Patent Grant
8244287
It has been known in microwave communications systems to simultaneously transmit two signals having polarizations orthogonal to each other or to selectively switch between signals of orthogonal polarization. In order to provide the ability to change polarization of microwave radios driving a common antenna, and to do so for a given radio independently of the other(s), it is known to use respective independent couplers between the radios and the antenna and to connect the independent couplers to the antenna through an ortho-mode transducer.
It has been known to provide a directional coupler capable of operation with two operational modes, propagating simultaneously or alternatively, such as described in Kurtz, U.S. Pat. No. 2,817,063, entitled “Balanced Slot Directional Coupler.”
In general, microwave couplers comprise coupled transmission lines. Telecommunications systems widely use waveguide, micro-strip, strip-line and coaxial couplers. One example of a microwave coupler comprises a first elongated waveguide section that is, for example, rectangular or circular in cross-section (transverse to the propagating direction of the electromagnetic wave) and that extends longitudinally in the wave's propagation direction, and a second elongated waveguide section that is also rectangular or circular in cross-section. Assuming a rectangular configuration, the rectangular cross-sectional dimensions of the two waveguide sections may be the same, and the sections are parallel with and adjacent to each other so that they share a common wall. The wall usually defines a single elongated through-slot aligned on the wall's longitudinal center line or a plurality of through-slots that are usually aligned on the wall's longitudinal center line and spaced apart about a quarter wavelength of the electromagnetic wave the coupler propagates. An electromagnetic wave in one of the waveguide sections excites the slots and thereby excites a corresponding electromagnetic wave in the other waveguide section. Such couplers are single-mode couplers when the rectangular cross-section, as is usually the case, does not support orthogonal propagation modes. At sufficiently high frequencies, however, a rectangular waveguide can support two modes. If the polarizations of the two modes are orthogonal to each other, the waveguide could be considered a dual-mode waveguide in such use.
The waveguide sections comprising the coupler can be modified, preferably to a square cross-section or to a rectangular cross-section with appropriate dimensions, as should be understood in this art, so that each section is capable of supporting orthogonal modes. If the slot configuration is also modified so that the row of slots (or elongated single slot) is offset from the center wall's longitudinal center line, and a parallel row of slots (or single slot) is added, for example where the two rows (or two single slots) are disposed symmetrically with respect to the center line, the slots can excite both orthogonal modes from one waveguide section so that both modes propagate in the other waveguide section, as described in Kurtz, U.S. Pat. No. 2,817,063. Because each of the two orthogonal modes in the first waveguide section couples to the same orthogonal mode in the second waveguide section, the first waveguide can simultaneously transmit both orthogonal modes and simultaneously couple both modes to the other waveguide without creating an interfering electromagnetic wave. In this sense, the coupler may be said to electrically isolate the two modes.
In microwave line-of-sight communication links, it is known to connect a single antenna, for example a reflector-type antenna, to a first radio unit and a second radio unit, so that either radio, or both simultaneously, may be used with the same antenna. In some such applications, the second radio is a back-up to the first radio, so that the second radio starts transmitting when the first radio fails to maintain radio communication, until the first radio is replaced. The two radios are connected to the antenna by a single mode coupler, as described above, in which: (a) the first radio is coupled to one end of the first waveguide section, (b) the second radio is coupled to one end of the second waveguide section, on the same end of the coupler as the first radio, (c) the antenna is coupled to the opposing end of the first waveguide section, and (d) the opposing end of the second waveguide section is terminated by a microwave-absorbing element to prevent undesirable microwave reflections Impedance matching is provided at the coupler ports at each radio and at the antenna, as should be understood by those skilled in this art. It is known to have radios operating at different polarizations connect to the same antenna via respective single mode couplers, where the single mode couplers connect to the antenna through an ortho-mode transducer.
It is also known to couple more than two radios to the same antenna, also using single mode coupling.
