Abstract:
To reduce the peak-to-average power ratio (PAPR) of a complex-valued digital baseband signal, the signal is mixed to an intermediate frequency and its real components extracted, to generate an intermediate-frequency real-valued digital signal. The intermediate frequency is one-quarter of a sampling rate of the complex-valued digital baseband signal. The intermediate-frequency real-valued digital signal is clipped and down-converted by one-quarter of the sampling rate.

Description:
TECHNICAL FIELD 
     The present embodiments relate generally to digital communications, and specifically to reducing the dynamic range of baseband signals. 
     BACKGROUND OF RELATED ART 
     The peak-to-average power ratio (PAPR) of a signal is defined as the ratio of the signal&#39;s peak power to its average power. PAPR is the square of the crest factor, which is defined as the signal&#39;s peak amplitude divided by its average value (i.e., its root-mean-squared, or RMS, value). A high PAPR and corresponding high crest factor reduce power-amplifier (PA) efficiency and increase the dynamic range for transmit-signal-processing stages, thus complicating the design of those stages. Dynamic range refers to the ratio of the maximum signal magnitude to the minimum signal magnitude. High PAPR is a known challenge in orthogonal frequency-division multiplexing (OFDM). 
     Accordingly, it is desirable to reduce the PAPR of digital baseband transmit signals, such as digital baseband OFDM transmit signals. The PAPR may be reduced by reducing the dynamic range. 
     One approach to dynamic range reduction, and thus to PAPR reduction, is clipping and filtering.  FIG. 1  is a block diagram of PAPR reduction circuitry  100  that clips and filters a digital baseband signal provided as input. The digital baseband signal is a quadrature-amplitude-modulation (QAM) baseband signal with in-phase (I in ) and quadrature (Q in ) components. This input signal is provided to an absolute-value module  102  and a multiplier  106 . The absolute-value module  102  determines the magnitude |in| of the input signal and provides the magnitude |in| to a lookup table (LUT)  104 . A clipping level A is also provided to the LUT  104 . The LUT  104  uses the magnitude |in| and the clipping level A to perform a lookup that returns a clipping factor c. The clipping factor c may be determined using the formula: 
     
       
         
           
             
               
                 
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     The multiplier  106  multiplies the input signal by the clipping factor c, thereby clipping the input signal. A low-pass filter (LPF)  108  filters the clipped input signal, resulting in an output signal of the PAPR reduction circuitry  100 . The output signal is a QAM baseband signal with in-phase (I out ) and quadrature (Q out ) components. 
     The clipping-and-filtering technique performed by the PAPR reduction circuitry  100  involves a large number of multiplications, because the QAM baseband signal is complex-valued. Accordingly, there is a need for clipping-and-filtering techniques that are computationally simple. 
     SUMMARY 
     In some embodiments, a method of reducing the PAPR of a signal includes mixing a first complex-valued digital baseband signal to an intermediate frequency and extracting real components to generate an intermediate-frequency real-valued digital signal. The intermediate frequency is one-quarter of a sampling rate of the first complex-valued digital baseband signal. The intermediate-frequency real-valued digital signal is clipped to generate a clipped intermediate-frequency digital signal, which is down-converted by one-quarter of the sampling rate. 
     In some embodiments, circuitry for reducing the peak-to-average power ratio (PAPR) of a signal includes a circuit to mix a first complex-valued digital baseband signal to an intermediate frequency and extract real components to generate an intermediate-frequency real-valued digital signal. The intermediate frequency is one-quarter of a sampling rate of the first complex-valued digital baseband signal. The circuitry also includes a multiplier to clip the intermediate-frequency real-valued digital signal, to generate a clipped intermediate-frequency digital signal. The circuitry further includes a down-converter to down-convert the clipped intermediate-frequency digital signal by one-quarter of the sampling rate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present embodiments are illustrated by way of example and are not intended to be limited by the figures of the accompanying drawings. 
         FIG. 1  is a block diagram of PAPR reduction circuitry that clips and filters a digital baseband signal. 
         FIGS. 2 and 3  are block diagrams of PAPR reduction circuitry in accordance with some embodiments. 
         FIGS. 4 and 5  are flowcharts showing methods of reducing the PAPR of a transmit signal in accordance with some embodiments. 
     
