CN116260523A - A Simplified Homologous Coherent System Based on Alamouti Coding - Google Patents

Abstract

The invention relates to a simplified homologous coherent system based on Alamouti coding. By utilizing Alamouti coding in combination with homologous coherence techniques, a simplification of the conventional coherence system is achieved. In terms of system architecture, the invention can replace the mixer with a low-cost coupler and save half the number of BPDs. In addition, the local oscillator laser at the receiving end can be saved by adopting the homologous coherence technology. In the aspect of DSP complexity, the receiving end does not need to process frequency offset and phase noise, and the complexity of the receiving end DSP is reduced to a great extent. In addition, the invention can recover the complete complex signal by only receiving the real part of X, Y polarized signal, so the IQ imbalance problem of the receiving end does not exist. The method for Alamouti coding the real part and the imaginary part of the signal can effectively avoid the IQ imbalance damage of the transmitting end. Therefore, the invention can realize a simplified coherent system insensitive to IQ imbalance damage of the transmitting end.

Description

A Simplified Homologous Coherent System Based on Alamouti Coding

technical field

The present invention relates to the technical field of high-speed optical signal processing, and more specifically, relates to a simplified homologous coherent system based on Alamouti coding.

Background technique

With the development of new broadband services such as the Internet of Things, cloud computing, and virtual reality, short-distance data center optical interconnection puts forward higher requirements for transmission capacity, while being constrained by cost and power consumption. Although the traditional intensity modulation direct detection (IM/DD) system has advantages in cost, power consumption and system complexity, it cannot realize multi-dimensional modulation and is affected by frequency selective fading caused by dispersion, which limits the system Transmission rate and transmission distance. In contrast, coherent systems have better receiving sensitivity and linearity, and can achieve multi-dimensional modulation. However, the traditional coherent system has the problems of high cost and system complexity. For data centers that are sensitive to cost and power consumption, it cannot be directly applied, and the traditional coherent system needs to be simplified. In order to realize a low-cost, low-complexity coherent system, some schemes have been proposed:

(1) Analog coherent technology: This method mainly uses an optical phase-locked loop to lock the frequency and phase of the local oscillator laser at the receiving end with the signal carrier transmitted from the transmitting end, so as to realize the same frequency of the transmission signal carrier and the local oscillator carrier In-phase, thus simplifying or even omitting the processing of frequency offset and phase noise at the receiving end. In this scheme, an optical frequency-locked phase-locked module is required at the receiving end, including a photodetector (PD), a mixer, a loop filter, a reference clock, a phase modulator, a band-pass filter, and the like. It can be seen that although this solution can simplify the complexity of the DSP at the receiving end, an additional optical module needs to be added at the receiving end, so the simplification brought by this solution still has certain limitations.

(2) Homologous coherent technology: This method mainly divides the optical carrier at the transmitting end into two, one is used to realize the modulation of the optical signal, and the other is transmitted to the receiving end through an additional optical fiber, which is used as a local oscillator light source. Beat frequency with the transmitted signal. Since the modulated signal and the transmission carrier are generated by the same light source, when the length of the transmission fiber of the signal and the carrier match, they can achieve strict same frequency and phase, so there is no need to process frequency offset and phase noise at the receiving end. This solution simplifies the DSP complexity of the receiving end by adding an additional transmission fiber, and the receiving end does not require a local oscillator laser. In the current solution, the receiver does not need a local oscillator laser, but still needs a traditional dual-bias coherent receiver. And in this solution, the signal will be damaged by the IQ imbalance at the transmitter and receiver at the same time, which will affect the transmission performance. Generally, complex algorithms need to be used to compensate for the IQ imbalance damage at the transceiver.

Analog coherent technology requires an additional optical frequency-locked phase-locked module to achieve frequency and phase locking of the local oscillator laser, so this solution is to a certain extent achieved by increasing the complexity of the traditional coherent system architecture to realize the DSP algorithm at the receiving end The complexity is simplified, and the reference clock in the module still has a certain phase noise, so it is difficult to achieve strict frequency offset and phase locking. In homologous coherent technology, except that the optical carrier of the local oscillator is replaced by the light source at the transmitting end of optical fiber transmission, the rest of the structure is basically the same as that of the traditional coherent system. Therefore, there will be an IQ imbalance problem at both the receiving and receiving ends, which seriously affects the transmission performance of the system, and complex digital signal processing algorithms need to be used to compensate.

Contents of the invention

In order to overcome the above-mentioned defects in the prior art, the present invention provides a simplified homologous coherent system based on Alamouti coding, which can effectively avoid the damage of IQ imbalance at the transmitting end, and can also effectively reduce the cost.

In order to solve the above-mentioned technical problems, the technical solution adopted in the present invention is: a signal processing method based on Alamouti coded simplified homologous coherent system, in the digital signal processing DSP at the transmitting end, at first the pseudo-random bit sequence is mapped into QAM symbols, Then generate two Orthogonal Frequency Division Multiplexing OFDM signals, perform Alamouti encoding on the real and imaginary parts of the generated OFDM signals, and add a cyclic prefix before each frame of OFDM signals; Perform dispersion pre-compensation; in the digital signal processing DSP at the receiving end, first synchronize the received signals of X polarization and Y polarization, then remove the cyclic prefix, then use the training sequence and Alamouti decoding to obtain channel parameters, and equalize the signal , and finally perform QAM inverse mapping on the equalized data, and calculate the bit error rate of the system.

