Abstract:
A Cartesian loop system for radio transmitters in which at least part of the baseband processing is carried out in the digital domain. Digital processing circuitry ( 250 ) combines a baseband input signal with a Cartesian feedback signal ( 206,207 ) to generate a forward signal. Analog circuitry ( 221, 201, 203, 204 ) converts the forward signal into a transmission output signal and generates the Cartesian feedback signal. Preferably the digital processing circuitry applies a phase shift process ( 208 ) to the Cartesian feedback signal before combination with the baseband input signal. The system is programmable allowing a single device to be used in a range of transmitters under different radio standards.

Description:
FIELD OF TEE INVENTION  
         [0001]    This invention relates to Cartesian loop systems with digital processing of baseband signals and in particular but not only to systems that are used in linearisation of radio transmitter equipment.  
         BACKGROUND TO THE INVENTION  
         [0002]    Standards relating to radio communications such as TETRA, UMTS and EDGE generally require a high degree of linearity in transmitter equipment to reduce noise between closely spaced radio channels. Linearisation of power amplifiers in transmitter equipment has been extensively researched and many techniques such as Cartesian loop, Polar loop, Envelope Elimination and Restoration, LINC and CALLUM have been produced.  
           [0003]    Linearity and bandwidth are traded off in these techniques, giving high linearity being possible over narrow bandwidth, with moderate linearity over a broader bandwidth. Most techniques also trade linearity for efficiency. Power amplifiers used in radio transmitters are more efficient when operated at higher power but then have lower linearity, particularly near their peak power ratings. These techniques are less satisfactory for mobile communications which require both high linearity and also high efficiency for longer battery life and lower weight  
           [0004]    The Cartesian loop technique involves negative feedback applied to a baseband input signal having inphase and quadrature components. The feedback signal is a measure of distortion introduced in the forward path of the loop, primarily by the amplifier, and is subtracted from the input signal in real time. This modifies the input signal with an error signal that tends to cancel the distortion at the output of the amplifier and accounts for changes in distortion over time. A phase shift is applied to counter RF delays around the loop.  
           [0005]    Cartesian loop systems are generally implemented in analog form which creates several practical disadvantages and reduces their suitability for radio equipment. The analog phase shifter is physically bulky and may introduce additional noise and distortion. Different channels usually require different phase shifts and different optimum settings. The circuit requires several extra ADC and DAC components for calibration of the phase shifter and DC offsets. Overall, analog Cartesian systems can be cumbersome to program and configure, and to implement in hardware.  
           [0006]    Predistortion is a digital alternative to Cartesian loop that is sometimes used, although it too has disadvantages. Predistortion involves a digital distortion characteristic that is complimentary to that of the amplifier and to other non-linear devices in the circuit. The characteristic is determined by a training sequence and then by ongoing adaptation to counter changes in non-linearity over time. A lookup table contains predistortion parameters that may be applied to the input signal in various ways.  
         SUMMARY OF THE INVENTION  
         [0007]    It is an object of the invention to provide improved Cartesian loop Systems for radio transmitters, or at least to provide alternatives to existing systems. In general terms, the baseband signals in these systems are at least partly processed by digital means.  
           [0008]    In one aspect the invention maybe said to consist in a Cartesian loop system for radio transmission equipment comprising: digital processing circuitry that combines a baseband input signal with a Cartesian feedback signal to generate a forward signal, coupled to analog circuitry that converts the forward signal into a transmission output signal and generates the Cartesian feedback signal. Preferably the digital processing circuitry applies a phase shift process to the Cartesian feedback signal before combination with the baseband input signal.  
