Abstract:
The present invention relates to a system for changing the radiation pattern shape of an antenna array  83; 88  during electrical tilting. The antenna array  83; 88  has multiple antenna elements  84,  and the system comprises a phase-shifting device  10; 20; 40; 85  provided with a primary port  11  configured to receive a transmit signal, and multiple secondary ports  12   1 - 12   4   ; 12  configured to provide phase shifted output signals to each antenna element  84.  The system further comprises a phase-taper device  20; 40; 85; 87  that changes phase taper over the antenna elements, and thus the beam shape, with tilt angle θ. The invention is adapted for use in down-link as well as up-link within a wireless communication system.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a system for adapting the beam-shape of an antenna in a wireless communication network. 
       BACKGROUND 
       [0002]    Variable beam tilt is an important tool for optimizing radio access networks for cellular telephony and data communications. By varying the main beam pointing direction of the base station antenna, both interference environment and cell coverage area can be controlled. 
         [0003]    Variable electrical beam tilt is conventionally performed by adding a variable linear phase shift to the excitation of the antenna elements, or groups of elements, by means of some phase-shifting device. For cost reasons, this phase-shifting device should be as simple and contain as few components as possible. It is therefore often realized using some kinds of variable delay lines. In the description, the terms “linear” and “non-linear” should be understood to refer to relative phase over multiple secondary ports of a multiport phase shifting network, and not the time or phase behaviour of a port in itself. 
         [0004]    Conventional multi-port phase shifters, with one primary port and a number N (N&gt;1) secondary ports, are implemented with linear progressive variable phase taper over the secondary ports. In addition to the linear progressive phase taper, fixed amplitude and phase tapers are often used as a means for generating a tapered nominal secondary port distribution. 
         [0005]      FIGS. 1   a  and  1   b  illustrate a conventional phase shifter  10 , with one primary port  11 , and the phase shifter generates in down-link linear progressive phase shifts over four secondary ports  12   1 - 12   4 . A variable-angle “delay board”  13  has multiple trombone lines  14 , one for each secondary port  12   1 - 12   4 . The trombones lines  14  are arranged at linearly progressive radii. By a proper choice of junction configurations, line lengths, and line impedance values, the nominal phase and amplitude taper of the phase shifter can be controlled, for example to achieve uniform phase over the secondary ports as indicated by “0” in  FIG. 1   a . By changing the delay line lengths (i.e. the length of the trombone lines  14 ), in this case by rotation of the delay board  13  relative to a fixed board  15 , the secondary ports  12   1 - 12   4  experience linear progressive phase shifts as indicated in  FIG. 1   b . In up-link, the secondary ports  12   1 - 12   4  receive signals from an antenna (not shown) which are combined within the phase shifter to a common receive signal at the primary port  11 . 
         [0006]    The use of non-linear phase-shifting devices for controlling electrical down tilt has been contemplated, such as mentioned in U.S. Pat. No. 5,798,675, by Drach, U.S. Pat. No. 5,801,600, by Butland et al. 
         [0007]    A system for tilt-dependent beam shaping using conventional linear phase shifters is disclosed in JP 2004 229220. The system has different beam width depending on the tilt angle, but this is achieved by a tilt angle control section ( 41 ) in combination with a vertical beam width control section ( 42 ) in the base station controller ( 4 ), see FIG. 6 in JP 2004 229220. 
