Abstract:
A multi frequency carrier transmitter comprises input means for receiving a plurality of different digital signals to be transmitted. The different signals are to be transmitted on different carrier frequencies. Amplifier means receive a composite signal comprising the different signals at the respective carrier frequencies and amplify the composite signal. Predistortion means predistort the plurality of digital signals prior to amplification of the composite signal by the amplification means. The predistortion provided by the predistortion means is subsequently altered in dependence on the difference between the input signals and the output at the amplifier means.

Description:
This application is a continuation of international application Ser. No. PCT/EP98/08445, filed Dec. 24, 1998. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a transmitter and a method of transmitting and in particular, but not exclusively, for use in a telecommunications network. 
   2. Description of the Related Art 
   In known wireless telecommunication networks  2  such as illustrated in  FIG. 1 , the area covered by the network  2  is divided into a plurality of cells  4 . Each cell  4  has associated therewith a base transceiver station  6 . Each base station  6  is arranged to communicate with terminals located in the cell  8  associated with that base station  6 . The terminals  8  may be mobile terminals which are arranged to move between the cells  4 . 
   Each base station  6  in the GSM (Global System for Mobile communications) is arranged to transmit N frequencies out of M available frequencies C 1  . . . CM, as illustrated in  FIG. 2   a . Each of the M frequencies is different and within the bandwidth allocated for signals transmitted by base stations. Each channel is divided into a plurality of sequential frames F one of which is shown in  FIG. 2   b . Each frame F is divided into eight slots S 0  . . . S 7 . The GSM standard is a time/frequency division multiple access system (F/TDMA) and accordingly signals will be transmitted by the base station to different mobile stations in different time slots. In other words, the base station will transmit signals to different mobile stations in different time slots at the same frequency. N is usually much less than M. 
   With known base transceiver stations a separate transmitter circuit is generally provided for each different frequency. For illustrative purposes a known base transceiver station is shown in  FIG. 3 . For the purpose of clarity only the transmit part  10  of the base station  12  is shown in  FIG. 3 . 
   The transmit part  10  comprises N different transmit paths  14 – 14 ′, one path being provided for each frequency. For clarity only the arrangement of one path  14  is shown but it should be appreciated that each path  14 – 14 ′ has the same structure. Each path  14 – 14 ′ comprises a modulator  16  which modulates the signal to be transmitted. The modulated signal is output to an amplifier  18  which amplifies the modulated signal. The output of the amplifier  18  is then filtered by a bandpass filter  20  which removes or attenuates unwanted noise and other emissions such as spurious signals and intermodulation products. 
   The output of each transmit path  14 – 14 ′ is connected to the input of a combiner  22  which combines the signals from each path  14 – 14 ′ to provide a multicarrier signal. The output of the combiner  22  is connected to an antenna  24  which transmits the multicarrier signal containing each of the N different channels to the terminals in the cell served by the base station. 
   It is desirable to reduce the number of transmit paths to one as this would significantly reduce the costs involved. However, if a single amplifier were to be used with a number of channels at the same time, that amplifier would need to be very linear to prevent the leakage of power into adjacent channels. The leakage of power into adjacent channels is undesirable in that a reduction in system capacity and/or a reduction in signal quality results. Linear amplifiers are known but even these amplifiers are not linear when operated near the peak capacity of the amplifier. 
   Several methods have been proposed for ensuring that amplifiers are linear. It is known, for example, from International Patent Application No. WO 97/30521 to use predistortion in a transmitter to try to ensure linearity. Predistortion is used to try to ensure that a transmitter is linear. Amplifiers introduce non linearity. With predistortion, a signal is distorted prior to being input to the amplifier which causes the distortion. The predistortion applied is the inverse of the distortion caused by the amplifier. Thus the output of the amplifier will be linear with respect to the signal prior to the application of the predistortion and the passing of that signal through the amplifier. The transmitter illustrated in this document is of the type where a different transmit path is provided for each channel. 
