# US 12,224,889 B2 · Adaptive equalization filter and method for updating filter coefficients

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- Number: US 12,224,889 B2
- Document ID: US12224889B2
- Status: Granted
- Kind: B2
- Application number: 18/038,194
- Filed: 2020-11-25
- Published: 2023-12-28
- Granted: 2025-02-11
- Earliest claimed: 2020-11-25
- Assignees: Nippon Telegraph and Telephone Corporation
- Applicants: Nippon Telegraph and Telephone Corporation
- Inventors: Etsushi Yamazaki; Seiji Okamoto; Masanori Nakamura; Kengo Horikoshi; Takeo Sasai; Yoshiaki Kisaka
- Web page: https://patentgrep.com/patent/US12224889B2

## CPC

- H04L 27/01: Equalisers {(baseband equalizers at the transmitter end H04L25/03343; in analogue transmission systems H04B3/04, H04B7/005)}
- H04B 10/61: Coherent receivers
- H04L 25/03057: {with a recursive structure (H04L25/03031 takes precedence)}
- H04L: TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION (arrangements common to telegraphic and telephonic communication H04M)
- H04B: TRANSMISSION

## Abstract

An aspect of the present invention is an adaptive equalization filter for an optical transmission system, including: a main signal filter; and a coefficient computation unit that updates the filter coefficient of the main signal filter. The pre-stage filter and the post-stage filter receive a sample of the main signal as an input. The pre-stage coefficient computation unit obtains the filter coefficient of the pre-stage filter by feedback control using the gradient method. The post-stage coefficient computation unit obtains the filter coefficient of the post-stage filter by feedforward control. The convolution computation unit obtains the filter coefficient of the main signal filter by convolution computation of the filter coefficient of the pre-stage filter obtained by the pre-stage coefficient computation unit and the filter coefficient of the post-stage filter obtained by the post-stage coefficient computation unit.

## Claims

### Claim 1

An adaptive equalization filter for an optical transmission system, the adaptive equalization filter comprising:
a main signal filter; and
a coefficient computator that updates a filter coefficient of the main signal filter,
wherein the coefficient computator includes:
a pre-stage filter and a post-stage filter that are connected in cascade, each having a sample of a main signal as an input;
a pre-stage coefficient computator that obtains a filter coefficient of the pre-stage filter by feedback control using a gradient method;
a post-stage coefficient computator that obtains a filter coefficient of the post-stage filter by feedforward control; and
a convolution computator that obtains the filter coefficient of the main signal filter by convolution computation of the filter coefficient of the pre-stage filter obtained by the pre-stage coefficient computator and the filter coefficient of the post-stage filter obtained by the post-stage coefficient computator.

### Claim 2 (depends on 1)

The adaptive equalization filter according to claim 1,
wherein the post-stage coefficient computator
obtains an inverse matrix of a covariance matrix of an input signal vector to the post-stage filter, and
obtains a correlation vector between a known signal symbol and the input signal vector to the post-stage filter
by using a plurality of sets of the input signal vector to the post-stage filter and the known signal symbol at a time position obtained by applying the post-stage filter to the input signal vector, and
applies the inverse matrix of the covariance matrix to the correlation vector to obtain the filter coefficient of the post-stage filter using a minimum mean square error method.

### Claim 3 (depends on 1)

The adaptive equalization filter according to claim 1,
wherein the post-stage filter is a filter having two inputs and two outputs, and
the post-stage coefficient computator obtains the filter coefficient of the post-stage filter, with one of coefficients of length M taps of two inputs of the post-stage filter denoted by hx[1], hx[2], . . . , and hx[M], and the other of the coefficients denoted by hy[1], hy[2], . . . , and hy[M], by selecting one or a plurality of tap time positions m from the M taps and performing, for each selected m, the steps of:
applying a filter coefficient at a tap time position other than the tap time position m to an input signal vector to the pre-stage filter to compute a provisional value y[m] of an output symbol of the post-stage filter;
calculating a complex error between a complex value of a known signal symbol corresponding to a time position of the provisional value y[m] and a complex value of the provisional value y[m];
using one or a plurality of sets of the complex error and an input signal vector to the post-stage filter used to compute the provisional value y[m] to
obtain an inverse matrix of a 2×2 covariance matrix of an input signal vector to the post-stage filter and
obtain a correlation vector between the complex error and the input signal vector to the post-stage filter; and
applying an inverse matrix of the 2×2 covariance matrix to the correlation vector to calculate hx[m] and hy[m] as a m-th tap coefficient, and
wherein m and M are positive integer.

### Claim 4 (depends on 2)

The adaptive equalization filter according to claim 2,
wherein the post-stage coefficient computator performs computation of multiplying each of the known signal symbols by a complex conjugate of a phase rotation amount predicted from an estimated frequency offset amount.

### Claim 5 (depends on 4)

The adaptive equalization filter according to claim 4,
wherein the post-stage coefficient computator further performs computation, for each of the known signal symbols, of
multiplication by a complex conjugate of a carrier phase compensation amount for a symbol for which the carrier phase compensation amount has already been estimated, and
multiplication by a complex conjugate of a representative value of a phase compensation amount of a symbol for which the carrier phase compensation amount has already been estimated for a symbol for which the carrier phase compensation amount has not been estimated yet.

### Claim 6 (depends on 1)

The adaptive equalization filter according to claim 1,
wherein the pre-stage coefficient computator obtains the filter coefficient of the pre-stage filter using the gradient method in a manner that an error between an output signal from the post-stage filter and a known signal decreases.

### Claim 7 (depends on 1)

The adaptive equalization filter according to claim 1,
wherein the pre-stage coefficient computator computes and updates the filter coefficient of the pre-stage filter by:
computing a complex error between a result of performing frequency offset compensation and carrier phase compensation on an output signal from the post-stage filter and a known symbol at a same time position;
calculating an inner product of an input sample to the pre-stage filter used to obtain the output signal from the post-stage filter and a coefficient of the post-stage filter, further applying the frequency offset compensation and the carrier phase compensation, and obtaining an update vector on a basis of a product of a complex conjugate of an application result and the complex error; and
multiplying the update vector by a step size.

### Claim 8

A filter coefficient update method for updating a filter coefficient of a main signal filter, wherein;
a pre-stage filter and a post-stage filter are connected in cascade with a sample of a main signal as an input,
a pre-stage coefficient computator obtains a filter coefficient of the pre-stage filter by feedback control using a gradient method,
a post-stage coefficient computator obtains a filter coefficient of the post-stage filter by feedforward control, and
a convolution computator obtains the filter coefficient of the main signal filter by convolution computation of the filter coefficient of the pre-stage filter obtained by the pre-stage coefficient computator and the filter coefficient of the post-stage filter obtained by the post-stage coefficient computator.