Abstract:
Integrating dual antennae into a single rigid assembly guarantees parallel alignment between the antennae and provides higher isolation with lower insertion loss than duplexing methods can achieve through a single antenna. The resulting higher performance at lower cost can benefit two-way communication systems using time division duplexing, frequency division duplexing, or polarization division duplexing; or combinations of these methods.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a non-provisional application claiming the benefit of provisional application No. 60/665,888, filed Mar. 28, 2005, entitled “Aligned Duplex Antennae with High Isolation”. 

   Related subject matter is also disclosed in U.S. provisional patent application 60/637,645, filed Dec. 20, 2004, entitled “High Definition Television Distribution Over Wireless Metropolitan Area Networks”. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not Applicable 
   REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISK APPENDIX 
   Not Applicable 
   TERMINOLOGY 
   By “duplex” is meant a channel which can carry information in both directions. 
   By “diplexer” is meant a device that separates or combines the radio frequency energy in two or more exclusive frequency bands to a single port. 
   By “radome” is meant an antenna cover made of material transparent to microwave radiation. 
   BACKGROUND OF THE INVENTION 
   This invention relates to the use of microwave antennae for duplex communications and radar. 
   Duplex communications (reception and transmission) through a single antenna requires separation of the transmitted and received signals, both for the protection of the sensitive receiver circuitry, and to prevent the transmissions from interfering with reception in (simultaneous) full-duplex applications. 
   When the duplex transmissions are sufficiently different in wavelength, diplexing or filtering can provide ports, each of which couple energy of primarily one channel. The degree to which power of the one wavelength is prevented from coupling to the port that is primarily for a different wavelength is termed its isolation. 
   Polarization can be used to separate receive and transmit signals. 
   In time division duplexing cases, where transmission and reception are not simultaneous, switchable attenuation can be provided between the receiver and the antenna. 
   Combinations of these methods can be used. For instance, separation in frequency and polarization can be employed where a single method is incapable of the desired isolation. 
   Otherwise, two antennae must be used for duplex operation, in which case both antennae must be aligned with the distant terminus of communication. Whereas an antenna connected to a receiver can be aligned by monitoring the received signal level, antennae not connected to receivers are more difficult to align for optimum performance. 
   The present invention integrates multiple antennae as a single rigid assembly guaranteeing alignment between these antennae and providing higher isolation with lower insertion loss than single antenna duplexing methods can achieve. 
   BRIEF SUMMARY OF THE INVENTION 
   One embodiment of the invention provides a rigid body shaped to provide separate dish antennae (i.e., dish reflectors) for collimated parallel microwave beams; with focal points at either end of the rigid body. 
   Very little signal leaks between these antennae; enabling them to be used simultaneously for receiving and transmitting. Even for time division duplexing applications, elimination of the switched attenuators gives the present invention the advantages of higher isolation and lower signal losses compared with current techniques. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       FIG. 1  shows an antenna assembly constructed according to the invention without a radome attached. 
       FIG. 2  shows an antenna assembly constructed according to the invention with a transparent radome attached. 
       FIG. 3  shows an antenna assembly constructed according to the invention with a half-cylinder back attached, a circular base for rotation, and covered by a cylindrical radome. 
       FIG. 4  shows two duplex antenna assemblies mounted back to back in accordance with the invention, a circular base for rotation, and covered by a cylindrical radome. 
       FIG. 5  shows duplex antenna assemblies ganged in accordance with the invention to increase channel capacity. 
       FIG. 6  shows a compact dual polarization antenna assembly constructed according to the invention. 
       FIG. 6A  shows a sectional side view of the antenna unit shown in  FIG. 6 . 
       FIG. 6B  shows a side view of a feedhorn and the location of two signal launchers with respect to the feedhorn. 
       FIG. 6C  shows a view of the feedhorn shown in  FIG. 6B  taken in the direction of the arrows  6 C- 6 C shown in  FIG. 6B . 
       FIG. 7  shows the antenna with rounded corners and flat radome. 
       FIG. 8  shows how two separate antenna units constructed according to the invention may be used in a cell phone backhauling application. 
       FIG. 9  shows a sectional side view of the antenna unit shown in  FIG. 7 . 
       FIG. 10  shows a sectional side view of a feedhorn assembly that may be used with antenna units constructed according to the invention. 
       FIG. 11  shows a disassembled view of the components used to construct the feedhorn assembly shown in  FIG. 10 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A single antenna is often used in a duplex communication system because it naturally aligns the received and transmitted beams. But the design effort, compromised specifications, and component cost to separate these signals can eclipse the antenna they serve. 
