Abstract:
The lineariser ( 1 ) comprises a digital signal processor (DSP) ( 100 ) for reducing distortion of the output of non-linear RF power amplifier ( 200 ). The DSP ( 100 ) implements a predistorter process ( 102 ) which predistorts the input to amplifier ( 200 ) in such a manner as to counter distortion imposed by the amplifier. Feedback from the amplifier output is sampled at ( 212 ) to provide a feedback signal for controlling the predistortion process. Additionally, a feedforward signal is combined with the amplifier output at ( 216 ) to further reduce distortion therein. The feedforward signal is derived by subtracting the input signal, at ( 112 ), from feedback from the output of amplifier ( 200 ) (which may contain residual distortion). The vector modulator ( 114 ) conditions the result of the subtraction process ( 112 ) to produce the feedforward signal. To ensure maximum cancellation of distortion in the output of amplifier ( 200 ), the vector modulator is adapted using feedback from the output of amplifier ( 200 ) (removed at  218 ) which is correlated with a reference signal produced by distorter ( 102 ). The reference signal contains components which correspond to some or all of the intermodulations distortion components which can be generated by amplifier ( 200 ).

Description:
FIELD OF THE INVENTION 
     This invention relates to an arrangement and a method for reducing distortion of an output signal provided by signal handling means in response to an input signal. 
     BACKGROUND OF THE INVENTION 
     Current and forthcoming telecommunications standards place an increasingly stringent requirement on the linearity of transceiver circuits, particularly given the proposed wide channel band-widths of handset transceivers compared to, for example, DAMPS, and PDC systems. In order to realise a power efficient transceiver design, some form of linearisation is therefore required. The linearisation arrangement itself should be low power, capable of broad band linearisation (up to 5 MHz for UMTS/UTRA), frequency flexible, preferably multiband, and capable of achieving and maintaining high levels of linearity improvement when used to reduce distortion caused by highly non-linear power amplifiers (eg class C amplifiers), such as may be used in transceiver circuits. 
     The trend in base station technology is towards adoption of “software radio” techniques, that is, base station architectures in which all of the modulation parameters, ramping, framing, etc. take place for all channels at base band in the digital domain. The combination of all channels, at appropriate frequency offsets from one another, can also be performed at base band and the whole channel spectrum up-converted to the transmission frequency in a single block for multi-carrier power amplification and transmission from a single antenna. 
     SUMMARY OF THE INVENTION 
     According to one aspect, the invention provides an arrangement for reducing distortion of an output signal provided by signal handling means in response to an input signal, the arrangement comprising feedforward means which derives a feedforward signal from the input signal and combines it with the output signal to reduce distortion thereof, and predistorting means which predistorts the input signal prior to the signal handling means to counter distortion caused by the signal handling means, wherein the predistorting means also derives a reference signal from the input signal for use in controlling the feedforward signal. An arrangement of this type can handle digital base band signals (such as may be provided by a software radio) and perform the necessary frequency conversion and amplification, with reduced distortion, to provide an output signal for transmission from an antenna. Further, this arrangement allows the benefits of input signal predistortion and feedforward correction of the output signal to be realised independently, thus combining the distortion reduction effects of each without any degradation of either. 
     Advantageously, the reference signal may contain components which correspond to some or all of the intermodulation distortion components which can be produced by the signal handling means. Preferably, the reference signal is generated in the digital domain. The reference signal may be created to an arbitrarily high degree of accuracy, which leads to improved control of the feedforward signal. 
     In a preferred embodiment, the arrangement additionally comprises control means for adapting the feedforward signal using feedback derived from the output signal. Preferably, this feedback is extracted from the output signal after its combination with the feedforward signal. The feedback may be correlated with the reference signal provided by the predistorting means to produce a control signal for the feedforward means. 
     In a preferred embodiment, the arrangement also comprises means for adapting the predistortion applied by the predistorting means using feedback derived from the output signal. 
