Patent Publication Number: US-2009232249-A1

Title: Peak suppressing apparatus, wireless transmitting apparatus, and window function generating apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2008-65174, filed on Mar. 14, 2008, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The present invention relates to a peak suppressing apparatus that suppresses peak components of a signal using a window function. The peak suppressing apparatus includes a technique related to a peak suppressing process performed in a wireless transmitting apparatus using a multi-carrier transmission system, such as an OFDM (orthogonal frequency divisional multiplexing) system or an OFDMA (orthogonal frequency divisional multiple access) system. 
     BACKGROUND 
     The multi-carrier transmission system divides data into a plurality of sub-carriers and transmits the divided sub-carriers in parallel. The multi-carrier transmission system may increase a symbol period, as compared to a single carrier transmission system, and reduce transmission deterioration using multiple paths. In addition, the OFDM system capable of effectively implementing the multi-carrier transmission transmits signals using a plurality of orthogonal sub-carriers. Therefore, the OFDM system may improve frequency use efficiency and achieve high-speed transmission. 
     The multi-carrier transmission system (such as the OFDM system) has been put into practical use in, for example, a terrestrial digital television or a wireless LAN, and has been applied to WiMAX (worldwide interoperability for microwave access), which is one of the communication standards, such as a mobile communication standard for mobile phones and a wireless data communication standard. The WiMAX (including mobile WiMAX) is standardized by IEEE802.16-2004/IEEE802.16e. In the following, an example of a wireless transmitting apparatus using the OFDM system as a transmission system is described. 
     The wireless transmitting apparatus performs inverse fast Fourier transform (IFFT) on sub-carrier signals that are orthogonal to each other to multiplex the frequency of each of the sub-carrier signals, thereby generating an OFDM modulation signal, and transmits the generated signal. The OFDM modulation signal generated by the IFFT process includes peak components. And, in the OFDM modulation signal, a PAPR (peak-to-average power ratio) tends to be increased, that is, peak transmission power tends to be considerably higher than average transmission power. When a signal with a high PAPR is transmitted, a transmission power amplifier (hereinafter, simply referred to as an ‘amplifier’) provided in the wireless transmitting apparatus is preferable to have high linearity over a wide dynamic range, in order to prevent the non-linear distortion of a transmission signal during signal amplification or power leakage to adjacent channels. 
     However, in general, the linearity and the efficiency of the amplifier are contrary to each other. Therefore, when high linearity is ensured over a wide dynamic range, power efficiency is lowered, and the power consumed by the wireless transmitting apparatus is increased. For this reason, a peak suppressing process of suppressing peak transmission power is performed in order to reduce the PAPR. 
     Further, when a peak component is suppressed, modulation accuracy is lowered. Therefore, a peak suppression method of minimizing the deterioration of the modulation accuracy using a window function has been known. The peak suppression method using a window function multiplies a peak point that is more than the threshold value of a suppression level by a correction coefficient (=a suppression level/the peak value of envelope of an input signal) to reduce the level of the peak point to the suppression level. In this case, the peak suppression method uses a window function as the correction coefficient in order to prevent the peak points from being discontinuous. The height (the level of the highest portion) of the window function is equal to (a suppression level/the peak value of the envelope of an input signal), and the magnitude of a start point is equal to that of an end point. The level of the peak point may be reduced to the suppression level (threshold value) while smoothly changing adjacent (front or rear of) peak points by multiplying the highest portion of the window function by the peak point of an input signal while synchronizing them. 
       FIG. 1  illustrates an example of the structure of a window-function-type peak suppressing unit of a wireless transmitting apparatus according to the related art. A window-function-type peak suppressing unit  10  illustrated in  FIG. 1  includes, for example, four window function generating units  13 - 1  to  13 - 4  (generically referred to as a window function generating unit  13 ) in order to process a maximum of four peak points detected within the time of the window width of a window function. When a peak point detecting unit  11  detects a peak point having a peak level that is more than a threshold value from a transmission signal subjected to IFFT, an allocating unit  12  allocates the peak point to one of the window function generating units  13  that performs no process. 
     A table (LUT)  134  of the window function generating unit  13  stores plural kinds of window functions having different window widths. Since a window width for effectively suppressing peak components depends on the frequency band width and the sampling rate of IFFT, the window width of the window function is changed depending on the frequency band width and the sampling rate of IFFT. Therefore, plural kinds of window functions having different window widths are prepared for the peak suppressing process. In particular, in a WiMAX communication system (WiMAX system), in order to correspond to various frequency band widths and sampling rates of various countries, the operation results of the window functions with window widths corresponding to various conditions are set in the wireless transmitting apparatus as a window function table in advance, and one of the window functions satisfying the current conditions is selected and used. 
     An address generating unit  133  generates read addresses for reading all the window functions of the tables, and outputs the window function stored in the table  134  to a selecting unit  135 . 
     A window width is set in a window width determining (setting) unit  132  in advance. The selecting unit  135  selects a window function corresponding to the window width set in the window width setting unit  132  and outputs the selected window function. 
     A window function height calculating unit  131  calculates a suppression amount (window function height) to reduce the peak level of a peak point to a suppression level. A multiplier  136  multiplies the selected window function by the calculated suppression amount. A multiplier  14  multiplies the output of the multiplier  136  of the window function generating unit  13 - 1  by the output of the multiplier  136  of the window function generating unit  13 - 2  to compose the window functions. A multiplier  15  multiplies the output of the multiplier  136  of the window function generating unit  13 - 3  by the output of the multiplier  136  of the window function generating unit  13 - 4  to compose the window functions. A multiplier  16  multiplies the output of the multiplier  14  by the output of the multiplier  15  to compose all the window functions generated by the window function generating units  13 - 1  to  13 - 4 . In this way, a window function for suppressing all peak points maximum of four, within the time of the window width is generated and then output to a multiplier  17 . The output of the multiplier  136  of the window function generating unit  13  allocated with no process is  1 . 
