Abstract:
Provided are an optical transmission apparatus and an optical access network for a wavelength-division multiplexing optical network with sub-carrier multiplex and sub-carrier multiple access schemes. The optical transmission apparatus includes: a multiplexer and/or demultiplexer demultiplexing M forward A band optical signals having wavelengths each comprising a plurality of sub-carriers and multiplexing M backward B band optical signals having wavelengths each comprising a plurality of sub-carriers; a plurality of optical power splitters splitting each of the M forward A band optical signals into N optical signals; a plurality of optical receivers receiving backward optical signals belonging to a C band; and M optical transmitters converting the backward optical signals in the C band into the M backward B band optical signals.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
   This application claims the benefit of Korean Patent Application No. 10-2005-0119064, filed on Dec. 7, 2005 and No. 10-2006-0037756, filed on Apr. 26, 2006 in the Korean Intellectual Property Office, the disclosures of which are incorporated herein in their entirety by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a wavelength-division multiplexing optical access network, and more particularly, to an optical transmission apparatus and an optical access network for a wavelength-division multiplexing optical network with both sub-carrier multiplex and sub-carrier multiple access schemes. 
   2. Description of the Related Art 
   Digital subscriber line (xDSL) technologies have been developed to provide data services to subscribers using existing telephone lines. Also, data service schemes have been suggested to provide data services through cable networks using coaxial cables. 
   Such existing data service technologies do not have problems in terms of supporting telephone traffic and Internet traffic. If ultrahigh speed services such as remote video conferences, high definition television (HDTV) services, remote education, remote diagnoses and treatments are generally available to subscribers, the existing data service technologies are limited by bandwidth and distance, and thus are not able to provide sufficient wideband, high quality services. 
   Optical access network technologies have been suggested in order to provide wideband, high quality services to users. In particular, wavelength-division multiplexing optical networks have been recently studied as optical access network technologies. 
   The wavelength-division multiplexing optical access networks are attractive as next generation access networks since the wavelength-division multiplexing optical access networks can provide a large amount of information to subscribers, and are highly secure. 
   However, the wavelength-division multiplexing optical access networks are costly in terms of installation and maintenance. 
   Different from Back-Bone networks having a small number of channels and high transmission speeds, in general the access networks require relatively low transmission speeds and include a large number of channels. Thus, new networks are required to be capable of accommodating a large number of subscribers using a small amount of communication resources, i.e., maximizing the number of subscribers per a single optical fiber, which as a result solves the cost reduction issue. 
   The schemes, each wavelength optical channel of wavelength division multiplexing optical access network joined with TDM or TDMA, sub-carrier multiplex and sub-carrier multiple access networks using a large number of sub-carriers to accommodate a large number of subscribers are considered as alternative options. 
   The advantages and disadvantages of time-division multiplex and time-division multiple access schemes and sub-carrier multiplex and sub-carrier multiple access schemes will now be described in more detail. 
   In the time-division multiplex and time-division multiple access schemes, data of a large number of subscribers is statistically multiplexed in order to efficiently use resources. 
   However, accurate synchronization is required, and the states and requests from subscribers affect the access of other subscribers, and packet overheads increase due to periodical polling and ranging. 
   In the sub-carrier multiplex and sub-carrier multiple access schemes, different sub-carriers are allocated to subscribers sharing an optical fiber, the information are transmitted through the allocated sub-carriers to each of subscribers, and at the receivers the information are filtered through sub-carrier band pass filters assigned to each of the subscribers. 
   Optical access networks using sub-carrier multiple access schemes do not suffer from the increase in overhead as in those using time division multiple access caused by timing synchronization, periodical polling, and ranging. However, optical beat interferences (OBIs) may occur in signal bands when receivers in the central base stations simultaneously receive a plurality of light sources. As a result, the OBIs lower a signal-to-noise ratio (SNR). 
   SUMMARY OF THE INVENTION 
   The present invention provides an optical transmission apparatus and an optical access network for a wavelength-division multiplexing optical network with a sub-carrier multiplex scheme and a sub-carrier multiple access scheme with which optical beat interferences (OBIs) at optical receivers in a central base station can be avoided and thus a signal-to-noise ratio (SNR) can be improved. 
