CN114070735A

CN114070735A - System for upgrading passive optical network based on combined code of Manchester code and PAM4

Info

Publication number: CN114070735A
Application number: CN202111286871.0A
Authority: CN
Inventors: 曹露芳; 卢旸; 李雪蒙; 徐凯强; 毕美华; 翟彦蓉
Original assignee: Hangzhou Dianzi University
Current assignee: Hangzhou Dianzi University
Priority date: 2021-11-02
Filing date: 2021-11-02
Publication date: 2022-02-18
Anticipated expiration: 2041-11-02
Also published as: CN114070735B

Abstract

The invention discloses a system for upgrading a passive optical network based on a combined code of Manchester codes and PAM4, wherein a central office comprises a new optical line terminal and an old optical line terminal; the remote node has an optical splitter; the optical network system comprises a new optical network unit and an old optical network unit; the new optical line terminal is connected with the first wavelength division multiplexer and the second wavelength division multiplexer; the old optical line terminal is connected with the first wavelength division multiplexer and the second wavelength division multiplexer; the first wavelength division multiplexer and the second wavelength division multiplexer are connected with the first optical circulator; the first optical circulator is connected with the first optical splitter through an optical fiber; the first optical splitter is connected to the old optical network unit and the new optical network unit, or the first optical splitter is connected to the old optical network unit, the first new optical network unit and the second optical splitter, and the second optical splitter is connected to the second new optical network unit, or the first optical splitter is connected to the old optical network unit and the second optical splitter, and the second optical splitter is connected to the new optical network unit. The invention ensures the normal communication of the new link and the old link in the upgrading process and realizes the smooth upgrading of the new PON.

Description

System for upgrading passive optical network based on combined code of Manchester code and PAM4

Technical Field

The present invention relates to a network architecture for upgrading an old Passive Optical Network (PON) to a next-generation Passive Optical Network (PON), and more particularly, to a network architecture capable of upgrading an originally existing old PON system to a next-generation PON system by using a combination code of a Manchester code and PAM4, thereby increasing a system bandwidth rate and maintaining a communication service of the old PON.

Background

PON is currently in widespread use. The PON technology is largely classified into a TDM-PON and a WDM-PON according to a method of sharing an optical fiber. In the TDM-PON, an optical fiber is mainly shared using Time Division Multiple Access (TDMA). However, in the TDM-PON, the uplink and downlink rates are fixed, and when a plurality of users share one PON network, the bandwidth provided to each user is reduced, and the increase of users is more variable. However, as the demand for bandwidth is rapidly increased due to the development of the internet, a large number of image and video services, TDM-PON technology is mature, and a higher rate TDM-PON is required. TDM-PON with higher bit rate and better performance is under study.

The rapid development of technology has led to the situation that PONs deployed in the past cannot meet the needs of people, and people need a higher-speed PON network to meet the needs of production and life. However, PON is relatively cost sensitive and its investment comes mainly from the installation of Optical Distribution Networks (ODNs). Redeploying a new PON link is costly, partial reconstruction of a deployed optical network unit is difficult, and central office reconstruction is simple. Smooth upgrades therefore require the new PON to re-use the deployed ODN to avoid re-deploying costly infrastructure, which requires that two generations of PONs can coexist during the upgrade phase without disrupting the service of the old PON.

New and old PONs typically occupy different wavelength ranges, so they can be separated by filters in new Optical Network Units (ONUs) or Wavelength Division Multiplexers (WDM) in the Optical Line Terminal (OLT). The filters and WDM act such that the crosstalk between the old downstream signal and the new downstream and upstream signals is very low. However, the old ONU is not equipped with a filter for the new PON wavelength, which is typically only used for downstream/upstream signal separation. The number of old ONUs is numerous and widely distributed, so the retrofitting cost of adding new filters to old ONUs is too high. Therefore, the main purpose of the upgrading method is to reduce or eliminate crosstalk between the downstream signals of the new PON and the downstream signals of the old PON during the upgrading process without changing the ODN and the old ONU.

