Transmission device, communication system, and transmission method

Abstract

Provided is a transmission device comprising: a polarization time block coding unit that performs space-time coding with respect to a transmission signal, thereby generating a polarization phase modulation signal; a quadrature phase signal generation unit that generates, for each polarized wave, a quadrature phase signal of a dual polarization phase modulation signal generated by the polarization time block coding unit; and a transmission unit that alternately transmits the dual polarization phase modulation signal and the quadrature phase signal of each polarized wave on the time axis.　

Description

Transmitting device, communication system, and transmitting method

[0001] The present invention relates to a transmitting device, a communication system, and a transmitting method.

[0002] In the intensity modulation-direct detection method used in optical subscriber networks, the wider the bandwidth of the transmitted signal light, the lower the receiving sensitivity of the signal light can be. Furthermore, the waveform of the signal light can be degraded due to chromatic dispersion of the signal light. To address these issues, the application of digital coherent receiving technology to receiving devices (user terminals) is being considered. A receiving device that applies digital coherent receiving technology can compensate for chromatic dispersion through digital signal processing and improve receiving sensitivity.

[0003] Fig. 20 is a diagram showing an example of the configuration of a communication system in a core network. The transmission device shown in Fig. 20 includes a dual-polarization IQ (In-Phase, Quadrature-Phase) modulator (DP (Dual-Polarization)-IQ modulator) as a transmitter.

[0004] As a result, the DP-IQ modulator generates a polarization phase modulated optical signal that carries information in the polarization and phase. In this way, the transmitter uses the polarization and phase of the light to superimpose information onto the signal light. In Figure 20, "PD" stands for photodetector, and "TIA" stands for transimpedance amplifier.

[0005] The receiving device includes a polarization / phase diversity receiver. The receiving device performs photoelectric conversion on the signal light separated into orthogonal polarizations and orthogonal phases and the local light output from the local light source using balanced receivers (e.g., multiple PDs in FIG. 20 ). The receiving device corrects chromatic dispersion, polarization rotation, and phase rotation that occur in the signal light transmitted through the transmission path by digital signal processing. Although the optical front-end configuration in the receiving device becomes complex, wideband and highly sensitive reception and long-distance transmission of the signal light are possible.

[0006] To make the access network more economical, a receiving device having a receiver with a simpler configuration than the polarization and phase diversity receiver shown in FIG. 20 is being considered.

[0007] Fig. 21 is a diagram showing an example of the configuration of a communication system in an access network (see Non-Patent Document 1). The transmitting device shown in Fig. 21 includes a DP-IQ modulator as a transmitter. Furthermore, the digital signal processing unit of the transmitting device shown in Fig. 21 performs Alamouti coding (polarization time block coding) as space-time coding on the transmission signal. Thus, in the configuration shown in Non-Patent Document 1, the digital signal processing unit of the transmitting device is provided with a function for performing space-time coding.

[0008] In addition, in the configuration disclosed in Non-Patent Document 1, a function for decoding space-time coding is provided in the digital signal processing unit of the receiving device, which enables the receiving device to receive signal light without adopting a polarization diversity configuration and without relying on polarization.

[0009] The receiving device shown in Figure 20 separates the I-axis phase from the Q-axis phase using the homodyne method. In the homodyne method, the receiving device must match the wavelength of the signal light with the wavelength of the local light. In contrast, the receiving device shown in Figure 21 separates the I-axis phase from the Q-axis phase using the heterodyne method. In the heterodyne method, a difference is created between the wavelength of the signal light and the wavelength of the local light, thereby down-converting the band of the signal light to an intermediate frequency band.

[0010] The heterodyne receiver in the receiving device includes an optical splitter (e.g., a 3 dB coupler), a local oscillator, and a single balanced receiver. The digital signal processing unit of the receiving device converts the analog-to-digital (AD) converted signal superimposed on the intermediate frequency band into a baseband signal. The digital signal processing unit of the receiving device performs Alamouti decoding on the baseband signal.

[0011] The configuration shown in Fig. 21 makes it possible to reduce the number of PDs and the number of AD converters to one-fourth compared to when the receiving device is equipped with the polarization / phase diversity receiver shown in Fig. 20. However, if a Nyquist filter is not used, an electrical bandwidth at least twice the signal bandwidth "B" is required.

[0012] Fig. 22 is a diagram showing an example of the configuration of a communication system that performs phase diversity reception (see Non-Patent Document 2). The transmitting device shown in Fig. 22 includes a single-polarization IQ modulator as a transmitter. The single-polarization IQ modulator transmits a phase-modulated signal (e.g., a signal subjected to m-ary Quadrature Amplitude Modulation (mQAM)) to the receiving device. The receiving device includes an optical branching device (e.g., a 3 dB coupler), a frequency-modulated light generator, and a balanced light source. The frequency-modulated light generator includes an external modulator on the output side of the local light source for phase diversity reception.

[0013] The frequency-modulated optical generator rotates the phase of the local oscillator light by 90 degrees at twice the symbol rate. Two consecutive samples converted from analog to digital at twice the symbol rate are considered as the I-axis and Q-axis signals in the phase diversity receiver. The I-axis and Q-axis signals are mapped to the IQ plane (complex plane). Note that the configuration shown in Figure 22 cannot achieve polarization diversity, so polarization diversity is performed in the optical front end. Therefore, the reduction in the number of balanced receivers and AD converters is only half that of a polarization / phase diversity receiver. Furthermore, the receiver requires an external phase modulator or a driver for directly modulating the phase of the local oscillator light source. Furthermore, the required signal bandwidth is 1.5B.

[0014] M. S. Faruk, H. Louchet, M. S. Erkilinc, et al., “DSP algorithms for recovering single-carrier Alamouti coded signals for PON applications,” Opt. Express 24, 24083-24091 (2016).Zhengxuan Li, Fan Yin, Xingang Huang, Zhuang Ma, Yingxiong Song, and Lilin Yi, “Demonstration of a 50G-PON with a 45-dB power budget using an IQ-interleaved coherent detection scheme”, Opt. Express 29, 32523-32534 (2021)P. Torres-Ferrera, G. Rizzelli, H. Wang, V. Ferrero and R. Gaudino, “Experimental Demonstration of 100 Gbps / λ C-Band Direct-Detection Downstream PON Using Non-Linear and CD Compensation with 29 dB+ OPL Over 0 Km-100 km”, in Journal of Lightwave Technology, vol. 40, no. 2, pp. 547-556, 15 Jan.15, 2022, doi: 10.1109 / JLT.2021.3129446.

[0015] As described above, conventional configurations have a problem in that polarization and phase diversity reception cannot be achieved with a simple receiver configuration. Specifically, the configuration shown in FIG. 20 has a complex receiver configuration, making it difficult to apply to user terminals in optical subscriber networks, which require cost-effectiveness. The configuration shown in FIG. 21 requires a receiving electrical bandwidth twice the symbol rate to perform heterodyne reception. The configuration shown in FIG. 22 can reduce the electrical bandwidth to 1.5 times the symbol rate, but requires twice as many components for polarization diversity reception as the configuration shown in FIG. 21. In addition, an external modulator is required to modulate the frequency of the local oscillator light source on the receiving side, making the receiver configuration more complex.

[0016] In view of the above circumstances, an object of the present invention is to provide a technique that can realize polarization and phase diversity reception with a simple receiver configuration.

[0017] One aspect of the present invention is a transmitting device comprising: a polarization time block coding unit that generates dual polarization phase-modulated signals by performing space-time coding on a transmission signal; a quadrature phase signal generation unit that generates, for each polarization, a quadrature phase signal of the dual polarization phase-modulated signals generated by the polarization time block coding unit; and a transmitting unit that alternately transmits the dual polarization phase-modulated signals and the quadrature phase signals of each polarization on the time axis.

[0018] One aspect of the present invention is a communication system comprising a transmitting device and one or more receiving devices, wherein the transmitting device comprises: a polarization time block coding unit that generates a dual polarization phase-modulated signal by performing space-time coding on a transmission signal; a quadrature phase signal generation unit that generates, for each polarization, a quadrature phase signal of the dual polarization phase-modulated signal generated by the polarization time block coding unit; and a transmitting unit that alternately transmits the dual polarization phase-modulated signal and the quadrature phase signal of each polarization on the time axis; and the one or more receiving devices receive the signal transmitted from the transmitting device and comprise: a quadrature phase signal restoration unit that restores two consecutive symbols of the received signal as a real axis component signal and an imaginary axis component signal; and a receiving digital signal processing unit that restores the transmission signal based on the restored real axis component signal and the imaginary axis component signal.