Based on the homologous coherent system, the present invention further proposes a simplified coherent scheme based on Alamouti coding, so as to realize the simplification of the coherent receiver and solve the IQ imbalance problem in the coherent system. The present invention performs Alamouti coding on the real part and imaginary part of the IQ signal at the transmitting end, so that the receiving end can recover a complete complex signal only by receiving the real part or the imaginary part of the signal. Therefore, the receiving end can replace the 90° mixer in the traditional coherent system with a low-cost 2×2 coupler, and can reduce the number of balanced photodetectors (BPD) by half, and realize the traditional coherent system to a large extent. Simplification of system architecture. In addition, since the proposed scheme only needs to receive the real part of the signal at the receiving end, there is no IQ imbalance problem at the receiving end, which can be effectively solved by Alamouti coding at the transmitting end.

Further, the transmitting end divides the light source emitted by the laser into two, one is used for signal modulation, and the other is transmitted to the receiving end as a local oscillator light source, the transmitting end generates two OFDM signals, and after passing through the digital-to-analog converter, Loaded into a dual-biased IQ modulator and modulated to X and Y polarizations respectively; after being transmitted through a single-mode fiber, an adjustable optical attenuator is used to adjust the received power of the signal; two polarization beam splitters are used at the receiving end to transmit the The signal and the carrier are divided into X and Y polarizations respectively. The X polarization component of the signal and the X component of the carrier are coupled through a 2×2 coupler and then received by a BPD to obtain the real part of the signal modulated to the X polarization; similarly , the Y polarization component of the signal and the Y component of the carrier are coupled by a 2×2 coupler and then received by a BPD, and the real part of the signal modulated to the Y polarization is obtained; the signal detected by the BPD is sent by an analog-to-digital converter Carry out analog-to-digital conversion, and then be processed by the digital signal processing DSP at the receiving end.

Further, the Alamouti encoding specifically includes: for the OFDM signal, the information at the first moment is repeatedly placed at the second moment, wherein the I-channel signal at the second moment is the negative value of the Q-channel signal at the first moment, and at the second moment The Q channel signal is the Q channel signal at the first moment, the imaginary part of the signal at the first moment is obtained from the real part of the signal at the second moment, and the receiving end only needs to receive the real part signal.

Further, when there is IQ imbalance damage at the transmitting end, the relationship between the received signal and the transmitted signal is expressed as follows:

Among them, s _XI1 , s _XQ1 , s _YI1 and s _YQ1 represent the real and imaginary signals of the X and Y polarizations transmitted at the first moment, and -s _XQ1 , s _XI1 , -s _YQ1 and s _YI1 represent the signals transmitted at the second moment The signals of the real and imaginary parts of X, Y polarization; s' _XI1 , s' _XI2 , s' _YI1 and s' _YI2 are expanded in the frequency domain as follows:

S' _XI1 ＝H _XI-XI S _XI1 +H _XQ-XI S _XQ1 +H _YI-XI S _YI1 +H _YQ-XI S _YQ1 (2)

S' _XI2 ＝-H _XI-XI S _XQ1 +H _XQ-XI S _XI1 -H _YI-XI S _YQ1 +H _YQ-XI S _YI1 (3)

S' _YI1 ＝H _XI-YI S _XI1 +H _XQ-YI S _XQ1 +H _YI-YI S _YI1 +H _YQ-YI S _YQ1 (4)

S' _YI2 ＝-H _XI-YI S _XQ1 +H _XQ-YI S _XI1 -H _YI-YI S _YQ1 +H _YQ-YI S _YI1 (5)

By calculating the channel parameters H _XI-XI , H _XQ-XI , H _YI-XI , H _YQ -XI , H _XI-YI , H _XQ-YI , H _YI-YI and H _YQ-YI in the above formula , that is, the transmitted signal can be recovered, and the equalization of the channel and the compensation of the IQ imbalance at the transmitting end can be realized.

Further, the calculation method of the channel parameters includes: constructing the training sequence, taking the Alamouti coding block as the unit, and at the first and second moments, only placing a code block after Alamouti coding in the X polarization, and then In the next two moments, only one encoded code block is placed on the Y polarization; then the relationship between the received training sequence and the transmitted training sequence is expressed as follows:

Expand the above formula in the frequency domain to get the following four sets of equations:

Use the training sequence to obtain the required eight channel parameters according to the relationship of the above four sets of equations; substitute the obtained channel parameters into the formulas (2)-(5), and use the zero-forcing algorithm to realize the channel equalization and summing of the received signal Compensation for IQ imbalance at the transmitter.

Compared with the proposed homologous coherent scheme, the present invention uses Alamouti coding on the real part and imaginary part of the transmitted signal on this basis, so that the receiving end only needs to receive the real part signals of X and Y polarizations to recover the Complete complex signal, so that the receiving end can save half the number of BPD, and can replace the 90° mixer with a low-cost 2×2 coupler, effectively reducing the complexity and cost of the receiver. In the present invention, in the OFDM system, the training sequences of X and Y polarizations are placed alternately in time to obtain 8 channel parameters when there is an IQ imbalance at the transmitting end for channel equalization and IQ imbalance at the transmitting end compensation. Therefore, the Alamouti-based simplified coherent system proposed by the present invention can effectively simplify the structure of a traditional coherent receiver, avoid IQ imbalance at the receiving end, and at the same time realize insensitivity to IQ imbalance damage at the transmitting end.

