Abstract:
Disclosed is a passive optical network of code division multiple access using error correction code, which includes a plurality of optical networks units (ONU) and a central office, assigns a pseudo-noise (PN) code to each ONU, and transfers optical signals modulated by the PN code between the central office and each ONU, the passive optical network comprising the ONU which inserts error correction codes into upstream signals, transmits the upstream signals to the central office, and controls frequencies of the upstream signals using a response of the central office for the transmitted error correction codes and the central office which receives the upstream signals code division-encoded, decodes the upstream signals, inserts error counts of the error correction code included the upstream signals into downstream signals for the ONU, and transmits the downstream signals to the ONU.

Description:
CLAIM OF PRIORITY  
       [0001]     This application claims priority to an application entitled “CDMA passive optical network using error correction code,” filed in the Korean Intellectual Property Office on Aug. 21, 2003 and assigned Serial No. 2003-58024, 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 an optical network, and more particularly to a passive optical network using a Code Division Multiple Access mode and an error correction code.  
         [0004]     2. Description of the Related Art  
         [0005]     A passive optical network (PON) has no active equipment except terminal equipment of the network outside the plant. An active optical network (AON) needs an active equipment outside the plant. Passive optical network are realized at a lower cost and are more easily maintained and managed than active optical networks.  
         [0006]     In general, passive optical networks are realized in stars, buses and point to point (PTP) configurations. The PTP network has a one to one structure between a central office (CO) and each subscriber. Thus, it doesn&#39;t require the protocol to distinguish between subscribers. However, the PTP network requires a very large cost for initial installation of the network. In addition, an individual and separated optical line needs to be physically constructed for each subscriber. In contrast, the star or bus network requires a separate protocol or separate physical method for discriminating between subscribers. However, the star or bus network has many resources that can be shared, thus, enabling lower costs for installation of the network.  
         [0007]     Particularly, Ethernet PONs, which are largely used for local access networks (LAN), can be constructed as a star or bus network. Thus, an Ethernet PON has a small initial construction cost and additionally can accept by many subscribers. In addition, asynchronous transmission multiplexing (ATM) PONs and wireless division multiplexing PON (WDM PONs) are used by many vendors.  
         [0008]     Optical communication networks that are formed in a star or bus configuration have a number of limitations in upstream transmission. Generally, in downstream transmission, the central office broadcasts information to all subscribers and each subscriber receives only their own information. However, upstream transmission require modes to discriminate between subscribers because the central office combines data from many subscribers. In order to discriminate each subscriber, various modes of upstream transmission, such as time division multiple access (TDMA), wavelength division multiple access (WDMA) and code division multiple access (CDMA), etc., have been suggested.  
         [0009]     Herein, TDMA mode enables each subscriber to send their own information during a predetermined time slot. It is also realized by a low cost light source, such as Fabry-Perot laser diode (FP-LD). However, the central office requires a burst mode receiver in TDMA mode. The burst mode receiver is an apparatus for receiving discontinuous optical signals. He discontinuous optical signals have intensities which change greatly due to difference between physical distances and signal losses according to subscribers. Typically, it is difficult to realize a high speed burst mode receiver, and thus it is difficult for networks using the burst mode receiver to conduct high speed operation.  
         [0010]     Also, WDM mode enables each subscriber to use a light source of a different wavelength. Therefore the central office discriminates each subscriber by the wavelength. Accordingly, the WDM does not have optical beat interference noise, which is generated when signals from a plurality of light sources having the same or a similar wavelength are incident on a single receiver. However, the WDM requires expensive light sources due to subscribers using light sources having different wavelengths. In addition, it also requires additional circuit devices for the stability of wavelengths.  
         [0011]     Lastly, CDMA mode enables subscribers to use different codes. This mode usually overcomes interference signals from other subscribers by means of orthogonal code and picks up specific subscribers.  
         [0012]      FIG. 1  illustrates a conventional passive optical network.  
         [0013]     As shown in  FIG. 1 , the passive optical network includes a central office  200 , a plurality of optical networks units  100 , and optical couplers  300 - 1  and  300 - 2 . The central office  200  includes a downstream signal processing unit and an upstream signal processing unit.  
