Patent Publication Number: US-2011058615-A1

Title: Preamble generating apparatus and method for digital direct transmission system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priorities of Korean Patent Application Nos. 10-2009-0084655 filed on Sep. 8, 2009, and 10-2010-0016333 filed on Feb. 23, 2010 in the Korean Intellectual Property Office, the disclosures of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a preamble generating apparatus and method for a digital direct transmission system, and more particularly, to a technique of generating a preamble, used for detecting the synchronization of a digital direct transmission system, through a combination of a pseudo-noise code, line coding, and spread coding to thus generate a preamble code having keen autocorrelation characteristics and low cross-correlation characteristics, while overcoming the problem of low frequency noise, thereby improving the performance of synchronization detection. 
     2. Description of the Related Art 
     A human body communication system, a technique of transmitting information through a human body unlike the existing wireline or wireless communication system, has a simplified structure by using the characteristics of a human body channel and uses a digital direct transmission scheme to minimize power consumption. 
     When a digital signal is directly transmitted by a system employing the digital direct transmission scheme, a spreading code is mainly used to overcome the problem of a low signal-to-noise ratio (SNR) and thus secure a desired bit error rate (BER). 
       FIG. 1  is a schematic block diagram showing the structure of a related art apparatus for generating a preamble for a digital direct transmission system. The related art preamble generating apparatus  100  includes a pseudo-noise (PN) code generation unit  110  and a spreading coder  120 . 
     The PN code used for detecting the synchronization of the digital direct transmission system must have keen autocorrelation characteristics and low cross-correlation characteristics. However, simply spreading the PN code generated by the PN code generation unit  110  of the related art preamble generating apparatus  100  by the spreading coder  120  cannot obtain such keen autocorrelation characteristics and low cross-correlation characteristics, leading to a failure to obtain the desired performance of synchronization detection. The degradation of the performance of synchronization detection makes a gain obtained through spreading useless. 
     Thus, in order to improve the performance of an overall system, a technique of generating a preamble code having keen autocorrelation characteristics and low cross-correlation characteristics, while overcoming the problem of low frequency noise, is required. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention provides a preamble generating apparatus and method for a digital direct transmission system capable of generating a preamble, used for detecting the synchronization of a digital direct transmission system, through a combination of a pseudo-noise code, line coding, and spread coding to thus generate a preamble code having keen autocorrelation characteristics and low cross-correlation characteristics, while overcoming the problem of low frequency noise, thereby improving the performance of synchronization detection. 
     According to an aspect of the present invention, there is provided a preamble generating apparatus for a digital direct transmission system including: a pseudo-noise (PN) code generation unit generating a PN code; a line coder performing line coding on the PN code received from the PN code generation unit; and a spreading coder performing spread coding on the line-coded PN code received from the line coder. 
     According to another aspect of the present invention, there is provided a preamble generating method for a digital direct transmission system including: generating a pseudo-noise (PN) code; performing line coding on the generated PN code; and performing spread coding on the line-coded PN code. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic block diagram showing the structure of the related art apparatus for generating a preamble for a digital direct transmission system; 
         FIG. 2  is a schematic block diagram showing the structure of an apparatus for generating a preamble for a digital direct transmission system according to an exemplary embodiment of the present invention; 
         FIG. 3  illustrates the generation of a preamble by the apparatus for generating a preamble for a digital direct transmission system according to an exemplary embodiment of the present invention; and 
         FIGS. 4   a  to  4   c  are graphs showing the performance of synchronization detection by preambles generated according to an exemplary embodiment of the present invention and the related art. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In describing the present invention, if a detailed explanation for a related known function or construction is considered to unnecessarily divert the gist of the present invention, such explanation will be omitted but would be understood by those skilled in the art. In the drawings, the shapes and dimensions may be exaggerated for clarity, and the same reference numerals will be used throughout to designate the same or like components. 
     It will be understood that when an element is referred to as being “connected with” another element, it can be directly connected with the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly connected with” another element, there are no intervening elements present. In addition, unless explicitly described to the contrary, the word “comprise” and variations such as “comprises” or “comprising,” will be understood to imply the inclusion of stated elements but not the exclusion of any other elements. 
       FIG. 2  is a schematic block diagram showing the structure of an apparatus for generating a preamble for a digital direct transmission system according to an exemplary embodiment of the present invention. A preamble generating apparatus  200  according to an exemplary embodiment of the present invention simultaneously uses a spreading code in order to overcome the problem of low frequency noise in a digital direct transmission system and a line coding scheme in order to generate a preamble code having keen autocorrelation characteristics and low cross-correlation characteristics required to perform synchronization detection. 
     The preamble generating apparatus  200  according to an exemplary embodiment of the present invention may include a pseudo-noise (PN) code generation unit  210 , a line coder  220 , and a spreading coder  230 . According to combinations of the elements, the PN code generation unit  210 , the line coder  220 , and the spreading coder  230  may be sequentially connected as shown in  FIG. 2(   a ) or the PN code generation unit  210 , the spreading coder  230  and the line coder  220  may be sequentially connected as shown in  FIG. 2(   b ). 
