A simplified homodyne system based on alamouti encoding

Abstract

The application relates to a simplified homodyne coherent system based on Alamouti coding. By utilizing Alamouti coding and combining with a homodyne technology, the traditional coherent system is simplified. In terms of system architecture, a low-cost coupler can be used to replace a frequency mixer, and half the number of BPDs is saved. In addition, the adoption of the homodyne technology can save the local oscillator laser at the receiving end. In terms of DSP complexity, the receiving end does not need to process frequency offset and phase noise, greatly reducing the complexity of the receiving end DSP. In addition, since the application only needs to receive the real part of the X and Y polarization signals to recover the complete complex signal, there is no IQ imbalance problem at the receiving end. The Alamouti coding mode of the real part and the imaginary part of the signal can effectively avoid the damage of the transmitting end IQ imbalance. Therefore, the application can realize the simplified coherent system which is not sensitive to the damage of the transmitting end IQ imbalance.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of high-speed optical signal processing, and more particularly, to a simplified homodyne coherent system based on Alamouti coding. BACKGROUND

[0002] With the development of new broadband services such as Internet of Things, cloud computing, virtual reality, etc., short-distance data center optical interconnection puts forward higher demand 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, limiting the transmission rate and transmission distance of the system. In contrast, the coherent system has better receiving sensitivity and linearity, and can realize multi-dimensional modulation. However, the traditional coherent system has the problems of high cost and system complexity, and cannot be directly applied to data centers which are sensitive to cost and power consumption, and needs to be simplified. In order to realize a low-cost and low-complexity coherent system, some schemes have been proposed:

[0003] (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 and phase of the transmission signal carrier and the local oscillator carrier, thereby simplifying or even omitting the processing of frequency offset and phase noise at the receiving end. In this scheme, the receiving end needs an optical frequency-locked phase-locked module, including a photodetector (PD), a frequency mixer, a loop filter, a reference clock, a phase modulator, a band-pass filter, etc. As can be seen, although this scheme can simplify the complexity of the receiving end DSP, the receiving end needs to add an additional optical module, so the simplification brought by this scheme still has certain limitations.

[0004] (2) Homodyne coherent technology: This method mainly divides the optical carrier at the transmitting end into two parts, one of which is used to modulate the optical signal, and the other is transmitted to the receiving end by an additional optical fiber and used as a local oscillator to beat with the transmission signal. Since the modulation signal and the transmission carrier are generated by the same light source, when the transmission fiber lengths of the signal and the carrier are matched, strict same frequency and phase can be realized, so there is no need to process the frequency offset and phase noise at the receiving end. This scheme simplifies the complexity of the receiving end DSP by adding an additional transmission fiber, and the receiving end does not need a local oscillator. At present, in this scheme, the receiving end no longer needs a local oscillator, but still needs a traditional dual-biased coherent receiver. In this scheme, the signal will be simultaneously affected by the IQ imbalance damage of the transmitting end and the receiving end, thereby affecting the transmission performance, and a complex algorithm is generally needed to compensate for the IQ imbalance damage of the transmitting and receiving ends.

[0005] The analog coherent technology needs to use an additional optical frequency locking module to realize the frequency and phase locking of the local laser, so the scheme realizes the simplification of the complexity of the DSP algorithm at the receiving end by increasing the complexity of the architecture of the traditional coherent system to some extent, and the reference clock in the module still has certain phase noise, so it is difficult to realize strict frequency offset and phase locking. In the homodyne coherent technology, except that the local optical carrier is replaced by the optical fiber transmission of the transmitting end light source, the rest of the structure is basically the same as the traditional coherent system. Therefore, the IQ imbalance problem exists in the transmitting end and the receiving end, which seriously affects the transmission performance of the system, and a complex digital signal processing algorithm needs to be used for compensation. SUMMARY

[0006] The application provides a simplified homodyne coherent system based on Alamouti coding, which can effectively avoid the damage of the IQ imbalance of the transmitting end and effectively reduce the cost.

[0007] To solve the above technical problems, the technical scheme adopted by the application is: a signal processing method of a simplified homodyne coherent system based on Alamouti coding, in the transmitting end digital signal processing DSP, first, a pseudo-random bit sequence is mapped into a QAM symbol, 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 in front of each frame of OFDM signal; according to the condition of the transmission optical fiber, finally, dispersion pre-compensation is performed at the transmitting end; in the receiving end digital signal processing DSP, first, the received X polarization and Y polarization signals are synchronized, then the cyclic prefix is removed, then the channel parameters are obtained by using the training sequence and Alamouti decoding, and the signal is equalized, finally, the equalized data is inversely mapped by QAM, and the bit error rate of the system is calculated.