     The multiplier  17  multiplies the transmission signal that is delayed by a predetermined amount of time by a delay unit  18  by the composed window function output from the multiplier  16 . In this way, the peak point of the transmission signal is suppressed. 
     For example, a technique related to the window-function-type peak suppressing process is proposed wherein the process of interpolating spectral peaks of data clipped by a Hanning window function using a quadratic function or a Gauss function (see Japanese Laid-open Patent Publication No. 6-295855). In addition, a technique related to the window-function-type peak suppressing process is proposed wherein the process of inserting a zero series into an A/D-converted data series so as to be approximate to a polynomial including a quadratic function, performing discrete Fourier transform to obtain amplitude values, and using three values near the maximum value among the amplitude values to calculate simultaneous polynomials of the quadratic function, thereby approximating the frequency of a beat signal (see Japanese Laid-open Patent Publication No. 2005-337825). 
     As described above, plural kinds of window functions having different window widths corresponding to the frequency bandwidth and the sampling rate of IFFT are set in the wireless transmitting apparatus in advance. However, this structure may not be applied to a wireless communication system (such as a WiMAX system) that does not have a predetermined frequency bandwidth or sampling rate. In addition, it is difficult to cope with a case in which the window width is changed to values other than the set values. 
     Therefore, the related art using a table having predetermined plural kinds of window functions stored therein has a problem in that it is difficult to flexibly correspond to various frequency band widths or sampling rates. 
     SUMMARY 
     According to an aspect of the embodiment discussed herein, a peak suppressing apparatus for suppressing peak components of a signal using a window function includes a detecting unit that detects the peak components which are more than a given threshold value, a window width setting unit that sets a window width of the window function, and a window function generating unit that generates the window function using quadratic functions by calculation on the basis of the peak levels of the peak components and the window width. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the embodiment, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example of the structure of a window-function-type peak suppressing unit of a wireless transmitting apparatus according to the related art; 
         FIG. 2  illustrates an example of the structure of a wireless transmitting apparatus according to the embodiment; 
         FIG. 3  illustrates an example of the structure of a window-function-type peak suppressing unit  40 ; 
         FIG. 4  illustrates a window function generated by a quadratic curve window function generating unit  44 ; 
         FIG. 5  illustrates the frequency characteristics of a quadratic curve window function; 
         FIGS. 6A to 6D  illustrate a method of generating a quadratic curve window function; 
         FIG. 7  illustrates an example of the structure of the quadratic curve window function generating unit  44  that generates a quadratic curve window function; and 
         FIGS. 8A to 8D  illustrate a quadratic curve window function when two peak points are detected within the time of a window width. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     Hereinafter, an example of a wireless transmitting apparatus of an embodiment is described with reference to the drawings. 
       FIG. 2  illustrates an example of the structure of a wireless transmitting apparatus according to the embodiment. A transmission signal generated by a baseband processing unit  100  is input to an RF processing unit  200 . A clip-type peak suppressing unit  20  performs a clip-type peak suppressing process that uniformly clips components that are more than a threshold value among the peak components included in the transmission signal. When the clip-type peak suppressing process is performed, radiation occurs beyond a frequency band. Therefore, a low-pass filter (LPF)  30  performs a band limiting process. When the band limiting process is performed, the amplitude of the clipped peak is increased to be more than the threshold value. Therefore, a window function-type peak suppressing unit  40  performs the peak suppressing process again. 
     In this embodiment, the window function-type peak suppressing unit  40  calculates a window function (hereinafter, referred to as a quadratic curve window function) generated by connecting three quadratic function curves (quadratic curves) such that the curves are continuous at change points for the detected peaks, and performs the peak suppressing process using the calculated window function, which is described below. 
     A distortion compensating unit  50  multiplies the transmission signal by characteristics (distortion compensation coefficient) opposite to the distortion characteristics of an amplifier (AMP)  400  in order to reduce the distortion of the output characteristics of the amplifier (AMP)  400 . The amplifier requires high power efficiency all the time, but the linearity and the efficiency of the amplifier are generally contrary to each other. Therefore, in order to improve the power efficiency using an amplifier having low linearity, a pre-distortion-type distortion compensating unit is used to compensate distortion. The pre-distortion-type distortion compensating unit compares a transmission signal before distortion compensation with a demodulated feedback signal to calculate an error therebetween, and calculates and updates a distortion compensation coefficient using the error. 
     The transmission signal whose distortion is compensated is converted into an analog signal by a DAC  300 , and the analog signal is amplified by the amplifier  400 . Then, the amplified signal is transmitted from an antenna. 
       FIG. 3  illustrates an example of the structure of the window-function-type peak suppressing unit  40 . When a peak point detecting unit  41  of the window-function-type peak suppressing unit  40  detects a peak point from the transmission signal, a window function height calculating unit  42  calculates a window function height H corresponding to a suppression amount required to reduce the peak level of the peak point to a suppression level. The window function height H is calculated by multiplying the suppression amount by a specified correction coefficient. 
     A window width L is set in a window width setting unit  43  in advance, and the window width setting unit  43  outputs the set window width L. In this embodiment, without selecting and setting one of the window widths of a limited number of window functions, a window function corresponding to the window width is calculated. Therefore, the window width setting unit  43  may set various window widths calculated according to the frequency bandwidth or the sampling rate of IFFT (inverse fast Fourier transform), without any restrictions. 
     A quadratic curve window function generating unit  44  generates and outputs a quadratic curve window function, and the structure and operation thereof are described below. A multiplier  45  multiplies the transmission signal that is delayed by a predetermined amount of time by a delay unit  46  by the window function generated by the quadratic curve window function generating unit  44 . In this way, the peak of the transmission signal is suppressed. 
       FIG. 4  illustrates the window function generated by the quadratic curve window function generating unit  44 . In this embodiment, the window function (quadratic curve window function) is generated by connecting three quadratic curves so as to be continuous with each other. For example, the window function is defined by the following three quadratic curves ( 1 ), ( 2 ), and ( 3 ). 
     
