Abstract:
Provided is a hybrid Passive Optical Network (PON) using a wireless communication. The hybrid PON includes a Central Office (CO) for transmitting downstream optical signals, a Remote Node (RN) for performing wavelength division demultiplexing of the downstream optical signals received from the CO and power-splitting each of the demultiplexed downstream optical signals to generate a plurality of downstream optical signals, transmitting the plurality of downstream optical signals to Optical Network Units (ONUs) of a corresponding group, generating a corresponding upstream optical signal modulated with wirelessly received upstream subcarriers of a corresponding group, and transmitting a plurality of generated upstream optical signals to the CO, and ONUs forming a plurality of groups for acquiring downstream subcarriers of a corresponding group from a corresponding downstream optical signal received from the RN, acquiring a corresponding downstream subcarrier by filtering downstream subcarriers of the group, and wirelessly transmitting a corresponding upstream subcarrier to the RN.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims the benefit of the earlier filing date, under 35 U.S.C. §119, to that patent application entitled “Hybrid Passive Optical Network Using Wireless Communication,” filed in the Korean Intellectual Property Office on Feb. 10, 2006 and assigned Serial No. 2006-13049, the contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention generally relates to a passive optical network, and in particular, to a hybrid passive optical network combining wavelength division multiplexing/subcarrier multiplexing types. 
         [0004]    2. Description of the Related Art 
         [0005]    Wavelength Division Multiplexed Passive Optical Network (WDM-PON) is gaining more attraction as a next-generation network for a future broadband communication service. The WDM-PON is a technique for transmitting a plurality of optical signals having different wavelengths over a single optical path in a wavelength band (e.g., 1300-1600 nm). With the demand for broadband services such as digital TVs (HDTVs), remote education, and video telephony from subscribers, the bandwidth required for each subscriber is increasing and a data transfer rate required for each subscriber can reach several hundreds of Mb/s. Thus, the WDM-PON is attractive because it allocates a separate wavelength to each subscriber. Since the WDM-PON has no limit in a bandwidth, it can provide a large bandwidth (transmission rates) of up to several gigabits per second (Gb/s) and has excellent security and protocol independence. However, the WDM-PON has not yet been commercialized due to high cost and research is being actively conducted on a low-cost WDM-PON. 
         [0006]    Subcarrier Multiplexing (SCM) is a technique for modulating a carrier with a data signal, such as a digital video signal, an analog video signal, or an Internet signal (hereinafter, the modulated carrier will be referred to as a subcarrier), generating an optical signal by modulating light of a predetermined wavelength with the subcarrier, and transmitting the generated optical signal. In a WDM/SCM-PON, a plurality of Optical Network Units (ONUs) transmits upstream optical signals of the same wavelength to a Central Office (CO) through a Remote Node (RN). The ONU refers to a device provided to a subscriber. Since SCM can use a large bandwidth of an optical fiber through a plurality of subcarriers, it can provide mass video and data services, provide the services to more subscribers using an optical amplifier and an optical Power Splitter (PS), and easily provide various kinds of services through subcarriers. Since all ONUs transmit upstream optical signals using a relatively inexpensive Fabry-Perot laser that is robust to Optical Beat Interference (OBI), wavelength management is easy in upstream and downstream transmission. However, the CO has to modulate a downstream optical signal using an expensive optical modulator having superior linearity because a large number of subcarriers have to be transmitted with a high signal to noise ratio for mass video and data services, and has to transmit a high-power downstream optical signal using an optical amplifier in order for an optical receiver included in each ONU to receive the high-power downstream optical signal. Moreover, since all the ONUs could share a single wavelength for downstream transmission, the CO divides a unit time band (hereinafter referred to as a cycle) for downstream transmission for allocation to each ONU and transmits a downstream optical signal to each ONU in an allocated time band (hereinafter referred to as a time slot). Thus, the amount of data transmitted to each ONU is limited. Furthermore, since all ONUs could share a single wavelength for upstream transmission, the CO divides a cycle for upstream transmission for allocation to each ONU and each ONU transmits an upstream optical signal in its allocated time slot. Thus, the amount of data transmitted by each ONU is limited. In other words, each ONU cannot transmit an upstream optical signal in another time slot except for its allocated time slot. 
         [0007]    Recently, a hybrid PON combining WDM/SCM types has been in the spotlight. In the hybrid PON, an RN (Remote Node) splits each downstream optical signal that is demultiplexed by a 1*N wavelength division multiplexer, by using a 1*N optical power splitter. At this time, a single downstream optical signal is modulated with M subcarriers. As a result, since M subcarriers can be acquired from each of N downstream optical signals, N*M subscribers can be served and therefore, a cost for each subscriber would be reduced by M times when compared to a general WDM-PON. 
         [0008]      FIG. 1  illustrates a typical hybrid PON  100  combining WDM/SCM types. The hybrid PON  100  includes a CO  110 , an RN  150 , and ONUs  190 - 1 - 1 - 190 -N-M included in first through N groups  180 - 1 - 180 -N. 
         [0009]    The CO  110  includes N optical transceivers (TRX)  120 - 1 - 120 -N and a first Wavelength Division Multiplexer (WDM)  130 . 
         [0010]    TRXs  120 - 1 - 120 -N have the same configuration and are sequentially connected to corresponding Demultiplexing Ports (DMPs) of the first WDM  130  based on one-to-one correspondence. The TRXs  120 - 1 - 120 -N output corresponding downstream optical signals and receive first through N th  upstream optical signals. The downstream optical signals have wavelengths λ 1 -λ N  and each of the downstream optical signals is modulated with M downstream subcarriers forming each group. In other words, first through M th  downstream subcarriers included in an N th  group have frequencies f 1 -f M  and are modulated with first through M th  downstream data signals included in an N th  group. The downstream subcarriers and the downstream data signals are all electric signals. The upstream optical signals have non-overlapping wavelengths λ (N+1) -λ 2N  and each of the upstream optical signals is modulated with M upstream subcarriers forming each group. In other words, first through M th  upstream subcarriers included in an N th  group have frequencies f 1 -f M  and are modulated with first through M th  upstream data signals included in an N th  group. The upstream subcarriers and the upstream data signals are all electric signals. 
         [0011]    With reference to the N th  TRX  120 -N, which is typical of all the other TRX illustrated herein, TRX  120 -N includes a Downstream Light Source (DLS)  122 -N, an upstream optical receiver (URX)  124 -N, and an Optical Coupler (CP)  126 -N. 
         [0012]    DLS  122 -N generates an downstream optical signal of a known wavelength, (in this exemplary case, the wavelength is λ N , an N th  wavelength) and outputs the N th  downstream optical signal to the N th  optical coupler  126 -N. The N th  downstream optical signal is modulated with first through M th  downstream subcarriers associated with the N th  group and the downstream subcarriers which have been modulated downstream data signals. 
         [0013]    URX  124 -N receives an N th  upstream optical signal from the N th  CP  126 -N and acquires the first through N th  upstream subcarriers to obtain the M upstream data signals. 
         [0014]    CP  126 -N includes first through third ports, in which the first port is connected with a Demultiplexing port of the first WDM  130 , the second port is connected with the URX  124 -N, and the third port is connected with the DLS  122 -N. The CP  126 -N outputs an N th  upstream optical signal input to the first port to the second port and outputs an N th  downstream optical signal input to the third port to the first port. 
         [0015]    As noted above, each of the TRX  120 - 1  through  120 -N is similar in construction and thus a detailed description of each TRX need not be presented herein. 
         [0016]    The first WDM  130  includes a Multiplexing Port (MP) and N DMPs, in which the MP is connected with a feeder fiber  140  and the first through N th  DMPs are connected with the first through N th  TRXs  120 - 1 - 120 -N based on one-to-one correspondence. The first WDM  130  de-multiplexes N upstream optical signals input to the MP and outputs the results on corresponding first through N th  DMPs based on one-to-one correspondence and further performs multiplexes the on N downstream optical signals input to the first through N th  DMPs to output the results to the MP. 
         [0017]    The RN  150  is connected to the CO  110  through the feeder fiber  140  and is further connected to each of the ONUs  190 - 1 - 1 - 190 -N-M of the first through N th  groups  180 - 1 - 180 -N through distribution optical fibers associated with the respective group. Each of the first through N th  groups  180 - 1 - 180 -N includes first through M th  distribution optical fibers. The RN  150  includes a second WDM  160  and first through N th  PSs  170 - 1 - 170 -N. 
         [0018]    The second WDM  160  has an MP and first through N th  DMPs, in which the MP is connected with the feeder fiber  140  and the first through N th  DMPs are connected with the first through N th  PSs  170 - 1 - 170 -N based on one-to-one correspondence. The second WDM  160  performs de-multiplexing on first through N th  downstream optical signals input to the MP to output the results to the first through N th  DMPs based on one-to-one correspondence and performs multiplexing of the first through N th  upstream optical signals input to the first through N th  DMPs to output the results to the MP. 
         [0019]    The first through N th  PSs  170 - 1 - 170 -N are connected with corresponding ones of the first through N th  DMPs of the second WDM  160 . 
         [0020]    Referring to the N th  PS  170 -N, which is typical of all the illustrated splitters, the N th  PS  170 -N has an Upstream Port (UP) and first through M th  Downstream Ports (DPs), in which the UP is connected with the N th  DMP of the second WDM  160  and the first through M th  DPs are connected with distribution optical fibers of the N th  group  180 -N based on a one-to-one correspondence. The N th  PS  170 -N power-splits an N th  downstream optical signal input to the UP (in this case, λ N ) into M portions and outputs the M portions to the first through M th  DPs. The N th  PS  170 -N combines M upstream optical signals input to the first through M th  DPs to output the result to the UP on a selected upstream optical signal (in this case, λ 2N ). 
         [0021]    As noted above, each of the power splitters  170  are identical in construction and there is no need to describe each one in detail herein. 
         [0022]    The ONUs  190 - 1 - 1 - 190 -N-M have the same configuration and each of the first through N th  groups  180 - 1 - 180 -N have the same configuration. Each of the ONU groups  180 - 1  through  180 -N include M ONUs (e.g.,  190 - 1 - 1 - 190 - 1 -M) that are connected with distribution optical fibers based on one-to-one correspondence. 
         [0023]    With reference to the M th  ONU of the N th  group  190 -N-M this ONU includes a Frequency Modulator (MOD)  191 -N-M, an upstream light source  192 -N-M, a 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 an upstream subcarrier corresponding to the M th  ONU of this N th  group, which is modulated with an M th  upstream data signal DN-M and has an M th  frequency (f m ). 
         [0025]    The M th  upstream light source  192 -N-M generates and outputs an N th  upstream optical signal that is modulated with an M th  subcarrier of the N th  group and has a wavelength, in this case, of λ 2N . 
         [0026]    The M th  downstream DRX  193 -N-M receives an N th  downstream optical signal from the M th  CP  195 -N-M and acquires first through M th  downstream subcarriers of an N th  group from the N th  downstream optical signal. 
         [0027]    The M th  BPF  194 -N-M outputs an M th  downstream subcarrier acquired by filtering the first through M th  downstream subcarriers of the N th  group input from the M th  downstream DRX  193 -N-M. The first through (M-1) th  downstream subcarriers are removed by the Mth BPF  194 -N-M. 
         [0028]    The M th  CP  195 -N-M includes first through third ports, in which the first port is connected with a corresponding distribution optical fiber of the N th  group  180 -N, the second port is connected with the M th  DRX  193 -N-M, and the third port is connected with the M th  upstream light source  192 -N-M. The M th  CP  195 -N-M outputs the N th  downstream optical signal input to the first port to the second port and outputs the N th  upstream optical signal input to the third port to the first port. 
         [0029]    However, the hybrid PON  100  has the following problems. 
         [0030]    First, the hybrid PON  100  can increase the number of subscribers by M times when compared to a general WDM-PON, but each ONU has to include a separate light source for upstream transmission. As a result, the number of upstream light sources also increases by M times and thus, a cost for implementing the entire PON increases. 
         [0031]    Second, when upstream optical signals output from different optical network devices of the same group are simultaneously input to each upstream optical receiver of the CO  110 , the performance of the entire PON may degrade due to Optical Beat Interference (OBI). At this time, it is assumed that at least one of the upstream optical signals has a wavelength error. In other words, a photodiode used as the upstream optical receiver has a square-law photo-detection property, which causes OBI. Optical current output form the photodiode due to the input of the optical signal is proportional to optical power and the optical power is expressed by a square of an optical field. Thus, when upstream optical signals of the same group having different wavelengths are input to the photodiode, a noise is generated around a frequency corresponding to a frequency difference. 
         [0032]    The following equations assume that first and second optical signals having different wavelengths are input to a photodiode at the same time. 
         [0000]        i ( t )= R*I ( t )= R·L{ε   2 ( t )}  (1) 
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         [0033]    where t indicates a time, i(t) indicates optical current, R indicates the responsivity of the photodiode, I(t) indicates optical power, ε(t) indicates an optical field, L{ε 2 (t)} indicates a function expressing I(t) by using ε(t) as a variable, I 1 (t) and I 2 (t) indicate powers of the first and second optical signals, I x (t) indicates the power of a noise, ω o1  and ω o2  indicate frequencies of the first and second optical signals, and φ 1  and φ 2  indicate frequencies of the first and second optical signals. 
         [0034]    The OBI is regarded as a serious problem to be solved in a hybrid PON combining WDM/SCM types, together with a cost for implementing the PON. 
       SUMMARY OF THE INVENTION 
       [0035]    It is, therefore, an object of the present invention to provide a hybrid PON combining WDM/SCM types, which is capable of minimizing OBI. 
         [0036]    It is another object of the present invention to provide a hybrid PON combining WDM/SCM types, which is capable of minimizing OBI and has a self-healing function. 
         [0037]    According to one aspect of the present invention, there is provided a hybrid Passive Optical Network (PON) using a wireless communication. The hybrid PON includes a Central Office (CO) for transmitting downstream optical signals, a Remote Node (RN) for performing wavelength division demultiplexing on the downstream optical signals received from the CO and power-splitting each of the demultiplexed downstream optical signals to generate a plurality of downstream optical signals, transmitting the plurality of downstream optical signals to Optical Network Units (ONUs) of a corresponding group, generating a corresponding upstream optical signal modulated with wirelessly received upstream subcarriers of a corresponding group, and transmitting a plurality of generated upstream optical signals to the CO, and ONUs forming a plurality of groups for acquiring downstream subcarriers of a corresponding group from a corresponding downstream optical signal received from the RN, acquiring a corresponding downstream subcarrier by filtering downstream subcarriers of the group, and further wirelessly transmitting a corresponding upstream subcarrier to the RN. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]    The above features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
           [0039]      FIG. 1  illustrates a typical hybrid PON combining WDM/SCM types; 
           [0040]      FIG. 2  illustrates a hybrid PON combining WDM/SCM types according to a first embodiment of the present invention; 
           [0041]      FIG. 3  illustrates in detail a Central Office (CO) illustrated in  FIG. 2 ; 
           [0042]      FIG. 4  illustrates a hybrid PON combining WDM/SCM types according to a second embodiment of the present invention; and 
           [0043]      FIG. 5  illustrates in detail a CO illustrated in  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0044]    Exemplary embodiments of the present invention will now be described in detail with reference to the annexed drawings. For the purposes of clarity and simplicity, a detailed description of known functions and configurations incorporated herein has been omitted for conciseness. 
         [0045]      FIG. 2  illustrates a hybrid PON  200  combining WDM/SCM types according to a first embodiment of the present invention, and  FIG. 3  illustrates in detail a Central Office (CO)  210  illustrated in  FIG. 2 . The hybrid PON  200  includes the CO  210 , a Remote Node (RN)  250 , first through N th  groups  280 - 1 - 280 -N, and Optical Network Units (ONUs)  290 - 1 - 1 - 290 -N-M. 
         [0046]    The CO  210  includes first through N th  optical transceivers (TRX)  220 - 1 - 220 -N and a first Wavelength Division Multiplexer (WDM)  230 , as described with regard to  FIG. 1 . 
         [0047]    The first through N th  TRXs  220 - 1 - 220 -N have the same configuration and are connected with first through N th  demultiplexing ports of the first WDM  230  based on one-to-one correspondence. The first through N th  TRXs  220 - 1 - 220 -N output first through N th  downstream optical signals and receive first through N th  upstream optical signals, respectively. The first through N th  downstream optical signals have first through N th  wavelengths λ 1 -λ N  and each of the first through N th  downstream optical signals is modulated with M downstream subcarriers forming each group. In other words, first through M th  downstream subcarriers included in an N th  group have first through M th  frequencies Df 1 -Df M  and are modulated with first through M th  downstream data signals included in an N th  group. The downstream subcarriers and the downstream data signals are all electric signals. The first through N th  upstream optical signals have wavelengths λ (N+1) -λ 2N  and each of the first through N th  upstream optical signals is modulated with M upstream subcarriers forming each group. In other words, first through M th  upstream subcarriers included in an N th  group have first through M th  frequencies Uf 1 -Uf N-M  and are modulated with first through M th  upstream data signals included in an N th  group. The upstream subcarriers and the upstream data signals are all electric signals. The N th  TRX  220 -N 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. 
         [0048]    Referring to  FIG. 3 , and with reference to the N th  TRX  220 -N, this TRX  220 -N includes DLS  222 -N that generates an N th  downstream optical signal of an N th  wavelength and outputs the N th  downstream optical signal to the N th  CP  226 -N, and the N th  downstream optical signal is modulated with first through M th  downstream subcarriers of an N th  group and the downstream subcarriers of the N th  group are modulated with first through M th  downstream data signals of an N th  group. The N th  DLS  222 -N may be a Fabry-Perot laser or a Distributed Feedback Laser Diode (DFB-LD). 
         [0049]    The N th  URX  224 -N receives an N th  upstream optical signal from the N th  CP  226 -N and acquires first through M th  upstream subcarriers of an N th  group and then first through M th  upstream data signals of an N th  group from the N th  upstream optical signal. The N th  URX  224 -N may be a combination of a photodiode for optoelectric conversion and a demultiplexer for frequency divisional demultiplexing. 
         [0050]    The N th  CP  226 -N has first through third ports, in which the first port is connected to an N th  DMP of the first WDM  230 , the second port is connected to the N th  URX  224 -N, and the third port is connected to the N th  DLS  222 -N. The N th  CP  226 -N outputs an N th  upstream optical signal input to the first port to the second port and outputs an N th  downstream optical signal input to the third port to the first port. 
         [0051]    The first WDM  230  includes a Multiplexing Port (MP) and first through N th  Demultiplexing Ports (DMPs), in which the MP is connected with a feeder fiber  240  and the first through N th  DMPs are connected with the first through N th  TRXs  220 - 1 - 220 -N based on one-to-one correspondence. The first WDM  230  performs de-multiplexing on first through N th  upstream optical signals received from the RN  250  to output the results to the first through N th  DMPs based on one-to-one correspondence and performs multiplexing on first through N th  downstream optical signals input to the first through N th  DMPs to output the results to the RN  250 . The first WDM  230  may be a 1*N arrayed waveguide grating (AWG). 
         [0052]    The RN  250 , (see  FIG. 2 ) is connected with the CO  210  through the feeder fiber  240  and is connected with the ONUs  290 - 1 - 1 - 290 -N-M of the first through N th  groups  280 - 1 - 280 -N through corresponding distribution optical fibers. Each of the first through N th  groups  280 - 1 - 280 -N includes M distribution optical fibers. The RN  250  includes a second WDM  260  and first through N th  distribution units (DUs)  270 - 1 - 270 -N. 
         [0053]    The second WDM  260  has an MP and first through N th  DMPs, in which the MP is connected with the feeder fiber  240  and the first through N th  DMPs are connected with the first through N th  DUs  270 - 1 - 270 -N based on a one-to-one correspondence. The second WDM  260  performs demultiplexing on first through N th  downstream optical signals received from the CO  210  to output the results to the first through N th  DUs  270 - 1 - 270 -N based on a one-to-one correspondence and performs multiplexing of the first through N th  upstream optical signals input from the first through N th  DUs  270 - 1 - 270 -N to output the results to the CO  210 . 
         [0054]    The first through N th  DUs  270 - 1 - 270 -N have the same configuration. With reference to the N th  DU  270 -N, which is typical of the remaining DUs, DU  270 -N includes a CP  272 -N, a Power Splitter (PS)  274 -N, an Upstream Light Source (ULS)  278 -N, and an upstream antenna  276 -N. 
         [0055]    The corresponding CP  272 -N includes first through third ports, in which the first port is connected with an N th  DMP of the second WDM  260 , the second port is connected with the N th  PS  274 -N, and the third port is connected with the N th  ULS  278 -N. The N th  CP  272 -N outputs an N th  downstream optical signal input from the second WDM  260  to the N th  PS  274 -N and outputs an N th  upstream optical signal input from the N th  ULS  278 -N to the second WDM  260 . 
         [0056]    The N th  PS  274 -N, which is typical of the remaining power splitters, includes an Upstream Port (UP) and first through M th  Downstream Ports (DPs), in which the UP is connected with the second port of the N th  CP  272 -N and the first through M th  DPs are connected with distribution optical fibers of the N th  group  280 -N based on a one-to-one correspondence. The N th  PS  274 -N power-splits an N th  downstream optical signal input from the N th  CP  272 -N into M signals and outputs the M signals to the first through M th  DPs. 
         [0057]    The N th  upstream antenna  276 -N is connected with an end of the N th  ULS  278 -N and outputs first through M th  upstream subcarriers of an N th  group received wirelessly from first through M th  ONUs  290 -N- 1 - 290 -N-M of the N th  group  280 -N to the N th  ULS  278 -N. 
         [0058]    One end of the N th  ULS  278 -N is connected with the N th  upstream antenna  276 -N and the other end is connected with the third port of the N th  CP  272 -N. The N th  ULS  278 -N generates the N th  upstream optical signal of wavelength (λ 2N ), which is modulated with the first through M th  upstream subcarriers, and outputs the N th  upstream optical signal to the N th  CP  272 -N. The N th  ULS  278 -N may be a Fabry-Perot laser. 
         [0059]    The ONUs  290 - 1 - 1 - 290 -N-M of the first through N th  groups  290 - 1 - 290 -N have the same configuration. In other words, the N th  group  280 -N includes first through M th  ONUs  290 -N- 1 - 290 -N-M that are connected with first through M th  distribution optical fibers of an N th  group  280 -N based on one-to-one correspondence. The M th  ONU  290 -N-M, which his typical of the remaining ONUs, of the N th  group  290 -N includes a downstream optical receiver (DRX)  292 -N-M, a Bandpass Filter (BPF)  294 -N-M for isolating a specific frequency (f m ), an M th  frequency modulator (MOD)  269 -N-M, and an M th  upstream antenna  298 -N-M. 
         [0060]    One end of the M th  DRX  292 -N-M is connected with an M th  distribution optical fiber of the N th  group  280 -N and the other end is connected with the N th  BPF  294 -N-M. The M th  DRX  292 -N-M acquires first through M th  downstream subcarriers of an N th  group from an N th  downstream optical signal received from the RN  250 . The M th  DRX  292 -N-M may be a photodiode for opto-electric conversion. 
         [0061]    The M th  BPF  294 -N-M receives first through M th  downstream subcarriers of an N th  group from the M th  DRX  292 -N-M and outputs an M th  downstream subcarrier acquired by filtering downstream subcarriers of the N th  group. The first through (M-1) th  downstream subcarriers are removed by the M th  BPF  294 -N-M. 
         [0062]    The M th  frequency modulator  296 -N-M is connected with the M th  upstream antenna  298 -N-M and generates an M th  subcarrier of an N th  group having an M th  upstream frequency of an N th  group, which is modulated with an M th  upstream data signal of an N th  group, to output the M th  subcarrier to the M th  antenna  298 -N-M. 
         [0063]    The M th  upstream antenna  298 -N-M transmits an M th  upstream subcarrier of an N th  group input from the M th  frequency modulator  296 -N-M to the RN  250  wirelessly. 
         [0064]      FIG. 4  illustrates a hybrid PON  300  combining WDM/SCM types according to a second embodiment of the present invention, and  FIG. 5  illustrates in detail a CO  310  illustrated in  FIG. 4 . The hybrid PON  300  has a similar configuration to the hybrid PON  200  of  FIG. 2  except that it further includes a self-healing means. The hybrid PON  300  includes the CO  310 , an RN  350 , and first through M th  ONUs  410 - 1 - 1 - 410 -N-M of first through N th  groups  410 - 1 - 410 -N. Hereinafter, a case where an M th  distribution optical fiber of an N th  group  400 -N, which connects the RN  350  and the M th  ONU  410 -N-M of the N th  group  410 - 1 , is broken will be taken as an example. 
         [0065]    Referring to  FIG. 3 , the CO  310  includes a P th  Downstream Light Source (DLS)  322 -P, first through N th  optical transceivers (TRX)  320 - 1 - 320 -N, and a first WDM  330 . 
         [0066]    The first through N th  TRX  320 - 1 - 320 -N have the same configuration and are connected with first through N th  Demultiplexing Ports (DPs) of the first WDM  330  based on a one-to-one correspondence. The first through N th  TRX  320 - 1 - 320 -N output first through N th  downstream optical signals and receive first through N th  upstream optical signals. The first through N th  downstream optical signals have first through N th  wavelengths λ 1 -λ N  and each of the first through N th  downstream optical signals is modulated with M or (M-1) downstream subcarriers forming each group. First through M th  downstream subcarriers included in an N th  group have first through M th  downstream frequencies Df 1 -Df M  and are modulated with first through M th  downstream data signals included in an N th  group. The downstream subcarriers and the downstream data signals are all electric signals. The first through N th  upstream optical signals have wavelengths λ (N+1) -λ 2N  and each of the first through N th  upstream optical signals is modulated with M upstream subcarriers forming each group. In other words, first through M th  upstream subcarriers included in an N th  group have first through M th  upstream frequencies Uf 1 -Uf M  and are modulated with first through M th  upstream data signals included in an N th  group. The upstream subcarriers and the upstream data signals are all electric signals. The N th  TRX  320 -N includes an N th  Downstream Light Source (DLS)  322 -N, an N th  upstream optical receiver (URX)  324 -N, and an N th  Optical Coupler (CP)  326 -N. 
         [0067]    The N th  DLS  322 -N generates an N th  downstream optical signal of an N th  wavelength and outputs the N th  downstream optical signal to the N th  CP  326 -N, and the N th  downstream optical signal is modulated with first through (M-1) th  downstream subcarriers of an N th  group and the downstream subcarriers of the N th  group are modulated with first through (M-1) th  downstream data signals of an N th  group. 
         [0068]    The N th  URX  324 -N receives an N th  upstream optical signal from the N th  CP  326 -N and acquires first through M th  upstream subcarriers of an N th  group and then first through M th  upstream data signals of an N th  group from the N th  upstream optical signal. 
         [0069]    The N th  CP  326 -N includes first through third ports, in which the first port is connected with an N th  DMP of the first WDM  330 , the second port is connected with the N th  URX  324 -N, and the third port is connected with the N th  DLS  322 -N. The N th  CP  326 -N outputs an N th  upstream optical signal input to the first port to the second port and outputs an N th  downstream optical signal input to the third port to the first port. 
         [0070]    The P th  DLS  322 -P is connected with a P th  DMP of the first WDM  330 . The P th  DLS  322 -P does not operate in a normal mode, but operates in a protection mode in which a failure occurs in a distribution optical fiber or an ONU and thus downstream transmission of a downstream subcarrier to the ONU having the failure is not possible. The P th  DLS  322 -P outputs a P th  downstream optical signal of a P th  wavelength λ P  to the first WDM  330 . The P th  downstream optical signal is modulated with the downstream subcarrier destined to the ONU having the failure. In the second embodiment of the present invention, since the M th  ONU  410 -N-M of the N th  group  410 - 1  has the failure, the P th  downstream optical signal is modulated with the M th  downstream subcarrier and the M th  downstream subcarrier is modulated with the M th  downstream data signal of an N th  group. 
         [0071]    The first WDM  330  includes an MP and first through P th  DMPs, in which the MP is connected with a feeder fiber  340 , the first through P th  DMPs are connected with the first through N th  TRX  320 - 1 - 320 -N, and the P th  DMP is connected with the P th  DLS  322 -P. The first WDM  330  performs de-multiplexing on first through N th  upstream optical signal input to the MP to output the results to the first through N th  DMPs based on a one-to-one correspondence and performs multiplexing on first through P th  downstream optical signals input to the first through P th  DMPs to output the results to the MP. 
         [0072]    The RN  350  is connected with the CO  310  through the feeder fiber  340  and is connected with ONUs  410 - 1 - 1 - 410 -N-M of the first through N th  groups  400 - 1 - 400 -N through corresponding distribution optical fibers. Each of the first through N t  groups  400 - 1 - 400 -N includes first through M th  distribution optical fibers. The RN  350  includes a second WDM  360 , first through N th  distribution units  370 - 1 - 370 -N, an optoelectric converter (O/E)  380 , and a downstream antenna  390 . 
         [0073]    The second WDM  360  has an MP and first through P th  DMPs, in which the MP is connected with the feeder fiber  340 , the first through N th  DMPs are connected with the first through N th  distribution units  370 - 1 - 370 -N based on a one-to-one correspondence, and the P th  DMP is connected with the O/E  380 . The second WDM  360  performs de-multiplexing on first through P th  downstream optical signals received from the CO  310  to output the results to the first through P th  DMPs and performs multiplexing on first through N th  upstream optical signals input from the first through N th  distribution units  370 - 1 - 370 -N to output the results to the CO  310 . 
         [0074]    The first through N th  distribution units  370 - 1 - 370 -N have the same configuration. The N th  distribution unit  370 -N power-splits an N th  downstream optical signal input from the second WDM  360  into M signals and outputs the M signals to the first through N th  ONUs  410 -N- 1 - 410 -N-M of the N th  group  410 -N. 
         [0075]    The N th  distribution unit  370 -N receives first through M th  upstream subcarriers of an N th  group wirelessly from the first through M th  ONUs  410 -N- 1 - 410 -N-M of the N th  group  410 -N and generates an N th  upstream optical signal having wavelength λ 2N , which is modulated with the first through M th  upstream subcarriers, to output the N th  upstream optical signal to the second WDM  360 . The N th  distribution unit  370 -N includes an N th  CP  372 -N, an N th  PS  374 -N, an N th  ULS  378 -N, and an N th  upstream antenna  376 -N. 
         [0076]    The N th  CP  372 -N includes first through third ports, in which the first port is connected with an N th  DMP of the second WDM  360 , the second port is connected with the N th  PS  374 -N, and the third port is connected with the N th  ULS  378 -N. The N th  CP  372 -N outputs an N th  downstream optical signal input from the second WDM  360  to the N th  PS  374 -N and outputs an N th  upstream optical signal input from the N th  ULS  378 -N to the second WDM  360 . 
         [0077]    The N th  PS  374 -N includes an Upstream Port (UP) and first through M th  Downstream Ports (DPs), in which the UP is connected with the second port of the N th  CP  372 -N and the first through M th  DPs are connected with distribution optical fibers of the N th  group  380 -N based on one-to-one correspondence. The N th  PS  374 -N power-splits an N th  downstream optical signal input from the N th  CP  372 -N into M signals and outputs the M signals to the first through M th  DPs. 
         [0078]    The N th  upstream antenna  376 -N is connected with an end of the N th  ULS  378 -N and outputs first through M th  upstream subcarriers of an N th  group received wirelessly from first through M th  ONUs  410 -N- 1 - 410 -N-M of the N th  group  410 -N to the N th  ULS  378 -N. 
         [0079]    One end of the N th  ULS  378 -N is connected with the N th  upstream antenna  376 -N and the other end is connected with the third port of the N th  CP  372 -N. The N th  ULS  378 -N generates the N th  upstream optical signal of the wavelength λ 2N  which is modulated with the first through M th  upstream subcarriers, and outputs the N th  upstream optical signal to the N th  CP  372 -N. 
         [0080]    One end of the O/E  380  is connected with the P th  DP of the second WDM  360  and the other end is connected with the downstream antenna  390 . The O/E  380  receives a P th  downstream optical signal from the second WDM  360  to perform optoelectric conversion of the P th  downstream optical signal and outputs an M th  downstream subcarrier of an N th  group, destined to the M th  ONU  410 -N-M of the N th  group  410 - 1  having a failure, to the downstream antenna  390 . 
         [0081]    The downstream antenna  390  wirelessly transmits the M th  downstream subcarrier of the N th  group input from the O/E  380  to the M th  ONU  410 -N-M of the N th  group  410 - 1 . 
         [0082]    The ONUs  410 - 1 - 1 - 410 -N-M of the first through N th  groups  410 - 1 - 410 -N-M have the same configuration, in which each of the first through N th  groups  410 - 1 - 410 -N-M includes first through M th  ONUs that are connected with distribution optical fibers of each group based on a one-to-one correspondence. The M th  ONU  410 -N-M of the N th  group  410 -N includes an M th  DRX  411 -N-M, an M th  BPF  412 -N-M, an M th  frequency modulator  413 -N-M, an M th  circulator  414 -N-M, an M th  upstream/downstream antenna  415 -N-M, and an M th  switch  416 -N-M. 
         [0083]    One end of the M th  DRX  411 -N-M is connected with an M th  distribution optical fiber of the N th  group  400 -N and the other end is connected with the M th  BPF  412 -N-M. In the normal mode, the M th  DRX  411 -N-M receives an N th  downstream optical signal from the M th  distribution optical fiber of the N th  group  400 -N and acquires downstream subcarriers of an N th  group from the N th  downstream optical signal. In the protection mode, the N th  downstream optical signal is not input to the M th  DRX  411 -N-M. 
         [0084]    One end of the M th  BPF  412 -N-M is connected with the M th  DRX  411 -N-M and the other end is connected with the M th  switch  416 -N-M. In the normal mode, the M th  BPF  412 -N-M receives downstream subcarriers of an N th  group from the M th  DRX  411 -N-M and outputs M th  downstream subcarriers acquired by filtering downstream subcarriers of the N th  group to the M th  switch  416 -N-M. The first through (M-1) th  downstream subcarriers are removed by the M th  BPF  412 -N-M. In the protection mode, downstream subcarriers of an N th  group are not input to the M th  BPF  412 -N-M. 
         [0085]    The M th  frequency modulator  413 -N-M is connected with a circulator and generates an M th  upstream subcarrier of an N th  group having an M th  upstream frequency of an N th  group, which is modulated with an M th  upstream data signal, to output the M th  upstream subcarrier to the M th  circulator  414 -N-M. 
         [0086]    The M th  upstream/downstream antenna  415 -N-M wirelessly transmits an M th  upstream subcarrier of the N th  group input from the M th  circulator  414 -N-M to the RN  350  and outputs an M th  downstream subcarrier of an N th  group received wirelessly from the M th  antenna r  415 -N-M to the M th  circulator  414 -N-M. 
         [0087]    The M th  circulator  414 -N-M includes first through third ports, in which the first port is connected with the M th  frequency modulator  413 -N-M, the second port is connected with the M th  upstream/downstream antenna  415 -N-M, and the third port is connected with the M th  switch  416 -N-M. The M th  circulator  414  outputs an M th  upstream subcarrier of an N th  group input to the first port to the upstream/downstream antenna  415  and outputs an M th  downstream subcarrier of an N th  group input to the second port to the M th  switch  416 -N-M. 
         [0088]    The M th  switch  416 -N-M includes first through third ports, in which the second port is connected with the other end of the M th  BPF  412 -N-M and the third port is connected with the third port of the M th  circulator  414 -N-M. The M th  switch  416 -N-M connects the first port and the second port in the normal mode and connects the first port and the third port in the protection mode. The M th  switch  416 -N-M outputs the M th  downstream subcarrier of the N th  group input to the second port to the first port in the normal mode and outputs the M th  downstream subcarrier of the N th  group input to the third port to the first port in the protection mode. 
         [0089]    The M th  ONU  410 -N-M of the Nth group  410 -N acquires M th  downstream data of an N th  group from an M th  downstream subcarrier of an N th  group output from the first port of the M th  switch  416 -N-M. 
         [0090]    To sense the occurrence of a failure, the CO  310  transmits the M th  downstream optical signal of the N th  group at predetermined intervals even if there is no downstream data to be transmitted to the M th  ONU  410 -N-M of the N th  group  410 -N. In addition, the M th  ONU  410 -N-M of the N th  group  410 -N senses the occurrence of the failure if a downstream optical signal is not output from the M th  switch  416 -N-M during a predetermined time period, and switches a connection state of the M th  switch  416 -N-M. The Mth ONU  410 -N-M of the Nth group  410 -N wirelessly transmits upstream data notifying the occurrence of the failure to the CO  310 . 
         [0091]    As described above, the hybrid PON according to the present invention wirelessly transmits upstream subcarriers generated by ONUs to an RN and the RN generates upstream optical signals modulated with the upstream subcarriers, by which the number of required upstream light sources can be reduced and thus a cost for implementing the entire PON can be reduced. Moreover, by using a single upstream light source for each upstream optical signal, Optical Beat Interference (OBI) can be minimized. 
         [0092]    Furthermore, in the hybrid PON, if a specific ONU cannot receive a downstream optical signal due to a failure, the RN wirelessly transmits a corresponding downstream subcarrier to the ONU, thereby implementing a self healing function. 
         [0093]    While the present invention has been 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.