Method for protecting a link in an optical network

Abstract

Enclosed herewith is a method for protecting a link in an optical network configured for transmitting digital data employing a predetermined modulation format which comprises a number of symbols in a constellation diagram. A binary address is associated with each symbol. The modulation format allows for a constellation distortion, according to which the relative positions of constellation points in the constellation diagram are varied in a predetermined way by a predetermined degree. The method the steps of: A) partitioning the traffic in two or more priority classes, B) mapping higher priority traffic to predefined bit positions within the binary symbol addresses, C) evaluating the quality of a predetermined protection link, D) determining a degree of distortion such that a desired transmission quality for the transmission of the traffic of the highest priority class or classes via said predetermined protection link and a desired transmission quality for the full traffic via said given link are simultaneously ensured, and E) employing said distorted constellation diagram for transmission of digital data over said given link.

Claims

1. A method for protecting a given link in an optical network, a signal being modulated based on a predetermined modulation format to carry traffic including digital data, wherein the modulation format is based on a constellation including a plurality of points in an n-dimensional Euclidean signal space with n1, each of the plurality of points representing a respective one of a plurality of symbols, each of the plurality of points has a relative position within the constellation, each of the plurality of symbols has a respective one of a plurality of binary addresses, and the predetermined modulation format allows for a constellation distortion, wherein relative positions of the plurality of the points in the constellation are varied in a predetermined manner by a predetermined degree, the method comprising the steps of: A) partitioning a first part of the traffic, the first part of the traffic having a higher priority than a second part of the traffic, B) mapping the first part of the traffic to predefined bit positions of selected binary addresses within the plurality of binary addresses, C) evaluating a quality of a predetermined protection link over which a portion of the traffic could be transmitted in case of a failure of the given link, D) determining a degree of distortion of the constellation wherein a desired transmission quality for transmission over the predetermined protection link of the first part of the traffic, and a desired transmission quality for transmission over the given link of the traffic to be transmitted over the given link, are simultaneously ensured, and E) employing a distorted constellation with the determined degree of distortion for transmission of the digital data over the given link.

2. The method of claim 1, wherein the predefined bit positions are bit positions associated with the at least one of the plurality of points of the constellation that have an error probability less than an average error probability of the bit positions of all the plurality of points of the constellation.

3. The method according to claim 1, further comprising a step F) that includes rerouting the traffic to the predetermined protection link in case of a failure of the given link.

4. The method according to claim 1, wherein in step C), the predetermined protection link is one of a plurality of alternative predetermined protection links that are evaluated for transmission quality, and wherein in step D), the degree of distortion of the constellation is determined such that the desired transmission quality is provided for one of the plurality of alternative predetermined protection links determined by step C) to have the worst transmission quality of the plurality of alternative predetermined protection links.

5. The method according to claim 1, wherein in the constellation distortion, the predetermined way of varying the relative positions of the at least a plurality of the constellation points comprises one or more of varying distances of a subset of adjacent ones of the constellation points in the constellation diagram, varying distances of a subset of the constellation points from a predefined position in the signal space which does not coincide with a constellation point, rotating a position of a subset of the constellation points with respect to an origin of the signal space.

6. The method according to claim 5, wherein the predefined position in the signal space corresponds to a center of mass of a subset of the constellation points, or is chosen such that upon the variation of the relative positions of the at least a plurality of the constellation points, an average power of a signal comprising the at least a plurality of the constellation points remains unchanged.

7. The method according to claim 1, wherein the constellation is in a two-dimensional plane, which comprises four quadrants, wherein in the binary addresses of the points, there are two predetermined bit positions which have identical values for each of a group of the plurality of points associated with the same quadrant, and wherein the first part of the traffic is mapped to the predetermined two bit positions.

8. The method according to claim 7, wherein the modulation format is based on a 16QAM constellation, and wherein in the constellation distortion, the predetermined manner of varying the relative positions of the at least a plurality of the constellation points comprises reducing distances between each of a group of the plurality of points within a same quadrant, while increasing a minimum distance between a first point within the group of the plurality of points and a second point of the group of plurality of points in a second quadrant, as compared to an even distribution of the plurality of points.

9. The method according to claim 7, wherein the modulation format is based on a 32QAM constellation, and wherein in the constellation distortion, the predetermined manner of varying the relative positions of the constellation points comprises: for each of a first four of the plurality of points, which are in one of the quadrants and are closest to an origin of the two-dimensional plane, reducing a respective distance between one of the plurality of points and a center of mass of the four points that are closest to the origin; and for each of a second four of the plurality of points, which are in the one of the quadrants and are farthest from the origin of the two-dimensional plane, increasing a respective distance between the one of the plurality of points and a closest one among the first four points that are closest to the origin.

