A coherent transmission method based on digital subcarrier multiplexing and a PON system

Abstract

This invention discloses a coherent transmission method and PON system based on digital subcarrier multiplexing, relating to the field of optical access network technology. The method includes: in the downlink transmission direction, the OLT transmitter generates an Alamouti-coded signal of M digital subcarriers, performs digital-to-analog conversion and dual polarization modulation, and transmits it as a downlink signal to the ONU receiver; the ONU receiver performs heterodyne coherent detection, analog-to-digital conversion, and digital signal processing, with the downlink signal occupying a single-sideband spectrum of the ONU's light source; in the uplink transmission direction, the ONU transmitter generates data of M digital subcarriers, performs digital-to-analog conversion, and then performs single-sideband modulation based on the ONU's light source, transmitting it as an uplink signal to the OLT receiver, with the uplink signal occupying another single-sideband spectrum; the OLT receiver performs analog-to-digital conversion and digital signal processing after coherent reception. This application enables bidirectional high-speed transmission, and uses the same light source for both transmission and reception at the ONU, which increases the capacity of the PON system, simplifies the ONU's structure and algorithm, and reduces system costs.

Description

Technical Field

[0001] This application relates to the field of optical access network technology, specifically to a coherent transmission method and PON system based on digital subcarrier multiplexing. Background Technology

[0002] In recent years, driven by 5G mobile internet, cloud networking, and high-definition video streaming services, the bandwidth demand for optical access networks has increased dramatically. EPON (Ethernet Passive Optical Network) and GPON (Gigabit-Capable PON) have already achieved transmission capabilities from 1G / 2.5G to 10G, and the ITU-T and IEEE have recently released standards for 25G PON and 50GPON. As PON develops towards 50Gb / s / λ and above, intensity modulation and direct detection technologies are struggling to meet power budget requirements in high-speed systems due to factors such as poor receiver sensitivity and power fading caused by dispersion at high speeds and long transmission distances.

[0003] Coherent reception is considered a strong candidate technology for PON beyond 100G due to its high spectral efficiency, high sensitivity, and high flexibility. Coherent technology utilizes digital signal processing techniques to achieve various compensations after the received signal is converted to the digital domain, but its complex structure also brings high cost and complexity to the ONU (Optical Network Unit) end.

[0004] The standard coherent detection scheme requires a local oscillator (LO) laser, two 90° polarization mixers, and finally, detection using four balanced photodetectors. Due to the low-cost requirements of PON systems, large-scale deployment in PON applications necessitates simplification of the coherent receiver to reduce costs. Summary of the Invention

[0005] This application provides a coherent transmission method and PON system based on digital subcarrier multiplexing, which can realize bidirectional high-speed transmission between the OLT and ONU ends, thereby increasing the capacity of the PON system and simplifying the structure and algorithm of the ONU to reduce system cost.

[0006] In a first aspect, embodiments of this application provide a coherent transmission method based on digital subcarrier multiplexing, the coherent transmission method based on digital subcarrier multiplexing comprising the following steps:

[0007] In the downlink transmission direction, the OLT transmitter generates an Alamouti coded signal with M digital subcarriers based on digital signal processing, performs digital-to-analog conversion on the Alamouti coded signal, and performs dual polarization modulation, which is then transmitted to the ONU receiver as a downlink signal.

[0008] The ONU receiver performs heterodyne coherent detection on the downlink signal, and performs analog-to-digital conversion and digital signal processing. The downlink signal occupies a single-sided spectrum of the ONU's light source.

[0009] In the uplink transmission direction, the ONU transmitter generates data of M digital subcarriers based on digital signal processing, performs digital-to-analog conversion, and then performs single-sideband modulation based on the ONU end light source, which is then sent to the OLT receiver as an uplink signal. The uplink signal occupies another single-sideband spectrum of the ONU end light source.

[0010] The OLT receiver performs analog-to-digital conversion and digital signal processing after coherent reception.

[0011] In conjunction with the first aspect, in one implementation, the OLT transmitter generates an Alamouti-coded signal with M digital subcarriers based on digital signal processing, including:

[0012] Map the M digital subcarriers and perform Alamouti 16QAM encoding;

[0013] The encoded signal is subjected to pulse shaping, pre-equalization and digital up-conversion processing;

[0014] Carrier aggregation is performed on the processed M digital subcarriers.

[0015] In conjunction with the first aspect, in one embodiment, the ONU receiver performs heterodyne coherent detection on the downlink signal and performs analog-to-digital conversion and digital signal processing, including:

[0016] A coupler is used to couple the downlink signal to the light source at the ONU terminal;

[0017] The coupled signal is detected using a balanced photodetector and converted into an electrical signal;

[0018] The electrical signal is converted from analog to digital using the ADC at the ONU receiver.

