PATENT ABSTRACT
An I/Q modulator processes a time-discrete I/Q signal comprising an I component and a Q component. The I/Q signal is based on a sampling frequency which is equal to four times a carrier frequency of a carrier signal onto which the I/Q signal is modulated. A predistorter of the modulator predistort the I and components with a predistortion signal, which depends on the I and Q components, so as to form a predistorted I component and, in temporal alternation therewith, a predistorted Q components. An adjuster of the modulator adjusts the signs of the predistorted I and Q components so that two temporally successive predistorted components have a first sign and two additional successive predistorted components, which follow the first-mentioned components in time, have a second sign which is the inverse of the first sign, so as to produce therefrom an output signal at an output of the modulator.

PATENT DESCRIPTION
FIELD OF THE INVENTION 
   The present invention relates to an I/Q modulator and, in particular, to an I/Q modulator for processing a time-discrete I/Q signal. 
   BACKGROUND OF THE INVENTION AND PRIOR ART 
   Conventional I/Q modulators are used in transmitting means for carrier-frequency transmission systems, e.g. transmitters for digital broadcasting, and in base stations for mobile communications. 
   One example of such a transmitting means is shown in  FIG. 4 . The transmitting means  400  comprises an I/Q modulator  402  with predistortion, the I/Q modulator  402  comprising a first input connected to the input of the transmitting means  400 , and an output. The first input of the I/Q modulator has an I/Q signal applied thereto. The output of the I/Q modulator is connected to a first input of a first mixer  404 . A second input  404  of the mixer is connected to an oscillator  406 . An output of the mixer  404  is connected to an input of an amplifier  408 . An output of the amplifier  408  is connected to an antenna  410 . The amplifier  408  and the antenna  410  have arranged between them a decoupling means  412  which is connected to the input of an attenuator  414 . An output of the attenuator  414  is connected to a first input of a second mixer  416 . A second input of the second mixer  416  is connected to the oscillator  406 . An output of the mixer  416  is connected to an input of an I/Q demodulator  418 . An output of the I/Q demodulator  418  is connected to a first input of a comparator  420 . A second input of the comparator  420  is connected to an output of a delay element  422 . An output of the comparator  420  is connected to a second input of the I/Q modulator  402 . An input of the delay element  422  is connected to the first input of the I/Q modulator  402 . The decoupling means  412 , the attenuator  414 , the second mixer  416 , the I/Q demodulator  418  and the comparator  420  define a feedback for determining the predistortion parameters. 
   In the following, the mode of operation of a transmitting means according to  FIG. 4  will be described briefly. An I/Q signal, which is e.g. a message-carrying baseband signal comprising an I component and a Q component, is modulated onto a carrier signal by means of the I/Q modulator. In order to compensate distortions of the first mixer  404  and of the amplifier  408 , the I/Q modulator carries out a predistortion of the I/Q signal in addition to the modulation. This is important especially when transmit signals with non-constant envelopes are used. The latter occur e.g. in cases in which amplitude-modulated instead of frequency-modulated signals are used so as to achieve a higher spectral efficiency of the modulation method. The non-constant envelope of the transmit signal causes in connection with the non-linearities of the first mixer  404  and of the amplifier  408  disturbances outside the useful frequency band. These disturbances are referred to as adjacent-channel emissions and should typically not exceed an application-specific limit value. The predistorted output signal of the I/Q modulator  402  is fed to the first mixer  404  in which the signal is up-converted with the aid of the oscillator  406 . The up-converted signal is then amplified by the amplifier  408 , e.g. a travelling wave tube, and sent to the antenna  410  and transmitted. 
   Part of the signal sent to the antenna  410  is previously tapped off by the decoupling means  412  and, for further processing, it is attenuated by the attenuator  414  so as to reverse the effect of the amplification of the amplifier  408 . The tapped-off attenuated signal is fed to the second mixer  416  for down-conversion. The down-converted signal is fed to the I/Q demodulator so as to be demodulated into an I/Q signal. The demodulated I/Q signal now carries the information on the distortion caused in the original I/Q signal by the first mixer  404  and the amplifier  408 . When this demodulated distorted I/Q signal is supplied to the comparator  420 , the comparison between the original I/Q signal and the distorted I/Q signal will provide the information indicating what predistortion of the I/Q modulator has to be chosen so that the distortions caused by the first mixer  404  and the amplifier  408  can be compensated for in the best possible way. 
   A feature which is important to the comparison is that the original I/Q signal is delayed in time by the delay element  422  prior to the comparison in the comparator  420  so that the original I/Q signal is actually the signal which caused the predistorted I/Q signal. This method of adjusting the predistortion of the I/Q modulator  402  in dependence upon a comparison is referred to as adaptive predistortion. 
   An example of such an adaptive predistortion is described in U.S. Pat. No. 5,049,832. U.S. Pat. No. 5,049,832 discloses an amplifier linearization of an amplifier circuit by adaptive predistortion in the case of which an input signal for a power amplifier of the amplifier circuit is derived from an input modulation signal of the amplifier circuit by predistortion, i.e. the input signal of the power amplifier is predistorted so as to achieve a linear amplification of the input signal by the power amplifier. 
   The predistortion of the input modulation signal is adjusted via a table, which is addressed in dependence upon the square of the amplitude of the input modulation signal, the contents of the table being continuously updated so that, when the input modulation signal is being distorted, variations of the distortion caused by the power amplifier can be taken into account through the table. 
     FIGS. 5A and 5B  show the components and  FIG. 5C  shows the overall configuration of a conventional I/Q modulator  500  with predistortion of the I/Q signal or baseband signal. 
     FIG. 5A  shows means  502  for applying an I/Q signal or baseband signal, which comprises an I component and an Q component, to a carrier signal, which comprises a first subcomponent, in this case a cosine component, and a subcomponent, in this case a sine component, which is substantially orthogonal to this first subcomponent, so as to produce an output signal y(t).
   y ( t )= i ( t )·cos ω 0   t−q ( t )·sin ω 0   t  with ω 0 =2 πf   0   equa. 1 
   Means  502  for applying an I/Q signal to a carrier signal comprises a first multiplier  506 , a second multiplier  508 , means  510  for producing a carrier signal and an adder  512 . 
   The first multiplier  506  comprises a first input, which is a first input of the means  502  for applying an I/Q signal to a carrier signal and which has the I component of the I/Q signal applied thereto, a second input, which is connected to a first output of the means  510  for producing a carrier signal, and an output which is connected to a first input of the adder  512 . 
   The second multiplier  508  comprises a first input, which is a second input of the means  502  for applying an I/Q signal to a carrier signal and which has the Q component of the I/Q signal applied thereto, a second input, which is connected to a second output of the means  510  for producing a carrier signal, and an output which is connected to an inverting second input of the adder  512 . 
   The means  510  for producing a carrier signal produces a carrier signal which can be represented as a complex function in the following way:
 
