JP2014014028A - Optical communication method, optical transmitter, optical receiver, and optical communication system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical communication method, an optical transmitter, an optical receiver and an optical communication system, capable of suppressing inter-pulse interference in a signal mark/space level, which is one factor of EVM (Error Vector Magnitude) deterioration, to generate a time-division multiplex signal of an identical wavelength in a light region, in a broadband ubiquitous network using a ubiquitous antenna system.SOLUTION: When modulating a pulsed optical carrier wave by a modulation signal in a ubiquitous antenna system to which broadband optical transmission is applied, the pulsed optical carrier wave is intensity-modulated after AM/FM converting the modulation signal, instead of direct intensity-modulation of the pulsed optical carrier wave by the modulation signal, so that receiver sensitivity is improved while using a simple optical receiver configuration.

Description

  The present invention relates to an optical communication method, an optical transmitter, an optical receiver, and an optical communication system in a broadband ubiquitous network.

  In recent years, the speed of optical access systems has been remarkably increased. In a FTTH (Fiber To The Home) system that provides high-speed communication services, 1GE-PON (Gigabit Ethernet (registered trademark) -Passive Optical Network) with a communication speed of 1 Gbps is currently used. However, in the future, studies on 10GE-PON capable of high-speed communication at 10 Gbps and the next generation PON system will be actively discussed in standardization organizations such as IEEE and FSAN (Full Service Access Network) with the aim of further speeding up. Has been. On the other hand, in the field of wireless communication, IEEE802.11n, which is a high-speed wireless LAN standard that realizes an effective speed of 100 Mbps or more while maintaining backward compatibility with IEEE802.11a / g, which is a conventional wireless LAN standard, and a maximum of 37. The LTE (Long Term Evolution) service, which is the standard of the 3.9th generation mobile communication that realizes high-speed communication of 5 Mbps, is used. Furthermore, as a future system, IMT-Advanced, which is being studied by the International Telecommunication Union-Radio communication sector (ITU-R), is assumed to realize the maximum 1 Gbps transmission using a new frequency band. In the next-generation wireless access, there is an increasing expectation for realizing a giga-class high-speed service.

  Among these, in particular, as a recent feature in wireless communication, wireless terminals have spread to many people, various wireless communication services such as wireless LAN and 3G are mixed, large capacity and real-time performance. For example, the traffic of requested data is increasing. To meet these requirements, femtocells covering a small communication area, RoF (Radio on Fiber) technology for transmitting radio signals over optical fiber while preserving the radio wave format, and high speed and large capacity using wavelength division multiplexing technology WDM-PON (Wavelength Division Multiplexing-Passive Optical Network) technology that realizes an FTTH system capable of communication, and MIMO (Multiple-Input Multiple-Output) technology that realizes higher radio transmission speed using space division multiplexing. Therefore, a system that enables flexible distributed arrangement of radio base stations, that is, a ubiquitous antenna system using broadband optical transmission has been proposed (for example, non-patent literature) See.).

  FIG. 1 shows a configuration of a ubiquitous antenna system to which broadband optical transmission is applied. This system applies RoF technology to a WDM-PON access network, and various radio wave formats are distributed between remote antennas of BS (Radio Base Station) 400 and CS (Center Station) 200 that are distributed. A number of femtocells that are transparently connected to the wireless service are configured (see, for example, Non-Patent Document 2). In FIG. 1, the number of antennas per BS and the number of MIMO antennas per cell are two, but it is not necessary to limit them to two in an actual system. With this system, it is possible to construct an access network that can use various wireless communication services for general purposes, and it is possible to obtain a high degree of flexibility for traffic and user movement. By providing various wireless communication services flexibly, Overall, large-capacity wireless communication of gigabit class can be realized.

JP 2010-245987 A

Kenji Miyamoto, Tatsuya Saikai, Takeshi Higashino, Katsutoshi Tsukamoto, Shozo Komaki, Takayoshi Tashiro, Kazutaka Hara, Tomohiro Taniguchi, Junichi Kani, Naoto Yoshimoto, Katsumi Iwatsuki, “Ubiquitous Antenna System Using Broadband Optical Transmission-AP Wavelength Multiplexing / Sector Service Time Multiplex Experiment- ", MWP Study Group, November 26, 2010. Katsumi Iwatuki, Jun-ichi Kani, Hiro Suzuki, Masamichi Fujiwara, “Access and Metro Networks based on WDM Technologies”, IE 22E. 11, pp. 2623-2630, 2004. Satoshi Ikeda, et al., "Wide-area optical video distribution using FM batch conversion technology", 2008 IEICE Electronics Society Conference, CS-5-4 Takato Kawatsuki, et al., “Transmission method of BS / CS110 ° signal by FM batch conversion method (3)”, 2007 IEICE Society Conference, B-8-19 M.M. Fujiwara, et al. , “Flatened optical multicarrier generation of 12.5 GHz spaced 256 channels based on sinusoidal amplification and phase hybrid modulation,” Electronics. 37, no. 15, 2001.