In one embodiment, a communication system has a first radio system having a first microwave radio, a second microwave radio, and a first antenna. A first dual mode coupler has a first dual mode transmission line extending between a first port and third port and a second dual mode transmission line extending between a second port and a microwave absorbing termination. A first microwave radio is coupled to the first port so that the first microwave radio is operable to at least one of outputting microwave signals into and receiving microwave signals from the first dual mode transmission line by the first port. The second microwave radio is coupled to the second port so that the second microwave radio is operable to at least one of outputting microwave signals into and receiving microwave signals from the second transmission line by the second port. The first antenna is coupled to the third port so that the first antenna is operable to at least one of outputting microwave signals into and receiving microwave signals from the first dual mode transmission line by the third port. The first dual mode transmission line is coupled to the second dual mode transmission line so that microwave signals in either of the first dual mode transmission line and the second dual mode transmission line propagates microwave signals in the other of the first dual mode transmission line and the second dual transmission line. A second radio system has a third microwave radio and a second antenna coupled to the third microwave radio. The first radio system and the second radio system are disposed in a geographic area so that one of the first antenna and second antenna radiates microwave radiation to the other of the first antenna and the second antenna.
In another embodiment, a radio and antenna system has a first microwave radio, a second microwave radio, and an antenna, and a dual mode coupler. The dual mode coupler has a first end, a second end opposite the first end, a first side extending between and generally transverse to the first end and the second end, a second side opposite the first side and extending between and generally transverse to the first end and the second end, a first port, a second, a third port, a first dual mode transmission line extending between the first port and the third port, and a second dual mode transmission line extending between the second port and a microwave absorbing termination. The first port is defined at the first side. The third port is defined in the second end. The second port is defined in the second side. The first microwave radio is coupled to the first port so that the first microwave radio is operable to at least one of outputting microwave signals into and receiving microwave signals from the first dual mode transmission line by the first port. The antenna is coupled to the third port so that the antenna is operable to at least one of outputting microwave signals into and receiving microwave signals from the first dual mode transmission line by the third port. The second microwave radio is coupled to the second port so that the second microwave radio is operable to at least one of outputting microwave signals into and receiving microwave signals from the second dual mode transmission line by the second port. The first dual mode transmission line is coupled to the second dual mode transmission line so that microwave signals in either of the first dual mode transmission line and the second dual mode transmission line propagates microwave signals in the other of the first dual mode transmission line and the second dual mode transmission line.
In another embodiment, a dual mode coupler for use in a radio and antenna system having a first microwave radio, a second microwave radio, and an antenna has a first end, a second end opposite the first end, a first side extending between and generally transverse to the first end and the second end, a second side opposite the first side and extending between and generally transverse to the first end and the second end, a first port at which microwave signals are receivable from or conveyable to the first microwave radio, a second port at which microwave signals are receivable from or conveyable to the second microwave radio, a third port at which microwave signals are receivable from or conveyable to the antenna, a first dual mode transmission line extending between the first port and the third port, and a second dual mode transmission line extending between the second port and a microwave absorbing termination. The first port is defined in the first side. The third port is defined in the second end. The second port is defined in the second side. The first dual mode transmission line is coupled to the second dual mode transmission line so that microwave signals in either of the first dual mode transmission line and the second mode transmission line propagates microwave signals in the other of the first dual mode transmission line and the second dual mode transmission line.
In another embodiment, a radio and antenna system has a first microwave radio, a second microwave radio, an antenna, and a dual mode coupler. The dual mode coupler has a first dual mode transmission line extending between a first port and a third port and a second dual mode transmission line extending between a second port and a microwave absorbing termination. The second dual mode transmission line has a first elongated section between the second port and a bend in the second dual mode transmission line and a second elongated section between the bend and the microwave absorbing termination. The first microwave radio is coupled to the port so that the first microwave radio is operable to at least one of outputting microwave signals into and receiving microwave signals from the first dual mode transmission line by the first port. The second microwave radio is coupled to the second port so that the second microwave radio is operable to at least one of outputting microwave signals into and receiving microwave signals from the second dual mode transmission line by the second port. The antenna is coupled to the third port so that the antenna is operable to at least one of outputting microwave signals into and receiving microwave signals from the first dual mode transmission line by the third port. The first dual mode transmission line is coupled to the second dual mode transmission line so that microwave signals in either of the first dual mode transmission line and the second dual mode transmission line propagates microwave signals in the other of the first dual mode transmission line and the second dual mode transmission line.
In another embodiment, a dual mode coupler for use in a radio and antenna system having a first microwave radio, a second microwave radio, and an antenna has a first port at which microwave signals are receivable from or conveyable to the first microwave radio, a second port at which microwave signals are receivable from or conveyable to the second microwave radio, a third port at which microwave signals are receivable from or conveyable to the antenna, a first dual mode transmission line extending between the first port and the third port, and a second dual mode transmission line extending between the second port and a microwave absorbing termination. The second dual mode transmission line comprises a first elongated section between the second port and a bend in the second dual mode transmission line and a second elongated section between the bend and the microwave absorbing termination. The first dual mode transmission line is coupled to the second dual mode transmission line so that microwave signals in either of the first dual mode transmission line and the second dual mode transmission line propagates microwave signals in the other of the first dual mode transmission line and the second dual mode transmission line.
