Abstract:
A method for controlling a radio frequency (RF) transmitter, the method having an integrating controller to produce a reference value of a first quality measure from a first error signal; producing an estimated value of the first quality measure relating to an actual value of the first quality measure; and supplying a tracking signal related to the estimated value of the first quality measure and the reference value of a first quality measure to the reference integrating controller.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. application Ser. No. 10/528,639, filed 2005Sep. 28, now pending, which was the National Stage of International Application No. PCT/EP03/10588 filed Sep. 23, 2003, which claims benefit of U.S. Provisional Application 60/500,427 filed Sep. 5, 2003 and U.S. Provisional Application 60/422,939 filed Oct. 31, 2002. This application incorporates by reference the entire disclosure of U.S. Provisional Application 60/500,427 and 60/422,939. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable 
       REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX 
       [0003]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0004]    The present invention relates to power control in telecommunications networks, and, in particular, in RF mobile telephone networks and systems. 
       BACKGROUND OF THE INVENTION 
       [0005]    Power control is important in mobile telephone networks, for example, because it is important to obtain desirably high capacity and efficiency, particularly in COMA systems. The variable that is controlled is called quality. Quality of the communication is controlled with reference to a quality measure such as BER (Bit Error Rate), FER (Frame Erasure Rate, BLER (Block Error Rate), number of iterations of a turbo decoder, or the average reliability of decision statistics. Below, for the sake of brevity, the quality measurement will be referred to as BLER. It will, however, be readily appreciated that BER or FER, or other quality measurement could be used. Usually an integrating controller is provided to achieve a steady state performance with zero control error. The control scheme used is cascade control, see for example  FIG. 1  of the accompanying drawings. The idea with cascade control is to make an inner control loop ( 2 ) much faster than an outer control loop ( 4 ). For transmission power control (TPC) the inner loop controls another quality measure such as for example the signal to interference ratio (SIR). The outer loop sets the SIR reference value SIRr for the inner loop. The goal of the outer loop is to control the SIR reference value to achieve a BLER that is equal to the BLER reference BLERr. To get a control system that in steady state achieves a BLER that is equal to the BLER reference, an integrating controller ( 9 ), which can be, for example, a PI controller, a PID controller, or a pure integrating controller, can be used. The cascade controller illustrated in  FIG. 1  comprises an inner control loop ( 2 ) and an outer control loop ( 4 ). Both control loops have an input of a received signal (y(k)). In the outer control loop  4 , the BLER is estimated in a BLER estimation unit ( 5 ) and compared with a BLER reference signal. A subtractor ( 7 ) calculates the difference between the reference signal and the BLER estimate to supply an input signal to an integrating controller ( 9 ). The integrating controller ( 9 ) produces a SIR reference signal. The SIR reference signal is compared with an SIR estimate from an SIR estimation unit ( 3 ) in the inner control loop ( 2 ). The difference between the SIR reference and the SIR estimate is supplied to a function, for example a step function ( 11 ) for determining a command u(k) that sets transmission power. More generally, the SIR estimate and the SIR reference value could both be supplied to a function that determines a command u(k) for setting the transmission power. 
         [0006]    A known problem with an integrating controller (such as a PI, PID, or pure integrating controller) is that it becomes unstable if the control signal saturates. This problem is often referred to as the windup problem. Transmission power control (TPC) saturation of the control signal corresponds to situations when the maximum (or minimum) transmitter power is used. The windup problem in the power control algorithms for third generation mobile telephony systems is well known. The specific problem of windup protection in WCDMA makes several additions to anti-windup schemes used in other areas necessary. As is well known, integrating controllers have the nice property of being able to achieve zero control error in steady state. As an example of an integrating controller, a continuous time PI-controller is shown in  FIG. 2 : Discrete time controllers have similar behaviour; see for example, Karl Johan Astrom and Tore Hagglund, “PID Controllers: Theory, Design and Tuning”, Instrument Society of America, Research Triangle Park, NC, second edition, 1995. A known problem with integrating controllers is that the integrator part turns unstable when the control signal saturates. This instability occurs because feedback from the process is needed to stabilize the controller, which is not open loop stable. In the case of transmission power control, saturation can occur when maximum (or minimum) transmission power is used. In this situation the transmission power can only be decreased (or increased in the case of a minimum), which can be seen as open loop operation of the integrator. As the controller is not open loop stable the controller state (the integrator, I-part) can start to build up a large state. This usually results in that it takes a long time for the control loop to start functioning again after the saturation state is left. This problem is usually referred to as the windup problem. 
       SUMMARY OF THE PRESENT INVENTION 
       [0007]    According to one aspect of the present invention there is provided a method for controlling a radio frequency (RF) transmitter, the method having an integrating controller to produce a reference value of a first quality measure from a first error signal; producing an estimated value of the first quality measure relating to an actual value of the first quality measure; and supplying a tracking signal related to the estimated value of the first quality measure and the reference value of a first quality measure to the reference integrating controller. 
         [0008]    According to another aspect of the present invention, there is provided a controller for controlling a radio frequency (RF) transmitter, the method having an integrating controller operable to produce a reference value of a first quality measure from a first error signal; an estimator operable to produce an estimated value of the first quality measure relating to an actual value of the first quality measure; and a tracking unit operable to supply a tracking signal related to the estimated value of the first quality measure and the reference value of a first quality measure to the reference integrating controller. 
