METHODS AND APPARATUS FOR SELECTIVE ATTENUATION IN AN OPTICAL COMMUNICATION USING ELECTROCHROMIC MATERIAL

Abstract: Disclosed methods and apparatus for processing optical signals in a communications network to adjust or reset the power level of a signal employ a body of electrochromic material disposed along an optical communication signal propagation path. An optical communication signal travelling along the propagation path is attenuated using the body of electrochromic material. The body preferably is switchable between at least two color states through selective application of a voltage potential in such a way as to affect the body, producing each of the at least two color states permitting at least attenuated passage through the body of the optical communication signal. Applications for the present invention include, for example, the selective attenuation of constituent WDM optical communication signals through the use of multiplexers and/or demultiplexers.

Description: METHODS AND APPARATUS FOR SELECTIVE ATTENUATION IN AN OPTICAL COMMUNICATION USING ELECTROCHROMIC MATERIAL Background The present invention relates to optical communication networks. More particularly, the present invention relates to methods and apparatus for processing optical communication signals, including selective attenuation of an optical communication signal using a body of electrochromic material. Over the past several years, optical links have increasingly become an important and fundamental part of modern communications networks. Optical communication networks typically comprise sources of electromagnetic radiation (or light), optical waveguides, optical couplers and other components such as, for example, switches, multiplexers, modulators and attenuators, which together operate to generate, carry and appropriately route optical communication signals along optical links of the communication network. Operating wavelengths for such commumcation networks may be on the order of approximately 0.85 microns for a local area network (LAN), for example, or may be longer for applications such as telecommunications. Wavelengths used in communication networks for long distance telecommunication transmission, for example, may be on the order of approximately 1.3 to 1.6 microns. Such networks may carry optical communication signals of a particular wavelength, or alternatively may carry optical communication signals that comprise a multiplicity of discrete wavelengths or bands of wavelengths. Networks that use one wavelength may require that the optical power at that wavelength be adjusted or reset along the route, either attenuated or amplified, in order to deliver to the ultimate receiver a power level lying within the receiver's 2 dynamic range. Similarly, networks that use multiple wavelengths of light, wavelength division networks, typically require that the relative power levels in at least certain ones of the various bands of wavelengths be adjusted or reset at frequent intervals along a given optical link to ensure that the network is operating with a power versus wavelength profile that is suitably flat. A suitably flat profile helps to ensure that each receiver sees a power level lying within its dynamic range. Periodic adjustment of this sort is particularly important in optical networks that employ optical amplifiers, as such amplifiers often possess a gain versus wavelength profile that does not have the desired level of flatness. Prior methods and systems for attenuating the optical signal to compensate for such non- ideal characteristics typically are relatively costly, cumbersome and prone to failure. An example prior art system employs a transparent film having gray scale thereon spanning in graduated fashion a range of densities from one end of the film to the other end. The film is manipulated relative to the light path using a mechanical stepper motor such that the film intersects the light path of interest at a particular gray scale point along the length of the film. The opacity of the film absorbs certain electromagnetic energy from a signal traveling along the light path, thus resulting in signal attenuation. The ability to selectively manipulate the film using a stepper motor, and in turn the degree of gray scale intersecting the light path, provides control over the degree of signal attenuation. The use of a stepper motor and film device in this manner, however, can be relatively cumbersome and costly. The system, with its moving parts, is also prone to mechanical failure. Electrochromic devices previously have been proposed for use in optical communications systems. U.S. Patent 4,245,883 issued to Johnson et al, for example, describes particular components which are electrically activated for use as switches, modulators, 3 attenuators, and mode selectors in networks of waveguides in optical communications systems. The attenuator described by Johnson et al., however, involves relatively expensive waveguide technology and attenuates light in a rather indirect manner. The attenuator disclosed by Johnson et al. attenuates radiation propagating in a waveguide through the use of a cladding layer of electrochromic material disposed parallel to the waveguide to absorb a certain fraction of energy. Such arrangements are less practical for many optical communications systems, for reasons of cost and