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WO2018198873A1 - Optical transmission method and optical transmission device - Google Patents

Optical transmission method and optical transmission device Download PDF

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Publication number
WO2018198873A1
WO2018198873A1 PCT/JP2018/015797 JP2018015797W WO2018198873A1 WO 2018198873 A1 WO2018198873 A1 WO 2018198873A1 JP 2018015797 W JP2018015797 W JP 2018015797W WO 2018198873 A1 WO2018198873 A1 WO 2018198873A1
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WO
WIPO (PCT)
Prior art keywords
signal
optical
light source
optical transmission
reception unit
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PCT/JP2018/015797
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French (fr)
Japanese (ja)
Inventor
中沢　正隆
勝美 岩月
俊彦 廣岡
吉田　真人
恵介 葛西
Original Assignee
国立大学法人東北大学
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Application filed by 国立大学法人東北大学 filed Critical 国立大学法人東北大学
Publication of WO2018198873A1 publication Critical patent/WO2018198873A1/en

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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/25—Arrangements specific to fibre transmission
    • H04B10/2507—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion
    • H04B10/2537—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion due to scattering processes, e.g. Raman or Brillouin scattering
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/25—Arrangements specific to fibre transmission
    • H04B10/2575—Radio-over-fibre, e.g. radio frequency signal modulated onto an optical carrier
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/60—Receivers
    • H04B10/61—Coherent receivers
    • H04B10/63—Homodyne, i.e. coherent receivers where the local oscillator is locked in frequency and phase to the carrier signal

Definitions

  • the present invention relates to an optical transmission method and an optical transmission apparatus.
  • Non-Patent Document 1 As mobile smartphone services become popular and mobile broadband services such as LTE (Long Term Evolution) progress, mobile communication traffic is rapidly increasing. Under such circumstances, research and development of a fifth generation mobile communication system (5G) as a next generation large-capacity mobile communication system has been energetically advanced in Japan and overseas (for example, see Non-Patent Document 1).
  • 5G fifth generation mobile communication system
  • the base station baseband unit (BBU: Base Band Unit) that performs transmission control and baseband signal processing is consolidated in one place, and the antenna radio unit (RRH: Remote Radio Head) responsible for antenna and high-frequency signal processing.
  • BBU Base Band Unit
  • RRH Remote Radio Head
  • C-RAN Centralized-Radio Access Network
  • the BBU and RRH mobile fronthaul
  • the BBU and RRH are connected by an optical fiber, and a radio signal transmitted and received from the antenna is superimposed on the light wave and transmitted.
  • a digital RoF (Radio over Fiber) system based on CPRI (Common Public Radio Interface) is widely used as a method of superimposing and transmitting a radio signal on a light wave (see, for example, Non-Patent Document 3).
  • CPRI Common Public Radio Interface
  • digital RoF since a radio signal is digitized and transmitted through an optical fiber, an optical transmission band approximately 16 times that of a radio signal is generally required.
  • 5G and higher-capacity wireless access systems assumed to have a wireless communication capacity of 10 / Gbit / s or higher, the transmission capacity required for the mobile fronthaul is 100 Gbit / s or higher.
  • TWDM-PON Time-and-Wavelength-Division-Multiplexing-Passive-Optical-Network
  • TDM-PON which is a conventional optical access network technology. So far, 10 Gbit / s, OOK (On-OffOnKeying) signal has been used.
  • a 40 Gbit / s transmission system that multiplexes four waves is realized (for example, see Non-Patent Document 5).
  • An efficient and economical mobile fronthaul optical network is indispensable in a 5G transmission system in which a large number of antennas (included in RRH) are arranged at high density. For that purpose, it is important to simply construct an optical transmission system, and it is required to reduce the number of optical / electronic components constituting the system as much as possible.
  • Digital coherent transmission system using multi-level signals is suitable for mobile fronthaul transmission where economy and efficiency are important.
  • digital coherent transmission requires a local light source on the receiving side in addition to the light source for transmission, and if the transmission and reception of the uplink and downlink are combined, four light sources are required per channel.
  • a highly accurate optical phase synchronization technique between the data signal and the local light source is indispensable.
  • a single-core bidirectional transmission method is used in which an upstream and downstream signal is transmitted to a single-core optical fiber in order to realize an economical transmission system by reducing the number of installed fibers.
  • the same single-core bidirectional transmission method is used in order to pursue economy and efficiency.
  • the backward Rayleigh scattered light of the upstream and downstream signals generated in the optical fiber transmission line is mixed as noise in the signal in the opposite direction.
  • transmission characteristics are greatly degraded, such as loss budget and transmission distance being limited.
  • the present invention is to solve such problems, and provides a backward Rayleigh scattering non-mixing type optical transmission method and an optical transmission apparatus that have a simple configuration, are highly economical, and have a large loss budget. Objective.
  • an optical transmission method transmits an upstream signal and a downstream signal between a first optical transmission / reception unit and a second optical transmission / reception unit via an optical fiber.
  • the laser light source arranged in the first optical transmission / reception unit is used as a light source for transmitting a downstream signal to the second optical transmission / reception unit, and the second optical transmission / reception unit
  • a laser light source disposed in the second optical transmission / reception unit as a light source for transmission of the upstream signal to the first optical transmission / reception unit.
  • Optical phase synchronization with the local light source and / or optical phase synchronization between the downstream signal and the local light source for receiving the downstream signal, and the upstream signal and downstream signal are transmitted at different frequencies. It is characterized by.
  • a pilot tone is superimposed on a data signal on the signal light transmitting side, and the signal light and the receiving local light source are transmitted via the pilot tone on the signal light receiving side.
  • the pilot tone, the upstream signal, and the downstream signal may be transmitted at different frequencies.
  • a plurality of pilot tones are superimposed on a data signal on the signal light transmitting side, and the signal light receiving side is routed through any one of the plurality of pilot tones.
  • the phase of the signal light and the local light source for reception is synchronized, and the difference frequency electric signal of the two pilot tones is extracted by detecting any two of the plurality of pilot tones.
  • the difference frequency electrical signal is connected to the reception local light source and used as a modulation signal for driving an optical modulator for modulating the output light of the local light source for reception or a reference signal for an optical phase locked loop,
  • a plurality of pilot tones, the uplink signal, and the downlink signal may be transmitted at different frequencies.
  • An optical transmission apparatus is an optical transmission apparatus for bidirectional transmission of an upstream signal and a downstream signal via an optical fiber between a first optical transceiver and a second optical transceiver.
  • the laser light source disposed in the first optical transmission / reception unit is a light source for transmitting a downstream signal to the second optical transmission / reception unit, and a station for receiving an upstream signal from the second optical transmission / reception unit.
  • the laser light source disposed in the second optical transmission / reception unit as a light emission source is a light source for transmitting an upstream signal to the first optical transmission / reception unit, and a downstream signal from the first optical transmission / reception unit
  • the laser light source disposed in the first optical transmitter / receiver or the second optical transmitter / receiver has a structure capable of injecting light from the outside, and the upstream signal is generated by injection locking. Between the upstream signal receiving local light source and the upstream signal Alternatively, it is configured to synchronize the phase between the downlink signal and the local light source for receiving the downlink signal, or the uplink signal is transmitted to the first optical transceiver or the second optical transceiver.
  • the first optical transmission / reception unit or the second optical transmission / reception unit includes a circuit that generates a pilot tone together with the upstream signal or the downstream signal, and the pilot tone.
  • a circuit that optically synchronizes the upstream signal and the local light source for receiving the upstream signal, or a circuit that optically synchronizes the downstream signal and the local light source for receiving the downstream signal, and the upstream signal and the The downlink signal and the pilot tone may be generated at different frequencies.
  • the first optical transceiver or the second optical transceiver includes a circuit that generates a plurality of pilot tones together with the uplink signal or the downlink signal, and the plurality of pilots A circuit that optically synchronizes the upstream signal and the local light source for receiving the upstream signal, or the optical phase synchronization of the downstream signal and the local light source for receiving the downstream signal, via any one of the tones. And a circuit for optically detecting any two of the plurality of pilot tones and extracting a difference frequency electric signal, and the upstream signal, the downstream signal, and the plurality of pilot tones are mutually connected. It may be configured to generate at different frequencies.
  • the first optical transmission / reception unit includes, for example, a base station baseband unit or an optical line termination device (Optical / Line Terminal), and the second optical transmission / reception unit includes For example, it preferably includes an antenna radio unit or an optical network device (Optical Network Unit).
  • An optical transmission method and an optical transmission apparatus serve as both a transmission light source and a local light source with a single laser, and transmit, for example, a pilot tone together with signal light as a phase reference signal for optical phase synchronization.
  • the light source can be used effectively, and at the same time, phase synchronization at the receiver can be realized by the optical circuit.
  • by transmitting upstream signals and downstream signals (and pilot tones when using pilot tones for optical phase synchronization) at different optical frequencies transmission performance deterioration due to backward Rayleigh scattered light is suppressed, and loss budget is achieved. Long-distance bidirectional transmission can be realized. Thereby, it is possible to provide a backward Rayleigh scattering non-mixing type optical transmission method and an optical transmission device with a simple configuration and high economic efficiency.
  • FIG. 12 is an explanatory diagram illustrating (a) frequency arrangement of downlink signals and (b) frequency arrangement of uplink signals of the optical transmission apparatus shown in FIG. 11. It is a block block diagram which shows the optical transmission apparatus in the 6th Embodiment of this invention.
  • the first to sixth embodiments of the present invention described below use a digital coherent optical transmission technology and an optical access network to perform high-speed wireless communication between the first optical transmission / reception unit and the second optical transmission / reception unit.
  • 1 shows an optical transmission method and an optical transmission apparatus for bidirectional transmission of a signal over a long distance via a single optical fiber.
  • the first optical transmission / reception unit comprises a base station baseband unit (BBU), and the second optical transmission / reception unit comprises an antenna radio unit (RRH).
  • BBU base station baseband unit
  • RRH antenna radio unit
  • 1 illustrates an optical transmission method and an optical transmission apparatus for performing mobile fronthaul transmission used in a mobile communication system such as 5G.
  • the base station baseband unit (BBU) is an optical line termination device and the antenna radio unit (RRH) is an optical line network device, so that only a mobile communication system such as 5G is used.
  • RRH antenna radio unit
  • it can be applied to a general optical access network system such as FTTH.
  • the reception methods in coherent transmission include homodyne detection and heterodyne detection.
  • homodyne detection the case where the frequency of the signal light and the local light is equal is called homodyne detection, and the case where the frequencies are different is called heterodyne detection.
  • the homodyne system is down-converted to the intermediate frequency band, the band is halved compared to the heterodyne system, and the noise band is also halved accordingly, so the reception sensitivity is improved by 3 dB compared to the heterodyne system.
  • the heterodyne system needs to be widened by the self-beat signal in addition to the signal band, whereas the homodyne system requires only half the signal band.
  • the homodyne system requires highly accurate phase synchronization in order to stabilize not only the frequency of local light but also the phase.
  • FIG. 1 shows the configuration of the optical transmission apparatus according to the first embodiment of the present invention.
  • a homodyne method is used for coherent detection.
  • a base station baseband unit (hereinafter referred to as BBU) 1 that performs baseband signal processing in a base station includes a laser light source 11, an IQ modulator 12, an optical modulator 13, an RF oscillator 14, an optical filter 15, and a 90-degree optical hybrid.
  • a circuit 16, a balanced photodetector 17, an A / D converter 18, a digital signal processor (DSP) 19, a D / A converter 20, and an optical circulator 21 are provided.
  • an antenna radio unit (hereinafter referred to as RRH) 3 that handles antennas and high-frequency signal processing includes a laser light source 31, an IQ modulator 32, an optical modulator 33, optical filters 34 and 35, a 90-degree optical hybrid circuit 36, a balanced Optical detector 37, A / D converter 38, digital signal processor (DSP) 39, D / A converter 40, optical amplifier 41, optical filter 42, optical circulator 43, optical detector 44, minute A peripheral 45 and an optical circulator 46 are provided.
  • the optical circulators 21, 43, and 46 output the signal light incident on the port 1 (P1) to the port 2 (P2) and the signal light incident on the port 2 (P2) to the port 3 (P3). It is designed to output.
  • the output light of the laser light source 11 oscillating at an optical frequency of f 1 [Hz] is branched into two, one as signal light for downlink from BBU1 to RRH3, and the other as upstream signal transmitted from RRH3 to BBU1. Each is used as a local light emission necessary for receiving the light.
  • the former is data-modulated by the IQ modulator 12.
  • the IQ modulator 12 is driven by baseband signals Ix, Qx, Iy, and Qy, and has a polarization multiplexing function.
  • two IQ modulators 12 may be used, and one may be data-modulated with Ix and Qx, the other with Iy and Qy, and then both may be polarization multiplexed.
  • the baseband signal may be supplied to the IQ modulator 12 after being amplified by a high frequency amplifier as necessary.
  • two signals of pilot tone 1 and pilot tone 2 are set up at a frequency separated from the data signal by ⁇ ⁇ f [Hz].
  • the signal light is composed of a data signal having a frequency f 1 [Hz] and two pilot tones having frequencies f 2 [Hz] and f 3 [Hz].
  • FIG. 2A shows the frequency arrangement of downlink signal data and pilot tones.
  • a signal on the low frequency side by ⁇ f [Hz] from f 1 is a pilot tone 1 (f 2)
  • a signal on the high frequency side is a pilot tone 2 (f 3).
  • the signal light on which the pilot tone is superimposed is incident on the port 1 (P1) of the optical circulator 21.
  • the signal light output from the port 2 of the optical circulator 21 propagates through the optical fiber transmission line 2 and is received by the RRH 3.
  • the signal after transmission is first incident on port 2 of circulator 46, and is output from port 3 to the receiving unit.
  • One of the two branched signal lights is input to a homodyne detection circuit including a 90-degree optical hybrid circuit 36 and a balanced photodetector 37.
  • the other is that the pilot tone is extracted by the optical filter 42, amplified by the optical amplifier 41 as necessary, and then input to the port 1 (P1) of the optical circulator 43, and the laser light source 31 from the port 2 (P2). Inject.
  • the phase of the laser light source 31 is synchronized with the phase of the pilot tone, that is, the phase of the downstream signal light.
  • the pilot tone used for injection locking may be either pilot tone 1 or pilot tone 2, but the following operation will be described on the assumption that the laser light source 31 is injection-locked to pilot tone 1 (f 2 [Hz]).
  • an optical comb signal in which a plurality of CW (Continuous Wave) lights at intervals of f clock [Hz] are arranged on the optical frequency axis is generated.
  • the homodyne-detected signal is converted into a digital signal by the A / D converter 38, subjected to signal processing such as polarization separation, demodulation, and adaptive equalization by the DSP circuit 39, and then again by the D / A converter 40.
  • Data signals Ix, Qx, Iy, Qy are output after being converted to analog signals.
  • the RRH 3 performs data modulation and polarization multiplexing on the output of the laser light source 31 by the IQ modulator 32 as in the case of BBU 1.
  • the pilot tone need not be superimposed on the data signal.
  • FIG. 2B shows the frequency arrangement of uplink signal data and pilot tones.
  • the signal light propagates through the optical fiber transmission line 2 and is received by the BBU 1.
  • the BBU 1 light having a frequency f 1 [Hz] branched from the laser light source 11 is incident on an optical modulator 13 driven by an electric signal of 2f clock [Hz], and optical intensity or optical phase modulation is performed.
  • the electrical signal of 2f clock [Hz] a clock signal used when generating a downlink baseband signal may be multiplied and used.