  In the ubiquitous antenna system to which the broadband optical transmission of FIG. 1 is applied, FIG. 2 shows a configuration when attention is paid to one cell. In this case, the number of upstream and downstream wavelengths is 2 (the total number of wavelengths is 4 for upstream / downstream), the number of BSs constituting one cell is 2, the number of antennas per BS is 2, and the number of MIMO antennas per cell is 2. In this system, a wavelength allocation multiplexing (WDM) technique is used to assign one wavelength to each BS. In addition, a plurality of MIMO signals are multiplexed on one channel, and signals are multiplexed to two adjacent cells sent to one BS, that is, signals of services different from sector multiplexing (for example, 2.4 GHz of IEEE802.11n) And a service division multiplexing (TDM: Time Division Multiplexing) technique is employed.

  FIG. 3 shows an example of the configuration of the CS 200 and the BS 400 in this system. 4 and 5 show time charts at each point in FIG.

  First, in the CS 200, coherent optical carriers (λ1, λ2) of two wavelengths are generated by LDs (Laser Diodes) 201-1 and 201-2, and wavelength division multiplexing is performed by the wavelength multiplexer 202 (a in FIG. 4). ). Then, pulsed carrier light is generated from a coherent carrier wave of each wavelength using an SPG (Sampling Pulse Generator) 203 and an optical pulse modulator 204 (b in FIG. 4). After splitting the pulsed carrier light of each wavelength using the wavelength demultiplexer 205, the pulsed carrier light of each wavelength is transmitted to the target cell by the branching optical coupler 206, and the MIMO signal in the adjacent cell, adjacent cell Branches for a 2.4 GHz band MIMO signal transmitted to the target cell, a 5 GHz band MIMO signal transmitted to the target cell, and a 5 GHz band MIMO signal transmitted to the adjacent cell (c in FIG. 4). For example, the wavelength demultiplexer 205 can be configured by AWG (Arrayed Waveguide Grating). Then, the branched pulsed carrier light is modulated by the IEEE802.11n signal by the signal modulator 207 (d in FIG. 4).

  Thereafter, the modulated light on which the 2.4 GHz band and 5 GHz band modulated signals to be transmitted to the adjacent cell are delayed by the delay circuit 209, and the modulated light to be transmitted to the target cell and the modulated light to be transmitted to the adjacent cell are combined in the optical multiplexer 210. And time-division multiplexed (e in FIG. 4) to obtain transmission signal light. Further, the modulated light in the 2.4 GHz band and the 5 GHz band is delayed by the delay circuit 211 and is time-division multiplexed by the optical multiplexer 212 to be transmitted signal light. The delay amounts are adjusted by the delay circuits 209 and 211 so that the delay circuit 209, the multiplexer 210, the delay circuit 211, and the multiplexer 212 can sequentially time-division multiplex four channels. Through the same process at the wavelength λ2, the transmission signal lights of λ1 and λ2 are wavelength division multiplexed by the wavelength multiplexer 213 and transmitted to the optical transmission line 500 (f in FIG. 4). For example, the wavelength multiplexer 213 can be composed of AWG.

  On the other hand, the transmission signal light from the CS 200 is wavelength-divided by the wavelength demultiplexer 300 and sent to the BS 400-1 and the BS 400-2, respectively (g in FIG. 5). For example, the wavelength demultiplexer 300 can be composed of AWG. Each BS (400-1, 400-2) performs optical-electrical conversion of an optical signal by a PD (Photo Diode) 403, and then electrically transmits pulse modulated signals of 2.4 GHz band and 5 GHz band to be transmitted to a target cell. Time division separation by switch 404 (h in FIG. 5). Thereafter, the modulated signal is restored from the pulsed modulated signal by a BPF (Band Pass Filter) 405 and transmitted from the antenna 406 (i in FIG. 5). Originally, each BS (400-1, 400-2) further has an electrical switch, a BPF, and an antenna for processing a pulse signal of 2.4 GHz band and 5 GHz band transmitted to adjacent cells. 3 is omitted.

  FIG. 3 shows a configuration in which a 2.4 GHz band modulated signal to be transmitted to the target cell and adjacent cells is branched by the coherent optical carrier of the LD 201-1 and modulated by each signal modulator 207. A configuration may be used in which the modulation signal in the 2.4 GHz band to the target cell and the adjacent cell is modulated using two LDs having the wavelength λ1. The optical pulse modulator 204 when generating the pulsed carrier light and the signal modulator 207 that modulates the pulsed carrier light with the modulation signal are intensity modulation. For example, the optical pulse modulator 204 and the signal modulator 207 are external modulators such as an LN-MZM (Lithium Niobate-Mach Zehnder Modulator) and an EAM (Electronic Absorption Modulator).