   According to an aspect of the present invention, there is provided an optical transmission apparatus for a wavelength-division multiplexing optical network, including: a multiplexer and/or demultiplexer demultiplexing M forward A band optical signals having wavelengths each comprising a plurality of sub-carriers and multiplexing M backward B band optical signals having wavelengths each including a plurality of sub-carriers; a plurality of optical power splitters splitting each of the M forward A band optical signals into N optical signals; a plurality of optical receivers receiving backward optical signals belonging to a C band; and M optical transmitters converting the backward optical signals in the C band into the M backward B band optical signals. 
   According to another aspect of the present invention, there is provided an optical access network for a wavelength-division multiplexing optical network, including a central base station, a local base station connected to the central base station through an optical fiber, and a plurality of subscriber nodes connected to the local base station. The optical access network may include: a plurality of optical transmitters positioned in the subscriber nodes, converting data into C band optical signals, and backward transmitting the C band optical signals; a plurality of optical receivers positioned in the local base station and receiving the C band optical signals; M optical transmitters converting the C band optical signals into M backward B band optical signals; a multiplexer multiplexing the M backward B band optical signals; a demultiplexer positioned in the central base station and demultiplexing the multiplexed M backward B band optical signals; M optical receivers converting the demultiplexed M backward B band optical signals into electric signals; a plurality of sub-carrier receivers filtering predetermined frequency band sub-carriers from the electric signals; and MODEM units demodulating the data from the sub-carriers. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
       FIG. 1  is a block diagram illustrating a transmission method and a structure of an optical access network for a wavelength-division multiplexing optical network with sub-carrier multiplex and sub-carrier multiple access schemes according to an embodiment of the present invention; 
       FIG. 2  is a block diagram illustrating a transmission method and a structure of an optical access network for a wavelength-division multiplexing optical network with sub-carrier multiplex and sub-carrier multiple access schemes according to another embodiment of the present invention; 
       FIG. 3  is a block diagram illustrating the optical access network illustrated in  FIG. 1  including a central base station according to an embodiment of the present invention; and 
       FIG. 4  is a block diagram illustrating the optical access network illustrated in  FIG. 1  including a central base station according to another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings. 
     FIG. 1  is a block diagram illustrating a transmission method and a structure of an optical access network for a wavelength-division multiplexing optical network with sub-carrier multiplex and sub-carrier multiple access schemes according to an embodiment of the present invention. Referring to  FIG. 1 , subscriber nodes  100  through  100 -N include modulation and/or demodulation (MODEM) units  101  through  101 -N, sub-carrier signal transmitters  102  through  102 -N, sub-carrier signal receivers  103  through  103 -N, C band optical transmitters  104  through  104 -N, and A band optical receivers  105  through  105 -N, modulate information concerning subscribers into signals, which can be transmitted to a network, and C and/or A band optical filters  106  through  106 -N. The MODEM units  101  modulate information concerning subscribers into signals transmittable to a network and demodulate the signals into subscriber information. The sub-carrier signal transmitters  102  through  102 -N convert intermediate frequencies of signals output from the MODEM units  101  through  101 -N into arbitrary sub-carrier frequencies. The sub-carrier signal receivers  103  through  103 -N convert signals in sub-carrier frequency bands into frequencies the MODEM units  101  through  101 -N may receive. The C band optical transmitters  104  through  104 -N convert signals output from the sub-carrier signal transmitters  102  through  102 -N into optical signals and transmit the optical signal. The A band optical receivers  105  through  105 -N receive A band forward optical signals and output sub-carrier signals. The C and/or A band optical filters  106  through  106 -N are connected to the C band optical transmitters  104  through  104 -N and the A band optical receivers  105  through  105 -N, and split and/or couple light having respective band wavelengths. 
   A local base station  110  includes C and/or A band optical filters  111  through  111 -N, optical receivers  112  through  112 -N, a B band optical transmitter  113 , an optical power splitter  114 , a B and/or A band optical filter  115 , and an optical multiplexer and/or demultiplexer  116 . 
