Abstract:
Disclosed is a hybrid PON including: a central office, remote terminal and a plurality of optical network units arranged in groups, the central office for outputting downstream optical signals, the remote node for wavelength-division-demultiplexing the downstream optical signals input from the central office, splitting the demultiplexed downstream optical signals, respectively, to generate multiple downstream optical signals, outputting the multiple downstream optical signals to optical network units of a corresponding group, generating corresponding upstream optical signals modulated into upstream subcarriers of a corresponding group input from the optical network units of the group, and outputting the generated upstream optical signals to the central office, and the optical network units for obtaining downstream subcarriers of a corresponding group from corresponding downstream optical signals input from the remote node, obtaining corresponding downstream subcarriers by filtering the downstream subcarriers of the group, and outputting corresponding upstream subcarriers to the remote node.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims the benefit of the earlier filing date, pursuant to 35 USC 119, to that patent application entitled “Hybrid PON” filed in the Korean Intellectual Property Office on Jan. 27, 2006 and assigned Serial No. 2006-9045, the contents of which are hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a Passive Optical Network (PON), and more particularly to a hybrid PON using Wavelength Division Multiplexing (WDM)/Subcarrier Multiplexing (SCM). 
         [0004]    2. Description of the Related Art 
         [0005]    Much interest has been focused on a WDM-PON as the next subscriber network for providing the future broadband communication service. A WDM-PON transmits multiple optical signals with different wavelengths through a single optical line in a wavelength range of 1300 to 1600 nm. Recently, as subscribers have required a broadband service including digital TV (HDTV), remote education and a picture phone, the bandwidth necessary per subscriber has been increasing. Based on a determination that a data rate per subscriber will reach several hundreds of Mb/s, much interest has been focused on a WDM-PON allocating a separate wavelength to each subscriber. A WDM-PON is advantageous in that it can not only provide a wide bandwidth of several Gb/s, but can also ensure excellent security and provide communication protocol independence. However, a WDM-PON has not yet been commercialized because it is still too expensive. Thus, research into a low-priced WDM-PON has been actively conducted. 
         [0006]    In an SCM scheme, a carrier is modulated into data signals, such as digital image signals, analog image signals and Internet signals (hereinafter, a modulated carrier referred to as a subcarrier), and optical signals generated by modulating light of a predetermined wavelength by using the subcarrier are transmitted. In a TDM/SCM PON, multiple Optical Network Units (ONUs) transmit upstream optical signals of the same wavelength to a Central Office (CO) through a Remote Node (RN). Herein, the ONU refers to an apparatus provided to a subscriber. In such an SCM scheme, a large amount of image and data services can be provided because it is possible to use the wide bandwidth of an optical fiber through multiple subcarriers. Further, it is possible to provide a service to many more subscribers by using an optical amplifier and a Power Splitter (PS), and to easily provide various types of services through subcarriers. Further, since all ONUs generally transmit upstream optical signals by using a low-priced Fabry-Perot laser tolerant to Optical Beat Interference (OBI), it is easy to manage wavelengths in upstream and downstream transmission. 
         [0007]    However, since it is necessary to transmit many subcarriers while maintaining a high Signal-to-Noise Ratio (SNR) for a large amount of image and data services, a Central Office must modulate downstream optical signals by using an expensive optical modulator with superior linearity, and must transmit high-power downstream optical signals by using an optical amplifier so that optical receivers provided in each ONU can receive the high-power downstream optical signals. Further, since all ONUs must share and use a single wavelength for downstream transmission, a CO divides a time domain (cycle) for downstream transmission, allocates the divided time domains to each ONU, and transmits corresponding downstream optical signals during the time domains (time slots) allocated to said each ONU. Therefore, the capacity of data transmitted to each ONU is restricted. Further, since all ONUs must share and use a single wavelength for upstream transmission, a CO divides a cycle for upstream transmission, allocates the divided cycles to each ONU, and each ONU transmits corresponding upstream optical signals during the time slots allocated to each ONU. Therefore, the capacity of data transmitted by each ONU is restricted. That is, each ONU cannot transmit upstream optical signals during time slots other than the allocated ones. 