  However, in the above configuration, pulsed carrier light having the same wavelength is time-division multiplexed in the optical region using an LD with high coherence. 6 and 7 show the results of measuring the extinction ratio of general LN-MZM and EAM, and the ON / OFF ratio of the modulation signal is about 20 dB. The OFF level of the signal is not completely 0 level. As shown in the CS configuration of FIG. 3, if the 2.4 GHz band modulated signal to the target cell and the adjacent cell is multiplexed at the same wavelength, interference occurs between the mark / space levels of the signal. For this reason, in the above configuration, there is a problem that reception sensitivity may be deteriorated by beat noise caused by inter-pulse interference at the mark / space level of the signal.

  Therefore, in order to solve the above-mentioned problems, the present invention provides a time division based on the same wavelength in the optical region, which is one of the factors that degrade EVM (Error Vector Magnetoide) in a broadband ubiquitous network using a ubiquitous antenna system. An object of the present invention is to provide an optical communication method, a transmission device, a reception device, and an optical communication system capable of suppressing inter-pulse interference at a mark / space level of a signal in order to generate a multiplexed signal.

  In order to solve the above-described problems, the present invention provides a ubiquitous antenna system to which broadband optical transmission is applied. When modulating a pulsed optical carrier with a modulated signal, the pulsed optical carrier is directly modulated with the modulated signal. Rather than doing so, intensity modulation is performed on the optical carrier pulsed after AM / FM conversion of the modulated signal, thereby improving reception sensitivity while using a simple optical receiver configuration.

  Specifically, the present invention generates pulsed carrier light by pulsing a coherent optical carrier wave with a sampling pulse, and generates an RF (Radio Frequency) signal by AM / FM conversion of a modulated signal having a different frequency for each channel. Then, the pulsed carrier light is branched by the number of channels, intensity modulated with the RF signal for each channel to generate intensity modulated light, and the intensity modulated light for each channel is time-division multiplexed as transmission signal light Transmitting and receiving the transmitted signal light to generate a received signal by opto-electrical conversion, time-separated RF signal pulsed from the received signal for each channel, and pulsed RF signal for each channel In this optical communication method, the RF signal for each channel is restored from the above, and the RF signal for each channel is FM / AM converted to reproduce the modulated signal.

  In the present invention, when generating pulsed carrier light, a plurality of coherent optical carriers having different wavelengths are wavelength-division multiplexed, then collectively pulsed with the sampling pulse, and wavelength-divided and separated for each wavelength. When transmitting as the pulsed carrier light, the transmission signal light for each wavelength is wavelength-division-multiplexed to obtain transmission signal light, and when generating a reception signal, the wavelength-division-multiplexed transmission signal light is wavelength-specific. After dividing and separating, the received signal may be converted into light and electricity.

  Specifically, in the present invention, a pulsed carrier light generation circuit that generates a pulsed carrier light by pulsing a coherent optical carrier wave with a sampling pulse, and an RF / RF signal obtained by AM / FM conversion of a modulation signal having a different frequency for each channel. An AM / FM conversion circuit for generating a signal and the pulsed carrier light from the pulsed carrier light generation circuit are branched by the number of channels, and intensity modulation is performed by the RF signal from the AM / FM conversion circuit for each channel. An intensity-modulated light generation circuit that generates intensity-modulated light, and a time-division multiplexing circuit that time-division-multiplexes the intensity-modulated light for each channel from the intensity-modulated light generation circuit and transmits the light as transmission signal light. It is an optical transmitter.

  In the present invention, the pulsed carrier light generation circuit wavelength-division-multiplexes a plurality of the coherent optical carriers having different wavelengths, and then collectively pulsates with the sampling pulse, and wavelength-division-separates each wavelength for each wavelength. The pulsed carrier light may be generated, and the time division multiplexing circuit may perform wavelength division multiplexing of the transmission signal light for each wavelength to obtain wavelength division multiplexed transmission signal light.

  The present invention relates to a receiving circuit that generates a received signal by opto-electrically converting received transmission signal light, and time division that separates an RF signal pulsed from the received signal from the receiving circuit for each channel. A separation circuit; an RF signal restoration circuit for restoring an RF signal for each channel from the pulsed RF signal for each channel from the time division separation circuit; and an RF signal for each channel from the RF signal restoration circuit. And an FM / AM conversion circuit that reproduces a modulated signal by FM / AM conversion.

  In the present invention, the receiving circuit may divide the wavelength-division-multiplexed transmission signal light for each wavelength and then perform optical-electrical conversion to obtain the received signal.

