Abstract:
A low sidelobe beam forming method and dual-beam antenna schematic are disclosed, which may preferably be used for 3-sector and 6-sector cellular communication system. Complete antenna combines 2-, 3- or -4 columns dual-beam sub-arrays (modules) with improved beam-forming network (BFN). The modules may be used as part of an array, or as an independent 2-beam antenna. By integrating different types of modules to form a complete array, the present invention provides an improved dual-beam antenna with improved azimuth sidelobe suppression in a wide frequency band of operation, with improved coverage of a desired cellular sector and with less interference being created with other cells. Advantageously, a better cell efficiency is realized with up to 95% of the radiated power being directed in a desired cellular sector.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a 35 U.S.C. §371 national stage application of PCT International Application No. PCT/US2009/006061, filed Nov. 12, 2009, which itself claims priority of Provisional Application U.S. Ser. No. 61/199,840 filed on Nov. 19, 2008 entitled Dual-Beam Antenna Array, the teaching of which are incorporated herein. The disclosure and content of both of which are incorporated herein by reference in their entireties. The above-referenced PCT International Application was published in the English language as International Publication No. WO2010/059786 A1 on May 27, 2010. 
    
    
     FIELD OF THE INVENTION 
     The present invention is generally related to radio communications, and more particularly to multi-beam antennas utilized in cellular communication systems. 
     BACKGROUND OF THE INVENTION 
     Cellular communication systems derive their name from the fact that areas of communication coverage are mapped into cells. Each such cell is provided with one or more antennas configured to provide two-way radio/RF communication with mobile subscribers geographically positioned within that given cell. One or more antennas may serve the cell, where multiple antennas commonly utilized and each are configured to serve a sector of the cell. Typically, these plurality of sector antennas are configured on a tower, with the radiation beam(s) being generated by each antenna directed outwardly to serve the respective cell. 
     In a common 3-sector cellular configuration, each sector antenna usually has a 65° 3 dB azimuth beamwidth (AzBW). In another configuration, 6-sector cells may also be employed to increase system capacity. In such a 6-sector cell configuration, each sector antenna may have a 33° or 45° AzBW as they are the most common for 6-sector applications. However, the use of 6 of these antennas on a tower, where each antenna is typically two times wider than the common 65° AzBW antenna used in 3-sector systems, is not compact, and is more expensive. 
     Dual-beam antennas (or multi-beam antennas) may be used to reduce the number of antennas on the tower. The key of multi-beam antennas is a beamforming network (BFN). A schematic of a prior art dual-beam antenna is shown in  FIG. 1A  and  FIG. 1B . Antenna  11  employs a 2×2 BFN  10  having a 3 dB 90° hybrid coupler shown at  12  and forms both beams A and B in azimuth plane at signal ports  14 . (2×2 BFN means a BFN creating 2 beams by using 2 columns). The two radiator coupling ports  16  are connected to antenna elements also referred to as radiators, and the two ports  14  are coupled to the phase shifting network, which is providing elevation beam tilt (see  FIG. 1B ). The main drawback of this prior art antenna as shown in  FIG. 1C  is that more than 50% of the radiated power is wasted and directed outside of the desired 60° sector for a 6-sector application, and the azimuth beams are too wide (150°@−10 dB level), creating interference with other sectors, as shown in  FIG. 1D . Moreover, the low gain, and the large backlobe (about −11 dB), is not acceptable for modern systems due to high interference generated by one antenna into the unintended cells. Another drawback is vertical polarization is used and no polarization diversity. 
     In other dual-beam prior art solutions, such as shown in U.S. Patent application U.S. 2009/0096702 A1, there is shown a 3 column array, but which array also still generates very high sidelobes, about −9 dB. 
     Therefore, there is a need for an improved dual-beam antenna with improved azimuth sidelobe suppression in a wide frequency band of operation, having improved gain, and which generates less interference with other sectors and better coverage of desired sector. 
     SUMMARY OF INVENTION 
     The present invention achieves technical advantages by integrating different dual-beam antenna modules into an antenna array. The key of these modules (sub-arrays) is an improved beam forming network (BFN). The modules may advantageously be used as part of an array, or as an independent antenna. A combination of 2×2, 2×3 and 2×4 BFNs in a complete array allows optimizing amplitude and phase distribution for both beams. So, by integrating different types of modules to form a complete array, the present invention provides an improved dual-beam antenna with improved azimuth sidelobe suppression in a wide frequency band of operation, with improved coverage of a desired cellular sector and with less interference being created with other cells. Advantageously, a better cell efficiency is realized with up to 95% of the radiated power being directed in a desired sector. The antenna beams&#39; shape is optimized and adjustable, together with a very low sidelobes/backlobes. 
