PON high signal-to-noise ratio modulation method

Abstract

The invention discloses a PON high signal-to-noise ratio modulation method, and belongs to the field of optical fiber communication. According to the invention, the in-band quantization noise obtained by the first Delta-Sigma modulator is modulated through a cascade method, and the in-band quantization noise of the original input optical signal is reduced. The in-band quantization noise obtained in the first modulator is extracted by using the decimation filter, and the quantization noise in the signal bandwidth is further reduced by oversampling the in-band quantization noise obtained in one modulator, so that the quantization noise generated by a subsequent in-band noise modulation part can be reduced. And the output signal of the third modulator and the output signal of the second quantization noise modulator generate a new signal through Gray coding. A new signal generated by Gray coding is aggregated with an output signal of the first modulator by using TDM (Time Division Multiplexing), and the aggregated optical signal is transmitted in the RCF, so that the quantization noise of an optical communication system is reduced, and the high signal-to-noise ratio transmission of the optical communication system is realized.

Description

Technical Field

[0001] The invention relates to a PON high signal-to-noise ratio modulation method, and belongs to the field of optical fiber communication. Background Art

[0002] In the past decade, the development of passive optical networks (PON) has provided the bandwidth required to meet the needs of modern information services such as cloud computing, high-speed Internet, and intelligent transportation systems. At the same time, fiber-optic communication systems based on intensity modulation and direct detection (IM / DD) have attracted widespread attention due to their strong anti-interference ability, high sensitivity, and good reliability, and are ideal for the next generation of passive optical networks. With the development of capacity expansion technology, the transmission capacity has approached the Shannon limit. Various network services have increased the capacity demand of the access network, and improving the transmission rate has become an important development trend of the next generation of passive optical networks. The emergence of mode division multiplexing (MDM) systems has provided a new idea for increasing capacity. However, in mode division multiplexing systems, there is random coupling between different modes, and the energy of one mode will be coupled to other modes, seriously affecting the performance of the communication system. Therefore, MDM-PON systems require a high signal-to-noise ratio to improve transmission performance.

[0003] However, for IM / DD transmission, in order to reduce the computational complexity, the resolution of the analog-to-digital converter at the receiving end is always set to a lower value. The output of the low-resolution analog-to-digital converter cannot accurately represent the original analog signal, resulting in quantization error and quantization noise. Especially in MDM-PON IM / DD transmission, the random coupling between different modes will seriously affect the performance of the communication system, and a high-performance analog-to-digital converter is usually required to equalize the signal impairment. However, the low-resolution analog-to-digital converter will seriously reduce the signal-to-noise ratio (SNR) of the MDM-PON signal. The Delta-Sigma Modulator (DSM) uses noise shaping technology to reduce in-band noise, which can be used to reduce the resolution requirements of the analog-to-digital converter and the complexity of the receiver signal processing algorithm. In order to further improve the signal-to-noise ratio of the system, a variety of Delta-Sigma modulators have been proposed, such as high-order Delta-Sigma modulators, multi-bit Delta-Sigma modulators, and multi-level Delta-Sigma modulators. In particular, for the multi-level Delta-Sigma modulator (CDSM), high-order noise shaping and good stability can be achieved, which is a good choice for achieving high signal-to-noise ratio transmission in the MDM-PON system. Summary of the invention

[0004] The purpose of the present invention is to provide a PON high signal-to-noise ratio modulation method, which is based on a cascaded Delta-Sigma modulator and uses a Delta-Sigma coding method to improve the signal-to-noise ratio of optical communication system signals, realize high-order signal transmission and noise shaping in the optical fiber communication system, and significantly improve the reliability of the optical communication system.