 e   jω     0     t =cos ω 0   t+j  sin ω 0   t   equa. 2
 
   The first multiplier  506  multiplies the first subcomponent of the carrier signal by the I component of the I/Q signal, as can be seen from the first multiplication of equation 1, so as to obtain a multiplied I component, and the second multiplier  508  multiplies the second subcomponent of the carrier signal by the Q component of the I/Q signal so as to obtain a multiplied Q component, as can be seen from the second multiplication of equation 1. 
   The adder  512  forms subsequent to the first multiplier  506  and the second multiplier  508  the difference between the multiplied I component and the multiplied Q component, as shown in equation 1, so as to produce the output signal y(t) of the means  502  for applying an I/Q signal to a carrier signal. 
   Also the I/Q signal is now represented as a complex function.
 
   x   ( t )= i ( t )+ jq ( t )  equa. 3
 
   The function of the means  502  for applying an I/Q signal to a carrier signal can be described by the following complex representation:
 
 y ( t )= Re{ x   ( t )· e   jω     0     t }  equa. 4
 
     FIG. 5B  shows a predistortion means  504  for predistorting an I/Q signal, i.e. an I component and a Q component of an I/Q signal. In a complex representation, the I/Q signal is predistorted by multiplication with a predistortion signal
     p   ( t )= p   1 ( t )+ jp   2 ( t )=ρ( t )· e   jφ(t)   equa. 5 
so as to obtain a predistorted I/Q signal.
     x     p ( t )=   x   ( t )·   p   ( t )= i   p ( t )+ jq   p ( t )  equa. 6   i   p ( t )= i ( t )· p   1 ( t )− q ( t )· p   2 ( t )  equa. 7   q   p ( t )= i ( t )·p 2 ( t )− q ( t )· p   1 ( t )  equa. 8 
   The predistortion means  504  comprises a first multiplier  514 , a second multiplier  516 , a third multiplier  518 , a fourth multiplier  520 , a first adder  522 , a second adder  524 , and means  526  for producing a predistortion signal. 
   The first multiplier  514  comprises a first input, which is connected to a first input of the predistortion means  504  and which has the I component of the I/Q signal applied thereto, and a second input, which is connected to a first output of the means  526  for producing a predistortion signal p(t) and which has applied thereto the first predistortion component p 1 (t) of the predistortion signal according to equation 5, and an output which is connected to a first input of the first adder  522 . 
   The second multiplier  516  comprises a first input, which is connected to a second input of the predistortion means  504  and which has the Q component of the I/Q signal applied thereto, and a second input, which is connected to a second output of the means  526  for producing a predistortion signal and which has the second predistortion component p 2 (t) of the predistortion signal p(t) applied thereto, and an output which is connected to an inverting second input of the first adder  522 . 
   The third multiplier  518  comprises a first input, which is connected to the second input of the predistortion means  504  and which has the Q component of the I/Q signal applied thereto, a second input connected to the first output of the means  526  for producing a predistortion signal, and an output connected to a first input of the second adder  524 . 
   The fourth multiplier  520  has an input, which is connected to the first input of the predistortion means  504  and which has the I component of the I/Q signal applied thereto, a second input connected to the second output of the means  526  for producing a predistortion signal, and an output connected to a second input of the second adder  524 . 
   An output of the first adder is a first output of the predistortion means  504  and an output of the second adder is a second output of the predistortion means  504 . 
   The means  526  for producing the predistortion signal supplies at the first output the first component p 1 (t) of the predistortion signal  p (t) and at the second output the second component p 2 (t) of the predistortion signal  p (t) depending on the I component i(t) of the I/Q signal and on the Q component q(t) of the I/Q signal, the I component being applied to a first input of the means  526  for producing the predistortion signal and the Q component of the I/Q signal being applied to a second input. 
   In the following, the mode of operation of the predistortion means  504  shown in  FIG. 5B  will be described briefly. The first adder  522  has the function of forming the difference indicated in equation 7, the first multiplier  514  carrying out the first multiplication occurring in equation 7 and the second multiplier  516  carrying out the second multiplication occurring in equation 7. The second adder  524  has the function of forming the sum indicated in equation 8, the third multiplier  518  carrying out the second multiplication occurring in equation 8 and the fourth multiplier  520  carrying out the first multiplication occurring in equation 8. 
     FIG. 5   c  shows the overall configuration of the conventional I/Q modulator  500  with predistortion of the I/Q signal comprising the means  502  for applying an I/Q signal to a carrier signal according to  FIG. 5A  and the predistortion means  504  according to  FIG. 5B . 
   The conventional I/Q modulator  500  according to  FIG. 5C  now supplies at its output, i.e. as a result of the addition of the adder  512 , the following output signal:
 
 y ( t )= Re{ x   ( t )·   p   ( t )· e   jω     0     t }  equa. 9
 
 y ( t )= Re{ x     p ( t )· e   jω     0     t   }=i   p ( t )·cos ω 0   t−q   p ( t )·sin ω 0   t   equa. 10
 
This is the predistorted I/Q signal modulated on a carrier signal.
 
     FIG. 6  shows an I/Q modulator  600  with predistortion of the carrier signal. In contrast to the I/Q modulator according to  FIGS. 5A , B, C, the carrier signal, instead of the I/Q signal, is now predistorted by a predistortion signal  p (t) so as to obtain a predistorted carrier signal.
     t     p ( t )=   p   ( t )· e   jω     0     t =ρ( t )· e   j[ω     0     t+φ(t)]   equa. 11 
   For an I/Q modulator with predistortion of the carrier signal, the output signal of this I/Q modulator is obtained on the basis of equation 9 and equation 11. 
   