         [0008]    Recently, a hybrid PON using a WDM scheme and an SCM scheme has attracted public attention. In a hybrid WDM/SCM PON, an Remote Node splits each downstream optical signal, which has been demultiplexed through a (1×N) wavelength division multiplexer, into multiple downstream optical signals by using a (1×M) Power Splitter. Herein, a single downstream optical signal has been modulated into M subcarriers. As a result, M subcarriers can be obtained from N downstream optical signals, respectively, so (N×M) subscribers can be accommodated. Thus, compared to a conventional WDM PON, the hybrid PON is expected to reduce the cost per subscriber. 
         [0009]      FIG. 1  is a block diagram illustrating a typical hybrid WDM/SCM PON. The hybrid PON  100  includes a CO  110 , an RN  150  and and a plurality of ONUs ( 190 - 1 - 1 ) to ( 190 -N-M) organized into a plurality of groups of ONUs ( 190 - 1 ) to ( 190 -N). 
         [0010]    The CO  110  includes first to N th  optical transceivers (TRXs) ( 120 - 1 - 120 -N), and a first wavelength division multiplexer  130 . 
         [0011]    The NTRXs ( 120 - 1 - 120 -N) each have the same construction, which are connected to first to N th  Demultiplexing Ports (DMPs) of the first wavelength division multiplexer  130  in a one-to-one fashion. The NTRXs ( 120 - 1 - 120 -N) output N downstream optical signals, respectively, and receive first to N th  upstream optical signals, respectively. The N downstream optical signals have wavelengths λ 1  to λ N , one wavelength being associated with a corresponding one of the N groups of ONUs. Each of the downstream optical signals is further modulated into M downstream subcarriers, each subcarrier associated with a corresponding with one of the ONUs with the corresponding group. The M downstream subcarriers have frequencies f 1  to f M , respectively, which are modulated into M downstream data signals. Both the downstream subcarriers and the downstream data signals are electrical signals. The N upstream optical signals have wavelengths λ (N+1)  to λ 2N ; each wavelength corresponding to one of the ONU groups. Each of the upstream optical signals is further modulated into M upstream subcarriers; each subcarrier associated with one of the ONUs in the corresponding group. The M upstream subcarriers have frequencies f 1 -f M , respectively, which have been modulated into M upstream data signals. Both the upstream subcarriers and the upstream data signals are electrical signals. 
         [0012]      FIG. 1  illustrates in further detail, with reference to N th  TRX ( 120 -N), a block diagram of a conventional transceiver. The description of the N th  TRX ( 120 -N) provided herein is typical of each of the remaining transceivers, and is thus applicable to each of the remaining tranceivers  120 - 1  through  120 -(N- 1 ). 
         [0013]    The N th  TRX ( 120 -N) includes a Downstream Light Source (DLS) ( 122 -N), a upstream optical receiver (URX) ( 124 -N) and an Optical Coupler (CP) ( 126 -N). 
         [0014]    The N th  DLS ( 122 -N) generates a downstream optical signal of an wavelength (λ N ) and outputs the downstream optical signal to the associated CP ( 126 -N). The downstream optical signal has been modulated into M downstream subcarriers that have been modulated into downstream data signals associated the N th  group of ONUs. 
         [0015]    The URX ( 124 -N) receives an upstream optical signal from CP ( 126 -N), and obtains upstream subcarriers and upstream data signals corresponding to the ONUs associated with the N th  group of ONUs ( 190 -N). 
         [0016]    CP ( 126 -N) has a first port connected to the N th  port of DMP of the first wavelength division multiplexer  130 , a second port connected to the URX ( 124 -N), and a third port connected to the DLS ( 122 -N). The CP ( 126 -N) outputs the N th  upstream optical signal, received at the first port, to the second port, and outputs the N th  downstream optical signal, received at the third port, to the first port. 