  The present invention is an optical communication system including the above-described optical transmission device and the above-described optical reception device.

  In the broadband ubiquitous network using a ubiquitous antenna system, the present invention provides a pulse at the mark / space level of a signal in order to generate a time-division multiplexed signal with the same wavelength in the optical region, which is one of the factors that degrade EVM. It is possible to provide an optical communication method, an optical transmitter, an optical receiver, and an optical communication system that can suppress inter-interference.

It is a figure explaining the broadband ubiquitous network by the ubiquitous antenna system to which broadband optical transmission is applied. It is a figure explaining the broadband ubiquitous network when the number of wavelengths is 2 (the number of wavelengths is 4 for uplink / downlink), the number of BS is 2, and the number of MIMO antennas per cell is 2. It is a figure explaining the structure of CS and BS of a ubiquitous antenna system. It is the figure which showed the time chart in each point of CS. It is the figure which showed the time chart in each point of BS. It is the figure which showed the result of having measured the extinction ratio of LN-MZM. It is the figure which showed the result of having measured the extinction ratio of EAM. It is a figure explaining the structure of the optical communication system which concerns on this invention. It is a figure explaining one form of the structure of an AM / FM converter. It is a figure explaining one form of the structure of an AM / FM converter. It is a figure explaining one form of the structure of an AM / FM converter. It is a figure explaining one form of the structure of an AM / FM converter. It is a figure explaining one form of the structure of FM / AM converter. It is a figure explaining the generation | occurrence | production principle of the interference between optical pulses. It is a figure explaining the structure of the optical communication system which concerns on this invention. It is a figure explaining the structure of the optical communication system which concerns on this invention. It is a figure explaining the structure of the optical communication system which concerns on this invention. It is a figure explaining a ubiquitous antenna system. It is a figure explaining the structure of the optical communication system which concerns on this invention. It is a figure explaining a ubiquitous antenna system. It is a figure explaining the structure of the optical communication system which concerns on this invention. It is a figure explaining a ubiquitous antenna system. It is a figure explaining the structure of the optical communication system which concerns on this invention.

Embodiments of the present invention will be described with reference to the drawings. In the present specification and drawings, the same reference numerals denote the same components.
The CS and BS are not limited to the transmission side and the reception side, and it is obvious to those skilled in the art that the technical idea of the present application can be realized even if the reception side and the transmission side are switched.
Further, the multiplexing number is not limited to the numerical values described in the description, and can be arbitrarily set.
In addition, the modulation signal described in the embodiment indicates a general radio signal transmitted by the RoF technology, and the frequencies described in the description (for example, 2.4 GHz band, 5 GHz band, etc.) are examples, It is not limited to these numerical values. The wireless type systems (for example, IEEE802.11n, MIMO, etc.) are also just examples, and are not limited to these.

(Embodiment 1)
This embodiment is an example in which the wavelength division multiplexing technique is not applied to transmission between CS and BS.

  FIG. 8 shows the configuration of the optical communication system according to the first embodiment. This optical communication system includes a CS 200-0 as an optical transmission device that modulates the intensity of an optical signal after AM / FM conversion of the modulation signal, and a BS 400 'as an optical reception device that reproduces the modulation signal. In FIG. 8, the number of time division multiplexing is set to two. First, a coherent optical carrier wave is generated in the LD 201 in the CS 200-0. The generated coherent optical carrier wave is pulsed by using a sampling pulse generation circuit (SPG) 203 and an optical pulse modulator 204. The LD 201, SPG 203, and optical pulse modulator 204 constitute a pulsed carrier light generation circuit.

  The AM / FM conversion circuit 215 performs AM / FM conversion on the Ch1 and Ch2 modulation signals, respectively. On the other hand, the branching optical coupler 206 splits the pulsed carrier light into two, and the intensity modulator 207 ′ generates intensity-modulated light by intensity-modulating the pulsed carrier light with the AM / FM converted Ch1 and Ch2 modulation signals. . The branched optical coupler 206 and the intensity modulator 207 'constitute an intensity modulated light generation circuit.

  The signal of the intensity-modulated light Ch1 is delayed by the delay circuit 209 and is multiplexed with the intensity-modulated light Ch2 by the optical multiplexer 210 to generate transmission signal light. In the delay circuit 209, the intensity-modulated light Ch1 and the intensity-modulated light Ch2 are combined so as not to overlap each other, and a time division multiplexing function is exhibited. Further, it is transmitted to the optical transmission line 500 as transmission signal light. The delay circuit 209 and the optical multiplexer 210 constitute a time division multiplexing circuit.