     
       
         
           
             
               
                 
                   
                     
                       
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   The I/Q modulator  600  in  FIG. 6  comprises a first multiplier  602 , a second multiplier  604 , a third multiplier  606 , a fourth multiplier  608 , means  610  for producing a carrier signal, an adder  612  and means  614  for producing a predistortion signal. 
   The first multiplier  602  comprises a first input, which is connected to a first input of the I/Q modulator  600  and which has the I component i(t) of the I/Q signal applied thereto, a second input connected to an output of the second multiplier  604 , and an output connected to a first input of the adder  612 . 
   The second multiplier  604  comprises a first input, which is connected to a first output of the means  614  for producing a predistortion signal and which has the magnitude ρ(t) of the predistortion signal  p (t) applied thereto, and a second input, which is connected to a first output of the means  610  for producing a carrier signal and which has a first subcomponent of the carrier signal, here a cosine function, applied thereto, and the output which is connected to the second input of the first multiplier  602 . 
   The third multiplier  606  comprises an input, which is connected to a second input of the I/Q modulator  600  and which has the Q component q(t) of the I/Q signal applied thereto, a second input connected to an output of the fourth multiplier  608 , and an output connected to an inverting second input of the adder  612 . 
   The fourth multiplier  608  comprises a first input, which is connected to a first output of the means  614  for producing a predistortion signal and which has the magnitude ρ(t) of the polar predistortion signal  p (t) applied thereto, a second input, which is connected to a second output of the means  610  for producing a carrier signal and which has a second subcomponent of the carrier signal, here the sine function, applied thereto, and the output which is connected to the second input of the third multiplier  606 . 
   The means  610  for producing a carrier signal comprises the above-mentioned first and the above-mentioned second output, which have applied thereto the first and second subcomponents of the carrier signal, here the cosine and sine components, and an input, which is connected to a second output of the means  614  for producing the predistortion signal and which has the phase φ(t) of the polar predistortion signal applied thereto. 
   Depending on at least the I component i(t) of the I/Q signal at a first input of the means  614  for producing a predistortion signal, which is connected to the first input of the I/Q modulator  600 , and the Q component q(t) of the I/Q signal at a second input of the means  614  for producing a predistortion signal, which is connected to the second input of the I/Q modulator  600 , the means  614  for producing a predistortion signal supplies at the first output thereof the magnitude ρ(t) of the predistortion signal  p (t) and at the second output thereof the phase φ(t) of the predistortion signal  p (t). 
   In the following, the mode of operation of the I/Q modulator  600  with predistortion of the carrier signal according to  FIG. 6  will be described briefly. The adder  612  has the function of forming the difference in equation 12. The first summand of equation 12 is produced by the first multiplier  602  and the second multiplier  604  and the second summand of equation 12 is produced by the third multiplier  606  and the fourth multiplier  608 . 
   The second multiplier  604  performs the second multiplication of the first summand of equation b  12 , i.e. the multiplication of the first subcomponent of the carrier signal with the magnitude of the predistortion signal, so as to produce a predistorted first subcomponent of the carrier signal, whereas the first multiplier  602  performs the first multiplication of the first summand of equation 12, i.e. the multiplication of the predistorted first subcomponent of the carrier signal with the I component of the I/Q signal. 
   The fourth multiplier  608  performs the second multiplication of the second summand of equation 12, i.e. the multiplication of the second subcomponent of the carrier signal with the magnitude of the predistortion signal, so as to obtain a predistorted second subcomponent of the carrier signal, and the third multiplier  606  performs the first multiplication of the second summand of equation 12, i.e. the multiplication of the predistorted second subcomponent of the carrier signal with the Q component of the I/Q signal. 
   One disadvantage of the conventional I/Q modulator  500  with predistortion of the I/Q signal according to  FIGS. 5A , B, C, and of the I/Q modulator  600  with predistortion of the carrier signal according to  FIG. 6  is that six and four multipliers, respectively, are required for realizing the I/Q modulators in circuitry. In the case of modern transmitting means the predistortion and the I/Q modulations are carried out digitally i.e. in a time-discrete manner. In view of the large bandwidth and the high precision demands of modern transmission methods, such as e.g. W-CDMA (CDMA=Code-Division Multiple Access), expensive, fast, digital multipliers having a high resolution of typically 14 bits are required for this purpose. 
   Another disadvantage is that, in view of the high numbers of multipliers, the number of gates and the power consumption of the conventional I/Q modulators according to  FIGS. 5A ,  5 B,  5 C and according to  FIG. 6  are very high. 
   SUMMARY OF THE INVENTION 
   It is the object of the present invention is to provide a simplified I/Q modulator for processing a time-discrete I/Q signal and a simplified method of processing a time-discrete I/Q signal. 
   In accordance with a first aspect of the present invention, this object is achieved by an I/Q modulator for processing a time-discrete I/Q signal comprising an I component and a Q component which is substantially orthogonal thereto, the I/Q signal being based on a sampling frequency which is equal to substantially four times a carrier frequency of a carrier signal onto which the I/Q signal is to be modulated, said I/Q modulator comprising: predistortion means for predistorting the I component and the Q component with a predistortion signal which depends on the I component and the Q component and which comprises a first predistortion component and a second predistortion component, the predistortion means being arranged for forming a predistorted I component as a difference between the I component multiplied by the first predistortion component and the Q component multiplied by the second predistortion component and for forming, in temporal alternation therewith, a predistorted Q component as a sum of the I component multiplied by the second predistortion component and of the Q component multiplied by the first predistortion component, so as to obtain a predistorted output signal and means for adjusting the signs of the temporally alternating predistorted I components and predistorted Q components of the predistorted output signal so that two temporally successive predistorted components have a first sign and two additional successive predistorted components, which follow said first-mentioned components in time, have a second sign, which is inverse to said first sign, so as to produce an output signal at an output of the I/Q modulator. 
