JP4784323B2 - Optical access network system - Google Patents

Description

  The present invention relates to an optical access network system for bidirectional communication between a business operator and a subscriber using a Code Division Multiplexing (CDM) method in a PON (Passive Optical Network).

  Attention has been focused on an optical access network system in which a provider (hereinafter sometimes referred to as a “center”) and a plurality of subscribers (hereinafter also referred to as “users”) are connected via a PON. ing. In the following description, the provider side device may be referred to as an optical line terminal device or OLT (Optical Line Terminal), and the subscriber side device may be referred to as an optical terminal device or ONU (Optical Network Unit).

  PON is an optical fiber transmission line that is connected to an optical multiplexer / demultiplexer, which is a passive element, and branches one optical fiber transmission line into multiple optical fiber transmission lines. A network that connects a plurality of optical terminal devices to each other (for example, see Non-Patent Document 1). By adopting PON for the network connecting the center and the user, the optical fiber transmission line between the center and the optical multiplexer / demultiplexer can be shared by a plurality of users, and the equipment cost can be reduced.

  In the conventional optical access network system using PON, a time division multiplexing (TDM) method is adopted, and the user assigned to each channel is identified by controlling the time slot of the TDM signal ( For example, see Non-Patent Document 2.) Here, an optical signal having a wavelength different from a signal from the user to the center (hereinafter, also referred to as “upstream signal”) and a signal from the center to the user (hereinafter also referred to as “downstream signal”). Is used. This is because the upstream signal and downstream signal share a single optical fiber transmission line, so that the upstream signal and downstream signal are identified based on the difference in wavelength. The upstream signal and the downstream signal are separated and combined by an optical bandpass filter, and the signal between each user and the center is multiplexed and demultiplexed by an optical multiplexer / demultiplexer.

  On the other hand, in an optical access network system using PON, a method of transmitting an uplink signal by a wavelength division multiplexing (WDM) system has also been studied (for example, see Non-Patent Document 3). However, in order to increase the number of channels to be multiplexed (in this case, the number of users), since the usable wavelength band has a finite width, it is necessary to narrow the wavelength interval assigned to adjacent channels. In order to narrow the wavelength interval in this way, the wavelength stability of the light source is necessary, and a lot of equipment costs are required to ensure this stability.

Therefore, it is desired to increase the number of channels to be multiplexed and substantially increase the transmission capacity without increasing the number of wavelengths to be used. As one of the methods, a method of performing transmission / reception between the center and the user by CDM transmission has been studied (for example, see Patent Documents 1 and 2). By adopting the CDM method for PON, the number of channels to be multiplexed (corresponding to the number of users) can be increased without increasing the number of wavelengths used.
Yokota, et al., “Optical Access System ATM-PON” Oki Electric Research and Development No.182, Vol. 67, No. 1, April 2000 Ian M. McGregor, et al. "Implementation of a TDM Passive Optical Network for Subscriber Loop Application", J. Lightwave Technology, Vol. 7, No. 11, November 1989 KW Lim, et al. "Fault Localization in WDM Passive Optical Network by Reusing Downstream Light Sources", IEEE Photonics Technology Letters Vol. 17, No. 12, December 2005 Special Table 2001-512919 JP 2004-282742 A

  However, in the PON optical access network system that adopts the CDM method, a part of the transmission signal transmitted from the ONU is reflected in the optical connector for connection with the optical multiplexer / demultiplexer provided in the optical transmission line, and the ONU There is a problem that a part of the transmission signal is mixed as a reflection noise in the reception signal received by. Due to this noise component, a situation occurs in which the received signal cannot be received correctly in the ONU.

  Therefore, there is a method of assigning different wavelengths to a signal transmitted from the ONU to the OLT and a signal transmitted from the OLT to the ONU. According to this method, it is necessary to double the types of wavelengths to be assigned to signals. Considering that the wavelength resources that can be used for optical communication are limited, it is not preferable to increase the types of wavelengths to be used.

  In addition, as a means for reducing the intensity of noise mixed in the received signal received by the ONU, an optical connector having a low reflectance as an optical connector for connection with the optical multiplexer / demultiplexer provided in the optical transmission path described above Can be considered. This optical connector is configured to perform optical connection by polishing and facing the end faces of optical fibers to be connected to each other so as to be inclined from a plane perpendicular to the light traveling direction. By adopting such a configuration, the attenuation of reflected light is suppressed to 60 dB or less.

  However, this low-reflection type optical connector is very expensive, and using it for connection between ONU, which is a user-side device, and an optical multiplexer / demultiplexer leads to high equipment cost of the optical access network system, It is not preferable. Since the number of ONUs that are user equipment is required by the number of users, the use of one expensive part as the ONU and a necessary part accompanying it greatly affects the equipment cost. On the other hand, since only one OLT is required as the center side device, the use of expensive parts does not significantly affect the equipment cost. Also, if the OLT that is the center side device fails, the entire system will not function. On the other hand, if the ONU that is the user side device fails, the function of the entire system is not lost. Therefore, while it is an important requirement that the cost of the components constituting the ONU is low, the high performance of the components constituting the OLT is an important requirement.

  Therefore, an object of the present invention is to reduce the S / N ratio of the received signal even if the reflected light component from the optical connector used to connect the optical multiplexer / demultiplexer to the ONU is mixed in the received signal of the ONU. To provide an optical access network system based on PON. This makes it possible to use a low-cost optical connector with a large reflected light component for connecting the optical multiplexer / demultiplexer and the ONU, which greatly contributes to lowering the equipment cost.

  The present invention is based on both code division multiplexing between an OLT installed on a provider side and N ONUs (N is a natural number of 2 or more) ONUs installed on a user side. The present invention relates to an optical access network system that performs optical communication. For the N ONUs, the first channel to the Nth channel are assigned in order. The OLT and N ONUs are coupled via an optical fiber transmission line, an optical multiplexer / demultiplexer, and N branch optical fiber transmission lines. The optical fiber transmission line is provided with an optical coupler at one end, and an OLT is coupled to the other end of the optical fiber transmission line. The optical fiber transmission line is branched into a plurality of branched optical fiber transmission lines by an optical coupler, and one ONU is coupled to each of the branched optical fiber transmission lines.

  Each of the OLT and N ONUs described above receives a transmission signal processing unit that encodes a signal to be transmitted and generates and outputs an encoded transmission signal, and an encoded reception signal that has been encoded and transmitted. A reception signal processing unit that decodes the encoded reception signal to extract and output the reception signal.

  In order to achieve the above-mentioned object, in the optical access network system of the present invention, a code set in the transmission signal processing unit including the ONU of the k-th channel (k is a natural number from 1 to N), The code set in the transmission signal processing unit provided in the OLT that encodes and outputs the downstream signal for the k channel ONU is different.

  The transmission signal processing unit provided in the kth channel ONU encodes and outputs an uplink signal that is a signal for transmission from the kth channel ONU to the OLT. On the other hand, the transmission signal processing unit provided in the OLT encodes and outputs a downstream signal that is a signal for transmission from the OLT toward the ONU of the k-th channel.

  Also, the code set in the transmission signal processing unit provided in the ONU of the p-th channel (p is a natural number from 1 to N) and the q-th channel (q is a natural number from 1 to N). It is preferable to set equal to the code set in the transmission signal processing unit provided in the OLT that encodes and outputs the downstream signal directed to the ONU. The transmission signal processing unit included in the ONU of the p-th channel encodes and outputs an upstream signal from the ONU of the p-th channel toward the OLT. On the other hand, the transmission signal processing unit included in the OLT encodes and outputs a downlink signal that is a signal for transmission from the OLT toward the ONU of the q-th channel.

  However, the pair (p, q) of the natural number p that defines the p-th channel and the natural number q that defines the q-th channel is (1, 2), (2, 3), (3, 4),. Limited to N combinations of (p, p + 1), ... (N-1, N) and (N, 1). For example, in the case of the optical access network system of the present invention having four ONUs, since N = 4, the pair (p, q) of the natural numbers p and q is (1, 2), (2, 3) Limited to 4 groups (3, 4) and (4, 1).

  That is, the code set in the transmission signal processing unit provided by the ONU of the first channel is equal to the code set in the transmission signal processing unit provided by the OLT that encodes and outputs the downstream signal directed to the ONU of the second channel. Set. The code set in the transmission signal processing unit provided by the ONU of the second channel is set equal to the code set in the transmission signal processing unit provided by the OLT that encodes and outputs the downlink signal directed to the ONU of the third channel. . The code set in the transmission signal processing unit provided by the third channel ONU is set equal to the code set in the transmission signal processing unit provided by the OLT that encodes and outputs the downstream signal directed to the fourth channel ONU. . The code set in the transmission signal processing unit provided by the fourth channel ONU is equal to the code set in the transmission signal processing unit provided by the OLT that encodes and outputs the downstream signal directed to the ONU of the first channel. Set. The same applies to cases other than N = 4.

  According to the optical access network system of the present invention, the code used for encoding the k-th channel upstream signal is different from the code used for encoding the k-th channel downstream signal.

  Therefore, even if a part of the upstream signal component sent from the k-th channel toward the OLT is reflected from the optical connector and mixed in the downstream signal transmitted from the OLT in the k-th channel, the upstream signal and the downstream signal are transmitted. Since the signal is encoded with a code different from the signal, the upstream signal component is not decoded when the downstream signal is decoded by the ONU of the k-th channel. Therefore, even if a part of the transmission signal transmitted from the ONU is reflected in the optical connector and a part of this transmission signal is mixed as reflected noise in the reception signal received by the ONU, due to this noise component, It is possible to avoid a situation in which the received signal cannot be received correctly in all ONUs.

  As described above, different codes for the number of channels are newly required in order to change the codes used for encoding the upstream signal and the downstream signal in each channel. As described above, the same effect can be realized by means of changing the wavelengths of the upstream signal and downstream signal, but a lot of equipment cost is required for ensuring the wavelength stability of the light source. In contrast, providing different codes for the number of channels has the advantage that, in principle, almost no new cost is generated.

  However, in order to prepare a large number of different codes, it is necessary to increase the code length. Therefore, it is desirable to devise a method that does not increase the number of codes to be prepared as much as possible. This is because by increasing the code length, new measures such as increasing the communication speed for transmission and reception may be required.

  That is, the bit rate of the signal that is encoded and transmitted in proportion to the code length increases. When the bit rate is high, distortion in the time waveform caused by chromatic dispersion in the transmission path becomes a problem as the transmission distance becomes longer. For this reason, for example, it may be necessary to take new measures such as the need to regenerate the time waveform by placing a repeater in the middle of the transmission path.

  Therefore, the code set in the transmission signal processing unit provided in the ONU of the p-th channel (p is a natural number from 1 to N) and the q-th channel (q is a natural number from 1 to N). It is preferable to set equal to the code set in the transmission signal processing unit provided in the OLT that encodes and outputs the downstream signal directed to the ONU. By doing so, the transmission signal processing unit provided with the OLT that encodes and outputs the code set for the transmission signal processing unit provided in the ONU of the k-th channel and the ONU of the k-th channel. A new code is not required while satisfying the above-described condition that the codes to be set are different from each other.

  As described above, the code set in the transmission signal processing unit provided by the ONU of the p-th channel and the transmission signal processing unit provided by the OLT that encodes and outputs the downlink signal directed to the ONU of the q-th channel. If the signs are set equal, the following is realized.

  That is, the codes for encoding the uplink and downlink signals of the first channel are different from each other, but the codes for decoding the downlink signals of the first channel and the codes for encoding the uplink signals of the second channel are the same. The codes for encoding the uplink and downlink signals of the second channel are different from each other, but the code for decoding the downlink signal of the second channel and the code for encoding the uplink signal of the third channel are the same. In general, the codes for encoding the upstream and downstream signals of the (N−1) th channel are different from each other, but the code for decoding the downstream signal of the (N−1) th channel and the upstream signal of the Nth channel are encoded. The symbol is the same. The codes for encoding the uplink and downlink signals of the Nth channel are different from each other, but the code for decoding the downlink signal of the Nth channel and the code for encoding the uplink signal of the first channel are the same.

  Thus, the number of required codes (N) is equal to the number of channels (N) equal to the number of users. For this reason, a new code is not required, and the code used for encoding the k-th channel uplink signal is different from the code used for encoding the k-th channel downlink signal. It is possible to satisfy the conditions. Therefore, by setting the code in the transmission signal processing unit of each channel in this way, it is possible to avoid a situation in which the reception signal cannot be received correctly in all ONUs.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Each figure shows an example of the configuration according to the present invention, and only schematically shows the arrangement relationship of each component to the extent that the present invention can be understood, and the present invention is limited to the illustrated example. is not. In the following description, specific materials and conditions may be used. However, these materials and conditions are only one of preferred examples, and are not limited to these. Moreover, in each figure, the same component is shown with the same number, and the overlapping description may be omitted. Further, in the schematic block configuration diagram shown below, the path of an optical signal such as an optical fiber is indicated by a thick line, and the path of an electrical signal is indicated by a thin line.

<Optical access network system>
The configuration and operation of the optical access network system will be described with reference to FIG. FIG. 1 is a schematic block diagram of an optical access network system. The optical access network system shown in FIG. 1 assumes a case where the number of subscribers (number of users) is 4, that is, the case where there are four optical terminal devices. However, the following explanation is valid.

  In FIG. 1, in order to identify a plurality of optical terminal devices, the optical terminal device assigned the first channel is represented as ONU-1, and the optical terminal device assigned the fourth channel is represented as ONU-4. . Although ONU-2 and ONU-3 are not shown, all of ONU-1 to ONU-4 have the same configuration. In the following description of the optical access network system, when the structure of the optical terminal device is described, it is generally expressed as the optical terminal device 10 and described.

  The optical access network system includes an optical line terminating device 100 that is a device installed on a provider side, and an optical terminal device (ONU-1 to ONU-4, hereinafter referred to as an optical terminal device 10) that is a device installed on a user side. The optical access network system performs bidirectional optical communication by code division multiplexing. The optical line termination device 100 and the optical terminal device 10 are coupled via an optical fiber transmission line 70, an optical multiplexer / demultiplexer 66, and a plurality of branched optical fiber transmission lines.

  The optical fiber transmission line 70 is provided with an optical coupler 66 at one end, and the optical line terminator 100 is coupled to the other end of the optical fiber transmission line 70. Further, the optical fiber transmission line 70 is branched into a plurality of branched optical fiber transmission lines by an optical coupler 66, and one optical terminal device is coupled to each of the branched optical fiber transmission lines. In FIG. 1, the optical fiber transmission line connecting ONU-1 is represented as a branched optical fiber transmission line 74, and the optical fiber transmission line connecting ONU-4 is represented as a branched optical fiber transmission line 76.

  Bidirectional optical communication by code division multiplexing is performed between each of the plurality of optical terminal devices (ONU-1 to ONU-4) and the optical line terminating device 100.

  The optical terminal device 10 includes an optical processing unit 12 and an electrical processing unit 14. The light processing unit 12 is a light emitting element 20 for converting the encoded transmission signal from the electric signal form to the optical signal form, and a light receiving element for converting the code division multiplexed signal from the optical signal form to the electric signal form. Has 18.

  The electrical processing unit 14 encodes the transmission signal to generate an encoded transmission signal in the form of an electrical signal, and the code converted from the optical signal form to the electrical signal form by the light receiving element 18 And a reception signal processing unit 22 for decoding the division multiplexed signal and extracting the reception signal.

The received signal processing unit 22 includes a decoding processing circuit 30 that performs processing for decoding the code division multiplexed signal, an automatic gain control (AGC) element 28, a clock signal reproduction circuit 34, and a frequency divider. 38. A delay circuit 40 is provided. The transmission signal processing unit 24 includes an encoding processing circuit 56 and a driver 60. Amplifier to the driver 60 (AMP: Amplifier) is used.

  On the other hand, similarly to the optical terminal device 10, the optical line termination device 100 includes an optical processing unit 102 and an electric processing unit 104. Similar to the optical processing unit 12 of the optical terminal device 10, the optical processing unit 102 includes a light emitting element 122 and a light receiving element 126. The electrical processing unit 104 includes a transmission signal processing unit 106, a reception signal processing unit 108, and a clock signal generation circuit 110.

  The transmission signal processing unit 106 includes an encoding processing circuit array 116 that includes encoding processing circuits in parallel, and a driver 120. The driver 120 uses an amplifier. Transmission signals output from the respective encoding processing circuits indicated by reference numerals 1 to 4 are combined by the electric signal combiner 118 and input to the driver 120.

  Further, the received signal processing unit 108 includes a decoding processing circuit row 132 including decoding processing circuits in parallel, and an automatic gain control element 128. The electric signal output from the automatic gain control element 128 is branched by the electric signal branching unit 130 and input to each of the decoding processing circuits indicated as decoding 1 to decoding 4 and decoded.

  A clock signal is supplied from the clock signal generation circuit 110 to the transmission signal processing unit 106 and the reception signal processing unit 108. The clock signal supplied from the clock signal generation circuit 110 is a clock signal serving as a reference for this optical access network system. In the optical terminal device 10, the clock signal recovery circuit 34 extracts this clock signal from the received code division multiplexed signal and uses it for decoding the code division multiplexed signal.

  The optical access network system having the above-described configuration shown in FIG. 1 operates as described below.

  First, the downlink signal will be described taking the first channel as an example. The downstream transmission signal of the first channel is input to an encoding processing circuit (encoding processing circuit represented as “code 1” in the first and second embodiments described later) that encodes the transmission signal, It is encoded and output as an encoded transmission electrical signal. The encoded transmission electric signal is multiplexed by the electric signal multiplexer 118 and input to the driver 120 as a code division multiplexed electric signal and amplified, and the amplified code division multiplexed electric signal is provided in the optical processing unit 102. It is converted into an optical signal by the light emitting element 122 and output as a code division multiplexed optical signal. As the light emitting element 122, for example, a semiconductor laser can be used.

  The code division multiplexed optical signal is input to the optical processing unit 12 of the optical terminal device 10 through the optical coupler 124 and the optical connector 72, and through the optical fiber transmission line 70, the optical connector 68, the optical multiplexer / demultiplexer 66, and the optical connector 61. Is done. The code division multiplexed optical signal input to the optical processing unit 12 is input to the light receiving element 18 via the optical coupler 16 included in the optical processing unit 12, and is converted into a code division multiplexed electric signal. Input to the electrical processing unit 14. As the light receiving element 18, for example, a photodiode can be used.

  The code division multiplexed electric signal input to the electric processing unit 14 is branched into two by an electric signal branching unit 26, and one is input to the clock signal regeneration circuit 34 and the other is input to the automatic gain control element 28. A clock signal is extracted from the code division multiplexed electric signal input to the clock signal reproduction circuit. Further, the code division multiplex electric signal input to the automatic gain control element 28 is arranged as a code division multiplex electric signal having a constant voltage value set in the automatic gain control element 28 regardless of its strength, The data is input to the decryption processing circuit 30. This constant voltage value is equal to the input level of an analog shift register that is a constituent element of the analog matched filter 44 included in the decoding processing circuit 30.

  The code division multiplexed electric signal whose voltage value has been adjusted is first decoded by an analog matched filter 44 provided in the decoding processing circuit 30 and input to the determination circuit 46. The decision circuit 46 extracts and outputs only the autocorrelation waveform component from the signal decoded by the analog matched filter 44. That is, the received signal generated from this autocorrelation waveform component is the signal component received by ONU-1 of the first channel.

  As described above, the downlink signal, that is, the transmission signal transmitted from the optical network unit 100 to the optical terminal device 10 is transmitted as a code division multiplexed optical signal that is encoded and multiplexed. Then, in the optical terminal device 10, the code division multiplexed optical signal is converted into a code division multiplexed electric signal and decoded. That is, all the decoding processes in the optical terminal device 10 are executed in the state of electrical signals.

  The optical access network system according to the present invention is characterized by an electric processing unit that encodes a transmission signal and decodes a reception signal, and most of the following description is mainly about the operation of the electric processing unit. A signal necessary for explaining the operation of the electric processing unit is an encoded transmission electric signal or a code division multiplexed electric signal. Therefore, in the following description, it is not distinguished whether it is an optical signal or an electric signal unless particularly necessary. In other words, the code division multiplexed optical signal or the code division multiplexed electric signal is expressed as a code division multiplexed signal without clearly indicating whether it is an electric signal or an optical signal.

  Next, the uplink signal will be described taking the first channel as an example. The transmission signal of the first channel (shown as “transmission signal input” of the optical terminal device 10 in FIG. 1) is a code provided in the transmission signal processing unit 24 of the electrical processing unit 14 of the optical terminal device 10. Is input to the encoding processing circuit 56, encoded, and output as an encoded transmission signal. The encoded transmission signal is input to the driver 60 and amplified, and the amplified encoded transmission signal is converted into an optical signal by the light emitting element 20 included in the optical processing unit 12. The light emitting element 20 can use a semiconductor laser.

  The encoded transmission signal is input to the optical multiplexer / demultiplexer 66 through the optical coupler 16 and the optical connector 61 to become a code division multiplexed signal, and is transmitted through the optical connector 68, the optical fiber transmission line 70, and the optical connector 72 to the optical line. Input to the optical processing unit 102 of the termination device 100. The code division multiplexed signal input to the optical processing unit 102 is input to the light receiving element 126 via the optical coupler 124 included in the optical processing unit 102 and converted into an electric signal, and the electric processing unit 104 of the optical line termination device 100 is converted into an electric signal. Is input. As the light receiving element 126, a photodiode can be used.

  The code division multiplexed signal input to the electric processing unit 104 is input to the automatic gain control element 128 included in the reception signal processing unit 108, and a constant voltage set in the automatic gain control element 128 regardless of its strength. It is arranged as an encoded received electrical signal having a value, and is input to a decoding processing circuit represented by “decoding 1” in the decoding processing circuit array 132. In this decoding processing circuit, processing similar to that of the decoding processing circuit 30 provided in the optical terminal device 10 is performed, and a signal transmitted from the ONU-1 of the first channel is generated and output.

<First embodiment>
A feature of the present invention is that, for each channel, a code is selected so that the code set in the transmission signal processing unit provided in the ONU is different from the code set in the transmission signal processing unit provided in the OLT. is there. With reference to FIG. 2, the configuration of the ONU in this case will be described as a first embodiment. FIG. 2 is a schematic block diagram of the ONU of the first embodiment. FIG. 2 is a diagram in which the configuration of the ONU portion of the optical access network system shown in FIG. 1 is extracted, and is shown in a simplified manner to the extent necessary for the following description.

  The electrical processing unit 14 shown in FIG. 1 corresponds to the electrical processing units 14-1 to 14-4 shown in FIG. 2, and the optical coupler 16 corresponds to the optical couplers 16-1 to 16-4 shown in FIG. The light receiving element 18 corresponds to the light receiving elements 18-1 to 18-4 shown in FIG. 2, and the light emitting element 20 corresponds to the light emitting elements 20-1 to 20-4 shown in FIG. 22 corresponds to the reception signal processing units 22-1 to 22-4 shown in FIG. 2, and the transmission signal processing unit 24 corresponds to the transmission signal processing units 24-1 to 24-4 shown in FIG.

  The ONU-1 to ONU-4 are connected to the optical coupler 66 via optical connectors 61, 62, 63, and 64, respectively. Each of the electrical processing units from ONU-1 to ONU-4 includes a reception signal processing unit and a transmission signal processing unit. In addition, reference numeral 1 is set in the reception signal processing unit 22-1 of ONU-1, and reference numeral 2 is set in the transmission signal processing unit 24-1. Similarly, the ONU-2 reception signal processing unit 22-2 is set with code 3 and the transmission signal processing unit 24-2 is set with code 4, respectively. The ONU-3 reception signal processing unit 22-3 5 is set for the transmission signal processing unit 24-3, and 6 is set for the reception signal processing unit 22-4 of the ONU-4. 8 are respectively set.

  On the other hand, as already described with reference to FIG. 1, the coding process in which the same code as that assigned to each of the ONU-1 to ONU-4 transmission signal processing units is also assigned to the OLT 100. In parallel, an encoding processing circuit array 116 including circuits in parallel and a decoding processing circuit to which the same codes as the codes allocated to the ONU-1 to ONU-4 received signal processing units are allocated are provided in parallel. A decoding processing circuit array 132 and an automatic gain control element 128 are provided.

  The flow of processing of uplink and downlink signals for ONU-1 to which the first channel is assigned will be described. First, the downlink signal will be described. The downlink signal of the first channel from OLT to ONU-1 is encoded with code 1 from OLT as an encoded transmission signal, code-division-multiplexed with the encoded transmission signals of other channels, and as a code-division multiplexed signal , Sent to ONU-1. This code division multiplexed signal is decoded by code 1 in the reception signal processing unit 22-1 provided in the electrical processing unit 14-1 of the ONU-1, and the reception signal of the first channel received by the ONU-1 is extracted. Is done. Also in ONU-2 to ONU-4, the downstream signal transmitted from the OLT is processed in the same manner.

  The second, third, and fourth channel downstream signals from OLT to ONU-2, 3, and 4 are encoded from OLT by codes 3, 5, and 7, respectively, and converted into encoded transmission signals. The signals are multiplexed and transmitted to ONU-2, 3 and 4 as code division multiplexed signals. This code division multiplexed signal is received signal processing units 22-2, 22-3 and 22-4 respectively provided by the electric processing units 14-2, 14-3 and 14-4 of ONU-2, 3 and 4. The received signals of the second, third and fourth channels, which are decoded by the codes 3, 5 and 7, respectively, and received by the ONUs 2, 3 and 4, respectively, are extracted.

  Next, the uplink signal will be described. The upstream signal of the first channel from ONU-1 to OLT is encoded by code 2 in the transmission signal processing unit 24-1 of ONU-1, and is encoded as a transmission signal together with the encoded transmission signals of other channels. It is code division multiplexed and transmitted to the OLT as a code division multiplexed signal. This code division multiplexed signal is decoded by code 2 in the reception signal processing unit 108 provided in the OLT electrical processing unit 104, and the reception signal of the first channel received by the OLT is extracted. The same processing is performed for the upstream signals of the second to fourth channels.

  From ONU-2, 3 and 4, the upstream signals of the 2nd, 3rd and 4th channels respectively directed to the OLT are encoded with codes 4, 6 and 8, respectively, and converted into encoded transmission signals. The signal is multiplexed and transmitted to the OLT as a code division multiplexed signal. This code division multiplexed signal is decoded by the codes 4, 6 and 8 in the received signal processing unit 108 included in the OLT electrical processing unit 104, and the received signals of the second, third and fourth channels received by the OLT, respectively. Extracted.

  As described above, the code (here, “code 1”) set in the reception signal processing unit 22-1 included in the ONU-1 and the code (here, “code 2”) set in the transmission signal processing unit 24-1. ) Is different. That is, the code used for encoding the upstream signal of the first channel (here, “code 2”) and the code used for encoding the downstream signal of the first channel (here, “code 1”) Is different.

  For this reason, in the first channel, the downlink signal (encoded with code 1) sent from the OLT is encoded with the uplink signal (encoded with code 2) sent from the first channel toward the OLT. ) Even if part of the component is reflected and mixed by the optical connector (here, optical connector 61), the upstream signal and downstream signal are encoded with different codes (code 2 and code 1 respectively). When this downstream signal is decoded by the ONU-1 of the first channel, the upstream signal component is not decoded.

  Therefore, even if part of the transmission signal transmitted from ONU-1 is mixed as reflected noise in the received signal received by ONU-1, the received signal cannot be received correctly by ONU-1 due to this noise component. That is, it is difficult for the situation that the decryption cannot be performed. The same applies to the upstream and downstream signals of the second to fourth channels. Some of the upstream signals mixed in the downstream signals of ONU-1, ONU-2, ONU-3, and ONU-4 are reflected from the optical connector 61, the optical connector 62, the optical connector 63, and the optical connector 64, respectively. The ONU-1 also reflects a part of the upstream signal slightly from optical connectors other than optical connector 61, that is, from optical connectors (optical connectors 62, 63, and 64) that are not directly connected to ONU-1. However, since the optical coupler 66 is provided on the way, the size is not so large as to be a problem. The same applies to each of ONU-2, ONU-3, and ONU-4.

  As described above, in each ONU, even if a part of the transmission signal transmitted to the received reception signal is mixed as reflection noise, it is unlikely that the reception signal cannot be received correctly. Therefore, the optical connector 68 shown in FIG. 1 does not need to use an expensive component of a low reflection type. Similarly, in the OLT, since the signs of the upstream signal and downstream signal of the same channel are different, even if a part of the transmission signal transmitted to the reception signal to be received is mixed as reflected noise, the reception signal cannot be received correctly. Things are hard to happen. For this reason, it is not necessary to use low-reflective type expensive parts for the optical connector 72 shown in FIG.

<Second embodiment>
With reference to FIG. 3, for each channel, select a code so that the code set in the transmission signal processing unit provided by the ONU is different from the code set in the transmission signal processing unit provided by the OLT. A selection method different from the first embodiment will be described.

  In this selection method, the code set in the transmission signal processing unit 24-1 included in the ONU-1 of the first channel is set as the code 2, and the downstream signal from the OLT to the ONU-2 of the second channel is also encoded as the code 2. And send it. The code set in the transmission signal processing unit 24-2 included in the ONU-2 of the second channel is set to code 3, and the downstream signal from the OLT to the ONU-3 of the third channel is also encoded with code 3 and transmitted. The code set in the transmission signal processing unit 24-3 included in the third channel ONU-3 is set to code 4, and the downstream signal from the OLT to the fourth channel ONU-4 is also encoded with code 4 and transmitted. The code set in the transmission signal processing unit 24-4 included in the fourth channel ONU-4 is set as code 1, and the downlink signal from the OLT toward the first channel ONU-1 is also encoded with code 1 and transmitted.

  Thus, by setting a code for encoding the transmission / reception signals of the first to fourth channels, the code set in the reception signal processing unit 22-1 included in the ONU-1 (here, "code 1") ) And the code (here, “code 2”) set in the transmission signal processing unit 24-1 are different. That is, the code used for encoding the upstream signal of the first channel (here, “code 2”) and the code used for encoding the downstream signal of the first channel (here, “code 1”) Is different.

  In addition, a code (here, “code 2”) set in the reception signal processing unit 22-2 included in the ONU-2 and a code (here, “code 3”) set in the transmission signal processing unit 24-2 Is different. That is, a code used for encoding the upstream signal of the second channel (here, “code 3”) and a code used for encoding the downstream signal of the second channel (here “code 2”) Is different.

  Similarly, a code used for encoding the upstream signal of the third channel (here, “code 4”) and a code used for encoding the downstream signal of the second channel (here “code 3”). ) Is different. Also, a code used for encoding the uplink signal of the fourth channel (here, “code 1”) and a code used for encoding the downlink signal of the fourth channel (here, “code 4”) Is different.

  For this reason, as in the case of the first embodiment, even if a part of the transmission signal is mixed as reflection noise in the reception signal, the reception signal cannot be correctly received due to this noise component, that is, the situation where decoding cannot be performed. Hard to happen.

  As described above, in each ONU, even if a part of the transmission signal transmitted to the received reception signal is mixed as reflection noise, it is unlikely that the reception signal cannot be received correctly. Therefore, the optical connector 68 shown in FIG. 1 does not need to use an expensive component of a low reflection type. However, it is necessary to adopt a low reflection type optical connector as the optical connector 72 set on the OLT side. This is because the same code is used in the transmission signal processing unit 106 and the reception signal processing unit 108, so that a part of the transmission signal from the transmission signal processing unit 106 is a signal received by the reception signal processing unit 108. Due to the mixing, a situation in which the received signal cannot be correctly received may occur.

  As described above, since only one OLT, which is the center side device, is required, only one optical connector 72 set in the front stage of the OLT is required. Therefore, the apparatus cost as a whole system is not greatly affected.

  The advantage of the second embodiment is that it does not require a new code, and is used for encoding the above k-channel uplink signal and the k-channel downlink signal. It is a point that the condition that the code is different can be satisfied.

<Encoding process>
A process for encoding a transmission signal will be described with reference to FIGS. 4A1 to 4C, taking the first channel as an example. 4A to 4C, the horizontal axis and the vertical axis are omitted, but the horizontal axis indicates the time axis, and the vertical axis indicates the signal strength. 4 (A1) and (A2) show the transmission signal and the encoded transmission signal of the first channel, respectively, and FIGS. 4 (B1) and (B2) show the transmission signal and the encoded transmission signal of the second channel, respectively. Show. FIG. 4C shows a time waveform of a code division multiplexed signal in which the encoded transmission signal of the first channel and the encoded transmission signal of the second channel are combined. In FIG. 4 (A1) to (C), the 0 level of the signal is indicated by a dashed line. The level 0 or higher is represented as “1”, and the level 0 or lower is represented as “−1”.

  The transmission signal of the first channel shown in FIG. 4 (A1) assumes the case of (1, 0, 1,...) And shows its time waveform. FIG. 4 (A2) assumes the code given by (1, 0, 0, 1) having a code length of 4, and the time of the encoded transmission signal of the first channel generated by encoding with this code The waveform is shown. Further, the transmission signal of the second channel shown in FIG. 4 (B1) assumes the case of (1, 1, 0,...) And shows its time waveform. FIG. 4 (B2) assumes the code given by (1, 0, 1, 0) having a code length of 4, and the time of the encoded transmission signal of the second channel generated by encoding with this code The waveform is shown.

  Here, the number of terms in the sequence consisting of “0” and “1” defining the code may be referred to as a code length. In this example, the number sequence that defines the code is (1, 0, 0, 1) or (1, 0, 1, 0), and since the number of terms in this number sequence is 4, the code length is 4. Become. In addition, a number sequence giving a code is called a code sequence, and each term “0” and “1” of the code sequence is sometimes called a chip. In addition, 0 and 1 themselves may be referred to as code values. The time width (also referred to as a time slot) assigned to one bit of the transmission signal is the reciprocal of the bit rate that is the transmission speed of the transmission signal.

  In encoding, 4 chips constituting a code are allocated to a time slot allocated to 1 bit of a transmission signal. That is, on the time axis, the encoded signal corresponding to the sequence (1, 0, 0, 1) or (1, 0, 1, 0) that defines the code within one bit of the transmission signal is completely contained. Are arranged on the time axis.

The meaning of encoding a transmission signal with a code having a code length of 4 is the product D of the transmission signal (hereinafter also referred to as “D”) and the encoded signal (hereinafter also referred to as “C”). This corresponds to finding xC. In the following description, when it is necessary to distinguish which channel corresponds to D or C, the number of channels is added. For example, D and C of the first channel are denoted as D ₁ and C ₁ , respectively. The same applies to the second channel and the like.

  Specifically, the encoding processing circuit for obtaining the product D × C includes EXNOR (exclusive) which is a gate circuit in which an inverter is connected to the output of an exclusive OR operation EXOR (exclusive or OR) gate.・ Noah) circuit is used. In this case, a transmission signal, an encoded transmission signal, and the like expressed as binary signals of 1 and 0 are converted into binary signals of 1 and -1. Specifically, the bias voltage of the transmission signal and the encoded transmission signal may be adjusted to change the center of the amplitude of these signals to the 0 V level.

  Since the transmission signal of the first channel shown in FIG. 4 (A1) is (1, 0, 1,...), If this is converted into a binary signal of 1 and -1, (1, -1, 1 , ...). Since the code used to encode the transmission signal of the first channel is (1, 0, 0, 1), when this is converted into a binary signal of 1 and -1, (1, -1,- 1, 1).

  The first bit of the transmission signal of the first channel is “1”, the second bit is “0”, and the third bit is “1”. Here, if the transmission signal of the first channel is encoded with the code given by (1, -1, -1, 1), the first bit "1" is (1, -1 , -1, 1), the second bit "-1" is encoded with the code given by (1, -1, -1, 1), and the third This means that the bit “1” is encoded with the code given by (1, −1, −1, 1). Although not shown, it is the same for the fourth and subsequent bits to be encoded.

  Since encoding the transmission signal D with the code C is equivalent to obtaining the product D × C, the first bit “1” of the transmission signal is (the first bit of D ( 1)) × C (1, -1, -1, 1) = (1 × 1, 1 × (-1), 1 × (-1), 1 × 1) = (1, -1, -1, 1) is encoded. The second bit “−1” of the transmission signal is (the second bit of D (−1)) × C (1, −1, −1, 1) = ((− 1) × 1 , (-1) x (-1), (-1) x (-1), (-1) x 1) = (-1, 1, 1, -1). The same applies to the third bit. Therefore, the encoded transmission signal obtained by encoding the transmission signal of the first channel shown in FIG. 4 (A1) is ((1, -1, -1, 1), (-1 , 1, 1, -1), (1, -1, -1, 1)) = (1, -1, -1, 1, -1, 1, 1, -1, 1, -1, -1 , 1, ...).

  In addition, the transmission signal of the second channel shown in FIG. 4 (B1) is obtained by converting the code (1, 0, 1, 0) into a binary signal of 1 and -1, (1, -1, 1, -1) The encoding is the same as in the case of the first channel. The first bit “1” of the transmission signal is (D 1st bit (1)) × C (1, −1, 1, −1) = (1 × 1, 1 × (− 1), 1 × 1, 1 × (-1) = (1, -1, 1, -1) Since the second bit of the transmission signal is also “1”, the second of D The th bit is also encoded as (1, -1, 1, -1).

  Since the third bit is “−1” (the third bit of D (−1)) × C (1, −1, 1, −1) = ((− 1) × 1, (− 1) × (-1), (-1) × 1, (-1) × (-1) = (-1, 1, -1, 1). The encoded transmission signal obtained by encoding the transmission signal of the second channel shown is ((1, -1, 1, -1), (1, -1, 1, -1) as described above. , (-1, 1, -1, 1)) = (1, -1, 1, -1, 1, -1, 1, -1, -1, 1, -1, 1, ...) Become.

  1st channel coded transmission signal (1, -1, -1, 1, -1, 1, 1, -1, 1, -1, -1, 1, ...) and 2nd channel coding The code division multiplexed signal given as the sum of the transmission signals (1, -1, 1, -1, 1, -1, 1, -1, -1, 1, -1, 1, ...) is ( 1 + 1, -1-1, -1 + 1, 1-1, -1 + 1, 1-1, 1 + 1, -1-1, 1-1, -1 + 1, -1-1, 1 + 1) = (+2, -2, 0, 0, 0, 0, 2, -2, 0, 0, -2, 2). Figure 4 (C) shows the time waveform of this code division multiplexed signal. Indicates.

  The code division multiplexed signal shown in FIG. 4C is converted into an optical signal and transmitted through the optical fiber transmission line. When it is received by the optical line terminal device or the optical terminal device, it is converted again into an electric signal and decoded to extract the received signal. Therefore, the absolute value of the amplitude of the time waveform of the code division multiplexed signal shown in FIG. 4 (C) has no essential meaning. Therefore, in the code division multiplexed signal shown in FIG. 4C, the center of the maximum and minimum values of the amplitude is set to 0 level, and the amplitude value is normalized to 1 (+1, -1, 0, 0, 0, 0, 1, -1, 0, 0, -1, 1) have the same meaning.

<Decryption process>
A process of decoding a code division multiplexed signal will be described with reference to FIGS. 5A to 5D, taking the first channel as an example. 5 (A) and 5 (B), the horizontal axis indicates the direction of the time axis and the vertical axis is omitted, but the direction of the vertical axis indicates the signal intensity. FIG. 5A shows a time waveform of a code division multiplexed signal input to an analog matched filter provided in the encoding processing circuit. The center of the maximum value and the minimum value of the amplitude of the code division multiplexed signal shown in FIG. 4 (C) is set to 0 level, and the amplitude value is normalized to 1. FIG. 5B shows a time waveform of a signal decoded and output by the analog matched filter. The horizontal axis in FIG. 5 (B) indicates the direction of the time axis. As described later, the signal output from the analog matched filter includes an autocorrelation waveform component that is a reception signal component of the optical terminal device of the received channel, and a cross-correlation waveform component that is received by other than the optical terminal device of the received channel. Is the sum of That is, the cross-correlation waveform component becomes a noise component.

  FIG. 5 (C1) shows a time waveform of a signal output after the threshold value is determined by the determination circuit. FIG. 5 (C2) shows a time waveform of a clock signal for latching the signal shown in FIG. 5 (C1). FIG. 5D shows a time waveform of a signal obtained by latching a signal output after the threshold determination shown in FIG. 5C1 is performed with the clock signal shown in FIG. 5C2. The signal shown in FIG. 5D is a received signal. In FIG. 5 (C1), (C2) and (D), the horizontal and vertical axes are omitted, but the horizontal axis indicates the time axis and the vertical axis indicates the signal strength. . Also, the 0 level of the signal is indicated by a one-dot broken line.

  The determination circuit that performs determination processing on the signal output from the analog matched filter corresponds to the determination circuit 46 included in the decoding processing circuit 30 illustrated in FIG. In FIG. 1, the signal output after the determination processing shown in FIG. 5 (C1) is included as a determination circuit 46 including a latch circuit for latching with the clock signal shown in FIG. 5 (C2). The latch circuit itself is not shown.

  The meaning of encoding the transmission signal corresponds to obtaining the product D × C of the transmission signal D and the encoded signal C as described above. On the other hand, receiving a code division multiplexed signal that has been encoded and transmitted and decoding the code division multiplexed signal corresponds to re-encoding the code division multiplexed signal with the same code.

The code division multiplexed signal includes a first channel encoded transmission signal (D ₁ × C ₁ ), a second channel encoded transmission signal (D ₂ × C ₂ ), and a third channel encoded transmission signal (D ₃ × C ₁ ). C ₃ ), etc., which is the sum of all encoded transmission signals to be multiplexed. Therefore, the code division multiplexed signal is represented by (D ₁ × C ₁ ) + (D ₂ × C ₂ ) + (D ₃ × C ₃ ) +. Decoding this code division multiplexed signal with code C ₁ assigned to the first channel means {(D ₁ × C ₁ ) + (D ₂ × C ₂ ) + (D ₃ × C ₃ ) + ... .} × C ₁ , that is, encoding the code division multiplexed signal with the code C ₁ .

That is, the time waveform of the signal decoded and output by the analog matched filter is {(D ₁ × C ₁ ) + (D ₂ × C ₂ ) + (D ₃ × C ₃ ) + ....} × C ₁ = (D ₁ × C ₁ ) × C ₁ + (D ₂ × C ₂ ) × C ₁ + (D ₃ × C ₃ ) × C ₁ + .... = D ₁ × C ₁ ² + (D ₂ x C ₂ x C ₁ ) + (D ₃ x C ₃ x C ₁ ) + .... Here, C ₁ ² = 1. This is because they are products of the same code, and all the chips constituting both codes have the same value, that is, “1” or “−1”. That is, when the calculation of C ₁ ² is seen for each chip of the code, 1 × 1 = 1 or (−1) × (−1) = 1 is always “1”. Therefore, the first term D ₁ × C ₁ ² representing the time waveform of the signal decoded and output by the above-mentioned analog matched filter becomes D ₁ , and the pulse D of each bit constituting the transmission signal of the first channel ₁ is played. That is, this component corresponds to the autocorrelation waveform component of the signal output after being decoded by the analog matched filter with respect to the transmission signal of the first channel.

On the other hand, the term following the second term representing the time waveform of the signal decoded and output by the analog matched filter is C ₁ × C _i ≠ 1 (where i = 2, 3,... Therefore, from the terms of (D ₂ × C ₂ ) × C ₁ and (D ₃ × C ₃ ) × C ₁ , the pulse D of each bit constituting the transmission signal of the second and third channels ₂ and D ₃ are not reproduced. That is, these components correspond to the cross-correlation waveform components of the signal output after being decoded by the analog matched filter with respect to the transmission signal of the first channel.

  As described above, according to the analog matched filter, it is possible to decode the code division multiplexed signal and reproduce the autocorrelation waveform component. In FIG. 5 (B), pulse components shown on the time axis (indicated by P and Q in FIG. 5 (B)) are autocorrelation waveform components. Further, the cross-correlation waveform component is a noise component that falls between broken lines shown above and below across the time axis. In FIG. 5 (B), since the shape of the cross-correlation waveform component is extremely complicated, the level of the maximum value and the minimum value is indicated by broken lines shown above and below the time axis, and the detailed shape is omitted. It is.

  The time waveform of the signal decoded and output by the analog matched filter shown in FIG. 5 (B) is processed by the decision circuit, and only the autocorrelation waveform component is extracted and the output signal is shown in FIG. 5 (C1). It is shown. The signal shown in FIG. 5 (C1) is latched by the clock signal shown in FIG. 5 (C2), and the received signal shown in FIG. 5 (D) is obtained.

  Next, the contents of the latch processing in the determination circuit will be described with reference to FIGS. 5 (C1), (C2), and (D). Since a known D flip-flop circuit or the like can be used as the latch circuit for performing the latch processing, description of the latch circuit itself is omitted. In this embodiment, MC100LVEL31 (manufactured by ON semiconductor) was used as the D flip-flop circuit.

  The time waveform shown in FIG. 5 (C1) is generated by processing the signal decoded and output by the analog matched filter shown in FIG. 5 (B) by a threshold processing circuit as will be described later. That is, the threshold processing circuit plays a role of converting the analog decoded signal shown in FIG. 5 (B) into a digital decoded signal shown in FIG. 5 (C1). Therefore, the time waveform shown in FIG. 5 (C1) is characterized in that a rectangular wave (rectangular pulse) appears corresponding to the autocorrelation waveform component of the decoded and output signal shown in FIG. 5 (B). . The amplitude of the rectangular pulse is defined by the threshold processing circuit, and the amplitudes of all the rectangular pulses appearing in FIG. 5 (C1) are constant. In FIG. 5 (C1), an example of this rectangular pulse is shown sandwiched between two downward arrows a and b respectively. As the threshold processing circuit, a suitable one from known comparators can be appropriately selected and used. In this example, MAX9600 (manufactured by MAXIM Integrated Products) was used.

  When the digital decoded signal shown in FIG. 5 (C1) and the clock signal shown in FIG. 5 (C2) are input to the D flip-flop circuit functioning as a latch circuit, the following processing is performed, and FIG. The received signal shown in 5 (D) is obtained.

  A rectangular pulse corresponding to the peak of the autocorrelation waveform of the digital decoded signal (for example, the moment shown by X in FIG. 5C2) is the rising edge of the clock signal shown in FIG. 5 (C2) is sandwiched between two downward arrows with a and b attached to each other.) If there is an intensity corresponding to “1” from the output terminal of the D flip-flop circuit The signal begins to be output. Then, until the next rising edge of the clock signal (the moment indicated by Y in FIG. 5 (C2)), a signal having an intensity corresponding to “1” continues to be output from the output terminal of the D flip-flop circuit. At the moment, the signal changes from the output terminal of the D flip-flop circuit to a signal having the intensity equivalent to “-1”.

  Similarly, the next time the signal having the intensity corresponding to “1” starts to be output from the output terminal of the D flip-flop circuit is the rising edge of the clock signal indicated by Z in FIG. 5 (C2). The output signal from the output terminal of the D flip-flop circuit changes to a signal having a strength corresponding to “-1” at the moment when the clock signal rises again (this moment deviates from FIG. 5 (C2)). ing.).

  As described above, when the rising signal of the clock signal is input to the D flip-flop circuit within the existence time of the rectangular pulse corresponding to the peak of the autocorrelation waveform of the digital decoded signal, it is shown in FIG. A rectangular pulse having an intensity corresponding to “1” of the received signal is generated. On the other hand, if the rising edge of the clock signal is input to the D flip-flop circuit outside the existence time of the rectangular pulse corresponding to the peak of the autocorrelation waveform of the digital decoded signal, the output terminal of the D flip-flop circuit The signal corresponding to “-1” is still output.

  As described above, the output terminal of the D flip-flop circuit is set to “1” in accordance with whether or not there is a rectangular pulse corresponding to the peak of the autocorrelation waveform of the digital decoded signal at the rising edge of the clock signal. A corresponding signal is output, or a signal corresponding to “−1” is output. As a result, the received signal is reproduced. The received signal shown in FIG. 5 (D) is a part of the transmission signal (1, -1, 1, ...) shown in FIG. 4 (A1). The part has been reproduced. In FIG. 5D, signal values “1” and “−1” are shown in parentheses in order to clearly indicate the portion corresponding to (1, −1, 1,...).

  As is clear from the above description, if there is no rectangular pulse corresponding to the peak of the autocorrelation waveform of the digital decoded signal at the moment of the rising edge of the clock signal, the reception signal shown in FIG. 5 (D) is generated. I can't. Therefore, it is necessary to adjust the relative positional relationship on the time axis between the digital decoded signal shown in FIG. 5 (C1) and the clock signal shown in FIG. 5 (C2). Adjustment of the relative positional relationship between the two will be described with reference to FIG.

  The code division multiplexed signal output from the light receiving element 18 is branched by the electric signal branching unit 26, and one of the branched signals is input to the clock signal recovery circuit 34, and the clock signal of the transmission rate frequency is recovered and output. . The clock signal having the transmission rate frequency is branched by the electric signal branching unit 36, and one of the branched signals is input to the frequency divider 38, converted into a clock signal having the base rate frequency, and output. Here, the transmission rate frequency refers to the frequency corresponding to the bit rate of the code division multiplexed signal, and the base rate frequency refers to the frequency corresponding to the bit rate of the transmission signal of each channel. That is, the frequency obtained by dividing the transmission rate frequency by the number of channels is the base rate frequency.

  The clock signal output from the frequency divider 38 is input to the delay circuit 40, and its phase is adjusted and output. The clock signal output from the delay circuit 40 is shown in FIG. 5 (C2). In other words, the position of the clock signal shown in FIG. 5C2 on the time axis can be adjusted by the delay circuit 40. This adjustment may be performed manually, but can also be performed automatically. One means for automating this adjustment is disclosed in an open patent publication (Japanese Patent Laid-Open No. 2005-33544).

  Here, the noise component included in the time waveform of the signal decoded and output by the analog matched filter shown in FIG. 5 (B) will be described. In the optical terminal device 10 of the optical access network system shown in FIG. 1, a transmission signal is output from the transmission signal processing unit 24, while a reception signal is input to the reception signal processing unit 22. Here, a code for encoding and a code for decoding are assigned to each channel. If these codes are the same, reflection noise generated in the optical connector 61 for connection to the optical coupler 66 provided in the optical transmission line is reflected in the received signal input to the received signal processing unit. The problem of mixing occurs.

  The mechanism of mixing this reflected noise into the received signal will be described with reference to FIG. FIG. 6 is a diagram for explaining how reflected light from the optical connector is mixed into the received signal processing unit. In FIG. 6, only the portions necessary for explaining the mechanism of mixing of reflected noise into the received signal are extracted from FIG. The transmission signal is output from the light emitting element 20 and input from the port 3 of the optical coupler 16, output from the port 1, and input to the optical connector 61. A part of the transmission signal is reflected from the optical connector 61 and input to the port 1 of the optical coupler 16. The reflected transmission signal is output from the port 2 of the optical coupler 16 and input to the light receiving element 18.

  Therefore, both the reception signal a input from the port 1 of the optical coupler 16 and output from the port 2 and a part b of the transmission signal reflected from the optical connector 61 are input to the light receiving element 18. A part b of this transmission signal is mainly input from port 3 of optical coupler 16 and output from port 1 is reflected from optical connector 61, input to port 1 again, and output from port 2 It has been done.

  With reference to FIGS. 7A and 7B, description will be made regarding a case where a signal in which a reception signal and a reflected transmission signal are mixed is decoded. FIGS. 7A and 7B are diagrams showing time waveforms of a signal obtained by decoding a signal in which a received signal and a reflected transmission signal are mixed. The horizontal axis indicates the direction of the time axis and the vertical axis is omitted, but the signal intensity is indicated in the direction of the vertical axis.

  FIG. 7 (A) shows the reception when the code for encoding the transmission signal with ONU and the code for encoding the transmission signal from OLT to ONU, that is, the reception signal with ONU, are equal in the same channel. The time waveform obtained by decoding the signal in which the signal and the reflected transmission signal are mixed is shown. FIG. 7 (B) shows the reception when the code for encoding the transmission signal with ONU and the code for encoding the transmission signal to ONU with OLT, that is, the code for encoding the reception signal with ONU, are different in the same channel. A time waveform obtained by decoding a signal in which a signal and a reflected transmission signal are mixed with the same code as that used to encode a received signal by OLT is shown.

In FIG. 7 (A), represents the peak of the autocorrelation waveform component of the received signal c _1, it is represented the peak of the autocorrelation waveform component of the transmission signal (reflected component optical connector) with c _2. In FIG. 7B, the peak of the autocorrelation waveform component of the received signal is represented by c, and the peak of the cross-correlation waveform component of the transmission signal (component reflected by the optical connector) is represented by r.

  7 (A) and 7 (B), the width equal to the absolute value of the peak value of the cross-correlation waveform component of the transmission signal (the component reflected by the optical connector) is shared by two broken lines parallel to the time axis. The time axis is shown.

In the same channel shown in FIG. 7 (A), when the code for encoding the transmission signal with ONU is equal to the code for encoding the transmission signal to ONU with OLT, that is, the reception signal with ONU, peak of the autocorrelation waveform of the received signal (is represented by c _1.) and (are expressed as c _2.) peak of the autocorrelation waveform of the transmission signal is overlapped, the difference between the two peak values, the net signal components This is indicated by S ₁ in FIG. 7 (A). The value of N giving the S / N ratio in this case is the peak value of the autocorrelation waveform of the transmission signal, and is indicated by N ₁ in FIG. 7 (A).

On the other hand, in the same channel shown in FIG. 7B, when the code for encoding the transmission signal with ONU is different from the code for encoding the transmission signal to ONU with OLT, that is, the reception signal with ONU. Is the difference between the peak (represented by c) value of the autocorrelation waveform and the peak (represented by r) value of the cross-correlation waveform component of the received signal, which is the net signal component. It is indicated by S ₂ in). The value of N giving the S / N ratio in this case is the peak value of the cross-correlation waveform component of the received signal, and is indicated by N ₂ in FIG. 7 (B).

As shown in FIGS. 7 (A) and (B), when the broken lines parallel to the two time axes are taken as a reference, S ₁ <S ₂ and N ₁ > N ₂ , so FIG. S / N ratio S ₂ / S when the code for encoding the transmission signal with ONU and the code for encoding the transmission signal to ONU with OLT, that is, the code for encoding the reception signal with ONU are different in the same channel shown in FIG. N ₂ is a code that encodes a transmission signal with ONU and a code that encodes a transmission signal to ONU with OLT, that is, a reception signal with ONU, in the same channel shown in FIG. S / N ratio when equal is greater than S ₁ / N ₁ . That is, (S ₂ / N ₂ )> (S ₁ / N ₁ ). Therefore, in the same channel, the S / N ratio is changed by making the code for encoding the transmission signal by ONU and the code for encoding the transmission signal to ONU by OLT, that is, the code for encoding the reception signal by ONU, different. Thus, it can be said that it is preferable to set the codes different from each other in this way.

<Analog matched filter>
In the optical access network system of the present invention, the code division multiplexed signal is decoded by the analog matched filter. Therefore, the configuration and operation of the analog matched filter will be described with reference to FIGS.

  8A and 8B are schematic block configuration diagrams of the analog matched filter. The analog matched filter outputs the output signals output from the analog shift register 140, the plus signal adder 142, the minus signal adder 144, the plus signal adder 142, and the minus signal adder 144, respectively. An analog adder 146 for adding and a low-pass filter 148 are provided. The plus signal adder 142 and the minus signal adder 144 include an amplifier 150 and an inverting amplifier 152, respectively. The amplifier 150 and the inverting amplifier 152 are shown with their peripheral circuits omitted.

  A code division multiplexed signal output from the automatic gain control element 28 is input to an input terminal indicated as data input. A clock signal having a transmission rate frequency branched by the electric signal branching device 36 is input to an input terminal indicated as clock input.

  The analog matched filter shown in FIG. 8 (A) is designed on the assumption that decoding is performed using a code given by a sequence (1, 0, 0, 1). That is, it is assumed that decoding is performed using the code assigned to the first channel of the first embodiment. The code given by the sequence (1, 0, 0, 1) can be said to be the code given by the sequence (1, -1, -1, 1) when the binary representation of "1" and "-1" is displayed. .

  Here, for simplicity, first, only the component of the first channel in the code division multiplexed signal will be taken up and described. The code division multiplexed signal also contains encoded transmission signals of channels other than the first channel, but these are encoded with a code different from the code assigned to the first channel. Does not play.

  The encoded transmission signal of the first channel shown in FIG. 4 (A2) has the same time waveform as the transmission signal of the first channel having the time waveform shown in FIG. 4 (A1) by the analog matched filter. The reproduction as a received signal will be described.

  The analog shift register 140 is a shift register (hereinafter referred to as “CCD shift register”) formed by a charge coupled device CCD (Charge Coupled Device) having four stages (shown in order from the input side as 1, 2, 3, 4). Register ")). That is, the analog shift register 140 is a 4-bit CCD shift register. In the first embodiment, since it is assumed that encoding is performed with a code having 4 chips (a code having a code length of 4), a 4-stage CCD shift register is used. Actually, since a code with a long code length such as a code with 16 or 32 chips is used, a CCD shift register with a large number of stages such as a 16 or 32 stage CCD shift register is used, but the principle described below is It is the same.

A clock signal having a transmission rate frequency is input to the clock input terminal of the CCD shift register 140. Further, a code division multiplexed signal (the encoded transmission signal shown in FIG. 4 (A2)) is input to the data input terminal of the CCD shift register 140. The input terminal of the first stage of the CCD shift register 140 shown in FIGS. 8A and 8B is indicated as D ₁ , and the output terminal is indicated as Q ₁ . In addition, the second, third, and fourth stage input terminals are denoted as D ₂ , D ₃ , and D ₄ , respectively, and the output terminals are denoted as Q ₂ , Q ₃ , and Q ₄ , respectively. Data input terminal of the CCD shift register 140 is connected to the input terminal D ₁ of the first stage.

  With reference to FIG. 8 (A), the principle of decoding the code division multiplexed signal of the first channel encoded with the code (1, -1, -1, 1) will be described.

First, the data input terminal D ₁ of the first stage CCD shift register, the code division multiplex signal, i.e., here, "1" (FIG encoded transmission signal of the first channel shown in FIG. 4 (A2) When 4 (A2) CS1 and the indicated time slot is set to 1.) is inputted in synchronization with the clock signal, the output "1" from the output terminal to Q ₁ first stage . Then, "-1" in the first channel of encoded transmission signal to the data input terminal D ₁ of the first stage (CS2 and indicated time slot of FIG. 4 (A2) is -1.) Of When input, “−1” is output from the first stage output terminal Q ₁ and “1” is output from the second stage output terminal Q ₂ in synchronization with the clock signal. Thus successively CS3 and indicated time slot, when a signal CS4 and the indicated time slot is inputted to the data input terminal D ₁ of the first stage, in synchronization with a clock signal, first from the first stage From the four-stage output terminals, the previously output signals are shifted one by one and output.

At the stage where all the chips of the encoded transmission signal existing in the time slot from CS1 to CS4 are all input from the data input terminal of the analog shift register 140, the output terminals of the first to fourth stages, Q ₁ , Q ₂ , Q _3, and Q ₄ output values (Q ₁ , Q ₂ , Q ₃ , Q ₄ ) are (1, -1, -1, 1). That is, the output values (Q ₁ , Q ₂ , Q ₃ , Q ₄ ) of the first stage to the fourth stage are the voltage values at the positions indicated by F, G, H, I in the analog shift register 140. appear.

  The voltage value at position F and the voltage value at position I are input to the plus signal adder 142, combined by the electrical signal combiner 154, and input to the amplifier 150. The voltage value at position F and the position I Is output as a signal corresponding to the sum of the voltage values of On the other hand, the voltage value at position G and the voltage value at position H are input to the negative signal adder 144, combined by the electrical signal combiner 156, and input to the inverting amplifier 152, and the voltage value of the position G And a voltage value corresponding to the sum of the voltage value at position H (a negative value) is converted into a positive voltage value and output.

  The output signal from the amplifier 150 and the output signal from the inverting amplifier 152 are combined by the analog adder 146 and input to the low-pass filter 148.

  The low-pass filter 148 filters the base rate frequency signal out of the signal output from the analog adder 146 and serves to block high frequency noise components.

The encoded transmission signal, just steps chips that are time slots from CS1 to CS4 is all inputted from the data input terminal of the analog shift register 140 Q _1, Q _2, Q from ₃ and the output terminal of Q ₄ Since the output values (Q ₁ , Q ₂ , Q ₃ , Q ₄ ) are (1, -1, -1, 1), the electric signal multiplexer 154 has a potential that is the potential at the positions F and I. 1 and the potential 1 are input, and the potential 2 is input to the amplifier 150. Further, the electric signal multiplexer 156 receives the electric potentials −1 and −1 which are the electric potentials at the G and H positions, and inputs the electric potential −2 to the inverting amplifier 152.

  Therefore, the amplifier 150 outputs a signal having a potential proportional to the potential 2 (here, the amplification factor is 1 for simplicity), and the inverting amplifier 152 inverts the potential −2 (here, the simple Therefore, the amplified signal of potential 2 is output, and both signals are combined by the analog adder 146 and passed through the low-pass filter 148 as the signal of potential 4, and the CCD shift register 140 Output from the data output terminal.

The next time the output value (Q ₁ , Q ₂ , Q ₃ , Q ₄ ) of the CCD shift register 140 becomes (1, -1, -1, 1) is the chip that exists in the time slots from CS9 to CS12 Are all input from the data input terminal of the analog shift register 140. At this time as well, a signal having a potential of 4 is output from the data output terminal of the CCD shift register 140.

When the output value (Q ₁ , Q ₂ , Q ₃ , Q ₄ ) of the CCD shift register 140 is different from (1, -1, -1, 1), the data output of the CCD shift register 140 A signal having a potential of 4 or higher is never output from the terminal, and the potential is always less than 4. This is because the output value (Q ₁ , Q ₂ , Q ₃ , Q ₄ ) of the CCD shift register 140 is different from (1, -1, -1, 1), for example, (-1, -1, 1, 1 ) Etc., it is clear if the same explanation as above is considered.

  Next, with reference to FIG. 8B, the principle of decoding the second channel code division multiplexed signal encoded with the code (1, -1, 1, -1) will be described. The difference between the analog matched filter shown in FIG. 8 (A) and the analog matched filter shown in FIG. 8 (B) is that the signal input to the amplifier 150 and the inverting amplifier 152 is in any position of F, G, H, and I. The difference between taking out from. In the analog matched filter shown in FIG. 8A, the input signal to the amplifier 150 is taken out from the positions F and I, and the input signal to the inverting amplifier 152 is taken out from the positions G and H. On the other hand, in the analog matched filter shown in FIG. 8B, the input signal to the amplifier 150 is taken out from the positions G and I, and the input signal to the inverting amplifier 152 is taken out from the positions F and H. . As described above, an arbitrary code having a code length of 4 can be set depending on whether the signal input to the amplifier 150 and the inverting amplifier 152 is extracted from F, G, H, or I.

  Code-division multiplexed signals also contain encoded transmission signals of channels other than the second channel, but these are encoded with a code different from the code assigned to the second channel. Does not play.

  The encoded transmission signal of the second channel shown in FIG. 4 (B2) has the same time waveform as the transmission signal of the second channel having the time waveform shown in FIG. 4 (B1) by the analog matched filter. The reproduction as a received signal will be described. Also in the analog matched filter shown in FIG. 8 (B), the decoding operation is basically the same as that of the analog matched filter shown in FIG. 8 (A).

First, the data input terminal D ₁ of the first stage of the CCD shift register, the code division multiplex signal, i.e., here, "1" (FIG encoded transmission signal of the second channel shown in FIG. 4 (B2) When 4 (B2) CS1 and the indicated time slot is set to 1.) is inputted in synchronization with the clock signal, the output "1" from the output terminal to Q ₁ first stage . Then, "-1" of the second channel of the encoded transmission signal to the data input terminal D ₁ of the first stage (CS2 and indicated time slot of FIG. 4 (B2) is set to -1.) Of When input, “−1” is output from the first stage output terminal Q ₁ and “1” is output from the second stage output terminal Q ₂ in synchronization with the clock signal. Thus successively CS3 and indicated time slot, when a signal CS4 and the indicated time slot is inputted to the data input terminal D ₁ of the first stage, in synchronization with a clock signal, first from the first stage From the four-stage output terminals, the previously output signals are shifted one by one and output.

At the stage where all the chips of the encoded transmission signal existing in the time slot from CS1 to CS4 are all input from the data input terminal of the analog shift register 140, the output terminals of the first to fourth stages, Q ₁ , Q ₂ , Q ₃ and Q ₄ output values (Q ₁ , Q ₂ , Q ₃ , Q ₄ ) are (−1, 1, −1, 1). That is, the output locations (Q ₁ , Q ₂ , Q ₃ , Q ₄ ) of the first to fourth stages are as voltage values at positions indicated by F, G, H, and I in the analog shift register 140. appear.

  The voltage value at position G and the voltage value at position I are input to the plus signal adder 142, combined by the electrical signal combiner 154, and input to the amplifier 150. Is output as a signal corresponding to the sum of the voltage values of On the other hand, the voltage value at position F and the voltage value at position H are input to the negative signal adder 144, combined by the electrical signal combiner 156, and input to the inverting amplifier 152, and the voltage value of the position F And a voltage value corresponding to the sum of the voltage value at position H (a negative value) is converted into a positive voltage value and output.

  The output signal from the amplifier 150 and the output signal from the inverting amplifier 152 are combined by the analog adder 146 and input to the low-pass filter 148.

The encoded transmission signal, just steps chips that are time slots from CS1 to CS4 is all inputted from the data input terminal of the analog shift register 140 Q _1, Q _2, Q from ₃ and the output terminal of Q ₄ Since the output values (Q ₁ , Q ₂ , Q ₃ , Q ₄ ) are (-1, 1, -1, 1), the electric signal multiplexer 154 has a potential that is a potential at the positions of G and I. 1 and the potential 1 are input, and the potential 2 is input to the amplifier 150. In addition, the electric signal multiplexer 156 receives the electric potentials −1 and −1 which are the electric potentials at the positions F and H, and inputs the electric potential −2 to the inverting amplifier 152.

  Therefore, a signal having a potential proportional to the potential 2 is output from the amplifier 150, and a signal having a potential 2 in which the potential -2 is inverted is output from the inverting amplifier 152. Both signals are combined by the analog adder 146. The signal having the potential 4 is output from the data output terminal of the CCD shift register 140 through the low-pass filter 148.

The next value of the output value (Q ₁ , Q ₂ , Q ₃ , Q ₄ ) of the CCD shift register 140 is (-1, 1, -1, 1) is the chip in the time slot from CS5 to CS8 Are all input from the data input terminal of the analog shift register 140. At this time as well, a signal having a potential of 4 is output from the data output terminal of the CCD shift register 140.

When the output value (Q ₁ , Q ₂ , Q ₃ , Q ₄ ) of the CCD shift register 140 is different from (-1, 1, -1, 1), the data output from the CCD shift register 140 A signal having a potential of 4 or higher is never output from the terminal, and the potential is always less than 4.

As described above, only when the output value (Q ₁ , Q ₂ , Q ₃ , Q ₄ ) of the CCD shift register 140 matches the set sign, the potential 4 from the data output terminal of the CCD shift register 140 Is output. This is a signal corresponding to the autocorrelation waveform. For example, in the time waveform of the signal obtained by decoding the encoded transmission signal of the first channel shown in FIG. 5B, the peaks indicated as P and Q are the data of the CCD shift register 140. This is a peak that appears at the moment when a signal having a potential of 4 is output from the output terminal.

<Determination circuit>
The configuration and operation of the determination circuit will be described with reference to FIGS. 9 (A) to 9 (C). FIG. 9 (A) is a schematic block configuration diagram of the determination circuit, and FIG. 9 (B) shows a time waveform of a decoded signal output from the analog matched filter. FIG. 9C shows a time waveform of a signal output after threshold determination. In FIGS. 9B and 9C, the horizontal axis indicates the time on an arbitrary scale, and the vertical axis is omitted, but the signal intensity is indicated on the vertical axis in an arbitrary scale.

  The time waveform shown in FIG. 9B corresponds to the time waveform of the signal output by decoding with the analog matched filter shown in FIG. 5B. Although FIG. 9 (B) and FIG. 5 (B) are apparently different, each figure is shown abstractly for convenience of explanation, and the time waveform of the actual signal is shown in FIG. 9 (B). close.

  The determination circuit includes a comparator 42 and a D flip-flop circuit 52. The decoded signal output from the analog matched filter shown in FIG. 9B is input from the analog data input terminal of the determination circuit to the input terminal (IN) of the comparator 42. On the other hand, a signal having a potential set as a threshold value is input from the threshold level input terminal (REF). This potential corresponds to the potential described as the threshold value in FIG.

  A signal having a potential corresponding to 1 is output from the output terminal (OUT) of the comparator 42 when the level of the signal input from the input terminal (IN) exceeds the threshold value. On the other hand, when the level of the signal input from the input terminal (IN) is below the threshold, a signal having a potential corresponding to 0 is output. Therefore, the time waveform of the signal output from the output terminal (OUT) of the comparator 42 is the time waveform shown in FIG. 9C. The time waveform shown in FIG. 9C corresponds to the time waveform shown in FIG. 5C1 described above.

  A signal having a time waveform shown in FIG. 9C is input to the input terminal (D) of the D flip-flop circuit 52. On the other hand, a clock signal is input to the clock signal input terminal (CLK) of the D flip-flop circuit 52. The clock signal input to the clock signal input terminal (CLK) is the clock signal shown in FIG. 5 (C2). That is, a threshold value input to the input terminal (D) is determined by this clock signal, and the output signal is latched. Since the principle of the latch operation has already been described, it will not be repeated here.

In FIG. 5 (C1), the widths of the rectangular pulses are shown to be equal, but actually the widths of the rectangular pulses are not equal as in the time waveform shown in FIG. 9 (C). However, since it is sufficient that the rising edge of the clock signal is included in the range of the width of the rectangular pulse, the widths of the rectangular pulses are not necessarily equal. However, the delay circuit 40 is set so that the rising edge of the clock signal input to the clock signal input terminal (CLK) falls within the rectangular pulse width (W ₁ and W ₂ ) shown in FIG. 9C. Therefore, it is necessary to adjust the position of the clock signal on the time axis.

<Third embodiment>
With reference to FIG. 10, the configuration and operation of the optical access network system of the third embodiment will be described. FIG. 10 is a schematic block diagram of an optical access network system according to the third embodiment. In the third embodiment, it is assumed that the number of subscribers is 16. The difference from the optical access network systems of the first and second embodiments is that the optical access network systems of the first and second embodiments have only one type of wavelength of signals used for transmission and reception, The optical access network system of the third embodiment is a so-called WDM system that uses four types of wavelengths as signal wavelengths.

Therefore, the configuration of the portion where the signal wavelength is λ ₁ is transmitted and received is the same as that of the optical access network system of the first embodiment. By using four types of wavelengths from λ ₁ to λ ₄ as signal wavelengths, the number of users has been expanded to 4 times the system (ONU-1 to ONU-16). That is, by utilizing the wavelength lambda ₁ in ONU-4 from ONU-1, using a wavelength lambda ₂ in ONU-8 from ONU-5, using the wavelength lambda ₃ in ONU-12 from ONU-9, ONU-13 to use the wavelength λ ₄ in the ONU-16 from. The codes assigned to ONU-1, 5, 9 and 13 can be made common. Similarly, codes assigned to ONU-2, 6, 10 and 14, codes assigned to ONU-3, 7, 11 and 15, and codes assigned to ONU-4, 8, 12 and 16 can be made common. . Further, the first channel (ch1) to the 16th channel (ch16) are associated with ONU-1 to ONU-16, respectively. Of course, the above-described code assignment and channel assignment to the ONU are merely examples, and are not limited thereto.

By increasing the number of wavelengths used as signal wavelengths from λ ₁ to λ ₄ , the optical couplers 66-1 and 66-4 corresponding to the optical coupler 66 in the optical access network system shown in FIG. Between the optical couplers 124-1 and 124-4 corresponding to the optical coupler 124, wavelength selective multiplexers / demultiplexers 50 and 54 having wavelength selectivity are required.

  As the wavelength selective multiplexer / demultiplexer having wavelength selectivity, for example, a WDM multiplexer / demultiplexer can be used. Further, it is also possible to use an apparatus in which an optical filter having a different transmission wavelength is installed in each port for outputting the optical coupler / branch having no wavelength selectivity and the branched light of the optical coupler / branch.

Also in the optical access network system of the third embodiment, the configuration of each part that transmits and receives signals with wavelengths λ ₁ to λ ₄ is the same as that of the optical access network system of the first embodiment. Even if the reflected light component from the optical connector used to connect the device to the ONU is mixed into the ONU received signal, the S / N ratio of the received signal does not decrease. it is obvious.

1 is a schematic block configuration diagram of an optical access network system. FIG. 1 is a schematic block configuration diagram of an optical terminal device according to a first embodiment. FIG. FIG. 6 is a schematic block configuration diagram of an optical terminal device according to a second embodiment. It is a figure where it uses for description of the process in which a transmission signal is encoded. It is a figure where it uses for description of the process in which a received signal is decoded. It is a figure where it uses for description of a mode that the reflected light from an optical connector mixes in a received signal processing part. It is a figure which shows about the relationship of the peak position of an autocorrelation waveform at the time of decoding the signal in which the received signal and the reflected transmission signal were mixed. It is a schematic block diagram of an analog matched filter. It is a figure used for description of the schematic block block diagram of a determination circuit, and its operation principle. FIG. 6 is a schematic block configuration diagram of an optical access network system according to a third embodiment.

Explanation of symbols

10: Optical terminal unit (ONU)
12, 102: Light processing section
14, 14-1, 14-2, 14-3, 14-4, 104: Electric processing section
16, 16-1, 16-2, 16-3, 16-4, 124, 124-1, 124-4: Optical coupler
18, 126: Light receiving element
20, 122: Light emitting element
22, 22-1, 22-2, 22-3, 22-4, 108: Received signal processor
24, 24-1, 24-2, 24-3, 24-4, 106: Transmission signal processor
26, 36, 130: Electric signal splitter
28, 128: Automatic gain control element
30: Decryption processing circuit
34: Clock signal recovery circuit
38: Divider
40: Delay circuit
42: Comparator
44: Analog matched filter
46: Judgment circuit
50, 54: Wavelength selective multiplexer / demultiplexer
52: D flip-flop circuit
56: Encoding processing circuit
60, 120: Driver (Amplifier)
61, 62, 63, 64, 68, 72, optical connector
66, 66-1, 66-4: Optical coupler
70: Optical fiber transmission line
74, 76: Branch optical fiber transmission line
100: Optical line terminator (OLT)
110: Clock signal generation circuit
116: Encoding circuit sequence
118, 154, 156: Electric signal multiplexer
132: Decoding processing circuit array
140: Analog shift register
142: Adder for positive signal
144: Adder for negative signal
146: Analog adder
148: Low-pass filter
150: Amplifier
152: Inverting amplifier

Claims (1)

An optical coupler is provided at one end of the optical fiber transmission line.
An optical line terminator coupled to the other end of the optical fiber transmission line and N branch optical fiber transmission lines formed by N branching (N is a natural number of 2 or more) formed by the optical coupler. Each comprising an optical terminal device coupled to each other,
The Nth channel is assigned in order from the first channel to the N optical terminal devices,
An optical access network system that performs bidirectional optical communication by code division multiplexing between the optical line termination device and the N optical terminal devices,
Each of the N optical terminal devices and the optical line terminating device each encodes a signal to be transmitted to generate and output a coded transmission signal, and a coded signal transmitted. A reception signal processing unit that receives a reception signal, decodes the encoded reception signal, extracts the reception signal, and outputs the received signal;
The k-th channel that encodes and outputs an uplink signal, which is a signal for transmission from the optical terminal device of the k-th channel (k is a natural number from 1 to N) to the optical line terminal device. a code to be set in the transmission signal processing unit optical terminal equipment comprises a signal the downlink signal and outputs the coding is for performing transmitted from the optical line terminal to the optical terminal of the k-th channel The codes set in the transmission signal processing unit provided in the optical line termination device are different from each other,
The p-th channel that encodes and outputs an upstream signal, which is a signal for transmission from the optical terminal device of the p-th channel (p is a natural number from 1 to N) to the optical line terminating device. A code set in the transmission signal processing unit provided in the optical terminal device;
The optical line terminator that encodes and outputs a downstream signal that is a signal for transmission from the optical line terminator to an optical terminal device of the q-th channel (q is a natural number from 1 to N). The optical access network system, wherein the codes set in the transmission signal processing unit are equal to each other .
Here, the pairs (p, q) of natural numbers p and q are (1, 2), (2, 3), (3, 4), ... (p, p + 1), ... (N- 1, N) and N pairs of (N, 1).