Abstract:
A method and apparatus for dynamically adjusting a power control loop that attempts to maintain either the transmit power of a signal sent from a base station to a mobile station or the signal to noise ratio of a signal sent from the mobile station to the base station above a minimum threshold. Location information that is representative of the distance and morphology between the mobile station and the base station is determined. The minimum threshold of the power control loop is then adjusted in accordance with the location information such that the minimum threshold varies as the distance and the morphology between the mobile station and the base station change. A transmitter sends the signal at a power level that is controlled by the power control loop with the adjusted minimum threshold.

Description:
BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     The present invention relates generally to mobile radio communication systems. More particularly, the present invention relates to power control systems for controlling the power level of signals sent on the forward links of a mobile radio communication system, and for controlling the signal to noise ratios of signals sent on the reverse links of a mobile radio communication system. Even more particularly, the present invention relates to a novel system and method that dynamically adjusts the parameters of forward and reverse link power control loops based upon the position and velocity of a mobile station. 
     II. Description of the Related Art 
     In mobile radio communication systems such as, for example, code division multiple access (CDMA) communication systems, the transmit power levels of signals sent between a base station and a mobile station are carefully controlled using power control loops. Typically, one power control loop is used to maintain the power level of signals on the forward link (i.e., signals sent from the base station to a mobile station) between a minimum and a maximum level. A different power control loop is similarly used to maintain the received signal to noise ratio of signals sent on the reverse link (i.e., the SNR measured at the base station of signals sent from a mobile station to the base station) between a minimum desired level and a maximum desired level. These power control loops typically adjust the transmit power of a signal upward or downward in fixed step wise increments (e.g., plus or minus 1 dB) in order to maintain either the power level or the signal to noise ratio of the signal between the minimum and maximum desired levels. 
     In early CDMA systems, forward link power control was typically accomplished using an open power control loop, whereas the reverse link power control system used a closed power control loop. Current CDMA systems use a closed power control loop for forward link power control and it is expected that future CDMA systems will continue to use a closed power control loop for forward link power control. 
     Each geographic cell in a mobile radio communication system is typically serviced by a base station associated with the cell. In current systems, the parameters of the forward link power control loop (i.e., the minimum power level, maximum power level, and step size associated with the forward link power control loop) and the limits of the reverse link power control loop (i.e., the minimum desired signal to noise ratio, maximum desired signal to noise ration, and step size associated with the reverse link power control loop) do not vary as the location and velocity of mobile units within the cell varies. 
     The transmit power requirements of signals sent between a base station and a mobile station vary as the mobile station moves within a cell. For example, as the mobile station moves closer to the base station, less transmit power is often required on the reverse link to achieve the same signal-to-noise ratio of the received signal at the base station. Thus, as the mobile station moves closer to the base station, the transmit power on the reverse link can often be reduced while maintaining the same signal-to-noise ratio of the received signal at the base station. In addition, less transmit power is also often required when the mobile station is stationary or moving slowly (as opposed to moving quickly) to achieve the same signal-to-noise ratio of the received signal at the base station. Thus, as the mobile station slows down, the transmit power on the reverse link can often be reduced while maintaining the same signal-to-noise ratio of the received signal at the base station. 
     The capacity of a given mobile radio communication system is limited by the transmit power available to the base station and the signal to noise ratio requirement on the reverse link. Thus, the capacity of the system could be increased if the base station transmit power and/or the signal to noise ratio on the reverse link in the system are reduced. In view of this, it would be desirable to have a power control system that could dynamically adjust the parameters of the forward and reverse link power control loops in response to the changing position and/or speed of a mobile station, so as to conserve transmit power and increase cell capacity. 
     SUMMARY OF THE INVENTION 
     The present invention maximizes the capacity of a given cell by adjusting one or both limits of a power control loop based on the position of a mobile station within the cell. In one embodiment, the present invention dynamically adjusts the parameters of a power control loop that attempts to maintain the transmit power of a signal sent from a base station to a mobile station above a minimum threshold. In this embodiment, location information that is representative of the distance and the morphology between the mobile station and the base station is determined. The minimum threshold of the power control loop is then adjusted in accordance with the location information such that the minimum threshold varies as the distance and the morphology between the mobile station and the base station change. A transmitter then transmits the signal at a power level that is controlled by the power control loop with the adjusted minimum threshold. This aspect of the invention can be used for controlling the minimum power level associated with the forward link power control loop. 