A full and enabling disclosure of the present invention, including the best mode thereof to one of the skill in the art, is set forth more particularly in the remainder of the specification, which makes reference to the accompanying figures, in which:
Repeat use of reference characters in the present specification and drawings is intended to represent same or analogous features or elements of one or more embodiments of the present disclosure.
Reference will now be made in detail to certain embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that modifications and variations can be made in the present invention without departing from the scope or spirit thereof. For instance, features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the present disclosure, including the appended claims.
An exemplary schematic illustration of a line-of-sight point-to-point wireless system 100 is shown in
Radio system 102 includes indoor microwave radio transceiver units 123 and 125 that communicate with outdoor microwave radio transceiver units 127 and 129, respectively, over local transmission lines 133 and 135. Indoor units 123 and 125 communicate with point of presence router 114 over backhaul 110. Router 114, in turn, communicates with the Internet 118 over telecommunications interface 120.
Radio system 104 includes indoor intermediate frequency (IF) transceiver units 124 and 126 that communicate with outdoor microwave radio transceiver units 128 and 130, respectively, over local transmission lines 137 and 139. As should be understood, the indoor units receive data, such as voice, ethernet or video data, and modulate an intermediate signal with such data for output to the outdoor units for transmission. Indoor units 124 and 126 communicate with the point of presence router 116 over backhaul 112. Router 116, in turn, communicates with the Internet 118 over telecommunications interface 122.
Outdoor radio units, for example transceivers, 127, 129, 128 and 130 are mounted on towers or other suitable structures so that antennas 11 and 13 are disposed in geographic area 106 in line-of-sight communication with each other, thereby facilitating communication via microwave radiation 108.
Transceivers 127 and 129 are coupled to antenna 11, and transceivers 128 and 130 are coupled to antenna 13, by respective dual-mode couplers 10, one or more exemplary constructions for which are provided in the discussion below. In the embodiment illustrated in
Router 114 may also switch such packets to indoor unit 125, again over backhaul 110. Indoor unit 125 forwards the packets to outdoor unit 129 over transmission line 135. Transceiver 129 communicates with transceiver 130 via the first coupler 10, antenna 11, microwave radiation 108, antenna 13 and the second coupler 10, at the same frequency at which transceiver 127 operates, but transceiver 129 provides the signal to first coupler 10 at a polarization orthogonal to the polarization at which transceiver 127 provides signals to the coupler, in this instance vertical. Transceiver 130 receives the packets and forwards them to indoor unit 124 via transmission line 139. Indoor unit 124 forwards the packets to router 116, which switches the packets to the desired upstream system via the Internet 118.
Because transceivers 127 and 129 provide signals to the first coupler 10 in respective polarizations that are orthogonal to each other, and because the first coupler 10 couples the orthogonally-polarized signals to antenna 11 with negligible (e.g. less than −40 dB) electrical interference such that the coupler may be considered to couple electromagnetic signals to the antenna in electrical isolation, transceivers 127 and 129 may simultaneously drive antenna 11 through first coupler 10, over the same frequency. Similarly, the second coupler 10 simultaneously couples the orthogonally polarized received signals to transceivers 128 and 130. Transceiver 128 is configured to receive horizontally polarized signals, and so receives the signals that originated from transceiver 127 but not those from transceiver 129. Transceiver 130 is configured to receive vertically polarized signals, and so receives the signals that originated from transceiver 129 but not those from transceiver 127. As should be understood, the systems may operate in the reverse direction, so that radio system 104 transmits to radio system 102, in the same manner Because transceivers 127 and 129 simultaneously transmit (or receive), and transceivers 128 and 130 simultaneously receive (or transmit), on the same frequency but orthogonal polarizations, system 100 may be described as a co-channel, dual-polarized application.
