Optical communication network system, communication method, station-side optical transmission device, and subscriber-side optical transmission device

Abstract

To provide an optical communication network system that reduces the scale of a circuit for compensating for dispersion and the like of a transmitted optical signal.SOLUTION: The present invention relates to an optical communication network system in which a central office optical transmission device and multiple subscriber-side optical transmission devices are connected by optical branching transmission lines. The central office optical transmission device according to the present invention includes fixed compensation means that compensates for a fixed amount of dispersion in a received signal and adaptive compensation means that compensates for distortion in an output signal of the fixed compensation means. The subscriber-side optical transmission device according to the present invention also includes fixed compensation means that compensates for a fixed amount of dispersion in a received signal and adaptive compensation means that compensates for distortion in an output signal of the fixed compensation means. Furthermore, each fixed compensation means compensates for dispersion that occurs when the optical branching transmission line is set to a distance ranging from longer than 0 to shorter than the maximum distance.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an optical communication network system, a communication method, a station-side optical transmission device, and a subscriber-side optical transmission device, and can be applied to subscriber-side optical network communications such as a PON (Passive Optical Network). [Background technology]

[0002] In recent years, the demand for communications has been rapidly increasing due to the development of mobile applications such as smartphones, the emergence of IoT (Internet of Things) technology, the increase in web conferences due to the spread of remote work, and the development of virtualization technologies such as the metaverse and digital twin, among other factors, and this has led to a demand for higher capacity transmission equipment in subscriber optical networks. Therefore, research is underway to utilize the development of digital signal processing (DSP) technology to apply optical transmission technology, which has traditionally been applied to backbone optical networks with transmission distances of hundreds to thousands of kilometers, to subscriber optical networks with transmission distances of at most 20 km.

[0003] In the field of digital signal processing technology, optical transceivers and DSP devices capable of achieving optical transmission speeds exceeding 100 Gb / s per line have already been commercially available. For example, a conventional DSP integrated circuit is shown in Non-Patent Document 1. The DSP integrated circuit described in Non-Patent Document 1 enables point-to-point (PtP) optical transmission, connecting two transmission devices in a paired fashion. Existing subscriber optical networks also use an optical transmission system called a PON. In a PON, one optical transceiver device at the central office is connected to multiple optical transceivers at subscribers via a partially shared optical fiber transmission line. Point-to-multipoint (PtMP) optical multiplexing / multiple access (OPM) technology is used for point-to-multipoint communications between the central office and multiple subscribers. Optical fiber, the transmission path of a PON, has a characteristic called dispersion, and light of different wavelengths propagates through the optical fiber at different phase velocities (see Non-Patent Document 2). Conventionally, a quantity called the dispersion parameter D is commonly used to represent the dispersion characteristics of optical fibers. The dispersion parameter D is expressed in units of ps / nm / km. Here, "ps" represents the propagation time difference between light with different wavelengths, "nm" represents the wavelength difference between the lights, and "km" represents the propagation distance. In other words, the dispersion parameter D represents the "propagation time difference between light with a unit wavelength difference propagating a unit distance."

[0004] In conventional PONs, the binary information time series transmitted and received in the electrical domain is converted into a time-series signal called a symbol sequence, which is suitable for optical transmission. The optical signal modulated with the symbol sequence signal is composed of a pulse time series, each with a pulse waveform corresponding to a symbol. When an optical signal propagates through a transmission line with dispersion characteristics, the various wavelengths of light that make up the pulses experience different propagation delays, causing the pulse waveforms to broaden, resulting in partial overlap between pulses in adjacent time slots at the receiving end. Such pulse overlap can cause reception errors in PONs. Therefore, the DSP device described in Non-Patent Document 1 is equipped with an equalization circuit in the receiving circuit that compensates for waveform distortion such as pulse broadening to reduce reception errors. Specifically, the DSP device described in Non-Patent Document 1 estimates the dispersion value generated in the optical transmission line based on the optical fiber material and transmission distance before the transmission system is put into operation. The equalization circuit is equipped with a fixed equalization means for compensating for the estimated dispersion value and an adaptive equalization means for compensating for residual dispersion, which is the error between the estimated dispersion value and the actual transmission line dispersion value.

[0005] Non-Patent Document 3 (Fig. 5) shows an example of a conventional FIR filter for dispersion compensation implemented using a DSP equalizer. The FIR filter in Non-Patent Document 3 weights and adds consecutive sample values, called the number of taps, of a time-series signal input in sampling time units, and sequentially outputs the weighted sum (convolution processing). The dispersion value that can be compensated for by the FIR filter (DSP equalizer) shown in Non-Patent Document 3 depends on the number of taps, etc. As shown in Non-Patent Document 4, dispersion compensation is possible if the maximum delay time that can be provided by the FIR filter is greater than the propagation time difference between the wavelength components at both ends of the optical signal spectrum. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] O. Ishida, et al., “Power Efficient DSP Implementation for 100G-and-Beyond Multi-Haul Coherent Fiber-Optic Communications,” OFC2016, W3G.3. [Non-patent document 2] Edited by Yoshihiro Konishi and written by Akiya Yamamoto, "Optical Fiber Communication Technology" (Section 3.2 Dispersion), Nikkan Kogyo Shimbun, pp.48-59 [Non-patent document 3] Kazuaki Kikuchi, "Adaptive Equalization Techniques for Digital Coherent Optical Receivers," IEICE Transactions on Information and Communication Engineers, Vol. J96-B, No. 3, pp. 212-219, 2013. [Non-patent document 4] Tianhua Xu, et al., “Chromatic dispersion compensation in coherent transmission system using digital filters,” Optics Express, vol. 18, no. 15, pp. 16243-16257, Jul. 19, 2010. Summary of the Invention [Problem to be solved by the invention]

[0007] However, in the past, when an FIR filter for compensating for dispersion in optical transmission was used in a DSP equalization circuit, in order to compensate for the effects of larger dispersion, it was necessary to use an FIR filter with a larger number of taps as the equalization circuit, as described above, which posed the problem of increasing the circuit size and power consumption of the DSP circuit.

[0008] In view of the above problems, there is a demand for an optical communication network system that can reduce the scale of the circuitry for compensating for dispersion and the like of transmitted optical signals. [Means for solving the problem]

[0009] The first invention is an optical communication network system in which a station-side optical transmission device and a plurality of subscriber-side optical transmission devices are connected by an optical branching transmission line, wherein the station-side optical transmission device has station-side fixed compensating means for compensating for a fixed amount of dispersion in a signal received from the optical branching transmission line, and station-side adaptive compensating means for compensating for distortion in an output signal of the station-side fixed compensating means, and each of the subscriber-side optical transmission devices has subscriber-side fixed compensating means for compensating for a fixed amount of dispersion in a signal received from the optical branching transmission line, and subscriber-side adaptive compensating means for compensating for distortion in an output signal of the subscriber-side fixed compensating means, and the station-side fixed compensating means and the subscriber-side fixed compensating means compensate for dispersion that occurs when the optical branching transmission line is set to a distance within a range from longer than 0 to shorter than a maximum distance.