         [0017]    The first wavelength division multiplexer  130  has a Multiplexing Port (MP) and first to N th  DMPs. The MP is connected to a feeder fiber  140  and the first to N th  DMPs are connected to the first to N th  TRXs ( 120 - 1 ) to ( 120 -N) in a one-to-one fashion. The first wavelength division multiplexer  130  wavelength-division-demultiplexes the N upstream optical signals received at port MP, and outputs the demultiplexed upstream optical signals to the corresponding DMPs in a one-to-one fashion. Further, the first wavelength division multiplexer  130  wavelength-division-multiplexes the N downstream optical signals the received at the N DMPs, and outputs the multiplexed downstream optical signals to the MP. 
         [0018]    The RN  150  is connected to the CO  110  through the feeder fiber  140 , which is connected to the ONUs ( 190 - 1 - 1 ) to ( 190 -N-M) through distribution fibers ( 180 - 1 - 1 ) to ( 180 -N-M) of the N groups of ONUs ( 180 - 1 ) to ( 180 -N). The distribution fibers in each group are constructed by M distribution fibers. The RN  150  includes a second wavelength division multiplexer  160  and first to N th  optical PSs ( 170 - 1 ) to ( 170 -N). 
         [0019]    The second wavelength division multiplexer  160  has an MP (multiplexing port) and N DMPs. The MP is connected to the feeder fiber  140  and the N DMPs are connected to corresponding optical PSs ( 170 - 1 ) to ( 170 -N) in a one-to-one fashion. The second wavelength division multiplexer  160  wavelength-division-demultiplexes the N downstream optical signals received at port MP, and outputs the demultiplexed upstream optical signals to the corresponding DMPs in a one-to-one fashion. Further, the second wavelength division multiplexer  160  wavelength-division-multiplexes the N upstream optical signals received at a corresponding DMP, and outputs the multiplexed downstream optical signals to the MP. 
         [0020]    The optical PSs ( 170 - 1 ) to ( 170 -N) are connected to the corresponding DMPs of the second wavelength division multiplexer  160  in a one-to-one fashion. 
         [0021]    With reference to optical splitter  170 -N, which is typical of each of the remaining splitters, optical PS ( 170 -N) has an Upstream Port (UP) and M Downstream Ports (DPs). The UP of splitter  170 -N is connected to the N th  DMP of the second wavelength division multiplexer  160 , and M DPs are\connected to associated distribution fibers ( 190 -N- 1 ) to ( 190 -N-M) of the N th  group ( 190 -N) in a one-to-one fashion. The N th  optical PS ( 170 -N) splits the received downstream optical signal received at UP to generate M optical signals, and outputs the M opticala corresponding one of the DPs. The N th  optical PS ( 170 -N) further combines M upstream optical signals input to the M DPs, and outputs the combined upstream optical signals to the UP. 
         [0022]    The groups of ONUs ( 190 - 1 )-( 190 -N) and ONUs ( 190 - 1 - 1  )-( 190 -N-M) each have the same construction, Hence, a description of one group of ONUs and one ONU is applicable to each of the remaining ones. Groups of ONUs ( 190 ) are connected to corresponding PSs ( 170 ) through fibers ( 180 ). Each fiber  180  connects M ONUs in an associated group through distribution fibers ( 180 -x- 1  through  180 -x-N) in a one-to-one fashion, where x represents a particular group. 
         [0023]    With reference to the M th  ONU ( 190 -N-M) of the N th  group ( 190 -N), this ONU includes a frequency Modulator (MOD) ( 191 -N-M, an Upstream Light Source, ULS ( 192 -N-M), an downstream optical receiver (DRX) ( 193 -N-M), a Bandpass Filter (BPF) ( 194 -N-M) and a CP ( 195 -N-M). 