         [0014]     The downstream signal processing unit of the central office  200  includes a pseudo-noise code (PN code) generator  108 , a CDMA encoder  106 , a combiner, and a light source of the downstream signal processing unit. The PN code generator  108  generates each PN code corresponding to each subscriber. The CDMA encoder  106  code division-encodes signals by means of PN codes. The combiner combines a plurality of code division-encoded signals.  
         [0015]     The upstream signal processing unit of the central office  200  includes a photo-electric converter  110 , a splitter of electric signal, a PN code generator  108 , and a CDMA decoder  107 .  
         [0016]     A plurality of optical networks units  100  include a downstream and upstream signal processing units.  
         [0017]     The downstream signal processing unit of the plurality of optical networks units  100  includes photo-electric converters  103 , PN code generators  105 , and CDMA encoders  101  which code division-encodes an electric signals from photo-electric converters  103  by means of signals of PN code generators  105 .  
         [0018]     The upstream signal processing unit comprises a light source for upstream transmission.  
         [0019]     As shown in  FIG. 1 , the CDMA passive optical network employs a different PN codes for each subscriber. In addition, the central office of the CDMA passive optical network decodes signals with the PN code assigned to each subscriber.  
         [0020]     The CDMA passive optical network does not require a burst mode receiver, as required by the TDMA passive optical network, nor an expensive distributed feedback (DFB) laser, as required by the WDMA passive optical network. This is because each subscriber is discriminated by their code in the CDMA passive optical network. However, when the CDMA passive optical network employs an inexpensive light source, such as a Fabry-Perot laser diode, the quality of signals is degraded due to an optical beat interference noise. The optical beat interference noise occurs when signals from a plurality of light sources having the same or a similar wavelength are incident on a single optical receiver, as shown in  FIG. 2 .  
         [0021]     Referring to  FIG. 2 , when optical signals having optical frequencies f0, f1, and f2 are incident on a single receiver, the optical beat interference noise occurs in frequency ranges |f0-f1|, |f1-f2|, and |f0-f2| corresponding to differences between frequencies of optical signals.  
         [0022]     As shown in  FIG. 2 , if the CDMA optical network doesn&#39;t control the wavelength of the light source, the frequencies of optical signals have similar values. Thus, optical beat interference noise is generated and degrades the quality of the signals.  
         [0023]     Generally, the signal quality degradation resulting from optical beat interference noise is more or less recovered during the procedure of demodulating codes. This is because the codes employed in the CDMA not only discriminate between subscribers but also protect against optical beat interference noise. Since, the signals have a correlation with the PN codes used during demodulation, the signals modulated by the PN codes are perfectly recovered. In contrast, optical beat interference noise has no correlation with the PN codes, thus, they are largely suppressed during the procedure of demodulation. At this time, the volume of the suppressed noise is related to a ratio of the transmission speed of the signals to transmission speed of the noise. The volume of the suppressed noise is called a processing gain. Thus, the passive optical network employing the CDMA mode not only identifies each subscriber but also overcomes the optical beat interference noise.  
         [0024]     However, the probability that the frequency of each light source is similar to that of another light source is proportionate to the number of subscribers. Therefore, the processor gain must be enhanced in order to level up the quality of signals to a specific value. To enhance the processor gain, the transmission speed of codes must be increased or the transmission speed of each subscriber must be decreased.  
         [0025]     Accordingly, in a conventional CDMA passive optical network, increases in the number of subscribers causes system performance to be greatly restricted by the optical beat interference noise, even though the CDMA passive optical network has the processor gain. Namely, the transmission speed of each subscriber must be lowered in order to acquire a high processor gain so that maximum transmission speed of each ONU are restricted. Also, as the number of subscribers increases, the processor gain is reduced, so that the number of ONUs to be connected to a single star network is greatly restricted.  
       SUMMARY OF THE INVENTION  
       [0026]     Accordingly, the present invention has been made to reduce or overcome the above-mentioned problems occurring in the prior art. An object of the present invention is to provide a passive optical network, for example a CDMA passive optical network, using error correction codes, which diminishes the degrading of signals resulting from optical beat interference noise. The passive optical network controls the wavelength of the light source in the optical network unit, and uses a demodulator and modulator using error correction codes and error detection function of error correction codes (forward error correction: FEC).  
         [0027]     An other object of the present invention to provide a passive optical network using error correction codes, which decreases the optical beat interference noise by actively controlling the light source of the optical network unit in order to increase the transmission speed of the optical network unit.  
         [0028]     An other object of the present invention to provide a passive optical network using error correction codes which can increase the number of subscribers connected with a single star network.  
         [0029]     In accordance with the principles of the present invention, a passive optical network is provided, comprising a plurality of optical networks units, each unit having an assigned code and transmits signals modulated by the code, wherein each unit is configured the use error correction codes with the transmitted signals, and control frequencies of the signals; and a central office configured to decode the signals from an optical network unit, and provides error counts of the error correction code to the optical networks unit. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]      FIG. 1  illustrates a conventional CDMA passive optical network;  
         [0031]      FIG. 2  illustrates an embodiment of optical beat interference noise in a CDMA passive optical network;  
         [0032]      FIG. 3  illustrates a CDMA passive optical network using error correction codes according to first embodiment of the present invention;  
         [0033]      FIG. 4 , illustrates a CDMA passive optical network using error correction codes according to second embodiment of the present invention; and  
         [0034]      FIG. 5 , is a flow chart for a method controlling current bias of upstream light source using error data in a CDMA passive optical network using error correction codes according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0035]     Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Note that the same or similar components in drawings are designated by the same reference numerals as far as possible even if they are shown in different drawings. In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear.  
         [0036]      FIG. 3  illustrates a CDMA passive optical network using error correction codes according to an embodiment of the present invention.  
         [0037]     In general, a CDMA passive optical network discriminates each subscriber using a CDMA mode and overcomes optical beat interference noise using a processor gain. However, as described above, as subscribers increase, conventional optical networks cannot sufficiently overcome the optical beat interference noise using only the processor gain. Therefore, as shown in  FIG. 3 , a CDMA passive optical network using error correction codes according to the present invention can additionally (1) overcome the optical beat interference noise by using error correction codes and (2) restrain generation of the optical beat interference noise by controlling an ONU light source through an error detection function of the error correction codes. The error correction codes causes an additional overhead to be put in data signals in the transmission section and causes errors of the data signals to be detected in the reception section. Also, the error correction codes can correct the transmission errors of the data signals provided a great number of errors are not generated during transmission. For example, the Reed-Soloman code with an overhead at a 7% of the total data can contribute to the 10 −12  bits error ratio even in the transmission environment where the 10 −4  bits error ratio occurs.  
         [0038]     As shown in  FIG. 3 , a passive optical network includes a central office  200 , optical networks units  100  and optical couplers  300 - 1  and  300 - 2 . The central office  200  includes an upstream signal processor unit and a downstream signal processor unit.  
         [0039]     The downstream signal processor unit of the central office  200  includes a multiplexer  301 , PN code generator  305 , and a code division encoder  303 , a combiner, and light source  306 . The multiplexer  301  multiplexes downstream signals and error signals of error correction codes. The code division encoder  303  code division encodes signals multiplexed by means of PN codes. The combiner combines a plurality of signals code division encoded.  
         [0040]     The upstream signal processing unit of the central office  200  includes a photo-electric converter  307 , a splitter of electric signals, a PN code generator  305 , a code division decoder  304  and an error correction code demodulator  302 .  
         [0041]     The optical network unit includes the downstream signal processing unit and the upstream signal processing unit.  
         [0042]     The downstream signal processing unit of the optical network unit includes a photo-electric converter  308 , a PN code generator  314 , a code division decoder  310 , demodulator  312 , and a demultiplexer  312 . The code division decoder  310  that code division-decodes by means of the signals from the PN code generator.  
         [0043]     The upstream signal processing unit includes an error correction code modulator  313 , a code division encoder  311 , a light source of upstream signal  309 , and a light source bias controller  315 .  
         [0044]     The following describes the operation of the passive optical network using error correction codes. The downstream transmission signal (e.g., Data 1 ) is multiplexed with error information (error count) from error correction code demodulator (FEC decoder) by the multiplexer  301  in the central office (CO)  200 . The multiplexed signal is code division encoded by each code which is assigned to each subscriber (e.g., PN code) and generated by the PN code generator  305 . It is combined with signals to be transmitted to other subscribers and input to the downstream light source (LD: Laser DIODE)  306 .  
         [0045]     The downstream light source  306  converts an electrical signal (the combination of code division encoded signal) into an optical signal. In addition, it transmits the optical signal through an optical fiber to the optical coupler  300 - 1 . Then the optical coupler  300 - 1  splits the optical signal to each subscriber.  
         [0046]     Each optical network unit (ONU)  100  converts the optical signal from the central office  200  into an electrical signal by the photo-electric converter (PD)  308 . It also demodulates the code division encoded signals by means of a code assigned to each subscriber generated from the PN code generator  314 . The demodulated signal includes the downstream signal and error information. The demodulated signal is split into the downstream signal and error information by the demultiplexer  312 . Herein, the error information is transmitted through the light source bias controller  315  to the upstream light source  309 , as described further below.  
         [0047]     The upstream transmission signal (e.g., Data) is error-corrected by the error correction code modulator (FEC encoder)  313 . It is also code division encoded by means of code assigned to each subscriber from the PN code generator  314  in optical network unit  100 . The code division-encoded signal is input to the upstream light source  309 , converted into optical signals, and transmitted through optical fiber to the optical coupler  300 - 2 . The optical coupler  300 - 1  couples the optical signal from each subscriber and transmits to the central office  200 .  
         [0048]     The central office  200  splits the signal input to the photo-electric converter  307  into signals for of the subscribers. It also decodes the code division encoded signals by means of a code (e.g., PN code) assigned to each subscriber from the PN code generator  305 . In addition, errors of the decoded signals generated during transmission are corrected through the error correction code demodulator (FEC decoder)  302 . The error information of the decoded signal is transmitted to the multiplexer  301 .  
         [0049]     The CDMA passive optical network can decrease signal errors caused by noise (e.g., reception apparatus noise, optical beat interference noise, etc.) generated during upstream transmission using the CDMA mode and error correction codes. The error correction codes includes the Reed-Solomon code, the BCH code, the turbo cord, and the LDPC code.  
         [0050]     However, if the light sources of all optical networks units  100  have the same wavelength, the above method cannot sufficiently overcome the optical beat interference noise. In order to supplement the above method, the CDMA passive optical network controls the optical signal frequency of the upstream light source  309  from each optical network unit  100  by means of the error information of the error correction codes.  
         [0051]     As described above, the downstream transmission signals (e.g., Data 1 , Data 2 , . . . Data N ) include error information. The error information is generated while the central office  200  demodulates the error correction coded upstream signals and monitors quality of the upstream signals.  
         [0052]     The central office  200  broadcasts the error information to each optical network unit  100  in the downstream transmission. Each optical network unit  100  receiving the error information detects the degrading degree of the signal quality. Therefore, the optical network unit  100  can control the frequencies of the upstream signals. As shown in  FIG. 3 , each optical network unit  100  splits the pure downstream signals and the error information using demultiplexer  312 . The split error information adjusts the optical frequency of the upstream light source  309  by controlling the bias current of the upstream light source  309  through the light source bias controller  315 . Generally, a Fabry-Perot laser or distribution feedback laser has an optical frequency variety coefficient corresponding to current changes about 0.2 GHz to 5 GHz/mA. The optical frequency of the upstream light source  309  changes in the range of several dB to tens of dB by controlling current having about 10 nA intensity. Thus, the optical beat interference noise is diminished by controlling the current. The detailed description of the process of controlling the current will be shown in  FIG. 5 .  
         [0053]      FIG. 4  illustrates a CDMA passive optical network using error correction codes according to second embodiment of the present invention.  
         [0054]      FIG. 4  shows that an error correction modulator  412  is located after a code division encoder  411 , in comparison with  FIG. 3 . Accordingly, in receiving the upstream signals in the central office  200 , an error correction demodulator  404  is located before a code division decoder  403 . The other structure shown in  FIG. 4  is similar with that shown in  FIG. 3 , and a detailed description of these elements is omitted here.  