[0010] The second present invention is a communication method performed by an optical communication network system in which a station-side optical transmission device and a plurality of subscriber-side optical transmission devices are connected by optical branching transmission lines, the station-side optical transmission device having station-side fixed compensating means and station-side adaptive compensating means, each of the subscriber-side optical transmission devices having subscriber-side fixed compensating means and subscriber-side adaptive compensating means, the station-side fixed compensating means compensates for a fixed amount of dispersion in a signal received from the optical branching transmission line, the station-side adaptive compensating means compensates for distortion in an output signal from the station-side fixed compensating means, the subscriber-side fixed compensating means compensates for a fixed amount of dispersion in a signal received from the optical branching transmission line, the subscriber-side adaptive compensating means compensates for distortion in the output signal from the subscriber-side fixed compensating means, and the station-side fixed compensating means and the subscriber-side fixed compensating means compensate for dispersion that occurs when the optical branching transmission line is set to a distance within a range from longer than 0 to shorter than a maximum distance.

[0011] The third aspect of the present invention is an optical transmission device at a central office connected to a plurality of optical transmission devices at subscriber offices via optical branching transmission lines, the optical transmission device comprising: a central office fixed compensating means for compensating for a fixed amount of dispersion in a signal received from the optical branching transmission line; and a central office adaptive compensating means for compensating for distortion in an output signal of the central office fixed compensating means, wherein the central office fixed compensating means compensates for dispersion that occurs when the optical branching transmission line is set to a distance within a range from longer than 0 to shorter than a maximum distance.

[0012] The fourth aspect of the present invention is a subscriber-side optical transmission device connected to a station-side optical transmission device by an optical branching transmission line, comprising: a subscriber-side fixed compensation means for compensating for a fixed amount of dispersion in a signal received from the optical branching transmission line; and a subscriber-side adaptive compensation means for compensating for distortion in an output signal of the subscriber-side fixed compensation means, wherein the subscriber-side fixed compensation means compensates for dispersion that occurs when the optical branching transmission line is set to a distance in the range from longer than 0 to shorter than the maximum distance.

[0013] A fifth aspect of the present invention is an optical communication network system in which a station-side optical transmission device and a plurality of subscriber-side optical transmission devices are connected by optical branching transmission lines, wherein the station-side optical transmission device comprises pre-distortion imparting means for imparting waveform distortion having an inverse characteristic to a fixed amount of dispersion to a signal to be sent to the optical branching transmission line, station-side fixed compensating means for compensating for the fixed amount of dispersion in a signal received from the optical branching transmission line, and station-side adaptive compensating means for compensating for distortion in an output signal of the station-side fixed compensating means, wherein each of the subscriber-side optical transmission devices comprises subscriber-side adaptive compensating means for compensating for distortion in the signal received from the optical branching transmission line, the pre-distortion imparting means imparting waveform distortion having an inverse characteristic to dispersion that occurs when the optical branching transmission line is set to a distance within a range from longer than 0 to shorter than a maximum distance, and the station-side fixed compensating means compensates for dispersion that occurs when the optical branching transmission line is set to a distance within a range from longer than 0 to shorter than the maximum distance.

[0014] A sixth aspect of the present invention is a communication method performed by an optical communication network system in which a station-side optical transmission device and a plurality of subscriber-side optical transmission devices are connected by optical branch transmission lines, the station-side optical transmission device having pre-distortion adding means, station-side fixed compensating means, and station-side adaptive compensating means, each of the subscriber-side optical transmission devices having subscriber-side adaptive compensating means, the pre-distortion adding means adding waveform distortion having an inverse characteristic to a fixed amount of dispersion to a signal to be sent to the optical branch transmission line, the station-side fixed compensating means compensating for the fixed amount of dispersion in a signal received from the optical branch transmission line, the station-side adaptive compensating means compensating for distortion in an output signal of the station-side fixed compensating means, and the subscriber-side adaptive compensating means compensating for distortion in the signal received from the optical branch transmission line, the pre-distortion adding means adding waveform distortion having an inverse characteristic to dispersion that occurs when the optical branch transmission line is set to a distance in a range from longer than 0 to shorter than a maximum distance, and the station-side fixed compensating means compensating for dispersion that occurs when the optical branch transmission line is set to a distance in a range from longer than 0 to shorter than the maximum distance.

[0015] A seventh aspect of the present invention is an optical transmission device at a central office connected to a plurality of optical transmission devices at subscriber offices via optical branching transmission lines, the device comprising: pre-distortion imparting means for imparting waveform distortion having an inverse characteristic to a fixed amount of dispersion to a signal to be sent to the optical branching transmission line; central office fixed compensating means for compensating for the fixed amount of dispersion in a signal received from the optical branching transmission line; and central office adaptive compensating means for compensating for distortion in an output signal of the central office fixed compensating means, wherein the pre-distortion imparting means imparts waveform distortion having an inverse characteristic to dispersion that occurs when the optical branching transmission line is set to a distance within a range from longer than 0 to shorter than a maximum distance, and the central office fixed compensating means compensates for dispersion that occurs when the optical branching transmission line is set to a distance within a range from longer than 0 to shorter than the maximum distance.

[0016] The eighth aspect of the present invention is a subscriber-side optical transmission device connected to a station-side optical transmission device by an optical branching transmission line, which comprises a subscriber-side adaptive compensation means for compensating for distortion of a received signal when the signal is received from the optical branching transmission line and has waveform distortion that is the inverse of a fixed amount of dispersion previously added in the station-side optical transmission device, and the fixed amount is the dispersion that occurs when the optical branching transmission line is set to a distance within a range from longer than 0 to shorter than a maximum distance. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide an optical communication network system that reduces the scale of the circuit for compensating for dispersion and the like of transmitted optical signals. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a block diagram showing the overall configuration of an optical communication network system according to a first embodiment. [Figure 2] FIG. 10 is a block diagram showing the overall configuration of an optical communication network system according to a second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0019] (A) First embodiment Hereinafter, a first embodiment of an optical communication network system, a communication method, a station-side optical transmission device, and a subscriber-side optical transmission device according to the present invention will be described in detail with reference to the drawings.

[0020] (A-1) Configuration of the First Embodiment FIG. 1 is a block diagram showing the overall configuration of an optical communication network system 1 according to the first embodiment.

[0021] The optical communication network system 1 includes an optical transmission device 402 on the station side and m (m is an integer equal to or greater than 2) subscriber-side optical transmission devices 401 (401-1 to 401-m) on the subscriber side. The optical transmission device 402 on the station side and the optical transmission devices 401 on the subscriber side are connected by an optical transmission path 600 (a PtMP optical transmission path; an optical fiber) that serves as an optical branch transmission path with multiple branches (m branches), and the optical transmission device 402 on the station side and the optical transmission devices 401 on the subscriber side transmit and receive optical signals in both directions. That is, in the optical communication network system 1, the optical transmission device 402 on the station side and the optical transmission devices 401 on the subscriber side are connected by a PON using the optical transmission path 600. In the following description, the direction from the optical transmission device 402 on the station side to the optical transmission device 401 on the subscriber side will be referred to as "downstream," and the direction from the optical transmission device 401 on the subscriber side to the optical transmission device 402 on the station side will be referred to as "upstream." In Fig. 1, the flow of downstream communication is indicated by a solid line, and the flow of upstream communication is indicated by a dashed line (the same applies to Fig. 2 of the second embodiment described later).

[0022] In the following, k (an integer from 1 to m) is a symbol that generalizes any subscriber k, and elements specific to the subscriber k are marked with "k." For example, the subscriber-side optical transmission device 401 of the subscriber k is represented as "subscriber-side optical transmission device 401-k."

