network.
• System10 also includes one or more transponder units (“transponders”)30 according to the present invention. Eachtransponder30 includes aconverter unit31 and adipole antenna system32 electrically coupled thereto, wherein the dipole antenna system has a dipole radiation characteristic the same as or substantially similar to that of an ideal dipole antenna.Transponder30 is discussed in greater detail below.
• System10 includes one or moreoptical fiber cables34 each optically coupled to head-end unit20. Eachoptical fiber cable34 includes one or more optical fiber RF transmission links36 optically coupled to respective one ormore transponders30. In an example embodiment, each optical fiberRF transmission link36 includes a downlinkoptical fiber36D and an uplinkoptical fiber36U. Example embodiments ofsystem10 include either single-mode optical fiber or multimode optical fiber for downlink and uplinkoptical fibers36D and36U. The particular type of optical fiber depends on the application ofsystem10, as well as the desired performance and cost considerations. For many in-building deployment applications, maximum transmission distances typically do not exceed 300 meters. The maximum length for the intended RoF transmission needs to be taken into account when considering using multi-mode optical fibers for downlink and uplinkoptical fibers36D and36U. For example, it has been shown that a 1400 MHz.km multi-mode fiber bandwidth-distance product is sufficient for 5.2 GHz transmission up to 300 meters. In an example embodiment, the present invention employs 50 μm multi-mode optical fiber for the downlink and uplinkoptical fibers36D and36U, and E/O converters (introduced below) that operate at 850 nm using commercially available vertical-cavity surface-emitting lasers (VCSELs) specified for 10 Gb/s data transmission.
• In an example embodiment, RoFpicocellular wireless system10 of the present invention employs a known telecommunications wavelength, such as 850 nm, 1300 nm, or 1550 nm. In another example embodiment,system10 employs other less common but suitable wavelengths such as 980 nm.
• Also shown inFIG. 1 is a local x-y-z Cartesian coordinate system C at eachdipole antenna system32 for the sake of reference, where the x-direction is into the paper and locally perpendicular tooptical fiber cable34. In an example embodiment,dipole antenna system32 is sufficiently stiff so thatoptical fiber cable34 is locally straight at the dipole antenna system location. In an example embodiment,dipole antenna system32 is located relatively far away from converter unit31 (e.g., 2 meters), while in other example embodiments the dipole antenna system is located relatively close to the converter unit (e.g., a few inches away), or even directly at the converter unit.
• Eachtransponder30 is adapted to form apicocell40 viadipole antenna system32 via electromagnetic transmission and reception when the transponder is addressed, e.g., receives a downlink optical signal SD′ from head-end unit20 and/or an uplink electromagnetic signal SU″ from aclient device46.Client device46, which is shown in the form of a computer as one example of a client device, includes an antenna48 (e.g., a wireless card) adapted to electromagnetically communicate with (i.e., address) the transponder andantenna system32 thereof.
• Dipole antenna system32 is adapted to formpicocell40 from adipole radiation pattern42 oriented perpendicular tooptical fiber cable34 at the location of the dipole antenna system. The term “locally perpendicular” is used herein to describe the orientation ofpicocell40 and/or the correspondingdipole radiation pattern42 relative tooptical fiber cable34 at the location ofdipole antenna system32.Dipole radiation pattern42 is thus centered about the local x-z plane P_XZ(viewed edge-on inFIG. 1 and illustrated as a dotted line). In an example embodiment, only a portion ofdipole radiation pattern42 is used forpicocell40, e.g., the portion below optical fiber cable34 (i.e., in the -z direction), as shown inFIG. 1.
• In an example embodiment,system10 is powered by apower supply50 electrically coupled to head-end unit20 via anelectrical power line52 that carries electrical power signals54.
Transponder Incorporated Into Optical Fiber Cable
