Patent Publication Number: US-6667714-B1

Title: Downtilt control for multiple antenna arrays

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to techniques for controlling the downtilt angle of phased-array antennas, such as those used in the base stations of wireless communication networks. 
     2. Description of the Related Art 
     In a conventional wireless communication network, communications with wireless units (e.g., mobile telephones) are supported by base stations, each configured with one or more antennas that provide communication coverage over an area surrounding the base station referred to as the base station cell. A typical base station cell may be divided into (e.g., three) sectors, with different antennas configured to support communications for the different sectors. In order to provide a relatively large cell size, base station antennas are typically configured at a higher height (e.g., on the tops of transmission towers) than the wireless units located within that cell. In order to communicate with wireless units located anywhere within a base station cell, including right next to the base station itself, base station antennas are typically configured with a downtilt angle to “point” the antennas down to provide the appropriate coverage. 
     One way to configure an antenna with a downtilt angle is to physically mount the antenna pointing at an angle below horizontal. Another way to achieve a downtilt angle is to use a phased-array antenna that can be pointed “electrically” by selecting appropriate phase shifts at the various antenna elements in the array. 
     FIG. 1 shows a block diagram of a conventional N-element, parallel-fed, fixed-phase, phased-array antenna  100 . Antenna  100  comprises a power splitter  102 , N phase shifters  104 , each phase shifter configured with a corresponding antenna element  106 , where the N phase shifters  104  are configured in parallel to power splitter  102 . Power splitter  102  receives an RF signal and distributes that RF signal to the N phase shifters  104  (e.g., splitting the signal power equally or in a shaped (e.g., cosine) manner among the different phase shifters). Each phase shifter  104   i  shifts the phase of its received portion of the RF signal by a particular fixed phase-shift angle φ i  and passes the resulting phase-shifted RF signal to its corresponding antenna element  106   i , which radiates that phase-shifted portion of the RF signal as a wireless electromagnetic (E-M) signal. 
     If the phase-shift angles φ at the N phase shifters  104  are selected appropriately, the resulting composite radiated E-M signal from the entire antenna array will form a uniform wavefront that propagates in a particular direction. As depicted in FIG. 1, to achieve a particular downtilt angle α, the element array of antenna  100  is configured with a progressive phase shift such that the phase-shift angle φ i  applied by each phase shifter  104   i  increases linearly from the first phase shifter  104   1  through the N th  phase shifter  104   N . 
     In general, the greater the number of antenna elements in the array, the more accurately and well-defined can be the coverage area (or footprint) of the antenna. This can be very important, especially in applications such as wireless communication systems, where base stations need to be distributed over a geographic area and configured with antennas that provide precise antenna footprints to ensure complete coverage over that geographic area with some overlap in adjacent antenna footprints to support handoffs for mobile wireless units, yet not with too much overlap in order to avoid undesirable interference between the signals of different wireless units. 
     Although FIG. 1 shows antenna  100  configured to transmit RF signals, antenna  100  can also be configured to receive RF signals, either at the same time as, or instead of, being configured to transmit RF signals, in which case, power splitter  102  (also) functions as a power combiner. 
     For relatively large downtilt angles and large arrays (e.g., more than four elements), the phase-shift angle φ i  for the last few phase shifters  104   i , where i=N, N−1, . . . , can become very large. This is not a problem for fixed-angle arrays. However, since the heights of base station antennas may vary from cell to cell, and the sizes of cells may vary from base station to base station, the magnitude of the downtilt angle will also typically vary from cell to cell. Moreover, the desired antenna footprint for a particular base station antenna may also vary over time, for example, as more base stations are configured within an existing covered geographic area. As such, it is not always practical to design base station antenna arrays with a fixed downtilt angle. 