The present invention also provides a simplified homologous coherent system based on Alamouti coding, including a transmitting-end DSP module and a receiving-end DSP module;

The DSP module at the transmitting end is used to map the pseudo-random bit sequence into a QAM symbol, then generate two OFDM signals, perform Alamouti encoding on the real part and the imaginary part of the generated OFDM signal, and A cyclic prefix is added before each frame of OFDM signal; according to the condition of the transmission fiber, dispersion pre-compensation is finally performed at the transmitting end;

The DSP module at the receiving end is used to synchronize the received X-polarized and Y-polarized signals, then remove the cyclic prefix, then use the training sequence and Alamouti decoding to obtain channel parameters, and equalize the signal, and finally equalize the QAM inverse mapping is performed on the data, and the bit error rate of the system is calculated.

Further, the transmitting end divides the light source emitted by the laser into two, one is used for signal modulation, and the other is transmitted to the receiving end as a local oscillator light source, the transmitting end generates two OFDM signals, and after passing through the digital-to-analog converter, Loaded into a dual-biased IQ modulator and modulated to X and Y polarizations respectively; after being transmitted through a single-mode fiber, an adjustable optical attenuator is used to adjust the received power of the signal; two polarization beam splitters are used at the receiving end to transmit the The signal and the carrier are divided into X and Y polarizations respectively. The X polarization component of the signal and the X component of the carrier are coupled through a 2×2 coupler and then received by a BPD to obtain the real part of the signal modulated to the X polarization; similarly , the Y polarization component of the signal and the Y component of the carrier are coupled by a 2×2 coupler and then received by a BPD, and the real part of the signal modulated to the Y polarization is obtained; the signal detected by the BPD is sent by an analog-to-digital converter Perform analog-to-digital conversion, and then be processed by the DSP module at the receiving end.

Further, when the DSP module at the transmitting end performs Alamouti encoding, for the OFDM signal, the information at the first moment is repeatedly placed at the second moment, wherein the I-channel signal at the second moment is the negative value of the Q-channel signal at the first moment , and the Q channel signal at the second moment is the I channel signal at the first moment, the imaginary part of the signal at the first moment is obtained from the real part of the signal at the second moment, and the receiving end only needs to receive the real part signal.

Further, when there is IQ imbalance damage at the transmitting end, the relationship between the received signal and the transmitted signal is expressed as follows:

Among them, s _XI1 , s _XQ1 , s _YI1 and s _YQ1 represent the real and imaginary signals of the X and Y polarizations transmitted at the first moment, and -s _XQ1 , s _XI1 , -s _YQ1 and s _YI1 represent the signals transmitted at the second moment The signals of the real and imaginary parts of X, Y polarization; s' _XI1 , s' _XI2 , s' _YI1 and s' _YI2 are expanded in the frequency domain as follows:

S' _XI1 ＝H _XI-XI S _XI1 +H _XQ-XI S _XQ1 +H _YI-XI S _YI1 +H _YQ-XI S _YQ1 (12)

S' _XI2 ＝-H _XI-XI S _XQ1 +H _XQ-XI S _XI1 -H _YI-XI S _YQ1 +H _YQ-XI S _YI1 (13)

S' _YI1 ＝H _XI-YI S _XI1 +H _XQ-YI S _XQ1 +H _YI-YI S _YI1 +H _YQ-YI S _YQ1 (14)

S' _YI2 ＝-H _XI-YI S _XQ1 +H _XQ-YI S _XI1 -H _YI-YI S _YQ1 +H _YQ-YI S _YI1 (15)

By calculating the channel parameters H _XI-XI , H _XQ-XI , H _YI-XI , H _YQ -XI , H _XI-YI , H _XQ-YI , H _YI-YI and H _YQ-YI in the above formula , that is, the transmitted signal can be recovered, and the equalization of the channel and the compensation of the IQ imbalance at the transmitting end can be realized.

Further, the calculation of the channel parameters includes: constructing the training sequence, taking the Alamouti coding block as the unit, at the first and second moments, only place a code block after Alamouti coding in the X polarization, and in the next In the two moments of , only one encoded code block is placed on the Y polarization; then the relationship between the received training sequence and the transmitted training sequence is expressed as follows:

Expand the above formula in the frequency domain to get the following four sets of equations:

Use the training sequence to obtain the required eight channel parameters according to the relationship of the above four sets of equations; substitute the obtained channel parameters into the formulas (12)-(15), and use the zero-forcing algorithm to realize the channel equalization and sum of the received signals Compensation for IQ imbalance at the transmitter.

Compared with the prior art, the beneficial effect is that the invention realizes the simplification of the traditional coherent system by using the Alamouti coding and combining the homologous coherent technology. Based on the homologous coherent scheme, the present invention can further reduce the complexity of the receiver. In terms of system architecture, compared with traditional coherent receivers, the present invention can use low-cost 2×2 couplers to replace 90° mixers, and save half the number of BPDs. In addition, the adoption of homologous coherent technology can save the local oscillator laser at the receiving end. In terms of DSP complexity, the receiver does not need to process frequency offset and phase noise, which greatly reduces the complexity of the DSP at the receiver. In addition, since the present invention only needs to receive the real part of the X and Y polarized signals to restore the complete complex signal, there is no IQ imbalance problem at the receiving end. However, the method of performing Alamouti coding on the real part and the imaginary part of the signal proposed by the present invention can effectively avoid the damage caused by IQ imbalance at the transmitting end. Therefore, the present invention can realize a simplified coherent system that is insensitive to IQ imbalance damage at the transmitting end.