[0019] The ONU receiver's DSP performs digital signal processing on the data after analog-to-digital conversion.

[0020] In conjunction with the first aspect, in one embodiment, the OLT receiver uses a light source with the same frequency as the ONU light source for coherent reception.

[0021] In conjunction with the first aspect, in one embodiment, the ONU end light source is a distributed feedback laser or an external cavity laser.

[0022] Secondly, embodiments of this application provide a coherent PON system based on digital subcarrier multiplexing, the coherent PON system based on digital subcarrier multiplexing includes an OLT end and an ONU end;

[0023] The OLT end includes an OLT transmitter and an OLT receiver, and the ONU end includes multiple ONUs, each of which includes an ONU transmitter and an ONU receiver;

[0024] In the downlink transmission direction, the OLT transmitter generates an Alamouti coded signal with M digital subcarriers based on digital signal processing, performs digital-to-analog conversion on the Alamouti coded signal, and performs dual polarization modulation, and sends it to the ONU receiver as a downlink signal;

[0025] The ONU receiver is used to perform heterodyne coherent detection on the downlink signal, and to perform analog-to-digital conversion and digital signal processing. The downlink signal occupies a single-sided spectrum of the ONU light source.

[0026] In the uplink transmission direction, the ONU transmitter generates data of M digital subcarriers based on digital signal processing, performs digital-to-analog conversion, and then performs single-sideband modulation based on the ONU end light source to send as an uplink signal to the OLT receiver. The uplink signal occupies another single-sideband spectrum of the ONU end light source.

[0027] The OLT receiver performs analog-to-digital conversion and digital signal processing after coherent reception.

[0028] In conjunction with the second aspect, in one implementation, the OLT transmitter generates an Alamouti-coded signal with M digital subcarriers based on digital signal processing, including:

[0029] Map the M digital subcarriers and perform Alamouti 16QAM encoding;

[0030] The encoded signal is subjected to pulse shaping, pre-equalization and digital up-conversion processing;

[0031] Carrier aggregation is performed on the processed M digital subcarriers.

[0032] In conjunction with the second aspect, in one embodiment, the ONU receiver includes an ONU light source, a heterodyne coherent receiver, an ONU receiver ADC, and an ONU receiver DSP, wherein the heterodyne coherent receiver includes a coupler and a balanced photodetector.

[0033] The coupler is used to couple the downlink signal and the ONU terminal light source;

[0034] The balanced photodetector is used to detect the coupled signal and convert it into an electrical signal;

[0035] The ONU receiver ADC is used to perform analog-to-digital conversion on the electrical signal;

[0036] The ONU receiver DSP is used to perform digital signal processing on the data after analog-to-digital conversion.

[0037] In conjunction with the second aspect, in one embodiment, the light source used for coherent reception at the OLT receiver has the same frequency as the light source at the ONU.

[0038] In conjunction with the second aspect, in one embodiment, the ONU end light source is a distributed feedback laser or an external cavity laser.

[0039] The beneficial effects of the technical solutions provided in this application include at least the following:

[0040] The coherent transmission method based on digital subcarrier multiplexing in this application involves the following steps: In the downlink transmission direction, the OLT transmitter generates an Alamouti-coded signal with M digital subcarriers based on digital signal processing, performs digital-to-analog conversion on the Alamouti-coded signal, and then performs dual polarization modulation, transmitting it as a downlink signal to the ONU receiver. The ONU receiver performs heterodyne coherent detection on the downlink signal, and then performs analog-to-digital conversion and digital signal processing. The downlink signal occupies one side of the spectrum of the ONU's light source. In the uplink transmission direction, the ONU transmitter generates data with M digital subcarriers based on digital signal processing, performs digital-to-analog conversion, and then performs single-sideband modulation based on the ONU's light source, transmitting it as an uplink signal to the OLT receiver. The uplink signal occupies the other side of the spectrum of the ONU's light source. The OLT receiver performs analog-to-digital conversion and digital signal processing after coherent reception.

[0041] This involves using Digital Subcarrier Multiplexing (DSCM) technology and simplified coherent reception technology in a Passive Optical Network (PON) system to achieve bidirectional high-speed transmission between the OLT and ONU ends. This not only increases system capacity, improves sensitivity and flexibility, but also allows for the use of only one light source at the ONU end for both transmission and reception, greatly simplifying the ONU's structure and algorithms and significantly reducing system costs. Attached Figure Description

[0042] Figure 1 This is a flowchart illustrating an embodiment of the coherent transmission method based on digital subcarrier multiplexing in this application;

[0043] Figure 2 This is a spectrum allocation diagram of uplink and downlink signals in this application;

[0044] Figure 3 This is a simplified diagram of the coherent receiver structure in this application;

[0045] Figure 4This is a flowchart of the DSP processing at the OLT transmitter and ONU receiver in this application.