[0019] One aspect of the present invention is a transmission method that generates dual polarization phase-modulated signals by performing space-time coding on a transmission signal, generates quadrature phase signals of the generated dual polarization phase-modulated signals for each polarization, and alternately transmits the dual polarization phase-modulated signals and the quadrature phase signals of each polarization on the time axis.

[0020] According to the present invention, it is possible to realize polarization and phase diversity reception with a simple receiver configuration.

[0021] 1 is a diagram illustrating an example of the configuration of a transmitting device in a conventional communication system. FIG. 2 is a diagram illustrating an example of the configuration of a receiving device in a conventional communication system. FIG. 3 is a diagram illustrating an example of the configuration of a transmitting device in a communication system of the first embodiment. FIG. 4 is a diagram illustrating the processing of a phase shift unit in the first embodiment. FIG. 5 is a diagram illustrating an example of the configuration of a receiving device in the communication system of the first embodiment. FIG. 6 is a diagram illustrating an example of the configuration of a receiving device in a modification 1 of the first embodiment. FIG. 7 is a diagram illustrating an example of the configuration of a receiving device in a modification 2 of the first embodiment. FIG. 8 is a diagram illustrating an example of the configuration of a receiving device in a modification 3 of the first embodiment. FIG. 9 is a diagram illustrating an example of the configuration of a transmitting device in a communication system of the second embodiment. FIG. 10 is a diagram illustrating the processing flow of a compensation coefficient calculation and storage unit and a transmission path characteristics compensation unit in the second embodiment. FIG. 11 is an image diagram relating to a transmitted signal, a received signal, and phase rotation caused by a frequency offset. FIG. 12 is a diagram illustrating an overview of processing in the third embodiment. FIG. 13 is a diagram illustrating an example of the configuration of a transmitting device in a communication system of the third embodiment. FIG. 14 is a diagram illustrating the processing flow of a phase rotation amount calculation and storage unit and a frequency offset compensation unit in the third embodiment. FIG. 15 is a diagram illustrating an example of the configuration of a receiving device in a modification 1 of the third embodiment. FIG. 16 is a diagram illustrating the processing at the time of initial authentication in the communication system. FIG. 17 is a diagram illustrating the processing at the time of initial authentication in the communication system. It is a diagram showing an example of the hardware configuration of a communication device in each embodiment. It is a diagram showing an example of the configuration of a communication system in a core network. It is a diagram showing an example of the configuration of a communication system in an access network. It is a diagram showing an example of the configuration of a communication system that performs phase diversity reception.

[0022] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0023] (Conventional Configuration) First, before explaining the configuration of the present invention, the configurations of a transmitting device and a receiving device in a conventional communication system that is compared with the present invention will be explained. Fig. 1 is a diagram showing an example of the configuration of a transmitting device 2 in a conventional communication system 1. Fig. 2 is a diagram showing an example of the configuration of a receiving device 4 in the conventional communication system 1. The transmitting device and receiving device in the conventional communication system shown in Figs. 1 and 2 are based on the configuration shown in Non-Patent Document 1.

[0024] The transmitter 2 includes a transmission digital signal processing unit 21, a DA converter 22, a light source 23, and a DP-IQ modulator 24. The transmission digital signal processing unit 21 performs predetermined signal processing (e.g., space-time coding) for transmitting the input transmission signal sequence. The transmission digital signal processing unit 21 includes an mQAM coding unit 211 and a polarization time block coding unit 212.

[0025] The mQAM encoder 211 acquires a predetermined transmission signal sequence from a higher-level device (not shown). The mQAM encoder 211 performs multi-level quadrature amplitude modulation encoding on the transmission signal sequence. As a result, the mQAM encoder 211 generates a time-series signal "St(t)". ​​The mQAM encoder 211 outputs the time-series signal "St(t)" to the polarization time block encoder 212.

[0026] The polarization time block coding unit 212 generates a dual polarization IQ modulated signal (a dual polarization phase modulated signal) by performing space-time coding on the time series signal “St(t)” output from the mQAM coding unit 211. For example, the polarization time block coding unit 212 generates a complex signal “S ｘ (t)" and a complex signal "S" corresponding to the Y polarization. Ｙ The polarization time block coding unit 212 generates the generated complex signal "S ｘ (t)" and the complex signal "S Ｙ(t)" is output to the DA converter 22. Here, a configuration for block coding between two consecutive symbols is shown. One symbol length is T [s].

[0027] The DA converter 22 converts the complex signal “S ｘ (t)" and the complex signal "S Ｙ The DA converter 22 performs digital-to-analog conversion on the complex signal "S ｘ The DA converter 22 outputs to the DP-IQ modulator 24 an analog signal "XI" of the I axis (real axis) of the X polarization and an analog signal "XQ" of the Q axis (imaginary axis) of the X polarization, which are obtained by performing digital-to-analog conversion on the complex signal "S Ｙ The analog signal "YI" of the I axis (real axis) of the Y polarization and the analog signal "YQ" of the Q axis (imaginary axis) of the Y polarization obtained by performing digital-to-analog conversion on "(t)" are output to the DP-IQ modulator 24.

[0028] The light source 23 outputs light of a predetermined wavelength to the DP-IQ modulator 24. The DP-IQ modulator 24 is composed of two IQ modulators (for example, a first IQ modulator and a second IQ modulator) and a polarization multiplexing unit. The DP-IQ modulator modulates the light output from the light source 23 based on the analog signal "XI" of the I component of the X polarization output from the DA converter 22, the analog signal "YI" of the Q component of the X polarization, the analog signal "YI" of the I component of the Y polarization, and the analog signal "YQ" of the Q component of the Y polarization.

[0029] The first IQ modulator generates an X-polarized optical signal by modulating the light output from the light source 23 based on an analog signal "XI" of the I component of the X polarization and an analog signal "XQ" of the Q component of the X polarization output from the DA converter 22. The first IQ modulator outputs the generated X-polarized optical signal to the polarization multiplexing unit. The second IQ modulator generates a Q-polarized optical signal by modulating the light output from the light source 23 based on an analog signal "YI" of the I component of the Y polarization and an analog signal "YQ" of the Q component of the Y polarization output from the DA converter 22. The second IQ modulator outputs the generated Q-polarized optical signal to the polarization multiplexing unit. The polarization multiplexing unit polarization-multiplexes the X-polarized optical signal output from the first IQ modulator and the Y-polarized optical signal output from the second IQ modulator.

[0030] As a result, the DP-IQ modulator 24 generates a polarization phase-modulated optical signal, and transmits the generated polarization phase-modulated optical signal to the receiving device 4 via the transmission path 3.

[0031] As shown in FIG. 1, the transmitter 2 transmits S １ and transmits S in Y polarization. ２ At time 2T following time T, the transmitter 2 transmits -S ２ The complex conjugate signal (-S ２ ＊ ) is superimposed on the Y polarization, and S １ The complex conjugate signal (-S １ ＊ ) are superimposed on the signal. This is considered to be one block code. The transmitter 2 then repeats the same modulation. The transmitter 2 transmits the above one block code using a transmitter corresponding to each polarization. The signal transmitted from the transmitter 2 undergoes fluctuations in the phase and vibration plane of each polarization along the transmission path 3. In the conventional optical fiber communication system described in Non-Patent Document 1, communication is performed using the configuration shown in Figure 21. At this time, the polarization deflection angle and phase angle of the signal light shift due to polarization mode dispersion, etc. The transmission path characteristics and received electric field for each polarization signal can be expressed using the Jones matrix as shown in the following equation (1).

[0032]

[0033] →E in formula (1) ｏｕｔ (→ is E ｏｕｔ (appended above) represents the vector of the received electric field after propagation through the transmission line, and E ｘ ｉｎ , E ｙ ｉｎ corresponds to the output electric field at the transmitter of the DP-IQ modulated signal.

[0034] The receiving device 4 includes a local light source 41, an optical splitter 42, two PDs 43-1 and 43-2, a TIA 44, an AD converter 45, and a received digital signal processing unit 46. The received digital signal processing unit 46 includes a baseband conversion processing unit 47, a serial-to-parallel conversion unit 48, an adaptive equalization filter 49, a parallel-to-serial conversion unit 50, and an mQAM decoding unit 51.