  • One optical CW light having a frequency f 4 [Hz] is extracted from the optical comb signal generated by the optical modulation at intervals of 2f clock [Hz] by using the optical filter 15, and this signal is used as local light emission in the BBU 1.
  • This signal is input together with the upstream signal (f 4 [Hz]) to a homodyne detection circuit composed of the 90-degree optical hybrid circuit 16 and the balanced photodetector 17.
  • the homodyne-detected signal is converted into a digital signal by the A / D converter 18, subjected to signal processing such as polarization separation, demodulation, and adaptive equalization by the DSP circuit 19, and then again by the D / A converter 20.
  • Data signals Ix, Qx, Iy, Qy are output after being converted to analog signals.
  • the two laser light sources 11 and 31 are phase-locked with each other by injection locking in the RRH 3, no optical phase-locked circuit is required in the BBU1.
  • the signal light and the local light input to the 90-degree optical hybrid circuit 16 or 36 pass through different optical paths, respectively. Therefore, as the optical path length varies (for example, the optical fiber length due to temperature change).
  • the phase between the data signal and local light fluctuates slightly at a slow speed. Such low-speed phase drift fluctuations are corrected by the DSP 19 or 39 of the receiving unit.
  • a downlink signal (f 1 [Hz]), pilot tone 1 (f 2 [Hz]), pilot tone 2 (f 3 [Hz]) transmitted from BBU1 to RRH3, and transmission from RRH3 to BBU1
  • the upstream signal (f 4 [Hz]) is assigned to a different frequency. Therefore, the backward Rayleigh scattered light of each signal generated in the optical fiber transmission line when these signals are transmitted is not mixed as noise in each signal light propagating forward.
  • the downstream signal (f 1 [ Hz]), pilot tone 1 (f 2 [Hz]), and pilot tone 2 (f 3 [Hz]) do not overlap with each other in frequency, so that the laser light source 31 of pilot tone 1 (f 2 [Hz]) This does not affect the injection locking operation or the demodulation of the downstream signal (f 1 [Hz]).
  • FIG. 3 shows a block diagram showing the configuration of the transmission system.
  • the same components as those in FIG. 1 are denoted by the same reference numerals.
  • a CW semiconductor laser having a line width of 8 kHz that oscillates at a wavelength of 1.55 ⁇ m (with an optical frequency of f 1 [Hz]) is used as the laser light source 11 in the BBU 1 .
  • the I and Q baseband data signals were generated by using an arbitrary waveform generator 22 composed of a DAC (Digital Analogue Converter) with a sampling speed of 65 GS / s and a DSP circuit.
  • DAC Digital Analogue Converter
  • the output light is modulated to generate a 5 Gbaud, 256 QAM data signal (f 1 [Hz]) and two pilot tones of f 2 [Hz] and f 3 [Hz].
  • the 256 QAM data signal output from the arbitrary waveform generator 22 is subjected to a root raised cosine Nyquist filter with a roll-off rate of 0.2, and the band is narrowed to 3 GHz.
  • a polarization multiplexing circuit 23 comprising an optical circuit composed of a polarization beam splitter and a delay circuit was used.
  • the generated 80 Gbit / s, polarization multiplexed 5 Gbaud, 256 QAM signal and two pilot tones propagate to the RRH3 by propagating the 26 km long SMF used as the optical fiber 2 with a transmission power of -5 dBm. To do.
  • the frequency relationship between the data signal and the pilot tone in this embodiment is the same as that shown in FIG.
  • a pilot tone signal having a frequency of f 2 [Hz] is extracted by the optical filter 42 and incident on the laser light source 31 to perform injection locking, whereby the laser light source 31 is phase-locked to the downstream data signal.
  • the laser light source 31 a CW semiconductor laser having a line width of about 200 kHz that oscillates in a wavelength band of 1.55 ⁇ m was used.
  • the data signal and the local light source are phase-synchronized with a phase noise of approximately 0.3 degrees.
  • the allowable phase noise obtained as the phase difference between the nearest symbols is approximately 2 degrees. Therefore, the injection locking circuit in this experiment has sufficient performance to demodulate the 256 QAM signal.
  • the output light of the laser light source 31 that oscillates at f 2 [Hz] by injection locking is incident on an optical modulator 33 driven by a sine wave of ⁇ f [Hz], and sidebands are generated by optical modulation.
  • an LN (LiNbO 3 ) light intensity modulator is used as the light modulator 33, but an LN optical phase modulator or an SSB (Single Side-Band) modulator may be used.
  • the clock signal of ⁇ f [Hz] is generated from the heterodyne beat signal of two pilot tones, but in this experiment, it is generated using the RF oscillator 47 for simplicity.
  • Low frequency side (f 4 [Hz]) and high frequency side (f 1 [Hz]) sidebands are extracted using optical filters 34 and 35 Each of them is used as an optical carrier for the upstream signal and local light for detecting the downstream data signal by homodyne detection.
  • the downstream data signal subjected to homodyne detection is A / D converted at a sampling rate of 40 GS / s and then demodulated off-line using the DSP 39. Note that the low-speed phase fluctuation of the homodyne detection signal caused by the signal light input to the 90-degree optical hybrid circuit 36 and the local light passing through different optical paths is corrected in the DSP 39.
  • the uplink signal generator in RRH3 generates polarization multiplexed 5 Gbaud, 256 QAM signals in the same manner as the downlink data signal, and transmits 26 km SMF to the BBU 1 side with a transmission power of -5 dBm.
  • the light having the frequency f 1 [Hz] branched from the laser light source 11 is converted into f 4 [Hz] by an optical frequency shifter including the optical modulator 13, the oscillator 14, and the optical filter 15. Is used as local light, and downstream signal light is subjected to homodyne detection.
  • a reference clock signal source that drives the arbitrary waveform generation device 22 may be used.
  • the detected signal is digitized by an A / D converter 17 having a sampling rate of 40 GS / s, and then demodulated off-line using a DSP 19. Even in the BBU 1, the phase fluctuation of the homodyne detection signal caused by the signal light and the local light passing through different optical paths occurs, and this is corrected in the DSP 19.
  • 4A and 4B are optical spectra of a downlink signal transmitted from BBU1 and an uplink signal transmitted from RRH3, respectively. These are measurements of output light from P2 of the circulator 21 or P2 of the circulator 46 in FIG. As shown in the figure, in this experiment, an uplink signal, a downlink signal, and two pilot tone signals are allocated and transmitted at different optical frequencies of f 1 to f 4 [Hz].
  • 4 (c) and 4 (d) are the optical spectra of the downstream signal and upstream signal after 26 km bidirectional transmission of SMF, and the output light from P3 of circulator 46 or P3 of circulator 21 in FIG. 1, respectively. It is the result of having measured.
  • FIGS. 5A and 5B are constellations of downstream signals and upstream signals, respectively. In any result, each symbol point having 8-bit information can be clearly separated, and it can be seen that the bit information can be accurately demodulated.
  • the frequency of the uplink signal generated by RRH3 is generated and transmitted at the same f 1 [Hz] as the downlink data signal, neither the uplink nor the downlink data signal can be demodulated.
  • the backward Rayleigh scattered light of downstream and upstream signals is mixed into the data signal light to be demodulated and propagated forward as noise, degrading the signal-to-noise ratio (S / N) of the data signal.
  • the frequency of the upstream signal is transmitted as f 3 [Hz] which is the same as the pilot tone 2 of the downstream signal, the downstream data signal cannot be demodulated.
  • the backward Rayleigh scattered light of the upstream signal is mixed as noise into the pilot tone 2 propagated forward, and the injection locking characteristic of the laser light source 31 is greatly deteriorated in the RRH 3.
  • uplink and downlink signals are assigned to different frequencies and the phase synchronization between data and local light is performed by light injection synchronization, a large-capacity mobile can be achieved with a simple transmission system.
  • a front hall can be realized.
  • FIG. 6 shows the configuration of the optical transmission apparatus according to the second embodiment of the present invention. Since this configuration is almost the same as that in FIG. 1, the same components as those in FIG. Further, the frequency arrangement of the uplink signal and the downlink signal in the present embodiment is the same as that in FIG. In this configuration, the local light emitted from the BBU 1 and RRH 3 is frequency-shifted so that the output frequencies of the laser light sources 11 and 31 become f 2 [Hz], respectively, and then supplied to the heterodyne detection circuit 24.
  • the frequency of the signal light and the local light is f 4 [Hz] and f 2 [Hz] in BBU1 and f 1 [Hz] and f 2 [Hz] in RRH 3, respectively, and the frequency of the IF (Intermediate Frequency) signal Is performing heterodyne detection with ⁇ f [Hz].
  • the configuration of the coherent detection circuit can be simplified, the number of parts can be reduced and the cost can be reduced.
  • the reception sensitivity is deteriorated by 3 dB compared to the homodyne detection, so care must be taken to ensure a loss budget.
  • FIG. 7 shows the configuration of the optical transmission apparatus according to the third embodiment of the present invention.
  • an optical phase-locked loop (OPLL) based on an OVCO (Optical Voltage Controlled Oscillator) system is used as an optical phase-locking technique instead of injection locking.
  • the frequency arrangement of the uplink signal and the downlink signal in the present embodiment is the same as that in FIG. Since the configuration of the BBU 1 is the same as that of the first embodiment, description thereof is omitted.
  • the RRH 3 includes an OPLL circuit including a narrow band electric filter 48, a mixer 49, a feedback circuit 50, an RF band voltage controlled oscillator (RF-VCO) 51, and an optical modulator 33.
  • RF-VCO RF band voltage controlled oscillator
  • a heterodyne beat signal (IF signal) between the laser light source 31 and the pilot tone 1 output from the balanced photodetector 37 is extracted using the narrowband electric filter 48.
  • the phase of the IF signal is compared with the phase of the clock signal (f clock [Hz]) generated from the two pilot tones of the downstream signal in a mixer (DBM: Double Balanced Mixer) 49, and the difference is an error voltage signal. Detected as The error signal is fed back to the RF-VCO 51 via the feedback circuit (loop filter) 50, so that the IF signal is always a highly stable signal synchronized with the clock signal.
  • the output light of the laser light source 31 is subjected to light intensity or optical phase modulation by the optical modulator 33 driven by the output signal (f clock [Hz]) from the RF-VCO 51, and the side of the frequency f 1 [Hz].
  • One band is extracted by the optical filter 35 and used as local light.
  • one side band of frequency f 4 [Hz] is extracted through the optical filter 34 and used as upstream signal light from RRH 3 to BBU 1.
  • the optical modulator 33 an LN intensity modulator, an LN phase modulator, an SSB modulator, or the like may be used as the optical modulator 33.
  • the DSP 19 corrects the slow phase fluctuation of the homodyne detection signal caused by the signal light incident on the 90-degree optical hybrid circuit 16 and the local light passing through different optical paths.
  • FIG. 8 shows a configuration when a heterodyne method is used as coherent detection in the present embodiment.
  • the same components as those in FIG. In this configuration the center frequency of the optical filter 15 in the BBU 1 and the center frequency of the optical filter 35 in the RRH 3 are set to f 2 [Hz], and the optical modulators 13 and 33 are used to output the outputs of the laser light sources 11 and 31. The frequency is shifted so that the frequency becomes f 2 [Hz]. This is supplied to the heterodyne detection circuit 24 as local light.
  • FIG. 9 shows the configuration of the optical transmission apparatus according to the fourth embodiment of the present invention.
  • part of the OPLL function is realized by digital signal processing in the third embodiment (FIG. 7).
  • the phase comparison realized by the four analog circuits of the photodetector 44, the frequency divider 45, the narrow band electric filter 48, and the mixer 49 is performed on the DSP 39, and the obtained error signal (
  • the digital signal is D / A converted by the D / A converter 40 and fed back to the RF-VCO 51 via the feedback circuit 50.
  • FIG. 9 shows an example of the homodyne system
  • the present embodiment can also be configured by the heterodyne system as shown in FIG. 10 as in the modification of the third embodiment (see FIG. 8). Is possible.
  • FIG. 11 shows the configuration of the optical transmission apparatus according to the fifth embodiment of the present invention.
  • the BBU 1 includes N transmitters for transmitting to each RRH 3 and N receivers for receiving an uplink signal from each RRH 3.
  • the configuration of each transceiver is shown in a simplified manner, but specifically, the one shown in any of the first to fourth embodiments may be used.
  • WDM wavelength division multiplexing
  • a different frequency is assigned to each RRH 3, and wavelength division multiplexing (WDM) transmission is performed in the optical fiber transmission line 2.
  • WDM grids with intervals of 2 ⁇ f [Hz] are defined, and uplink signals are assigned to even-numbered frequency channels (f 2 , f 4 ,... F (2N) , N are natural numbers).
  • ⁇ f [Hz] may be equal to the modulation frequency of the data signal.
  • the downlink data signal is assigned to an odd-numbered frequency channel (f 1 , f 3 ,... F (2N ⁇ 1) , N is a natural number) and transmitted.
  • the WDM grid is defined as a WDM grid with a frequency interval of 2 ⁇ f [Hz] that differs from the WDM grid by ⁇ f [Hz] (f 1 ′, f 2 ′... f N ′, N are natural numbers).
  • a pilot tone for the signal is assigned.
  • Downstream signals are combined from N transmitters by a WDM multiplexer 4, WDM transmitted through an optical fiber transmission line 2, and distributed to N RRHs 3 by a power splitter 6.
  • a desired WDM signal is selected by the optical filter 7 whose center frequency is set to f 1 , f 3 ,... F (2N ⁇ 1) and received by the coherent detection circuit 8a.
  • Each RRH 3 includes a laser light source 31 of frequencies f 2 , f 4 ,... F (2N) , and the coherent detection circuit 8a is configured by any of the methods shown in the first to fourth embodiments.
  • the coherent detection circuit 8a includes, for example, the optical modulator 33, the optical filters 34 and 35, the 90-degree optical hybrid circuit 36, the balanced photodetector 37, the A / D converter 38, and the like in the first embodiment shown in FIG.
  • a digital signal processor (DSP) 39, a D / A converter 40, an optical amplifier 41, an optical filter 42, an optical circulator 43, a photodetector 44, and a frequency divider 45 can be used. It is possible to omit the optical filter 7 and simultaneously select and demodulate the WDM signal in the coherent detection circuit 8a.
  • the upstream signal is multiplexed from N RRHs 3 by a power splitter 6, WDM transmitted through an optical fiber transmission line 2, and separated into N different frequencies by a WDM demultiplexer 5 in the BBU 1.
  • Each receiver uses the laser light source 11 as a local light source and performs coherent detection by any of the methods shown in the first to fourth embodiments.
  • the coherent detection circuit 8b includes, for example, the optical modulator 13, the RF oscillator 14, the optical filter 15, the 90-degree optical hybrid circuit 16, the balanced photodetector 17, A / A of the first embodiment shown in FIG.
  • a D converter 18, a digital signal processor (DSP) 19, and a D / A converter 20 can be used.
  • FIG. 13 shows the configuration of the optical transmission apparatus according to the sixth embodiment of the present invention.
  • the downlink signal is distributed to each RRH 3 by the WDM demultiplexer 9 instead of the power splitter 6, and each RRH 3 is separated for each wavelength.
  • a signal is sent.
  • each RRH 3 does not require a wavelength selection element such as the optical filter 7.