     FIG. 2 shows a block diagram of a conventional N-element, parallel-fed, variable-phase, phased-array antenna  200 . Like antenna  100  of FIG. 1, antenna  200  comprises a power splitter  202 , N phase shifters  204 , each with a corresponding antenna element  206 , where the N phase shifters  204  are configured in parallel to power splitter  202 . In antenna  200 , however, the N phase shifters  204  are configured as part of a phase-shifter assembly  208 , which is configured to a motor  210 , which is in turn configured to a controller  212 . 
     Controller  212  receives phase control signals that determine how to control the operations of motor  210 , which in turn drives phase-shifter assembly  208 . Phase-shifter assembly  208  is typically a mechanical device with movable components (as driven by motor  210 ) whose movements affect the electro-magnetic characteristics (e.g., line length) of the various phase shifters  204  to change the magnitude of the phase-shift angle φ i  applied by each phase shifter  204   i  in a controlled manner. 
     Because the downtilt angle can be varied in a controllable manner, a single antenna design can be used for different base stations having different antenna heights that require different and varying downtilt angles. One advantage of parallel-fed, variable-phase antennas, such as antenna  200 , is that they can be implemented with minimum insertion phase (i.e., phase difference) between adjacent antenna elements. For example, if the progressive phase shift needs to be 17 degrees in order to achieve a downtilt angle α of 4 degrees, then this can be achieved using parallel-fed phase shifters, where the difference in phase-shift angle φ between adjacent antenna elements  206   i  and  206   i+1  is simply (φ i+1 −φ i )=17°. 
     Because the insertion phase can be minimized, parallel-fed, phased-array antennas can have relatively wide bandwidths. Typical wireless communication networks use different frequency bands for uplink (i.e., wireless unit to base station) and downlink (i.e., base station to wireless unit) communications. If the bandwidth of parallel-fed, phased-array antennas can be large enough, a single antenna array may be able to support both the uplink and downlink frequency bands. In that case, a single phased-array antenna can be used to both transmit downlink signals to the wireless units and receive uplink signals from the wireless units. 
     Unfortunately, for large ranges in downtilt angle (e.g., greater than 4 degrees) and large arrays (e.g., more than eight elements), the last few phase shifters (e.g.,  204   N ,  204   N−1 , . . .) of parallel-fed antenna  200  can become impractical to realize, because those phase shifters must be able to provide a relatively large range of phase-shift angles φ (e.g., from as small as 0 degrees for a zero downtilt angle to as large as 180 degrees for a downtilt angle of 4 degrees). In order to avoid this problem, series-fed phased-array antennas are typically used. 
     FIG. 3 shows a block diagram of a conventional N-element, series-fed, variable-phase, phased-array antenna  300 . Like antenna  200  of FIG. 2, antenna  300  comprises a power splitter  302 , a phase-shifter assembly  308  with N phase shifters  304 , each with a corresponding antenna element  306 , a motor  310  that drives phase-shifter assembly  308  and a controller  312  that controls motor  310 . Unlike antenna  200 , however, the N phase shifters  304  in phase-shifter assembly  308  are configured in series with (N−1) power couplers  314  within a power-splitter assembly  302 . As indicated in FIG. 3, the outgoing RF signal received by power-splitter assembly  302  is split by the first coupler  314   1  into two RF signals: one of which is phase-shifted by the first phase shifter  304   1  by a phase-shift angle φ 1  for radiation by the first antenna element  306   1  and the other of which is transmitted to the second phase shifter  304   2 , which applies a phase-shift angle φ 2 . In a typical implementation where phase-shift angle φ 1  is always zero, phase shifter  304   1  can be omitted. The phase-shifted RF signal from phase shifter  304   2  is then further split by the second coupler  314   2  into two RF signals: one of which is transmitted by the second antenna element  306   2  and the other of which is transmitted to the third phase shifter  304   3 , which applies a further phase-shift angle φ 3  to the already phase-shifted RF signal. The phase-shifted RF signal from phase shifter  304   3  is then further split by the third coupler  314   3  into two RF signals: one of which is transmitted by the third antenna element  306   3  and the other of which is transmitted to the fourth phase shifter (not shown), which applies a fourth phase-shift angle φ 4  to the twice phase-shifted RF signal. Since phase-shift angles are additive, the RF signal radiated by the third antenna element  306   3  has a total phase shift equal to the sum of the phase-shift angles applied by the second and third phase shifters  304   2  and  304   3  or (φ 2 +φ 3 ). 
