Abstract:
A wireless receiver which is used for a digital signal transmission system to wirelessly transmit a digital signal by packetizing and modifying it, selectively sets the shortest arithmetical bit length satisfying a required communication quality when performs demodulation arithmetical processing to demodulate a digital signal to be packet-transmitted, inputs a demodulation arithmetical result by the arithmetical bit length to calculate an error vector magnitude value that is a measure indicating a difference between the arithmetical result and a known ideal result, predicts a bit error rate by using the EVM value as an evaluation criterion, selects an arithmetical bit length by which the bit error rate becomes optimum, and executes the demodulating arithmetical processing by the selected arithmetical bit length.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2006-079124, filed Mar. 22, 2006, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a wireless receiver and its demodulating method which utilizes, for example, a wireless local area network (LAN), and more specifically, relates to a technique which reduces power consumption at a demodulating circuit used for a processing device of a digital signal. 
         [0004]    2. Description of the Related Art 
         [0005]    The wireless receiver is driven by a battery for giving weight to its portability. Therefore, especially, in the processing device of the digital signal, the reduction in the power consumption in the demodulating circuit is desired. 
         [0006]    A concrete configuration of a conventional technique is disclosed by Jpn. Pat. Appln. KOKAI Publication No. 2002-051016 is disclosed so as to satisfy this desire. The wireless receiver described in the aforementioned patent document enables changing an arithmetical bit length of digital signal processing and demodulates a digitized receiving signal by an instructed arithmetical bit length. The wireless receiver estimates a communication line situation from the demodulation result, and obtains the shortest arithmetical bit length satisfying a required communication quality on the basis of the estimation result to instruct the bit length to demodulating processing. 
         [0007]    By the way, the configuration disclosed by the foregoing patent document measures a carrier-to-noise ratio (CNR) from the preamble of the receiving signal, dramatically changes the arithmetical bit length of the digital signal processing by using a branch table of a CNR versus arithmetical bit length using the conditions satisfying the required quality created from a pre-simulation (additive white Gaussian noise [AWGN]), and then obtains the shortest arithmetical bit length satisfying the required communication quality.  FIG. 7  illustrates an applying range of the shortest bit length satisfying a CNE versus bit error ratio (BER) characteristic (BER=10 −3 ) under an AWGN environment at each bit length.  FIG. 8  illustrates a CNR versus bit-length branch table created on the basis of the CNR versus BER characteristic illustrated in  FIG. 7 . 
         [0008]    However, the aforementioned method presumes that a propagation environment is in an ideal state, and it does not take a variation in an actual propagation environment. Therefore, especially, the environment having been causing multipath fading poses a problem that an error operation is caused because the relation between the CNE and the arithmetical bit length possible to satisfy an actual required quality does not coincide with the pre-created branch table and a bit error characteristic is degraded. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    An object of the present invention is to provide a wireless receiver and its demodulating method which can appropriately select an arithmetical bit length without being affected by a variation in propagation environment, thereby, can effectively decrease power consumption in demodulating processing. 
         [0010]    According to the present invention, there is provided a wireless receiver which is used for a digital signal transmission system wirelessly transmitting a digital signal by packetizing and modulation it, and selectively sets the shortest arithmetical bit length satisfying a required communication quality in conducting demodulation arithmetic processing to demodulate the digital signal to be packet-transmitted, the wireless receiver comprises an error vector magnitude (EVM) calculation unit configured to calculate an EVM value that is a measure indicating the difference from a known ideal result by inputting a demodulation arithmetic result by the arithmetical bit length, and an arithmetical bit-length selection unit configured to predict a bit error rate by using the EVM value as an evaluation criterion and for selecting an arithmetical bit length by which the bit error rate becomes optimum, and performs the demodulation arithmetic processing by the selected arithmetical bit length. 
         [0011]    In addition, according to the present invention, there is provided a demodulating method of the wireless receiver which is used for a digital signal transmission system wirelessly transmitting the digital signal by packetizing and modulation it, the demodulating method comprises demodulating the digital signal to be packetized and transmitted by the specified arithmetical bit length, calculating an EVM value that is a measure indicating the difference from a known ideal result by inputting a demodulating arithmetical result from the demodulating arithmetical result, selecting an arithmetical bit length by which the bit error rate becomes optimum, and specifying the selected arithmetical bit length into the demodulating. 