[0023] In this embodiment, it is assumed that a subscriber-side host 700 (700-1 to 700-m) is connected to the downstream side (subscriber side) of each subscriber-side optical transmission device 401 (401-1 to 401-m). The subscriber-side host 700 corresponds to a subscriber-side communication device such as a router, PC, or switching hub. Also, in this embodiment, it is assumed that a termination device 800 of a core-side network 900 is connected to the upstream side of the station-side optical transmission device 402. The termination device 800 corresponds to a network device such as a router or switching hub. That is, in this embodiment, the optical communication network system 1 relays and transmits communications between each subscriber-side host 700 and the core-side network 900.

[0024] In this embodiment, the optical transmission line 600 is divided into a downstream optical transmission line 610 for transmitting downstream optical signals and an upstream transmission line 620 for transmitting upstream optical signals, but the optical transmission line may be shared between the upstream and downstream directions by wavelength division duplexing. Also, the optical transmission line 600 in this embodiment is assumed to have a star topology as shown in Figure 1, but may also have a bus topology.

[0025] The downstream optical transmission line 610 is branched into m paths by a splitter 613. The upstream optical transmission line 620 is also branched into m paths by a splitter 623.

[0026] 1, in the downstream optical transmission path 610, a portion connected to the optical transmission device 402 on the station side is designated as a downstream common optical transmission path 611, and a portion connected to the optical transmission device 401 (401-1 to 401-m) on the subscriber side is designated as a downstream branch optical transmission path 612 (612-1 to 612-m). Also in FIG. 1, in the upstream optical transmission path 620, a portion connected to the optical transmission device 402 on the station side is designated as an upstream common optical transmission path 621, and a portion connected to the optical transmission device 401 (401-1 to 401-m) on the subscriber side is designated as an upstream branch optical transmission path 622 (622-1 to 622-m). For example, an upstream branch optical transmission path 622-k and a downstream branch optical transmission path 612-k are connected to the optical transmission device 401-k on the subscriber side (k is any one of 1 to m).

[0027] In Figure 1, for the sake of convenience, elements that are not related to the essential parts of the present invention, such as means for binary information processing that constitute the optical communication network system 1 (e.g., optical components such as light sources and photodetectors, framers, error correction processing means), are omitted from the illustration.

[0028] Next, the internal configuration of the subscriber-side optical transmission device 401 will be described.

[0029] In this embodiment, the description will be made on the assumption that all of the subscriber-side optical transmission devices 401-1 to 401-m have the same configuration. Here, an example of the internal configuration of the subscriber-side optical transmission device 401-k will be described.

[0030] 1, the subscriber-side optical transmission device 401-k includes a subscriber-side symbol mapper 211-k, a subscriber-side digital-to-analog conversion means 212-k, a subscriber-side analog-to-digital conversion means 221-k, a subscriber-side fixed equalization means 422-k, a subscriber-side adaptive equalization means 223-k, and a subscriber-side symbol demapper 224-k. In each subscriber-side optical transmission device 401, the subscriber-side symbol mapper 211-k and the subscriber-side digital-to-analog conversion means 212-k are components related to upstream communications, and the subscriber-side analog-to-digital conversion means 221-k, the subscriber-side fixed equalization means 422-k, the subscriber-side adaptive equalization means 223-k, and the subscriber-side symbol demapper 224-k are components related to downstream communications.

[0031] Next, the configuration of the subscriber-side optical transmission device 401-k regarding upstream communication will be described.

[0032] The subscriber-side symbol mapper 211-k converts the binary information sequence (upstream signal) sent from the subscriber-side host 700 into a digital symbol sequence and outputs it.

[0033] The subscriber-side analog / digital conversion means 221-k converts the symbol sequence of the upstream data (upstream signal) output from the subscriber-side symbol mapper 211-k into an analog signal (an optical signal that can be sent over a PON) and sends it over the connected upstream tributary optical transmission path 622-k.

[0034] Next, the configuration of the subscriber-side optical transmission device 401-k for downstream communication will be described.

[0035] The subscriber-side analog / digital conversion means 221-k converts the analog signal sent from the downstream branch optical transmission line 612-k into a digital signal and outputs it.

[0036] The subscriber-side fixed equalization means 422-k uses a DSP equalization circuit to perform a fixed amount (constant amount) of dispersion compensation on the digital signal output by the subscriber-side analog-to-digital conversion means 221-k (hereinafter, this dispersion compensation by the DSP equalization circuit will be referred to as "fixed equalization" or "fixed compensation"). Note that the amount of dispersion performed by the subscriber-side fixed equalization means 422-k is assumed to be the same in each subscriber-side optical transmission device 401-k. The amount of dispersion compensated for by the subscriber-side fixed equalization means 422-k will be described in detail later.

[0037] The subscriber-side adaptive equalization means 223-k compensates for residual dispersion in the output signal (digital signal) of the subscriber-side fixed equalization means 422-k using a DSP equalization circuit (hereinafter, dispersion compensation by this DSP equalization circuit will be referred to as "adaptive equalization" or "adaptive compensation"). The subscriber-side adaptive equalization means 223-k performs processing to compensate for the error between the dispersion value compensated for by fixed equalization in the subscriber-side fixed equalization means 422-k and the dispersion value actually occurring in the downstream optical transmission line 610.

[0038] The subscriber-side symbol demapper 224-k converts the symbol sequence output by the subscriber-side adaptive equalization means 223-k into a binary information sequence and outputs it to the subscriber-side host 700.

[0039] Next, the configuration of the optical transmission device 402 on the station side will be described.

[0040] 1, the optical transmission device 402 on the station side includes optical transmission device 401-k on the station side, optical transmission device 402 ...

[0041] In the optical transmission device 402, the analog / digital converter 341, the fixed equalizer 442, the adaptive equalizer 443, and the symbol demapper 244 (244-1 to 244-m) are components related to upstream communication, and the symbol mappers 231 (231-1 to 231-m), the digital / analog converter 332, and the time division multiplexer 350 are components related to downstream communication.

[0042] Next, the configuration of the optical transmission device 402 on the station side regarding downstream communication will be described.

[0043] In the optical transmission device 402, the binary information sequence sent from the termination device 800 of the core network 900 is distributed to each subscriber (each subscriber-side optical transmission device 401) and supplied to the corresponding optical transmission device 231. For example, a binary information sequence that is transmission information addressed to the optical transmission device 401-k (subscriber k) is distributed to the optical transmission device 231-k.

[0044] Each station symbol mapper 231 converts the supplied binary information sequence into a digital symbol sequence and supplies it to the time division multiplexing means 350. In other words, all symbol sequences addressed to all subscribers (downstream communication symbol sequences) are supplied to the time division multiplexing means 350.

[0045] The time division multiplexing means 350 time division multiplexes the symbol sequences addressed to all subscribers and supplies the result to the central office digital / analog conversion means 332 .

[0046] The station side digital / analog conversion means 332 converts the supplied symbol sequence into an analog signal and outputs it to the downstream common optical transmission line 611 .

[0047] As described above, in the optical transmission device 402 on the central office side, the transmission signals (downstream signal symbol series) addressed to all subscribers are time-division multiplexed by the time-division multiplexing means 350, and optically modulated using one digital / analog conversion means 332.

[0048] Next, the configuration of the optical transmission device 402 for upstream communication will be described.

[0049] The analog signal transmitted from the upstream common optical transmission line 621 is supplied to the station side analog / digital conversion means 341 .