     Advantageously, the feedforward signal is produced by performing a subtraction on the input signal and a feedback signal derived from the output signal. Preferably, this feedback signal is taken from the output signal before its combination with the feedforward signal. The subtraction process preferably uses the input signal prior to its predistortion. The subtraction may be performed in the digital domain, or, alternatively, in the analogue domain. 
     According to another aspect, the invention provides a method for reducing distortion of an output signal provided by signal handling means in response to an input signal, the method comprising a feed forward step of deriving a feed forward signal from the input signal and combining it with the output signal to reduce distortion thereof, a predistorting step comprising predistorting, using predistorting means, the input signal prior to the signal handling means to counter distortion caused by the signal handling means, and a generating step of generating a reference signal from the input signal using the predistorting means for use in controlling the feed forward signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     By way of example only, some embodiments of the invention will now be described with reference to the accompanying drawings; in which: 
     FIG. 1 is a block diagram illustrating a lineariser operating on a non-linear RF power amplifier; 
     FIGS. 2 a  and  2   b  are block diagrams illustrating the lineariser of FIG. 1 in more detail; 
     FIG. 3 is a block diagram of a lineariser operating on a non-linear RF power amplifier, and 
     FIG. 4 is a block diagram of a lineariser operating on a non-linear RF power amplifier. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to FIG. 1, the lineariser, generally indicated  1 , comprises a digital signal processor (DSP)  100  which controls the manipulation of the input signal to, and the output signal from, radio frequency (RF) power amplifier  200  so as to linearise, i.e reduce distortion of, the output of the amplifier  200 . 
     The input signal to the DSP  100  is provided by a software radio system and comprises a digital, base band, quadrature-format signal comprising in phase (I) and quadrature (Q) channel signals. In the Figures, quadrature-format signals are indicated by heavy black arrows. Within the DSP  100 , the quadrature-format input signal is provided to predistorter  102  which provides a predistorter version of the input signal to quadrature upconverter  104 . Upconverter  104  mixes the input signal with a signal having a first frequency from a first local oscillator  106  to produce a predistorted input signal which has been upconverted to the intermediate frequency (IF) band. This signal is converted into an analogue signal at  202  and band pass filtered at  204 . The output of filter  204  is supplied to mixer  206  for up conversion to the RF band. At mixer  206 , the filter output is mixed with a signal having a second frequency from the local oscillator  208  to produce the RF predistorted input signal. This signal is band pass filtered at  210  prior to being supplied to amplifier  200 . The predistortion applied to the input signal helps to counter distortion introduced by the non-linear amplifier  200 . A portion of the amplifier output is removed at coupler  212  and is fed back to the DSP  100  for control purposes as will be described later. 
     The main portion of the output of amplifier  200  continues through coupler  212  and delay element  214  (the function of which will be described later) to coupler  216 . At coupler  216 , a feedforward signal is combined with the output of amplifier  200  in order to further reduce the distortion caused by the amplifier. The twice linearised output of amplifier  200  is supplied from coupler  216  as an RF output for transmission from an antenna. Coupler  218  removes a portion of this RF output signal to provide a feedback signal for controlling the linerisation process, as will be described later. 
     The predistorter  102  distorts the input signal in such a way as to counter distortion which will be caused by the non-linear amplifier  200 . The predistorter  102  may be of any appropriate type. For example, it may comprise a polynominal predistorter which functions by creating a distortion which is added into the input signal to predistort it. The distortion signal itself is generated from the input signal and comprises various harmonics of the input signal, generated by mixing the input signal with itself the requisite number of times. Alternatively, for example, the predistorter could be a lookup table based predistorter which retrieves coefficients from a lookup table which correspond to amplitude and frequency values of the input signal. The retrieved coefficients are multiplied with, typically, amplitude values of frequency components of the input signal in order to generate a predistorted input signal. 
     The control aspects relating to the predistortion process will now be described. The portion of the output of amplifier  200  which is fed by coupler  212  is down converted at mixer  220  by mixing it with the signal having the second frequency from local oscillator  208 . The output of mixer  220  is band pass filtered at  222  and the resulting IF band signal is converted to a digital signal by analogue to digital converter (ADC)  224  and supplied to DSP  100 . This digital IF band feedback signal is down converted to base band at quadrature down converter  108 . The quadrature format base band feedback signal is then supplied to predistorter controller  110  which adapts the characteristics of the predistortion applied to the input signal by predistorter  102  in order to minimise residual distortion appearing in the signal fed back from coupler  212 . 
