FR3134266A1 - DENSE IQ CODING METHOD AND DEVICE FOR WDM COMMUNICATION SYSTEM ON OPTICAL FIBER - Google Patents

Abstract

The present invention relates to a WDM transmission method and device over dual polarization optical fiber. The transmission method uses specific IQ coding to combat the effects of PDL. The modulation symbols to be transmitted on the polarization states of the wavelengths undergo a decomposition into real values and imaginary values (220). A real vector composed by the concatenation of these real values and these imaginary values is then constructed. A first invertible linear transformation, represented by a dense real matrix, is applied (230) to the real vector thus obtained to provide a transformed real vector. Complex transmission symbols are formed by combining I/Q (240) of the components of the transformed vector, the transmission symbols then modulating the different polarization states of the WDM channels. Fig. 2

Description

DENSE IQ CODING METHOD AND DEVICE FOR WDM COMMUNICATION SYSTEM ON OPTICAL FIBER

The present invention relates to the field of communications over optical fibers and more particularly to wavelength multiplexing or WDM ( Wavelength D i vision Multiplexing ) communications.

State of the prior art

Commonly used optical fiber WDM communication systems achieve transmission rates of the order of several Tb/s. Different types of WDM systems are known in the state of the art, some being defined in wavelengths (CWDM for Coarse Wavelength Division Multiplexing ) and others, more recent, being defined in frequencies (DWDM for Dense WDM). The difference between CWDM and DWDM systems is essentially the spacing between transmission channels. When the transmission channels are contiguous or even overlapping, we respectively speak of a WDM superchannel and a Nyquist superchannel . The term WDM will be used in the following in its general sense and will cover the different types of systems mentioned above.

The use of high modulation orders as well as multiplexing on orthogonal polarizations have made it possible to further increase the capacity of these communication systems, but this progress now comes up against various limitations.

First of all, the increase in the density of WDM transmission channels and correlatively the rapprochement of the subcarriers, leads to an increase in the level of inter-channel interference or ICI ( Inter Channel Interference ). This interference can be combatted by adopting an ideally rectangular shaping of the channels in the frequency domain, in other words by a waveform according to a sync function in the time domain (so-called Nyquist shaping). Of course, in practice the shaping is imperfect and residual inter-channel interference remains.

Then, different dispersion phenomena such as chromatic dispersion or CD ( Chromatic Dispersion ), polarization dispersion or PMD ( Polarization Mode Dispersion ) and polarization-dependent attenuation or PDL ( Polarization Dependent Loss ) increase the error rate. (BER) in the different channels. However, if the first two can be compensated digitally on reception, the last cannot be due to its non-unitary nature, which degrades the performance of WDM transmission systems in terms of BER depending on the bit rate. , and therefore transmission capacity.

It was proposed in Elie Awad's thesis entitled "Emerging space-time coding techniques for optical fiber transmission systems", published in 2015, to use spatio-temporal coding techniques to combat the degradation of transmission capacity due to the PDL. However, these coding techniques complicate the transmitter and the receiver since the block of information symbols to be transmitted is coded over several successive transmission intervals or TTIs ( Time Transmission Interval s ) and, more generally, over several channel uses. or CUs ( Channel Uses ).

A precoding method on orthogonal polarizations to combat the reduction in capacity due to PDL was described in the article by C. Zhu et al. entitled “Improved polarization dependent loss tolerance for polarization multiplexed coherent optical systems by polarization pairwise coding” published in J. Lightwave Technology, vol. 34 no. 8, pages 1746-1753, 2016.

This method of precoding on orthogonal polarizations was illustrated schematically in .

The information symbols (binary words) to be transmitted are converted into symbols of a modulation constellation in the modulators -area with symbol 110-1 and 110-2. The modulation symbols obtained, are then subject to an angle rotation in the complex plane by means of the respective rotation modules 120-1 and 120-2 to obtain rotated symbols, . The real part of the first rotated symbol and the real part of the second rotated symbol are combined in 130-1 to provide a first transmission symbol, , carried by a first polarization component (for example a state of horizontal polarization). Similarly, the imaginary part of the first rotated symbol and the imaginary part of the second rotated symbol are combined at 130-2 to provide a second transmission symbol , carried by a second polarization component orthogonal to the first (for example a state of vertical polarization).

The light signal whose orthogonal polarization components have been respectively modulated by the emission symbols , is then transmitted over the optical fiber.

The precoding method described in this article, however, only applies to a single-carrier transmission system and not to a WDM transmission system.

An object of the present invention is therefore to propose a WDM transmission method over optical fiber, as well as an associated transmission device, which makes it possible to achieve high transmission capacities despite the PDL and the interference between adjacent channels while requiring only a single transmission channel use to transmit a block of information symbols.