         [0024]    The MOD ( 191 -N-M) generates and outputs a subcarrier with a frequency (f m ), which is modulated into an M th  upstream data signal (D N-M ). 
         [0025]    The ULS ( 192 -N-M) generates and outputs an upstream data signal which is modulated into an M th  subcarrier on a (2N) th  wavelength. 
         [0026]    The DRX ( 193 -N-M) receives a downstream optical signal from the CP ( 195 -N-M), and obtains associated downstream subcarriers. 
         [0027]    The BPF ( 194 -N-M) receives the downstream subcarriers and outputs a downstream subcarrier obtained by filtering the downstream subcarriers. The remaining (i.e., first to (M-1) th ) downstream subcarriers are removed by the M th  BPF ( 194 -N-M). 
         [0028]    The CP ( 195 -N-M) has a first port connected to the associated distribution fiber ( 180 -N-M) of the associated group ( 180 -N), a second port connected to the DRX ( 193 -N-M), and a third port connected to the ULS ( 192 -N-M). The M th  CP ( 195 -N-M) outputs the N th  downstream optical signal, which is received at the first port, to the second port, and outputs the N th  upstream optical signal, which is received at the third port, to the first port. 
         [0029]    However, the WDM/SCM hybrid PON  100  as described above has the following problems. 
         [0030]    First, the hybrid PON  100  can increase the number of subscribers by M times, as compared to a conventional WDM PON, but each of the ONUs ( 190 - 1 - 1 ) to ( 190 -N-M) must have a separate ULS. Therefore, the number of ULSs increases by M times, which results in an increase in the cost required to construct an entire optical subscriber network. 
         [0031]    Second, when upstream optical signals output from different optical network devices of the same group are simultaneously input to each URX included in the CO  110 , the entire performance of an optical subscriber network may greatly deteriorate due to an Optical Beat Interference (OBI). The OBI occurs when two or more of lasers are operating simultaneously and components of their optical spectra too close in wavelength, wherein these components can beat at a receiver and generate noise. Herein, it is assumed that at least one of the upstream optical signals has a wavelength error. That is, a photodiode used as the URX has square-law photo-detection property which may cause an OBI. Since optical current output from the photodiode by optical signal input is proportional to optical power, and the optical power is expressed by the square of an optical field, when upstream optical signals with different wavelengths of the same group are input to the photodiode, an OBI may occur at a frequency corresponding to the difference among the wavelengths. 
         [0032]    Equations 1 and 2 are given on an assumption that first and second optical signals with different wavelengths are simultaneously input to a photodiode. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
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             where t denotes time,
           i(t) denotes optical current,   R denotes the degree of response of a photodiode,   l(t) denotes optical power,   ε(t) denotes optical field,   L{ε 2 (t)} denotes a function using ε(t) as a replacement variable for l(t), l 1 (t) and l 2 (t) denote power of the first and second optical signals,   I x (t) denotes power of an OBI,   ω 01  and ω 02  denote frequencies of the first and second optical signals, and   φ 1  and φ 2  denote frequencies of the first and second optical signals.   
         
           
         
       
     
         [0042]    The OBI has been recognized as an important issue in a WDM/SCM hybrid PON, together with the cost required to construct an entire network. 
       SUMMARY OF THE INVENTION 
       [0043]    Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art and provides additional advantages, by providing a low-cost WDM/SCM hybrid PON capable of minimizing an OBI. 
