Patent Publication Number: US-2005129404-A1

Title: Apparatus for providing broadcasting service through overlay structure in WDM-PON

Description:
This application claims the priority of Korean Patent Application Nos. 2003-89360, filed on Dec. 10, 2003 and 2004-74217, filed on Sep. 16, 2004 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 an apparatus for providing a broadcasting service through an overlay structure in a wavelength division multiplexing passive optical network (WDM-PON).  
      2. Description of the Related Art  
      Since a WDM-PON system allocates a wavelength per user, the WDM-PON system is expected as a system suitable to provide a next generation fiber to the home (FTTH) service, which has a flexibility for various information services provided to each user.  
      The WDM-PON system has advantages as follows: 1) it is possible to provide a high speed service of more than 1 Gbps to each user since a dedicated wavelength is allocated to each user; 2) it is possible to expand the number of subscribers since the WDM-PON has a lower wavelength splitting loss as comparing with an optical power splitting of time division multiple access (TDMA) PON; and 3) complex control circuits for bandwidth control and timing synchronization is unnecessary since a plurality of users do not share a band in time.  
      A method for a subscribers network to accommodate a convergence service of data communication and broadcasting(C&amp;B) has been being studied. Since WDM-PON structures suggested till now provide a separate large bandwidth to each subscriber, an in-band C&amp;B integration method within a communication channel is presumed.  
      The in-band C&amp;B integration method is considered as an optimal method for a convergence service in the WDM-PON system in terms of efficiency of a communication channel usage. However, it is predicted that it is difficult to deploy commercial services on the in-band integration method in the near future due to conflicts between communication providers and broadcasting providers and current communication laws. Considering this problem, an overlay method of providing a communication service and a broadcasting service via separate logical communication channels can be currently considered as an alternative plan.  
     SUMMARY OF THE INVENTION  
      The present invention provides an apparatus for providing a communication service and a broadcasting service through an overlay structure in a WDM-PON.  
      According to an aspect of the present invention, there is provided an apparatus for providing a broadcasting service through an overlay structure in a WDM-PON, the apparatus comprising: a first grating section receiving a multiplexed signal of N data communication optical wavelength signals and a broadcasting optical wavelength signal, which have separate wavelengths, transmitted from an optical line terminal (OLT) and wavelength-demultiplexing the multiplexed signal; a mirror reflecting the broadcasting optical wavelength signal wavelength-demultiplexed by the first grating section; and a second grating section receiving the reflected broadcasting optical wavelength signal and splitting it to all output ports to subscribers.  
      According to another aspect of the present invention, there is provided an apparatus for providing a broadcasting service through an overlay structure in a WDM-PON, the apparatus comprising: a first grating section receiving a multiplexed signal of N data communication optical wavelength signals and a broadcasting optical wavelength signal, which have separate wavelengths, transmitted from an OLT and wavelength-demultiplexing the multiplexed signal; a second grating section multiplexing N upstream data optical wavelength signals and transmitting the multiplexed signal to the OLT; a mirror reflecting the broadcasting optical wavelength signal wavelength-demultiplexed by the first grating section; and a third grating section receiving the reflected broadcasting optical wavelength signal and splitting it to all subscriber ports.  
      According to another aspect of the present invention, there is provided an apparatus for providing a broadcasting service through an overlay structure in a WDM-PON, the apparatus comprising: an arrayed-waveguide grating (AWG) demultiplexing a multiplexed signal of N data communication optical wavelength signals and a broadcasting optical wavelength signal, which have separate wavelengths, transmitted from an OLT and transmitting the demultiplexed signals to all subscriber ports; and an optical power splitter splitting a signal focused on a focal position of the broadcasting optical wavelength signal in the AWG in order to split the broadcasting optical wavelength signal to a plurality of broadcasting optical wavelength signal output ports, wherein the split broadcasting optical wavelength signals are feedbacked to the AWG in order to evenly transmit the split broadcasting optical wavelength signals to the output ports. 