     As mentioned above, distortion appearing in the output of amplifier  200  is also countered by a feedforward signal introduced at coupler  216 . The process by which the feedforward signal is produced and controlled will now be described. The quadrature format input signal provided to DSP  100  is supplied to a subtractor  112 . Subtractor  112  also receives from downconverter  108  the sampled output of amplifier  200  containing residual distortion. Subtractor  112  subtracts the input signal from the feedback signal to produce a residual distortion signal. This signal is received by vector modulator  114  which applies appropriate delay and weighting of gain and phase. The vector modulator  114  conditions the residual distortion signal, creating a feedforward signal which is added at coupler  216  to further reduce (cancel) distortion in the output of amplifier  200 . The feedforward signal produced by vector modulator  114  is quadrature upconverted at  116  using the signal of the first frequency from local oscillator  106  to provide an IF band signal which is then converted to the analogue domain by DAC  226 . The IF feedforward signal is passed by band pass filter  228  to mixer  230  where it is upconverted to the RF band by mixing with the signal having the second frequency from local oscillator  208 . The output of mixer  230  is band pass filtered at  232 , and the resulting RF band feedforward signal is subjected to appropriate amplification at  234  to provide the feedforward signal for cancelling distortion in the output of amplifier  200 . Delay element  214  is provided prior to coupler  216  to ensure that the signal issued by amplifier  200  is synchronised with the feedforward signal. 
     In order to adapt the feedforward signal to the varying nature of the distortion in the output of amplifier  200 , coupler  218  removes a portion of the twice linearised amplifier output for use in feedback control of the conditioning applied to the residual distortion signal by vector modulator  114 . The feedback from coupler  218  is frequency downconverted at mixer  236  by mixing it with the signal having the second frequency from local oscillator  208 . The IF output of mixer  236  is then mixed, at  238 , with a reference signal from DSP  100  as part of a correlation process. The reference signal generated within DSP  100  is provided by the predistorter  102 . 
     Using the input signal, the predistorter  102  creates a reference signal containing only components corresponding to intermodulation distortion signals which could be created by amplifier  200 , and containing substantially no main signal (i.e original input signal) energy. Since the reference signal is being created digitally, it may be created to an arbitrarily high degree of accuracy, which leads to refined results from the correlation process involving mixer  238  (which will be described in more detail shortly). The reference signal produced by predistorter  102  is quadrature upconverted to the IF band at  118  by mixing it with a signal from local oscillator  120 . The signal provided by local oscillator  120  has a frequency which is displaced from the first frequency, i.e the frequency of the signal provided by local oscillator  106 , by an audio frequency (AF) amount. The IF band reference signal is converted to the analogue domain by DAC  240  and supplied to an input of mixer  238 . 
     The output of mixer  238  is converted to the digital domain by ADC  242  and supplied to quadrature down converting process  122  in DSP  100 . At quadrature down converter  122 , the output of ADC  242  is mixed with a signal from local oscillator  124 . The signal provided by this local oscillator has a frequency equal to the audio frequency offset of oscillator  120 . The output signals produced by quadrature down converter  122  are integrated at  126  to provide dc control signals for the conditioning applied by vector modulator  114 . The integrator  126  provides non-zero signals corresponding to any base band input it receives. The integrator  126  receives base band input if there is any residual distortion in the feedback from  218  which corresponds in frequency (once frequency conversion processes  118 ,  236  and  122  have been taken into account) to (components of) the reference signal provided by predistorter  102 . 
     FIG. 2, which is divided into FIGS. 2 a  and  2   b , illustrates the lineariser of FIG. 1 in more detail. For example, the I and Q quadrature signals comprising the input signal to the DSP  100  are shown individually. The predistorter  102  of FIG. 1 comprises two independent distorters  102  I and  102  Q, each operating on a respective one of the I and Q channel input signals. The predistorters  102  I and Q are controlled by independent feedback and control mechanisms  110  I and  110  Q, respectively, which receive respective I and Q feedback signals derived from the feedback signal from coupler  212  by quadrature down converter  108 . 