• FIG. 2 is a schematic close-up view of an example embodiment oftransponder30 as incorporated intooptical fiber cable34. In an example embodiment,optical fiber cable34 includes anouter coating58. As mentioned above,transponder30 includes aconverter unit31.Converter unit31 includes an electrical-to-optical (E/O)converter60 adapted to convert an electrical signal into a corresponding optical signal, and an optical-to-electrical (O/E)converter62 that converts an optical signal into a corresponding electrical signal. E/O converter60 is optically coupled to aninput end70 of uplinkoptical fiber36U and O/E converter62 is optically coupled to anoutput end72 of downlinkoptical fiber36D.
• In an example embodiment,optical fiber cable34 includeselectrical power line52, andconverter unit31 includes aDC power converter80 electrically coupled to the electrical power line and to E/O converter60 and O/E converter62.DC power converter80 is adapted to change the voltage levels and provide the power required by the power-consuming components inconverter unit31. In an example embodiment,DC power converter80 is either a DC/DC power converter, or an AC/DC power converter, depending on the type ofpower signal54 carried byelectrical power line52. In an example embodiment,electrical power line52 includes twoelectrical wires52A and52B connected toDC power converter80.
• As discussed above,dipole antenna system32 is electrically coupled toconverter unit31. In an example embodiment,dipole antenna system32 includes one or more antenna elements (“antennas”)33. In the example embodiment shown inFIG. 2,antenna system32 includes a receivingantenna33R electrically coupled to E/O converter60 via a firstRF cable section90 and a transmittingantenna33T electrically coupled to E/O converter62 via a secondRF cable section90. In an example embodiment, the one ormore antennas33 is/are made of or include sections of wire. One or moreRF cable sections90 are used in example embodiments of the present invention to connect corresponding one ormore antennas33 to theconverter unit31. In an example embodiment,dipole antenna system32 supports multiple frequency bands. Additionally, the diversity principle can be used to send the same information through statistically independent channels.
Example Transponder Converter Unit
• FIG. 3 is a detailed schematic diagram of an example embodiment ofconverter unit31 anddipole antenna system32 for thetransponder30 of the present invention. In the example embodiment ofFIG. 3, E/O converter60 includes alaser100 optically coupled to aninput end70 of uplinkoptical fiber36U, a bias-T unit106 electrically coupled to the laser, anamplifier110 electrically coupled to the bias-T unit, and aRF filter114 electrically coupled to the amplifier and to the correspondingRF cable section90. Also in an example embodiment, O/E converter62 includes aphotodetector120 optically coupled to output end72 of downlinkoptical fiber36D, anamplifier110 electrically coupled to the photodetector, and aRF filter114 electrically coupled to the amplifier and to the correspondingRF cable section90. In an example embodiment,laser100 is adapted to deliver sufficient dynamic range for one or more RoF applications. Examples of suitable lasers for E/O converter60 include laser diodes, distributed feedback (DFB) lasers, Fabry-Perot (FP) lasers, and VCSELs.
• In the operation oftransponder30 ofFIG. 3, a downlink optical signal SD′ traveling in downlinkoptical fiber36D exits this optical fiber atoutput end72 and is received byphotodetector120.Photodetector120 converts optical signal SD′ into a corresponding electrical signal SD, which is then amplified byamplifier110 and then filtered byRF filter114. Electrical signal SD is then fed via the correspondingRF cable section90 to transmittingantenna33T, which converts electrical signal SD into a corresponding electromagnetic signal SD″, which then travels to one ormore client devices46 within the corresponding picocell40 (FIG. 1).
