WO2022174363A1

WO2022174363A1 - Mimo equalization with weighted coefficients update

Info

Publication number: WO2022174363A1
Application number: PCT/CN2021/076668
Authority: WO
Inventors: Fabio PITTALA; Changsong Xie; Xianhua Liu; Yongheng Huang
Original assignee: Huawei Technologies Co Ltd
Current assignee: Huawei Technologies Co Ltd
Priority date: 2021-02-17
Filing date: 2021-02-17
Publication date: 2022-08-25
Anticipated expiration: 2023-08-17
Also published as: EP4233210A1; EP4233210A4

Abstract

A signal processing device (1) processes a receiver signal (20) that has two components which are, or derive from, two polarization components of a received optical signal (23). The receiver signal comprises a plurality of frames (41-45). Each frame comprises a training sequence ( "TS" ) and a payload sequence ( "Data" ). The payload sequence comprises N sections. A channel state information, CSI, estimator (10) determines CSI of each frame based on the training sequence of the frame. A 2x2 MIMO equalizer equalizes each section of the payload sequence of a current frame based on the CSI of L reference frames among the plurality of frames, wherein the CSI of the L reference frames is weighted. Thus, highly effective equalization is achieved.

Description

MIMO EQUALIZATION WITH WEIGHTED COEFFICIENTS UPDATE

TECHNICAL FIELD

The invention relates to a signal processing device and method, especially concerned with equalization of received signals.

BACKGROUND

Amongst other technologies, optical ground wire (OPGW) cables have been used on overhead power transmission lines for lightning protection. This allows for a cheap deployment of optical fiber cables, which are placed in metal tubes in the center of the cable, shielded by one or more layers of armoring wires. However, in recent years, field and laboratory measurements indicated that the magnetic field generated along the axis of the conductors caused by lightning current changes the state of polarization, SOP, of light in the fiber very rapidly.

This becomes an issue when using coherent optical polarization multiplexed transmission schemes of 100+Gb/s. An initial assessment of the potential impact on the tracking abilities of coherent digital signal processing, DSP, integrated circuits was made, where the SOP speed due to lightning strikes was estimated to be of the order of a few Mrad/s. A recent field trial confirmed this phenomenon and measured SOP speeds up to 5.1 Mrad/s. In recent laboratory experiments, fast SOP speeds exceeding 8 Mrad/swere measured.

Since most optical transceivers containing commercial DSPs have been designed to withstand SOP changes mainly caused by mechanical vibrations, the SOP tracking capabilities are often only in the hundreds of krad/s. Therefore, the industry arrived at a consensus that lightning strikes into OPGW cables can lead to traffic interruptions in metro and long-haul networks.

A 2x2 multiple-input multiple-output (MIMO) equalizer transforms a two-component receiver signal into an equalized two-component receiver signal. The two components of the receiver signal (before and after equalization) are associated with two polarization components of the received optical signal. Mathematically, the equalizer can be represented as a 2x2 matrix (equalizing matrix) which transforms two input signals into two output signals. Ideally, the 2x2 equalizing matrix inverts a 2x2 channel matrix (often denoted H) associated with the optical transmission channel. The receiver signal may be the received optical signal itself, or it may derive from the optical signal; for example, the receiver signal may be a baseband signal generated by coherent detection of the optical signal.

The polarization tracking capability of the DSP depends on the 2x2 MIMO equalizer adaptation and/or update speed as it determines how fast the equalizer can follow transient polarization changes. The adaptation scheme can be either implemented using blind or data-aided approaches. In case of blind adaptation, the update relies solely on signal statistics, in particular on the expected signal constellation, while the data-aided adaptation uses pre-defined training sequences for channel estimation, CE, that are inserted at the transmitter at the cost of signaling overhead. With regard to the adaptation speed, both approaches have their advantages and disadvantages: The blind scheme, enables “continuous” adaptation without introducing signaling overhead. However, it requires a feedback loop, which limits the update speed. The data-aided scheme, can be implemented in a feed-forward structure and does not need a feedback loop. However, its adaptation speed depends on the repetition rate of the training sequence which is eventually limited by the required signaling overhead.