   Reference is also made to the following papers: Linearization of Multi-Carrier Power Amplifiers by Johansson, Mattsson and Faulkner (IEEE 1993); Linearization of RF multicarrier amplifiers using Cartesian Feedback by Johansson and Sundstrom (Electronics Letters Jul. 7, 1994); and Linearization of Wideband RF Power Amplifiers by Johansson and Faulkner. These documents disclose a transmitter which uses Cartesian feedback to correct for the non linearity of the amplifier. A signal from the output of the power amplifier is fedback and demodulated. The demodulated signal is subtracted from the input signals to provide an error signal. The error signal is used to drive the modulator and the amplifier. The feedback is performed in the analogue domain. In these documents, a separate Cartesian feedback module is provided for each channel. It should be appreciated that Cartesian feedback is by its nature a narrow band linearization technique. 
   SUMMARY OF THE INVENTION 
   It is an aim of embodiments of the present invention to provide a transmitter which can have only a single transmit path and which solves or at least mitigates the problems caused by a non linear amplifier. 
   According to one aspect of the present invention, there is provided a multi frequency carrier transmitter comprising input means for receiving a plurality of different digital signals to be transmitted, said different signals to be transmitted on different carrier frequencies; amplifier means for receiving a composite signal comprising said different signals at the respective carrier frequencies and amplifying said composite signal; and predistortion means for predistorting said plurality of digital signals prior to amplification of the composite signal by said amplification means, said predistortion provided by said predistortion means being subsequently altered in dependence on the difference between said input signals and the output at said amplifier means. 
   Accordingly, embodiments of the present invention provide a multi carrier transmitter which is able to deal with the problems of nonlinearity by using predistortion. The predistortion can be implemented more simply than the feedback method discussed in the prior art. 
   The input means are preferably arranged to separately receive each of the different signals. Combiner means are preferably provided between the input means and the amplifier means for combining said plurality of different signals to provide a composite signal. This may be the composite signal input to the amplifier means or may be a composite signal at a lower, intermediate frequency. In the latter case, that composite signal at a lower frequency would be upconverted to provide the composite signal input to the amplifier means. 
   Preferably the predistortion means are arranged to individually predistort each of the different signals before the plurality of different signals are combined by said combiner. This has the advantage that it is easy to ensure that different predistortion coefficients appropriate for the respective carrier frequency are applied to each of the signals which will be at the respective different carrier frequencies. 
   Alternatively, the predistortion means may be arranged to predistort the composite signal after the plurality of signals have been combined by the combiner. 
   A feedback path may be arranged between the amplifying means and the predistorting means. Preferably, the predistorting means is arranged to compare the output from the amplifying means from the feedback path with the signals received by the receiving means and to provide, if necessary, at least one new predistortion value to be applied to at least one subsequent signal received by the receiving means. In this way, the predistorting means can adapt to changes in the operating conditions of the amplifier means. 
   Preferably, means are provided in the feedback path for separating the output of the amplifying means into a plurality of different signals. The predistorting means is preferably arranged to compare each of the separated signals with the corresponding signals received from the input means and to determine if the at least one predistortion value needs to be altered. 
   Alternatively, the predistorting means is arranged to compare the composite signal from the amplifying means with a plurality of different signals to provide, if necessary, the at least one new predistortion value. 
   The predistortion means are preferably arranged to provide a plurality of predistortion coefficients, at least one predistortion coefficient being provided for each multi carrier frequency. Different predistortion coefficients are preferably provided for each multi carrier frequency regardless of the way in which the predistortion is achieved in practice. 
   Preferably, the predistortion coefficients for each multicarrier frequency take into account characteristics of other of the said multicarrier frequencies. Those characteristics can include one or more of the following: frequency; and distortion caused by the amplifier means. 
   The amplifier means is preferably a nonlinear amplifier. The predistortion means is preferably arranged to compensate for the nonlinearity of the phase and/or amplitude of the output of the amplifier. 
   A digital to analogue converter may be provided for converting the plurality of signals to analogue form before said signals are amplified by said amplifier means. 
   The transmitter is preferably incorporated in a base station or alternatively in a mobile station. 