  An example of the configuration of the AM / FM conversion circuit is shown in FIG. 9 (see, for example, Non-Patent Document 3). In the configuration of FIG. 9, the output light from the laser diode (FM-LD) is FM-modulated with the modulation signal to generate FM modulated light. Further, the FM modulated light and the output light from the local LD (Local-LD) are combined and received by a photodiode to generate a beat, whereby the modulated signal is AM / FM converted to generate an RF signal. . The center frequency of the converted RF signal is stabilized by controlling the difference between the center frequency of the FM-LD output light and the center frequency of the Local-LD output light to be constant (Δf).

  Another example of the configuration of the AM / FM conversion circuit is shown in FIG. 10 (see, for example, Non-Patent Document 4). In the configuration of FIG. 10, a carrier wave from a laser diode (LD) is input to a phase modulator (PM-MOD), and FM modulation light is generated by PM modulation with a modulation signal. Further, the FM modulated light and the output light from the local LD (Local-LD) are combined and received by a photodiode to generate a beat, whereby the modulated signal is AM / FM converted to generate an RF signal. . The center frequency of the converted RF signal is stabilized by controlling the difference between the center frequency of the carrier wave from the LD and the center frequency of the output light of the Local-LD to be constant (Δf). Since this configuration does not require FM modulation by the LD, it is possible to use a low-cost LD and to easily control the center frequency between the LDs.

  Another example of the configuration of the AM / FM conversion circuit is shown in FIG. In this configuration, in FIG. 10, an LD and a Local-LD are used between a plurality of AM / FM conversion circuits. The basic principle for generating the RF signal is the same as the configuration of FIG. Since the number of control mechanisms for the center frequency between the LDs can be reduced, the cost can be further reduced as compared with the configuration of FIG.

  Another example of the configuration of the AM / FM conversion circuit is shown in FIG. In this configuration, in FIG. 10, an LD and a Local-LD are used between a plurality of AM / FM conversion circuits. In this configuration, first, the center frequency difference between the LD carrier wave and the Local-LD output light is set to Δf, and both are combined by the coupler (optical spectrum (1)). A solid line and a dotted line represent the optical spectra of the LD output light and the Local-LD output light, respectively. For convenience, the solid line is described as the low frequency side and the dotted line is described as the high frequency side, but the frequency relationship may be reversed. Next, intensity modulation and phase modulation are performed on the combined LD carrier wave and Local-LD output light by an intensity modulator (IM-MOD) and a phase modulator (PM-MOD), respectively. At this time, if the modulation frequency and the modulation index are optimized, the LD carrier wave and the Local-LD output light can be converted into multicarriers at intervals of 2Δf (see Non-Patent Document 5 for the modulation frequency and modulation index setting). (Light spectrum (2)). The solid line and the dotted line represent the optical spectrum in which the LD carrier wave and the Local-LD output light are converted into multicarriers, respectively. ). The frequency interval between adjacent optical spectra (dotted line and solid line) is Δf. Next, the optical carrier separates the carrier wave of the multicarrier LD and the output light of the multicarrier Local-LD. For example, a circular Mach-Zehnder filter (MZ) can be used as this optical filter, but other optical filters may be used (light spectrums (3) and (4)). Next, the multicarriers (3) and (4) are each separated into single carriers by an optical filter. For example, an arrayed waveguide grating (AWG) can be applied to this optical filter, but other optical filters may also be used (optical spectra (5), (6), (7), (8), (9 ), (10)). Among these, the single carrier (optical spectrum (5), (7), (9)) generated from the carrier wave of the LD is input to each different phase modulator (PM-MOD), and different modulation signals are used for this. FM modulated light is generated by PM modulation, and this is combined with a single carrier (optical spectrum (6), (8), (10)) generated from the output light of the Local-LD. By receiving light and generating a beat, each modulation signal is individually AM / FM converted. Note that the combination of carriers that are combined and input to the photodiode is the frequency difference of the optical spectrum is Δf as shown in (5) and (6), (7) and (8), and (9) and (10). Choose something. Since this configuration can reduce the number of LD, Local-LD, and center frequency control mechanisms between the LDs, it has the effect of further reducing the cost compared to the configuration of FIG. In addition, this configuration has different frequency carriers for each modulation signal ((5), (6), (7) generated from LD, or (6), (8), (10) generated from Local-LD). ) Is used, there is an effect that interference due to multiple reflection does not occur compared to the configuration of FIG.

  Note that the AM / FM conversion circuit can be applied to other configurations than those described above.

  In the BS 400 ′, a PD (Photo Diode) 403 constituting a reception circuit generates a reception signal by performing optical-electrical conversion on transmission signal light. Thereafter, the received signal is branched, and only the pulsed RF signal of the desired channel is taken out by the electric switch 404 constituting the time division separation circuit.

