JP2010283509A - Transmitter for optical communication and receiver for optical communication - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a transmitter for optical communication and a receiver for optical communication for a system for optical communication, which reduce a circuit scale of the receiver by adding a known signal line to a transmission signal and reduce a circuit scale by further utilizing a part of a known circuit. <P>SOLUTION: In a transmitter for optical communication for a system for optical communication performing coherent optical communications putting information on an amplitude and a phase of a transmission signal to be transmitted, and performing equalization processing by frequency domain equalization to the transmission signal on a receiving side, a transmission means transmitting the transmission signal includes: a first signal processing means inserting into the transmission signal a first continuous known signal line (PilotA) for an initial stage synchronous acquisition of receiver on a receiver side at the time of a communication start; and a second signal processing means inserting into the interval between the transmission signals periodically a second known signal line (PilotB) for synchronous tracking on the receiver side. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to optical communication for an optical communication system that performs coherent optical communication in which information is transmitted with amplitude and phase of a transmission signal and performs equalization processing by frequency domain equalization on the transmission signal on the reception side. The present invention relates to an optical transmitter and an optical communication receiver.

  For example, in next-generation optical communication related to a polarization multiplexed optical transmission / reception system disclosed in the following patent document, for further high-speed transmission, multiple values that place information on the amplitude and phase of a transmission signal instead of conventional intensity modulation Modulation / demodulation techniques are attracting attention. Optical communication networks are mainly used for long-distance transmission, but waveform distortion due to polarization mode dispersion PMD (Polarization Mode Dispersion) and wavelength dispersion CD (Chromatic Dispersion) becomes a problem.

  Similarly, even in land mobile radio communications where multilevel modulation / demodulation technology has been put to practical use, waveform distortion due to frequency selective fading due to multiple phase propagation paths is a problem, and equalization technology has been studied as a countermeasure. ing. In recent years, studies are underway to apply equalization techniques used in wireless communications to optical communications. In particular, a frequency domain equalization technique FDE (Frequency Domain Equalization) is attracting attention as an equalization technique that can relatively reduce the circuit scale even during high-speed transmission.

In optical communication, as the waveform spread increases due to long-distance transmission, the number of FIR filter stages required for equalization in the time domain in the receiver also increases. Since the FIR filter is a convolution operation, a processing amount stages of FIR filter is N times increases on the order of N ^2. On the other hand, since FDE performs equalization in the frequency domain, time domain convolution can be processed by frequency domain multiplication, and implementation of DFT (Discrete Fourier Transformation) and IDFT (Inverse DFT) is required. However, the circuit scale can be reduced.

JP 2008-263590 A

  Conventionally, when DFT is performed on received data, since the received data is discontinuous, CP (Cyclic Prefix) corresponding to delay dispersion is periodically added to improve data continuity so that there is no problem with DFT. A method of retaining a part is taken. In this case, a decrease in data throughput due to the addition of CP becomes a problem. Therefore, a method of cutting out data having a desired DFT length from the DFT output so as not to lose continuity without adding a CP is also being studied. Therefore, the effective DFT length is shorter than the DFT length mounted on the circuit. That is, it is necessary to mount a DFT having an effective DFT length or more, and this method has a problem that the circuit scale of the DFT increases.

  In particular, the DFT is often implemented by FFT (Fast Fourier Transformation) having a small circuit scale, but the FFT that is relatively easy to implement is a power of two, so the DFT processing is performed. For this purpose, it is necessary to implement an FFT having a size twice the number of assumed input signals.

  Further, in order to satisfy the sampling theorem, the receiver side performs sampling at twice or more the symbol rate. When the FDE is operated before symbol synchronization, an FFT having an input signal number that is several times the number of oversampling is required.

  The present invention makes it possible to reduce the circuit scale of the receiver by adding a known signal sequence to the transmission signal, and further to reduce the circuit scale by diverting a part of the circuit. An object of the present invention is to provide an optical communication transmitter and an optical communication receiver for an optical communication system.