   According to a second aspect of the present invention, there is provided a multi carrier frequency transmission method comprising the steps of receiving a plurality of different digital signals to be transmitted, said different signals to be transmitted on different carrier frequencies; combining said plurality of different signals to provide a composite signal comprising the different signals at the respective carrier frequencies; and amplifying said composite signal, wherein the method further comprises the steps of: predistorting the plurality of different digital signals prior to amplification of the composite signal by the amplification means; and altering the predistortion applied to subsequent signals in dependence on the difference between said different signals and the amplified signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the present invention and as to how the same may be carried into effect, reference will now be made by way of example to the accompanying drawings in which: 
       FIG. 1  shows a typical wireless telecommunications network; 
       FIG. 2   a  shows the M available frequencies for transmitting to terminals by base stations; 
       FIG. 2   b  shows the structure of a frame; 
       FIG. 3  shows the transmit part of a known base station; 
       FIG. 4  shows the transmit part of a first base station embodying the present invention; 
       FIG. 5  shows the transmit part of a second base station embodying the present invention; 
       FIG. 6  shows the transmit part of a third base station embodying the present invention; 
       FIG. 7  shows a predistorter, which can be used in the first and second embodiments, in more detail; 
       FIG. 8  shows a second predistorter, which can be used in the first and second embodiments, in more detail; 
       FIG. 9  shows a third predistorter which can be used in the third embodiments, in more detail; and 
       FIG. 10  shows a channelizer. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference will now be made to  FIG. 4  which shows a first embodiment of the present invention. In particular, the transmit part  102  of the base station  100  is shown. The base station  100  is in a cellular system of the type shown in  FIG. 1 . 
   The transmit part  102  of the base station  100  comprises N channel encoders  104  where N is the number of different frequencies on which the base station  100  will transmit signals. Each channel encoder  104  receives the data to be transmitted on the particular frequency associated with the given channel encoder  104 . The data is placed in a suitable form for transmission. This may include digitising the data, encoding the data, interleaving the data and/or any other step required to be carried out. As mentioned hereinbefore, each channel is on a different frequency after the modulator. The signals output by the channel encoders  104  are in digital form and are at a baseband frequency. Coding, interleaving etc is performed on the digital baseband signals. 
   The output of each channel encoder  104  is input to a respective digital modulator  106 . Each digital modulator  106  also receives two outputs from the adaptive predistorter  108 . The first output from the predistorter  108  provides one or more amplitude coefficients which are used to predistort the signal which is output by the digital modulator to compensate for the non linearity of an amplifier  122  with respect to amplitude. The second output from the predistorter  108  provides one or more phase coefficients which are used to predistort the signal which is output by the modulator  106  to compensate for phase distortion provided by the amplifier  122 . The amplifier  122  will be described in more detail hereinafter. 
   The digital modulators  106  are arranged to modulate the output of the respective channel encoders  104  according to the modulation method use by the telecommunications system of which the base station  100  forms a part. In the case of the GSM standard the modulators  106  will perform Gaussian minimum shift keying (GMSK). In general terms the modulators  106  take the baseband input from the respective channel encoder  104 , pass that input through a shaping filter and mix the signal with a carrier signal at an intermediate frequency which is higher than the baseband frequency but less than the frequency at which the signals are transmitted to thereby provide a modulated output. 
   The predistortion to compensate for the phase and amplitude distortion introduced by the amplifier  122  can take place at the same time. The predistortion can be performed during or more preferably after the modulation has been carried out by the modulator  106 . 
   The output of each of the digital modulators  106  are input to a signal combiner  11  and to the adaptive predistortor  108 . The combiner  110  combines the outputs of all of the modulators  106  to provide a single output. That output contains the signals for each of the frequencies. It should be appreciated that the output of the combiner  110  provides a single stream of digital words which represent the amplitude of the composite signal at each sampling instant. The output of the combiner  110  is input to a digital to analogue converter  112  which converts the digital wideband signal output by the combiner to an analogue signal. 
   The output of the digital to analogue converter  112  is input to a first bandpass filter  114  which removes signal components generated by the digital to analogue converter  112  which are outside the intermediate frequency band. The output of the first bandpass filter  114  is input to a first mixer  116 . The first mixer  116  also receives an input from a first local oscillator  118 . The output of the first bandpass filter  114  is thus mixed by the first mixer  116  with the output of the first local oscillator  118  to provide an output containing the N signals, each of which is at the radio frequency at which that signal is to be transmitted. Each of the N signals will thus be at a different radio frequency. The frequency of the first local oscillator  118  is thus selected to allow the intermediate frequency signals to be upconverted to the required radio frequencies by the first mixer  116 . 