     Similar power splitting and phase shifting is repeated for each antenna element until the last coupler  314   N−1  is reached. Coupler  314   N−1  splits its received RF signal into two RF signals: one of which is transmitted by antenna element  306   N−1  with a total phase shift of (φ 2 +φ 3 +. . . +φ N−1 ) and the other of which is transmitted to the last phase shifter  304   N , which applies a final phase-shift angle φ N  to the already multiply phase-shifted RF signal before passing the resulting RF signal to the last antenna element  306   N , whose radiated signal has a total phase shift of (φ 2 +φ 3 +. . . +φ N−1 +φ N ). 
     Because the various phase shifters  304  and power couplers  314  are configured in series (rather than in parallel as in antennas  100  and  200 ) and since phase shifts are additive, each preceding phase shifter in the series only needs to apply a fraction of the overall phase shift for each antenna element  306  to achieve the desired progressive phase shift for the overall antenna array. As a result, a series-fed, variable-phase, phased-array antenna such as antenna  300  can be designed to provide a wide range of downtilt angles, since each phase shifter needs only to provide a fraction of the overall phase range and is therefore more easily realized. 
     Unfortunately, however, series-fed antenna designs often do not provide minimum insertion phase. For example, to achieve a progressive phase shift of 17 degrees over an antenna array, the difference in phase shift φ between adjacent antenna elements  306   i  and  306   i+1  may be (φ i+1 −φ i )=377°, where excess phase in the design is padded by 360 degrees. Over the size of the array, this larger insertion phase makes the phase change rate vary faster as a function of frequency, thereby making the array more narrow in bandwidth. For large arrays (e.g., six elements or more), it is very difficult to achieve a bandwidth wide enough to cover both the uplink and downlink frequency bands for conventional wireless communication networks. As a result, two separate antenna arrays may be needed to support communications between a base station and the corresponding wireless units, with one antenna array designed for the uplink frequency band and the other antenna array designed for the downlink frequency band. In order to support both the uplink and the downlink communications for each wireless unit, the footprints of these uplink and downlink antenna arrays need to be the same and, as a result, their respective downtilt angles need to be able to be coordinated to achieve such common coverage areas. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an apparatus for simultaneously controlling the downtilt angles of two (or more) different variable-phase phased-array antennas, such as those used for uplink and downlink communications at a base station of a wireless communication network. Because the uplink and downlink frequency bands in typical wireless communication networks are different, for a common downtilt angle, the progressive phase shifts will be different for the uplink and downlink antennas. The present invention preferably takes those differences into account to achieve coordinated control over downtilt angle for the two different antenna arrays. 
     In one embodiment, the present invention is an apparatus for simultaneously controlling downtilt angles of two or more arrays of antenna elements, comprising (a) for each array, a power splitter and a phase-shifter assembly configured to control the progressive phase shifts between successive elements in the array; (b) a common linkage connected to one or more movable components of each phase-shifter assembly; (c) a common motor configured to the linkage to convert motion of the common motor into motion of the linkage; and (d) a controller configured to control the motion of the common motor, wherein the motion of the common motor causes the motion of the linkage which simultaneously moves the one or more components within each phase-shifter assembly to change the progressive phase shifts between successive elements in the corresponding array, thereby simultaneously changing the downtilt angles of the two or more arrays in a coordinated fashion. 
