Abstract:
Provided is an optical Code Division Multiple Access (CDMA) transmitting apparatus and method for transmitting bipolar data in an optical CDMA system. The optical CDMA transmitting apparatus and method of the present research has a simple structure, including only one optical CDMA encoder and one optical modulator. This technology minimizes multiple access interference by using modified pseudo-noise code in the encoding process, thus improving the optical CDMA transmission performance. The optical CDMA transmitting apparatus includes: an optical CDMA encoding means for encoding lights from the outside into a code or a complement code of the code; and an optical modulation means for transmitting the code or the complement code of the code which is selected in the optical CDMA encoding means based on the polarity (either ‘0’ or ‘1’) of the data inputted from the outside.

Description:
FIELD OF THE INVENTION 
   The present invention relates to an optical Code Division Multiple Access (CDMA) system; and, more particularly, to an optical CDMA transmitting apparatus and method for transmitting bipolar data in an optical CDMA system. 
   DESCRIPTION OF RELATED ART 
   Conventionally, the use of modified pseudo-noise (PN) codes have been limited to a temporal domain Code Division Multiple Access (CDMA) system, which uses electric signals in consideration of their electrical characteristics, and not used in a spectral domain. 
   In conventional optical CDMA systems, an encoder uses two optical modulators and one electric switch to transmit bipolar data. If the data is ‘1’, the CDMA transmitting apparatus transmits PN codes or Walsh codes, or if the data is ‘0’, it transmits a complement code to the codes used when the data are ‘1’. Here, the modified PN code is not applied thereto. Moreover, the structure of the encoder for transmitting bipolar codes is complicated, and the use of an electric switch has dropped the data transmission rate. 
   SUMMARY OF THE INVENTION 
   It is, therefore, an object of the present invention to provide an optical Code Division Multiple Access (CDMA) transmitting apparatus and method which has one simple structured optical CDMA encoder and one optical modulator. 
   It is another object of the present invention to provide an optical CDMA transmitting apparatus and method having a simple-structured optical CDMA encoder and an optical modulator, the apparatus and method which can improve the optical CDMA transmission characteristics by using modified pseudo-noise (PN) codes in the encoding process to minimize the detrimental interference between CDMA channels which is called multiple access interference. In accordance with an aspect of the present invention, there is provided an optical Code Division Multiple Access (CDMA) transmitting apparatus for transmitting bipolar data, including: an optical CDMA encoding unit for encoding a light from the outside into a code and a complement code of the code to generate an encoded light into the code and an encoded light into the complement code; and an optical modulation unit for selecting and transmitting one of the encoded lights into the code and the complement code of the code based on the polarity (‘0’ or ‘1’) of the data inputted from the outside. 
   In accordance with another aspect of the present invention, there is provided an optical CDMA transmitting apparatus for transmitting bipolar data, including: an optical modulation unit for outputting a light from the outside through a different output terminal based on the polarity (‘0’ or ‘1’) of data inputted from the outside; and an optical CDMA encoding unit for receiving the light outputted from a first output terminal of the optical modulation unit, and encoding the light into a code; and receiving the light outputted from a second output terminal of the optical modulation unit, and encoding the light into a complement code of the code. 
   In accordance with another aspect of the present invention, there is provided an optical CDMA transmitting apparatus for transmitting bipolar data, including: a first light source, which is turned on based on data inputted from the outside, for outputting a light; a second light source, which is turned on in opposition to the first light source, for outputting a light; and an optical CDMA encoding unit for receiving the light outputted from the second light source, encoding the light into a code, and receiving the light outputted from the first light source, encoding the light into a complement code of the code, and then transmitting the encoded light into the code or the complement code. 
   In accordance with another aspect of the present invention, there is provided an optical CDMA transmitting method used in an optical CDMA transmitting apparatus for transmitting bipolar data, including the steps of: a) encoding a light into a code and a complement code of the code; and b) selectively transmitting the encoded light into the code or the complement code based on the polarity (‘0’ or ‘1’) of data to be transmitted by using one optical modulator. 
   In accordance with another aspect of the present invention, there is provided an optical CDMA transmitting method used in an optical CDMA transmitting apparatus for transmitting bipolar data, including the steps of: a) outputting a light through a different output terminal based on the polarity (‘0’ or ‘1’) of data to be transmitted; and b) encoding the light outputted through a first output terminal into a code between the lights outputted in the step a), and encoding the light outputted through a second output terminal into a complement code of the code between the lights outputted in the step a), and then transmitting the code or the complement code. 
