WO2019109973A1

WO2019109973A1 - Multi-level quadrature amplitude modulation with geometric shaping

Info

Publication number: WO2019109973A1
Application number: PCT/CN2018/119530
Authority: WO
Inventors: Jianjun Yu; Hung-Chang Chien; Yan Xia; Yufei Chen
Original assignee: Zte Corporation
Priority date: 2017-12-06
Filing date: 2018-12-06
Publication date: 2019-06-13

Abstract

An optical transmitter apparatus includes a light source that generates a light wave as an optical carrier wave, a modulator that modulates data to be transferred as an optical multi-level quadrature amplitude modulation (QAM) signal using a geometric shaping scheme, a symbol mapper that assigns data to symbols, and a processor in communication with the symbol mapper to decide positions of the symbols and perform the assignment between the data and the symbols, memory for storing one or more instructions configured to be executed by the processor. The instructions include instructions for minimizing a number of different bits between symbols that are more likely to cause wrong QAM decoding decision at a receiver, and instructions for shifting constellation points in a way that decreases a distance between adjacent symbols that are different by one bit and increases a distance between adjacent symbols that are different by two or more bits.

Description

MULTI-LEVEL QUADRATURE AMPLITUDE MODULATION WITH GEOMETRIC SHAPING

CROSS REFERENCE TO RELATED APPLICATIONS

This patent document claims the benefit of priority under 35 U.S.C. §119 (a) and the Paris Convention of International Patent Application No. PCT/CN2017/114811, filed on December 6, 2017. The entire content of the before-mentioned patent application is incorporated by reference as part of the disclosure of this document.

TECHNICAL FIELD

The present document relates to optical communication systems.

BACKGROUND

Multi-level modulation schemes, such as M-quadrature amplitude modulation (QAM) , are regarded as a promising candidate to enable next generation bandwidth-efficient communication solutions. Uniformly spaced QAM formats have been widely used in fiber-optics transmission systems. However, a gap exists between the channel capacity and the highest achievable rate of the uniformly spaced format. In order to compensate for this gap, non-uniform constellation mapping can be employed.

SUMMARY

The present document discloses, among other things, techniques for reducing the possibility of making wrong QAM decoding decisions at the receiver side by using a modified mapping algorithm. In another advantageous aspect, geometrically shaped QAM signal can be processed in a way that minimizes the number of different bits between adjacent points that are more likely to interfere with each other and thus more likely to cause wrong QAM decoding decision at a receiver side.

In one example aspect, an optical transmitter apparatus includes a light source that generates a light wave as an optical carrier wave, a modulator that modulates, using the optical carrier wave, data to be transferred in a form of an optical multi-level quadrature amplitude modulation (QAM) signal using a geometric shaping scheme, a symbol mapper that assigns data to symbols, and a processor in communication with the symbol mapper to decide positions of the symbols and perform the assignment between the data and the symbols, memory for storing one or more instructions configured to be executed by the processor. The instructions include instructions for minimizing a number of different bits between symbols that are more likely to cause wrong QAM decoding decision at a receiver, and instructions for shifting the optical multi-level QAM constellation points in a way that decreases a first distance between adjacent symbols that are different by one bit and increases a second distance between adjacent symbols that are different by two or more bits.

In another example aspect, a method of mapping data to be transferred in a form of a geometrically shaped (GS) optical multi-level quadrature amplitude modulation (QAM) signal, comprising: generating multi-bit data to be mapped to symbols of GS multi-level QAM constellation, arranging the symbols according to GS QAM constellation, minimizing a number of different bits between symbols that are more likely to cause wrong QAM decoding decision at a receiver, and shifting the GS multi-level QAM constellation points in a way that decreases a first distance between adjacent symbols that are different by one bit and increases a second distance between adjacent symbols that are different by two or more bits.

These and other aspects, and their implementations and variations are set forth in the drawings, the description and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example optical communication network.

FIG. 2 illustrates an example optical signal transmitter.