Description of drawings

In Fig. 1, (a) is the structure of the dual-polarization OFDM signal after Alamouti encoding, and (b) is the structure of the training sequence.

FIG. 2 is a diagram of a simulation device in Embodiment 2 and a schematic flow diagram of a DSP at a transceiver end.

Fig. 3 is a schematic diagram of the influence of the time delay of the transmitting end on the system performance.

Fig. 4 is a schematic diagram showing the impact of amplitude mismatch at the transmitting end on system performance.

Fig. 5 is a schematic diagram showing the influence of phase mismatch at the transmitting end on system performance.

Figure 6 is the bit error rate performance comparison under different optical signal-to-noise ratio conditions (a) optical back-to-back transmission (b) 80km optical fiber transmission.

Detailed ways

The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. The present invention will be described in one of the embodiments below in conjunction with specific embodiments. Wherein, the accompanying drawings are only for illustrative purposes, showing only schematic diagrams, rather than physical drawings, and should not be construed as limitations on this patent; in order to better illustrate the embodiments of the present invention, some parts of the accompanying drawings will be omitted, Enlargement or reduction does not represent the size of the actual product; for those skilled in the art, it is understandable that certain known structures and their descriptions in the drawings may be omitted.

In the description of the present invention, it should be understood that if the orientation or positional relationship indicated by the terms "upper", "lower", "left", "right" etc. is based on the orientation or positional relationship shown in the drawings, only It is for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, so the terms describing the positional relationship in the drawings are only for illustrative purposes , should not be construed as a limitation on this patent, and those of ordinary skill in the art can understand the specific meanings of the above terms according to specific situations. In addition, if there are descriptions involving "first", "second" and so on in the embodiments of the present invention, the descriptions of "first", "second" and so on are only for descriptive purposes, and should not be interpreted as indicating or implying Its relative importance or implicitly indicates the number of technical features indicated. Thus, the features defined as "first" and "second" may explicitly or implicitly include at least one of these features. In addition, the meaning of "and/or" appearing in the whole text includes three parallel schemes, taking "A and/or B" as an example, including scheme A, scheme B, or schemes in which A and B are satisfied at the same time.

Example 1:

This embodiment provides a signal processing method for a simplified homologous coherent system based on Alamouti coding. In the digital signal processing DSP at the transmitting end, the pseudo-random bit sequence is first mapped into a QAM symbol, and then two OFDM symbols are generated. For OFDM signal, the real part and imaginary part of the generated OFDM signal are Alamouti encoded, and a cyclic prefix is added before each frame of OFDM signal; according to the condition of the transmission fiber, dispersion pre-compensation is finally performed at the transmitting end; digital signal at the receiving end In the processing of DSP, the received X-polarized and Y-polarized signals are first synchronized, then the cyclic prefix is removed, and then the channel parameters are obtained by using the training sequence and Alamouti decoding, and the signal is equalized, and finally the equalized data is QAM reversed. Mapping, and calculate the bit error rate of the system.

Figure 1(a) shows the signal structure of dual polarization Orthogonal Frequency Division Multiplexing (OFDM) after Alamouti encoding. As shown in the figure, the information at the first moment will be placed repeatedly at the second moment, where the I-channel signal at the second moment is the negative value of the Q-channel signal at the first moment, and the Q-channel signal at the second moment is the signal at the first moment I signal. Therefore, the imaginary part of the signal at the first moment can be obtained from the real part of the signal at the second moment, so that the receiving end only needs to receive the real part signal, so as to further simplify the structure of the coherent receiver. When there is IQ imbalance damage at the transmitting end, the relationship between the received signal and the transmitted signal can be expressed as follows:

Among them, s _XI1 , s _XQ1 , s _YI1 and s _YQ1 represent the real and imaginary signals of the X and Y polarizations transmitted at the first moment, and -s _XQ1 , s _XI1 , -s _YQ1 and s _YI1 represent the signals transmitted at the second moment The signals of the real part and the imaginary part of the X and Y polarizations; it can be seen from formula (1) that the channel can be expressed as a 4×4 matrix. Since in the proposed system, the receiving end only needs to receive the real part of the signal, it only needs to analyze s' _XI1 , s' _XI2 , s' _YI1 and s' _YI2 in formula (1), s' _XI1 , s' _XI2 , s' _YI1 and s' _YI2 are expanded in the frequency domain as follows:

S' _XI1 ＝H _XI-XI S _XI1 +H _XQ-XI S _XQ1 +H _YI-XI S _YI1 +H _YQ-XI S _YQ1 (2)

S' _XI2 ＝-H _XI-XI S _XQ1 +H _XQ-XI S _XI1 -H _YI-XI S _YQ1 +H _YQ-XI S _YI1 (3)

S' _YI1 ＝H _XI-YI S _XI1 +H _XQ-YI S _XQ1 +H _YI-YI S _YI1 +H _YQ-YI S _YQ1 (4)

S' _YI2 ＝-H _XI-YI S _XQ1 +H _XQ-YI S _XI1 -H _YI-YI S _YQ1 +H _YQ-YI S _YI1 (5)

It can be known from the above formulas that only the channel parameters H _XI-XI , H _XQ-XI , H _YI-XI , H _YQ -XI , H _XI-YI , H _XQ-YI , H _YI-YI and H _{YQ- YI} , the transmitted signal can be restored to achieve channel equalization and compensation for IQ imbalance at the transmitting end.