[0050] The station side analog / digital conversion means 341 converts the analog signal sent from the upstream common optical transmission line 621 into a digital signal and outputs it.

[0051] The station-side fixed equalization means 442 performs fixed equalization (compensates for a fixed amount of dispersion using a DSP equalization circuit) on the digital signal output from the station-side analog / digital conversion means 341 and outputs the result. Details of the dispersion value compensated for by the station-side fixed equalization means 442 will be described later.

[0052] The station-side adaptive equalization means 443 adaptively equalizes the output signal of the station-side fixed equalization means 442 (compensating for residual dispersion, etc., using a DSP equalization circuit) and outputs the result as a symbol sequence. The station-side adaptive equalization means 443 performs processing such as compensating for the error between the dispersion value compensated for by fixed equalization in the station-side fixed equalization means 442 and the dispersion value actually occurring in the upstream optical transmission path 620. The station-side adaptive equalization means 443 distributes the output symbol sequence to each subscriber and supplies it to the station-side symbol demapper 244 of the corresponding subscriber. For example, the station-side adaptive equalization means 443 supplies a symbol sequence based on a signal supplied from subscriber k (subscriber-side optical transmission device 401-k) to the station-side symbol demapper 244-k.

[0053] Each optical line terminal (OWB) symbol demapper 244 converts the supplied symbol sequence into a binary information sequence and outputs it. At this time, the optical line terminal (OWB) symbol demapper 244-k outputs the binary information sequence as the binary information sequence transmitted from subscriber k (subscriber-side optical transmission device 401-k). The optical line terminal (OWB) multiplexes the binary information sequences obtained from the optical line terminal (OWB) symbol demappers 244-1 to 244-m (i.e., the binary information sequences from all subscribers), and transmits the multiplexed sequence to the termination device 800 of the core-side network 900.

[0054] In this embodiment, the station-side fixed equalization means 442 and the subscriber-side fixed equalization means 422 are collectively referred to simply as "fixed equalization means" or "fixed compensation means." Also, in this embodiment, the station-side adaptive equalization means 443 and the subscriber-side adaptive equalization means 223 are collectively referred to simply as "adaptive equalization means" or "adaptive compensation means."

[0055] (A-2) Operation of the First Embodiment Next, the operation of the optical communication network system 1 according to the first embodiment will be described.

[0056] First, the upstream communication process for the subscriber-side optical transmission device 401-k (subscriber k) in the optical communication network system 1 will be described.

[0057] The subscriber-side symbol mapper 211-k of the subscriber-side optical transmission device 401-k converts the binary information sequence transmitted from the subscriber-side host 700-k into a digital symbol sequence and supplies the digital symbol sequence to the subscriber-side digital-to-analog conversion means 212-k. The subscriber-side analog-to-digital conversion means 221-k then converts the transmitted symbol sequence into an analog signal and transmits it to the upstream branch optical transmission line 622-k. The analog signal transmitted to the branch optical transmission line 622-k is supplied to the central office analog-to-digital conversion means 341 of the central office optical transmission device 402 via the upstream common optical transmission line 621. The central office analog-to-digital conversion means 341 converts the analog signal transmitted from the upstream common optical transmission line 621 into a digital signal and supplies the digital signal to the central office fixed equalization means 442. The central office fixed equalization means 442 performs fixed equalization on the transmitted digital signal and supplies the digital signal to the central office adaptive equalization means 443. The station-side adaptive equalization means 443 adaptively equalizes the supplied signal to generate a symbol sequence and supplies it to the station-side symbol demapper 244-k. The station-side symbol demapper 244-k converts the supplied symbol sequence into a binary information sequence and sends it to the termination device 800 of the core-side network 900.

[0058] Next, downstream communication processing for the subscriber-side optical transmission device 401-k (subscriber k) in the optical communication network system 1 will be described.

[0059] In the optical transmission device 402, a binary information sequence addressed to subscriber k (subscriber-side optical transmission device 401-k) transmitted from the terminal device 800 of the core network 900 is supplied to the optical transmission device 402-k. The optical transmission device 402-k converts the supplied binary information sequence into a digital symbol sequence and supplies the digital symbol sequence to the time division multiplexing means 350. The time division multiplexing means 350 time-division multiplexes the supplied symbol sequence and supplies the multiplexed symbol sequence to the optical transmission device 402-k. The optical transmission device 402-k converts the supplied symbol sequence into an analog signal and transmits it to the optical transmission device 402-k. The analog signal transmitted to the downstream common optical transmission path 611 is supplied to the subscriber-side analog-to-digital conversion means 221-k of the optical transmission device 401-k via the downstream branch optical transmission path 612-k. The subscriber-side analog / digital conversion means 221-k converts the analog signal transmitted from the downstream branch optical transmission path 612-k into a digital signal and supplies it to the subscriber-side fixed equalization means 422-k. The subscriber-side fixed equalization means 422-k performs fixed equalization on the supplied digital signal and supplies it to the subscriber-side adaptive equalization means 223-k. The subscriber-side adaptive equalization means 223-k adaptively equalizes the supplied digital signal to generate a symbol sequence and supplies it to the subscriber-side symbol demapper 224-k. The subscriber-side symbol demapper 224-k converts the supplied symbol sequence into a binary information sequence and supplies it to the subscriber-side host 700-k.

[0060] Next, the processes relating to adaptive equalization and fixed equalization in the optical communication network system 1 will be described.

[0061] Conventionally, the optical transmission distance (maximum possible transmission distance) in a PON is specified by standardization organizations such as the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) and the Institute of Electrical and Electronics Engineers (IEEE) as a maximum of 20 km. Here, this optical transmission distance (maximum possible transmission distance) is represented as "Lmax." Also, here, the optical transmission path distance between the optical transmission device 402 at the central office and subscriber k (the optical transmission device 401-k at the central office) is represented as "Lk." In this case, the optical signal received by the optical transmission device 401-k at the central office in downstream communication is distorted due to the cumulative dispersion D×Lk that occurs in the transmission path. Similarly, the optical signal transmitted by the optical transmission device 401-k at the kth subscriber in upstream communication and received by the optical transmission device 402 at the central office is distorted due to the cumulative dispersion D×Lk that occurs in the transmission path.

[0062] Here, the cumulative dispersion amount (hereinafter, also simply referred to as "dispersion amount" or "dispersion value") that can be compensated by the subscriber-side fixed equalization means 422-k of each subscriber-side optical transmission device 401 is denoted as "Sfix" (assuming that Sfix in all subscriber-side optical transmission devices 401 has the same value), and the influence of the dispersion generated in the output signal of the subscriber-side fixed equalization means 422-k of the subscriber-side optical transmission device 401-k in the downstream communication (hereinafter, referred to as "residual dispersion amount" or "residual dispersion value") is denoted as "Sres_k". And here, when Sfix = D×Lmax / 2 is set, Sres_k = D(Lk - Lmax / 2). In the subscriber-side optical transmission device 401-k, for the output signal of the subscriber-side fixed equalization means 422-k, the subsequent subscriber-side adaptive equalization means 223-k performs adaptive equalization to compensate for the residual dispersion Sres_k. Similarly, in the station-side optical transmission device 402, when the cumulative dispersion amount that can be compensated by the station-side fixed equalization means 442 is set as Sfix, in the upstream communication, for the signal from subscriber k (subscriber-side optical transmission device 401-k), the influence of the dispersion generated in the output signal of the station-side fixed equalization means 442 becomes Sres_k when expressed as the cumulative dispersion value. In the station-side optical transmission device 402, the output signal of the station-side fixed equalization means 442 is adaptively equalized by the station-side adaptive equalization means 443 to compensate for the residual dispersion Sres_k.