         [0008]    Traditionally, base station antennas have had a variable beam tilt range of approximately one beamwidth. This together with the fact that most mobile connections today are circuit switched voice with a fixed requirement on bit-rates, has not triggered any interest in improving the Signal-to-interference+noise ratio (SINR) close to the antenna. Normally it is good enough. 
         [0009]    For particular cell configurations, e.g. highly placed antennas in combination with small cells, the need for using antennas with large beam tilt is greater. For antennas with conventional narrow elevation beam radiation patterns, the large beam tilt causes users close to the base station to experience a lower path gain than users close to the cell border, since the difference in path loss for the near and far users is smaller than the difference in directive antenna gain. For packet-based data communication this is not optimal usage of the available power. Therefore, for antennas with large beam tilt, some degree of radiation pattern null-fill below the main beam, or even some cosec-like beam-shaping is desirable. 
         [0010]    In large cells, on the other hand, when no or small beam tilt is employed, the antenna pattern should be optimized for maximum peak gain. The path gain for the users at the cell border will anyway be smaller than for users closer to the base station because the path loss varies rapidly with vertical observation angle in the case of large cells and observation angles close to the horizon. 
       SUMMARY 
       [0011]    An object with the present invention is to provide a system that allows a radiation pattern of an antenna to be optimized both for high maximum gain at small tilt angles, and high degree of null filling below the main beam at large tilt angles. 
         [0012]    A solution to the object is achieved by providing a system for changing the beam shape of an antenna, preferably having multiple antenna elements arranged in an array, in dependency of a tilt angle. Electric tilting is achieved by including a phase-shifting device that will provide phase shifts over secondary ports from the phase-shifting device. A phase-taper device provides changed phase taper over the antenna elements with tilt angle. 
         [0013]    An advantage with the present invention is that a single antenna may be used in an adaptive system, to fulfil the need for increasing the quality of a communication link and thus increase the bit rate associated with one or more simultaneous users, by maintaining an optimal antenna pattern, which depends on the distance to the base station. 
         [0014]    Further objects and advantages will become apparent for a skilled person from the detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIGS. 1   a  and  1   b  show a linear phase shifter. 
           [0016]      FIGS. 2   a  and  2   b  show a first embodiment of a non-linear phase shifter. 
           [0017]      FIGS. 3   a  and  3   b  show diagrams illustrating phase shifts from the linear and non-linear phase shifters. 
           [0018]      FIG. 4  shows a second embodiment of a non-linear phase shifter. 
           [0019]      FIG. 5  shows antenna element excitation at 0° beam tilt. 
           [0020]      FIG. 6  shows antenna element excitation at 9° beam tilt. 
           [0021]      FIGS. 7   a - 7   d  show elevation radiation patterns utilizing the present invention. 
           [0022]      FIG. 8  shows a wireless telecommunication network having base stations including the present invention. 
           [0023]      FIG. 9  schematically illustrates the tilt dependent beam shape according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    A base station, including an antenna with multiple antenna elements, is arranged within a cell, where the characteristics of the antenna determine the size of the cell and the cell coverage area all else being equal. To accomplish the same signal strength in the entire cell, independent of the distance to the base station, the antenna gain G(θ) divided by the path loss L(θ) should be constant in the cell, as a function of observation angle θ: 
         [0000]    
       