[0046] Figure 5 This is a structural block diagram of an embodiment of a coherent PON system based on digital subcarrier multiplexing according to this application. Detailed Implementation

[0047] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present application.

[0048] The terms "comprising" and "having," and any variations thereof, in the specification, claims, and accompanying drawings of this application are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not limited to the listed steps or units, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to such process, method, product, or apparatus. The terms "first," "second," and "third," etc., are used to distinguish different objects, etc., and do not indicate a sequence, nor do they limit "first," "second," and "third" to different types.

[0049] In the description of the embodiments of this application, terms such as "exemplary," "for example," or "for instance" are used to indicate examples, illustrations, or explanations. Any embodiment or design described as "exemplary," "for example," or "for instance" in the embodiments of this application should not be construed as being more preferred or advantageous than other embodiments or designs. Specifically, the use of terms such as "exemplary," "for example," or "for instance" is intended to present the relevant concepts in a concrete manner.

[0050] In the description of the embodiments of this application, unless otherwise stated, " / " means "or". For example, A / B can mean A or B. The "and / or" in the text is merely a description of the relationship between related objects, indicating that there can be three relationships. For example, A and / or B can mean: A exists alone, A and B exist simultaneously, and B exists alone. In addition, in the description of the embodiments of this application, "multiple" means two or more.

[0051] In some processes described in the embodiments of this application, multiple operations or steps are included in a specific order. However, it should be understood that these operations or steps may not be executed in the order they appear in the embodiments of this application, or they may be executed in parallel. The sequence number of the operation is only used to distinguish different operations, and the sequence number itself does not represent any execution order. In addition, these processes may include more or fewer operations, and these operations or steps may be executed sequentially or in parallel, and these operations or steps may be combined.

[0052] To make the objectives, technical solutions, and advantages of this application clearer, the embodiments of this application will be described in further detail below with reference to the accompanying drawings.

[0053] In a first aspect, embodiments of this application provide a coherent transmission method based on digital subcarrier multiplexing.

[0054] In one embodiment, reference is made to Figure 1 , Figure 1 This is a schematic flowchart of an embodiment of the coherent transmission method based on digital subcarrier multiplexing according to this application. Figure 1 As shown, the coherent transmission method based on digital subcarrier multiplexing includes:

[0055] S1. In the downlink transmission direction, the OLT (Optical Line Terminal) transmitter generates an Alamouti coded signal with M digital subcarriers based on digital signal processing, performs digital-to-analog conversion on the Alamouti coded signal, and performs dual polarization modulation, and sends it to the ONU receiver as a downlink signal.

[0056] It is worth noting that Alamoti coding and the shift from intrinsic detection to heterodyne detection can effectively reduce the number of components, allowing the ONU receiver to contain only a simple digital-to-analog converter (DAC), a balanced photodetector (BPD), and an analog-to-digital converter (ADC). This greatly simplifies the receiver structure and enables polarization-independent single-polarization coherent reception.

[0057] Digital subcarrier multiplexing (DSCM) uses digital signal processing to distribute the transmitted signal's spectrum across several narrower digital carriers. This method uses a similar bandwidth to single-carrier modulation (using the same modulation format and total transmission capacity), but offers advantages such as better tolerance to fiber nonlinearities and spectral flexibility. Furthermore, coherent systems based on DSCM are particularly well-suited for point-to-multipoint network architectures. Introducing frequency division multiplexing (FDM) into coherent PON systems through DSCM allows a single ONU to modulate and demodulate only one subcarrier, reducing the bandwidth requirements of the ONU and effectively lowering costs.

[0058] S2 and ONU receivers perform heterodyne coherent detection on the downlink signal, and perform analog-to-digital conversion and digital signal processing. The downlink signal occupies a single-sided spectrum of the light source at the ONU end.

[0059] In this embodiment, in the downlink transmission direction, the OLT transmitter uses Alamouti 16QAM encoded data to perform dual-polarization IQ modulation on the optical carrier, and the ONU receiver uses simplified coherence for single-polarization reception.

[0060] Specifically, the signal from the OLT transmitter is processed by digital signal processing to generate an Alamouti-coded 16QAM signal with M digital subcarriers. The generated four signals are then converted from digital to analog by a DAC and modulated by a dual-polarization IQ modulator.