   Any portion of a dish reflector works to focus a collimated beam parallel to that original dish&#39;s axis. Although segmented antennae have been used to reduce the size of antenna arrays, the foci in these designs usually cluster in front of the center of the antenna. 
   By increasing the distance, and hence the isolation, between the foci, the present invention combines partial dishes,  10  and  11  in  FIG. 1 , to abut on their rims,  14 , spacing apart their foci (and feedhorns  15  and  16 ) the full length of the assembly. 
   In the preferred embodiment formed from a single piece of metal, the variety of angles and curves in this configuration serve to stiffen the assembly, guaranteeing the alignment of the reflectors. 
   In the preferred embodiment a metal plate,  21 , fastened to the back of the assembly confers more rigidity; and creates a Faraday cage suitable for housing electronic circuitry. With its large surface area, such a housing can dissipate heat well. 
   In the preferred embodiment, the curvature of the reflector is chosen so that its rim,  14 , obstructs the line between the foci located in the feedhorns  15  and  16 . Additional isolation can be achieved with the addition of a reflective plate in the plane at  14  which bisects the line between the foci. 
   In the preferred embodiment, an exponential horn ( 15  and  16 ) with circular cross section and an exit angle of 90 degrees and phase center at the focus illuminates the parabolic reflector ( 10  and  11 ). The projected disk fills most of the reflector. Thus, for example, the unit shown in  FIG. 1  can generate a transmitted beam and receive a separate beam in the following manner: feedhorn  15  transmits a beam to dish  10 , and dish  10  reflects that beam thus forming the transmitted beam; and dish  11  reflects a separate received beam towards feedhorn  16 . Electronics generate a signal that causes feedhorn  15  to transmit its beam towards dish  10 , and also processes the beam received by feedhorn  16 . Similarly, the two dishes  10 ,  11  can be used to transmit two independent beams (in which case the dishes  10 ,  11  reflect separate beams received from the feedhorns  15 ,  16 ), or to receive two independent beams (in which case the dishes  10 ,  11  reflect separate received beams towards feedhorns  15 ,  16 ). 
   Not needing a diplexer or transmit-receive switch, the feedhorns ( 15  and  16 ) can interface directly to the transmitter and receiver electronics at  17  and  18  respectively, avoiding switch and diplexer losses. If the electronics at  17  and  18  do frequency conversions, then lower frequency signals (as opposed to microwaves) can be routed through coaxial cables in the posts  19  and  20  to connectors or to electronic circuitry within the assembly. This can significantly reduce costs compared with routing microwave signals through waveguides. 
   Rather than extend the parabolic reflectors ( 10  and  11 ) to areas where they are not illuminated by the feedhorns ( 15  and  16 ), the preferred embodiment truncates those surfaces at areas  12  and  13 . Although the flat top areas,  29 , shown in  FIG. 5  would suffice, the preferred embodiment shown in  FIGS. 1 and 2  truncates to an inverted parabolic cylinder to limit the depth of drawing if the antenna assembly is to be formed by stamping or casting. 
   A radome in the shape of the cylinder just described can be fitted to the assembly to shield it from the effects of weather. In the preferred embodiment, the rims at  14  are higher than the feedhorns  15  and  16  and their electronics  17  and  18 . Hence the radome,  22  in  FIG. 2 , encloses the assembly,  25 . 
   The radar embodiments shown in  FIGS. 3 and 4  truncate to a right cylinder with circular cross section. 
   Radar 
   Active remote sensing, such as weather radar, is a focused, duplex application for the present invention.  FIG. 3  shows the antenna assembly,  25 , standing upright on a rotary bearing,  24 , with a half-cylinder back,  26 . The envelope of the assembly fitting within a cylinder allows it to rotate while covered by a cylindrical radome,  23 . 
     FIG. 4  shows two antenna assemblies,  25  and  35 , mounted back to back and standing on a rotary bearing,  24 , for use in a radar system. Multiple frequency bands could be scanned by the device. If one antenna assembly,  25 , is inclined relative to the other,  35 , then two cones of sky can be scanned by the device. As above, the envelope of the assembly fitting within a cylinder allows it to rotate while covered by a cylindrical radome,  23 . 
   Minimal Area 
   Rent on antenna towers being proportional to an antenna&#39;s silhouette area,  FIG. 7  shows a duplex antenna,  25 , with its four corners rounded to reduce its area. The unit shown in  FIG. 7  is formed from two sheets of metal that are fixed together. A first sheet is stamped to form the two dishes, and a second sheet  31  is stamped to form an enclosure. The enclosure defines upright walls that surround the dishes as well as a flat base, or backplate. The feedhorns are mounted on the enclosure (e.g., instead of on posts). The unit can be covered with a flat radome,  32 , sealing the duplex antenna unit. 
   The units shown in  FIGS. 1 and 7  each include a flat base (e.g., shown in  FIG. 1  at flat metal plate  21 ). The flat base advantageously simplifies mounting the antenna unit on a tower. Prior art dish antennas are generally mounted at the dish&#39;s center to the tower, which disadvantageously produces only a small area of contact between the dish and the tower and also allows wind to stress the antenna mounting. In contrast to the prior art, the flat base provided by antenna units constructed according to the invention significantly increases the area of contact between the antenna unit and the tower. 