[0005] The objective of the present invention is achieved through the following technical solutions:

[0006] A PON high signal-to-noise ratio modulation method disclosed in the present invention comprises the following steps:

[0007] Step 1: Delta-Sigma modulation method is divided into quantization, oversampling, noise shaping and filtering. When sampling the optical communication input signal, the quantization noise is dispersed in the entire signal bandwidth. When the sampling frequency of the Delta-Sigma modulation method is much higher than the signal bandwidth, the quantization noise is evenly distributed in the entire sampling bandwidth, reducing the quantization noise in the signal bandwidth. Noise shaping squeezes the in-band quantization noise out of the signal band by setting the noise transfer function NTF, thereby improving the signal-to-noise ratio. The low-pass filter LPF is used to filter out the out-of-band quantization noise at the receiving end.

[0008] Step 1.1: Delta-Sigma modulation method is divided into quantization, oversampling, noise shaping and filtering. In the sampling of optical communication input signal, the quantization noise is dispersed in the entire signal bandwidth. When the sampling frequency of Delta-Sigma modulation method is much higher than the signal bandwidth, the quantization noise is evenly distributed in the entire sampling bandwidth, reducing the quantization noise in the signal bandwidth. The oversampling ratio OSR represents the ratio of sampling rate to signal bandwidth:

[0009]

[0010] Where: f s is the sampling rate and BW is the signal bandwidth.

[0011] Step 1.2: Noise shaping squeezes the in-band quantization noise out of the signal band by setting the noise transfer function NTF, thereby improving the signal-to-noise ratio. The q-order noise transfer function NTF of the Delta-Sigma modulation method is expressed as:

[0012]

[0013] Where m is the zero point and n is the pole, which is determined by the oversampling ratio OSR, the modulator order and the maximum radius of the noise transfer function NTF pole.

[0014] Step 1.3: Use a low-pass filter (LPF) to filter out the out-of-band quantization noise at the receiving end. However, there is still quantization noise in the signal frequency band that affects the transmission of the optical signal.

[0015] The sampling frequency being much higher than the signal bandwidth means that the sampling frequency is more than 2 times higher than the signal bandwidth.

[0016] Step 2: Use a multi-stage Delta-Sigma modulator to extract the in-band quantization noise obtained from the first Delta-Sigma modulator, and use a filter to extract the in-band quantization noise obtained from the first Delta-Sigma modulator. After quantization and noise shaping of the extracted in-band quantization noise, the generated signal and the output signal of the first Delta-Sigma modulator are transmitted through time division multiplexing TDM. By subtracting the quantization noise in the signal at the receiving end, the in-band noise is eliminated, thereby improving the signal-to-noise ratio of the optical communication system.

[0017] Step 3: Construct a high signal-to-noise ratio Delta-Sigma modulation method, modulate the in-band quantization noise obtained in step 2 through a cascaded Delta-Sigma modulation method, and reduce the in-band quantization noise. The transmitter of the cascaded Delta-Sigma modulation method includes two parts: signal modulation and in-band noise modulation. The signal modulation part is quantization, oversampling, and noise shaping. The in-band quantization noise of the cascaded Delta-Sigma modulation method is obtained from the first Delta-Sigma modulator, and the in-band quantization noise obtained from the first Delta-Sigma modulator is extracted by a decimation filter, which helps to reduce the quantization noise generated by the subsequent in-band noise modulation part. The obtained quantization noise is the input of the in-band noise modulation part. The decimation filter is composed of an anti-aliasing FIR filter and a downsampler. The quantization noise of the second quantization noise Delta-Sigma modulator is extracted by a decimation filter, and the output signal of the third Delta-Sigma modulator is obtained after amplification, quantization, oversampling and noise shaping. The output signal D3 of the third Delta-Sigma modulator and the output signal D2 of the second quantization noise Delta-Sigma modulator are Gray-coded to generate a new signal D4. The new signal generated by Gray coding is aggregated with the output signal D1 of the first Delta-Sigma modulator using time division multiplexing TDM to obtain an aggregated optical signal, which is transmitted in the ring optical fiber RCF. At the receiving end, the received signal is restored to signals D1', D'2 and D3', where D'2 and D3' are decoded by a time division multiplexer. After the signals D1', D'2 and D3' pass through a low-pass filter, low-pass filtered signals S1', S2' and S3' are obtained respectively. S3' is attenuated using the same ratio as the amplification ratio in the above-mentioned transmitting end to obtain the quantization noise Q′2 of the second Delta-Sigma modulator. The quantization noise generated by the first Delta-Sigma modulator is S'2-Q'2. The original signal of the quantization noise Q′1 of the first Delta-Sigma modulator is restored to S1′-Q1′, that is, high signal-to-noise ratio Delta-Sigma coded optical communication is realized.