   In accordance with another aspect of the present invention, there is provided an optical CDMA transmitting method used in an optical CDMA transmitting apparatus for transmitting bipolar data, including the steps of: a) outputting a first light by turning on a first light source based on data to be transmitted; b) outputting a second light by turning on a second light source in opposition to the step a) based on the data to be transmitted; and c) encoding the second light into a code, and encoding the first light into a complement code of the code, and then transmitting the code or the complement code. 
   Meanwhile, to achieve another object of the present invention, the optical CDMA transmitting apparatus and method of this invention performs encoding by using a filter having an arrangement of wavelengths for the lights to be reflected or to be transmitted based on a modified pseudo-noise code. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and features of the present invention will become apparent from the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which: 
       FIG. 1  is an exemplary diagram showing modified pseudo-noise (PN) codes in accordance with the present invention; 
       FIG. 2  is a block diagram illustrating an optical Code Division Multiple Access (CDMA) transmitting apparatus and method for transmitting bipolar data by using a 2×1 optical modulator in accordance with an embodiment of the present invention; 
       FIG. 3  is an exemplary diagram illustrating data encoded in an optical CDMA transmitting apparatus in accordance with the embodiment of the present invention; 
       FIG. 4  is a block diagram illustrating an optical CDMA transmitting apparatus and method for transmitting bipolar data by using a 1×2 optical modulator in accordance with another embodiment of the present invention; 
       FIG. 5  is a block diagram showing an optical CDMA transmitting apparatus and method for transmitting bipolar data by using two light sources in accordance with another embodiment of the present invention; 
       FIG. 6  is a block diagram describing an optical CDMA encoder in accordance with yet another embodiment of the present invention; and 
       FIG. 7  is a block diagram depicting an optical CDMA receiving apparatus adopting a balanced detection method in accordance with an embodiment of the present invention, the receiving apparatus being capable of receiving bipolar data. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Other objects and aspects of the invention will become apparent from the following description of the embodiments with reference to the accompanying drawings, which is set forth hereinafter. 
     FIG. 1  is an exemplary diagram showing modified pseudo-noise (PN) codes in accordance with the present invention. In a conventional PN code, the number of ‘0’ and the number of ‘1’ are always different by one. Here, due to the difference between the number of ‘0’ and the number of ‘1’, interference occurs between the different Code Division Multiple Access (CDMA) channels in an optical CDMA system where bipolar data are transmitted and/or received using PN codes. 
   However, as illustrated in  FIG. 1 , the numbers of ‘0’ and ‘1’ can be the same by adding a stuffed bit ‘0’ to an arbitrary position of a PN code. Then, the interference between the different CDMA channels disappears in an optical CDMA system using bipolar data. Here, it does not matter where to put the stuffed bit in the PN code, as long as it is placed in the same column that other stuffed bits are placed in all CDMA channel codes. 
   To make a decoder of the optical CDMA system receive the bipolar data transmitted thereto, the encoder of the optical CDMA system transmits the encoded light into a CDMA code Cn, such as PN code or Walsh code, if the data to be transmitted is ‘1’, and if the data is ‘0’, it transmits the encoded light into a complement code  Cn  of the CDMA code Cn. 
   Referring to  FIG. 2  is a block diagram illustrating an optical CDMA transmitting apparatus and method for transmitting bipolar data by using a 2×1 optical modulator in accordance with an embodiment of the present invention, and  FIG. 3  is an exemplary diagram illustrating data encoded in an optical CDMA transmitting apparatus in accordance with an embodiment of the present invention. 
   Referring to  FIG. 2 , the optical CDMA transmitting apparatus of the present invention includes: an optical CDMA encoder  23  for encoding a light from an external light source  20  into a code and a complement code of the code by assigning the center wavelengths of optical fiber Bragg grating (FBG) filters based on a code; and a 2×1 optical modulator  25  for selecting and transmitting the encoded light into a code in the optical CDMA encoder  23  or the complement code of the code to an optical CDMA receiving apparatus (see  FIG. 7 ) based on the polarity (‘0’ or ‘1’) of data inputted from a data generator  24 , which is to be transmitted. 
   The optical CDMA encoder  23  includes an optical circulator  21  and optical fiber Bragg grating (FBG) filters  22 . The optical circulator  21  transmits the light from the light source  20  to the optical FBG filters  22  and transmits the encoded light into a code Cn, which is encoded by reflecting the light in the optical FBG filters  22 , to the 2×1 optical modulator  25 . 