   High Capacity 
   High capacity backhauling applications may require operating transmitters and receivers in multiple frequency bands. Where the expense or signal losses of diplexers are unacceptable, duplex antennae can be ganged as shown in  FIG. 5 . High isolation can be achieved by putting reflective baffles,  27 , between adjacent duplex units,  28  and  38 . High isolation is usually necessary only between transmit and receive feedhorns. Thoughtful organization, such as putting all the transmitters on one side and all the receivers on the other, can eliminate most need for baffles. 
     FIG. 8  shows an example of how two antenna units  105 ,  205  constructed according to the invention may be used in a cellular telephone backhauling application. Each of antenna units  105 ,  205  is similar to the units shown in  FIGS. 1 and 2 . Specifically, unit  105  defines two partial dish reflectors  110 ,  111  separated by a rim  114 . A feedhorn  115  is disposed such that the focus of dish  110  is within feedhorn  115 , and a feedhorn  116  is disposed such that the focus of dish  111  is located within feedhorn  116  (the focus of each dish may preferably be located within, or just behind, the dish&#39;s associated feedhorn at the point at which the impedance of the feedhorn matches the impendence of free space). Unit  105  is enclosed within a protective radome  122 . Similarly, unit  205  defines two partial dish reflectors  210 ,  211  separated by a rim  214 . A feedhorn  215  is disposed such that the focus of dish  210  is located within feedhorn  215 , and a feedhorn  216  is disposed such that the focus of dish  211  is located within feedhorn  216 . Unit  205  is enclosed within a protective radome  222 . Unit  105  is mounted on a cell tower  101 , whereas unit  205  is mounted on a tower at a telephone central office  201 . 
   In unit  105 , reflector  110  is used to generate a transmitted beam  150 . In unit  205 , reflector  210  is used to generate a transmitted beam  250 . In unit  105 , reflector  111  is used to receive the beam  250  (generated by unit  205 ). In unit  205 , reflector  211  is used to receive the beam  150  (generated by unit  105 ). 
   In operation, the cell tower  101  and the central office  201  communicate (via antenna units  105 ,  205 ) to enable cell phone use. At any given time, cell tower  101  is in communication with a plurality of cell phones. Radio equipment located in the equipment container (or “hut”) under tower  101  collects information transmitted by that plurality of cell phones and transmits it to central office  201  via transmitted beam  150 . Similarly, information to be transmitted to the plurality of cell phones is transmitted from radio equipment in the central office  201  to tower  101  via beam  250 . Equipment in the hut of tower  101  uses the information contained in beam  250  to generate the signal that it broadcasts to the plurality of cell phones. 
   In one type of prior art backhauling application, the cell tower included a single dish antenna that was used (a) to generate a beam that was transmitted to the central office and (b) to receive a beam that was transmitted from the central office (similarly, the central office included a single dish antenna that was used to (a) generate a beam that was transmitted to the cell tower and (b) to receive a beam that was transmitted from the cell tower). Such systems suffered because they had to use a single dish antenna for both transmitted and received beams. Such systems used either time division or frequency division multiplexing. In such time division multiplexing systems, only one location (e.g., the central office or the cell tower) can transmit at a time limiting aggregate capacity. Also, such frequency division multiplexing systems use larger bandwidth and are therefore inherently more expensive. 
   In another type of prior art backhauling application, the cell tower included two separate dish antennae (one for transmit and one for receive) and the central office also included two separate dish antennae (again, one for transmit and one for receive). Such systems suffered because they required two pairs of antennae to be separately aligned (i.e., ( 1 ) cell tower transmit dish and central office receive dish and ( 2 ) central office transmit dish and cell tower receive dish). 
   In contrast to the prior art, in the system shown in  FIG. 8 , no single dish is used for both transmit and receive, only a single alignment is performed, and neither time division nor frequency division multiplexing is required. Since dishes  110 ,  111  of unit  105  are formed in a single rigid body, they can be constructed so as to insure that the beams transmitted by dish  110  and received by dish  111  are parallel. Similarly, since dishes  210 ,  211  of unit  205  are formed in a single rigid body, they can be constructed so as to insure that the beams transmitted by dish  210  and received by dish  211  are parallel. As will be appreciated, the direction of a beam transmitted by a dish (e.g., dish  110 ) is defined by the axis, or ray, along which the beam has maximum intensity, and similarly, the direction of a beam received by a dish (e.g., dish  111 ) is defined by the axis, or ray, to which the feedhorn has maximum sensitivity. The transmitted and received beams (e.g., by dishes  110 ,  111 ) are parallel if the axes associated with those beams are parallel. The transmit and receive axes for units  105 ,  205  are illustrated in  FIG. 8 . Also, the shape of the dishes  110 ,  111 ,  210 ,  211  insure that the beams transmitted or received by them are highly focused. 
