Patent Application
20150295649
In fiber optics communications, fiber nonlinearity is always a serious issue that limits the signal's reach/performance. In coherent optical communications, the signal reach and performance suffers the trade-off between linear and nonlinear distortion, i.e., at low transmitting power, the signal would sink into the linear noise while at high power, the signal phase would be strongly disturbed, and adversely affect the signal due to the fiber Kerr effect. Currently, many commercial optical systems utilize the polarization-division-multiplexing quadrature-phase-shift-keying (PDM-QPSK) format, which carries 4 bits/symbol per optical carrier. Facing the continuous increase in the demand for capacity, high quadrature amplitude modulation (QAM) formats, such as 16 QAM, have been developed. For instance, with the upgrade from PDM-QPSK to PDM-16 QAM, the bits/symbol per carrier could be increased from 4 to 8, thus doubling the capacity. However, the price of using 16 QAM is the reduced signal reach/performance due to its vulnerability against noise. While increasing the transmitting power may raise the signal to noise ratio, this unfortunately would also introduce significant nonlinear distortions that would compromise the signal.
It would be beneficial to reduce and/or overcome fiber nonlinearities to extend the reach of 16 QAM signaling.
In some embodiments of the present invention, an apparatus is disclosed for reducing link fiber nonlinearities. The apparatus includes memory having stored therein computer executable instructions, and a processor executing computer-executable instructions stored in the memory. The executable instructions include decomposing a high order QAM input into a plurality of sub-components. The instructions include applying a plurality of logical operations to the plurality of sub-components. The instructions include determining a non-linear compensation term based on the applying the plurality of logical operations to the plurality of sub-components.
In other embodiments, a method for reducing link fiber nonlinearities executed by a processor is disclosed. The method includes decomposing a high order QAM input into a plurality of sub-components. The method includes applying a plurality of logical operations to the plurality of sub-components. The method includes determining a non-linear compensation term based on the applying the plurality of logical operations to the plurality of sub-components.
In still other embodiments of the present invention, a system for reducing link fiber nonlinearities is disclosed. The system includes a communication network. The system includes a plurality of transmitters and a plurality of receivers coupled to the communication network, wherein at least one of a transmitter or a receiver is configured for reducing link fiber nonlinearities by decomposing a high order QAM input into a plurality of sub-components, applying a plurality of logical operations to the plurality of sub-components, and determining a non-linear compensation term based on the applying the plurality of logical operations to the plurality of sub-components.
These and other objects and advantages of the various embodiments of the present disclosure will be recognized by those of ordinary skill in the art after reading the following detailed description of the embodiments that are illustrated in the various drawing figures.
The accompanying drawings, which are incorporated in and form a part of this specification and in which like numerals depict like elements, illustrate embodiments of the present disclosure and, together with the description, serve to explain the principles of the disclosure.
Reference will now be made in detail to the various embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings. While described in conjunction with these embodiments, it will be understood that they are not intended to limit the disclosure to these embodiments. On the contrary, the disclosure is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the disclosure as defined by the appended claims. Furthermore, in the following detailed description of the present disclosure, numerous specific details are set forth in order to provide a thorough understanding of the present disclosure. However, it will be understood that the present disclosure may be practiced without these specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the present disclosure.
Accordingly, embodiments of the present invention provide for reducing fiber nonlinearity for coherent optical higher order QAM (e.g., 16QAM) communications by decomposing each QAM symbol into QPSK symbols to avoid the use of complex multiplications.
Some portions of the detailed descriptions which follow are presented in terms of procedures, steps, logic blocks, processing, and other symbolic representations of operations on data bits that can be performed on computer memory. These descriptions and representations are the means used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. A procedure, computer generated step, logic block, process, etc., is here, and generally, conceived to be a self-consistent sequence of steps or instructions leading to a desired result. The steps are those requiring physical manipulations of physical quantities, and refer to the action and processes of a computing system, or the like, including a processor configured to manipulate and transform data represented as physical (electronic) quantities within the computer system's registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices.
Flowcharts of examples of methods for reducing fiber nonlinearity are provided, according to one or more embodiments of the present invention. Although specific steps are disclosed in the flowcharts, such steps are exemplary. That is, embodiments of the present invention are well-suited to performing various other steps or variations of the steps recited in the flowcharts. Also, embodiments described herein may be discussed in the general context of computer-executable instructions residing on some form of computer-readable storage medium, such as program modules, executed by one or more computers or other devices. By way of example, and not limitation, the software product may be stored in a nonvolatile or non-transitory computer-readable storage media that may comprise non-transitory computer storage media and communication media. Generally, program modules include routines, programs, objects, components, data structures, etc., that perform particular tasks or implement particular abstract data types. The functionality of the program modules may be combined or distributed as desired in various embodiments.