     The PN code generation unit  210 , which generates a PN code, may be implemented as one of the existing maximal code generator, nonmaximal code generator, gold code generator and Kasami code generator. The generation of the maximal code, nonmaximal code, gold code, and Kasami code is known to a person having skill in the art, so a detailed description thereof will be omitted. 
     The line coder  220  receives the PN code from the PN code generation unit  210  and performs line coding thereupon, and in this case, the line coder  220  may perform line coding according to one of the existing NRZ (non-return-to-zero), Unipolar RZ, Bi-Phase-Level, Bi-Phase-Mark, Bi-Phase-Space, and Delay Modulation schemes. The various schemes are known to a person having skill in the art, so a detailed description thereof will be omitted. 
     The spreading coder  230  receives the PN code which has been line-coded by the line coder  220 , and performs spreading coding on the received PN code. For example, the spreading coder  230  may perform spread coding according to a Walsh coding scheme. 
     Meanwhile, as shown in  FIG. 2(   b ), the line coder  220  and the spreading coder  230  may be interchanged such that the spreading coder  230  receives the PN code generated by the PN code generation unit  210  and performs spread coding thereon and the line coder  220  receives the PM code which has been spread-coded by the spreading coder  230  and performs line coding thereon. 
     Also, in  FIG. 2 , it is illustrated that the line coder  220  and the spreading coder  230  are separated. However, because the line coder  220  and the spreading coder  230  perform linear arithmetic operation, respectively, they may be implemented so as to be integrated into a single element to perform both line coding and spread coding together. Also, the results of the PN code generation unit  210  or the results of the line coder  220  may be implemented as a ROM table, whereby a corresponding result may be output according to an input. 
       FIG. 3  illustrates the generation of a preamble by the apparatus for generating a preamble for a digital direct transmission system according an exemplary embodiment of the present invention. Specifically,  FIG. 3  illustrates a detailed example of generating a preamble by the preamble generating apparatus as shown in  FIG. 2(   a ). 
     First, the PN code generation unit  210  may generate a PN code by using a generative polynomial expression, for example, ‘P(z)=z 7 +z 6 + 1 ’. In detail, the PN code generation unit  210  may include seven shift registers  211  and a single XOR calculator  212  in order to generate a 128-bit PN code, and the results may be expressed hexidecimally as follows: 
     P(z)=“8106147916753E87126D6F634BB9957E” 
     Thereafter, the line coder  220  may perform line coding on the PN code which has been generated by the PN code generation unit  210  according to a bi-phase level scheme  221 , among various line coding schemes. In detail, the line coder  220  receives the output of the PN code generation unit  210  and converts 0 into 01 and 1 into 10,thus performing line coding. 
     Then, the spreading coder  230  performs spread coding on the PN code, which has been line-coded by the line coder  220 , according to a Walsh coding scheme. In detail, the spreading coder  230  receives the output of the line coder  220  and converts 0 into 0101 and 1 into 1010,thus performing spread coding. 
     Meanwhile, when the line coder  200  and the spreading coder  230  are implemented so as to be integrated into a single configuration (i.e., a line and spreading coder), the line and spreading coder receives the output of the PN code generation unit  210  and immediately converts 0 into 01011010 and 1 into 10100101, thus simultaneously performing line coding and spread coding on the PN code. 
       FIGS. 4   a  to  4   c  are graphs showing the performance of synchronization detection by preambles generated according to an exemplary embodiment of the present invention and the related art. In  FIGS. 4   a  to  4   c , 128PN×8Spread indicates the case of using a preamble generated by the related art preamble generating apparatus, and 128PNxMCx4Spread indicates the case of using a preamble generated by the preamble generating apparatus according to an exemplary embodiment of the present invention. 
     Here, the preamble generating apparatus according to an exemplary embodiment of the present invention generates a preamble according to the embodiment illustrated in  FIG. 3 . Also, the related art preamble generating apparatus uses the same generative polynomial expression as that of the present invention as shown in  FIG. 1  and converts 0 into 01010101 and 1 into 10101010, thus performing spread coding. 
       FIG. 4   a  is a graph showing the autocorrelation characteristics of the preamble generated according to an exemplary embodiment of the present invention and the preamble generated according to the related art.  FIG. 4   b  shows an enlargement of only the part of synchronization detection time. It is noted from  FIG. 4   b  that a maximum correlation distance according to the related art is  262 , while that according to the present invention is  643 . Also, as a result, as shown in  FIG. 4   c , it is noted that the present invention attains an SNR margin of about 8 dB from a 99% detection probability. 
     As set forth above, according to exemplary embodiments of the invention, because a preamble is generated through a combination of a pseudo-noise (PN) code, line coding, and spread coding, a preamble code having keen autocorrelation characteristics and low cross-correlation characteristics can be generated, while overcoming the problem of low frequency noise, and thus, the performance of detecting synchronization of the digital direct transmission system can be improved. 
     While the present invention has been shown and described in connection with the exemplary embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.