  • pilot tones In each of the above embodiments, the case where two pilot tones are used has been exemplified. Instead, three or more pilot tones are superimposed on the data signal on the signal light transmission side to receive the signal light. The phase of the signal light and the receiving local light source is synchronized via any one of these pilot tones on the side, and any two of these pilot tones are detected by light detection. The difference frequency electrical signal of two pilot tones is extracted, and the difference frequency electrical signal is used as a modulation signal for driving an optical modulator that modulates the output light of the local light source for reception or a reference signal for an optical phase locked loop. May be. In this case, these pilot tones, upstream signals and downstream signals are transmitted at different frequencies.
  • a single pilot tone is superimposed on the data signal on the signal light transmission side, and the phase of the signal light and the local light source for reception is synchronized via the pilot tone on the signal light reception side, and for reception.
  • the modulation signal for driving the optical modulator that modulates the output light from the local light source and the reference signal for the optical phase locked loop may be generated by a separately prepared oscillator.
  • the pilot tone, the upstream signal, and the downstream signal are transmitted at different frequencies.
  • the present invention is a non-mixed backward Rayleigh scattering method for long-distance bidirectional transmission of a radio signal between a BBU and an RRH with a large loss budget via an optical fiber in a mobile fronthaul.
  • Type optical transmission method and optical transmission apparatus can be provided.
  • the present invention is characterized by using digital coherent transmission technology as its optical transmission method, and has high affinity with wireless signals in terms of its coherence, thus realizing an efficient and economical optical / wireless access network. it can.
  • the present invention can also be used in a general optical access network system such as FTTH using an optical fiber.
  • BBU Base Station Baseband
  • SYMBOLS 11
  • Laser light source 12
  • Optical modulator 14 (RF) oscillator 15
  • Optical filter 16 90 degree optical hybrid circuit 17
  • Balanced photodetector 18
  • Digital signal processing circuit (DSP) 20
  • D / A converter 21
  • Optical circulator 22
  • Arbitrary waveform generator 23
  • Polarization multiplexing circuit 24
  • Heterodyne detection circuit Optical fiber transmission line
  • Antenna radio section (RRH) Reference Signs List 31
  • Laser light source 32
  • IQ modulator 33
  • Optical modulator 34 Optical filter 35
  • Optical filter 36 90 degree optical hybrid circuit
  • Balanced optical detector 38
  • a / D converter 39
  • Digital signal processing circuit (DSP) 40
  • D / A Converter 41
  • Optical Amplifier 42
  • Optical Filter 43
  • Optical Circulator 44
  • Optical Detector 45

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  • Electromagnetism (AREA)
  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
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  • Optical Communication System (AREA)

Abstract

[Problem] To provide a Rayleigh back scattering non-mixing type optical transmission method and optical transmission device which achieve high economy with a simple configuration and have a large loss budget. [Solution] A wireless signal is transmitted between a base station baseband unit 1 and an antenna wireless unit 3 via an optical fiber 2. A laser light source 11 disposed in the base station baseband unit 1 is used as a light source for transmitting a downlink signal to the antenna wireless unit 3 and also used as a station light emitting source for receiving an uplink signal from the antenna wireless unit 3. A laser light source 31 disposed in the antenna wireless unit 3 is used as a light source for transmitting an uplink signal to the base station baseband unit 1 and also used as a station light emitting source for receiving a downlink signal from the base station baseband unit 1. The uplink signal and the downlink signal are transmitted with mutually different frequencies. In the base station baseband unit 1 and/or the antenna wireless unit 3, optical phase synchronization between signal light and the station light emitting source for reception is performed by an injection synchronization method or by means of an optical phase synchronization loop.

Description

光伝送方法および光伝送装置Optical transmission method and optical transmission device
　本発明は、光伝送方法及び光伝送装置に関するものである。 The present invention relates to an optical transmission method and an optical transmission apparatus.
　スマートフォンの普及とLTE（Long Term Evolution）に代表されるモバイルブロードバンドサービスの進展に伴い、移動通信トラフィックが急増している。このような中、次世代の大容量モバイル通信システムとして第５世代移動通信システム（５Ｇ）の研究開発が国内外で精力的に進められている（例えば、非特許文献１参照）。 As mobile smartphone services become popular and mobile broadband services such as LTE (Long Term Evolution) progress, mobile communication traffic is rapidly increasing. Under such circumstances, research and development of a fifth generation mobile communication system (5G) as a next generation large-capacity mobile communication system has been energetically advanced in Japan and overseas (for example, see Non-Patent Document 1).
　本伝送システムでは、伝送制御やベースバンド信号処理を行う基地局ベースバンド部（BBU: Base Band Unit）を１ヵ所に集約し、アンテナ及び高周波信号処理を担うアンテナ無線部（RRH: Remote Radio Head）を分散配置するC-RAN（Centralized-Radio Access Network）の適用が検討されている（例えば、非特許文献２参照）。C-RANの構成ではBBUとRRH間（モバイルフロントホール）は光ファイバで接続され、アンテナより送受信される無線信号を光波に重畳して伝送する。 In this transmission system, the base station baseband unit (BBU: Base Band Unit) that performs transmission control and baseband signal processing is consolidated in one place, and the antenna radio unit (RRH: Remote Radio Head) responsible for antenna and high-frequency signal processing. Application of C-RAN (Centralized-Radio Access Network) that distributes and distributes is considered (for example, see Non-Patent Document 2). In the C-RAN configuration, the BBU and RRH (mobile fronthaul) are connected by an optical fiber, and a radio signal transmitted and received from the antenna is superimposed on the light wave and transmitted.
　無線信号を光波に重畳して伝送する手法としては、CPRI（Common Public Radio Interface）によるデジタルRoF（Radio over Fiber）方式が広く用いられている（例えば、非特許文献３参照）。デジタルRoFでは、無線信号をデジタル化して光ファイバ伝送するため、一般に無線信号の１６倍程度の光伝送帯域が要求される。10 Gbit/s以上の無線通信容量が想定される５Ｇ及びそれ以降の大容量無線アクセスシステムでは、モバイルフロントホールに要求される伝送容量は100 Gbit/s以上となる。 A digital RoF (Radio over Fiber) system based on CPRI (Common Public Radio Interface) is widely used as a method of superimposing and transmitting a radio signal on a light wave (see, for example, Non-Patent Document 3). In digital RoF, since a radio signal is digitized and transmitted through an optical fiber, an optical transmission band approximately 16 times that of a radio signal is generally required. In 5G and higher-capacity wireless access systems assumed to have a wireless communication capacity of 10 / Gbit / s or higher, the transmission capacity required for the mobile fronthaul is 100 Gbit / s or higher.
　モバイルフロントホールの大容量化を実現する光伝送方式の一つとして、TWDM-PON（Time and Wavelength Division Multiplexing Passive Optical Network）が検討されている（例えば、非特許文献４参照）。本方式は、従来の光アクセスネットワーク技術であるTDM-PONに波長多重伝送技術を適用して大容量化を図るものであり、これまでに10 Gbit/s, OOK（On-Off Keying）信号を４波多重する40 Gbit/s伝送システムが実現されている（例えば、非特許文献５参照）。 TWDM-PON (Time-and-Wavelength-Division-Multiplexing-Passive-Optical-Network) is being studied as one of the optical transmission systems that realizes a large mobile fronthaul capacity (for example, see Non-Patent Document 4). This system is intended to increase the capacity by applying wavelength multiplexing transmission technology to TDM-PON, which is a conventional optical access network technology. So far, 10 Gbit / s, OOK (On-OffOnKeying) signal has been used. A 40 Gbit / s transmission system that multiplexes four waves is realized (for example, see Non-Patent Document 5).
　一方、近年の光通信では、デジタルコヒーレント方式を用いた大容量光伝送技術が急速に進展している。本伝送においては、無線通信と同様に多値変調が採用されており、高度なデジタル信号処理技術を用いてキャリヤ位相推定、偏波分離、波形歪の補正および復調処理を行うことが大きな特徴である。これまで、１波長当たり100 Gbit/sの通信容量を有する25 Gbaud, QPSK（Quadrature Phase Shift Keying）伝送システムが商用化されており、基幹伝送系への導入が始まっている（例えば、非特許文献６参照）。本光伝送においては、データ信号の多値度を増大することによって、波長多重伝送方式を用いることなくシンプルな構成で100 Gbit/sを越える大容量通信を実現することができる。また、低い変調速度でも高速伝送が実現できるため、比較的安価である低速な電子デバイスを用いて伝送系を構築することが可能であり、経済的な面でもメリットが大きい。 On the other hand, in recent optical communication, a large-capacity optical transmission technology using a digital coherent method is rapidly progressing. This transmission employs multi-level modulation in the same way as wireless communication, and is characterized by carrier phase estimation, polarization separation, waveform distortion correction, and demodulation processing using advanced digital signal processing technology. is there. To date, 25 Gbaud, QPSK (Quadrature Phase Shift Keying) transmission systems having a communication capacity of 100 Gbit / s per wavelength have been commercialized, and their introduction into backbone transmission systems has begun (for example, non-patent literature) 6). In this optical transmission, by increasing the multilevel of the data signal, it is possible to realize a large-capacity communication exceeding 100 Gbit / s with a simple configuration without using a wavelength division multiplexing transmission system. In addition, since high-speed transmission can be realized even at a low modulation speed, it is possible to construct a transmission system using a relatively inexpensive low-speed electronic device, which is very advantageous in terms of economy.
　このような特徴から、将来予想される無線アクセスネットワークのさらなる大容量化・高度化に対応するために、モバイルフロントホールにおいてもデジタルコヒーレント光伝送を適用することに高い関心が寄せられている。また、モバイルフロントホールを利用した５Ｇなどの移動通信システムに限らず、光ファイバを利用したFTTH（Fiber To The Home）などの一般的な光アクセスネットワークシステムについても、さらなる経済性や効率性の観点から、モバイルフロントホールでの技術を利用できると考えられる。 Because of these characteristics, there is a great interest in applying digital coherent optical transmission to mobile fronthauls in order to cope with further increases in capacity and advancement of radio access networks expected in the future. In addition to 5G mobile communication systems that use mobile fronthauls, general optical access network systems such as FTTH (Fiber To The Home) that use optical fiber are also considered to be more economical and efficient. Therefore, it is considered that the technology in the mobile fronthaul can be used.
　多数のアンテナ（RRHに含まれる）が高密度に配置される５Ｇ伝送システムにおいては、効率的かつ経済的なモバイルフロントホール光ネットワークが不可欠となる。そのためには、光伝送系をシンプルに構築することが重要であり、これを構成する光・電子部品の点数を出来るだけ削減することが求められる。 An efficient and economical mobile fronthaul optical network is indispensable in a 5G transmission system in which a large number of antennas (included in RRH) are arranged at high density. For that purpose, it is important to simply construct an optical transmission system, and it is required to reduce the number of optical / electronic components constituting the system as much as possible.
　経済性、効率性が重要視されるモバイルフロントホール伝送には、多値信号を用いたデジタルコヒーレント伝送方式が好適である。しかしながら、デジタルコヒーレント伝送は、送信用光源に加えて受信側で局発光源が必要であり、上り回線・下り回線の各送受信を合計すると、１チャネルあたり４台の光源が必要となる。また、多値データ信号を高精度に復調するためには、データ信号と局発光源との高精度な光位相同期技術が不可欠となる。 Digital coherent transmission system using multi-level signals is suitable for mobile fronthaul transmission where economy and efficiency are important. However, digital coherent transmission requires a local light source on the receiving side in addition to the light source for transmission, and if the transmission and reception of the uplink and downlink are combined, four light sources are required per channel. In addition, in order to demodulate a multilevel data signal with high accuracy, a highly accurate optical phase synchronization technique between the data signal and the local light source is indispensable.
　モバイルフロントホール光伝送系においては、敷設ファイバ数を削減して経済的な伝送システムを実現するため、一芯の光ファイバに上りと下り両方向の信号を伝送させる一芯双方向伝送方式が用いられる。また、FTTHなどの一般的な光アクセスネットワークシステムでも、経済性や効率性を追求するため、同様の一芯双方向伝送方式が用いられる。このような伝送形態においては、光ファイバ伝送路中で生じる上り・下りそれぞれの信号の後方レイリー散乱光が、反対方向の信号にノイズとして混入する。その結果、ロスバジェットや伝送距離が制限される等、伝送特性が大きく劣化する。 In the mobile fronthaul optical transmission system, a single-core bidirectional transmission method is used in which an upstream and downstream signal is transmitted to a single-core optical fiber in order to realize an economical transmission system by reducing the number of installed fibers. . Also, in general optical access network systems such as FTTH, the same single-core bidirectional transmission method is used in order to pursue economy and efficiency. In such a transmission form, the backward Rayleigh scattered light of the upstream and downstream signals generated in the optical fiber transmission line is mixed as noise in the signal in the opposite direction. As a result, transmission characteristics are greatly degraded, such as loss budget and transmission distance being limited.
　本発明は、このような課題を解決するためのものであり、簡便な構成で経済性が高く、且つロスバジェットの大きい後方レイリー散乱非混合型の光伝送方法および光伝送装置を提供することを目的とする。 The present invention is to solve such problems, and provides a backward Rayleigh scattering non-mixing type optical transmission method and an optical transmission apparatus that have a simple configuration, are highly economical, and have a large loss budget. Objective.
　かかる目的を達成するために、本発明に係る光伝送方法は、第１の光送受信部と第２の光送受信部との間で上り信号および下り信号を、光ファイバを介して一芯双方向伝送させるための光伝送方法において、前記第１の光送受信部に配置されたレーザ光源を、前記第２の光送受信部への下り信号の伝送用光源として用いると共に、前記第２の光送受信部からの上り信号の受信用局発光源として用い、前記第２の光送受信部に配置されたレーザ光源を、前記第１の光送受信部への上り信号の伝送用光源として用いると共に、前記第１の光送受信部からの下り信号の受信用局発光源として用い、前記第１の光送受信部および／または前記第２の光送受信部において、注入同期法又は光位相同期ループを用いて、前記上り信号と前記上り信号の受信用局発光源との光位相同期、および／または、前記下り信号と前記下り信号の受信用局発光源との光位相同期を行い、前記上り信号と前記下り信号は互いに異なる周波数で伝送することを特徴とする。 In order to achieve such an object, an optical transmission method according to the present invention transmits an upstream signal and a downstream signal between a first optical transmission / reception unit and a second optical transmission / reception unit via an optical fiber. In the optical transmission method for transmitting, the laser light source arranged in the first optical transmission / reception unit is used as a light source for transmitting a downstream signal to the second optical transmission / reception unit, and the second optical transmission / reception unit And a laser light source disposed in the second optical transmission / reception unit as a light source for transmission of the upstream signal to the first optical transmission / reception unit. As a local light source for receiving a downstream signal from the optical transmitter / receiver unit, in the first optical transmitter / receiver unit and / or the second optical transmitter / receiver unit, using the injection locking method or the optical phase locked loop, Signal and the upstream signal Optical phase synchronization with the local light source and / or optical phase synchronization between the downstream signal and the local light source for receiving the downstream signal, and the upstream signal and downstream signal are transmitted at different frequencies. It is characterized by.
　また、本発明に係る光伝送方法において、信号光の送信側でデータ信号にパイロットトーンを重畳し、前記信号光の受信側で前記パイロットトーンを介して前記信号光と前記受信用局発光源との間の位相を同期させ、前記パイロットトーンと前記上り信号および前記下り信号とは互いに異なる周波数で伝送してもよい。 In the optical transmission method according to the present invention, a pilot tone is superimposed on a data signal on the signal light transmitting side, and the signal light and the receiving local light source are transmitted via the pilot tone on the signal light receiving side. The pilot tone, the upstream signal, and the downstream signal may be transmitted at different frequencies.
