Abstract:
The present invention retunes adjustable base station antennas of a wireless communications system in order to improve performance metrics that are determined by measurements and that are associated with sub-sectors within the wireless communications system. The present invention calculates and applies control signals to the adjustable antennas to improve the performance in accordance with the performance metrics. Thus, the present invention obviates much of the labor that is required for retuning the wireless communications system with changing radio conditions.

Description:
TECHNICAL FIELD 
   This invention relates to a wireless communications system that monitors and adjusts a base station antenna&#39;s signal broadcast radiation pattern in order to improve its performance. 
   BACKGROUND OF THE INVENTION 
   A wireless communications system is engineered to serve a desired level of traffic according to radio transmission characteristics that are assumed to provide homogeneous signal strength coverage over a defined geographic area and are assumed to be time invariant. However, radio conditions between mobile subscriber units and serving base stations change with time, which degrades the performance of the wireless communications system (possibly substantially). Degradation of the wireless communications system&#39;s performance can be manifested in a number of ways. Examples include an increased dropped call rate and an increased frame error rate. The wireless communications system may require periodic “retuning” of base station antennas in order to maintain the engineered performance objectives. Each retuning of a base station may require that a technician physically travel to a base station&#39;s location. The effort associated with retuning is amplified by the number of base stations (which may be in the hundreds) associated with the wireless communications system. Thus, the task of retuning the wireless communications system is labor-intensive, time-consuming and expensive. 
   Additionally, radio characteristics are usually not homogeneous within a serving area of a base station antenna. Within the serving area, factors such as buildings, foliage, terrain and weather are not homogeneous, causing radio characteristics not to be homogeneous. Moreover, these factors may change with time, e.g. new buildings are constructed within the service area and the leaves of trees grow and fall with the seasons of the year. These phenomena cause “holes” in the radio coverage area. Increasing the signal strength in the direction of the hole can compensate for the deficiency. 
   With the prior art, “drive tests” are periodically conducted in order to detect holes in radio frequency (RF) coverage. Drive tests require that technicians operate mobile subscriber units while traversing routes and collecting measurements within the coverage area of the wireless communications system. The measurements are typically stored on a recording medium attached to the mobile subscriber unit. The measurements are subsequently analyzed to evaluate the RF coverage as provided by the wireless communications system. 
   A base station serves a region called a cell, which is further partitioned into sectors. The base station serves multiple sectors of a cell with each sector corresponding to a base station antenna. Because RF characteristics may not be homogeneous within a sub-region (sub-sector) of a sector, each base station antenna (sector) may require adjustments that are dependent upon a subregion of the given sector. Periodic retuning (that is typical with the prior art) must therefore account for the heterogeneous nature of RF characteristics. Thus, the wireless industry has a definite and urgent need for an invention that allows a wireless service provider to automatically retune the base station antennas within the wireless communications system in order to provide better service at a lower cost. 
   SUMMARY OF THE INVENTION 
   The present invention enables a service provider of a wireless communications system to measure and retune the radiation patterns of base station antennas without labor-intensive effort that is typical with the prior art. The present invention includes both apparatus and methods in which measurements are collected and in which performance metrics are derived from the measurements and analyzed so that an adjustable base station antenna can be controlled. Examples of adjustable base station antennas include linear array antennas and narrow beam antenna configurations as disclosed in the exemplary embodiment. The performance metrics are derived from measurements that are associated with subregions (sub-sectors) within the sector of the serving base station antenna. In order to accomplish this association, an approximate location of a mobile subscriber unit is determined at the time of a measurement. Control signals that are applied to adjustable base station antennas are calculated so that the performance metrics of the sub-sector are improved within the constraints limiting the degradation of performance metrics of other subsectors. 