    
    
     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  illustrates wavelength allocation in a WDM-PON providing a broadcasting service through an overlay structure;  
       FIG. 2A  is a first configuration of a WDM-PON providing a broadcasting service through an overlay structure;  
       FIG. 2B  illustrates in detail an example of a WDM MUX (WDM DMX) shown in  FIG. 2A  according to a first embodiment of the present invention;  
       FIG. 2C  illustrates in detail another example of the WDM MUX (WDM DMX) shown in  FIG. 2A  according to a second embodiment of the present invention;  
       FIG. 3A  is a second configuration of a WDM-PON providing a broadcasting service through an overlay structure;  
       FIG. 3B  illustrates in detail an example of a WDM MUX (WDM DMX) shown in  FIG. 3A  according to a third embodiment of the present invention;  
       FIG. 3C  illustrates in detail another example of the WDM MUX (WDM DMX) shown in  FIG. 3A  according to a fourth embodiment of the present invention;  
       FIG. 4  illustrates wavelength allocation in a WDM-PON providing a broadcasting service through an overlay structure when two wavelengths are allocated for broadcasting;  
       FIG. 5A  is a third configuration of a WDM-PON providing a broadcasting service through an overlay structure;  
       FIG. 5B  illustrates in detail an example of a WDM MUX (WDM DMX) shown in  FIG. 5A  according to a fifth embodiment of the present invention;  
       FIG. 5C  illustrates in detail another example of the WDM MUX (WDM DMX) shown in  FIG. 5A  according to a sixth embodiment of the present invention;  
       FIG. 6A  illustrates in detail a WDM MUX (WDM DMX) for providing a multicast broadcasting service according to a seventh embodiment of the present invention;  
       FIG. 6B  illustrates in detail a WDM MUX (WDM DMX) for providing a multicast broadcasting service according to a eighth embodiment of the present invention;  
       FIG. 7A  is a schematic configuration of an SCM/WDM-PON; and  
       FIG. 7B  illustrates in detail the SCM/WDM-PON, which provides a broadcasting service through an overlay structure, shown in  FIG. 7A . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Hereinafter, the present invention will now be described more fully with reference to the accompanying drawings, in which embodiments of the invention are shown. Like reference numbers are used to refer to like elements through at the drawings.  
       FIG. 1  illustrates wavelength allocation in a WDM-PON providing a broadcasting service through an overlay structure.  
      A broadcasting optical wavelength λ B  is allocated in the middle of wavelength bands (λ 1 , . . . , λ N  for downstream, λ N+1 , . . . , λ 2N  for upstream) allocated for downstream and upstream data communication.  
       FIG. 2A  is a first configuration of a WDM-PON providing a broadcasting service through an overlay structure according to a first embodiment of the present invention.  
      An OLT  21  receives a broadcasting optical wavelength λ B  on which broadcasting signals are carried from a broadcast server  20 , multiplexes the broadcasting optical wavelength λ B  with downstream data communication optical wavelengths λ 1 , . . . , λ N , and transmits the multiplexed wavelengths to subscribers, i.e., an optical network terminal (ONT) # 1  through an ONT #N. The broadcasting optical wavelength λ B  input to a WDM multiplexer/demultiplexer (MUX/DMX)  22  is split to all subscriber ports  221 ,  222 , . . . ,  22   n , and the downstream data communication optical wavelengths λ 1 , . . . , λ N  are transmitted to relevant subscribers by being wavelength-demultiplexed and transferred to relevant subscriber ports.  
      Upstream data communication optical wavelengths λ N+1 , . . . , λ 2N  on which upstream data input from ONTs are carried are multiplexed by the WDM MUX/DMX  22  and transmitted to the OLT  21 . The WDM MUX/DMX  22  corresponds to a remote node in the WDM-PON. Here, each subscriber port includes an optical fiber  23  for transmitting a data communication optical wavelength to bi-direction and an optical fiber  24  for transmitting a broadcasting optical wavelength to uni-direction, and these two optical fibers are wrapped in one two-core optical cable and connected to each ONT. Therefore, since each ONT performs diplex transmission in which an optical filter for separating data communication optical wavelengths and a broadcasting optical wavelength is unnecessary, costs can be reduced comparing with a triplex ONT providing a similar service.  
      As described above, a core component for realizing this embodiment is the WDM MUX/DMX  22 , and a configuration of the WDM MUX/DMX  22  for providing a broadcasting service through an overlay structure is a core point.  
       FIG. 2B  illustrates in detail an example of the WDM MUX/DMX  22  shown in  FIG. 2A  according to a first embodiment of the present invention.  
      Data communication optical wavelengths λ 1 , . . . , λ N  and a broadcasting optical wavelength λ B  input from the OLT  21  are wavelength-demultiplexed by a first grating section. After wavelength-demultiplexing, the data communication optical wavelengths λ 1 , . . . , λ N  are directly transmitted to relevant subscriber ports, and the broadcasting optical wavelength λ B  is reflected to a second grating section by a mirror. The reflected broadcasting optical wavelength λ B  is split to all subscriber ports by the second grating section.  