The present invention is defined by a WDM transmission method over optical fiber with polarization duality, intended to transmit, during channel use, symbols belonging to a modulation constellation in the complex plane, being the number of WDM channels used for transmission, said WDM transmission method being original in that:

said symbols undergo a separation into real part and imaginary part to provide a real vector ( ) size formed by the real parts of these symbols and the imaginary parts of these same symbols;

an invertible linear transformation represented by a dense real matrix of size is applied to the real vector to provide a transformed real vector;

Complex emission symbols are obtained by performing an IQ combination of components of a first set of components of the real vector transformed respectively with the components of a second set of components of the transformed real vector, the first and second sets being disjoint, each complex transmission symbol modulating a first state and a second polarization state of a WDM channel.

The real vector is typically formed by the concatenation of a first vector composed of the real parts of the modulation symbols and a second vector composed of the imaginary parts of these same symbols.

Preferably, the first set of components of the transformed real vector is composed of the first components of this vector and that the second set of components of the transformed real vector is composed of the last components of this vector.

Advantageously, the characteristic polynomial of the dense real matrix does not have real roots.

According to a preferred embodiment, the dense real matrix is a rotation matrix in space .

The invention also relates to a WDM transmission device over optical fiber with polarization duality, intended to transmit, during channel use, symbols belonging to a modulation constellation in the complex plane, being the number of WDM channels used for transmission, said device being original in that it includes:

a first module configured to separate each of said symbols into a real part and an imaginary part to provide a real vector ( ) size formed by the real parts of these symbols and the imaginary parts of these same symbols;

a second linear combination module configured to apply an invertible linear transformation, represented by a dense real matrix of size , to the real vector to provide a transformed real vector;

a third IQ combination module configured to respectively combine components of a first set of components of the real vector transformed with components of a second set of components of the transformed real vector, the first and second sets being disjoint, so as to generate complex transmission symbols, each complex transmission symbol modulating a first state and a second polarization state of a WDM channel.

The first module is typically configured to form said real vector by concatenating a first vector composed of the real parts of the modulation symbols and a second vector composed of the imaginary parts of these same symbols.

Preferably, the third module is configured so that the first set of components of the transformed real vector is composed of the first components of this vector and that the second set of components of the transformed real vector is composed of the last components of this vector.

Advantageously, the characteristic polynomial of the dense real matrix does not have real roots.

According to a preferred embodiment, the dense real matrix is a rotation matrix in space .

Other characteristics and advantages of the invention will appear on reading a preferred embodiment of the invention, described with reference to the attached figures among which:

, already described, schematically represents an optical fiber transmission device using pre-coding on two orthogonal polarizations;

schematically represents a WDM transmission device over optical fiber with dense IQ coding according to a general embodiment of the invention;

schematically represents a WDM transmission device over optical fiber with dense IQ coding according to a preferred embodiment of the invention;

show the gain provided by a WDM transmission device according to the invention for different hypotheses of WDM channels.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

We will subsequently consider a WDM transmission system over optical fiber and assume that this fiber is classically affected by PDL attenuation, in other words that the different states of polarization (SOP) in the fiber do not undergo the same attenuation. It is recalled that PDL attenuation is generally introduced by optical elements between fiber sections, in particular doped fiber optical amplifiers (EDFA) which create energy losses and fluctuations in optical signal to noise ratio or OSNR (Optical Signal to Noise Ratio). However, dispersive effects in the fiber such as chromatic dispersion (CD) and polarization dispersion (PMD) will be ignored since these effects can be effectively corrected by channel equalization in the DSP of the receiver.

The effect of PDL attenuation for a WDM channel (and a single spatial mode) can be expressed by the matrix applying to the two states of polarization:

Or is the gain matrix, is the polarization rotation matrix and is the birefringence matrix with defining the value of PDL, , with And .

The WDM transmission system uses a plurality of WDM channels (wavelengths or subcarriers), each WDM channel being associated with two polarization states. Thus, at each transmission moment, in other words at each use of the channel, the transmission system can transmit modulation symbols, one symbol being transmitted per polarization state and per WDM channel. The number is chosen generally high, of the order of several dozen or even several hundred. In any case .

The idea underlying the present invention is to separate the real parts and the imaginary parts of the different modulation symbols and to subject all of the real and imaginary parts of the different symbols to an invertible linear transformation. We thus carry out an averaging of the PDL attenuation on the different polarization states and the different WDM channels.

There schematically represents a WDM transmission device over optical fiber according to a general embodiment of the invention.

The data to be transmitted at each transmission interval is in the form of information symbols, e.g. words -areas with Or is the cardinal of the modulation alphabet. The modulation alphabet may in particular be an alphabet -QAM.

The information symbols may themselves result from source coding and/or channel coding, in a manner known per se.