The existing PON upgrading method includes subcarrier modulation, synchronization pulse interpolation, and spectrum shaping coding (SSLC).

Among the above-mentioned existing methods, there are complicated implementations and high costs, in which the SSLC is considered as a cost-effective method, the encoding suppresses low-frequency components of the new PON signal that can generate crosstalk with respect to the old PON signal, and the upgrading method can be operated only by software without requiring additional equipment, and thus is economical and flexible. In SSLC, Manchester code is simple to generate and detect, and the best low-frequency suppression effect is shown. But has the disadvantage that the coding efficiency is only 50%, resulting in a low effective bit rate for the new PON signal.

Disclosure of Invention

Aiming at the problems of complexity, high cost, low effective bit rate of signals, large PON bandwidth limitation and the like in the prior art, the invention provides a network architecture for upgrading an old Passive Optical Network (PON) to a next-generation passive optical network based on a combined code of Manchester codes and PAM 4. The system structure can keep normal transmission of the old signal and can realize the transmission of the high-speed new signal under the condition of not changing the prior PON network.

The invention adopts the following technical scheme:

a system for upgrading a passive optical network based on a combined code of Manchester code and PAM4, comprising:

a central office: the system comprises a new optical line terminal and an old optical line terminal;

the remote node: having a beam splitter;

an optical network system: the optical network unit comprises a new optical network unit and an old optical network unit;

the new optical line terminal is connected with the first wavelength division multiplexer and the second wavelength division multiplexer; the old optical line terminal is connected with the first wavelength division multiplexer and the second wavelength division multiplexer; the first wavelength division multiplexer and the second wavelength division multiplexer are connected with the first optical circulator; the first optical circulator is connected with the first optical splitter through an optical fiber; the first optical splitter is connected to the old optical network unit and the new optical network unit, or the first optical splitter is connected to the old optical network unit, the first new optical network unit and the second optical splitter, and the second optical splitter is connected to the second new optical network unit, or the first optical splitter is connected to the old optical network unit and the second optical splitter, and the second optical splitter is connected to the new optical network unit.

Preferably, in the network architecture, the old optical line terminal OLT includes a downstream NRZ data transmitting module and an upstream NRZ data receiving module, and the downstream signal is modulated by using a common OOK. The downlink NRZ data transmitting module comprises a light source, a Mach-Zehnder modulator and an NRZ data generating module. The uplink NRZ data receiving module comprises a photoelectric detector, a low-pass filter and a signal processing module.

Preferably, the new optical line terminal OLT comprises a downlink Manchester-PAM4 data transmitting module and an uplink NRZ data receiving module, and the downlink signal is modulated by four-level Manchester-PAM 4. The system comprises a downlink Manchester-PAM4 data transmitting module, an uplink NRZ data receiving module and a downlink control module, wherein the downlink Manchester-PAM4 data transmitting module comprises a light source, a four-level Mach-Zehnder modulator, a Manchester-PAM4 encoder and an NRZ data sequence, and the uplink NRZ data receiving module comprises a photoelectric detector, a low-pass filter and a signal processing module.

The novel optical line terminal comprises an NRZ data sequence, a Manchester-PAM4 encoder, a light source, a four-level Mach-Zehnder modulator, a signal processing module, a low-pass filter and a photoelectric detector, wherein the NRZ data sequence is connected with the four-level Mach-Zehnder modulator through the Manchester-PAM4 encoder, the light source is connected with the Mach-Zehnder modulator, and the four-level Mach-Zehnder modulator is connected with the first wavelength division multiplexer; the second wavelength division multiplexer is connected with the photoelectric detector, and the photoelectric detector is connected with the signal processing module through the low-pass filter.

Preferably, a single mode fiber SMF is used for transmitting the uplink and downlink signals.

Preferably, the old optical network unit ONU comprises a downstream NRZ data receiving module, an upstream NRZ data transmitting module, and an optical circulator, and the upstream signal is modulated by a common OOK.