FIG. 3 illustrates a constellation diagram of geometrically shaped 16QAM.

FIG. 4A illustrates mutual information (MI) of standard 16QAM and GS-16QAM as function of signal-to-noise ratio (SNR)

FIG. 4B illustrates bit error rate (BER) of standard 16QAM and GS-16QAM as function of SNR.

FIG. 5 is a table showing joint probabilities of transmitted symbols and received symbols.

FIG. 6A illustrates a normal constellation bitmap schedule of GS-16QAM.

FIG. 6B illustrates a modified constellation bitmap schedule of GS-16QAM in accordance with an implementation of the present document.

FIG. 7 illustrates BER as function of SNR for a normal constellation bitmap schedule of GS-16QAM and a modified constellation bitmap schedule of GS-16QAM illustrated in FIGS. 6A and 6B.

FIG. 8 illustrates shifted constellation points of the modified constellation bitmap schedule of GS-16QAM in accordance with an implementation of the present document.

FIG. 9 illustrates BER as function of SNR for (1) standard 16QAM, (2) normal constellation bitmap schedule of GS-16QAM illustrated in FIG. 6A, (3) modified constellation bitmap schedule of GS-16QAM illustrated in FIG. 6B, and (4) shifted constellation points of the modified constellation bitmap schedule of GS-16QAM illustrated in FIG. 8.

FIG. 10 is a flow chart representation of an example optical communication method.

FIG. 11 is a flow chart representation of another example optical communication method.

FIG. 12 illustrates an example configuration of the optical communication network.

DETAILED DESCRIPTION

Quadrature Amplitude Modulation (QAM) is a form of signal modulation where information is encoded in both the amplitude and phase of a series of signal pulse such as an optical wave. In order to satisfy the demand for capacity, there has been a focus on increasing spectral efficiency by employing multi-level modulation formats. A way to further increase spectral efficiency is by means of signal shaping. For example, geometrical shaping may arrange constellation points non-uniformly to provide better noise tolerance and higher transmission capacity for optical fiber communication systems.

FIG. 1 illustrates an example optical communication network 10 in which an optical signal transmitter 12 and an optical signal receiver 16 communicate with each other via an optical transmission channel 14. The optical signal transmitter 12 may include circuitry configured to convert electrical input signals to optical signals. The optical transmission channel 14 may include optical fibers that extend in length from several hundred feet (e.g., last mile drop) to several thousands of kilometers (e.g., long haul networks) . The optical signals that have passed the optical transmission channel 14 may be transmitted through intermediate optical equipment such as amplifiers, repeaters, switches, etc., which are not shown in FIG. 1 for clarity. The optical signal receiver 16 may include circuitry configured to perform the actual reception of the optical signals and convert the optical signals into electrical signals. In some implementations, the optical signal transmitter 12 may generate geometrically shaped (GS) multi-level QAM optical signals. For example, the optical signal transmitter 12 may provide GS 16QAM optical transmission over the optical transmission channel 14.

FIG. 2 illustrates an example optical signal transmitter 12 in which a light wave is modulated by electrical GS multi-level QAM signal. In an implementation, the optical signal transmitter 12 includes a light source 200, a modulator 202, and a geometrically shaped multi-level QAM signal generator 204. The light source 102 may generate a carrier light wave that will be modulated by electrical GS multi-level PAM signal at the modulation circuit 202. For example, a continuous wave light generated at the light source 200 is modulated at the modulator 202 with a binary data sequence that is non-uniformly mapped to symbols at the geometrically shaped multi-level QAM signal generator 204.

FIG. 3 illustrates a constellation diagram of geometrically shaped 16QAM based on iterative polar quantization (IPQ) algorithm. The symbol constellation is obtained by quantizing data signals while minimizing quantization mean squared error (QMSE) . For example, the IPQ algorithm may consist of a non-uniform scalar quantization of the amplitude and a uniform scalar quantization of the phase. Here, the constellation diagram has one point (i.e., constellation index point “0” ) at a center, six points (i.e., constellation index points “1” through “6” ) along an inner ring, and eight points (i.e., constellation index point “7” through “15” ) along an outer ring.