         [0012]    Advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0013]    The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
           [0014]      FIG. 1  is an exemplary block diagram illustrating a configuration in the case in which the present invention is applied to a wireless LAN receiver in a quatrature phase shift keying-orthogonal frequency division multiplex (QPSK-OFDM) system; 
           [0015]      FIG. 2  is an exemplary view illustrating a CNR-BER characteristic measured from a QPSK-OFDM system receiver for each packet under a multipath fading environment; 
           [0016]      FIG. 3  is an exemplary view illustrating an EVM versus BER characteristic measured from the QPSK-OFDM system receiver for each packet under the multipath fading environment; 
           [0017]      FIG. 4  is an exemplary schematic view illustrating an outline of a bit-length changing method by using an EVM as an evaluation criterion for a packet transmission OFDM system used for a wireless LAN; 
           [0018]      FIG. 5  is an exemplary view illustrating an example of a bit-length changing rule by using an EVR evaluation criterion; 
           [0019]      FIG. 6  is an exemplary flowchart illustrating a flow of concrete processing in the case in which a bit-length changing method is achieved by software; 
           [0020]      FIG. 7  is an exemplary view illustrating an allocation range of the shortest bit length satisfying a CNR versus BER characteristic and a preset quality (BER=10 −3 ) under an AWGN environment as for a description of a conventional technique; and 
           [0021]      FIG. 8  is an exemplary view illustrating a CNR versus bit length branch table created on the basis of the CNR versus BER characteristic illustrated in  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    Hereinafter, embodiments of the present invention will be described with reference to the drawings in detail. 
         [0023]      FIG. 1  is a block diagram depicting a configuration in the case in which the present invention is applied to a wireless LAN receiver in a quatrature phase shift keying-orthogonal frequency division multiplex (QPSK-OFDM) system represented by IEEE 802.11a/g standard. In  FIG. 1 , a radio frequency (RF) signal received at a receiving antenna  11  is amplified by an RF module  12 , and converted into a digital signal through an analog-to-digital converter  13  to be input in an OFDM modulator  14  after being converted into a base-band OFDM signal by orthogonal detection. 
         [0024]    The OFDM modulator  14  includes a demodulator  141 , a channel predicting unit  142 , an error vector magnitude (EVM) calculating unit  143 , and a bit length selecting unit  144 . 
         [0025]    After inputting the base-band OFDM signal input to the OFDM demodulator  14  to a low-pass filter  14 A and removing unnecessary frequency components, the demodulating unit  141  conducts fast Fourier transform through an FFT circuit  14 B to demultiplex the OFDM signal for each sub-carrier, and calculates backward amplitude-phase characteristics of transmission paths for each channel by a channel equalizing circuit  14 C to restore it to an original transmission signal. 
         [0026]    The channel predicting unit (arithmetical bit length fix)  142  predicts a defined channel by a prescribed frequency width for an FFT arithmetical result and the prediction result is transferred to the channel equalizing circuit  14 C to be supplied to channel section processing. 
         [0027]    The EVM calculating unit  143  calculates the EVM value to be an evaluation measure of arithmetical bit length selection for the transmission signal demodulated by the demodulator  141  (detailed will be mentioned later), and the EVM value calculated herein is sent to the bit length selecting unit  144 . 
         [0028]    The bit length selecting unit  144  predicts the BER by using the EVM value as the evaluation criterion to select the arithmetical bit length by which the BER becomes optimum, the selected arithmetical bit length herein is specified for the low-pass filter  14 A, FET circuit  14 B and channel equalizing circuit  14 C of the demodulating unit  141 . 
         [0029]    In the foregoing configuration, the following will describe its processing contents. 
         [0030]    At first, the wireless LAN receiver regarding the invention uses the evaluation criterion of the EVM for selecting the arithmetical bit length. The EVM is expressed by the following equation (1). 
         [0000]    
       
         
           
             
               
                 
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         [0031]    Wherein, z(k) is a channel-equalized QPSK demodulation signal (complex number with I, and Q components) generated form an EVM calculation symbol, R(k) is a known ideal signal (complex number with I, and Q components), and M is the number of OFDM sub-carriers. The calculation equation (1) is not limited to the case in which the transmission signal is the QPSK signal, and it goes the same as the case of a quadrature amplitude modulation (QAM) signal. 
         [0032]    That is to say, the EVM is a measure to indicate the difference between an ideal waveform and a measured waveform, it is given by a distance between a receiving signal vector having the I and Q components of a demodulated output.  FIG. 2  and  FIG. 3  illustrate the CNR versus BER characteristic and the EVM versus BER characteristic in bit lengths ( 6 ,  7 , and  8 ) measured for each packet from the QPSK-OFDM system receiver under the multipath fading environments (delay spread 50 ns and 18-wave independent Rayleigh fading model), respectively. The CNR versus BER characteristics, as shown in  FIG. 2 , are uneven for each bit length. Conversely, the EVR versus BER characteristics, as shown in  FIG. 3 , have less variations for each bit length, given almost in a relation of one to one, and even if the bit lengths are different from one another, the characteristics are excellent. Therefore, using the EVM as the evaluation criterion enables prediction of the BER with high reliability. 