           [0009]    In another aspect the invention consists in a method of linearising a radio transmitter comprising: (a) receiving a baseband input signal for transmission, (b) digitally combining the input signal with a Cartesian feedback signal to generate a modified signal, (c) upconverting and amplifying the modified signal to generate a radio frequency output signal, and (d) generating the Cartesian feedback signal from the radio frequency output signal. Preferably the the Cartesian feedback signal is digitally phase shifted before combination with the baseband input signal  
       
    
    
     LIST OF FIGURES  
       [0010]    Preferred embodiments of the invention will be described with respect to the accompanying drawings, of which:  
         [0011]    [0011]FIG. 1A shows an analog Cartesian loop system,  
         [0012]    [0012]FIG. 1B shows a radio transmitter including the analog Cartesian system,  
         [0013]    [0013]FIG. 2 shows a Cartesian loop system with digital processing of the baseband signal,  
         [0014]    [0014]FIG. 3 shows a Cartesian loop system with digital processing of the baseband and modulation stages, using an intermediate frequency  
         [0015]    [0015]FIG. 4 shows a possible Weaver modulation path for FIGS. 2 and 3,  
         [0016]    [0016]FIG. 5 shows a Cartesian loop system with digital processing of the baseband and modulation stages, without an intermediate frequency,  
         [0017]    [0017]FIGS. 6A, 6B show alternative digital processing stages for FIGS. 2, 3 and  5 ,  
         [0018]    [0018]FIG. 7 shows a phase shift stage in the digital processing,  
         [0019]    [0019]FIG. 8 shows the system of FIG. 2 including predistortion,  
         [0020]    [0020]FIG. 9 shows the system of FIG. 3 including predistortion,  
         [0021]    [0021]FIG. 10 shows the system of FIG. 5 including predistortion, and  
         [0022]    [0022]FIG. 11 shows the system of FIG. 2 including envelope elimination and restoration. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0023]    Referring to the drawings it will be appreciated that Cartesian loop systems according to the invention can be implemented in various forms to meet a wide range of standards required by radio communication equipment. These embodiments are given by way of example only, and parts of different embodiments can be combined in different ways. Many features of the systems such as modulation, demodulation, RF amplification, digital to analog and analog to digital conversion come in many forms that will be well known to skilled readers and need not be described in detail.  
         [0024]    [0024]FIG. 1 A shows a conventional Cartesian loop system with analog circuitry, as briefly described above. A baseband input signal with analog quadrature components I and Q is used to modulate a carrier or local oscillator signal LO which is then amplified and output as a radio frequency signal RO. A feedback signal FB from the output is demodulated to form quadrature components which are subtracted from the input signal in real time to reduce overall distortion in the output  
         [0025]    In FIG. 1A quadrature modulator  101  and demodulator  102  can operate in conventional ways. An RF amplifier  103  preferably operates at peak power for high efficiency, but thereby with increased non-linearity. Coupler  104  creates the feedback signal from the output of the amplifier. Attenuator  105  sets a suitable amplitude in the feedback signal. Adders  106 ,  107  combine quadrature components of the feedback signal in antiphase with respective components of the input signal to reduce the non-linearity. Phase shifter  108  is required to shift the phase of the carrier between modulator  101  and demodulator  102  in order to accommodate delays around the loop. Loop filters  109  determine the bandwidth and gain of the loop and reduce noise. Buffers  110  set signal levels for the adders.  
         [0026]    [0026]FIG. 1B shows how the analog Cartesian loop circuit of FIG. 1A is typically used in a radio transmitter. Quadrature components I and Q of a digital baseband signal DB are formed in the digital processor  121 . Calibration and configuration functions required to operate the loop are cased out by a digital processor  122 , such as determination of DC offsets and phase shift estimation. The digital processors are connected to the analog Cartesian loop circuit by digital to analog and analog to digital converters, DACs  123  and ADC  124 .  
         [0027]    [0027]FIG. 2 shows a Cartesian loop system using a combination of digital and analog circuitry that can be implemented in a radio transmitter more effectively than a fully analog system. A digital processing stage  250  combines the quad components I, Q of the baseband input signal with respective components of the feedback signal, and preferably carries out several other requirements of the loop such as phase shifting and compensation for DC offsets. A range of digital processor devices are suitable such as DSP, FPGA or ASIC devices, for example. The baseband signals are coupled between digital and analog parts of the system through DAC and ADC devices that are now part of the loop. The DAC devices are linearised in the forward path and may be less highly specified than those in FIG. 1B.  
         [0028]    In FIG. 2 the analog circuitry includes quadrature modulator  201  and demodulator  202 , power amplifier  203 , coupler  204  and an attenuator  205  which can be substantially similar to those of FIG. 1A. The baseband input components I, Q are upconverted by the modulator  201  to the frequency of the local oscillator signal LO and added for amplification and transmission. Conversely the feedback signal is donwnconverted and separated into quadrature components by the demodulator  202 .  
         [0029]    In FIG. 2 the digital processor  250  includes combiners  206 ,  207  that carry out real time subtraction of the feedback signal from the input signal, a phase shifter  208  that is now implemented precisely in baseband, either in the forward path or feedback path of the loop, and a filter  209  for stabilisation at a desired loop gain. Both the phase shifter and filter are readily implemented and calibrated by digital programming. A digital phase shift adds no noise to the loop and has no insertion loss, unlike the conventional analog phase shift approach. Calibration may take place once on startup or periodically as required. Digital baseband components I, Q output by the processor  250  in the forward path of the loop are sampled by DACs  220  at frequency F s  and passed in analog form to anti-alias filters  221 . Quadrature components of the analog feedback signal from demodulator  202  are passed to anti-alias filters  230  and then to ADCs for sampling at F s  and input in digital form to the processor.  