         [0044]    In accordance with one aspect of the present invention, there is provided a hybrid Passive Optical Network (PON) including a central office, a remote terminal and a plurality of optical network units arranged in a plurality of optical network unit groups, the central office for outputting downstream optical signals, the remote node for wavelength-division-demultiplexing the downstream optical signals input from the central office, splitting the demultiplexed downstream optical signals, respectively, to generate multiple downstream optical signals, outputting the multiple downstream optical signals to optical network units of a corresponding group, generating corresponding upstream optical signals modulated into upstream subcarriers of a corresponding group input from the optical network units of the group, and outputting the generated upstream optical signals to the central office, and the optical network units for obtaining downstream subcarriers of a corresponding group from corresponding downstream optical signals input from the remote node, obtaining corresponding downstream subcarriers by filtering the downstream subcarriers of the group, and outputting corresponding upstream subcarriers to the remote node. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0045]    The above features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
           [0046]      FIG. 1  is a block diagram illustrating a typical WDM/SCM hybrid PON; 
           [0047]      FIG. 2  is a block diagram illustrating a WDM/SCM hybrid PON according to a preferred embodiment of the present invention; and 
           [0048]      FIG. 3  is a block diagram illustrating the detailed construction of the CO illustrated in  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0049]    An exemplary embodiment of the present invention will be described in detail herein below with reference to the accompanying drawings. For the purposes of clarity and simplicity, a detailed description of known functions and configurations incorporated herein will be omitted as it may obscure the subject matter of the present invention. 
         [0050]      FIG. 2  is a block diagram illustrating a WDM/SCM hybrid PON according to a preferred embodiment of the present invention, and  FIG. 3  is a block diagram illustrating the detailed construction of the CO illustrated in  FIG. 2 . The hybrid PON  200  includes a CO  210 , an RN  250  and ONUs ( 300 - 1 - 1 ) to ( 300 -N-M) in N groups ( 300 - 1 ) to ( 300 -N). 
         [0051]    The CO  210  includes N optical transceivers (TRXs) ( 220 - 1 ) to ( 220 -N), and a first wavelength division multiplexer  230 . 
         [0052]    Each N TRXs ( 220 - 1 ) to ( 220 -N) have the same construction, which are connected to corresponding Demultiplexing Ports (DMPs) of the first wavelength division multiplexer  230  in a one-to-one fashion. The NTRXs ( 220 - 1 ) to ( 220 -N) each output a downstream optical signal and receive corresponding upstream optical signals, The downstream optical signals are represented as wavelengths λ 1  to λ N , and each of the downstream optical signals is modulated into M downstream subcarriers constituting a corresponding group. The M downstream subcarriers have frequencies represented as f 1  to f M . Both the downstream subcarriers and the downstream data signals are electrical signals. 
         [0053]    The upstream optical signals have (N+1) th  to (2N) th  wavelengths λ (N+1)  to λ 2N , and each of the upstream optical signals is modulated into M upstream subcarriers constituting a corresponding group. The M upstream subcarriers have frequencies which are modulated into M upstream data signals constituting a corresponding group, respectively. Both the upstream subcarriers and the upstream data signals are electrical signals. 
         [0054]    As discussed previously, the transceivers are identically constructed and thus only a single one need be discussed in detail to provide one skilled in the art sufficient information to practice the invention discloses. With reference to  FIG. 3 , the N th  TRX ( 220 -N), this transceiver includes an N th  Downstream Light Source (DLS) ( 222 -N), an N th  upstream optical receiver (URX) ( 224 -N) and an N th  Optical Coupler (CP) ( 226 -N). All of the M frequencies may be radio frequencies. 
         [0055]    The N th  DLS ( 222 -N) generates an downstream optical signal of an N th  wavelength and outputs the downstream optical signal to the N th  CP ( 226 -N). The downstream optical signal is modulated into downstream subcarriers of a corresponding group, and these downstream subcarriers are modulated into downstream data signals of the group, respectively. In one aspect, it is possible to use a Febry-Perot laser or a Distribute feedback Laser Diode (DFB-LD) as the N th  DLS ( 222 -N). 
         [0056]    The N th  URX ( 224 -N) receives an upstream optical signal from the CP ( 226 -N), and sequentially obtains upstream subcarriers and upstream data signals from the N th  upstream optical signal. The N th  URX ( 224 -N) may use a combination of a photodiode for photoelectric conversion and a demultiplexer for frequency division demultiplexing. 