• Similarly, receivingantenna33R receives electromagnetic uplink signal SU″ from one ormore client devices46 withinpicocell40 and converts each such signal to a corresponding electrical signal SU. This electrical travels over the correspondingRF cable section90 and is signal is fed toRF filter114, which filters the signal and passes it along toamplifier110, which amplifies the signal. Electrical signal SU then travels to bias-T unit106, which conditions electrical signal SU—i.e., combines a DC signal with the electrical RF signal so it can drive (semiconductor)laser100 above threshold using a DC current source (not shown) and independently modulate the power around its average value as determined by the provided DC current. The conditioned electrical signal SU then travels tolaser100, which converts the electrical signal to an corresponding optical signal SU″ that is sent to head-end unit20 for processing.
• Transponders30 of the present invention differ from the typical access point device associated with wireless communication systems in that the preferred embodiment of the transponder has just a few signal-conditioning elements and no digital information processing capability. Rather, the information processing capability is located remotely in head-end unit20. This allowstransponder30 to be very compact and virtually maintenance free. In addition, the preferred example embodiment oftransponder30 consumes very little power, is transparent to RF signals, and does not require a local power source, as described below. Moreover, ifsystem10 needs to be changed (e.g., upgraded), the change can be performed at head-end unit20 without having to change or otherwise altertransponders30.
Example Transponder Configurations
• In an example embodiment oftransponder30 such as shown inFIG. 3,dipole antenna system32 includes one ormore antennas33, such as receivingantenna33R and a transmittingantenna33T. In an example embodiment,antennas33R and33T are or include respective wires oriented locally parallel to optical fiber cable34 (i.e., along the y-axis). The ability ofdipole antenna system32 to lie along the direction ofoptical fiber cable34 allows for easy integration of the dipole antenna system into the optical fiber cable relative to other types of direction antennas, such as patch antennas. In an example embodiment,dipole antenna system32 includes a circuit-based dipole antenna, such as available over the Internet from Winizen Co., Ltd., Kyounggi-do 429-250, Korea.
• FIG. 4 is a schematic diagram of an example embodiment of thetransponder30 of the present invention similar to that shown inFIG. 3, but whereindipole antenna system32 includes just asingle antenna33 that both transmits and receives electromagnetic signals. Intransponder30 ofFIG. 4,converter unit31 includes a RF signal-directingelement130, such as a circulator, electrically coupled tosingle antenna33 via a thirdRF cable section90.
• FIG. 5 is a schematic diagram of an example embodiment of E/O converter unit60 inconverter unit31, wherein the E/O converter includes a number ofamplifiers100 electrically connected tophotodetector120. Eachamplifier100 is adapted to amplify a different frequency in electrical signal SD. This allows for parallel conditioning of different frequency bands withintransponder30. A variety of other multi-frequency amplification and antenna system arrangements are also possible for E/O converter60, as well as for O/E converter62.
Example Dipole Antenna System Configurations
• Thetransponder30 of the present invention is capable of supporting numerous configurations ofdipole antenna system32.FIG. 6 is a schematic diagram of an example embodiment oftransponder30 with adipole antenna system32 that includes threedifferent antennas33 electrically connected to apower divider210 via respectiveRF cable sections90.Power divider210 in turn is electrically coupled toconverter unit31 via a correspondingRF cable section90.
• FIG. 7 is a schematic diagram of an example embodiment oftransponder30 with adipole antenna system32 similar to that shown inFIG. 6, but that includes a plurality ofpower dividers210 arranged along aRF cable section90, with each power divider branching off anantenna33. An advantage of the example embodiment oftransponder30 ofFIG. 7 is that if the optical fiber cable is deployed in a building and oneantenna33 is obstructed (say, by an air conditioning duct), another antenna can still send and receive electromagnetic signals.