In digital communications, modulation techniques using higher order QAM modulation rates provide for increased data rates and higher levels of spectral efficiency. However these higher order QAM modulation techniques are susceptible to nonlinear noise and interference. Embodiments of the present invention as described below are configured to reduce fiber nonlinearity for coherent optical QAM communications by avoiding multiplication operations.
In general, the behavior of fiber nonlinearity is dominated by 3rd order distortion, meaning that its mathematical form can be described by the multiplications of any three symbols in the time-domain. The fiber nonlinear model can be described as in Eqn. 1, as follows:
E′(n)−E(n)+Σp,qCpqE(n+p)E(n+q)E′(n+p+q) (1)
In Eqn. 1, E(n) and E′(n) are symbol-spaced discrete time representations of signals without (e.g., E(n)) and with nonlinear distortions.
Further, in Eqn. 1 and as shown in
However, the above technique employed for the QPSK format is unsuitable for use with higher capacity QAM formats (e.g., 16 QAM). That is, since 16 QAM includes more complex symbols, e.g., {±1±j, ±1±3j, ±3±j, ±3±3j}, the products of E(n+p)E(n+q)E′(n+p+q) cannot be obtained by simply swapping the real and imaginary portions or inverting the polarity of the real or imaginary portion, therefore resulting in numerous multiplications for Eqn. 1. To overcome fiber nonlinearities, a new multiplier-free nonlinear compensation to reduce the link fiber nonlinearities for 16 QAM is introduced in embodiments of the present invention. According to one aspect, a multiplier-free compensation method against fiber nonlinearities is provided for higher order coherent optical QAM (e.g., 16 QAM). According to one or more embodiments, this method decomposes each QAM symbol into QPSK symbols to avoid the use of complex multiplications, thus simplifying the implementation.
At 210, the method includes decomposing a high order QAM input into a plurality of sub-components. Orders greater than or equal to four for the QAM input (e.g., 4 QAM, 8 QAM, 16 QAM, 32 QAM, 64 QAM, etc.) realize the greatest benefit by reducing execution complexity when determining the compensation for nonlinearity in the QAM input.
At 220, the method includes applying a plurality of logical operations to the plurality of sub-components. In particular, the logical operations do not include a multiplication operation to reduce execution complexity thereby realizing greater speed in execution. For instance, logical operations include AND, OR, NOR, etc., which are implementable within hardware based logic circuits. In one embodiment, the sub-components include QPSK symbols. In another embodiment, the sub-components include 4 QAM symbols.
At 230, the method includes determining a total nonlinear compensation term based on the applying the plurality of logical operations to the plurality of sub-components. In particular, the total nonlinear term is calculated and/or estimated by determining and/or solving the one or more nonlinear terms of the total nonlinear compensation term, wherein each nonlinear term is associated with logical operations. Term selection is dependent on the acceptable execution cost. That is, a more accurate estimation of the total nonlinear compensation term requires execution of more nonlinear terms, whereas a less accurate estimation of the total nonlinear compensation term requires execution of less nonlinear terms. In many cases, accuracy is not sacrificed even though the execution cost is less, as will be described below.
In particular,
E(n)=2A(n)+B(n) (2)
As shown in
As such, the product of the nonlinear term E(n+p)E(n+q)E′(n+p+q) shown in Eqn. 1 can be expanded into the summation of eight products of QPSK. In particular,
In this case, the nonlinear term E(n+p)E(n+q)E′(n+p+q) 405 can be obtained by evaluating each QPSK product (e.g., terms 410, 420, 430, and 440) with logical operations. The result is scaled by a factor of the power of 2 (e.g., 1, 2, 4, 8, etc.). In one embodiment, the scaling is performed by shifting the bit (e.g., shifting n bits to the left to multiply by 2″). This result is then summed (e.g., using adders [not shown]).
In one embodiment, since each term 410, 420, 430, and 440 has a different coefficient (e.g., power of two), this means that their impact on the benefit of performing nonlinear compensation would be different. That is, terms having a greater impact on the nonlinear compensation may be kept (e.g., terms with coefficients greater than 4). Also, terms having a lesser impact on the nonlinear compensation may be discarded (e.g., terms with coefficients smaller than 4, such as those terms with 1 and 2). As such, the plurality of sub-components is filtered by discarding at least one term having a smaller coefficient. Of course, the selection of the value of kept coefficients can vary, such that a coefficient of 2 may be kept, or a coefficient of 4 may be discarded, etc. In the case where terms with coefficients of 1 and 2 are determined to have lesser impacts on the determination of the nonlinearity compensation, terms in block 490 may be discarded, such as terms 430 and 440. In this manner, the complexity of determining the nonlinear term and its compensation can be reduced as much as, and even greater than, fifty percent for each product.