In order to obtain the above 8 channel parameters, the training sequence is constructed, as shown in Figure 1(b), with the Alamouti code block as the unit, at the first and second moments, only one code block after Alamouti code is placed in the X polarization , and in the next two moments, only one encoded code block is placed on the Y polarization; then the relationship between the received training sequence and the transmitted training sequence is expressed as follows:

Expand the above formula in the frequency domain to get the following four sets of equations:

Use the training sequence to obtain the required eight channel parameters according to the relationship of the above four sets of equations; substitute the obtained channel parameters into the formulas (2)-(5), and use the zero-forcing algorithm to realize the channel equalization and summing of the received signal Compensation for IQ imbalance at the transmitter.

Compared with the proposed homologous coherent scheme, the present invention uses Alamouti coding on the real part and imaginary part of the transmitted signal on this basis, so that the receiving end only needs to receive the real part signals of X and Y polarizations to recover the Complete complex signal, so that the receiving end can save half the number of BPD, and can replace the 90° mixer with a low-cost 2×2 coupler, effectively reducing the complexity and cost of the receiver. In the present invention, in the OFDM system, the training sequences of X and Y polarizations are placed alternately in time to obtain 8 channel parameters when there is an IQ imbalance at the transmitting end for channel equalization and IQ imbalance at the transmitting end compensation. Therefore, the Alamouti-based simplified coherent system proposed by the present invention can effectively simplify the structure of a traditional coherent receiver, avoid IQ imbalance at the receiving end, and at the same time realize insensitivity to IQ imbalance damage at the transmitting end.

Example 2

Fig. 2 has provided the simulation device figure of the present invention and the DSP flow diagram of receiving end. The simulation is realized by co-simulation of MATLAB and VPI. The transmitting end divides the light source emitted by the laser into two, one is used for signal modulation, and the other is transmitted to the receiving end as a local oscillator light source (LO). Two 25GHz OFDM signals are generated by the off-line DSP at the transmitting end. After passing through a digital-to-analog converter (DAC) with a sampling rate of 64GSa/s, they are loaded into a dual-biased IQ modulator (DP-IQM) and modulated to X and Y polarizations respectively. superior. After 80km of single-mode optical fiber transmission, the receiving power of the signal is adjusted by using an adjustable optical attenuator (ROP). The receiving end uses two polarization beam splitters (PBS) to divide the transmitted signal and carrier into X and Y polarizations respectively, and the X polarization component of the signal and the X component of the carrier are coupled by a 2×2 coupler and then received by a BPD , to get the real part of the signal modulated onto the X polarization. Similarly, the real part of the Y-polarized signal can be obtained. The signal detected by the BPD is converted by an analog-to-digital converter (ADC) with a sampling rate of 64GSa/s, and then processed by an off-line DSP at the receiving end.

(1) Transmitter DSP

In the DSP at the transmitting end, the pseudo-random bit sequence is first mapped into 16QAM symbols, and then two 25GHz OFDM signals are generated. The real and imaginary parts of the generated OFDM signal are Alamouti coded, and a cyclic prefix is added before each frame of OFDM signal. For the case of transmission fiber, dispersion pre-compensation is finally performed at the transmitting end.

(2) Receiver DSP

At the receiving end, the received signals of X polarization and Y polarization are first synchronized, and then the cyclic prefix is removed. Then use the training sequence and Alamouti decoding to get the channel parameters, and equalize the signal. Finally, 16QAM inverse mapping is performed on the equalized data, and the bit error rate of the system is calculated.

Result analysis

Based on the simulation setup shown in Figure 2, the performance of the 25GHz OFDM signal in the proposed simplified coherent system is simulated. First, in the case of optical back-to-back transmission, the robustness of the simplified coherent system to the IQ imbalance damage at the transmitter is discussed after the I-channel and Q-channel signals at the transmitter are encoded by Alamouti. The optical signal-to-noise ratio of the system is set to 25dB. Figure 3 shows the influence on the bit error rate of the system when there is only delay deviation at the transmitting end. It can be seen from the figure that with the change of the delay deviation between the I and Q channels, the bit error rates of X polarization and Y polarization remain almost unchanged, and are the same as the bit error rate when there is no delay deviation . It can be seen that Alamouti coding at the transmitting end can effectively avoid delay deviation damage of the transmitter.

Figure 4 shows the impact on system performance when there is amplitude mismatch between I and Q signals at the transmitting end. As shown in Figure 4, the bit error rate remains constant throughout and is the same as it would be without the amplitude mismatch. The simulation results show that the Alamouti coding based on the I and Q signals at the transmitter can make the system insensitive to the amplitude mismatch at the transmitter.

