Method for precoding to mitigate nonlinear distortions and precoder for performing the same

A method for precoding to mitigate nonlinear distortions and a precoder for performing the same are disclosed. The precoder for mitigating distortions of a communication signal may include a filter configured to generate a filtering signal based on a third signal and filter coefficients corresponding to a selected signal generated based on a first signal, a second signal, and the third signal, and a modulo operator configured to generate the third signal by performing a modulo operation on the second signal, wherein the second signal is generated based on the first signal and the filtering signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Korean Patent Application No. 10-2021-0034676 filed on Mar. 17, 2021, in the Korean Intellectual Property Office, the entire disclosure of which is incorporated herein by reference for all purposes.

BACKGROUND

1. Field of the Invention

One or more example embodiments relate to a precoding method for mitigating nonlinear distortions and a precoder for performing the same.

2. Description of Related Art

Recently, it has become necessary to implement low cost, small size short distance optical links used in massive constructions such as a data center, a metro network, an optical access network. In addition, as a modulation method, pulse amplitude modulation (PAM), one of the direct detection modulation methods, is being widely used. More particularly, PAM-4 has been adopted as a modulation method for IEEE 802.3bs, which is a 400 Gbps ethernet standard, in consideration of power consumption and implementation complexity.

In case of a direct detection method, interference between symbols such as inter-symbol interference (ISI) may occur due to a bandwidth limitation of an electronic device. In addition, since the optical devices being used are low cost, nonlinear distortion may occur. The distortion is an amplitude-dependent skew according to a level or an amplitude of a signal and an amplitude-dependent noise.

Conventionally, a pre-equalizer such as a Tomlinson-Harashima Precoder (THP) is used in a transmitter to reduce ISI generated due to a bandwidth limitation. This is a modified method of mitigating ISI by a feed-forward equalizer (FFE) in a receiver. In a conventional THP structure, a modulo operator is added to a FFE filter used in the receiver.

Since a conventional THP may remove post-cursor ISI based on a channel response characteristic of a channel, it may be used in a transmitter of a system with a small bandwidth capability. A filter used in a THP may be designed to have a linear characteristic and configured with same channel response characteristic regardless of a level of a signal as a filter. Thus, it may be difficult to solve the problem of linear distortions occurring at each level of the signal.

The above description has been possessed or acquired by the inventor(s) in the course of conceiving the present disclosure and is not necessarily an art publicly known before the present application is filed.

SUMMARY

Example embodiments provide a precoding method for mitigating nonlinear distortions of a signal as well as inter-symbol interference (ISI).

However, the technical aspects are not limited to the aforementioned aspects, and other technical aspects may be present.

According to an aspect, there is provided a precoder for mitigating distortions of a communication signal including a filter configured to generate a filtering signal based on a third signal and filter coefficients corresponding to a selected signal generated based on a first signal, a second signal, and the third signal, and a modulo operator configured to generate the third signal by performing a modulo operation on the second signal, wherein the second signal may be generated based on the first signal and the filtering signal.

The filter may include a level detector configured to generate the selected signal, a plurality of delay elements configured to output delayed signals by delaying the third signal, a plurality of selectors configured to select and output the filter coefficients based on the selected signal, a plurality of multipliers configured to multiply the third signal and the delayed signals by the filter coefficients, and an adder configured to output the filtering signal by adding multiplication results obtained by the plurality of multipliers.

The level detector may include an adder configured to generate a level signal based on the first to third signals and a comparator configured to output the selected signal based on the level signal and a plurality of comparison signals.

The plurality of delay elements may include a first delay element configured to output a first delayed signal by delaying the third signal and a second delay element configured to output a second delayed signal by delaying the first delayed signal.

The plurality of selectors may include a first selector configured to output a first filter coefficient corresponding to the third signal, a second selector configured to output a second filter coefficient corresponding to the first delayed signal, and a third selector configured to output a third filter coefficient corresponding to the second delayed signal.

According to an aspect, there is provided a precoder for mitigating distortions of a communication signal, including a filter configured to generate a filtering signal based on a third signal and filter coefficients respectively corresponding to a plurality of selected signals generated based on a first signal, a second signal, and the third signal, and a modulo operator configured to generate the third signal by performing a modulo operation on the second signal, wherein the second signal is generated based on the first signal and the filtering signal.

The filter may include a level detector configured to generate the plurality of selected signals, a plurality of delay elements configured to output delayed signals by delaying the third signal, a plurality of selectors configured to select and output the filter coefficients based on the plurality of selected signals respectively, a plurality of multipliers configured to multiply the third signal and the delayed signals by the filter coefficients, and an adder configured to output the filtering signal by adding multiplication results obtained by the plurality of multipliers.

The level detector may include an adder configured to generate a level signal based on the first to third signals, a plurality of delay elements configured to output delayed signals by delaying the level signal, and a plurality of comparators configured to output the plurality of selected signals based on the level signal and the delayed level signals respectively.

According to an aspect, there is provided a method for precoding to mitigate distortions of a communication signal, including generating a filtering signal based on a third signal and filter coefficients selected based on a selected signal generated based on a first signal, a second signal, and the third signal, and generating the third signal by performing a modulo operation on the second signal, wherein the second signal is generated based on the first signal and the filtering signal.

The generating of the filtering signal may include generating the selected signal; outputting delayed signals by delaying the third signal, selecting and outputting the filter coefficients based on the selected signal, multiplying the third signal and the delayed signals by the filter coefficients, and outputting the filtering signal by adding multiplication results.

The generating of the selected signal may include generating a level signal based on the first to third signals and outputting the selected signal based on the level signal and a plurality of comparison signals.

The outputting by delaying may include outputting a first delayed signal by delaying the third signal and outputting a second delayed signal by delaying the first delayed signal.

The selecting and outputting of the filter coefficients may include outputting a first filter coefficient corresponding to the third signal, outputting a second filter coefficient corresponding to the first delayed signal, and outputting a third filter coefficient corresponding to the second delayed signal.

The outputting of the filtering signal may include generating a plurality of selected signals, outputting the delayed signals by delaying the third signal, selecting and outputting the filter coefficients based on the plurality of selected signals respectively, multiplying the third signal and the delayed signal by the filter coefficients, and outputting the filtering signal by adding multiplication results.

The generating of the plurality of selected signals may include generating a level signal based on the first to third signals, outputting delayed level signals by delaying the level signal, and outputting the plurality of selected signals based on the level signal and the delayed level signals respectively.

DETAILED DESCRIPTION

It should be noted that if it is described that one component is “connected”, “coupled”, or “joined” to another component, a third component may be “connected”, “coupled”, and “joined” between the first and second components, although the first component may be directly connected, coupled, or joined to the second component.

Hereinafter, example embodiments will be described in detail with reference to the accompanying drawings. When describing the example embodiments with reference to the accompanying drawings, like reference numerals refer to like components and a repeated description related thereto will be omitted.

FIG.1illustrates a communication system according to an example embodiment.

A communication system10may include a precoder100to mitigate a nonlinear distortion and inter-symbol interference (ISI) in a signal being transmitted and received. The precoder100may be implemented at a transmitter of the communication system10. A demodulator may be implemented at a receiver corresponding to the precoder100.FIG.1illustrates only a configuration to mitigate a nonlinear distortion and ISI in the communication system10. Thus, it shall be understood that descriptions of general components of the communication system10are omitted.

The precoder100may remove the ISI occurring in the communication system10using an inverse function H(Z)−1 of a channel response H(Z) through a filter200. The precoder100may limit a maximum swing value transmitted for stabilizing the filter200to remove post-cursor ISI by performing a modulo operation Mod 2M by a modulo operator130.

The precoder100may output a third signal Ynby performing precoding when a first signal Xnis input. The filter200may generate a filtering signal Znbased on the third signal Yn. A mixer110may generate a second signal Xn′ based on the first signal Xnand the filtering signal Zn. The second signal Xn′ may be a signal removing the filtering signal Znfrom the first signal Xn. The modulo operator130may generate the third signal Ynby performing a modulo operation on the second signal Xn′. The third signal Ynmay be a signal transmitted to the receiver.

The demodulator150may restore a received signal rnreceived from the receiver. The received signal rnmay be a signal including noise when passing through a channel. A modulo operator153may restore the received signal rnby performing the modulo operation Mod 2M on the received signal rn.

The precoder100may mitigate a nonlinear distortion occurring in the communication system10by selecting filter coefficients having nonlinear characteristics based on a level of a signal. The filter200may compensate for the nonlinear distortion by selecting the filter coefficients to generate the filtering signal Znbased on the first signal Xn, the second signal Xn′ and the third signal Yn.

FIG.2illustrates an example of the precoder ofFIG.1.

The precoder100may include the modulo operator130and the filter200. The filter200may include a level detector300, a plurality of delay elements210-1and210-2, a plurality of selectors230-1to230-3, a plurality of multipliers250-1to250-3, and an adder270.

The level detector300may generate a selected signal Sel based on the first signal Xn, the second signal Xn′, and the third signal Yn. The level detector300may generate a level signal Lnbased on the first signal Xn, the second signal Xn′ and the third signal Ynand may output the selected signal Sel based on a level value of the level signal Ln. For example, the selected signal Sel may have a value of 1 to m (m is a natural number) according to a level value of the level signal Ln.

The selectors230-1to230-3may output filter coefficients based on the selected signal Sel. For example, the selectors230-1to230-3may output filter coefficients corresponding to the selected signal Sel among a plurality of filtering signals h41to h4m, h31to h3m, and h21to h2m. For example, when the selected signal Sel is k (k is a natural number greater than or equal to 1 and less than or equal to m), the selectors230-1to230-3may output the filter coefficients h4k, h3k, and h2krespectively.

The delay elements210-1and210-2may output delayed signals by delaying the third signal Yn. For example, the first delay element210-1may output a first delayed signal Yn-1by delaying the third signal Ynby a unit of time and the second delay element210-2may output a second delayed signal Yn-2by delaying the first delayed signal Yn-1by a unit of time. That is, the second delayed signal Yn-2may be an output signal of the third signal Yndelayed by two units of time.

The multipliers250-1to250-3may multiply the third signal Yn, the first delayed signal Yn-1and the second delayed signal Yn-2by the filter coefficient and may output multiplication results. For example, the first multiplier250-1may multiply the second delayed signal Yn-2by the filter coefficient output from the first selector230-1and may output a multiplication result, the second multiplier250-2may multiply the first delayed signal Yn-1by the filter coefficient output from the second selector230-2and may output a multiplication result, and the third multiplier250-3may multiply the third signal Ynby the filter coefficient output from the third selector230-3and may output a multiplication result.

The adder270may add the multiplication results obtained by the multipliers250-1to250-3and output the filtering signal Zn. For example, the adder270may output the filtering signal Znby adding all multiplication results output from the first multiplier250-1to the third multiplier250-3respectively. For example, when the selected signal Sel is k so that filter coefficients output from the selectors230-1to230-3are the filter coefficient h4k, h3k, and h2krespectively, the filtering signal Znmay be expressed by Equation 1.
Zn=h2kYn+h3kYn-1+h4kYn-2[Equation 1]

FIG.3illustrates the level detector ofFIG.2.

The level detector300may include an adder310to generate a level signal Lnbased on a first signal Xn, a second signal Xn, and a third signal Ynand a comparator330to output the selected signal Sel based on the level signal Lnand a plurality of comparison signals Lth1to Lthm.

The level signal Lnoutput from the adder310may be expressed by Equation 2.
Ln=Xn−Xn′+Yn[Equation 2]

The comparator330may determine which comparison signal among the plurality of comparison signals Lth1to Lthmhas a same level value as the level signal Ln. For example, when a difference of level values between the level signal Lnand a comparison signal Lthkis less than or equal to a threshold value, the comparator330may determine that the level signal Lnhas a same level value as the comparison signal Lthk.

The comparator330may output the selected signal Sel corresponding to the comparison signal Lthkwhich is determined to have a same level value as the level signal Ln. For example, the comparator330may output k as the selected signal Sel corresponding to the comparison signal Lthkhaving a same level value as the level signal Ln.

When the first signal Xnis a pulse amplitude modulation-4 (PAM-4) signal, the level signal Lnmay include four signal levels as shown inFIG.6A. The plurality of comparison signals Lth1to Lthmmay correspond to six signal levels as shown inFIG.6B, and in this case, the selected signal Sel may be output as any one of 1 to 6.

FIG.4illustrates another example of the precoder ofFIG.1andFIG.5illustrates the level detector ofFIG.4.

The precoder100may include the filter200to generate the filtering signal Znusing three selected signals Sel1to Sel3. Since the precoder100ofFIG.4may be same as the precoder100ofFIG.2except for the level detector300, a duplicate description of the configuration to perform the same operation is omitted.

The level detector300may further include a plurality of delay elements350-1and350-2. The delay elements350-1and350-2may output delayed signals by delaying the level signal Lnoutput from the adder310. For example, the first delay element350-1may output a first delayed level signal Ln-1by delaying the level signal Lnby a unit of time and the second delay element350-2may output a second level signal Ln-2by delaying the first delayed level signal Ln-1by a unit of time. That is, the second delayed level signal Ln-2may be a delayed signal of the level signal Lnby two units of time.

The level detector300may further include a plurality of comparators330-1to330-3. The respective comparators330-1to330-3may perform the same operation as the comparator330ofFIG.3. For example, the comparators330-1to330-3may output the first selected signal Sel1, the second selected signal Sel2, and the third selected signal Sel3respectively based on the level signal Ln, the first delayed level signal Ln-1, and the second delayed level signal Ln-2.

The first selected signal Sel1, the second selected signal Sel2, and the third selected signal Sel3may be input to the corresponding selectors230-1to230-3. That is, since the selected signals Sel1to Sel3different from each other may be input to each of the selectors230-1to230-3, the selectors230-1to230-3may output filter coefficients different from each other and the filtering signal Znmay be generated based on the different filter coefficients. For example, when the selected signals Sel1to Sel3are p, q, and r (p, q, and r are natural numbers greater than or equal to 1 and less than or equal to m) respectively, the filtering signal Znmay be expressed by Equation 3.
Zn=h2pYn+h3qYn-1+h4rYn-2[Equation 3]

The above-described devices may be configured to act as one or more software modules in order to perform the operations of the above-described examples, or vice versa.