   The output of the first mixer  116  is input to a second bandpass filter  120  which removes any signal components introduced by the first mixer  116  which are outside the radio frequency band of the signals to be transmitted. 
   The output of the second bandpass filter  120  is input to the power amplifier  122  which amplifies the received input. The output of the amplifier  122  is input to a third bandpass filter  124  which is arranged to filter out any signal components introduced by the amplifier  122  which fall outside the radio frequency band of the signals to be transmitted. The output of the third bandpass filter  124  is passed to an antenna  126  which transmits the signals to the terminals in the cell associated with the base station. 
   Between the amplifier  122  and the third bandpass filter  124 , there is arranged a coupler  128  which is arranged to sample a small part of the signals which are transmitted. In other words, the coupler  128  allows a small proportion of the signals to be transmitted to be sampled. The coupler  128  outputs an analogue sample of the output signal which is typically below the power of the input to the third bandpass filter  124 . 
   The output of the coupler  128  is input to a second mixer  132  which receives a signal from a second local oscillator  134 . The signals which are received from the coupler  128  are in the radio frequency band. The signal from the second local oscillator  134  is at a frequency such that when the signals from the coupler  128  are mixed with signal from the local oscillator  134 , the output of the second mixer  132  consists of the signals which were transmitted but at the intermediate frequencies. The intermediate frequencies output by the second mixer  132  may but not necessarily be the same as the intermediate frequencies input to the first mixer  116 . Accordingly, the frequency of the signals provided by the first and second oscillators  118  and  134  may be the same. In that case, a single oscillator may be provided. 
   The output of the second mixer  132  is input to a fourth bandpass filter  136  which filters out any signal components introduced by the second mixer  132  which are outside the intermediate frequency band. The output of the fourth bandpass filter  136  is input to an analogue to digital converter  138  which is arranged to convert the analogue signals to digital form. 
   The digital signals are output by the analogue to digital converter  138  to a channelizer  140 . The channelizer  140  divides the signals into the N different frequencies corresponding to the frequencies on which those signals were transmitted. The channelizer  140  is able to distinguish the N signals as they are all at different frequencies. The channelizer  140  provides N outputs, one for each of the frequencies. Each of the N outputs provided by the channelizer  140  are input to the adaptive predistorter  108 . The channelizer  140  can be provided in any suitable way including a bank of digital downconverters and fast fourier transform circuitry. 
   The adaptive predistorter  108  thus receives for each channel a version of the signal before it passes through the combiner  110  and subsequent elements of transmit part  102  of the base station  100  (from the respective digital modulator) and a version of the signal after it has passed through the transmit part  102  (from the channelizer  140 ). The predistorter  108  thus compares like versions of the same signal eg, both signals will be modulated at the same frequencies. The adaptive predistorter  108  is arranged to compare these signals. If the coefficients which the predistorter  108  outputs to the given digital modulator are good then the signal which is transmitted should be the same as the respective signal which is input to the combiner  110 . Ideally the only differences should be in scale of the signals. If the signals differ, the predistorter  108  arranged to calculate new predistortion coefficients which take into account the results of the comparison. 
   Thus, the predistorter  108  compares for each frequency the signal which was transmitted with the signal output by the respective digital modulator  106 . For each frequency it is determined if the current predistortion coefficients are acceptable and if not calculates new coefficients to be used for that channel. Predistortion is thus performed individually for each frequency prior to signal combining. The predistortion coefficients for each frequency are determined based on feedback from the corresponding frequency as transmitted. The predistortion coefficients are arranged to compensate for non-linearity with respect to amplitude and also phase distortion. The phase and amplitude of the signals of the digital modulators  106  are altered in accordance with these coefficients. How these coefficients are used will depend on the modulation method. In the simplest cases, the coefficients could be added to the signal or used as multipliers. 