  The present invention relates to optical communication for an optical communication system that performs coherent optical communication in which information is transmitted with amplitude and phase of a transmission signal and performs equalization processing by frequency domain equalization on the transmission signal on the reception side. A transmitter for transmitting the transmission signal, wherein a first known signal sequence for acquiring receiver initial synchronization at the receiver side that is continuous at the start of communication is inserted into the transmission signal; Optical communication, comprising: signal processing means; and second signal processing means for periodically inserting a second known signal sequence between the transmission signals for synchronization tracking on the receiver side. It is in the transmitter etc.

  In this invention, it is possible to reduce the circuit scale of the receiver by adding a known signal sequence to the transmission signal, and it is also possible to reduce the circuit scale by diverting a part of the circuit. An optical communication transmitter and an optical communication receiver for an optical communication system can be provided.

It is a figure which shows an example of the transmission signal transmitted from the time of a communication start with the transmitter for the systems for optical communication by this invention. It is a figure which shows an example of a structure of the transmitter for optical communications in Embodiment 1 of this invention. It is a figure which shows an example of a structure of the receiver for optical communications in Embodiment 1 of this invention. It is a figure which shows an example of a structure of the transmitter for optical communications in Embodiment 2 of this invention. It is a figure which shows an example of a structure of the receiver for optical communications in Embodiment 3 of this invention. It is a figure which shows an example of a structure of the transmitter for optical communications in Embodiment 4 of this invention. It is a figure which shows another example of the transmission signal transmitted from the time of a communication start with the transmitter for the systems for optical communication by this invention.

In the present invention, the transmitting side has a function of transmitting a known signal sequence in addition to the data signal sequence, and the receiver side uses an FDE FFT and IDFT circuit to reproduce a known signal sequence and a replica of the known signal sequence in the frequency domain. Perform correlation calculation. When a known signal sequence is transmitted as a receiver initial synchronization acquisition sequence on the transmission side, initial synchronization is obtained by performing a correlation operation with the known signal sequence in the frequency domain before data detection. Further, frequency deviation estimation, phase estimation, and transmission path estimation are performed from the correlation calculation result of the known signal sequence. The transmission path information is used for FDE weighting coefficient calculation.
When performing polarization multiplexing, timing information, frequency deviation, and phase for each polarization are similarly obtained from the correlation calculation result of the known signal sequence by inserting different known signal sequences for each polarization.
The transmission path information is used for FDE weighting coefficient calculation.

Since the correlation operation with the known signal sequence is performed using the FFT and IFFT shared with the FDE, it is not necessary to prepare a separate correlator and the circuit scale can be reduced.
It is possible to detect the FFT start timing from the correlation calculation result with the known signal series, and it is not necessary to prepare an FFT having a larger number of input signals than necessary, and the circuit scale can be reduced. Further, it is possible to estimate the frequency deviation / phase from the correlation calculation result with the known signal sequence.
When performing polarization multiplexing, it is possible to obtain timing information, frequency deviation, and phase for each polarization from the correlation calculation result of known signal sequences by inserting different known signal sequences for each polarization. It is.
The transmission path information can be used for FDE weighting coefficient calculation.
It is possible to reduce the circuit scale rather than acquiring synchronization by blind operation.

  Hereinafter, a transmitter and a receiver for an optical communication system according to the present invention will be described with reference to the drawings according to each embodiment.

Embodiment 1 FIG.
An optical communication system according to Embodiment 1 of the present invention will be described. In the present invention, in coherent optical communication in which information is transmitted with the amplitude and phase of a transmission signal transmitted, the receiver is applied to a configuration in which the transmission signal is equalized by FDE.

  FIG. 1 shows an example of a transmission signal transmitted from the start of communication by a transmitter for an optical communication system according to the present invention. At the start of transmission, a known signal sequence (Pilot signal) is transmitted in order to capture the initial timing of the receiver and correct the receiver frequency deviation. In this configuration, when the number of input signals of the FFT circuit for the receiver FDE is N (N is a power of 2), the known signal sequence is a repetitive signal having a known sequence length N (N is a natural number). The known signal sequence is a PN (Pseudo Noise) sequence such as an orthogonal M sequence or an orthogonal Gold code. Data (data signal sequence) transmission is started after transmission of the known signal sequence. After starting data transmission, a known signal sequence is periodically inserted for timing tracking of the receiver and frequency deviation / phase tracking. A known signal sequence used for initial synchronization acquisition by the former receiver is Pilot A, and a known signal sequence used for synchronization tracking of the receiver is Pilot B. Pilot B adds L samples after the known signal sequence of sequence length N (signal sequence of sequence length L) to the beginning of the known signal sequence in order to maintain the continuity of the known signal sequence when performing FFT. (L is a natural number, L ≦ N). Note that L has a time length corresponding to delay dispersion during communication.