     In another embodiment, the present invention is an antenna system for a base station of a wireless communication network, comprising (a) an uplink array of antenna elements; (b) a downlink array of antenna elements; (c) an uplink power-combiner and an uplink phase-shifter assembly configured to control progressive phase shifts between successive array elements in the uplink array; (d) a downlink power-splitter and a downlink phase-shifter assembly configured to control progressive phase shifts between successive array elements in the downlink array; (e) a common linkage connected to one or more movable components of both the uplink and downlink phase-shifter assemblies; (f) a common motor configured to the linkage to convert motion of the common motor into motion of the linkage; and (g) a controller configured to control the motion of the common motor, wherein the motion of the common motor causes the motion of the linkage which simultaneously moves the one or more components within the uplink and downlink power-splitter/phase-shifter assemblies to simultaneously change the progressive phase shifts between successive elements in the uplink and downlink arrays, thereby simultaneously changing the downtilt angles of the uplink and downlink arrays in a coordinated fashion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other aspects, features, and advantages of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which: 
     FIG. 1 shows a block diagram of a conventional N-element, parallel-fed, fixed-phase, phased-array antenna; 
     FIG. 2 shows a block diagram of a conventional N-element, parallel-fed, variable-phase, phased-array antenna; 
     FIG. 3 shows a block diagram of a conventional N-element, series-fed, variable-phase, phased-array antenna; 
     FIG. 4 shows a block diagram of an antenna system for a base station of a wireless communication network, according to one embodiment of the present invention; 
     FIG. 5 shows a schematic diagram of a base station tower configured with the uplink and downlink antennas of the antenna system of FIG. 4; and 
     FIG. 6 shows a schematic diagram of an integrated uplink power-splitter/phase-shifter assembly for the uplink antenna of FIG.  4  and an integrated downlink power-splitter/phase-shifter assembly for the downlink antenna of FIG. 4 configured with a common linkage, according to one embodiment of the present invention in which each phased-array antenna has four antenna elements. 
    
    
     DETAILED DESCRIPTION 
     FIG. 4 shows a block diagram of an antenna system  400  for a base station of a wireless communication network, according to one embodiment of the present invention. Antenna system  400  comprises two different N-element, series-fed, variable-phase, phased-array antennas: uplink antenna  401   U  configured to receive RF signals in the uplink frequency band from one or more wireless units, and downlink antenna  401   D  configured to transmit RF signals in the downlink frequency band to the same one or more wireless units. FIG. 5 shows a schematic diagram of a base station tower  502  configured with uplink antenna  401   U  and downlink antenna  401   D  of antenna system  400  of FIG.  4 . 
     As shown in FIG. 4, each phased-array antenna in antenna system  400  has a power-splitter assembly  402  with N−1 couplers  414 , a phase-shifter assembly  408  with N phase shifters  404 , each phase shifter configured with a corresponding antenna element  406 , where the N−1 couplers  414  are configured in series with the N phase shifters  404 , analogous to that described for antenna  300  of FIG.  3 . Note that, for uplink antenna  401   U , power-splitter assembly  402   U  functions as a “power-combiner” assembly. 
     In addition, antenna system  400  has a controller  412 , which controls the rotational motion of a motor  410 , which drives a mechanical linkage  409 , which in turn is connected to drive the positions of movable components within both phase-shifter assemblies  408   U  and  408   D  to simultaneously change the downtilt angles for both the uplink and downlink antennas  401   U  and  401   D , respectively. Thus, a single electro-mechanical actuator (comprising controller  412 , motor  410 , and linkage  409 ) is used to control and coordinate changes in the downtilt angles for both the uplink and downlink antennas. 