[0018] Beneficial effects:

[0019] 1. A PON high signal-to-noise ratio modulation method disclosed in the present invention modulates the in-band quantization noise obtained by the first Delta-Sigma modulator through a cascade method, thereby reducing the in-band quantization noise of the original input optical signal, thereby reducing the resolution requirements for the analog-to-digital converter and the complexity of the receiver DSP algorithm.

[0020] 2. The quantization noise of the multi-level Delta-Sigma modulation method will be distributed in the entire sampling bandwidth. Compared with the multi-level Delta-Sigma modulation method, the PON high signal-to-noise ratio modulation method disclosed in the present invention is based on a cascaded Delta-Sigma modulator, and uses a decimation filter to extract the in-band quantization noise obtained in the first Delta-Sigma modulator. The quantization noise in the signal bandwidth is further reduced by oversampling the in-band quantization noise obtained in a Delta-Sigma modulator, which helps to reduce the quantization noise generated by the subsequent in-band noise modulation part.

[0021] 3. A PON high signal-to-noise ratio modulation method disclosed in the present invention is based on cascaded Delta-Sigma modulators, and the output signal of the third Delta-Sigma modulator and the output signal of the second quantization noise Delta-Sigma modulator are combined by Gray coding to generate a new signal. The new signal generated by Gray coding is aggregated with the output signal of the first Delta-Sigma modulator by time division multiplexing TDM to obtain an aggregated optical signal, which is transmitted in a ring optical fiber RCF, which helps to reduce the quantization noise of the optical communication system and realize high signal-to-noise ratio transmission of the optical communication system. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 (a) Schematic diagram of the spectrum of the MDSM scheme; (b) Schematic diagram of the spectrum of the CDSM scheme; (c) quantization noise comparison diagram of the MDSM scheme and the CDSM scheme.

[0023] Figure 2 is the frequency response and pole-zero diagram of NTF, where Figure 2 (a) is the frequency response, Figure 2 (b) is the pole-zero diagram.

[0024] Figure 3 The present invention discloses a flow chart of a PON high signal-to-noise ratio modulation method.

[0025] Figure 4 Figure 2 is the relationship between the bit error rate and the received optical power (ROP) in the MDM-PON IM / DD transmission system with a 2km RCF when (a) l=2 and (b) l=3.

[0026] Figure 5 (a) DMT waveform recovered in the transmit / receive time domain; (b) enlarged view of the constellation part of the 16777216QAM transmitter; (c) constellation diagram recovered at the 16777216QAM receiver.

[0027] Figure 6 A system block diagram of an OAM optical communication system provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0028] The present invention will be described in detail below with reference to the accompanying drawings and embodiments. The technical problems solved by the technical solution of the present invention and the beneficial effects are also described. It should be noted that the described embodiments are only intended to facilitate the understanding of the present invention and do not have any limiting effect on it.

[0029] In MDM-PON transmission, in order to reduce the computational complexity, the ADC resolution at the receiving end is always set to a lower value. However, the output of the low-resolution ADC cannot accurately represent the original analog signal, and a high-performance ADC is required to equalize the signal impairment.