   Since each of units  105 ,  205  transmit and receive parallel beams, once units  105 ,  205  are aligned to insure proper reception of one of the beams (e.g.,  150 ), the units  105 ,  205  will have automatically been aligned to also insure proper reception of the other beam (e.g.,  250 ). 
   Also, since each of the units  105 ,  205  provides a high degree of isolation between the two beams  150 ,  250 , these two beams may use the same frequency. Thus, frequency division multiplexing need not be used. Also, since two independent beams  150 ,  250  are transmitted simultaneously, time division multiplexing is also unnecessary. 
   The beams  150 ,  250  in  FIG. 8  are shown as diverging beams. It will be appreciated that the angle of divergence shown in  FIG. 8  is greater than the actual angle of divergence for beams transmitted by units  105 ,  205 . However, the beam transmitted by unit  105  will generally have diverged enough by the time it reaches unit  205  so as to completely encompass unit  205  (as shown generally in  FIG. 8 ). Similarly, the beam transmitted by unit  205  will generally have diverged enough by the time it reaches unit  105  so as to completely encompass unit  105  (as shown generally in  FIG. 8 ). The amount of divergence experienced by the beam by the time the beam reaches the next antenna unit is of course a function of the distance between the two units  105 ,  205 . The maximum distance achievable between units  105 ,  205  is a function of several parameters such as dish size, transmit power, and frequency. About one to three kilometers is a typical distance between units  105 ,  205 . 
   It also will be appreciated that use of units  105 ,  205  also simplifies radio equipment connected to the antenna units. Such radio equipment generally includes (a) an “indoor unit”, which is located inside a building, such as the cell tower hut, and is therefore shielded from the outside environment, and (b) an “outdoor unit”, which is located very near the feedhorn and is therefore at least partly exposed to the outside environment. As an example of the simplification provided by the invention, prior art outdoor units designed for use with time division multiplexing schemes included a receiver protect switch that isolated the outdoor unit&#39;s receive circuitry when the outdoor unit&#39;s transmitter was operating. Similarly, such prior art outdoor units also included a transmit power switch which connected the outdoor unit&#39;s transmitter to the antenna during only defined transmit time intervals. Outdoor unit&#39;s designed for use with antenna units constructed according to the invention need neither the receiver protect switch nor the transmit power switch (i.e., since the radio&#39;s transmitter is continuously coupled to a transmit dish, such as dish  110 , and since the radio&#39;s receiver is continuously coupled to a receive dish, such as dish  111 ). Also, since the transmitter portion of such an outdoor unit couples (via a feedhorn) to one dish and the receiver portion of such an outdoor unit couples (via another feedhorn) to a different dish, such outdoor units constructed in accordance with the invention can simultaneously transmit and receive at the same frequency. 
     FIG. 9  shows a sectional side view of the antenna unit shown in  FIG. 7 . As shown, the outdoor unit, which communicates with the feedhorns, can be located in interior space between the enclosure and the reflectors. It may be advantageous to coat interior surfaces with radio frequency absorbent foam, while preferably the exterior surfaces of the dishes are left bare. When mounting the antenna unit (e.g., on a tower), it may also be advantageous to orient the unit so that the dish used for transmit is above the dish used for receive. 
   Table 1 below shows physical dimensions for three example embodiments of antenna units constructed according to the invention (such as the ones shown in  FIGS. 1 ,  7 , and  9 ). The “Bounds” and “Area” are the length and width, and area, respectively, of a unit that includes two dishes (such as dishes  10 ,  11  of  FIG. 1 ). The “Area/Dish” is the area of a single dish (e.g.,  10  of  FIG. 1 ) of the unit. The table shows the gain and beam width (or beam angle) associated with each of the three example embodiments for five different operating frequencies. It will be appreciated that each dish (e.g., dish  10  of  FIG. 1 ) is an offset antenna. As is well known in the art of offset antennas, the surface of the dish ideally tracks a theoretical surface that is defined by rotating a parabola about an axis of rotation. The parabola of the dish is also ideally matched to the curvature of the feedhorn. The axis of rotation extends from the focal point to the point on the theoretical surface that is closest to the focal point. Again, as is well known in the art of offset antennas, the actual dish only covers part of that theoretical surface. The portions of the theoretical surface that are omitted from the dish are selected so as to offset the transmit (or receive) axis from the axis of rotation (so as to prevent the feedhorn from obstructing the beam) and to maximize the amount of beam energy that can be transmitted between the feedhorn and dish. In antenna units constructed according to the invention, the focal length of the parabola (which defines the ideal location of the feedhorn) is preferably selected so that the rims of the dishes obstruct a straight line between the two feedhorns (e.g., as shown in  FIG. 1 , the rim  14  obstructs a straight line between feedhorns  15 ,  16 ). 
   
