EP-4742561-A2 - PILOT-ASSISTED FIBER LENGTH ESTIMATION

EP4742561A2EP 4742561 A2EP4742561 A2EP 4742561A2EP-4742561-A2

Abstract

A pilot-assisted fiber length estimation (FLE) technique estimates chromatic dispersion for optical signals. The pilot-assisted FLE technique is agnostic to the modulation format of the optical payload and is resilient to polarization effects, bandwidth limitations, and signal shaping, thus enabling consistent performance across a wide range of operating conditions. A transmitter periodically inserts a known pilot sequence into an optical signal, and a receiver processes the pilot sequence to estimate the length of the optical fiber, or to estimate chromatic dispersion of the optical fiber, which is related to the fiber length.

Inventors

Campos, Nestor Daniel
Martinez Balsa, Agustin
Olmos Rebellato, Marcos Sebastian
MORERO, DAMIAN ALFONSO
CARRER, HUGO SANTIAGO
HUEDA, MARIO RAFAEL

Assignees

Marvell Asia Pte Ltd

Dates

Publication Date: 20260513
Application Date: 20251017

Claims (15)

A method for estimating a length of an optical fiber for chromatic dispersion compensation, the method comprising: receiving, from the optical fiber, an optical signal that includes a pilot sequence, the pilot sequence comprising a repeated pattern; at a plurality of steps corresponding to different fiber lengths, applying a chromatic dispersion (CD) equalizer to the pilot sequence; for at least some of the plurality of steps, computing a correlation-based metric based on the pilot sequence; and selecting one of the plurality of steps that maximizes the correlation-based metric, wherein the selected step corresponds to an estimated fiber length of the optical fiber.
The method of claim 1, wherein the optical signal comprises a first polarization and a second polarization, and the pilot sequence comprises the repeated pattern in the first polarization and in the second polarization.
The method of claim 1 or 2, wherein a data payload follows the pilot sequence.
The method of one of claims 1 to 3, further comprising: selecting a setting of a digital filter for the CD equalizer based on the selected step or the estimated length of the optical fiber; compensating CD of the received optical signal using the digital filter of the CD equalizer; and outputting a filtered optical signal.
The method of one of claims 1 to 4, wherein, for a particular step of the plurality of steps, the method comprises: receiving a plurality of the pilot sequences, wherein the pilot sequences is included periodically in the optical signal; for each of the plurality of pilot sequences, computing one of a plurality of correlation-based metrics; calculating an average of the plurality of correlation-based metrics; wherein the average of the plurality of correlation-based metrics is used to as the correlation-based metric for the particular step of the plurality of steps.
The method of one of claims 1 to 5, wherein, for a particular step of the plurality of steps, applying the CD equalizer to the pilot sequence comprises: configuring the CD equalizer to compensate for a candidate chromatic dispersion value corresponding to the particular step; and processing the received pilot sequence using the configured equalizer to generate an output signal for metric computation.
The method of claim 6, wherein an equalized symbol from the CD equalizer for a polarization j is represented as z j [k], and computing the correlation-based metric comprises: computing a differential signal d j [k] of the output signal z j [k] ; and calculating a cost function between two disjointed sections of the differential signal d j [k] , wherein the cost function is the correlation-based metric, and/or wherein the differential signal is calculated according to: d j k = z j k ⋅ z j ∗ k − 1 wherein z j [ k ] is the symbol for a polarization j at time k, and z j ∗ k − 1 is the complex conjugate of the symbol from a previous time step, and/or wherein the cost function is calculated according to: C j k = ℝ ∑ n = 0 W − 1 d j ∗ k − W + n ⋅ d j k + n wherein W is a window length corresponding to a length of the pattern in the pilot sequence, and n is an index of summation across the window length.
The method of one of claims 1 to 7, wherein the plurality of steps have a first step size, the method further comprising: identifying a range of step sizes based on computed values of the correlation-based metric; at a second plurality of steps corresponding to different fiber lengths, applying the chromatic dispersion (CD) equalizer to the received pilot sequence, wherein the second plurality of steps have a second step size that is smaller than the first step size; for each of the second plurality of steps, computing the correlation-based metric based on the pilot sequence; and selecting one of the second plurality of steps that maximizes the correlation-based metric.
A method for chromatic dispersion (CD) compensation of an optical signal received over an optical fiber, the method comprising: receiving, from the optical fiber, an optical signal comprising two polarizations, the optical signal comprises multiple instances of a pilot sequence at periodic intervals on both polarizations of the optical signal, wherein the pilot sequence comprises a repeated pattern, and portions of a data signal are between instances of the pilot sequence; selecting a setting for CD compensation based on received instances of the pilot sequence in the optical signal; and compensating CD in the optical signal according to the selected setting.
The method of claim 9, wherein, at a particular instance of the pilot sequence, the pilot sequence comprises two instances of the repeated pattern, or wherein a first length of the pilot sequence is less than 0.1% of a second length of a portion of the data signal between two adjacent instances of the pilot sequence, or wherein the pilot sequence is quadrature phase-shift keying (QPSK) encoded.
The method of claim 9 or 10, wherein selecting a setting for CD compensation comprises: performing a first search for a CD value based on a first range of CD values and a first step size between consecutive CD values in the first range; and performing a second search for the CD value based on a second range of CD values, wherein the second range is smaller than the first range, and a second step size between consecutive CD values in the second range, wherein the second step size is smaller than the first step size; wherein the selected setting is based on a result of the second search.
The method of one of claims 9 to 11, wherein selecting the setting for CD compensation comprises: computing a plurality of cost functions based on the pilot sequence, wherein different ones of the plurality of cost functions correspond to different test CDs; and selecting the test CD associated with a maximum value of the plurality of cost functions.
An integrated circuit (IC) for compensating chromatic dispersion (CD) of an optical signal, the IC comprising: a front end component to receive an optical signal from an optical fiber coupled to the IC, the optical signal comprising a pilot sequence at periodic intervals on the optical signal, the pilot sequence comprising a repeated pattern; a fiber length estimator (FLE) configured to determine a length of the optical fiber coupled to the IC based on the pilot sequence, and determine a CD setting for the IC; and a CD equalizer (CDE) configured to compensate for CD in the received optical signal based on the CD setting.
The IC of claim 13, wherein the IC further comprises a transmit module configured to periodically insert the pilot sequence into an optical signal.
The IC of claim 13, wherein the FLE is configured to: instruct the CDE to apply a first CD setting to the optical signal comprising the pilot sequence; calculate a first metric based on a first output signal from the CD applying the first CD setting; instruct the CDE to apply a second CD setting to the optical signal comprising the pilot sequence; calculate a second metric based on a second output signal from the CD applying the second CD setting; and determine the length of the optical fiber based on the first metric and the second metric, or wherein the FLE comprises a pilot synchronizer configured to identify the pilot sequence in an output signal from the CDE, or wherein the optical signal comprises a first polarization and a second polarization, and the pilot sequence comprises the repeated pattern in the first polarization and in the second polarization, or wherein a first length of the pilot sequence is less than 0.1% of a second length of a portion of data signal between two adjacent instances of the pilot sequence, or wherein the pilot sequence is quadrature phase-shift keying (QPSK) encoded.

Description

Priority Application This patent application claims priority to and/or receives benefit from U.S. Provisional Application No. 63/709,268, titled, "Pilot-assisted Fiber Length Estimation", filed on October 18, 2024. The U.S. Provisional Application is hereby incorporated by reference in its entirety. Background High-speed, high-bandwidth communication systems are integral to modem computing and networking applications. These systems are designed to facilitate efficient and reliable data transmission over various media or communication links, including optical fibers, copper cables, and wireless channels. Advances in communication technologies, such as signal modulation, error correction, and clock recovery, can ensure data integrity, reduce latency, and maintain synchronization across devices. Brief Description of the Drawings Embodiments will be readily understood by the following detailed description in conjunction with the accompanying drawings. To facilitate this description, like reference numerals designate like structural elements. Embodiments are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings. FIG. 1 illustrates an exemplary electronics system, according to some embodiments of the disclosure.FIG. 2 illustrates an implementation of a digital signal processor in a transceiver, according to some embodiments of the disclosure.FIG. 3 illustrates an example of an optical signal with a periodically inserted pilot sequence, according to some embodiments of the disclosure.FIG. 4 illustrates correlation windows for calculating a cost function based on the pilot sequence, according to some embodiments of the disclosure.FIG. 5 depicts a flow chart illustrating a method for estimating a fiber length and compensating for chromatic dispersion based on the estimate, according to some embodiments of the disclosure.FIG. 6 shows two plots illustrating data for the coarse search and the fine search, according to some embodiments of the disclosure.FIG. 7 depicts a flow chart illustrating a method for searching for a fiber length based on pilot sequences, according to some embodiments of the disclosure.FIG. 8 depicts a flow chart illustrating a method for calculating a correlation-based metric based on pilot sequences, according to some embodiments of the disclosure. Detailed Description Overview As artificial intelligence (AI) applications continue to evolve, they demand unprecedented data processing speeds and bandwidth capabilities to support their complex algorithms and massive datasets. Digital signal processors (DSPs), such as optical DSPs and coherent DSPs, can enable high-bandwidth optical interconnects for AI infrastructure. In particular, the DSPs can enable low-latency, high-performance, and energy-efficient data transfer. These DSPs can offer seamless connectivity across AI, cloud computing, enterprise systems, and 5G infrastructure. The DSPs can implement one or more instances of digital equalizers. Digital equalizers can mitigate signal distortion caused by channel impairments such as chromatic dispersion (CD). These equalizers can adjust the amplitude and phase of incoming signals to compensate for frequency-dependent attenuation, phase shifts, and group delay introduced by the transmission medium. In some implementations, a chromatic dispersion equalizer can be configured as a finite impulse response (FIR) filter or other digital filter architecture, and may be tuned to compensate for a range of dispersion values based on signal characteristics. In coherent optical receivers, chromatic dispersion compensation is typically performed using a digital filter configured to reverse the frequency-dependent group delay introduced by the transmission fiber. Blind search techniques are often used to estimate chromatic dispersion. In such techniques, the receiver applies a set of candidate equalizer settings, evaluates a cost function for each, and selects settings to minimize the cost function. Cost functions used for blind search may include autocorrelation metrics, peak-to-average power ratio (PAPR), or timing tone analysis. These methods can be sensitive to modulation format and may require reconfiguration when the payload changes. For example, PAPR-based techniques tend to degrade when probabilistic constellation shaping (PCS) is used, as the statistical properties of the shaped data may resemble those of a dispersed signal. Other approaches may struggle under bandwidth-limited conditions or in the presence of polarization mode dispersion (PMD), leading to inaccurate estimates or latency at startup. To address these limitations, the present disclosure describes a pilot-assisted fiber length estimation technique that can estimate chromatic dispersion for optical signals for any modulation scheme. Pilot-assisted fiber length estimation (PAFLE) is also resilient to polarization effects, bandwidth limitations, and signal shaping, thus enabling consistent perf