  An example of the configuration of the optical communication transmitter in the first embodiment is shown in FIG. The mapping unit 201 maps the transmission data (bit sequence) to the IQ complex plane. For example, mapping is performed in accordance with a modulation method such as BPSK (Binary Phase Shift Keying), QPSK (Quadrature Phase Shift Keying), or 16QAM (Quadrature Amplitude Modulation). The selector 202 switches between the data signal and the pilot signal. By the switching operation of the selector 202, the above-described known signal series Pilot A and Pilot B are generated. Pilot signal 1 generation section 203 generates a known signal sequence in the time domain. Since it is a known signal sequence, it can also be stored in a ROM (Read Only Memory). The D / A converters 204 (a) and 204 (b) convert digital signals into analog signals. The EO converters 205 (a) and 205 (b) convert electrical signals into optical signals. The optical oscillator 206 is a source oscillation laser for optical orthogonal modulation. The optical I / Q modulator 207 performs orthogonal modulation on the optical signals from the EO converters 205 (a) and 205 (b) at the frequency of the optical oscillator 206 and outputs the result.

  An example of the configuration of the optical communication receiver in the first embodiment is shown in FIG. The A / D converter 301 converts an analog signal input from the optical front end into a digital signal. An OE converter (not shown) that receives an optical signal from a transmitter and converts it into an analog electrical signal is provided at the front stage of the optical front end. The frequency converter 302 converts the digital signal into a baseband signal. If the input of the A / D converter 301 is an IF (Intermediate Frequency) signal, the signal is converted into a complex signal I / Q (In-Phase / Quadrature Phase) by orthogonal demodulation at the frequency converter 302. In addition, it is possible to have two A / D converters 301 and make an I / Q signal already at the AD input terminal. A solid line in FIG. 3 indicates a complex signal.

  The timing control unit 303 has a function of delaying the FFT input signal in accordance with the FFT start timing, which is detected by a later circuit described later. The FFT circuit 304 is an FFT with N input signals, and performs a fast Fourier transform. The complex multiplier 305 performs complex multiplication with the frequency domain weighting coefficient calculated to perform frequency domain equalization. The selector 306 switches between a data signal (data signal sequence) and a pilot signal (known signal sequence) in the transmission signal. The weighting factor calculation unit 307 calculates a weighting factor for FDE from the data signal or the pilot signal. An MMSE (Minimum Mean Square Error) algorithm or an LMS (Least Mean Square) algorithm is used for the calculation of the weighting coefficient. The IFFT circuit 308 performs an inverse Fourier transform. The selector 309 switches between a data signal and a pilot signal, similarly to the selector 306. The complex multiplier 310 performs complex multiplication with a PN code replica (frequency domain) generated by a PN code replica (frequency domain) generator 311 (or a ROM (Read Only Memory) storing the PN code replica (frequency domain)). I do. The timing detector 312 detects the data transmission start timing and the FFT start timing from the IFFT result of the Pilot signal. The frequency deviation detector 313 detects the frequency deviation from the IFFT result of the Pilot signal and outputs it to the frequency converter 302.

  Note that each selector of the present invention has a function of switching according to a predetermined signal or state (the same applies hereinafter).

  The operation in the first embodiment will be described. The transmitter starts to transmit Pilot A at the start of communication. Data transmission is started after the Pilot A PN code is repeated a predetermined number of times.

  On the receiver side, the selector 306 is in the initial state set to the complex multiplier 310 side with the PN code replica generated by the PN code replica (frequency domain) generator 311 and the selector 309 is set to the timing detector 312 side. Start initial capture of. If Pilot A has arrived as expected, the autocorrelation waveform of the PN code can be observed with the IFFT output, and a signal is detected by determining the threshold value of the peak value. This function is performed by the timing detection unit 312.