　また、本発明に係る光伝送方法において、信号光の送信側でデータ信号に複数のパイロットトーンを重畳し、前記信号光の受信側で前記複数のパイロットトーンのうち、いずれか１本を介して前記信号光と前記受信用局発光源との位相を同期させると共に、前記複数のパイロットトーンのうち、いずれか２本を光検出することで前記２本のパイロットトーンの差周波電気信号を抽出し、前記差周波電気信号を、前記受信用局発光源に接続され、前記受信用局発光源の出力光を変調する光変調器を駆動する変調信号または光位相同期ループの基準信号として用い、前記複数のパイロットトーンと前記上り信号および前記下り信号とは、互いに異なる周波数で伝送してもよい。 Further, in the optical transmission method according to the present invention, a plurality of pilot tones are superimposed on a data signal on the signal light transmitting side, and the signal light receiving side is routed through any one of the plurality of pilot tones. The phase of the signal light and the local light source for reception is synchronized, and the difference frequency electric signal of the two pilot tones is extracted by detecting any two of the plurality of pilot tones. The difference frequency electrical signal is connected to the reception local light source and used as a modulation signal for driving an optical modulator for modulating the output light of the local light source for reception or a reference signal for an optical phase locked loop, A plurality of pilot tones, the uplink signal, and the downlink signal may be transmitted at different frequencies.
　本発明に係る光伝送装置は、第１の光送受信部と第２の光送受信部との間で上り信号および下り信号を、光ファイバを介して一芯双方向伝送させるための光伝送装置において、前記第１の光送受信部に配置されたレーザ光源が、前記第２の光送受信部への下り信号の伝送用光源であると共に、前記第２の光送受信部からの上り信号の受信用局発光源であり、前記第２の光送受信部に配置されたレーザ光源が、前記第１の光送受信部への上り信号の伝送用光源であると共に、前記第１の光送受信部からの下り信号の受信用局発光源であり、前記第１の光送受信部もしくは前記第２の光送受信部に配置された前記レーザ光源は、外部から光を注入できる構造を有し、注入同期によって前記上り信号と前記上り信号の受信用局発光源との間の位相もしくは前記下り信号と前記下り信号の受信用局発光源との間の位相を同期するよう構成されている、または、前記第１の光送受信部もしくは前記第２の光送受信部に、前記上り信号と前記上り信号の受信用局発光源との間の位相もしくは前記下り信号と前記下り信号の受信用局発光源との間の位相を同期させるための電圧制御型発振器及び光位相同期ループが配置されていることを特徴とする。このとき、前記上り信号と前記下り信号とを、互いに異なる周波数で生成するよう構成されていることが好ましい。 An optical transmission apparatus according to the present invention is an optical transmission apparatus for bidirectional transmission of an upstream signal and a downstream signal via an optical fiber between a first optical transceiver and a second optical transceiver. The laser light source disposed in the first optical transmission / reception unit is a light source for transmitting a downstream signal to the second optical transmission / reception unit, and a station for receiving an upstream signal from the second optical transmission / reception unit. The laser light source disposed in the second optical transmission / reception unit as a light emission source is a light source for transmitting an upstream signal to the first optical transmission / reception unit, and a downstream signal from the first optical transmission / reception unit The laser light source disposed in the first optical transmitter / receiver or the second optical transmitter / receiver has a structure capable of injecting light from the outside, and the upstream signal is generated by injection locking. Between the upstream signal receiving local light source and the upstream signal Alternatively, it is configured to synchronize the phase between the downlink signal and the local light source for receiving the downlink signal, or the uplink signal is transmitted to the first optical transceiver or the second optical transceiver. A voltage-controlled oscillator and an optical phase-locked loop for synchronizing a phase between a signal and a receiving local light source of the upstream signal or a phase between the downstream signal and the receiving local light source of the downstream signal It is arranged. At this time, it is preferable that the uplink signal and the downlink signal are generated at different frequencies.
　本発明に係る光伝送装置において、前記第１の光送受信部または前記第２の光送受信部は、前記上り信号または前記下り信号と共にパイロットトーンを生成する回路と、前記パイロットトーンを介して、前記上り信号と前記上り信号の受信用局発光源とを光位相同期する回路または前記下り信号と前記下り信号の受信用局発光源とを光位相同期する回路とを有し、前記上り信号と前記下り信号と前記パイロットトーンとを、互いに異なる周波数で生成するよう構成されていてもよい。 In the optical transmission apparatus according to the present invention, the first optical transmission / reception unit or the second optical transmission / reception unit includes a circuit that generates a pilot tone together with the upstream signal or the downstream signal, and the pilot tone. A circuit that optically synchronizes the upstream signal and the local light source for receiving the upstream signal, or a circuit that optically synchronizes the downstream signal and the local light source for receiving the downstream signal, and the upstream signal and the The downlink signal and the pilot tone may be generated at different frequencies.
　また、本発明に係る光伝送装置において、前記第１の光送受信部または前記第２の光送受信部は、前記上り信号または前記下り信号と共に複数のパイロットトーンを生成する回路と、前記複数のパイロットトーンのうちいずれか１本を介して、前記上り信号と前記上り信号の受信用局発光源とを光位相同期する回路または前記下り信号と前記下り信号の受信用局発光源とを光位相同期する回路と、前記複数のパイロットトーンのうちいずれか２本を光検出して差周波電気信号を抽出する回路とを有し、前記上り信号と前記下り信号と前記複数のパイロットトーンとを、互いに異なる周波数で生成するよう構成されていてもよい。 Further, in the optical transmission apparatus according to the present invention, the first optical transceiver or the second optical transceiver includes a circuit that generates a plurality of pilot tones together with the uplink signal or the downlink signal, and the plurality of pilots A circuit that optically synchronizes the upstream signal and the local light source for receiving the upstream signal, or the optical phase synchronization of the downstream signal and the local light source for receiving the downstream signal, via any one of the tones. And a circuit for optically detecting any two of the plurality of pilot tones and extracting a difference frequency electric signal, and the upstream signal, the downstream signal, and the plurality of pilot tones are mutually connected. It may be configured to generate at different frequencies.
　本発明に係る光伝送方法および光伝送装置で、前記第１の光送受信部は、例えば、基地局ベースバンド部または光回線終端装置（Optical Line Terminal）から成り、前記第２の光送受信部は、例えば、アンテナ無線部または光回線ネットワーク装置（Optical Network Unit）から成ることが好ましい。 In the optical transmission method and the optical transmission apparatus according to the present invention, the first optical transmission / reception unit includes, for example, a base station baseband unit or an optical line termination device (Optical / Line Terminal), and the second optical transmission / reception unit includes For example, it preferably includes an antenna radio unit or an optical network device (Optical Network Unit).
　本発明に係る光伝送方法及び光伝送装置は、１台のレーザで送信用光源および局発光源の両方の役割を担わせ、光位相同期の位相基準信号として、例えばパイロットトーンを信号光と共に伝送することにより、光源を有効利用できると同時に、受信器での位相同期を光回路で実現することができる。また、上り信号および下り信号を（光位相同期用のパイロットトーンを用いる場合にはパイロットトーンも）互いに異なる光周波数で伝送することによって、後方レイリー散乱光による伝送性能の劣化を抑制し、ロスバジェットの大きい長距離双方向伝送を実現することができる。これにより、簡便な構成で且つ経済性の高い、後方レイリー散乱非混合型の光伝送方法および光伝送装置を提供することができる。 An optical transmission method and an optical transmission apparatus according to the present invention serve as both a transmission light source and a local light source with a single laser, and transmit, for example, a pilot tone together with signal light as a phase reference signal for optical phase synchronization. By doing so, the light source can be used effectively, and at the same time, phase synchronization at the receiver can be realized by the optical circuit. In addition, by transmitting upstream signals and downstream signals (and pilot tones when using pilot tones for optical phase synchronization) at different optical frequencies, transmission performance deterioration due to backward Rayleigh scattered light is suppressed, and loss budget is achieved. Long-distance bidirectional transmission can be realized. Thereby, it is possible to provide a backward Rayleigh scattering non-mixing type optical transmission method and an optical transmission device with a simple configuration and high economic efficiency.
本発明の第１の実施形態における光伝送装置（ホモダイン方式）を示すブロック構成図である。It is a block block diagram which shows the optical transmission apparatus (homodyne system) in the 1st Embodiment of this invention. 本発明の第１の実施形態における光伝送装置の、（ａ）下り信号の周波数配置、（ｂ）上り信号の周波数配置を示す説明図である。It is explanatory drawing which shows (a) frequency arrangement | positioning of a downstream signal and (b) frequency arrangement | positioning of an upstream signal of the optical transmission apparatus in the 1st Embodiment of this invention. 本発明の第１の実施形態における光伝送装置の、伝送実験の実験系を示すブロック図である。It is a block diagram which shows the experimental system of the transmission experiment of the optical transmission apparatus in the 1st Embodiment of this invention. 図３に示す伝送実験における、（ａ）ＢＢＵ１からＲＲＨ３へ伝送する下り信号の光スペクトル、（ｂ）ＲＲＨ３からＢＢＵ１へ伝送する上り信号の光スペクトル、（ｃ）伝送後の下り信号の光ペクトル、（ｄ）伝送後の上り信号の光スペクトルである。In the transmission experiment shown in FIG. 3, (a) the optical spectrum of the downstream signal transmitted from BBU1 to RRH3, (b) the optical spectrum of the upstream signal transmitted from RRH3 to BBU1, (c) the optical spectrum of the downstream signal after transmission, (D) The optical spectrum of the upstream signal after transmission. 図３に示す伝送実験における、（ａ）光ファイバ伝送後の下り信号の256 QAMコンスタレーション、（ｂ）光ファイバ伝送後の上り信号の256 QAMコンスタレーションである。In the transmission experiment shown in FIG. 3, (a) a 256 QAM constellation of a downstream signal after optical fiber transmission, and (b) a 256 QAM constellation of an upstream signal after optical fiber transmission. 本発明の第２の実施の形態における光伝送装置（ヘテロダイン方式）を示すブロック構成図である。It is a block block diagram which shows the optical transmission apparatus (heterodyne system) in the 2nd Embodiment of this invention. 本発明の第３の実施の形態における光伝送装置（ホモダイン方式）を示すブロック構成図である。It is a block block diagram which shows the optical transmission apparatus (homodyne system) in the 3rd Embodiment of this invention. 本発明の第３の実施の形態における光伝送装置の、ヘテロダイン方式を用いる変形例を示すブロック構成図である。It is a block block diagram which shows the modification which uses the heterodyne system of the optical transmission apparatus in the 3rd Embodiment of this invention. 本発明の第４の実施の形態における光伝送装置（ホモダイン方式）を示すブロック構成図である。It is a block block diagram which shows the optical transmission apparatus (homodyne system) in the 4th Embodiment of this invention. 本発明の第４の実施の形態における光伝送装置の、ヘテロダイン方式を用いる変形例を示すブロック構成図である。It is a block block diagram which shows the modification which uses the heterodyne system of the optical transmission apparatus in the 4th Embodiment of this invention. 本発明の第５の実施の形態における光伝送装置を示すブロック構成図である。It is a block block diagram which shows the optical transmission apparatus in the 5th Embodiment of this invention. 図１１に示す光伝送装置の、（ａ）下り信号の周波数配置、（ｂ）上り信号の周波数配置を示す説明図である。FIG. 12 is an explanatory diagram illustrating (a) frequency arrangement of downlink signals and (b) frequency arrangement of uplink signals of the optical transmission apparatus shown in FIG. 11. 本発明の第６の実施の形態における光伝送装置を示すブロック構成図である。It is a block block diagram which shows the optical transmission apparatus in the 6th Embodiment of this invention.
　以下、図面に基づいて、本発明の実施の形態について説明する。
　以下に示す第１～第６の本発明の実施の形態は、デジタルコヒーレント光伝送技術ならびに光アクセスネットワークを用いて、第１の光送受信部と第２の光送受信部との間で高速な無線信号を、一芯の光ファイバを介して長距離双方向伝送させるための光伝送方法および光伝送装置を示している。 Hereinafter, embodiments of the present invention will be described with reference to the drawings.
The first to sixth embodiments of the present invention described below use a digital coherent optical transmission technology and an optical access network to perform high-speed wireless communication between the first optical transmission / reception unit and the second optical transmission / reception unit. 1 shows an optical transmission method and an optical transmission apparatus for bidirectional transmission of a signal over a long distance via a single optical fiber.
　以下に示す第１～第６の本発明の実施の形態では、第１の光送受信部が基地局ベースバンド部（BBU）から成り、第２の光送受信部がアンテナ無線部（RRH）から成っており、５Ｇなどの移動通信システムで利用されるモバイルフロントホール伝送を行うための光伝送方法および光伝送装置を示している。しかし、これらの本発明の実施の形態は、基地局ベースバンド部（BBU）を光回線終端装置とし、アンテナ無線部（RRH）を光回線ネットワーク装置とすることにより、５Ｇなどの移動通信システムだけでなく、FTTHなどの一般的な光アクセスネットワークシステムにも適用可能である。 In the following first to sixth embodiments of the present invention, the first optical transmission / reception unit comprises a base station baseband unit (BBU), and the second optical transmission / reception unit comprises an antenna radio unit (RRH). 1 illustrates an optical transmission method and an optical transmission apparatus for performing mobile fronthaul transmission used in a mobile communication system such as 5G. However, in these embodiments of the present invention, the base station baseband unit (BBU) is an optical line termination device and the antenna radio unit (RRH) is an optical line network device, so that only a mobile communication system such as 5G is used. In addition, it can be applied to a general optical access network system such as FTTH.
　なお、コヒーレント伝送における受信方式には、ホモダイン検波とヘテロダイン検波とがある。具体的には、信号光と局発光の周波数が等しい場合をホモダイン検波、異なる場合をヘテロダイン検波と呼ぶ。ホモダイン方式は、中間周波数帯にダウンコンバートした際、その帯域がヘテロダイン方式に比べて半分となり、それに伴い雑音帯域も半分となるため、受信感度がヘテロダイン方式より３ｄＢ改善される。また、光検出器の帯域に関しても、ヘテロダイン方式では信号帯域に加え、セルフビート信号の分だけ広くとる必要があるのに対して、ホモダイン方式では信号帯域の半分だけで良い。一方、ホモダイン方式は、局発光の周波数だけではなく位相も安定化させるために、高精度な位相同期が必要となる。 Note that the reception methods in coherent transmission include homodyne detection and heterodyne detection. Specifically, the case where the frequency of the signal light and the local light is equal is called homodyne detection, and the case where the frequencies are different is called heterodyne detection. When the homodyne system is down-converted to the intermediate frequency band, the band is halved compared to the heterodyne system, and the noise band is also halved accordingly, so the reception sensitivity is improved by 3 dB compared to the heterodyne system. Also, with respect to the band of the photodetector, the heterodyne system needs to be widened by the self-beat signal in addition to the signal band, whereas the homodyne system requires only half the signal band. On the other hand, the homodyne system requires highly accurate phase synchronization in order to stabilize not only the frequency of local light but also the phase.
