Abstract:
A method for cell control in a wireless network is disclosed, which comprises providing a base transceiver station (BTS) with a plurality of antennas receiving a plurality of feed signals, respectively, varying one or more parameters of the plurality of feed signals according to a plurality of predetermined criteria for synthesizing a desired antenna radiation pattern, wherein a coverage area of the BTS is adjusted.

Description:
RELATE BACK 
       [0001]    The present invention claims priority to U.S. Provisional application 60/808,318 filed May 24, 2006. 
     
     BACKGROUND 
       [0002]    The present invention relates generally to wireless communication systems, and more particularly to dynamic cell control. 
         [0003]    Cell control, i.e., cell blossoming, cell wilting or cell breathing, is a process with which the cell coverage is deliberately tweaked during operations. Cell control is one of the key network optimization processes aiming to improve network performances. For example, a cell wilting is a network gradually reduces a cell&#39;s coverage to take it out of service for maintenance without causing dropped calls. Cell blossom is adding a new cell into the network by gradually increasing its coverage without overloading active terminals. Cell breathing is used to dynamically shuffle users among cells to help better balance traffics among the cells. 
         [0004]    A conventional cell control method employs forward power control. For example, given a total power allocated for a round of traffic, the coverage radian can be estimated based on a path loss, which, in turn, is mainly determined by following factors: base transceiver station (BTS) height, carrier frequency, morphology of the coverage area, and terminal heights. If the coverage needs be reduced (cell wilting or breathe), the network may reduce the power allocated for the traffic. The amount of power reduction can be estimated from the relationship of transmitter power vs. coverage radian. The transmitter power vs. coverage radian relationship can be established theoretically or experimentally. The cell control via forward power control needs be carried out synchronically among multiple cells in a cluster to avoid leaving coverage holes in the coverage area. This makes the network system design complicated 
         [0005]    Therefore, what is desired is simple cell control method that can be carried out dynamically and in real time. 
       SUMMARY 
       [0006]    In view of the foregoing, the present invention discloses a new method for cell control in a wireless network. In one aspect of the present invention, the method comprises providing a base transceiver station (BTS) with a plurality of antennas receiving a plurality of feed signals, respectively, varying one or more parameters of the plurality of feed signals according to a plurality of predetermined criteria for synthesizing a desired antenna radiation pattern, wherein a coverage area of the BTS is adjusted. 
         [0007]    In another aspect of the present invention, the method comprises providing a base transceiver station (BTS) with a plurality of antennas consecutively receiving a first and second plurality of feed signals, respectively, varying one or more parameters of the first plurality of feed signals according to a first plurality of predetermined criteria for synthesizing a first desired antenna radiation pattern, and varying one or more parameters of the second plurality of feed signals according to a second plurality of predetermined criteria for synthesizing a second desired antenna radiation pattern, wherein the first and second desired antenna radiation patterns covers a first and second areas either next to or substantially overlap each other. 
         [0008]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings in which like reference numerals refer to similar elements. 
           [0010]      FIG. 1  illustrates conventional cells with omni-directional coverage area in a wireless network. 
           [0011]      FIGS. 2A and 2B  illustrate cells with antenna radiation pattern synthesis in a wireless network according to an embodiment of the present invention. 
           [0012]      FIG. 3  is a flow chart illustrating steps taken to synthesize antenna radiation patterns according to the embodiment of the present invention. 
       
    
    
     DESCRIPTION 
       [0013]    The following will provide a detailed description of a cell control method that controls cells&#39; coverage though real-time antenna radiation pattern synthesis, which allows for two-dimensional cell coverage control and improves system performances. This method can be used alone or combined with the conventional cell control method of varying BTS transmitter power. 
         [0014]      FIG. 1  illustrates conventional cells with omni-directional coverage area in a wireless network  100 . Base transceiver stations (BTSs)  110 ,  120  and  130  cover omni-directional areas  112 ,  122  and  132 , respectively. Terminals  114  and  134  are in the omni-directional areas  112  and  132 , respectively, and therefore served by the BTSs  110  and  130 , respectively. A terminal  124  is in an overlapping area of the omni-directional areas  112  and  122 , and therefore can be served by both BTSs  110  and  120 . For the wireless network  100  with conventional omni-directional cells, cell control may be achieved through varying forward transmission power. But the shape of the coverage areas  112 ,  122  and  132  are always circular. Varying forward transmission power results only in diameter changes of the circular coverage areas  112 ,  122  and  132 . When neighboring cells require different coverage areas, diameter change is apparently less efficient to have all the areas covered. 