In all cases, the information symbols are respectively converted into modulation symbols in modulators -area with symbol 210-1,…,210-2N. The odd indices of these symbols correspond to a first polarization state and the even indices to a second polarization state, orthogonal to the first. Each of these modulation symbols, noted below , is then subjected to a decomposition into a real part and an imaginary part in the I/Q separation module, 220.

The real parts and the imaginary parts form a real vector of which is supplied to the linear combination module 230. In the figure, the real vector is obtained by separately grouping the real parts and the real parts of the modulation symbols , either . However, in general the vector can be obtained by concatenating in any way the real parts and the imaginary parts of these symbols, either Or represents any permutation of components.

This first module, 230, combines the elements of by means of an invertible linear transformation, , represented by a matrix , linear group of dimension on , to provide a transformed vector, , In . The linear transformation is chosen such that the matrix (representative of in the canonical basis of ) is dense (or full), that is to say it does not contain any zeros. Advantageously, the matrix is chosen such that its characteristic polynomial has no root in , in other words such that it has no space of its own. This property ensures effective mixing of the components of the vector and subsequently an averaging of the PDL.

The transformed vector, , can be expressed in the following form:

Or , ,…, are linear forms on .

THE first elements and last elements of are then combined two by two in an I/Q combination module, 240, to give a complex vector, of dimension :

More generally, we can form a first partial transformed vector, size by selecting vector components and a second partial transformed vector, , also sized ,by selecting the remaining components, the complex vector then being obtained as .

In any case, the complex elements of the vector are then respectively used to modulate the polarization states of WDM subcarriers/wavelengths.

There schematically represents a WDM transmission device over optical fiber according to a preferred embodiment of the invention.

Modules 310-1,…,310-N, 320, 330, 340 respectively perform the same functions as modules 210-1, 210-N, 220, 230, 240 of the .

This embodiment is a particular case of that represented in Fig. 2 in that the linear transformation here is a rotation in space . Note that the fact that the matrix must be full immediately excludes trivial rotation matrices Or Or is the size identity matrix .

Furthermore, the dimension of the space being even, the rotation matrix does not have its own (invariant) space.

Here again, the vector can be obtained by concatenating in any way the real parts and the imaginary parts of the modulation symbols . Likewise, the complex vector can be obtained as from partial transformed vectors constructed by selecting for each a set of components of the transformed vector , the sets of components associated with these two vectors being disjoint.

Finally, the complex elements of the vector are respectively used to modulate the polarization states of WDM subcarriers/wavelengths.

In the embodiments presented in Figs. 2 and 3, dense I/Q coding is applied to all WDM channels. However, alternatively, dense I/Q coding can be applied by block of channels, non-invertible linear transformations, by rotations which can be chosen distinct from one channel to another.

Finally, although the present invention has been presented in the context of an optical fiber with polarization state duality, those skilled in the art will understand that the dense IQ coding method described above can be applied in the case of a single polarization state.

In all cases, the received optical signal is demultiplexed into WDM channel (wavelength or subcarrier) and into polarization state and then equalized to compensate for chromatic dispersion (CD).

According to a first variant, a channel estimation and a corresponding equalization can be carried out sub-band by sub-band. According to a second variant, the channel estimation and the corresponding equalization can be carried out globally on the MIMO channel . In both cases, the channel estimation could be based on pilot symbols. In particular, CAZAC (Constant Amplitude Zero Auto Correlation) sequences can be used for this purpose, for example Zadoff-Chu sequences.

In the case of MIMO channel equalization , the symbols transmitted by the transmission device can be estimated using a MIMO decoder using an ML ( Maximum Likelihood ) estimate or more simply a ZF ( Zero Forcing ) estimate aimed at multiplying the signal received by the pseudo-inverse of the channel matrix, namely Or size is the estimated matrix of the MIMO channel. Alternatively, in sub-band by sub-band equalization, the estimation of the transmitted symbols is carried out from matrices , , each of these matrices corresponding to a sub-band. It should be noted that this operation does not include inverting the linear transformation represented by the matrix .

After separation of the real and imaginary parts of each of the components of , we construct from these components a real vector, , size . For example, if the embodiment illustrated in Fig. 2 or Fig. 3 was used for transmission, we can form a first vector made up of the real parts and a second vector consisting of imaginary parts, then concatenate the first vector and the second vector to obtain the real vector .

We then apply the inverse orthogonal transformation to vector to get a vector , then the inverse of the permutation applied to the emission on its components.

For example, when the real vector was obtained by grouping the real parts and the imaginary parts of the modulation symbols, the first components of the vector give an estimate of the real parts and the last components give an estimate of the imaginary parts transmitted modulation symbols.

Figs. 4A-4B show the gain provided by a WDM transmission device according to the invention for different hypotheses of WDM channels.