   Numerous other advantages and features of the present invention will become readily apparent from the detailed description of the invention and the embodiments thereof, from the claims, and from the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates an architecture of a wireless communications system; 
       FIG. 2  illustrates partitioning the coverage of two essentially abutting cells with each cell served by a base station as shown in  FIG. 1 ; 
       FIG. 3  shows further partitioning of a cell&#39;s sector into sub-sectors in which the sector is served through the base station antenna shown in  FIG. 1 ; 
       FIG. 4  illustrates a coverage of a base station antenna configured with a narrow beam signal broadcast radiation pattern for each sub-sector; 
       FIG. 5  illustrates apparatus controlling an adjustable base station antenna serving a sector of a cell; 
       FIG. 6  shows a flow diagram operative in the apparatus of  FIG. 5  for collecting measurements that are associated with each sub-sector; 
       FIG. 7  shows a flow diagram for analyzing measurements for each sub-sector and for controlling an adjustable base station antenna of a sector; and 
       FIG. 8  shows a flow diagram for analyzing measurements for each sub-sector and for incrementally controlling and adjusting an adjustable base station antenna associated with a sector. 
   

   DETAILED DESCRIPTION 
     FIG. 1  illustrates the architecture of a wireless communications system  10  serving mobile subscriber units  100 ,  101 , and  102  by base stations  103 ,  104 , and  105  through base station antennas  106 ,  107 , and  108  utilizing radio channels  109 ,  110 , and  111 , respectively. Of course, a wireless communications system such as  10  may serve many thousands of mobile subscriber units; however, this fact does not affect the intrinsic nature of the invention disclosed herein. Base stations  103 ,  104 , and  105  serve mobile subscriber units located within the corresponding cell associated with the coverage area. Base stations  103 ,  104 , and  105  are connected to mobile switching center (MSC)  112 , which is connected to public switching telephone network (PSTN)  113 , locating processor  114 , and antenna controller  115 . PSTN  113  allows MSC  112  to establish incoming and outgoing calls with mobile subscriber units  100 ,  101 , and  102 . Locating processor  114 , in response to commands from MSC  112 , provides information about the location of mobile subscriber units  100 ,  101 , and  102 . 
   Locating processor  114  and antenna controller  115  may be physically separated from MSC  112 , physically situated within MSC  112 , or physically distributed throughout wireless communications system  10 . In the exemplary embodiment of the present invention, locating processor  114  and antenna controller  115  are both distributed within wireless communications system  10 . In the exemplary embodiment, location processor  114  is the same as or similar to the one described in U.S. Pat. No. 5,963,866, issued to Palamara, et al., and assigned to Lucent Technologies, Inc. Location processor  114  determines the location of a mobile subscriber unit by transmitting an audit signal, receiving a confirmation signal, time stamping the confirmation signal, and processing information from time stamping. The present invention allows for other approaches in order to locate a mobile subscriber unit. One alternative is for MSC  112  to request a position determining entity (PDE) for location information of a mobile subscriber unit (such as mobile subscriber unit  100 ,  101 , or  102 ). The PDE determines the precise position or geographic location of a wireless subscriber unit when the wireless subscriber unit starts a call or while the mobile subscriber unit is engaged in a call. (Telecommunications Industry Association TR-45, PN-3890 , Enhanced Wireless  9-1-1 Phase 2.) 
   Antenna controller  115  causes the adjustment of base station antennas  106 ,  107 , and  108  by applying control signals  116  and  117 ,  118  and  119 , and  120  and  121 , respectively. Control signals  116 ,  117 ,  118 ,  119 ,  120 , and  121  are coupled to attenuators  516  and  522 , phase shifters  517  and  523 , attenuators  518  and  524 , phase shifters  519  and  525 , attenuators  520  and  526 , and phase shifters  521  and  527  as shown in  FIG. 5 . 
     FIG. 2  illustrates two essentially abutting cells  200  and  201 . Each cell is served by a base station such as base stations  103 ,  104 , or  105 . The coverage area of cell  200  is partitioned into sectors  202 ,  203 , and  204 ; the coverage area of cell  201  is partitioned into sectors  205 ,  206 , and  207 . In the disclosed exemplary embodiment, each sector covers an angular width of approximately 120 degrees; however, the present invention supports angular widths having different values. Each sector is associated with an antenna such as base station antennas  106 ,  107 , or  108 . 