     In a further embodiment, the present invention dynamically adjusts the parameters of a power control loop that attempts to maintain the signal to noise ratio of a signal sent from the mobile station to the base station above a minimum threshold. In this embodiment, location information that is representative of the distance and the morphology between the mobile station and the base station is determined. The minimum threshold of the power control loop is then adjusted in accordance with the location and the morphology information such that the minimum threshold varies as the distance and the morphology between the mobile station and the base station change. A transmitter then transmits the signal at a power level that is controlled by the power control loop with the adjusted minimum threshold. This aspect of the invention can be used for controlling the minimum desired signal to noise ratio (i.e., the minimum desired signal to noise ratio of a reverse link signal received at the base station) in a reverse link power control loop. 
     In accordance with further alternative embodiments, the present invention also determines velocity information that is representative of the velocity of the mobile station. In these embodiments, the minimum threshold of a power control loop is adjusted based on the velocity information alone or, alternatively, the minimum threshold of the power control loop is adjusted based on both the velocity information and the current position of the mobile station. This aspect of the invention can also be used for controlling the minimum thresholds associated with both the forward and reverse link power control loops, and is especially useful in mixed wireless networks where there are both fixed position wireless users (also referred to as wireless local loop users) and mobile wireless users (i.e., wireless users that change position.) In such mixed wireless networks, adjustment of the minimum thresholds associated with both the forward and reverse link power control loops based on the velocity of wireless user ensures efficient allocation of system resources. 
     In accordance with a still further aspect, the maximum threshold of a forward link power control loop (i.e., the power level below which the power control loop attempts to maintain the transmit power of the signal sent from the base station) is also adjusted in accordance with the location information (and optionally the velocity information) such that the maximum threshold varies as the distance and the morphology between the mobile station and the base station (and optionally the velocity of the mobile station) vary. 
     In accordance with yet a further aspect, the maximum threshold of a reverse link power control loop (i.e., the maximum desired signal to noise ratio below which the power control loop attempts to maintain the signal to noise ratio of a reverse link signal received at a base station) is also adjusted in accordance with the location information (and optionally the velocity information) such that the maximum threshold varies as the distance and the morphology between the mobile station and the base station (and optionally the velocity of the mobile station) vary. 
     In accordance with a still further aspect, the step size of a power control loop (i.e., the step wise power increment used in the power control loop to adjust the transmit power of the signal upward or downward) is also adjusted in accordance with the velocity information such that the step size varies as the velocity of the mobile station varies. In accordance with this aspect, the step size used by the power control loop is preferably reduced for stationary or slow moving mobile stations, and the step size is increased when a mobile station begins moving more quickly. This aspect of the invention can be used for controlling the step size of either a forward link or reverse link power control loop. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the invention can be appreciated, a more particular description of the invention briefly described above will be rendered by reference to a specific embodiment thereof which is illustrated in the appended drawings. Understanding that these drawings depict only a typical embodiment of the invention and are not therefore to be considered limiting of its scope, the invention and the presently understood best mode thereof will be described and explained with additional specificity and details through the use of the accompanying drawings in which: 
     FIG. 1 is a flow diagram of a method for adjusting the minimum and maximum limits of the forward and reverse link power control loops in a mobile radio communication system, in accordance with a preferred embodiment of the present invention. 
     FIG. 2 is a map showing a cell with several geographical regions each of which has a different minimum power threshold associated therewith for controlling the transmit power of reverse link signals transmitted from such regions to a base station, in accordance with a preferred embodiment of the present invention. 
     FIG. 3 is a block diagram showing the components of an exemplary CDMA mobile station used to implement the power control systems of the present invention. 
     FIG. 4 is a block diagram showing the components of an exemplary CDMA base station used to implement the power control systems of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, there is shown a flow diagram of a method for adjusting the minimum limits, maximum limits, and step size increments of the forward and reverse link power control loops in a mobile radio communication system, in accordance with a preferred embodiment of the present invention. In step  110 , a position P(x, y) of the mobile station is determined using, for example, measurements from both the Global Positioning Satellite (GPS) system and the system infrastructure. The position of the mobile station is preferably determined in this step using differential GPS and in accordance with the method disclosed in U.S. patent application Ser. No. 09/040,501, entitled “System and Method for Determining The Position of a Wireless CDMA Transceiver,” filed Mar. 17, 1998, which is owned by the assignee of the present invention, and the contents of which is incorporated herein in their entirety by reference. In an alternate embodiment, the position of the mobile station can be determined using at least three (and preferably more) terrestrial base stations in the system infrastructure. 
     In step  120 , the motion of the mobile station is modeled (preferably using past positional measurements) in order to estimate the current velocity (or a range of potential velocities) of the mobile station. Map information, which places the mobile station on a street or highway, which is oriented in a known direction, may also be used to model the current velocity of the mobile station. Filtering methods can further enhance this trajectory estimation technique. A good example is to use a Kalman filter to adaptively track the mobile trajectory and hence predict its dynamic state in terms of speed and position. 
     In step  130 , the position of the mobile station is applied to a look up table associated with a cell (such as Table I below) in order to determine the upper and lower limits of the forward and reverse link power control loops. 
     