complexity. SUMMARY OF THE INVENTION The present invention relates to new methods and apparatus for processing optical communication signals in a communications network, which include the use of an electrochromic medium to provide optical signal attenuation. An optical signal corresponding to voice and/or data information and traveling in a communications network is selectively attenuated through the use of a body of electrochromic material that is disposed to intersect the propagation path. The amount by which optical network traffic is attenuated can be varied through variable application of an electric field in such a way as to affect the body of electrochromic material. The applied electric field operates to affect a change in the color or opacity of the electrochromic material, and thus the degree of attenuation that is provided by the electrochromic medium. The optical signal passes through the electrochromic medium, where it is attenuated, and is thereafter communicated to the commumcation network for further processing. The present invention provides for selective attenuation of optical communication signals in a flexible, efficient, reliable, and cost-effective manner. The present invention also 4 may, in the appropriate applications, operate to relax requirements imposed upon optical amplifiers relative to the flatness of the amplifier's gain versus wavelength profile. These and other objects of the present invention will be apparent to those of skill in the art from the description of the preferred embodiment that follows. BRIEF DESCRIPTION OF THE DRAWING Preferred embodiments of the present invention are described herein with reference to the drawing wherein: FIGURE 1 illustrates schematically, in side view, an electro-optic attenuation device in accordance with one example embodiment of the present invention; FIGURE 2 illustrates an end and partial cross-sectional view of the electro-optic attenuation device embodiment shown in Figure 1 ; FIGURE 3 illustrates schematically a first example communication network link of the present invention that provides selective attenuation of an optical signal traveling along the link; FIGURE 4 shows schematically a first example embodiment of an optical transport system of the present invention that provides selective attenuation of constituent optical signals of a wavelength division multiplexed signal; FIGURE 5 shows schematically a second example embodiment of an optical transport system of the present invention that provides selective attenuation of constituent optical signals of a wavelength division multiplexed signal; FIGURE 6 shows schematically a third example embodiment of an optical transport system of the present invention that provides selective attenuation of constituent optical signals of a wavelength division multiplexed signal; and 5 FIGURE 7 illustrates schematically a second example communication network link of the present invention that provides selective attenuation of constituent optical signals of a wavelength division multiplexed signal traveling on the link. DESCRIPTION OF THE PREFERRED EMBODIMENT Generally, the electrochromic effect involves the ability to adjust the relative color of a material through chemical changes in an electrochromic material, these changes being caused by introducing into the body of the material a number of ions supplied by the adjacent conductive layer, the density of ions so supplied being controlled by the intensity of an applied electric field. For instance, a layer of electrochromic material and a compatible ion conducting layer may be together sandwiched between two layers of transparent conductors. Electrochromic materials may include a suitable inorganic material, such as WO3 for example, or a suitable organic material. The ion conducting layer may be realized using, for example, an electrolyte or an ion rich solid. The two conductors which effectively sandwich the layer of electrochromic material and the ion conducting layer may be, for example, layers of indium tin oxide (ITO). A relatively small potential difference (typically on the order of a few volts) applied across the conductors, for example, induces an electric field that causes the electrochromic material to electrochemically react with the ion conducting layer. This reaction causes a change in the amount of light that is absorbed by the material, and in turn a change in the relative color or opaqueness of the electrochromic layer. By appropriately varying the applied voltage potential, the electrochromic material may be controllable between at least a first state that passes some or all of incident light, and a second state that results in a complete or at least partial reduction of the level of incident light that passes through the layer relative to the first state. Preferred embodiments of the present invention are shown in the Figures. Figures 1 and 6 2, for example, provide schematic illustrations of an electro-optic attenuation device 10 constructed in accordance with one example embodiment of the present invention. In particular, a substrate 11, constructed from transparent silica or other suitable material, supports an electrochromic sandwich 12 that comprises an electrochromic region 14, an adjacent conductive region 16, and transparent electrodes 18 and 20. Substrate 11, region 14, region 16, electrode 18 