Preferably, the new optical network unit ONU comprises a downstream Manchester-PAM4 data receiving module, an upstream NRZ data sending module, and an optical circulator, wherein the 1 st port of the circulator is connected to the transmitting end, the 2 nd port is connected to the optical filter, the 3 rd port is connected to the receiving end, and the upstream signal is modulated by using a common OOK. The downstream Manchester-PAM4 data receiving module comprises an optical filter, a photoelectric detector, a low-pass filter and a signal processing module, and the upstream NRZ data sending module comprises a light source, a Mach-Zehnder modulator and an upstream NRZ data generating module.

The novel optical network unit comprises an optical filter, a photoelectric detector, a low-pass filter, a signal processing module, a light source, an uplink NRZ data generation module and a Mach-Zehnder modulator, wherein the light source and the uplink NRZ data generation module are connected with the Mach-Zehnder modulator, and the Mach-Zehnder modulator is connected with the 1 st port of the second optical circulator; the signal processing module is connected with the 3 rd port of the second optical circulator after sequentially passing through the photoelectric detector and the low-pass filter; and the 2 nd port of the second optical circulator is connected with an optical filter.

Wherein, the light source wavelength of new and old OLT is different.

The network system structure of the invention does not change the old ONU link when upgrading, and the new PON signal wavelength and the old PON signal wavelength are different.

The invention also provides an upgrading method of a new and old PON upgrading system based on the combined code modulation of Manchester codes and PAM4, which comprises the following steps:

in the downstream direction:

step 1: adding a new OLT into the system, converting the digital signal into a Manchester-PAM4 data format after the digital signal is subjected to line coding on software in the new OLT, and setting the data bit rate to be 12.5 Gb/s;

step 2: the existing old OLT data format in the system is NRZ, and the data bit rate is set to be 1.25 Gb/s;

and step 3: the digital signals in the step 1 and the step 2 are used as electric input signals of optical loads after passing through an electric pulse generator, and are modulated onto optical carriers through an optical amplitude modulator to form optical carrier signals;

and 4, step 4: and the new OLT signal and the old OLT signal are combined by a wavelength division multiplexer and then transmitted in a 25km single-mode optical fiber, and the combined signal is divided into a plurality of parts by an optical splitter and transmitted to each ONU.

And 5: in the old ONU, the optical signal passes through a photoelectric detector and a low-pass filter and then is demodulated into an old signal; in the new ONU, the light is demodulated into a new signal through a photoelectric detector and a low-pass filter after the old wavelength is filtered by an optical filter.

Step 6: when all old ONUs in the network are upgraded to new ONUs, the mapping relation of data to the Manchester-PAM4 modulation format is released through software operation. As can be seen from fig. 10, when there is a mapping relationship, four bits are required to represent two bits of information, and when the mapping relationship is cancelled, four bits can represent four bits of information, thereby further increasing the transmission rate.

Preferably, in step 1, the Manchester-PAM4 data format is generated: this is the data generated by the mapping. The mapping table is shown in fig. 10. And in combination with a four-level Mach-Zehnder modulator, modulating the Manchester-PAM4 data format into a Manchester-PAM4 waveform.

Preferably, in

steps

1 and 2, different OLT transmitting signal wavelengths are different from each other, and coexistence in the line is realized by using wavelength division multiplexing.

Preferably, in step 3, the optical amplitude modulator is a mach-zehnder modulator, and the extinction ratio is 14dB, which is common.

Preferably, in step 4, the two signals benefit from being in different frequency bands, so that the two signals can coexist and transmit without interfering with each other.

Preferably, in step 5, since there is no optical filter in the old ONU, the obtained signal is a new signal and an old signal, but the new signal is a high-frequency signal, and the bandwidth of the low-pass filter of the old ONU is limited, so that the new signal can be automatically filtered; because the new ONU has the optical filter, the old wavelength is firstly filtered, and then the new wavelength signal is received.