       
         
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 Position 
                   
                 Reverse Link PC Loop 
               
               
                 Information 
                 Forward Link PC 
                 Limits 
               
             
          
           
               
                 Lon- 
                   
                 Loop Limits 
                   
                 Desired 
               
             
          
           
               
                 gi- 
                 Lat- 
                 Power 
                 Power 
                 Desired SNR 
                 SNR 
               
               
                 tude 
                 itude 
                 Ceiling 
                 Floor 
                 Maximum 
                 Minimum 
               
               
                   
               
               
                 x 1   
                 y 1   
                 PMAX1 FL   
                 PMIN1 FL   
                 SNR_MAX1 RL   
                 SNR_MIN1 RL   
               
               
                 x 2   
                 y 2   
                 PMAX2 FL   
                 PMIN2 FL   
                 SNR_MAX2 RL   
                 SNR_MIN2 RL   
               
               
                 x 3   
                 y 3   
                 PMAX3 FL   
                 PMIN3 FL   
                 SNR_MAX3 RL   
                 SNR_MIN3 RL   
               
               
                 x 4   
                 y 4   
                 PMAX4 FL   
                 PMIN4 FL   
                 SNR_MAX4 RL   
                 SNR_MIN4 RL   
               
               
                   
               
             
          
         
       
     
