Patent Publication Number: US-2005118966-A1

Title: Simple Crest Factor reduction technique for multi-carrier signals

Description:
BACKGROUND OF INVENTION  
      The present invention relates to a Crest Factor reduction circuit to boost the out put power of a multi-carrier wireless RF amplifier. The Crest Factor reduction circuit input could be baseband, intermediate frequency (IF), or RF signal and its output is the Crest Factor reduced RF signal as a new input to the amplifier. In any wireless communication system one of the critical components is the power amplifier. This component has a major contribution in cost, power consumption, and size of the system. The main reason is the requirement of wireless radio communication system for linear amplifiers. The higher the linearity, the higher the power consumption, cost and size. In order to minimize the cost, size and power consumption there is a need for techniques that overcome this problem. This invention conquers these challenges by using a simple and accurate Crest Factor reduction module used at the input to the amplifier.  
     SUMMARY OF INVENTION  
      According to the invention, a low-cost RF Crest Factor reduction circuit, for use with multi-carrier RF amplifier, uses a plurality of simple and accurate circuits in conjunction with intelligent signal processing to improve power handling of the multi-carrier RF amplifier. By intelligent, it is meant that the Crest Factor reduction module has features of removing the unwanted signals after applying the crest factor reduction function. The Crest Factor reduction module uses the amplifier input which could be a baseband, an IF or RF signal as its input and conditions the input before applying to the multi-carrier amplifier. The conditioning or Crest Factor reduction helps to boost the power handling of the amplifier or acts more linearly. The inputs to the Crest Factor reduction should be within a limit that can be handled by the Crest Factor reduction module.  
      In a particular embodiment, the Crest Factor reduction unit comprises a multi-carrier transmitter and a multi-carrier broadband receiver, a signal processing, and a clock generator. The receiver and transmitter convert the baseband, IF, or RF signal to digital baseband and the digital baseband signal to RF. The signal processor performs the signal conditioning as well as performs the initial calibration, and transmitter and receiver control.  
      The invention will be better understood by reference to the following detailed description in conjunction with the accompanying drawings.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is an overall block diagram of the a amplifier with a booster using Crest Factor reduction  
       FIG. 2  is the block diagram of the Crest Factor reduction module  
       FIG. 3  is the block diagram of the digital processing unit of Crest Factor reduction module  
       FIG. 4  is the block diagram of the digital signal processing block performing the Crest Factor reduction  
       FIG. 5  is the detail block diagram of Crest Factor reduction  
    
    
     DESCRIPTION OF THE SPECIFIC EMBODIMENTS  
      In a first preferred embodiment the Crest Factor reduction circuit monitors the signal strength of the multi-carrier input signal channels using the input receiver and finds the frequency and channel number of the input signals. In a second preferred embodiment of the invention, the Crest Factor reduction circuit uses sub-harmonic sampling to convert multi-carrier RF or IF signals to digital baseband signal. In a third preferred embodiment the input signal is conditioned or Crest Factor reduced using the multi-carrier baseband signal. In a fourth embodiment the digital baseband signal is further down converted to produce the individual carrier baseband signal. In a fifth embodiment the multi-carrier signal is amplitude clipped or limited either in analog or digital domain. In a sixth embodiment the individual baseband signals are individually filtered and up converted to reconstruct the multi-carrier digital baseband signal.  
      Referring to  FIG. 1 , a Crest Factor reduction circuit diagram is illustrated. The systems receive its inputs from wireless transmitter  100 . The output of the Crest Factor reduction circuit  200  is applied to the input of the amplifier. The Crest Factor reduction circuit performs the following functions: 
          1. Finds the frequencies and channel numbers of the multi-carrier wireless transmitter output  100 .     2. Reduce the Crest Factor of the input signal  100  before applying to amplifier.     3. Adaptively adjust the gain in the signal paths to keep the total gain from input to output of the Crest Factor reduction zero.        

       FIG. 2  illustrates the detail block diagram of the Crest Factor reduction circuit unit. The received signal from multi-carrier wireless transmitter  100  is applied to multi-carrier receiver  201  to produce signal  400 . The output of the multi-carrier receiver  201  is applied to signal processing block  202  for digital signal processing which is Crest Factor reduction and filtering of baseband representation of each carrier. The output of signal processing block  202  the Crest Factor reduced signal  401  is applied to multi-carrier transmitter  203  to create the input signal  101  for the multi-carrier amplifier. Clock generator  205  produces all the clocks necessary for the Crest Factor reduction circuit and the power supply block  204  produce all the voltages necessary for the Crest Factor reduction circuit.  
       FIG. 3  shows the detail block diagram of the Crest Factor reduction signal processing block  202 . The receiver block  201  output  400  is applied to analog to digital converter (in case the signal is RF, IF, or baseband ) block  500  to produce the digital signal  410 . If the signal is RF or IF the analog to digital conversion is based on sub-harmonic sampling. The output of the analog to digital converter  500  is applied to the DSP block  501  for down conversion and decimation to produce “m” sample per symbol. In case the signal is a multi-carrier baseband the signal may need to be interpolated or decimated to produce the right number of samples per symbol. If the signal is baseband but in bit format the up conversion function of  501  is used. The signal is converted to symbol domain with desired samples per symbol first and then each channel is up converted to its baseband frequency to produce multi-carrier baseband. The DSP block  501  also performs the Crest Factor reduction and produce signal  411 . The Crest Factor reduced signal  411  is applied to up converter and interpolator  503  to produce the up converted and interpolated signal  412 . Signal  412  is applied to digital to analog converter  503  to produce the analog signal  401  for the multi-carrier transmitter block  203 .  
       FIG. 4  shows the block diagram of the Crest Factor reduction block  502 . The multi-carrier baseband signal  410  from the main multi-carrier receiver has its amplitude clipped by amplitude clipping block  510  to produced amplitude limited multi-carrier signal  420 . The amplitude limited signal  420  is down converted to single carrier baseband signals by block  511  to produce the baseband representative of each individual carrier. The individual single carrier baseband signals  421  are filtered by filter block  512  to produce the filtered signals  422 . The filtered signals  422  are applied to block  513  to reconstruct the multi-carrier baseband signal  411 .  
       FIG. 5  shows the detail block diagram of the Crest Factor reduction circuit. The multi-carrier baseband signal  410  from the receiver is applied to amplitude clipping block  510  to produce amplitude limited multi-carrier signal  420 . The amplitude limited signal  420  is applied to down converters  601 ,  602 , and  603  to produce the baseband signal of each carrier  701 ,  711 , and  721 . The second input to down converters  601 ,  602 , and  603  are supplied by NCOs  661 ,  662 , and  663 . The baseband representative of each carrier then is applied to Low Pass Filters (LPF)  611 ,  612 , and  613  to filter unwanted signals. The filtered baseband representative of each carrier  702 ,  712 , and  722  is applied to up converter blocks  651 ,  652 , and  653 . The other signal used by up converter is supplied by NCOs  681 ,  682 , and  683 . The up converted signals  706 ,  716 , and  726  are then combined in block  600  to produced the new multi-carrier baseband signal  411 . In  FIG. 5  only a multi-carrier with  3  carrier is shown. This approach can be applied to unlimited number of carriers.