In the uplink direction:

step 1: the new ONU and the old ONU transmit upstream signals with different wavelengths through the modulator and are jointly transmitted in the SMF. The new and old ONUs have different wavelengths, and all the new ONUs are the same wavelength.

Step 2: when the combined signal is transmitted to the CO, the upstream signals with different wavelengths are separated and transmitted to the new OLT and the old OLT through the WDM2 for receiving processing.

Preferably, the modulator in the new and old ONUs is a second-order mach-zehnder modulator.

The method and the network system structure for upgrading the old PON to the next-generation PON based on the combined code of the Manchester code and the PAM4 have the advantages that:

1) the Manchester-PAM4 signal is adopted as a signal format of the new PON link, and the format effectively reduces low-frequency components, so that crosstalk caused to the old PON link is small.

2) Compared with other signals with low-frequency suppression capability formats, such as Manchester encoding signals, the Manchester-PAM4 signal has the encoding efficiency of 100%, and the signal transmission capability can be effectively improved.

3) Thanks to the Manchester-PAM4 being a four-level signal, it is possible to achieve a transition to a PAM4 format signal without altering the hardware device to operate solely by software.

Drawings

The invention is described in detail below with reference to the drawings and the detailed description.

Fig. 1 is a schematic diagram of a network architecture for upgrading an old PON to a next-generation PON based on a combined code of a Manchester code and PAM 4.

Fig. 2 is a schematic diagram of an internal structure of an old OLT.

Fig. 3 is a schematic diagram of the internal structure of the new OLT.

Fig. 4 is a schematic diagram of the inside of a new ONU.

Fig. 5 shows a new ONU downstream Manchester-PAM4 data reception module.

Fig. 6 shows a new ONU upstream NRZ data transmission module.

Fig. 7 is an eye diagram when new/old downlink signals coexist, (a): old downstream NRZ format signal eye, (b): the new Manchester-PAM4 format downlink signal eye diagram.

FIG. 8 is a schematic diagram of encoding of Manchester-PAM4 and the like.

Fig. 9 is a graph of BER when new and old signals coexist. In the figure: the horizontal axis represents the received optical power, and the vertical axis BER represents the bit error rate, wherein the comparison of the BER performance of the receiving end respectively comprises: NRZ signal at 1.25Gb/s and Manchester-PAM4 signal at 12.5Gb/s transmitted over 25km single mode fiber.

Fig. 10 is a table of mapping of binary data to Manchester-PAM4 encoded data.

Detailed Description

The present invention will be described in detail with reference to specific examples. The following examples will assist those skilled in the art in further understanding the invention, but are not intended to limit the invention in any way. It should be noted that variations and modifications can be made by persons skilled in the art without departing from the spirit of the invention. All falling within the scope of the present invention.

As shown in fig. 1, the network architecture system for upgrading an old pon to a next-generation pon based on a combined code of a Manchester code and PAM4 in the present embodiment includes: the optical network system comprises a central office CO with an optical line terminal, a single mode fiber SMF and an optical network system.

The central office comprises a new optical line terminal OLT, an old optical line terminal OLT, a wavelength division multiplexer WDM and an optical circulator. The downlink data transmitting modules (corresponding to the downlink NRZ data transmitting module in FIG. 2 and the downlink Manchester-PAM4 data transmitting module in FIG. 3) of the new and old optical line terminals are connected with a wavelength division multiplexer WDM1, the wavelength division multiplexer WDM1 is connected to the 1 st port of the optical circulator, the uplink NRZ data receiving modules (corresponding to the uplink NRZ data receiving modules in FIGS. 2 and 3) of the new and old optical line terminals are connected with a wavelength division multiplexer WDM2, the wavelength division multiplexer WDM2 is connected to the 3 rd port of the optical circulator, and the 2 nd port of the optical circulator is connected with a 25km optical fiber SMF.