   Each of the C and/or A band optical filters  111  through  111 -N is connected to each of the C and/or A band optical filters  106  through  106 -N through a strand of optical fiber to split and/or couple light having C and/or A band wavelengths. The optical receivers  112  through  112 -N receive C band subscriber optical signals split by the C and/or A band optical filters  111  through  111 -N. The B band optical transmitter  113  couples subscriber sub-carrier signals output from the optical receivers  112  through  112 -N into an optical signal and outputs the optical signal. The optical power splitter  114  is connected to the C and/or A band optical filters  111  through  111 -N to split A band forward optical signals. The B and/or A band optical filter  115  is connected to the optical power splitter  114  and the B band optical transmitter  113  to split and/or couple light having B and/or A band wavelengths. The optical multiplexer and/or demultiplexer  116  is connected to the B and/or A band optical filter  115  to wavelength-division multiplex and/or demultiplex light having B and/or A band wavelengths. 
   Since a point-to-point transmission is performed between the subscriber nodes  100  through  100 -N and the local base station  110 , low-priced Fabry-perot laser diodes (FP-LDs) may be used as subscriber light sources in the C band optical transmitters  104  through  104 -N. 
     FIG. 2  is a block diagram illustrating a transmission method and a structure of an optical access network for a wavelength-division multiplexing optical network with sub-carrier multiplex and sub-carrier multiple access schemes according to another embodiment of the present invention. In the present embodiment, the sub-carrier signal transmitters  102  through  102 -N illustrated in  FIG. 1  may be positioned in a local base station  110  illustrated in  FIG. 2 . 
   Subscriber nodes  100  through  100 -N include MODEM units  101  through  101  -N, sub-carrier signal receivers  103  through  103 -N, C band optical transmitters  104  through  104 -N, A band optical receivers  105  through  105 -N, and C and/or A band optical filters  106  through  106 -N. 
   The MODEM units  101  through  101 -N modulate information concerning subscribers into transmittable signals and demodulate the transmitted signals into subscriber information. The sub-carrier signal receivers  103  through  103 -N receive and convert signals in sub-carrier frequency bands into frequencies the MODEM units  101  through  101 -N may receive. The C band optical transmitters  104  through  104 -N convert signals output from the MODEM units  101  through  101 -N into optical signals and transmit the optical signals. The optical receivers  105  through  105 -N receive A band forward optical signals and output sub-carrier signals. The C and/or A band optical filters  106  through  106 -N are connected to the C band optical transmitters  104  through  104 -N and the A band optical receivers  105  through  105 -N to split and/or couple light having respective band wavelengths. 
   A local base station  110  includes C and/or A band optical filters  111  through  111 -N, optical receivers  112  through  112 -N, sub-carrier signal transmitters  217  through  217 -N, a B band optical transmitter  113 , an optical power splitter  114 , a B and/or A band optical filter  115 , and an optical multiplexer and/or demultiplexer  116 . 
   Each of the C and/or A band optical filters  111  through  111 -N is connected to each of the C and/or A band optical filters  106  through  106 -N through a strand of optical fiber to split and/or couple light having C and/or A band wavelengths. The optical receivers  112  through  112 -N receive C band subscriber optical signals split by the C and/or A band optical filters  111  through  111 -N. The sub-carrier signal transmitters  217  through  217 -N convert intermediate frequencies of signals output from the optical receivers  112  through  112 -N into sub-carrier frequencies. The B band optical transmitter  113  couples signals output from the sub-carrier signal transmitters  217  through  217 -N into an optical signal and transmit the optical signal. The optical power splitter  114  is connected to the C and/or A band optical filters  111  through  111 -N to split A band forward optical signals. The B and/or A band optical filter  115  is connected to the optical power splitter  114  and the B band optical transmitter  113  to split and/or couple light having B and/or A band wavelengths. The optical multiplexer and/or demultiplexer  116  is connected to the B and/or A band optical filter  115  to wavelength-division multiplex and/or demultiplex light having wavelengths in the B and A bands. 
   The optical access network illustrated in  FIG. 1  including a central base station according to an embodiment of the present invention will be described with reference to  FIGS. 3 and 4 . 