Most of the DSP application specific circuits (ASICs) available in the market use the 2x2 MIMO equalizer based on gradient algorithms, such as the constant-modulus algorithm or the decision-directed least-mean-square, DD-LMS, algorithm. Thus, considering also the other processing algorithms, 100G DP-QPSK DSP-ASICs can be designed such to track SOP rotation speeds up to a few hundred of krad/s, covering most of the SOP transients occurring in buried fiber links and some cases of lightning. For higher-order quadrature amplitude modulation, QAM, the SOP tracking capability of such DSPs drastically reduces. DSPs based on 2x2 MIMO channel estimation improve the performance of the 2x2 MIMO equalizer, but ultra-fast SOP rotation speed due to lightning on aerial fiber links cannot be compensated, unless an unpractical training-overhead is used. In addition, state-of-the-art training-aided 2x2 MIMO channel estimation induces a non-uniform bit error distribution. In fact, looking at a frame of bits: a lower number of bit errors is observed near the training-sequence used for channel estimation with increasing number of bit errors when moving apart from the point of the channel estimation. This might be an issue for state-of-the art FEC decoders leading to long burst of post-FEC errors.

SUMMARY

According to a first aspect of the invention, a signal processing device is configured to process a receiver signal that has two components which are, or derive from, two polarization components of a received optical signal. The receiver signal comprises a plurality of frames, each of the frames in turn comprising a training sequence and a payload sequence. The payload sequence comprises N sections, N being 2 or greater than 2. The signal processing device comprises a channel state information, CSI, estimator configured to determine, for each frame, CSI based on the training sequence of the frame. The signal processing device further comprises a two-times-two (2x2) multiple input multiple output (MIMO) equalizer. The 2x2 MIMO equalizer is a filter with two inputs and two outputs. The two inputs and two outputs are associated with the two polarization components of the optical signal, respectively (i.e. each input and each output relates to one polarization) . The 2x2 MIMO equalizer is configured to equalize each of the N sections of the payload sequence of a current frame based on the CSI of L reference frames among the plurality of frames. L is at least 2. The equalizing comprises weighting the CSI of the L reference frames. This makes the equalization more effective.

According to a first embodiment of the first aspect, the equalizer is configured to weight the CSI in accordance with a temporal distance between the training sequence of a current frame and a current section of the payload sequence of the current frame. This further increases the equalization effectivity.

According to a second advantageous embodiment of the first aspect, the L reference frames are consecutive. Additionally, or alternatively, half of the training sequences of the reference frames are before the current payload sequence and half are after. This leads to an especially efficient equalization.

According to a third embodiment of the first aspect, the equalizer is configured to use different weighting factors for weighting the CSI of different sections of the N sections of each payload sequence. This further increases the equalization effectivity.

According to a fourth embodiment of the first aspect, the equalizer is configured to weight CSI of training sequence closer to a current section of the payload sequence heavier than further from the current section of the payload sequence. Also, this measure further increases equalization effectivity.

According to a fifth embodiment of the first aspect, the signal processing device further comprises a buffer, which is configured to buffer a signal derived from the receiver signal, while the channel estimator determines the CSI. This allows for using the optimal coefficient for each section of the payload sequence, and therefore further increases the equalization effectivity.

According to a sixth embodiment of the first aspect, N is 2 or 3 or 4 or 5 or 6 or 7 or 8 or 9 or 10 or at least 3 or at least 4 or at least 5 or at least 6 or at least 7 or at least 8 or at least 9, or at least 10. This allows for a very flexible equalization.

According to a seventh embodiment of the first aspect, L is 2 or 3 or 4 or 5 or 6 or 7 or 8 or 9 or 10 or at least 3 or at least 4 or at least 5 or at least 6 or at least 7 or at least 8 or at least 9, or at least 10. This further increases the flexibility of the equalization.

According to an eighth embodiment of the first aspect, the equalizer is configured to adaptively determine N and/or L and/or the weighting factors for weighting the CSI. This further increases the effectivity of the equalization.

According to a ninth embodiment of the first aspect, the equalizer is configured to perform a statistical analysis of the receiver signal and adaptively determine N, and/or L, and/or the weighting coefficients based upon the statistical analysis. This further increases the equalization effectivity.

According to a tenth embodiment of the first aspect, the channel is a 2x2 multiple-input multiple-output, MIMO, channel. The CSI are 2x2 MIMO channel estimation coefficients, and the equalization filter is a 2x2 MIMO filter. This allows the use of the signal processing device with a wide range of communication systems.