10. The method according to claim 7, wherein the modulation format employs an 8QAM constellation, and wherein in the constellation distortion, the predetermined manner of varying the relative positions of the group of the plurality of points comprises rotating, by an angle about an origin of the two-dimensional plane, each of four constellation points farthest away from the origin, the angle defining a degree of variation.

11. The method of claim 1, wherein the traffic to be transmitted comprises at least one of the following: 200 G traffic to be transmitted using a single DP-16QAM optical carrier partitioned into two 100 G signals, wherein one of the two 100 G signals is for traffic belonging to the first part of the traffic, 300 G traffic to be transmitted using a super channel comprising two DP-8QAM carriers, single-carrier 400 G traffic to be transmitted using 32QAM or 64QAM.

12. The method of claim 1, wherein the method further comprises mapping a first FlexEthernet stream to a bit set in one of the plurality of binary addresses of one of the plurality of symbols that is a different bit set to which a second FlexEthernet stream is mapped in the one of the plurality of binary addresses of the symbol.

13. A control device for controlling the protection of a given link in an optical network over which a transmitter is operative to transmit to a receiver a signal being modulated based on a predetermined modulation format to carry traffic including digital data, wherein the modulation format is based on a constellation including a plurality of points in an n-dimensional Euclidean signal space with n>1, each of the plurality of points representing a respective one of a plurality of symbols, each of the plurality of points has a relative position within the constellation, each of the plurality of symbols has a respective one of a plurality of binary addresses, and the predetermined modulation format allows for a constellation distortion wherein relative positions of the plurality of the points in the constellation are varied in a predetermined manner by a predetermined degree, the transmitter further operative to partition a first part of the traffic, the first part of the traffic having a higher priority than a second part of the traffic, and map the first part of the traffic to predefined bit positions of selected binary addresses within the plurality of binary addresses, the control device comprising hardware or a combination of hardware and software, wherein the control device is operative to evaluate a quality of a predetermined protection link over which a portion of the traffic could be transmitted in case of a failure of the given link, determine a degree of distortion of the constellation wherein a desired transmission quality for transmission over the predetermined protection link of the first part of the traffic, and a desired transmission quality for transmission over the given link of the traffic the transmitter is operative to transmit over the given link, are simultaneously ensured, and control the transmitter and the receiver to employ a distorted constellation diagram with the determined degree of distortion for transmission of the digital data over the given link.

14. The control device of claim 13, wherein the control device is operatively connected to each of the transmitter and the receiver.

15. The control device of claim 13, wherein the constellation diagram is two-dimensional and comprises four quadrants, in the binary addresses of the constellation points there are two predetermined bit positions which have identical values for each constellation point within the same quadrant, and traffic partitioned into the class of higher priority is mapped to the predetermined two bit positions.

16. A system for transmitting, over a given link, a signal being modulated based on a predetermined modulation format to carry traffic including digital data, wherein the modulation format is based on a constellation including a plurality of points in an n-dimensional Euclidean signal space with n>1, each of the plurality of points representing a respective one of a plurality of symbols, each of the plurality of points has a relative position within the constellation, each of the plurality of symbols has a respective one of a plurality of binary addresses, and the predetermined modulation format allows for a constellation distortion wherein relative positions of the plurality of the points in the constellation are varied in a predetermined by a predetermined degree, the system comprising a transmitter and a control device, wherein the transmitter is operative to partition a first part of the traffic, the first part of the traffic having a higher priority than a second part of the traffic, map the first part of the traffic to predefined bit positions of selected binary addresses within the plurality of binary addresses, receive instructions from the control device to employ the distorted constellation diagram with a determined degree of distortion for transmission of the digital data, and employ a distorted constellation diagram with the determined degree of distortion for transmission of the digital data, wherein the control device is operative to evaluate a quality of a predetermined protection link over which a portion of the traffic could be transmitted in case of a failure of the given link, determine the degree of distortion of the constellation wherein a desired transmission quality is simultaneously ensured for transmission of the traffic over the given link, and transmission over the predetermined protection link of the first part of the traffic partitioned, and control the transmitter to employ a distorted constellation diagram with the determined degree of distortion for transmission of digital data over the given link.