FIG. 4A illustrates mutual information (MI) of standard 16QAM and GS-16QAM as function of signal-to-noise ratio (SNR) . As can be seen here, GS-16QAM outperforms uniformly distributed or standard 16QAM. FIG. 4B illustrates bit error rate (BER) of standard 16QAM and GS-16QAM as function of SNR. Although the GS-16QAM based on IPQ has a better MI value, the BER thereof is higher than that of standard-16QAM.

FIG. 5 is a table showing joint probabilities of transmitted symbols and received symbols. To optimize the constellation bitmap of GS-16QAM, constellation points with higher error probability are found first according to one-to-one correspondence of transmitted/received symbol joint probability matrix. Referring to this table and FIG. 3, the most error-prone constellation point is a center point “0, ” followed by six points of the inner ring, and the least error-prone constellation points are points “8, ” “11, ” and “14” of the outer ring. In addition, when the center point “0” is transmitted, the six points of the inner ring can be wrongly decided at a receiver side. When data corresponding to the constellation points of the inner ring is transmitted, the probability of making a wrong QAM decoding decision at the receiver side is highest at adjacent points of the inner ring, followed by the center point, and several closer points of outer ring can also be wrongly decided. For example, when data corresponding to the constellation point “1” is transmitted, the probability of making a wrong QAM decoding decision at the receiver side is highest at points “2” and “6, ” followed by point “0, ” and data corresponding to

points

7 and 8 can also be wrongly decided. As for the outer points, the closer inner points are most likely to be wrongly decided, followed by two adjacent points of the outer ring.

According to an implementation of the present document, it is possible to reduce the possibility of making wrong QAM decoding decisions at the receiver side by using a modified mapping algorithm. When multi-bit data mapped to corresponding constellation points, the modified mapping algorithm minimizes the number of different bits of multi-bit data between constellation points that are more likely to cause wrong QAM decoding decision at the receiver side. For example, for six constellation points of inner ring, only one bit can be different between adjacent points. One bit (or two bits) can be different between the center point and six points of inner ring. For six outer points (e.g., points “7, ” “9, ” “10, ” “12, ” “13, ” and “15” ) , which are closer to six points of inner ring, only one bit can be different from the corresponding inner points (e.g., points “7” and “1” ) . For the remaining three points (e.g., points “8, ” “11, ” and “14” ) , the number of bits that are different from their closer points should be as small as possible.