[0063] Here, when Lk > Lmax / 2, in the optical communication network system 1, the influence of the dispersion generated by optical transmission is partially compensated by the fixed equalization means (station-side fixed equalization means 442 and subscriber-side fixed equalization means 422-k), and the remaining influence can be compensated by the adaptive equalization means (station-side adaptive equalization means 443 and subscriber-side adaptive equalization means 223-k). On the other hand, when Lk < Lmax / 2, in the optical communication network system 1, the influence of the dispersion generated by optical transmission is overcompensated by the fixed equalization means (station-side fixed equalization means 442 and subscriber-side fixed equalization means 422-k), and the overcompensated influence can be compensated by the adaptive equalization means (station-side adaptive equalization means 443 and subscriber-side adaptive equalization means 223-k). Therefore, the sign of the residual dispersion Sres_k is reversed between the two cases (when Lk > Lmax / 2 and when Lk < Lmax / 2).

[0064] Here, since 0<=Lk<=Lmax, the range of the absolute value |Sres_k| of the residual dispersion Sres_k is 0<=|Sres_k|<=|Sfix|. Therefore, the number of taps of the FIR filter required for residual dispersion compensation in the subscriber-side adaptive equalization means 223-k and the station-side adaptive equalization means 443 is equal to or less than the number of taps of the subscriber-side fixed equalization means 422-k and the station-side fixed equalization means 442.

[0065] The adaptive equalization process by the subscriber-side adaptive equalizer 223-k and the central office adaptive equalizer 443 may be designed to compensate for waveform distortion caused by factors other than dispersion that are not dependent on the optical transmission distance. For example, the adaptive equalization process by the subscriber-side adaptive equalizer 223-k and the central office adaptive equalizer 443 is suitable for compensating for time-varying waveform distortion (e.g., waveform distortion caused by fluctuations in the sampling clock, waveform distortion caused by fluctuations in the polarization state of the optical signal, etc.). The subscriber-side adaptive equalizer 223-k and the central office adaptive equalizer 443 require a certain number of taps to compensate for the above-mentioned time-varying waveform distortion, regardless of the optical transmission distance. In the subscriber-side adaptive equalizer 223-k and the central office adaptive equalizer 443, if the number of taps of the FIR filter required for dispersion compensation is equal to or less than the number of taps of the FIR filter required to compensate for the above-mentioned waveform distortion, it is possible to simultaneously compensate for dispersion and waveform distortion caused by factors other than dispersion, regardless of the transmission distance.

[0066] Next, the number of taps N of the FIR filter required for dispersion compensation in each adaptive equalization means (subscriber side adaptive equalization means 223-k and station side adaptive equalization means 443) will be considered.

[0067] First, let the wavelength width of the signal spectrum be Δλ and the transmission distance be L. From the definition of the dispersion parameter, the pulse spreading time due to the influence of dispersion is D can be calculated using the following formula (1). τ D =DLΔλ …(1)

[0068] Furthermore, if the speed of light in a vacuum is c, the frequency is f, and the wavelength is λ, then λ can be calculated using the following equation (2). λ=c / f …(2)

[0069] Furthermore, the relationship between the frequency width Δf and Δλ of the signal spectrum is as shown in the following equation (3). Here, it is assumed that the signal carrier wavelength λ0 is sufficiently larger than the spectrum width Δλ. Therefore, τ D can be expressed as the following equation (4).

number

[0070] Here, the frequency width of the signal spectrum is the symbol rate R sym On the other hand, if we assume that the number of taps for the FIR filter is N and that it operates at a clock speed equal to the symbol rate, the maximum time difference τ FIR can be expressed by the following equation (6).

number

[0071] Here, in each adaptive equalization means (subscriber side adaptive equalization means 223-k and station side adaptive equalization means 443), τ FIR ≧|τ D When |, dispersion can be guaranteed, and the required number of taps N can be expressed by the following equation (7).

number

[0072] Next, specific examples of the required number of taps N will be described.

[0073] Here, the speed of light c = 3 × 10 8Let L be "m / s" and the signal light wavelength λ0 = 1550 [nm]. Generally, in a PON, a standard optical fiber (SMF: Single Mode Fiber) with a dispersion parameter D = +17 "ps / nm / km" at a wavelength of 1550 [nm] is laid, so here too, it is assumed that the optical fiber constituting the optical transmission line 600 has a dispersion parameter D = +17 "ps / nm / km" at a wavelength of 1550 [nm]. Also, here, it is assumed that L = Lmax / 2 = 10 [km]. Furthermore, in order to achieve signal transmission of over 100 Gb / s, a multi-level modulation method and polarization multiplexing are used, and the symbol rate R sym = 50 [Gbaud]. Substituting the above parameters into equation (7), N≧7.807, so the required number of taps N is 8 or more. Furthermore, if the DSP in each adaptive equalization means (subscriber-side adaptive equalization means 223-k and station-side adaptive equalization means 443) is operated with a double oversampling clock, the required number of taps N is 16 or more.

[0074] From the above, it can be seen that the number of taps required by the fixed equalization means (station-side fixed equalization means 442 and subscriber-side fixed equalization means 422-k) and the adaptive equalization means (subscriber-side adaptive equalization means 223-k and station-side adaptive equalization means 443) can be made approximately the same (for example, about 8 to 16). Specifically, for example, if the number of taps of the FIR filter used in the fixed equalization means (station-side fixed equalization means 442 and subscriber-side fixed equalization means 422-k) is set to 16 and the number of taps of the FIR filter used in the adaptive equalization means (subscriber-side adaptive equalization means 223-k and station-side adaptive equalization means 443) is set to 16, dispersion compensation can be performed by the fixed equalization means, and other waveform distortions can also be compensated for by the adaptive equalization means.

[0075] (A-3) Effects of the First Embodiment According to the first embodiment, the following effects can be achieved.

[0076] In the optical communication network system 1 of the first embodiment, each fixed equalization means (the station-side fixed equalization means 442 and the subscriber-side fixed equalization means 422-k) is configured to compensate for half the dispersion that occurs when Lk is set to the maximum value (Lmax). As a result, in the optical communication network system 1 of the first embodiment, by making the number of taps required for the fixed equalization means and the adaptive equalization means (the subscriber-side adaptive equalization means 223-k and the station-side adaptive equalization means 443) approximately the same, dispersion compensation is possible with the fixed equalization means, and other waveform distortions can also be compensated for with the adaptive equalization means.

[0077] Furthermore, in conventional PONs, the central office (OLT) needed to have fixed equalization means and adaptive equalization means for the number of subscribers (ONUs), but in the optical communication network system 1 of the first embodiment, the processing contents of the fixed equalization means and adaptive equalization means for each subscriber can be uniform (conventionally, fixed equalization according to the transmission distance had to be performed for each subscriber, but in the first embodiment, there is no need to separate them for each subscriber), so they can be combined into one and the circuit scale (DSP circuit scale) can be reduced. This leads to effects such as reduced equipment costs for the central office fixed equalization means 442 and reduced power consumption.