     FIG. 3  shows further partitioning sector  203  into sub-sectors  301 ,  302 ,  303 , and  304 . In the disclosed exemplary embodiment of the present invention, the angular coverage of each sub-sector is approximately 30 degrees. However, embodiments of the present invention are not limited to this angular value. A base station antenna (such as base station antenna  106 ,  107 , or  108 ) serves an entire sector (such as sector  203 ). The radiation pattern of the associated antenna is controlled by adjusting the antenna characteristics of each sub-sector. In a first antenna configuration of the present invention, the base station antenna associated with sector  203  is a linear array antenna. A linear array antenna comprises at least one antenna element. For each antenna element, the control signals are adjusted both in amplitude and in phase. Linear array antennas are known to one skilled in the art and are discussed in detail by a number of references. (Hansen, R. C.,  Phased Array Antennas , John Wiley and Sons, Inc., 1998, pp. 47–105; Rudge, A. W., et al.,  The Handbook of Antenna Design , Peter Peregrinus Ltd., 1986, pp. 695–834.) 
     FIG. 4  shows a second antenna configuration of the present invention. The base station antenna associated with sector  203  is an array of antenna elements, each antenna element providing coverage for a sub-sector and each antenna element having a narrow beam radiation pattern. Narrow beam radiation patterns  405 ,  406 ,  407 , and  408  correspond to sub-sectors  301 ,  302 ,  303 , and  304 , respectively. Antenna controller  115  individually adjusts each narrow beam radiation pattern  405 ,  406 ,  407 , and  408 . 
     FIG. 5  shows the apparatus comprising antenna controller  115 , which is applicable to both the first and the second antenna configurations of the present invention. A base station antenna, which is associated with a sector, comprises antenna elements  501 ,  502 , and  503 . Antenna elements  501 ,  502 , and  503  are connected to duplexers  507 ,  508 , and  509  through coaxial cables  504 ,  505 , and  506 , respectively. Duplexers  507 ,  508 , and  509  enable antenna elements  501 ,  502 , and  503  to support both transmitted signals and received signals. 
   The transmitted signal to antenna element  501  is coupled to transmitter  513  through phase shifter  517 , attenuator  516 , and amplifier  510 ; the transmitted signal to antenna element  502  is coupled to transmitter  513  through phase shifter  519 , attenuator  518 , and amplifier  511 . The signal transmitted to antenna element  503  is coupled to transmitter  513  through phase shifter  521 , attenuator  520 , and amplifier  512 . 
   The signal received from antenna element  501  is coupled to receiver  515  through phase shifter  523 , attenuator  522 , and combiner  514 ; the received signal from antenna element  502  is coupled to receiver  515  through phase shifter  525 , attenuator  524 , and combiner  514 ; and the received signal from antenna element  503  is coupled to receiver  515  through phase shifter  527 , attenuator  526 , and combiner  514 . Combiner  514  sums the received signals from antenna elements  501 ,  502 , and  503 . In the first antenna configuration of the present invention, three antenna elements are shown, although linear array antennas may utilize a different number of antenna elements in other antenna configurations. In the discussion herein, N antenna elements are implied. 
   Phase shifters  517 ,  519 ,  521 ,  523 ,  525 , and  527  and attenuators  516 ,  518 ,  520 ,  522 ,  524 , and  526  are controlled by antenna controller  115 . The value of a control signal that is applied to each phase shifter and attenuator is determined by antenna controller  115  and coupled to the associated device. Thus, in  FIG. 1 , antenna controller determines the values M.sub. 1 , P.sub. 1 , M.sub. 2 , P.sub. 2 , M.sub.N, and P.sub.N, corresponding to control signals  116 ,  117 ,  118 ,  119 ,  120 , and  121 , respectively, and coupled to attenuators  516  and  522 , phase shifters  517  and  523 , attenuators  518  and  524 , phase shifters  519  and  525 , attenuators  520  and  526 , and phases shifter  521  and  527 , respectively. 