     
       
             
             
           
             
             
             
             
           
             
             
           
             
             
             
             
           
             
             
           
             
             
             
             
           
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
           
         
             
                 
               TABLE 1 
             
           
           
             
                 
                 
             
             
                 
               Bounds 
             
           
        
         
             
                 
               64. cm * 32. cm 
               96. cm * 48. cm 
               127. cm * 64. cm 
             
           
        
         
             
                 
               Area 
             
           
        
         
             
                 
               1810. cm 2   
               4072. cm 2   
               7238. cm 2   
             
           
        
         
             
                 
               Area/Dish 
             
           
        
         
             
                 
               796. cm 2   
               1791. cm 2   
               3184. cm 2   
             
           
        
         
             
                 
                 
               Beam 
                 
               Beam 
                 
               Beam 
             
             
                 
               Gain 
               Width 
               Gain 
               Width 
               Gain 
               Width 
             
             
                 
                 
             
           
        
         
             
               15. GHz: 
               32.7 decibels 
               3.60 degrees 
               36.3 decibels 
               2.40 degrees 
               38.8 decibels 
               1.80 degrees 
             
             
               18. GHz: 
               34.3 decibels 
               3.00 degrees 
               37.8 decibels 
               2.00 degrees 
               40.3 decibels 
               1.50 degrees 
             
             
               23. GHz: 
               36.5 decibels 
               2.36 degrees 
               40.0 decibels 
               1.56 degrees 
               42.5 decibels 
               1.17 degrees 
             
             
               26. GHz: 
               37.5 decibels 
               2.08 degrees 
               41.0 decibels 
               1.38 degrees 
               43.5 decibels 
               1.04 degrees 
             
             
               38. GHz: 
               40.8 decibels 
               1.42 degrees 
               44.3 decibels 
               .095 degrees 
               46.8 decibels 
               0.71 degrees 
             
             
                 
             
           
        
       
     
   