In an extreme case where only the first sub term of the nonlinear term E(n+p)E(n+q)E′(n+p+q) 405 remains, the process may turn to a “degenerate” method, which immediately degenerates each 16 QAM symbol to a single QPSK at the expense of a certain amount of inaccuracy.
In another embodiment, the QAM decomposition method depicted in
In particular, both transmitter 600 and receiver 700 are each capable of decomposing a high order QAM input into a plurality of sub-components, applying a plurality of logical operations to the plurality of sub-components, and determining a non-linear compensation term based on the application of a plurality of logical operations to the plurality of sub-components, which were decomposed from the input signal. For instance, these operations can be performed within the digital signal processor (DSP) based transceivers located within system 500.
As shown in
Processor 625 and memory 627 may be included within block 620 specifically configured for reducing link fiber nonlinearities. For example, block 620 is inserted between the signal source E(n) 610 and the digital-to-analog converter (DAC) 660. More specifically, block 620 reduces and/or compensates for link fiber nonlinearities by first determining the nonlinearity term ΔE(n) at calculator 630, and then subtracting the nonlinearity term from the input signal E(n). The filtered term is received by a pre-equalizer 650 that may be located between block 620 and the DAC 660. After conversion, the signal is modulated by modulator 670 and transmitted over the network.
As shown in
Processor 725 and memory 727 may be included within block 720 specifically configured for reducing link fiber nonlinearities. For example, block 720 may be located right after the regular equalizer 710 that receives the signal after phase recovery, and outputs the equalized signal R(n). A slicer 730 receives the equalized signal R(n) and is configured for mitigating the noise impact, and more importantly, to ensure that all the symbols are received to the original symbol positions so that QAM decomposition and logical operations can be employed for nonlinearity compensation.
After slicer 730, the nonlinear term ΔE(n) is determined by estimator 740. Thereafter, the nonlinear term compensator 750 subtracts the nonlinear term ΔE(n) from the equalized signal R(n).
Thus, according to embodiments of the present disclosure, systems and methods are described for reducing link fiber nonlinearities in optical networks.
While the foregoing disclosure sets forth various embodiments using specific block diagrams, flowcharts, and examples, each block diagram component, flowchart step, operation, and/or component described and/or illustrated herein may be implemented, individually and/or collectively, using a wide range of hardware, software, or firmware (or any combination thereof) configurations. In addition, any disclosure of components contained within other components should be considered as examples because many other architectures can be implemented to achieve the same functionality.
The process parameters and sequence of steps described and/or illustrated herein are given by way of example only and can be varied as desired. For example, while the steps illustrated and/or described herein may be shown or discussed in a particular order, these steps do not necessarily need to be performed in the order illustrated or discussed. The various example methods described and/or illustrated herein may also omit one or more of the steps described or illustrated herein or include additional steps in addition to those disclosed.
While various embodiments have been described and/or illustrated herein in the context of fully functional computing systems, one or more of these example embodiments may be distributed as a program product in a variety of forms, regardless of the particular type of computer-readable media used to actually carry out the distribution. The embodiments disclosed herein may also be implemented using software modules that perform certain tasks. These software modules may include script, batch, or other executable files that may be stored on a computer-readable storage medium or in a computing system. These software modules may configure a computing system to perform one or more of the example embodiments disclosed herein. One or more of the software modules disclosed herein may be implemented in a cloud computing environment. Cloud computing environments may provide various services and applications via the Internet. These cloud-based services (e.g., software as a service, platform as a service, infrastructure as a service, etc.) may be accessible through a Web browser or other remote interface. Various functions described herein may be provided through a remote desktop environment or any other cloud-based computing environment.
Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as may be suited to the particular use contemplated.
Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.
Embodiments according to the present disclosure are thus described. While the present disclosure has been described in particular embodiments, it should be appreciated that the disclosure should not be construed as limited by such embodiments, but rather construed according to the below claims.
This application claims priority to and the benefit of U.S. Provisional Patent Application No. 61/978,678, filed on Apr. 11, 2014, entitled “Fiber Nonlinearity compensation for Optical 19QAM [sic],” the disclosure of which is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61978678 | Apr 2014 | US |
