Three-dimensional 16QAM constellation diagram based on geometric shaping and digital communication method

Abstract

The invention discloses a three-dimensional 16QAM constellation diagram based on geometric shaping and a digital communication method, and relates to the field of communication, in a three-dimensional space, 16 constellation points are distributed on the surface of a three-dimensional sphere with the radius of 2.5, and 8 constellation points are distributed on each of the upper hemispherical surface and the lower hemispherical surface; a regular pentagon parallel to the XOY plane is formed by five constellation points in the upper hemispherical surface, the center of the regular pentagon coincides with the projection of the sphere center on the XOY plane, and one edge of the regular pentagon is parallel to the XOZ plane; the other three constellation points form an equilateral triangle parallel to the XOY plane, the center of the equilateral triangle coincides with the projection of the sphere center on the XOY plane, and the vertical bisector of one edge of the equilateral triangle is parallel to the YOZ plane; and rotating the eight constellation points of the upper semi-spherical surface by 180 degrees along the center of the sphere in the direction parallel to the YOZ plane to obtain the distribution of the constellation points of the lower semi-spherical surface. According to the method, the channel noise tolerance and the overall transmission performance of the system are improved, the bit error rate is reduced, and the method has an important application prospect in a high-tolerance and high-performance transmission system.

Description

Technical Field

[0001] This invention relates to the field of communication technology, specifically to a three-dimensional 16QAM constellation diagram based on geometric shaping and a digital communication method. Background Technology

[0002] With the rapid development of communication technology, the demand for data transmission rates and spectral efficiency is constantly increasing. However, traditional two-dimensional constellation diagram transmission technology is gradually approaching its theoretical limit in terms of spectral efficiency, making it difficult to meet future requirements for higher bandwidth. Traditional two-dimensional constellation diagrams increase data transmission rates by increasing the constellation order, but this method significantly reduces the minimum Euclidean distance between constellation points, thereby increasing inter-symbol interference. Especially in complex real-world communication environments, such as those with noise, multipath fading, and phase noise, this interference is further aggravated, leading to a significant increase in bit error rate and severely impacting the performance of the communication system.

[0003] To address this issue, the design of three-dimensional constellation diagrams has emerged. Compared to traditional two-dimensional constellation diagrams, three-dimensional constellation diagrams, by introducing a third dimension, can transmit more data within the same bandwidth, thus significantly improving spectral efficiency. Furthermore, three-dimensional constellation diagrams possess stronger anti-interference capabilities. By optimizing the layout of constellation points, mutual interference between symbols can be effectively reduced, thereby lowering the bit error rate and improving the overall system performance. The design of three-dimensional constellation diagrams not only better copes with complex channel environments, such as multipath fading and phase noise, but also enhances the stability of the system in high-speed mobile scenarios. Therefore, three-dimensional constellation diagrams are considered one of the important development directions for future communication technologies, and they will play a crucial role in 5G and 6G communication networks, providing strong technical support for achieving more efficient and reliable wireless communication.

[0004] Currently, most 3D constellation diagrams designed for 16QAM modulation formats are simple tetrahedral and hexahedral structures. On the one hand, their simplicity allows for further optimization; on the other hand, the tetrahedral structure, with its limited number of faces, restricts expansion, resulting in sparse constellation point distribution and higher average energy during high-order modulation. Conversely, the hexahedral structure, with its excessive number of points, increases the number of long-distance constellation points, making it difficult to guarantee the minimum Euclidean distance and reducing power efficiency. Therefore, a novel 3D constellation diagram is needed to achieve a balance between average power and minimum Euclidean distance, while further improving transmission performance. Summary of the Invention

[0005] To address the problems of traditional two-dimensional constellation diagrams and existing three-dimensional constellation diagram structures, this invention provides a three-dimensional 16QAM constellation diagram and digital communication method based on geometric shaping.

[0006] This invention, in designing a three-dimensional 16QAM constellation diagram, adjusts the positions of constellation points using geometric shaping techniques to maximize the minimum Euclidean distance, thereby reducing inter-symbol interference and improving the system's noise immunity. Simultaneously, a Gray-like symbol mapping rule is employed when allocating bits to constellation points to mitigate the impact of bit error propagation. Through its designed three-dimensional 16QAM constellation diagram, this invention solves the problem of balancing data transmission rate and noise immunity in two-dimensional constellation diagrams, thus improving the overall performance of the communication system.

[0007] The technical solution adopted by this invention to solve the technical problem is as follows:

[0008] This invention provides a three-dimensional 16QAM constellation diagram based on geometric shaping, the structure of which is as follows:

[0009] In three-dimensional space, 16 constellation points are distributed on the surface of a three-dimensional sphere with a radius of 2.5, with 8 constellation points distributed on each of the upper and lower hemispheres. In the upper hemisphere, 5 constellation points form a regular pentagon parallel to the XOY plane, with its center coinciding with the projection of the center of the three-dimensional sphere onto the XOY plane, and one of its sides parallel to the XOZ plane. The remaining 3 constellation points form an equilateral triangle parallel to the XOY plane, with its center coinciding with the projection of the center of the three-dimensional sphere onto the XOY plane, and the perpendicular bisector of one of its sides parallel to the YOZ plane. The constellation point distribution on the lower hemisphere is obtained by rotating the 8 constellation points on the upper hemisphere by 180 degrees around the center of the three-dimensional sphere in a direction parallel to the YOZ plane.

[0010] In a preferred embodiment, in the upper hemisphere, the distance between the vertex of the regular pentagon and its center is 2.41, and the regular pentagon intersects the surface of the three-dimensional sphere at 5 constellation points.

[0011] In a preferred embodiment, the equilateral triangle in the upper hemisphere has a side length of 2 and intersects the surface of the three-dimensional sphere at 3 constellation points.

[0012] In a preferred embodiment, the five vertices of the regular pentagon in the lower hemisphere are located on the perpendicular bisectors of the five sides of the regular pentagon in the upper hemisphere.

[0013] In a preferred embodiment, the distance from each constellation point of the regular pentagon to its nearest constellation point is 2.

[0014] This invention provides a symbolic mapping method for a three-dimensional 16QAM constellation diagram based on geometric shaping, which specifically includes the following steps:

[0015] If the sign of each constellation point of a regular pentagon differs by 1 bit from the sign of its two nearest constellation points, then the signs of the constellation points of the regular pentagon will differ by 1 bit in turn. If the sign of each constellation point of an equilateral triangle differs by 1 bit from the sign of the constellation point of its nearest regular pentagon, then the signs of the constellation points of the equilateral triangle will differ by 2 bits or 4 bits.

[0016] This invention provides a digital communication method, implemented using the aforementioned geometrically shaped three-dimensional 16QAM constellation diagram, which specifically includes the following steps:

[0017] To construct an orthogonal frequency division multiplexing (OFDM) communication system, the input bitstream data (i.e., the binary plaintext sequence) is first converted from serial to parallel, and then symbol mapping is performed using the three-dimensional 16QAM constellation diagram. Subsequently, IFFT processing is applied, and pilot signals are inserted and cyclic prefixes are added to complete the OFDM. At the transmitter, the electrical signal is modulated to the laser carrier emitted by the laser by the modulator, and then propagated through the optical fiber channel. At the receiver, coherent detection demodulates the optical signal, performs serial-to-parallel conversion, removes the cyclic prefix, and performs frequency domain conversion based on FFT. Finally, the demodulation mapping reconstructs the plaintext bitstream through inverse symbol mapping, and finally outputs the data through parallel-to-serial conversion.

[0018] As a preferred embodiment, the symbol mapping rule is as follows: set the symbol of each constellation point of a regular pentagon to differ from the symbol of its two nearest constellation points by 1 bit, then the symbols of each constellation point of the regular pentagon differ by 1 bit in turn; set the symbol of each constellation point of an equilateral triangle to differ from the symbol of the constellation point in its nearest regular pentagon by 1 bit, then the symbols of each constellation point of the equilateral triangle differ by 2 bits or 4 bits.

[0019] The beneficial effects of this invention are:

[0020] 1. Improve noise immunity;

[0021] To address the trade-off between data transmission rate and noise immunity in traditional two-dimensional constellation diagrams, this invention increases the dimensions of the constellation diagram, resulting in a three-dimensional 16QAM constellation diagram that, while reducing average power compared to traditional two-dimensional constellation diagrams, increases the minimum Euclidean distance between constellation points, thereby improving the noise immunity of the communication system.

[0022] 2. Reduced bit error rate;

[0023] Based on the designed three-dimensional 16QAM constellation diagram structure, this invention proposes a Gray code-like symbol mapping method to minimize the number of bits between adjacent constellation points and reduce the bit error rate during transmission.

[0024] 3. Improved transmission performance;

[0025] The three-dimensional 16QAM constellation diagram designed in this invention has lower average power and a larger minimum Euclidean distance compared to traditional two-dimensional constellation diagrams and existing three-dimensional constellation diagram structures. Therefore, it has better transmission characteristics and has important application prospects in high-tolerance, high-performance transmission systems. Attached Figure Description

[0026] Figure 1 This is a flowchart of the process of the present invention.

[0027] Figure 2 This invention provides a design flowchart for a three-dimensional 16QAM constellation diagram based on geometric shaping.

[0028] Figure 3 This invention relates to a three-dimensional 16QAM constellation diagram based on geometric shaping.

[0029] Figure 4 This invention relates to a three-dimensional 16QAM constellation map and constellation point mapping map based on geometric shaping.

[0030] Figure 5 This invention presents a transmission flowchart of a three-dimensional 16QAM constellation diagram based on geometric shaping in an orthogonal frequency division multiplexing communication system.

[0031] Figure 6 Comparison diagrams of different three-dimensional 16QAM constellation structures.

[0032] Figure 7 A comparison of bit error rate curves for different three-dimensional 16QAM constellation diagrams. Detailed Implementation

[0033] The present invention will be further described in detail below with reference to the accompanying drawings.

[0034] like Figure 1 As shown, the present invention mainly includes three aspects: a three-dimensional 16QAM constellation diagram design based on geometric shaping, a symbol mapping rule design similar to Gray code, and the implementation of digital communication in an orthogonal frequency division multiplexing communication system using a three-dimensional 16QAM constellation diagram based on geometric shaping designed in this invention.

[0035] This invention designs a three-dimensional 16QAM constellation diagram based on geometric shaping. Addressing the trade-off between data transmission rate and noise immunity in traditional two-dimensional constellation diagrams, this invention increases the dimensionality of the constellation diagram. This results in a lower average power compared to traditional two-dimensional constellation diagrams, while simultaneously increasing the minimum Euclidean distance between constellation points, thus improving the noise immunity of the communication system. Furthermore, based on the designed three-dimensional 16QAM constellation diagram, this invention designs a Gray code-like symbol mapping rule, resulting in a smaller bit difference between adjacent constellation points, reducing the bit error rate during transmission. Finally, the transmission performance of the designed three-dimensional 16QAM constellation diagram is analyzed through simulation. The simulation results show that the designed three-dimensional 16QAM constellation diagram can effectively improve the performance of communication systems and has significant application prospects in future high-capacity, high-speed, and high-tolerance communication fields.

[0036] In a first aspect, the present invention provides a three-dimensional 16QAM constellation diagram based on geometric shaping.

[0037] This invention provides a three-dimensional 16QAM constellation diagram based on geometric shaping, the specific structure of which is as follows:

[0038] In three-dimensional space, 16 constellation points are distributed on the surface of a three-dimensional sphere with a radius of 2.5, with 8 constellation points distributed on the upper hemisphere and 8 on the lower hemisphere. For example... Figure 2 As shown in (a), a regular pentagon is formed in the upper hemisphere by five constellation points. The five vertices of this pentagon correspond to the five constellation points. The pentagon is parallel to the XOY plane in three-dimensional space. The center of the pentagon coincides with the projection of the center of the three-dimensional sphere onto the XOY plane. One side of the pentagon is parallel to the XOZ plane. The distance from each vertex of the pentagon to its center is 2.41, and the pentagon intersects the surface of the three-dimensional sphere formed by the three-dimensional constellation diagram at five constellation points.

[0039] like Figure 2 As shown in (b), in the upper hemisphere, the remaining three constellation points form an equilateral triangle. The three vertices of this equilateral triangle correspond to the three constellation points. This equilateral triangle is parallel to the XOY plane in three-dimensional space. The center of this equilateral triangle coincides with the projection of the center of the three-dimensional sphere onto the XOY plane. The perpendicular bisector of one side of this equilateral triangle is parallel to the YOZ plane. The side length of this equilateral triangle is 2, and it intersects the surface of the three-dimensional sphere formed by the three-dimensional constellation diagram at three constellation points.

[0040] like Figure 2As shown in (c), by treating the eight constellation points in the upper hemisphere as a whole and rotating them 180 degrees around the center of the three-dimensional sphere in a direction parallel to the YOZ plane, the distribution of constellation points in the lower hemisphere can be obtained. At this point, the five vertices of the regular pentagon in the lower hemisphere lie on the perpendicular bisectors of the five sides of the regular pentagon in the upper hemisphere (for example, the first vertex of the regular pentagon in the lower hemisphere lies on the perpendicular bisector of the first side of the regular pentagon in the upper hemisphere, the second vertex lies on the perpendicular bisector of the second side of the regular pentagon in the upper hemisphere, and so on, with the fifth vertex lying on the perpendicular bisector of the fifth side of the regular pentagon in the lower hemisphere). Similarly, the five vertices of the regular pentagon in the upper hemisphere lie on the perpendicular bisectors of the five sides of the regular pentagon in the lower hemisphere. The distance from each constellation point corresponding to a vertex of the regular pentagon to its nearest constellation point is 2. The final design of this invention yields a three-dimensional 16QAM constellation diagram based on geometric shaping, as shown below. Figure 3 As shown.

[0041] Secondly, the present invention provides a symbolic mapping method for a three-dimensional 16QAM constellation diagram based on geometric shaping.

[0042] This invention provides a symbolic mapping method for three-dimensional 16QAM constellation diagrams based on geometric shaping, the specific implementation process of which is as follows:

[0043] Based on a geometrically shaped 3D 16QAM constellation diagram structure, this invention proposes a symbol mapping method for this structure. Since the 3D 16QAM constellation diagram designed in this invention does not conform to the Gray code mapping rules, this invention designs a Gray code-like symbol mapping rule based on its structural characteristics. By setting the minimum possible difference in the number of bits between adjacent constellation points, the anti-interference capability of the 3D 16QAM constellation diagram during transmission is improved.

[0044] Specifically, in the designed 3D 16QAM constellation diagram based on geometric shaping, the signs of the constellation points corresponding to each vertex of a regular pentagon differ by 1 bit from the signs of their two nearest neighboring constellation points (for example, if the constellation point L corresponding to vertex L of a regular pentagon is L, and the two nearest neighboring constellation points L are M and N, then constellation point L differs by 1 bit from constellation point M, and constellation point L differs by 1 bit from constellation point N). Thus, the signs of the constellation points (between the 10 constellation points in the two regular pentagons) of the two pentagons formed are successively 1 bit apart. The signs of the constellation points corresponding to each vertex of an equilateral triangle differ by 1 bit from the signs of the constellation points in their nearest neighboring regular pentagon, and the signs of the constellation points corresponding to each vertex of an equilateral triangle differ by 2 bits or 4 bits.

[0045] Comparing the geometrically shaped 3D 16QAM constellation diagram designed in this invention with traditional 2D constellation diagrams and existing 3D constellation diagrams, the 3D 16QAM constellation diagram designed in this invention exhibits the lowest average power under the same minimum Euclidean distance condition. While maintaining the same average power, the 3D 16QAM constellation diagram designed in this invention has the largest minimum Euclidean distance, thus achieving a balance between average power and minimum Euclidean distance. Subsequently, the 3D 16QAM constellation diagram structure designed in this invention is applied to an orthogonal frequency division multiplexing (OFDM) communication system, and the transmission performance of the 3D 16QAM constellation diagram structure designed in this invention under different channel conditions is analyzed through simulation. Compared with traditional 2D constellation diagrams and existing 3D 16QAM constellation diagram structures, the 3D 16QAM constellation diagram designed in this invention exhibits a lower bit error rate and higher noise immunity under the same channel conditions.

[0046] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0047] With the relative positions between constellation points remaining unchanged, the minimum Euclidean distance will change as the three-dimensional constellation diagram is enlarged or reduced. To facilitate subsequent comparison and calculation, this embodiment fixes the length of the minimum Euclidean distance to 2.

[0048] According to the design method of the present invention, in three-dimensional space, 16 constellation points are distributed on the surface of a three-dimensional sphere with a radius of 2.5, with 8 constellation points distributed on the upper and lower hemispheres respectively. The 8 constellation points on the upper hemisphere consist of two parts: one part is a regular pentagon formed by 5 constellation points, which is parallel to the XOY plane, and the projection of its geometric center onto the XOY plane coincides with the center of the three-dimensional sphere, with one side parallel to the XOZ plane. All vertices of this regular pentagon are located on the surface of the three-dimensional sphere, such as... Figure 2 As shown in (a); the other part is an equilateral triangle with a side length of 2 formed by 3 constellation points. This equilateral triangle is also parallel to the XOY plane, and the projection of its geometric center onto the XOY plane coincides with the center of the three-dimensional sphere. Furthermore, the perpendicular bisector of one side is parallel to the YOZ plane, and all vertices of this equilateral triangle lie on it. Figure 2 As shown in (b), the eight constellation points of the lower hemisphere are obtained by rotating the eight constellation points of the upper hemisphere as a whole around the center of the sphere in a direction parallel to the YOZ plane by 180 degrees. After rotation, each vertex of the regular pentagon of the lower hemisphere lies on the perpendicular bisector of the corresponding side of the regular pentagon of the upper hemisphere, as shown in (b). Figure 2As shown in (c).

[0049] When the radius of the three-dimensional sphere is 2.5, the minimum Euclidean distance between constellation points is 2, and the overall average power of the three-dimensional 16QAM constellation diagram is 6.25. When the average power of the three-dimensional 16QAM constellation diagram is 1, the minimum Euclidean distance is 0.83, which is larger than that of existing three-dimensional constellation diagram structures. Furthermore, due to its spherical boundary, the overall gain of the three-dimensional 16QAM constellation diagram is 1.083.

[0050] This embodiment designs symbol mapping rules based on the designed three-dimensional 16QAM constellation diagram. In Gray mapping, any two adjacent codes differ by only one bit. However, the three-dimensional 16QAM constellation diagram structure in this embodiment does not conform to this rule. Therefore, the design must minimize the number of bits that differ between symbols of adjacent constellation points to reduce the overall bit error rate of the three-dimensional 16QAM constellation diagram. The designed constellation point mapping positions are as follows: Figure 4 As shown in Table 1, the mapping relationship between the corresponding constellation point positions and symbols is shown in Table 1.

[0051] Table 1. Mapping Relationship Between Constellation Point Locations and Symbols

[0052]

[0053]

[0054] To verify the performance of the three-dimensional 16QAM constellation diagram structure designed in this embodiment, an OFDM-PON simulation system incorporating the designed three-dimensional 16QAM constellation diagram structure was constructed. Its workflow is as follows: Figure 5 As shown, the input bitstream data (binary plaintext sequence) is first converted from serial to parallel, and then symbol mapping is performed using the designed three-dimensional 16QAM constellation structure. Subsequently, it is processed by IFFT (Inverse Fast Fourier Transform), and pilot signals are inserted and a cyclic prefix (CP) is added to complete OFDM (Orthogonal Frequency Division Multiplexing). At the transmitter, the electrical signal is modulated to the 1550nm laser carrier emitted by the laser, and propagated through a 50km single-mode fiber channel. At the receiver, coherent detection demodulates the optical signal, followed by serial-to-parallel conversion, CP removal, and FFT-based frequency domain transformation. Finally, the demodulated mapping reconstructs the plaintext bitstream through inverse symbol mapping, and the data is output after parallel-to-serial conversion.

[0055] The constellation diagram structure compared in this embodiment is as follows: Figure 6 As shown, where Figure 6 (a) is the traditional two-dimensional 16QAM constellation diagram structure. Figure 6 (b) is a three-dimensional constellation structure based on a regular hexahedron (Rectangular Constellation Map, RCM). Figure 6 (c) Shows a 3D constellation map (Double Hexahedron Constellation Map, DHCM) with nested double cubes. Figure 6 (d) shows the three-dimensional 16QAM constellation diagram structure designed in this embodiment, which is called a regular pentagonal constellation map (RPCM).

[0056] The relationship between bit error rate and optical signal-to-noise ratio obtained through simulation system analysis is as follows: Figure 7 As shown, by Figure 7 It can be seen that the three-dimensional 16QAM constellation structure designed in this embodiment has a lower bit error rate under the same optical signal-to-noise ratio conditions. The bit error rate decision threshold is 3.8 × 10⁻⁶. -3 Under the given conditions, the optical signal-to-noise ratio (SNR) of the three-dimensional 16QAM constellation structure designed in this embodiment is 8.5 dB, while the optical SNRs of the RCM and DHCM constellation structures are 10.5 dB and 11 dB, respectively. It can be seen that the optical SNR of the three-dimensional 16QAM constellation structure designed in this embodiment can reach more than 2 dB under the same bit error rate, and has better noise immunity.

[0057] In summary, this embodiment designs a geometrically shaped three-dimensional 16QAM constellation structure, which, compared to traditional two-dimensional constellation diagrams and existing three-dimensional constellation diagrams, exhibits lower average power and a larger minimum Euclidean distance, thus providing better transmission characteristics. Furthermore, this embodiment also presents symbol mapping rules for the three-dimensional 16QAM constellation structure, resulting in a lower bit error rate. Subsequently, simulations were conducted on systems with different constellation diagram structures under a 50km OFDM-PON simulation system for comparison. The results show that the geometrically shaped three-dimensional 16QAM constellation structure designed in this invention has better transmission performance.

[0058] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A three-dimensional 16QAM constellation diagram based on geometric shaping, characterized in that, Its structure is as follows: In three-dimensional space, 16 constellation points are distributed on the surface of a three-dimensional sphere with a radius of 2.5, with 8 constellation points distributed on each of the upper and lower hemispheres. In the upper hemisphere, 5 constellation points form a regular pentagon parallel to the XOY plane, with its center coinciding with the projection of the center of the three-dimensional sphere onto the XOY plane, and one of its sides parallel to the XOZ plane. The remaining 3 constellation points form an equilateral triangle parallel to the XOY plane, with its center coinciding with the projection of the center of the three-dimensional sphere onto the XOY plane, and the perpendicular bisector of one of its sides parallel to the YOZ plane. The constellation point distribution on the lower hemisphere is obtained by rotating the 8 constellation points on the upper hemisphere by 180 degrees around the center of the three-dimensional sphere in a direction parallel to the YOZ plane.

2. The three-dimensional 16QAM constellation diagram based on geometric shaping according to claim 1, characterized in that, In the upper hemisphere, the distance between the vertex of the regular pentagon and its center is 2.41, and the regular pentagon intersects the surface of the three-dimensional sphere at 5 constellation points.

3. The three-dimensional 16QAM constellation diagram based on geometric shaping according to claim 1, characterized in that, In the upper hemisphere, the equilateral triangle has a side length of 2 and intersects the surface of the three-dimensional sphere at 3 constellation points.

4. The three-dimensional 16QAM constellation diagram based on geometric shaping according to claim 1, characterized in that, The five vertices of the regular pentagon in the lower hemisphere are located on the perpendicular bisectors of the five sides of the regular pentagon in the upper hemisphere.

5. The three-dimensional 16QAM constellation diagram based on geometric shaping according to claim 1, characterized in that, The distance from each constellation point of the regular pentagon to its nearest constellation point is 2.

6. The symbol mapping method for a three-dimensional 16QAM constellation diagram based on geometric shaping as described in any one of claims 1-5, characterized in that, Includes the following steps: If the sign of each constellation point of a regular pentagon differs by 1 bit from the sign of its two nearest constellation points, then the signs of the constellation points of the regular pentagon will differ by 1 bit in turn. If the sign of each constellation point of an equilateral triangle differs by 1 bit from the sign of the constellation point of its nearest regular pentagon, then the signs of the constellation points of the equilateral triangle will differ by 2 bits or 4 bits.

7. A digital communication method, characterized in that, The method employs the geometrically shaped 3D 16QAM constellation diagram as described in any one of claims 1-5, characterized by comprising the following steps: To construct an orthogonal frequency division multiplexing (OFDM) communication system, the input bitstream data (i.e., the binary plaintext sequence) is first converted from serial to parallel, and then symbol mapping is performed using the three-dimensional 16QAM constellation diagram. Subsequently, IFFT processing is applied, and pilot signals are inserted and cyclic prefixes are added to complete the OFDM. At the transmitter, the electrical signal is modulated to the laser carrier emitted by the laser by the modulator, and then propagated through the optical fiber channel. At the receiver, coherent detection demodulates the optical signal, performs serial-to-parallel conversion, removes the cyclic prefix, and performs frequency domain conversion based on FFT. Finally, the demodulation mapping reconstructs the plaintext bitstream through inverse symbol mapping, and finally outputs the data through parallel-to-serial conversion.

8. The digital communication method according to claim 7, characterized in that, The symbol mapping rule is as follows: set the symbol of each constellation point of a regular pentagon to differ from the symbol of its two nearest constellation points by 1 bit, then the symbols of each constellation point of the regular pentagon differ by 1 bit in turn; set the symbol of each constellation point of an equilateral triangle to differ from the symbol of the constellation point of its nearest regular pentagon by 1 bit, then the symbols of each constellation point of the equilateral triangle differ by 2 bits or 4 bits.

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