17. A system for receiving, from a given link, a signal being modulated based on a predetermined modulation format to carry traffic including digital data, wherein the modulation format is based on a constellation including a plurality of points in an n-dimensional Euclidean signal space with n>1, each of the plurality of points representing a respective one of a plurality of symbols, each of the plurality of points has a relative position within the constellation, each of the plurality of symbols has a respective one of a plurality of binary addresses, and the predetermined modulation format allows for a constellation distortion wherein relative positions of the plurality of the points in the constellation are varied in a predetermined by a predetermined degree, the system comprising a receiver and a control device, wherein the receiver is operative to receive instructions from the control device to employ the distorted constellation diagram with a determined degree of distortion for receiving digital data, and employing a distorted constellation diagram with the determined degree of distortion for receiving digital data, wherein the control device is operative to evaluate a quality of a predetermined protection link over which a portion of the traffic could be transmitted in case of a failure of the given link, determine the degree of distortion of the constellation wherein a desired transmission quality is simultaneously ensured for transmission of the traffic over the given link, and transmission over the predetermined protection link of the first part of the traffic partitioned, and control the receiver to employ a distorted constellation diagram with the determined degree of distortion for transmission of digital data over the given link.

18. The system of claim 16, wherein the predefined bit positions are bit positions associated with the at least one of the plurality of points of the constellation that have an error probability less than an average error probability of the bit positions of all the plurality of points of the constellation.

19. The system of claim 16, wherein in case of failure of the given link, the system reroutes the traffic to the predetermined protection link.

20. The system of claim 16, wherein the predetermined protection link is one of a plurality of alternative predetermined protection links the control device is operative to evaluate for transmission quality, and wherein the degree of distortion of the constellation diagram is determined such that the desired transmission quality is provided for a one of the plurality of predetermined protection links determined by the evaluation to have the worst transmission quality of the plurality of alternative predetermined protection links.

Description

SHORT DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows a Gray labeled 16QAM constellation diagram without distortion on the left and with distortion on the right,

(2) FIG. 2 shows a transmitter and a receiver for use in the method of the invention,

(3) FIG. 3 shows the required OSNR at a pre-FEC BER of 2.10.sup.2 for non-uniform DP-16QAM with a symbol rate of 30.7 GHz,

(4) FIG. 4 is a diagram illustrating the derivation of a required degree of distortion of a constellation diagram,

(5) FIG. 5 is a diagram showing the reach of the strong bits and the week bits as a function of the degree of distortion of the constellation diagram,

(6) FIG. 6 shows a Gray labeled 8QAM constellation diagram without distortion on the left and with distortion on the right,

(7) FIG. 7 shows a quasi-Gray labeled 32QAM constellation diagram distorted according to two independent distortion parameters.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(8) For the purposes of promoting an understanding of the principles of the invention, reference will now be made to a preferred embodiment illustrated in the drawings, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated apparatus and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur now or in the future to one skilled in the art to which the invention relates.

(9) For the sake of exemplification we give a detailed description of the method of the invention for the important case that the given link or working path carries a payload of about 200 Gb/s, shortly referred to as a 200G client, over a single optical carrier. The modulation format of choice for this application is considered to be DP-16QAM.

(10) The left side of FIG. 1 shows a uniform 16QAM two-dimensional constellation diagram with Gray labeling: The binary addresses of any two symbols at minimum Euclidean distance differ exactly in one position. In this shown example, the two rightmost bits identify the quadrant, whereas the two leftmost bits determine the symbol in the quadrant on the in-phase (I)-quadrature (Q) plane. With the adopted normalization, the average of the four constellation points in the first quadrant, referred to as a center of mass herein and marked by a cross, lies at (2, 2). In each quadrant the distance between the projection of every symbol and the projection of the center is equal to 1 both on the I and Q axes. The situation is referred to as the uniform or non-distorted constellation.

(11) The right side of FIG. 1 shows a non-uniform, distorted 16QAM constellation where has been decreased to 0.75. Herein, is a geometrical parameter that indicates the degree of distortion referred to above, and is also referred to as distortion parameter herein. In this case the distance between symbols belonging to different quadrants is enhanced at the expense of the intra-quadrant distances. A reduction of hence increases the error resilience on the two rightmost bit positions and degrades that of the two leftmost bit positions in the binary address. Since the two rightmost bits are better protected, they are also referred to as strong bits herein, and the two leftmost bits will be referred to as weak bits. Note that this distortion of the constellation is an example of the varying of the distances of a subset of constellation points from a predefined position in the signal space which does not coincide with a constellation point referred to in the introductory portion of the specification, where the predefined position corresponds to the center of mass of the four constellation points in each quadrant. The position of the center of mass could be further shifted such as to keep the average power of the corresponding signal constant.

(12) In the described example, it shall be assumed that the 200 G signal is partitioned in a high-priority and a low-priority 100 G client according to step A) referred to above.