The optical network system comprises a remote node RN, an optical fiber and an optical network unit ONU; the remote node RN is an optical splitter, ports of the optical splitter depend on the number of optical network units, and the ports of the optical splitter are connected with a plurality of old ONUs, new optical network units ONU or next-layer (one or more) optical splitters. When optical network units ONU are newly added in the structure, the optical network units ONU can be directly connected with the optical splitter, and the number of the optical network units ONU can be further increased by increasing the number of the optical splitters. In the downlink direction, OLT signals with different wavelengths are transmitted to an optical splitter through wavelength division multiplexing and are divided into a plurality of parts, wherein one part of the signals are transmitted to an old ONU for demodulation, and the other part of the signals are transmitted to a new ONU for demodulation. In the uplink direction, ONU signals with different wavelengths are coupled and transmitted, and then are respectively transmitted to corresponding OLTs for demodulation through demultiplexing.

The optical line terminal OLT is divided into an old optical line terminal and a new optical line terminal, wherein the old optical line terminal comprises a downlink NRZ data transmitting module and an uplink NRZ data receiving module, and downlink signals adopt common OOK modulation. The uplink and downlink modules of the old optical line terminal are currently mainstream modules for transmitting/receiving NRZ signals, and in the embodiment, the downlink rate is 1.25 Gb/s. The new optical line terminal comprises a downlink Manchester-PAM4 data transmitting module and an uplink NRZ data receiving module (corresponding to figure 3), and downlink signals are modulated by a four-level Manchester-PAM 4. The new optical line terminal transmitting module adopts Manchester-PAM4 coding as shown in figure 3, and the transmitting speed is improved to 12.5 Gb/s. The uplink rates of the new and old optical network units are all 1.25 Gb/s. As shown in fig. 3, the new optical line terminal OLT includes an NRZ data sequence, a Manchester-PAM4 encoder, a light source, a four-level mach-zehnder modulator, a signal processing module, a low-pass filter, and a photodetector, where the NRZ data sequence is connected to the mach-zehnder modulator through the Manchester-PAM4 encoder, the light source is connected to the mach-zehnder modulator, and the mach-zehnder modulator is connected to the wavelength division multiplexer WDM 1. The process of downlink transmission signal processing: the NRZ data sequence is encoded by a Manchester-PAM encoder into a Manchester-PAM4 signal, which is then modulated onto an optical carrier by a mach-zehnder modulator for information transmission. The wavelength division multiplexer WDM2 is connected with the photoelectric detector, and the photoelectric detector is connected with the signal processing module through the filter. The uplink signal receiving and processing process comprises the following steps: the optical signal is converted into an electric signal from the signal after uplink transmission through a photoelectric conversion detector, and then the interference of a high-speed downlink wavelength signal is filtered through a low-pass filter with a limited receiving bandwidth, wherein the received signal is an uplink NRZ data signal. In the data generation module, the old optical line termination module generates an NRZ signal, and the new optical line termination module generates a Manchester-PAM4 signal. The downstream Mandchester-PAM4 signal in fig. 1 corresponds to the signal encoded by the Manchester-PAM4 encoder of the NRZ data sequence in the downstream Manchester-PAM4 data transmission module in fig. 3. The downstream Mandchester-PAM4 signal of FIG. 1 is input to the signal processing module of FIG. 4; the upstream NRZ is input to the upstream NRZ data transmission module of fig. 6.