     In one embodiment, the power control loop values stored in the table are determined empirically by collecting data reflecting the power requirements of a mobile user as the user moves to different locations within a cell. For example, the data can be collected by giving a mobile station with position location capability to a postman and monitoring the power requirements of the mobile station as the post man moves about the cell making postal deliveries. In this embodiment, the forward link power levels and the reverse link signal to noise ratio are monitored and stored at different locations in the cell as the mobile station moves about the cell. Thereafter, for each location or region in the cell being monitored, the SNR_MAX RL  and SNR_MIN RL  values—which represent the minimum and maximum desired signal to noise ratios associated with the reverse link power control loop to be applied at the location or region—are select by, for example, adding and subtracting predetermined margins from the reverse link signal to noise ratio of the reverse link signal that was measured at the base station when the mobile station was in the location or region. In other words, the SNR_MAX RL  value for the region will be set by adding a predetermined margin of dB to the reverse link signal to noise ratio that was measured at the base station when the mobile station was in the location or region, and the SNR_MIN RL  value for the region will be set by subtracting a predetermined margin of dB from the reverse link signal to noise ratio that was measured at the base station when the mobile station was in the location or region. Similarly, for each location or region in the cell being monitored, the PMAX FL  and PMIN FL  values—which represent the power ceiling and floor of the forward link power control loop to be applied at the location or region—are selected by, for example, adding and subtracting predetermined power margins from the measured forward link power level in the location or region. In other words, the PMAX FL  value for the region will be set by adding a predetermined margin of dB to the forward link power level that was measured when the mobile station was in the location or region, and the PMIN FL  value for the region will be set by subtracting a predetermined margin of dB from the forward link power level that was measured when the mobile station unit was in the location or region. 
     In step  130 , after the PMAX FL , PMIN FL , SNR_MAX RL  and SNR_MIN RL  values are selected from the look up table, the values are adjusted based on the velocity of the mobile station. This aspect of the invention recognizes that low mobility users (e.g., mobile stations moving at less than 5 mph or pedestrian speed) will experience less fading than higher mobility users and that, as a result, the forward and reverse links associated with lower mobility users can operate within tighter power control limits. In accordance with this aspect of the invention, in cases where the user is moving below a predetermined speed such as 5 mph, a predetermined amount (e.g., 2 dB) is subtracted from the PMAX FL  and SNR_MAX RL  values and a predetermined amount (e.g., 2 dB) is added to the PMIN FL  and SNR_MIN RL  values before the PMAX FL , PMIN FL , SNR_MAX RL  and SNR_MIN RL  values are applied to the power control loops in steps  140  and  150 . This aspect of the invention similarly recognizes that high mobility users (e.g., mobile stations moving at more than 35 mph) will experience more fading than lower mobility users and that, as a result, the forward and reverse links associated with such higher mobility users should operate within wider power control limits. In accordance with this aspect of the invention, in cases where the user is moving above a predetermined speed such as, for example, 35 mph, a predetermined amount (e.g., 2 dB) is added to the PMAX FL  and SNR_MAX RL  values and a predetermined amount (e.g., 2 dB) is subtracted from the PMIN FL  and SNR_MIN RL  values before the PMAX FL , PMIN FL , SNR_MAX RL  and SNR_MIN RL  values are applied to the power control loops in steps  140  and  150 . 
     Adjustment of the PMAX FL , PMIN FL , SNR_MAX RL  and SNR_MIN RL  based on the velocity of the mobile station as is done in step  130  is especially important in mixed wireless networks where there are both fixed position wireless users (also referred to as wireless local loop users) and mobile wireless users (i.e., wireless users that change position.) In such mixed wireless networks, adjustment of the minimum thresholds associated with both the forward and reverse link power control loops based on the velocity of the wireless users ensures efficient allocation of system resources. For purposes of the present invention, the term “mobile station” shall refer to a mobile station such as mobile station  300  shown in FIG. 3, regardless of whether the user of such station is a fixed position wireless user or a mobile wireless user. 
     In step  130 , the velocity of the mobile station is also applied to a second look up table (such as Table II below) in order to determine the power adjustments step sizes to be used in the forward and reverse link power control loops: 
     
       
         
               
               
               
             
           
               
                 TABLE II 
               
               
                   
               
               
                   
                 Forward Link PC 
                 Reverse Link PC Loop 
               
               
                 Velocity Information 
                 Loop Step Size 
                 Step size 
               
               
                   
               
             
             
               
                 0 &lt; Velocity &lt; v 1   
                 STEP1 FL   
                 STEP1 RL   
               
               
                 v 1  &lt; Velocity &lt; v 2   
                 STEP2 FL   
                 STEP2 RL   
               
               
                 v 2  &lt; Velocity 
                 STEP3 FL   
                 STEP3 RL   
               
               
                   
               
             
          
         
       
     