(13) In the following step B), the high-priority traffic is mapped to the strong bits and the low-priority traffic is mapped to the weak bits. This is illustrated in FIG. 2, in which a transmitter 10 and a receiver 12 are shown. As further shown in FIG. 2, the transmitter 10 comprises two identical encoders A and B at reference sign 14, two interleavers A and B at reference sign 16 and a mapper 18. The receiver 12 comprises a demapper 20, de-interleavers A and B at reference sign 22 and decoders A and B at reference sign 24.

(14) The high-priority and low-priority bit streams, b.sub.A and b.sub.B respectively, are separately encoded by the two identical encoders A and B shown at reference sign 14. Each encoded stream is distributed by the corresponding interleaver 16 between two different inputs of the mapper, corresponding to different bit positions in the binary address. With reference to FIG. 1, the binary positions are numbered from right to left from 1 to 4. The receiver 12 implements the corresponding sequence of inverse operations.

(15) Further shown in FIG. 2 is a control device 8 which is operatively coupled with the transmitter 10 and the receiver 12. The control device 8 can be realized in software, in hardware or both. In particular, the control device 8 is capable for carrying out the above-mentioned method steps C) and D), and to communicate to the transmitter 10 and the receiver 12 the degree of distortion of the constellation that shall be employed.

(16) FIG. 3 shows simulation results for DP-16QAM over a DP additive white Gaussian noise (AWGN) channel. A symbol rate of 30.7 GHz is chosen to transport the whole 200 G traffic and accommodate for 15% FEC overhead and for pilot symbols employed for non-differential transmission. The required optical signal-to-noise ratio (ROSNR), defined over a noise bandwidth of 12.5 GHz, for a pre-FEC bit error rate (BER) of 2.10.sup.2, which is assumed to be the FEC threshold, is plotted as a function of the geometry parameter or distortion parameter both for the strong and the weak bits. The dotted line shows also the ROSNR for a conventional approach with uniform 16QAM when a single FEC code is applied through bit-interleaving to all four bit positions. In FIG. 3, it is seen that by reducing , or in other words increasing the degree of distortion, the resilience of the strong bits is increased while sacrificing the performance of the weak bits.

(17) Next, according to step C) referred to above, the quality of a predetermined protection link via which a part of the traffic could be transmitted in case of failure of the given link or working path is determined. In particular, this comprises determining the available optical signal-to-noise ratio (OSNR) on the working path and the worst-case protection path. Herein, the worst-case protection path is the protection path among a set of predetermined alternative protection paths providing the worst transmission quality. For the sake of exemplification it shall be assumed that that these values are 19 dB and 14 dB, respectively, as shown by the additional horizontal lines in FIG. 4.

(18) Proceeding with step D), the geometry parameter , or in other words, the degree of distortion, is determined such as to ensure that the pre-FEC BER is equal or better than the desired threshold both on the working path and the worst protection path. As indicated by the additional vertical lines in FIG. 4, allowed values of the geometry parameter range roughly from 0.775 to 0.817. According to step E) the distorted constellation diagram with the determined degree of distortion, i.e. with 0.775<<0.817, is then employed for the transmission of digital data over the working path. In case of link failure, the traffic is rerouted according to step F) without changing the modulation format, and maintaining the geometry parameter or degree of distortion . Having chosen the geometry parameter in the allowable range, it is ensured that the receiver is still capable of detecting the high-priority traffic with the desired performance without reconfiguration of the coding and modulation scheme.

(19) Although the proposed solution was exemplified over an idealized AWGN channel model, it works without fundamental modifications also under realistic channel conditions. FIG. 5 shows the performance of non-uniform DP-16QAM over a nonlinear fiber-optic link consisting of several 80 km spans of standard single-mode fiber (SSMF) connected through erbium-doped fiber amplifiers (EDFAs). Again a non-differential transmission and 15% overhead FEC with a BER threshold of 2.Math.10.sup.2 are assumed. The performance of 96 wavelength division multiplexed carriers with 50 GHz spacing is evaluated, assuming a system margin of 3 dB, an implementation penalty of 2 dB and an EDFA noise figure of 5 dB. The nonlinear interference caused by the fiber is evaluated according to the semi-analytical GN-model described by A. Carena, V. Curri, G. Bosco, P. Poggiolini, and F. Forghieri in the article Modeling of the impact of non-linear propagation effects in uncompensated optical coherent transmission links, IEEE Journal of Lightwave Technology, volume 30, number 10, pp. 1524-1539 (2012). A launch power of 3 dBm per channel is assumed, which corresponds to optimum performance.