   In accordance with a second aspect of the present invention, this object is achieved by An I/Q modulator for processing a time-discrete I/Q signal comprising an I component and a Q component which is substantially orthogonal thereto, the I/Q signal being based on a sampling frequency which is equal to substantially four times a carrier frequency of a carrier signal onto which the I/Q signal is to be modulated, said I/Q modulator comprising: a first multiplier for multiplying the I component by a predistorted first subcomponent of the carrier signal so as to obtain a multiplied I component; a second multiplier for multiplying the Q component by a predistorted second subcomponent of the carrier signal so as to obtain a multiplied Q component; an adder for adding the multiplied I component and the inverted multiplied Q component; and predistortion means for predistorting a carrier signal so as to produce a predistorted carrier signal, which comprises first and second predistorted subcomponents, from a predistortion signal which depends on the I component and on the Q component and which comprises a first predistortion component and a second predistortion component, said predistortion means being arranged for selecting, in temporal alternation, the first predistortion component and the second predistortion component as predistorted first subcomponent of the carrier signal, wherein in a group of four temporally successive selected predistortion components for the predistorted first subcomponent of the carrier signal the signs of the second and third selected predistortion components of said group are inverted, and said predistortion means being additionally arranged for selecting, in temporal alternation, the second predistortion component and the first predistortion component as predistorted second subcomponent of the carrier signal, wherein in a group of four temporally successive selected predistortion components for the predistorted second subcomponent of the carrier signal the signs of the third and fourth selected predistortion components of said group are inverted, and wherein the predistorted first subcomponent of the carrier signal is equal to the predistortion component at a time instant at which the predistorted second subcomponent of the carrier signal is not equal. 
   In accordance with a third aspect of the present invention, this object is achieved by a method of processing a time-discrete I/Q signal comprising an I component and a Q component which is substantially orthogonal thereto, the I/Q signal being based on a sampling frequency which is equal to substantially four times a carrier frequency of a carrier signal onto which the I/Q signal is to be modulated, said method comprising the following steps: predistorting the I component and the Q component with a predistortion signal which depends on the I component and the Q component and which comprises a first predistortion component and a second predistortion component, the predistortion being carried out such that a predistorted I component is formed as a difference between the I component multiplied by the first predistortion component and the Q component multiplied by the second predistortion component and that, in temporal alternation therewith, a predistorted Q component is formed as a sum of the I component multiplied by the second predistortion component and of the Q component multiplied by the first predistortion component, so as to obtain a predistorted output signal; and adjusting the signs of the temporally alternating predistorted I components and predistorted Q components of the predistorted output signal so that two temporally successive predistorted components have a first sign and two additional successive predistorted components, which follow said first-mentioned components in time, have a second sign, which is inverse to said first sign, so as to produce an output signal. 
   In accordance with a fourth aspect of the present invention, this object is achieved by a method of processing a time-discrete I/Q signal comprising an I component and a Q component which is substantially orthogonal thereto, the I/Q signal being based on a sampling frequency which is equal to substantially four times a carrier frequency of a carrier signal onto which the I/Q signal is to be modulated, said method comprising the following steps: multiplying the I component by a predistorted first subcomponent of the carrier signal so as to obtain a multiplied I component; multiplying the Q component by a predistorted second subcomponent of the carrier signal so as to obtain a multiplied Q component; adding the multiplied I component and the inverted multiplied Q component; and predistorting the carrier signal so as to produce a predistorted carrier signal, which comprises first and second predistorted subcomponents, from a predistortion signal which depends on the I component and on the Q component and which comprises a first predistortion component and a second predistortion component, said predistortion being carried out such that the first predistortion component and the second predistortion component are selected, in temporal alternation, as predistorted first subcomponent of the carrier signal, wherein in a group of four temporally successive selected predistortion components for the predistorted first subcomponent of the carrier signal the signs of the second and third selected predistortion components of said group are inverted, and said predistortion being additionally carried out such that the second predistortion component and the first predistortion component are selected, in temporal alternation, as predistorted second subcomponent of the carrier signal, wherein in a group of four temporally successive selected predistortion components for the predistorted second subcomponent of the carrier signal the signs of the third and fourth selected predistortion components of said group are inverted, and wherein the predistorted first subcomponent of the carrier signal is equal to the predistortion component at a time instant at which the predistorted second subcomponent of the carrier signal is not equal. 
   The present invention is based on the finding that, by selecting a specific sampling ratio of the digital signals in a time-discrete i.e. digital realization of an I/Q modulator, the structure of the digital I/Q modulator can be simplified substantially due to the symmetry properties of the orthogonal subcomponents of the carrier signal, which has the I/Q signal applied thereto. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following, preferred embodiments of the present invention will be explained in detail making reference to the drawings enclosed, in which: 
       FIGS. 1A  and B show the components and  FIG. 1C  shows the overall configuration of a first embodiment of a digital I/Q modulator with predistortion of the I/Q signal according to the present invention; 
       FIG. 2  shows a second embodiment of a digital I/Q modulator with predistortion of the carrier signal according to the present invention; 
       FIG. 3  shows a third embodiment of a digital I/Q modulator with predistortion of the carrier signal according to the present invention; 
       FIG. 4  shows a conventional transmitting means comprising an I/Q modulator with predistortion; 
       FIGS. 5A , B show the components and  FIG. 5C  shows the overall configuration of a conventional I/Q modulator with predistortion of the I/Q signal; and 
       FIG. 6  shows an I/Q modulator with predistortion of the carrier signal. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   When an I/Q modulator is digitally realized, all the signals are represented by sampled values at intervals T A =1/f A , wherein f A  is the sampling rate and wherein the time t=nT A  and the phase Ω 0 =ω 0 T A . n is the time index. 
   An I/Q modulator with predistortion of the I/Q signal according to  FIGS. 5A , B, C is then described by the following equations in the time-discrete case:
 