     The values stored in Table II preferably reflect that a smaller step size will be required to adequately maintain the power or signal to noise ratio of a signal within predetermined limits in cases where the mobile station is moving more slowly. Where v 1  and v 2  are equal to 5 mph and 35 mph, respectively, exemplary values for STEP1 RL , STEP2 RL  and STEP3 RL  are 0.25 dB, 0.5 dB and 1.0 dB, and exemplary values of for STEP1 FL , STEP2 FL  and STEP3 FL  are _, _ and 1 times the normal power increment step size on the forward link power control loop. Use of this aspect of the invention reduces the power fluctuations on the forward and reverse links by keeping at a minimum the size of each power adjustment made by the power control loops. 
     Although in the embodiment described above, the power control loop parameters were adjusted in step  130  by predetermined amounts based on the velocity of the mobile station, it will be understood by those skilled in the art that the particular amounts used to adjust the power control loop parameters represent a matter of design choice and that optimal values may be determined using empirical data for a given cell. 
     In steps  140  and  150 , the PMAX FL , PMIN FL , SNR_MAX RL  and SNR_MIN RL  values from the first look up table (which have been adjusted based on the mobility of the mobile station) are applied to the forward and reverse link power control loops and used as the upper and lower power limits of these control loops. Similarly, the STEP RL  and STEP FL  values from the second look up table are applied to the forward and reverse link power control loops and used as the step size used for power adjustment by these control loops. The process shown in FIG. 1 is preferably repeated at regular intervals so that the power control loops are regularly updated as a mobile station moves about a cell and/or its mobility varies. 
     Although in the preferred embodiment of the present invention, the PMAX FL , PMIN FL , SNR_MAX RL  and SNR_MIN RL  values from the look up table are adjusted based on the mobility (or velocity) of the mobile station before being applied to the forward and reverse link power control loops, in alternative embodiments the PMAX FL , PMIN FL , SNR_MAX RL  and SNR_MIN RL  values from the look up table may be applied to the forward and reverse link power control loops without any adjustment based on the mobility (or velocity) of the mobile station. In further alternate embodiments of the present invention, the PMAX FL , PMIN FL , SNR_MAX RL  and SNR_MIN RL  values may be adjusted based only on the mobility or velocity of the mobile station (and not the location of the mobile station within the cell) before being applied to the forward and reverse link power control loops by making the PMAX FL , PMIN FL , SNR_MAX RL  and SNR_MIN RL  values in the look up table location independent (i.e., in these embodiments the values in the table are set so that they do not vary based on position of the mobile station within the cell). 
     Referring now to FIG. 2, there is shown a map of a cell  200  with several geographical regions (R 0 , R 1 , R 2  and R 3 ) each of which has different minimum and maximum thresholds associated therewith for controlling the transmit power of forward and reverse link signals transmitted from such regions to a base station. In this embodiment, the limits of the forward and reverse link power control loops vary in a stepwise manner based on regions within the cell. Thus, one set of PMAX FL , PMIN FL , SNR_MAX RL  and SNR_MIN RL  values will be extracted from the look up table in step  130  whenever the mobile station is operating in region R 0 , a second set of PMAX FL , PMIN FL , SNR_MAX RL  and SNR_MIN RL  values will be extracted from the look up table whenever the mobile station is operating in region R 1 , and so on. It will be understood by those skilled in the art that the PMAX FL , PMIN FL , PSN_MAX RL  and SNR_MIN RL  values could alternatively vary in a more continuous or less regular manner as a function of the distance of the mobile station from the center of the cell. 
     Referring now to FIG. 3, there is a block diagram showing the components of an exemplary code division multiple access (CDMA) mobile station  300  used for implementing the power control systems of the present invention. The mobile station includes an antenna system  330 , which is coupled through diplexer  332  to analog receiver  334  and transmit power amplifier  336 . Antenna system  330  and diplexer  332  are of standard design and permit simultaneous reception and transmission through one or more antennae. Antenna system  330  optionally includes one antenna for voice traffic, and a separate antenna for receiving GPS signals. Antenna system  330  collects signals transmitted to the mobile station from one or more base stations and the GPS system, and provides the signals though diplexer  332  to analog receiver  334 . Receiver  334  is also provided with an analog to digital converter (not shown). Receiver  334  receives RF signals from diplexer  332 , amplifies and frequency downconverts the signals, and provides a digitized output signal to digital data receivers  340 ,  342  and to search receiver  344 . It will be understood that, although in the embodiment in FIG. 3 only two digital data receivers are shown, a low performance mobile station might have only a single digital data receiver while higher performance units will have two or more digital data receivers to permit diversity reception. The outputs of receivers  340  and  342  are provided to diversity and combiner circuitry  348  which time adjusts the two streams of data received from the receivers  340  and  342 , adds the streams together and decodes the result. Details concerning the operation of the digital data receivers  340 ,  342 , the search receiver  344 , and the diversity combiner and decoder circuitry  348  are described in U.S. Pat. No. 5,101,501 entitled “Method and Apparatus for Providing A Soft Handoff In Communications In A CDMA Cellular Telephone System”, which is assigned to the assignee of the present invention and incorporated herein by reference. 