FIG. 6A illustrates a normal constellation bitmap schedule of GS-16QAM, and FIG. 6B illustrates a modified constellation bitmap schedule of GS-16QAM in accordance with an implementation of the present document. Referring to FIG. 6A, “0000” is assigned to the center point “0, ” and “0001, ” and then “0010, ” “0011, ” “0100, ” “0101, ” and “0110” are assigned to constellation points of the inner ring “1, ” “2, ” “3, ” “4, ” “5, ” and “6” labeled as FIG. 3, respectively. In addition, “0111, ” “1000, ” “1001, ” “1010, ” “1011, ” “1100, ” “1101, ” “1110, ” and “1111” are assigned to constellation points of the outer ring “7, ” “8, ” “9, ” “10, ” “11, ” “12, ” “13, ” “14, ” and “15” labeled as FIG. 3, respectively. Referring to FIG. 6B, a modified mapping is applied to GS-16QAM. The modified mapping includes minimizing the number of different bits between adjacent points that are more likely to interfere with each other and thus more likely to cause wrong QAM decoding decision at the receiver. In an implementation, symbols of inner ring (e.g., GS-16 QAM has six symbols in the inner sing) are arranged such that only one bit is different between adjacent inner ring symbols. The modified constellation bitmap schedule of GS-16QAM can be implemented as follows: “0000” is assigned to the center point “0” ; “0101, ” “0001, ” “0011, ” “0010, ” “0110, ” and “0100” are assigned to the points of the inner ring “1, ” “2, ” “3, ” “4, ” “5, ” and “6” labeled as FIG. 3, respectively; and “1101, ” “1111, ” “1001, ” “1011, ” “0111, ” “1010, ” “1110, ” “1000, ” and “1100” are assigned to outer ring points “7, ” “8, ” “9, ” “10, ” “11, ” “12, ” “13, ” “14, ” and “15” labeled as FIG. 3, respectively. In the inner ring of GS-16QAM, six symbols with smaller value ( “0101, ” “0001, ” “0011, ” “0010, ” “0110, ” and “0100” ) are arranged. A constellation point corresponding to “0101” is arranged next to constellation points corresponding to “0001” and “0100, ” and a constellation point corresponding to “0010” is arranged next to constellation points corresponding to “0011” and “0110. ” Constellation points corresponding to “0011” and “0001” are arranged next to each other, and constellation points corresponding to “0110 and “0100” are arranged next to each other. Remaining constellation points are arranged in the outer ring such that two inner and outer ring symbols are arranged at closest positions to each other. For GS-16QAM, a constellation point of the outer ring corresponding to “1101” is arranged at an outer ring position closest to a constellation point corresponding to the inner ring symbol “0101, ” and a constellation point of the outer ring corresponding to “1001” is arranged at an outer ring position closest to a constellation point corresponding to the inner ring symbol “0001, ” and a constellation point of the outer ring corresponding to “1011” is arranged at an outer ring position closest to a constellation point corresponding to the inner ring symbol “0011, ” and a constellation point of the outer ring corresponding to “1010” is arranged at an outer ring position closest to the inner ring symbol “0010, ” and a constellation point of the outer ring corresponding to “1110” is arranged at an outer ring position closest to a constellation point corresponding to the inner ring symbol “0110. ” The remaining three symbols (e.g., for GS-16QAM, symbols corresponding to “0111, ” “1000, ” and “1111” ) are arranged such that the number of bits that are different from their closer points should be as small as possible. As illustrated in FIG. 6B, a constellation point corresponding to “0111” is arranged between constellation points corresponding to “1010” and “1011, ” and a constellation point corresponding to “1000” is arranged between constellation points corresponding to “1110” and “1100, ” and a constellation point corresponding to “1111” is arranged between constellation points corresponding to “1001” and “1101. ”

FIG. 7 shows BER as function of SNR for a normal constellation bitmap schedule of GS-16QAM and a modified constellation bitmap schedule of GS-16QAM shown in FIGS. 6A-6B. As can be seen when looking at soft-decision forward error correction (SD-FEC) threshold, the modified constellation mapping shown in FIG. 6B can achieve a better SNR (e.g., by around 0.6dB) compared to the constellation mapping in FIG. 6A. As can be seen when when looking at hard-decision forward error correction (HD-FEC) threshold, the modified constellation mapping shown in FIG. 6B can achieve a better SNR (e.g., by around 0.6dB) compared to the constellation mapping in FIG. 6A.

FIG. 8 illustrates adjusted constellation points of GS-16QAM in accordance with an implementation of the present document. As discussed above, two bits may be different between the center point and some of the six constellation points of inner ring, and two or more bits may be different between the constellation points of the outer ring and their adjacent constellation points. In an implementation of the present document, the modified constellation mapping of GS-16QAM may further reduce SNR by adjusting the positions of constellation points in a way that decreases the distances between 1-bit different adjacent constellation points and increases the distances between 2-bit and 3-bit different adjacent constellation points. In FIG. 8, original constellation points and shifted constellation points are depicted. Three points “0101, ” “0110, ” and “0011” in FIG. 6B (i.e., points 1, 3 and 5 in FIG. 3) , which are 2-bit different from the center point, are shifted away from the center point (i.e., point 0 in FIG. 3 and 0000 in FIG. 6B) by 0.145 times the distance between the original constellation points and the center point. The other three points “0001, ” “0100, ” and “0010” in FIG. 6B, which are 1-bit different from the center point, are shifted away from the center point by 0.042 times the distance between the original constellation points and the center point. For the constellation points in the outer ring, the phases of points are shifted clockwise or counterclockwise by 0.04 radian according to the number of different bits of adjacent points. In an implementation, constellation points “1111, ” “1011, ” “0111, ” and “1110” in FIG. 6B (i.e., points 8, 10, 11, and 13 in FIG. 3) are shifted clockwise, and constellation points “1001, ” “1010, ” and “1000” in FIG. 6B (i.e., points 9, 12, and 14 in FIG. 3) are shifted counterclockwise.