       
         
           
             
               
                 
                   
                     
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     As illustrated in  FIG. 4 , the quadratic curve window function generating unit  44  may generate a window function using the quadratic curves by connecting three quadratic curves so as to be continuous at change points. 
       FIG. 5  illustrates the frequency characteristics of a quadratic curve window function. In order to evaluate the frequency characteristics of the quadratic curve window function (waveform a represented by a bold solid line), the frequency characteristics of a Hanning window function (waveform b represented by a dotted line) and the frequency characteristics of a Bartlett window function (waveform c represented by a thin solid line) are also illustrated in  FIG. 5 . 
     The frequency characteristics of the quadratic curve window function are sufficiently practical, as compared to the frequency characteristics of the Hanning window function that is generally used for a peak suppressing process. In addition, the Bartlett window function is calculated by a relatively easy operation. However, the Bartlett window function has a high sidelobe and is not used for the peak suppressing process. This embodiment proposes a method of calculating a quadratic curve window function having good frequency characteristics suitable for the peak suppressing process using a simple arithmetic circuit. 
       FIGS. 6A to 6D  illustrate a method of generating a quadratic curve window function. The quadratic curve window function is generated by accumulating a delta function (impulse function) S satisfying the following conditions three times (triple integral). When the delta function S is δ(t−T) at a time t, a time T (T 1 , T 2 , T 3 , and T 4 ) corresponds to the change point of the quadratic curve, and a pulse having a height h is generated at the times T 1 , T 2 , T 3 , and T 4 . 
     
