A low complexity digital predistorter based on frequency domain comb pilot channel estimation

Abstract

The application belongs to the technical field of optical communication, and particularly relates to a low-complexity digital pre-equalizer based on a frequency-domain comb pilot channel estimation. The application comprises a data sending module, a channel response testing module and a digital signal pre-equalization module. The data sending module comprises an initial comb signal generation and a frequency component phase rotation. The channel response testing module comprises a signal frequency difference estimation and recovery and a channel amplitude-frequency and phase-frequency response testing. The digital signal pre-equalization module comprises a filter coefficient generation and a sending signal convolution pre-equalization. The application analyzes the channel amplitude-frequency and phase-frequency responses by sending a frequency-domain comb pilot signal, and generates a pre-equalizer for pre-compensating the channel response at a sending end. The application has the characteristics of high granularity, high precision, robust transmission signal system and the like, and can be applied to various scenes such as polarization multiplexing long-distance single-carrier coherent optical transmission and mode multiplexing short-distance intensity modulation direct detection systems.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of optical fiber communication, and particularly relates to a digital pre-equalizer. BACKGROUND

[0002] In the 5G era and the coming 6G era, data center interconnection (DCI) is facing increasing data traffic demand due to applications such as artificial intelligence, machine learning and edge computing. Although coherent technology has made progress in short-distance applications, due to cost-effectiveness and low-power characteristics, intensity modulation direct detection (IM / DD) systems using multi-level pulse amplitude modulation (PAM) are still the preferred solution for 400 / 800 GbE. However, bandwidth limitation has become a major challenge for high-speed O-band transmission. Severe bandwidth limitation will cause severe high-frequency fading to the signal, causing severe inter-symbol interference, resulting in a serious decline in signal quality at the receiving end. Therefore, how to effectively solve the reduction of high-speed optical signal quality caused by bandwidth limitation has become a hot topic in high-speed optical communication research.

[0003] To alleviate bandwidth limitation, researchers have proposed filtering schemes such as adaptive post-equalization, pre-equalization (Pre-Eq) technology to solve the signal impairment caused by bandwidth limitation. The traditional pre-equalization (Pre-Eq) technology based on feedback filtering can effectively compensate for the transmission channel response by analyzing the post-equalization filter coefficients, but it is complex and unstable in channels with severe frequency-selective fading. Therefore, how to implement a low-complexity digital pre-equalizer based on frequency-domain comb pilot channel estimation is a crucial task. SUMMARY

[0004] In view of the above, the purpose of the present application is to provide a low-complexity digital pre-equalizer based on frequency-domain comb pilot channel estimation suitable for bandwidth-limited high-speed data center optical interconnection.

[0005] The present invention provides a low-complexity digital pre-equalizer based on frequency domain comb pilot channel estimation for bandwidth-limited high-speed data center optical interconnection systems. It not only has the adaptive advantages of traditional pre-equalization schemes based on feedback filtering, but also can improve the accuracy of channel measurement by detecting frequency domain comb signals, and further improve the robustness to frequency-selective fading caused by broadband limitations. In order to be able to measure the channel response of the transmission signal with high precision, the present invention adds random phase rotation at each frequency point, and further reduces the peak-to-average power ratio of the transmitted test signal by adjusting the phase rotation value, thereby avoiding the influence of the nonlinear noise of the channel on the measurement of the channel response. By performing frequency component analysis on the received frequency domain comb signal, the pre-equalizer can effectively measure the accurate channel amplitude-frequency response and phase-frequency response, thereby realizing channel response pre-compensation for the transmission signal at the transmitting end, overcoming the frequency-selective fading caused by the transmission channel. The low-complexity digital pre-equalizer based on frequency domain comb pilot channel estimation can be applied to a variety of optical communication transmission scenarios, especially in data center optical interconnection scenarios that are severely limited by bandwidth:

[0006] The low-complexity digital pre-equalizer based on frequency domain comb pilot channel estimation provided by the present invention specifically includes three parts: a data sending module, a channel response test module, and a digital signal pre-equalization module. Specifically:

[0007] (1) The data transmission module includes two parts: initial comb signal generation and frequency component phase rotation;

[0008] In the initial comb signal generation part, the transmitter first generates an initial frequency domain comb signal x for testing the channel response. comb_ini , which is at a specific frequency f comb_i Generate a frequency component a of a certain amplitude comb_i , i=1,2,…,m;as shown in formula (1)

[0009] As shown:

[0010]

[0011] Among them, f comb_i (i=1,2,…,m) is the frequency vector of the frequency domain comb signal, a comb_i (i=1,2,…,m) is the amplitude signal of the frequency component of the frequency domain comb signal, and m is the number of frequency components contained in the frequency domain comb signal. The number of frequency domain comb signals will affect the granularity of the channel response test, and thus affect the overall performance of the algorithm.