Table 1 lists coordinates and bitmap of new GS-16QAM signal constellation in accordance with an implementation of the present document.

Table 1

FIG. 9 illustrates BER as function of SNR for (1) standard 16QAM, (2) normal constellation bitmap schedule of GS-16QAM illustrated in FIG. 6A, (3) modified constellation bitmap schedule of GS-16QAM illustrated in FIG. 6B, and (4) shifted constellation points of the modified constellation bitmap schedule of GS-16QAM illustrated in FIG. 8. As can be seen when looking at SD-FEC threshold, the shifted constellation points of the modified constellation bitmap schedule of GS-16QAM illustrated in FIG. 8 shows the best SNR compared to the standard 16QAM, the normal constellation bitmap schedule of GS-16QAM illustrated in FIG. 6A, and the modified constellation bitmap schedule of GS-16QAM illustrated in FIG. 6B. Likewise, as can be seen when looking at HD-FEC threshold, the shifted constellation points of the modified constellation bitmap schedule of GS-16QAM illustrated in FIG. 8 shows the best SNR compared to the standard 16QAM, the normal constellation bitmap schedule of GS-16QAM illustrated in FIG. 6A, and the modified constellation bitmap schedule of GS-16QAM illustrated in FIG. 6B.

FIG. 10 is a flow chart representation of an example optical communication method. At step 1002, multi-bit data is generated to be mapped to symbols of geometrically shaped QAM constellation. An optical signal transmitter, in which a light wave is modulated by electrical GS multi-level QAM signal, maps the multi-bit data to symbols arranged according to geometrically shaped QAM constellation at step 1004. At step 1006, the optical signal transmitter assigns data to symbols based on modified constellation bitmap schedule based on various implementation examples of the present document. For example, at step 1006, the optical signal transmitter minimizes a number of different bits between symbols that are more likely to cause wrong QAM decoding decision at a receiver. At step 1008, the optical signal transmitter shifts the GS-multi-level QAM constellation points in a way that decreases a first distance between one-bit-different adjacent symbols and increases a second distance between two-or-more-bit-different adjacent symbols.

In order to reduce the possibility of making wrong QAM decoding decisions at the receiver side, the present document provides various implementations of the modified mapping algorithm. The modified mapping algorithm includes minimizing the number of different bits between adjacent points that are more likely to interfere with each other and thus more likely to cause wrong QAM decoding decision. In an implementation, symbols of inner ring (e.g., GS-16 QAM has six symbols in the inner sing) are arranged such that only one bit is different between adjacent inner ring symbols. For example, in the inner ring of GS-16QAM where six symbols with smaller value ( “0101, ” “0001, ” “0011, ” “0010, ” “0110, ” and “0100” ) are arranged, a symbol corresponding to “0101” is arranged next to symbols corresponding to “0001” and “0100, ” and a symbol corresponding to “0010” is arranged next to symbols corresponding to “0011” and “0110. ” At the same time, the symbols corresponding to “0011” and “0001, ” respectively, are arranged next to each other, and the symbols corresponding to “0110 and “0100, ” respectively, are arranged next to each other. Remaining symbols are arranged in the outer ring such that two inner and outer ring symbols are arranged at closest positions to each other. For GS-16QAM, the outer ring symbol “1101” is arranged at the closest outer ring position to the inner ring symbol “0101, ” and the outer ring symbol “1001” is arranged at the closest outer ring position to the inner ring symbol “0001, ” and the outer ring symbol “1011” is arranged at the closest outer ring position to the inner ring symbol “0011, ” and the outer ring symbol “1010” is arranged at the closest outer ring position to the inner ring symbol “0010, ” and the outer ring symbol “1110” is arranged at the closest outer ring position to the inner ring symbol “0110. ” The remaining three symbols (e.g., for GS-16QAM, symbols corresponding to “0111, ” “1000, ” and “1111” ) are arranged such that the number of bits that are different from their closer points should be as small as possible. For example, the symbol corresponding to “0111” is arranged between the symbols corresponding to “1010” and “1011, ” and the symbol corresponding to “1000” is arranged between the symbols corresponding to “1110” and “1100, ” and the symbol corresponding to “1111” is arranged between the symbols corresponding to “1001” and “1101. ”