  Further, the timing detection unit 312 acquires the FFT start timing from the peak position of the autocorrelation waveform, and the frequency deviation detection unit 313 detects the frequency deviation from the phase of the autocorrelation waveform. The timing detection unit 312 detects the data transmission start timing by observing the correlation waveform exceeding the signal detection threshold for a predetermined number of times, and starts processing the data by combining the FFT start timing and the data transmission start timing. . At the start of data processing, the selector 306 is set to the complex multiplication unit 305 side with the FDE weight coefficient, and the selector 309 is set to the LLR (Log Likelihood Ratio) calculation side.

  The data after synchronous detection by FDE is output to an LLR unit (not shown) and a log likelihood ratio is calculated, and then decoded by a decoding processing unit (not shown). Note that the insertion of the initial synchronization acquisition sequence Pilot A is not indispensable, and even acquisition of synchronization tracking Pilot B requires time, but acquisition of synchronization is possible.

Embodiment 2. FIG.
An optical communication system according to Embodiment 2 of the present invention will be described. An example of the configuration of the optical communication transmitter in the second embodiment is shown in FIG. The same or corresponding parts as those in the above embodiment are denoted by the same reference numerals and description thereof will be omitted (the same applies hereinafter). 4 differs from the first embodiment shown in FIG. 2 in the configuration of a pilot signal 1 generation unit (ROM 1) 203. In FIG. In FIG. 4, 401 is a short code generation unit, 402 is a long code generation unit, and multipliers 403 (a) and 403 (b) multiply a short code and a long code to generate a pilot signal.

  In the second embodiment, with the above configuration, the pilot A in the first embodiment is a long code (signal sequence) of sequence length N × M (N and M are natural numbers) obtained by repeating a known signal sequence of sequence length N M times. In addition, a signal sequence obtained by repeatedly multiplying a short code (known signal sequence) having a sequence length N is used, and data transmission is started after transmission of the long code. The configuration on the receiver side is basically the same as that shown in FIG. 3, and the start of data transmission is detected from the phase of the long code being processed on the receiver side. If the short code and long code are binary sequences of 0 and 1, the above multiplication is exclusive OR.

Embodiment 3 FIG.
An optical communication system according to Embodiment 3 of the present invention will be described. An example of the configuration of the optical communication receiver in the third embodiment is shown in FIG. In FIG. 5, the following circuit is added to the optical communication receiver shown in FIG. The complex multiplier 501 performs complex multiplication of the PN code replica (time domain) after timing control and the received signal (Pilot signal). The selector 502 is set (switched) to the complex multiplier 501 side at the timing of receiving the Pilot signal. The switching timing of the selector 502 is set based on the FFT start timing detected by the timing detection unit 312. The PN timing control unit 503 controls the timing at which the complex multiplier 501 multiplies the received signal by the PN code. The timing is given from the timing detector 312. Reference numeral 504 denotes a PN code replica (time domain) generator (a ROM (Read Only Memory) storing a PN code replica (time domain) is also possible). In the case of a ROM, a PN code replica (time domain) may be stored together in the ROM 311. Further, the configuration on the transmitter side may basically be that of any of the above-described embodiments shown in FIGS.

  The operation in the third embodiment will be described. In the third embodiment, after obtaining the FFT start timing by the means described in the first embodiment, the selector 306 is set on the complex multiplier 305 side with the FDE weight coefficient, and the complex multiplication is performed during the reception of the pilot signal. 501, PN timing controller 503, and PN code replica (time domain) generator 504 are operated. With this operation, the FDE weighting factor calculation unit 307 can perform weighting factor calculation on the pilot signal portion in which the pilot modulation pattern is solved, and the processing amount of the weighting factor calculation unit 307 can be reduced.