[0078] Furthermore, in conventional PONs, each subscriber (ONU) had to be provided with fixed equalization means and adaptive equalization means that performed processing according to the transmission distance between the OLT and the subscriber, but in the optical communication network system 1 of the first embodiment, the processing contents of the fixed equalization means and adaptive equalization means in each subscriber-side optical transmission device 401 can be made uniform, just like in the central office, so the configuration of the subscriber-side optical transmission device 401 can be unified into one type, facilitating mass production of the subscriber-side optical transmission device 401. This leads to effects such as reduced equipment costs for the subscriber-side optical transmission device 401 in the entire optical communication network system 1.

[0079] (B) Second embodiment A second embodiment of an optical transmission device and an optical communication network system according to the present invention will be described below in detail with reference to the drawings.

[0080] (B-1) Configuration of the second embodiment Fig. 2 is a block diagram showing the overall configuration of an optical communication network system 1A according to the second embodiment. In Fig. 2, the same or corresponding parts as those in Fig. 1 are denoted by the same or corresponding reference numerals.

[0081] The following describes the optical communication network system 1A of the second embodiment, focusing on the differences from the first embodiment.

[0082] The optical communication network system 1A of the second embodiment differs in that the station-side optical transmission device 402 and the subscriber-side optical transmission device 401 (401-1 to 401-m) are replaced with the station-side optical transmission device 402A and the subscriber-side optical transmission device 401A (401A-1 to 401A-m).

[0083] The subscriber-side optical transmission device 401A-k differs from the first embodiment in that the subscriber-side fixed equalization means 422-k is omitted and the output signal of the subscriber-side analog / digital conversion means 221-k is directly supplied to the subscriber-side adaptive equalization means 223-k.

[0084] The optical transmission device 402A on the optical line side differs from the first embodiment in that it additionally includes an optical line side pre-equalization means 522. In the optical transmission device 402A on the optical line side, an output signal from the time division multiplexing means 350 is supplied to the optical line side pre-equalization means 522. In the optical transmission device 402A on the optical line side, an output signal from the optical line side pre-equalization means 522 is supplied to the optical line side digital-to-analog conversion means 332.

[0085] As described above, the optical communication network system 1A of the second embodiment has the same configuration as the first embodiment with respect to upstream communication, but with respect to downstream communication, the optical transmission device 402A is configured to have a function corresponding to the subscriber-side fixed equalization means 422 that the subscriber-side optical transmission device 401 in the first embodiment has as an optical transmission device 522 on the optical transmission device side.

[0086] (B-2) Operation of the Second Embodiment Next, the operation of the optical communication network system 1A according to the first embodiment will be described, focusing on the differences from the first embodiment.

[0087] As described above, in the second embodiment, the processing of the station side pre-equalization means 522 in downstream communication is different, so the operation of this station side pre-equalization means 522 will be mainly described.

[0088] In downstream communication, for optical transmission to the subscriber-side optical transmission device 401A-k, the station-side optical transmission device 402A imparts cumulative dispersion of Sfix=-D×Lmax / 2 to the optical signal using the station-side pre-equalization means 522. That is, the station-side optical transmission device 402A transmits an optical signal with a waveform that has been pre-distorted (an optical signal to which distortion corresponding to the distortion occurring in the downstream optical transmission path 610 has been imparted). In other words, the station-side pre-equalization means 522 uses a DSP equalization circuit to pre-impart waveform distortion to the optical signal to be transmitted, the waveform distortion having the opposite characteristics to the dispersion compensated for by the subscriber-side fixed equalization means 422-k in the first embodiment through fixed equalization (hereinafter, the process of imparting distortion by this DSP equalization circuit is referred to as "pre-equalization" or "distortion imparting"). Hereinafter, the station-side pre-equalization means 522 will also be simply referred to as "pre-equalization means" or "pre-distortion imparting means."

[0089] The sum of the distances of the downstream common optical transmission line 611 and the downstream tributary optical transmission line 612-k is Lk, and an accumulated dispersion of D×Lk is imparted to the optical signal there, and at the input to the k-th subscriber-side optical transmission device 401A-k, the optical signal is affected by the accumulated dispersion Sres_k=D(Lk-Lmax / 2), causing waveform distortion. The subscriber-side adaptive equalization means 223-k can compensate for this accumulated dispersion Sres_k.

[0090] Here, since 0<=Lk<=Lmax, the range of the absolute value |Sres_k| of the residual dispersion Sres_k is 0<=|Sres_k|<=|Sfix|. Therefore, the number of taps of the FIR filter required for residual dispersion compensation in the subscriber-side adaptive equalization means 223-k is equal to or less than the number of taps of the office-side fixed equalization means 442. Therefore, as in the first embodiment, in downstream communications in the optical communication network system 1A, if the number of taps of the FIR filter required for dispersion compensation is equal to or less than the number of taps of the FIR filter required to compensate for such waveform distortion, waveform distortion due to dispersion, etc., can be compensated for regardless of the transmission distance.

[0091] (B-3) Effects of the Second Embodiment According to the second embodiment, in addition to the effects of the first embodiment, the following effects can be achieved.

[0092] In the optical communication network system 1A of the second embodiment, the fixed equalization means is integrated into the optical transmission device 402A at the central office, thereby reducing the overall DSP circuit size, which leads to effects such as reduced costs for the devices constituting the entire optical communication network system 1A and reduced power consumption.

[0093] In the optical communication network system 1A of the second embodiment, the configuration (DSP circuit scale, etc.) of the subscriber-side optical transmission device 401A is simplified, thereby achieving effects such as reduction in hardware size and power consumption. This improvement is useful for a point-to-multipoint communication network in which the installation environment of the subscriber-side device is harsh, such as "small installation volume" or "poor power supply situation."

[0094] (C) Other embodiments The present invention is not limited to the above-described embodiments, and may include modified embodiments such as those exemplified below.

[0095] (C-1) In the optical communication network systems 1 and 1A of the above embodiments, an FIR filter that performs convolution processing in the time domain (time-domain equalization, TDE) was used to realize the equalization means (adaptive equalization means and fixed equalization means) in a DSP circuit. An equivalent method to convolution processing is frequency-domain equalization (FDE), which performs a discrete Fourier transformation (DFT) on an input signal, multiplies it by a frequency transfer function, and performs an inverse discrete Fourier transform (IDFT) on it to return it to a time-domain signal format and output it. DFT / IDFT processes the input signal sequence while reading it in block units. In such block processing, a method known as the overlap method is generally used, in which a certain block is read and processed, and then a portion of the sequence at the end of that block is read again (i.e., overlap) and a new sequence is read in to form the next block and process it, so that inconsistencies with the signal obtained by equivalent TDE do not occur at the boundary between successive blocks. The overlap sequence length is related to the number of taps in the equivalent TDE, and the greater the number of taps in the equivalent TDE, the longer the overlap sequence length. Storage elements are required to hold the overlap sequence, which increases the circuit size. Furthermore, the longer the overlap sequence length, the larger the block size and the number of DFT / IDFT points. Therefore, when using FDE means in a DSP circuit, the greater the amount of dispersion to be compensated for, the larger the circuit size.

[0096] From the above, it can be seen that the same effect can be obtained even if the above-mentioned FDE means is applied to the equalization means (each adaptive equalization means and fixed equalization means) in the optical communication network systems 1 and 1A of the above-mentioned embodiments. In other words, in the optical communication network systems 1 and 1A of the above-mentioned embodiments, the equalization means (each adaptive equalization means and fixed equalization means) is not limited to TDE or the like, and FDE may be applied.