[0008] The application further provides a simplified coherent scheme based on Alamouti coding on the basis of the homodyne coherent system, to realize the simplification of the coherent receiver and solve the IQ imbalance problem in the coherent system. The application encodes the real part and the imaginary part of the IQ signal at the transmitting end by Alamouti, so that the receiving end only needs to receive the real part or the imaginary part of the signal to restore the complete complex signal. Therefore, the receiving end can use a low-cost 2*2 coupler to replace the 90° mixer in the traditional coherent system, and can reduce the number of balanced photodetectors (BPD) by half, thereby greatly simplifying the architecture of the traditional coherent system. 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, and the IQ imbalance problem at the transmitting end can be effectively solved by the Alamouti encoding at the transmitting end.

[0009] Further, the transmitting end divides the light source emitted by the laser into two parts, one of which is used for signal modulation, and the other of which 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 after digital-to-analog conversion, and are modulated onto X and Y polarizations, respectively. After transmission through a single-mode optical fiber, the receiving power of the signal is adjusted using an adjustable optical attenuator. The receiving end divides the transmitted signal and the carrier into X and Y polarizations, respectively, using two polarization beam splitters. The X polarization component of the signal and the X component of the carrier are coupled through a 2x2 coupler and received by a BPD to obtain the real part of the signal modulated onto the X polarization. Similarly, the Y polarization component of the signal and the Y component of the carrier are coupled through a 2x2 coupler and received by a BPD to obtain the real part of the signal modulated onto the Y polarization. The signal detected by the BPD is subjected to analog-to-digital conversion by an analog-to-digital converter, and then processed by a digital signal processing DSP in the receiving end.

[0010] Further, the Alamouti encoding specifically includes: for the OFDM signal, the second time repeats the information of the first time, wherein the I channel signal of the second time is the negative of the Q channel signal of the first time, and the Q channel signal of the second time is the Q channel signal of the first time. The imaginary part of the first time signal is obtained from the real part of the second time signal, and the receiving end only needs to receive the real part signal.

[0011] Further, when there is an IQ imbalance damage in the transmitting end, the relationship between the received signal and the transmitted signal is represented as follows:

[0012]

[0013] wherein s XI1 , s XQ1 , s YI1 and s YQ1 represent the real and imaginary parts of the X and Y polarizations of the first time transmitted signal, -s XQ1 , s XI1 , -s YQ1 and s YI1 represent the real and imaginary parts of the X and Y polarizations of the second time transmitted signal; s' XI1 , s' XI2 , s' YI1 and s' YI2 are expanded in the frequency domain as follows:

[0014] S' XI1 = H XI-XI S XI1 + H XQ-XI S XQ1 + H YI-XI S YI1 + H YQ-XIS YQ1 (2)

[0015] S' XI2 = -H XI-XI S XQ1 +H XQ-XI S XI1 -H YI-XI S YQ1 +H YQ-XI S YI1 (3)

[0016] S' YI1 = H XI-YI S XI1 +H XQ-YI S XQ1 +H YI-YI S YI1 +H YQ-YI S YQ1 (4)

[0017] S' YI2 = -H XI-YI S XQ1 +H XQ-YI S XI1 -H YI-YI S YQ1 +H YQ-YI S YI1 (5)

[0018] 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 recovered, the channel can be equalized and the compensation of the IQ imbalance at the transmitting end can be realized.

[0019] Further, the method for calculating the channel parameters comprises: constructing a training sequence, taking Alamouti coding blocks as units, placing one Alamouti coded block only on the X polarization at the first and second time instants, and placing one coded block only on the Y polarization at the next two time instants; and the relationship between the received training sequence and the transmitted training sequence is represented as follows:

[0020]

[0021] Expanding the above formula in the frequency domain, the following four groups of equations are obtained:

[0022]

[0023]

[0024]

[0025]

[0026] The eight channel parameters required are obtained according to the relationship of the above four groups of equations by using the training sequence; the obtained channel parameters are substituted into formulas (2)-(5), and the channel equalization of the received signal and the compensation of the transmitting end IQ imbalance are realized by using the zero-forcing algorithm.

[0027] Compared with the homologous coherent scheme that has been proposed, the application uses Alamouti encoding on the real part and the imaginary part of the transmitting signal, so that the receiving end can recover the complete complex signal only by receiving the real part signals of X and Y polarizations, thereby saving half the number of BPDs and replacing the 90° mixer with a low-cost 2*2 coupler, effectively reducing the complexity and cost of the receiver. The application places the training sequences of X and Y polarizations alternately in time in the OFDM system to obtain eight channel parameters when there is transmitting end IQ imbalance, for channel equalization and compensation of the transmitting end IQ imbalance. Therefore, the simplified coherent system based on Alamouti proposed in the application can effectively simplify the structure of the traditional coherent receiver, avoid the receiving end IQ imbalance, and realize the insensitivity to the transmitting end IQ imbalance.