(20) FIG. 5 shows the maximum reach both for the strong and the weak bits as a function of the geometry parameter . Additionally, the dotted line indicates the reach of a conventional transmission system with uniform 16QAM and a single FEC code applied to all four bit positions. It is apparent that the nonlinear fiber effects do not alter the qualitative trend observed in FIG. 3. Once again, by tuning one can optimize the performance of strong and weak bits according to the characteristics of protection and working paths.

(21) Note that after step F), some parameters of the receiver may still need resynchronization or adaptation. In particular the accumulated chromatic dispersion (CD) and the polarization mode dispersion (PMD) over the protection link are typically different from the working link. The resynchronization of the CD compensator (not shown) at the receiver 12 could be either triggered externally together with the reconfiguration of the cross-connects (not shown) or initiated automatically by a locally generated alarm. The PMD compensator is continuously adapted at run-time and therefore reacts automatically to the new channel conditions. The benefit of the approach of the invention stems from the fact that with the current transponder technology, the adaptation of the receiver parameters is much faster (roughly by two orders of magnitude) than a reconfiguration of the coding and modulation scheme.

(22) According to the invention, in case of link failure, the high-priority client experiences only a short interruption due to failure detection time, reconfiguration of the cross-connects and resynchronization of the receiver: its protection mechanism is completely implemented in the optical layer. On the contrary, the low-priority traffic is dropped at the optical link layer and its protection is fully delegated to the higher layers. As a consequence, low-priority traffic is likely to undergo a longer downtime, consistent with the state of the art.

(23) In the previous example, the 200G traffic transmitted over a single DP-16QAM optical carrier was partitioned in two 100G signals with different priorities. This application addresses in a natural way the problem of transporting two optical data units 4 (ODU4), which are standard 100 G client signals defined in the optical transport network (OTN) multiplexing hierarchy introduced in the ITU-T recommendation G.709/Y.1331 (February 2012).

(24) Other advantageous embodiments of the invention relate to the transport of 300 G traffic over a super-channel consisting of two DP-8QAM carriers, single carrier 400 G transmission using 32QAM, or 400 G transmission over a single 64QAM carrier.

(25) In all these cases, by distorting the symbol constellation, the protection level of distinct ODU4 (100 G) clients can be altered. However, various embodiments of the invention allow also a different granularity of the traffic classes when the transport equipment implements traffic aggregation. This becomes particularly attractive in conjunction with the FlexEthernet project started by the Optical Internetworking Forum (OIF) with the aim of introducing flexible rate connections between routers. Using FlexEthernet-aware transport equipment, in one embodiment one can map individual FlexEthernet streams to different bit-sets in the binary address of the constellation symbols. For example, one can partition 250 G traffic transported over a single DP-16QAM carrier into two 125 G FlexEthernet clients with different priorities or 150G traffic transported over a single DP-8QAM carrier into a 100 G and a 50G client. Further examples are easily conceivable in view of the present disclosure.

(26) While in the embodiment above, only DP-16QAM modulation formats have been discussed in detail, the invention is by no means limited to this. Square mQAM constellations, like 64QAM, can be treated similarly to 16QAM by clustering their symbols around their center of mass in each quadrant. If necessary, each group of four neighboring points can be further clustered around their respective center of mass, and the center of mass may optionally be shifted.

(27) According to a further embodiment, the left side of FIG. 6 shows a uniform 8QAM constellation with quasi-Gray labeling. By rotating the outer constellation points in clockwise direction, the constellation on the right side of FIG. 6 is obtained, where the two leftmost bits are strengthened at the expense of the rightmost bit.

(28) According to a further embodiment, FIG. 7 shows a quasi-Gray labeled 32QAM constellation together with two possible distortion parameters .sub.1 and .sub.2 that control its geometry. With the adopted normalization, for .sub.1=.sub.2=1 the uniform constellation is obtained. By decreasing .sub.1, for the four constellation points in each quadrant that are closest to the origin of the two-dimensional plane, the respective distance from their center of mass is reduced, which strengthens the quadrant bits 3 and 4 (numbering from right to left) at the expense of the rightmost bits 1 and 2. By increasing .sub.2, for the four constellation points in each quadrant that are the farthest from the origin of the two-dimensional plane, the respective distance from the closest one among the four constellation points closest to the origin is increased, which protects the leftmost bit 5 at the expense of all other bits.

(29) Alternative geometry parameters for the constellations mentioned above and for further symbol constellations can be determined the framework of alternative embodiments. Although a preferred exemplary embodiment is shown and specified in detail in the drawings and the preceding specification, these should be viewed as purely exemplary and not as limiting the invention. It is noted in this regard that only the preferred exemplary embodiment is shown and specified, and all variations and modifications should be protected that presently or in the future lie within the scope of protection of the invention as defined in the claims.