• FIG. 8 is a schematic diagram of an example embodiment oftransponder30 with adipole antenna system32 similar to that shown inFIG. 7, wherein a portion of the antenna system lies outside ofcable coating58. InFIG. 8,antenna33 ofantenna system32 is shown arranged outside ofcable coating58. In an example embodiment, anexternal covering220, such as a shrink wrapper, is applied tocable coating58 to secure and protect the portion ofantenna system32 that lies outside of the cable coating.
• FIG. 9 is a schematic diagram of an example embodiment oftransponder30 with adipole antenna system32 similar to that shown inFIG. 8, whereinconverter unit31 anddipole antenna system32 are both outside ofcable coating58 and optionally covered byexternal covering220.
• FIG. 10 is a schematic diagram of an example embodiment oftransponder30 with adipole antenna system32 that includes twopairs234 and235 ofwire antennas33, with each antenna connected toconverter unit33 via a correspondingRF cable section90. Antenna pairs234 and235 may be designed, for example, to transmit and receive at the 5.2 GHz and 2.4 GHz frequency bands, respectively (i.e., the IEEE 802 a/b/g standard frequency bands). The judicious use ofRF cable sections90 in this example embodiment mitigates fading and shadowing effects that can adversely affect thedipole radiation pattern42 and thus the size and shape ofpicocell40.
RoF System with Transponder
• FIG. 11 is a detailed schematic diagram of an example embodiment ofsystem10 ofFIG. 1, showing the details of an example embodiment of head-end unit20. In an example embodiment, head-end unit20 includes acontroller250 that provides electrical RF signals SD for a particular wireless service or application. In an example embodiment,controller250 includes a RF signal modulator/demodulator unit270 for modulating/demodulating RF signals, adigital signal processor272 for generating digital signals, a central processing unit (CPU)274 for processing data and otherwise performing logic and computing operations, and amemory unit276 for storing data. In an example embodiment,controller250 is adapted to provide WLAN signal distribution as specified in the IEEE 802.11 standard, i.e., in the frequency range from 2.4 to 2.5 GHz and from 5.0 to 6.0 GHz. In an example embodiment,controller250 serves as a pass-through unit that merely coordinates distributing electrical RF signals SD and SU from and tooutside network24 or betweenpicocells40.
• Head-end unit20 includes one or more converter pairs66 each having an E/O converter60 and an O/E converter62. Eachconverter pair66 is electrically coupled tocontroller250 and is also optically coupled to corresponding one ormore transponders30. Each E/O converter60 inconverter pair66 is optically coupled to aninput end76 of a downlinkoptical fiber36D, and each O/E converter62 is optically coupled to anoutput end74 of an uplinkoptical fiber36U.
• In an example embodiment of the operation ofsystem10 ofFIG. 11,digital signal processor272 incontroller250 generates a downlink digital RF signal S1. This signal is received and modulated by RF signal modulator/demodulator270 to create a downlink electrical RF signal (“electrical signal”) SD designed to communicate with one ormore client devices46 in picocell(s)40. Electrical signal SD is received by one or more E/O converters60, which converts this electrical signal into a corresponding optical signal SU′, which is then coupled into the corresponding downlinkoptical fiber36D atinput end76. It is noted here that in an example embodiment optical signal SD′ is tailored to have a given modulation index. Further, in an example embodiment the modulation power of E/O converter60 is controlled (e.g., by one or more gain-control amplifiers, not shown) in order to vary the transmission power fromdipole antenna system32, which is the main parameter that dictates the size of the associatedpicocell40. In an example embodiment, the amount of power provided todipole antenna system32 is varied to define the size of the associatedpicocell40.