     Because the uplink and downlink frequency bands are different in conventional wireless communication networks, the progressive phase shift needed to achieve a particular downtilt angle α U  for uplink antenna  401   U  will typically be different from the progressive phase shift needed to achieve the equivalent downtilt angle α D  for downlink antenna  401   D . This implies that the phase-shift angles φ applied by the various corresponding phase shifters  404  will differ between the upper and lower phase-shifter assemblies  408   U  and  408   D . For example, the phase-shift angle φ 2   U  applied by the second phase-shifter  404   U2  in phase-shifter assembly  408   U  of uplink antenna  401   U  will typically be different from the phase-shift angle φ 2   D  applied by corresponding phase shifter  404   D2  in phase-shifter assembly  408   D  of downlink antenna  401   D . (In a typical implementation where phase-shift angles φ 1   U  and φ 1   D  are both always zero, phase shifters  404   U1  and  404   D1  can both be omitted.) 
     In preferred embodiments of the present invention, the different progressive phase-shift values are taken into account when designing phase-shifter assemblies  408   U  and  408   D , such that motion of motor  410  is translated into equivalent changes in the two downtilt angles α U  and α D . In particular, the two phase-shift assemblies will typically have different geometries and/or different electrical characteristics to achieve the two different progressive phase shifts. Note that, in most embodiments, what is desired is that the uplink and downlink antennas have substantially the same downtilt angle so that they achieve the same footprints. This might enable the downtilt angle to be set efficiently based on only one set of measurements. For example, field testing could be limited to measurement of received signal strength throughout the cell for downlink transmission from the base station to a test mobile. Since the uplink and downlink downtilt angles will be known to be equivalent, actual test confirmation of adequate downlink coverage will imply that adequate uplink coverage is also achieved. 
     In alternative embodiments, for example, where the uplink and downlink antennas are mounted at substantially different heights on a base station tower or where different coverage patterns are desired, different downtilt angles may be needed for the uplink and downlink antennas to achieve the same antenna footprints. In such cases, the different required downtilt angles are taken into consideration when designing phase-shifter assemblies  408   U  and  408   D.    
     In preferred embodiments, linkage  409  is a rigid structure that is connected to motor  410  through one or more gear boxes that translate rotational motion of motor  410  into uniform translational motion of the movable components within both the uplink and downlink phase-shifter assemblies. Alternatively, the different progressive phase-shift values can also be taken into account when designing mechanical linkage  409 , such that rotational motion of motor  410  is translated into non-uniform translational motion by linkage  409  for uplink antenna  401   U  and for downlink antenna  401   D.    
     FIG. 6 shows a schematic diagram of an integrated uplink power-splitter/phase-shifter assembly  602   U  for uplink antenna  401   U  and an integrated downlink power-splitter/phase-shifter assembly  602   D  for downlink antenna  401   D  of FIG. 4 configured to a common linkage  409 , according to one embodiment of the present invention in which each phased-array antenna has four antenna elements  406 . Each integrated assembly  602  integrates the power-splitting functionality of one of the power-splitter assemblies  402  of FIG. 4 with the phase-shifting functionality of the corresponding phase-shifter assembly  408 . Each integrated assembly  602  comprises a series of dielectric wedges  604  sandwiched between a microstrip conductor  606  and a lower, conducting, ground plane (not shown), where each dielectric wedge  604  is connected to linkage  409 , which controls the “depth” of insertion of each dielectric wedge  604  between the corresponding microstrip conductor  606  and the ground plane. 
     Each integrated power-splitter/phase-shifter assembly shown in FIG. 6 is an air dielectric suspended microstrip line realized in sheet metal and based on a dielectric wedge, series-fed, phase-shifter assembly that is described in further detail in U.S. Pat. No. 5,940,030. Another suitable type of integrated power-splitter/phase-shifter assembly for the present invention is the sliding-short, reflection-mode, series-fed, phase-shifter assembly, which is another type of air dielectric suspended microstrip line realized in sheet metal and is described in U.S. patent application Nos. 09/148,442, filed on Sep. 4, 1998, and 09/148,449, filed on Sep. 4, 1998. Both of these two types of phase-shifter assemblies combine the N−1 couplers (i.e.,  414  in FIG. 4) of a power-splitter assembly and the N phase-shifters (i.e.,  404  in FIG. 4) of a phase-shifter assembly into a single integrated device that provides the functions of both power splitting (or combining) and series-fed phase shifting. 