      That is, the first grating section operates as a MUX/DMX of input optical wavelengths, and the second grating section operates as a splitter splitting a broadcasting wavelength to subscriber ports.  
      Data communication optical wavelengths λ N+1 , . . . , λ 2N  on which upstream data are carried are input from the ONTs, wavelength-multiplexed by the first grating section, and transmitted to the OLT  21  via a one-core optical cable. A bulk grating component or an Echelle grating component of an integrated optic type may be used as the first/second grating sections, and the latter is used in this embodiment.  
       FIG. 2C  illustrates in detail another example of the WDM MUX (WDM DMX)  22  shown in  FIG. 2A  according to a second embodiment of the present invention.  
      A configuration shown in  FIG. 2C  adopts an arrayed-waveguide grating (AWG). Data communication optical wavelengths λ 1 , . . . , λ N  and a broadcasting optical wavelength λ B  are demultiplexed by the AWG and transmitted to subscriber ports, and data communication optical wavelengths λ N+1 , . . . , λ 2N  are multiplexed by the AWG and transmitted to the OLT  21 . Here, since multiplexing and demultiplexing must be performed using one AWG, a free spectral range of the AWG is matched to using wavelength bands λ 1 , . . . , λ N  and λ B . If it is assumed that a grating order corresponding to the data communication optical wavelengths λ N+1 , . . . , λ 2N  is m, a grating order corresponding to the data communication optical wavelengths λ 1 , . . . , λ N  is m−1.  
      After a signal focused on a λ B  focal position to split the broadcasting optical wavelength λ B  to λ B  output ports  27  is split by an optical power splitter  28 .  
      The AWG and the optical power splitter  28  can be manufactured using a semiconductor process after they are integrated on a single substrate made of silicon or silica and waveguides are formed using a substance such as polymer, silica, or silicon nitride.  
       FIG. 3A  is a second configuration of a WDM-PON providing a broadcasting service through an overlay structure.  
      Each of subscriber ports  331 ,  332 , . . . ,  33   n  transmits data communication optical wavelengths λ 1 , . . . , λ N  and λ N+1 , . . . , λ 2N  and a broadcasting optical wavelength λ B  to bi-direction using one optical fiber (single-core optical cable). Since the single-core optical cable is used, each ONT needs an optical filter for separating the data communication optical wavelengths λ 1 , . . . , λ N  and the broadcasting optical wavelength λ B  unlike the method suggested in  FIG. 2A . Since the other configuration and operations are equal to those of  FIG. 2A , the other description is omitted.  
       FIG. 3B  illustrates in detail an example of the WDM MUX (WDM DMX)  22  shown in  FIG. 3A  according to a third embodiment of the present invention.  
      Input multiplexed optical wavelengths λ 1 , . . . , λ N  and λ B  are wavelength-demultiplexed by a first grating section. The data communication optical wavelengths λ 1 , . . . , λ N  are directly transmitted to relevant subscriber ports. The broadcasting optical wavelength λ B  is diffracted by the first grating section and reflected to a third grating section by a mirror. The reflected broadcasting optical wavelength λ B  is split(copied) to all subscriber ports by the third grating section.  
      Upstream data communication optical wavelengths λ N+1 , . . . , λ 2N  are input from ONTs, wavelength-multiplexed by a second grating section, and transmitted to an OLT  21  via a single-core optical cable.  
       FIG. 3C  illustrates in detail another example of the WDM MUX (WDM DMX)  22  shown in  FIG. 3A  according to a fourth embodiment of the present invention.  
      Like the configuration of  FIG. 2C , a configuration suggested in  FIG. 3C  adopts an AWG. After a signal focused on a λ B  focal position to split a broadcasting optical wavelength λ B  to λB output ports is split by an optical power splitter  28 , the split broadcasting optical wavelengths λ B  are feedbacked to the AWG via λ B  feedback ports  29 . In order to evenly split the broadcasting optical wavelength λ B  to the λ B  output ports, an interval of the λ B  feedback ports  29  is set same as an interval of the λ B  output ports, and each position of the λ B  feedback ports  29  can be obtained by an AWG design principle.  
