A passive wavelength division multiplexer / demultiplexer, a front transmission model and a 5G front transmission method

Abstract

The present application relates to a kind of passive wave division multiplexer / demultiplexer, front model and 5G front method, passive wave division multiplexer / demultiplexer includes primary filter and secondary filter group, the secondary filter group includes first group of secondary filter and second group of secondary filter, the primary filter is connected with the first group of secondary filter by dispersion compensation type optical fiber, and the primary filter is connected with the second group of secondary filter by optical fiber.The technical scheme provided by the present application is to solve that a set of passive wave division multiplexer / demultiplexer can transmit 12-way 25G optical signal, to reduce the demand amount of front optical fiber, to reduce cost.

Description

Technical Field

[0001] This invention belongs to the field of transmission and communication technology, specifically relating to a passive wavelength division multiplexing / demultiplexer, a fronthaul model, and a 5G fronthaul method. Background Technology

[0002] 5G fronthaul refers to the transmission between DU (Distributed Unit) and AAU (Active Antenna Unit) in a 5G radio access network.

[0003] Currently, 5G fronthaul primarily employs WDM technology, using passive wavelength division multiplexing / demultiplexing devices to combine multiple optical signals using different wavelengths into a single optical fiber for transmission. For example, ... Figure 6 As shown, a typical 5G base station has 3 AAUs, and there are 6 transmission signals from the DU to the AAU. A 6-channel multiplexer / demultiplexer is used on both the DU and AAU sides to combine the 6 transmission signals between the DU and AAU into a single optical fiber for transmission. The AAUs are mainly attached to existing towers. Currently, there are 2 to 3 5G base stations at different frequencies on a single tower, and this number may increase in the future as services expand. The fronthaul interface rate of 5G base stations is mainly 25G, and each frequency point of the 5G base station has 6 fronthaul interfaces, as shown in Table 1.

[0004]

[0005] Table 1

[0006] Due to cost considerations, passive wavelength division multiplexers / demultiplexers generally employ CWDM (coarse wavelength division) technology and DML (direct modulation laser). CWDM supports 18 wavelengths. Since the dispersion coefficient of commonly used G.652 fiber increases with wavelength, and the dispersion tolerance of 25G DML is only about 60 ps / nm, current 5G fronthaul generally only uses the first 6 wavelengths of CWDM (the dispersion of the 6th wavelength for a 10km fiber link is 60 ps / nm), as shown in Table 2. From the 7th wavelength onwards, due to the excessively large dispersion coefficient of the fiber, the inexpensive DML cannot be used.

[0007]

[0008] Table 2

[0009] The fronthaul signal rate of 5G base stations is mostly 25G, and the fronthaul distance is usually no more than 10km. The existing passive wavelength division multiplexing / demultiplexing 5G fronthaul system only supports the combination of 6 25G fronthaul signals in a single optical fiber for transmission, which is called a 6-in-1 system. Although a 12-in-1 system is also used in current 5G fronthaul, due to the limitation of fiber dispersion, only the first 6 wavelengths in the 12-in-1 system can still be used for 25G fronthaul. Wavelengths 7 to 12 can only be used for 10G fronthaul, and waves 7 to 12 cannot meet the requirements for 25G fronthaul.

[0010] Currently, one set of passive wavelength division multiplexing / demultiplexing unit can only meet the fronthaul requirements of one frequency point 5G station. However, there are often multiple frequency point 5G stations on one tower, which requires the use of multiple sets of passive wavelength division multiplexing / demultiplexing units, thus requiring multiple optical fiber cores from DU to AAU. Summary of the Invention

[0011] To address all or part of the aforementioned problems, the present invention aims to provide a passive wavelength division multiplexer / demultiplexer, a fronthaul model, and a 5G fronthaul method, so as to solve the problem that one set of passive wavelength division multiplexer / demultiplexer can transmit 12 channels of 25G optical signals, thereby reducing the amount of fronthaul fiber required, thus reducing costs and facilitating maintenance by staff.

[0012] According to a first aspect of the present invention, a passive wavelength division multiplexing / demultiplexer is provided, comprising a primary filter and a secondary filter bank, wherein the secondary filter bank comprises a first group of secondary filters and a second group of secondary filters, the primary filter being connected to the first group of secondary filters via a dispersion-compensating optical fiber, and the primary filter being connected to the second group of secondary filters via an optical fiber.

[0013] Furthermore, both the first group of secondary filters and the second group of secondary filters include N secondary filters. The primary filter is provided with a first port, a second port, and a third port, and the secondary filter is provided with a fourth port, a fifth port, and a sixth port.

[0014] Furthermore, the third port of the first-stage filter is connected to the fourth port of the second-stage filter in the first group that outputs the maximum or minimum wavelength via the dispersion-compensating optical fiber, and the second port of the first-stage filter is connected to the fourth port of the second-stage filter in the second group that outputs the maximum or minimum wavelength via the optical fiber; the N second-stage filters in different groups are connected sequentially according to the output or received wavelength from largest to smallest or smallest to largest, and the fifth port of adjacent second-stage filters is connected to the fourth port via optical fiber.

[0015] Furthermore, the wavelength of the primary filter transmitted to or received from the first group of secondary filters is 1391nm~1491nm, and the wavelength of the primary filter transmitted to or received from the second group of secondary filters is 1271nm~1371nm.

[0016] Furthermore, it also includes a first interface that is connected to the first port via an optical fiber.

[0017] Furthermore, it also includes 2N second interfaces, each of the sixth ports being connected to the corresponding second interface via optical fiber.

[0018] Further, N=6; the first interface is a line-side interface, and the signal received or transmitted by the first interface is a wavelength of 1271nm~1491nm; the second interface is a device-side interface, and each of the second interfaces sequentially receives or transmits any wavelength of 1271nm~1491nm with an interval of 20nm and without repetition.

[0019] Furthermore, the dispersion-compensated optical fiber has a dispersion value of -45ps / nm to -50ps / nm, and the optical fiber is G.652 or G.657 optical fiber.

[0020] According to a second aspect of the present invention, a fronthaul model is provided, including an AAU, a DU, and a fronthaul fiber. The AAU is connected to a first passive wavelength division multiplexing / demultiplexer, and the DU is connected to a second passive wavelength division multiplexing / demultiplexer. The first passive wavelength division multiplexing / demultiplexer and the second passive wavelength division multiplexing / demultiplexer are connected via the fronthaul fiber. The first passive wavelength division multiplexing / demultiplexer and / or the second passive wavelength division multiplexing / demultiplexer employ the passive wavelength division multiplexing / demultiplexer described above.

[0021] According to a third aspect disclosed in this invention, a 5G fronthaul method is provided, including the fronthaul model described above, wherein the transmission distance between the AAU and the DU is S;

[0022] The primary filter and the first group of secondary filters are connected by optical fiber to a common type of passive wavelength division multiplexer / demultiplexer;

[0023] When S≤3.0km, both the first passive wavelength division multiplexer / demultiplexer and the second passive wavelength division multiplexer / demultiplexer adopt the ordinary type of passive wavelength division multiplexer / demultiplexer;

[0024] When 3.0km < S ≤ 6.5km, either the first passive wavelength division multiplexing / demultiplexing unit or the second passive wavelength division multiplexing / demultiplexing unit adopts the dispersion-compensated passive wavelength division multiplexing / demultiplexing unit described above, and the other adopts the ordinary passive wavelength division multiplexing / demultiplexing unit described above.

[0025] When 6.5 < S ≤ 10.0 km, both the first passive wavelength division multiplexer / demultiplexer and the second passive wavelength division multiplexer / demultiplexer adopt the dispersion-compensated passive wavelength division multiplexer / demultiplexer described above.

[0026] The technical solution provided by the present invention has the following advantages:

[0027] (1) The first-level filter and the first group of second-level filters are connected through dispersion-compensated optical fiber. Dispersion compensation is performed by connecting to the dispersion-compensated optical fiber with negative dispersion to ensure that the total dispersion of the 1391nm~1491nm wavelength transmitted in the link is within the dispersion tolerance of DML, thereby realizing high-speed and large-capacity communication.

[0028] (2) The first-level filter and the second-level filter are connected by optical fiber. Since the wavelength of the optical signal transmitted by the second-level filter is 1271nm~1371nm, the total dispersion value is within the dispersion tolerance of DML, so dispersion compensation is not required. Therefore, ordinary optical fiber connection can be used to achieve the effect of high-speed and large-capacity communication, which can save costs.

[0029] (3) It has the characteristics of simple structure, low cost and easy maintenance. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the passive wavelength division multiplexing / demultiplexer in Embodiment 1 of the present invention;

[0031] Figure 2 This is a schematic diagram of a typical passive wavelength division multiplexer / demultiplexer.

[0032] Figure 3 This is a schematic diagram of the front-pass model when the front-pass distance exceeds 3.0km but does not exceed 6.5km.

[0033] Figure 4 This is a schematic diagram of the front-pass model when the front-pass distance exceeds 6.5km but does not exceed 10.0km.

[0034] Figure 5 This is a schematic diagram of the front-pass model when the front-pass distance does not exceed 3.0km;

[0035] Figure 6 This is a schematic diagram of an existing 5G fronthaul structure based on passive wavelength division multiplexing (WDM) technology.

[0036] Figure labeling: First-stage filter 1; First port 11; Second port 12; Third port 13; Second-stage filter group 2; First group of second-stage filters 21; Second group of second-stage filters 22; Second-stage filter 23; Fourth port 231; Fifth port 232; Sixth port 233; Dispersion-compensated fiber 3; First interface 4; Second interface 5; DU 6; AAU 7; Fronthaul fiber 8; First passive wavelength division multiplexer / demultiplexer 9; Second passive wavelength division multiplexer / demultiplexer 10. Detailed Implementation

[0037] To better understand the purpose, structure, and function of this invention, the following description, in conjunction with the accompanying drawings, provides a more detailed account of a passive wavelength division multiplexing / demultiplexer, a fronthaul model, and a method for using the fronthaul model.

[0038] Example 1

[0039] like Figure 1 As shown, a passive wavelength division multiplexing / demultiplexing device according to an embodiment of the present invention is described. This passive wavelength division multiplexing / demultiplexing device includes a primary filter 1 and a secondary filter bank 2. The secondary filter bank 2 includes a first group of secondary filters 21 and a second group of secondary filters 22. The primary filter 1 and the first group of secondary filters 21 are connected via a dispersion-compensating optical fiber 3, and the primary filter 1 and the second group of secondary filters 22 are connected via optical fibers.

[0040] To further explain, the primary filter 1 and the first group of secondary filters 21 are connected via dispersion-compensated optical fiber 3. By connecting to the negative dispersion-compensated optical fiber 3, dispersion compensation is performed to ensure that the total dispersion value of the optical signal transmitted in the link with a wavelength of 1391nm~1491nm is within the dispersion tolerance of DML, thereby achieving high-speed, high-capacity communication. The primary filter 1 and the second group of secondary filters 22 are connected via optical fiber. Since the wavelength of the optical signal transmitted by the second group of secondary filters 22 is 1271nm~1371nm, the maximum dispersion value is within the dispersion tolerance of DML, so dispersion compensation is not required. Therefore, using ordinary optical fiber connection can achieve the effect of high-speed, high-capacity communication. Thus, 12 channels of 25G optical signals can be transmitted simultaneously in one fronthaul optical fiber 8, reducing the demand for fronthaul optical fiber 8 by 50% and saving costs.

[0041] Dispersion-compensated fiber 3 has a large negative dispersion value. By connecting to the negative dispersion-compensated fiber 3, dispersion compensation is performed to ensure that the total dispersion value in the link is within the dispersion tolerance of DML, thereby achieving high-speed and high-capacity communication.

[0042] like Figure 1As shown, in this embodiment, the first group of secondary filters 21 and the second group of secondary filters 22 each include N secondary filters 23. The primary filter 1 is provided with a first port 11, a second port 12 and a third port 13. The secondary filter 23 is provided with a fourth port 231, a fifth port 232 and a sixth port 233. The first port 11 and the second port 12 are located on the same side, and the fourth port 231 and the fifth port 232 are located on the same side.

[0043] To further explain, when the device is used as a demultiplexer, the first port 11 and the fourth port 231 are input terminals, the second port 12 and the fifth port 232 are reflection terminals, and the third port 13 and the sixth port 233 are output terminals; when the device is used as a multiplexer, the first port 11 and the fourth port 231 are output terminals, the second port 12 and the fifth port 232 are reflection terminals, and the third port 13 and the sixth port 233 are input terminals.

[0044] In this embodiment, the third port 13 of the first-stage filter 1 is connected to the fourth port 231 of the second-stage filter 23 in the first group of second-stage filters 21, which outputs the maximum or minimum wavelength, through a dispersion-compensating optical fiber 3. The second port 12 of the first-stage filter 1 is connected to the fourth port 231 of the second-stage filter 23 in the second group of second-stage filters 22, which outputs the maximum or minimum wavelength, through an optical fiber. The N second-stage filters 23 in different groups are connected sequentially according to the output or received wavelength from largest to smallest or smallest to largest. The fifth port 232 and the fourth port 231 of adjacent second-stage filters 23 are connected through an optical fiber.

[0045] In this embodiment, the wavelength of the first-stage filter 1 transmitted to or received from the first group of second-stage filters 21 is 1391nm~1491nm, and the wavelength of the first-stage filter 1 transmitted to or received from the second group of second-stage filters 22 is 1271nm~1371nm.

[0046] To further explain, the optical signal in the optical fiber includes 12 wavelengths from 1271nm to 1491nm. The specific wavelengths filtered by the first-stage filter 1 are 1391nm to 1491nm, and the specific wavelengths filtered by the 12 second-stage filters 23 are 1271nm, 1291nm, 1311nm, 1331nm, 1351nm, 1371nm, 1391nm, 1411nm, 1431nm, 1451nm, 1471nm, and 1491nm, respectively.

[0047] To further explain, the third port 13 of the first-stage filter 1 is connected to the fourth port 231 of the second-stage filter 23 in the first group of second-stage filters 21, which outputs the maximum or minimum wavelength, via a dispersion-compensating optical fiber 3. The wavelength output or input from the third port 13 is 1391nm~1491nm. After filtering a specific wavelength, the second-stage filter 23 connected to the first-stage filter 1, preferably, filters a wavelength of 1391nm or 1491nm, while other wavelengths are reflected sequentially from the fifth port 232 to the fourth port 231 of the connected second-stage filter 23. This second-stage filter 23 filters a specific wavelength, and the other second-stage filters 23 in this group filter specific wavelengths in conjunction with the above... The principle is the same as described above, so it will not be repeated. The second port 12 of the first-stage filter 1 is connected to the fourth port 231 of the second-stage filter 23, which outputs the maximum or minimum wavelength of the second-stage filter 22, through an optical fiber. The wavelength reflected from or out of the fourth port 231 is 1271nm~1371nm. After filtering a specific wavelength, the second-stage filter 23 connected to the first-stage filter 1 preferably filters the wavelength of 1271nm or 1371nm. Other wavelengths are reflected sequentially from the fifth port 232 to the fourth port 231 of the second-stage filter 23 connected to it. The second-stage filter 23 filters a specific wavelength. The other second-stage filters 23 can be obtained in the same way, so it will not be repeated.

[0048] Preferably, the wavelengths filtered by the secondary filter 23 are sequentially from large to small or from small to large, which facilitates inspection and maintenance by staff.

[0049] Further explanation: Tables 2 and 3 show that when the wavelength is 1271nm~1371nm, the dispersion coefficient of G.652 optical fiber is between -2.8ps / nm·km and 6ps / nm·km. When the transmission distance is 10.0km, the dispersion value of the 1371nm wavelength is 60ps / nm. The existing dispersion tolerance of DML is generally 60ps / nm. When DML is set, the dispersion value of wavelengths smaller than 1371nm is within the dispersion tolerance of DML. This can ensure that the total dispersion value in the link is within the dispersion tolerance of DML, thereby realizing high-speed and high-capacity communication.

[0050] When the wavelength is 1391nm~1491nm, the dispersion coefficient of G.652 optical fiber is between 7.6ps / nm·km and 14.5ps / nm·km. When the transmission distance is 10km, the dispersion value of this band is greater than 60ps / nm. Therefore, the third port 13 of the first-stage filter 1 is connected to the fourth port 231 of the second-stage filter 23 in the first group of second-stage filters 21, which outputs the maximum or minimum wavelength, through dispersion-compensated optical fiber 3. The dispersion value of dispersion-compensated optical fiber 3 is negative. Therefore, by connecting the negative dispersion dispersion-compensated optical fiber 3, dispersion compensation is performed to ensure that the total dispersion value in the link is within the dispersion tolerance of DML, thereby realizing high-speed and high-capacity communication.

[0051] like Figure 1 As shown, in this embodiment, a first interface 4 is also included, which is connected to the first port 11 via an optical fiber.

[0052] In this embodiment, there are also 2N second interfaces 5, and each sixth port 233 is connected to the corresponding second interface 5 via optical fiber.

[0053] In this embodiment, N=6; the first interface 4 is a line-side interface, and the signal received or transmitted by the first interface 4 is a wavelength of 1271nm~1491nm; the second interface 5 is a device-side interface, and each second interface 5 sequentially receives or transmits any wavelength of 1271nm~1491nm with an interval of 20nm and without repetition.

[0054] To further explain, the first interface 4 is a line-side interface used to connect to the fronthaul fiber 8. Since wavelength combining is transmitted through the fronthaul fiber 8, the wavelength transmitted or received by the line-side interface is 1271nm~1491nm. The second interface 5 is a device-side interface used to connect to AAU7 or DU6. The 12 optical signals of different wavelengths in AAU7 or DU6 are connected to the 12 device-side interfaces according to their wavelengths.

[0055] To further explain, the passive wavelength division multiplexer / demultiplexer in this embodiment uses CWDM (coarse wavelength division) technology. CWDM supports 18 wavelengths, from 1271nm to 1611nm. Since the dispersion coefficient of the 1511nm-1611nm wavelength range is too high, this band will not be used for optical signal transmission in this embodiment. Therefore, within the 1271nm-1491nm wavelength range, there are 12 wavelengths, each spaced 20nm apart, used for optical signal transmission, with the 1271nm wavelength being one of them. The dispersion coefficient of wavelengths from ~1371nm is relatively small, and its dispersion value is within the dispersion tolerance of DML. This ensures that the total dispersion value in the link is within the dispersion tolerance of DML, thereby achieving high-speed and high-capacity communication. However, the dispersion coefficient of wavelengths from 1391nm to 1491nm is relatively large. Therefore, it is necessary to connect dispersion-compensating fiber 3 for dispersion compensation to solve the problem of the large total dispersion value in the link and ensure that the total dispersion value in the link is within the dispersion tolerance of DML, thereby achieving high-speed and high-capacity communication.

[0056] To further clarify, the wavelengths for receiving or transmitting at the 12 device-side interfaces are 1271nm, 1291nm, 1311nm, 1331nm, 1351nm, 1371nm, 1391nm, 1411nm, 1431nm, 1451nm, 1471nm, and 1491nm.

[0057] In this embodiment, the dispersion value of the dispersion-compensated fiber 3 is -45ps / nm to -50ps / nm, and the fiber is G.652 fiber or G.657 fiber.

[0058] As further explained in Table 3, the dispersion value of dispersion-compensated fiber 3 is -45ps / nm to -50ps / nm. When the transmission distance does not exceed 10km, the maximum dispersion value in the link is 144.7ps / nm, which is greater than the dispersion tolerance of DML. Only when passive wavelength division multiplexers / demultiplexers with dispersion-compensated fiber 3 are used on both sides of AAU7 and DU6 will the maximum dispersion value in the entire link be less than the dispersion tolerance of DML, and the total dispersion value in the link will be within the dispersion tolerance of DML.

[0059]

[0060] Table 3

[0061] Example 2

[0062] like Figure 3 and Figure 4 As shown, the fronthaul model in this embodiment includes AAU7, DU6, and fronthaul fiber 8. AAU7 is connected to the first passive wavelength division multiplexer / demultiplexer 9, and DU6 is connected to the second passive wavelength division multiplexer / demultiplexer 10. The first passive wavelength division multiplexer / demultiplexer 9 and the second passive wavelength division multiplexer / demultiplexer 10 are connected through the fronthaul fiber 8. The first passive wavelength division multiplexer / demultiplexer 9 and / or the second passive wavelength division multiplexer / demultiplexer 10 adopt the passive wavelength division multiplexer / demultiplexer in Embodiment 1.

[0063] Example 3

[0064] The 5G fronthaul method in this embodiment includes a fronthaul model, where the transmission distance between AAU7 and DU6 is S.

[0065] The primary filter 1 and the first group of secondary filters 21 are connected by optical fiber to form a common type of passive wavelength division multiplexer / demultiplexer;

[0066] When S≤3.0km, both the first passive wavelength division multiplexer / demultiplexer 9 and the second passive wavelength division multiplexer / demultiplexer 10 are ordinary passive wavelength division multiplexers / demultiplexers.

[0067] When 3.0km < S ≤ 6.5km, either the first passive wavelength division multiplexer / demultiplexer 9 or the second passive wavelength division multiplexer / demultiplexer 10 adopts a dispersion-compensated passive wavelength division multiplexer / demultiplexer, and the other adopts a conventional passive wavelength division multiplexer / demultiplexer.

[0068] When 6.5 < S ≤ 10.0 km, both the first passive wavelength division multiplexer / demultiplexer 9 and the second passive wavelength division multiplexer / demultiplexer 10 are dispersion-compensated passive wavelength division multiplexers / demultiplexers.

[0069] To further explain, DML is usually set in AAU7 and DU6, and the dispersion tolerance of DML is generally 60ps / nm;

[0070] Among them, such as Figure 2 The first-stage filter 1 and the first group of second-stage filters 21 in the ordinary passive wavelength division multiplexing / demultiplexer shown are connected by optical fiber. The other structures are the same as those of the dispersion-compensated passive wavelength division multiplexing / demultiplexer, and the wavelengths transmitted or received by the dispersion-compensated passive wavelength division multiplexing / demultiplexer are the same, which are 1271nm, 1291nm, 1311nm, 1331nm, 1351nm, 1371nm, 1391nm, 1411nm, 1431nm, 1451nm, 1471nm, and 1491nm.

[0071] like Figure 5 As shown, when S≤3.0km, the maximum dispersion value of the link is 43.4ps / nm, which is less than the dispersion tolerance of DML. Therefore, at both the AAU7 and DU6 ends, the following method is adopted. Figure 2 The ordinary passive wavelength division multiplexer / demultiplexer shown can achieve stable transmission.

[0072] like Figure 3 As shown, when 3.0km < S ≤ 6.5km, the maximum dispersion value of the link is 94.1ps / nm, which is greater than the dispersion tolerance of DML. Therefore, only dispersion-compensated passive wavelength division multiplexers / demultiplexers need to be used at the AAU7 or DU6 end. For cost savings, the other end should preferably use a... Figure 2 The ordinary passive wavelength division multiplexer / demultiplexer shown has a dispersion value of -45ps / nm to -50ps / nm for the dispersion-compensated fiber 3. After dispersion compensation, the dispersion value of the link is 49.1ps / nm to 44.1ps / nm, which is less than the dispersion tolerance of DML, so stable transmission can be achieved.

[0073] like Figure 4 As shown, when 6.5 < S ≤ 10.0 km, the maximum dispersion value of the link is 144.7 ps / nm, which is greater than the dispersion tolerance of DML. Therefore, dispersion-compensated passive wavelength division multiplexers / demultiplexers need to be used at both the AAU7 and DU6 ends. Since the dispersion value of the dispersion-compensated fiber 3 is -45 ps / nm to -50 ps / nm, the dispersion value of the link after dispersion compensation is 54.7 ps / nm to 44.7 ps / nm, which is less than the dispersion tolerance of DML, thus achieving stable transmission.

[0074] It should be noted that, unless otherwise stated, the technical or scientific terms used in this application should have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0075] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be covered within the scope of the claims and specification of the present invention. In particular, as long as there is no structural conflict, the various technical features mentioned in the embodiments can be combined in any way. The present invention is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A passive wavelength division multiplexing / demultiplexer, characterized in that, The system includes a primary filter and a secondary filter bank. The secondary filter bank includes a first group of secondary filters and a second group of secondary filters. The third port of the primary filter is connected to the fourth port of the secondary filter in the first group that outputs the maximum or minimum wavelength via a dispersion-compensated optical fiber. The second port of the primary filter is connected to the fourth port of the secondary filter in the second group that outputs the maximum or minimum wavelength via an optical fiber. N secondary filters in different groups are connected sequentially according to the output or received wavelength from largest to smallest or smallest to largest. The fifth and fourth ports of adjacent secondary filters are connected via optical fibers. Wherein, the wavelength of the first-stage filter transmitted to or received from the first group of second-stage filters is 1391nm~1491nm, and the wavelength of the first-stage filter transmitted to or received from the second group of second-stage filters is 1271nm~1371nm; dispersion compensation is performed by connecting a dispersion-compensated optical fiber with negative dispersion to ensure that the total dispersion value of the optical signal transmitted in the link with a wavelength of 1391nm~1491nm is within the dispersion tolerance of DML.

2. The passive wavelength division multiplexing / demultiplexer according to claim 1, characterized in that, Both the first group of secondary filters and the second group of secondary filters include N secondary filters. The primary filter is provided with a first port, a second port and a third port, and the secondary filter is provided with a fourth port, a fifth port and a sixth port.

3. The passive wavelength division multiplexing / demultiplexer according to claim 2, characterized in that, It also includes a first interface that is connected to the first port via an optical fiber.

4. The passive wavelength division multiplexing / demultiplexer according to claim 3, characterized in that, It also includes 2N second interfaces, each of the sixth ports being connected to the corresponding second interface via optical fiber.

5. The passive wavelength division multiplexing / demultiplexer according to claim 4, characterized in that, The number N is 6; the first interface is a line-side interface, and the signal received or transmitted by the first interface is a wavelength of 1271nm~1491nm; the second interface is a device-side interface, and each second interface sequentially receives or transmits any wavelength of 1271nm~1491nm with an interval of 20nm and without repetition.

6. The passive wavelength division multiplexing / demultiplexing unit according to any one of claims 1-5, characterized in that, The dispersion-compensated optical fiber has a dispersion value of -45ps / nm to -50ps / nm, and the optical fiber is G.652 or G.657 optical fiber.

7. A fronthaul model, comprising an AAU, a DU, and a fronthaul fiber, wherein the AAU is connected to a first passive wavelength division multiplexing / demultiplexing unit (WDM), the DU is connected to a second passive wavelength division multiplexing / demultiplexing unit (WDM), and the first WDM and the second WDM are connected via the fronthaul fiber, characterized in that, The first passive wavelength division multiplexing / demultiplexer and / or the second passive wavelength division multiplexing / demultiplexer adopts the passive wavelength division multiplexing / demultiplexer as described in any one of claims 1-6.

8. A 5G fronthaul method, characterized in that, The forward transmission model as described in claim 7 is included, wherein the transmission distance between the AAU and the DU is S; The primary filter and the first group of secondary filters are connected by optical fiber to a common type of passive wavelength division multiplexer / demultiplexer; When S≤3.0km, both the first passive wavelength division multiplexer / demultiplexer and the second passive wavelength division multiplexer / demultiplexer adopt the ordinary type of passive wavelength division multiplexer / demultiplexer; When 3.0km < S ≤ 6.5km, either the first passive wavelength division multiplexing / demultiplexing unit or the second passive wavelength division multiplexing / demultiplexing unit adopts the passive wavelength division multiplexing / demultiplexing unit described in any one of claims 1-6, and the other adopts the ordinary passive wavelength division multiplexing / demultiplexing unit described above. When 6.5 < S ≤ 10.0 km, both the first passive wavelength division multiplexer / demultiplexer and the second passive wavelength division multiplexer / demultiplexer adopt the passive wavelength division multiplexer / demultiplexer described in any one of claims 1-6.

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