US20070292136A1

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

Info

Publication number: US20070292136A1
Application number: US11/454,581
Authority: US
Inventors: Michael Sauer; Andrey Kobyokov
Original assignee: Corning Optical Communications LLC
Current assignee: Corning Research and Development Corp
Priority date: 2006-06-16
Filing date: 2006-06-16
Publication date: 2007-12-20
Also published as: US20070292137A1; EP2033343A2; US7590354B2; CN101473568A; WO2007146428A3; WO2007146428A2

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 of FIG. 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 in FIG. 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 in FIG. 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 in FIG. 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 in FIG. 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 of FIG. 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 RoF picocellular wireless system 10 according to the present invention. System 10 includes a head-end unit 20 adapted to transmit, receive and/or process RF optical signals. In an example embodiment, head-end unit 20 is operably coupled to an outside network 24 via a network link 25, and the head-end unit serves as a pass-through for RF signals sent to and from the outside network.
System 10 also includes one or more transponder units (“transponders”) 30 according to the present invention. Each transponder 30 includes a converter unit 31 and a dipole antenna system 32 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. Transponder 30 is discussed in greater detail below.
System 10 includes one or more optical fiber cables 34 each optically coupled to head-end unit 20. Each optical fiber cable 34 includes one or more optical fiber RF transmission links 36 optically coupled to respective one or more transponders 30. In an example embodiment, each optical fiber RF transmission link 36 includes a downlink optical fiber 36D and an uplink optical fiber 36U. Example embodiments of system 10 include either single-mode optical fiber or multimode optical fiber for downlink and uplink optical fibers 36D and 36U. The particular type of optical fiber depends on the application of system 10, 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 uplink optical fibers 36D and 36U. 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 uplink optical fibers 36D and 36U, 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, RoF picocellular wireless system 10 of the present invention employs a known telecommunications wavelength, such as 850 nm, 1300 nm, or 1550 nm. In another example embodiment, system 10 employs other less common but suitable wavelengths such as 980 nm.
Also shown in FIG. 1 is a local x-y-z Cartesian coordinate system C at each dipole antenna system 32 for the sake of reference, where the x-direction is into the paper and locally perpendicular to optical fiber cable 34. In an example embodiment, dipole antenna system 32 is sufficiently stiff so that optical fiber cable 34 is locally straight at the dipole antenna system location. In an example embodiment, dipole antenna system 32 is located relatively far away from converter unit 31 (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.
Each transponder 30 is adapted to form a picocell 40 via dipole antenna system 32 via electromagnetic transmission and reception when the transponder is addressed, e.g., receives a downlink optical signal SD′ from head-end unit 20 and/or an uplink electromagnetic signal SU″ from a client device 46. Client device 46, which is shown in the form of a computer as one example of a client device, includes an antenna 48 (e.g., a wireless card) adapted to electromagnetically communicate with (i.e., address) the transponder and antenna system 32 thereof.
Dipole antenna system 32 is adapted to form picocell 40 from a dipole radiation pattern 42 oriented perpendicular to optical fiber cable 34 at the location of the dipole antenna system. The term “locally perpendicular” is used herein to describe the orientation of picocell 40 and/or the corresponding dipole radiation pattern 42 relative to optical fiber cable 34 at the location of dipole antenna system 32. Dipole radiation pattern 42 is thus centered about the local x-z plane P_XZ(viewed edge-on in FIG. 1 and illustrated as a dotted line). In an example embodiment, only a portion of dipole radiation pattern 42 is used for picocell 40, e.g., the portion below optical fiber cable 34 (i.e., in the -z direction), as shown in FIG. 1.
In an example embodiment, system 10 is powered by a power supply 50 electrically coupled to head-end unit 20 via an electrical power line 52 that carries electrical power signals 54.