［第１の実施形態］
　本発明の第１の実施形態における光伝送装置の構成を、図１に示す。本実施形態では、コヒーレント検波としてホモダイン方式を用いる。基地局においてベースバンド信号処理を行う基地局ベースバンド部（以下、BBUと称する）１は、レーザ光源１１、ＩＱ変調器１２、光変調器１３、ＲＦ発振器１４、光フィルタ１５、９０度光ハイブリッド回路１６、平衡光検出器１７、Ａ／Ｄ変換器１８、デジタル信号処理回路（DSP: Digital Signal Processor）１９、Ｄ／Ａ変換器２０、光サーキュレータ２１を備える。一方、アンテナ及び高周波信号処理を担うアンテナ無線部（以下、RRHと称する）３は、レーザ光源３１、ＩＱ変調器３２、光変調器３３、光フィルタ３４及び３５、９０度光ハイブリッド回路３６、平衡光検出器３７、Ａ／Ｄ変換器３８、デジタル信号処理回路（DSP: Digital Signal Processor）３９、Ｄ／Ａ変換器４０、光増幅器４１、光フィルタ４２、光サーキュレータ４３、光検出器４４、分周器４５、光サーキュレータ４６を備える。なお、光サーキュレータ２１、４３、４６は、ポート１（Ｐ１）へ入射された信号光をポート２（Ｐ２）へ出力し、ポート２（Ｐ２）へ入射された信号光をポート３（Ｐ３）へ出力するようになっている。 [First Embodiment]
FIG. 1 shows the configuration of the optical transmission apparatus according to the first embodiment of the present invention. In this embodiment, a homodyne method is used for coherent detection. A base station baseband unit (hereinafter referred to as BBU) 1 that performs baseband signal processing in a base station includes a laser light source 11, an IQ modulator 12, an optical modulator 13, an RF oscillator 14, an optical filter 15, and a 90-degree optical hybrid. A circuit 16, a balanced photodetector 17, an A / D converter 18, a digital signal processor (DSP) 19, a D / A converter 20, and an optical circulator 21 are provided. On the other hand, an antenna radio unit (hereinafter referred to as RRH) 3 that handles antennas and high-frequency signal processing includes a laser light source 31, an IQ modulator 32, an optical modulator 33, optical filters 34 and 35, a 90-degree optical hybrid circuit 36, a balanced Optical detector 37, A / D converter 38, digital signal processor (DSP) 39, D / A converter 40, optical amplifier 41, optical filter 42, optical circulator 43, optical detector 44, minute A peripheral 45 and an optical circulator 46 are provided. The optical circulators 21, 43, and 46 output the signal light incident on the port 1 (P1) to the port 2 (P2) and the signal light incident on the port 2 (P2) to the port 3 (P3). It is designed to output.
　f1 [Hz]の光周波数で発振するレーザ光源１１の出力光は、２分岐され、一方はBBU１からRRH３への下り回線用の信号光として、もう一方はRRH３からBBU１へ伝送してきた上り信号の受信に必要な局発光として、それぞれ用いる。前者は、ＩＱ変調器１２によってデータ変調される。ここでＩＱ変調器１２は、ベースバンド信号Ix, Qx, Iy, Qyで駆動され、偏波多重機能を備えている。あるいは２台のＩＱ変調器１２を用い、１台をIx, Qx、もう１台をIy, Qyでデータ変調し、その後両者を偏波多重させてもよい。ベースバンド信号は、必要に応じて高周波増幅器で増幅された後、ＩＱ変調器１２に供給されてもよい。ここで、データ信号から±Δf [Hz]だけ離れた周波数にパイロットトーン１およびパイロットトーン２の２本の信号を立てておく。その結果、信号光は、周波数f1 [Hz]のデータ信号および周波数f2 [Hz]、f3 [Hz]の２本パイロットトーンで構成される。下り信号のデータとパイロットトーンの周波数配置を、図２（ａ）に示す。ここでは、f1よりΔf [Hz]だけ低周波数側の信号をパイロットトーン１（ｆ２）、高周波数側の信号をパイロットトーン２（ｆ３）としている。 The output light of the laser light source 11 oscillating at an optical frequency of f 1 [Hz] is branched into two, one as signal light for downlink from BBU1 to RRH3, and the other as upstream signal transmitted from RRH3 to BBU1. Each is used as a local light emission necessary for receiving the light. The former is data-modulated by the IQ modulator 12. Here, the IQ modulator 12 is driven by baseband signals Ix, Qx, Iy, and Qy, and has a polarization multiplexing function. Alternatively, two IQ modulators 12 may be used, and one may be data-modulated with Ix and Qx, the other with Iy and Qy, and then both may be polarization multiplexed. The baseband signal may be supplied to the IQ modulator 12 after being amplified by a high frequency amplifier as necessary. Here, two signals of pilot tone 1 and pilot tone 2 are set up at a frequency separated from the data signal by ± Δf [Hz]. As a result, the signal light is composed of a data signal having a frequency f 1 [Hz] and two pilot tones having frequencies f 2 [Hz] and f 3 [Hz]. FIG. 2A shows the frequency arrangement of downlink signal data and pilot tones. Here, a signal on the low frequency side by Δf [Hz] from f 1 is a pilot tone 1 (f 2), and a signal on the high frequency side is a pilot tone 2 (f 3).
　パイロットトーンが重畳された信号光は、光サーキュレータ２１のポート１（Ｐ１）へ入射される。光サーキュレータ２１のポート２より出力された信号光は、光ファイバ伝送路２を伝搬し、RRH３で受信される。RRH３では、伝送後の信号は、まずサーキュレータ４６のポート２に入射され、ポート３より受信部へ出力される。２分岐された信号光の一方は、９０度光ハイブリッド回路３６および平衡光検出器３７で構成されるホモダイン検波回路へ入力される。もう一方は、光フィルタ４２によってパイロットトーンが抽出され、これを必要に応じて光増幅器４１で増幅した後、光サーキュレータ４３のポート１（Ｐ１）へ入力し、ポート２（Ｐ２）からレーザ光源３１に注入する。パイロットトーンが十分なパワーを有し、且つその周波数がレーザ光源３１の発振周波数に十分近いとき（正確には、パイロットトーンのパワー及び周波数が、レーザ光源３１のロッキングレンジの範囲にあるとき）、注入同期現象により、レーザ光源３１の位相はパイロットトーンの位相、即ち下り信号光の位相に同期する。注入同期に用いるパイロットトーンは、パイロットトーン１、パイロットトーン２のいずれでも構わないが、ここではパイロットトーン１（f2 [Hz]）にレーザ光源３１が注入同期するとして以降の動作を説明する。 The signal light on which the pilot tone is superimposed is incident on the port 1 (P1) of the optical circulator 21. The signal light output from the port 2 of the optical circulator 21 propagates through the optical fiber transmission line 2 and is received by the RRH 3. In RRH3, the signal after transmission is first incident on port 2 of circulator 46, and is output from port 3 to the receiving unit. One of the two branched signal lights is input to a homodyne detection circuit including a 90-degree optical hybrid circuit 36 and a balanced photodetector 37. The other is that the pilot tone is extracted by the optical filter 42, amplified by the optical amplifier 41 as necessary, and then input to the port 1 (P1) of the optical circulator 43, and the laser light source 31 from the port 2 (P2). Inject. When the pilot tone has sufficient power and its frequency is sufficiently close to the oscillation frequency of the laser light source 31 (more precisely, when the power and frequency of the pilot tone are within the rocking range of the laser light source 31) Due to the injection locking phenomenon, the phase of the laser light source 31 is synchronized with the phase of the pilot tone, that is, the phase of the downstream signal light. The pilot tone used for injection locking may be either pilot tone 1 or pilot tone 2, but the following operation will be described on the assumption that the laser light source 31 is injection-locked to pilot tone 1 (f 2 [Hz]).
　注入同期され、f2 [Hz]の光周波数で発振するレーザ光源３１の出力はfclock（=Δf）[Hz]のRF信号で駆動される光変調器３３に入力される。ここで光強度または光位相変調を行うことで、fclock [Hz]間隔の複数のCW（Continuous Wave）光が光周波数軸上に並ぶ光コム信号が生成される。fclock [Hz]のRF信号は、伝送されてきたパイロットトーン１とパイロットトーン２とを光検出器４４に入力し、本光検出器４４より出力される周波数2 fclock（=f3-f2）[Hz]のヘテロダインビート電気信号を、分周器４５を用いて１／２分周することで得られる。生成された光コム信号を２分岐し、一方では、光フィルタ３４を介して周波数f4（=f2-Δf）[Hz]のCW光を一本抽出し、これをRRH３からBBU１への上りの信号光として用いる。もう一方では、光フィルタ３５を介して周波数f1 [Hz]のCW光を一本抽出し、これを局発光として下り信号光とともに、９０度光ハイブリッド回路３６および平衡光検出器３７で構成されるホモダイン検波回路へ入力する。ホモダイン検波された信号は、Ａ／Ｄ変換器３８でデジタル信号に変換され、DSP回路３９で偏波分離、復調、適応等化等の信号処理を行った後、Ｄ／Ａ変換器４０で再びアナログ信号へ変換され、データ信号Ix, Qx, Iy, Qyが出力される。 The output of the laser light source 31 that is injection-locked and oscillates at an optical frequency of f 2 [Hz] is input to an optical modulator 33 that is driven by an RF signal of f clock (= Δf) [Hz]. Here, by performing optical intensity or optical phase modulation, an optical comb signal in which a plurality of CW (Continuous Wave) lights at intervals of f clock [Hz] are arranged on the optical frequency axis is generated. For the RF signal of f clock [Hz], the transmitted pilot tone 1 and pilot tone 2 are input to the photodetector 44 and the frequency 2 f clock (= f 3 −f) output from the photodetector 44 is output. 2 ) Obtained by frequency-dividing [Hz] heterodyne beat electric signal by 1/2 using frequency divider 45. The generated optical comb signal is split into two, and on the other hand, a single CW light having a frequency f 4 (= f 2 −Δf) [Hz] is extracted via the optical filter 34, and this is extracted from RRH 3 to BBU 1. Used as signal light. On the other hand, one CW light having a frequency f 1 [Hz] is extracted through an optical filter 35, and this is used as a local light and is composed of a 90-degree optical hybrid circuit 36 and a balanced photodetector 37 together with a downstream signal light. Input to the homodyne detection circuit. The homodyne-detected signal is converted into a digital signal by the A / D converter 38, subjected to signal processing such as polarization separation, demodulation, and adaptive equalization by the DSP circuit 39, and then again by the D / A converter 40. Data signals Ix, Qx, Iy, Qy are output after being converted to analog signals.
　一方、上り信号については、RRH３において、BBU１と同様に、レーザ光源３１の出力をＩＱ変調器３２によってデータ変調および偏波多重する。なお、このとき、データ信号にはパイロットトーンを重畳させなくてよい。上り信号のデータとパイロットトーンの周波数配置を、図２（ｂ）に示す。 On the other hand, for the upstream signal, the RRH 3 performs data modulation and polarization multiplexing on the output of the laser light source 31 by the IQ modulator 32 as in the case of BBU 1. At this time, the pilot tone need not be superimposed on the data signal. FIG. 2B shows the frequency arrangement of uplink signal data and pilot tones.
　信号光は、光ファイバ伝送路２を伝搬し、BBU１で受信される。BBU１では、レーザ光源１１から分岐された周波数f1 [Hz]の光を、2fclock [Hz]の電気信号で駆動される光変調器１３へ入射し、光強度または光位相変調を行う。ここで、2fclock [Hz]の電気信号としては、下りのベースバンド信号を生成する際に使用されるクロック信号を逓倍して用いてもよい。光変調によって生成された2fclock [Hz]間隔の光コム信号より、光フィルタ１５を用いて周波数f4 [Hz]のCW光を一本抽出し、本信号をBBU１における局発光として用いる。本信号を上り信号（f4 [Hz]）と共に、９０度光ハイブリッド回路１６および平衡光検出器１７で構成されるホモダイン検波回路へ入力する。ホモダイン検波された信号は、Ａ／Ｄ変換器１８でデジタル信号に変換され、DSP回路１９で偏波分離、復調、適応等化等の信号処理を行った後、Ｄ／Ａ変換器２０で再びアナログ信号へ変換され、データ信号Ix, Qx, Iy, Qyが出力される。 The signal light propagates through the optical fiber transmission line 2 and is received by the BBU 1. In the BBU 1 , light having a frequency f 1 [Hz] branched from the laser light source 11 is incident on an optical modulator 13 driven by an electric signal of 2f clock [Hz], and optical intensity or optical phase modulation is performed. Here, as the electrical signal of 2f clock [Hz], a clock signal used when generating a downlink baseband signal may be multiplied and used. One optical CW light having a frequency f 4 [Hz] is extracted from the optical comb signal generated by the optical modulation at intervals of 2f clock [Hz] by using the optical filter 15, and this signal is used as local light emission in the BBU 1. This signal is input together with the upstream signal (f 4 [Hz]) to a homodyne detection circuit composed of the 90-degree optical hybrid circuit 16 and the balanced photodetector 17. The homodyne-detected signal is converted into a digital signal by the A / D converter 18, subjected to signal processing such as polarization separation, demodulation, and adaptive equalization by the DSP circuit 19, and then again by the D / A converter 20. Data signals Ix, Qx, Iy, Qy are output after being converted to analog signals.
　本実施形態では２つのレーザ光源１１、３１は、RRH３における注入同期によって互いに位相同期されているため、BBU１においては、光位相同期回路は不要である。しかしながら、コヒーレント検波を行う際、９０度光ハイブリッド回路１６または３６へ入力される信号光および局発光は、それぞれ異なる光路を通るため、光路長の変動に伴って（例えば温度変化に伴う光ファイバ長の変動）、データ信号と局発光間の位相はゆっくりとした速度で僅かに変動する。このような低速な位相ドリフト変動は、受信部のDSP１９または３９にて補正を行う。 In the present embodiment, since the two laser light sources 11 and 31 are phase-locked with each other by injection locking in the RRH 3, no optical phase-locked circuit is required in the BBU1. However, when performing coherent detection, the signal light and the local light input to the 90-degree optical hybrid circuit 16 or 36 pass through different optical paths, respectively. Therefore, as the optical path length varies (for example, the optical fiber length due to temperature change). The phase between the data signal and local light fluctuates slightly at a slow speed. Such low-speed phase drift fluctuations are corrected by the DSP 19 or 39 of the receiving unit.
　本実施形態においては、BBU１からRRH３へ伝送する下り信号（f1 [Hz]）、パイロットトーン１（f2 [Hz]）、パイロットトーン２（f3 [Hz]）と、RRH３からBBU１へ伝送する上り信号（f4 [Hz]）をそれぞれ異なる周波数に割り当てている。そのため、これらの信号を伝送した際に光ファイバ伝送路中で生じるそれぞれの信号の後方レイリー散乱光は、前方へ伝搬するそれぞれの信号光にノイズとしての混入することがない。例えば、RRH３からBBU１へ向けて伝送した上り信号（f4 [Hz]）の後方レイリー散乱光がRRH３の受信部へ戻って来ても、BBU１からRRH３へ伝送されてきた下り信号（f1 [Hz]）、パイロットトーン１（f2 [Hz]）およびパイロットトーン２（f3 [Hz]）と周波数的に互いに重なることがないため、パイロットトーン１（f2 [Hz]）のレーザ光源３１への注入同期動作や下り信号（f1 [Hz]）の復調に影響を及ぼすことはない。 In this embodiment, a downlink signal (f 1 [Hz]), pilot tone 1 (f 2 [Hz]), pilot tone 2 (f 3 [Hz]) transmitted from BBU1 to RRH3, and transmission from RRH3 to BBU1 The upstream signal (f 4 [Hz]) is assigned to a different frequency. Therefore, the backward Rayleigh scattered light of each signal generated in the optical fiber transmission line when these signals are transmitted is not mixed as noise in each signal light propagating forward. For example, even if the backward Rayleigh scattered light of the upstream signal (f 4 [Hz]) transmitted from RRH 3 to BBU 1 returns to the receiving unit of RRH 3, the downstream signal (f 1 [ Hz]), pilot tone 1 (f 2 [Hz]), and pilot tone 2 (f 3 [Hz]) do not overlap with each other in frequency, so that the laser light source 31 of pilot tone 1 (f 2 [Hz]) This does not affect the injection locking operation or the demodulation of the downstream signal (f 1 [Hz]).