and electrode 20 of device 10 preferably are adjacent, substantially parallel, planar layers. Graded index lenses 22 and 24 are disposed on either side of the sandwich 12 as shown in Figure 1. Graded index lenses 22 and 24 each communicate optically with optical fibers 26 and 28, respectively. Optical fibers 26 and 28 each comprise a length of an optical core 30 and 31, respectively, that is surrounded by a cladding material 32 and 33, respectively. The cladding material 32 and 33 is shown partially removed in Figure 1 to more clearly illustrate the inner cores 30 and 31. Electrical leads 34 and 36 extend from electrodes 18 and 20, respectively, and preferably connect to a variable voltage source 37 that is adapted to apply various voltage potentials across electrodes 18 and 20 as desired. A casing 38 is used in this example embodiment to substantially encapsulate the sandwich 12 and graded index lenses 22 and 24, although a portion of the casing 38 is shown removed in Figure 1 to clearly illustrate interior components of the device 10. This casing 38, which may be constructed from a polymer or other suitable material, serves as a protective exterior for at least a portion of the electro-optic attenuation device 10. Casing 38 has apertures 40, 42, 44, and 46 through which pass fiber 26, fiber 28, lead 34 and lead 36, respectively. Region 14 of sandwich 12 is a layer comprising a suitable inorganic electrochromic material such as WO 3 for example, or a suitable organic electrochromic material. Region 16 is an ion conducting layer such as, for example, an electrolyte or an ion rich solid. Electrodes 18 7 and 20 each comprise a transparent conductive coating of suitable transparent electrode material. Indium tin oxide (ITO), zinc oxide and tin oxide are example transparent electrode materials. Leads 34 and 36 communicate electrically with electrodes 18 and 20, and are used to establish a desired potential difference across, and resultant electric field between, the electrodes 18 and 20. The thickness of the sandwich 12 and range of voltages applied to the electrodes 18 and 20, two factors which can affect the range and degree of transmission and attenuation provided by device 10, are preferably selected so that the device 10 satisfies the operating requirements of the particular application. Further, particular applications of the present invention may warrant the selection of alternative materials and/or material relationships within device 10 that are apparent to persons of ordinary skill in the art. Referring now to Figures 1 and 2, the present invention is preferably used to process an optical signal in an optical link of a communications network, such as a local area network or telecommunications network for example. In Figure 1, the combination of lens 22 and lens 24, as well as associated fibers 26 and 28, operate to define and establish a propagation path 48 along which a digital or analog optical communication signal travels. An optical communication signal travels along fiber 26 towards lens 22 along path 48. Lens 22 collimates the light of the optical signal traveling in fiber 26. The collimated light is then delivered to the planar electrochromic sandwich 12 and, more particularly, to an incident planar surface 13 of the region 14 through substrate 11 and electrode 18 along path 48. Path 48 preferably is substantially perpendicular to the planar sandwich 12 and the incident planar surface 13 of the region 14. Electrochromic sandwich 12 is disposed along and intersects the path 48 such that sandwich 12 effectively bisects the path 48 into a first portion 48a and a second portion 48b. The optical signal thus traveling along path 48 is passed through the sandwich 12, including the 8 region 14, where it may be attenuated by absorption. Selective application of an appropriate voltage potential V across, and resultant electric field between, electrodes 18 and 20 is used to effect the desired color or opacity (i.e., electrochromic or attenuating effect) in sandwich 12 and, in particular, region 14. In this regard it is preferred that the region 14 be switchable between at least two color states to provide for variable attenuation. An attenuated optical signal is thereafter received in lens 24 following passage through electrode 20, and communicated along path 48 into fiber 28 for further processing in the communication link and/or communication network. Such further processing may include, for example, downstream reception and detection of the optical signal to facilitate the successful transfer of voice and/or data information to its intended destination. Figure 3 depicts an example communication network link 60 comprising a transmitter 62, a receiver 64 and an intermediately disposed electro-optic attenuation device 10 that, on a first side, communicates through optical medium 66 with the transmitter 62 and, on a second side, communicates through optical medium 68 with the receiver 64. Selective attenuation of an optical signal traveling along the link 60 is provided by application of the appropriate voltage potential across electrodes 18 and 20 of device 10, as described above. Link 60 may form a part of a larger communication network such as, for example, a local area network or local and/or long distance telecommunication network. Figure 4 is a schematic illustration of an example