Patent Publication Number: US-7724839-B2

Title: Multilevel LINC transmitter

Description:
This application claims the benefit of U.S. Provisional Application No. 60/807,952, filed on Jul. 21, 2007. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a LINC transmitter and, in particular, to a multilevel LINC transmitter. 
   2. Description of the Related Art 
   To prolong battery life of mobile handset devices, demands on power efficiency of wireless mobile communication systems have become more important. In general, the most power hungry device in a transceiver is a power amplifier which has nonlinear characteristics. In addition, modulation of non-constant-envelope signals demands high linearity of a power amplifier. As a result, a trade off between linearity and power efficiency in a wireless transmitter is necessary. 
   Various PA linearization techniques have been adopted to improve linearity and power efficiency of wireless transmitters. Linear amplification with nonlinear components (LINC) is a transmitter architecture which increases linearity and power efficiency of a wireless transmitter. Due to accurate signal processing and insensitivity to process variation, a digital LINC architecture is more suitable for modern process technologies. 
     FIG. 1  is a block diagram of a conventional LINC architecture. As shown in  FIG. 1 , an input signal S(t) of the LINC  100  is a varying envelope signal. A signal separator  110  receives and divides the input signal S(t) into two constant-envelope signals S 1  and S 2 . Subsequently, two power amplifiers PA 1  and PA 2  respectively amplify the constant-envelope signals S 1  and S 2 . Since a nonlinear power amplifier can amplify a constant-envelope signal linearly, two power efficient nonlinear power amplifiers are used in such architecture. Finally, the two amplified signals are combined by a power combiner  120 . Thus, a linearly amplified signal is obtained at an output of the power combiner  120 . 
   The input of the LINC system is a varying-envelope signal S(t),
 
 S ( t )= A ( t )· e   jφ(t)  
 
wherein A(t) denotes the signal envelope and φ(t) is signal phase. In the phasor diagram shown in  FIG. 2A , the varying-envelope signal S(t) is split into a set of constant-envelope signals, S 1 (t) and S 2 (t),
 
                         S   ⁡     (   t   )       =       ⁢       1   2     ⁡     [         S   1     ⁡     (   t   )       +       S   2     ⁡     (   t   )         ]                   =       ⁢       1   2     ⁢       r   0     ⁡     [       ⅇ     j   ⁡     (       φ   ⁡     (   t   )       +     θ   ⁡     (   t   )         )         +     ⅇ     j   ⁡     (       φ   ⁡     (   t   )       -     θ   ⁡     (   t   )         )           ]                       
And an out-phasing angle θ(t) is expressed as
 