       
         
           
             
               
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       FIG. 6A  is a timing chart illustrating the delta function S (S 1 , S 2 , S 3 , and S 4 ) satisfying the above-mentioned conditions. At the times T 1 , T 2 , T 3 , and T 4 , pulses having heights h 1 , h 2 , h 3 , and h 4  (hereinafter, referred to as pulses h 1 , h 2 , h 3 , and h 4 ) are output. In this case, according to the above-mentioned expression, the pulse h 2  is minus two times the pulse h 1 , the pulse h 3  is two times the pulse h 1 , and the pulse h 4  is equal to the negative of the pulse h 1 . 
       FIG. 6B  illustrates the operation results when the delta function S illustrated in  FIG. 6A  is accumulated for each clock. The pulse h 1  is output at the time T 1 , and the delta function S 1  is zero for a period from the time T 1  to the time T 2 . Therefore, the operation result is h 1  when T 1 ≦t&lt;T 2 . Then, the pulse h 2 =−2×h 1  is output at the time T 2 , and the delta function S 2  is zero for a period from the time T 2  to the time T 3 . Therefore, the operation result is h 1 +h 2 =−h 1  when T 2 ≦t&lt;T 3 . Then, the pulse h 3 =2×h 1  is output at the time T 3 , and the delta function S 3  is zero for a period from the time T 3  to the time T 4 . Therefore, the operation result is −h 1 +2h 1 =h 1  when T 3 ≦t&lt;T 4 . Then, the pulse h 4 =−h 1  is output at the time T 4 , and the operation result at the time T 4  is h 1 −h 1 =0. 
       FIG. 6C  illustrates the operation results obtained by accumulating the operation results illustrated in  FIG. 6B  for each clock. In  FIG. 6C , when T 1 ≦t&lt;T 2 , the pulse height h 1  is added to each clock. When T 2 ≦t&lt;T 3 , the pulse height h 1  is subtracted from each clock (−h 1  is added to each clock). When T 3 ≦t&lt;T 4 , the pulse height h 1  is added to each clock. In this way, the operation results illustrated in  FIG. 6C  are obtained. 
       FIG. 6D  illustrates the operation results obtained by accumulating the operation results illustrated in  FIG. 6C  for each clock. As illustrated in  FIG. 6D , four delta functions S (S 1 , S 2 , S 3 , and S 4 ) are accumulated three times to obtain a quadratic curve window function. 
       FIG. 7  illustrates an example of the structure of the quadratic curve window function generating unit  44  that generates the quadratic curve window functions illustrated in  FIGS. 6A to 6D . First, the quadratic curve window function generating unit  44  generates the delta functions S 1 , S 2 , S 3 , and S 4 . Specifically, a pulse height calculating unit  62  calculates a base pulse (h 1 ) corresponding to a peak suppression amount on the basis of the width L and the height H of the window function according to the above-mentioned expression, and outputs the pulse h 1  at the time T 1  (output of the delta function S 1 ). 
     A delay unit  63  delays the pulse h 1  by L/4, and a computing unit  64  multiplies the height of the pulse h 1  by −2 to output the pulse h 2  at the time T 2  (output of the delta function S 2 ). 
     A delay unit  65  further delays the pulse h 1  delayed by the delay unit  63  by L/2, and a computing unit  66  multiplies the height of the pulse h 1  by 2 to output the pulse h 3  at the time T 3  (output of the delta function S 3 ). 
     A delay unit  67  further delays the pulse h 1  delayed by the delay unit  65  by L/4, and a computing unit  68  multiplies the height of the pulse h 1  by −1 to output the pulse h 4  at the time T 4  (output of the delta function S 4 ). The computing units  64 ,  66 , and  68  may be adders. 
     An adder  69  adds the output (pulse h 3 ) of the computing unit  66  and the output (pulse h 4 ) of the computing unit  68 , and an adder  70  adds the output (pulses h 3  and h 4 ) of the adder  69  and the output of the computing unit  64  (pulse h 2 ). An adder  71  adds the (pulses h 2 , h 3 , and h 4 ) of the adder  70  and the output (pulse h 1 ) of the pulse height calculating unit  62 , and outputs the added result. That is, the adder  71  outputs the pulses h 1 , h 2 , h 3 , and h 4  at the times T 1 , T 2 , T 3 , and T 4 , respectively.  FIG. 6A  corresponds to an output P 1  of the adder  71 . 
     The output P 1  of the adder  71  is input to the accumulator  72  and accumulated. An output P 2  of the accumulator  72  is input to the accumulator  73  and accumulated. An output P  3  of the accumulator  73  is input to the accumulator  74  and accumulated. An output P 4  of the accumulator  74  is a quadratic curve window function. That is, the quadratic curve window function is generated by accumulating the output P 1  of the adder  71  three times. The output P 2  of the accumulator  72  corresponds to  FIG. 6B , the output P 3  of the accumulator  73  corresponds to  FIG. 6C , and the output P 4  of the accumulator  74  corresponds to  FIG. 6D . 
     The quadratic curve window function generating unit  44  according to this embodiment illustrated in  FIG. 7  performs operations using only the adders without using a multiplier. Therefore, the quadratic curve window function generating unit  44  may have a circuit structure that is smaller than one multiplier module. As a result, the quadratic curve window function generating unit has a simple circuit structure and a very small operation load. In addition, the quadratic curve window function generating unit  44  may directly calculate a window function having a given window width without storing the operation results of a limited number of window functions as a table. Further, in the structure illustrated in  FIG. 7 , the window width may be set for every four clocks. 
     When a plurality of peak points are detected within the time of the window width, each peak point is processed by the structure illustrated in  FIG. 7 , and the output P 4  of the accumulator  74  in the last stage is a window function obtained by composing quadratic curve window functions corresponding to the peak points. Therefore, in this embodiment, the quadratic curve window function generating unit  44  may not have a circuit structure corresponding to the number of peak points generated within the time of the window width, and there is no limitation in the number of peak points. 
       FIGS. 8A to 8D  illustrate quadratic curve window functions when two peak points are detected within the time of the window width.  FIG. 8A  illustrates a delta function generated for each peak. In  FIG. 8A , pulses h 1 - 1 , h 2 - 1 , h 3 - 1 , and h 4 - 1  generate a window function that suppresses a first peak, and pulses h 1 - 2 , h 2 - 2 , h 3 - 2 , and h 4 - 2  generate a window function that suppresses a second peak. Similar to the above, these pulses are accumulated three times.  FIG. 8B  illustrates the operation results obtained by the first accumulating process,  FIG. 8C  illustrates the operation results obtained by the second accumulating process, and  FIG. 8D  illustrates the operation results obtained by the third accumulating process. Similar to  FIGS. 6A to 6D , the output P 1  of the adder  71  corresponds to  FIG. 8A , the output P 2  of the accumulator  72  corresponds to  FIG. 8B , the output P 3  of the accumulator  73  corresponds to  FIG. 8C , and the output P 4  of the accumulator  74  corresponds to  FIG. 8D . As illustrated in  FIG. 8D , the output P 4  of the accumulator  74  is a window function obtained by composing quadratic curve window functions corresponding to the two peaks. 
     Therefore, in this embodiment, not preparing the window function generating units  44  corresponding to the number of peak points that are expected to occur within the window width, one window function generating unit  44  may correspond to a plurality of peak points. Therefore, in this embodiment, it is possible to significantly reduce the size of a circuit. In addition, it is possible to simplify the structure of the window function generating unit, and the size of a circuit to about one-tenth the size of a window function generating circuit according to the related art. 
     According to the above-mentioned apparatus and method, it is possible to calculate a window function using quadratic functions. In this way, it is possible to simplify the structure of a window function generating circuit. Therefore, when various window widths are set as in a WiMAX system, it is possible to generate a window function corresponding to a set window width, without restrictions in the window width, and flexibly cope with a change in the window width. 
     The window function generating unit according to this embodiment may be applied to various apparatuses using a window function as well as a peak suppressing process of a wireless transmitting apparatus.