Abstract:
The invention specifies a transmission arrangement, particularly for mobile radio, which has a homodyne design and in which a feedback path is routed from the input or from the output of the vector modulator ( 10, 11, 12, 14 ) via a switch ( 20 ) and an A/D converter ( 19 ) back to the digital signal processing ( 3 ) in baseband ( 1 ). As a result, it is a particularly simple matter to detect offsets and amplitude mismatches and to correct them without difficulty in a digital signal processor ( 3 ). This allows both significantly improved carrier frequency suppression and significantly improved sideband suppression to be achieved with little complexity. The mobile radio transmitter described is particularly suitable for use in modern mobile radio systems such as GSM EDGE and UMTS.

Description:
REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of PCT/DE03/02477, which was not published in English, which claims the benefit of the priority date of German Patent Application No. DE 102 34 657.7, filed on Jul. 29, 2002, the contents of which both are herein incorporated by reference in their entireties.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to a transmission arrangement, particularly for mobile radio.  
       BACKGROUND OF THE INVENTION  
       [0003]     In mobile radio, the use of heterodyne transmission architectures is increasingly being replaced by the use of homodyne architectures with direct conversion from baseband to a radio frequency.  
         [0004]     If the baseband signal is in the form of a complex-value signal which has been split into an inphase component and a quadrature component which is orthogonal thereto, then a quadrature modulator or vector modulator is normally provided for converting the frequency of the modulation signal to a carrier frequency. A vector modulator normally comprises two Gilbert mixer cells. These up-convert the useful signal to a carrier frequency. In this case, a carrier signal generated by a frequency generator is supplied to one of the two radio-frequency mixers or multipliers in unaltered form and to the other with a phase shift of 90°. A summing element connected downstream of the two mixers combines the output signals with one another to form a transmission signal.  
         [0005]     With a modulator of this kind, it is desirable to obtain sufficient carrier suppression and sideband suppression.  
         [0006]     However, the carrier suppression is normally impaired by offset voltages, which may arise as a result of unavoidable component mismatches during mass production, for example. In addition, inadequate insulation between the carrier signal input and the signal output of the modulator may result in crosstalk by the carrier signal and hence in further impairment of the carrier suppression.  
         [0007]     The ever greater bandwidth requirement, the increasing data rates, the demands for ever lower drawn current and the introduction of new modulation methods are continually increasing the demands on carrier suppression in a modulator. Taking the mobile radio standard GSM (Global System for Mobile communication) as a basis, the modulation method 8-PSK (Phase Shift Keying) provided in line with the GSM EDGE system increases the linearity demands and the demands on carrier suppression.  
         [0008]     Inadequate sideband suppression may normally arise firstly from a discrepancy in the ideal phase difference of 90° for the carrier signals for the vector modulator. Secondly, any existing amplitude mismatch in the useful signal components may also result in impairment of the sideband suppression. The document U.S. Pat. No. 4,243,955 specifies a method for improved carrier suppression in modulation systems. In this case, in addition to the actual carrier signal a quadrature signal is generated and is added to the modulated radio-frequency signal at the output of a modulator. Any carrier signal which shows through is suppressed on account of the opposing phases.  
         [0009]     A further method for avoiding carrier crosstalk is specified in the document U.S. Pat. No. 5,574,994. In this document, a modulated, radio-frequency transmission signal is attenuated on the basis of the power gain in a feedback path, is down-converted to baseband, is amplified and is added to the inphase and quadrature paths in baseband as appropriate.  
         [0010]     In addition, trimming methods during manufacture are also known, which involve spectrum analyzers being used to improve the carrier and sideband suppression by trimming the baseband signals. Furthermore, the offset voltages in the modulator circuits can be reduced by virtue of appropriate enlargement of the surface areas of the components used and by virtue of layout measures during circuit design.  
         [0011]     It is also a known practice to measure, with a Schottky diode as a power detector, the radio-frequency modulation signal following amplification with a power amplifier. The resultant analog signal is then converted into a digital signal to ascertain the carrier suppression for trimming the baseband signals with a digital signal processor by means of computation.  