Transponder Incorporated Into Optical Fiber Cable

FIG. 2 is a schematic close-up view of an example embodiment of transponder 30 as incorporated into optical fiber cable 34. In an example embodiment, optical fiber cable 34 includes an outer coating 58. As mentioned above, transponder 30 includes a converter unit 31. Converter unit 31 includes an electrical-to-optical (E/O) converter 60 adapted to convert an electrical signal into a corresponding optical signal, and an optical-to-electrical (O/E) converter 62 that converts an optical signal into a corresponding electrical signal. E/O converter 60 is optically coupled to an input end 70 of uplink optical fiber 36U and O/E converter 62 is optically coupled to an output end 72 of downlink optical fiber 36D.
In an example embodiment, optical fiber cable 34 includes electrical power line 52, and converter unit 31 includes a DC power converter 80 electrically coupled to the electrical power line and to E/O converter 60 and O/E converter 62. DC power converter 80 is adapted to change the voltage levels and provide the power required by the power-consuming components in converter unit 31. In an example embodiment, DC power converter 80 is either a DC/DC power converter, or an AC/DC power converter, depending on the type of power signal 54 carried by electrical power line 52. In an example embodiment, electrical power line 52 includes two electrical wires 52A and 52B connected to DC power converter 80.
As discussed above, dipole antenna system 32 is electrically coupled to converter unit 31. In an example embodiment, dipole antenna system 32 includes one or more antenna elements (“antennas”) 33. In the example embodiment shown in FIG. 2, antenna system 32 includes a receiving antenna 33R electrically coupled to E/O converter 60 via a first RF cable section 90 and a transmitting antenna 33T electrically coupled to E/O converter 62 via a second RF cable section 90. In an example embodiment, the one or more antennas 33 is/are made of or include sections of wire. One or more RF cable sections 90 are used in example embodiments of the present invention to connect corresponding one or more antennas 33 to the converter unit 31. In an example embodiment, dipole antenna system 32 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 of converter unit 31 and dipole antenna system 32 for the transponder 30 of the present invention. In the example embodiment of FIG. 3, E/O converter 60 includes a laser 100 optically coupled to an input end 70 of uplink optical fiber 36U, a bias-T unit 106 electrically coupled to the laser, an amplifier 110 electrically coupled to the bias-T unit, and a RF filter 114 electrically coupled to the amplifier and to the corresponding RF cable section 90. Also in an example embodiment, O/E converter 62 includes a photodetector 120 optically coupled to output end 72 of downlink optical fiber 36D, an amplifier 110 electrically coupled to the photodetector, and a RF filter 114 electrically coupled to the amplifier and to the corresponding RF cable section 90. In an example embodiment, laser 100 is adapted to deliver sufficient dynamic range for one or more RoF applications. Examples of suitable lasers for E/O converter 60 include laser diodes, distributed feedback (DFB) lasers, Fabry-Perot (FP) lasers, and VCSELs.
In the operation of transponder 30 of FIG. 3, a downlink optical signal SD′ traveling in downlink optical fiber 36D exits this optical fiber at output end 72 and is received by photodetector 120. Photodetector 120 converts optical signal SD′ into a corresponding electrical signal SD, which is then amplified by amplifier 110 and then filtered by RF filter 114. Electrical signal SD is then fed via the corresponding RF cable section 90 to transmitting antenna 33T, which converts electrical signal SD into a corresponding electromagnetic signal SD″, which then travels to one or more client devices 46 within the corresponding picocell 40 (FIG. 1).
Similarly, receiving antenna 33R receives electromagnetic uplink signal SU″ from one or more client devices 46 within picocell 40 and converts each such signal to a corresponding electrical signal SU. This electrical travels over the corresponding RF cable section 90 and is signal is fed to RF filter 114, which filters the signal and passes it along to amplifier 110, which amplifies the signal. Electrical signal SU then travels to bias-T unit 106, which conditions electrical signal SU—i.e., combines a DC signal with the electrical RF signal so it can drive (semiconductor) laser 100 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 to laser 100, which converts the electrical signal to an corresponding optical signal SU″ that is sent to head-end unit 20 for processing.
Transponders 30 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 unit 20. This allows transponder 30 to be very compact and virtually maintenance free. In addition, the preferred example embodiment of transponder 30 consumes very little power, is transparent to RF signals, and does not require a local power source, as described below. Moreover, if system 10 needs to be changed (e.g., upgraded), the change can be performed at head-end unit 20 without having to change or otherwise alter transponders 30.