 i   p ( n )= i ( n )· p   1 ( n )− q ( n )· p   2 ( n )  equa. 13
 
 q   p ( n )= i ( n )· p   2 ( n )+ q ( n )· p   1 ( n )  equa. 14
 
 y ( n )= i   p ( n )·cos Ω 0   n−q   p ( n )·sin Ω 0   n   equa. 15
 
   Making use of time-discrete signals, these equations result from equations 7, 8 and 10 for the I/Q modulator with predistortion of the I/Q signal. 
   An I/Q modulator with predistortion of the carrier signal according to  FIG. 6  is, however, described in a time-discrete manner by the following equation:
 
 y ( n )= i ( n )·ρ( n )·cos [Ω 0   n +φ( n )]− q ( n )·ρ( n )·sin [Ω 0   n +φ( n )]  equa. 16
 
   Making use of time-discrete signals, this equation results from equation 12 for an I/Q modulator with predistortion of the carrier signal. 
   Since the orthogonal functions cosine and sine are here used for the subcomponents of the carrier signal, the symmetry properties i.e. the periodic properties of these functions can be used for digitally realizing the I/Q modulators. When the sampling rate f A  is chosen to be equal to four times the carrier frequency f 0 , the following equation is obtained: 
                   Ω   0     =         ω   0     ⁢     T   A       =         2   ⁢   π   ⁢           ⁢     f   0         f   A       ⁢     =       f   A     =     4   ⁢     f   0           ⁢     ·     π   2                   equa   .           ⁢   17               
and, consequently:
 cos Ω 0   n = . . . ,1,0,−1,0, . . . for  n = . . . ,0,1,2,3, . . .   equa. 18 sin Ω 0   n = . . . ,0,1,0,−1, . . . for  n = . . . ,0,1,2,3, . . .   equa. 19 
   When this selected sampling frequency f A  is taken into account in equation 15 for the output signal of an I/Q modulator  500  with predistortion of the I/Q signal according to  FIG. 5C , the following equation is obtained for the output signal in the case of this sampling rate: 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         
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   As can be seen from a comparison of equation 20 and equation 21, the means  502  for applying an I/Q signal to a carrier signal according to  FIG. 5A  is simply implemented as means for adjusting the signs. 
     FIGS. 1A  and B show the components and  FIG. 1C  shows the overall configuration of a first embodiment of an I/Q modulator  100  with predistortion of the I/Q signal, said I/Q modulator  100  following from these considerations and comprising a predistortion means  102  according to  FIG. 1A  and means  104  for adjusting the signs according to  FIG. 1B . 
     FIG. 1A  shows the predistortion means  102  of the I/Q modulator with predistortion of the I/Q signal according to the present invention. The predistortion means  102  comprises a first multiplexer  106 , a second multiplexer  108 , an inverter  110 , means  112  for producing a predistortion signal, a first multiplier  114 , a second multiplier  116 , an adder  118  and a control unit  119 . 
   The first multiplexer  106  comprises a first input connected to a first input of the predistortion means  102 , which has the I component of the I/Q signal applied thereto, a second input connected to a second input of the predistortion means  102 , which has the Q component of the I/Q signal applied thereto, and an output which is connected to a first input of the first multiplier  114 . 
   The second multiplexer  108  comprises a first input connected to the first input of the predistortion means  102 , a second input connected to an output of the inverter  110 , and an output connected to a first input of a second multiplier  116 . The inverter  110  additionally comprises an input which is connected to the second input of the predistortion means  102 . 
   The means  112  for producing a predistortion signal comprises a first input connected to the first input of the predistortion means  102 , a second input connected to the second input of the predistortion means  102 , a first output connected to a second input of the first multiplier  114 , and a second output connected to a second input of the second multiplier  116 . 
   The first multiplier  114  additionally comprises an output connected to a first input of the adder  118 , and the second multiplier  116  additionally comprises an output connected to a second input of the adder  118 . The adder  118  comprises an output which constitutes the output of the predistortion means  102  having the output signal applied thereto. 
   In the following, the mode of operation of the predistortion means  102  according to  FIG. 1A  will be described briefly. The adder  118  produces alternately according to equation 21 at the output of the predistortion means  102  either the I component i p  or the Q component q p  of the predistortion I/Q signal according to equation 6. In so doing, the adder  118  alternately executes the subtraction according to equation 13 or the addition according to equation 14; for the subtraction according to equation 13, the inverter  110  is switched into the signal path for the second summand of equation 13. 
   The first multiplexer  106 , the first multiplier  114  and the means  112  for producing a predistortion signal produce alternately in dependence upon the time index n either the first summand of equation 13 or the second summand of equation 14, which each contain the first predistortion component p 1 (n) of the predistortion signal p(n), i.e. in equation 13 the predistortion of the I component i(n) of the I/Q signal by the first predistortion component p 1 (n) of the predistortion signal and in equation 14 the predistortion of the Q component q(n) of the I/Q signal by the first predistortion component p 1 (n) of the redistortion signal. The multiplication of the first predistortion component p 1 (n) of the predistortion signal with either the I component or the Q component is executed by the first multiplier  114 . The selection of either the I component or the Q component for the multiplication of the first multiplier  114  is executed by the first multiplexer  106 , which is controlled by a control function m(n) of the control unit  119  depending in the time index n. In dependence upon the control function m(n), the first multiplexer  106  selects among its inputs the input which is addressed by the result of the control function, i,e. the multiplexer  106  selects in dependence upon the control function either the I component at the “0” input, the first input of the first multiplexer  106  which is selected for the result “zero” of the control function, or the Q component at the “1” input, the second input of the first multiplexer  106 , which is selected for the result “one” of the control function. 
   The second multiplexer  108 , the second multiplier  116 , the inverter  110  and the means  112  for producing a predistortion signal produce again alternately and in dependence upon the time index n either the second summand of equation 13 or the first summand of equation 14, which each contain the second predistortion component p 2 (n) of the predistortion signal p(n), i.e. in equation 13 the predistortion of the Q component and in equation 14 the predistortion of the I component of the I/Q signal by the second predistortion component p 2 (n) of the predistortion signal. The multiplication of the second predistortion component p 2 (n) of the predistortion signal with either the I component or the Q component is carried out by the second multiplier  116 . The selection of either the Q component or the I component of the I/Q signal for the multiplication of the second multiplier  116  is carried out by the second multiplexer  108  which is also controlled by the control function m(n) of the control unit  119  in synchronism with the first multiplexer  106 . In dependence upon the control function m(n) of the control unit  119 , the second multiplexer  108  selects among its inputs the input which is addressed by the result of the control function, i.e. either I component of the I/Q signal at the “1” input or first input of the second multiplexer  108 , when the control function m(n) provides the result “one”, or the inverted Q component of the I/Q signal at the “0” input or second input of the second multiplexer  108 , when the control function m(n) provides the result “zero”. The inverter  110  before the “0” input of the second multiplexer  108  provides the negative sign of the subtraction according to equation 13 taking place alternately in the adder  118 . The control function m(n) of the control unit  119  for controlling the first multiplexer  106  and the second multiplexer  108  is given by the following equation:
 
 m ( n )= n mod2= . . . ,0,1,0,1, . . . for  n = . . . ,1,2,3, . . .   equa. 22
 
n mod 2 is the divide remainder of the whole-number division (div) of n divided by 2 (n div 2).
 
   Finally, it should be pointed out that the means  112  for producing a predistortion signal p(n), i.e. the first predistortion component p 1 (n) and the second predistortion component p 2 (n), respectively adjusts the predistortion signal in dependence upon the I component and the Q component of the predistortion signal. 
     FIG. 1B  shows the means  104  for adjusting the signs of the first embodiment of an I/Q modulator with predistortion of the I/Q signal according to the present invention. As has already been mentioned hereinbefore, the means  502  for applying an I/Q signal to a carrier signal according to  FIG. 5A  is simply implemented as a means for adjusting the signs, so as to obtain from the output signal v(n) according to equation 21 of the predistortion means  102  of  FIG. 1A  the output signal y(n) according to equation 20 of the I/Q modulator  100  with predistortion of the I/Q signal according to  FIG. 1C . 
   The means  104  for adjusting the signs comprises a multiplexer  120  having a first and a second input and an output, and an inverter  122  having an input and an output. 
   The first input of the multiplexer  120  is directly connected to an input of the means  104  for adjusting the signs, whereas the second input of the multiplexer  120  is connected to the output of the inverter  122 . The output of the multiplexer  120  is an output of the means  104  for adjusting the signs. Also the inverter  122  has its input connected to the input of the means  104  for adjusting the signs. 
   In the following, the function of the means  104  for adjusting the signs will be described briefly. Depending on the result of a control function s(n) of a control unit  123 , the multiplexer  120  selects either the first input or “0” input, which is associated with the result “zero” of the control function s(n), or the second input or “1” input, which is associated with the result “one” of the control function s(n). In dependence upon the input selected, the output signal v(n) of the predistortion means  102  is either inverted, i.e. the sign is reversed, or it is applied unchanged to the output of the means  104  for adjusting the signs, so as to supply the output signal of the I/Q modulator. 
   The control function s(n) of the control unit  123 , which controls the multiplexer  120 , can be derived from the comparison of equations 20 and 21, where the control function s(n) must map equation 21 on equation 20.
 
 s ( n )=[( n+ 1)div 2]mod 2= . . . ,0,1,1,0, . . . for  n = . . . ,0,1,2,3, . . .   equa. 23
 
   The function div in this equation is a whole-number division without any divide remainder, whereas the function mod is the divide remainder of a whole-number division div. 
     FIG. 1C  finally shows the overall configuration of the I/Q modulator  100  which comprises the predistortion means  102  according to  FIG. 1A  and the means  104  for adjusting the signs according to  FIG. 1B . 
   The advantage of the I/Q modulator  100  according to  FIG. 1C  is to be seen in the fact that it only requires two multipliers, instead of six multipliers, as shown in  FIG. 5C  for the conventional I/Q modulator with predistortion of the I/Q signal, and that, consequently, the number of gates as well as the power consumption are reduced. 
   Taking into account the selection of the sampling frequency according to equation 17 and the resultant consequences according to equations 18 and 19 in equation 16 for the output signal of an I/Q modulator with predistortion of the carrier signal and in equation 11 for the predistorted carrier signal, the following equations are obtained:
 
 t   1 ( n )=ρ( n )·cos[Ω 0   n +φ( n )]= . . .  p   1 (0),− p   2 (1),− p   l (2),p 2 (3), . . .   equa. 24
 
 t   2 ( n ) =ρ( n )·sin[Ω 0   n +φ( n )]= . . . , p   2 (0), p   1 (1),− p   2 (2),− p   1 (3), . . .   equa. 25
 
 y ( n )= i ( n )· t   1 ( n )− q ( n )· t   2 ( n )  equa. 26
 
wherein
 
p 1 ( n )=ρ( n )·cosφ( n ), p   2 ( n )=ρ( n )·sin φ( n )  equa. 27
 
     FIG. 2  shows a second embodiment of an I/Q modulator  200  with predistortion of the carrier signal following from these considerations. The I/Q modulator  200  comprises a first multiplexer  202 , a second multiplexer  204 , a first multiplier  206 , a second multiplier  208 , an adder  210 , a first inverter  212 , a second inverter  214 , means  216  for producing a predistortion signal, and a control unit  217 . The first multiplexer  202 , the second multiplexer  204 , the first inverter  212 , the second inverter  214 , the means  216  for producing a predistortion signal and the control unit  217  define a predistortion means  201  for predistorting a carrier signal. 
   The first multiplexer  202  comprises a first input, which is connected to a first output of the means  216  for producing a predistortion signal and which has a first predistortion component p 1 (n) applied thereto, a second input, which is connected to a second output of the means  216  for producing a predistortion signal and which has a second predistortion component p 2 (n) applied thereto, a third output connected to an output of the first inverter  212 , a fourth input connected to an output of the second inverter  214 , and an output connected to a first input of the first multiplier  206 . 
   The second multiplexer  204  comprises a first input connected to the output of the second inverter  214 , a second input connected to the first output of the means  216  for producing a predistortion signal, a third input connected to the second output of the means  216  for producing a predistortion signal, a fourth input connected to the output of the first inverter  212 , and an output connected to a first input of the second multiplier  208 . 
   The first multiplier  206  additionally comprises a second input, which is connected to a first input of the I/Q modulator  200  and which has the I component i(n) of the I/Q signal applied thereto, and an output connected to a first input of the adder  210 . The second multiplier additionally comprises a second input, which is connected to a second input of the I/Q modulator  200  and which has the Q component q(n) of the I/Q signal applied thereto, and an output connected to a second input of the adder  210 . 
   The adder  210  comprises, in addition to the first and the second input, also an output which is an output of the I/Q modulator  200  having the output signal y(n) of the I/Q modulator applied thereto. The means  216  for producing a predistortion signal additionally comprises a first input connected to the first input of the I/Q modulator  200 , and a second input connected to the second input of the I/Q modulator  200 . In addition, an input of the first inverter  212  is connected to the first output of the means  216  for producing a predistortion signal, and an input of the second inverter  214  is connected to the second output of the means  216  for producing a predistortion signal. 
   In the following, the function of the I/Q modulator  200  according to  FIG. 2  will be described briefly. The adder  210  forms the difference according to equation 26 so as to produce the output signal y(n) of the I/Q modulator  200 . The first multiplier  206  produces the signal at the first input of the adder  210 , which is described by the first summand of equation 26 as product of the I component i(n) of the I/Q signal and of the first predistorted subcomponent t 1 (n) of the carrier signal, whereas the second multiplier  208  produces the signal at the second input of the adder  210 , which is described by the second summand of equation 26 as product of the Q component q(n) of the I/Q signal and of the second predistorted subcomponent t 2 (n) of the carrier signal. 
   The first multiplexer  202  produces the first predistorted subcomponent t 1 (n) of the carrier signal which is described by equation 24. As can be seen from equation 24, the predistortion component of the predistortion signal varies with the time index n, i.e. either the first predistortion component p 1 (n) or the second predistortion component p 2 (n) determining the first predistorted subcomponent t 1 (n) of the carrier signal is selected, and also the sign of the respective predistortion component varies with the time index n. The function of the first predistorted subcomponent of the carrier signal in dependence upon n can be realized by a multiplexer with four inputs and one output, i.e. the first multiplexer  202  in  FIG. 2 , which is controlled by a control function l(n) of the control unit  217 . Depending on the result of this control function l(n), the respective input of the first multiplexer  202  assigned to this result is selected and applied to the output of the first multiplexer  202 . If the result of the control function is “zero”, the “0” input, i.e. the first input of the first multiplexer  202 , is selected, which has the predistortion component p 1 (n) applied thereto. If the result of the control function is “one”, the “1” input, i.e. the second input of the first multiplexer  202 , is selected, which has the second predistortion component p 2 (n) applied thereto. If the result of the control function is “two”, the “2” input, i.e. the third input of the first multiplexer  202 , is selected, which has applied thereto the first predistortion component −p 1 (n) inverted by the first inverter  212 . If the result of the control function is “three”, the “3” input, i.e. the fourth input of the first multiplexer  202  is selected, which has applied thereto the second predistortion component −p 2 (n) inverted by the second inverter  214 . It follows that the assignment of the first predistorted subcomponent t 1 (n) of the carrier signal in equation 24 to the first predistortion component p 1 (n) and the second predistortion component p   2 (n) of the predistortion signal in dependence upon n is described by the control function l(n).
 
 l ( n )= n  mod 4  equa. 28
 
The function mod is the divide remainder of the whole-number division (div).
 