         [0057]    The CP ( 226 -N) has a first port connected to the N th  DMP of the first wavelength division multiplexer  230 , a second port connected to the URX ( 224 -N), and a third port connected to the DLS ( 222 -N). The CP ( 226 -N) outputs the N th  upstream optical signal, which is received at the first port, to the second port, and further outputs the downstream optical signal, which is input to the third port, to the first port. 
         [0058]    The first wavelength division multiplexer  230  has a Multiplexing Port (MP) and N DMPs. The MP is connected to a feeder fiber  240  and the N DMPs are sequentially connected to the corresponding TRXs ( 220 - 1 ) to ( 220 -N) in a one-to-one fashion. The first wavelength division multiplexer  230  wavelength-division-demultiplexes the N upstream optical signals input to the MP, and sequentially outputs the demultiplexed upstream optical signals to the first to N th  DMPs in a one-to-one fashion. Further, the first wavelength division multiplexer  230  wavelength-division-multiplexes the N downstream optical signals input to the corresponding DMP, and outputs the multiplexed downstream optical signals to the MP. Herein, it is possible to use a (1×N) Arrayed Waveguide Grating (AWG) as the first wavelength division multiplexer  230 . 
         [0059]    The RN  250  (see  FIG. 2 ) is connected to the CO  210  through the feeder fiber  240 , which is connected to the ONUs ( 300 - 1 - 1 ) to ( 300 -N-M) of the N groups ( 300 - 1 ) to ( 300 -N) through both distribution fibers ( 280 - 1 - 1 ) to ( 280 -N-M) of the corresponding groups ( 280 - 1 ) to ( 280 -N) and electrical lines ( 290 - 1 - 1 ) to ( 290 -N-M) of the corresponding groups ( 290 - 1 ) to ( 290 -N). The distribution fibers in each group are constructed by the first to M th  distribution fibers, and the electrical lines in each group are constructed by the first to M th  electrical lines. It is possible to use conventional coaxial cables as the electrical lines ( 290 - 1 - 1 ) to ( 290 -N-M). The RN  250  includes a second wavelength division multiplexer  260  and N Distribution Units (DUs) ( 270 - 1 ) to ( 270 -N). 
         [0060]    The second wavelength division multiplexer  260  has an MP and N DMPs. The MP is connected to the feeder fiber  240  and the N DMPs are sequentially connected to a corresponding DUs ( 270 - 1 ) to ( 270 -N) in a one-to-one fashion. The second wavelength division multiplexer  260  wavelength-division-demultiplexes the N downstream optical signals input to the MP, and sequentially outputs the demultiplexed upstream optical signals to an associated DMP in a one-to-one fashion. Further, the second wavelength division multiplexer  260  wavelength-division-multiplexes the N upstream optical signals input to the corresponding DMP, and outputs the multiplexed downstream optical signals to the MP. 
         [0061]    The DUs ( 270 - 1 ) to ( 270 -N) each have the same construction, which are sequentially connected to corresponding DMPs of the second wavelength division multiplexer  260  in a one-to-one fashion. The N th  DU ( 270 -N) includes an N th  CP ( 272 -N), an N th  PS ( 274 -N), an N th  Frequency Combiner (CB) ( 276 -N) and an N th  ULS ( 278 -N). 
         [0062]    The N th  CP ( 272 -N) has a first port connected to the N th  DMP of the second wavelength division multiplexer  260 , a second port connected to the N th  PS ( 274 -N), and the third port connected to the N th  ULS ( 278 -N). The N th  CP ( 272 -N) outputs the N th  downstream optical signal, which is received at the first port, to the second port, and outputs the N th  upstream optical signal, which received at the third port, to the first port. 
