Abstract:
The present invention presents an improved power control scheme for RF power amplifiers. The gain control signal used to control the power amplifier is subjected to pre-distortion (Hpre) before being supplied to the power amplifier, in order to reduce variations in control loop gain.

Description:
This patent application claims the benefit of priority from U.S. Provisional Patent Application Ser. No. 60/301,909 filed on Jun. 29, 2001. This application incorporates by reference the entire disclosure of U.S. Provisional Patent Application Ser. No. 60/301,909. 

   The present invention relates to telecommunications systems, and in particular to power control in mobile telephone systems. 
   BACKGROUND OF THE INVENTION 
   The requirements for output power control in mobile telephones can often be difficult to achieve. The requirements for GSM, for example, can be found in the ETSI specification “05.05  Digital cellular telecommunications system; Radio transmission and reception ”. There are three critical parameters concerning the transmitter output power.
         Output power level during a constant power part (“mid part”) of a transmitted burst.   Power vs. time, i.e. output power during up-ramping and down-ramping parts of a transmitted burst.   Spectrum due to switching (up- and down-ramping).       

   Several output power classes are specified in the 05.05 document. These power levels should be kept within well-defined tolerances. 
   The “power vs. time” requirements state that the transmitted power should fit within a specified template, of output power versus time. The template can be illustrated as a graph of power vs. time. Adjusting the telephone parameters so that they fit the power vs. time template can be a very time-consuming task during development and critical during manufacturing. 
   The spectrum due to switching requirement means that the spectrum caused by the ramping (switching) process should fit in a specified spectrum mask. It is therefore necessary to have a “good” (reliable) power vs. time behaviour, not only to fulfil the power vs. time template but also to avoid spectrum contamination. 
   It is to be noted that, although the GSM system is used as an example, the ideas presented in this specification could be used in any TDMA (Time Division Multiple Access) system, or any system that requires fast and/or accurate power control, such as CDMA. 
   Arranging the power control so that the telephone fits the power vs. time template and the spectrum due to switching mask, can be a very time-consuming task. In production, good yield is necessary. 
   In  FIG. 1  of the accompanying drawings, the principle of today&#39;s power control solution is shown. A power amplifier  1  is connected to receive an RF input RF in . The power amplifier operates to output an amplified RF signal RF out  to an antenna  2 , as is known and understood. 
   In order to control the power output of the power amplifier, the current, I c , used by the PA (Power Amplifier)  1 , is measured (through a resistor R). This current value provides an indirect measurement of the PA output power. The measurement provided by voltage x 4 =RI c , is fed back for comparison with an input control signal x 1  in an error detection unit  3 . A difference (or error signal), x 2 , is filtered by a loop filter, H LP , to produce a control signal x 3 , which is used for controlling the PA RF output power. Signal x 3  is often called V apc  (apc=amplifier power control). Ideally, the measurement signal x 4  should track the input control signal x 1 . 
   The total transfer function for the control system (H tot =x 4 /x 1 ) can be found from the following:
 
 x   4   =x   3   H   PA   =x   2   H   LP   H   PA =( x   1   −x   4 ) H   LP   H   PA   (1)
 
 x   4 (1 +H   LP   H   PA )= x   1   H   LP   H   PA   (2)
 
   
     
       
         
           
             
               
                 
                   H 
                   tot 
                 
                 = 
                 
                   
                     
                       x 
                       4 
                     
                     
                       x 
                       1 
                     
                   
                   = 
                   
                     
                       
                         H 
                         LP 
                       
                       ⁢ 
                       
                         H 
                         PA 
                       
                     
                     
                       1 
                       + 
                       
                         
                           H 
                           LP 
                         
                         ⁢ 
                         
                           H 
                           PA 
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 3 
                 ) 
               
             
           
         
       
     
   