         [0009]    It is emphasized that the term “comprises” or “comprising” is used in this specification to specify the presence of stated features, integers, steps or components, but does not preclude the addition of one or more further features, integers, steps or components, or groups thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  illustrates a controller for a third generation mobile system; 
           [0011]      FIG. 2  illustrates an integrating controller; 
           [0012]      FIG. 3  illustrates a simplified model of a quality control process; 
           [0013]      FIG. 4  illustrates a controller according to an exemplary embodiment of the present invention; and 
           [0014]      FIG. 5  illustrates the PI controller of  FIG. 2  with a tracking signal input. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0015]    To explain the present invention, a simplified model for the quality control process is illustrated in  FIG. 3 . The SIR control loop is modelled as a saturation ( 2 ˜) and a delay ( 23 ). 
         [0016]    The process that maps SIR to BLER is modelled as a static function ( 25 ). This is not important for the invention and can be modelled by any suitable means. A problem is that SIR and BLER cannot be directly measured. SIR and BLER can only be estimated. This is indicated in  FIG. 3  with two estimation noises v k  and w k . BLER is usually estimated by evaluating the CRC flags of received blocks for a period of time. 
         [0017]    In WCDMA systems, SIR is usually estimated by using so-called pilot symbols transmitted from the base station. Pilot symbols are predetermined symbols that are known to both the base station and the mobile terminal. By observing how the pilot symbols are received in the mobile terminal, the SIR can be estimated. The estimation is split in two parts, estimation of signal power, and estimation of interference power. The signal power is estimated by observing with what power the pilot symbols are received. The interference power is estimated by observing how large variation that is seen in the received pilot symbols. The estimated SIR is then calculated as the ratio of the signal power estimate and the interference power estimate. 
         [0018]    When transmission power saturates (i.e. when the maximum or minimum transmission power is used) the actual and estimated SIR will no longer follow SIRr (SIR reference). In the tracking approach of the present invention the difference between the estimated SIR and SIRr is calculated and fed back to stabilize the integrating controller. If v k  is small, the difference will be small, except when the transmission power is saturated. 
         [0019]      FIG. 4  is a schematic illustration of a tracking solution according to an exemplary embodiment of the present invention. The components of  FIG. 3  are shown, namely the saturation ( 21 ), delay ( 23 ) and the static mapping function ( 25 ). A reference SIR (SIR r ) is input to this model to produce an SIR estimate (SIR est ) and a BLER estimate (BLER est ). 
         [0020]    A reference BLER (BLER r ) is supplied via a log function ( 31 ) to a subtractor ( 33 ). Also supplied the subtractor ( 33 ) is the BLER estimate, via a log function ( 41 ), so that the subtractor ( 33 ) produces an error e in the desired quality measure, the error being equal to the difference between the reference BLER and the estimated BLER. The log functions are introduced to ensure that the control loop behaves in a linear fashion, and is not important for the invention. 
         [0021]    The tracking solution is illustrated by elements ( 35 ), ( 37 ), and ( 39 ). A controller  35  (C-BLER) receives as one input the error signal e. The controller also receives a tracking signal e s , The controller produces a signal representing the reference SIR for supply to the SIR control loop. The reference SIR signal is also supplied, via a delay element  37 , to a subtractor  39  which produces the tracking signal by subtracting the delayed reference SIR signal SIR r  from the estimated SIR signal SIR est . 
         [0022]    A PI-controller with tracking signal input to the integrator is shown in  FIG. 5 . As before, a continuous time loop is shown, but a discrete time version is easily derived and would have similar behaviour. 
         [0023]      FIG. 5  illustrates an exemplary controller  35  in more detail. As can be seen, the controller includes a gain element  43  of gain K which receives an input e and supplies an output e*K to an adder  44 . The error signal e is also supplied to a component  45  having a transfer function K/Ti (where Ti is the integration time) whose output is supplied to an adder  46 . A second input of the adder  46  is provided by the output from a second component  49  having a transfer function I/T t  (where T t  is the tracking time) as supplied with the error signal e s . The output of the adder  46  is integrated by the integrator  47  (I/s) and supplied to the adder  44 . The output of the adder  44  gives the reference SIR signal. It can be seen that the controller  35  provides the following transfer function as given in equation 1. 
         [0000]    
       
         
           
             
               
                 
                   
                     SIR 
                     r 
                   
                   = 
                   
                     
                       e 
                       * 
                       K 
                     
                     + 
                     
                       
                         1 
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                         ( 
                         
                           
                             
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         [0024]    An alternative implementation would be to use the estimated tracking signal e s  to do “conditional integration”. In such an implementation the integrator part is not updated if e s  is larger than a threshold, i.e. if |e s |&gt;e threshold  the integrator is not updated. This solution also prevents the integrator state to build up a large value in scenarios of power saturation. 
         [0025]    One exemplary implementation of the tracking arrangement includes to filter e s  and use a dead zone. 
         [0026]    This makes the impact of estimation errors smaller in the case when power is not saturated. The classical implementation of a dead-zone is a block with the following function (input: u, output: y, dead-zone parameter: U d ): 
         [0000]    
       
         
           
             
               
                 
                   y 
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         [0027]    The invention is a new application of the tracking approach to the windup problem. The major improvement compared to existing approaches are that the saturation is estimated by comparing SIR r  and SIR est  to produce a tracking signal e_s. The invention is applicable to transmission power control systems in both the up-link and the down-link.