[0035] The local light source 41 outputs local light to the optical splitter 42. The optical splitter 42 splits the input signal light and the local light output from the local light source 41 to the PDs 43-1 and 43-2. ​​The PDs 43-1 and 43-2 output current signals corresponding to the intensities of the signal light and the local light to the TIA 44. The TIA 44 outputs a voltage signal corresponding to the current signal to the AD converter 45. The AD converter 45 converts the voltage signal into a digital signal "S ｒｘ (t)".

[0036] The receiver 4 is assumed to have a heterodyne configuration in which the wavelength difference between the local light source 41 and the signal light is set to correspond to the symbol rate B, and the beat component of the signal light and the local light is received in the intermediate frequency band. At this time, if the transmitter 2 does not perform Nyquist filter processing, the received signal band will be 2B. Therefore, in order to receive the signal without degradation in signal quality in the receiver 4, the AD converter 45 performs AD conversion at a sampling rate four times the symbol rate B. The received signal is the product of the received electric field vector and the local light. Note that if the local light is in an arbitrary polarization state, the Jones matrix can be considered to include components representing the difference in relative polarization state and frequency between the local light and the signal light.

[0037] The digital signal "S" converted by the AD converter 45 ｒｘSince "(t)" is an intermediate frequency band signal, it needs to be converted to a baseband signal. The baseband conversion processing unit 47 is made up of a signal source 471, a 90-degree phase shift unit 472, and mixers 473-1 and 473-2. The signal source 471 outputs a sine wave corresponding to the intermediate frequency. The 90-degree phase shift unit 472 shifts the phase of the sine wave output from the signal source 471 by 90 degrees (shifts it by 90 degrees).

[0038] The mixer 473-1 receives the digital signal "S ｒｘ (t)" and the sine wave output from the signal source 471. The mixer 473-1 digitally processes the sine wave to generate a digital signal "S ｒｘ (t)) to convert it to baseband. As a result, the mixer 473-1 converts the real axis signal "Real (S ｒｘ The mixer 473-2 generates a digital signal "S ｒｘ (t)" and a sine wave whose phase has been shifted by 90 degrees by the 90-degree phase shift unit 472. The mixer 473-2 digitally processes the sine wave whose phase has been shifted by 90 degrees to generate a digital signal "S ｒｘ (t)) to convert it to baseband. As a result, the mixer 473-2 converts the imaginary axis signal "Image (S ｒｘ (t)) is generated.

[0039] The baseband conversion processing unit 47 also includes a low pass filter (LPF) for removing high frequency noise. The serial-to-parallel conversion unit 48 performs serial-to-parallel conversion on the signal converted to the baseband. The serial-to-parallel conversion unit 48 converts consecutive symbols "S" into "S" from the signal converted to the baseband. ｒｘ１ (t)" and "S ｒｘ２ (t)" is distributed by serial-parallel conversion.

[0040] The adaptive equalization filter 49 receives the input symbol "S ｒｘ１ (t)" and "S ｒｘ２(t)". ​​Specifically, the adaptive equalization filter 49 is an adaptive equalization filter with a 2x2 MIMO (Multi-Input Multi-Output) configuration as described in Non-Patent Document 1. The adaptive equalization filter 49 performs adaptive equalization processing on the input symbol "S ｒｘ１ (t)" and "S ｒｘ２ The inverse matrix of the above transmission path characteristics (Jones matrix) is applied to "(t)".

[0041] The parallel-serial converter 50 performs parallel-to-serial conversion on the signal after adaptive equalization processing. This makes it possible to decode the signal as a transmission signal "St'(t)". ​​The mQAM decoder 51 performs multilevel quadrature amplitude modulation (mQAM) decoding processing on the signal after parallel-to-serial conversion. This restores the transmission signal "St(t)".

[0042] The above is the configuration of the transmitter 2 and receiver 4 in the conventional communication system 1. In the following, a configuration for realizing polarization and phase diversity reception with a simpler configuration than the receiver 4 shown in FIG.

[0043] 3 and 4, the configuration of a communication system 1a according to the first embodiment will be described. The communication system 1a includes a transmitter 2a, a transmission path 3, and a receiver 4a. The transmission path 3 includes an optical fiber. In the first embodiment, optical communication between the transmitter 2a and the receiver 4a will be described as an example; however, radio wave communication may be used between the transmitter 2a and the receiver 4a. That is, the communication system 1a may be an optical communication system (optical fiber communication system) or a wireless communication system (spatial diversity transmission / reception wireless system). Although multilevel quadrature amplitude modulation (mQAM) will be described as an example of the modulation method used by the transmitter 2a, this method is not dependent on the modulation method.

[0044] (Configuration of Transmitter 2a) Figure 3 is a diagram showing an example configuration of the transmitter 2a in the communication system 1a of the first embodiment. The transmitter 2a includes a transmit digital signal processing unit 21a, a DA converter 22, a light source 23, and a DP-IQ modulator 24. The transmit digital signal processing unit 21a performs predetermined signal processing (e.g., space-time coding) for transmitting an input transmit signal sequence. The transmit digital signal processing unit 21a includes an mQAM encoder 211, a polarization time block encoder 212, two replicators 213a-1 and 213a-2, a quadrature phase signal generator 214a, and two parallel-to-serial converters 215a-1 and 215a-2.

[0045] The configuration of the transmission digital signal processing unit 21a differs from that of the transmission digital signal processing unit 21 in that it includes duplicating units 213a-1 and 213a-2, a quadrature phase signal generating unit 214a, and parallel-serial converting units 215a-1 and 215a-2. The other configuration of the transmission digital signal processing unit 21a is the same as that of the transmission digital signal processing unit 21. The following description will focus on the differences from the transmission digital signal processing unit 21.

[0046] The replicator 213a replicates the input signal. The replicator 213a-1 replicates the complex signal “S ｘ The replicator 213a-1 receives the complex signal "S ｘ For example, the replicator 213a-1 replicates the complex signal "S ｘ (t)" is replicated into two complex signals and output to the quadrature signal generating unit 214a via different paths.

[0047] The replicator 213a-2 replicates the complex signal "S Ｙ The replicator 213a-2 receives the input complex signal "S Ｙ For example, the replicator 213a-2 replicates the complex signal "S Ｙ (t)" is replicated into two complex signals and output to the quadrature signal generating unit 214a via different paths.

[0048] The quadrature phase signal generator 214a generates quadrature phase signals, each having an orthogonal phase, for each polarization based on the complex signals output from each duplicator 213a via a different path. The quadrature phase signal generator 214a includes two phase shifters 216a-1 and 216a-2. The phase shifter 216a shifts the phase of the input signal by 90 degrees or −90 degrees.

[0049] The quadrature phase signal generator 214a generates the complex signal "S ｘ (t)) to the parallel-serial conversion unit 215a-1 as it is, and outputs the complex signal "S ｘ The phase of the signal (t) is shifted by 90 degrees or −90 degrees by a phase shifter 216a-1 and output to a parallel-to-serial converter 215a-1.

[0050] The quadrature phase signal generator 214a generates the complex signal "S Ｙ (t)) to the parallel-serial conversion unit 215a-2 as it is, and outputs the complex signal "S Ｙ The phase of the signal (t) is shifted by 90 degrees or −90 degrees by a phase shifter 216a-2 and output to a parallel-to-serial converter 215a-2.

[0051] As described above, the quadrature phase signal generator 214a performs a process of shifting the phase of one of the sequences of the replicated complex signals for each polarization. As a result, the quadrature phase signal generator 214a generates a quadrature phase signal for each polarization. In the following description, it is assumed that the phase of the complex signal is shifted by −90 degrees by the phase shifter 216a.

[0052] FIG. 4 is a diagram for explaining the processing of the phase shift unit 216a in the first embodiment. FIG. 4 shows an example of signal transition on the IQ plane. As shown in FIG. 4, the phase shift unit 216a １ ” is phase shifted by -90 degrees to obtain the complex signal “S １ For QPSK (Quadrature Phase-Shift Keying) signals, １If j=1+j, the phase shift unit 216a outputs the phase-shifted signal to the parallel-to-serial conversion unit 215a.