   Minimizing the difference between x 1  and x 4  would provide an ideal control loop. This means that x 4 /x 1 ≈1, or H LP H PA &gt;&gt;1. 
   Ideally, the transfer function H PA =x 4 /x 3  should be constant(=I c /V apc ). However, in practice, this is not generally the case. As illustrated in  FIG. 2 , the transfer function of the feedback loop typically varies, i.e. the feedback loop gain varies. This variation is due to the variation of the PA transfer function H PA  with the control voltage V apc . Thus, the maximum achievable error reduction of the control system will vary. In the  FIG. 2  example, the loop is practically “open” for low V apc  and high V apc  values, causing poor tracking ability in the control system. For medium V apc  values however, the tracking ability is good, since the loop gain is high. 
   The non-constant behaviour of H PA  will also result in implementation difficulties for the loop filter since the risk of instability is high. The reason for this is that the loop filter must have sufficient gain to achieve good error reduction and fast control even at low or high V apc  values (where H PA  is small). On the other hand, this means increased risk for instability at medium V apc  values (where H PA  is large). 
   SUMMARY OF THE PRESENT INVENTION 
   The invention presented in this document adds a biasing pre-distortion block to the control loop shown in  FIG. 1 . By doing this, the behaviour of the PA control loop will have less loop gain variation, since the gain variations of H PA  is compensated for. 
   Distinguishing properties of the presented solutions are: 
   The power vs. time mask (in GSM specified in 05.05) should be more straightforward to fulfil, since variations in the loop gain due to variations in H PA  are reduced. This will make implementation of suitable loop filters easier. 
   Since power vs. time will be easier to control, this will also mean that it is easier to do the up-ramping and down-ramping in such a way that the spectral contamination is held low. Thus, the requirements on spectrum due to switching (in GSM specified in 05.05) will be easier to fulfil. 
   Although the GSM system is used as an example, the ideas presented in this report could be used in any TDMA (Time Division Multiple Access) system, or any systems that require fast and/or accurate power control. 
   It is emphasised 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 
       FIG. 1  illustrates a prior art power amplifier and control circuit; 
       FIG. 2  illustrates a transfer function of the control circuit of  FIG. 1 ; 
       FIG. 3  illustrates a power amplifier and control circuit embodying one aspect of the present invention; 
       FIG. 4  illustrates respective transfer functions of parts of the circuit in  FIG. 3 ; 
       FIG. 5  illustrates a derivative of one of the transfer functions shown in  FIG. 4 ; 
       FIG. 6  illustrates control loop gain in a practical example of the circuit of  FIG. 3 ; 
       FIG. 7  illustrates a power amplifier and control circuit embodying another aspect of the present invention; and 
       FIG. 8  illustrates a power amplifier and control circuit embodying another aspect of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 3  is a block diagram of a power amplifier and control circuit embodying one aspect of the invention. The  FIG. 3  embodiment is similar to the circuit of  FIG. 1 . However, the  FIG. 3  circuit includes an extra functional block, H pre , in the feedback control loop. The extra block H pre  introduces an additional term in the transfer function of the feedback control loop. The overall transfer function of the feedback control loop can be found from the following, with reference to  FIG. 3 : 
                         x   5     =       ⁢       x   4     ⁢     H   PA                   =       ⁢       x   3     ⁢     H   LP     ⁢     H   PA                   =       ⁢       x   2     ⁢     H   pre     ⁢     H   LP     ⁢     H   PA                   =       ⁢       (       x   1     -     x   5       )     ⁢     H   pre     ⁢     H   LP     ⁢     H   PA                     (   4   )                 x   5 (1 +H   pre   H   LP   H   PA )= x   1   H   pre   H   LP   H   PA   (5) 
   Which gives: 
   
     
       
         
           
             
               
                 
                   H 
                   tot 
                 
                 = 
                 
                   
                     
                       x 
                       5 
                     
                     
                       x 
                       1 
                     
                   
                   = 
                   
                     
                       
                         H 
                         pre 
                       
                       ⁢ 
                       
                         H 
                         LP 
                       
                       ⁢ 
                       
                         H 
                         PA 
                       
                     
                     
                       1 
                       + 
                       
                         
                           H 
                           pre 
                         
                         ⁢ 
                         
                           H 
                           LP 
                         
                         ⁢ 
                         
                           H 
                           PA 
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 6 
                 ) 
               
             
           
         
       
     
   
   To produce an ideal feedback control loop, the difference between x 1  and x 5  should be minimized. This means that x 5 /x 1 ≈1 or in other words H pre H LP H PA &gt;&gt;1. 
   As discussed above, H PA  is not constant. However, it is desirable to make the transfer function H tot =x 5 /x 1  constant. If the loop filter (H LP ) is assumed to be linear (i.e. the gain is independent of input signal), H tot  can be made constant by choosing H pre =kH PA   −1 , where k is a constant. With H pre =kH pa   −1  in equation (6), this gives: 
   
     
       
         
           
             
               
                 
                   H 
                   tot 
                 
                 = 
                 
                   
                     kH 
                     LP 
                   
                   
                     1 
                     + 
                     
                       kH 
                       LP 
                     
                   
                 
               
             
             
               
                 ( 
                 7 
                 ) 
               
             
           
         
       
     
   