    
    
     Like reference numerals refer to corresponding parts throughout the drawings and specification. 
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth such as examples of specific components, circuits, and processes to provide a thorough understanding of the present disclosure. Also, in the following description and for purposes of explanation, specific nomenclature is set forth to provide a thorough understanding of the present embodiments. However, it will be apparent to one skilled in the art that these specific details may not be required to practice the present embodiments. In other instances, well-known circuits and devices are shown in block diagram form to avoid obscuring the present disclosure. The term “coupled” as used herein means connected directly to or connected through one or more intervening components or circuits. Any of the signals provided over various buses described herein may be time-multiplexed with other signals and provided over one or more common buses. Additionally, the interconnection between circuit elements or software blocks may be shown as buses or as single signal lines. Each of the buses may alternatively be a single signal line, and each of the single signal lines may alternatively be buses, and a single line or bus might represent any one or more of a myriad of physical or logical mechanisms for communication between components. The present embodiments are not to be construed as limited to specific examples described herein but rather to include within their scope all embodiments defined by the appended claims. 
     The number of multiplications involved in reducing the dynamic range and PAPR of a signal may be reduced with respect to the clipping-and-filtering technique of  FIG. 1  by avoiding clipping complex-valued signals. 
       FIG. 2  is a block diagram of PAPR reduction circuitry  200  in accordance with some embodiments. The PAPR reduction circuitry  200  is described with reference to  FIG. 4 , which is a flowchart showing a method  400  of reducing the PAPR (and dynamic range) of a transmit signal in accordance with some embodiments. The PAPR reduction circuitry  200  is implemented in a transmitter, such that the transmitter performs the method  400 . 
     A first complex-valued digital baseband signal is provided ( 402 ) as an input to the PAPR reduction circuitry  200 . For example, the first complex-valued digital baseband signal is a first QAM baseband signal with in-phase (I in ) and quadrature (Q in ) components. This signal is provided to a mixer  202 , which mixes ( 404 ) the first complex-valued digital baseband signal to an intermediate frequency that is one-quarter of a sampling rate (i.e., f s /4) of the first complex-valued digital baseband signal. The mixer  202  thus up-converts the first complex-valued digital baseband signal to the intermediate frequency of one-quarter of the sampling rate, thereby changing the center frequency of the signal but not its sampling rate. A real-component-extraction module  204  receives the output of the mixer  202  and extracts ( 406 ) its real components, thus generating an intermediate-frequency real-valued digital signal. 
     The first complex-valued digital baseband signal is also provided to an absolute-value module  102 , which determines ( 408 ) the signal&#39;s magnitude, denoted as link. The absolute-value module  102  provides the magnitude |in| to a LUT  104 , which also receives a specified clipping level A. (Alternatively, the absolute-value module  102  determines another metric that corresponds to the signal&#39;s magnitude. For example, the absolute-value module  102  may determine the square of the signal&#39;s magnitude, |in| 2 .) The LUT  104  determines ( 410 ) a clipping factor c using the magnitude |in| and the clipping level A (e.g., in accordance with equation 1), by performing ( 412 ) a lookup based on the values of |in| and A. Performing a lookup in a LUT  104  is merely one way of determined the clipping factor c (e.g., of performing the calculation of equation 1); others are possible. 
     A multiplier  206  clips ( 414 ) the intermediate-frequency real-valued digital signal by multiplying the intermediate-frequency real-valued digital signal by the clipping factor c. A mixer  208  down-converts ( 416 ) the clipped intermediate-frequency real-valued digital signal by one-quarter of the sampling rate (i.e., f s /4) to generate a second complex-valued digital baseband signal. A low-pass filter (LPF)  210  filters ( 418 ) the second complex-valued digital baseband signal. The output of the LPF filter  210 , and thus of the PAPR reduction circuitry  200 , may be a QAM baseband signal with in-phase (I out ) and quadrature (Q out ) components. The LPF filter  210  functions by analogy to the LPF  108 , although it may have a different number of taps (e.g., more taps) than the LPF  108 . 
     In some embodiments, the functionality of the mixer  202 , real-component-extraction module  204 , and mixer  208  are achieved through sample selection. The functionality of these components thus may be achieved without performing multiplication. 
       FIG. 3  is a block diagram of PAPR reduction circuitry  300 , which is an example of the PAPR reduction circuitry  200  ( FIG. 2 ), in accordance with some embodiments. The PAPR reduction circuitry  300  achieves the functionality of the mixer  202  and real-component-extraction module  204  using a first sample-selection module  302  and achieves the functionality of the mixer  208  using a second sample-selection module  304 . The PAPR reduction circuitry  300  is described with reference to  FIG. 5 , which is a flowchart showing a method  500  of reducing the PAPR (and dynamic range) of a transmit signal in accordance with some embodiments. The method  500  is an example of the method  400  ( FIG. 4 ). The PAPR reduction circuitry  300  is implemented in a transmitter, such that the transmitter performs the method  500 . 
