Multi-carrier modulation scheme based on the reconstruction of orthogonal chirp multiplexing signals under the KK relationship

Abstract

The present invention discloses a multi - carrier modulation scheme for reconstructing orthogonal chirp multiplexing signals based on the KK relationship. By constructing a single - sideband OCDM modulation signal and using digital up - conversion technology, the real part and the imaginary part of the frequency - converted OCDM signal satisfy the Kramers - Kroning relationship. At the receiving end, a KK receiver is adopted to reconstruct the real - part and imaginary - part information of the OCDM signal, ultimately improving the receiving sensitivity of the communication system.

Description

Technical Field

[0001] The present invention relates to the technical fields of orthogonal chirp multiplexing technology and Kramers-Kroning receiver technology, and particularly relates to a multi-carrier modulation scheme for reconstructing orthogonal chirp multiplexing signals based on the KK relationship. Background Art

[0002] With the breakthrough achievements of silicon-based photonic devices compatible with CMOS technology strongly promoting the development of on-chip optical interconnection communication systems, on-chip optical interconnection communication systems have the advantages of high integration, light weight, low power consumption, etc., and are the development trend of future large-capacity and high-integration communication systems. And in the face of the urgent demand problem of the transmission capacity of communication systems caused by the rapid growth of network data traffic today, high-order modulation formats and efficient digital signal processing algorithms have emerged, promoting the development of high-speed and large-capacity optical fiber communication systems. Orthogonal frequency division multiplexing (OFDM) systems based on multi-carrier modulation have high spectral efficiency, and coherent optical-OFDM systems can effectively resist dispersion problems in optical channels. However, in intensity modulation direct detection (IM / DD)-OFDM systems, information is modulated on different subcarriers in the frequency domain, and subcarriers at high frequencies are more likely to be affected by dispersion and power fading, affecting communication quality.

[0003] Recently, orthogonal chirp multiplexing (OCDM) has gradually become one of the modulation technologies attracting wide attention because it can well cope with frequency selective fading in IM / DD-OFDM systems and has better anti-interference ability. OCDM is an upgrade of traditional chirp spread spectrum technology (CSS). Traditional CSS modulates information on chirp signals with linear frequency modulation. At the cost of greatly increasing the signal bandwidth, CSS signals obtain high anti-interference ability. Therefore, the spectral efficiency of traditional CSS technology is low and it is not suitable for current high-speed and large-capacity optical communication systems. OCDM technology transmits signals using a group of orthogonal chirp subcarriers at the Nyquist rate within a given bandwidth, making the most of the spectral efficiency of the system, and at the same time using the spectral characteristics of chirp waveforms to improve the system's resistance to dispersion damage.

[0004] In an IM / DD-OFDM system, it is necessary to transform the orthogonal chirp signal in the frequency domain to the time domain during the offline data signal processing (DSP) process. And considering that the IM / DD system only supports one-dimensional optical intensity modulation, the time-domain orthogonal chirp signal must be further processed to obtain a one-dimensional orthogonal chirp signal. Digital frequency conversion technology can realize a one-dimensional orthogonal chirp signal for an IM / DD communication system. Through digital up-conversion, the real part of the frequency-converted signal contains the real part and the imaginary part of the original complex orthogonal chirp signal. The real part of the frequency-converted signal is retained and used for modulation transmission in the IM / DD system. However, the imaginary part information of the frequency-converted signal is completely discarded.

[0005] Different from traditional intensity modulation direct detection technology, single-sideband modulation signals have only half of the baseband signal spectrum in the frequency domain, with higher spectral efficiency. When the double-sideband modulation technology of optical signals uses square intensity detection at the receiving end, it will be affected by frequency fading caused by dispersion, resulting in signal damage. And when there is a lack of corresponding dispersion compensation measures, it severely limits the maximum transmission distance of the signal in the optical fiber. In terms of cost, the single-sideband modulation method only increases the complexity of the transmitting end, and the receiving end still only needs a single photodetector to receive. However, the single-sideband modulation technology has the problem of signal-to-signal beat interference (SSBI). Since the Kramers-Kroning (KK) receiver scheme was proposed in 2016 to eliminate the signal-to-noise ratio deterioration problem caused by SSBI, the KK receiver scheme has low requirements for the carrier signal power ratio (CSPR) and high receiver sensitivity.