Example Transponder Configurations

In an example embodiment of transponder 30 such as shown in FIG. 3, dipole antenna system 32 includes one or more antennas 33, such as receiving antenna 33R and a transmitting antenna 33T. In an example embodiment, antennas 33R and 33T are or include respective wires oriented locally parallel to optical fiber cable 34 (i.e., along the y-axis). The ability of dipole antenna system 32 to lie along the direction of optical fiber cable 34 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 system 32 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 the transponder 30 of the present invention similar to that shown in FIG. 3, but wherein dipole antenna system 32 includes just a single antenna 33 that both transmits and receives electromagnetic signals. In transponder 30 of FIG. 4, converter unit 31 includes a RF signal-directing element 130, such as a circulator, electrically coupled to single antenna 33 via a third RF cable section 90.
FIG. 5 is a schematic diagram of an example embodiment of E/O converter unit 60 in converter unit 31, wherein the E/O converter includes a number of amplifiers 100 electrically connected to photodetector 120. Each amplifier 100 is adapted to amplify a different frequency in electrical signal SD. This allows for parallel conditioning of different frequency bands within transponder 30. A variety of other multi-frequency amplification and antenna system arrangements are also possible for E/O converter 60, as well as for O/E converter 62.

Example Dipole Antenna System Configurations

The transponder 30 of the present invention is capable of supporting numerous configurations of dipole antenna system 32. FIG. 6 is a schematic diagram of an example embodiment of transponder 30 with a dipole antenna system 32 that includes three different antennas 33 electrically connected to a power divider 210 via respective RF cable sections 90. Power divider 210 in turn is electrically coupled to converter unit 31 via a corresponding RF cable section 90.
FIG. 7 is a schematic diagram of an example embodiment of transponder 30 with a dipole antenna system 32 similar to that shown in FIG. 6, but that includes a plurality of power dividers 210 arranged along a RF cable section 90, with each power divider branching off an antenna 33. An advantage of the example embodiment of transponder 30 of FIG. 7 is that if the optical fiber cable is deployed in a building and one antenna 33 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 of transponder 30 with a dipole antenna system 32 similar to that shown in FIG. 7, wherein a portion of the antenna system lies outside of cable coating 58. In FIG. 8, antenna 33 of antenna system 32 is shown arranged outside of cable coating 58. In an example embodiment, an external covering 220, such as a shrink wrapper, is applied to cable coating 58 to secure and protect the portion of antenna system 32 that lies outside of the cable coating.
FIG. 9 is a schematic diagram of an example embodiment of transponder 30 with a dipole antenna system 32 similar to that shown in FIG. 8, wherein converter unit 31 and dipole antenna system 32 are both outside of cable coating 58 and optionally covered by external covering 220.
FIG. 10 is a schematic diagram of an example embodiment of transponder 30 with a dipole antenna system 32 that includes two pairs 234 and 235 of wire antennas 33, with each antenna connected to converter unit 33 via a corresponding RF cable section 90. Antenna pairs 234 and 235 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 of RF cable sections 90 in this example embodiment mitigates fading and shadowing effects that can adversely affect the dipole radiation pattern 42 and thus the size and shape of picocell 40.