   In other words, by introducing a pre-distortion block, H pre =kH PA   −1 , in the feedback control loop, the overall loop gain can be made to be constant (i.e. independent of the power amplifier V apc =x 4 ).  FIG. 4  illustrates such ideal pre-distortion using H pre . In the ideal case, H pre ˜x 4 /x 5 . This can be seen to mean that H pre  is proportional to the inverse of the derivative of the function I c  vs. V apc . The function and its derivative are illustrated in  FIG. 5 . 
   However, it is not always necessary to eliminate completely variations in the loop gain by achieving perfect pre-distortion. In a solution for mobile telephones, for example, it could be acceptable to use an implementation that simply reduces the loop gain variations by a desired amount.  FIG. 6  illustrates the PA transfer function (H PA ) and the resulting overall transfer function (H pre H PA ) following use of the pre-distortion block in a practical embodiment. 
   The pre-distortion function H pre  can be implemented or calculated in several ways. 
   For example, by varying the gain H PA  of the PA  1  in real time, a circuit solution in the analogue domain can be used to determine H pre . An analogue circuit which has x 4  and x 5  as inputs can be used to determine H pre  such that variations in H TOT  are reduced. The signal “gain control” in  FIG. 7  sets the gain of H pre . 
   Alternatively, H pre  can be calculated in the digital domain whilst varying the H PA  in real time. Again, both x 4  and x 5  are used as inputs for deciding proper gain control signal to supply to the pre-distortion block. The gain control signal is calculated to minimise variations in H TOT . As seen in  FIG. 7 , the predistortion gain control network  4  together with the predistortion block H pre  form a signal processing unit/means and may be either an analog or digital signal processing unit. 
   Alternatively, a burst-based learning solution can be used. In such a case, the gain of the predistortion block is first set constant (=1) during one burst. Values of x 4  and x 5  are sampled (collected) during this burst. The PA characteristic, H PA , is thereby obtained and a suitable H pre {x 5 } (or H pre {x 4 }) is then used during the call, for all bursts that have the same nominal (“mid-part”) power as the one that the H pre {x 5 } (or H pre {x 4 }) was meant for. When a “new” power level is requested for the first time during a call, the procedure is repeated, ie. x 4  and x 5  are collected and a H pre {x 5 } (or H pre {x 4 }) for this power level is calculated. H pre {x 5 } (or H pre {x 4 }) for different power levels are stored in a memory (table). When the H pre {x 5 } (or H pre {x 4 }) is to be used, first the memory address containing the table is addressed to find the data H pre {x 5 } (or H pre {x 4 }) associated with the power level that is to be sent. Then, during transmission of the burst, the elements in the table (on the memory address) are addressed with x 5  (or x 4 ). 
   As an alternative x 1  could be used to address the elements in a table containing H pre {x 1 } The learning principle is the same as above, with the exception that not only values of x 4  and x 5  are sampled (collected) during the burst but also x 1 . 
   As a further alternative, a learning procedure can be used during manufacture of the device concerned (telephone, for example). Tables, addressed with x 4  and/or x 5 , are used for deciding proper gain in the pre-distortion block. For each power level (and with H pre =constant=1), values of x 4  and x 5  are sampled (collected). Thus, the PA characteristic, H PA , is obtained and a suitable H pre {x 5 } (or H pre {x 4 }) for each power level can be computed. H pre {x 5 } (or H pre {x 4 }) for different power levels are stored in a memory (table). When the H pre {x 5 } (or H pre {x 4 }) is to be used, first the memory address containing the table is addressed to find the data H pre {x 5 } (or H pre {x 4 }) associated with the power level that is to be sent. Then, during transmission of the burst, the elements in the table (on this memory address) are address with x 5  (or x 4 ). 
   As an alternative, x 1  could be used to address the elements in a table containing H pre {x 1 }. The “learn-up” principle is the same as described above, with the exception that not only values of x 5  and x 5  are sampled (collected) during the burst, but also x 1 . 
   The learning procedure can also be used to produce tables for reference during use. The gain H pre  in the pre-distortion block then depends on what power level the telephone is requested to transmit on during the “mid part” of the burst. 
     FIG. 7  shows the principle of using the real time or burst variation of the H pre , to reduce loop gain variations due to gain variations in H PA . 
   In  FIG. 8 , the principles of using the tabular methods are illustrated. The gain of H pre  is varied in a manner that has been determined by the learning procedure described above. The memory is associated with the H pre  block. The gain control points to a particular memory address and each memory address is associated with a table. Specifically, x1 or x4 or x5 points to a element of that particular table. In other words, there are tables associated with the memory address, which in turn is associated with the memory. 
   Reducing variations in feedback loop gain means that the power vs. time mask in TDMA systems (in GSM this is specified in the 05.05 ETSI document) will probably be easier to fulfil. This will make implementation of suitable loop (filter) easier. 
   Since power vs. time will be easier to control, up-ramping and down-ramping will be possible in such a way that the spectral contamination remains low. Thus, the requirements on spectrum due to switching (in GSM this is specified in the 05.05 ETSI document) will probably be easier to fulfil.