Patent Publication Number: US-9838049-B2

Title: Method and frequency agile pre-distorted transmitter using programmable digital up and down conversion

Description:
TECHNICAL FIELD 
     The present invention generally relates to pre-distorted transmitters capable to transmit signals using digital down and up conversions that are programmable to accommodate a plurality of frequency bands. 
     BACKGROUND 
     Radio transmitters in most communication systems are required to be “linear”, meaning that their output signal must accurately reproduce a frequency spectrum of a corresponding input signal. A non-linearity in the input/output relationship causes the output signal to spread onto adjacent radio frequencies and interfere with other radio channels. Pre-distortion is a technique of cancelling an initial non-linearity by altering the input signal to produce an overall linear input/output relationship. In a pre-distorted transmitter, a pre-distortion function applied to the input signal is adjusted based on a feedback signal. Since the distortion primarily occurs in the power amplifier, the signal acquired after the power amplifier is fed back and compared with the initial signal as part of the pre-distortion process. 
     Currently available pre-distorted transmitters are generally designed to operate for a single frequency band or a small range of transmitted frequencies (e.g. LTE band 1 or 2110-2170 MHz). Both a forward path and a feedback path in these pre-distorted transmitters are effectively tuned to the same frequency band, and cannot be deployed to support other frequency bands. The pre-distorted transmitters&#39; operation in a single (narrow) frequency band of interest occurs as a result of a design which includes, for example, various tuned circuits (e.g., narrow band filters), a limited range of operation of components (e.g., voltage controlled oscillators that only tune only over a few hundred MHz), and amplitude and phase variation over frequency which makes linearization (e.g., pre-distortion) difficult over a broad range of frequencies. 
     The number and range of available frequency bands around the world has recently increased significantly. The currently available pre-distorted transmitters require that they be redesigned, modified and re-banded to operate in a new frequency range. This situation leads to many radio communication systems which are expensive to design and to support on an on-going basis. 
     Accordingly, it would desirable to have a frequency agile pre-distorted transmitter capable of being effectively tuned for plural frequency bands, thereby avoiding the afore-described problems and drawbacks. 
     SUMMARY 
     According to one exemplary embodiment, a transmitter operable over a wide range of frequencies including a plurality of predetermined frequency bands includes a baseband processing unit, a programmable digital up-converter, a digital to analog converter, an analog filter, a power amplifier, an analog to digital converter, and a programmable digital down-converter. The baseband processing unit is configured to receive a complex input signal and a feedback output signal, and to output a pre-distorted baseband signal based on said complex input signal and said feedback signal. The programmable digital up-converter is connected to the baseband processing unit, and is configured to receive the pre-distorted baseband signal, and to convert the pre-distorted baseband signal into a digital signal having a spectral content which is selected based on at least one first control input received from the baseband processing unit. The digital to analog converter is connected to the programmable digital up-converter, and is configured to receive the digital signal and to convert the digital signal in an analog signal. The analog filter is connected to the digital to analog converter, and is configured to receive the analog signal, and to output a filtered analog signal including substantially only components having frequencies in a selected one of the frequency bands. The power amplifier is connected to the analog filter, and is configured to receive the filtered analog signal, and to amplify the filtered analog signal. The analog to digital converter is configured to receive an analog feedback signal corresponding to the amplified filtered analog signal, to sample the analog feedback signal, and to output a digital feedback signal. The programmable digital down-converter is connected to the analog to digital converter and the baseband processing unit, and is configured to receive the digital feedback signal, and to convert the digital feedback signal to a lower sample rate based on at least one second control input, into the feedback signal, which is provided to the baseband processing unit. 
     According to one exemplary embodiment, a transmitter operable over a wide range of frequencies including a plurality of predetermined frequency bands includes a baseband processing unit, one or more forward signal paths, and a single feedback signal path. The baseband processing unit is configured to receive one or more complex baseband input signals, to output one or more pre-distorted signals corresponding to the one or more complex baseband input signals. The one or more forward signal paths are connected to the baseband processing unit, and are configured to digitally up-convert a respective one of the one or more complex baseband input signals, to convert the up-converted signal into an analog signal, amplify the analog signal and to broadcast the analog signal. The single feedback signal path is configured to receive an analog signal which corresponds to the one or more broadcasted signals, to convert analog signal into a digital signal at a fixed sampling rate, to down-sample the digital signal digitally and to output the down sampled signal to the baseband processing unit. 