     A first QAM signal with in-phase (I in ) and quadrature (Q in ) components is provided ( 502 ) as an input to the PAPR reduction circuitry  300 . The first QAM signal is an example of the first complex-valued digital baseband signal of the method  400  ( FIG. 4 ). The I in  component includes a first stream of samples: 
     x I [0], x I [1], x I [2], . . . 
     and the Q in  component includes a second stream of samples: 
     x Q [0], x Q [1], x Q [2], . . . 
     where the bracketed numbers index the samples in each stream and thus indicate the temporal ordering of the samples in each stream. 
     The first sample-selection module  302  generates ( 504 ) a third stream of samples: 
     x I [0], −x Q [1], −x I [2], x Q [3], x I [4], . . . 
     To generate the third stream of samples, the first sample-selection module  302  thus selects ( 506 ) samples from the first and second streams in an alternating manner: x I [0] is selected, followed by x Q [1], followed by x I [2], followed by x Q [3], followed by x I [4], and so on. Unselected samples from the first and second streams are discarded ( 508 ) and thus not included in the third stream: x Q [0] is discarded, as are x I [1], x Q [2], x I [3], x Q [4], and so on. The signs of alternating pairs of the selected samples are inverted ( 510 ): the signs of x Q [1] and x I [2] are inverted (i.e., multiplied by −1), while the signs of x Q [3] and x I [4] are not inverted. 
     The third stream of samples is an example of the intermediate-frequency real-valued digital signal of the method  400  ( FIG. 4 ). Generating the third stream of samples is an example of mixing ( 404 ,  FIG. 4 ) the first complex-valued digital baseband signal and extracting ( 406 ,  FIG. 4 ) its real components. 
     The absolute-value module  102  determines ( 512 ) the magnitude |in| of the first QAM signal. The LUT  104  determines ( 514 ) the clipping factor c based on the magnitude |in| and the clipping level A (e.g., using equation 1). 
     The multiplier  206  clips ( 518 ) the third stream of samples by multiplying the samples of the third stream by the clipping factor c. The multiplier  206  thus generates a fourth stream of samples, which is an example of the clipped intermediate-frequency real-valued digital signal of the method  400  ( FIG. 4 ). The fourth stream of samples may be represented as: 
     z[0], z[1], z[2], . . . 
     The second sample-selection module  304  generates a second QAM signal with I and Q components, each of which includes a respective stream of samples. The stream of samples for the I component of the second QAM signal is: 
     y I [0]=z[0], y I [1]=0, y I [2]=−z[2], y I [3]=0, y I [0]=z[4], . . . 
     while the stream of samples for the Q component of the second QAM signal is: 
     y Q [0]=0, y Q [1]=−z[1], y Q [2]=0, y Q [3]=z[3], y Q [4]=0, . . . 
     To generate the second QAM signal, the second sample-selection module  304  thus assigns ( 522 ) respective samples of the fourth stream to either the I component or the Q component of the second QAM signal, such that successive samples of the fourth stream are divided between the I and Q components of the second QAM signal in an alternating manner. For example, z[0] is assigned to the I component, z[1] is assigned to the Q component, z[2] is assigned to the I component, z[3] is assigned to the Q component, z[4] is assigned to the I component, and so on. The second sample-selection module  304  inserts ( 524 ) zeros into the I and Q components, such that the zeros alternate with the samples assigned from the fourth stream, as shown above. The second sample-selection module  304  also inverts ( 526 ) the sign of alternating ones of the samples assigned from the fourth stream in each of the I and Q components. For example, the sign of z[2] is inverted (i.e., multiplied by −1) in the I component, while the signs of z[0] and z[4] are not. In the Q component, the sign of z[1] is inverted while the sign of z[3] is not, as shown above. 
     The LPF  210  filters ( 528 ) the second QAM signal. The output of the LPF filter  210 , and thus of the PAPR reduction circuitry  300 , is a QAM baseband signal with in-phase (I out ) and quadrature (Q out ) components. 
     The PAPR reduction circuitry  300  and method  500  reduce the dynamic range of transmit signals, and thus reduce PAPR and the crest factor, in a computationally simple manner. The sample selection modules  302  and  304  function without performing any arithmetic operations. The multiplicative clipping performed by the multiplier  206  operates on a real-valued signal as opposed to a complex-valued signal, thereby reducing the number of multiplications as compared to the PAPR reduction circuitry  100  ( FIG. 1 ). Also, half of the samples of the second QAM signal are zeros, which simplifies the filtering performed by the LPF  210 . 
     While the methods  400  and  500  include a number of operations that appear to occur in a specific order, it should be apparent that the methods  400  and  500  can include more or fewer operations. Operations can be executed serially or in parallel, performance of two or more operations may overlap, and two or more operations may be combined into a single operation. For example, all of the operations of the methods  400  and/or  500  may be performed in parallel in an on-going manner as a transmit signal is processed. 
     In the foregoing specification, the present embodiments have been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the disclosure as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense.