JP2014120788A - Communication system and control method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To allow each ONU to evenly enjoy a throughput extension effect of the overall system by the application of adaptive modulation in a communication system having a fixed distance between an OLT and an ONU.SOLUTION: The number of effective information bits is determined according to fit to the distance between the OLT and the ONU, and an optical signal is transmitted in an allocated transmission permission time, to thereby actualize evenness of a transmittable band in each ONU.

Description

  Due to the increasing needs for high-speed access services, FTTH (Fiber To The Home) is spreading worldwide. Most of the FTTH service accommodates a plurality of subscriber side devices (ONU: Optical Network Unit) by time division multiplexing (TDM) with one accommodation station side device (OLT: Optical Line Terminal), It is provided by a PON (Passive Optical Network) system which is excellent in economy.

  In the PON system, it is required that all ONUs having different distances from the OLT communicate with each other with a certain quality or more. In optical fiber transmission, signal quality deteriorates due to waveform distortion caused by attenuation of optical signal intensity, chromatic dispersion, etc. as the transmission distance increases. Therefore, when the signal speed of optical signals transmitted and received by all ONUs in the PON system is equal, OLT All ONUs can communicate if the ONU with the longest distance from can communicate.

が成り立つが、ＧＥ−ＰＯＮ（非特許文献１参照。）、Ｇ−ＰＯＮ（非特許文献２参照。）等の関連するＴＤＭ−ＰＯＮではｍ＝２であり、信号速度とシンボル速度は等しい。 Therefore, the signal speed in the related TDM-PON is fixed to a value at which the farthest ONU can communicate, and the number of information bits that each ONU can transmit / receive per unit time is equal. The structural example of TDM-PON relevant to FIG. 1 is shown. ONU _k (k = 1, 2,..., K and K are integers of 2 or more) having different distances from the OLT are connected to a single OLT. The signal speed of the optical signal transmitted and received by each ONU is fixed to the signal speed B [bit / s] at which the ONU having the longest distance from the OLT can communicate. S and m in the figure represent the symbol rate and the modulation level, respectively.

However, in the related TDM-PON such as GE-PON (see Non-Patent Document 1) and G-PON (see Non-Patent Document 2), m = 2, and the signal rate and the symbol rate are equal.

  In uplink communication of TDM-PON, each ONU transmits an optical signal intermittently only during the transmission allowable time allocated to each ONU based on the dynamic bandwidth allocation calculation in the OLT. Is preventing. In the related TDM-PON, since the number of information bits that each ONU can transmit / receive per unit time is equal as described above, the transmission allowance is made so that the total time allowed to transmit the optical signal within a certain period is equal between the ONUs. By allocating time, the amount of information transmitted by each ONU within the period becomes equal, and bandwidth fairness between the ONUs is realized.

IEEE Standard 802.3ah ITU-T Recommendation G. 984.2

  In the related technology, in TDM-PON, the signal speed is fixed to a value at which the ONU farthest from the OLT can communicate, but the ONU near the distance from the OLT is originally a higher signal speed. Communication with is possible. On the other hand, in mobile communication, a 3.5G system (HSPA: High Speed) using an adaptive modulation technique that varies the signal speed for each terminal according to the distance to the radio base station, the radio propagation environment, and the like that change from moment to moment. Packet Access) has been put into practical use. In this method, the number of information bits that can be transmitted / received per unit time is improved by improving the modulation multi-level number and symbol rate of signals transmitted / received by terminals that are close to the radio base station or terminals that have a good radio propagation environment. By increasing the throughput, the throughput of the entire system is expanded.

  By applying adaptive modulation technology to TDM-PON and increasing the speed of optical signals transmitted and received by ONUs that are close to the OLT, the throughput of the entire system can be expanded in the same manner as mobile communication. However, in mobile communication, the distance between each terminal and the radio base station and the radio propagation environment change due to the movement of the terminal, so that all terminals can enjoy an increase in signal speed by adaptive modulation. Since the distance between each ONU and the OLT is fixed in the PON, only a specific ONU having a short distance from the OLT can enjoy an increase in signal speed by adaptive modulation.

  Therefore, according to the related bandwidth fairing method for uplink communication in which the transmission allowable time is allocated so that the total time allowed to transmit the optical signal within the fixed period is equal between the ONUs, the fixed period is determined according to the distance from the OLT. There is a problem that unfairness occurs in the amount of information that can be sent within.

  It is an object of the present invention to determine the number of effective information bits in accordance with the distance between the OLT and each ONU, and to equalize the bandwidth that can be transmitted by each ONU by transmitting an optical signal within the allocated allowable transmission time. And

  In order to achieve the above object, the present invention determines the number of effective information bits that can be transmitted per unit time, and the transmission allowable time to each ONU so as to be equal to the ratio of the reciprocal of the number of effective information bits of each ONU. Assign. As a result, the ONU can transmit the amount of information fairly.

Specifically, the communication system of the present invention is a communication system in which a parent node and a child node are connected,
The parent node determines the number of valid information bits that each child node can transmit per unit time according to the distance to the child node, and is set to be equal to the ratio of the reciprocal of the number of valid information bits of each child node. It has an allocation function that allocates the allowable transmission time to child nodes,
The child node has a transmission function of transmitting an optical signal having the number of effective information bits determined by the allocation function to the parent node during the transmission allowable time allocated by the allocation function.

Specifically, the control method of the communication system of the present invention is:
A control method of a communication system in which a parent node and a plurality of child nodes are connected,
The number of valid information bits that each child node can transmit per unit time is determined according to the distance from the parent node to the child node, and each child node is set to be equal to the ratio of the reciprocal number of the valid information bits of each child node. An allocation procedure to allocate time allowed for transmission to the node;
A transmission procedure is performed in which an optical signal having the number of effective information bits determined by the allocation procedure is transmitted from the child node to the parent node during the transmission allowable time allocated by the allocation procedure.

Specifically, the control device of the communication system
A control device of a communication system in which a parent node and a plurality of child nodes are connected,
The parent node determines the number of valid information bits that each child node can transmit per unit time according to the distance to the child node, and is set to be equal to the ratio of the reciprocal of the number of valid information bits of each child node. An allocation function for allocating the allowable transmission time to the child node is provided.

Specifically, the control program for the communication system is:
A communication system control program in which a parent node and a plurality of child nodes are connected,
The number of valid information bits that each child node can transmit per unit time is determined according to the distance from the parent node to the child node, and each child node is set to be equal to the ratio of the reciprocal number of the valid information bits of each child node. An allocation procedure to allocate time allowed for transmission to the node;
Causing the computer to execute a transmission procedure for transmitting the optical signal having the number of effective information bits determined by the allocation procedure from the child node to the parent node during the transmission allowable time allocated by the allocation procedure.

Specifically, the computer-readable recording medium is
Records the control program of the communication system.

In the communication system of the present invention,
The optical signal is an optical modulation signal,
The allocation function determines the number of effective information bits by determining the modulation level of the optical signal of each child node,
The transmission function may transmit an optical signal modulated with a modulation multi-level number determined by the allocation function to a parent node.

In the communication system of the present invention,
The allocation function determines the number of valid information bits by determining a symbol rate of an optical signal of each child node;
The transmission function may transmit an optical signal having a symbol rate determined by the allocation function to a parent node.

In the communication system of the present invention,
The optical signal is an optical encoded signal,
The allocation function determines the number of effective information bits by determining an encoding method of an optical signal of each child node;
The transmission function may transmit an optical signal of an encoding method determined by the allocation function to a parent node.

In the communication system of the present invention,
The optical signal is an optical OFDM signal;
The allocation function determines the number of effective information bits by determining the number of subcarriers of the optical signal of each child node;
The transmission function may transmit the number of subcarriers determined by the allocation function to the parent node.

  The above inventions can be combined as much as possible.

  According to the present invention, the number of effective information bits is determined in accordance with the distance between the OLT and each ONU, and an optical signal is transmitted within the allocated allowable transmission time, thereby realizing fairness of the bandwidth that can be transmitted by each ONU. can do.

It is a figure explaining the structure of TDM-PON relevant to this invention. It is a figure explaining the structure of the TDMA system which concerns on Embodiment 1. FIG. It is a figure explaining the concept of the dynamic band allocation which concerns on Embodiment 1. FIG. It is a figure explaining the structure of the TDMA system which concerns on Embodiment 2. FIG. It is a figure explaining the concept of the dynamic band allocation which concerns on Embodiment 2. FIG. It is a figure explaining the structure of the OFDMA system which concerns on Embodiment 5. FIG. It is a figure explaining the structure of the OFDMA system which concerns on Embodiment 6. FIG.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

  The communication system according to the present embodiment is a one-to-many connection communication system in which a parent node and a plurality of child nodes are connected, and includes a TDMA, an OFDMA optical system, and a fixed wireless system. The operation method of the communication system according to the present embodiment has an assignment procedure and a transmission procedure in order. The parent node has an allocation function for executing the allocation procedure, and each child node has a transmission function for executing the transmission procedure. The assignment function and the transmission function may be realized by causing a computer to execute a program. The program may be recorded on a computer-readable recording medium.

  The allocation function determines the number of effective information bits that each child node can transmit per unit time according to the distance from the parent node to the child node, and is equal to the ratio of the reciprocal number of the effective information bits of each child node. Is assigned a permissible transmission time to each child node. In addition, the transmission function transmits an optical signal having the number of effective information bits determined by the allocation function to the parent node during the transmission allowable time allocated by the allocation function. An optical signal is a signal transmitted and received between a parent node and a child node, and includes an optical modulation signal, an optical encoding signal, and an optical OFDM signal.

(Embodiment 1)
In this embodiment, the optical signal is an optical modulation signal, the allocation function determines the number of effective information bits by determining the modulation level of the optical signal of each child node, and the transmission function modulates the modulation determined by the allocation function. An optical signal modulated with a multi-valued number is transmitted to the parent node.
FIG. 2 shows a configuration example of a time division multiple access (TDMA) system which is an optical communication system according to the first embodiment. Via a fiber optic transmission line and optical multiplexing / demultiplexing means, ONU _{k as} a child node (k = 1, 2,..., K, K is an integer of 2 or more) is connected to a single OLT as a parent node TDM-PON.

  Examples of the optical multiplexing / demultiplexing means include an optical fiber and an optical coupler created by a PLC (Planar Lightwave Circuit). In FIG. 2, one optical multiplexing / demultiplexing means is connected between each ONU and OLT, but two or more optical multiplexing / demultiplexing means may be cascaded.

The ONU _k intermittently transmits the symbol rate S [Symbol / s] and the modulation multi-value number m _k only during the transmission allowable time allocated to itself based on the dynamic bandwidth allocation calculation in the OLT. While S is equal for all ONUs regardless of the value of _k , m _k is adaptively set to the maximum value at which ONU _k can communicate with the OLT with a quality of a certain level or higher. At this time, the number of information bits B _k that can be transmitted per unit time by the ONU _k is expressed by Equation (2).

Since the modulation multi-level number m _k depends on the distance between the OLT and the ONU and the characteristics of the optical fiber transmission line, the monitoring signal is periodically transmitted and received between the OLT and the ONU and the loss between the OLT and the ONU. Or by measuring the amount of chromatic dispersion and calculating the reception characteristics with respect to the modulation multi-level number. When the distance between the OLT and the ONU and the characteristics of the optical fiber transmission line do not change greatly in time, the values determined at the time of initial registration of the ONU can be fixedly used.

  When the intensity of the optical signal is constant, the distance between adjacent symbols on the optical electric field plane becomes narrower as the modulation multi-level number increases. For this reason, in order to obtain a certain level of quality (= distance between adjacent symbols) when receiving an optical signal having a large modulation multilevel number, it is required that the intensity of the optical signal input to the receiver is large. In addition, the larger the modulation multi-level number, the weaker the tolerance to waveform distortion caused by chromatic dispersion or the like. Therefore, in order to obtain a certain quality or more when receiving an optical signal, the waveform distortion is required to be small. .

Here, in optical fiber transmission, the shorter the transmission distance, the smaller the attenuation of the intensity of the optical signal and the smaller the waveform distortion caused by wavelength dispersion or the like. Therefore, in an ONU that is close to the OLT, there is a possibility that the modulation bit number m _k can be set to a large value and the number of information bits B _k that can be transmitted per unit time can be increased.

  Therefore, for example, a modulation format such as binary phase modulation (BPSK: Binary Phase-Shift-Keying), quaternary phase modulation (QPSK: Quadrature Phase-Shift-Keying), 8-level phase modulation (8PSK),. A different optical signal is time-division multiplexed for each ONU that is a transmission source and input to the OLT.

The dynamic bandwidth allocation calculation, the ONU _k is the total time T _k which is allowed to transmit the optical signal within a predetermined period, so as to satisfy the equation (3), the ratio of T _k is 1 symbol between ONU The transmission allowable time is assigned so as to be equal to the ratio of the reciprocal of the number of information bits log ₂ m _k that can be transmitted by

The number of times that ONU _k can be assigned the allowable transmission time within a predetermined period is N _k times (N _k is an integer equal to or greater than 1), and the nth transmission time (n = 1, 2,..., N _k ) FIG. 3 shows a conceptual diagram of dynamic bandwidth allocation when the height is t _k (n). The sum for each ONU of the rectangular area with the number of information bits log ₂ m _k that can be transmitted in one symbol and the length t _k (n) of the allowable transmission time is equal between the ONUs.

From Expressions (2) and (3), since the amount of information transmitted by each ONU represented by the product of T _k and B _k within the period is equal, the bandwidth fairness between the ONUs by the dynamic bandwidth allocation described above. Can be realized. Furthermore, since the modulation level of the optical signal is maximized for each ONU within a range in which communication with a certain quality or more is possible, the ONU having the longest distance from the OLT is the same as the related TDM-PON. Compared with the case where the signal speed is fixed to the value B _min at which communication is possible, the throughput of the entire system can be expanded, and the equation (4) is established.

  That is, according to the present embodiment, in the TDMA system in which the distance between each child node and the parent node is fixed, the effect of increasing the throughput of the entire system by applying adaptive modulation can be enjoyed fairly between all child nodes. In a WDM / TDMA system capable of extending the system band without increasing the signal speed per wavelength by applying wavelength division multiplexing (WDM), the adaptive setting of the same modulation multi-level number and By the dynamic bandwidth allocation calculation, the effect of increasing the throughput of the entire system can be enjoyed fairly among all the child nodes sharing the bandwidth.

(Embodiment 2)
In this embodiment, the allocation function determines the number of valid information bits by determining the symbol rate of the optical signal of each child node, and the transmission function transmits the optical signal of the symbol rate determined by the allocation function to the parent node. .

FIG. 4 shows a configuration example of a TDMA system that is an optical communication system in the second embodiment. The difference from the TDM-PON (FIG. 2) which is the configuration example of the first embodiment is that the child node ONU _k (k = 1, 2,..., K, K is an integer of 2 or more) is the parent. For an optical signal that is intermittently transmitted only during the transmission allowable time allocated to itself based on the dynamic bandwidth allocation calculation in the node OLT, the modulation multilevel number m is equal in all ONUs regardless of the value of k, The symbol rate S _k [Symbol / s] is that the ONU _k is adaptively set to the maximum value that allows communication with the OLT that is the parent node with a certain quality or higher. The number of information bits B _k that ONU _k can transmit per unit time is expressed by equation (5).

Since the symbol rate S _k depends on the distance between the OLT and the ONU and the characteristics of the optical fiber transmission line, the monitoring signal is periodically transmitted and received between the OLT and the ONU, and the loss or wavelength between the OLT and the ONU. It is determined by measuring the amount of dispersion and calculating the reception characteristic with respect to the symbol rate. When the distance between the OLT and the ONU and the characteristics of the optical fiber transmission line do not change greatly in time, the values determined at the time of initial registration of the ONU can be fixedly used.

  The higher the symbol rate of the optical signal, the wider the signal band and the influence of high frequency noise components. Therefore, when receiving an optical signal, it is input to the receiver in order to obtain a certain quality (= signal to noise ratio). The intensity of the optical signal is required to be high. Further, since the higher the symbol speed, the weaker the tolerance to waveform distortion caused by chromatic dispersion or the like, the waveform distortion is required to be small in order to obtain a certain quality or higher when receiving an optical signal. Here, in optical fiber transmission, the shorter the transmission distance, the smaller the attenuation of the intensity of the optical signal and the smaller the waveform distortion caused by wavelength dispersion or the like.

Therefore, in an ONU that is close to the OLT, there is a possibility that the symbol rate S _k can be set to a fast value and the number of information bits B _k that can be transmitted per unit time can be increased. Therefore, for example, different optical signals are time-division multiplexed and input to the OLT for each ONU having a symbol rate such as 1.25 G Symbol / s, 2.5 G Symbol / s, 10 G Symbol / s,... .

The dynamic bandwidth allocation calculation, the ONU _k is the total time T _k which is allowed to transmit the optical signal within a predetermined period, so as to satisfy the equation (6), T _k ratio unit time between ONU allocating transmission allowable time to be equal to the ratio of the inverse of the symbol number S _k that can be transmitted per.

The number of times that ONU _k can be assigned the allowable transmission time within a predetermined period is N _k times (N _k is an integer equal to or greater than 1), and the nth transmission time (n = 1, 2,..., N _k ) FIG. 5 shows a conceptual diagram of dynamic bandwidth allocation when the height is t _k (n). The sum for each ONU of the rectangular area having the number of symbols S _k that can be transmitted per unit time and the length t _k (n) of the allowable transmission time is equal between the ONUs.

From Expressions (5) and (6), since the amount of information transmitted by each ONU represented by the product of T _k and B _k within the period is equal, the bandwidth fairness between the ONUs by the dynamic bandwidth allocation described above. Can be realized. Furthermore, since the symbol rate of the optical signal is maximized for each ONU within a range where communication with a certain quality or more is possible, the ONU having the longest distance from the OLT communicates in the same manner as the related TDM-PON. Compared with the case where the signal speed is fixed to the possible value _Bmin , the throughput of the entire system can be expanded, and the expression (4) in the first embodiment is established.

  That is, according to the present embodiment, in the TDMA system in which the distance between each child node and the parent node is fixed, the effect of increasing the throughput of the entire system by applying adaptive modulation can be enjoyed fairly among all the child nodes. Even in a WDM / TDMA system that can expand the system bandwidth without increasing the signal rate per wavelength by applying WDM, the throughput of the entire system can be obtained by adaptive setting of the symbol rate and dynamic bandwidth allocation calculation. The effect of expansion can be enjoyed fairly among all child nodes sharing the bandwidth.

(Embodiment 3)
In this embodiment, the optical signal is an optical encoded signal, the allocation function determines the number of effective information bits by determining the encoding method of the optical signal of each child node, and the transmission function is determined by the allocation function. An encoding type optical signal is transmitted to the parent node.

In the TDMA system, which is an optical communication system according to the third embodiment, ONU _k that is a child node (k = 1, 2,..., K, K is an integer of 2 or more) is dynamic in an OLT that is a parent node. For an optical signal that is intermittently transmitted only during the transmission allowable time allocated to itself based on the band allocation calculation, the modulation multi-value number m and the symbol rate S [Symbol / s] are fixed regardless of the value of k, While the number of information bits B (= S × log ₂ m) that can be transmitted per unit time is the same for all ONUs, the encoding method in PCS (Physical Coding Sublayer) can be selected adaptively for each ONU. The ratio R _{k of the} number of bits before and after encoding differs depending on the encoding method.

As encoding in PCS, Ethernet (registered trademark) performs data conversion of 8B / 10B, 64B / 66B, etc. in order to prevent the same logic from continuing. 8B / 10B is a method of converting 8-bit information into 10 bits, and the number of bits is increased by 25% compared to actual data (R _k = 1.25). On the other hand, since 64B / 66B converts 64-bit information into 66 bits, the number of bits to be increased is about 3% of actual data.

  In the PCS, forward error correction (FEC) is also performed. FEC is a scheme that enables data to be detected and corrected on the receiving side by adding redundancy to the data on the transmitting side. Due to the FEC coding gain, it is possible to communicate with a quality of a certain level or higher even at a signal speed where it is impossible to communicate with a quality of a certain level or higher when FEC is not used.