[0028] The application also provides a simplified homologous coherent system based on Alamouti encoding, comprising a transmitting end DSP module and a receiving end DSP module.

[0029] The transmitting end DSP module is used for mapping a pseudo-random bit sequence into a QAM symbol, then generating two orthogonal frequency division multiplexing (OFDM) signals, Alamouti encoding the real part and the imaginary part of the generated OFDM signals, and adding a cyclic prefix in front of each frame of OFDM signal; finally, according to the condition of the transmission optical fiber, performing dispersion pre-compensation at the transmitting end.

[0030] The receiving end DSP module is used for synchronizing the received X-polarization and Y-polarization signals, then removing the cyclic prefix, then obtaining channel parameters by using a training sequence and Alamouti decoding, equalizing the signals, finally performing QAM inverse mapping on the equalized data, and calculating the system bit error rate.

[0031] Further, the transmitting end divides the light source emitted by the laser into two parts, one of which 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 into a dual bias IQ modulator after digital-to-analog conversion, and are modulated onto X and Y polarizations, respectively. After transmission through a single-mode fiber, the receiving power of the signal is adjusted by using an adjustable optical attenuator. The receiving end divides the transmitted signal and the carrier into X and Y polarizations, respectively, by using two polarization beam splitters. The X polarization component of the signal and the X component of the carrier are coupled by a 2x2 coupler and then received by a BPD to obtain the real part of the signal modulated onto the X polarization. Similarly, the Y polarization component of the signal and the Y component of the carrier are coupled by a 2x2 coupler and then received by a BPD to obtain the real part of the signal modulated onto the Y polarization. The signal detected by the BPD is converted into digital signals by an analog-to-digital converter, and then processed by a DSP module in the receiving end.

[0032] Further, when the transmitting end DSP module performs Alamouti encoding, for the OFDM signal, the second time information is placed in the first time, wherein the I signal of the second time is the negative value of the Q signal of the first time, and the Q signal of the second time is the I signal of the first time. The imaginary part of the first time signal is obtained from the real part of the second time signal, and the receiving end only needs to receive the real part signal.

[0033] Further, when there is an IQ imbalance damage in the transmitting end, the relationship between the received signal and the transmitted signal is represented as follows:

[0034]

[0035] wherein s XI1 , s XQ1 , s YI1 and s YQ1 represent the real and imaginary parts of the X and Y polarizations of the first time transmitted signal, -s XQ1 , s XI1 , -s YQ1 and s YI1 represent the real and imaginary parts of the X and Y polarizations of the second time transmitted signal; s' XI1 , s' XI2 , s' YI1 and s' YI2 are expanded in the frequency domain as follows:

[0036] S' XI1 = H XI-XI S XI1 + H XQ-XI S XQ1 + H YI-XI S YI1 + H YQ-XI SYQ1 (12)

[0037] S' XI2 = -H XI-XI S XQ1 +H XQ-XI S XI1 -H YI-XI S YQ1 +H YQ-XI S YI1 (13)

[0038] S' YI1 = H XI-YI S XI1 +H XQ-YI S XQ1 +H YI-YI S YI1 +H YQ-YI S YQ1 (14)

[0039] S' YI2 = -H XI-YI S XQ1 +H XQ-YI S XI1 -H YI-YI S YQ1 +H YQ-YI S YI1 (15)

[0040] 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 recovered, the channel can be equalized and the compensation of the IQ imbalance at the transmitting end can be realized.

[0041] Further, the calculation of the channel parameters comprises: constructing a training sequence, taking Alamouti coding blocks as units, placing one Alamouti coded block only on the X polarization at the first and second time instants, and placing one coded block only on the Y polarization at the next two time instants; the relationship between the received training sequence and the transmitted training sequence is represented as follows:

[0042]

[0043] Expanding the above formula in the frequency domain, the following four groups of equations are obtained:

[0044]

[0045]

[0046]

[0047]

[0048] The eight channel parameters required are obtained according to the relationship of the above four groups of equations by using the training sequence; the obtained channel parameters are substituted into formulas (12)-(15), and the channel equalization of the received signal and the compensation of the transmitter IQ imbalance are realized by using the zero-forcing algorithm.