[0053] The parallel-serial conversion unit 215a performs parallel-serial conversion on the input signal. The parallel-serial conversion unit 215a-1 converts the complex signal "S ｘ (t)" and the phase-shifted complex signal "S ｘ (t)-90°" and a continuous signal "S ｘ The parallel-to-serial conversion unit 215a-2 converts the complex signal "S (t)" output from the quadrature phase signal generation unit 214a into a complex signal "S Ｙ (t)" and the phase-shifted complex signal "S Ｙ (t)-90°" and a continuous signal "S Ｙ (t)" to the DA converter 22. ｘ (t)" and continuous signal "S Ｙ (t)" is also called an IQ interleaved signal.

[0054] The DA converter 22 performs digital-to-analog conversion on the IQ interleaved signal output from the parallel-to-serial conversion unit 215a at a modulation speed twice the symbol rate B, and outputs the result. ｘ The DA converter 22 outputs to the DP-IQ modulator 24 an analog signal "XI" of the I axis (real axis) of the X polarization and an analog signal "XQ" of the Q axis (imaginary axis) of the X polarization obtained by performing digital-to-analog conversion on the continuous signal "S Ｙ The analog signal "YI" of the I axis (real axis) of the Y polarization and the analog signal "YQ" of the Q axis (imaginary axis) of the Y polarization obtained by performing digital-to-analog conversion on "(t)" are output to the DP-IQ modulator 24.

[0055] The DP-IQ modulator 24 modulates the light output from the light source 23 based on the analog signal "XI" of the I component of the X polarization, the analog signal "YI" of the Q component of the X polarization, the analog signal "YI" of the I component of the Y polarization, and the analog signal "YQ" of the Q component of the Y polarization, which are output from the DA converter 22. In this way, the DP-IQ modulator 24 generates a polarization phase-modulated optical signal. The DP-IQ modulator 24 transmits the generated polarization phase-modulated optical signal to the receiving device 4 via the transmission path 3. For example, the DP-IQ modulator 24 alternately transmits both polarization phase-modulated signals of each polarization and quadrature phase signals of each polarization on the time axis.

[0056] As shown in FIG. 3, the transmitter 2a transmits S １ and S １ Transmit -90°, Y polarization S ２ and S ２ At time 2T following time T, the transmitter 2a transmits -S ２ The complex conjugate signal (-S ２ ＊ ) and -S ２ The complex conjugate signal (-S ２ ＊ ) with a phase shift of −90° (−S ２ ＊ -90°) is superimposed on the Y-polarized wave, and S １ The complex conjugate signal (-S １ ＊ ) and -S １ The complex conjugate signal (-S １ ＊ ) with a phase shift of −90° (−S １ ＊ -90°). This is considered to be one block code. After that, the transmitter 2a repeats the same modulation. The transmitter 2 transmits the above one block code using a transmitter corresponding to each polarization. By transmitting the I-axis signal and the Q-axis signal in a time-division manner within one symbol time length in this way, IQ diversity reception on the time axis is possible on the receiving side, and by using it in combination with polarization time block code, reception independent of polarization is also possible.

[0057] When the transmitting device 2a shown in FIG. 3 is applied to a wireless communication system, a frequency conversion unit that up-converts a signal to an intermediate frequency is provided after each parallel-serial conversion unit 215a.

[0058] 5 is a diagram showing an example of the configuration of the receiving device 4a in the communication system 1a according to the first embodiment. The receiving device 4a includes a local light source 41, an optical splitter 42, two PDs 43-1 and 43-2, a TIA 44, an AD converter 45, and a received digital signal processing unit 46a. The received digital signal processing unit 46a includes a serial-to-parallel conversion unit 48, an adaptive equalization filter 49, a parallel-to-serial conversion unit 50, an mQAM decoding unit 51, and a quadrature phase signal restoration unit 52a.

[0059] The received digital signal processing unit 46a differs in configuration from the received digital signal processing unit 46 in that it does not include a baseband conversion processing unit 47 and that it newly includes a quadrature phase signal restoration unit 52a. The other configuration of the received digital signal processing unit 46a is the same as that of the received digital signal processing unit 46. The following description will focus on the differences from the received digital signal processing unit 46.

[0060] The receiver 4a is assumed to have an intradyne configuration in which the wavelength difference between the local light source 41 and the signal light is matched and the beat component of the signal light and the local light is received in the baseband frequency band. The signal bandwidth is assumed to be B, which corresponds to the symbol rate. In this case, the receiver 4a receives an IQ interleaved signal at a rate twice the symbol rate on the time axis. Therefore, the AD converter 45 samples at a rate of 2B or higher, twice the received signal bandwidth. The following explanation uses an example of double oversampling. Since the transmitter 2a generates a signal with a phase shift of -90 degrees, this corresponds to receiving an I-axis signal during the first half (e.g., 0 to T / 2) of one symbol time T, and a Q-axis signal during the second half (e.g., T / 2 to T).

[0061] Therefore, the quadrature phase signal restoration unit 52a converts the digital signal "S ｒｘOf the two consecutive samples of "(t)", the first sample is regarded as an I-component signal and the second sample as a Q-component signal, and a complex signal is restored. The quadrature phase signal restoration unit 52a outputs the restored complex signal to the serial-to-parallel conversion unit 48. The subsequent processing is the same as that shown in FIG. 2.

[0062] According to the communication system 1a configured as described above, the transmitting device 2a includes a polarization time block coding unit 212 that generates dual polarization phase-modulated signals by performing space-time coding on the transmission signal, a quadrature phase signal generating unit 214a that generates, for each polarization, a quadrature phase signal of the generated dual polarization phase-modulated signals, and a DP-IQ modulator 24 that alternately transmits the dual polarization phase-modulated signals of each polarization and the quadrature phase signal of each polarization on the time axis.

[0063] In this way, the transmitting device 2a generates a quadrature phase signal for each polarization, which enables the receiving device 4a to realize polarization and phase diversity reception with a simple configuration consisting of a local light source 41, an optical branching device 42 (3 dB optical splitter), and a PD 43 (balanced receiver).

[0064] Furthermore, in the communication system 1a, the transmission device 2a, which has a complex configuration, can be shared by multiple users, thereby reducing the cost of the entire communication system.

[0065] (Variation 1 of the First Embodiment) In the receiving device 4a described above, it is desirable that the sampling phase of the sampling point at which the AD converter 45 samples is adjusted so that the center of the IQ interleaved signal is sampled. Therefore, the receiving device 4a may be provided with a function for adjusting the sampling phase either before or after the quadrature phase signal restoration unit 52a. FIG. 6 is a diagram showing an example configuration of a receiving device 4b according to Variation 1 of the first embodiment. The receiving device 4b includes a local oscillator 41, an optical splitter 42, two PDs 43-1 and 43-2, a TIA 44, an AD converter 45, and a received digital signal processing unit 46b. The receiving device 4b includes the received digital signal processing unit 46b instead of the received digital signal processing unit 46a.

[0066] The received digital signal processing unit 46b includes a serial-to-parallel conversion unit 48, an adaptive equalization filter 49, a parallel-to-serial conversion unit 50, an mQAM decoding unit 51, a quadrature phase signal restoration unit 52a, and a sampling clock phase compensation unit 53b. The sampling clock phase compensation unit 53b converts the digital signal "S ｒｘ The sampling phase of the AD converter 45 may be adjusted based on the signal "(t)." This configuration enables polarization and phase diversity reception without heterodyne reception and in combination with polarization time block coding.

[0067] The optical receiver placed after the optical splitter 42 shown in Figure 6 may be a single PD 43 rather than a balanced receiver with two PDs 43. In this type of transmission / reception configuration, the transmission rate required on the transmitting side is higher than in the conventional technology, but the receiving electrical bandwidth can be kept low. Furthermore, since the transmitting side is expected to be used as a transmitter for a master device in a point-to-multipoint communication system, sharing it among multiple user devices can reduce the impact on the cost requirements of the communication system.

[0068] (Second Modification of First Embodiment) The receiving device 4a may be configured to reduce the error function of the adaptive equalization filter 49. Fig. 7 is a diagram showing an example configuration of a receiving device 4c in the second modification of the first embodiment. The receiving device 4c includes a local oscillator 41, an optical splitter 42, two PDs 43-1 and 43-2, a TIA 44, an AD converter 45, and a receiving digital signal processing unit 46c. The receiving device 4c includes the receiving digital signal processing unit 46c instead of the receiving digital signal processing unit 46a.