  For FEC, a Reed-Solomon code, a turbo code, or the like is used. A code with a higher error correction strength and a larger coding gain has a higher rate of increase in the number of bits due to the coding. Therefore, different optical signals are time-division multiplexed and input to the OLT for each ONU whose PCS encoding method, such as 8B / 10B, 64B / 66B, etc., data conversion method, FEC application status, FEC code type, etc. Is done.

The dynamic bandwidth allocation calculation, when the total time ONU _k is allowed to transmit the optical signal to T _k in a predetermined period, so as to satisfy the equation (7), the encoding ratio of T _k between ONU allocating transmission allowable time to be equal to the ratio of the specific R _k of the number of bits before and after.

Since the number of information bits B that can be transmitted per unit time is the same for all ONUs, the amount of information B × T _{k that} is transmitted within the period is proportional to R _k, and equation (8) holds.

  Here, each term of Equation (8) represents the effective information amount obtained by subtracting the increment due to encoding from the information amount transmitted by each ONU within the period. It can be seen that sex is realized. Since the encoding method in the PCS can be selected for each ONU, a plurality of ONUs whose data conversion methods such as 8B / 10B and 64B / 66B are not unified can be accommodated in the same system while maintaining bandwidth fairness.

Also, the presence or absence of FEC applied to each ONU within it is possible to communicate at a certain level or higher quality, since it is possible to select the type of FEC code, and the minimum required ratio R _k of the number of bits before and after the coding The signal rate can be increased with redundancy, and the error correction strength is such that the ONU with the longest distance from the OLT can communicate as in the related TDMA system, and the ratio of the number of bits before and after encoding is high. Compared to the case where the FEC code of R _max is applied to all ONUs, the throughput of the entire system can be expanded, and the formula (9) is established.

  That is, according to the present embodiment, in the TDMA system in which the distance between each child node and the parent node is fixed, the effect of expanding the throughput of the entire system by adaptively selecting the encoding method in the PCS is not necessarily the data conversion method in the PCS. Can be enjoyed fairly among all child nodes that are not unified.

  Even in a WDM / TDMA system that can expand the system bandwidth without increasing the signal speed per wavelength by applying WDM, the adaptive selection of the same encoding method and the dynamic bandwidth allocation calculation can be used to The effect of increasing the throughput can be enjoyed fairly among all child nodes sharing the bandwidth and not necessarily having a unified data conversion method in the PCS.

(Embodiment 4)
The TDMA system, which is an optical communication system according to the fourth embodiment, is dynamic in an OLT in which ONU _k (k = 1, 2,..., K, K is an integer of 2 or more) as a child node is a parent node. At least two of the modulation multi-level number m _k , the symbol rate S _k [Symbol / s], and the encoding scheme for an optical signal that is transmitted intermittently only within the transmission allowable time allocated to itself based on the band allocation calculation. Can be set / selected adaptively for each ONU.

When setting m _k and S _k values, whether or not FEC is applied, and selecting the type of FEC code, ONU _k is the combination of ONU _k that can communicate with the OLT that is the parent node with a certain quality or higher. The number of effective information bits B ′ _{k obtained} by subtracting the increment due to encoding from the number of information bits that can be transmitted per unit time is set / selected so as to be maximized. If the ratio of the number of bits before and after encoding is R _k , B ′ _k is expressed by equation (10).

Therefore, a different optical signal is time-division multiplexed for each ONU from which at least two of the modulation multi-level number m _k , the symbol rate S _k [Symbol / s], and the encoding method are transmitted, and are input to the OLT.

The dynamic bandwidth allocation calculation, the total time ONU _k is allowed to transmit the optical signal when the T _k in a predetermined period, so as to satisfy the equation (11), T _k ratio unit time between ONU allocating transmission allowable time to be equal to the ratio of the reciprocal of the effective number of information bits B _'k that can be transmitted per.

Here, each term of the expression (11) represents an effective information amount obtained by subtracting the increment due to encoding from the information amount transmitted by each ONU within the period. It can be seen that sex is realized. Furthermore, since the number of valid information bits that each ONU can transmit per unit time is maximized for each ONU within a range in which communication with a certain quality or more is possible, the OLT is similar to the related TDMA system. Compared to the case where the number of valid information bits that can be transmitted per unit time is fixed to the value B ′ _min that enables communication of the ONU with the longest distance, the throughput of the entire system can be expanded, and Expression (12) is established.

  That is, according to the present embodiment, in the TDMA system in which the distance between each child node and the parent node is fixed, the effect of expanding the throughput of the entire system due to the application of adaptive modulation is not necessarily unified in the data conversion method in the PCS. Can be enjoyed fairly among other child nodes.

  Even in a WDM / TDMA system that can expand the system band without increasing the signal speed per wavelength by applying WDM, the same symbol rate, adaptive setting of the modulation multi-level number, and adaptive encoding scheme By the selection and the dynamic bandwidth allocation calculation, the effect of expanding the throughput of the entire system can be enjoyed fairly among all the child nodes sharing the bandwidth and not necessarily having a unified data conversion method in the PCS.