         
           
             
               
                 G 
                  
                 
                   ( 
                   θ 
                   ) 
                 
               
               
                 L 
                  
                 
                   ( 
                   θ 
                   ) 
                 
               
             
             = 
             
               C 
               = 
               
                 const 
                 . 
               
             
           
         
       
     
         [0025]    However, the constant C changes with cell configuration, i.e. antenna installation height and cell size, which in turn means that the optimal antenna radiation pattern changes with beam tilt angle, as illustrated in  FIGS. 7   b - 7   d , lines  71 . The tilt dependent radiation pattern can be accomplished by changing the phase taper over the antenna with tilt-angle, e.g. by providing a non-linear phase shifter as described in connection with  FIGS. 2   a ,  2   b ,  3   b  and  4 . The non-linear phase shifter facilitates different phase tapers for different beam tilt angles, and thus will provide tilt-dependent beam shape of the antenna. 
         [0026]    The terms “phase shift” and “time delay” are used interchangeably in the following description and it should be understood that these terms refer to equivalent properties in the present context, except if otherwise noted. 
         [0027]    An essential part of the invention is to provide non-linear phase taper over the secondary ports of a phase shifter network. A method for achieving this is to use a multi-secondary port true time delay network in which the relative delay line lengths are, in general, non-linearly progressive. A true time delay network generates frequency-dependent phase shifts, a property which makes it particularly suitable for antenna applications, such as beam-steering. 
         [0028]    The principle idea of a first embodiment of a non-linear phase shifter  20 , in down-link, is illustrated in  FIGS. 2   a  and  2   b  using a true time delay network, similar to the one illustrated in  FIGS. 1   a  and  1   b . The key property of the delay network (and the method as such) is to provide non-linear relative time delays over the secondary ports, by arranging trombone lines  24  (in this particular embodiment) in a non-periodic fashion on a delay board  23 . By a proper choice of junction configurations, line lengths, and line impedance values, the nominal phase and amplitude taper of the true time delay network with non-linear delay dependence can be controlled, for example to achieve uniform phase over the secondary ports as indicated by “0” at the secondary ports  12   1 - 12   4  in  FIG. 2   a . In contrast with the true time delay network in  FIG. 1 , changes in the delay line lengths by rotation of the delay board relative to a fixed board  25  produces non-linear progressive time delays (and, hence, phase shifts) over the secondary ports  12   1 - 12   4 , as indicated by “φ 1 ”, “φ 2 ”, “φ 3 ”, and “φ 4 ” in  FIG. 2   b . In up-link, the secondary ports  12   1 - 12   4  of the phase shifter  20  receive signals from an antenna (not shown) which are non-linearly time-delayed and combined within the phase shifter to a common receive signal at the primary port  11 . 
         [0029]    As a non-limiting example, the phase-shifts from a linear and a non-linear true time delay network in down-link are compared in  FIGS. 3   a  and  3   b  for different rotations (see legend) of the delay board  13  and  23 , respectively. In  FIG. 3   a , the phase advance (relative phase) over the secondary ports  12   1 - 12   4  is linear with delay board  13  rotation, which manifests itself as straight lines  30 ,  31 ,  32  and  33  for a given board rotation. This means that for any given delay board rotation, the relative phase values (between secondary port n and port  1 ) are 
         [0000]      Δφ n =( n− 1)Δφ=( n− 1) kα,    
         [0000]    where n is the secondary port number, α is the board rotation angle, and k is a constant that depends on implementation aspects, for example wave number of transmission lines and radial separation of the trombones  14 . 
         [0030]    The non-linear phase advance (relative phase) over the secondary ports  12   1 - 12   4  of a non-linear true time delay network is illustrated in  FIG. 3   b . In  FIG. 3   b , the phase advance (relative phase) over the secondary ports  12   1 - 12   4  is non-linear when rotating the delay board  23 , which manifests itself as one straight line  35  for 0° rotation and three non-straight lines  36 ,  37  and  38  for a given board rotation ≠0°. Thus, the relative phase values are not identical, i.e., 
         [0000]      φ n −φ n−1 ≠φ n+1 −φ n , for at least one n, nε{ 2 , N− 1} 
         [0000]    wherein N is the number of delay branches. In  FIG. 3   b , the phase of delay branch  3  varies faster than twice that of branch  2  when the board angle changes. 
         [0031]      FIG. 4  shows a second embodiment of a non-linear phase shifter  40 . This delay line network is based on translation (rather than rotation) of the delay board  43  relative a fixed board  45 . The delay network trombone lines  44  are shown with equal lengths, but they could also have different lengths (both the lines on the delay board  43  and the lines on the fixed board  45 ). 