[0069] The received digital signal processing unit 46c includes a serial-to-parallel conversion unit 48, an adaptive equalization filter 49c, a parallel-to-serial conversion unit 50, an mQAM decoding unit 51, a quadrature phase signal restoration unit 52a, and a sampling clock phase compensation unit 53c. The adaptive equalization filter 49c feeds back the processing result to the sampling clock phase compensation unit 53c. The sampling clock phase compensation unit 53c changes the compensation value based on the feedback result from the adaptive equalization filter 49c. Specifically, the sampling clock phase compensation unit 53c changes the compensation value so as to reduce the error function of the adaptive equalization filter 49c.

[0070] (Third Modification of First Embodiment) The receiving device 4a may be configured to compensate for transmission path characteristics such as chromatic dispersion that occur in the transmission path 3, and a frequency offset that occurs due to the frequency difference between the local light output from the local oscillator 41 and the signal light. Fig. 8 is a diagram showing an example configuration of a receiving device 4d in the third modification of the first embodiment. The receiving device 4d includes a local oscillator 41, an optical splitter 42, two PDs 43-1 and 43-2, a TIA 44, an AD converter 45, and a received digital signal processing unit 46d. The receiving device 4d includes the received digital signal processing unit 46d instead of the received digital signal processing unit 46a.

[0071] The reception digital signal processing unit 46d includes a serial-parallel conversion unit 48, an adaptive equalization filter 49c, a parallel-serial conversion unit 50, an mQAM decoding unit 51, a quadrature-phase signal restoration unit 52a, a sampling clock phase compensation unit 53c, a transmission path characteristic compensation unit 54d, and a frequency offset compensation unit 55d. The transmission path characteristic compensation unit 54d is disposed after the quadrature-phase signal restoration unit 52a and compensates for chromatic dispersion occurring in the transmission path 3 with respect to the complex signal restored by the quadrature-phase signal restoration unit 52a. The frequency offset compensation unit 55d is disposed after the transmission path characteristic compensation unit 54d and compensates for a frequency offset occurring due to the frequency difference between the signal light and the local light output from the local oscillator 41 with respect to the complex signal whose transmission path characteristics have been compensated for by the transmission path characteristic compensation unit 54d. Here, the transmission path characteristic refers to the characteristics of the path along which light or radio waves are transmitted between the transmitting device 2a and the receiving device 4d. When the communication system 1d is an optical communication system, the transmission path characteristics are expressed by, for example, chromatic dispersion. When the communication system 1d is a wireless communication system, the transmission path characteristics are expressed by, for example, a transfer function. The same applies to the following embodiments.

[0072] Second Embodiment In a second embodiment, the transmission path characteristics are compensated in the transmission device to simplify the receiver signal processing configuration and remove inter-block interference associated with time block coding. Generally, in time block coding, characteristics are degraded due to signal interference occurring between blocks. Therefore, signals interfere with each other between IQ signals of IQ interleaving and between polarization time block codes due to the influence of the transmission path characteristics. Therefore, in the second embodiment, a configuration for suppressing the influence of the transmission path characteristics in the transmission device will be described.

[0073] The configuration of a communication system 1e in the second embodiment will be described using Figures 9 and 10. The communication system 1e includes a transmitter 2e, a transmission path 3, and a receiver 4a. In the second embodiment, optical communication between the transmitter 2e and the receiver 4a will be described as an example, but radio wave communication may also be performed between the transmitter 2e and the receiver 4a. That is, the communication system 1e may be an optical communication system (optical fiber communication system) or a wireless communication system (spatial diversity transmission / reception wireless system). Although multilevel quadrature amplitude modulation (mQAM) will be described as an example of the modulation method used by the transmitter 2e, this method is not dependent on the modulation method.

[0074] (Configuration of Transmitter 2e) Figure 9 is a diagram showing an example of the configuration of the transmitter 2e in the communication system 1e of the second embodiment. The transmitter 2e includes a transmit digital signal processing unit 21e, a DA converter 22, a light source 23, and a DP-IQ modulator 24. The transmit digital signal processing unit 21e performs predetermined signal processing (e.g., space-time coding, compensation for transmission path characteristics, etc.) for transmitting the input transmission signal sequence. The transmit digital signal processing unit 21e includes an mQAM encoder 211, a polarization time block encoder 212, two replicators 213a-1 and 213a-2, a quadrature phase signal generator 214a, two parallel-to-serial converters 215a-1 and 215a-2, a compensation coefficient calculation and storage unit 217e, and two transmission path characteristic compensators 218e-1 and 218e-2.

[0075] In the following description, it is assumed that the transmitter 2e compensates for chromatic dispersion. A possible method for compensating for the inverse characteristics of chromatic dispersion is a configuration in which the inverse characteristics of chromatic dispersion are applied to a transmission signal sequence by a digital signal processing circuit, as described in Non-Patent Document 3. When applying the technology described in Non-Patent Document 3, a polarization time block code and an IQ interleaved signal are used in combination, and therefore, as shown in FIG. 9, a transmission path characteristic compensator 218e for each polarization is provided after the parallel-to-serial converters 215a-1 and 215a-2.

[0076] The compensation coefficient calculation and storage unit 217e stores a compensation coefficient table. The compensation coefficient table is a table that stores filter coefficients corresponding to the inverse characteristics of the transmission path characteristics (e.g., chromatic dispersion characteristics) generated by the transmission path 3 connecting each receiving device 4a that will be the communication destination and the DP-IQ modulator 24. The filter coefficients corresponding to each receiving device 4a that will be the communication destination may be coefficients that have been measured in advance, or may be obtained during initial authentication, as described below. The compensation coefficient calculation and storage unit 217e may calculate a filter coefficient for each receiving device 4a that will be the communication destination using a theoretical formula similar to that described in Non-Patent Document 3, based on information such as the dispersion coefficient and wavelength of the transmission path 3.

[0077] The compensation coefficient calculation and storage unit 217e switches the coefficients to be set in the transmission path characteristic compensation unit 218e according to the destination receiving device 4a, based on communication destination information (e.g., data transmission timing and identification information of the destination receiving device 4a) notified from a MAC (Media Access Control) unit or externally. For example, when the timing for transmitting data included in the communication destination information arrives, the compensation coefficient calculation and storage unit 217e acquires a filter coefficient associated with the identification information of the destination receiving device 4a from a compensation coefficient table. The compensation coefficient calculation and storage unit 217e sets the acquired filter coefficients in the transmission path characteristic compensation units 218e-1 and 218e-2.

[0078] The same coefficients are used for each polarization. That is, the compensation coefficient calculation and holding unit 217e sets the same filter coefficients for the transmission path characteristics compensating units 218e-1 and 218e-2. If there is only one receiving device 4a at the other end of communication, the compensation coefficient calculation and holding unit 217e does not need to switch the filter coefficients.

[0079] The transmission path characteristic compensator 218e is configured with a linear filter on the time axis or the frequency axis. The transmission path characteristic compensator 218e applies the inverse of the transmission path characteristic to the transmission signal sequence using a linear filter whose filter coefficients are set by the compensation coefficient calculation and storage unit 217e. For example, the transmission path characteristic compensator 218e applies the inverse of the chromatic dispersion characteristic to the transmission signal sequence using a linear filter whose filter coefficients are set by the compensation coefficient calculation and storage unit 217e. Below, a configuration in which the inverse of the chromatic dispersion characteristic is applied will be described as an example. The transmission path characteristic compensator 218e-1 applies the inverse of the chromatic dispersion characteristic to the X-polarized transmission signal sequence. The transmission path characteristic compensator 218e-2 applies the inverse of the chromatic dispersion characteristic to the Y-polarized transmission signal sequence.

[0080] The processing flow of the compensation coefficient calculation / holding unit 217e and the transmission path characteristic compensating unit 218e in the second embodiment will be described. FIG. 10 is a diagram for explaining the processing flow of the compensation coefficient calculation / holding unit 217e and the transmission path characteristic compensating unit 218e in the second embodiment. As shown in FIG. 10, it is assumed that the identification information of the receiving device 4a#1 and the transmission timing t1 are notified to the compensation coefficient calculation / holding unit 217e as communication destination information. In this case, at time t1, the compensation coefficient calculation / holding unit 217e refers to the compensation coefficient table and obtains a filter coefficient #1 corresponding to the identification information of the receiving device 4a#1. The compensation coefficient calculation / holding unit 217e sets the obtained filter coefficient #1 in each transmission path characteristic compensating unit 218e. As a result, a signal that has been dispersion compensated by a linear filter in which the filter coefficient #1 is set in each transmission path characteristic compensating unit 218e can be transmitted to the receiving device 4a#1.