[0097] In the first embodiment, regarding the dispersion amount Sfix (hereinafter also referred to as the "fixed compensation amount") compensated by each fixed equalization means (the network-side fixed equalization means 442 and the subscriber-side fixed equalization means 422-k), it was set as half of the dispersion amount that occurs when Lk is the maximum value (Lmax) (that is, the dispersion amount that occurs when Lk is Lmax / 2). However, the fixed compensation amount is not limited to this, and it may be set as the dispersion amount that occurs when Lk is a transmission distance longer than 0 and shorter than the maximum (Lmax) (0 < Lk < Lmax). That is, the fixed compensation amount Sfix may be set as the dispersion amount that occurs within the range of 0 < Lk < Lmax.

[0098] Here, when the transmission distance corresponding to Sfix [ps / nm] is set as x [Km] (0 < x < Lmax), the dispersion amount (hereinafter also referred to as the "adaptive compensation amount") Saeq that should be compensated by each adaptive equalization means (the network-side adaptive equalization means 443 and the subscriber-side adaptive equalization means 223-k) is considered. In the following, when the dispersion amount is expressed as D A with a subscript A, it will be explained that the subscript represents the transmission distance on the optical transmission path 600. That is, D A indicates the dispersion amount that occurs when the transmission distance is A.

[0099] Here, the dispersion amount that the signal immediately after reception is affected by in the receiving device of the optical signal is D x Therefore, the dispersion amount that the signal after fixed equalization is affected by is D x + Sfix. Thus, Saeq = -(D x + Sfix). For example, here, if Sfix = -D Lmax / 2 is set, then Saeq = D (Lmax / 2-x) If Sfix = 0 (that is, the dispersion amount that occurs when the transmission distance is 0 km) is set, then Saeq = -D x If Sfix = -D Lmax is set, then Saeq = D (Lmax-x)It becomes as follows. That is, as described above, since 0 <= x <= Lmax, the possible values of the dispersion amount Saeq given by adaptive equalization are -DLmax <= Saeq <= DLmax. Note that since x and Lmax are distances, they are non - negative values, and D is a positive value in the wavelength band (around 1.55 μm) usually used in optical fiber communication. Therefore, in fixed equalization for the purpose of dispersion compensation, Sfix becomes a negative value. In other words, Sfix has a value with the opposite sign (i.e., a negative value) to the dispersion corresponding to the transmission distance in the range of 0 < Lk < Lmax.

[0100] Here, when the transmission distances corresponding to the fixed compensation amount Sfix are changed to 0, Lmax / 2, and Lmax (that is, when the fixed compensation amount Sfix is changed to 0, -D Lmax / 2 , -D Lmax ), the number of taps of the DSP equalization circuit used in each fixed equalization means and each adaptive equalization means is examined by the same calculation formula as in the above - described embodiment.

[0101] When the transmission distance is Lmax / 2 (Sfix = -D Lmax / 2 ), the number of taps required for each fixed equalization means is 8, and the number of taps required for each adaptive equalization means is also 8.

[0102] When the transmission distance is 0 (Sfix = 0), the number of taps required for each fixed equalization means is 1, and the number of taps required for each adaptive equalization means is 15. Note that in the fixed equalization means, the number of taps being 1 means that the signal passes through directly, that is, the input signal is output as it is.

[0103] When the transmission distance is Lmax (Sfix = -D Lmax ), the number of taps required for each fixed equalization means is 15, and the number of taps required for each adaptive equalization means is also 15. In this case, since the adaptive equalization means needs to correspond to all transmission distances (all distances less than or equal to Lmax), it is necessary to use the maximum value among the number of taps of adaptive equalization obtained for each transmission distance as the number of taps required for adaptive equalization.

[0104] As described above, in this example, it can be seen that according to the fixed compensation amount Sfix adapted to the fixed equalization means, the number of taps required for the fixed equalization means and the adaptive equalization means varies between 1 and 15 respectively. Also, from the above, when the transmission distance corresponding to the fixed equalization is set to Lmax / 2, it can be seen that the number of taps of the fixed equalization means and the adaptive equalization means are arranged in a well-balanced and efficient configuration. On the other hand, when the transmission distance corresponding to the fixed equalization is extremely short, such as 0, the number of taps of the fixed equalization means becomes the minimum (number of taps 1), but there is a bias that the dispersion compensation must be performed only by the adaptive equalization means, so the total number of taps required for the entire optical communication network system 1 is larger than when the transmission distance is set to Lmax / 2. Also, when the transmission distance corresponding to the fixed equalization exceeds Lmax / 2 significantly, such as when the transmission distance is Lmax, since it is necessary to set the maximum value among the number of taps of the adaptive equalization obtained for each transmission distance as the number of taps required for the adaptive equalization (because it is necessary to cope with overcompensation by the fixed equalization), the total number of taps required for the entire optical communication network system 1 is larger than when the transmission distance is set to Lmax / 2.

[0105] From the above, in the first embodiment, the transmission distance corresponding to the fixed compensation amount Sfix for each fixed equalization means varies between a distance longer than 0 and shorter than Lmax (that is, 0 < Sfix < -D Lmax even when it varies), it can be seen that appropriate adaptive equalization can be achieved by arranging a DSP equalization circuit with the number of taps (the number of taps that can compensate for the residual dispersion and waveform distortion due to overcompensation at the output of the fixed equalization means) corresponding to it for each adaptive equalization means.

[0106] Also, similarly in the second embodiment, for the local-side pre-equalization means 522, a process of pre-applying waveform distortion with characteristics opposite to the dispersion that occurs when the transmission distance Lk is set to be longer than 0 and shorter than the maximum (Lmax) (0 < Lk < Lmax) in the same manner as the above fixed equalization means may be performed, and a DSP equalization circuit with the number of taps corresponding to it may be arranged in the subscriber-side adaptive equalization means 223-k.

Explanation of Reference Numerals

[0107] 1, 1A... Optical communication network system, 211, 211-1 to 211-m, 211-k... Subscriber side symbol mapper, 212-1 to 212-m, 212-k... Subscriber side digital / analog conversion means, 221, 221-1 to 221-m, 221-k... Subscriber side analog / digital conversion means, 223, 223-1 to 223-m, 223-k... Subscriber side adaptive equalization means , 224, 224-1 to 224-m, 224-k... subscriber side symbol demapper, 231, 231-1 to 231-m, 231... station side symbol mapper, 244, 244-1 to 244-m, 244-k... station side symbol demapper, 332... station side digital / analog conversion means, 341... station side analog / digital conversion means, 350... time division multiplexing means, 401, 401-1 to 401- m, 401-k, 401A, 401A-1 to 401A-m, 401A-k... subscriber-side optical transmission devices, 402, 402A... central office-side optical transmission devices, 422, 422-1 to 422-m, 422-k... subscriber-side fixed equalization means, 442... central office-side fixed equalization means, 443... central office-side adaptive equalization means, 522... central office-side pre-equalization means, 600... optical transmission path, 610... downstream optical transmission path, 611... downstream common optical Transmission paths 612, 612-1 to 612-m, 612-k... downstream tributary optical transmission paths, 613, splitter, 620... upstream optical transmission paths, 621... upstream common optical transmission paths, 622, 622-1 to 622-m, 622-k... upstream tributary optical transmission paths, 623... splitter, 700, 700-1 to 700-m, 700-k... subscriber side hosts, 800... termination equipment, 900... core side network