[0006] In view of the above problems, this application proposes a solution.

[0007] The proposed solution of the present invention is a multi-carrier modulation optical communication system based on the reconstruction of orthogonal chirp multiplexed signals based on the Kramers-Kroning relationship. It constructs a single-sideband OCDM modulation signal, and uses digital up-conversion technology to make the real part and the imaginary part of the frequency-converted OCDM signal satisfy the Kramers-Kroning relationship. The receiving end uses a KK receiver to reconstruct the real part and the imaginary part information of the OCDM signal, improving the receiving sensitivity of the communication system. Summary of the Invention

[0008] Object of the Invention: The object of the present invention is to provide a multi-carrier modulation scheme based on the reconstruction of orthogonal chirp multiplexed signals under the KK relationship. By constructing a single-sideband OCDM modulation signal and using digital up-conversion technology to make the real part and the imaginary part of the frequency-converted OCDM signal satisfy the Kramers-Kroning relationship, a KK receiver is used at the receiving end to reconstruct the real part and the imaginary part information of the OCDM signal, and finally improve the receiving sensitivity of the communication system.

[0009] Technical Solution: The multi-carrier modulation scheme based on the reconstruction of orthogonal chirp multiplexed signals under the KK relationship described in the present invention specifically includes the following steps:

[0010] S1: In an optical communication system, after the original binary bit stream undergoes serial-to-parallel conversion, it passes through a QAM mapping module to generate constellation symbols;

[0011] S2: Through an orthogonal chirp multiplexed OCDM modulation module, the constellation symbols are modulated onto a group of orthogonal chirp waveforms to generate an OCDM signal. At this time, the OCDM signal is a complex-valued signal, and the orthogonality between the chirp waveforms is used to improve the transmission efficiency and dispersion resistance;

[0012] S3: After adding a cyclic prefix to the modulated OCDM signal through an orthogonal chirp multiplexing OCDM up-conversion module, perform digital domain up-conversion to obtain a single-sideband OCDM complex-valued signal that satisfies the minimum phase condition for the OCDM signal by loading virtual carriers;

[0013] S4: Pass the single-sideband OCDM complex-valued signal obtained in S3 through digital-to-analog conversion and an IQ modulator to load it onto an optical wave for transmission in an optical fiber channel;

[0014] S5: At the receiving end, receive the single-sideband OCDM signal transmitted through the optical fiber channel in S4 through a photodetector PD, perform analog-to-digital conversion, and then send it to an offline data signal processing DSP for processing;

[0015] S6: In the offline data signal processing DSP, extract the phase information from the signal amplitude through a KK receiver to reconstruct the OCDM complex-valued signal;

[0016] S7: Restore the original information through an orthogonal chirp multiplexing OCDM demodulation module and a demapping module.

[0017] Preferably, the QAM mapping module in S1 maps the original bit information into high-order constellation symbols, using the high-order constellation symbols to carry more bit information and improve the transmission rate.

[0018] Preferably, the orthogonal chirp multiplexing OCDM modulation module in S2 is based on a group of orthogonal chirp subcarriers, using the orthogonal chirp subcarriers within a certain bandwidth to transmit information. Among them, the orthogonal chirp multiplexing modulation is implemented through a discrete Fresnel transform DFnT. The orthogonal chirp baseband signal after the discrete inverse Fresnel transform is expressed as:

[0019]

[0020] where represents the discrete inverse Fresnel transform operation, and x(k) represents the k-th chirp symbol of the modulation. According to formula (1), it can be seen that the discrete Fresnel transform needs to distinguish between odd and even cases;

[0021] During the orthogonal frequency division multiplexing modulation process, the discrete Fourier transform of the (m,n)th element can be expressed as:

[0022]

[0023] And further from formula (1), is expressed as:

[0024]

[0025] Thus, the discrete Fresnel transform matrix can be composed of a discrete Fourier transform matrix and an additional quadratic phase, and the additional quadratic phases are respectively:

[0026]

[0027] And:

[0028]

[0029] Therefore, the discrete Fresnel transform can be replaced by the discrete Fourier transform.