   The second multiplexer  204  in  FIG. 2  produces the second predistorted subcomponent t 2 (n) of the carrier signal, which is described by equation 25. As can be seen from equation 25, the predistortion component of the predistortion signal varies with the time index n, i.e. either the first predistortion component p 1 (n) or the second predistortion component p 2 (n) constituting part of the predistortion signal and determining the second predistorted subcomponent of the carrier signal, is selected, and also the sign of the respective predistortion component varies with the time index n. The function of the second predistorted subcomponent t 2 (n) of the carrier signal in dependence upon n can again be realized by a multiplexer with four inputs and one output, i.e. here the second multiplexer  204  in  FIG. 2 , which is again controlled by the control function l(n) of the control unit  217  in synchronism with the first multiplexer  202 . Depending on the result of this control function l(n), the respective input of the second multiplexer  204  assigned to this result is selected and applied to the output of the second multiplexer  204 . If the result of the control function l(n) is “zero”, the “0” input, i.e. the first input of the second multiplexer  204 , is selected, which has applied thereto the second predistortion component −p 2 (n) inverted by the second inverter  214 . If the result of the control function is “one”, the “1” input, i.e. the second input of the second multiplexer  204 , is selected, which has the first predistortion component p 1 (n) applied thereto. If the result of the control function is “two”, the “2” input, i.e. the third input of the second multiplexer  204 , is selected, which has the second predistortion component p 2 (n) applied thereto. If the result of the control function is “three”, the “3” input, which is the fourth input of the second multiplexer  204 , is selected; this input has applied thereto the first predistortion component −p 1 (n) inverted by the first inverter  212 . 
   Finally, it should also be pointed out that the means  216  for producing a predistortion signal in  FIG. 2  produces the predistortion signal p(n), i.e. the first predistortion component p 1 (n) and the second predistortion component p 2 (n), in dependence upon at least the I and the Q components i(n), q(n) of the I/Q signal. The first predistortion component p 1 (n) is applied to the first output of the means  216  for producing the predistortion signal, and the second predistortion component p 2 (n) is applied to the second output of the means  216  for producing the predistortion signal. 
   An advantage of the digital I/Q modulator  200  according to  FIG. 2  is to be seen in the fact that, in comparison with the conventional I/Q modulator with predistortion of the I/Q signal according to  FIG. 5C , it requires only two, instead of six, multipliers and that, consequently, the number of gates as well as the power consumption are reduced. 
     FIG. 3  shows a third embodiment of a digital I/Q modulator with carrier predistortion, which can be derived from the I/Q modulator  200  of  FIG. 2 . The I/Q modulator  300  of  FIG. 3  comprises, similar to the I/Q modulator  100  of  FIG. 1C  and  FIGS. 1A , B, a predistortion means  302  and means  304  for adjusting the signs. The means  304  for adjusting the signs is here not described, since it is identical to the means  104  for adjusting the signs according to  FIG. 1B  and since the multiplexer  320  thereof also uses the control function s(n) according to equation 23. In the following, the configuration and the function of the predistortion means  302  will, however, be described. 
   The predistortion means  302  of the I/Q modulator  300  comprises a first multiplexer  306 , a second multiplexer  308 , an inverter  310 , means  312  for producing a predistortion signal, a first multiplier  314 , a second multiplier  316 , an adder  318  and a control unit  319 . 
   The first multiplier  306  comprises a first input, which is connected to a first output of the means  312  for producing a predistortion signal and which has the first predistortion component p 1 (n) of the predistortion signal applied thereto, a second input, which is connected to a second output of the means  312  for producing a predistortion signal and which has the second predistortion component p 2 (n) of the predistortion signal applied thereto, and an output connected to a first input of the first multiplier  314 . 
   The second multiplier  308  comprises a first input connected to an output of the inverter  310 , a second input connected to the first output of the means  312  for producing a predistortion signal, and an output connected to a first input of the second multiplier  316 . 
   The first multiplier  314  additionally comprises a second input, which is connected to a first input of the I/Q modulator  300  and which has the I component i(n) of the I/Q signal applied thereto, and an output which is connected to a first input of the adder  318 . The second multiplier  316  additionally comprises a second input, which is connected to a second input of the I/Q modulator  300  and which has the Q component q(n) of the I/Q signal applied thereto, and an output which is connected to a second input of the adder  318 . The adder  318  comprises an output which is an output of the predistortion means  302 , and produces from the signals at the first and second inputs thereof the output signal of the predistortion means  302 . 
   The means  312  for producing a predistortion signal, i.e. the first predistortion component p 1 (n) and the second predistortion component p 2 (n), additionally comprises a first input, which is connected to the first input of the I/Q modulator  300  and which has the I component of the I/Q signal applied thereto, and a second input, which is connected to the second input of the I/Q modulator and which has the Q component of the I/Q signal applied thereto. Finally, an input of the inverter  310  is connected to the second output of the means  312  for producing a predistortion signal. 
   The adder  318  performs at a time instant or time index n the subtraction according to equation 13 and at another, subsequent time instant the addition according to equation 14 so that, as can be seen from equation 21, the output of the predistortion means  302  has alternately applied thereto either the I component i p (n) or the Q component q p (n) of the predistorted I/Q signal according to equation 6. 
   It follows that the first multiplier  314  alternately performs the multiplication of the first summand according to equation 13, i.e. the multiplication of the I component i(n) of the I/Q signal with the first predistortion component p 1 (n) of the predistortion signal, and the multiplication of the first summand according to equation 14, i.e. the multiplication of the I component i(n) of the I/Q signal with the second predistortion component p 2 (n) of the predistortion signal. 
   In a similar way, the second multiplier  316  alternately performs the multiplication of the second summand according to equation 13, i.e. the multiplication of the Q component q(n) of the I/Q signal with the second predistortion component p 2 (n) of the predistortion signal, and the multiplication of the second summand according to equation 14, i.e. the multiplication of the Q component q(n) of the I/Q signal with the first predistortion component p 1 (n) of the predistortion signal. 
   The first multiplier  306  performs the selection of the predistortion component, here p 1 (n) or p 2 (n), used for the first summands of equation 13 and equation 14, i.e. it causes the above-described alternating multiplication of the first multiplier  314 . Depending on the value of a control function m(n) of the control unit  319 , the first multiplexer  306  selects either the first or the second input of the first multiplexer  306  and thus either the first or the second predistortion component of the predistortion signal. If the value of the control function m(n) depends on n zero, the “0” input, i.e. the first input of the first multiplexer  306 , which has the first predistortion component p 1 (n) applied thereto, will be selected. If the value of the control function is “one”, the “1” input, i.e. the second input of the first multiplexer  306 , which has the second predistortion component p 2 (n) applied thereto, will be selected. The control function m(n) for alternating equation 13 with equation 14 corresponds to the control function m(n) according to equation 22. 
   The second multiplier  308  performs the selection of the predistortion component, here p 2 (n) or p 1 (n), used for the second summands of equation 13 and equation 14, i.e. it causes the above-described alternating multiplication of the second multiplier  316 . Depending on the value of the control function m(n) of the control unit  319 , the second multiplexer  308  selects, in synchronism with the first multiplexer  306 , either the first or the second input of the second multiplexer  308 . If the value of the control function m(n) depends on n zero, the “0” input, i.e. the first input of the second multiplexer  308 , which has the inverted second predistortion component −p 2 (n) of the predistortion signal applied thereto, will be selected. If the value of the control function is “one”, the “1” input, i.e. the second input of the second multiplexer  308 , which has the first predistortion component p l (n) of the predistortion signal applied thereto, will be selected. 
   An advantage of the I/Q modulator  300  with carrier predistortion according to the third embodiment of the present invention is to be seen in the fact that, in comparison with a conventional I/Q modulator with predistortion of the I/Q signal, it comprises only two, instead of six, multipliers, and that, consequently, the number of gates as well as the power consumption are reduced. 
   The means  112 ,  216  and  312  for producing a predistortion signal in  FIGS. 1 ,  2  and  3 , which serve to produce the predistortion signal, i.e. the first predistortion component and the second predistortion component, produce the predistortion signal  p (t) at least in dependence upon the I and Q components i(t), q(t) of the I/Q signal.
 
   p   ( t )=   p [     x   ( t )]=   p [i ( t ), q ( t )]  equa. 29
 
   The respective means for producing a predistortion signal may be a table which, depending on the condition of the I/Q signal, i.e. the amplitude of the I/Q signal, is addressed so as to output the first and second predistortion components. The table can, however, also be addressed with other optional parameters. In the case of the example of a transmitting means according to  FIG. 4 , which can have installed therein the I/Q modulator, these other optional parameters may take into account e.g. the temperature dependence, the ageing properties, power variations etc. of the transmitting means and, primarily, of the transmitter amplifier included therein. The table increases in size in accordance with the number of additional optional parameters. The table may also be a dynamic table comprising variable tabular values. The contents of this dynamic table can, e.g. in the case of the transmitting means of  FIG. 4 , be adjusted in dependence upon a comparison between an original I/Q signal, which has been fed to the distorting elements of the transmitting means following the I/Q modulator, and a signal outputted by these elements, so as to effect the optimum dynamic adjustment of the predistortion of the I/Q signal by means of said table, i.e. by means of the predistortion signal, at any time. As has already been described in  FIG. 4 , this is carried out e.g. via a feedback and is referred to as adaptive predistortion of the I/Q signal. 
   Due to their reduced number of multipliers, the I/Q modulators with predistortion of the I/Q signal and with carrier predistortion according to the present invention offer substantial structural advantages in comparison with conventional I/Q modulators with predistortion of the I/Q signal. Structurally simplified and energy-efficient I/Q modulators can be realized.