embodiment of an optical transport system 80 of the present invention that may be used to provide selective attenuation of constituent optical signals in a wavelength division multiplexed communications link. The system 80 comprises an optical line 82 through which a wavelength division multiplexed optical communication signal is received. Optical line 82 communicates the received signal to 9 demultiplexer 84. Constituent optical signals are each delivered by demultiplexer 84 to respective optical lines 1, 2, ... L. Each of optical lines 1, 2, ... L communicates the respective constituent signal to a device 10 which, in turn, is used to provide a level of attenuation that is particularly suited for the constituent signal, as described above for example. The constituent signal is thereafter provided to the corresponding line of optical lines 1', 2', ... L'. In this manner each of the constituent signals of a wavelength division multiplexed optical communication signal may be independently adjusted or reset within the optical transport system 80 to the extent necessary to ensure a suitably flat power versus wavelength profile. Figure 5 is a schematic illustration of another example embodiment of an optical transport system 90 of the present invention that also may be used to provide selective attenuation of constituent optical signals in a wavelength division multiplexed communications link. The system 90 comprises an array of optical lines 1, 2, ... M through which constituent optical communication signals are respectively received. Each of the optical lines 1, 2, ... M communicates the received constituent signal to a device 10 which is used to provide a level of attenuation that is particularly suited for the signal. The signal is thereafter delivered through the corresponding line of optical lines 1', 2', ... M' to multiplexer 92. Multiplexer 92, in turn, multiplexes the various signals to form a wavelength division multiplexed optical signal for communication on optical line 94. Again, each of the constituent signals of the wavelength division multiplexed optical communication signal may be independently adjusted or reset within the optical transport system 90 to the extent necessary to ensure a suitably flat power versus wavelength profile. Figure 6 is a schematic illustration of yet another embodiment of an optical transport system 100 of the present invention that may be used to provide selective attenuation of 10 constituent optical signals in a wavelength division multiplexed communications link. The system 100 comprises an optical line 102 through which a wavelength division multiplexed optical communication signal is received. Optical line 102 communicates the received signal to demultiplexer 104. Constituent optical signals are each delivered by demultiplexer 104 to respective optical lines 1, 2, ... N. Each of optical lines 1, 2, ... N communicates the respective constituent signal to a device 10 which is used to provide a level of attenuation that is particularly suited for the constituent signal. The constituent signal is thereafter provided to multiplexer 106 through the corresponding line of optical lines V, V, ... N'. Multiplexer 106, in turn, multiplexes the various signals to form a wavelength division multiplexed optical signal for communication on optical line 108. Each of the constituent signals of the wavelength division multiplexed optical communication signal may be independently adjusted or reset within the optical transport system 100 to the extent necessary to ensure a suitably flat power versus wavelength profile. Figure 7 depicts an example wavelength division multiplexed optical communication network link 110 comprising a transmitter 112, a receiver 114, and an intermediately disposed optical transport system 116 that, on a first side, communicates through optical medium 118 with the transmitter 112 and, on a second side, communicates through optical medium 120 with the receiver 114. In one embodiment, optical transport system 116 might be, for example, optical transport system 80 from Figure 4, wherein system 80 might further communicate respective constituent signals to receiver 114 and other receivers (not shown) in parallel. In another embodiment, optical transport system 116 might be, for example, optical transport system 90 from Figure 5, wherein system 90 might receive respective constituent signals from transmitter 114 and other transmitters (not shown) in parallel. Further, in yet another 1 1 embodiment, optical transport system 116 might be, for example, optical transport system 100 from Figure 6, wherein both lines 118 and 120 carry a wavelength division multiplexed optical communication signal. In this way the optical transport system 116 may provide selective attenuation of constituent optical signals traveling along the link 110. Link 110 may form a part of a larger communication network such as, for example, a local area network or local and/or long distance telecommunication network. Although certain embodiments of the invention have been described and illustrated herein, it will be readily apparent to those of ordinary skill in the art that a number of modifications and substitutions can be made to the preferred example methods and apparatus disclosed and described herein without departing from the true spirit and scope of the invention.