[0081] Thereafter, it is assumed that the identification information of the receiving device 4a#2 and the transmission timing t2 are notified to the compensation coefficient calculation and holding unit 217e as communication destination information. In this case, at time t2, the compensation coefficient calculation and holding unit 217e refers to the compensation coefficient table and acquires a filter coefficient #2 corresponding to the identification information of the receiving device 4a#2. The compensation coefficient calculation and holding unit 217e sets the acquired filter coefficient #2 in each transmission path characteristics compensator 218e. As a result, the filter coefficient of the linear filter in each transmission path characteristics compensator 218e is changed from filter coefficient #1 to filter coefficient #2. Then, a signal that has been dispersion compensated by the linear filter in which filter coefficient #2 is set in each transmission path characteristics compensator 218e can be transmitted to the receiving device 4a#2.

[0082] Subsequently, it is assumed that the identification information of the receiving device 4a#3 and transmission timing t3 are notified to the compensation coefficient calculation and holding unit 217e as communication destination information. In this case, at time t3, the compensation coefficient calculation and holding unit 217e refers to the compensation coefficient table and acquires filter coefficient #3 corresponding to the identification information of the receiving device 4a#3. The compensation coefficient calculation and holding unit 217e sets the acquired filter coefficient #3 in each transmission path characteristic compensator 218e. As a result, the filter coefficient of the linear filter in each transmission path characteristic compensator 218e is changed from filter coefficient #2 to filter coefficient #3. Then, a signal that has been dispersion compensated by the linear filter in which filter coefficient #3 is set in each transmission path characteristic compensator 218e can be transmitted to the receiving device 4a#3.

[0083] As described above, the transmitting device 2e transmits signals while switching the filter coefficient for each communication destination. Note that the receiving device 4a in the second embodiment may be configured to include a transmission path characteristic compensator that compensates for the transmission path characteristics and remove residual dispersion, which is the difference between the theoretical formula and the actual propagation path characteristics.

[0084] The communication system 1e according to the second embodiment configured as described above can achieve the same effects as those of the first embodiment. Furthermore, in the communication system 1e according to the second embodiment, the transmission device 2e compensates for the transmission path characteristics. This simplifies the configuration of the receiving device 4a. Furthermore, it is possible to suppress inter-block interference associated with time block coding.

[0085] Third Embodiment In a third embodiment, a configuration for compensating for a loss of orthogonality caused by a frequency offset corresponding to the wavelength difference between signal light and local light in a transmitting device will be described.

[0086] When an IQ-interleaved signal is IQ-decoded by a receiving device, if a frequency offset exists, a phase rotation corresponding to the frequency offset occurs between adjacent I and Q signals. Figure 11 shows an image diagram of the phase rotation caused by the transmitted signal, the received signal, and the frequency offset. For simplicity of explanation, Figure 11 illustrates an example in which the X-polarized wave of the transmitted signal matches the polarization of the local oscillator light.

[0087] If the amount of phase rotation occurring over half the symbol length T (T / 2) is φ, it can be seen that the phase difference between adjacent IQ interleaved signals deviates from -90° by φ. In this method, in which the transmitter generates orthogonal signals by shifting the phase by -90°, this frequency offset is a factor that disrupts the orthogonality of the IQ signals. Therefore, in the third embodiment, as shown in FIG. 12 , this disruption of orthogonality is compensated for by applying a phase that is the opposite of the amount of phase rotation that occurs between the IQ interleaved signals due to the frequency offset in the transmitter. As a result, as shown in FIG. 12 , although the frequency offset between adjacent symbols remains, orthogonality can be maintained between the IQ interleaved signals. Note that while FIG. 12 shows a configuration in which phase rotation is applied to the signal to be phase-shifted (e.g., the signal input to the phase shift unit 216a), the phase rotation only needs to be applied as a relative phase difference between the IQ signals. Therefore, a configuration in which the phase of the signal not to be phase-shifted (e.g., the signal not input to the phase shift unit 216a) is changed may also be used.

[0088] The configuration of a communication system 1f according to the third embodiment will be described below with reference to Figures 13 and 14. The communication system 1f includes a transmitter 2f, a transmission path 3, and a receiver 4a. In the third embodiment, optical communication between the transmitter 2f and the receiver 4a will be described as an example, but radio wave communication may also be performed between the transmitter 2f and the receiver 4a. That is, the communication system 1f may be an optical communication system (optical fiber communication system) or a wireless communication system (spatial diversity transmission / reception wireless system). Although multilevel quadrature amplitude modulation (mQAM) will be described as an example of the modulation method used by the transmitter 2f, this method is not dependent on the modulation method.

[0089] (Configuration of Transmitter 2f) Figure 13 is a diagram showing an example configuration of a transmitter 2f in a communication system 1f according to the third embodiment. The transmitter 2f includes a transmit digital signal processor 21f, a DA converter 22, a light source 23, and a DP-IQ modulator 24. The transmit digital signal processor 21f performs predetermined signal processing (e.g., space-time coding, frequency offset compensation, etc.) for transmitting an input transmit signal sequence. The transmit digital signal processor 21f includes an mQAM encoder 211, a polarization time block encoder 212, two replicators 213a-1 and 213a-2, a quadrature phase signal generator 214a, two parallel-to-serial converters 215a-1 and 215a-2, a phase rotation amount calculator / storer 220f, and two frequency offset compensators 221f-1 and 221f-2.

[0090] As shown in FIG. 13, the transmitting device 2f includes a frequency offset compensator 221f for each polarization in front of the parallel-to-serial converters 215a-1 and 215a-2.

[0091] The phase rotation amount calculation and storage unit 220f stores a frequency offset compensation table. The frequency offset compensation table stores the amount of phase rotation for each receiving device 4a that will be the communication partner. The phase rotation amount calculation and storage unit 220f switches the amount of compensation (phase difference) to be compensated for in the frequency offset compensation unit 221f according to the receiving device 4a that will be the communication partner, based on communication partner information (e.g., data transmission timing, identification information and wavelength difference of the receiving device 4f that will be the communication partner) notified from the MAC unit or externally. For example, when the timing for transmitting data included in the communication partner information arrives, the phase rotation amount calculation and storage unit 220f acquires the amount of phase rotation associated with the identification information of the receiving device 4f that will be the communication partner from the frequency offset compensation table. The phase rotation amount calculation and storage unit 220f switches the amount of compensation (phase difference) to be compensated for in the frequency offset compensation unit 221f-1 and the frequency offset compensation unit 221f-2 based on the acquired amount of phase rotation. If there is only one receiving device 4d at the other end of communication, the phase rotation amount calculation and storage unit 220f does not need to switch the compensation amount (phase difference).

[0092] The frequency offset compensator 221f compensates for the frequency offset of the complex signal using the phase rotation amount output from the phase rotation amount calculation and storage unit 220f. Ｘ (t)" or the phase-shifted complex signal "S Ｘ The frequency offset compensator 221f-2 rotates the phase of the complex signal "S (t) -90°" output from the quadrature phase signal generator 214a to compensate for the frequency offset. Ｙ (t)" or the phase-shifted complex signal "S Ｙ (t) -90°" to compensate for the frequency offset.

[0093] The processing flow of the phase rotation amount calculation and storage unit 220f and the frequency offset compensation unit 221f in the third embodiment will be described. FIG. 14 is a diagram for explaining the processing flow of the phase rotation amount calculation and storage unit 220f and the frequency offset compensation unit 221f in the third embodiment. As shown in FIG. 14, it is assumed that the identification information of the receiving device 4d#1, the transmission timing t1, and the wavelength difference #1 are notified to the phase rotation amount calculation and storage unit 220f as communication destination information. In this case, at time t1, the phase rotation amount calculation and storage unit 220f refers to the frequency offset compensation table and obtains the phase rotation amount #1 corresponding to the identification information of the receiving device 4d#1. The phase rotation amount calculation and storage unit 220f notifies each frequency offset compensation unit 221f of the obtained phase rotation amount #1. As a result, each frequency offset compensation unit 221f can transmit a signal (signal with phase difference #1) whose phase has been rotated by the notified phase rotation amount #1 to the receiving device 4d#1.