     An output signal is provided from the decoder  348  to control processor  346 . The output signal from the decoder will include, for example, any pilot signals from the base stations that will be used for making positional measurements, timing signals received from the GPS system, and other information such as modeling information and base station position information sent from a base station to the mobile unit. In response to this information, the control processor  346  determines the forward and/or reverse link power control parameters in accordance with the method shown in FIG.  1  and the reverse link power control parameters are used with the power control unit in the mobile station to control the power level of the reverse link signals from the mobile station. All the steps shown in FIG. 1 are preferably implemented in software on control processor  346 ; however, it will be understood by those skilled in the art that many of these steps could alternatively be implemented in the system infrastructure. 
     Referring now to FIG. 4, there is shown a block diagram of the components of an exemplary CDMA base station  400  used for implementing the forward and reverse link power control systems of the present invention. At the base station, two receiver systems are utilized with each having a separate antenna and analog receiver for diversity reception. In each of the receiver systems, the signals are processed identically until the signals undergo a diversity combination process. The elements within the dashed lines correspond to elements corresponding to the communications between the base station and one mobile station. Referring still to FIG. 4, the first receiver system is comprised of antenna  460 , analog receiver  462 , searcher receiver  464  and digital data receivers  466  and  468 . The second receiver system includes antenna  470 , analog receiver  472 , searcher receiver  474  and digital data receiver  476 . Cell-site control processor  478  is used for signal processing and control. Among other things, cell site processor  478  monitors the signals sent to and received from a mobile station in order to determine the position information used in the present, invention, and this processor is also preferably used for performing the modeling of the mobile velocity in step  120 . Cell site processor  478  optionally determines the forward and/or reverse link power control parameters in accordance with the method shown in FIG. 1, and the forward link power control parameters are used with the power control unit in the base station to control the power level of the forward link signals from the base station. 
     Both receiver systems are coupled to diversity combiner and decoder circuitry  480 . A digital link  482  is used to communicate signals from and to a base station controller or data router under the control of control processor  478 . Signals received on antenna  460  are provided to analog receiver  462 , where the signals are amplified, frequency translated and digitized in a process identical to that described in connection with the mobile station analog receiver. The output from the analog receiver  462  is provided to digital data receivers  466  and  468  and searcher receiver  464 . The second receiver system (i.e., analog receiver  472 , searcher receiver  474  and digital data receiver  476 ) processes the received signals in a manner similar to the first receiver system. The outputs of the digital data receivers  466 ,  476  are provided to diversity combiner and decoder circuitry  480 , which processes the signals in accordance with a decoding algorithm. Details concerning the operation of the first and second receiver systems and the diversity combiner and decoder  980  are described in U.S. Pat. No. 5,101,501 entitled “Method and Apparatus for Providing A Soft Handoff In Communications In A CDMA Cellular Telephone System”, incorporated above. Signals for transmission to mobile units are provided to a transmit modulator  484  under the control of processor  478 . Transmit modulator  484  modulates the data for transmission to the intended recipient mobile station. 
     Although the present invention has been described in connection with mobile radio communication systems that use CDMA modulation for transmitting signals between the mobile station and the base stations in the system, it will be understood by those skilled in the art that the teachings of the present invention could be applied in mobile radio communications that use other modulation methods such as, for example, time division multiple access modulation, for communicating between the mobile station and base stations in the system. 
     The previous description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to the embodiments described above will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of inventive faculty. Thus, the present invention is not intended to be limited to the methods and apparatuses shown herein but is to be accorded the widest scope consistent with the claims set forth below.