[0030] Example 1

[0031] like Figure 3 As shown, a PON high signal-to-noise ratio modulation method disclosed in this embodiment is specifically implemented in the following steps:

[0032] Step 1: Set the OSR of DSM1 to 8 and the quantization bit to 2 to generate the PAM4 signal D1. The order of DSM is 6, the oversampling ratio is 8, and the maximum radius of the NTF pole is 0.95. After running the closed-loop analysis of the noise shaping (CLANS) function, the NTF expression can be expressed as follows based on the generated zeros and poles:

[0033]

[0034] The frequency response and pole-zero diagram of NTF are shown in Figure 2 (a) and (b). In the zero-pole diagram, the six zeros are located on the unit circle and distributed within the signal frequency band, making the noise power within the band lower. At the same time, the poles are distributed within the unit circle, making the system more stable. The frequency response curve shows the noise shaping performance, with a lower response within the signal frequency band, suppressing quantization noise.

[0035] Step 2: In the signal modulation part, a system block diagram of an OAM optical communication system provided in the embodiment is as follows: Figure 6 As shown. Use DSM1 to convert the input DMT signal S1 into a first-order digital signal D1, as shown Figure 3As shown. The OSR of DSM1 is 8. The quantization noise P1 is obtained by S1-D1 as the input of the in-band noise modulation part. Then P1 passes through the decimation filter, which is composed of an anti-aliasing FIR filter and a downsampler. The output signal Q1 of the decimation filter is the in-band noise of D1. The sampling rate of the output Q1 of the decimation filter is 1 / M times the original sampling rate. The output Q1 is the input of DSM2 and is also the in-band noise of the received DMT signal S1' corresponding to D1. Therefore, Q1 is reconstructed at the receiving end, the in-band noise of the received signal S1' is eliminated, and the ultra-high-order QAM signal is restored. In the in-band noise modulation part, the signal Q1 is amplified to meet the input requirements of DSM2, and the generated D2 consists of 2 pulse amplitudes, which only contain +1 and -1 levels, that is, OOK signal. Similarly, the second quantization error P2 is extracted by S2-D2. Then after amplification, quantization, oversampling, and noise shaping, D3 is obtained. Finally, the outputs D2 and D3 are encoded into D4. D1 and the coded signal D4 are then aggregated through time division multiplexing (TDM). The receiving end is the inverse process of modulation, and the DMT signal is restored using high-order QAM. At the receiving end, the received signal is restored to D1', D2' and D3', of which D2' and D3' are restored by decoding. After low-pass filtering, S1', S2' and S3' are obtained respectively. S2'-Q2' is the quantization noise generated by DSM2, so Q1 can be obtained more accurately at the receiving end. In addition, S1'-Q1' can further eliminate the in-band noise of the recovered DMT signal S1'.

[0036] Step 3: Generate a pseudo-random binary sequence (PRBS) at the transmitter, and then perform standard QAM mapping. The QAM signal is converted into a DMT signal with an FFT size of 1024, in which 900 subcarriers are loaded with data. In the CDSM modulator, the OSR of the 2-bit DSM1 is set to 8 and quantized into a PAM4 signal. The generated PAM4 signal is subjected to quantization noise extraction and filtering to obtain the quantization noise. The OSR of the 1-bit DSM2 and the 1-bit DSM3 is set to 6 and quantized into an OOK signal to improve the effective utilization of the spectrum. The output OOK signals of DSM2 and DSM3 are encoded into PAM4 signals, and the output signal of DSM1 is time-division multiplexed for transmission. After time-division multiplexing and upsampling, the electrical signal is generated by an arbitrary waveform generator (AWG) with a sampling rate of 120GSa / s and amplified by an electronic amplifier (EA). The Mach-Zehnder modulator (MZM) is used to modulate the generated 40Gbaud PAM4 electrical signal onto the optical carrier. An external cavity laser (ECL) is used to generate optical carriers with a wavelength of 1550.12nm. The optical signal is divided into two branches by an optical coupler (OC) and amplified by an erbium-doped fiber amplifier (EDFA), one of which uses a 10m SMF delay for data mode decorrelation. The two beams are injected into a spatial light modulator (SLM) through a collimator (col) and a linear polarizer (LP), and the polarization direction of the light is adjusted to convert it into an OAM beam with l=+2 and l=+3. The polarization direction is optimized by adjusting the half-wave plate (HWP), and the two OAM beams are merged into a single beam by the PBS. The multiplexed OAM beam is converted into a circular polarization state by a quarter-wave plate (QWP) and coupled into a 2.3km RCF.