   A microcontroller  142  is provided which provides control signals to the predistorter  108 , the combiner  110 , the channelizer  140  and the first and second local oscillators  118  and  134 . The microcontroller  142  is arranged to control the frequency of the signals generated by the oscillators  118  and  134 . 
   The embodiment of  FIG. 4  has the advantage that each of the individual channels are input to the adaptive predistorter on the input side and the individual channels are received from a channeliser on the output side. Therefore, the predistortion coefficients that are calculated by the adaptive predistorter are based on all of the carriers. This allows optimal performance to be obtained since all the information is available. 
   The combiner  110  requires information on the frequency if an inverse fast fourier transform combining technique is required. The algorithm performed by the adaptive predistorter  108  may also, as discussed hereinbefore utilise frequency information when calculating the predistortion coefficients. 
   Reference will now be made to  FIG. 7  which shows a first predistorter which can be used in the arrangement of  FIG. 4 . The predistortor  108   a  comprises n subtracters  202  each of which receives a respective one of the outputs from the digital modulators  106 . Each subtracter also receives a input from the channeliser  140 . Each subtracter  202  receives the output from the modulator  106  corresponding to a given user and the output from the channeliser  140  which also corresponds to that user. The subtracter  202  subtracts one of the outputs from the other to thus determine the difference between those signals. The output of each subtracter  202  thus constitutes a difference signal which is input to an adaption algorithm block  204  which on the basis of the input received from the subtractors  202  calculates, if necessary, new amplitude and phase predistortion coefficients. These amplitude and phase coefficients are output to a look-up table  200  where they are stored. The values from the look-up table  200  are supplied, as shown in  FIG. 4 , to the respective digital modulators  106 . The adaption algorithm block  204  may also receive an input from the microcontroller  142  which provides information as to the frequency of the signals. This information can be used by the adaption algorithm block  204  to calculate the amplitude and phase predistortion coefficients. 
   Reference will now be made to  FIG. 5  which shows a second embodiment of the present invention. The embodiment shown in  FIG. 5  is similar to the embodiment shown in  FIG. 4  and accordingly only the differences between the first and second embodiments will be described. 
   The adaptive predistorter  108  does not, as in the first embodiment, have outputs connected to the digital modulators  106 . The output of the combiner  110  is connected to the input of the predistorter  108 . The output of the predistorter  108  is connected to the input of the digital to analogue convertor. The predistorter  108  thus predistorts the multicarrier output of the combiner  110  and outputs a predistorted multicarrier signal. The feedback path is the same in the first and second embodiments and accordingly the predistorter  108  operates in a similar manner to that shown in the first embodiment. However, phase correction is not applied and the amplitude correction is the same for all frequencies. This solution is simple but may not provide the required performance in all cases. 
   Reference will now be made to  FIG. 8  which shows a second predistorter  108   b  which can be used in the second embodiment shown in  FIG. 5 . This second predistorter  108  comprises n subtractors  208  each of which receives an output from a respective one of the digital modulators. Each subtracter  208  also receives a corresponding input from the channelizer  140  as in the first embodiment. Each subtracter  208  thus receives two modulated signals. The two signals received by each subtracter  208  are subtracted one from the other to generate a difference signal. This output difference signal is input to the adaption algorithm block  204  which is the same as discussed in relation to the first predistorter of  FIG. 7 . As with the first predistorter of  FIG. 7 , the adaption algorithm block  204  also receives an input from the microcontroller  142  with information concerning the frequency. The adaption algorithm block  204  provides amplitude coefficients on its output which are output to the look-up table  200 . Unlike the embodiment shown in  FIG. 7 , the look-up table receives the multicarrier signal from the combiner  110  which defines an address in the look-up table. Accordingly, the multicarrier signal is modified in accordance with the amplitude values from the look-up table and output to the digital to analogue convertor  112 . 