     In one aspect of the present invention, an antenna is achieved by utilizing a M×N BFN, such as a 2×3 BFN for a 3 column array and a 2×4 BFN for a 4 column array, where M≠N. 
     In another aspect of the invention, 2 column, 3 column, and 4 column radiator modules may be created, such as a 2×2, 2×3, and 2×4 modules. Each module can have one or more dual-polarized radiators in a given column. These modules can be used as part of an array, or as an independent antenna. 
     In another aspect of the invention, a combination of 2×2 and 2×3 radiator modules are used to create a dual-beam antenna with about 35 to 55° AzBW and with low sidelobes/backlobes for both beams. 
     In another aspect of the invention, a combination of 2×3 and 2×4 radiator modules are integrated to create a dual-beam antenna with about 25 to 45° AzBW with low sidelobes/backlobes for both beams. 
     In another aspect of the invention, a combination of 2×2, 2×3 and 2×4 radiator modules are utilized to create a dual-beam antenna with about 25 to 45° AzBW with very low sidelobes/backlobes for both beams in azimuth and the elevation plane. 
     In another aspect of the invention, a combination of 2×2 and 2×4 radiator modules can be utilized to create a dual-beam antenna. 
     All antenna configurations can operate in receive or transmit mode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A, 1B, 1C and 1D  shows a conventional dual-beam antenna with a conventional 2×2 BFN; 
         FIG. 2A  shows a 2×3 BFN according to one embodiment of the present invention which forms 2 beams with 3 columns of radiators; 
         FIG. 2B  is a schematic diagram of a 2×4 BFN, which forms 2 beams with 4 columns of radiators, including the associated phase and amplitude distribution for both beams; 
         FIG. 2C  is a schematic diagram of a 2×4 BFN, which forms 2 beams with 4 columns of radiators, and further provided with phase shifters allowing slightly different AzBW between beams and configured for use in cell sector optimization; 
         FIG. 3  illustrates how the BFNs of  FIG. 1A  can be advantageously combined in a dual polarized 2 column antenna module; 
         FIG. 4  shows how the BFN of  FIG. 2A  can be combined in a dual polarized 3 column antenna module; 
         FIG. 5  shows how the BFNs of  FIG. 2B  or  FIG. 2C  can be combined in dual polarized 4 column antenna module; 
         FIG. 6  shows one preferred antenna configuration employing the modular approach for 2 beams each having a 45° AzBW, as well as the amplitude and phase distribution for the beams as shown near the radiators; 
         FIG. 7A  and  FIG. 7B  show the synthesized beam pattern in azimuth and elevation planes utilizing the antenna configuration shown in  FIG. 6 ; 
         FIGS. 8A and 8B  depicts a practical dual-beam antenna configuration when using 2×3 and 2×4 modules; and 
         FIGS. 9-10  show the measured radiation patterns with low sidelobes for the configuration shown in  FIG. 8A  and  FIG. 8B . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 2A , there is shown one preferred embodiment comprising a bidirectional 2×3 BFN at  20  configured to form 2 beams with 3 columns of radiators, where the two beams are formed at signal ports  24 . A 90° hybrid coupler  22  is provided, and may or may not be a 3 dB coupler. Advantageously, by variation of the splitting coefficient of the 90° hybrid coupler  22 , different amplitude distributions of the beams can be obtained for radiator coupling ports  26 : from uniform (1-1-1) to heavy tapered (0.4-1-0.4). With equal splitting (3 dB coupler) 0.7-1-0.7 amplitudes are provided. So, the 2×3 BFN  20  offers a degree of design flexibility, allowing the creation of different beam shapes and sidelobe levels. The 90° hybrid coupler  22  may be a branch line coupler, Lange coupler, or coupled line coupler. The wide band solution for a 180° equal splitter  28  can be a Wilkinson divider with a 180° Shiffman phase shifter. However, other dividers can be used if desired, such as a rat-race 180° coupler or 90° hybrids with additional phase shift. In  FIG. 2A , the amplitude and phase distribution on radiator coupling ports  26  for both beams Beam 1 and Beam 2 are shown to the right. Each of the 3 radiator coupling ports  26  can be connected to one radiator or to a column of radiators, as dipoles, slots, patches etc. Radiators in column can be a vertical line or slightly offset (staggered column). 