     According to another exemplary embodiment, a method of broadcasting signals over a wide range of frequencies including a plurality of predetermined frequency bands is provided. The method includes generating a pre-distorted baseband signal and up-sampling digitally the pre-distorted signal to convert the pre-distorted baseband signal into a digital signal having spectral content which is selected based on at least one first control input. Further the method includes converting the digital signal into an analog signal, filtering the analog signal to include substantially only components having frequencies in a selected one of the frequency bands, and amplifying the filtered analog signal. The method further includes converting an analog feedback signal corresponding to the amplified filtered analog signal, into a digital feedback signal. The method also includes down-sampling the digital feedback signal to a lower sample rate based on at least one second control input, to output a feedback signal, wherein the pre-distorted baseband signal is generated from a complex input signal using a pre-distortion function adjustable based on the feedback signal. 
     According to another exemplary embodiment, a method of manufacturing a transmitter operable over a wide range of frequencies including a plurality of predetermined frequency bands includes mounting a programmable digital up-converter, a digital to analog converter, and an analog filter, along a transmit path on a substrate or a circuit board, between a baseband processing unit configured to output a pre-distorted signal and a power amplifier. The method further includes mounting an attenuator, an analog to digital converter and a programmable digital down-converter, along a feedback path on the substrate or the circuit board, between the power amplifier and the baseband processing unit. The programmable digital up-converter is configured to convert a pre-distorted baseband signal, into a digital signal based on at least one first control input. The programmable digital down-converter is configured to convert a digital feedback signal to a lower sample rate based on at least one second control input, into a feedback signal. The baseband processing unit is configured to generate the pre-distorted baseband signal based on a complex input signal using a pre-distorting function, which is adjustable based on the feedback signal. 
     It is an objective of the exemplary embodiments to overcome some of the deficiencies discussed in the previous section and to provide a frequency agile pre-distorted transmitter capable of being effectively configured for plural frequency bands. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. In the drawings: 
         FIG. 1  is a schematic diagram of a pre-distorted transmitter according to an exemplary embodiment; 
         FIG. 2  is a schematic diagram of a programmable digital up-converter included in an apparatus according to an exemplary embodiment; 
         FIG. 3  is a schematic diagram of a programmable digital down-converter included in an apparatus according to an exemplary embodiment; 
         FIG. 4  is a flow chart of a method performed to achieve frequency agile pre-distorted transmission according to an exemplary embodiment; and 
         FIG. 5  is a flow chart of a method of manufacturing a frequency agile pre-distorted transmitter according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The following description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to the terminology and structure of pre-distorted transmitters. However, the embodiments to be discussed next are not limited to these systems but may be applied to other existing radio communication systems. 
     Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily all referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. 
     According to exemplary embodiments, a wideband transmitter can be implemented using a programmable digital up-converter in a transmit path, and a programmable digital down converter in a feedback path. By using a digital down-converter in the feedback path, errors which would otherwise be caused by analog components (e.g., due to time, voltage, and/or temperature variations) are not promulgated back through signals on the transmit path. Moreover by using programmable digital converters in the transmit path and the feedback path, the response of the transmission system can be of the same quality across a wide range of frequency bands. By way of contrast, converters which employ analog components may be tuned to have optimal response in a single frequency band, but will behave differently (thus, introducing errors) in other frequency bands of operation. 
     According to an exemplary embodiment illustrated in  FIG. 1 , a frequency agile pre-distorted transmitter  100  is configured to operate over a wide range of frequencies including a plurality of predetermined frequency bands. The transmitter  100  includes a baseband processing unit  110  configured to receive a complex input signal and a feedback signal, and to output a pre-distorted baseband signal. Thus, the baseband processing unit  110  is the start point of a transmission signal path  104  and the end point of a feedback signal path  106 . Although the pre-distorted baseband signal is a baseband signal, note that the feedback signal may be a baseband signal or an intermediate frequency (IF) signal. 
     The pre-distorted baseband signal output by the baseband processing unit  110  is a pre-distorted signal obtained by applying a pre-distortion function. Applying the pre-distortion function is intended to provide a linear relationship between the complex input signal and the signal at the end of the transmission path. The higher the quality of the feedback signal, the more capable the baseband processing unit  110  will be to achieve this linear relationship. In contrast with the conventional pre-distorted transmitter, in which the feedback path has to be retuned when the frequency band is changed, the frequency agile pre-distorted transmitter  100  is capable of providing the same high quality feedback signal for all of the covered frequency bands. The pre-distortion function may be initially a predetermined function (based, e.g., on modeling the power amplifier), but can be adjusted based on the comparison of the complex input signal with the feedback signal. 