  A BPF (Band Pass Filter) 405 constituting the RF signal restoration circuit restores the RF signal for each channel by deleting the alias component. The FM / AM conversion circuit 407 performs FM / AM conversion on the RF signal for each channel to reproduce the original modulation signal. The modulated signal is transmitted from the antenna 406.

  Here, an example of the configuration of the FM / AM conversion circuit is shown in FIG. 13 (see, for example, Non-Patent Document 3). The configuration shown here uses a delay line. The RF signal that has been subjected to AM / FM conversion is branched into two, and a delay is given to one of them, and an AND (logical sum) is taken to obtain the original RF signal. To the modulated signal. The configuration of the FM / AM conversion circuit can be applied to other configurations.

  When the modulated signal is intensity-modulated and time-division multiplexed, as shown in FIG. 14, inter-pulse interference (ISI: Intersymbol Interference) that causes beat noise occurs due to leakage of the preceding pulse and is reproduced by BS 400 ′. There was a problem that the characteristics of the modulated signal deteriorated.

  In this embodiment, each modulated signal is AM / FM converted before being time division multiplexed. For this reason, since the time division multiplexed signal with the same wavelength in the optical region, which is one of the factors that degrade the EVM, is generated, inter-pulse interference at the mark / space level of the signal can be suppressed.

  Further, the characteristic deterioration due to the multiple optical reflection in the fiber transmission section can be similarly suppressed.

  Further, since the amplitude of the AM / FM converted RF signal is constant, there is no fluctuation in amplitude due to the state of the modulation signal in the section from the CS intensity modulator to the BPF of the BS, and linearity can be ensured directly. There is an effect that the characteristics are improved in terms of ensuring linearity rather than intensity modulation of the modulation signal.

(Embodiment 2)
FIG. 15 shows the configuration of the optical communication system according to the second embodiment. This optical communication system includes a CS 200-1 as an optical transmission device that modulates the intensity of an optical signal after AM / FM conversion of the modulation signal, and a BS 400 ′ as an optical reception device that reproduces the modulation signal. In FIG. 15, the number of time-division multiplexing is set to two. First, a coherent optical carrier wave having a plurality of wavelengths is generated by the LD 201 in the CS 200-1. The generated coherent optical carrier wave is wavelength division multiplexed by the wavelength multiplexer 202. The wavelength-division multiplexed coherent optical carrier wave is pulsed at once using a sampling pulse generation circuit (SPG) 203 and an optical pulse modulator 204. After pulsing, the wavelength demultiplexer 205 performs wavelength division to generate pulsed carrier light for each wavelength. The LD 201, the wavelength multiplexer 202, the SPG 203, the optical pulse modulator 204, and the wavelength demultiplexer 205 constitute a pulsed carrier light generation circuit.

  The AM / FM conversion circuit 215 performs AM / FM conversion on the Ch1 and Ch2 modulation signals, respectively. On the other hand, the branching optical coupler 206 splits the pulsed carrier light into two, and the intensity modulator 207 ′ generates intensity-modulated light by intensity-modulating the pulsed carrier light with the AM / FM converted Ch1 and Ch2 modulation signals. . The branched optical coupler 206 and the intensity modulator 207 'constitute an intensity modulated light generation circuit.

  The signal of the intensity-modulated light Ch1 is delayed by the delay circuit 209 and is multiplexed with the intensity-modulated light Ch2 by the optical multiplexer 210 to generate transmission signal light. In the delay circuit 209, the intensity-modulated light Ch1 and the intensity-modulated light Ch2 are combined so as not to overlap each other, and a time division multiplexing function is exhibited. Further, the wavelength multiplexer 213 multiplexes similarly generated transmission signal lights of other wavelengths and transmits them to the optical transmission line 500 as transmission signal lights that are wavelength division multiplexed. The delay circuit 209 and the optical multiplexer 210 constitute a time division multiplexing circuit.

  In the BS 400 ′, the transmission signal light is wavelength-divided and separated for each wavelength by the wavelength demultiplexer 300. As shown in FIG. 15, the wavelength demultiplexer 300 may be provided outside the BS 400 '.

  A PD (Photo Diode) 403 constituting the reception circuit generates a reception signal by performing photoelectric conversion on the transmission signal light. Thereafter, the received signal is branched, and only the pulsed RF signal of the desired channel is taken out by the electric switch 404 constituting the time division separation circuit.

  A BPF (Band Pass Filter) 405 constituting the RF signal restoration circuit restores the RF signal for each channel by deleting the alias component. The FM / AM conversion circuit 407 performs FM / AM conversion on the RF signal for each channel to reproduce the original modulation signal. The modulated signal is transmitted from the antenna 406.

  In this embodiment, a plurality of coherent optical carriers having different wavelengths are wavelength-division multiplexed, and then pulsed together with a sampling pulse, and wavelength-division-divided for each wavelength to obtain pulsed carrier light for each wavelength. The modulator can be reduced.