   With the second antenna configuration, each antenna element has a narrow beam radiation pattern, corresponding to a specific angular region of the associated sector (i.e. sub-sector). Consequently, phase shifters  517 ,  519 ,  521 ,  523 ,  525 , and  527  are not required because phase adjustment is not necessary. Equivalently, phase shifters  517 ,  519 ,  521 ,  523 ,  525 , and  527  can induce equal values of phase shift. 
   In the exemplary embodiment of the present invention, the control signal values of a given antenna element associated with the transmit path are the same as with the receive path. However, since the frequency of the transmitted signal is usually different from the frequency of the received signal, the radio characteristics of the receive path and the transmit path are different. If the differences are substantial, it may be necessary that the control signal values associated with the transmit path and the receive path be different. The present invention supports such cases. 
   With a uniform spaced linear antenna array for isotropic antenna elements (in which the radiation pattern is uniform in all directions), the radiation pattern is determined by
         F(theta)=sum from i=1 to N {A.sub.i*exp(j*2*pi*(i−1)*d/lambda*sin(theta)} (1),
 
where F is a value representing the amplitude and phase of the radiation pattern, theta is the angle of observation with respect to broadside, N is the number of antenna elements, i corresponds to the ith antenna element, A is the coefficient associated with the ith antenna element, lambda is the wavelength of RF operation, d is the spacing between antenna elements, and pi is approximately 3.14159. (Rudge, A. W., et al.,  The Handbook of Antenna Design , Peter Peregrinus Ltd., 1986, pp. 699–697.) The control signal value M.sub.i of the ith attenuator is determined from the magnitude of A.sub.i and the control signal value of the ith phase shifter is determined from the phase of A.sub.i.
       

   Equation 1 can be extended to cases in which the antenna elements are not isotropic (as characterized by any directional antenna such as a dipole antenna) by multiplying the radiation pattern determined in Equation 1 by the radiation pattern of a directional antenna element, assuming that all antenna elements of the linear array antenna are the same. This assumption simplifies the solution to Equation 1 because spatial periodicity is introduced. Because the magnitude and not the phase of the received signal is important, only the absolute value of F(theta) in Equation 1 needs to be determined. Both the variables N and d are known from a given antenna array. The absolute values of F(theta) are precalculated for different values of A.sub.i from which a lookup table is formed. If measurements indicate that signal strength in a given direction, i.e. absolute value of F(theta) needs to modified, values of A.sub.i can be retrieved from the lookup table that corresponds to the change in F(theta). However, one skilled in the art appreciates the fact that basic principles of electromagnetics must be observed. For example, if the total radiated power (integrated over all values of theta) is constant, then if the absolute value of F(theta) is increased in one direction, then the absolute value of F(theta) must be decreased in some other directions. 
   In the exemplary embodiment disclosed herein, the control signal values of each antenna element apply to all calls being served by a given sector at a given time. (Even though  FIG. 1  shows one mobile subscriber unit being served by a base station, e.g. mobile subscriber unit  100  served by base station  103 , a plurality of mobile subscriber units are typically served by a given sector.) In other words, control signal values are not determined for each specific call as may be the case for “smart antennas.” The present determines the resulting updated control signal values for the given sector. These updated control signal values are used until the control values are recalculated during a subsequent study period. With the present invention, measurements include accumulated peg counts determined by call processing (service measurements and call processing failures). 
   Service measurements are typically counts associated with normal call processing including frame error rates on the forward radio channel (downlink) and the reverse radio channel (uplink), calls blocked on either the forward or reverse radio channel, and handoff failures. Call processing failures are additional counts that are generated for specific calls that cannot be sustained such as a dropped call. Measurements generated by either source are accumulated over the study period (e.g. one-hour). 