[0012] In the frequency component phase rotation part, the initial frequency domain comb signal performs a certain phase rotation on the frequency component at each frequency point to reduce the peak-to-average power ratio of the generated comb signal. The final generated frequency domain comb pilot signal xcomb As shown in the following formula (2):

[0013]

[0014] θ comb_i (i = 1, 2, …, m) is a frequency component rotation phase vector of the frequency domain comb signal;

[0015] After the completion of the comb signal generation, the pilot signal will be output to the transmission channel, and the test of the amplitude and phase response of the transmission system is completed.

[0016] (ii) Channel response test module, comprising: signal frequency difference estimation and recovery, channel amplitude and phase response test;

[0017] In the signal frequency difference estimation and recovery part, the receiving end, after receiving the frequency domain comb pilot signal, first passes through frame synchronization, intercepts a complete frame, and then uses fast Fourier transform to the frequency domain. By analyzing the offset of each frequency component, the frequency deviation f o of the received signal at the receiving end can be quickly estimated. o After obtaining the frequency deviation value f comb_afFOE , the frequency offset compensation is performed on the received signal to obtain the frequency domain comb signal y comb after recovery of the frequency offset. As shown in the following formula:

[0018]

[0019] y comb is the received frequency domain comb signal after channel transmission;

[0020] In the channel amplitude and phase response test part, after the completion of the corresponding frequency offset compensation, the modulus value of the frequency domain signal is found according to the preset frequency point f comb_i of the sending end, and the corresponding modulus value p comb_i and the coarse phase value are obtained. At this time, the amplitude frequency response of the system composed of the modulus value p comb_i has been obtained.

[0021] For the coarse phase value , the fine phase value is obtained by frequency correction according to the following formula:

[0022]

[0023] After correction, the fine phase value is shifted and phase-angled to simplify and eliminate phase jump:

[0024]

[0025] Wherein, and:

[0026]

[0027] After that, the obtained It is the phase frequency response of the system.

[0028] (Three) digital signal pre-equalization module, comprising: filter coefficient generation, sending signal convolution pre-equalization;

[0029] In the filter coefficient generation part, the pre-equalization filter tap coefficient length is L, and according to the obtained system amplitude frequency and phase frequency response, the corresponding sending end pre-equalization coefficient is generated through inverse response, as shown in the following formula:

[0030]

[0031] The sending signal convolution pre-equalization part is shown in the accompanying Figure 1 .

[0032] The to-be-sent signal u and W are convolved, and the pre-equalized signal v is obtained:

[0033] v=u⊙W (6)

[0034] The technical features and performance advantages of the present application mainly include:

[0035] (1) The number of frequency test components contained in the digital pre-equalizer based on the frequency domain comb-shaped pilot channel estimation of the present application, the length of the pre-equalizer tap coefficient generated and the specific phase rotation value of each frequency component should be determined according to the specific situation, which will have a crucial influence on the performance of the overall pre-equalization compensation frequency selected fading.

[0036] (2) The present application has strong flexibility and universality. It can be applied to intensity modulation direct detection transmission or coherent transmission optical communication system, and is suitable for various transmission scenarios including long-distance backbone network transmission, short-distance access network direct detection transmission and optical wireless transmission scenarios.

[0037] (3) The present application has extremely low computational complexity and extremely high channel robustness. The equalizer avoids the channel response obtained by complex post-equalization through the method of frequency domain component analysis, and realizes the effective reduction of the computational complexity required by pre-equalization. At the same time, since the present application can realize accurate channel response test at each frequency point, it has very high robustness to actual transmission channels.

[0038] The present application sends a frequency domain comb pilot signal to analyze channel amplitude and phase frequency response, to generate a pre-equalizer for pre-compensating channel response at a sending end; has high granularity, high precision, signal mode robustness, etc., and can be applied to polarization multiplexing long distance single carrier coherent optical transmission, mode multiplexing short distance intensity modulation direct detection system and other various scenes. BRIEF DESCRIPTION OF DRAWINGS

[0039] Figure 1 A low complexity digital pre-equalizer based on frequency domain comb pilot channel estimation.

[0040] Figure 2 A related amplitude and phase frequency response graph of the present application, wherein 13 is the corresponding transmission channel amplitude and phase frequency response graph, 14 is the generated pre-equalizer amplitude and phase frequency graph, 15 is the received signal spectrum graph without pre-equalization, 16 is the pre-equalization filter spectrum graph, and 17 is the received signal spectrum graph with equalization.

[0041] In the figure, 1 is an initial frequency domain comb pilot signal x comb_ini , 2 is a frequency component phase rotation module, 3 is a comb pilot signal x comb after frequency component rotation, 4 is an actual transmission system, 5 is a received comb pilot signal, 6 is a received end comb pilot signal frequency offset module, 7 is a frequency offset estimation value f o , 8 is a comb pilot signal y comb_afFOE after frequency offset recovery, 9 is a channel amplitude and phase frequency characteristic measurement module, 10 is a frequency point module value p comb_i , 11 is a frequency point coarse phase value 12 is a phase frequency correction module. DETAILED DESCRIPTION

[0042] The present application provides a low complexity digital pre-equalizer based on frequency domain comb pilot channel estimation for bandwidth limited high speed data center optical interconnection, which can compensate for high frequency fading caused by system bandwidth limitation, and can effectively recover frequency selective fading caused by non-perfect device response. The equalizer can be applied to various optical communication transmission systems including coherent or intensity modulation direct detection systems, and is illustrated by taking an intensity modulation direct detection system as an example.

[0043] Specifically includes three parts of "data sending", "channel response test", "digital signal pre-equalization":

[0044] (1) Data sending, including:

[0045] The principle of the "initial comb signal generation" part is shown in the attached Figure 1 .

[0046] The sending end first generates an initial frequency-domain comb signal x comb_ini at certain frequency f comb_i to test the channel response comb_i , which has a certain amplitude a o at certain frequency f o , as shown in the following formula

[0047]

[0048] The principle of the "frequency component phase rotation" part is shown in the following figure Figure 1 .

[0049] The initial frequency-domain comb signal rotates the phase of the frequency component at each frequency point to reduce the peak-to-average power ratio of the generated comb signal. The final frequency-domain comb pilot signal is shown in the following formula

[0050]

[0051] After the generation of the comb signal, the pilot signal is output to the transmission channel to complete the test of the amplitude and phase responses of the transmission system.

[0052] (2) The "channel response test" part includes:

[0053] The principle of the "signal frequency difference estimation and recovery" part is shown in the following figure Figure 1 .

[0054] The received pilot signal is first subjected to frame synchronization. After a complete frame is intercepted, it is converted to the frequency domain using fast Fourier transform. By analyzing the offset of each frequency component, the frequency offset f o received at the receiving end can be quickly estimated. After obtaining the frequency offset value f o , the frequency offset compensation of the received signal is completed, as shown in the following formula

[0055]

[0056] The principle of the "channel amplitude and phase response test" part is shown in the following figure Figure 1 .

[0057] After completing the corresponding frequency offset compensation, the modulus value of the frequency-domain signal is found to have the highest value at the frequency f comb_i preset by the sending end. The corresponding modulus value p comb_i and coarse phase value are obtained. At this time, the amplitude response of the system composed of the modulus value p comb_i has been obtained.

[0058] The coarse phase value needs to be frequency-corrected to obtain the fine phase value

[0059]

[0060] After correction, the fine phase value needs to be adjusted Simplify the phase angle and eliminate phase jumps.

[0061]

[0062] in and:

[0063]

[0064] Afterwards, the obtained This is the phase-frequency response of the system. The measured classic amplitude-frequency and phase-frequency responses of the transmission system are shown in the attached figure. Figure 2 As shown in (19), due to the bandwidth limitation of the transmission system, the system response shows very obvious high-frequency fading.

[0065] (3) “Digital signal pre-equalization” section, including:

[0066] The principle of "filter coefficient generation" is as shown in the attached Figure 1 shown.

[0067] Based on the obtained system amplitude-frequency and phase-frequency responses, this solution generates the corresponding transmitter pre-equalization coefficients through the inverse response, as shown in the following formula:

[0068]

[0069] The obtained classic spectrum response of the pre-equalization filter is shown in the attached figure. Figure 2 As shown in (20) and (22), in order to achieve the fidelity transmission of the high-frequency components of the signal, the spectrum response of the pre-equalization filter shows a very obvious high-frequency enhancement.

[0070] The principle of "transmit signal convolution pre-equalization" is as shown in the attached Figure 1 As shown. Convolving the signal u to be sent with W can complete the pre-equalized signal v:

[0071] v=u⊙W (6)

[0072] The spectrum of the signal without digital pre-equalization and the signal with digital pre-equalization after transmission is as shown in the attached figure. Figure 2 As shown in (21) and (23), digital pre-equalization enhances the high-frequency components in advance at the transmitting end to combat the high-frequency fading of the transmission channel and achieve signal transmission with high spectral flatness.

[0073] The present application utilizes a frequency domain comb pilot signal and a high-precision channel response estimation algorithm to realize a high-performance low-complexity digital pre-equalizer of a bandwidth-limited high-speed data center optical interconnection system, effectively solving the damage to signal quality caused by the bandwidth limitation of traditional optoelectronic devices. Meanwhile, the encoder greatly improves the tolerance of the channel response through high-precision high-granularity frequency point response analysis, greatly improves the pre-equalization performance, and provides a low-complexity reliable solution for super-high-speed data center optical interconnection.

Claims

1. A low-complexity digital pre-equalizer based on frequency-domain comb pilot channel estimation, characterized in that: Specifically include: The module consists of three parts: data transmission module, channel response test module, and digital signal pre-equalization module. The data sending module includes two parts: initial comb signal generation and frequency component phase rotation; In the initial comb signal generation part, the transmitter first generates an initial frequency domain comb signal x for testing the channel response. comb_ini , which is at a specific frequency f comb_i Generate a frequency component a of a certain amplitude comb_i , i=1,2,…,m; as shown in the following formula (1): Among them, f comb_i is the frequency vector of the frequency domain comb signal, a comb_i is the frequency component amplitude signal of the frequency domain comb signal, and m is the number of frequency components contained in the frequency domain comb signal; In the frequency component phase rotation part, the initial frequency domain comb signal performs a certain phase rotation on the frequency component at each frequency point to reduce the peak-to-average power ratio of the generated comb signal. The final generated frequency domain comb pilot signal x comb , as shown in the following formula (2): θ comb_i is the rotation phase vector of the frequency component of the frequency domain comb signal, i=1,2,…,m; After the comb signal generation is completed, the pilot signal will be output to the transmission channel to complete the test of the amplitude-frequency and phase-frequency response of the transmission system; The channel response test module includes two parts: signal frequency difference estimation and recovery, and channel amplitude-frequency and phase-frequency response testing; In the signal frequency difference estimation and recovery part, after the receiving end receives the frequency domain comb pilot signal, it first undergoes frame synchronization, intercepts a complete frame, and then uses fast Fourier transform to convert it to the frequency domain. By analyzing the offset of each frequency component, the frequency deviation f received at the receiving end is quickly estimated. o ; Get the frequency deviation value f o After that, the received signal is frequency offset compensated to obtain the frequency domain comb signal y after the frequency offset is restored. comb_afFOE , as shown below: y comb is the received frequency domain comb signal after channel transmission; In the channel amplitude-frequency and phase-frequency response test, after completing the corresponding frequency offset compensation, the modulus of the frequency domain signal is calculated based on the frequency point f preset by the transmitter. comb_i Find the highest value and get its corresponding modulus value p comb_i and coarse phase value At this time, the modulus value p comb_i The amplitude-frequency response of the constructed system has been obtained; For the coarse phase value The frequency is corrected according to the following formula to obtain the fine phase value. After correction, the fine phase value Perform phase angle simplification to eliminate phase jumps: in, and: Afterwards, the obtained This is the phase-frequency response of the system; The digital signal pre-equalization module includes two parts: filter coefficient generation and transmission signal convolution pre-equalization; In the filter coefficient generation part, the tap coefficient length of the pre-equalization filter is set to L. According to the obtained system amplitude-frequency and phase-frequency responses, the corresponding transmitter pre-equalization coefficient is generated through the inverse response, as shown in the following formula: In the transmission signal convolution pre-equalization part, the to-be-transmitted signal u and W are convolved to obtain the pre-equalized signal v: v=u⊙W (6) .

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