      Referring to  FIG. 3C , a region in which waveguides are crossed each other exists. However, according to experiments or theories, if a crossing angle between waveguides is above around 30°, a coupling between waveguides can be ignored. The AWG and the optical power splitter  28  can be manufactured using a semiconductor process after they are integrated on a single substrate made of silicon or silica and waveguides are formed using a material such as polymer, silica, or silicon nitride. Since the λ B  feedback ports  29  are located below a data input port, data communication optical wavelengths λ 1 , . . . , λ N  and the broadcasting optical wavelength λ B  can be simultaneously transmitted to subscriber ports.  
       FIG. 4  illustrates wavelength allocation in a WDM-PON providing a broadcasting service through an overlay structure when two wavelengths are allocated for broadcasting.  FIG. 4  is equal to  FIG. 1  but two allocated broadcasting optical wavelengths.  FIG. 4  shows that two wavelengths are allocated for broadcasting. However, it will be understood by those skilled in the art that more than two wavelengths can be allocated for broadcasting. A broadcasting service can be expanded by allocating a plurality of wavelengths.  
       FIG. 5A  is a third configuration of a WDM-PON providing a broadcasting service through an overlay structure.  
      An OLT  21  receives broadcasting optical wavelengths λ B1  and λ B2  on which broadcasting signals are carried from a broadcast server  20 , multiplexes the broadcasting optical wavelengths λ B1  and λ B2  with downstream data communication optical wavelengths λ 1 , . . . , λ N  and transmits the multiplexed wavelengths λ 1 , . . . , λ B1  and λ B2  to subscribers, i.e., an ONT # 1  through an ONT #N, through a WDM DMX  22 . The broadcasting optical wavelengths λ B1  and λ B2  input to the WDM DMX  22  are split to all subscriber ports  511 ,  512 , . . . ,  51   n , and the downstream data communication optical wavelengths λ 1 , . . . , λ N , are transmitted to relevant subscribers by being wavelength-demultiplexed and transferred to relevant subscriber ports.  
      Upstream data communication optical wavelengths λ N+1 , . . . , λ 2N  input from the ONTs are multiplexed by the WDM MUX  22  and transmitted to the OLT  21 . Each subscriber port includes an optical fiber  52  for transmitting a data communication optical wavelength to bi-direction and optical fibers  53  and  54  for transmitting respective broadcasting optical wavelengths λ B1  and λ B2  to uni-direction, and these three optical fibers are wrapped in one three-core optical cable and connected to each ONT. Therefore, since each ONT performs diplex transmission in which an optical filter for separating data communication optical wavelengths and a broadcasting optical wavelength is unnecessary, costs can be reduced comparing with a triplex ONT providing a similar service.  
       FIG. 5B  illustrates in detail an example of a WDM MUX (WDM DMX) shown in  FIG. 5A  according to a fifth embodiment of the present invention.  
      Data communication optical wavelengths λ 1 , . . . , λ N  and broadcasting optical wavelengths λ B1  and λ B2  input from the OLT  21  are wavelength-demultiplexed by a first grating section. After wavelength-demultiplexing, the data communication optical wavelengths λ 1 , . . . , λ N  are directly transmitted to relevant subscriber ports, the broadcasting optical wavelength λ B1  is reflected to a second grating section by a first mirror, and the broadcasting optical wavelength λ B2  is reflected to a third grating section by a second mirror. The reflected broadcasting optical wavelength λ B1  and λ B2  are split to all subscriber ports by the second grating section and the third grating section.  
      That is, the first grating section operates as a MUX/DMX of input optical wavelengths, the second grating section operates as a splitter splitting the broadcasting wavelength λ B1  to all subscriber ports, and the third grating section operates as a splitter splitting the broadcasting wavelength λ B2  to all subscriber ports.  
      Data communication optical wavelengths λ N+1 , . . . , λ 2N  on which upstream data are carried are input from the ONTs, wavelength-multiplexed by the first grating section, and transmitted to the OLT  21  via a one-core optical cable. As described in  FIG. 2A , a bulk grating component or an Echelle grating component of an integrated optic type may be used as the first through third grating sections, and the latter is used in this embodiment.  
       FIG. 5C  illustrates in detail another example of the WDM MUX (WDM DMX) shown in  FIG. 5A  according to a sixth embodiment of the present invention.  
      Like the configuration of  FIG. 2C , a configuration suggested in  FIG. 5C  adopts an AWG. Data communication optical wavelengths λ 1 , . . . , λ N  and broadcasting optical wavelength λ B1  and λ B2  are demultiplexed by the AWG and transmitted to subscriber ports, and data communication optical wavelengths λ N+1 , . . . , λ 2N  are multiplexed by the AWG and transmitted to the OLT  21 . Here, since multiplexing and demultiplexing must be performed using one AWG, a free spectral range of the AWG is matched to using wavelength bands λ 1 , . . . , λ N , λ B1  and λ B2 . If it is assumed that a grating order corresponding to the data communication optical wavelengths λ N+1 , . . . , λ 2N  is m, a grating order corresponding to the data communication optical wavelengths λ 1 , . . . , λ N  is m−1.  