It was assumed that the value of PDL, was identical for all WDM channels and equal to 3dB, polarization rotation, was equal to .

The chromatic dispersion coefficient was equal to 17 and the polarization dispersion coefficient was equal to 0.05 .

The optical fiber was made up of 10 sections of 100 km each, an optical amplifier with constant gain in wavelength being provided between consecutive sections. The symbol rate was 28 Gbaud and the modulation constellation was 16-QAM.

Reception equalization was achieved using sub-band ZF equalization.

Fig. 4A gives the bit error rate (BER) as a function of the optical signal-to-noise ratio (OSNR) in the fiber for sub-bands. The sub-bands corresponding to the different WDM channels were non-overlapping and the signals transmitted in each sub-band were shaped by a raised cosine root filter with a roll-off factor of 0.1, which ensured an absence of interference between channels (ICI).

Curve 410 gives the bit error rate as a function of the signal-to-noise ratio for a PDL value equal to 3dB, in the case of a classic WDM transmission without coding.

Curve 420 gives the BER for a B2B ( Back to Back) transmission, i.e. a direct transmission, without fiber between the transmitter and the receiver.

We note that the curve 410 shows a performance loss of 2.5 dB in terms of signal to noise ratio (OSNR) compared to direct transmission, 420.

On the other hand, when a dense I/Q coding method according to the present invention is applied, we observe, at 430, a gain of approximately 1.2 dB compared to the uncoded case. The loss is then only around 1 dB compared to a B2B transmission.

There again gives the bit error rate (BER) as a function of the optical signal-to-noise ratio (OSNR) in the fiber but this time the sub-bands overlap (super-Nyquist WDM). Due to cross-channel interference, performance is degraded compared to the situation shown in . However, dense I/Q coding saves about 1 dB of SNR compared to the uncoded case.

Claims (10)

WDM transmission method over optical fiber with polarization duality, intended to transmit, during channel use, symbols belonging to a modulation constellation in the complex plane, being the number of WDM channels used for transmission, characterized in that:
said symbols undergo a separation into real part and imaginary part (220) to provide a real vector ( ) size formed by the real parts of these symbols and the imaginary parts of these same symbols;
an invertible linear transformation (230) represented by a dense real matrix of size is applied to the real vector to provide a transformed real vector;
complex emission symbols are obtained by performing a combination IQ (240) of components of a first set of components of the real vector transformed respectively with the components of a second set of components of the transformed real vector, the first and second sets being disjoint, each complex transmission symbol modulating a first state and a second polarization state of a WDM channel. WDM transmission method over optical fiber with polarization duality according to claim 1, characterized in that said real vector is formed by the concatenation of a first vector composed of the real parts of the modulation symbols and of a second vector composed by the imaginary parts of these same symbols. WDM transmission method over optical fiber with polarization duality according to claim 1 or 2, characterized in that the first set of components of the transformed real vector is composed of the first components of this vector and that the second set of components of the transformed real vector is composed of the last components of this vector. WDM transmission method over optical fiber with polarization duality according to one of the preceding claims, characterized in that the characteristic polynomial of the dense real matrix does not have real roots. WDM transmission method over optical fiber with polarization duality according to claim 4, characterized in that the dense real matrix is a rotation matrix in space . WDM transmission device over optical fiber with polarization duality, intended to transmit, during channel use, symbols belonging to a modulation constellation in the complex plane, being the number of WDM channels used for transmission, characterized in that it includes:
a first module configured to separate each of said symbols into a real part and an imaginary part (220) to provide a real vector ( ) size formed by the real parts of these symbols and the imaginary parts of these same symbols;
a second linear combination module (230) configured to apply an invertible linear transformation, represented by a dense real matrix of size , to the real vector to provide a transformed real vector;
a third IQ combination module (240) configured to respectively combine components of a first set of components of the real vector transformed with components of a second set of components of the transformed real vector, the first and second sets being disjoint, so as to generate complex transmission symbols, each complex transmission symbol modulating a first state and a second polarization state of a WDM channel. WDM transmission device over optical fiber with polarization duality according to claim 6, characterized in that the first module is configured to form said real vector by concatenating a first vector composed of the real parts of the modulation symbols and a second vector composed by the imaginary parts of these same symbols. WDM transmission device over optical fiber with polarization duality according to claim 6 or 7, characterized in that the third module is configured so that the first set of components of the transformed real vector is composed of first components of this vector and that the second set of components of the transformed real vector is composed of the last components of this vector. WDM transmission device on optical fiber with polarization duality according to one of claims 6 to 8, characterized in that the characteristic polynomial of the dense real matrix does not have real roots. WDM transmission device on optical fiber with polarization duality according to claim 9, characterized in that the dense real matrix is a rotation matrix in space .