According to an eleventh embodiment of the first aspect, the signal processing device is configured to compensate dynamic changes of a state of polarization, SOP, of the receiver signal, and/or polarization-mode dispersion and/or chromatic dispersion, and/or frequency response of the channel. This allows for an especially flexible use of the signal processing device.

According to a twelfth embodiment of the first aspect, the channel is a channel in a coherent optical transmission system. This allows for a flexible use of the device.

According to a second aspect of the present invention, a communication device comprising a before-described signal processing device is provided. This allows for an efficient communication in a communication system.

According to a third aspect of the invention, a method of processing a receiver signal is provided, the received signal having two components which are, or derive from, two polarization components of an optical signal received via a channel. The receiver signal comprises a plurality of frames, each of the frames comprising a training sequence, and a payload sequence. The payload sequence comprises N sections. N is 2 or greater. The signal processing method comprises determining, for each frame, CSI based on the training sequence of the frame, and equalizing each of the N sections of the payload sequence of a current frame based on the CSI of L reference frames of the frames, wherein L is at least 2. Equalizing the current section comprises weighting the CSI of the L reference frames. This allows for an especially effective equalization.

According to a fourth aspect of the invention, a computer program comprises a program code for performing the above described method when the computer program runs on a computer.

As a fifth aspect of the invention, a computer-readable medium carries program code for performing the above described method when the program is executed by a computer or by a digital signal processor.

Accordingly, an object of the present invention is to provide an apparatus and method, which allow for an increased quality of an equalization, performed on the reception side of a transmission.

The object is solved by the features of the independent claims. Any embodiments that are not within the scope of protection of the independent claims are to be understood as examples helpful for understanding the invention, but not as embodiments of the invention.

Generally, it has to be noted that all arrangements, devices, elements, units and means and so forth described in the present application could be implemented by software or hardware elements or any kind of combination thereof. Furthermore, the devices may be processors or may comprise processors, wherein the functions of the elements, units and means described in the present applications may be implemented in one or more processors. All steps which are performed by the various entities described in the present application as well as the functionality described to be performed by the various entities are intended to mean that the respective entity is adapted to or configured to perform the respective steps and functionalities. Even if in the following description or specific embodiments, a specific functionality or step to be performed by a general entity is not reflected in the description of a specific detailed element of that entity which performs that specific step or functionality, it should be clear for a skilled person that these methods and functionalities can be implemented in respect of software or hardware elements, or any kind of combination thereof.

BRIEF DESCRIPTION OF DRAWINGS

The present invention is in the following explained in detail in relation to embodiments of the invention in reference to the enclosed drawings, in which

Fig. 1 shows a first embodiment of the inventive signal processing device in a block diagram;

Fig. 2 shows a second embodiment of the inventive signal processing device;

Fig. 3 shows a third embodiment of the inventive signal processing device as part of a first embodiment of the inventive communication device;

Fig. 4 shows an example of a receiver signal;

Fig. 5 shows reception results using a state-of-the-art equalization;

Fig. 6 shows an example receiver signal with highlighted segments of the payload sequence as employed by a fourth embodiment of the inventive signal processing device;

Fig. 7 shows reception results using a fifth embodiment of the inventive signal processing device, and

Fig. 8 shows an embodiment of the inventive signal processing method in a flow diagram.

DESCRIPTION OF EMBODIMENTS

First, we demonstrate the general construction and function of a most basic embodiment of the inventive signal processing device with respect to Fig. 1. Along Fig. 2 and Fig. 3, further construction details of further embodiments of the inventive signal processing device and the inventive communication device are shown and described. Along Fig. 4 and Fig. 5, a state-of-the-art equalization is briefly explained, while along Fig. 6 and Fig. 7, further details of the equalization using a further embodiment of the inventive signal processing device are described. Finally, along Fig. 8, the detailed function of an embodiment of the inventive method is described. Similar entities and reference numbers in different figures have been partially omitted.

In Fig. 1, a first embodiment of the inventive signal processing device 1 is shown. The signal processing device comprises a channel state information estimator 10 connected to an equalizer 11. The signal processing device 1 receives a receiver signal 20. The receiver signal 20 comprises two components which correspond to a first polarization and a second polarization of a received optical signal, respectively. The received optical signal is an optical signal that has been transmitted over a channel, e.g. an optical fiber. The receiver signal 20 may be the received optical signal itself, or it may be a baseband signal generated from the received optical signal, e.g. by coherent detection. The receiver signal comprises a plurality of frames, each of the frames comprising a training sequence (TS) and a payload sequence (PS) , the payload sequence comprising N sections, N being 2 or greater than 2.

The frames are successive, non-overlapping segments of the receiver signal. The receiver signal is fed to the channel state information estimator 10, which determines, for each frame, a channel state information, CSI of the frame based on the training sequence of the frame. This channel state information CSI 21 is handed on to the equalizer 11, which equalizes at least two of the N sections of the payload sequence of the current frame based on the CSI 21 of L reference frames among the plurality of the frames. L is at least 2. The equalizing by the equalizer comprises weighting the CSI 21 of the L reference frames. The equalizer 11 moreover is configured to output an equalized receiver signal 22, comprising the equalized payload segments.

In Fig. 2, a further embodiment of the signal processing device 1 is shown. Here, the signal processing device 1 additionally comprises a buffer 12 which is also fed with the receiver signal 20. The buffer 12 buffers the receiver signal 20, while the CSI estimator 10 performs the CSI estimation. The buffer 12 therefore time-delays the receiver signal 20 resulting in the time-delayed receiver signal 20'. The time delay amount is set so that all CSI of the payload sequence taken into account for a specific payload section are available, when the respective payload section is equalized.

The time-delayed receiver signal 20' as well as the CSI 21 are supplied to the equalizer 11, which performs the equalization of the time-delayed receiver signal 20' using the CSI information determined by the CSI estimator 10. Apart from the specific functions of the buffer 12, the function is identical to the embodiment of Fig. 1.

Therefore, the equalization is performed using a number of L reference frames as a basis for the CSI. Therein, half (i.e. L/2) of the training sequences of the L reference frames come before the current payload sequence and half (i.e. L/2) of the training sequences of the L reference frames come after the current payload sequence.

Preferably, the equalizer uses different weighting factors for weighting the CSI for different sections of the N section of each payload sequence. Moreover, preferably the equalizer is configured to weight CSI of training sequences that are closer (i.e. closer in time) to a current section of the payload sequence more heavily than CSI of training sequences that are further away (in time) from the current section of the payload sequence.

The number of payload sections within one payload sequence can be 2 or 3 or 4 or 5 or 6 or 7 or 8 or 9 or 10 or at least 3 or at least 4 or at least 5 or at least 6 or at least 7 or at least 8 or at least 9 or at least 10. Moreover, the number of reference frames L can be 2, 3, 4, 5, 6, 7, 8, 9, 10, at least 3, at least 4, at least 5, at least 6, at least 7, at least 8, at least 9, or at least 10.

Moreover, N and L can be adaptively determined. Also, an adaptive determining of the weighting factors used for the different payload sections is possible.

The equalizer may additionally perform a statistical analysis of the receiver signal and adaptively determine N and/or L and/or the weighting factors based on the statistical analysis.

The equalization can compensate for effects such as dynamic changes of a state of polarization of the optical signal, polarization-mode dispersion, chromatic dispersion, or a frequency response of the channel.

Moreover, in Fig. 3, a third embodiment of the signal processing device 1 is shown. Here, the signal processing device 1 is shown as part of an embodiment of the communication device 30. The communication device 30 comprises an optocoupler 31, which is connected to the signal processing device 1, especially to the channel estimator 10 and the buffer 12. Moreover, the communication device 30 comprises a symbol detector 32, which is connected to the signal processing device 1, especially to the equalizer 11.

The communication device 30 receives an optical signal 23, which is converted to the receiver signal 20 by the optocoupler 31. In this embodiment, the receiver signal 20 is a baseband signal. The baseband signal may be provided in an analog form or in a digital form, depending on the design of the optocoupler 31. The receiver signal 20 is fed to the signal processing device 1, as described above with reference to Fig. 2. Alternatively, the signal processing device 1 of Fig. 1 could be employed here.

The equalized receiver signal 22 is fed to the symbol detector 32, which detects reception symbols in the equalized receiver signal 22.

Fig. 4 schematically illustrates an example of a receiver signal 40. The receiver signal 40 comprises

frames

41, 42, 43, 44, 45. The signal may comprise further frames (not shown) before or after the frames 41-45, or between

frames

41 and 42, or between

frames

42 and 43. Each frame comprises a training sequence “TS” and a payload sequence “Data” . In this example, the training sequence comes before the payload sequence in each frame. In another example (not shown) , the training sequence may come after the payload sequence in each frame. From each training sequence, a CSI may be determined. The CSI are labeled here as H ₀, H _(L/2) , H _(L/2) +1, H _L-1.

Moreover, in Fig. 4, above the exemplary receiver signal 40, a resulting bit error rate 46 dependent upon the position within the respective payload sequence, when using only a single CSI per payload sequence, is shown. In the middle of the payload sequence, the bit error rate has a relatively low value, while it significantly rises towards the beginning and end of the payload sequence. This is due to the fact that the channel constantly fluctuates. Therefore, using only a single CSI for performing an equalization leads to an optimal equalization only for a single point within the payload sequence, while all other points of the payload sequence are sub-optimally equalized.

This is further exemplified in Fig. 5, where the resulting bit error rate 50 and the average bit error rate 51 throughout one payload sequence are shown.

In Fig. 6, a further exemplary receiver signal 40 is shown. Here, the payload sequence is divided into three

sections

60, 61 and 62. The points corresponding to the payload sections 60-62 are indicated with arrows in the bit error rate depicted above the receiver signal 40.

According to the present invention, a filter update is performed for each

payload section

60, 61 and 62, resulting in an optimized equalization for each section, i.e. for different sections of the payload sequence.

The CSI of each of the L reference frames may be weighted depending on the position of the current payload section within the payload sequence. For example, for the first payload section 60, CSI derived from training sequences before the payload section 60 can be given higher weights than CSI derived from training sequences further away from the payload section 60. Therefore, the CSI obtained from training sequences before the current frame may be given higher weight than that obtained from training sequences after the current frame. For example, for the payload section 61, CSI coefficients derived from training sequences before and after the current frame may be weighted equally. For example, for the payload section 62, CSI coefficients derived from training sequences after the current frame may be given higher weights than those from before the current frame.

In one example of an embodiment, a channel matrix H _WGT for equalizing a section of a payload sequence is computed as a weighted sum as follows:

where

α (0<α≤1) and μ _l (0≤μ _l≤1) are two pre-determined or user-selectable weighting factors, L is the number of reference frames,

l is a frame index,

k is the number of the current payload sequence, and

H _l is the 2x2 channel matrix (i.e. the CSI) determined from the training sequence l.

The elements of a 2x2 equalizing matrix can be computed from the channel matrix H _WGT using known methods such as a minimum mean square error algorithm or zero-forcing.

An equalized receiver signal is generated by applying the 2x2 equalizing matrix to the receiver signal.

Since this method is based on weighted averaging of consecutive channel estimations it is possible to compensate in particular for dynamic changes of the state-of-polarization rotation. Further effects, such as polarization-mode dispersion, chromatic-dispersion and amplitude frequency response of the optical channel can also be compensated for.

The weighted channel estimation cost function can be adaptively changed based on an actual SOP rotation speed to trade-off B2B performance (quasi static channel) with SOP tracking-mode (time-varying channel) .

In Fig. 7, a resulting bit rate 70 and a resulting average bit rate 71 by using a number of N=3 are shown. It is readily visible in comparison to Fig. 5, that a significant reduction of the bit error rate is achieved.

Finally, in Fig. 8, an embodiment of the signal processing method is shown.

In a first step 100, an optical signal is converted into a receiver signal, or the optical signal itself is taken as a receiver signal. The receiver signal comprises a plurality of frames, each of the frames comprising a training sequence and a payload sequence. The payload sequence comprising N sections, N being 2 or greater than 2.

In a second step 101, for each frame, a CSI of the frame based on the training sequence of the frame is determined.

In a third step 102, a weighting of the CSI of L reference frames is performed, wherein L is greater than or equal to 2.

In a fourth step 103, each of the N sections of the payload sequence of a current frame are equalized based on the weighted CSI of the L reference frames of the frames. Since the device and method very closely correspond, we point out that all features of the method and device are applicable to the other, and herewith disclosed.

The invention is not limited to the examples and especially not to a specific number of N, L, or specific weighting factors. Also, the invention is not restricted to a specific type of receiver signal. The characteristics of the exemplary embodiments can be used in any advantageous combination.

The invention has been described in conjunction with various embodiments herein. However, other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word “comprising “does not exclude other elements or steps and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in usually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the internet or other wired or wireless communication systems.

Claims

A signal processing device (1) for processing a receiver signal (20) having two components, the two components being, or deriving from, two polarization components of a received optical signal (23) , the receiver signal comprising a plurality of frames (41-45) , each of the frames comprising a training sequence ( “TS” ) and a payload sequence ( “Data” ) , the payload sequence comprising N sections (60, 61, 62) , N being 2 or greater than 2,

wherein the signal processing device comprises:

a channel state information, CSI, estimator (10) configured to determine, for each frame, CSI (21) based on the training sequence of the frame; and

a 2x2 MIMO equalizer (11) configured to equalize each of the N sections of the payload sequence of a current frame based on the CSI of L reference frames among the plurality of the frames, wherein L is greater than or equal to 2,

wherein equalizing a current section among the N sections of the payload sequence of the current frame comprises weighting the CSI of the L reference frames.
The signal processing device of claim 1,

wherein the weighting the CSI of the L reference frames comprises, for each reference frame among the L reference frames:

weighting the CSI of the reference frame in accordance with a temporal distance between the training sequence of the reference frame and the current section of the payload sequence of the current frame.
The signal processing device of claim 1 or 2,

wherein the L reference frames are consecutive, and/or

wherein L/2 of the training sequences of the L reference frames come before the current payload sequence, and L/2 of the training sequences of the L reference frames come after the current payload sequence.
The signal processing device of claim 1 or 2,

wherein the equalizer is configured to use different weighting factors for weighting the CSI for different sections of the N sections of each payload sequence.
The signal processing device of any of the claims 1 to 4,

wherein the weighting the CSI of the L reference frames comprises:

weighting CSI determined from a training sequence that is closer to the current section more heavily than CSI determined from a training sequence that is further away from the current section of the payload sequence.
The signal processing device of any of the claims 1 to 5,

further comprising a buffer, configured to buffer a signal derived from the receiver signal while the channel estimator determines the CSI.
The signal processing device of any of the claims 1 to 6,

wherein N is 2 or 3 or 4 or 5 or 6 or 7 or 8 or 9 or 10, or at least 3 or at least 4 or at least 5 or at least 6 or at least 7 or at least 8 or at least 9 at least 10.
The signal processing device of any of the claims 1 to 7,

wherein L is 2, or 3, or 4 or 5 or 6 or 7 or 8 or 9 or 10, or at least 3 or at least 4 or at least 5 or at least 6 or at least 7or at least 8 or at least 9 at least 10.
The signal processing device of any of the claims 1 to 8,

wherein the equalizer is configured to adaptively determine N and/or L and/or weighting factors for weighting the CSI.
The signal processing device of any of the claims 1 to 8,

wherein the equalizer is configured to:

analyze statistics of the receiver signal, and

adaptively determine N, and/or L, and/or weighting factors for weighting the CSI based on the statistics of the receiver signal.
The signal processing device of any of the claims 1 to 10,

wherein the channel is a 2x2 multiple input multiple output, MIMO, channel and the CSI comprises 2x2 MIMO channel estimation coefficients.
The signal processing device of any of the claims 1 to 11,

wherein the signal processing device is configured to compensate:

dynamic changes of a state of polarization, SOP, of the receiver signal, and/or

polarization-mode dispersion, and/or

chromatic dispersion, and/or

frequency response of the channel.
The signal processing device of any of the claims 1 to 12,

wherein the channel is a channel in a coherent optical transmission system.
A communication device comprising the signal processing device of any of the claims 1 to 13.
A method of processing a receiver signal (20) having two components, the two components being, or deriving from, two polarization components of a received optical signal (23) , the receiver signal comprising a plurality of frames, each of the frames comprising a training sequence ( “TS” ) and a payload sequence ( “Data” ) , the payload sequence comprising N sections, N being 2 or greater than 2,

wherein the method comprises:

determining, for each frame, channel state information, CSI, based on the training sequence of the frame,

equalizing each of the N sections of the payload sequence of a current frame based on the CSI of L reference frames among the plurality of frames, wherein L is greater than or equal to 2,

wherein equalizing a current section among the N sections of the payload sequence of the current frame comprises weighting the CSI of the L reference frames.
A computer program comprising a program code for performing the method according to claim 15 when the computer program runs on a computer.
A computer readable medium carrying a program code which causes a processor to perform the method of claim 15 when the program is executed by the processor.