[0049] Compared with the prior art, the beneficial effects are that: the application realizes the simplification of the traditional coherent system by using Alamouti encoding and combining with the homodyne coherent technology. On the basis of the homodyne coherent scheme, the application can further reduce the complexity of the receiver. In terms of system architecture, compared with the traditional coherent receiver, the application can replace the 90° mixer with a low-cost 2*2 coupler, and saves half the number of BPD. In addition, the adoption of the homodyne coherent technology can save the local oscillator laser at the receiving end. In terms of DSP complexity, the receiving end does not need to process the frequency offset and phase noise, greatly reducing the complexity of the receiving end DSP. In addition, since the application only needs to receive the real part of the X and Y polarization signals to recover the complete complex signal, there is no IQ imbalance problem at the receiving end. The way of Alamouti encoding of the real and imaginary parts of the signal proposed by the application can effectively avoid the damage of the transmitter IQ imbalance. Therefore, the application can realize the simplified coherent system which is insensitive to the damage of the transmitter IQ imbalance. BRIEF DESCRIPTION OF DRAWINGS

[0050] Figure 1 Fig. (a) is the Alamouti encoded dual polarization OFDM signal structure, and (b) is the training sequence structure.

[0051] Figure 2 Fig. is the simulation device diagram and the transceiver end DSP flowchart of Example 2.

[0052] Figure 3 Fig. is the schematic diagram of the influence of the transmitter time delay on the system performance.

[0053] Figure 4 Fig. is the schematic diagram of the influence of the transmitter amplitude mismatch on the system performance.

[0054] Figure 5 Fig. is the schematic diagram of the influence of the transmitter phase mismatch on the system performance.

[0055] Figure 6are the bit error rate performance comparisons under different optical signal-to-noise ratio conditions (a) optical back-to-back transmission (b) 80 km optical fiber transmission. DETAILED DESCRIPTION

[0056] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments. The present application will be described below in one of the embodiments with reference to the specific embodiments. In the drawings, only the schematic diagrams are shown, not the physical diagrams, and cannot be understood as a limitation of the present patent; in order to better illustrate the embodiments of the present application, some components in the drawings may be omitted, enlarged or reduced, and do not represent the size of the actual product; for those skilled in the art, it is understandable that some well-known structures and their descriptions in the drawings may be omitted.

[0057] In the description of the present application, it should be understood that the orientation or positional relationship indicated by the terms "upper", "lower", "left", "right" and the like is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present application and simplifying the description, and does not indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, therefore the terms describing the positional relationship in the drawings are only for illustrative purposes, and cannot be understood as a limitation of the present patent, for those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances. In addition, if the present application embodiments have descriptions involving "first", "second" and the like, the "first", "second" and the like are only for description purposes, and cannot be understood as indicating or implying the relative importance of the indicated technical features or implicitly indicating the number of the indicated technical features. Therefore, the features limited by "first", "second" can explicitly or implicitly include at least one of the features. In addition, "and / or" appearing throughout the text means that three parallel schemes are included, for example, "A and / or B" includes A scheme, or B scheme, or A and B schemes are satisfied at the same time.

[0058] Example 1:

[0059] The embodiment provides a signal processing method of a simplified homodyne coherent system based on Alamouti coding, in the digital signal processing (DSP) of a transmitting end, first, a pseudo-random bit sequence is mapped into a QAM symbol, then two orthogonal frequency division multiplexing (OFDM) signals are generated, the real part and the imaginary part of the generated OFDM signals are subjected to Alamouti coding, and a cyclic prefix is added in front of each frame of OFDM signals; according to the condition of a transmission optical fiber, finally, dispersion pre-compensation is performed at the transmitting end; in the digital signal processing (DSP) of a receiving end, first, the received X polarization and Y polarization signals are synchronized, then the cyclic prefix is removed, then the channel parameters are obtained by using a training sequence and Alamouti decoding, and the signals are equalized, finally, the equalized data is subjected to QAM inverse mapping, and the bit error rate of the system is calculated.

[0060] Figure 1 Fig. a) shows the structure of Alamouti coded dual polarization orthogonal frequency division multiplexing (OFDM) signals. As shown in the figure, the information of the first time is repeatedly placed at the second time, wherein the I channel signal of the second time is the negative value of the Q channel signal of the first time, and the Q channel signal of the second time is the I channel signal of the first time. Therefore, the imaginary part of the signal of the first time can be obtained from the real part of the signal of the second time, 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 a transmitting end IQ imbalance damage, the relationship between the received signal and the transmitting signal can be expressed as follows:

[0061]

[0062] wherein s XI1 , s XQ1 , s YI1 and s YQ1 represent the real part and the imaginary part of the X polarization and Y polarization signals transmitted at the first time, -s XQ1 , s XI1 , -s YQ1 and s YI1 represent the real part and the imaginary part of the X polarization and Y polarization signals transmitted at the second time; it can be seen from formula (1) that the channel can be expressed as a 4x4 matrix. Since in the proposed system, the receiving end only needs to receive the real part signal, only s' XI1 , s' XI2 , s' YI1 and s' YI2 in formula (1) need to be analyzed, s' XI1 , s' XI2 , s' YI1 and s' YI2 are expanded in the frequency domain as follows:

[0063] S' XI1 = HXI-XI S XI1 +H XQ-XI S XQ1 +H YI-XI S YI1 +H YQ-XI S YQ1 (2)

[0064] S' XI2 =-H XI-XI S XQ1 +H XQ-XI S XI1 -H YI-XI S YQ1 +H YQ-XI S YI1 (3)

[0065] S' YI1 =H XI-YI S XI1 +H XQ-YI S XQ1 +H YI-YI S YI1 +H YQ-YI S YQ1 (4)

[0066] S' YI2 =-H XI-YI S XQ1 +H XQ-YI S XI1 -H YI-YI S YQ1 +H YQ-YI S YI1 (5)

[0067] From the above formulas, we can see that we only need to get the channel parameter 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 of IQ imbalance at the transmitting end.

[0068] In order to obtain the above 8 channel parameters, the training sequence is constructed, such as Figure 1 As shown in (b), taking the Alamouti code block as the unit, at the first and second moments, only one Alamouti-coded code block is placed on the X polarization, and at the next two moments, only one coded code block is placed on the Y polarization. The relationship between the received training sequence and the transmitted training sequence is expressed as follows:

[0069]

[0070] The above formula is expanded in the frequency domain to obtain the following four groups of equations:

[0071]

[0072]

[0073]

[0074]

[0075] The required eight channel parameters are obtained according to the relationship of the above four groups of equations using the training sequence; the obtained channel parameters are substituted into formulas (2)-(5), and the channel equalization of the received signal and the compensation of the transmit end IQ imbalance are realized using the zero-forcing algorithm.

[0076] Compared with the homologous coherent scheme that has been proposed, the present application uses Alamouti encoding on the real part and the imaginary part of the transmit signal on this basis, so that the receiving end only needs to receive the real part signal of the X and Y polarizations to recover the complete complex signal, thereby enabling the receiving end to save half the number of BPDs, and enabling a low-cost 2x2 coupler to replace the 90° mixer, effectively reducing the complexity and cost of the receiver. The present application obtains eight channel parameters when there is transmit end IQ imbalance by alternately placing the training sequences of the X and Y polarizations in time in the OFDM system, for channel equalization and compensation of the transmit end IQ imbalance. Therefore, the simplified coherent system based on Alamouti proposed by the present application can effectively simplify the structure of the traditional coherent receiver, avoid the IQ imbalance of the receiving end, and at the same time realize insensitivity to the transmit end IQ imbalance damage.

[0077] Embodiment 2

[0078] Figure 2The simulation device diagram of the present invention and the DSP flow chart of the transceiver are given. The simulation is realized by joint simulation of MATLAB and VPI. The transmitting end divides the light source emitted by the laser into two, one for signal modulation and the other transmitted to the receiving end as a local oscillator (LO). The transmitting end generates two 25GHz OFDM signals by offline DSP. After passing through a digital-to-analog converter (DAC) with a sampling rate of 64GSa / s, they are loaded into a dual-bias IQ modulator (DP-IQM) and modulated to X and Y polarizations respectively. After being transmitted through 80km of single-mode optical fiber, the received power of the signal is adjusted by a variable optical attenuator (ROP). The receiving end uses two polarization beam splitters (PBS) to separate the transmitted signal and 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 2×2 coupler and received by a BPD to obtain the real part of the signal modulated to the X polarization. Similarly, the real part of the Y polarization signal can be obtained. The signal after BPD detection is converted to digital by an analog-to-digital converter (ADC) with a sampling rate of 64GSa / s and then processed by the offline DSP at the receiving end.

[0079] (1) Transmitter DSP

[0080] The DSP at the transmitter first maps the pseudorandom bit sequence into 16QAM symbols, then generates two 25 GHz OFDM signals. The real and imaginary parts of the generated OFDM signals are Alamouti-encoded, and a cyclic prefix is ​​added to each OFDM frame. Finally, dispersion precompensation is performed at the transmitter for transmission fibers.

[0081] (2) DSP at the receiving end

[0082] At the receiver, the received X- and Y-polarization signals are first synchronized, and the cyclic prefix is ​​removed. Next, the training sequence and Alamouti decoding are used to determine the channel parameters, and the signal is equalized. Finally, 16QAM demapping is performed on the equalized data, and the system bit error rate is calculated.