Claims: 12 I claim: 1. A method for processing an optical signal in a communications link, comprising the steps of: establishing a propagation path along which an optical communication signal travels; providing a body of electrochromic material that intersects the path; passing through the body of electrochromic material an optical communication signal traveling along the path to attenuate the signal; and communicating the attenuated optical communication signal for further processing in the communication link. 2. The method for processing an optical signal in a communications link as set forth in claim 1, wherein the signal traveling along the path is a constituent optical signal that is demultiplexed from a wavelength division multiplexed (WDM) optical communication signal. 3. The method for processing an optical signal in a communications link as set forth in claim 1, further comprising the step of multiplexing the attenuated signal into a wavelength division multiplexed (WDM) optical communication signal. 4. The method for processing an optical signal in a communications link as set forth in claim 1 , further comprising the steps of: passing the optical communication signal through a first transparent electrode 13 disposed proximate the body prior to passing the signal through the body; and passing the optical communication signal tlirough a second transparent electrode disposed proximate the body after passing the signal through the body. 5. The method for processing an optical signal in a communications link as set forth in claim 1 , further comprising the steps of: passing the optical communication signal through a first graded index lens disposed proximate the body prior to passing the signal through the body; and passing the optical communication signal through a second graded index lens disposed proximate the body after passing the signal through the body. 6. The method for processing an optical signal in a communications link as set forth in claim 1, wherein the optical communication signal comprises a telecommunication signal. 7. The method for processing an optical signal in a communications link as set forth in claim 6, wherein the body is switchable between at least two color states. 8. The method for processing an optical signal in a communications link as set forth in claim 1, wherein the optical communication signal comprises a local area network (LAN) signal. 9. A method for processing an optical signal in a communications link, comprising 14 the steps of: providing a body of electrochromic material, the body being switchable between at least two color states; applying a voltage potential proximate the body to effect one of the at least two color states of the body; delivering an optical communication signal to an incident planar surface of the body along a propagation path that is substantially perpendicular to the planar surface; passing the signal through the body to attenuate the signal; and communicating the attenuated optical communication signal for further processing in the communication link. 10. The method for processing an optical signal in a communications link as set forth in claim 9, wherein the signal is a constituent optical signal that is demultiplexed from a wavelength division multiplexed (WDM) optical communication signal. 11. The method for processing an optical signal in a communications link as set forth in claim 9, further comprising the step of multiplexing the attenuated signal into a wavelength division multiplexed (WDM) optical communication signal. 12. The method for processing an optical signal in a communications link as set forth in claim 9, further comprising the step of passing the optical communication signal through a first transparent electrode disposed adjacent the body prior to passing the signal through the body. 15 13. The method for processing an optical signal in a communications link as set forth in claim 12, further comprising the step of passing the optical communication signal through a second transparent electrode disposed adjacent the body after passing the signal through the body. 14. The method for processing an optical signal in a communications link as set forth in claim 9, further comprising the step of passing the optical communication signal through a first graded index lens disposed adjacent the body prior to passing the signal through the body. 15. The method for processing an optical signal in a communications link as set forth in claim 14, further comprising the step of passing the optical communication signal through a second graded index lens disposed adjacent the body after passing the signal through the body. 16. The method for processing an optical signal in a communications link as set forth in claim 9, further comprising the steps of: passing the optical communication signal through both a first graded index lens and a first transparent electrode prior to passing the signal through the body, the first lens and first electrode both being disposed proximate the body; and passing the optical communication signal through both a second transparent electrode and a second graded index lens after passing the signal through the body, the 16 second lens and second electrode both being disposed proximate the body, wherein the voltage potential is applied across the first and second electrodes. 17. The method for processing an optical signal in a communications link as set forth in claim 9, wherein the optical communication signal comprises a telecommunication signal. 18. The method for processing an optical signal in a communications link as set forth in claim 9, wherein the optical communication signal comprises a local area network (LAN) signal. 19. A method for processing an optical signal in a communications link, comprising the steps of: demultiplexing a first wavelength division multiplexed (WDM) optical communication signal into its constituent optical signals; providing a body of electrochromic material, the body being switchable between at least two color states; applying a voltage potential proximate the body to effect one of the at least two color states of the body; delivering a demultiplexed constituent optical communication signal to an incident surface of the body; passing the demultiplexed constituent optical signal through the body to attenuate the signal; 17 multiplexing the attenuated constituent optical signal into a second WDM optical communication signal; and communicating the second WDM signal for further processing in the communication link. 20. The method for processing an optical signal in a communications link as set forth in claim 19, wherein the incident surface of the body is a planar surface and wherein the demultiplexed constituent optical signal is delivered to the incident planar surface along a propagation path that is substantially peφendicular to the incident planar surface. 