Claims

1. In an optical communication network system in which a station-side optical transmission device and a plurality of subscriber-side optical transmission devices are connected by optical branching transmission lines, the optical transmission device at the station side, a station-side fixed compensation means for compensating for a fixed amount of dispersion in the signal received from the optical branch transmission line; a station-side adaptive compensation means for compensating for distortion in the output signal of the station-side fixed compensation means, Each of the subscriber-side optical transmission devices a subscriber-side fixed compensation means for compensating for a fixed amount of dispersion in a signal received from the optical branch transmission line; a subscriber-side adaptive compensation means for compensating for distortion in the output signal of the subscriber-side fixed compensation means, The station-side fixed compensation means and the subscriber-side fixed compensation means compensate for dispersion that occurs when the optical branch transmission line is set to a distance in a range from longer than 0 to shorter than the maximum distance. An optical communication network system comprising:

2. 2. The optical communication network system according to claim 1, wherein said station-side fixed compensation means and said subscriber-side fixed compensation means compensate for dispersion that occurs when said optical branch transmission line is set to half its maximum distance.

3. 3. The optical communication network system according to claim 2, wherein the station-side fixed compensation means, the station-side adaptive compensation means, the subscriber-side fixed compensation means, and the subscriber-side adaptive compensation means all perform dispersion compensation processing using FIR filters.

4. 4. The optical communication network system according to claim 3, wherein the station-side fixed compensation means, the station-side adaptive compensation means, the subscriber-side fixed compensation means, and the subscriber-side adaptive compensation means all perform dispersion compensation processing using FIR filters with the same number of taps.

5. 5. The optical communication network system according to claim 4, wherein the FIR filters provided in the station-side fixed compensation means, the station-side adaptive compensation means, the subscriber-side fixed compensation means, and the subscriber-side adaptive compensation means are all configured using DSP equalization circuits.

6. A communication method performed in an optical communication network system in which a station-side optical transmission device and a plurality of subscriber-side optical transmission devices are connected by optical branching transmission lines, comprising: the optical transmission device at the station side includes a fixed compensation unit and an adaptive compensation unit at the station side; Each of the subscriber-side optical transmission devices has a subscriber-side fixed compensation means and a subscriber-side adaptive compensation means, the station-side fixed compensation means compensates for dispersion in the signal received from the optical branch transmission line by a fixed amount; the station-side adaptive compensation means compensates for distortion in the output signal of the station-side fixed compensation means; the subscriber-side fixed compensation means compensates for a fixed amount of dispersion in the signal received from the optical branch transmission line; the subscriber-side adaptive compensation means compensates for distortion in the output signal of the subscriber-side fixed compensation means; The station-side fixed compensation means and the subscriber-side fixed compensation means compensate for dispersion that occurs when the optical branch transmission line is set to a distance in a range from longer than 0 to shorter than the maximum distance. A communication method comprising:

7. In a central office optical transmission device connected to a plurality of subscriber-side optical transmission devices via optical branching transmission lines, a station-side fixed compensation means for compensating for a fixed amount of dispersion in the signal received from the optical branch transmission line; a station-side adaptive compensation means for compensating for distortion in the output signal of the station-side fixed compensation means, The station-side fixed compensation means compensates for dispersion that occurs when the optical branch transmission line is set to a distance in the range from 0 to a maximum distance.

1. A station-side optical transmission device.

8. In a subscriber-side optical transmission device connected to a station-side optical transmission device by an optical branching transmission line, a subscriber-side fixed compensation means for compensating for a fixed amount of dispersion in a signal received from the optical branch transmission line; a subscriber-side adaptive compensation means for compensating for distortion in the output signal of the subscriber-side fixed compensation means, The subscriber-side fixed compensation means compensates for dispersion that occurs when the optical branch transmission line is set to a distance in the range from longer than 0 to shorter than the maximum distance. A subscriber-side optical transmission device characterized by:

9. In an optical communication network system in which a station-side optical transmission device and a plurality of subscriber-side optical transmission devices are connected by optical branching transmission lines, the optical transmission device at the station side, a pre-distortion imparting means for imparting a waveform distortion having a fixed amount of dispersion and an inverse characteristic to the signal to be sent to the optical branch transmission line; a station-side fixed compensation means for compensating for a fixed amount of dispersion in the signal received from the optical branch transmission line; a station-side adaptive compensation means for compensating for distortion in the output signal of the station-side fixed compensation means, Each of the subscriber-side optical transmission devices is a subscriber-side adaptive compensation means for compensating for distortion of the signal received from the optical branch transmission line; the pre-distortion imparting means imparts waveform distortion having an inverse characteristic of dispersion that occurs when the optical branch transmission line is set to a distance in a range longer than 0 and shorter than a maximum distance, The station-side fixed compensation means compensates for dispersion that occurs when the optical branch transmission line is set to a distance in the range from 0 to a maximum distance. An optical communication network system comprising:

10. A communication method performed in an optical communication network system in which a station-side optical transmission device and a plurality of subscriber-side optical transmission devices are connected by optical branching transmission lines, comprising: the optical transmission device at the station side includes a pre-distortion applying means, a station side fixed compensation means, and a station side adaptive compensation means; Each of the subscriber-side optical transmission devices has a subscriber-side adaptive compensation means, the pre-distortion imparting means imparts a waveform distortion having an inverse characteristic to a fixed amount of dispersion to the signal to be sent to the optical branch transmission line; the station-side fixed compensation means compensates for a fixed amount of dispersion in the signal received from the optical branch transmission line; the station-side adaptive compensation means compensates for distortion in the output signal of the station-side fixed compensation means; the subscriber-side adaptive compensation means compensates for distortion in the signal received from the optical branch transmission line; the pre-distortion imparting means imparts waveform distortion having an inverse characteristic of dispersion that occurs when the optical branch transmission line is set to a distance in a range longer than 0 and shorter than a maximum distance, The station-side fixed compensation means compensates for dispersion that occurs when the optical branch transmission line is set to a distance in the range from 0 to a maximum distance. A communication method comprising:

11. In a central office optical transmission device connected to a plurality of subscriber-side optical transmission devices via optical branching transmission lines, a pre-distortion imparting means for imparting a waveform distortion having a fixed amount of dispersion and an inverse characteristic to the signal to be sent to the optical branch transmission line; a station-side fixed compensation means for compensating for a fixed amount of dispersion in the signal received from the optical branch transmission line; a station-side adaptive compensation means for compensating for distortion in the output signal of the station-side fixed compensation means, the pre-distortion imparting means imparts waveform distortion having an inverse characteristic of dispersion that occurs when the optical branch transmission line is set to a distance in a range longer than 0 and shorter than a maximum distance, The station-side fixed compensation means compensates for dispersion that occurs when the optical branch transmission line is set to a distance in the range from 0 to a maximum distance.

1. A station-side optical transmission device.

12. In a subscriber-side optical transmission device connected to a station-side optical transmission device by an optical branching transmission line, a subscriber-side adaptive compensation means for compensating for the distortion of a signal received from the optical branch transmission line, the signal having a waveform distortion that is an inverse characteristic of a fixed amount of dispersion previously imparted by the station-side optical transmission device; The fixed amount is the dispersion that occurs when the optical branch transmission line is set to a distance that is longer than 0 and shorter than the maximum distance. A subscriber-side optical transmission device characterized by:

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