Embodiment 4 FIG.
An optical communication system that performs polarization multiplexing transmission according to Embodiment 4 of the present invention will be described. An example of the configuration of the optical communication transmitter in the fourth embodiment is shown in FIG. As shown in FIG. 6, the configuration basically includes two transmission systems of the first embodiment shown in FIG. Although the transmission system of FIG. 1 is provided for each polarization, the case of two polarizations is shown here. The selector 701 selects transmission data to be mapped for each polarization. Pilot signal 2 generation section 703 (which can be configured by ROM) generates a pilot signal different from pilot signal 1 generation section 203. The pilot signal 1 generation unit 203 and the pilot signal 2 generation unit 703 can generate by changing only the initial shift amount of one primitive polynomial from two identical primitive polynomials having a preferred pair (better combination) relationship. For example, two Gold codes are assigned. Further, the configuration on the receiver side may basically be, for example, the configuration of any of the above-described embodiments shown in FIGS. 3 and 5 provided for each polarization.

  The operation in the fourth embodiment will be described. The fourth embodiment is premised on a MIMO (Multiple Input Multiple Output) environment including polarization multiplexing when applied to optical communication and to wireless communication. As shown in FIG. 7, when two transmission sequences are polarization-multiplexed, each pilot signal is multiplied by a different PN code, so that the receiver can easily separate the polarization in the pilot signal section. Become. For example, by assigning two Gold codes that can be generated by changing only the initial shift amount of one of the primitive polynomials from two identical primitive polynomials that have a preferred pair relationship to the Pilot signal, Polarization separation is possible. The pilot signal after separation facilitates transmission path estimation, frequency deviation estimation, phase estimation, and timing estimation for each polarization.

  The A / D converter (301) on the receiver side may perform oversampling more than twice in order to satisfy the sampling theorem. In this case, the number of FFT input signals becomes 2N, and the input signal is doubled. It is necessary to implement an FFT with a number. By detecting the FFT start timing based on the Pilot signal before starting the reception data detection processing, the sampling frequency can be decimated to ½, and the FFT circuit scale can be reduced.

  In each of the above embodiments, the above FFT circuit and IFFT circuit can be replaced with a DFT circuit and an IDFT circuit, respectively (the DFT circuit and the IDFT circuit include the FFT circuit and the IFFT circuit, respectively). is there). Further, although each of the above embodiments has been described on the assumption that it is applied to optical communication, it can also be applied to wireless communication using radio waves. Further, the present invention is not limited to the above embodiments, and includes all possible combinations of these embodiments.

  In each embodiment, the first and second signal processing means include a selector 202, a pilot signal 1 generation unit 203, a short code generation unit 401, a long code generation unit 402, and multipliers 403 (a) and 403 ( b), the pilot signal 2 generator 703 and the like. The discrete Fourier transform means is constituted by an FFT circuit 304 and a DFT circuit. The complex multiplier 310 includes multiplication means 310 that multiplies the known signal sequence transmitted to the known signal sequence after the discrete Fourier transform by a replica signal of the known signal sequence in the frequency domain obtained by performing the discrete Fourier transform. Further, the discrete inverse Fourier transform means is constituted by an IFFT circuit 308 and an IDFT circuit. Further, a timing detection / control unit that detects the reception timing of the known signal sequence from the signal after the discrete inverse Fourier transform and controls the input to the discrete Fourier transform unit includes a timing detection unit 312 and a timing control unit 303. . Also, a weighting factor calculation / accumulation means for calculating a weighting factor for frequency domain equalization from the transmission signal after the discrete Fourier transform and integrating the weighting factor on the transmission signal after the discrete Fourier transform includes a weighting factor calculation unit 307 and a complex multiplier 305. Consists of. Then, based on the reception timing of the known signal sequence detected by the timing detection / control unit, an arithmetic unit that starts complex multiplication in the time domain of the replica signal of the same sequence as the known signal sequence and the transmitted known signal sequence, A complex multiplier 501, a selector 502, a PN timing control unit 503, and a PN code replica (time domain) generator 504 are configured.