21. The method for processing an optical signal in a communications link as set forth in claim 19, wherein the body of electrochromic material is disposed intermediate two transparent electrodes across which the voltage potential is applied. 22. The method for processing an optical signal in a communications link as set forth in claim 19, wherein the body of electrochromic material is disposed intermediate two graded index lenses. 23. The method for processing an optical signal in a communications link as set forth in claim 19, further comprising the step of passing the demultiplexed constituent optical signal through a first graded index lens disposed adjacent the body prior to passing the signal through the body. 24. The method for processing an optical signal in a communications link as set forth in claim 23, further comprising the step of passing the demultiplexed constituent optical signal through a second graded index lens disposed adjacent the body after passing the signal through the body. 25. The method for processing an optical signal in a communications link as set forth in claim 19, further comprising the steps of: passing the demultiplexed constituent optical signal through both a first graded index lens and a first transparent electrode prior to passing the signal through the body, the first lens and first electrode both being disposed proximate the body; and passing the constituent optical signal through both a second transparent electrode and a second graded index lens after passing the signal through the body, the second lens and second electrode both being disposed proximate the body, wherein the body of electrochromic material is disposed intermediate the first and second electrodes and wherein the voltage potential is applied across the first and second electrodes. 26. The method for processing an optical signal in a communications link as set forth in claim 19, wherein the first and second WDM signals comprise telecommunication signals. 27. The method for processing an optical signal in a communications link as set 19 forth in claim 19, wherein the first and second WDM signals comprise local area network (LAN) signals. 28. An attenuating device for use in processing an optical signal in a communications link, comprising: first and second graded index lenses disposed to establish a propagation path for an optical communication signal; and a body of electrochromic material disposed intermediate the first and second graded index lenses to intersect the propagation path, the body being switchable between at least two color states that each permit at least attenuated passage through the body of an optical communication signal traveling along the propagation path. 29. The attenuating device set forth in claim 28, further comprising a first transparent electrode disposed intermediate the first graded index lens and the body of electrochromic material, and further comprising a second transparent electrode disposed intermediate the body of electrochromic material and the second graded index lens. 30. The attenuating device set forth in claim 28, further comprising a first fiber optic cable that communicates optically with the first graded index lens, and further comprising a second fiber optic cable that communicates optically with the second graded index lens. 31. The attenuating device set forth in claim 28, wherein the body of electrochromic 20 material has an incident planar surface that is substantially peφendicular to the propagation path. 32. The attenuating device set forth in claim 28, wherein the first graded index lens is connected to receive a constituent optical signal that is demultiplexed from a wavelength division multiplexed (WDM) optical communication signal. 33. The attenuating device set forth in claim 28, wherein the second graded index lens is connected to communicate an optical signal attenuated by passage through the body to a multiplexer for multiplexing the attenuated signal into a wavelength division multiplexed (WDM) optical communication signal. 34. The attenuating device set forth in claim 28, wherein the first graded index lens is connected to receive a constituent optical signal that is demultiplexed from a first wavelength division multiplexed (WDM) optical communication signal, and wherein the second graded index lens is connected to communicate the attenuated constituent optical signal to a multiplexer for multiplexing the attenuated signal into a second wavelength division multiplexed (WDM) optical communication signal. 35. The attenuating device set forth in claim 28, wherein the device is connected to process an optical telecommunication signal. 36. The attenuating device set forth in claim 28, wherein the device is connected to 21 process an optical local area network (LAN) signal. 37. A device for attenuating an optical telecommunications signal, comprising: a first waveguide connected to receive an optical signal that represents a digital telecommunication signal; a second waveguide disposed to cooperate with the first waveguide to establish a propagation path through the first and second waveguides for the received optical signal; a variable source; and an electrochromic device electrically connected to the variable source and disposed intermediate the first and second waveguides to intersect the propagation path such that an incident planar surface of the electrochromic device is substantially peφendicular to the propagation path, wherein an electrochromic medium of the electrochromic device intersects the propagation path and is responsive to a voltage potential established at the device by the variable source to attenuate the received optical signal as the signal passes through the electrochromic medium. 38. A method for processing an optical signal in a communications link, comprising the steps of: establishing a propagation path along which the optical signal travels; providing an electrochromic region to intersect the propagation path; and attenuating the optical signal by passing the signal through the electrochromic region.

Inventors: GREEN PAUL ELIOT JR, GREEN, PAUL, ELIOT, JR.
Applicants: GREEN PAUL ELIOT JR, TELLABS OPERATIONS INC, GREEN, PAUL, ELIOT, JR., TELLABS OPERATIONS, INC.
Application year: 1999
Main classification: H04B10
IPC codes: H04B10, H04J14