       FIG. 2B  is a schematic diagram of a bidirectional 2×4 BFN  30  according to another preferred embodiment of the present invention, which is configured to form 2 beams with 4 columns of radiators and using a standard Butler matrix 38 as one of the components. The 180° equal splitter  34  is the same as the splitter  28  described above. The phase and amplitudes for both beams Beam 1 and Beam 2 are shown in the right hand portion of the figure. Each of 4 radiator coupling ports  40  can be connected to one radiator or to column of radiators, as dipoles, slots, patches etc. Radiators in column can stay in vertical line or to be slightly offset (staggered column). 
       FIG. 2C  is a schematic diagram of another embodiment comprising a bidirectional 2×4 BFN at  50 , which is configured to form 2 beams with 4 columns of radiators. BFN  50  is a modified version of the 2×4 BFN  30  shown in  FIG. 2B , and includes two phase shifters  56  feeding a standard 4×4 Butler Matrix 58. By changing the phase of the phase shifters  56 , a slightly different AzBW between beams can be selected (together with adjustable beam position) for cell sector optimization. One or both phase shifters  56  may be utilized as desired. 
     The improved BFNs  20 ,  30 ,  50  can be used separately (BFN  20  for a 3 column 2-beam antenna and BFN  30 ,  50  for 4 column 2-beam antennas). But the most beneficial way to employ them is the modular approach, i.e. combinations of the BFN modules with different number of columns/different BFNs in the same antenna array, as will be described below. 
       FIG. 3  shows a dual-polarized 2 column antenna module with 2×2 BFN&#39;s generally shown at  70 . 2×2 BFN  10  is the same as shown in  FIG. 1A . This 2×2 antenna module  70  includes a first 2×2 BFN  10  forming beams with −45° polarization, and a second 2×2 BFN  10  forming beams with +45° polarization, as shown. Each column of radiators  76  has at least one dual polarized radiator, for example, a crossed dipole. 
       FIG. 4  shows a dual-polarized 3 column antenna module with 2×3 BFN&#39;s generally shown at  80 . 2×3 BFN  20  is the same as shown in  FIG. 2A . This 2×3 antenna module  80  includes a first 2×3 BFN  20  forming beams with −45° polarization, and a second 2×3 BFN  20  forming beams with +45° polarization, as shown. Each column of radiators  76  has at least one dual polarized radiator, for example, a crossed dipole. 
       FIG. 5  shows a dual-polarized 4 column antenna module with 2×4 BFN&#39;s generally shown at  90 . 2×4 BFN  50  is the same as shown in  FIG. 2C . This 2×4 antenna module  80  includes a first 2×4 BFN  50  forming beams with −45° polarization, and a second 2×4 BFN  50  forming beams with +45° polarization, as shown. Each column of radiators  76  has at least one dual polarized radiator, for example, a crossed dipole. 
     Below, in  FIGS. 6-10 , the new modular method of dual-beam forming will be illustrated for antennas with 45 and 33 deg., as the most desirable for 5-sector and 6-sector applications. 
     Referring now to  FIG. 6 , there is generally shown at  100  a dual polarized antenna array for two beams each with a 45° AzBW. The respective amplitudes and phase for one of the beams is shown near the respective radiators  76 . The antenna configuration  100  is seen to have 3 2×3 modules  80  is and two 2×2 modules  70 . Modules are connected with four vertical dividers  101 ,  102 ,  103 ,  104 , having 4 ports which are related to 2 beams with +45° polarization and 2 beams with −45° polarization), as shown in  FIG. 6 . The horizontal spacing between radiators columns  76  in module  80  is X3, and the horizontal spacing between radiators in module  70  is X2. Preferably, dimension X3 is less than dimension X2, X3&lt;X2. However, in some applications, dimension X3 may equal dimension X2, X3=X2, or even X3&gt;X2, depending on the desired radiation pattern. Usually the spacings X2 and X3 are close to half wavelength (λ/2), and adjustment of the spacings provides adjustment of the resulting AzBW. The splitting coefficient of coupler  22  was selected at 3.5 dB to get low Az sidelobes and high beam cross-over level of 3.5 dB. 