[0030] Preferably, the orthogonal chirp multiplexing OCDM up-conversion module in S3 performs digital domain up-conversion on the OCDM signal, retains the real part information after digital frequency conversion for intensity modulation transmission, and at the same time makes the real part and the imaginary part of the OCDM signal satisfy the Kramers-Kroning relationship. Specifically:

[0031] Determine the baud rate B of the signal, and perform digital domain frequency conversion operation on the OCDM signal using a virtual carrier. The signal after frequency conversion is expressed as:

[0032]

[0033] where j represents the imaginary unit, f s = B / 2, the constant A represents the amplitude of the virtual carrier, and |A| > max(|s(t)|) to meet the minimum phase condition;

[0034] Considering that the baseband OCDM signal is a complex-valued signal, it is further deduced that:

[0035]

[0036] The real part and the imaginary part of the signal after frequency conversion are respectively expressed as: s fr (t) and s fi (t). According to the Hilbert transform local product theorem, perform the Hilbert transform on s fr (t) to obtain:

[0037]

[0038] After digital domain frequency conversion, the imaginary part of the OCDM signal is the Hilbert transform of the real part, satisfying the Kramers-Kroning relationship.

[0039] Preferably, when receiving the single-sideband OCDM signal at the receiving end in S5, it is necessary to remove the stray light through a filter.

[0040] Preferably, in S6, the KK receiver extracts the phase information from the signal amplitude and reconstructs the OCDM complex-valued signal specifically as:

[0041] S6.1: Oversample the signal and use the square root as the signal amplitude information;

[0042] S6.2: Take the logarithm and perform Hilbert transform on the oversampled signal to obtain the signal phase information;

[0043] S6.3: Reconstruct the OCDM complex-valued signal using the amplitude information and the phase information.

[0044] Beneficial effects: By adopting a group of orthogonal chirp signals, the present application improves the dispersion resistance and multipath problems of the system; by performing single-sideband modulation on the orthogonal chirp signals and constructing the Kramers-Kroning relationship for the real and imaginary parts after modulation, the receiving end uses the KK receiver to extract the phase information from the signal amplitude information and reconstructs the orthogonal chirp signal, improving the receiving sensitivity of the system. Description of the Drawings

[0045] Figure 1 is a schematic diagram of the overall solution of the present application;

[0046] Figure 2 is a schematic diagram of a group of orthogonal chirp subcarriers in the present application;

[0047] Figure 3 is a schematic diagram of the OCDM modulation process in the present application;

[0048] Figure 4 is a flowchart of the KK receiver algorithm in the present application. Detailed Embodiments

[0049] The following further elaborates on the present invention in combination with specific embodiments, as Figure 1 shown in the schematic diagram of the solution. In this embodiment, the following steps are specifically included:

[0050] S1: In an optical communication system, after the original binary bit stream undergoes serial-to-parallel conversion, it passes through a QAM mapping module to generate constellation symbols. Specifically, the QAM mapping module maps the original bit information into high-order constellation symbols, using the high-order constellation symbols to carry more bit information, thereby improving the transmission rate.

[0051] S2: Through the orthogonal chirp multiplexing OCDM modulation module, modulate the constellation symbols onto a group of orthogonal chirp waveforms as Figure 2 shown to generate an OCDM signal. As Figure 3 shown in the schematic diagram of the OCDM modulation process, the OCDM signal at this time is a complex-valued signal, using the orthogonality between the chirp waveforms to improve the transmission efficiency and dispersion resistance. Specifically:

[0052] The orthogonal chirp division multiplexing OCDM modulation module is based on a set of orthogonal chirp subcarriers, and uses the orthogonal chirp subcarriers within a certain bandwidth to transmit information. Among them, the orthogonal chirp division multiplexing modulation is implemented through the discrete Fresnel transform DFnT. The orthogonal chirp baseband signal after the discrete inverse Fresnel transform is expressed as:

[0053]

[0054] Where represents the discrete inverse Fresnel transform operation, and x(k) represents the k-th chirp symbol of the modulation. According to formula (1), it can be seen that the discrete Fresnel transform needs to distinguish between odd and even cases;

[0055] During the orthogonal frequency division multiplexing modulation process, the discrete Fourier transform of the (m,n)th element can be expressed as:

[0056]

[0057] And further from formula (1), is expressed as:

[0058]

[0059] Thus, the discrete Fresnel transform matrix can be composed of the discrete Fourier transform matrix and the additional quadratic phase. The additional quadratic phases are respectively:

[0060]

[0061] And:

[0062]

[0063] Therefore, the discrete Fresnel transform can be replaced by the discrete Fourier transform.