             θ   ⁡     (   t   )       =       cos     -   1       ⁡     (       A   ⁡     (   t   )         r   0       )             
Both S 1 (t) and S 2 (t) are on a circle with a radius r 0 . In a conventional LINC transmitter, r 0  is a constant scale factor predefined by a system designer. Because input range of an inverse cosine function is [−1, 1], selection of r 0  needs to satisfy the formula:
   r   0 ≧max( A ( t )) 
     FIG. 2B  illustrates the signals after amplification. The amplified signals are expressed as G·S 1 (t) and G·S 2 (t), where G is voltage gain of the power amplifiers. The two amplified signals are combined by a power combiner to obtain a signal √{square root over (2)}G·S(t) which is a linear amplification of the input signal S(t). Because of the out-phasing technique, LINC achieves linear amplification with two power efficient nonlinear power amplifiers. 
   BRIEF SUMMARY OF THE INVENTION 
   An embodiment of a multilevel LINC transmitter comprises a multilevel signal component separator, a phase modulator block, and an RF block. The multilevel signal component separator comprises a multilevel scaler and converts an input signal to phase signals. The phase modulator block is coupled to the multilevel signal component separator. The RF block comprises a plurality of power amplifiers coupled to the phase modulator block and the multilevel scaler and a power combiner coupled to the power amplifiers. 
   The invention provides a multilevel LINC transmitter with a multilevel scaler in a multilevel signal component separator thereof. The multilevel scaler dynamically adapts a scale factor according to the input signal and therefore the out-phasing angle is adjustable. As a result, high power efficiency and linearity are achieved. 
   A detailed description is given in the following embodiments with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
       FIG. 1  is a block diagram of a conventional LINC architecture; 
       FIGS. 2A and 2B  are respectively phasor diagrams of a signal and components thereof before and after amplification; 
       FIG. 3  is a block diagram of a multilevel LINC transmitter according to an embodiment of the invention; 
       FIGS. 4A and 4B  are respectively phasor diagrams showing out-phasing angles of single-level and multilevel scaling techniques; 
       FIGS. 5A and 5B  are respectively a detailed phasor diagram and a generalized phasor diagram showing out-phasing angles of multilevel scaling techniques; 
       FIG. 6  is a schematic diagram showing signal envelope distribution in WCDMA; 
       FIG. 7  is a block diagram of a multilevel scaler  313  in  FIG. 3 ; 
       FIG. 8  is a block diagram of the envelope modulator  340  in  FIG. 3 ; 
       FIG. 9A  is a schematic diagram of VDD-to-PM distortion which degrades linearity; 
       FIG. 9B  is a schematic diagram showing characteristics of the distortion compensator  350  in  FIG. 3 ; and 
       FIG. 9C  is a schematic diagram showing constant phase of the output signal of the multilevel LINC transmitter with different PA supply votlages. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
     FIG. 3  is a block diagram of a multilevel LINC transmitter according to an embodiment of the invention. The multilevel LINC transmitter  300  comprises a multilevel signal component separator  310 , a phase modulator block  320 , and an RF block  330 . The multilevel signal component separator  310  comprises a polar converter  311 , a multilevel scaler  313  coupled to the polar converter  311 , an inverse cosine module  315  coupled to the multilevel scaler  313 , and a phase calculator  317  coupled to the polar converter  311  and the inverse cosine module  315 . The polar converter  311  receives and converts the input signal S(t) to polar form. Then, an envelope signal A(t) is scaled by a multilevel scaler  313  and the inverse cosine module  315  generates an out-phasing angle θ′(t). Thereafter, the phase calculator  317  generates phase signals φ(t)+θ′(t) and φ(t)−θ′(t). In other words, the multilevel signal component separator  310  converts the input signal S(t) into phase signals φ(t)+θ′(t) and φ(t)−θ′(t). The phase modulator block  320  comprises two phase modulators  321  coupled to the multilevel signal component separator  310 . The RF block  330  comprises a plurality power amplifiers  331  coupled to the phase modulator block  320  and the multilevel scaler  313  and a power combiner  333  coupled to the power amplifiers  331 . 
   In an embodiment of the invention, a Wilkinson power combiner is adopted in a LINC transmitter, however, scope of the invention is not limited thereto. Other hybrid couplers, lossless Wilkinson power combiner, Chireix-outphasing combiner, or the like are also applicable to the invention. For a Wilkinson power combiner, efficiency η(t) thereof is defined as,
 
η( t )=cos 2  θ( t )
 
It is noted that η(t) is high when θ(t) is low. When the out-phasing angle θ(t) is substituted by the formula disclosed previously, the efficiency η(t) is expressed as,
 
             θ   ⁡     (   t   )       =       cos     -   1       ⁡     (       A   ⁡     (   t   )         r   0       )             
As a result, to utilize high power efficiency of a Wilkinson power combiner, the value of r 0  must be close to and not less than the maximum of A(t).
 
   Rather than the conventional scaling technique using single-level r 0 , the multilevel scaler  313  in  FIG. 3  reduces θ(t) such that high Wilkinson power combiner efficiency is achieved. A 2-level design example is illustrated in  FIG. 4B . When A(t) is much smaller than r 0 , the multilevel scaler adapts scale factor from r 0  to r 1 ., and out-phasing angle. θ′(t) in  FIG. 4B  is much smaller than the conventional out-phasing angle θ(t) in  FIG. 4A . Thus, the multilevel scaling technique enhances Wilkinson combiner efficiency. The multilevel scaling technique can be generalized to N levels in  FIG. 5A , and R N  is a general expression for multilevel scaling as shown in  FIG. 5B , where R N =r k , for r k+1 &lt;A(t)≦r k  k=0, 1, . . . , N−1, where r N =0, r 0 =max(A(t)). The definition of out-phasing angle θ′(t) in multilevel scaling technique is modified as 
   
     
       
         
           
