Multilevel detection method and multilevel detector for bipolar pulse amplitude modulation signals

Abstract

The application discloses a multi-stage detection method and a multi-stage detector for a bipolar pulse amplitude modulation (BPAM) signal, which is suitable for a BPAM modulation-direct detection system, especially for a short-distance high-speed data transmission scene, aims to solve the problem of high direct detection complexity caused by various signal distortions in the prior art, and compensates for the three types of damages, i.e., a transmitter response, a diode square law detection and a receiver response, in stages, including a receiver response compensation step, a polarity recovery step and a transmitter-fiber response compensation step, so that the application is a low-complexity detection scheme capable of compensating for multi-type distortion coupling. The application realizes decoupling and stage-by-stage compensation of complex damages, is easy to realize in practice, has high robustness, and significantly reduces the calculation complexity of a detection module.

Description

Technical Field

[0001] This invention relates to optical communication technology, specifically to a multi-level detection technology for bipolar pulse amplitude modulation (BPAM) signals. Background Technology

[0002] With the continuous development of artificial intelligence technology, data traffic for short-distance applications is exploding. To meet this demand, the transmission rate of intra-DCIs (Intra-Data Center Interconnects) is moving towards 1.6Tb / s or even 3.2Tb / s. Due to its low cost and simple architecture, intensity modulation direct detection (IMDD) transmission schemes are highly favored in intra-DCIs. To support terabit-level transmission rates, wavelength division multiplexing (WDM) with 8 or 16 channels has become the mainstream solution. However, dense channel multiplexing brings integration challenges, and the increase in the number of channels leads to a proportional increase in the number of components, affecting economic efficiency. Therefore, increasing the data rate per channel is a more promising solution.

[0003] To improve single-channel rate, higher-order complex modulation formats can be used to enhance spectral efficiency. However, in IMDD systems, traditional methods suffer from low spectral efficiency because they only detect signal strength and lose phase information. To improve spectral efficiency, bipolar pulse amplitude modulation (BPAM) and oversampling reception schemes have been proposed.

[0004] BPAM signals retain the positive and negative polarities of the signal during modulation, allowing each symbol to carry more information. Direct detection refers to the receiver detecting only the intensity of the optical signal, i.e., the square of the amplitude of the photoelectric field. To recover the signal phase information lost in direct detection, the signal after direct detection is generally oversampled by a factor of two and processed using the Maximum Likelihood Detection (MLD) algorithm. The complexity of MLD increases exponentially with the modulation order and the link memory length. For example, for BPAM-16 signals, the complexity of MLD reaches as high as 16 when the channel memory length exceeds 8. 8 However, this approach is impractical for real-world applications. To reduce complexity, a detection scheme combining serial interference cancellation and channel memory length reduction has been proposed, which partially alleviates the problem of excessive complexity. However, in practice, the non-ideal characteristics of the transceiver and distortion effects such as the square-law detection of the photodiode are coupled together, leading to difficulties in overall channel modeling, complex experimental implementation, and limited performance. Summary of the Invention

[0005] The technical problem to be solved by this invention is to provide an easy-to-implement, low-complexity multi-level detection scheme for the IMDD system interconnected within a data center, where the polarity of the BPAM signal is lost after passing through the diode and is affected by the coupling of various distortion effects in the system, resulting in excessive complexity of the traditional MLD on the receiver side.

[0006] The technical solution adopted by the present invention to solve the above-mentioned technical problems is a multi-level detection method for bipolar pulse amplitude modulation signals, characterized by comprising the following steps:

[0007] Receiver response compensation steps: The signal sampled at twice the symbol rate from the receiver response output is input into the feedforward equalizer T / 2-FFE with twice the symbol sampling rate. After compensating the receiver response, the feedforward equalizer T / 2-FFE outputs the power estimate of the odd and even sampled symbols.

[0008] Polarity recovery step: Using the interpolation relationship between the power estimates of odd and even sampling symbols, the polarity of the sampling symbols at a single sampling rate is recovered through maximum likelihood estimation, thus obtaining the samples sampled at a single sampling rate;

[0009] Transmitter-fiber response compensation steps: Input the sampled at a single sampling rate into a feedforward equalizer (T-FFE) at a single symbol sampling rate. The feedforward equalizer (T-FFE) compensates for the transmitter-fiber response and outputs a bipolar pulse amplitude modulated electrical signal to complete distortion coupling compensation.

[0010] Specifically, before the receiver response compensation step, the receiver frequency response is measured, and the frequency response of the transmitter and fiber optic channel is estimated.

[0011] The tap values ​​of the feedforward equalizer T / 2-FFE are obtained from the T / 2-FFE response; the T / 2-FFE response is the inverse function of the receiver frequency response;

[0012] The tap values ​​of the feedforward equalizer T-FFE are obtained from the T-FFE response; the T-FFE response is the inverse function of the frequency response of the transmitter and the fiber optic channel.

[0013] Simultaneously, a multi-level detector for implementing the above method is provided, including a receiver response compensation unit, a polarity recovery unit, and a transmitter-fiber response compensation unit;

[0014] The receiver response compensation unit is used to input the signal sampled at twice the symbol rate from the receiver response output to the feedforward equalizer T / 2-FFE with twice the symbol sampling rate. After compensating the receiver response, the feedforward equalizer T / 2-FFE outputs the power estimate of the odd and even sampled symbols.

[0015] The polarity recovery unit is used to recover the polarity of the sampling symbols at a single sampling rate by using the interpolation relationship between the power estimates of odd and even sampling symbols and the maximum likelihood estimation, thus obtaining the samples sampled at a single sampling rate.

[0016] The transmitter-fiber response compensation unit is used to input the sampled at a single sampling rate into the feedforward equalizer T-FFE at a single symbol sampling rate. The feedforward equalizer T-FFE compensates the transmitter-fiber response and outputs a bipolar pulse amplitude modulated electrical signal to complete distortion coupling compensation.

[0017] Specifically, the receiver response compensation unit receives the known receiver frequency response, which is obtained through measurement; the transmitter-fiber response compensation unit receives the known frequency responses of the transmitter and fiber optic channel, which are obtained through estimation.

[0018] The receiver response compensation unit sets the tap values ​​of the feedforward equalizer T / 2-FFE based on the T / 2-FFE response; the T / 2-FFE response is the inverse function of the receiver frequency response.

[0019] The transmitter-fiber response compensation unit sets the tap values ​​of the feedforward equalizer T-FFE based on the T-FFE response; the T-FFE response is the inverse function of the frequency response of the transmitter and the fiber optic channel.

[0020] The multi-level detection scheme of this invention decouples three main types of damage: transmitter-fiber response, nonlinear effects caused by the square law of photodiode, and receiver response, and compensates for them in stages to replace complex damage compensation.

[0021] The beneficial effects of this invention are: it achieves decoupling and phased compensation for complex damage, is easy to implement and has high robustness, and significantly reduces the computational complexity of the detection module. Attached Figure Description

[0022] Figure 1 This is a block diagram of the BPAM transmission system and multi-level detection module of the present invention.

[0023] Figure 2 This is a block diagram illustrating an implementation of the BPAM transmission system of the present invention. Detailed Implementation

[0024] The multi-level detection scheme in this embodiment decouples three main types of damage: transmitter-fiber response, nonlinear effects caused by the square law of photodiode, and receiver response. It then performs receiver response compensation, polarity recovery, and transmitter-fiber response compensation in stages.

[0025] like Figure 1 As shown, at the transmitting end, the bipolar pulse amplitude modulation (BPAM-16) symbol is transmitted via the transmitter response h. tx (t) is then converted into a bipolar optical signal via electro-optical conversion and transmitted to the optical fiber; at the receiving end, the bipolar optical signal {s} from the optical fiber... e ,s oThe detection electrical signal {|s} is obtained through direct detection after photoelectric conversion. e | 2 ,|s o | 2},{|s e | 2 ,|s o | 2} Receiver response h rx (t) then yields a signal {r} sampled at twice the symbol rate. e ,r o}.in, e 'o' indicates an even-numbered position in the bipolar region, and 'o' indicates an odd-numbered position in the bipolar region. e ,r o The input is fed into a multi-level detection module to perform graded compensation for the multi-level damage coupling effect of the system link.

[0026] The multi-stage detection module includes a receiver response compensation unit, a polarity recovery unit, and a transmitter-fiber response compensation unit. Before compensation, the multi-stage detection module first uses a spectrum analyzer or vector analyzer to measure the receiver frequency response for compensation. The frequency response of the transmitter and fiber channel is estimated using the recursive least squares (RLS) method.

[0027] The multi-level detection module compensates for the multi-level damage coupling effect in the system links by performing the following steps:

[0028] Receiver response compensation steps: The receiver response compensation unit samples the signal {r} at twice the symbol rate. e ,r o The receiver response is compensated using a feedforward equalizer (T / 2-FFE) with twice the symbol sampling rate. Where r e r represents the even-numbered sampling points of a signal sampled at twice the symbol rate. o T / 2-FFE represents the odd-numbered sampling points of a signal sampled at twice the symbol rate. It indicates that the tap interval of the equalizer is half the symbol period T, meaning it's a feedforward equalizer with a sampling rate twice the symbol rate. The twice-symbol-rate samples output by T / 2-FFE are alternately divided into even and odd parts, representing the power estimates of even and odd-numbered sampled symbols. The feedforward equalizer T / 2-FFE response is the inverse function of the measured receiver frequency response. When the T / 2-FFE response is known, the tap values ​​of the T / 2-FFE can be calculated by performing an inverse discrete Fourier transform on the T / 2-FFE frequency response.

[0029] The power estimate of the odd and even sampled symbols output after equalization is Corresponding to the optical signal {s} at the receiving end e ,s oThe power estimate for} lost the corresponding polarity information. Therefore, polarity recovery is required.

[0030] Polarity recovery step: The polarity recovery unit utilizes odd / even sampling symbols The interpolation relationship between the two symbols is used to estimate the polarity of the sampled symbols using the maximum likelihood algorithm. An interpolation relationship exists for optical signals sampled at twice the symbol rate. interpolation parameter h p The power can be estimated by sending a known training sequence. The known training sequence is a BPAM-16 symbol x of a certain length transmitted by the transmitter. Furthermore, the power estimate of the odd-even sampled symbols can be derived. The interpolation relationship between them is:

[0031]

[0032] Where n is random noise, approximately following a Gaussian distribution. Using the constraint relationship between odd and even sampled symbols in equation (1), the polarity of any odd or even symbol is estimated using the maximum likelihood algorithm. The example uses the estimated even sampled symbols... For example, polarity The polar variable sequence c′ that maximizes the objective function in equation (2):

[0033]

[0034] Wherein, the constant σ 2 This represents the variance of the noise n, which can be set to 1 without affecting the polarity estimation result. Error metric c′ m ∈{-1,1} represents polarity variables. k and m are the ordinal variables in the symbol sequence. Equation (2) defines the polarity variable sequence c′=[c1′,…,c′ that minimizes the overall error metric (maximizes the objective function). m [,...] as polarity estimates use The symbol can be recovered. polarity:

[0035]

[0036] The example only restores the polarity of the even-numbered sampled signs, using As sample values ​​for single-symbol-rate sampling. Similarly, it is also possible to recover only the polarity of odd-sampled symbols, using Sample values ​​as single symbol rate sampling.

[0037] Transmitter-fiber response compensation steps: The transmitter-fiber response compensation unit restores the polarity of the sample. The transmitter-fiber response is compensated using a feedforward equalizer (T-FFE) at a single symbol sampling rate. With the aid of a training sequence x, the frequency responses of the transmitter and fiber channel can be estimated. The feedforward equalizer T-FFE response is the inverse function of the estimated transmitter and fiber channel frequency responses. When the T-FFE response is known, the tap values ​​of the T-FFE can be calculated by performing an inverse discrete Fourier transform on the T / 2-FFE frequency response.

[0038] Because BPAM signals are affected by the diode square rate detection effect during reception, resulting in a spectrum broadening of twice the original, the receiver needs to sample at twice the symbol rate. Therefore, receiver response compensation should also use a double symbol sampling rate equalizer (T / 2-FFE). However, for transmitter-fiber response compensation, a single symbol sampling rate equalizer (T-FFE) is sufficient.

[0039] Example: 1-kilometer transmission of a 28Gbaud BPAM-16 signal.

[0040] Implementation platform such as Figure 2 As shown, the implementation platform consists of two parts: an offline digital signal processing system and an optical transmission system for direct intensity modulation detection. The offline digital signal processing is performed on a computer, and the digital signal generated by the computer is transmitted to a digital-to-analog converter (DAC) or acquired from an analog-to-digital converter (ADC). The transmitting end of the optical transmission system includes a DAC, an RF amplifier, a Mach-Zehnder modulation (MZM) amplifier, and a laser. The receiving end includes an optical attenuator, a photodiode, and an ADC.

[0041] The specific implementation steps are as follows:

[0042] 1. Receiver parameter measurement:

[0043] The frequency response of the ADC is measured individually using a spectrum analyzer or vector analyzer, and the inverse function of this response is used for receiver response compensation in multi-stage detectors.

[0044] 2. Signal Generation

[0045] At the transmitter, a random bit sequence is mapped to 28 Gbaud BPAM-16 symbols, with the first 3000 symbols used as the training sequence x. This is followed by pre-emphasis, raised cosine pulse shaping with a roll-off factor of 0.3, and resampling. The signal is then fed into a DAC to generate an analog waveform. The output waveform is amplified by an RF amplifier and used to drive the MZM. Simultaneously, a continuous laser signal with a wavelength of 1550 nm and an optical power of 14.5 dBm is input to the MZM, with a maximum output optical power of approximately 7 dBm. The MZM modulator bias voltage is set to zero optical power to generate a bipolar optical signal (BPAM signal).

[0046] 3. Transmission and reception:

[0047] After the modulated optical signal is transmitted through a 1-kilometer single-mode optical fiber, the received optical power is adjusted by a variable optical attenuator. Subsequently, the optical signal is subjected to square-law detection by a 50 GHz bandwidth photodiode, and finally sampled by an ADC at ≥2 times the symbol rate and processed offline.

[0048] 4. Offline signal processing:

[0049] The sampled signal passes through a Bessel filter and undergoes clock recovery and downsampling to two samples per symbol. The downsampled samples are then sent to a multi-stage detector for processing. The specific process and parameter settings for the multi-stage detector are as follows:

[0050] a) Receiver response compensation: For samples sampled at twice the symbol rate (representing even-numbered and odd-numbered sampling points respectively), a feedforward equalizer T / 2-FFE at twice the symbol sampling rate is used to compensate the receiver response.

[0051] b) Polarity recovery: The polarity of the sample symbol is estimated using the maximum likelihood estimation algorithm based on the interpolation relationship between even and odd samples. Here, only the polarity of the even (or odd) sample points needs to be recovered to obtain signal samples at a single sampling rate.

[0052] c) Transmitter-fiber response compensation: For single samples with restored polarity, the transmitter-fiber response is compensated by a feedforward equalizer (T-FFE) with a single symbol sampling rate.

[0053] The multi-stage detector outputs BPAM-16 symbol estimates, which are then demapped and the bit error rate is calculated.

[0054] The embodiments described herein are merely examples of a 1-kilometer transmission system using a 28 Gbaud BPAM-16 signal to illustrate the main technical concept and features of the present invention, namely, how the multi-level detection scheme and modules are applied in practice. However, they are not intended to limit the implementation of the present invention. Any modifications, equivalent transformations, and improvements made to this application should be included within the scope of protection of the claims of this invention.

Claims

1. A method of multi-level detection for a bipolar pulse amplitude modulated signal, characterized in that, The method comprises the following steps: Receiver response compensation step: the twice symbol rate sampled signal {r e , o} from the receiver response output is input to a feed forward equalizer T / 2-FFE operating at twice the symbol sampling rate; the feed forward equalizer T / 2-FFE outputs the power estimate of the even and odd sampled symbols r e denotes the even position sample point of the twice symbol rate sampled signal, r o denotes the odd position sample point of the twice symbol rate sampled signal, is the power estimate of the even sampled symbols, is the power estimate of the odd sampled symbols; Polarity recovery step: Using the interpolation relationship between the power estimates of odd and even sampled symbols, the polarity of the sampling symbols at a single sampling rate is recovered through maximum likelihood estimation, thus obtaining the samples sampled at a single sampling rate; where the power estimates of odd and even sampled symbols... The interpolation relationship between them is: wherein n is a random noise, approximately following a Gaussian distribution; h p is an interpolation parameter; The specific method for recovering the sampling symbol polarity at the single sampling rate through the maximum likelihood estimation is: Even sample symbols as single sample rate sample symbols of polarity to make the polarity variable sequence c' at which the objective function in the following equation takes a maximum value where the constant σ 2 denotes the variance of the random noise n, the kth error metric c′ m denotes a polar variable; k and m are index variables in the symbol sequence; the polar variable sequence c' = [c1',..., c'm'] ; and m ...]. The transmitter-fiber response compensation unit is configured to input the samples at the single sampling rate into a feedforward equalizer T-FFE at the single symbol sampling rate, and the feedforward equalizer T-FFE compensates the transmitter-fiber response to output a bipolar pulse amplitude modulation electrical signal after distortion coupling compensation.

2. The method of claim 1, wherein, Before the receiver response compensation unit, the measurement of the receiver frequency response and the estimation of the frequency response of the transmitter and the fiber channel are performed.

3. The method of claim 2, wherein, The tap value of the feedforward equalizer T / 2-FFE is obtained according to the T / 2-FFE response; the T / 2-FFE response is the inverse function of the receiver frequency response; The tap value of the feedforward equalizer T-FFE is obtained according to the T-FFE response; the T-FFE response is the inverse function of the frequency response of the transmitter and the fiber channel.

4. A multi-stage detector for a bipolar pulse amplitude modulated signal, characterized by, The method comprises a receiver response compensation unit, a polarity recovery unit and a transmitter-fiber response compensation unit. The receiver response compensation unit is configured to input a signal of a double symbol rate sample from a receiver response output into a feedforward equalizer T / 2-FFE operating at a double symbol rate, and the feedforward equalizer T / 2-FFE compensates the receiver response and outputs a power estimation value of an even sample symbol r e represents an even position sample point of the double symbol rate sample signal, r o represents an odd position sample point of the double symbol rate sample signal, is a power estimation value of an even sample symbol, is a power estimation value of an odd sample symbol; The polarity recovery unit is used to recover the polarity of the sampling symbols at a single sampling rate by utilizing the interpolation relationship between the power estimates of odd and even sampling symbols through maximum likelihood estimation, thus obtaining the samples sampled at the single sampling rate; wherein, the power estimates of odd and even sampling symbols... The interpolation relationship between them is: wherein n is a random noise, approximately following a Gaussian distribution; h p is an interpolation parameter; The specific method for recovering the sampling symbol polarity at the single sampling rate through the maximum likelihood estimation is: Even sample symbols as single sample rate sample symbols of the polarities to make the polarities variable sequence c' when the objective function in the following formula takes the maximum value Wherein, the constant σ 2 The k-th error measure represents the variance of random noise n. c′ m ∈{-1,1} represents a polar variable; k and m are ordinal variables in the symbol sequence; the polar variable sequence c′=[c1′,…,c′ m ,…]; The transmitter-fiber response compensation unit is configured to input the samples at the single sampling rate into a feedforward equalizer T-FFE at the single symbol sampling rate, and the feedforward equalizer T-FFE compensates the transmitter-fiber response to output a bipolar pulse amplitude modulation electrical signal after distortion coupling compensation.

5. The multi-stage detector of claim 4, wherein, The receiver response compensation unit receives the known receiver frequency response, and the receiver frequency response is obtained through measurement; the transmitter-fiber response compensation unit receives the known frequency response of the transmitter and the fiber channel, and the frequency response of the transmitter and the fiber channel is obtained through estimation.

6. The multi-stage detector of claim 5, wherein, The tap value of the feedforward equalizer T / 2-FFE is obtained according to the T / 2-FFE response; the T / 2-FFE response is the inverse function of the receiver frequency response; The tap value of the feedforward equalizer T-FFE is obtained according to the T-FFE response; the T-FFE response is the inverse function of the frequency response of the transmitter and the fiber channel. The tap value of the feedforward equalizer T / 2-FFE is obtained according to the T / 2-FFE response; the T / 2-FFE response is the inverse function of the receiver frequency response; The tap value of the feedforward equalizer T-FFE is obtained according to the T-FFE response; the T-FFE response is the inverse function of the frequency response of the transmitter and the fiber channel.

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