         [0032]      FIG. 5  shows an element excitation of a 15 element linear antenna array, optimized for maximum gain and a suppression of the upper sidelobes to −20 dB. This element excitation produces the radiation pattern in  FIG. 7   a , i.e. 0° beam tilt. In prior art techniques, linearly progressive phase is added to the phase taper shown in  FIG. 5  to achieve different tilt angles, θ tilt . 
         [0033]      FIG. 6  shows the element excitation for 9° beam tilt, where the amplitude taper is the same as for 0° beam tilt, but the phase taper has been optimized for null-filling, in accordance with the present invention. This excitation produces the radiation pattern with 9° beam tilt in  FIG. 7   d.    
         [0034]    For beam tilt angles between 0° and 9°, the phase excitation is found by a linear interpolation of the phase excitations at 0° and 9°. Some of these radiation patterns  70  are shown in  FIGS. 7   b  and  7   c , with the beam tilt changing 3° for each subplot. For comparison, the relative path loss  71 , normalized at beam peak, is shown in the same plots. The relative path loss changes with beam tilt angle θ tilt . 
         [0035]    The invention is not limited to the example with constant cell illumination described above, but is applicable in all cases where it is desirable, for one reason or another, to have a radiation pattern that changes with beam tilt angle. Furthermore, the invention is not limited to linear antenna arrays, but may also be implemented in a base station having a non-linear antenna array. 
         [0036]    The present invention allows the antenna pattern to be optimized both for high maximum gain at small tilt angles, and for good coverage (high degree of null filling) close to the antenna at large tilt angles θ tilt . 
         [0037]      FIG. 8  shows a wireless telecommunication system  80 , exemplified using GSM standard, including a first base station BS 1 . The first base station BS 1  is connected via a first base station controller BSC 1  to a core network  81  of the telecommunication system  80 . A uniform linear antenna array  83  comprises in this embodiment six antenna elements  84 . Secondary ports  12  of a non-linear phase shifter  85  is connected to each antenna element  84  of the uniform linear antenna array  83 , and a primary port  11  of the phase shifter  85  is connected to the first base station BS 1 . The first base station controller BSC 1  controls the variable beam tilt by changing the position of a non-linear delay board, as previously described in connection with  FIGS. 2   a ,  2   b  and  4 , and thereby altering the beam shape of a beam from the uniform linear antenna array  83 . 
         [0038]    The telecommunication system  80  also includes a second base station BS 2 . The second base station BS 2  is connected via a second base station controller BSC 2  to the core network  81 . A non-uniform linear antenna array  88  comprises in this embodiment four antenna elements  84 , not necessarily cross polarized as illustrated. Secondary ports  12  of a linear phase shifter  10  (prior art) are connected, via a phase-taper device  87  that changes the phase taper over the antenna elements with tilt angle θ tilt , to each antenna elements  84  of the non-linear antenna array  88 . A primary port  11  of the phase shifter  10  is connected to the second base station BS 2 . The second base station controller BSC 2  controls the variable beam tilt by changing the position of a linear delay board, as previously described in connection with  FIGS. 1   a  and  1   b , and thereby altering the beam shape of a beam from the non-uniform linear antenna array  88 . 
         [0039]    It should be noted that the antenna array may have uniformly, or non-uniformly, arranged antenna elements  84 , and cross polarized antenna elements are only shown as a non-limiting example and other types of antenna elements may naturally be used without deviating from the scope of the invention. Furthermore, antenna elements operating in different frequency bands may be interleaved without departing from the scope of the claims. 
         [0040]    The illustrated telecommunication system (GSM) should be considered as a non-limiting example, and other wireless telecommunication standards, such as WCDMA, WiMAX, WiBro, CDMA2000, etc. may implement the described invention without deviating from the scope of the invention. Some of the described parts of the GSM system, e.g. base station controller BSC 1  and BSC 2  may be omitted in certain telecommunication standards, which is obvious for a skilled person in the art. 
         [0041]      FIG. 9  illustrates an antenna array  83  arranged in an elevated position, such as in a mast  90 . A non-linear phase shifter  85  is connected to the antenna array  83  (as described in connection with  FIG. 8 ) and is controlled by a base station controller BSC 1 . A non-tilted beam  91  (corresponding to the 0° plot in  FIG. 7   a ) is illustrated in  FIG. 9  together with a tilted beam  92  (corresponding to the 9° plot in  FIG. 7   d ). 
         [0042]    Although the invention has been described in detail using down-link, the skilled person in the art may readily adapt the teachings for up-link, as is mentioned above.