[0083] Result Analysis

[0084] based on Figure 2 The simulation setup shown simulates the performance of a 25 GHz OFDM signal in the proposed simplified coherent system. First, the simulation investigates the robustness of the simplified coherent system to IQ imbalance impairments at the transmitter, after Alamouti encoding of the I and Q signals in a back-to-back transmission scenario. The optical signal-to-noise ratio (OSNR) of the system is set to 25 dB. Figure 3The impact of transmitter-side delay skew on the system bit error rate is shown. As the figure shows, as the I / Q delay skew changes, the bit error rates for both X and Y polarizations remain nearly constant, and are identical to those without delay skew. This demonstrates that Alamouti encoding effectively mitigates the effects of transmitter delay skew.

[0085] Figure 4 The impact of amplitude mismatch between the I and Q signals at the transmitter on system performance is given. Figure 4 As shown in Figure 3, the bit error rate remains constant and is the same as when there is no amplitude mismatch. Simulation results show that Alamouti coding based on the transmitter I and Q signals can make the system insensitive to transmitter amplitude mismatch.

[0086] Figure 5 The impact of transmitter phase mismatch alone on bit error rate performance is discussed. Simulation results show that as the degree of transmitter phase mismatch increases, the system's bit error rate remains essentially the same as when no phase mismatch exists. These results demonstrate that the proposed simplified coherent receiver based on Alamouti coding is robust to transmitter I / Q delay, amplitude mismatch, and phase mismatch impairments.

[0087] Figure 6 (a) and Figure 6 (b) shows the comparison of the bit error rate performance when there are different degrees of IQ imbalance damage at the transmitter and when there is no IQ imbalance damage under the conditions of optical back-to-back transmission and 80km optical fiber transmission. Under the conditions of optical back-to-back transmission, the amplitude mismatch and phase mismatch at the transmitter are set to 3dB and 10° respectively, and the bit error rate performance is compared when there are 4ps, 8ps and 12ps delays. At the same time, the bit error rate curve when there is no IQ imbalance damage is given. Figure 6 As shown in Figure (a), when there is a 4ps delay, a 3dB amplitude mismatch, and a 10° phase mismatch, the BER curve essentially overlaps with the curve without IQ imbalance. As delay increases, the BER performance gradually deteriorates, but at the hard-decision FEC threshold, it remains similar to the BER performance without IQ imbalance. Figure 6 Figure (b) shows a comparison of bit error rate performance after transmission over 80 km of optical fiber, with the amplitude and phase mismatch at the transmitter fixed at 3 dB and 10°, respectively. As can be seen from the figure, when there is a 2 ps delay, a 3 dB amplitude mismatch, and a 10° phase mismatch, the bit error rate curves essentially overlap with those without IQ imbalance. When the delay reaches 4 ps, the bit error rate curve shifts slightly upward, but at the hard-decision forward error correction coding threshold, the bit error rate performance remains similar to that without IQ imbalance.

[0088] In summary, simulations have demonstrated that the proposed simplified coherent system based on Alamouti coding is robust to transmitter IQ imbalance impairments, while also simplifying the architecture and DSP of the traditional coherent system.

[0089] Example 3

[0090] This embodiment provides a simplified homologous coherent system based on Alamouti coding, including a transmitting end DSP module and a receiving end DSP module;

[0091] The transmitter DSP module is used to map the pseudo-random bit sequence into QAM symbols, then generate two orthogonal frequency division multiplexing (OFDM) signals. The real and imaginary parts of the generated OFDM signals are Alamouti-encoded and a cyclic prefix is ​​added to each OFDM frame. Finally, dispersion pre-compensation is performed at the transmitter depending on the conditions of the transmission fiber.

[0092] The receiving-end DSP module synchronizes the received X- and Y-polarization signals, removes the cyclic prefix, uses the training sequence and Alamouti decoding to obtain channel parameters, equalizes the signal, performs QAM inverse mapping on the equalized data, and calculates the system's bit error rate.

[0093] The transmitter splits the laser light source into two: one for signal modulation and the other for transmission to the receiver as a local oscillator light source. The transmitter generates two OFDM signals, which, after passing through a digital-to-analog converter, are loaded into a dual-bias IQ modulator and modulated into the X and Y polarizations, respectively. After transmission through a single-mode fiber, an adjustable optical attenuator is used to adjust the received power of the signal. The receiver uses two polarization beam splitters to separate the transmitted signal and 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 2×2 coupler and received by a beam splitter (BPD), obtaining the real part of the signal modulated onto the X polarization. Similarly, the Y-polarization component of the signal and the Y component of the carrier are coupled through a 2×2 coupler and received by a beam splitter (BPD), obtaining the real part of the signal modulated onto the Y polarization. The signal detected by the BPD undergoes analog-to-digital conversion by an analog-to-digital converter and is then processed by the DSP module at the receiver.