  When the modulated signal is intensity-modulated and time-division multiplexed, as shown in FIG. 14, inter-pulse interference (ISI: Intersymbol Interference) that causes beat noise occurs due to leakage of the preceding pulse and is reproduced by BS 400 ′. There was a problem that the characteristics of the modulated signal deteriorated.

  In this embodiment, each modulated signal is AM / FM converted before being time division multiplexed. For this reason, since the time division multiplexed signal with the same wavelength in the optical region, which is one of the factors that degrade the EVM, is generated, inter-pulse interference at the mark / space level of the signal can be suppressed.

  Further, the characteristic deterioration due to the multiple optical reflection in the fiber transmission section can be similarly suppressed.

  Further, since the amplitude of the AM / FM converted RF signal is constant, there is no fluctuation in amplitude due to the state of the modulation signal in the section from the CS intensity modulator to the BPF of the BS, and linearity can be ensured directly. There is an effect that the characteristics are improved in terms of ensuring linearity rather than intensity modulation of the modulation signal.

(Embodiment 3)
FIG. 16 shows the configuration of the optical communication system according to the third embodiment. The present optical communication system includes a CS 200-2 and a BS 400 ′. The CS 200-2 and the CS 200-1 of the second embodiment are such that the CS 200-2 generates a coherent optical carrier wave by applying an optical frequency comb (Optical Frequency Comb Generator) 201 ′ as an alternative to a plurality of LDs 201 having different wavelengths. Is different. The process after the generation of the coherent optical carrier wave is the same as that described in the second embodiment.

  In the configuration of the present embodiment, since an optical frequency comb is used to generate a plurality of coherent optical carriers having different wavelengths, LDs having different wavelengths can be eliminated.

(Embodiment 4)
FIG. 17 shows the configuration of the optical communication system according to the fourth embodiment. This optical communication system includes CS 200-3 and BS 400 ″. This optical communication system has a configuration in which the wavelength division multiplexing number is L, the sector multiplexing number is M, and the service multiplexing number is N. At each wavelength. The number of time division multiplexes, that is, the number of modulation signal channels, is the product of the number of sector multiplexes and the number of service multiplexes, and can be expressed as M × N, for example, L = 2, M = 2, and N = 2 in the configuration of FIG. Correspondingly, the number of time division multiplexing at each wavelength is M × N = 4.

  After intensity-modulating the pulsed carrier light by the intensity modulator 207 ′, each intensity-modulated light is time-division multiplexed by a time division multiplexing circuit (TDM) 220. As described in the first embodiment, a time division multiplexing circuit may be configured by the delay circuit and the optical multiplexer.

  In the following embodiments, the time division multiplexing circuit 220 is described instead of the delay circuit and the optical multiplexer. Subsequent to the time division multiplexing circuit is the same as that described in the second embodiment.

(Embodiment 5)
FIG. 18 shows the configuration of the ubiquitous antenna system when the number of antennas per cell is three. In this case, the number of antennas per BS 400 ″ is 6, and therefore the number of multiplexed sectors is M = 6. FIG. 19 shows the configuration of CS 200-3 and BS 400 ″ in FIG. The wavelength division multiplexing number L is determined by the number of BSs to be arranged. Further, if the services to be multiplexed are signals of 2.4 GHz band and 5 GHz band of IEEE802.11n as in the case of FIG. 3, the number of service multiplexes is N = 2, and therefore the number of time division multiplexes is M × N = 12 That is, this embodiment corresponds to the case where M × N = 12 in the fourth embodiment.

(Embodiment 6)
FIG. 20 shows the configuration of the ubiquitous antenna system when the number of antennas per cell is four. In this case, the number of antennas per BS 400 ″ is 4, and therefore the number of multiplexed sectors is M = 4. FIG. 21 shows the configuration of CS 200-3 and BS 400 ″ in FIG. The wavelength division multiplexing number L is determined by the number of BSs to be arranged. Further, if the services to be multiplexed are signals of 2.4 GHz band and 5 GHz band of IEEE802.11n as in the case of FIG. 3, the number of service multiplexes is N = 2, and therefore the number of time division multiplexes is M × N = 8 That is, this embodiment corresponds to the case where M × N = 8 in the fourth embodiment.

(Embodiment 7)
FIG. 22 shows the configuration of the ubiquitous antenna system when the number of antennas per cell is six. In this case, the number of antennas per BS 400 ″ is 3, and therefore the number of multiplexed sectors is M = 3. FIG. 23 shows the configuration of CS 200-3 and BS 400 ″ in FIG. The wavelength division multiplexing number L is determined by the number of BSs to be arranged. Further, if the services to be multiplexed are signals of 2.4 GHz band and 5 GHz band of IEEE802.11n as in the case of FIG. 3, the number of service multiplexes is N = 2, and therefore the number of time division multiplexes is M × N = 6 That is, this embodiment corresponds to the case where M × N = 6 in the fourth embodiment.