[0064] S3: After adding a cyclic prefix to the modulated OCDM signal through the orthogonal chirp division multiplexing OCDM up-conversion module, perform digital domain up-conversion, and use the loaded virtual carrier to make the OCDM signal satisfy the minimum phase condition of the single-sideband OCDM complex signal; specifically, the orthogonal chirp division multiplexing OCDM up-conversion module performs digital domain up-conversion on the OCDM signal, retains the real part information after digital frequency conversion for intensity modulation transmission, and at the same time makes the real part and the imaginary part of the OCDM signal satisfy the Kramers-Kroning relationship. The steps are as follows:

[0065] Determine the baud rate B of the signal, and perform digital domain frequency conversion operation on the OCDM signal using the virtual carrier. The signal after frequency conversion is expressed as:

[0066]

[0067] where j represents the imaginary unit, and f s = B / 2, the constant A represents the amplitude of the virtual carrier, and |A| > max(|s(t)|) to satisfy the minimum phase condition;

[0068] Considering that the baseband OCDM signal is a complex-valued signal, it is further deduced that:

[0069]

[0070] The real and imaginary parts of the signal after frequency conversion are respectively expressed as: s fr (t) and s fi (t). According to the Hilbert transform local product theorem, the Hilbert transform of s fr (t) is obtained as:

[0071]

[0072] After digital domain frequency conversion, the imaginary part of the OCDM signal is the Hilbert transform of the real part, satisfying the Kramers-Kroning relationship.

[0073] S4: The single-sideband OCDM complex-valued signal obtained in S3 is loaded onto the optical wave through digital-to-analog conversion and an IQ modulator, and is transmitted in the optical fiber channel.

[0074] S5: At the receiving end, the single-sideband OCDM signal transmitted through the optical fiber channel in S4 is received by the photodetector PD. At this time, stray light needs to be removed through a filter, and then after analog-to-digital conversion, it is sent to the offline data signal processing DSP for processing.

[0075] S6: In the offline data signal processing DSP, the phase information is extracted from the signal amplitude through a KK receiver, and the OCDM complex-valued signal is reconstructed, which specifically includes the following steps:

[0076] S6.1: Oversample the signal and take the square root as the signal amplitude information;

[0077] S6.2: Take the logarithm and perform the Hilbert transform on the oversampled signal to obtain the signal phase information;

[0078] S6.3: Reconstruct the OCDM complex-valued signal using the amplitude information and the phase information.

[0079] S7: As Figure 4 shown, perform orthogonal chirp demodulation on the OCDM complex-valued signal reconstructed by the KK receiver. The demodulation process is the reverse of the modulation process. Through the reverse order operations of demodulation and QAM demapping, the original bit information is finally restored.

Claims

1. A multi - carrier modulation scheme based on the reconstruction of orthogonal chirp multiplexing signals under the KK relationship, characterized in that: Specifically, it includes the following steps: S1: In an optical communication system, after the original binary bit stream undergoes serial-to-parallel conversion, it passes through a QAM mapping module to generate constellation symbols. S2: Through an orthogonal chirp multiplexing OCDM modulation module, the constellation symbols are modulated onto a group of orthogonal chirp waveforms to generate an OCDM signal. At this time, the OCDM signal is a complex-valued signal, and the orthogonality between the chirp waveforms is utilized to improve the transmission efficiency and dispersion resistance. S3: Through an orthogonal chirp multiplexing OCDM up-conversion module, a cyclic prefix is added to the modulated OCDM signal and then digital-domain up-conversion is performed. By loading virtual carriers, a single-sideband OCDM complex-valued signal that satisfies the minimum-phase condition is obtained for the OCDM signal. S4: The single-sideband OCDM complex-valued signal obtained in S3 is converted from digital to analog and loaded onto an optical wave through an IQ modulator, and then transmitted in an optical fiber channel. S5: At the receiving end, the single-sideband OCDM signal transmitted through the optical fiber channel in S4 is received by a photodetector PD, and after analog-to-digital conversion, it is sent to an offline data signal processing DSP for processing. S6: In the offline data signal processing DSP, the phase information is extracted from the signal amplitude through a KK receiver to reconstruct the OCDM complex-valued signal. S7: The original information is restored through an orthogonal chirp multiplexing OCDM demodulation module and a demapping module.