[0094] Specifically, when the DSP module at the transmitting end performs Alamouti encoding, for the OFDM signal, it repeats the information of the first moment at the second moment, where 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 signal.

[0095] When there is a transmitting end IQ imbalance damage, the relationship between the receiving signal and the transmitting signal is expressed as follows:

[0096]

[0097] where s XI1 , s XQ1 , s YI1 and s YQ1 represent the real and imaginary parts of the X and Y polarized signals sent at the first time, -s XQ1 , s XI1 , -s YQ1 and s YI1 represent the real and imaginary parts of the X and Y polarized signals sent at the second time; s' XI1 , s' XI2 , s' YI1 and s' YI2 are expressed as follows in the frequency domain:

[0098] S' XI1 = H XI-XI S XI1 + H XQ-XI S XQ1 + H YI-XI S YI1 + H YQ-XI S YQ1 (12)

[0099] S' XI2 = -H XI-XI S XQ1 + H XQ-XI S XI1 - H YI-XI S YQ1 + H YQ-XI S YI1 (13)

[0100] S' YI1 = H XI-YI S XI1 + H XQ-YI S XQ1 + H YI-YI S YI1 + H YQ-YI S YQ1 (14)

[0101] S' YI2 = -H XI-YI S XQ1 + H XQ-YI S XI1 - H YI-YI S YQ1 + H YQ-YI S YI1 (15)

[0102] 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 recovered, the channel equalization and the compensation of the IQ imbalance of the transmitting end are realized.

[0103] The calculation of the channel parameters comprises the following steps: constructing the training sequence, taking the Alamouti encoding block as a unit, placing one Alamouti encoded code block on the X polarization at the first and second time, and placing one encoded code block on the Y polarization at the next two time; the relationship between the received training sequence and the transmitted training sequence is shown as follows:

[0104]

[0105] The above formula is expanded in the frequency domain to obtain the following four groups of equations:

[0106]

[0107]

[0108]

[0109]

[0110] The eight required channel parameters are obtained according to the relationship of the above four groups of equations by using the training sequence; the obtained channel parameters are substituted into the formula (12)-(15), and the channel equalization of the received signal and the compensation of the IQ imbalance of the transmitting end are realized by using the zero-forcing algorithm.

[0111] Obviously, the above embodiments of the present application are only examples for clearly illustrating the present application, and are not intended to limit the implementation manners of the present application. Any modification, equivalent replacement and improvement made on the basis of the above description for those skilled in the art should be included in the protection scope of the present application.

Claims

1. A signal processing method for a simplified homodyne coherent system based on Alamouti coding, characterized in that, In the transmitting end digital signal processing (DSP), firstly, a pseudo-random bit sequence is mapped into QAM symbols, then two orthogonal frequency division multiplexing (OFDM) signals are generated, the real and imaginary parts of the generated OFDM signals are subjected to Alamouti encoding, and a cyclic prefix is added in front of each frame of OFDM signal; According to the condition of transmission optical fiber, finally, dispersion pre-compensation is performed at the transmitting end; in the receiving end digital signal processing (DSP), firstly, the received X-polarization and Y-polarization signals are synchronized, then the cyclic prefix is removed, then the channel parameters are obtained by using the training sequence and Alamouti decoding, the signals are equalized, finally, the equalized data is subjected to QAM inverse mapping, and the system bit error rate is calculated; wherein, the Alamouti encoding specifically comprises: for the OFDM signal, the information of the first time is repeatedly placed in the second time, wherein the I signal of the second time is the negative value of the Q signal of the first time, and the Q signal of the second time is the I signal of the first time, the imaginary part of the first time signal is obtained from the real part of the second time signal, and the receiving end only needs to receive the real part signal.

2. The signal processing method for Alamouti coding-based simplified homodyne coherent system according to claim 1, characterized in that, The transmitting end divides the light source emitted by the laser into two parts, one of which is used for signal modulation, and the other of which 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 after passing through a digital-to-analog converter to modulate X and Y polarizations respectively; after transmission 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 on 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 on the Y polarization; the signals detected by the BPD are subjected to analog-to-digital conversion by an analog-to-digital converter, and then processed by the receiving end digital signal processing (DSP). 3.The signal processing method of Alamouti coding-based simplified homogenous coherent system according to claim 1, characterized in that, When there is transmitting end IQ imbalance damage, the relationship between the received signal and the transmitting signal is represented as follows: where s XI1 , s XQ1 , s YI1 and s YQ1 represent the real and imaginary parts of the X and Y polarized signals transmitted at the first time instant; -s XQ1 , s XI1 , -s YQ1 and s YI1 represent the real and imaginary parts of the X and Y polarized signals transmitted at the second time instant; 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, the transmitted signal can be recovered, the channel equalization and the compensation of the IQ imbalance at the transmitting end can be realized.