     Uplink microstrip conductor  606   U  is configured to receive the different RF signals received at the different antenna elements  406   U  of uplink antenna  401   U  from the wireless units and provide a phase-shifted, combined receive (RX) RF signal. Analogously, downlink microstrip conductor  606   D  is configured to accept a transmit (TX) RF signal and provide differently phase-shifted RF signals to the various transmit antenna elements  406   D  of downlink antenna  401   D  for propagation to the wireless units. Impedance transformations due to line-width changes control the magnitude ratios for the power-splitting (or combining) function for the individual antenna array elements. Between successive antenna elements, a solid dielectric wedge  604  is introduced in place of the air, underneath the suspended conducting line. By altering the effective dielectric constant, the effective line length is changed, thereby changing the progressive phase shift between the successive antenna elements. The position (i.e., depth of insertion) of each dielectric wedge  604  between the corresponding microstrip conductor  606  and the ground plane determines the amount of dielectric material located between the microstrip conductor and the ground plane, which in turn determines the amount of phase shift applied to the RF signal at that location along the microstrip conductor. By controlling the depth of insertion (i.e., by controlling the motion of the wedges configured to linkage  409 ), the progressive phase shift and therefore the downtilt angle of the antenna can be controlled. 
     As represented in FIG. 6, rotational (or linear) motion of motor  410  (which is preferably a linear stepper motor) is translated into translational motion of linkage  409  by a suitable gear box  608 . Translational motion of linkage  409  (i.e., left-to-right motion in FIG. 6) moves more of each dielectric wedge  604  (right in FIG. 6) between microstrip conductor  606  and the ground plane (and vice versa), thereby affecting the electromagnetic characteristics for signals propagating along microstrip conductor  606 . In particular, moving dielectric wedges  604  changes the amount of phase shift applied to the RF signal as it propagates along microstrip conductor  606 . By carefully selecting the thickness, size, shape (e.g., the taper of the wedges), and position of each dielectric wedge  604 , as well as the size and shape of the corresponding microstrip conductor  606 , the amount of phase shift applied by the various wedges and therefore the overall progressive phase shift of the integrated power-splitter/phase-shifter assembly can be accurately controlled for the entire range of motion of linkage  409 . Note that in the exemplary embodiment of FIG. 6, the shapes of the upper and lower microstrip conductors  606   U  and  606   D  are different to take into account differences between the uplink and downlink frequency ranges. In alternative embodiments, the thicknesses, sizes, shapes, and positions of the dielectric wedges  604  may also vary from wedge to wedge and from antenna to antenna, either in addition to or instead of the differing shapes of the microstrip conductors  606 . 
     Although FIG. 5 shows the uplink antenna  401   U  configured above the downlink antenna  401   D , it will be understood that the present invention can be implemented with alternative configurations, including those with the downlink antenna above the uplink antenna and those with the uplink and downlink antennas configured side-by-side. Moreover, although FIG. 4 shows uplink and downlink antennas  401   U  and  401   D  both with N antenna elements, it will be understood that the present invention can be implemented with uplink and downlink arrays having differing numbers of antenna elements. 
     Although the present invention has been described in the context of series-fed, variable-phase, phased-array antennas, it will be understood that the present invention could also be implemented for parallel-fed, variable-phase, phased-array antennas. Moreover, although the present invention has been described in the context of simultaneously controlling two variable-phase, phased-array antennas, one for transmitting downlink signals and one for receiving uplink signals, it will be understood that, in general, the present invention can be implemented to simultaneously control two or more variable-phase, phased-array antennas, where each different antenna may be differently used for transmitting only, receiving only, or both transmitting and receiving. 
     It will be further understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain the nature of this invention may be made by those skilled in the art without departing from the scope of the invention as expressed in the following claims.