FIG. 11 is a flow chart representation of another example optical communication method. The modified constellation mapping is conducted in the following say. At step 1102, multi-bit data is generated to be mapped to geometrically shaped QAM symbols. At step 1104, the symbol corresponding to data with only ＂0＂ bit (data bit corresponding to the smallest number) is assigned to the center point. At 1106, symbols in inner ring corresponding to data with smaller value are arranged such that only one bit is different between adjacent inner ring symbols. Remaining symbols corresponding to data with larger value are arranged to outer ring (step 1108) . If an inner ring symbol point and an outer ring symbol point are at closest positions to each other, those inner and outer ring symbols are arranged such that only one bit is different between an inner ring symbol and an outer ring symbol (at step 1112) . Remaining outer ring symbols are arranged such that the number of bits different from adjacent inner ring symbol (s) is as little as possible. Next, the positions of constellation points are adjusted such that the distance between one-bit-different adjacent symbols decreases and the distance between two-or-more-bit-different adjacent symbols increases. In this way, the possibility of making wrong QAM decoding decisions at the receiver side may be minimized.

FIG. 12 illustrates an example configuration of the optical communication network. an optical transmitter apparatus 120 includes a light source (not illustrated in FIG. 12) that generates a light wave as an optical carrier wave, a modulator (not illustrated in FIG. 12) that modulate data to be transferred in a form of an optical multi-level quadrature amplitude modulation (QAM) signal using a geometric shaping scheme, a symbol mapper 1202 that assigns data to symbols, and a processor 1204 in communication with the symbol mapper to decide positions of the symbols and perform the assignment between the data and the symbols, memory 1206 for storing one or more instructions configured to be executed by the processor 1204. The instructions include instructions for minimizing a number of different bits between symbols that are more likely to cause wrong QAM decoding decision at a receiver, and instructions for shifting constellation points in a way that decreases a distance between adjacent symbols that are different by one bit and increases a distance between adjacent symbols that are different by two or more bits.

The instructions for minimizing the number of different bits between symbols that are more likely to cause wrong QAM decoding decision include instructions for dividing symbols into groups of symbols belonging to a center point, an inner ring, and an outer ring of constellation, respectively. The instructions for minimizing the number of different bits between symbols that are more likely to cause wrong QAM decoding decision include instructions for identifying bits of data to assign data with smaller numeric values to the center point and the inner ring and assign data with larger numeric values to the outer ring. The instructions for minimizing the number of different bits between symbols that are more likely to cause wrong QAM decoding decision include instructions for assigning symbols in the inner ring such that only one bit is different between adjacent inner ring symbols. The instructions for minimizing the number of different bits between symbols that are more likely to cause wrong QAM decoding decision include instructions for assigning symbols in the outer ring such that only one bit is different between one inner symbol point and one outer ring symbol point that are at closest positions to each other. The processor 1204 may further execute instructions for assigning data with only zero bit to the center point. The processor 1204 may further execute instructions for assigning remaining data to symbols in the outer ring in a way that minimizes a number of bits different from one or more adjacent symbols in the inner ring. Here, the modulation scheme of the optical multi-level quadrature amplitude modulation signal is geometrically shaped (GS) 16QAM. In that case, the adjacent symbols that are different by two bits from the center point are shifted away from the center point by 0.145 times a distance between their original constellation points and the center point. In addition, the adjacent symbols that are different by one bit from the center point are shifted away from the center point by 0.042 times the distance between their original constellation points and the center point.