Figure 5 discusses the impact on BER performance in the presence of only transmitter phase mismatch. The simulation results show that with the increase of the degree of transmitter phase mismatch, the bit error rate of the system is basically the same as the bit error rate when there is no phase mismatch. The above results show that the proposed simplified coherent receiver based on Alamouti coding is robust to I, Q delay, amplitude mismatch and phase mismatch impairments at the transmitter.

Figure 6(a) and Figure 6(b) respectively show the performance comparison of bit error rate when there are different degrees of IQ imbalance damage at the transmitting end and when there is no IQ imbalance damage in the case of optical back-to-back transmission and 80km optical fiber transmission . Under the condition of optical back-to-back transmission, set the amplitude mismatch and phase mismatch of the transmitter to 3dB and 10° respectively, compare the bit error rate performance with 4ps, 8ps and 12ps time delay, and give the result that there is no IQ imbalance Bit error rate curve when damaged. It can be seen from Figure 6(a) that when there is a delay of 4ps, an amplitude mismatch of 3dB and a phase mismatch of 10°, the bit error rate curve basically coincides with the curve when there is no IQ imbalance. As the delay increases, the bit error rate performance will gradually deteriorate, but at the threshold of hard decision forward error correction coding, it is still close to the bit error rate performance without IQ imbalance damage. Figure 6(b) shows the bit error rate performance comparison after transmitting 80km optical fiber. At this time, the amplitude mismatch and phase mismatch at the transmitting end are fixed at 3dB and 10°. It can be seen from the figure that when there is a 2ps delay, 3dB amplitude mismatch and 10° phase mismatch, the bit error rate curve basically coincides with the curve when there is no IQ imbalance. When the time delay is 4ps, the BER curve shifts slightly upwards, but at the threshold of hard-decision FEC coding, it is still close to the BER performance when there is no IQ imbalance damage.

In summary, the simulation can verify that the proposed simplified coherent system based on Alamouti coding is robust to the IQ imbalance damage at the transmitter, and at the same time realizes the simplification of the architecture and DSP of the traditional coherent system.

Example 3

This embodiment provides a simplified homologous coherent system based on Alamouti coding, including a transmitting-end DSP module and a receiving-end DSP module;

The DSP module at the transmitting end is used to map the pseudo-random bit sequence into QAM symbols, and then generate two OFDM signals, perform Alamouti encoding on the real and imaginary parts of the generated OFDM signals, and in each frame Add a cyclic prefix before the OFDM signal; according to the condition of the transmission fiber, finally perform dispersion pre-compensation at the transmitting end;

The DSP module at the receiving end is used to synchronize the received signals of X polarization and Y polarization, then remove the cyclic prefix, then use the training sequence and Alamouti decoding to obtain channel parameters, and equalize the signal, and finally perform equalization on the equalized data QAM inverse mapping, and calculate the bit error rate of the system.

Among them, the transmitting end divides the light source emitted by the laser into two, one is used for signal modulation, and the other is transmitted to the receiving end as a local oscillator light source. The transmitting end generates two OFDM signals, which are loaded after passing through the digital-to-analog converter. Into the dual-biased IQ modulator to modulate to the X and Y polarizations respectively; after being transmitted through a single-mode fiber, an adjustable optical attenuator is used to adjust the received power of the signal; the receiving end uses two polarization beam splitters to transmit the signal and the carrier are divided into X and Y polarizations respectively, the X polarization component of the signal and the X component of the carrier are coupled through a 2×2 coupler and then received by a BPD, and the real part of the signal modulated to the X polarization is obtained; similarly, The Y polarization component of the signal and the Y component of the carrier are coupled by a 2×2 coupler and then received by a BPD, and the real part of the signal modulated to the Y polarization is obtained; the signal detected by the BPD is processed by an analog-to-digital converter The analog-to-digital conversion is then processed by the receiver DSP module.

Specifically, when the DSP module at the transmitting end performs Alamouti encoding, for the OFDM signal, the information at the first moment is repeatedly placed at the second moment, where the I-channel signal at the second moment is the negative value of the Q-channel signal at the first moment, and the second The Q channel signal at the second moment is the I channel signal at the first moment, the imaginary part of the signal at the first moment is obtained from the real part of the signal at the second moment, and the receiving end only needs to receive the real part signal.

When there is an IQ imbalance damage at the transmitting end, the relationship between the received signal and the transmitted signal is expressed as follows:

Among them, s _XI1 , s _XQ1 , s _YI1 and s _YQ1 represent the real and imaginary signals of the X and Y polarizations transmitted at the first moment, and -s _XQ1 , s _XI1 , -s _YQ1 and s _YI1 represent the signals transmitted at the second moment The signals of the real and imaginary parts of X, Y polarization; s' _XI1 , s' _XI2 , s' _YI1 and s' _YI2 are expanded in the frequency domain as follows:

S' _XI1 ＝H _XI-XI S _XI1 +H _XQ-XI S _XQ1 +H _YI-XI S _YI1 +H _YQ-XI S _YQ1 (12)

S' _XI2 ＝-H _XI-XI S _XQ1 +H _XQ-XI S _XI1 -H _YI-XI S _YQ1 +H _YQ-XI S _YI1 (13)

S' _YI1 ＝H _XI-YI S _XI1 +H _XQ-YI S _XQ1 +H _YI-YI S _YI1 +H _YQ-YI S _YQ1 (14)

S' _YI2 ＝-H _XI-YI S _XQ1 +H _XQ-YI S _XI1 -H _YI-YI S _YQ1 +H _YQ-YI S _YI1 (15)

By calculating the channel parameters H _XI-XI , H _XQ-XI , H _YI-XI , H _YQ -XI , H _XI-YI , H _XQ-YI , H _YI-YI and H _YQ-YI in the above formula , that is, the transmitted signal can be recovered, and the equalization of the channel and the compensation of the IQ imbalance at the transmitting end can be realized.