[0037] At the receiving end, the multiplexed OAM mode is separated into two beams by LP and BS and converted into linearly polarized beams by QWP. These beams are then converted into Gaussian beams using a vortex phase plate (VPP) with opposite topological charge. The waveforms are recorded at 256GSa / s using a real-time oscilloscope and processed by an offline DSP module. The offline DSP includes resampling, low-pass filter, synchronization, PAM-HD, decimation filter, DMT demodulation, and bit error rate calculation.

[0038] The discriminated symbols are compared with the transmitted symbols, and the QPSK symbol bit error rate is calculated to obtain the bit error rate performance of the proposed high signal-to-noise ratio multi-stage Delta-Sigma modulator under the optical signal-to-noise ratio. The equalization results based on the high signal-to-noise ratio multi-stage Delta-Sigma modulator are compared with the MSE and bit error rate performance of the SGD MIMO equalizer and the Adam MIMO equalizer under three equalization conditions. Figure 4As shown. Keeping the bit rate unchanged, analyze the effect of different DSM optimizations to restore the DMT signal. The NTF of the MDSM and TDSM schemes on the DSM modulated PAM4 signal and the NTF after modulation are set as formula 3. The CDSM scheme that transmits 16,777,216 QAM signals is better than the other two schemes in terms of bit error rate, as shown in Figure 4 As shown. Under the 15% FEC (forward error correction) threshold, the gains of the two modes (l = +2 and l = +3) are improved by about 4dB and 3dB respectively compared with the second-order DSM scheme. When rop is 0-6dBm, whether l = +2 or l = +3, the bit error rate of the TDSM and MDSM schemes cannot reach below 4.2×10-2 to support 16,777,216 QAM signaling. Therefore, this embodiment can achieve 240Gbit / s 16777216QAM signal transmission.

[0039] Figure 5 (a) is a comparison between the DMT waveform at the transmitter and the restored waveform when ROP is 6dBm. The waveforms are basically the same. Figure 5 (b) and Figure 5 (c) are the constellation diagram at the transmitting end and the recovered constellation diagram at the receiving end, respectively. The CDSM scheme can accurately recover the original 16777216QAM signal.