As discussed above, various implementations of the present document can reduce the possibility of making wrong QAM decoding decisions at the receiver side by using a modified mapping algorithm. By using the modified mapping algorithm, geometrically shaped QAM signal can be processed in a way that minimizes the number of different bits between adjacent constellation points that are more likely to interfere with each other and thus more likely to cause wrong QAM decoding decision at the receiver side.

The disclosed and other embodiments, algorithms, modules and the functional operations described in the present document can be implemented in digital electronic circuitry, or in computer software, firmware, or hardware, including the structures disclosed in the present document and their structural equivalents, or in combinations of one or more of them. The disclosed and other embodiments can be implemented as one or more computer program products, i.e., one or more modules of computer program instructions encoded on a computer readable medium for execution by, or to control the operation of, data processing apparatus. The computer readable medium can be a machine-readable storage device, a machine-readable storage substrate, a memory device, a composition of matter effecting a machine-readable propagated signal, or a combination of one or more them. The term “processor” encompasses all apparatus, devices, and machines for processing data, including by way of example a programmable processor, a computer, or multiple processors or computers. The apparatus can include, in addition to hardware, code that creates an execution environment for the computer program in question, e.g., code that constitutes processor firmware, a protocol stack, a database management system, an operating system, or a combination of one or more of them. A propagated signal is an artificially generated signal, e.g., a machine-generated electrical, optical, or electromagnetic signal, that is generated to encode information for transmission to suitable receiver apparatus.

In implementing the modified mapping algorithm discussed in the present document, a computer program (also known as a program, software, software application, script, or code) can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program does not necessarily correspond to a file in a file system. A program can be stored in a portion of a file that holds other programs or data (e.g., one or more scripts stored in a markup language document) , in a single file dedicated to the program in question, or in multiple coordinated files (e.g., files that store one or more modules, sub programs, or portions of code) . A computer program can be deployed to be executed on one computer or on multiple computers that are located at one site or distributed across multiple sites and interconnected by a communication network.

The processes and logic flows described in the present document can be performed by one or more programmable processors executing one or more computer programs to perform functions by operating on input data and generating output. The processes and logic flows can also be performed by, and apparatus can also be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit) .

Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer. Generally, a processor will receive instructions and data from a read only memory or a random access memory or both. The essential elements of a computer are a processor for performing instructions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto optical disks, or optical disks. However, a computer need not have such devices. Computer readable media suitable for storing computer program instructions and data include all forms of non-volatile memory, media and memory devices, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto optical disks; and CD ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in, special purpose logic circuitry.

While the present document contains many specifics, these should not be construed as limitations on the scope of an invention that is claimed or of what may be claimed, but rather as descriptions of features specific to particular embodiments. Certain features that are described in present document in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable sub-combination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a sub-combination or a variation of a sub-combination. Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results.

Only a few examples and implementations are disclosed. Variations, modifications, and enhancements to the described examples and implementations and other implementations can be made based on what is disclosed.

Claims

An optical transmitter apparatus, comprising:

a light source that generates a light wave as an optical carrier wave;

a modulator that modulates, using the optical carrier wave, data to be transferred in a form of an optical multi-level quadrature amplitude modulation (QAM) signal using a geometric shaping scheme;

a symbol mapper that assigns data to symbols of the optical multi-level QAM signal; and

a processor in communication with the symbol mapper to decide positions of the symbols and perform the assignment between the data and the symbols;

memory for storing one or more instructions configured to be executed by the processor, including:

instructions for minimizing a number of different bits between symbols that are more likely to cause wrong QAM decoding decision at a receiver; and