［第１の実施形態で行った伝送実験の実施例］
　第１の実施形態で行った、80 Gbit/s、偏波多重5 Gbaud、256 QAM (Quadrature Amplitude Modulation)信号のSMF (Single Mode Fiber) 26 km双方向伝送実験について説明する。 [Example of transmission experiment performed in the first embodiment]
The SGF (Single Mode Fiber) 26 km bidirectional transmission experiment of the 80 Gbit / s, polarization multiplexed 5 Gbaud, 256 QAM (Quadrature Amplitude Modulation) signal performed in the first embodiment will be described.
　伝送系の構成を示すブロック図を、図３に示す。なお、本図において、図１と同じ構成要素には同じ符号を付している。本伝送実験では、BBU１において、レーザ光源１１として波長1.55 μmで発振する（光周波数f1 [Hz]とする）線幅8 kHzのCW半導体レーザを用いた。I、Qベースバンドデータ信号は、サンプリング速度65 GS/sのDAC（Digital Analogue Converter）とDSP回路とから成る任意波形生成装置２２を用いて生成した。本装置から出力される5 Gbaud, 256 QAMベースバンド信号と、12 GHz（＝Δf＝fclock）の正弦波信号（パイロットトーン信号として用いる）とをＩＱ変調器１２へ入力してレーザ光源１１の出力光を変調し、5 Gbaud, 256 QAMデータ信号（f1 [Hz]）と、f2 [Hz]およびf3 [Hz]の２本のパイロットトーンを生成している。ここで、任意波形生成装置２２から出力される256 QAMデータ信号には、ロールオフ率が0.2のroot raised cosine Nyquist filterを施しており、その帯域を3 GHzに狭窄化している。偏波多重には、偏波ビームスプリッタと遅延回路とで構成される光回路から成る偏波多重回路２３を用いた。生成した80 Gbit/s、偏波多重5 Gbaud、256 QAM信号と２本のパイロットトーンは、-5 dBmの伝送パワーで、光ファイバ２として用いた長さ26 kmのSMFを伝搬しRRH３へ到達する。なお、本実施例におけるデータ信号とパイロットトーンの周波数関係は、図２（ａ）と等しい。 FIG. 3 shows a block diagram showing the configuration of the transmission system. In the figure, the same components as those in FIG. 1 are denoted by the same reference numerals. In this transmission experiment, a CW semiconductor laser having a line width of 8 kHz that oscillates at a wavelength of 1.55 μm (with an optical frequency of f 1 [Hz]) is used as the laser light source 11 in the BBU 1 . The I and Q baseband data signals were generated by using an arbitrary waveform generator 22 composed of a DAC (Digital Analogue Converter) with a sampling speed of 65 GS / s and a DSP circuit. A 5 Gbaud, 256 QAM baseband signal output from this apparatus and a 12 GHz (= Δf = f clock ) sine wave signal (used as a pilot tone signal) are input to the IQ modulator 12 and the laser light source 11 The output light is modulated to generate a 5 Gbaud, 256 QAM data signal (f 1 [Hz]) and two pilot tones of f 2 [Hz] and f 3 [Hz]. Here, the 256 QAM data signal output from the arbitrary waveform generator 22 is subjected to a root raised cosine Nyquist filter with a roll-off rate of 0.2, and the band is narrowed to 3 GHz. For polarization multiplexing, a polarization multiplexing circuit 23 comprising an optical circuit composed of a polarization beam splitter and a delay circuit was used. The generated 80 Gbit / s, polarization multiplexed 5 Gbaud, 256 QAM signal and two pilot tones propagate to the RRH3 by propagating the 26 km long SMF used as the optical fiber 2 with a transmission power of -5 dBm. To do. The frequency relationship between the data signal and the pilot tone in this embodiment is the same as that shown in FIG.
　RRH３では、まず周波数f2 [Hz]のパイロットトーン信号を光フィルタ４２で抽出し、これをレーザ光源３１へ入射して注入同期を行うことで、レーザ光源３１を下りのデータ信号に位相同期する。レーザ光源３１としては、波長1.55 μm帯で発振する線幅200 kHz程度のCW半導体レーザを用いた。本実験では、およそ0.3 度の位相雑音でデータ信号と局発光源との位相同期を行っている。256 QAM信号の場合、最隣接するシンボル間の位相差として求められる許容位相雑音は、およそ2度である。したがって、本実験における注入同期回路は、256 QAM信号を復調するに十分な性能を有している。注入同期によってf2 [Hz]で発振するレーザ光源３１の出力光を、Δf [Hz]の正弦波で駆動される光変調器３３に入射し、光変調によってサイドバンドの生成を行っている。本実験では、光変調器３３としてLN (LiNbO3)光強度変調器を用いたが、LN光位相変調器やSSB (Single Side-Band）変調器を用いても良い。また、第１の実施形態では、Δf [Hz]のクロック信号は２本のパイロットトーンのヘテロダインビート信号から生成しているが、本実験では簡単のため、RF発振器４７を用いて生成した。f2 [Hz]から12 GHz（=Δf [Hz]）低い周波数側（f4 [Hz]）および高い周波数側（f1 [Hz]）のサイドバンドを、光フィルタ３４および３５を用いて抽出し、それぞれを上り信号の光キャリヤ、下りのデータ信号をホモダイン検波する局発光として用いている。ホモダイン検波された下りのデータ信号は、40 GS/sのサンプリング速度でＡ／Ｄ変換され、その後DSP３９を用いてオフラインで復調される。なお、９０度光ハイブリッド回路３６へ入力する信号光と局発光とが異なる光路を通ることに起因するホモダイン検波信号の低速な位相変動は、DSP３９内で補正を行っている。 In the RRH 3, first, a pilot tone signal having a frequency of f 2 [Hz] is extracted by the optical filter 42 and incident on the laser light source 31 to perform injection locking, whereby the laser light source 31 is phase-locked to the downstream data signal. . As the laser light source 31, a CW semiconductor laser having a line width of about 200 kHz that oscillates in a wavelength band of 1.55 μm was used. In this experiment, the data signal and the local light source are phase-synchronized with a phase noise of approximately 0.3 degrees. In the case of a 256 QAM signal, the allowable phase noise obtained as the phase difference between the nearest symbols is approximately 2 degrees. Therefore, the injection locking circuit in this experiment has sufficient performance to demodulate the 256 QAM signal. The output light of the laser light source 31 that oscillates at f 2 [Hz] by injection locking is incident on an optical modulator 33 driven by a sine wave of Δf [Hz], and sidebands are generated by optical modulation. In this experiment, an LN (LiNbO 3 ) light intensity modulator is used as the light modulator 33, but an LN optical phase modulator or an SSB (Single Side-Band) modulator may be used. In the first embodiment, the clock signal of Δf [Hz] is generated from the heterodyne beat signal of two pilot tones, but in this experiment, it is generated using the RF oscillator 47 for simplicity. f 2 [Hz] to 12 GHz (= Δf [Hz]) Low frequency side (f 4 [Hz]) and high frequency side (f 1 [Hz]) sidebands are extracted using optical filters 34 and 35 Each of them is used as an optical carrier for the upstream signal and local light for detecting the downstream data signal by homodyne detection. The downstream data signal subjected to homodyne detection is A / D converted at a sampling rate of 40 GS / s and then demodulated off-line using the DSP 39. Note that the low-speed phase fluctuation of the homodyne detection signal caused by the signal light input to the 90-degree optical hybrid circuit 36 and the local light passing through different optical paths is corrected in the DSP 39.
　RRH３における上り信号生成部では、下りのデータ信号と同様の方法で偏波多重5 Gbaud、256 QAM信号を生成し、-5 dBmの伝送パワーで、26 kmのSMF をBBU１側へ伝送する。BBU１の受信部では、レーザ光源１１から分岐された周波数f1 [Hz]の光を、光変調器１３、発振器１４、光フィルタ１５から成る光周波数シフタによってf4 [Hz]に変換し、これを局発光として用いて、下りの信号光をホモダイン検波する。ここで、発振器１４の代わりとして、任意波形生成装置２２を駆動する基準クロック信号源を用いてもよい。検波後の信号は、40 GS/sのサンプリング速度のＡ／Ｄ変換器１７でデジタル化され、その後DSP１９を用いてオフラインで復調される。BBU１においても、信号光と局発光とが異なる光路を通ることに起因するホモダイン検波信号の位相変動が生じるが、これはDSP１９内で補正している。 The uplink signal generator in RRH3 generates polarization multiplexed 5 Gbaud, 256 QAM signals in the same manner as the downlink data signal, and transmits 26 km SMF to the BBU 1 side with a transmission power of -5 dBm. In the receiving unit of the BBU 1 , the light having the frequency f 1 [Hz] branched from the laser light source 11 is converted into f 4 [Hz] by an optical frequency shifter including the optical modulator 13, the oscillator 14, and the optical filter 15. Is used as local light, and downstream signal light is subjected to homodyne detection. Here, instead of the oscillator 14, a reference clock signal source that drives the arbitrary waveform generation device 22 may be used. The detected signal is digitized by an A / D converter 17 having a sampling rate of 40 GS / s, and then demodulated off-line using a DSP 19. Even in the BBU 1, the phase fluctuation of the homodyne detection signal caused by the signal light and the local light passing through different optical paths occurs, and this is corrected in the DSP 19.
　図４（ａ）および（ｂ）は、それぞれBBU１から送信される下り信号およびRRH３から送信される上り信号の光スペクトルである。これらはそれぞれ、図１のサーキュレータ２１のＰ２またはサーキュレータ４６のＰ２からの出力光を測定したものである。図のように本実験では、f1～f4 [Hz]のそれぞれ異なる光周波数に、上り信号、下り信号および２本のパイロットトーン信号を割り当てて伝送している。図４（ｃ）および（ｄ）は、SMFを26 km双方向伝送した後の下り信号および上り信号の光スペクトルであり、それぞれ、図１のサーキュレータ４６のＰ３またはサーキュレータ２１のＰ３からの出力光を測定した結果である。本図から、光ファイバ伝送路中で生じた上りおよび下り信号の後方レイリー散乱光が、前方へ伝送したそれぞれの信号に混入している様子がわかる。例えば、図４（ｃ）では、RRH３へ伝送された下り信号にRRH３よりBBU１へ向けて送信した上り信号（f4 [Hz]）の後方レイリー散乱光が混入していることがわかる。なお、本実験では、光サーキュレータ２１および４６のＰ１からＰ３への漏れ光量（クロストーク）は十分に小さいことを確認している。 4A and 4B are optical spectra of a downlink signal transmitted from BBU1 and an uplink signal transmitted from RRH3, respectively. These are measurements of output light from P2 of the circulator 21 or P2 of the circulator 46 in FIG. As shown in the figure, in this experiment, an uplink signal, a downlink signal, and two pilot tone signals are allocated and transmitted at different optical frequencies of f 1 to f 4 [Hz]. 4 (c) and 4 (d) are the optical spectra of the downstream signal and upstream signal after 26 km bidirectional transmission of SMF, and the output light from P3 of circulator 46 or P3 of circulator 21 in FIG. 1, respectively. It is the result of having measured. From this figure, it can be seen that the backward Rayleigh scattered light of the upstream and downstream signals generated in the optical fiber transmission line is mixed in each signal transmitted forward. For example, in FIG. 4C, it can be seen that the downstream signal transmitted to RRH3 is mixed with the backward Rayleigh scattered light of the upstream signal (f 4 [Hz]) transmitted from RRH3 toward BBU1. In this experiment, it was confirmed that the amount of light leakage (crosstalk) from P1 to P3 of the optical circulators 21 and 46 is sufficiently small.
　図５に光ファイバ伝送後の偏波多重5 Gbaud、256 QAM信号の復調結果示す。図５（ａ）および（ｂ）は、それぞれ下り信号および上り信号のコンスタレーションである。いずれの結果も８ビットの情報を有する各シンボル点が明確に分離できており、ビット情報を正確に復調できていることがわかる。 Figure 5 shows the demodulation result of polarization multiplexed 5 Gbaud and 256 QAM signals after optical fiber transmission. FIGS. 5A and 5B are constellations of downstream signals and upstream signals, respectively. In any result, each symbol point having 8-bit information can be clearly separated, and it can be seen that the bit information can be accurately demodulated.
　本実施例において、例えばRRH３で生成する上り信号の周波数を、下りのデータ信号と同じf1 [Hz]で生成して伝送すると、上り、下りいずれのデータ信号も復調することが出来なくなる。これは、下り、上りそれぞれの信号の後方レイリー散乱光が互いにノイズとして前方へ伝搬した復調対象となるデータ信号光に混入し、データ信号のS/N （Signal-to-Noise Ratio）を劣化させるためである。また、上り信号の周波数を下り信号のパイロットトーン２と同じf3 [Hz]として伝送させた場合には、下りのデータ信号の復調が不可となる。この場合は、上り信号の後方レイリー散乱光が前方へ伝搬したパイロットトーン２へノイズとして混入し、RRH３においてレーザ光源３１の注入同期特性を大きく劣化させるためである。 In this embodiment, for example, if the frequency of the uplink signal generated by RRH3 is generated and transmitted at the same f 1 [Hz] as the downlink data signal, neither the uplink nor the downlink data signal can be demodulated. This is because the backward Rayleigh scattered light of downstream and upstream signals is mixed into the data signal light to be demodulated and propagated forward as noise, degrading the signal-to-noise ratio (S / N) of the data signal. Because. In addition, when the frequency of the upstream signal is transmitted as f 3 [Hz] which is the same as the pilot tone 2 of the downstream signal, the downstream data signal cannot be demodulated. In this case, the backward Rayleigh scattered light of the upstream signal is mixed as noise into the pilot tone 2 propagated forward, and the injection locking characteristic of the laser light source 31 is greatly deteriorated in the RRH 3.
　以上のように、上り、下り信号を互いに異なる周波数に割り当て、且つ光注入同期によってデータと局発光との位相同期を行う本発明に係る伝送方式を用いることにより、簡便な伝送系で大容量モバイルフロントホールを実現することが出来る。 As described above, by using the transmission method according to the present invention in which uplink and downlink signals are assigned to different frequencies and the phase synchronization between data and local light is performed by light injection synchronization, a large-capacity mobile can be achieved with a simple transmission system. A front hall can be realized.