• Optical signal SD′ travels over downlinkoptical fiber36D to anoutput end72 and is processed as described above in connection withsystem10 ofFIG. 2 to return an uplink optical signal SU″. Optical signal SU″ is received at head-end unit20, e.g., by O/E converter62 in theconverter pair66 that sent the corresponding downlink optical signal SD′. O/E converter62 converts optical signal SU′ back into electrical signal SU, which is then processed. Here, in an example embodiment “processed” includes one or more of the following: storing the signal information inmemory unit276; digitally processing or conditioning the signal incontroller250; sending the electrical signal SU, whether conditioned or unconditioned, on to one or moreoutside networks24 via network links25; and sending the signal to one ormore client devices46 within the same orother picocells40. In an example embodiment, the processing of signal SU includes demodulating this electrical signal in RF signal modulator/demodulator unit270, and then processing the demodulated signal indigital signal processor272.
• FIG. 12 is a schematic diagram illustrating a typical prior artpicocellular coverage area44P formed by a conventional picocellular wireless system that formssymmetric picocells40P. Note that such picocells are traditionally represented as hexagons because they can be shown as tiling a given space without gaps.Picocell coverage area44P requires sevenoptical fiber cables34P that employ conventional transponders having omnidirectional antennas. The conventional optical fiber cables are optically coupled to a conventional head-end station20P. The dashed-line outer box B inFIG. 12 represents the approximate boundary forpicocell coverage area44P.
• FIG. 13 is a schematic diagram of apicocellular coverage area44 based on the RoFpicocellular wireless system10 of the present invention that includestransponders30 according to the present invention. The number oftransponders30 and thus the number ofpicocells40 that formpicocellular coverage area44 are approximately equal to theprior art system10P ofFIG. 12. Dashed-line outer box B is provided inFIG. 13 to show that the size ofpicocellular coverage area44 is about the same as that forpicocellular coverage area44P ofFIG. 12. However, eachtransponder30 of the present invention forms anelongate picocell40 with a long axis A_Pperpendicular to the local y-direction ofoptical fiber cable34 at the correspondingdipole antenna system32.Transponders30 of the present invention thus form apicocellular coverage area44 made up ofelongate picocells40 by virtue of orienting the dipole radiation pattern locally perpendicular to the optical fiber cable at the location of eachdipole antenna system32. The elongate shape ofpicocells40 allowssystem10 to cover substantially the same picocellular coverage area asarea44P but using only threeoptical fiber cables34—a reduction of over 50% as compared to the optical fiber cabling needed for the traditionalpicocellular coverage area44P ofFIG. 12. The use of the about the same number oftransponders30 allows forpicocells40 operating at the same frequency to be maximally separated.
• Picocells40 are elongate becausedipole antenna32 has an asymmetric (elliptical) power distribution in the local x-y plane due to the different power decay rate in the different directions.FIG. 14 is a plot of RF power (dBm) emitted bydipole antenna system32 vs. the distance (m) from the antenna along both the x-direction (curve300) and the y-direction (curve302).Curve302 indicates that along the direction of the optical fiber cable (y-direction), the decay is fast, so that one can packtransponders30 more densely along the optical fiber cable without increasing the picocell-to-picocell crosstalk.
• Omnidirectional antennas, such as vertical dipole antennas, typically have a relatively shallow RF power decay rate similar tocurve300 inFIG. 14. Consequently,picocells40 formed from such antennas are prone to cross-talk. Directive antennas, such as microstrip patches, can have an asymmetric radiation pattern in the x-y plane that can create asymmetric cells. However, these antennas require proper alignment in space. Thedipole antenna system32 of the present invention produces predictable radiation patterns without any orientation tuning of individual antennas. This is because thedipole antenna system32 is incorporated into (or onto)optical fiber cable34 in a manner that allows for the picocell location and orientation to be determined by orientingoptical fiber cable34 rather than orienting individual antennas per se. This makesoptical fiber cable34 easier to manufacture and deploy relative to using other more complex directional dipole antenna systems.
• It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Claims (20)