             
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   To maximize the Wilkinson power combiner efficiency, optimal scale factors of each level r k  need to be determined in advance. Since multilevel scale factors are used in the LINC transmitter, Wilkinson power combiner efficiency formula is modified as 
             η   ⁡     (   t   )       =         (       A   ⁡     (   t   )         R   N       )     2     .             FIG. 6  shows envelope distribution of WCDMA where A(t) is a probability function. To acquire an expected value of η(t), the envelope A(t) is divided into several regions, illustrated in  FIG. 5 . Then, the expectation value of each region is summed to derive E(η)(t)),
 
               E   ⁡     (     η   ⁡     (   t   )       )       =       ∑     k   =   0       k   =     N   -   1         ⁢       ∫     r   k       r     k   +   1         ⁢         p   ⁡     (     A   ⁡     (   t   )       )       ·       (       A   ⁡     (   t   )         r   k       )     2       ⁢           ⁢     ⅆ     A   ⁡     (   t   )                 ,         
wherein p(A(t)) is a probability density function of A(t), r k  is a value of a kth level scale factor, N is a number of a scale factor level, and max(A(t)) is a maximum input signal envelope. To maximize the Wilkinson power combiner efficiency, E(η(t)) is differentiated such that
 
                 ∂     E   ⁡     (     η   ⁡     (   t   )       )           ∂       P   k   ′     ⁡     (   t   )           =   0     ,         
whrerein k=0, 1, . . . , N. As a result, an optimal set of R N  is obtained. With the optimal set of R N , the multilevel scaler dynamically adapts R N  close to and no lower than the envelope A(t).
 
     FIG. 7  is a block diagram of a multilevel scaler  313  in  FIG. 3 . The multilevel scaler  313  comprises a slicer  510  and a ROM  530  coupled to the slicer  510 . The slicer  510  is used to select and output a specific r k  to the inverse cosine module  315 . Preferably, the slicer  510  comprises a comparator. The comparator determines in which range the envelope A(t) is and which r k  should be selected according thereto. The ROM  530  stores the optimal set of R N . 
   Moreover, the multilevel LINC transmitter according to an embodiment of the invention further comprises an envelope modulator  340  coupled to the multilevel scaler  313  and the power amplifiers  331 , as shown in  FIG. 3 .  FIG. 8  is a block diagram of the envelope modulator  340  in  FIG. 3 . The envelope modulator  340  comprises a digital to analog converter (DAC)  341  coupled to the multilevel scaler  313 , a low pass filter (LPF)  343  coupled between the DAC  341 , and a low low drop-out (LDO) regulator  345  coupled between the LPF  343  and the power amplifier  331 . An input signal of the envelope modulator  340  is a digital control signal from the multilevel scaler  313 . The digital-to-analog converter (DAC)  341  converts the control signal to an analog signal. Then the analog control signal passes through the low pass filter (LPF)  343 . Finally, the highly power efficient low drop out (LDO) regulator  345  ensures a robust power supply voltage to the PA  331 . Due to RC delay, the control signal path group delay and the phase path delay are different. An additional delay compensator is inserted in a phase path, between the phase modulator block and the RF block, to partially overcome distortion due to RC delay. 
   Additionally, the multilevel LINC transmitter according to an embodiment of the invention further comprises a distortion compensator  350  coupled between the multilevel signal component separator  310  and the envelope modulator  340 . Since adjustment of the supply voltage of two RF power amplifiers  331  introduces another distortion, VDD-to-PM distortion, a distortion compensator  350  is incorporated in the multilevel LINC transmitter to compensate VDD-to-PM distortion.  FIG. 9A  is a schematic diagram of VDD-to-PM distortion which degrades linearity. To correct VDD-to-PM distortion, a digital distortion compensator  350  with characteristics shown in  FIG. 9B  is incorporated in the multilevel LINC transmitter. Thus, phase of the output signal remains constant even with different PA supply voltages, as shown in  FIG. 9C . 
   Moreover, the multilevel LINC transmitter according to an embodiment of the invention further comprises a temperature sensor  360 . Since temperature variation may result in different VDD-to-PM distortion, a temperature sensor  360  is incorporated in the the multilevel LINC transmitter such that VDD-to-PM distortion is compensated. 
   The invention provides a multilevel LINC transmitter with a multilevel scaler in a multilevel signal component separator thereof. The multilevel scaler dynamically adapts a scale factor according to the input signal and therefore the out-phasing angle is adjustable. As a result, high power efficiency and linearity are achieved. 
   While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.