(Other embodiments)
In the above embodiment, for convenience, the network topology between CS and BS is described as a bus type, but is not limited thereto. For example, as shown in Patent Document 1, a similar effect can be obtained even in a ring configuration or star configuration topology. The delay line described in the embodiment may be an optical delay circuit or a transmission line as long as it gives a delay, and does not depend on the configuration or material.

100: Mobile operator core network 200, 200-0, 200-1, 200-2, 200-3: Central station (CS)
201, 201-1, 201-2, 201-L: Laser diode (LD)
201 ′: optical frequency comb 202: wavelength multiplexer 203: sampling pulse generation circuit (SPG)
204: optical pulse modulator 205: wavelength demultiplexer 206: branching optical coupler 207: signal modulator 207 ′: intensity modulator 209: delay circuit 210: optical multiplexer 211: delay circuit 212: optical multiplexer 213: wavelength multiplexing Wave 215: AM / FM conversion circuit 220: Time division multiplexing circuit (TDM)
300: Wavelength demultiplexers 400, 400 ′, 400 ″, 400-1, 400-2: Radio base stations (BS)
403: Photodiode (PD)
404: Electric switch 405: BPF
406: Antenna 407: FM / AM conversion circuit 500: Optical transmission line 600: Mobile terminal (UE)

Claims (8)

Pulse coherent optical carrier with sampling pulse to generate pulsed carrier light,
An RF signal is generated by AM / FM conversion of a modulation signal having a different frequency for each channel,
The pulsed carrier light is branched by the number of channels, and intensity modulated with the RF signal for each channel to generate intensity modulated light,
The intensity-modulated light for each channel is time-division multiplexed and transmitted as transmission signal light,
The received transmission signal light is photoelectrically converted to generate a reception signal,
RF signal pulsed from the received signal is time-division separated for each channel,
Restore the RF signal for each channel from the pulsed RF signal for each channel;
An optical communication method for reproducing the modulated signal by FM / AM converting the RF signal for each channel. When generating pulsed carrier light, a plurality of the coherent optical carriers having different wavelengths are wavelength-division multiplexed, then pulsed together with the sampling pulse, and wavelength-division-separated for each wavelength, and the pulsed carrier for each wavelength. Light and
When transmitting, the transmission signal light for each wavelength is wavelength division multiplexed into transmission signal light,
2. The optical communication according to claim 1, wherein, when generating a reception signal, the wavelength-division multiplexed transmission signal light is wavelength-division-separated for each wavelength and then optical-electrically converted into the reception signal. Method. A pulsed carrier light generation circuit that generates a pulsed carrier light by pulsing a coherent optical carrier with a sampling pulse;
An AM / FM conversion circuit for generating an RF signal by AM / FM converting a modulation signal having a different frequency for each channel;
Intensity modulated light that branches the pulsed carrier light from the pulsed carrier light generation circuit by the number of channels and generates intensity modulated light by intensity-modulating the RF signal from the AM / FM conversion circuit for each channel. A generation circuit;
A time-division multiplexing circuit that time-division-multiplexes the intensity-modulated light for each channel from the intensity-modulated light generation circuit and transmits it as transmission signal light; and
An optical transmission device comprising: The pulsed carrier light generation circuit wavelength-division-multiplexes a plurality of the coherent optical carriers having different wavelengths, and then pulsates them collectively with the sampling pulse, and wavelength-separates each wavelength to perform the pulsed carrier for each wavelength. Produce light,
4. The optical transmission apparatus according to claim 3, wherein the time division multiplexing circuit wavelength-division-multiplexes the transmission signal light for each wavelength to obtain wavelength-division multiplexed transmission signal light. A receiving circuit that generates a reception signal by photoelectrically converting the received transmission signal light; and
A time division separation circuit for time division separating the RF signal pulsed from the reception signal from the reception circuit for each channel;
An RF signal restoration circuit for restoring an RF signal for each channel from the pulsed RF signal for each channel from the time division separation circuit;
An FM / AM conversion circuit for FM / AM converting the RF signal for each channel from the RF signal restoration circuit to reproduce a modulated signal;
An optical receiver comprising:   6. The optical receiving apparatus according to claim 5, wherein the receiving circuit performs wavelength-division separation on the wavelength-division-multiplexed transmission signal light for each wavelength, and then performs optical-electrical conversion to obtain the received signal.   An optical communication system comprising the optical transmission device according to claim 3 and the optical reception device according to claim 5.   An optical communication system comprising the optical transmission device according to claim 4 and the optical reception device according to claim 6.

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