(Embodiment 5)
In this embodiment, the optical signal is an optical OFDM signal, the allocation function determines the number of effective information bits by determining the number of subcarriers and the modulation multi-level number of the optical signal of each child node, and the transmission function is allocated. The number of subcarriers whose functions are determined and the optical signal having the modulation multi-level number are transmitted to the parent node.

FIG. 6 shows a configuration example of an Orthogonal Frequency Multiple Access (OFDMA) system that is an optical communication system according to the fifth embodiment. Via a fiber optic transmission line and optical multiplexing / demultiplexing means, ONU _{k as} a child node (k = 1, 2,..., K, K is an integer of 2 or more) is connected to a single OLT as a parent node OFDM-PON. Examples of the optical multiplexing / demultiplexing means include an optical fiber and an optical coupler created by PLC. In FIG. 6, one optical multiplexing / demultiplexing means is connected between each ONU and OLT, but two or more optical multiplexing / demultiplexing means may be cascaded.

  In OFDM transmission, digital signal processing (DSP: Digital Signal Processing) and digital / analog (DFP) including serial-parallel conversion, symbol mapping, inverse fast Fourier transform (IFFT), parallel-serial conversion, and the like are performed on the transmission side. / A) Outputs an optical OFDM signal obtained by intensity modulation or optical I / Q modulation of the optical carrier wave using the OFDM signal generated by the conversion.

  On the receiving side, the OFDM signal obtained by direct detection or optical I / Q detection of the optical OFDM signal is subjected to analog / digital (A / D) conversion, serial parallel conversion, and fast Fourier transform (FFT). The data is demodulated by a DSP comprising symbol demapping, parallel serial conversion, and the like. Since multi-carrier transmission using a plurality of subcarriers and a low symbol rate, it has a feature that it is highly resistant to transmission distortions such as chromatic dispersion and polarization mode dispersion, and the transmission distance can be extended.

The ONU _k intermittently transmits an optical OFDM signal in which only the subcarriers allocated at the same time are modulated only during the transmission allowable time allocated to itself based on the dynamic band allocation calculation in the OLT.

The subcarrier symbol rate S [Symbol / s] is the same for all ONUs regardless of the value of _k , while at least one of the modulation multi-level number m _k and the coding scheme can be set / selected adaptively for each ONU. is there. setting the value of m _k, the presence or absence of FEC applied at the time of selection of the type of FEC code, in combination data from ONU _k can be demodulated at a certain level or higher quality in OLT which is the parent node, the ONU _k The number of effective information bits b ′ _{k obtained} by excluding the increment due to encoding from the number of information bits that can be transmitted per unit time using one subcarrier is set / selected.

Of the J subcarriers whose frequencies are f _{1 to} f _J (J is an integer of 2 or more), the number of subcarriers assigned to ONU _k at time t is _denoted by J _k (t), and ONU _k When the ratio of the number of bits before and after encoding is R _k , the effective information bit number B ′ _k (t) that ONU _k can transmit per unit time at each time is expressed by Expression (13).

Modulation level _{m k} and the presence or absence of FEC application, the type of FEC code is dependent on the characteristics of the distance and the optical fiber transmission line between the OLT and the ONU, periodic monitoring signals between the OLT and the ONU Is determined by measuring the loss and chromatic dispersion amount between the OLT and the ONU, and calculating the reception characteristics with respect to the modulation multi-level number. When the distance between the OLT and the ONU and the characteristics of the optical fiber transmission line do not change greatly in time, the setting / selection determined at the time of initial registration of the ONU can be used in a fixed manner.

  When the intensity of the optical signal is constant, for each subcarrier separated by FFT, the distance between adjacent symbols on the electric field plane becomes narrower as the modulation multi-level number increases. Therefore, the intensity of the optical signal input to the receiver is large in order to obtain a certain quality (= inter-symbol inter-symbol distance) when receiving an optical OFDM signal with a large modulation multi-level number for each subcarrier. Is required.

Also, the greater the modulation multi-level number, the weaker the tolerance to waveform distortion due to chromatic dispersion, etc., so that a small waveform distortion is required to obtain a certain level of quality when receiving an optical OFDM signal. . Here, in optical fiber transmission, the shorter the transmission distance, the smaller the attenuation of the intensity of the optical signal and the smaller the waveform distortion caused by wavelength dispersion or the like. Therefore, in an ONU that is close to the OLT, the modulation multi-level number m _k is set to a high value, and the effective number obtained by subtracting the increment due to encoding from the number of information bits that can be transmitted per unit time using one subcarrier There is a possibility that the number of information bits b ′ _k can be increased.

  Since the optical OFDM signal from each ONU allowed to be transmitted at the same time is combined and input to the OLT, the modulation format and encoding method of each subcarrier is determined by direct detection or optical I / Q detection in the OLT. A different OFDM signal is obtained for each ONU from which at least one of them is a transmission source.

In the dynamic band allocation calculation, as expressed by the equation (14), between ONUs having an area ∫J _K (t) dt composed of the number of subcarriers allocated to the ONU _k and the allowable transmission time within a certain period. The subcarriers and the transmission allowance time are allocated so that the ratio of is equal to the ratio of the reciprocal of log ₂ m _k / R _K.