［第２の実施形態］
　本発明の第２の実施形態における光伝送装置の構成を、図６に示す。本構成は、図１とほぼ同一であるため、図１と同じ構成要素には同じ符号を付して重複する説明を省略する。また、本実施形態における上り信号、下り信号の周波数配置は、図２と同じである。本構成では、BBU１およびRRH３における局発光は、それぞれレーザ光源１１および３１の出力の周波数がf2 [Hz]となるように周波数シフトした後、ヘテロダイン検波回路２４に供給される。即ち、BBU１では、信号光および局発光の周波数がそれぞれf4 [Hz]、f2 [Hz]、RRH３ではそれぞれf1 [Hz]、f2 [Hz]となり、IF（Intermediate Frequency）信号の周波数がΔf [Hz]となるヘテロダイン検波を行っている。 [Second Embodiment]
FIG. 6 shows the configuration of the optical transmission apparatus according to the second embodiment of the present invention. Since this configuration is almost the same as that in FIG. 1, the same components as those in FIG. Further, the frequency arrangement of the uplink signal and the downlink signal in the present embodiment is the same as that in FIG. In this configuration, the local light emitted from the BBU 1 and RRH 3 is frequency-shifted so that the output frequencies of the laser light sources 11 and 31 become f 2 [Hz], respectively, and then supplied to the heterodyne detection circuit 24. That is, the frequency of the signal light and the local light is f 4 [Hz] and f 2 [Hz] in BBU1 and f 1 [Hz] and f 2 [Hz] in RRH 3, respectively, and the frequency of the IF (Intermediate Frequency) signal Is performing heterodyne detection with Δf [Hz].
　本構成では、コヒーレント検波回路の構成が簡便化できるため、部品点数が削減され、コストが低減できる。その一方で、ヘテロダイン検波方式は、ホモダイン検波に比べて受信感度が3 dB劣化するため、ロスバジェットの確保に注意を要する。 In this configuration, since the configuration of the coherent detection circuit can be simplified, the number of parts can be reduced and the cost can be reduced. On the other hand, in the heterodyne detection method, the reception sensitivity is deteriorated by 3 dB compared to the homodyne detection, so care must be taken to ensure a loss budget.
［第３の実施形態］
　本発明の第３の実施形態における光伝送装置の構成を、図７に示す。本構成は、光位相同期の手法として、注入同期ではなくOVCO（Optical Voltage Controlled Oscillator）方式に基づいた光位相同期ループ（OPLL: Optical Phase-Locked Loop）を用いる。また、本実施形態における上り信号、下り信号の周波数配置は、図２と同じである。BBU１の構成は、第１の実施形態と同じであるため、説明を省略する。RRH３では、狭帯域電気フィルタ４８、ミキサ４９、フィードバック回路５０、RF帯電圧制御型発振器（RF-VCO: Voltage Controlled Oscillator）５１、光変調器３３で構成されるOPLL回路を備える。 [Third Embodiment]
FIG. 7 shows the configuration of the optical transmission apparatus according to the third embodiment of the present invention. In this configuration, an optical phase-locked loop (OPLL) based on an OVCO (Optical Voltage Controlled Oscillator) system is used as an optical phase-locking technique instead of injection locking. Further, the frequency arrangement of the uplink signal and the downlink signal in the present embodiment is the same as that in FIG. Since the configuration of the BBU 1 is the same as that of the first embodiment, description thereof is omitted. The RRH 3 includes an OPLL circuit including a narrow band electric filter 48, a mixer 49, a feedback circuit 50, an RF band voltage controlled oscillator (RF-VCO) 51, and an optical modulator 33.
　下り信号をRRH３でホモダイン検波するには、まず平衡光検出器３７より出力されるレーザ光源３１とパイロットトーン１とのヘテロダインビート信号（IF信号）を、狭帯域電気フィルタ４８を用いて抽出する。IF信号の位相は、ミキサ（DBM: Double Balanced Mixer）４９において、下り信号の２本のパイロットトーンから生成されるクロック信号（fclock [Hz]）の位相と比較され、その差が誤差電圧信号として検出される。この誤差信号を、フィードバック回路（ループフィルタ）５０を介してRF-VCO５１に帰還することにより、IF信号は常にクロック信号と同期した高安定な信号となる。最後に、レーザ光源３１の出力光を、RF-VCO５１からの出力信号（fclock[Hz]）で駆動される光変調器３３によって光強度または光位相変調し、周波数f1 [Hz]のサイドバンド１本を光フィルタ３５で抽出し、これを局発光として用いる。また、光フィルタ３４を介して周波数f4 [Hz]のサイドバンドを１本抽出し、これをRRH３からBBU１への上りの信号光として用いる。ここで、光変調器３３としては、LN強度変調器、LN位相変調器、SSB変調器などを用いてよい。 In order to perform homodyne detection of the downstream signal with RRH 3, first, a heterodyne beat signal (IF signal) between the laser light source 31 and the pilot tone 1 output from the balanced photodetector 37 is extracted using the narrowband electric filter 48. The phase of the IF signal is compared with the phase of the clock signal (f clock [Hz]) generated from the two pilot tones of the downstream signal in a mixer (DBM: Double Balanced Mixer) 49, and the difference is an error voltage signal. Detected as The error signal is fed back to the RF-VCO 51 via the feedback circuit (loop filter) 50, so that the IF signal is always a highly stable signal synchronized with the clock signal. Finally, the output light of the laser light source 31 is subjected to light intensity or optical phase modulation by the optical modulator 33 driven by the output signal (f clock [Hz]) from the RF-VCO 51, and the side of the frequency f 1 [Hz]. One band is extracted by the optical filter 35 and used as local light. Further, one side band of frequency f 4 [Hz] is extracted through the optical filter 34 and used as upstream signal light from RRH 3 to BBU 1. Here, as the optical modulator 33, an LN intensity modulator, an LN phase modulator, an SSB modulator, or the like may be used.
　本実施形態では、２つのレーザ光源１１、３１は、RRH３におけるOPLL回路によって互いに位相同期されているため、BBU１においては、光位相同期回路は不要である。しかしながら、９０度光ハイブリッド回路１６へ入射される信号光と局発光とがそれぞれ異なる光路を通ることに起因する、ホモダイン検波信号の低速な位相揺らぎは、DSP１９にて補正を行う。 In this embodiment, since the two laser light sources 11 and 31 are phase-synchronized with each other by the OPLL circuit in RRH 3, no optical phase synchronization circuit is required in BBU1. However, the DSP 19 corrects the slow phase fluctuation of the homodyne detection signal caused by the signal light incident on the 90-degree optical hybrid circuit 16 and the local light passing through different optical paths.
　本実施形態において、コヒーレント検波としてヘテロダイン方式を用いる場合の構成を、図８に示す。尚、本変形例では、図６と同じ構成要素には同じ符号を付して重複する説明を省略する。本構成では、BBU１における光フィルタ１５の中心周波数とRRH３における光フィルタ３５の中心周波数とをf2 [Hz]に設定し、光変調器１３および３３を用いて、レーザ光源１１、３１の出力の周波数がf2 [Hz]となるように周波数シフトする。これを、局発光としてヘテロダイン検波回路２４に供給する。 FIG. 8 shows a configuration when a heterodyne method is used as coherent detection in the present embodiment. In this modification, the same components as those in FIG. In this configuration, the center frequency of the optical filter 15 in the BBU 1 and the center frequency of the optical filter 35 in the RRH 3 are set to f 2 [Hz], and the optical modulators 13 and 33 are used to output the outputs of the laser light sources 11 and 31. The frequency is shifted so that the frequency becomes f 2 [Hz]. This is supplied to the heterodyne detection circuit 24 as local light.
［第４の実施形態］
　本発明の第４の実施形態における光伝送装置の構成を、図９に示す。本構成は、第３の実施形態（図７）において、OPLLの機能の一部をデジタル信号処理で実現したものである。具体的には、RRH３においては光検出器４４、分周器４５、狭帯域電気フィルタ４８、ミキサ４９の４つのアナログ回路で実現していた位相比較をDSP３９上で行い、得られた誤差信号（デジタル信号）をＤ／Ａ変換器４０によってＤ／Ａ変換し、フィードバック回路５０を介してRF-VCO５１に帰還させている。これにより、図７と比較して、構成を簡素化することができる。なお、図９はホモダイン方式の例を示しているが、本実施形態においても、第３の実施形態の変形例（図８参照）と同様に、図１０のようにヘテロダイン方式で構成することも可能である。 [Fourth Embodiment]
FIG. 9 shows the configuration of the optical transmission apparatus according to the fourth embodiment of the present invention. In this configuration, part of the OPLL function is realized by digital signal processing in the third embodiment (FIG. 7). Specifically, in RRH3, the phase comparison realized by the four analog circuits of the photodetector 44, the frequency divider 45, the narrow band electric filter 48, and the mixer 49 is performed on the DSP 39, and the obtained error signal ( The digital signal is D / A converted by the D / A converter 40 and fed back to the RF-VCO 51 via the feedback circuit 50. Thereby, a structure can be simplified compared with FIG. Although FIG. 9 shows an example of the homodyne system, the present embodiment can also be configured by the heterodyne system as shown in FIG. 10 as in the modification of the third embodiment (see FIG. 8). Is possible.
［第５の実施形態］
　本発明の第５の実施形態における光伝送装置の構成を、図１１に示す。本実施形態では、１台のBBU１にＮ台のRRH３が接続されている場合を想定している。そのため、BBU１は、各RRH３へ送信するためのＮ台の送信器と、各RRH３からの上り信号を受信するためのＮ台の受信器を備えている。同図では各送受信器の構成を簡素化して示しているが、具体的には第１～第４のいずれかの実施形態で示したものを用いればよい。 [Fifth Embodiment]
FIG. 11 shows the configuration of the optical transmission apparatus according to the fifth embodiment of the present invention. In the present embodiment, it is assumed that N RRHs 3 are connected to one BBU 1. For this reason, the BBU 1 includes N transmitters for transmitting to each RRH 3 and N receivers for receiving an uplink signal from each RRH 3. In the figure, the configuration of each transceiver is shown in a simplified manner, but specifically, the one shown in any of the first to fourth embodiments may be used.
　本構成では、各RRH３に異なる周波数を割り当て、光ファイバ伝送路２中を波長多重（WDM: Wavelength Division Multiplexing）伝送させている。その周波数配置の一例を、図１２に示す。ここでは2Δf [Hz]間隔のWDMグリットを定義し、偶数番目の周波数チャネル（f2, f4, …f(2N), Nは自然数）に上り信号を割り当てている。例えば、Δf [Hz]は、データ信号の変調周波数と等しくてもよい。一方、下りのデータ信号は、奇数番目の周波数チャネル（f1, f3, …f(2N-1), Nは自然数）に割り当てて伝送する。また、WDMグリットとは絶対周波数がΔf [Hz]だけ異なる周波数間隔2Δf [Hz]のWDMグリットを定義し（f1’, f2’…fN’, Nは自然数とする）、ここに下り信号のパイロットトーンを割り当てている。 In this configuration, a different frequency is assigned to each RRH 3, and wavelength division multiplexing (WDM) transmission is performed in the optical fiber transmission line 2. An example of the frequency arrangement is shown in FIG. Here, WDM grids with intervals of 2Δf [Hz] are defined, and uplink signals are assigned to even-numbered frequency channels (f 2 , f 4 ,... F (2N) , N are natural numbers). For example, Δf [Hz] may be equal to the modulation frequency of the data signal. On the other hand, the downlink data signal is assigned to an odd-numbered frequency channel (f 1 , f 3 ,... F (2N−1) , N is a natural number) and transmitted. Also, the WDM grid is defined as a WDM grid with a frequency interval of 2Δf [Hz] that differs from the WDM grid by Δf [Hz] (f 1 ′, f 2 ′… f N ′, N are natural numbers). A pilot tone for the signal is assigned.
　下り信号は、Ｎ台の送信器からWDM合波器４で合波され、光ファイバ伝送路２をWDM伝送し、パワースプリッタ６でＮ台のRRH３に分配される。各RRH３では、中心周波数がf1, f3, …f(2N-1)に設定された光フィルタ７によって所望のWDM信号を選択し、コヒーレント検波回路８ａで受信する。各RRH３は、周波数f2, f4, …f(2N)のレーザ光源３１を備えており、コヒーレント検波回路８ａは、第１～第４の実施形態で示したいずれかの方法で構成する。コヒーレント検波回路８ａは、例えば、図１に示す第１の実施形態の、光変調器３３、光フィルタ３４及び３５、９０度光ハイブリッド回路３６、平衡光検出器３７、Ａ／Ｄ変換器３８、デジタル信号処理回路（DSP: Digital Signal Processor）３９、Ｄ／Ａ変換器４０、光増幅器４１、光フィルタ４２、光サーキュレータ４３、光検出器４４、分周器４５で構成することができる。光フィルタ７を省略し、コヒーレント検波回路８ａにおいてWDM信号の選択と復調とを同時に行うことも可能である。 Downstream signals are combined from N transmitters by a WDM multiplexer 4, WDM transmitted through an optical fiber transmission line 2, and distributed to N RRHs 3 by a power splitter 6. In each RRH 3 , a desired WDM signal is selected by the optical filter 7 whose center frequency is set to f 1 , f 3 ,... F (2N−1) and received by the coherent detection circuit 8a. Each RRH 3 includes a laser light source 31 of frequencies f 2 , f 4 ,... F (2N) , and the coherent detection circuit 8a is configured by any of the methods shown in the first to fourth embodiments. The coherent detection circuit 8a includes, for example, the optical modulator 33, the optical filters 34 and 35, the 90-degree optical hybrid circuit 36, the balanced photodetector 37, the A / D converter 38, and the like in the first embodiment shown in FIG. A digital signal processor (DSP) 39, a D / A converter 40, an optical amplifier 41, an optical filter 42, an optical circulator 43, a photodetector 44, and a frequency divider 45 can be used. It is possible to omit the optical filter 7 and simultaneously select and demodulate the WDM signal in the coherent detection circuit 8a.
　一方、上り信号は、Ｎ台のRRH３からパワースプリッタ６で合波され、光ファイバ伝送路２をWDM伝送し、BBU１中のWDM分波器５でＮ個の異なる周波数に分離される。各受信器では、レーザ光源１１を局発光源として用い、第１～第４の実施形態で示したいずれかの方法によりコヒーレント検波を行う。この場合のコヒーレント検波回路８ｂは、例えば、図１に示す第１の実施形態の、光変調器１３、RF発振器１４、光フィルタ１５、９０度光ハイブリッド回路１６、平衡光検出器１７、Ａ／Ｄ変換器１８、デジタル信号処理回路（DSP: Digital Signal Processor）１９、Ｄ／Ａ変換器２０で構成することができる。 On the other hand, the upstream signal is multiplexed from N RRHs 3 by a power splitter 6, WDM transmitted through an optical fiber transmission line 2, and separated into N different frequencies by a WDM demultiplexer 5 in the BBU 1. Each receiver uses the laser light source 11 as a local light source and performs coherent detection by any of the methods shown in the first to fourth embodiments. In this case, the coherent detection circuit 8b includes, for example, the optical modulator 13, the RF oscillator 14, the optical filter 15, the 90-degree optical hybrid circuit 16, the balanced photodetector 17, A / A of the first embodiment shown in FIG. A D converter 18, a digital signal processor (DSP) 19, and a D / A converter 20 can be used.
［第６の実施形態］
　本発明の第６の実施形態における光伝送装置の構成を、図１３に示す。本構成は、第５の実施形態（図１１）において、下り信号の各RRH３への分配をパワースプリッタ６の代わりにWDM分波器９で行っており、各RRH３へは波長ごとに分離された信号が送られる。その結果、各RRH３は、光フィルタ７等の波長選択素子が不要となる。 [Sixth Embodiment]
FIG. 13 shows the configuration of the optical transmission apparatus according to the sixth embodiment of the present invention. In this configuration, in the fifth embodiment (FIG. 11), the downlink signal is distributed to each RRH 3 by the WDM demultiplexer 9 instead of the power splitter 6, and each RRH 3 is separated for each wavelength. A signal is sent. As a result, each RRH 3 does not require a wavelength selection element such as the optical filter 7.