         [0055]      FIG. 5 , is a flow chart for a method controlling current bias of upstream light source using error information in a CDMA passive optical network using error correction codes according to an embodiment of the present invention.  
         [0056]     The error information is checked from each of three current values, for example: (1) a present bias current value Ion, (2) a current value Ion+dl obtained by adding a predetermined variance to the present bias current value, and (3) a current value Ion−dl by subtracting a predetermined variance from the present bias value. The three values are compared with each other to determine the direction in which the bias current of the light source  309  is controlled. In this manner the quality of signal can be improved.  
         [0057]     As shown in  FIG. 5 , initial values are stored for the present bias current Ion, the predetermined variance dl, an error information value ε, a minimum bias current value Imin, and a maximum bias current value Imax, in step  501 .  
         [0058]     In step  502 , a check is conducted to determine whether the data is transmitted or not. If the data is transmitted, the bias current is set to the present bias current (Ion). At operation time, the upstream signals are transmitted to the central office  200 .  
         [0059]     In steps  504  to  516 , the transmission is established and then the central office  200  sends error information to the optical network unit  100 . The optical network unit  100  checks using the error information whether the upstream signals normally are transmitted to the central office  100  or not.  
         [0060]     In steps  504  to  506 , if the error information is larger than a specific value (error count &lt;ε) because the optical beat interference noise is large, then the first error information ε0 of the present bias current Ion are stored and the present bias current is added to the predetermined variance Ion +dl.  
         [0061]     In step  508  and  509 , the second information ε+of the added present bias current is restored, and the present bias current is subtracted to 2 by dl (that is, Ion−2 dl) and the third error information value ε− is re-measured and stored.  
         [0062]     In the same manner, the three error information values, ε0, ε+, and ε− are extracted. The upstream light source current is moved to the direction having the smallest value of the three error information values.  
         [0063]     Moreover, after the comparison between three error information values, ε0, ε+, and ε−, if the minimum value and the maximum value are ε+ and ε−, respectively, then it is checked whether the sum Ion+dl is larger than a maximum current value Imax. If it is larger, the bias current is set as Ion, in step  516 , but if not, the bias current is set as the sum Ion +dl, in step  516 .  
         [0064]     Meanwhile, after comparing the three error information values ε0, ε+, and ε−, if the minimum and the maximum value are not ε+, and ε−, respectively, in step  510 , then it is checked whether minimum and maximum are ε− and ε+, respectively instep  511 .  
         [0065]     Also, as a result of the procedure in step  511 , if the minimum and the maximum value are ε− and ε+, respectively, then it is checked whether Ion−dl is smaller than minimum value Imin. If it is smaller, the bias current is set as Ion in step  514 , and if not, the bias current is set as Ion−dl in step  513 .  
         [0066]     Also, as a result of the procedure in step  511 , if the minimum and the maximum value are not ε−, and ε+, respectively, then the bias current is set as Ion in step  514 .  
         [0067]     In the same manner, it is confirmed whether the newly set bias current is suitable for the operation of the system, in order to avoid mis-conduction of system and trouble of light source owing to over-current and low-current. The above method is repeatedly conducted while controlling bias current of the light source so that system performance is not degraded from the optical beat interference noise.  
         [0068]     As described above, the present invention overcomes the degrading of signals influenced by the optical beat interference noise by using CDMA mode and error correction codes.  
         [0069]     In particular, the present invention diminishes the optical beat interference noise by controlling the frequencies of the optical signals. Controlling the frequencies of the optical signals is conducted using an error detection function of error correction codes. Thus, transmission speed of optical networks units in passive optical communication networks (realized by a light source having a low cost) is improved. In addition, the number of subscribers is increased, so that the passive optical communication networks have a lower cost of initial construction and extensibility.  
         [0070]     While the invention has been shown and described with reference to certain 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. Consequently, the scope of the invention are not to be limited to the exemplary embodiments, but should be defined by the appended claims and equivalents thereof.