2. The multi-carrier modulation scheme for reconstructing an orthogonal chirp multiplexing signal based on the KK relationship according to claim 1, wherein: The QAM mapping module in S1 maps the original bit information into high-order constellation symbols, and uses the high-order constellation symbols to carry more bit information to improve the transmission rate.

3. The multi-carrier modulation scheme for reconstructing orthogonal chirp multiplexed signals based on the KK relationship according to claim 1, wherein: The orthogonal chirp multiplexing OCDM modulation module in S2 is based on a group of orthogonal chirp subcarriers, and uses the orthogonal chirp subcarriers within a certain bandwidth to transmit information. Among them, the orthogonal chirp multiplexing modulation is realized through a discrete Fresnel transform DFnT. The orthogonal chirp baseband signal after the discrete inverse Fresnel transform is expressed as: where represents the discrete inverse Fresnel transform operation, and x(k) represents the k-th chirp symbol of the modulation. According to formula (1), it can be seen that the discrete Fresnel transform needs to distinguish between odd and even cases; In the orthogonal frequency division multiplexing modulation process, the discrete Fourier transform of the (m,n)th element can be expressed as: And further from formula (1), is expressed as: Thus, the discrete Fresnel transform matrix can be composed of the discrete Fourier transform matrix and an additional quadratic phase, and the additional quadratic phases are respectively: And: Therefore, the discrete Fresnel transform can be replaced by the discrete Fourier transform.

4. The multi - carrier modulation scheme for reconstructing orthogonal chirp multiplexing signals based on the KK relationship according to claim 1, characterized in that: The orthogonal chirp multiplexing OCDM up-conversion module in S3 performs digital-domain up-conversion on the OCDM signal, retains the real part information after digital frequency conversion for intensity modulation transmission, and at the same time makes the real part and the imaginary part of the OCDM signal satisfy the Kramers-Kroning relationship. Specifically: Determine the baud rate B of the signal, and perform digital-domain frequency conversion operation on the OCDM signal using virtual carriers. The signal after frequency conversion is expressed as: where j represents the imaginary unit, and f s = B / 2, the constant A represents the amplitude of the virtual carrier, and |A| > max(|s(t)|) to satisfy the minimum phase condition; Considering that the baseband OCDM signal is a complex-valued signal, it is further deduced that: The real part and the imaginary part of the signal after frequency conversion are respectively expressed as: s fr (t) and s fi (t). According to the local product theorem of the Hilbert transform, performing the Hilbert transform on s fr (t) gives: After digital-domain frequency conversion, the imaginary part of the OCDM signal is the Hilbert transform of the real part, satisfying the Kramers-Kroning relationship.

5. The multi-carrier modulation scheme for reconstructing orthogonal chirp multiplexed signals based on the KK relationship according to claim 1, characterized in that: When receiving the single-sideband OCDM signal at the receiving end in S5, it is necessary to remove the stray light through a filter.

6. The multi - carrier modulation scheme for reconstructing orthogonal chirp multiplexing signals based on the KK relationship according to claim 1, wherein: In S6, the process of extracting the phase information from the signal amplitude through the KK receiver and reconstructing the OCDM complex-valued signal is specifically: S6.1: Oversample the signal and take the square root as the signal amplitude information; S6.2: Take the logarithm and perform Hilbert transform on the oversampled signal to obtain the signal phase information; S6.3: Reconstruct the OCDM complex-valued signal using the amplitude information and the phase information.

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