[0040] The specific description above further illustrates the purpose, technical solutions and beneficial effects of the invention in detail. It should be understood that the above is only a specific embodiment of the present invention and does not limit the scope of protection of the present invention. Any modifications, replacements, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A PON high signal-to-noise ratio modulation method, characterized in that: The following steps are included: Step 1: The Delta-Sigma modulation method is divided into quantization, oversampling, noise shaping and filtering. When sampling the optical communication input signal, the quantization noise is dispersed in the entire signal bandwidth; when the sampling frequency of the Delta-Sigma modulation method is much higher than the signal bandwidth, the quantization noise is evenly distributed in the entire sampling bandwidth, reducing the quantization noise in the signal bandwidth; Noise shaping squeezes the in-band quantization noise out of the signal band by setting the noise transfer function NTF, thus improving the signal-to-noise ratio; Use a low-pass filter LPF to filter out the out-of-band quantization noise at the receiving end; Step 2: extracting in-band quantization noise using a multi-stage Delta-Sigma modulator, the in-band quantization noise is obtained from the first Delta-Sigma modulator, and extracting the in-band quantization noise obtained from the first Delta-Sigma modulator using a filter; after quantizing and noise shaping the extracted in-band quantization noise, the generated signal and the output signal of the first Delta-Sigma modulator are transmitted through time division multiplexing TDM; by subtracting the quantization noise in the signal at the receiving end, the in-band noise is eliminated, thereby improving the signal-to-noise ratio of the optical communication system; Step 3: construct a high signal-to-noise ratio Delta-Sigma modulation method, modulate the in-band quantization noise obtained in step 2 through a cascaded Delta-Sigma modulation method, and reduce the in-band quantization noise; the transmitter of the cascaded Delta-Sigma modulation method includes two parts: signal modulation and in-band noise modulation; The signal modulation part includes quantization, oversampling and noise shaping; the in-band quantization noise of the cascaded Delta-Sigma modulation method is obtained from the first Delta-Sigma modulator, and the in-band quantization noise obtained from the first Delta-Sigma modulator is extracted by a decimation filter, which is helpful to reduce the quantization noise generated by the subsequent in-band noise modulation part; the obtained quantization noise is the input of the in-band noise modulation part; the decimation filter is composed of an anti-aliasing FIR filter and a downsampler; the quantization noise of the second quantization noise Delta-Sigma modulator is extracted by the decimation filter, and the quantization noise is amplified, quantized, oversampled and noise shaped to obtain the output signal of the third Delta-Sigma modulator; the output signal D3 of the third Delta-Sigma modulator and the output signal D2 of the second quantization noise Delta-Sigma modulator are combined by Gray coding to generate a new signal D 4; Use time division multiplexing TDM to generate a new signal for Gray coding and aggregate it with the output signal D1 of the first Delta-Sigma modulator to obtain an aggregated optical signal, and the aggregated optical signal is transmitted in the ring optical fiber RCF; at the receiving end, the received signal is restored to signals D′1, D′2 and D′3, among which D′2 and D′3 are decoded by a time division multiplexer; after the signals D′1, D′2 and D′3 are passed through a low-pass filter, low-pass filtered signals S′1, S'2 and S′3 are obtained respectively; S′3 is attenuated at the same ratio as the amplification ratio in the above-mentioned transmitting end to obtain the quantization noise Q′2 of the second Delta-Sigma modulator; the quantization noise generated by the first Delta-Sigma modulator is S'2-Q'2; the original signal of the quantization noise Q′1 of the first Delta-Sigma modulator is restored to S′1-Q′1, that is, high signal-to-noise ratio Delta-Sigma coded optical communication is realized.

2. A PON high signal-to-noise ratio modulation method as claimed in claim 1, characterized in that: The implementation method of step one is: Step 1.1: Delta-Sigma modulation method is divided into quantization, oversampling, noise shaping and filtering. In the sampling of optical communication input signal, quantization noise is dispersed in the entire signal bandwidth; when the sampling frequency of Delta-Sigma modulation method is much higher than the signal bandwidth, quantization noise is evenly distributed in the entire sampling bandwidth, reducing the quantization noise in the signal bandwidth; oversampling ratio OSR represents the ratio of sampling rate to signal bandwidth: Where: f s is the sampling rate, BW is the signal bandwidth; Step 1.2: Noise shaping squeezes the in-band quantization noise out of the signal band by setting the noise transfer function NTF, thereby improving the signal-to-noise ratio; the q-order noise transfer function NTF of the Delta-Sigma modulation method is expressed as: Where m is the zero point and n is the pole, which is determined by the oversampling ratio OSR, the modulator order and the maximum radius of the noise transfer function NTF pole; Step 1.3: Use a low-pass filter (LPF) to filter out the out-of-band quantization noise at the receiving end.

3. A PON high signal-to-noise ratio modulation method as claimed in claim 1 or 2, characterized in that: The sampling frequency being much higher than the signal bandwidth means that the sampling frequency is more than 2 times higher than the signal bandwidth.

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