Equation (15) can be derived from Equations (13) and (14).

Here, each term of the equation (15) represents an effective information amount obtained by subtracting the increment due to encoding from the information amount transmitted by each ONU within the period, and therefore, the bandwidth fairness between the ONUs by the dynamic bandwidth allocation described above. It can be seen that sex is realized. Further, the number of effective information bits b ′ obtained by subtracting the increment by encoding from the number of information bits that each ONU can transmit per unit time using one subcarrier within a range in which communication with a certain quality or higher is possible. _{Since k} is maximized for each ONU, the throughput of the entire system can be increased as compared to the case where the ONU having the longest distance from the OLT is fixed to a value b ′ _min that enables communication, and Equation (16) can be expanded. It holds.

  In other words, according to the present embodiment, in the OFDMA system in which the distance between each child node and the parent node is fixed, the effect of expanding the throughput of the entire system by applying adaptive modulation is not necessarily unified in the data conversion method in the PCS. Can be enjoyed fairly among other child nodes.

  Even in a WDM / OFDMA system that expands the system band without increasing the signal speed per wavelength by applying WDM, the same adaptive modulation multi-level setting, adaptive selection of coding scheme and dynamic By the bandwidth allocation calculation, the effect of increasing the throughput of the entire system can be enjoyed fairly among all child nodes sharing the bandwidth and not necessarily having a unified data conversion method in the PCS.

(Embodiment 6)
In this embodiment, the optical signal is an optical OFDM signal, the allocation function determines the number of effective information bits by determining the number of subcarriers and the symbol rate of the optical signal of each child node, and the transmission function is the allocation function. An optical signal having the determined number of subcarriers and symbol rate is transmitted to the parent node.

FIG. 7 shows a configuration example of an OFDMA system that is an optical communication system according to the sixth embodiment. In OFDM-PON (FIG. 6) which is a configuration example of the fifth embodiment, only a single ONU is allowed to transmit at the same time in uplink communication. For an optical OFDM signal transmitted by a child node ONU _k (k = 1, 2,..., K, K is an integer of 2 or more), the subcarrier modulation multi-level number m _k and the subcarrier symbol rate S are transmitted. _At least one of _k [Symbol / s], the number of subcarriers J _k, and the coding scheme can be set / selected adaptively for each ONU.

When setting the values of m _k , S _k , and J _k , whether to apply FEC, and selecting the type of FEC code, among the combinations in which ONU _k can communicate with the OLT that is the parent node with a certain quality or higher, The number of effective information bits B ′ _{k obtained} by excluding the increment due to encoding from the number of information bits that ONU _k can transmit per unit time is set / selected. When the ratio of the number of bits before and after encoding in ONU _k is R _k , the number of effective information bits B ′ _k that ONU _k can transmit per unit time is expressed by Expression (17).

Modulation multi-level number m _k , symbol rate S _k [Symbol / s], number of subcarriers J _k , presence / absence of application of FEC, type of FEC code, distance between OLT and ONU, characteristics of optical fiber transmission path Therefore, the monitoring signal is periodically transmitted and received between the OLT and the ONU, the loss and chromatic dispersion amount between the OLT and the ONU are measured, and the reception characteristic for the modulation multi-level number is calculated. The When the distance between the OLT and the ONU and the characteristics of the optical fiber transmission line do not change greatly in time, the setting / selection determined at the time of initial registration of the ONU can be used in a fixed manner.

  Assuming that the intensity of the optical signal is constant with respect to the modulation multilevel number, for each subcarrier separated by FFT, the distance between adjacent symbols on the electric field plane becomes narrower as the modulation multilevel number increases. Therefore, the intensity of the optical signal input to the receiver is large in order to obtain a certain quality (= inter-symbol inter-symbol distance) when receiving an optical OFDM signal with a large modulation multi-level number for each subcarrier. Is required.

Also, the greater the modulation multi-level number, the weaker the tolerance to waveform distortion due to chromatic dispersion, etc., so that a small waveform distortion is required to obtain a certain level of quality when receiving an optical OFDM signal. . Here, in optical fiber transmission, the shorter the transmission distance, the smaller the attenuation of the intensity of the optical signal and the smaller the waveform distortion caused by wavelength dispersion or the like. Therefore, in an ONU that is close to the OLT, there is a possibility that the modulation multi-value number m _k can be set to a high value and the number of effective information bits B ′ _k that can be transmitted per unit time can be increased.

  Regarding the symbol rate, the higher the symbol rate, the wider the signal band and the influence of high frequency noise components. To obtain a certain level of quality (= signal-to-noise ratio) when receiving an optical OFDM signal, it is necessary to send to the receiver. The intensity of the input optical signal is required to be large.

  Further, since the higher the symbol rate, the weaker the tolerance to waveform distortion caused by chromatic dispersion or the like, the waveform distortion is required to be small in order to obtain a certain quality or higher when receiving an optical OFDM signal.

Here, in optical fiber transmission, the shorter the transmission distance, the smaller the attenuation of the intensity of the optical signal and the smaller the waveform distortion caused by wavelength dispersion or the like. Therefore, in an ONU that is close to the OLT, there is a possibility that the symbol rate S _k can be set to a high value and the number of information bits B _k that can be transmitted / received per unit time can be increased.