The calculation of the channel parameters includes: constructing the training sequence, taking the Alamouti coding block as the unit, at the first and second moments, only place a code block after Alamouti coding at the X polarization, and in the next two moments, Only one code block after encoding is placed on the Y polarization; then the relationship between the received training sequence and the transmitted training sequence is expressed as follows:

Expand the above formula in the frequency domain to get the following four sets of equations:

Use the training sequence to obtain the required eight channel parameters according to the relationship of the above four sets of equations; substitute the obtained channel parameters into the formulas (12)-(15), and use the zero-forcing algorithm to realize the channel equalization and sum of the received signals Compensation for IQ imbalance at the transmitter.

Apparently, the above-mentioned embodiments of the present invention are only examples for clearly illustrating the present invention, rather than limiting the implementation of the present invention. For those of ordinary skill in the art, other changes or changes in different forms can be made on the basis of the above description. It is not necessary and impossible to exhaustively list all the implementation manners here. All modifications, equivalent replacements and improvements made within the spirit and principles of the present invention shall be included within the protection scope of the claims of the present invention.

Claims (10)

1. A signal processing method for a simplified homologous coherent system based on Alamouti coding, characterized in that, in a digital signal processing DSP at a transmitting end, a pseudo-random bit sequence is first mapped into a QAM symbol, and then two orthogonal frequency division multiplexing OFDM signals are generated, the real part and the imaginary part of the generated OFDM signal are Alamouti-coded, and a cyclic prefix is added before each frame of OFDM signal; according to the conditions of the transmission optical fiber, dispersion pre-compensation is finally performed at the transmitting end; in a digital signal processing DSP at a receiving end, the received X-polarization and Y-polarization signals are first synchronized, and then the cyclic prefix is removed, and then the channel parameters are obtained by using a training sequence and Alamouti decoding, and the signal is equalized, and finally the equalized data is subjected to QAM inverse mapping, and the bit error rate of the system is calculated. 2. The signal processing method of the simplified homologous coherent system based on Alamouti coding according to claim 1 is characterized in that the transmitting end divides the light source emitted by the laser into two, one for signal modulation, and the other for transmission to the receiving end as a local oscillator light source, the transmitting end generates two OFDM signals, which are loaded into a dual-bias IQ modulator and modulated to X and Y polarizations respectively after passing through a digital-to-analog converter; after being transmitted through a single-mode optical fiber, an adjustable optical attenuator is used to adjust the received power of the signal; the receiving end uses two polarization beam splitters to divide the transmitted signal and the carrier into X and Y polarizations respectively, the X polarization component of the signal and the X component of the carrier are coupled through a coupler and then received by a BPD to obtain the real part of the signal modulated to the X polarization; similarly, the Y polarization component of the signal and the Y component of the carrier are coupled through a coupler and then received by a BPD to obtain the real part of the signal modulated to the Y polarization; the signal detected by the BPD is converted into digital by an analog-to-digital converter, and then processed by a digital signal processing DSP at the receiving end. 3. According to claim 1, the signal processing method of a simplified homologous coherent system based on Alamouti coding is characterized in that the Alamouti coding specifically includes: for an OFDM signal, the information of the first moment is repeated at the second moment, wherein the I-path signal at the second moment is the negative value of the Q-path signal at the first moment, and the Q-path signal at the second moment is the I-path signal at the first moment, the imaginary part of the signal at the first moment is obtained from the real part of the signal at the second moment, and the receiving end only needs to receive the real part of the signal. 4. The signal processing method of the simplified homologous coherent system based on Alamouti coding according to claim 3 is characterized in that when there is IQ imbalance damage at the transmitting end, the relationship between the received signal and the transmitted signal is expressed as follows:

Among them, s _XI1 , s _XQ1 , s _YI1 and s _YQ1 represent the real and imaginary signals of X and Y polarizations sent at the first moment, -s _XQ1 , s _XI1 , -s _YQ1 and s _YI1 represent the real and imaginary signals of X and Y polarizations sent at the second moment; s' _XI1 , s' _XI2 , s' _YI1 and s' _YI2 are expanded in the frequency domain as follows: S _{' XI1} ＝H _{XI-XI S XI1} +H _{XQ - XI S} S' _XI2 ＝-H _{XI-XI S XQ1 +} H _{XQ - XI S} S _{' YI1} ＝H _{XI-YI S XI1} +H _{XQ - YI S} S' _YI2 ＝-H _XI-YI S _XQ1 +H _XQ-YI S _XI1 -H _YI-YI S _YQ1 +H _YQ-YI S _YI1 (5) By calculating the channel parameters H _XI-XI , H _XQ-XI , H _YI-XI , H _YQ-XI , H _XI-YI , H _XQ-YI , H _YI-YI and H _YQ-YI in the above formula, the transmitted signal can be restored to achieve channel equalization and compensation of IQ imbalance at the transmitting end. 5. According to the signal processing method of the simplified homologous coherent system based on Alamouti coding as claimed in claim 4, it is characterized in that the method for calculating the channel parameters comprises: constructing the training sequence, taking the Alamouti coding block as the unit, placing only one code block after Alamouti coding on the X polarization at the first and second moments, and placing only one code block after coding on the Y polarization at the next two moments; the relationship between the received training sequence and the transmitted training sequence is expressed as follows:

Expanding the above equation in the frequency domain yields the following four sets of equations:

The training sequence is used to obtain the required eight channel parameters according to the relationship between the above four sets of equations; the obtained channel parameters are substituted into formulas (2)-(5), and the zero forcing algorithm is used to achieve channel equalization of the received signal and compensation of the IQ imbalance at the transmitting end. 6. A simplified homologous coherent system based on Alamouti coding, characterized by comprising a transmitting end DSP module and a receiving end DSP module; The transmitting end DSP module is used to map the pseudo-random bit sequence into QAM symbols, then generate two orthogonal frequency division multiplexing OFDM signals, perform Alamouti encoding on the real and imaginary parts of the generated OFDM signals, and add a cyclic prefix before each frame of OFDM signal; finally, perform dispersion pre-compensation at the transmitting end according to the conditions of the transmission optical fiber; The receiving end DSP module is used to synchronize the received X-polarized and Y-polarized signals, remove the cyclic prefix, obtain the channel parameters using the training sequence and Alamouti decoding, equalize the signal, perform QAM inverse mapping on the equalized data, and calculate the system bit error rate. 7. The simplified homologous coherent system based on Alamouti coding according to claim 6 is characterized in that the transmitting end divides the light source emitted by the laser into two, one for signal modulation, and the other is transmitted to the receiving end as a local oscillator light source, the transmitting end generates two OFDM signals, which are loaded into a dual-bias IQ modulator and modulated to X and Y polarizations respectively after passing through a digital-to-analog converter; after being transmitted through a single-mode optical fiber, an adjustable optical attenuator is used to adjust the received power of the signal; the receiving end uses two polarization beam splitters to divide the transmitted signal and the carrier into X and Y polarizations respectively, the X polarization component of the signal and the X component of the carrier are coupled through a coupler and then received by a BPD to obtain the real part of the signal modulated to the X polarization; similarly, the Y polarization component of the signal and the Y component of the carrier are coupled through a coupler and then received by a BPD to obtain the real part of the signal modulated to the Y polarization; the signal detected by the BPD is converted into digital by an analog-to-digital converter and then processed by a DSP module at the receiving end. 8. According to the simplified homologous coherent system based on Alamouti coding as described in claim 6, it is characterized in that when the DSP module at the transmitting end performs Alamouti coding, for the OFDM signal, it repeats the information at the first moment at the second moment, wherein the I-path signal at the second moment is the negative value of the Q-path signal at the first moment, and the Q-path signal at the second moment is the I-path signal at the first moment, the imaginary part of the signal at the first moment is obtained from the real part of the signal at the second moment, and the receiving end only needs to receive the real part of the signal. 9. The simplified homologous coherent system based on Alamouti coding according to claim 8, characterized in that when there is IQ imbalance damage at the transmitting end, the relationship between the received signal and the transmitted signal is expressed as follows:

Among them, s _XI1 , s _XQ1 , s _YI1 and s _YQ1 represent the real and imaginary signals of X and Y polarizations sent at the first moment, -s _XQ1 , s _XI1 , -s _YQ1 and s _YI1 represent the real and imaginary signals of X and Y polarizations sent at the second moment; s' _XI1 , s' _XI2 , s' _YI1 and s' _YI2 are expanded in the frequency domain as follows: S _{' XI1} ＝H _{XI-XI S XI1} +H _{XQ - XI S} S' _XI2 ＝-H _{XI-XI S XQ1 +} H _{XQ - XI S} S _{' YI1 ＝} H _XI-YI S _XI1 +H _{XQ - YI S} S' _YI2 ＝-H _XI-YI S _XQ1 +H _XQ-YI S _XI1 -H _YI-YI S _YQ1 +H _YQ-YI S _YI1 (15) By calculating the channel parameters H _XI-XI , H _XQ-XI , H _YI-XI , H _YQ-XI , H _XI-YI , H _XQ-YI , H _YI-YI and H _YQ-YI in the above formula, the transmitted signal can be restored to achieve channel equalization and compensation of IQ imbalance at the transmitting end. 10. The simplified homologous coherent system based on Alamouti coding according to claim 9 is characterized in that the calculation of the channel parameters comprises: constructing a training sequence, taking an Alamouti coding block as a unit, placing only one Alamouti coded code block on the X polarization at the first and second moments, and placing only one coded code block on the Y polarization at the next two moments; the relationship between the received training sequence and the transmitted training sequence is expressed as follows:

Expanding the above equation in the frequency domain yields the following four sets of equations:

The training sequence is used to obtain the required eight channel parameters according to the relationship between the above four sets of equations; the obtained channel parameters are substituted into formulas (12)-(15), and the zero forcing algorithm is used to achieve channel equalization of the received signal and compensation of the IQ imbalance at the transmitting end.

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