WO2018198873A1 - Procédé de transmission optique et dispositif de transmission optique - Google Patents

Procédé de transmission optique et dispositif de transmission optique 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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Prior art keywords
signal
optical
light source
optical transmission
reception unit
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PCT/JP2018/015797
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English (en)
Japanese (ja)
Inventor
中沢　正隆
勝美 岩月
俊彦 廣岡
吉田　真人
恵介 葛西
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国立大学法人東北大学
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Publication of WO2018198873A1 publication Critical patent/WO2018198873A1/fr

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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

Abstract

La présente invention aborde le problème de la réalisation d'un procédé de transmission optique de type sans mélange par rétrodiffusion de Rayleigh et un dispositif de transmission optique qui réalisent une économie élevée avec une configuration simple et ont un budget de perte important. La solution selon l'invention porte sur un signal sans fil qui est transmis entre une unité en bande de base de station de base (1) et une unité sans fil d'antenne (3) par le biais d'une fibre optique (2). Une source de lumière laser (11) disposée dans l'unité de bande de base de station de base (1) est utilisée en tant que source de lumière pour transmettre un signal de liaison descendante à l'unité sans fil d'antenne (3) et également utilisée en tant que source d'émission de lumière de station pour recevoir un signal de liaison montante provenant de l'unité sans fil d'antenne (3). Une source de lumière laser (31) disposée dans l'unité sans fil d'antenne (3) est utilisée en tant que source de lumière pour transmettre un signal de liaison montante à l'unité de bande de base de station de base (1) et également utilisée en tant que source d'émission de lumière de station pour recevoir un signal de liaison descendante en provenance de l'unité de bande de base de station de base (1). Le signal de liaison montante et le signal de liaison descendante sont transmis avec des fréquences mutuellement différentes. Une synchronisation de phase optique entre la lumière de signal et la source d'émission de lumière de station pour la réception est effectuée dans l'unité de bande de base de station de base (1) et/ou l'unité sans fil d'antenne (3) par un procédé de synchronisation d'injection ou au moyen d'une boucle de synchronisation de phase optique.
PCT/JP2018/015797 2017-04-28 2018-04-17 Procédé de transmission optique et dispositif de transmission optique WO2018198873A1 (fr)

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