     Pre-distortion of signals to be transmitted is, per se, known in the art and, therefore, not described in more detail here. 
     The pre-distorted baseband signal is up-converted as a digital signal by a programmable digital up-converter  120  connected to the baseband processing unit  110 . The digital signal has a spectral content which is selected based on at least one first control input received from the baseband processing unit  110  via control line  124 . The at least one first control input provided to the programmable digital up-converter includes at least one of: filter information associated with a selected one of the predetermined frequency bands, a complex tuner frequency and an up-sampling parameter N associated with the selected one of the predetermined frequency bands. 
     An example of a programmable digital up-converter  120  is described in U.S. Patent Application Publication 2010/0098191, the disclosure of which is incorporated herewith by reference in its entirety.  FIG. 2  is a schematic diagram of a programmable digital up-converter  200  which may be included in a pre-distorted transmitter (e.g., the programmable digital up-converter  120  in the frequency agile pre-distorted transmitter  100  in  FIG. 1 ) according to an exemplary embodiment. The programmable digital up-converter  200  according to this exemplary embodiment includes a complex tuner  210 , a digital up-sampler  220 , an complex channel selection filter  230  and a quadrature modulator  240 , which may be arranged between a baseband processing unit (e.g.,  110  in  FIG. 1 ) and a digital to analog converter (e.g.,  130  in  FIG. 1 ). 
     Due to the presence of a digital up-sampler (e.g.,  220  in  FIG. 2 ) the programmable digital up converter  120  converts a baseband input signal into a digital signal having a spectral content which is selected based on at least one first control input. For example, an up-sampling parameter N used by the digital up-sampler  220  may depend on a selected frequency band for the signal to be transmitted, as well as a sampling rate used in the transmit path. The programmable digital up-converter  120  may receive the up-sampling parameter N, or other information associated with the selected frequency band, from the baseband processing unit  110 , via a control path  124 , which is different than the transmission signal path  104 . 
     Thus, the digital up-sampler  220  may generate a digital signal that includes N values (i.e., samples) for each sample in a digital signal input to the up-sampler  220 , where N is larger than or equal to 2 . In order to simplify the hardware, the added samples may, for example, be zero values requiring no calculation. Since N is typically a power of 2, a rate change filter may be included in the programmable digital up-converter to bridge a sample rate of the signal input to the programmable digital up-converter, and a desired predetermined sample rate of the signal output from the programmable digital up-converter, which is used by the DAC. The rate change filter is a device configured to output a digital signal having a sample rate which is a fraction of the sample rate of a digital signal input to the rate change filter. 
     The digital signal output from the programmable digital up-converter  120  is then converted into an analog signal by the digital to analog converter (DAC)  130 , which operates at a predetermined DAC rate. To operate at this predetermined DAC rate, the DAC  130  receives a local oscillator signal having a predetermined frequency corresponding to the predetermined DAC rate, for example, from a local oscillator  140 . 
     The analog signal output by the DAC  130 , is then filtered by the analog filter  150 , and the filtered analog signal is input to a power amplifier (PA)  160  prior to being transmitted via an antenna  170 . The analog filter  150  filters the analog signal to output only components with frequencies within all or a part of a single Nyquist zone. For example, if the DAC  130  operates at 2.5 GSps, the analog filter  150  can isolate most of the first Nyquist zone (0 to 1.25 GHz) and hence supports all bands within that region, e.g., 450 to 960 MHz. 
     A feedback analog signal corresponding to the amplified filtered signal is fed back to the baseband processing unit  110  via the feedback signal path  106 . The feedback analog signal may be received from the PA  160  via an attenuator  165 . 
     Along the feedback signal path  106 , the feedback analog signal is converted to a digital feedback signal by an analog to digital converter (ADC)  180  operating at a predetermined ADC rate. This predetermined ADC rate may be the same as the predetermined DAC rate of the DAC  130 . However, the invention is not limited by this feature. In order to perform the analog to digital conversion at the predetermined ADC rate, the ADC  180  uses a local oscillation signal. If the predetermined ADC rate is the same as the predetermined DAC rate, the ADC  180  may use the same local oscillator  140  as the DAC  130 . Using the same local oscillator  140  is advantageous because any variation of the local oscillation frequency affects equally the transmit signal path  104  (i.e., the DAC  130 ) and the feedback signal path  106  (i.e., the ADC  180 ). Alternatively, however, two different local oscillators may be used to provide local oscillation (clock) signals to DAC  130  and ADC  180 , respectively. The predetermined ADC and DAC rates may, for example, be larger than 1 GSps. 
     The digital feedback signal output by the ADC  180  is then input to a programmable digital down-converter  190 .  FIG. 3  is a schematic diagram of a programmable digital down-converter  300  usable in various embodiments of frequency agile pre-distorted transmitters. The digital down converter  300 , which is connected between an analog to digital converter (e.g.,  180  in  FIG. 1 ) and a baseband processing unit (e.g.,  110  in  FIG. 1 ) includes a quadrature demodulator  310 , a complex channel selection filter  320 , a digital down-sampler  330 , and a complex baseband tuner  340 . The complex baseband tuner  340  may alternatively be included in the baseband processing unit  110 , and in this case, the signal output by the programmable digital down-converter has an intermediate frequency (IF), which may be zero. The functionality of the complex channel selection filter  320  and the digital down-sampler  330  may be achieved by using a polyphase filter. The polyphase filter may also include the quadrature demodulator. 
     Due to the presence of a digital down-sampler (e.g.,  330  in  FIG. 3 ) the programmable digital down converter  190  converts the digital feedback signal, into the feedback signal having a lower sample rate depending on at least one second control input. The at least one second control input provided to the programmable digital down-converter  190  includes, for example, at least one of: filter information associated with a selected one of the predetermined frequency bands, complex tuner frequency and a down-sampling parameter N associated with the selected one of the predetermined frequency bands. The programmable digital down-converter  190  may receive the down-sampling parameter N or other information on the selected frequency band from the baseband processing unit  110 , via a control path  126 , which is different than the feedback signal path  106 . 
     The digital down-sampler  330  may generate a down-sampled signal that includes one sample for each N samples in a digital signal input to the down-sampler  330 , where N is larger than or equal to 2. Since N is preferred to be a power of 2, a rate change filter (as described above) may be included in the programmable digital down-converter to bridge a predetermined sample rate of the signal input to the programmable digital down-converter divided by N, to the desired sample rate of the signal output from the programmable digital down-converter. For example, suppose (as a purely illustrative numerical example) that the input sample rate to the programmable digital down-converter is 2488.32 MSps, N=8, and the output sample rate from the programmable digital down-converter (without a rate change filter) is 311.04, but the desired output sample rate is 245.76. To bridge these latter two values, a rate change filter performing 64/81 re-sampling can be provided at any point in the signal processing chain shown in  FIG. 3 , but preferably after the complex baseband tuner  340 . 
     If there is minimal interference (e.g. something other than the transmitted signal) from the antenna  170  coupled back in the feedback signal path  106 , then a filter is not required prior to the ADC  180  on the feedback signal path  106 . All required filtering may then be achieved digitally during the down-conversion process, for example, by judicious selection of the polyphase filter coefficients in the programmable digital down-converter  190 . However, optionally, an analog feedback filter  175  may be included in the pre-distorted transmitter  100 , to protect the ADC  180  from overload due to interfering signals. 
     A pre-distorted transmitter including a digital up-converter and a digital down-converter according to exemplary embodiments ensures a nominally flat frequency response (magnitude and linear phase) and repeatability (manufacturing). As the frequency response of the digital up-converter and the digital down-converter is fully defined by the digital signal processing, a frequency agile pre-distorted transmitter can be developed. In other words, a pre-distorted transmitter including a digital up-converter and a digital down-converter is configured to be operated for plural predetermined frequency bands within a wide frequency range. 
     An important advantage of the pre-distorted transmitter including a programmable digital up-converter and a programmable digital down-converter is that it enables a frequency agile pre-distorted transmitter thereby substantially reducing the number of radios required to support the large number of frequency bands around the world. For example, on January 2010, there were 23 bands below 2.2 GHz designated for use in LTE (Long Term Evolution) communication systems. With the DAC/ADC sampling at 2.5 GSps, a single transmitter design, according to exemplary embodiments, having two different image selection filters could be used to cover all the frequency bands. 