　尚、以上の各実施形態においては、パイロットトーンを２本用いる場合を例示したが、これに代えて、信号光の送信側でデータ信号に３本以上のパイロットトーンを重畳し、信号光の受信側でこれらのパイロットトーンのうち、いずれか１本を介して信号光と受信用局発光源との位相を同期させると共に、これらのパイロットトーンのうち、いずれか２本を光検出することで当該２本のパイロットトーンの差周波電気信号を抽出し、当該差周波電気信号を、受信用局発光源の出力光を変調する光変調器を駆動する変調信号や光位相同期ループの基準信号として用いてもよい。この場合、これらのパイロットトーンと上り信号および下り信号とは、互いに異なる周波数で伝送される。あるいは、信号光の送信側でデータ信号に単一のパイロットトーンを重畳し、信号光の受信側でこのパイロットトーンを介して信号光と受信用局発光源との位相を同期させると共に、受信用局発光源の出力光を変調する光変調器を駆動する変調信号や光位相同期ループの基準信号は、別途用意した発振器により発生させてもよい。この場合、このパイロットトーンと上り信号および下り信号とは、互いに異なる周波数で伝送される。 In each of the above embodiments, the case where two pilot tones are used has been exemplified. Instead, three or more pilot tones are superimposed on the data signal on the signal light transmission side to receive the signal light. The phase of the signal light and the receiving local light source is synchronized via any one of these pilot tones on the side, and any two of these pilot tones are detected by light detection. The difference frequency electrical signal of two pilot tones is extracted, and the difference frequency electrical signal is used as a modulation signal for driving an optical modulator that modulates the output light of the local light source for reception or a reference signal for an optical phase locked loop. May be. In this case, these pilot tones, upstream signals and downstream signals are transmitted at different frequencies. Alternatively, a single pilot tone is superimposed on the data signal on the signal light transmission side, and the phase of the signal light and the local light source for reception is synchronized via the pilot tone on the signal light reception side, and for reception. The modulation signal for driving the optical modulator that modulates the output light from the local light source and the reference signal for the optical phase locked loop may be generated by a separately prepared oscillator. In this case, the pilot tone, the upstream signal, and the downstream signal are transmitted at different frequencies.
　以上詳細に説明したように、本発明は、モバイルフロントホールにおいて、BBUとRRHとの間で無線信号を、光ファイバを介して大きなロスバジェットで長距離双方向伝送するための後方レイリー散乱非混合型の光伝送方法および光伝送装置を提供することができる。本発明は、その光伝送方式としてデジタルコヒーレント伝送技術を用いることを特徴とし、そのコヒーレンスの点で無線信号と高い親和性を有することから、効率的且つ経済性の高い光・無線アクセスネットワークを実現できる。また、本発明は、光ファイバを利用したFTTHなどの一般的な光アクセスネットワークシステムでも、利用することができる。 As described in detail above, the present invention is a non-mixed backward Rayleigh scattering method for long-distance bidirectional transmission of a radio signal between a BBU and an RRH with a large loss budget via an optical fiber in a mobile fronthaul. Type optical transmission method and optical transmission apparatus can be provided. The present invention is characterized by using digital coherent transmission technology as its optical transmission method, and has high affinity with wireless signals in terms of its coherence, thus realizing an efficient and economical optical / wireless access network. it can. The present invention can also be used in a general optical access network system such as FTTH using an optical fiber.
　１　基地局ベースバンド部（BBU）
　　１１　レーザ光源
　　１２　ＩＱ変調器
　　１３　光変調器
　　１４　（RF）発振器
　　１５　光フィルタ
　　１６　９０度光ハイブリッド回路
　　１７　平衡光検出器
　　１８　Ａ／Ｄ変換器
　　１９　デジタル信号処理回路（DSP）
　　２０　Ｄ／Ａ変換器
　　２１　光サーキュレータ
　　２２　任意波形生成装置
　　２３　偏波多重回路
　　２４　ヘテロダイン検波回路
　２　光ファイバ伝送路
　３　アンテナ無線部（RRH）
　　３１　レーザ光源
　　３２　ＩＱ変調器
　　３３　光変調器
　　３４　光フィルタ
　　３５　光フィルタ
　　３６　９０度光ハイブリッド回路
　　３７　平衡光検出器
　　３８　Ａ／Ｄ変換器
　　３９　デジタル信号処理回路（DSP）
　　４０　Ｄ／Ａ変換器
　　４１　光増幅器
　　４２　光フィルタ
　　４３　光サーキュレータ
　　４４　光検出器
　　４５　分周器
　　４６　光サーキュレータ
　　４７　RF発振器
　　４８　狭帯域電気フィルタ
　　４９　ミキサ
　　５０　フィードバック回路
　　５１　RF帯電圧制御型発振器（RF-VCO）
　４　WDM合波器
　５　WDM分波器
　６　パワースプリッタ
　７　光フィルタ
　８ａ，８ｂ　コヒーレント検波回路
　９　WDM分波器
  1 Base Station Baseband (BBU)
DESCRIPTION OF SYMBOLS 11 Laser light source 12 IQ modulator 13 Optical modulator 14 (RF) oscillator 15 Optical filter 16 90 degree optical hybrid circuit 17 Balanced photodetector 18 A / D converter 19 Digital signal processing circuit (DSP)
20 D / A converter 21 Optical circulator 22 Arbitrary waveform generator 23 Polarization multiplexing circuit 24 Heterodyne detection circuit 2 Optical fiber transmission line 3 Antenna radio section (RRH)
Reference Signs List 31 Laser light source 32 IQ modulator 33 Optical modulator 34 Optical filter 35 Optical filter 36 90 degree optical hybrid circuit 37 Balanced optical detector 38 A / D converter 39 Digital signal processing circuit (DSP)
40 D / A Converter 41 Optical Amplifier 42 Optical Filter 43 Optical Circulator 44 Optical Detector 45 Frequency Divider 46 Optical Circulator 47 RF Oscillator 48 Narrow Band Electrical Filter 49 Mixer 50 Feedback Circuit 51 RF Band Voltage Controlled Oscillator (RF-VCO) )
4 WDM multiplexer 5 WDM demultiplexer 6 Power splitter 7 Optical filter 8a, 8b Coherent detection circuit 9 WDM demultiplexer

Claims (8)

  1. 　第１の光送受信部と第２の光送受信部との間で上り信号および下り信号を、光ファイバを介して一芯双方向伝送させるための光伝送方法において、
    　前記第１の光送受信部に配置されたレーザ光源を、前記第２の光送受信部への下り信号の伝送用光源として用いると共に、前記第２の光送受信部からの上り信号の受信用局発光源として用い、
    　前記第２の光送受信部に配置されたレーザ光源を、前記第１の光送受信部への上り信号の伝送用光源として用いると共に、前記第１の光送受信部からの下り信号の受信用局発光源として用い、
    　前記第１の光送受信部および／または前記第２の光送受信部において、注入同期法又は光位相同期ループを用いて、前記上り信号と前記上り信号の受信用局発光源との光位相同期、および／または、前記下り信号と前記下り信号の受信用局発光源との光位相同期を行い、
    　前記上り信号と前記下り信号は互いに異なる周波数で伝送することを
    　特徴とする光伝送方法。     In an optical transmission method for transmitting an upstream signal and a downstream signal between a first optical transmission / reception unit and a second optical transmission / reception unit via a single fiber bidirectionally,
    The laser light source disposed in the first optical transmitter / receiver is used as a light source for transmitting the downstream signal to the second optical transmitter / receiver, and the local light for receiving the upstream signal from the second optical transmitter / receiver is used. Used as a source,
    The laser light source disposed in the second optical transmitter / receiver is used as a light source for transmitting the upstream signal to the first optical transmitter / receiver, and the local light for receiving the downstream signal from the first optical transmitter / receiver is used. Used as a source,
    In the first optical transceiver and / or the second optical transceiver, using an injection locking method or an optical phase locked loop, optical phase synchronization between the upstream signal and the local light source for receiving the upstream signal, And / or optical phase synchronization between the downstream signal and the local light source for receiving the downstream signal,
    The optical transmission method, wherein the uplink signal and the downlink signal are transmitted at different frequencies.
  2. 　信号光の送信側でデータ信号にパイロットトーンを重畳し、
    　前記信号光の受信側で前記パイロットトーンを介して前記信号光と前記受信用局発光源との位相を同期させ、
    　前記パイロットトーンと前記上り信号および前記下り信号とは、互いに異なる周波数で伝送することを
    　特徴とする請求項１記載の光伝送方法。     A pilot tone is superimposed on the data signal on the signal light transmission side,
    Synchronize the phase of the signal light and the local light source for reception via the pilot tone on the reception side of the signal light,
    The optical transmission method according to claim 1, wherein the pilot tone, the uplink signal, and the downlink signal are transmitted at different frequencies.
  3. 　信号光の送信側でデータ信号に複数本のパイロットトーンを重畳し、
    　前記信号光の受信側で前記複数のパイロットトーンのうち、いずれか１本を介して前記信号光と前記受信用局発光源との位相を同期させると共に、
    　前記複数のパイロットトーンのうち、いずれか２本を光検出することで前記２本のパイロットトーンの差周波電気信号を抽出し、前記差周波電気信号を、前記受信用局発光源に接続され、前記受信用局発光源の出力光を変調する光変調器を駆動する変調信号または光位相同期ループの基準信号として用い、
    　前記複数のパイロットトーンと前記上り信号および前記下り信号とは、互いに異なる周波数で伝送することを
    　特徴とする請求項１記載の光伝送方法。     Multiple pilot tones are superimposed on the data signal on the signal light transmission side,
    While synchronizing the phase of the signal light and the local light source for reception via any one of the plurality of pilot tones on the reception side of the signal light,
    Extracting the difference frequency electrical signal of the two pilot tones by optically detecting any two of the plurality of pilot tones, and connecting the difference frequency electrical signal to the local light source for reception; Used as a modulation signal for driving an optical modulator that modulates the output light of the local light source for reception or a reference signal for an optical phase locked loop,
    The optical transmission method according to claim 1, wherein the plurality of pilot tones, the uplink signal, and the downlink signal are transmitted at different frequencies.
  4. 　前記第１の光送受信部は、基地局ベースバンド部または光回線終端装置から成り、
    　前記第２の光送受信部は、アンテナ無線部または光回線ネットワーク装置から成ることを
    　特徴とする請求項１乃至３のいずれか１項に記載の光伝送方法。     The first optical transmission / reception unit includes a base station baseband unit or an optical line termination device,
    4. The optical transmission method according to claim 1, wherein the second optical transmission / reception unit includes an antenna radio unit or an optical line network device. 5.
  5. 　第１の光送受信部と第２の光送受信部との間で上り信号および下り信号を、光ファイバを介して一芯双方向伝送させるための光伝送装置において、
    　前記第１の光送受信部に配置されたレーザ光源が、前記第２の光送受信部への下り信号の伝送用光源であると共に、前記第２の光送受信部からの上り信号の受信用局発光源であり、
    　前記第２の光送受信部に配置されたレーザ光源が、前記第１の光送受信部への上り信号の伝送用光源であると共に、前記第１の光送受信部からの下り信号の受信用局発光源であり、
    　前記第１の光送受信部もしくは前記第２の光送受信部に配置された前記レーザ光源は、外部から光を注入できる構造を有し、注入同期によって前記上り信号と前記上り信号の受信用局発光源との間の位相もしくは前記下り信号と前記下り信号の受信用局発光源との間の位相を同期するよう構成されている、または、前記第１の光送受信部もしくは前記第２の光送受信部に、前記上り信号と前記上り信号の受信用局発光源との間の位相もしくは前記下り信号と前記下り信号の受信用局発光源との間の位相を同期させるための電圧制御型発振器及び光位相同期ループが配置されていることを
    　特徴とする光伝送装置。     In the optical transmission device for transmitting the upstream signal and the downstream signal between the first optical transmission / reception unit and the second optical transmission / reception unit via a single fiber bidirectionally,
    The laser light source disposed in the first optical transmission / reception unit is a light source for transmitting a downstream signal to the second optical transmission / reception unit, and local light for receiving an upstream signal from the second optical transmission / reception unit The source
    The laser light source disposed in the second optical transmission / reception unit is a light source for transmission of the upstream signal to the first optical transmission / reception unit, and the local light for receiving the downstream signal from the first optical transmission / reception unit The source
    The laser light source disposed in the first optical transmitter / receiver or the second optical transmitter / receiver has a structure capable of injecting light from the outside, and receives local light for receiving the upstream signal and the upstream signal by injection locking. Configured to synchronize the phase with the source or the phase between the downstream signal and the local light source for receiving the downstream signal, or the first optical transceiver or the second optical transceiver A voltage controlled oscillator for synchronizing a phase between the upstream signal and the local light source for receiving the upstream signal or a phase between the downstream signal and the local light source for receiving the downstream signal; An optical transmission device comprising an optical phase-locked loop.
  6. 　前記第１の光送受信部または前記第２の光送受信部は、前記上り信号または前記下り信号と共にパイロットトーンを生成する回路と、前記パイロットトーンを介して、前記上り信号と前記上り信号の受信用局発光源とを光位相同期する回路または前記下り信号と前記下り信号の受信用局発光源とを光位相同期する回路とを有し、
    　前記上り信号と前記下り信号と前記パイロットトーンとを、互いに異なる周波数で生成するよう構成されていることを
    　特徴とする請求項５記載の光伝送装置。     The first optical transmission / reception unit or the second optical transmission / reception unit is configured to generate a pilot tone together with the upstream signal or the downstream signal, and to receive the upstream signal and the upstream signal via the pilot tone. A circuit that optically synchronizes the local light source or a circuit that optically synchronizes the downstream signal and the local light source for receiving the downstream signal,
    The optical transmission device according to claim 5, wherein the uplink signal, the downlink signal, and the pilot tone are generated at different frequencies.
  7. 　前記第１の光送受信部または前記第２の光送受信部は、前記上り信号または前記下り信号と共に複数のパイロットトーンを生成する回路と、前記複数のパイロットトーンのうちいずれか１本を介して、前記上り信号と前記上り信号の受信用局発光源とを光位相同期する回路または前記下り信号と前記下り信号の受信用局発光源とを光位相同期する回路と、前記複数のパイロットトーンのうちいずれか２本を光検出して差周波電気信号を抽出する回路とを有し、
    　前記上り信号と前記下り信号と前記複数のパイロットトーンとを、互いに異なる周波数で生成するよう構成されていることを
    　特徴とする請求項５記載の光伝送装置。     The first optical transmission / reception unit or the second optical transmission / reception unit includes a circuit that generates a plurality of pilot tones together with the uplink signal or the downlink signal, and any one of the plurality of pilot tones, A circuit that optically synchronizes the upstream signal and the local light source for receiving the upstream signal, or a circuit that optically synchronizes the downstream signal and the local light source for receiving the downstream signal, and among the plurality of pilot tones A circuit for detecting any two of them and extracting a difference frequency electrical signal;
    The optical transmission device according to claim 5, wherein the upstream signal, the downstream signal, and the plurality of pilot tones are generated at different frequencies.
  8. 　前記第１の光送受信部は、基地局ベースバンド部または光回線終端装置から成り、
    　前記第２の光送受信部は、アンテナ無線部または光回線ネットワーク装置から成ることを
    　特徴とする請求項５乃至７のいずれか１項に記載の光伝送装置。
          The first optical transmission / reception unit includes a base station baseband unit or an optical line termination device,
    The optical transmission device according to any one of claims 5 to 7, wherein the second optical transmission / reception unit includes an antenna radio unit or an optical line network device.
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