         [0030]    [0030]FIG. 3 shows an alternative Cartesian loop system using a combination of digital and analog circuitry that can be implemented in a radio transmitter for linearisation. The system has many similarities with the system of FIG. 2, except that the modulation and demodulation functions are now also carried out by the digital processor, at a relatively low intermediate frequency. This has an advantage that these functions are now carried out more accurately, in addition to the phase shift and stabilisation, but a disadvantage in that at least one final mixing stage is still required in the analog circuitry and an image that requires additional filtering is now generated. Alternatively however, the Weaver method as indicated in FIG. 4 may be used instead to generate the output signal at radio frequency without also generating an image.  
         [0031]    In FIG. 3 the digital processor  350  includes combiners  306 ,  307  that carry out real time subtraction of the feedback signal from the input signal, a phase shifter  308  that is again implemented precisely in baseband, either in the forward path or feedback path of the loop, and a filter  309  for stabilization. Both the phase shifter and filter are readily implemented and calibrated by digital programming. Quadrature modulator  301  and demodulator  302  are also now included as digital processing functions.  
         [0032]    In FIG. 3 the analog circuitry includes a frequency upconverter  360  and downconverter  361  that operate with a local oscillator signal LO, image filter  370 , power amplifier  303 , coupler  304  and an attenuator  305 . The digital signal output by the processor  350  in the forward path of the loop is sampled by DAC  320  at frequency F s  that is more than twice the intermediate frequency of the modulator, and passed in analog form to anti-alias filter  321 . The upconverter  360  then mixes the signal with a local oscillator signal LO followed by image filter  370  and amplifier  303  before transmission. The analog feedback signal from downconverter  361  is passed to anti-alias filter  330  and then to ADC  331  for sampling at F s  and input in digital form to the processor.  
         [0033]    [0033]FIG. 4 indicates a Weaver subsystem that might be used in modifications of the systems in FIGS.  2  or  3 , particularly the forward path of FIG. 2 and the reverse path of FIG. 3. The digital quadrature modulator  401  generates a modulated signal at frequency F IF  which is passed to a Weaver modulator in which an analog quadrature modulator  411  produces a modulated signal at F C-IF . This allows use of the same local oscillator in either of the feedback paths in FIGS.  2  or  3 . The digital and analog portions of the path are coupled by DACs  420 . Further description of the Weaver method can be found in  Communication Systems, S Haykin,  2 nd  edition, J Wiley and Sons, 1983, pp 145,146, 171.  
         [0034]    [0034]FIG. 5 shows a further alternative Cartesian loop system using a combination of digital and analog circuitry. The modulation and demodulation functions are carried out by the digital processor as in FIG. 3, but now at a relatively high frequency. These functions are carried out accurately in addition to the phase shift and DC compensation but without need of a further mixing stage because the modulation function takes place at the required frequency for transmission.  
         [0035]    In FIG. 5 the digital processor  550  includes combiners  506 ,  507  that carry out real time subtraction of the feedback signal from the input signal, a phase shifter  508  in either the forward path or feedback path of the loop, and a filter  509  for stabilisation. Both the phase shifter and filter are readily implemented and calibrated by digital programming. Quadrature modulator  501  and demodulator  502  are also included as digital processing functions.  
         [0036]    In FIG. 5 the analog circuitry includes power amplifier  503 , coupler  504  and an attenuator  505 . The digital signal output by the processor  550  is sampled by DAC  520  at rate F s (DAC) to produce a series of images at multiples of the rate, one of which is the required transmission frequency. Anti-alias filter  521  removes the unwanted images. The analog feedback signal is passed from the coupler  504  and attenuator  505  to anti-alias filter  530  and then to ADC  531  for sampling at F s (ADC) and input in digital form to the processor. F s (DAC) may be an integer multiple of the F s (ADC). Generally when F s (DAC) is greater than F s (ADC) an interpolator can be included in the feedback path before ADC  531  to change the effective sampling rate and reduce alias products resulting from the different sampling rates.  
         [0037]    [0037]FIGS. 6A, 6B show alternative parts of the baseband processing that may take place in the digital processors  250 , 350 , 550 . Quadrature components I, Q of the input baseband signal are combined with components I F , Q F  of the feedback baseband signal, to produce components I′, Q′ of the signal in the forward path. The feedback components are subtracted from the input components to create components e I , e Q  of an error signal that cancels distortion at the output of the loop. Corrections are generally applied to counter both delays and DC offset by devices around the loop, either before or after combination of the input signal with the feedback signal. In FIG. 6A a phase shift is applied to the feedback signal and a DC correction is applied to the input signal before combination of the feedback and input signals. In FIG. 6B the phase shift is applied to the combined signal in the forward path.  
         [0038]    In FIGS. 6A, 6B the feedback components I F , Q F  are added 180° out of phase to the input components I, Q by combiners  606 ,  607 . Alternatively the action may be considered as an error summation in which a forward signal containing small input signal components and error components e I , e Q  is generated. Phase shift block  608  acts on either the feedback components or the forward signal components, as will be described with more detail in relation to FIG. 7. The magnitude of the phase shift is determined by an estimation block  611 , generally based on known properties of the Cartesian circuit, set during manufacture or on power up, perhaps modified by periodic updates when in operation. DC offsets are determined in estimation block  612  and subtracted from the input components I, Q by combiners  613 ,  614 . Loop filter blocks  615  are generally necessary for stabilisation while gain blocks  616  are generally optional.  
         [0039]    [0039]FIG. 7 indicates operation of the digital phase shifter  608  in FIGS. 6A, 6B. The phase shift can be considered as rotation of the vector formed by quadrature components of the particular signal. Delay in the loop effectively rotates the signal. If the vector of the feedback signal is not aligned with the vector of the input signal then signal components do not cancel and the loop becomes unstable. Correct alignment leaves the error signal and a small signal component. Phase shift of a signal I 1 , Q 1  to I 2 , Q 2  by angle θ can be carried out by a matrix operation as follows:  
         [           I   2               Q   2           ]     =       [             cos                 θ                 -   sin                   θ                      sin                 θ               cos                 θ             ]          [           I   1               Q   1           ]                             
 
         [0040]    The phase shift operation can also include compensation for gain imbalance and DC offset effects. If g and d are parameters required to equalise I, Q amplitude and DC imbalances, then the matrix operation can be expanded as follows:  
         [           I   2               Q   2           ]     =         [             g                 cos                 θ                 -   g                   sin                 θ                      sin                 θ               cos                 θ             ]          [           I   1               Q   1           ]       +     [           d   1               d   2           ]                             
 
         [0041]    [0041]FIGS. 8, 9,  10  show how the systems of FIGS. 2, 3,  5  may be enhanced by combination of both Cartesian loop and predistortion techniques. Predistortion modifies the forward signal in the loop so that the combined characteristic of the predistorter and the power amplifier is linear. The characteristic of the amplifier changes with time and environment so an adaptive process is commonly used to update parameters required by the predistorter. The error signal created by the Cartesian loop is also predistorted, so that the predistorter linearises the amplifier and the Cartesian loop further linearises the system overall. It is alternatively possible to predistort the Cartesian feedback signal or the input signal.  
         [0042]    In FIGS. 8, 9,  10  the Cartesian loop systems are readily modified by variation in the digital processing without need of additional analog components. Most of the digital and analog elements can remain the same. Predistortion blocks  280 ,  380 ,  580  respectively are added in the forward path of digital processors  251 ,  351 ,  551 . Adaption blocks  281 ,  381 ,  581  respectively are also added to update parameters required by the predistortion blocks. Existing phase shift blocks  208 ,  308 ,  508  can remove the need for phase effects to be calculated in the predistortion or adaption blocks.  
         [0043]    [0043]FIG. 11 shows how the system of FIG. 2 may be enhanced by combination of Cartesian loop and Envelope Elimination and Restoration techniques. EER divides the forward path to create an envelope signal and a phase signal, being polar rather than Cartesian components of the baseband signal. The envelope signal modulates the power supply of the amplifier while the phase signal has a constant amplitude and is amplified efficiently in a linear fashion. An envelope feedback path is usually added. EER is relatively simple and popular but does not achieve high linearisation. Delay lines are also required to compensate differences between the envelope and phase signal paths. Cartesian feedback can assist or replace envelope feedback, and reduce phase distortion effects due to high envelope modulation indexes and mismatched delays.  
         [0044]    In FIG. 11 the Cartesian loop system has been modified with both digital and analog elements. Digital envelope and phase generation blocks  290 ,  291  produce the envelope and phase signals in the forward path of processor  252 . An analog phase modulator  292  implemented as a quadrature modulator or a phase lock loop, for example, upconverts the phase signal to radio frequency before amplification. An amplitude modulator  293  such as a switching mode amplifier varies the voltage applied to the amplifier  203  according to the envelope signal. Alternatively the gate or base of the amplifier may be dynamically biased. The envelope and phase generation blocks are now inside the Cartesian loop and their specifications can be relaxed along with other elements normally outside the loop in analog Cartesian systems.