     In the feedforward mechanism, the I and Q input signals are independently delayed and adjusted for amplitude and phase under the control of feedback control process  128 . The adjusted I and Q input signals so produced are then handled in parallel, and are subjected to the same processes, and hence the processing of the I channel adjusted input signal only will now be described. The adjusted I channel input signal is subtracted at  112  I from the I channel component of the feedback from  212  in order to produce an I channel residual distortion signal. This signal is amplitude adjusted by available gain element  130  I, which is under the control of a signal from integrator  126  I (corresponding to integrator  126  of the FIG.  1 ). The Q channel adjusted input signal is processed in a similar manner by elements  112  Q,  130  Q and  126  Q. Variable gain elements  130  I and Q constitute a vector modulator. The I and Q outputs of the vector modulator are combined by quadrature upconverter  116  using mixers  132  I and Q. 
     Whilst the positioning of subtraction process  112  within the DSP  100  in the embodiment of FIGS. 1 and 2 permits ideal subtraction to be performed, it does introduce the potential for a delay being introduced in the subsequent digital processing and DAC. This delay must be matched by a delay in the main path ( 214  following amplifier  200 ), and the resulting delay at the amplifier output may be significant in terms of size and loss, depending on the application involved. 
     To avoid such a delay, the subtraction process can be performed in a higher frequency band, either at IF (digitally or, preferably, in the analogue domain) or at RF. The latter arrangement is shown in FIG.  3 . 
     In FIG. 3, the subtraction process  244  has been removed from the DSP  100  and now operates on RF band signals. Two vector modulators  246  and  248  are shown in FIG.  3 . Together, these correspond to vector modulator  114  of FIG.  1 . Vector modulator  246  conditions the quadrature input signal in the same way as the delay, gain and phase adaptation process is controlled by process  128  in FIG. 2 a . The conditioned input signal produced by vector modulator  246  is upconverted to the IF band and then to the RF band, and is then supplied to process  244  where it is subtracted from the feedback signal from coupler  212 . The output of process  244  is a signal containing residual distortion components, and this is conditioned by vector modulator  248 , which corresponds to vector modulator elements  130  I and Q of FIG. 2 a . The output of vector modulator  248  is the feedforward signal which is introduced to the output of amplifier  200  at coupler  216 . It is also to be noted that the feedback signal from coupler  212  is no longer used in merely down converted form as a feedback signal for distorter controller  110 . Instead, the residual distortion signal produced by subtraction process  244  is down converted to base band and supplied to the predistortion controller  110  and feedforward loop controller  134 . The feedforward loop controller  134  corresponds to feedback and control processes  110  I and Q of FIG. 2 a.    
     As mentioned above, the subtraction process  244  could be implemented at other points in the system. For example, the subtraction process could be implemented after upconversion to the IF band  116  or after DAC  224 . 
     A further embodiment, shown in FIG. 4, uses an AF offset frequency based technique to control the generation of the control signals for vector modulator  246 . This embodiment is especially useful if fixed predistortion is to be applied, as it entirely removes the need for wide band ADC. The predistorter  102  is nevertheless still capable of providing the reference signal for ideal correlation with the feedback from coupler  218 . Returning to the control of the vector modulator  246 , the residual distortion signal output by subtraction process  244  is down converted at mixer  250  to the IF band. Mixer  252  then correlates the IF band residual distortion signal from mixer  250  with an IF band version of the input signal produced by quadrature upconversion process  136  using AF offset oscillator  120 . The output of mixer  252  is down converted to base band in the DSP  100  and integrated at  138  to provide control signals for vector modulator  246 . If one considers the frequencies employed by the various local oscillators, it will be apparent that integrator  138  produces non-zero output when it receives DC input indicative of residual distortion in the output of subtraction process  244  matching the frequency or frequencies of the system input signals.