The components of wireless system 100 shown in
The components of wireless system 100 shown in
The components of wireless system 100 shown in
Similarly, transceiver 130 does not transmit (or receive) simultaneously with transceiver 128. Router 116 normally drives transceiver 128 through indoor unit 126. Indoor unit 124 monitors the operation of transceiver 128 and indoor unit 126 via communication with indoor unit 126, as indicated at 141. If indoor unit 124 detects a fault with transceiver 128, indoor unit 126 or link 137, such that transceiver 128 no longer transmits (or receives) through second coupler 10, indoor unit 124 sends a notice signal to router 116 through backhaul 112, and router 116 thereafter drives transceiver 130 via indoor unit 124. Transceiver 130 transmits (or receives) at the same frequency as transceiver 128 and provides/receives signals to and from second coupler 10 with the same polarization as transceiver 128. The system shown in
The components of wireless system 100 shown in
It should be understood that the two radio systems may vary with respect to each other and may, for example, use different dual-mode couplers between the respective transceivers and antennas. As should be understood, radio units may be transmit-only or receive-only radios, or may be dual purpose transceiver radios, depending on the needs of the system. Thus, while the presently described examples refer to transceiver radios, it should be understood that the radio systems could use transmit-only radios or receive-only radios. Moreover, although line-of-sight communication systems are described above with respect to
Referring to
First waveguide section 12, second waveguide section 14 and center wall 16 are secured together by, for example, bolts (not shown) or other suitable means such as brazing. The waveguide sections have generally rectangular outer dimensions, although such dimensions may vary as desired, and are preferably made of metal such as aluminum or a non-metal material, such as a polymer with a highly electrically conductive coating such as silver or copper. It should be understood that the waveguide sections may be made of any material as desired, provided that surfaces in contact with electromagnetic waves are highly electrically conductive.
Dual mode waveguide transmission line 18 extends from a first radio port 22 at a first end 24 of coupler 10 to an antenna port 26 at the coupler body's opposite end 28. In second waveguide section 14, second dual mode transmission line 20 extends from a second radio port 30 to a microwave absorbing element 32 at second coupler body end 28.
In describing microwave transmission lines 18 and 20 as dual-mode, it should be understood that the present disclosure refers to transmission lines over which microwave signals having modes with orthogonal polarization may propagate simultaneously in electrical isolation. As should be understood, microwave transmission lines are usually constructed to propagate a single mode only. The dimensions of such a transmission line are in a specific range compared to the free space wavelength of the transmitted radiation. At high frequencies, however, the waveguide transmission line dimensions may be sufficiently large such that higher modes, which can have orthogonal polarizations, can travel along the same line and, in such instance, could be considered dual mode. Thus, a dual mode transmission line refers to a transmission line with dimensions that operably support two propagating modes that have orthogonal polarizations. It should also be understood that evanescent modes are not considered in determining whether a transmission line is a dual mode line, since such modes decay quickly as a function of distance along the direction of propagation.
The long, straight central portions of transmission lines 18 and 20 open toward, and are aligned in parallel with, each other. They are separated by center wall 16 and, more particularly, a dual row of slots 34 defined in and through the center wall. Wall 16 is preferably a thin metal plate. The plate's thickness may be determined on a case-by-case basis with regard to manufacturability and performance. As should be understood, greater thickness in the common wall of a coupler tends to degrade coupling uniformity but improve isolation and return loss, while very thin walls can be difficult to manufacture. The construction and dimensions of the common wall of the coupler are not peculiar to a dual-mode coupler, as compared to a single-mode coupler, and are therefore not discussed further herein.
Referring also to
Each of the two coupling waveguides has two propagating modes, the polarizations of which are orthogonal to each other. Considering the distribution of electrical and magnetic fields of each mode, it can be shown that the symmetry of the coupling apertures/slots with respect to one of the common wall centerlines (the one parallel to the length of the waveguides in the coupling section) causes the isolation between the two modes. In other words, theoretically, changes in energy of one mode has no, or in practice very small, effect on that of the other mode.
Although the presently-described embodiment uses rectangular slots that are aligned longitudinally in the direction of propagation, it should be understood that slots of other shapes may be employed. In this one preferred embodiment, however, there are two rows of such slots that are symmetrical with respect to the plane perpendicular to plate 16 and including centerline 36, such that each pair of opposing slots across the two rows are also symmetric, about a plane perpendicular to plate 16 and perpendicular to centerline 36, with respect to each adjacent pair of such slots.