     Referring to  FIG. 7A , there is shown at  110  a simulated azimuth patterns for both of the beams provided by the antenna  100  shown in  FIG. 6 , with X3=X2=0.46λ and 2 crossed dipoles in each column  76 , separated by 0.87λ As shown, each azimuth pattern has an associated sidelobe that is at least −27 dB below the associated main beam with beam cross-over level of −3.5 dB. Advantageously, the present invention is configured to provide a radiation pattern with low sidelobes in both planes. As shown in  FIG. 7B , the low level of upper sidelobes  121  is achieved also in the elevation plane (&lt;−17 dB, which exceeds the industry standard of &lt;−15 dB). As it can be seen in  FIG. 6 , the amplitude distribution and the low sidelobes in both planes are achieved with small amplitude taper loss of 0.37 dB. So, by selection of a number of 2×2 and 2×3 modules, distance X2 and X3 together with the splitting coefficient of coupler  22 , a desirable AzBW together with desirable level of sidelobes is achieved. Vertical dividers  101 , 102 , 103 , 104  can be combined with phase shifters for elevation beam tilting. 
       FIG. 8A  depicts a practical dual-beam antenna configuration for a 33° AzBW, when viewed from the radiation side of the antenna array, which has three (3) 3-column radiator modules  80  and two (2) 4-column modules  90 . Each column  76  has 2 crossed dipoles. Four ports  95  are associated with 2 beams with +45 degree polarization and 2 beams with −45 degree polarization. 
       FIG. 8B  shows antenna  122  when viewing the antenna from the back side, where 2×3 BFN  133  and 2×4 BFN  134  are located together with associated phase shifters/dividers  135 . Phase shifters/dividers  135 , mechanically controlled by rods  96 , provide antenna  130  with independently selectable down tilt for both beams. 
       FIG. 9  is a graph depicting the azimuth dual-beam patterns for the antenna array  122  shown in  FIG. 8A, 8B , measured at 1950 MHz and having 33 deg. AzBW. 
     Referring to  FIG. 10 , there is shown at  140  the dual beam azimuth patterns for the antenna array  122  of  FIG. 8A, 8B , measured in the frequency band 1700-2200 MHZ. As one can see from  FIGS. 9 and 10 , low side lobe level (&lt;20 dB) is achieved in very wide (25%) frequency band. The Elevation pattern has low sidelobes, too (&lt;−18 dB). 
     As can be appreciated in  FIGS. 9 and 10 , up to about 95% of the radiated power for each main beam, Beam 1 and Beam 2, is directed in the desired sector, with only about 5% of the radiated energy being lost in the sidelobes and main beam portions outside the sector, which significantly reduces interference when utilized in a sectored wireless cell. Moreover, the overall physical dimensions of the antenna  122  are significantly reduced from the conventional 6-sector antennas, allowing for a more compact design, and allowing these sector antennas  122  to be conveniently mounted on antenna towers. Three (3) of the antennas  122  (instead of six antennas in a conventional design) may be conveniently configured on an antenna tower to serve the complete cell, with very little interference between cells, and with the majority of the radiated power being directed into the intended sectors of the cell. 
     For instance, the physical dimensions of 2-beam antenna  122  in  FIG. 8A, 8B  are 1.3×0.3 m, the same as dimensions of conventional single beam antenna with 33 deg. AzBW. 
     In other designs based on the modular approach of the present invention, other dual-beam antennas having a different AzBW may be achieved, such as a 25, 35, 45 or 55 degree AzBW, which can be required for different applications. For example, 55 and 45 degree antennas can be used for 4 and 5 sector cellular systems. In each of these configurations, by the combination of the 2×2, 2×3 and 2×4 modules, and the associated spacing X2, X3 and X4 between the radiator columns (as shown in  FIGS. 6 and 8A ), the desired AzBW can be achieved with very low sidelobes and also adjustable beam tilt. Also, the splitting coefficient of coupler  22  provides another degree of freedom for pattern optimization. In the result, the present invention allows to reduce azimuth sidelobes by 10-15 dB in comparison with prior art. 
     Though the invention has been described with respect to a specific preferred embodiment, many variations and modifications will become apparent to those skilled in the art upon reading the present application. For example, the invention can be applicable for radar multi-beam antennas. The intention is therefore that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.