         [0033]    As mentioned above, the conventional wireless receiver disclosed by Jpn. Pat. Appln. KOKAI Publication No. 2002-051016 uses the CNR for predicting a communication situation, so that a problem is produced, wherein a large error occurs in the prediction result under the multipath fading environment, and error operations in changing bit lengths is caused to deteriorate a bit error characteristic. In contrast, the wireless LAN receiver using the EVM as the evaluation criterion calculates the EVM on the basis of the output signal from the demodulating unit  141  having arithmetical units  14 A to  14 C of variable bit lengths. The wireless receiver evaluates the arithmetical bit length by including the arithmetical error accompanied by multipath interference, or bit-length changes of the receiver other than noise components in a communication path. Like this, the EVM being calculated on the basis of the demodulation performance of the actual receiver, a change in bit length is possible with high accuracy. 
         [0034]    In terms of a packet transmission OFDM system for use in the wireless LAN, an outline of a bit-length changing method in which the EVM is used as the evaluation criterion is shown in  FIG. 4 . The method shown in  FIG. 4  replaces the head of data symbol of a packet to a known search symbol for EVM calculation to transmit it on a transmission side. In  FIG. 4 , Wmax, W(t), and Ws(t) are bit lengths of a preamble, a search symbol and a data symbol applied to the arithmetical units  14 A to  14 C of the receiver, respectively. A receiving side calculates the EVM from the search symbol W(t) of an input packet to decide W(t+1) setting a bit length Ws(t) to apply to a data symbol immediately following the search symbol and a bit length of the next packet, based on the prescribed search rule. 
         [0035]      FIG. 5  illustrates an example of a hit-length changing rule using the EVM evaluation criterion. In this example, the rule increases and decreases the bit length so that the preset quality becomes BER=10 −3 . The branch value of the EVM is generated on the basis of the EVR versus BER characteristic shown in  FIG. 3 , and changing the branch value allows changing the preset quality. The bit length Ws(t) reflecting the EVM measured result measured in a packet, the bit length Ws(t) may be changed by following a communication characteristic even if the communication characteristic is rapidly varied for each packet. 
         [0036]    When the technique disclosed in the Jpn. Pat. Appln. KOKAT Publication No. 2002-051016 is applied to the packet mode OFDM, a single bit length becomes to be applied to the whole of the packet, and the bit length cannot be changed dynamically. In contrast, the receiver of the configuration described above may selectively apply bit lengths independent from one another for the three of the preamble, search symbol, and data symbol inside the packet. Therefore, even when the communication situations for the packet of the last reception and for the currently receiving packet are extremely different form each other, the receiver regarding the invention may immediately respond to the situation by measuring the demodulation performance of the receiver in receiving the search symbol. 
         [0037]    Having described about the hardware configuration shown in  FIG. 1  herein, the whole or a part of the OFDM demodulator  14  may be achieved by software.  FIG. 6  depicts the flow of the concrete processing for achieving the foregoing bit-length changing method by software. 
         [0038]    In  FIG. 6 , an equation of W=Wmax is given as an initial condition. At first, the changing method determines a packet input (step S 11 ), and when the packet input is made, the method discriminates a bit length Wmax after starting the reception of the preamble (step S 12 ), discriminates the search symbol following the preamble to obtain its bit length W(t) (step S 13 ), and conducts EVM calculation by the equation (1) (step S 14 ). Next, the method decides the bit length Ws(t) for the data symbol following the search symbol, and decides the bit length W(t+1) for the search symbol of the next packet, based on the result of the EVM calculation (step S 15 ). The method discriminates the data symbol depending on the bit length Ws(t) decided in this way (step S 16 ). After this, the method repeatedly conducts the processing in the steps coming after the step S 11 . The proceeding of the processing like such a manner enables achieving the processing equivalent to that in the case of the hardware by means of the software. 
         [0039]    The present invention is not limited to each of the aforementioned embodiments as they are, and in an implementation phase, this invention may be embodied in modifications of constituent elements without departing from the sprit or scope of the general inventive concept thereof. For instance, in the aforementioned embodiments, the present invention having been described in the case in which it is applied to the wireless LAN receiver of the QPSK-OFDM system, the invention is also applicable to, for example, a digital television broadcasting receiver employing a continuous mode OFDM system. This invention is also applicable to a BPSK-OFDM system, 16-QAM-OFDM system, 64-QAM-OFDM system, and the like as for its transmission system in a similar manner. Further, the invention is applicable not only to the OFDM system, but also applicable to, for example, a code division multiplex (CDM) system, or a time division multiplex (TDM) system in the like manner. Other than this, various types of the invention can be formed by appropriately combining a plurality of constituent elements. Some of the elements, for example, may be omitted from the whole of the constituent elements shown in the embodiments above. Further, the constituent elements over different embodiments may be appropriately combined. 
         [0040]    Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.