   Reference is now made to  FIG. 10  which shows a possible construction of the channeliser  140 . The channeliser comprises n blocks  220 , with one block provided for each frequency. Each block of the channeliser  220  comprises an input for receiving the output of the analogue to digital converter  138  and a frequency control signal from the microcontroller  142 . The output from the analogue to digital converter  138  is input to first and second multipliers  222  and  224 . The first multiplier  222  produces the I or in-band, component of the input signal whilst the second multiplier  224  produces the Q, or quadrature-phase component of the input signal. Each of the multipliers  222  and  224  receives a frequency from the numerically controlled oscillator  226 . The frequency of the oscillator  226  is controlled by the frequency control input. The output of the oscillator  226  is connected directly to one of the multipliers  222  whilst the output for the second multipliers  224  is connected to a phase shifter  228  which shifts the phase of the output signal from the oscillator  226  by 90°. The phase shifted output signal of the oscillator  226  is input to the second multipliers  224 . 
   The output of each of the first and second multipliers  222  and  224  are connected to filters  230  and  232  which are arranged to filter out signal components introduced by the respective multipliers as well as other carriers within the multicarrier signal. 
   Each element  220  thus provides two outputs, one for the I component and one for the Q component for each frequency. 
   The frequency information provided by the microcontroller  142  is used by the predistorter  108  in the third embodiment to reconstruct the composite signal using any appropriate method. The methods require frequency information. One example is the inverse fast fourier transform technique which creates a spectral representation of the signal sampled in the time domain. 
   Reference will now be made to  FIG. 6  which shows a third embodiment of the present invention. As with the second embodiment of the present invention, only the differences between the first and third embodiments will be discussed. 
   The channelizer  140  of  FIG. 4  has been omitted from the embodiment of  FIG. 6 . The output of the analogue to digital converter  138  is thus connected directly to the input of the predistorter  108 . As information is only available on the composite signal, the predistorter is arranged to create a reference. 
   Reference will now be made to  FIG. 9  which shows a third embodiment of a predistorter which can be used in the embodiment of  FIG. 6 . The third predistorter  108 c receives the outputs from the digital modulators  106 . The output of each of the digital modulators  106  is input to a combiner  212  which combines the outputs of the modulators  200  to provide a single signal which is input to a single subtracter  214 . The single subtracter  214  also receives the output of the analogue to digital converter  138 . The single subtracter  214  provides a difference signal as a result of subtracting one of the outputs from the combiner  212  and the analogue to digital converter  138  from the other. This difference signal is input to an adaption algorithm block  216  which calculates, if necessary new amplitude and phase distortion coefficients. As with the first two predistorters, the adaption algorithm block  216  receives an input from the microcontroller  146  with information concerning the frequency. This information is also input to the combiner  212 . The new values are stored in the look-up table 200. 
   In the previously described embodiments, the output from the digital modulators are input to the adaptive predistorter. In an alternative embodiment, the output of the channel encoders may be output to the adaptive predistorter instead. In those circumstances, the channeliser may need also to demodulate the signals so that the adaptive predistortor compares like with like. 
   The embodiments which calculate the predistortion coefficients for each carrier can be modified so that the frequency, distortion etc on the other carriers are taken into consideration. 
   In the embodiments where the composite signal is predistorted, the combiner output consists of amplitude words, the correction occurring by adjusting the magnitude of the words. Nonlinearities in the amplifier output could be compensated by correcting the magnitude of the combiner output words with an inverse function. The correction may thus be dependent on the particular combiner output. This might occur by indexing a look-up table with the combiner output word and outputting the respective contents to the digital to analogue converter. 
   In the embodiments of the present invention described hereinbefore, only one set of intermediate frequencies is used. However in alternate embodiments of the present invention, there may be a further stage where the frequencies of the signals at a first set of intermediate frequencies may be upconverted to a second set of intermediate frequencies which are greater than the first set of intermediate frequencies but less than the radio frequency. 
   Whilst the embodiment of the present invention has been described in the context of the GSM standard it should be appreciated that embodiments of the present invention are applicable to any other TDMA system, any system using spread spectrum techniques such as code division multiple access (CDMA), frequency division multiple access (FDMA) or hybrids of any of these methods. For example GSM is an FDMA/TDMA hybrid. 
   It should be appreciated that although the preferred embodiment of the present invention has been described in the context of a base transceiver station, embodiments of the present invention are also applicable to mobile stations and terminals which are arranged to communicate with the base station. Embodiments of the present invention can be used with any suitable transmitter which is sending signals at more than one frequency at the same time in both wireless and wired environments.