Low-complexity digital pre-equalizer based on frequency domain comb pilot channel estimation

Abstract

The invention belongs to the technical field of optical communication, and particularly relates to a low-complexity digital pre-equalizer based on frequency domain comb pilot channel estimation. The device comprises a data sending module, a channel response test module and a digital signal pre-equalization module, the data sending module comprises an initial comb signal generation part and a frequency component phase rotation part; the channel response test module comprises a signal frequency difference estimation and recovery part and a channel amplitude-frequency and phase-frequency response test part; the digital signal pre-equalization module comprises a filtering coefficient generation part and a sending signal convolution pre-equalization part; according to the invention, a frequency domain comb pilot signal is sent to analyze channel amplitude-frequency and phase-frequency response, so that a pre-equalizer for pre-compensating channel response at a sending end is generated; the method 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, mode multiplexing short-distance intensity modulation direct detection systems and the like.

Description

Technical Field

[0001] The invention belongs to the technical field of optical fiber communication, and in particular relates to a digital pre-equalizer. Background Art

[0002] In the 5G era and with 6G on the horizon, Data Center Interconnect (DCI) faces an increasing demand for data traffic due to applications such as artificial intelligence, machine learning, and edge computing. Although coherent technology has made progress in short-distance applications, the intensity-modulated direct detection (IM / DD) system using multi-level pulse amplitude modulation (PAM) is still the preferred solution for 400 / 800GbE due to its cost-effectiveness and low power consumption. However, bandwidth limitation has become a major challenge for high-speed O-band transmission. Severe bandwidth limitation will cause severe high-frequency fading of the signal, causing severe inter-symbol interference, resulting in a serious degradation of the signal quality at the receiving end. Therefore, how to effectively solve the degradation 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 limitations, researchers have proposed filtering solutions such as adaptive post-equalization and pre-equalization (Pre-Eq) technology to address signal damage caused by bandwidth limitations. Traditional pre-equalization (Pre-Eq) technology based on feedback filtering reconstructs the transmission channel response by analyzing the post-equalization filter coefficients. Although it can effectively compensate, 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 critical task. Summary of the invention

[0004] In view of the above situation, the object of the present invention 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 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, and 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 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 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. 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 the rotation phase vector of the frequency component of the frequency domain comb signal;

[0015] 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.

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

[0017] In the signal frequency difference estimation and recovery part, after receiving the frequency domain comb pilot signal, the receiving end first undergoes frame synchronization, intercepts a complete frame, and then uses fast Fourier transform to transform it into the frequency domain. By analyzing the offset of each frequency component, the frequency deviation f received at the receiving end can be 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:

[0018]

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

[0020] 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 according to 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.

[0021] For the coarse phase value The frequency is corrected according to the following formula to obtain the fine phase value.

[0022]

[0023] After correction, the fine phase value Simplify the phase angle and eliminate phase jumps:

[0024]

[0025] in, and:

[0026]

[0027] Afterwards, the obtained This is the phase-frequency response of the system.

[0028] (iii) a digital signal pre-equalization module, including: filter coefficient generation, transmission signal convolution pre-equalization;

[0029] 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:

[0030]

[0031] Send signal convolution pre-equalization part, see attached Figure 1 shown.

[0032] Convolve the signal u to be sent with W to obtain the pre-equalized signal v:

[0033] v=u⊙W (6)

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

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

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

[0037] (3) The present invention 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 effectively reduces the computational complexity required for pre-equalization. At the same time, since the present invention can realize accurate channel response testing at each frequency point, it has very high robustness for the actual transmission channel.

[0038] The present invention analyzes the channel amplitude-frequency and phase-frequency responses by sending frequency-domain comb pilot signals to generate a pre-equalizer for pre-compensating the channel response at the transmitting end. The present invention has the characteristics of high granularity, high precision, and robust transmission signal format. The present invention can be applied to various scenarios such as polarization-multiplexed long-distance single-carrier coherent optical transmission and mode-multiplexed short-distance intensity modulation direct detection systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 The block diagram of a low-complexity digital pre-equalizer based on frequency domain comb pilot channel estimation.

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

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

[0042] 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, which can effectively recover the frequency-selective fading caused by imperfect device response while compensating for the high-frequency fading caused by system bandwidth limitation. The equalizer can be applied to a variety of optical communication transmission systems including coherent or intensity modulation direct detection systems, etc., and is described by taking the application in intensity modulation direct detection system as an example.

[0043] Specifically, it includes three parts: "data transmission", "channel response test" and "digital signal pre-equalization":

[0044] (1) Data transmission, including:

[0045] The principle of "initial comb signal generation" is as follows Figure 1 shown.

[0046] The transmitter first generates an initial frequency domain comb signal x for testing the channel response. comb_ini , which at a specific frequency f comb_i Generate a frequency component a of a certain amplitude comb_i , as shown below

[0047]

[0048] The principle of "Frequency Component Phase Rotation" is as follows Figure 1 shown.

[0049] The initial frequency domain comb signal will perform 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 is shown in the following formula:

[0050]

[0051] 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.

[0052] (2) “Channel response test” section, including:

[0053] The principle of "Signal frequency difference estimation and recovery" is as follows Figure 1 shown.

[0054] The received pilot signal is first frame synchronized, and after intercepting a complete frame, it is transformed into the frequency domain using fast Fourier transform. By analyzing the offset of each frequency component, the frequency deviation f received at the receiving end can be quickly estimated. o , get the frequency deviation value f o After that, the frequency offset compensation for the received signal is completed, as shown in the following formula:

[0055]

[0056] The principle of the "Channel Amplitude-Frequency and Phase-Frequency Response Test" is as follows: Figure 1 shown.

[0057] After completing the corresponding frequency offset compensation, the modulus value of the frequency domain signal is calculated according to 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.

[0058] For the coarse phase value It is necessary to perform frequency correction according to the following formula to obtain a fine phase value

[0059]

[0060] After correction, the fine phase value needs to be 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 response of the system shows very obvious high-frequency attenuation.

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

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

[0067] According to the obtained system amplitude-frequency and phase-frequency responses, this scheme generates the corresponding transmitter pre-equalization coefficient 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 follows 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 after digital pre-equalization at the receiving end 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 counteract the high-frequency fading of the transmission channel and achieve signal transmission with high spectral flatness.

[0073] The present invention uses frequency domain comb pilot signals and high-precision channel response estimation algorithms to realize a high-performance, low-complexity digital pre-equalizer for bandwidth-limited high-speed data center optical interconnection systems, effectively solving the damage to signal quality caused by the bandwidth limitation of traditional optoelectronic devices. At the same time, the encoder greatly improves the tolerance for channel response through high-precision and high-granularity frequency response analysis, and greatly improves the pre-equalization performance, providing a low-complexity and reliable solution for ultra-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 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: signal frequency difference estimation and recovery, channel amplitude-frequency and phase-frequency response test; 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 transform it into 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 according to 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 Simplify the phase angle and 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) .

Images