Between port 30 and the main, elongated central portion of waveguide transmission line 20, and between port 22 and the elongated central portion of waveguide transmission line 18, transmission lines 20 and 18 define respective curved portions 38 and 40. The curved portions permit ports 22 and 30 to be located on opposite lateral sides of coupler 10 that extend between and generally transverse to ends 24 and 28. This allows microwave transceivers 127 and 129 to be disposed on the sides, rather than behind, the coupler, thereby achieving a more compact system structure. Although shown in phantom for purposes of clarity, it will be understood that transceiver 127 is coupled to port 30, and transceiver 129 is coupled to port 22, and antenna 11 is coupled to port 26, by suitable adapters, as discussed in more detail below. It has been known in the prior art to have bends in waveguide transmission lines using mitered corners. In dual-mode waveguides, such mitered corners have been constructed using multiple surface reflectors implemented as ridged surfaces or a plurality of parallel wires. As shown in the present figures, however, corners 38 and 40 are formed as smooth, continuous curved sections having a radius tuned to provide desirable performance. The smooth surface corners cause the device to have different return losses for the two modes. By tuning the corner radius, however, the waveguide transmission line may preferably be optimized to the lowest return loss over the bandwidth range of the propagating energy. Such optimum radius is typically less than the width of the square waveguide section, but this is not required. A radius as large as possible may be preferred, but results in a less compact device.
Microwave termination 32 is of a conical shape and is made of electromagnetic wave-absorbing material, for example ECCOSORB MF-117, available from Emerson & Cuming Microwave Products, Inc. of Randolph, Mass. The microwave termination may, however, comprise different material and shapes for the same purpose, as should be understood by those skilled in the art. For example, the microwave termination may comprise a stepped taper or a pyramid taper, preferably symmetric with respect to orthogonal planes that are, respectively, parallel and perpendicular to plate 16 and that include the centerline of the waveguide in which the microwave termination is placed.
In another embodiment, microwave transmission line 20 includes a second curved section (similar to curved section 38) just before microwave-absorbing termination 32. The second curved section continues to a 180 degree turn so that microwave termination 32 lies parallel to the elongated section of microwave transmission line 20. This may reduce the length of the coupler, whether or not the overall length of microwave transmission line 20 is reduced, and microwave transmission line 18 is shortened accordingly. This increases the width of coupler 10 but may decrease its length.
Such an embodiment is illustrated in
Dual mode waveguide transmission line 18 extends from a first radio port 22 at a first end 24 of coupler 10 to an antenna port 26 at the coupler body's opposite end 28. In second waveguide section 14, second dual mode transmission line 20 extends from a second radio port 30 to a microwave absorbing element 32. Unlike the embodiment shown in
In the embodiment shown in
It should be understood that the particular dimensions of the waveguide transmission lines may be determined as desired for a given configuration. For example, by methods such as experiment or electromagnetic simulation, the size, shape and distance between slots 34 can be determined to provide required or desired coupling values, isolation between radio ports, isolation between orthogonal propagating modes and polarizations, and impedance matching. Similar methods may also be employed to design the dual mode waveguide corners and the microwave termination. Further, it is possible to design the coupling section so that the coupling value for the two propagating modes in the waveguide, and therefore the microwave coupler, are different. Such configuration can provide high isolation between the two modes and, therefore, polarizations. Although it will therefore be understood that the dimensions and configurations of the coupler may vary, in one preferred embodiment the length of each side of each of the two dual polarization waveguide transmission lines 18 and 20 (if filled with air) is about sixty percent of the free space wavelength of the propagating energy. The center-to-center distance (in the direction of wave propagation) between two adjacent coupling apertures 34 is approximately one quarter of the propagating energy in the dual mode waveguide transmission lines. As noted above, the optimum radius for the dual mode corners is typically less than the size of the pertaining square waveguide.
Microwave coupler 10 maybe fabricated, for example, by CNC milling Middle plate 16, if of sufficiently small thickness, can be fabricated by etching. As should be understood, precision in manufacturing may be desirable to meet electrical specifications where tight tolerances are required.
