WO2018198873A1

WO2018198873A1 - Optical transmission method and optical transmission device

Info

Publication number: WO2018198873A1
Application number: PCT/JP2018/015797
Authority: WO
Inventors: 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
Priority date: 2017-04-28
Filing date: 2018-04-17
Publication date: 2018-11-01
Also published as: JP2020109887A

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

Optical transmission method and optical transmission device

The present invention relates to an optical transmission method and an optical transmission apparatus.

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).

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.

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.

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).

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.

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.

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. 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.

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.

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.

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.

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.

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.

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.

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.

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.

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).

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.

It is a block block diagram which shows the optical transmission apparatus (homodyne system) in the 1st Embodiment of this invention. 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. 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. 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. 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. It is a block block diagram which shows the optical transmission apparatus (heterodyne system) in the 2nd Embodiment of this invention. It is a block block diagram which shows the optical transmission apparatus (homodyne system) in the 3rd Embodiment of this invention. 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. It is a block block diagram which shows the optical transmission apparatus (homodyne system) in the 4th Embodiment of this invention. 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. It is a block block diagram which shows the optical transmission apparatus in the 5th Embodiment of this invention. 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.

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.

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.

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.

[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.

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).

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]).

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.

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.

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.

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.

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]).

[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.

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.

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.

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.

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.

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.

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.

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.

[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].

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.

[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.

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.

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.

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.

[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.

[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.

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.

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.

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.

[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.

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.

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 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

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.
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.
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.
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.
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.
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.
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.
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.