     FIG. 10  shows a cross section of an example feedhorn assembly that can be used with the invention.  FIG. 11  shows how the feedhorn assembly shown in  FIG. 10  may be constructed from two cast aluminum components, A, B. 
   Another advantage of the present invention is that the feedhorns need not be disposed in the center of the dish as is typically done in the prior art. The location of the feedhorns shown e.g., in  FIG. 1  prevents them from obstructing the beams transmitted and received by the dishes of the unit. 
   Polarization 
   Perpendicular polarizations permit overlapped dual antennae which are more compact yet have large separation between the foci. In  FIG. 6 , one parabolic dish reflector  601  is formed from rigid wires running along the length of the base; and another parabolic dish reflector  602  is formed from rigid wires running along the width of the base. Each dish  601 ,  602  reflects a beam having a polarization that is orthogonal to the polarization of the beam reflected by the other dish. The supports for the wires are not shown, but both dishes  601 ,  602  are integrated into a single rigid body. The simplest way to integrate each wire into the rigid body is to bend each wire at the perimeter of the dish shape such that each wire includes two downwardly extending ends (not shown) and by attaching both (downwardly extending) ends of each wire (e.g., by welding or adhesives) to the flat base. A feedhorn  603  is located at the focal point of dish  601 , and a feedhorn  604  is located at the focal point of dish  602 .  FIG. 6A  shows a side view of the unit shown in  FIG. 6 . As shown in  FIG. 6A , the two dishes  601 ,  602  intersect and overlap with one another thus reducing the spacing between the two feedhorns  603 ,  604 . Although spacing between feedhorns  603 ,  604  is reduced (e.g., as compared with the feedhorns shown in  FIG. 1 ), feedhorn  603  is outside of the conical beam reflected by dish  602 , and similarly feedhorn  604  is outside of the conical beam reflected by dish  601 . 
   It will be appreciated that the arrangement shown in  FIGS. 6 and 6A  allows a doubling of the data to be transmitted or received by a dish of any given size. That is, a dish of diameter D is generally used to transmit or receive a single beam. However, in the arrangement shown in  FIGS. 6 and 6A , the distance between the feedhorns is only slightly larger than D, and yet the arrangement can be used to handle two independent beams. That is, the arrangement shown in  FIGS. 6 and 6A  can (a) transmit two independent beams; (b) receive two independent beams; or (c) transmit one beam and receive a beam that is independent from the transmitted beam. 
   With reference to  FIG. 1 , another way to advantageously use polarization is to provide two signal launchers in feedhorn  15  and two signal receivers in the other feedhorn  16 . The two signal launchers are configured so as to produce beams aimed at dish  10  with orthogonal polarizations, and similarly, the two signal receivers are configured so as to receive beams with orthogonal polarizations from dish  11 .  FIGS. 6B and 6C  show an example of how the signal launchers (or the signal receivers) can be configured. As shown, two wires  71 ,  72 , are disposed orthogonally to one another behind the rear opening  70  of the feedhorn. It will be understood that each of these wires is connected to circuitry in outdoor unit, and that each of these wires can function as a signal launcher (to generate a signal that is transmitted from the feedhorn to the dish) or as a signal receiver (to receive a signal from the dish). It will be appreciated that equipping feedhorn  15  with two signal launchers and feedhorn  16  with two signal receivers allows the data transmitted and received by the unit shown in  FIG. 1  to be doubled (i.e., since each dish either transmits or receives two independent beams at orthogonal polarization angles). 
   Applications 
   Reducing the cost of customer-premises equipment is a requirement for providing television services to consumers using the Local Multipoint Distribution Service (LMDS) bands. Provisional Patent Application U.S. 60/637,654, “High Definition Television Distribution over Wireless Metropolitan Area Networks”, filed Dec. 20, 2004 by Jaffer, et al describes such a point-to-multipoint (PMP) system which would benefit from the cost reductions resulting from use of the present invention. 
   The present invention can reduce the cost of fixed wireless duplex point-to-point (PTP) links. PMP and PTP applications include broadband Internet connections, mobile cellular infrastructure, cellular telephone backhaul, CATV backhaul, CATV and carrier last-mile access, fixed network connections, private network connections, disaster recovery, and public transportation and utility connections. 
   Other changes, embodiments or substitutions made by one skilled in the art according to the present invention is considered within the scope of the present invention which is not to be limited by the claims which follow.