     FIG. 6  illustrates a call flow in which measurements are accumulated and grouped with respect to sub-sector for each of the disclosed exemplary embodiment. Step  600  activates the process at the beginning of the study period. In step  601  if a specific call event (e.g. a dropped call, blocked call, excessive forward frame error rate, or excessive reverse frame error rate) is detected, MSC  112  requests that locating processor  114  determine the location of mobile subscriber unit  100 ,  101 , or  102  in step  602 . If locating processor  114  cannot determine the location of the mobile subscriber unit, MSC  112  uses a last determined location of the mobile subscriber unit  100 ,  101 , or  102 . MSC  112  uses the location information obtained in step  602  to associate and accumulate the measurement obtained in step  601  with a specific sub-sector. The corresponding counter is incremented in step  604 . This process is continued over the entire study period for all call events. Step  605  determines if the study period is complete. If not, step  601  is repeated; otherwise, step  606  exits the routine (i.e. the study period has ended). The routine in  FIG. 6  provides a collection of counters representing performance metrics associated with each of the sub-sectors. 
     FIG. 7  shows a flow diagram (which may be used by either of the antenna configurations) for analyzing the measurements that are collected by the process shown in  FIG. 6 . The process starts in step  700 . In step  701 , it is determined if a first performance metric of a specific sub-sector, as calculated from the counters from the process of  FIG. 6 , requires an improvement. As an example, dropped calls can be collected for each sub-sector in the process of  FIG. 6 . These measurements can be normalized by the number of calls associated with the given sub-sector to provide a dropped call rate performance metric. If the dropped call rate performance metric is above a threshold (i.e. there are too many dropped calls), as predetermined by the service provider, step  701  indicates that the first performance metric associated with the given sub-sector requires an improvement. In step  702 , updated control values associated with the sub-sectors of the associated sector are determined (e.g. the dropped call rate needs to be reduced). Step  702  must operate within practical constraints of the wireless communications system. For example, if a sub-sector requires an increase of power to improve the first performance metric and if the total power of the associated sector is constant, then power must be allocated from the other sub-sectors and reallocated to the given sub-sector. As another example, increasing the power for one sub-sector may increase the interference to another sub-sector associated with another sector of the base station or of a neighboring base station. Consequently, a second performance metric associated with another sub-sector may be degraded. Step  703  determines if the degradation is within a threshold set by the service provider. If this is the case, step  704  reduces the improvement of the first performance metric. The base station antenna is adjusted in step  705  by applying control signals  116 ,  117 ,  118 ,  119 ,  120 , and  121  to the apparatus shown in  FIG. 5  and the routine is exited in step  706 . 
     FIG. 8  shows an alternative flow diagram for analyzing the measurements collected by the process shown in  FIG. 6 . Either of the antenna configurations may use the process shown in  FIG. 8  in lieu of the process shown in  FIG. 7  As in  FIG. 7 , performance metrics are analyzed for each sub-sector; however, the base station antenna is controlled in an incremental manner. Step  800  initiates the process. As in step  701 , step  801  determines if a first performance metric needs to be improved for a given sub-sector. In step  802 , if any affected sub-sector needs an improvement, then the routine is exited in step  803 . An affected sub-sector is a neighboring sub-sector for which a second performance metric is degraded below an acceptable level with the improvement of the first performance metric. The antenna is adjusted in an incremental manner by step  804  so all affected sub-sectors are degraded within an acceptable level. In step  804 , values for control signals  116 ,  117 ,  118 ,  119 ,  120 , and  121  are calculated and are applied to the apparatus shown in  FIG. 5 . The routine is exited in step  805 . The process in  FIG. 6  is repeated for a subsequent study period, and the process in  FIG. 8  is re-executed causing a subsequent incremental adjustment if necessary. 
   Processing, in accordance with the flow diagrams shown in  FIGS. 6 ,  7 , and  8  may be implemented at MSC  112 , base stations  106 ,  107 , and  108 , or distributed across these entities. 
   It is to be understood that the above-described embodiment is merely an illustrative principle of the invention and that many variations may be devised by those skilled in the art without departing from the scope of the invention. It is, therefore, intended that such variations be included with the scope of the claims.