instructions for shifting the optical multi-level QAM constellation points in a way that decreases a first distance between adjacent symbols that are different by one bit and increases a second distance between adjacent symbols that are different by two or more bits.
The apparatus of claim 1, wherein the instructions for minimizing the number of different bits between symbols that are more likely to cause wrong QAM decoding decision include instructions for dividing symbols into groups of symbols belonging to a center point, an inner ring, and an outer ring of constellation, respectively.
The apparatus of claim 2, wherein the instructions for minimizing the number of different bits between symbols that are more likely to cause wrong QAM decoding decision include instructions for identifying bits of data to assign data with smaller numeric values to the center point and the inner ring and assign data with larger numeric values to the outer ring.
The apparatus of claim 3, wherein the instructions for minimizing the number of different bits between symbols that are more likely to cause wrong QAM decoding decision include instructions for assigning symbols in the inner ring such that only one bit is different between adjacent inner ring symbols.
The apparatus of claim 4, wherein the instructions for minimizing the number of different bits between symbols that are more likely to cause wrong QAM decoding decision include instructions for assigning symbols in the outer ring such that only one bit is different between one inner symbol point and one outer ring symbol point that are at closest positions to each other.
The apparatus of claim 1, further comprising instructions for assigning data with only zero bit to the center point.
The apparatus of claim 6, further comprising instructions for assigning remaining data to symbols in the outer ring in a way that minimizes a number of bits different from one or more adjacent symbols in the inner ring.
The apparatus of claim 1, wherein a modulation scheme of the optical multi-level quadrature amplitude modulation signal is geometrically shaped (GS) 16QAM.
The apparatus of claim 8, wherein the adjacent symbols that are different by two bits from the center point are shifted away from the center point by 0.145 times a distance between their original constellation points and the center point.
The apparatus of claim 9, wherein the adjacent symbols that are different by one bit from the center point are shifted away from the center point by 0.042 times a distance between their original constellation points and the center point.
A method of mapping data to be transferred in a form of a geometrically shaped (GS) optical multi-level quadrature amplitude modulation (QAM) signal, comprising:

generating multi-bit data to be mapped to symbols of GS multi-level QAM constellation;

arranging the symbols according to the GS multi-level QAM constellation;

minimizing a number of different bits between symbols that are more likely to cause wrong QAM decoding decision at a receiver; and

shifting the GS multi-level QAM constellation points in a way that decreases a first distance between adjacent symbols that are different by one bit and increases a second distance between adjacent symbols that are different by two or more bits.
The method of claim 11, wherein the minimizing of the number of different bits between symbols that are more likely to cause wrong QAM decoding decision include dividing symbols into groups of symbols belonging to a center point, an inner ring, and an outer ring of constellation, respectively.
The method of claim 12, wherein the minimizing of the number of different bits between symbols that are more likely to cause wrong QAM decoding decision include identifying bits of data to assign data with smaller numeric values to the center point and the inner ring and assign data with larger numeric values to the outer ring.
The method of claim 13, wherein the minimizing of the number of different bits between symbols that are more likely to cause wrong QAM decoding decision include assigning symbols in the inner ring such that only one bit is different between adjacent inner ring symbols.
The method of claim 14, wherein the minimizing of the number of different bits between symbols that are more likely to cause wrong QAM decoding decision include assigning symbols in the outer ring such that only one bit is different between one inner symbol point and one outer ring symbol point that are at closest positions to each other.
The method of claim 11, further comprising assigning data with only zero bit to the center point.
The method of claim 16, further comprising assigning remaining data to symbols in the outer ring in a way that minimizes a number of bits different from one or more adjacent symbols in the inner ring.
The method of claim 11, wherein a modulation scheme of the optical multi-level quadrature amplitude modulation signal is geometrically shaped (GS) 16QAM.
The method of claim 11, wherein the adjacent symbols that are different by two bits from the center point are shifted away from the center point by 0.145 times a distance between their original constellation points and the center point.
The method of claim 11, wherein the adjacent symbols that are different by one bit from the center point are shifted away from the center point by 0.042 times a distance between their original constellation points and the center point.