         [0015]      FIGS. 2A and 2B  illustrate cells with antenna radiation pattern synthesis in a wireless network  200  according to an embodiment of the present invention. Referring to  FIG. 2A , cell coverage areas  210 ,  220  and  230  are provided by corresponding BTSs (not shown), respectively, through antenna radiation pattern synthesis. Angle spans  215 ,  225  and  235  represent the radiation pattern spanning of the coverage areas  210 ,  220  and  230 , respectively. Assuming each coverage area  210 ,  220  or  230  has approximately equal amount of traffic loads, then the radiation patterns span an equal 120° in an azimuth plane, i.e., the three coverage areas  210 ,  220  and  230  are approximately equal in shape and size. 
         [0016]    Referring to  FIG. 2B , coverage areas  250 ,  260  and  270  in a wireless network  240  are of different sizes. Angle spans  255 ,  265  and  275  represent radiation pattern spanning of the coverage areas  250 ,  260  and  270 , respectively. Angle spans  255  and  265  are equally at 150°, and angle span  275  is at 60°. Therefore, the coverage area  330  is significantly smaller than the coverage areas  250  and  260 . In this case, traffic density in coverage area  270  is higher than in other two coverage areas  250  and  260 . That is when the coverage area  270  is overloaded with traffic while other two coverage areas  250  and  260  are underloaded; the angle span  275  of the overloaded coverage area  270  is reduced, while the angle spans  255  and  265  of the other coverage areas  250  and  260 , respectively, are increased. As a result, some of the terminals that previously belong to the overloaded coverage area  270  are now being shuffled to the underloaded coverage areas  250  and  260 . This will balance out the traffic loads among the three coverage areas  250 ,  260  and  270 . Apparently adjusting the shapes and sizes of cell coverage areas through antenna radiation pattern synthesis is more efficient than adjusting diameters of circular cell coverage areas shown in  FIG. 1 . 
         [0017]    Antenna radiation pattern synthesis is more achievable in a BTS with multiple antennas. For a BTS with an antenna array, traffics are received and sent through beamformings. For all uplink traffics (data sent from terminals to the BTS), the BTS antenna array will beamform them via either maximum ratio combining (MRC) or equal gain combining (EGC), or log likely-hood (LLC) combining or any other combining schemes. For downlink traffics, there are two types of beamformings. For traffic that is intended for a particular terminal, BTS will beamform the traffic to that particular terminal. However, for traffics that are intended for a group of terminals over a large coverage area, or the traffics are intended for a particular terminal but the terminal is not fixed and may be at any locations over a large coverage area (uncertainty of its spatial signature), BTS needs to send the traffic to a large designated coverage area. This can be achieved by synthesizing a radiation pattern that covers the desired area. For example, as shown in  FIG. 1 , if the desired coverage area is a circle with the BTS in the center, the synthesized BTS antenna radiation pattern for broadcast messages will be an omni-directional. 
         [0018]    In order to synthesize a BTS&#39;s radiation pattern to a desired shape and size, parameters such as magnitudes and phases of the BTS&#39;s feed signals need to be set in such a way that the difference between the resulted (synthesized) pattern and a desired radiation pattern is minimized. For example, for a circular array with λ/2 spacing, where λ is a wave length of the signal, an omni-directional radiation pattern can be achieved by setting the same magnitude and phase for all feed signals. For a linear array with 8 elements and λ/2 spacing, a radiation pattern spanning 120° in the azimuth plane, as shown in  FIG. 2A , can be achieved with the feed signals that have the same magnitude but different phases as follows: 
         [0019]    Phase=[−312°, −208°, −52°, 0°, 0°, −52°, −208°, −312°]. 
         [0020]    For the same linear array, a radiation pattern spanning 40° in the azimuth plane, as shown in  FIG. 2B , can be achieved with the feed signals that have the same magnitude and the following phases: 
         [0021]    Phase=[135°, 100°, 30°, 0°, 0°, 30°, 100°, 135°]. 
         [0022]    Although synthesizing only two spanning angles through phase adjustments are disclosed here, one having skill in the art would have no difficulties to synthesizing other spanning angles through different phases adjustments as well as magnitude adjustments. 