         [0063]    The N th  PS ( 274 -N) has an Upstream Port (UP) and M Downstream Ports (DPs). The UP is connected to a port of CP ( 272 -N), and the M DPs are sequentially connected to the distribution fibers ( 280 -N- 1 ) to ( 280 -N-M) of the corresponding group ( 280 -N) in a one-to-one fashion. The N th  PS ( 274 -N) splits the a received downstream optical signal input to the UP to generate M number of N th  downstream optical signals, and outputs the M number of N th  downstream optical signals to a corresponding one of the M DPs. 
         [0064]    The N th  CB ( 276 -N) has a UP and M DPs. The UP is connected to the N th  ULS ( 278 -N), and the first to M th  DPs are sequentially connected to the electrical lines ( 290 -N- 1 ) to ( 290 -N-M) of the corresponding N th  group ( 290 -N) in a one-to-one fashion. The N th  CB ( 276 -N) combines the first to M th  upstream subcarriers input to the first to M th  DPs and outputs the combined upstream subcarriers to the UP. 
         [0065]    The N th  ULS ( 278 -N) is connected to the UP of the N th  CB ( 276 -N) at one end thereof, and is connected to the third port of the N th  CP ( 272 -N) at the other end thereof. The N th  ULS ( 278 -N) generates the N th  upstream optical signal with an (2N) th  wavelength, which is modulated into the first to M th  upstream subcarriers, and outputs the N th  upstream optical signal to the N th  CP ( 272 -N). It is possible to use a Fabry-Perot laser as the N th  ULS ( 278 -N). 
         [0066]    The ONUs ( 300 - 1 - 1 ) to ( 300 -N-M) each have the same construction, and the connect of each of the ONUs in each group are also constructed the same. The ONUs in each group are sequentially connected to distribution fibers of a corresponding group in a one-to-one fashion, and are sequentially connected to electrical lines of the corresponding group in a one-to-one fashion. The M th  ONU ( 300 -N-M) of the N th  group ( 300 -N) includes an M th  MOD ( 302 -N-M), an M th  downstream optical receiver (DRX) ( 304 -N-M), and an M th  Bandpass Filter (BPF) ( 306 -N-M). 
         [0067]    The M th  MOD ( 302 -N-M) is connected to the M th  electrical line ( 290 -N)-M of the N th  group ( 290 -N). The M th  MOD ( 302 -N-M) generates an M th  subcarrier with an M th  frequency, which is modulated into an M th  upstream data signal, and outputs the M th  subcarrier to the M th  electrical line ( 290 -N)-M. 
         [0068]    The M th  DRX ( 304 -N-M) is connected to the distribution fiber ( 280 -N)-M of the N th  group ( 280 -N) at one end thereof, and is connected to the M th  BPF ( 306 -N-M) at the other end thereof. The M th  DRX ( 304 -N-M) receives an N th  downstream optical signal from the distribution fiber ( 280 -N-M) of the N th  group ( 280 -N), and obtains downstream subcarriers of the N th  group from the N th  downstream optical signal. The M th  DRX ( 304 -N-M) may use a combination of a photodiode for photoelectric conversion and a demultiplexer for frequency division demultiplexing. 
         [0069]    The M th  BPF ( 306 -N-M) receives the downstream subcarriers of the N th  group from the M th  DRX ( 304 -N-M), and outputs an M th  downstream subcarrier obtained by filtering the downstream subcarriers of the N th  group. In this case, the first to (M-1) th  downstream subcarriers are removed by the M th  BPF ( 306 -N-M), except for the M th  downstream subcarrier. 
         [0070]    According to a WDM/SCM hybrid PON based on the present invention as described above, subcarriers generated by ONUs are transmitted to an RN through electrical lines, and the RN generates upstream optical signals modulated into the subcarriers, so that the required number of ULSs may be greatly reduced and thus the cost required to construct an entire optical subscriber network may also be greatly reduced. Further, one ULS is used for each upstream optical signal, so that it is possible to minimize OBI. 
         [0071]    Although a preferred embodiment of the present invention has been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims, including the full scope of equivalents thereof.