         [0012]     The methods described have the common drawback that a relatively high level of circuit complexity is required in order to implement said methods in integrated circuits.  
       SUMMARY OF THE INVENTION  
       [0013]     The following presents a simplified summary in order to provide a basic understanding of one or more aspects of the invention. This summary is not an extensive overview of the invention, and is neither intended to identify key or critical elements of the invention, nor to delineate the scope thereof. Rather, the primary purpose of the summary is to present one or more concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.  
         [0014]     The present invention is directed to a transmission arrangement, particularly for mobile radio, which allows the carrier and sideband suppression to be improved with little complexity.  
         [0015]     The transmission arrangement of the present invention comprises a baseband signal processing unit that is operable to condition a useful signal comprising an inphase and a quadrature path or component with a respective digital/analog converter. The arrangement also includes a frequency converter with an inphase input, which is coupled to the inphase path of the baseband signal processing unit, and with a quadrature input, which is coupled to the quadrature path of the baseband signal processing unit. The frequency converter also has an output for providing a modulated signal which has been derived from the useful signal. The arrangement further comprises a feedback path comprising an analog/digital converter with an input connected to the inphase input and to the quadrature input of the frequency converter via a respective switch. The analog/digital converter further includes an output connected to a control input of the baseband signal processing unit and such output influences the inphase component or the quadrature component of the useful signal.  
         [0016]     For the carrier trimming which is required for carrier suppression, the present invention involves the offset voltages and the currents in the inphase and quadrature paths at the input of the up-conversion frequency changer or converter or the signal at the output of the up-conversion frequency changer or converter being respectively detected with an analog/digital converter.  
         [0017]     The offset voltages in the modulator can be trimmed by virtue of the digital baseband signal processing involving a digital offset being added to the amplitude of the inphase signal component in baseband or to the quadrature component of the useful signal in baseband. In this case, trimming the modulator&#39;s offset voltage brings about the desired carrier suppression.  
         [0018]     The magnitude of the sideband suppression is increased by virtue of a respective voltage of the same amplitude being applied in the inphase path and in the quadrature path in a calibration mode in baseband. Again, the A/D converter in the feedback path is used to detect an amplitude mismatch between the inphase and quadrature paths at the mixer input or at the output of the mixer. The trimming is effected on the basis of the measured signals by altering the amplitude, that is to say the gain in the inphase path or in the quadrature path of the baseband signal processing unit.  
         [0019]     Preferably, the ascertained values may be stored for the useful signal transmission mode with modulated baseband.  
         [0020]     The trimming may be effected either before a radio-frequency signal is transmitted or during the transmission mode. If the trimming is to take place during the transmission mode, then the feedback path is connected to the inphase and quadrature inputs of the frequency converter.  
         [0021]     Since the offset voltages have thus been reduced but are simultaneously dependent on the size of the components used, the present invention advantageously allows smaller components, particularly transistors and resistors, to be used. The relatively high carrier suppression which is required for GSM EDGE and UMTS (Universal Mobile Telecommunications Standard) can be attained by means of the modulator trimming which is possible with the present invention. In this case, it is advantageously possible to dispense with radio-frequency detectors such as Schottky diodes, directional couplers etc. In addition, there is advantageously no need for complex manual trimming of the mobile radio transmitter during production.  
         [0022]     The measured signal being fed back is preferably measured at the inphase and quadrature inputs of the frequency converter, which is preferably in the form of a vector modulator, at the inputs of the multiplier cells in question. To this end, the feedback path containing the A/D converter is preferably connected to the useful signal inputs of the two mixer cells in the modulator.  
         [0023]     Preferably, feedback and A/D conversion are effected by tapping off a signal which is proportional to the respective signal component to be detected at the input of the radio-frequency mixer.  
         [0024]     With further preference, a current input for supplying a current corresponding to the respective useful signal component has a portion of these currents tapped off and measured with the A/D converter.  
         [0025]     The currents are preferably tapped off in a respective current mirror for feeding the mixer cells with the IQ signal components, a further preference being that the output transistor in the current mirror is produced in duplicate and the supplementary transistor has an output connection which is coupled to the A/D converter. As explained, switches are preferably provided for the coupling.  
         [0026]     To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]     The invention is explained below using exemplary embodiments with reference to a plurality of drawings, in which:  
         [0028]      FIG. 1  is a block diagram illustrating an exemplary embodiment of the present invention with reference to a transmission arrangement,  
         [0029]      FIG. 2  is a schematic diagram illustrating an exemplary circuit of the vector modulator of  FIG. 1  with currents preferably tapped off at the mixer inputs, and  
         [0030]      FIG. 3  is a block diagram illustrating an exemplary, alternative embodiment of the transmission arrangement from  FIG. 1  according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]      FIG. 1  shows a mobile radio transmitter with a homodyne architecture in line with the present principle. It comprises a baseband block  1  and a radio-frequency block  2 .  
         [0032]     The baseband block  1  contains a digital baseband signal processing unit  3  with an inphase output and a quadrature output to which an inphase signal path I and a quadrature signal path Q are connected for the purpose of transmitting complex-value signal components of the useful signal. The baseband signal processing unit  3  is in the form of a digital signal processor. The two outputs respectively have digital/analog converters  4 ,  5  connected to them in the I and Q paths, the output sides of said converters having a respective low-pass filter  6 ,  7  connected downstream of them. The outputs of the low-pass filters  6 ,  7  are of balanced design for the purpose of carrying differential signals and form inphase and quadrature outputs of the baseband block  1 .  
         [0033]     These outputs of the baseband block have voltage/current converters  8 ,  9  connected to them both in the inphase and in the quadrature signal path I, Q, said voltage/current converters being arranged in the radio-frequency block  2 . The outputs of these voltage/current converters are connected to the first inputs of a respective radio-frequency mixer  10 ,  11  in a vector modulator  10 ,  11 ,  12 ,  14 . Second inputs on the radio-frequency mixers  10 ,  11 , which are of balanced design, like the first inputs of the mixers  10 ,  11 , are connected via a frequency divider  12  to a voltage-controlled oscillator  13  which operates as a carrier frequency generator. The outputs of the mixer cells  10 ,  11  are connected to inputs on a summing element  14 , whose balanced output forms the output of the radio-frequency block  2 .  
         [0034]     Coupling elements  15 , a downstream balanced/unbalanced converter  16  which converts the balanced signal into an unbalanced signal and a power output stage  17  are used to couple an antenna  18  to the output of the vector modulator and hence to the output of the summing element  14 . The vector modulator accordingly comprises the frequency divider  12 , the two frequency mixers  10 ,  11  and the summing element  14 .  
         [0035]     In addition, the present invention provides a feedback path which comprises an analog/digital converter  19 . The input of the A/D converter  19  is connected via a total of four switches  20  to respective associated balanced first signal inputs on the two mixers  10 ,  11 . Hence, a total of four signals, in the present case current signals, are tapped off and are converted into a digital signal using the A/D converter. This signal, which may comprise a plurality of or partly combined single signals, is supplied to the digital signal processor  3  in order to influence the inphase or quadrature path so as to correct the offsets. Alternatively or in addition, instead of offsets in the inphase or quadrature signal, it is also possible to adjust the amplitudes in the inphase or quadrature path, so that in addition to or alternatively to carrier suppression an improved sideband suppression effect is attained with the tapped-off and digitized current data.  
         [0036]     In line with  FIG. 1 , any offset which is present in the audio-frequency linear baseband signal is measured and trimmed directly in the vector modulator  10 ,  11 ,  12 ,  14  right at the input to the combination of the baseband signal using the quadrature transistors in the mixers. The breakthrough of the carrier signal, which arises as a result of offsets in the baseband signal, and the unwanted modulator sideband, which arises as a result of amplitude mismatches between the inphase and quadrature paths, can be significantly reduced in this manner. The circuit complexity required for this is particularly low in this case.  
         [0037]      FIG. 2  shows the radio-frequency mixers  10 ,  11  (designed for balanced signal processing) from  FIG. 1 , including summing element  14 , using an exemplary circuit diagram. In this case, the balanced or differential signal processing means that the modulator contains a total of four mixer cells  21 ,  22 ,  23 ,  24  (constructed using MOS circuitry) which each comprise two n-channel MOS field-effect transistors whose sources are connected directly to one another in pairs. A carrier signal or local oscillator signal LO which has been split into inphase and quadrature components and is respectively supplied as a differential signal is supplied to these field-effect transistors at the interconnected gate connections on a total of four input terminals, as is customary in quadrature modulators. For this, the four input terminals have the four-way frequency divider  12  (not shown in  FIG. 2 ) from  FIG. 1  connected to them.  
         [0038]     The eight drain connections of the mixer cells  21 ,  22 ,  23 ,  24  are combined with one another in signal-summing fashion to form the balanced output  30 , as is customary for vector modulators.  
         [0039]     The useful signal (again in the form of inphase and quadrature components in differential form), which is already in the form of a current signal, is supplied at the source nodes (which are likewise coupled to one another) of the four mixer cells  21 ,  22 ,  23 ,  24  on input terminals, of which there are likewise four, via a respective current mirror  25 ,  26 ,  27 ,  28  whose output transistor is connected to the respective common source node of the associated mixer cell. The current mirrors  25  to  28  comprise a respective n-channel field-effect transistor, which is connected up as a diode and operates as an input transistor, and a respective output transistor, whose gate connection is connected to the gate connection of the associated transistor connected up as a diode. The sources of the input and output transistors in the current mirrors are connected directly to a reference potential connection  29 .  
         [0040]     A particular feature of the present vector modulator is that the respective gate connection of a supplementary transistor  31 ,  32 ,  33 ,  34  which is used for tapping off and feeding back current is connected to the gate connections of the output transistors in the current mirrors  25  to  28 . The supplementary transistors, like the current-mirror transistors, are in the form of n-channel field-effect transistors and have their sources connected directly to reference potential connection  29 .  
         [0041]     The drain connections of the supplementary transistors  31 ,  32 ,  33 ,  34  form the current-tapping connections of the modulator, which are connected to the feedback path in  FIG. 1  via switches  20  at A/D converter  19 . The transistors  31 ,  32  form the balanced inphase output IOUT and the transistors  33 ,  34  form the likewise balanced quadrature output QOUT of the vector modulator at the inputs of the multipliers.  
         [0042]     The differential baseband signals which are split into complex-value components are supplied via the current mirrors  25  to  28  to the quadrature mixer transistors  21  to  24 , where they are mixed with the local oscillator signal. The parallel output transistors  31  to  34  are used to tap off a portion of the baseband currents, and the voltage drop across a respective resistor (not shown here) is measured with an A/D converter. During layout development, these transistors  31  to  34  preferably form a unit with the output transistors in the current mirrors  25  to  28  in order to ensure a narrow pairing tolerance for the transistors.  
         [0043]     It can clearly be seen that the signals can be tapped off without complex additional components such as Schottky diodes and directional couplers. Advantageously, the invention can therefore be implemented with little complexity and is particularly suitable for mass production of mobile radios.  
         [0044]      FIG. 3  shows an alternative embodiment of the transmission arrangement from  FIG. 1 . This largely matches the block diagram from  FIG. 1  in terms of design and action, and the description thereof will therefore not be repeated in this respect at this juncture. The only difference is the connection of the feedback path containing the analog/digital converter  19 , which, in line with  FIG. 3 , is not connected to the inputs of the mixer cells  10 ,  11  via four switches  20  but rather to an output on the vector modulator  10 ,  11 ,  12 ,  14 , more precisely on the combination element  14  at the output of the vector modulator. The feedback path is connected at this point and is routed via a switch  35  and via the A/D converter  19  to the digital baseband signal processing unit  3 .  
         [0045]     While the invention has been illustrated and described with respect to one or more implementations, alterations and/or modifications may be made to the illustrated examples without departing from the spirit and scope of the appended claims. In particular regard to the various functions performed by the above described components or structures (assemblies, devices, circuits, systems, etc.), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component or structure which performs the specified function of the described component (e.g., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary implementations of the invention. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description and the claims, such terms are intended to be inclusive in a manner similar to the term “comprising”.