1. A transponder for a radio-over-fiber (RoF) optical fiber cable, comprising:

an electrical-to-optical (E/O) converter;

an optical-to-electrical (O/E) converter; and

a dipole antenna system operably coupled to the E/O converter and the O/E converter and arranged relative to the optical fiber cable so as to create an elongate picocell in a direction locally perpendicular to the optical fiber cable when the transponder is addressed.

2. The transponder ofclaim 1, wherein the dipole antenna includes a transmitting antenna formed from a first wire electrically coupled to the O/E converter, and a receiving antenna formed from a second wire electrically coupled to the E/O converter, wherein the first and second wires are arranged locally parallel to the optical fiber cable.

3. The transponder ofclaim 1, wherein the optical fiber cable has an outer coating, and wherein at least a portion of the transponder resides outside of the outer coating.

4. The transponder ofclaim 1, wherein the dipole antenna system has includes one or more power dividers and corresponding one or more antenna elements electrically coupled to respective power dividers.

5. The transponder ofclaim 1, wherein the E/O converter and the O/E converter constitute a converter unit, and wherein the dipole antenna system includes one or more wires electrically coupled to the converter unit via respective one or more radio-frequency (RF) cable sections.

6. A radio-over-fiber (RoF) picocellular wireless system, comprising:

a head-end unit adapted to send and receive optical RF signals;

one or more transponders according toclaim 1; and

one or more optical fiber cables that include the one or more transponders and that optically couple the head-end unit to each transponder.

7. The system ofclaim 6, wherein each optical fiber cable includes, for each transponder:

a downlink optical fiber optically coupled to the head-end unit and to the transponder O/E converter; and

an uplink optical fiber optically coupled to the head-end unit and to the transponder E/O converter.

8. The system ofclaim 7, wherein each optical fiber cable includes an electrical power line adapted to provide electrical power to each transponder in the corresponding optical fiber cable.

9. A transponder for forming a picocell as part of a radio-over-fiber (RoF) system having an optical fiber cable optically connected to a head-end unit, comprising:

a converter unit adapted to convert electrical signals to optical signals and vice versa; and

a dipole antenna system arranged relative to the optical fiber cable so as to create a picocell formed by creating a dipole radiation field directed perpendicular to the optical fiber cable at the dipole antenna system location.

10. The transponder ofclaim 9, wherein the dipole antenna system includes one or more antenna elements each electrically coupled to the converter unit via corresponding one or more radio-frequency (RF) cable sections.

11. The transponder ofclaim 9, wherein the optical fiber cable includes an outer coating, and wherein at least a portion of the transponder resides outside of the outer coating.

12. The transponder ofclaim 11, wherein some or all of the dipole antenna system resides outside of the outer coating.

13. A radio-over-fiber (RoF) picocellular wireless system, comprising:

one or more transponders according toclaim 9;

a head-end unit adapted to send and receive optical RF signals;

one or more optical fiber cables each having at least one transponder and corresponding one or more optical fiber RF communication links that optically couple the one or more transponders to the head-end unit; and

wherein the one or more transponders form a picocellular coverage area made up of elongate picocells formed by each transponder.

14. The system ofclaim 13, wherein the head-end unit is adapted to send and transmit optical RF signals having different frequencies, and the dipole antenna system is adapted to transmit and receive electromagnetic signals having the different frequencies.

15. The system ofclaim 13, further including:

a power supply operably connected to the head-end unit via an electrical power line that runs through the one or more optical fiber cables so as to provide electrical power to each transponder.

16. The system ofclaim 13, wherein each optical fiber cable has an outer coating, and at least a portion of some or all of the one or more transponders reside outside of the outer coating.

17. A method of forming an elongate picocell for a radio-over fiber (RoF) system that includes an optical fiber cable, comprising:

transmitting optical RF signals to a transponder via an optical fiber RF communication link in the optical fiber cable;

converting the optical signals to electrical RF signals at the transponder;

converting the electrical signals to electromagnetic RF signals at the transponder using a dipole antenna system that creates the elongate picocell in a direction locally perpendicular to the optical fiber cable.

18. The method ofclaim 17, wherein the optical fiber cable has an outer coating and including providing at least a portion of the dipole antenna system outside of the outer coating.

19. The method ofclaim 17, including performing the acts therein for multiple transponders so as to form a picocellular coverage area made up of multiple elongate picocells.

20. The method ofclaim 17, including orienting the picocell by orienting the optical fiber cable.

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