   The optical FBG filters  22  return to the optical circulator  21  the encoded light into the code Cn, which is obtained by reflecting the light, and the returned light is transmitted to the 2×1 optical modulator  25  through the optical circulator  21 ; and directly transmits to the 2×1 optical modulator  25  the encoded light into a complement code  Cn  of the code, which is obtained by transmitting the light inputted from the optical circulator  21 . 
   As described above, the optical CDMA encoder  23  can be embodied using optical FBG filters  22  having reflection wavelengths of λ 0 , λ 1 , λ 2  and λ 5 . When the optical FBG filters  22  are used, the light reflected from the optical FBG filters  22  have wavelengths of λ 0 , λ 1 , λ 2  and λ 5 , and the light transmitted through the optical FBG filters  22  have wavelengths of λ 3 , λ 4 , λ 6  and λ 7 . Here, as illustrated in  FIGS. 2 and 3 , the light reflected from the optical FBG filters  22  are encoded into a code Cn (11100100)  31 , and the light transmitted through the optical FBG filters  22  are encoded into a complement code  Cn  (00011011)  32  of the code Cn  31 . 
   To describe the operation of the optical CDMA encoder  23 , the light from the light source  20  are transmitted through the optical FBG filters  22  and encoded into the complement code  Cn . Then, the complement code is transmitted from one output terminal of optical FBG filters  22  to one input terminal of the 2×1 optical modulator  25 . Between the lights, the light reflected in the optical FBG filters  22  are encoded into a code Cn. The code Cn is transmitted from another output terminal of optical FBG filters  22  to another input terminal of the 2×1 optical modulator  25  through the optical circulator  21 . 
   Subsequently, if the data inputted from the data generator  24  is ‘1’, the 2×1 optical modulator  25  outputs the encoded light into the code Cn, and if the data is ‘0’, it outputs the encoded light into the complement code  Cn  of the code Cn to the optical CDMA receiving apparatus (see  FIG. 7 ), selectively. This way, the optical CDMA receiving apparatus can receive bipolar data. 
   Meanwhile, a system having no interference between different CDMA channels can be formed as follows. First, an optical CDMA encoder  23  is formed by assigning the wavelengths of the optical FBG filters  22  based on a modified PN code as shown in  FIG. 1 , and then an optical CDMA transmitting apparatus is formed adopting the optical CDMA encoder  23  and the 2×1 optical modulator  25 , as illustrated in  FIG. 2 . If the optical CDMA transmitting apparatus is coupled with the optical CDMA receiving apparatus (see  FIG. 7 ), which will be described later on, the system without multiple access interference is completed. An optical CDMA transmitting apparatus in accordance with another embodiment of the present invention can be used in the system in the same way. 
     FIG. 4  is a block diagram illustrating the optical CDMA transmitting apparatus and method for transmitting bipolar data by using a 1×2 optical modulator in accordance with another embodiment of the present invention. As illustrated in  FIG. 4 , the optical CDMA transmitting apparatus of another embodiment of the present invention includes a 1×2 optical modulator  41  for receiving a light from a light source  20  and outputting them through different output terminals based on the polarity (‘0’ or ‘1’) of the data that are inputted from a data generator  24 ; and an optical CDMA encoder  23  for receiving the light transmitted from one output terminal of the 1×2 optical modulator  41 , encoding them into a code (which is a code encoded based on the wavelength assignment of the optical FBG filters), or receiving the light from another output terminal of the 1×2 optical modulator  41 , encoding them into a complement code of the code, and then transmitting the code or the complement code to the optical CDMA receiving apparatus (see  FIG. 7 ). 
   The optical CDMA encoder  23  includes the optical circulator  21  and the optical FBG filters  22 . The optical circulator  21  receives the light outputted from one output terminal of the 1×2 optical modulator  41 , inputs them to the optical FBG filters  22  in the reverse direction, and outputs the encoded light into a code Cn, which is obtained by encoding the light reflected in the optical FBG filters  22 , through an output terminal of the optical circulator. The optical circulator  21  also outputs a complement code  Cn  of the code Cn, which is obtained by encoding the light transmitted from the optical modulator in the optical FBG filters  22 . 