     Programmable digital up-converters and programmable digital down-converters of various architectures according to these exemplary embodiments are suitable to be implemented as integrated circuits and take advantage of the continuous miniaturization. Moreover, the fewer components require less board area. 
     Because both the transmission signal path (e.g.,  104 ) and the feedback path (e.g.,  106 ) are linear according to the exemplary embodiments, it becomes easier to build apparatuses including multi-input power amplifiers (i.e., with multiple forward paths). Moreover, the feedback path of the architecture may be used for several forward paths. 
     The flow chart of a method  400  of transmitting signals over a wide range of frequencies including a plurality of predetermined frequency bands according to an exemplary embodiment is illustrated in  FIG. 4 . The method  400  may be performed, for example, by the transmitter  100  illustrated in  FIG. 1 . At S 405 , the method  405  includes generating a pre-distorted baseband signal. At S 410 , the method  400  includes digitally up-sampling the pre-distorted baseband signal to convert the pre-distorted baseband signal into a digital signal having a spectral content which is selected based on at least one first control input. The method  400  includes converting the digital signal into an analog signal, at S 420 , filtering the analog signal to include substantially only components having frequencies in a selected one of the frequency bands, at S 430 , and amplifying the filtered analog signal at S 440 . The operations at S 410 , S 420 , S 430  and S 440  are operations related to a transmission signal path (e.g.,  104 ) in a transmitter, towards an emitting antenna (e.g.,  170 ) that is configured to transmit the amplified filtered analog signal. 
     At S 450 , the method  400  further includes converting an analog feedback signal corresponding to the amplified filtered analog signal into a digital feedback signal. The analog feedback signal may be an attenuated version of the amplified analog signal. At S 460 , the method  400  includes down-sampling the digital feedback signal to a lower sample rate based on at least one second control input, to output a feedback signal. The pre-distorted baseband signal is generated from a complex input signal using a pre-distortion function adjustable based on the feedback signal. 
     The flow chart of a method  500  of manufacturing a transmitter operable over a wide range of frequencies including a plurality of predetermined frequency bands according to an exemplary embodiment is illustrated in  FIG. 5 . At S 510 , the method  500  includes mounting a programmable digital up-converter, a digital to analog converter, and an analog filter, along a transmission path on a substrate or a circuit board, between a baseband processing unit configured to output a pre-distorted signal and a power amplifier. Further, at S 520 , the method  500  includes mounting an attenuator, an analog to digital converter and a programmable digital down-converter along a feedback path on the substrate or the circuit board, between the power amplifier and the baseband processing unit. The programmable digital up-converter in the method  500  is configured to convert a pre-distorted baseband signal into a digital signal based on at least one first control input. The programmable digital down-converter in the method  500  is configured to convert a digital feedback signal to a lower sample rate based on at least one second control input, into a feedback signal. The baseband processing unit is configured to generate the pre-distorted baseband signal based on a complex input signal using a pre-distorting function, which is adjustable based on the feedback signal. 
     The method  500  may further include mounting a local oscillator on the substrate or the circuit board, and connecting the local oscillator to the analog to digital converter and the digital to analog converter. 
     The method  500  may also include mounting an analog feedback filter on the substrate or the circuit board, between the attenuator and the analog to digital converter, or before the attenuator. The method  500  may include connecting the programmable digital up-converter and the programmable digital down-converter to the baseband processing unit via control paths other than the transmission path and the feedback path, the control paths being configured to allow the baseband processing unit to provide the at least one first control input and the at least one second control input to the programmable digital up-converter and the programmable digital down-converter, respectively. 
     The mounting of the of the programmable digital up-converter at S 510  may include mounting a complex baseband tuner, a digital up-sampler, a complex channel selection filter and a quadrature modulator between the baseband processing unit and the digital to analog converter, on the transmission path. 
     The mounting of the of the programmable digital down-converter at S 520  may include mounting a quadrature demodulator, a complex channel selection filter, a digital down converter and a complex baseband tuner between the analog to digital converter and the baseband processing unit, along the feedback path. Alternatively, in another embodiment, the mounting of the programmable digital down-converter at S 520  may include mounting a quadrature demodulator, a polyphase filter and a complex baseband tuner between the analog to digital converter and the baseband processing unit, along the feedback path. 
     Although the features and elements of the present exemplary embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein.