Patent Publication Number: US-7212583-B2

Title: Transmission of signal

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a method and a transmitter in a telecommunication network. Specifically, the invention aims to reduce PAPR (Peak-to-Average Power Ratio) in QAM modulated signal transmission. 
   2. Description of the Related Art 
   In radio systems, a data-carrying base-band signal is modulated with the carrier wave. In PSK (Phase Shift Keying) modulation, the phase of the carrier waveform is changed depending on the symbol value in the information signal. In QAM (Quadrature Amplitude Modulation), both the phase and the amplitude are changed depending on the symbol values. 
   QAM is an excellent modulation method from the point of view of efficiency but suffers from high PAPR. A high PAPR will need a power amplifier with a high 1-dB compression point in order to transmit the waveform undistorted even if the average power of the signal is much lower. This will lead to an expensive solution. If the signal is clipped by the amplifier, unacceptable spectral leakage to the adjacent channels will occur. 
   SUMMARY OF THE INVENTION 
   It is the object of this invention to provide a solution which reduces the PAPR in conjunction with QAM modulation without causing spectral leakage. The invention relates to a data transmission method used in conjunction with QAM modulation in a radio system, the method comprising steps of filtering data symbols to be transmitted with a first transmit filter to form a waveform, comparing the waveform to a predetermined threshold value of power level, clipping power peaks that exceed the predetermined threshold value from the waveform to form a clipped waveform, filtering the clipped waveform with a receive filter to form an impaired signal, filtering the impaired signal with a second transmit filter to form a modulated signal, and up-converting the modulated signal to a transmission frequency. 
   The invention also relates to a data transmission method used in conjunction with QAM modulation in a radio system, the method comprising steps of filtering data symbols to be transmitted with a first transmit filter to form a waveform, comparing the waveform to a predetermined threshold value of power level, clipping power peaks that exceed the predetermined threshold value from the waveform, multiplying the clipped power peaks with an impulse response of a receive filter to form an impaired signal, subtracting the impaired signal from the data symbols to be transmitted to form subtracted data symbols, filtering the subtracted data symbols with a second transmit filter to form a modulated signal, and up-converting the modulated signal to a transmission frequency. 
   The invention also relates to a radio transmitter using QAM modulation, comprising a first transmit filter for filtering data symbols to be transmitted to form a waveform, means for comparing the waveform to a predetermined threshold value, means for clipping power peaks that exceed the predetermined threshold value from the waveform to form a clipped waveform, a receive filter for filtering the clipped waveform to form an impaired signal, a second transmit filter for filtering the impaired signal to form a modulated signal, and means for up-converting the modulated signal to a transmission frequency. 
   The invention also relates to a radio transmitter using QAM modulation, comprising a first transmit filter for filtering data symbols to be transmitted to form a waveform, means for comparing the waveform to a predetermined threshold value, means for clipping power peaks that exceed the predetermined threshold value from the waveform to form a clipped waveform, means for multiplying the clipped power peaks with an impulse response of a receive filter to form an impaired signal, means for subtracting the impaired signal from the data symbols to be transmitted to form subtracted data symbols, a second transmit filter for filtering the subtracted data symbols to form a modulated signal, and means for up-converting the modulated signal to a transmission frequency. 
   Embodiments of the invention are described in the dependent claims. 
   The object of the invention is achieved by injecting a small impairment into the symbols to be transmitted. This impairment is selected so that the PAPR is reduced. The amount of PAPR reduction can be controlled in the invention by controlling the threshold value determining the degree of clipping. The threshold value corresponds to the desired PAPR of the output signal and can be set according to the amplifier capabilities. Besides this impairment, there are no side effects like spectrum regrowth in the adjacent channels in a solution according to the invention. 
   Generally, in transmission, the data symbols are up-sampled well above the Nyquist rate, that is, at a rate at least twice the signal bandwidth before the signal is fed to the transmit filter. The transmit filter produces the initial waveform, which is then clipped in order to cut the highest power peaks in the signal. In the invention, before providing the signal to the final transmit filter, impairment is injected into the clipped signal by exposing the signal to two extra filtering stages, that is, one transmit filter and one receive filter, which in principle are contrary to each other. The first filtering stage is performed before clipping and the second filtering stage after the clipping phase. By these additional filtering phases, a small impairment is injected into the data-carrying signal, thereby reducing power peaks in the signal. The impairment is due to the fact that the error introduced by the clipping is spread out among a few neighbour symbols. 
   In one embodiment, the two filtering stages can be realized by using a transmit filter and a receive filter, which together form a raised cosine filter. In another embodiment, instead of the receive filter, an impulse response of a receive filter is used, thereby reducing the need for filters from three to two in the transmission chain. 
   The method and system of the invention provide several advantages. For instance, the invention reduces PAPR significantly with minimal degradation since there is no spectral leaking to adjacent channels. Due to the reduced PAPR, smaller and more cost-effective amplifiers can be used. The solution is particularly attractive in uplink, since the size of the power amplifier is a very important factor in the terminal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following, the invention will be described in greater detail with reference to the preferred embodiments and the accompanying drawings, in which 
       FIG. 1  shows an embodiment of the method according to the invention, 
       FIG. 2  shows 16-QAM modulation constellation and the effect of the invention on the constellation, 
       FIG. 3  illustrates the effect of the invention on the waveform, 
       FIG. 4  shows the principle in clipping of the waveform, 
       FIG. 5  shows a prior art transmitter structure, 
       FIG. 6  illustrates another prior art transmitter, 
       FIG. 7  illustrates one embodiment of a transmitter according to the invention, 
       FIG. 8  shows another embodiment of a transmitter according to the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows one example of the method according to the invention. All figures show only the I channel because the operations in the Q channel are identical to those in the I channel. In method step  102  the data symbols are up-sampled. In the up-sampling process, zero valued symbols are added to the information stream in order to provide a data stream with a sampling rate higher than the Nyquist rate. If, for example, the original symbols are [1,−1,−1,1] in the I channel, in the up-sampling process zeroes are added in-between the data symbols. The up-sampled data stream can then be for instance [1,0,0,0,−1,0,0,0,−1,0,0,0,1,0,0,0]. 
   In step  104 , the up-sampled data symbols are filtered in a first filtering stage. The filtering is performed in a transmit filter, which outputs an information-carrying base-band waveform. In method step  106 , the waveform is clipped so that the highest power peaks are eliminated from the filtered wavefrom. In practice, clipping can be done so that the filtered waveform is sampled and such samples that exceed the value of a predetermined threshold value for the power get the threshold value. 
   In step  108 , the clipped waveform is filtered in a second filtering stage and the filtered signal is down-sampled  110 . The filtering in this stage is in principle inverse to the filtering in step  104  and consists of a receive filter with the idea to recover the symbol representation of the original data. However, the filtering performed in steps  104  and  108  cause impairment to the symbols that were to be transmitted originally. Because of the up/down sampling processes the symbol values has to be properly scaled in  112 . 
   The impaired symbols are fed to the transmit filter in step  114 . The filtered signal is up-converted in step  116  to provide a modulated waveform. In up-conversion, the filtered waveform is modulated with the carrier wave to form a high-frequency radio signal. The radio frequency signal is amplified and fed to one or more transmission antennas for radio transmission. 
     FIG. 2  shows a 16-QAM modulation constellation. The modulation scheme takes 4 bits as input, and so 16 different values in the amplitude-phase coordinate system can be presented. Two constellation points, corresponding to bit combinations 0000 and 0010, are marked in  FIG. 2 . The traditional 16-QAM modulation uses dot-like constellation values one marked with reference number  200  is shown. In the invention, a small impairment is introduced into modulation points. The dot-like point value  200  does not necessarily apply anymore, but instead the amplitude-phase values are determined from a larger circle  202 , however so that the centre point of area  202  is the exact modulation point  200 . 
     FIG. 3  illustrates the difference when a traditional 64-QAM modulation and the invention are used. The x axis illustrates time and the y axis shows the power level of the signal. In  FIG. 3 , signal  300  is an original transmission signal and signal  302  is the signal when the PAPR reduction according to the invention is used. 
     FIG. 4  shows the basic principle in clipping of the waveform. The power of I and Q channels is taken in blocks  400  and  402 , respectively. The powers are summed in summing means  404 . The summed power is compared in comparing means  406  to a predetermined threshold value. The comparison result is transferred to multiplexing means  410 . Multiplexer  410  can choose either the non-scaled signal or a scaled signal depending on the comparison result. If the predetermined threshold value is not exceeded, the original I and Q channels are maintained. If the sum exceeds the predetermined threshold value, the I and Q signals are clipped proportionally in scaling means  408 A– 408 B. The scaling can be performed taking a square root of the quotient “threshold/peak”. In the following figures, clipping operation shown in  FIG. 4  is shown as a single block. Furthermore, in the following figures, all filtering operations are performed separately and identically for I and Q channels. 
     FIG. 5  illustrates one prior art radio transmitter structure showing only parts essential to clarifying the invention. Data  500  contains information bits to be transmitted by the transmitter, such as data or control information. Up-sampling is performed in block  502  and the information bits are modulated by the transmit filter  504 . Up-conversion block  506  combines a high-frequency carrier wave and the waveform which carries the user information bits. The modulated radio-frequency signal is amplified by an amplifier  508  before transmission. 
     FIG. 6  differs from the transmitter shown in  FIG. 5  in that the information-carrying waveform is clipped by clipping block  600  before conversion of the signal to the radio frequency. The solution shown in  FIG. 5  suffers from high PAPR, and it cannot be used in practical cases. The clipping shown in  FIG. 6  diminishes the requirements for the power amplifier  508  but the clipped signal has a tendency to leak to adjacent channels. 
     FIG. 7  shows one embodiment of the invention. Data  700  is received in a means for up-sampling  702 . The up-sampling means  702  forwards up-sampled data to a first means for filtering  704 . The filtering means  704  can be implemented for instance as a SRRC filter (square-root-raised-cosine). The SRRC filter can be adapted for instance to 16-QAM modulation, when four information bits are inputted into the filter and a waveform is output to these four bits. 16-QAM is shown here only as an example of the modulation method, and the invention can also be applied to other QAM modulation schemes. After the first filtering step, the created waveform is clipped by a means for clipping  706 . After clipping, in the transmitter, the signal is next handled by a second means for filtering  708 . This second filtering means  708  can be an inverse filter compared to the first filtering means  704 , meaning that they together form a root-raised- cosine filter. The filter, which has low pass characteristics, will average the signal so that the peak will be diminished and spread among the neighbouring symbols. After the second filtering means  708 , the signal is down-sampled in a means for down-sampling  710 . The purpose in performing the first and second filtering stages around the clipping stage is that impairment is injected into data symbols  700 . 
   The down-sampled symbols are scaled in scaling means  712  due to the up-sampling and fed to a third filtering means  714 . The filtered signal is then up-converted to a transmission frequency in up-converting means  716  and amplified in power amplifying means  718  before radio transmission. 
     FIG. 8  shows another embodiment of the invention. In a means for multiplying  800 , the clipped signal portion K is multiplied with the impulse response of a receive filter. The impulse response to multiplying means  800  is provided by means  802  for providing an impulse response. The original signal is delayed in delaying means  806 , and from this delayed original signal, the impaired signal is subtracted in subtracting means  804 . The embodiment of  FIG. 8  differs from the embodiment of  FIG. 7  is that one filter fewer is used. 
   The following equations illustrate the mathematical background behind the solution shown in  FIG. 8 . The alternative solution in  FIG. 8  is derived from the basic solution in  FIG. 7  for the case when one peak of the transmit filter exceeds the threshold value. In the following, s[n] denotes symbols obtained from block  700 , x[n] denotes the signal from the clipping means  706 , and y[n] is the signal where the effect of the receiving signal has been taken into consideration. 
   Equation (1) shows thus the clipped signal 
   
     
       
         
           
             
               
                 
                   
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   K is the clipped part of the signal, h t  is the impulse response of the transmitting filter and m is the time instant for the clipping. The index i runs through all taps of the transmit filter. The δ is the unit sample sequence and denotes an impulse at time m. In the transmitting filter, convolution is calculated between the data symbols and the impulse response of the transmitting filter. 
   The signal after the receive filter  708  is shown by equation (2), 
   
     
       
         
           
             
               
                 
                   
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   h r  is the impulse response of the receive filter and the index j runs through all taps of the filter. K is the amount of clipping that was carried out at time m. By interchanging the order of indices equation (2) can be rewritten (3). 
   
     
       
         
           
             
               
                 
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 rc=h   t   *h   r , then  (4)
 
   equation (3) can be rewritten in form of equation (5) 
   
     
       
         
           
             
               
                 
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   Because the convolution of s[ ] and rc[ ] is a delayed version of s[ ] after downsampling, equation (6) is obtained
 
 y[n]=s[n+D]−Kh   r   [n−m].   (6)
 
   Thus, the signal y[n] that is input to a transmitting filter  714  is a signal from which the clipped signal portion multiplied by the impulse response of the receive filter is subtracted. Thus, the resulting signal y[n] is the delayed original signal subtracted by the impulse response of the receive filter weighted with the amount of clipping performed at time m. In order to modify the impairment produced by the clipping, the signal can also be formed by subtracting the impulse response of some other optimally selected filter instead of the receive filter. 
   The invention can be realized by using filters or by modifying the signals with impulse responses of the filters. Besides the filters, software, ASIC or separate logic components can be used for implementation of features in the invention. 
   Even though the invention is described above with reference to an example according to the accompanying drawings, it is clear that the invention is not restricted thereto but it can be modified in several ways within the scope of the appended claims.