[0094] Subsequently, suppose that the identification information of receiving device 4d#2, transmission timing t2, and wavelength difference #2 are notified to the phase rotation amount calculation and holding unit 220f as communication partner information. In this case, at time t2, the phase rotation amount calculation and holding unit 220f refers to the frequency offset compensation table and obtains the phase rotation amount #2 corresponding to the identification information of receiving device 4d#2. The phase rotation amount calculation and holding unit 220f notifies each frequency offset compensator 221f of the obtained phase rotation amount #2. As a result, each frequency offset compensator 221f can transmit a signal (signal with phase difference #2) whose phase has been rotated by the notified phase rotation amount #2 to receiving device 4d#2.

[0095] Subsequently, suppose that the identification information of receiving device 4d#3, transmission timing t3, and wavelength difference #3 are notified to the phase rotation amount calculation and holding unit 220f as communication partner information. In this case, at time t3, the phase rotation amount calculation and holding unit 220f refers to the frequency offset compensation table and obtains the phase rotation amount #3 corresponding to the identification information of receiving device 4d#3. The phase rotation amount calculation and holding unit 220f notifies each frequency offset compensator 221f of the obtained phase rotation amount #3. As a result, each frequency offset compensator 221f can transmit a signal (signal with phase difference #3) whose phase has been rotated by the notified phase rotation amount #3 to receiving device 4d#3.

[0096] As described above, the transmitting device 2f transmits signals while switching the amount of phase rotation for each communication partner.

[0097] Since the phase rotation due to the frequency offset remains in the receiving device 4a in the third embodiment, it is necessary to include a frequency offset compensating unit 55d shown in Fig. 8. For frequency offset compensation, a method can be considered in which a training signal is applied and time-frequency processing such as FFT (Fast Fourier Transform) is used to detect the degree of frequency deviation on the frequency axis from the received signal.

[0098] The communication system 1f according to the third embodiment configured as described above can achieve the same effects as those of the first embodiment. Furthermore, the communication system 1f according to the third embodiment compensates for the loss of orthogonality caused by a frequency offset in the transmitting device 2f. This simplifies the configuration of the receiving device 4a.

[0099] (Variation 1 of the Third Embodiment) To compensate for the loss of orthogonality due to the frequency offset, the difference between the frequency of the signal light and the frequency of the local light must be less than a predetermined value. Therefore, the receiving device 4 a may adjust the oscillation frequency of the local light source 41 (the frequency of the local light) based on the detected frequency offset so that the difference between the frequency of the signal light and the frequency of the local light becomes less than the predetermined value.

[0100] 15 is a diagram showing an example of the configuration of a receiving device 4f in Modification 1 of the third embodiment. The receiving device 4f includes a local oscillator 41, an optical splitter 42, two photodiodes 43-1 and 43-2, a TIA 44, an AD converter 45, and a received digital signal processing unit 46f. The received digital signal processing unit 46f includes a serial-to-parallel converter 48, an adaptive equalization filter 49, a parallel-to-serial converter 50, an mQAM decoder 51, a quadrature phase signal restoration unit 52a, a sampling clock phase compensation unit 53c, a transmission path characteristic compensation unit 54d, a frequency offset compensation unit 55d, and a DA converter 56f.

[0101] 15 shows, as an example, a configuration in which a DA converter 56f is further provided in addition to the configuration of the receiving device 4d shown in FIG. 8. Note that the received digital signal processing unit 46f does not necessarily have to include the sampling clock phase compensating unit 53c and the transmission path characteristics compensating unit 54d. The DA converter 56f may convert the digital signal representing the frequency offset amount detected by the frequency offset compensating unit 55 into an analog signal representing the frequency offset amount. The DA converter 56f may output the analog signal to the local oscillator light source 41. The local oscillator light source 41 may adjust the oscillation frequency (frequency of the local oscillator light) based on the analog signal representing the frequency offset amount.

[0102] (Second Modification of Third Embodiment) The frequency offset compensator 55d may be operated based on known information such as a frequency difference measured in advance by another device.

[0103] (Third Modification of the Third Embodiment) The communication system 1f may be operated in combination with the second embodiment. That is, the transmitting device 2f in the communication system 1f may be configured to switch the filter coefficient and the compensation amount (phase difference) for each communication destination. In this case, the transmitting device 2f further includes a compensation coefficient calculation / holding unit 217e and two transmission path characteristic compensators 218e-1 and 218e-2. For example, the transmitting device 2f further includes two transmission path characteristic compensators 218e-1 and 218e-2 in a stage subsequent to the parallel-to-serial conversion unit 215a.

[0104] (Fourth Embodiment) In the second and third embodiments, a configuration was described in which the influence of at least one of a frequency offset and transmission path characteristics was reduced in a transmitting device. However, in either configuration, information on the transmission path characteristics and the mutual optical frequency difference between the transmitting device and the receiving device, which is the communication partner, must be known. Therefore, in the fourth embodiment, a configuration will be described in which information on the transmission path characteristics and the mutual optical frequency difference is acquired during initial authentication when the transmitting device and the receiving device start communication.

[0105] In the fourth embodiment, during initial authentication, the master device transmits an initial authentication signal corresponding to the transmission path characteristics and optical frequency difference assumed by the communication system. Based on the information of the initial authentication frame that matches the characteristics and can be received from the authentication signal, the user device transmits an authentication request signal via an upstream signal or a separate information path. The following explanation takes the case where the communication system is an optical communication system as an example.

[0106] 16 and 17 are diagrams illustrating the processing performed during initial authentication in a communication system. FIG. 16 illustrates a one-to-one connection between a master station and user devices, while FIG. 17 illustrates a one-to-many connection between a master station and user devices. In FIGS. 16 and 17, the direction from the master station to the user devices is the downlink direction, and the direction from the user devices to the master station is the uplink direction. In FIGS. 16 and 17, the master station includes a transmitter, a receiver, and a multiplexer / demultiplexer. The transmitter included in the master station is one of transmitters 2a, 2e, and 2f. In FIGS. 16 and 17, the user device includes a transmitter, a receiver, and a multiplexer / demultiplexer. The receiver included in the user device is one of receivers 4a to 4d and 4f.

[0107] When compensation for dispersion is performed by the transmitting device as in the second embodiment, the transmitting device of the master station transmits frames whose characteristics have been modified by an arbitrary division number of the maximum dispersion amount assumed in the network (process (1) in FIGS. 16 and 17 ). The numbers of the downstream signals shown in FIG. 16 are not numbers for identifying user devices, but are the above-mentioned dispersion amount allocation number L (L is an integer equal to or greater than 1). The numbers of the downstream signals shown in FIG. 17 are not numbers for identifying user devices, but are the above-mentioned frequency offset amount division number M (M is an integer equal to or greater than 1). The user device receives an arbitrary frame (process (2) in FIGS. 16 and 17 ). Thereafter, the user device transmits an authentication request signal to the master station (process (3) in FIGS. 16 and 17 ). The master station receives and registers the authentication request signal (process (4) in FIGS. 16 and 17 ).

[0108] The above operations (1) to (4) correspond to, for example, the discovery operation of a PON (Passive Optical Network) system. A similar scheme is also used when performing frequency offset compensation as described in the third embodiment. For example, the transmitting device of the master station transmits a downstream authentication frame in which the expected frequency offset amount is divided by an arbitrary division number M (process (1) in FIG. 17). Then, the above operations (2) to (4) are performed. Note that when both chromatic dispersion compensation and frequency offset are compensated for in advance, the transmitting device of the master station transmits an authentication signal with a combination (L x M) that covers all patterns of both the dispersion amount and the frequency offset amount.

[0109] In the initial authentication frame transmitted in this embodiment, the dispersion compensation amount and frequency offset amount for the frame are stored within the frame, and the user device notifies the master device of the values ​​at which it was able to receive the frame using an authentication signal, etc. In subsequent communications, the master station device associates the information notified from the user device with user information (user device identification information), and when communicating with the associated user device, performs normal communications as described in the second or third embodiment using the notified values.

[0110] The information on the frames that were decodable by the user equipment may be read and utilized for receiving signal compensation and correction of the local oscillator 41. Figure 18 is a diagram showing an example configuration of a receiving device 4g in a communication system 1g according to the fourth embodiment. The receiving device 4g includes a local oscillator 41, an optical splitter 42, two photodiodes 43-1 and 43-2, a TIA 44, an AD converter 45, and a receiving digital signal processing unit 46g. The receiving digital signal processing unit 46g includes a serial-to-parallel converter 48, an adaptive equalization filter 49, a parallel-to-serial converter 50, an mQAM decoder 51, a quadrature phase signal restoration unit 52a, a sampling clock phase compensation unit 53c, a transmission path characteristic compensation unit 54d, a frequency offset compensation unit 55d, a DA converter 56f, and a stored information reading unit 57g.