The optical network unit ONU is divided into an old optical network unit and a new optical network unit, and the difference between the old optical network unit and the new optical network unit is that an optical filter capable of filtering wavelengths is arranged in the new optical network unit, so that the wavelength selection function is realized, and the receiving rate and the transmitting rate are improved. The old optical network unit comprises a downlink NRZ data receiving module and an uplink NRZ data sending module, and uplink signals are modulated by common OOK. The new optical network unit comprises a downlink Manchester-PAM4 data receiving module and an uplink NRZ data sending module (the downlink Manchester-PAM4 data receiving module corresponds to the graph 5, and the uplink NRZ data sending module corresponds to the graph 6), and uplink signals are modulated by common OOK. As shown in fig. 4, the new optical network unit ONU includes an optical filter, a photodetector, a filter, a signal processing module, a light source, an uplink NRZ data generating module, and a modulator, where the light source and the uplink NRZ data generating module are both connected to the modulator, and the modulator is connected to the 1 st port of the optical circulator; the signal processing module is connected with the 3 rd port of the optical circulator after passing through the filter and the photoelectric detector in sequence; the 2 nd port of the optical circulator is connected with an optical filter. The uplink signal transmitting and processing process comprises the following steps: and the upstream NRZ data generation module is modulated to an optical carrier emitted by the light source through the Mach-Zehnder modulator to carry out information transmission. The downlink signal receiving and processing process comprises the following steps: the signals transmitted by the optical fiber pass through an optical filter to filter the wavelength of the downlink old PON, so that the ONU of the new PON can filter the old PON signals. The optical filter mainly functions to filter out the unwanted old wavelength signal and receive the target new wavelength signal. The Manchester-PAM4 modulated signal is a four-level signal, and each Manchester-PAM4 symbol is divided into two complementary parts. The signal is capable of suppressing low frequency components and maintaining the coding efficiency at 100%.

When the PON link is additionally provided with deployment and upgrading equipment, the new OLT adopting a Manchester-PAM4 modulation mode and the new ONU capable of receiving the new signal in the Manchester-PAM4 format are increased in speed to upgrade the downlink bandwidth of the user. When only new ONU exists in the line, the Manchester-PAM4 format is changed to PAM4 to further upgrade the downstream bandwidth of the user.

In the downlink direction, the downlink signal transmitted by the old OLT and the downlink signal transmitted by the new OLT are multiplexed by WDM1 and transmitted in a single-mode optical fiber together, and are transmitted to the optical network unit after being equally divided by the optical splitter, the old optical network unit can only receive low-speed NRZ signals due to the limited bandwidth of a filter, and high-speed and high-frequency Manchester-PAM4 signals are automatically filtered, so that the new and old signal separation operation in the old optical network unit is realized. In the new optical network unit, because the receiving end of the new optical network unit is provided with the optical filter, the old optical network wavelength can be directly filtered, and therefore only new wavelength signals are received by the new optical network unit. The feasibility of the network architecture is verified through simulation, as shown in fig. 7 and fig. 9. Fig. 7(a) is an eye diagram of an NRZ signal received by an old optical network unit, and (b) is a Manchester-PAM4 signal received by a new optical network unit. As can be seen from fig. 9, the minimum received optical power (error rate 10E-3) for the old NRZ signal is-33 dBm, and the minimum received optical power for the new Manchester-PAM4 signal is-20 dBm. This received optical power meets the power budget of the new optical network. The feasibility of the network architecture of the invention is verified by simulation.

Further, in the uplink direction, the new and old ONUs transmit uplink signals with respective wavelengths, and the uplink signals are coupled by the combiner and transmitted in a single-mode optical fiber, and when the central office is in use, different signals are sent to different OLTs by demultiplexing through WDM 2. Thereby realizing uplink signal transmission. The uplink transmission uses the existing wavelength division multiplexing technology.

When the upgrade is completed and only a new ONU is in the system, the old OLT, the wavelength division multiplexer and other equipment used for distinguishing new and old signals in the central office CO can be deleted; and the mapping relation from PAM4 in the new OLT to signals in the Manchester-PAM4 format is released, for example: four bits are originally transmitted to represent two bits of information, and after the mapping relation is canceled, four bits can represent four bits of information. So far, the new OLT transmits a PAM4 signal, and the system downlink bandwidth is further improved.

The embodiment adopts a network system based on Manchester-PAM4 to upgrade the old passive optical network to the next-generation passive optical network, and has the following advantages:

1) the Manchester-PAM4 was higher in frequency and had less effect on the old signal.