     FIG. 3  is a block diagram illustrating the optical access network illustrated in  FIG. 1  including a central base station according to an embodiment of the present invention. Referring to  FIG. 3 , a central base station  330  includes MODEM units  331  through  331 -N, sub-carrier signal transmitters  332  through  332 -N, sub-carrier signal receivers  333  through  333 -N, A band optical transmitters  334  through  334 -M, optical receivers  335  through  335 -M, an optical multiplexer  336 , an optical demultiplexer  337 , and an A and/or B band optical filter  338 . The MODEM units  331  through  331 -N operate as the MODEM units  101  through  101 -N of the subscriber nodes  100  through  100 -N as illustrated in  FIG. 1 . The sub-carrier signal transmitters  332  through  332 -N convert intermediate frequencies of signals output from the MODEM units  331  through  331 -N into arbitrary/preassigned sub-carrier frequencies. The sub-carrier signal receivers  333  through  333 -N convert signals in sub-carrier frequency bands into frequencies the MODEM units  331  through  331 -N may receive. The A band optical transmitter  334  converts signals output from the sub-carrier signal transmitters  332  through  332 -N into optical signals and transmits the optical signals. The optical receiver  335  receives a backward B band optical signal and outputs sub-carrier signals. The optical multiplexer  336  is connected to the A band optical transmitters  334  through  334 -M to wavelength-division multiplex lights having A band wavelengths. The optical demultiplexer  337  is connected to the optical receivers  335  through  335 -M to demultiplex lights having B band wavelengths. The A and/or B band optical filter  338  is connected to the optical multiplexer  336  and the optical demultiplexer  337  to split and/or couple light having A and/or B band wavelengths. 
     FIG. 4  is a block diagram illustrating the optical access network illustrated in  FIG. 1  including a central base station according to another embodiment of the present invention. Referring to  FIG. 4 , a central base station  430  includes MODEM units  431  through  431 -N, sub-carrier signal transmitters  432  through  432 -N, sub-carrier signal receivers  433  through  433 -N, A band optical transmitters  434  through  434 -M, optical receivers  435  through  435 -M, A and/or B band optical filters  436  through  436 -M, and an optical multiplexer and/or demultiplexer  437 . 
   The MODEM units  431  through  431 -N operate as the MODEMs  101  through  101 -N of the subscriber nodes  100  through  100 -N illustrated in  FIG. 1 . The sub-carrier signal transmitters  432  through  432 -N convert intermediate frequencies of signals output from the MODEM units  431  through  431 -N into arbitrary/preassigned sub-carrier frequencies. The sub-carrier signal receivers  433  through  433 -N convert signals in sub-carrier frequency bands into frequencies the MODEM units  431  through  431 -N may receive. The A band optical transmitters  434  through  434 -M convert signals output from the sub-carrier signal transmitters  432  through  432 -N into optical signals and transmit the optical signals. The optical receivers  435  through  435 -M receive backward B band optical signals and output sub-carrier signals. The A and/or B band optical filters  436  through-M are connected to the A band optical transmitters  434  through  434 -M and the optical receivers  435  through  435 -M to split and/or couple lights having A and/or B band wavelengths. The optical multiplexer and/or demultiplexer  437  is connected to the A and/or B band optical filters  436  through  436 -M to wavelength-division multiplex and/or demultiplex light having A and B band wavelengths. 
   The optical access network illustrated in  FIG. 2  including the local base station  110 , in which the sub-carrier signal receivers  102  through  102 -N are positioned, may include the central base stations  330  or  430  illustrated in  FIGS. 3  or  4 . 
   As described above, according to the present invention, an optical access network can reduce installation and maintenance costs by using a wavelength-division multiplex optical network with sub-carrier multiplex and sub-carrier multiple access schemes and a bi-directional optical communication technique for performing backward and/or forward communications through a strand of optical fiber. 
   In the case of the sub-carrier multiple access scheme, a local base station can receive a plurality of subscriber sub-carrier optical signals and transmit the plurality of subscriber sub-carrier optical signals to an optical transmitter. Thus, optical beat interferences (OBIs) do not occur in receivers of a central base station so as to improve a signal-to-noise ratio. As a result, signal quality can be increased, and a larger number of subscribers can be accommodated. 
   Since the OBIs do not occur, a number of subscribers limited by thermal noise can be accommodated. 
   The invention can also be embodied as computer readable codes on a computer readable recording medium. The computer readable recording medium is any data storage device that can store data that can be thereafter read by a computer system. Examples of the computer readable recording medium include read-only memory (ROM), random-access memory (RAM), CD-ROMs, magnetic tapes, floppy disks, optical data storage devices, and carrier waves (such as data transmission through the Internet). The computer readable recording medium can also be distributed over network coupled computer systems so that the computer readable code is stored and executed in a distributed fashion. 
   While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.