         [0023]    The antenna radiation pattern synthesis follows the general practice of optimization procedure, and all techniques used in optimization can be readily used here. A difference is that the optimization objective is the desired radiation pattern. In addition, some optimization constraints may also be added. For example, it may be required that all antennas have the same output power. This would make the feed signals for all antennas have the same magnitude. Another example of adding constraints is that the gain ripple of the synthesized pattern needs be less than certain threshold. 
         [0024]      FIG. 3  is a flow chart  300  illustrating steps taken to synthesize antenna radiation patterns in more generic terms according to the embodiment of the present invention. In order to synthesize a desired antenna radiation pattern by a multi-antenna BTS, the BTS first sets magnitudes of feed signals based on prior knowledge in step  310 . Then it is followed by phase setting on the feed signals also according to prior knowledge in step  320 . These feed signals may be optimized, such as adding output power constraints, in step  330 . Finally the adjusted feed signals are transmitted through their corresponding antennas in step  340 . Note that the magnitudes and phases of the feed signals of each antenna element can be set in a baseband, radio frequency (RF) or a combination of the both. 
         [0025]    The antenna radiation pattern synthesis method may be combined with the forward power control method to control cell coverage areas. Forward power allocation determines the coverage along radian while the antenna radiation pattern determines the coverage in the angular dimension. With the forward power control and antenna radiation synthesis, cell coverage area may be shaped the in two dimensions. This will considerably improve the cell control. 
         [0026]    There are many applications of the dynamic cell control. Some of them are described below. A first application is traffic balances. The dynamic traffic control is used to better balance the traffics among cells. The cell control needs to be dynamic and in real time. The traffics are monitored for all cells, and if unbalanced traffics are detected, the over loaded cell will reduce its cell coverage along radian or angular or both, and the under loaded cells will increase its cell coverage along radian or angular or both. The coverage change along radian is done through power control, while the coverage change along angular is done through radiation pattern synthesis. 
         [0027]    A second application is cell wilt. The cell control can be used to wilt cells out for the purpose of maintenance or repair. 
         [0028]    A third application is cell blossom. The cell control can be used to add new cells into the network. 
         [0029]    A fourth application is network planning. The antenna radiation pattern synthesis can help to improve the cell planning. After an initial cell planning and testing, some coverage holes in the coverage area may be discovered. The coverage holes can be covered up by tweaking the radiation patterns through antenna radiation pattern synthesis. 
         [0030]    A fifth application is cell splitting. The cell control can be used for the purpose of dynamic cell splitting. Dynamic cell splitting is done through antenna radiation pattern synthesis. For example, if the cell needs to be split into two virtual smaller cells, one could synthesize a radiation pattern that has two distinctive beams (split pattern) that their aggregated beam pattern covers the original coverage area. Each beam covers a sub-region of the original coverage area. The resource assigned for these two sub-regions could be the same or different. Splitting cells via split radiation beams is a technique used to improve frequency reuse within cell and among cells. For example, if a 120° cell is split into two 60° cells, the same spectrum can be used twice within the cell, one for terminals in one beam, and the other for terminals in the other beam. The cell splitting is dynamic and in real time, varying with time and traffic condition. 
         [0031]    A sixth application is diversity. The antenna radiation pattern synthesis can be used for the purpose of diversity. For example, a BTS synthesizes two radiation patterns that are different in spatial characteristics (like different DOA) and send and receive the same messages via both the radiation patterns. In a case of fading channel, one copy of the message may be faded out. But chances that the two copies of the messages are faded out at the same time are small, so a terminal has a better chance to detect one good copy of the message. 
         [0032]    A seventh application is antenna reuse and carrier overlay. With the antenna radiation pattern synthesis, it is possible to use the same set of antennas for multiple coverage areas. For example, an initial 120° coverage area is running out its capacity, and the wireless network needs to overlay the initial cell with another carrier. One way to do this is to synthesize a 60° antenna radiation pattern for the initial carrier and a 60° radiation pattern for the new carrier. These two 60° radiation patterns have their bore sights separated by 60° and cover the original 120° coverage area. Benefits of antenna radiation pattern synthesis in this case include that the same antennas are reused, and since the two carriers are not overlapped in space, there will be less hand off between these two carriers, resulting in better overall performance. 
         [0033]    The above illustration provides many different embodiments or embodiments for implementing different features of the invention. Specific embodiments of components and processes are described to help clarify the invention. These are, of course, merely embodiments and are not intended to limit the invention from that described in the claims. 
         [0034]    Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.