      In order to split the broadcasting optical wavelengths λ B1  and λ B2  to λ B1  output ports  57  and λ B2  output ports  58 , signals focused on a λ B1  focal position and a λ B2  focal position are split by an optical power splitter  59 . The AWG and the optical power splitter  59  can be manufactured using a semiconductor process after they are integrated on a single substrate made of silicon or silica and waveguides are formed using a material such as polymer, silica, or silicon nitride.  
       FIG. 6A  illustrates in detail a WDM MUX (WDM DMX) for providing a multicast broadcasting service according to a seventh embodiment of the present invention.  
      While the broadcasting services in the embodiments described above adopt a broadcasting method of providing a broadcasting service to all subscribers, the present embodiment adopts a WDM MUX/DMX structure for a multicasting method of providing a broadcasting service to only specific subscribers. A basic configuration and operations are equal to those described in  FIG. 2B  but a usage of an N×N on/off optical switch. That is, a broadcasting service is provided to only specific subscribers by inserting the N×N on/off optical switch on optical paths of broadcasting optical wavelengths λ B  between a second grating section and ouput ports to users. A thermo-optic switch, a mechanical switch, or an acousto-optic switch can be used as the N×N on/off optical switch, and the N×N on/off optical switch is combined with a WDM MUX/DMX  22  as a hybrid type.  
       FIG. 6B  illustrates in detail a WDM MUX (WDM DMX) for providing a multicast broadcasting service according to a eighth embodiment of the present invention.  
      A basic configuration and operations are equal to those described in  FIG. 2C  but a usage of on/off optical switches  68 . That is, a broadcasting service is provided to only specific subscribers by inserting the on/off optical switches  68  at λ B  output ports  67 . The on/off optical switches  68  are preferably a thermo-optic switch type, which can be integrated on a silicon or silica substrate. A substance, which can be used to manufacture the thermo-optic switch, is silica, polymer, or silicon nitride.  
      The methods of providing a broadcasting service through an overlay structure described above have advantages described above. However, the number of subscribers who can be accommodated per optical fiber is limited by the number of multiplexed optical wavelengths. Therefore, the methods can be a good solution for subscribers needing a large bandwidth more than 1 Gbps. However, the methods are unnecessary for subscribers needing a narrow bandwidth of around 100 Mbps.  
      Considering this problem, the present invention suggests a method of modulating data on a radio frequency (RF) carrier between hundreds MHz and one point some GHz and transmitting the modulated data by being carried on an optical wavelength. This method is called a sub-carrier multiplexing access (SCMA) method, and independent communication channels can be formed by allocating different RF carriers to the same optical wavelength using this method. Since the number of subscribers to be accommodated increases as many as a multiple number of the number of RF carriers allocated per optical wavelength, a subscriber accommodation capacity can be expanded.  
       FIG. 7A  is a schematic configuration of an SCM/WDM-PON.  
      In the present configuration, a multiplexing density increases by dividing a same optical wavelength into RF carriers. A plurality of subscribers uses the same optical wavelength by locating an optical power splitter between a WDM MUX/DMX  22  and the subscribers. The subscribers using the same optical wavelength have separate communication channels by using different RF carriers f 1 , . . . , f m .  
       FIG. 7B  illustrates in detail the SCM/WDM-PON, which provides a broadcasting service through an overlay structure, shown in  FIG. 7A . A basic configuration and operations are equal to those described in  FIG. 2A  but a usage of optical power splitters. That is, a plurality of ONTs (m ONTs in  FIG. 7B ) use the same optical wavelength by inserting the optical power splitters between a WDM MUX/DMX  22  and subscribers.  
      As described above, the present invention can cost-effectively provide a broadcasting channel of an overlay type to subscribers with advantages of a WDM-PON. The present invention also can be applied to a network in which communication channels are formed by dividing a same optical wavelength into RF carriers once more.  
      While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The preferred embodiments should be considered in descriptive sense only and not for purposes of limitation. Therefore, the scope of the invention is defined not by the detailed description of the invention but by the appended claims, and all differences within the scope will be construed as being included in the present invention.