RoF System with Transponder

FIG. 11 is a detailed schematic diagram of an example embodiment of system 10 of FIG. 1, showing the details of an example embodiment of head-end unit 20. In an example embodiment, head-end unit 20 includes a controller 250 that provides electrical RF signals SD for a particular wireless service or application. In an example embodiment, controller 250 includes a RF signal modulator/demodulator unit 270 for modulating/demodulating RF signals, a digital signal processor 272 for generating digital signals, a central processing unit (CPU) 274 for processing data and otherwise performing logic and computing operations, and a memory unit 276 for storing data. In an example embodiment, controller 250 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, controller 250 serves as a pass-through unit that merely coordinates distributing electrical RF signals SD and SU from and to outside network 24 or between picocells 40.
Head-end unit 20 includes one or more converter pairs 66 each having an E/O converter 60 and an O/E converter 62. Each converter pair 66 is electrically coupled to controller 250 and is also optically coupled to corresponding one or more transponders 30. Each E/O converter 60 in converter pair 66 is optically coupled to an input end 76 of a downlink optical fiber 36D, and each O/E converter 62 is optically coupled to an output end 74 of an uplink optical fiber 36U.
In an example embodiment of the operation of system 10 of FIG. 11, digital signal processor 272 in controller 250 generates a downlink digital RF signal S1. This signal is received and modulated by RF signal modulator/demodulator 270 to create a downlink electrical RF signal (“electrical signal”) SD designed to communicate with one or more client devices 46 in picocell(s) 40. Electrical signal SD is received by one or more E/O converters 60, which converts this electrical signal into a corresponding optical signal SU′, which is then coupled into the corresponding downlink optical fiber 36D at input end 76. 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 converter 60 is controlled (e.g., by one or more gain-control amplifiers, not shown) in order to vary the transmission power from dipole antenna system 32, which is the main parameter that dictates the size of the associated picocell 40. In an example embodiment, the amount of power provided to dipole antenna system 32 is varied to define the size of the associated picocell 40.
Optical signal SD′ travels over downlink optical fiber 36D to an output end 72 and is processed as described above in connection with system 10 of FIG. 2 to return an uplink optical signal SU″. Optical signal SU″ is received at head-end unit 20, e.g., by O/E converter 62 in the converter pair 66 that sent the corresponding downlink optical signal SD′. O/E converter 62 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 in memory unit 276; digitally processing or conditioning the signal in controller 250; sending the electrical signal SU, whether conditioned or unconditioned, on to one or more outside networks 24 via network links 25; and sending the signal to one or more client devices 46 within the same or other picocells 40. In an example embodiment, the processing of signal SU includes demodulating this electrical signal in RF signal modulator/demodulator unit 270, and then processing the demodulated signal in digital signal processor 272.
FIG. 12 is a schematic diagram illustrating a typical prior art picocellular coverage area 44P formed by a conventional picocellular wireless system that forms symmetric picocells 40P. Note that such picocells are traditionally represented as hexagons because they can be shown as tiling a given space without gaps. Picocell coverage area 44P requires seven optical fiber cables 34P that employ conventional transponders having omnidirectional antennas. The conventional optical fiber cables are optically coupled to a conventional head-end station 20P. The dashed-line outer box B in FIG. 12 represents the approximate boundary for picocell coverage area 44P.
FIG. 13 is a schematic diagram of a picocellular coverage area 44 based on the RoF picocellular wireless system 10 of the present invention that includes transponders 30 according to the present invention. The number of transponders 30 and thus the number of picocells 40 that form picocellular coverage area 44 are approximately equal to the prior art system 10P of FIG. 12. Dashed-line outer box B is provided in FIG. 13 to show that the size of picocellular coverage area 44 is about the same as that for picocellular coverage area 44P of FIG. 12. However, each transponder 30 of the present invention forms an elongate picocell 40 with a long axis A_Pperpendicular to the local y-direction of optical fiber cable 34 at the corresponding dipole antenna system 32. Transponders 30 of the present invention thus form a picocellular coverage area 44 made up of elongate picocells 40 by virtue of orienting the dipole radiation pattern locally perpendicular to the optical fiber cable at the location of each dipole antenna system 32. The elongate shape of picocells 40 allows system 10 to cover substantially the same picocellular coverage area as area 44P but using only three optical fiber cables 34—a reduction of over 50% as compared to the optical fiber cabling needed for the traditional picocellular coverage area 44P of FIG. 12. The use of the about the same number of transponders 30 allows for picocells 40 operating at the same frequency to be maximally separated.
Picocells 40 are elongate because dipole antenna 32 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 by dipole antenna system 32 vs. the distance (m) from the antenna along both the x-direction (curve 300) and the y-direction (curve 302). Curve 302 indicates that along the direction of the optical fiber cable (y-direction), the decay is fast, so that one can pack transponders 30 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 to curve 300 in FIG. 14. Consequently, picocells 40 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. The dipole antenna system 32 of the present invention produces predictable radiation patterns without any orientation tuning of individual antennas. This is because the dipole antenna system 32 is incorporated into (or onto) optical fiber cable 34 in a manner that allows for the picocell location and orientation to be determined by orienting optical fiber cable 34 rather than orienting individual antennas per se. This makes optical fiber cable 34 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

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 of claim 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 of claim 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 of claim 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 of claim 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 to claim 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 of claim 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 of claim 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 of claim 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 of claim 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 of claim 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 to claim 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 of claim 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 of claim 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 of claim 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 of claim 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 of claim 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 of claim 17, including orienting the picocell by orienting the optical fiber cable.