   The optical FBG filters  22  transmit to the optical circulator  21  the encoded light into a code Cn, which is obtained by reflecting the light inputted in the reverse direction from the optical circulator  21  and transmit to the optical circulator  21  the encoded light into the complement code  Cn , which is obtained by transmitting the light outputted from another output terminal of the 1×2 optical modulator  41 . 
   As described in  FIG. 2 , the optical CDMA encoder  23  can be embodied using an optical FBG filters  22  having the wavelengths of λ 0 , λ 1 , λ 2  and λ 5 . 
   Hereinafter, the operation of the optical CDMA transmitting apparatus will be described in accordance with another embodiment of the present invention. If the data inputted from the data generator  24  is ‘1’, the 1×2 optical modulator  41  receives the light from the light source  20  and outputs them to the optical circulator  21  of the optical CDMA encoder  23  through one output terminal (i.e., the output terminal in the lower position of the 1×2 optical modulator  41  in  FIG. 4 ). If the data inputted from the data generator  24  is ‘0’, it outputs them to the optical FBG filters  22  of the optical CDMA encoder  23  through another output terminal (i.e., the output terminal in the upper position of the 1×2 optical modulator  41  in  FIG. 4 ). 
   The light outputted from one output terminal when the input data is ‘1’ are inputted to the optical FBG filters  22  through the optical circulator  21  of the optical CDMA encoder  23 . The light reflected in the optical FBG filters  22  are encoded into a code Cn, and then the encoded light is outputted to the output terminal  26  again through the optical circulator  21 . Meanwhile, the light outputted from another output terminal when the input data is ‘0’ are transmitted to the optical FBG filters  22  of the optical CDMA encoder  23 , and encoded into a complement code  Cn  of the code Cn, and then the encoded light to the complement code is outputted to the output terminal  26  through the optical circulator  21 . Here, the signal outputted from the output terminal  26  are the same as those of  FIG. 3 , if the optical FBG filters  22  have the same wavelength assignment of  FIG. 2 . 
     FIG. 5  is a block diagram showing an optical CDMA transmitting apparatus and method for transmitting bipolar data by using two light sources in accordance with another embodiment of the present invention. As shown in the drawing, the optical CDMA transmitting apparatus of another embodiment includes a data generator  24 , a first light source  52 , a second light source  53 , and an optical CDMA encoder  23 . The first light source  52  is turned on based on the data inputted from a data generator  24  to output lights. The second light source  53  is turned on in opposition to the first light source  52  based on the data inputted from the data generator  24  to output lights. The optical CDMA encoder  23  receives the light outputted from the second light source  53 , encoding the light into a code based on the wavelength assignment of the optical FBG filters  22 , or receives the light from the first light source  52 , encodes them into a complement code of the code, and then transmits the code or complement code to the optical CDMA receiving apparatus (see  FIG. 7 ). Here, the first and second light sources  52  and  53  are turned on in opposition to each other by an inverter  51 . 
   The optical CDMA encoder  23  includes an optical circulator  21  and optical FBG filters  22 . The optical circulator  21  receives the light outputted from the second light source  53  and inputs them to the optical FBG filters  22  in the reverse direction to output the encoded light into a code Cn, which is obtained from the light reflected in the FBG filters  22 , through the output terminal  26 . It also outputs the encoded light into a complement code  Cn  of the code Cn, which is obtained from the light transmitted through the optical FBG filters  22 , through the output terminal  26 . The optical FBG filters  22  transmits to the optical circulator  21  the encoded light into the code Cn, which is obtained by reflecting the light inputted in the reverse direction from the optical circulator  21 , and transmits to the optical circulator  21  the encoded light into the complement code  Cn  of the code Cn, which is obtained by transmitting the light outputted from the first light source  52 . 
   As described in  FIG. 2 , the optical CDMA encoder  23  can be embodied using the optical FBG filters  22  having wavelengths of λ 0 , λ 1 , λ 2  and λ 5 . Here, the data outputted from the output terminal  26  are the same as those of  FIG. 3 , if the optical FBG filters  22  used here have the same wavelength assignment of  FIG. 2 . 