4. The signal processing method for Alamouti coding-based simplified homodyne coherent system according to claim 3, characterized in that, The channel parameter calculation method comprises: constructing a training sequence, taking an Alamouti encoding block as a unit, placing an Alamouti encoded code block only on the X polarization in the first and second time, and placing an encoded code block only on the Y polarization in the next two time; the relationship between the received training sequence and the transmitted training sequence is represented as follows: The above four groups of equations are obtained by expanding the above formula in the frequency domain: The obtained channel parameters are substituted into formulas (2)-(5), and the zero-forcing algorithm is used to realize channel equalization of the received signal and compensation of the transmitting end IQ imbalance.

5. A simplified homodyne system based on Alamouti coding, characterized in that, It comprises a transmitting end DSP module and a receiving end DSP module. The transmitting end DSP module is used for mapping pseudo-random bit sequence into QAM symbol, then generating two orthogonal frequency division multiplexing (OFDM) signals, and adding cyclic prefix in front of each frame of OFDM signal; According to the condition of transmission optical fiber, finally, dispersion pre-compensation is performed at the transmitting end; when performing Alamouti encoding, for the OFDM signal, the second-time information is placed in the first-time, wherein the I signal of the second time is the negative value of the Q signal of the first time, and the Q signal of the second time is the I signal of the first time, and the imaginary part of the first-time signal is obtained from the real part of the second-time signal; the receiving end only needs to receive the real part signal; The receiving end DSP module is used for synchronizing the received X polarization and Y polarization signals, then removing the cyclic prefix, then obtaining channel parameters by using training sequence and Alamouti decoding, and then equalizing the signals, finally, performing QAM inverse mapping on the equalized data, and calculating the system bit error rate.

6. The Alamouti code based simplified homodyne system of claim 5, wherein, The transmitting end divides the light source emitted by the laser into two parts, one of which is used for signal modulation, and the other of which 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 after passing through a digital-to-analog converter, and are modulated onto X and Y polarizations, respectively; after transmission 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 onto 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 onto the Y polarization; the signals detected by the BPD are analog-to-digital converted by an analog-to-digital converter, and then processed by the receiving end DSP module.

7. The Alamouti code based simplified homodyne system of claim 5, wherein, When there is transmitting end IQ imbalance damage, the relationship between the received signal and the transmitting signal is represented as follows: where s XI1 , s XQ1 , s YI1 and s YQ1 represent the real and imaginary parts of the X, Y polarized signals transmitted at the first time instant; -s XQ1 , s XI1 , -s YQ1 and s YI1 represent the real and imaginary parts of the X, Y polarized signals transmitted at the second time instant; 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, the transmitted signal can be recovered, the channel equalization and the compensation of the IQ imbalance at the transmitting end can be realized.

8. The Alamouti code based simplified homodyne system of claim 7, wherein, The calculation of the channel parameters includes: constructing a training sequence, taking Alamouti encoding block as a unit, placing an Alamouti encoded code block only on the X polarization in the first and second times, and placing an encoded code block only on the Y polarization in the next two times; the relationship between the received training sequence and the transmitted training sequence is represented as follows: The above four groups of equations are obtained by expanding the above formula in the frequency domain: The eight required channel parameters are obtained by using the training sequence according to the relationship of the above four groups of equations; the obtained channel parameters are substituted into formulas (12)-(15), and the zero-forcing algorithm is used to realize channel equalization of the received signal and compensation of the transmitting end IQ imbalance. The transmitting end divides the light source emitted by the laser into two parts, one of which is used for signal modulation, and the other of which 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 after passing through a digital-to-analog converter, and are modulated onto X and Y polarizations, respectively; after transmission 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 onto 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 onto the Y polarization; the signals detected by the BPD are analog-to-digital converted by an analog-to-digital converter, and then processed by the receiving end DSP module. When there is transmitting end IQ imbalance damage, the relationship between the received signal and the transmitting signal is represented as follows: The calculation of the channel parameters includes: constructing a training sequence, taking Alamouti encoding block as a unit, placing an Alamouti encoded code block only on the X polarization in the first and second times, and placing an encoded code block only on the Y polarization in the next two times; the relationship between the received training sequence and the transmitted training sequence is represented as follows: The above four groups of equations are obtained by expanding the above formula in the frequency domain: The eight required channel parameters are obtained by using the training sequence according to the relationship of the above four groups of equations; the obtained channel parameters are substituted into formulas (12)-(15), and the zero-forcing algorithm is used to realize channel equalization of the received signal and compensation of the transmitting end IQ imbalance.

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