[0111] 18 shows, as an example, a configuration in which a stored information reading unit 57g is further included in the configuration of the receiving device 4f shown in FIG. 15. The received digital signal processing unit 46g does not necessarily have to include at least one of the sampling clock phase compensator 53c, the transmission path characteristics compensator 54d, the frequency offset compensator 55d, or the DA converter 56f. The stored information reading unit 57g reads, from a memory (not shown), information on either the dispersion compensation amount or the frequency offset amount of a frame decoded by the user equipment. The stored information reading unit 57g outputs the read information on either the dispersion compensation amount or the frequency offset amount to the transmission path characteristics compensator 54d or the frequency offset compensator 55d, thereby utilizing it for receiving signal compensation and correction of the local oscillator 41.

[0112] If frequency correction of the local oscillator 41 or dispersion compensation is performed only on the receiving side, the master station device will not be able to receive the compensated signal, so the master station device notifies the master station device whether or not correction or compensation is performed on the receiving side, and the master station device switches to an operation where compensation is not performed when communicating with the user device. Note that if frequency correction of the local oscillator 41 is not performed and information is used for compensation in the frequency offset compensator 55d in signal processing, transmission compensation must be performed to ensure IQ orthogonality.

[0113] 19 is a diagram showing an example of the hardware configuration of the communication device 5 in each embodiment. The example of the hardware configuration of the communication device 5 corresponds to the example of the hardware configuration of each of the transmitting device and receiving device in each embodiment.

[0114] The communication device 5 is realized as software by a processor 101, such as a CPU (Central Processing Unit), executing a program stored in a storage device 103 having a non-volatile recording medium (non-transitory recording medium) and a memory 102. The program may be recorded on a computer-readable recording medium. Examples of computer-readable recording media include portable media such as a flexible disk, a magneto-optical disk, a ROM (Read Only Memory), and a CD-ROM (Compact Disc Read Only Memory), and non-transitory recording media such as a hard disk or a solid state drive (SSD) built into a computer system. The communication unit 104 executes predetermined communication processing.

[0115] The communication device 5 may be realized using hardware including an electronic circuit (electronic circuit or circuitry) using, for example, an LSI (Large Scale Integrated circuit), an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array).

[0116] Although an embodiment of the present invention has been described in detail above with reference to the drawings, the specific configuration is not limited to this embodiment, and includes designs within the scope of the gist of the present invention.

[0117] The present invention is applicable to communication systems such as optical communication systems and wireless communication systems.

[0118] REFERENCE SIGNS LIST 1, 1a, 1b, 1c, 1d, 1e, 1f, 1g...communication system, 2a, 2e, 2f...transmitting device, 3...transmission path, 4a, 4b, 4c, 4d, 4f, 4g...receiving device, 21a, 21e, 21f...transmitting digital signal processing unit, 22...DA converter, 23...light source, 24...DP-IQ modulator, 41...local oscillator light source, 42...optical splitter, 43, 43-1 to 43-2...PD, 44...TIA, 45...AD converter, 46a, 46b, 46c, 46d, 46f, 46g...receiving digital signal processing unit, 48...serial-to-parallel conversion unit, 49, 49c...adaptive equalization filter, 50...parallel-to-serial conversion unit, 51...mQAM decoding unit, 52a...quadrature phase signal restoration unit 53b, 53c...Sampling clock phase compensation unit, 54d...Transmission path characteristic compensation unit, 55d...Frequency offset compensation unit, 56f...DA converter, 57g...Stored information reading unit, 211...mQAM encoding unit, 212...Polarization time block encoding unit, 213a, 213a-1 to 213a-2...Replication unit, 214a...Quadrature phase signal generation unit, 215a, 215a-1 to 215a-2...Parallel-serial conversion unit, 216a, 216a-1 to 216a-2...Phase shift unit, 217e...Compensation coefficient calculation and storage unit, 218a, 218e-1 to 218e-2...Transmission path characteristic compensation unit, 220f...Phase rotation amount calculation and storage unit, 221f, 221f-1 to 221f-2...Frequency offset compensation unit

Claims

1. A transmitting device comprising: a polarization time block coding unit that generates dual polarization phase-modulated signals by performing space-time coding on a transmission signal; a quadrature signal generation unit that generates, for each polarization, a quadrature signal of the dual polarization phase-modulated signals generated by the polarization time block coding unit; and a transmitting unit that alternately transmits the dual polarization phase-modulated signals and the quadrature signals of each polarization on the time axis.

2. The transmitting device according to claim 1, wherein the two polarization phase-modulated signals include a polarization phase-modulated signal of an X polarization and a polarization phase-modulated signal of a Y polarization, and further comprising: a first duplicating unit that duplicates the polarization phase-modulated signal of the X polarization; a second duplicating unit that duplicates the polarization phase-modulated signal of the Y polarization; a first parallel-to-serial converting unit that performs parallel-to-serial conversion on the polarization phase-modulated signal of the X polarization and the quadrature phase signal of the polarization phase-modulated signal of the X polarization generated by the quadrature phase signal generating unit; and a second parallel-to-serial converting unit that performs parallel-to-serial conversion on the polarization phase-modulated signal of the Y polarization and the quadrature phase signal of the polarization phase-modulated signal of the Y polarization generated by the quadrature phase signal generating unit.

3. The transmitting device according to claim 2, wherein the quadrature signal generation unit generates a quadrature signal of the X-polarized polarization phase-modulated signal by adjusting the phase of a portion of the X-polarized polarization phase-modulated signal copied by the first duplicating unit so that the phase of the portion is orthogonal to the phase of the other X-polarized polarization phase-modulated signals, and generates a quadrature signal of the Y-polarized polarization phase-modulated signal by adjusting the phase of a portion of the Y-polarized polarization phase-modulated signal copied by the second duplicating unit so that the phase of the portion is orthogonal to the phase of the other Y-polarized polarization phase-modulated signals.

4. The transmitting device according to claim 2 or 3, further comprising: a first transmission path characteristic compensation unit that compensates for transmission path characteristics of the output signal of the first parallel-to-serial conversion unit; and a second transmission path characteristic compensation unit that compensates for transmission path characteristics of the output signal of the second parallel-to-serial conversion unit.

5. The transmitting device according to claim 4, wherein the first transmission path characteristic compensating unit and the second transmission path characteristic compensating unit compensate for the transmission path characteristics by switching the filter coefficients set in the filters for compensating for the transmission path characteristics depending on the communication partner.

6. The transmitting device according to claim 2 or 3, further comprising: a first frequency offset compensating unit that compensates for a frequency offset in either the X-polarized polarization phase-modulated signal or the quadrature-phase signal of the X-polarized polarization phase-modulated signal; and a second frequency offset compensating unit that compensates for a frequency offset in either the Y-polarized polarization phase-modulated signal or the quadrature-phase signal of the Y-polarized polarization phase-modulated signal.

7. The transmitting device according to claim 6, wherein the first frequency offset compensating unit and the second frequency offset compensating unit compensate for the frequency offset by switching the amount of phase rotation depending on the communication partner.

8. A communication system comprising a transmitting device and one or more receiving devices, wherein the transmitting device comprises: a polarization time block coding unit that generates dual polarization phase-modulated signals by performing space-time coding on the transmission signal; a quadrature phase signal generation unit that generates, for each polarization, a quadrature phase signal of the dual polarization phase-modulated signals generated by the polarization time block coding unit; and a transmitting unit that alternately transmits the dual polarization phase-modulated signals and the quadrature phase signals of each polarization on the time axis; and the one or more receiving devices comprise: a quadrature phase signal restoration unit that receives the signal transmitted from the transmitting device and restores two consecutive symbols of the received signal as a real axis component signal and an imaginary axis component signal; and a receiving digital signal processing unit that restores the transmission signal based on the restored real axis component signal and imaginary axis component signal.

9. A transmission method comprising: generating dual polarization phase-modulated signals by performing space-time coding on a transmission signal; generating quadrature phase signals of the generated dual polarization phase-modulated signals for each polarization; and alternately transmitting the dual polarization phase-modulated signals and the quadrature phase signals of each polarization on the time axis.

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