  With regard to the number of subcarriers, the greater the number of subcarriers, the wider the signal band and the influence of high frequency noise components. Therefore, in order to obtain a certain quality (= signal to noise ratio) when receiving an optical OFDM signal The intensity of the optical signal input to the receiver is required to be high.

Further, since the wider the signal band, the stronger the influence of chromatic dispersion and the like, it is required that the chromatic dispersion amount of the optical fiber transmission line is small in order to obtain a certain level of quality when receiving the optical OFDM signal. Here, in optical fiber transmission, the shorter the transmission distance, the smaller the attenuation of the intensity of the optical signal and the smaller the chromatic dispersion amount. Therefore, the ONU close the distance between the OLT, the number J _k of sub-carriers increases, it may be possible to increase the effective number of information bits B _'k that can be transmitted per unit time.

  Therefore, a different optical OFDM signal is time-division multiplexed for each ONU whose transmission format is at least one of the modulation format, symbol rate, number of subcarriers, and coding scheme of each subcarrier, and is input to the OLT.

In the dynamic bandwidth allocation calculation, if the total time during which the ONU _k is allowed to transmit the optical OFDM signal within a certain period is T _k , the ONU _k is set between the ONUs so as to satisfy the expression (11) in the fourth embodiment. The allowable transmission time is allocated so that the ratio of T _k of the two becomes equal to the ratio of the reciprocal of the number B ′ _k of valid information bits that can be transmitted per unit time.

Since each term in Expression (11) represents the effective information amount obtained by subtracting the increment due to encoding from the information amount transmitted by each ONU within the period, the bandwidth fairness between the ONUs is realized by the dynamic bandwidth allocation described above. You can see that Furthermore, since the number of valid information bits that each ONU can transmit per unit time is maximized for each ONU within a range where communication with a certain quality or more is possible, the ONU with the longest distance from the OLT communicates. Compared to the case where the number of valid information bits that can be transmitted per unit time is fixed to the possible value B ′ _min , the throughput of the entire system can be expanded, and the expression (12) in the fourth embodiment is established.

  In other words, according to the present embodiment, in the OFDMA system in which the distance between each child node and the parent node is fixed, the effect of expanding the throughput of the entire system by applying adaptive modulation is not necessarily unified in the data conversion method in the PCS. Can be enjoyed fairly among other child nodes.

  Even in a WDM / OFDMA system that expands the system band without increasing the signal speed per wavelength by applying WDM, the same adaptive modulation multi-level number, symbol rate, subcarrier number setting, and encoding method By adaptive selection and dynamic bandwidth allocation calculation, the effect of increasing the throughput of the entire system can be enjoyed fairly among all the child nodes sharing the bandwidth and not necessarily having a unified data conversion method in the PCS.

  The present invention can be applied to the information communication industry.

Claims (6)

A communication system in which a parent node and a plurality of child nodes are connected,
The parent node determines the number of valid information bits that each child node can transmit per unit time according to the distance to the child node, and each child node is equal to the ratio of the reciprocal number of each valid information bit. Has an allocation function that allocates the allowable transmission time to
The child node has a transmission function of transmitting an optical signal having the number of effective information bits determined by the allocation function to the parent node during a transmission allowable time allocated by the allocation function.
Communications system. The allocation function determines the number of effective information bits by determining the number of modulation levels of the optical signal of each child node, or
The allocation function determines the number of valid information bits by determining a symbol rate of an optical signal of each child node, or
The allocation function determines the number of effective information bits by determining an encoding method of an optical signal of each child node, or
The allocation function determines the number of effective information bits by determining the number of subcarriers of the optical signal of each child node;
The communication system according to claim 1. A control device of a communication system in which a parent node and a plurality of child nodes are connected,
The parent node determines the number of valid information bits that each child node can transmit per unit time according to the distance to the child node, and is set to be equal to the ratio of the reciprocal of the number of valid information bits of each child node. It has an allocation function that allocates the allowable transmission time to child nodes.
Communication system control device. A control method of a communication system in which a parent node and a plurality of child nodes are connected,
The number of valid information bits that each child node can transmit per unit time is determined according to the distance from the parent node to the child node, and each child node is set to be equal to the ratio of the reciprocal number of the valid information bits of each child node. An allocation procedure to allocate time allowed for transmission to the node;
A transmission procedure for transmitting an optical signal having the number of valid information bits determined by the allocation procedure from a child node to a parent node during a transmission allowable time allocated by the allocation procedure;
A control method for a communication system comprising: A communication system control program in which a parent node and a plurality of child nodes are connected,
The number of valid information bits that each child node can transmit per unit time is determined according to the distance from the parent node to the child node, and each child node is set to be equal to the ratio of the reciprocal number of the valid information bits of each child node. An allocation procedure to allocate time allowed for transmission to the node;
A transmission procedure for transmitting an optical signal having the number of valid information bits determined by the allocation procedure from a child node to a parent node during a transmission allowable time allocated by the allocation procedure;
A communication system control program for causing a computer to execute.   A computer-readable recording medium on which the control program for the communication system according to claim 5 is recorded.

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