     FIG. 6  is a block diagram describing an optical CDMA encoder in accordance with yet another embodiment of the present invention. It shows an example of an optical CDMA encoder formed by using a spatial filter  62  and diffraction gratings  61  and  63 . As shown in the drawing, the optical CDMA encoder  23  includes an optical circulator  21 , a first diffraction grating  61 , a spatial filter  62  and a second diffraction grating  63 . The optical circulator  21  transmits the light from an external light source to a first diffraction grating  61 , and outputs the encoded light into a complement code of the code, which is obtained by reflecting the light in the spatial filter  62 . The first diffraction grating  61  divides the lights inputted from the optical circulator  21  for different paths based on the wavelength and transmits the divided lights to a spatial filter  62 , and combines the lights reflected in the spatial filter  62  to generate an encoded light into the complement code of the code, and then transmits to the optical circulator  21  the encoded light into the complement code of the code. The spatial filter  62  transmits to the first diffraction grating  61  the encoded light into the complement code of the code by reflecting the lights inputted from the first diffraction grating  61 , and transmitting to a second diffraction grating  63  the encoded light into the code by transmitting the lights inputted from the first diffraction grating  61 . The second diffraction grating  63  combines the lights which are obtained by transmitting the light through the spatial filter  62 , to generate an encoded lights into the code, and outputs the encoded light. 
   The lights inputted through the optical circulator  21  are divided for different paths based on the wavelength in the first diffraction grating  61 . Then, some of the lights having specific wavelengths are reflected, and the others pass through the spatial filter. Here, if the wavelengths for transmission and those for reflection are predetermined in the arrangement of the spatial filter  62  based on the modified pseudo-noise (PN) code in  FIG. 1 , the result is that the lights reflected from the spatial filter  62  have the wavelengths of λ 3 , λ 4 , λ 6  and λ 7 , and the lights transmitted through the spatial filter  62  have the wavelengths of λ 0 , λ 1 , λ 2  and λ 5 . And then the transmitted lights are encoded into the code and outputted as a code Cn (11100100), and the reflected lights are encoded into a complement code of the code and outputted as a complement code  Cn  (00011011) in the spatial filter  62 . 
   Consequently, the optical CDMA encoder (i.e., reference numeral  23  of  FIG. 6 ) using the spatial filter  62  and diffraction gratings  61  and  63  outputs the same result as the optical CDMA encoder adopting the optical FBG filters, described in  FIGS. 2 ,  4  and  5 . Therefore, when the optical CDMA encoder is applied to the optical CDMA transmitting apparatus of  FIGS. 2 ,  4  and  5 , the result comes out the same. 
   As shown from the above description, it is possible to use the optical FBG filters or the spatial filter, or other thin film filters, in forming the optical CDMA encoder. 
     FIG. 7  is a block diagram depicting an optical CDMA receiving apparatus adopting a balanced detection method in accordance with an embodiment of the present invention, the receiving apparatus being capable of receiving bipolar data. The optical CDMA receiving apparatus adopting balanced detection method can receive the bipolar data transmitted from the optical CDMA transmitting apparatus. 
   As shown in  FIG. 7 , the optical signals transmitted through the optical FBG filters  71  of the optical CDMA decoder  72  among the received optical signals are decoded into a complement code  Cd , and the decoded light into the complement code  Cd  is inputted into a first optical detector  73 . The optical signals reflected from the optical FBG filters  71  are decoded into a code Cd, and the decoded light into the code Cd is inputted into a second optical detector  74 . Here, if the encoding code Cn and the decoding code Cd are the same and if the data transmitted from the optical CDMA transmitting apparatus is ‘1’, all received optical signals are inputted to the second optical detector  74 . If the data transmitted from the optical CDMA transmitting apparatus is ‘0’, all received optical signals are inputted to the first optical detector  73 . Therefore, bipolar data can be received by inputting the difference between the electric signals outputted from the first and second optical detectors  73  and  74 . 
   Meanwhile, if the encoding code Cn and the decoding code Cd are not the same, the received and decoded optical signals are divided and inputted to the two optical detectors  73  and  74 , regardless of the data transmitted from the optical CDMA transmitting apparatus. In case where a modified PN code is used, the intensities of the optical signals inputted to the two optical detectors  73  and  74  are the same. Therefore, the difference between the electric signals outputted from the first and second optical detectors  73  and  74  becomes zero, thus not affecting other CDMA channels. 
   The optical CDMA transmitting apparatus of the present invention, described above, can be formed using one optical CDMA encoder and one optical modulator. Also, in the present invention, it is possible to construct a system without multiple access interference by first forming the optical CDMA transmitting apparatus using a modified PN code and then forming an optical CDMA system using the optical CDMA transmitting apparatus. The technology of the present invention can improve the transmission rate by forming an optical CDMA encoder without using an electrical switch, which is different from conventional technologies. 
   While the present invention has been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.