In operation, transceiver 129 is coupled to port 22 so that transceiver 129 outputs microwave signals into dual polarization transmission line 18. Antenna 11 is coupled to port 26 so that this signal excites the antenna, which thereby radiates microwave radiation 108 (
In the event transceiver 129 fails or is otherwise disabled or disconnected, (assuming a configuration as described above with regard to
More specifically, as described above with regard to
The waves excite slots 34, which in turn excite horizontally and vertically polarized waves in transmission line 18. Because the two rows of slots 34 are symmetric with respect to centerline 36 (
In the receiving function, assume antenna 11 receives signals from antenna 13 carrying both vertically-polarized and horizontally-polarized energy. The antenna inputs signals to transmission line 18 that, in turn, propagates both vertically and horizontally-polarized waves. As discussed above, this excites both vertically and horizontally-polarized waves in transmission line 20. Assume transceiver 127 is coupled to coupler 10 at port 30 to provide vertically-polarized signals to, and receive vertically-polarized signals from, the coupler and that transceiver 129 is coupled to coupler 10 at port 22 to provide horizontally-polarized signals to, and receive horizontally-polarized signals from, the coupler. Transceiver 127 thereby receives the vertically-polarized signal, and transceiver 129 receives the horizontally-polarized signal. Assume also that the vertically-polarized signals arise from vertically-polarized signals input to the other coupler 10 by transceiver 128 and that the horizontally-polarized signals arise from horizontally-polarized radiation in radiation 108 driven by horizontally-polarized signals input to the other coupler 10 by transceiver 130. In this manner, transceivers 128 and 127 communicate with each other, and transceivers 130 and 129 communicate with each other. Antenna 13 and system 104 may receive signals from antenna 11 and system 102 in the same manner
As noted herein, however, these examples are provided by way of explanation only, and the present disclosure encompasses other dual-mode coupler configurations and communications arrangements.
In order to change the polarization of signals from one of the radio/transceiver units 127 and/or 129, the respective radio(s) may be rotated by 90° or, as should be understood, by adding a 90° waveguide twist between the radio unit and the microwave coupler. If the radio unit is capable of inherently transmitting in dual polarizations, such mechanical adjustments need not be made.
Depending on the type of antenna and, in the case of a reflector antenna, its feeding structure, the two different and substantially orthogonal polarizations may be linear (for example, horizontal and vertical) or circular (i.e., right hand circular and left hand circular).
Adaptors (not shown) are used to mount the radio units and the antenna to the dual mode coupler. For example, radio units used in line-of-sight radio links usually have rectangular waveguide ports, whereas reflector-type antennas usually have circular-type waveguide ports. Thus, if dual mode microwave coupler 10 has square waveguide ports, in one embodiment rectangular-to-square waveguide adaptors may be used to couple the radios to the ports, and a circular-to-square waveguide adaptor may be used to mate the antenna port to the square coupler port. To facilitate the rotation of a radio, to thereby rotate polarization, each radio may be coupled to the coupler by a rectangular-to-circular adapter (with the rectangular connection connected to the radio's rectangular port) connected to a circular-to-square adapter (with the square connection at the coupler's square port). The circular-to-circular connection at the adapters facilitates relative rotation between the radio and the coupler. The principal criteria for designing couplers is good impedance matching, i.e., sufficiently low reflection of electromagnetic signals due to mismatch between the transmission lines, and also isolation between the dual modes expected to be transmitted and/or received. The design of waveguide port adaptors should be understood by those skilled in the art and is, therefore, not further discussed herein.
Mechanical provisions such as extension of metal blocks and brackets, may be added to secure the coupler to the radio units and the antenna.
While one or more preferred embodiments of the invention have been described, it should be understood that any and all equivalent realizations of the present invention are included within the scope and spirit thereof. For example, while the above-described microwave transmission lines are filled with air, it should be understood that the transmission lines may be loaded with a dielectric material having a dielectric constant higher than that of air, to thereby reduce coupler size, although at the cost of reducing maximum bandwidth. Thus, it should be understood that the embodiments described are presented by way of example only and are not intended as limitations upon the present invention. It should be understood by those of ordinary skill in this art that the present disclosure is not limited to these embodiments since modifications can be made. Therefore, it is contemplated that any and all such embodiments are included within the scope of the present disclosure.
| Number | Name | Date | Kind |
|---|---|---|---|
| 2817063 | Kurtz | Dec 1957 | A |
| 4795993 | Park et al. | Jan 1989 | A |
| 5003617 | Epsom et al. | Mar 1991 | A |
| 5243357 | Koike et al. | Sep 1993 | A |
| 5329285 | McCandless | Jul 1994 | A |
| 6041283 | Sigmar et al. | Mar 2000 | A |
| 6313714 | Junker et al. | Nov 2001 | B1 |
| 7053849 | Pike et al. | May 2006 | B1 |
| Number | Date | Country |
|---|---|---|
| 2760899 | Sep 1998 | FR |
| WO 02075839 | Sep 2002 | WO |
| WO 03012916 | Feb 2003 | WO |
| WO 2004049497 | Jun 2004 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20110105019 A1 | May 2011 | US |