  201 mapping unit, 202 selector, 203 Pilot signal 1 generation unit, 204 (a), 204 (b) D / A converter, 205 (a), 205 (b) EO converter, 206 optical oscillator, 207 optical I / Q modulator, 301 A / D converter, 302 frequency converter, 303 timing controller, 304 FFT circuit, 305 complex multiplier, 305 complex multiplier, 306 selector, 307 weight coefficient calculator, 308 IFFT circuit, 309 selector , 310 complex multiplier, 311 PN code replica (frequency domain) generator, 312 timing detector, 313 frequency deviation detector, 401 short code generator, 402 long code generator, 403 (a), 403 (b) multiplication 501 Complex multiplier 502 Selector 503 PN timing controller 504 PN code Rika (time domain) generator, 701 a selector, 703 Pilot signal 2 generating unit.

Claims (7)

An optical communication transmitter for an optical communication system that performs coherent optical communication that transmits information with amplitude and phase of a transmission signal and performs equalization processing by frequency domain equalization on the transmission signal on the receiving side. There,
Transmission means for transmitting the transmission signal,
First signal processing means for inserting a first known signal sequence for acquisition of receiver initial synchronization at the receiver side continuous at the start of communication into the transmission signal;
Second signal processing means for periodically inserting a second known signal sequence between the transmission signals for synchronization tracking at the receiver side;
A transmitter for optical communication, comprising: Light for optical communication system that performs coherent optical communication that transmits information with amplitude and phase of a transmission signal and performs polarization multiplexing transmission that performs equalization processing by FDE on the transmission signal on the reception side A transmitter for communication,
Transmission means for each polarization for transmitting the transmission signal,
First signal processing means for inserting a first known signal sequence for acquisition of initial receiver synchronization at the receiver side continuous at the start of communication into the transmission signal;
Second signal processing means for periodically inserting a second known signal sequence between the transmission signals for synchronization tracking on the receiver side;
A transmitter for optical communication, comprising:   3. The optical communication transmitter according to claim 1, wherein the first known signal sequence is a repetitive signal of a known signal sequence having a sequence length N (N is a natural number).   The second known signal sequence has a length of a sequence length L (L is a natural number (L ≦ N)) corresponding to a delay dispersion at the time of communication from the end of the known signal sequence to a known signal sequence of sequence length N. 3. The transmitter for optical communication according to claim 1, wherein a signal sequence is cut out and added to the head of the known signal sequence having the sequence length N.   The first signal processing means uses the first known signal sequence as a sequence length N × M signal sequence obtained by repeating a known signal sequence of sequence length N M times (N and M are natural numbers), and a sequence length of N Repeating and multiplying the known signal sequence, or calculating the exclusive OR of the signal sequence and the known signal sequence when the signal sequence and the known signal sequence are binary sequences indicated by 1 and 0 The transmitter for optical communication according to claim 1, wherein the transmitter is obtained. An optical communication receiver for an optical communication system that performs coherent optical communication that transmits information with amplitude and phase of a transmission signal and performs equalization processing by frequency domain equalization on the transmission signal on the reception side. There,
Receiving means for receiving a transmission signal and performing a correlation operation between a frequency domain equalization and a known signal sequence included in the transmission signal, or receiving means for each polarization when performing polarization multiplexing transmission,
Discrete Fourier transform means used for the frequency domain equalization for performing discrete Fourier transform on the received signal;
Multiplication means for multiplying the known signal sequence after the discrete Fourier transform by a replica signal of the known signal sequence in the frequency domain obtained by performing discrete Fourier transform on the transmitted known signal sequence;
Discrete inverse Fourier transform means also used for the frequency domain equalization for performing discrete inverse Fourier transform on the known signal sequence after multiplication;
Timing detection / control means for detecting the reception timing of the known signal sequence from the signal after the discrete inverse Fourier transform and controlling the input to the discrete Fourier transform means;
A receiver for optical communication, comprising: Weighting factor calculation / integration means for calculating a weighting factor for frequency domain equalization from the transmission signal after the discrete Fourier transform and integrating the weighting factor for the transmission signal after the discrete Fourier transform;
Based on the reception timing of the known signal sequence detected by the timing detection / control unit, an arithmetic unit that starts complex multiplication in the time domain of the known signal sequence and the replica signal of the same sequence as the transmitted known signal sequence;
The optical communication receiver according to claim 6, further comprising:

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