2) Manchester-PAM4 can be switched by software to PAM4 encoding.

3) The network system structure upgrades the equipment in the central office, adds new ONU in the line without changing the old ONU in a large scale, thereby having low cost and high feasibility.

The invention discloses a network system for upgrading an old Passive Optical Network (PON) to a next-generation passive optical network based on Manchester-PAM4, belonging to the field of network upgrading. Wherein the old passive optical network comprises a Central Office (CO) having an old Optical Line Terminal (OLT); a Remote Node (RN) having an optical splitter and a plurality of old Optical Network Units (ONUs) connected by the optical splitter. All devices are connected by Single Mode Fiber (SMF). The network architecture further comprises a new Optical Line Terminal (OLT) of the next-generation PON, a Wavelength Division Multiplexer (WDM) for multiplexing and demultiplexing new and old wavelength signals, and a plurality of new Optical Network Units (ONUs) connected through optical splitters. While sharing the SMF. The modulation mode of the downlink signal of the new OLT adopts Manchester-PAM4, and the Manchester and PAM4 are combined, so that low-frequency components are inhibited, crosstalk in coexistence is reduced, and the coding efficiency is kept at 100%. The invention ensures the normal communication of the new link and the old link in the upgrading process, can finally realize the smooth upgrading of the new PON, has low cost and easy operation, and is suitable for being used in the upgrading of the PON.

It will be apparent to those skilled in the art that the foregoing summary of the invention is not intended to describe each illustrated embodiment or every implementation of the present invention, and that various modifications and substitutions can be made thereto without departing from the spirit and scope of the present invention. Therefore, it is intended that the present invention cover the modifications, substitutions and equivalents of this invention provided they come within the scope of the appended claims and their equivalents.

Claims

1. The system for upgrading the passive optical network based on the combined code of Manchester code and PAM4 is characterized by comprising the following steps:

the remote node: having a beam splitter;

2. The system for upgrading the passive optical network based on the combined code of the Manchester code and the PAM4 as claimed in claim 1, wherein the new optical line terminal comprises a PRBS pseudo-random sequence, a Manchester-PAM4 encoder, a light source, a Mach-Zehnder modulator, a signal processing module, a filter and a photodetector, the PRBS pseudo-random sequence is connected with the Mach-Zehnder modulator through the Manchester-PAM4 encoder, the light source is connected with the Mach-Zehnder modulator, and the Mach-Zehnder modulator is connected with the first wavelength division multiplexer; the second wavelength division multiplexer is connected with the photoelectric detector, and the photoelectric detector is connected with the signal processing module through the filter.

3. The system for upgrading the passive optical network based on the combined code of the Manchester code and the PAM4 of claim 1, wherein the new optical network unit comprises an optical filter, a photoelectric detector, a filter, a signal processing module, a light source, an upstream NRZ data generating module and a modulator, the light source and the upstream NRZ data generating module are both connected with the modulator, and the modulator is connected with the 1 st port of the second optical circulator; the signal processing module is connected with the 3 rd port of the second optical circulator after passing through the filter and the photoelectric detector in sequence; and the 2 nd port of the second optical circulator is connected with an optical filter.

4. A system for upgrading a passive optical network based on a combined code of Manchester code and PAM4 according to any of claims 1-3, wherein the new optical line termination has a different wavelength of light source than the old optical line termination.

5. The system for upgrading the passive optical network based on the combined code of the Manchester code and the PAM4 of claim 2, wherein in the new optical line terminal, the digital signal is converted into a Manchester-PAM4 data format after being subjected to line coding in software, and the data bit rate is set to be 12.5 Gb/s.

6. The system for upgrading the passive optical network based on the combined code of the Manchester code and the PAM4 as claimed in claim 5, wherein the Manchester-PAM4 data format is generated by mapping, and the Manchester-PAM4 data format is modulated into a Manchester-PAM4 waveform in combination with a four-level Mach-Zehnder modulator.