[0061] The light source uses a low-cost source with a frequency of f1, such as a distributed feedback laser (DFB). Alternatively, a better source, such as an external cavity laser (ECL), can be used. A schematic diagram of the modulated optical spectrum is shown below. Figure 2 As shown in the diagram, M subcarriers carried by a single wavelength are transmitted downlink and then received at the ONU by a simplified coherent receiver. The structure of the simplified coherent receiver is as follows. Figure 3 As shown, only a 3dB coupler and a balanced photodetector (BPD) are needed. Simultaneously, a low-cost light source (ONU-side light source) with an output frequency of f2 is sufficient for coherent reception. The ONU receiver performs heterodyne coherent detection on the downlink Alamouti-coded signal, followed by ADC conversion and DSP processing, enabling polarization-independent data reception.

[0062] Specifically, the downlink signal and the ONU light source are coupled using a coupler; the coupled signal is detected using a balanced photodetector and converted into an electrical signal; the electrical signal is converted from analog to digital using the ONU receiver's ADC; and finally, the converted data is processed by the ONU receiver's DSP.

[0063] It is worth noting that in this embodiment, only one light source needs to be set at the ONU end. Because heterodyne coherent detection is adopted, the ONU end light source can use one side spectrum for coherent reception in the downlink transmission direction, and the other side spectrum of the ONU end light source can be used for modulation in the uplink transmission direction. Thus, the structure and algorithm of the ONU can be greatly simplified, and the system cost can be greatly reduced.

[0064] See Figure 4As shown, the DSP at the OLT transmitter includes Alamouti 16QAM encoding, M subcarrier mapping, pulse shaping, pre-equalization, digital up-conversion, and subcarrier aggregation. The DSP at the ONU receiver includes frequency offset estimation using the residual carrier method, phase recovery, subcarrier demultiplexing, data down-conversion, data synchronization, Alamouti equalization, and QAM demodulation.

[0065] S3. In the uplink transmission direction, the ONU transmitter generates data of M digital subcarriers based on digital signal processing, performs digital-to-analog conversion, and then performs single-sideband modulation based on the ONU end light source to send the uplink signal to the OLT receiver. The uplink signal occupies another single-sideband spectrum of the ONU end light source.

[0066] The S4 and OLT receivers perform analog-to-digital conversion and digital signal processing after coherent reception.

[0067] In this embodiment, in the uplink transmission direction, the ONU transmitter uses DP-16QAM data to perform single-sideband modulation on the optical carrier, and the OLT receiver uses standard coherent reception.

[0068] Specifically, the ONU transmitter sends DP-16QAM data with M digital subcarriers, using a dual-polarization IQ modulator for single-sideband modulation. The modulation light source uses the same light source at frequency f2 as the downlink receiver. Because standard dual-polarization coherent modulation reception is used, the required bandwidth is half that of the downlink Alamouti encoded signal. The OLT receiver uses standard coherent reception and requires another local light source at frequency f2 as the LO for coherent reception. The received electrical signal is converted by an ADC and then processed by a DSP.

[0069] Because digital subcarrier multiplexing is used, both uplink and downlink transmission use M subcarriers. Therefore, each time slot can be dynamically allocated to the ONU according to its needs. For example, in time slot 1, subcarriers 1 and 2 are allocated to ONU1, and subcarriers 3 and 4 are allocated to ONU2. In time slot 2, subcarriers 1, 2, and 3 are allocated to ONU2, and subcarrier 4 is allocated to ONU2.

[0070] The bidirectional TFDM-PON architecture implemented using the above scheme employs simplified coherent reception at the ONU end and uses only one light source for both transmission and reception. This not only maintains the advantages of coherent systems—high speed, large capacity, and high power budget—but also significantly reduces costs. Furthermore, it achieves bidirectional transmission on a single wavelength, improving spectrum utilization. The TFDM structure also provides great flexibility in network spectrum allocation.

[0071] In summary, the coherent transmission method based on digital subcarrier multiplexing in this application, in the downlink transmission direction, involves the OLT transmitter generating an Alamouti-coded signal of M digital subcarriers based on digital signal processing, performing digital-to-analog conversion on the Alamouti-coded signal, and then performing dual polarization modulation, transmitting it as a downlink signal to the ONU receiver. The ONU receiver performs heterodyne coherent detection on the downlink signal, and then performs analog-to-digital conversion and digital signal processing, wherein the downlink signal occupies one side of the spectrum of the ONU's light source. In the uplink transmission direction, the ONU transmitter generates data of M digital subcarriers based on digital signal processing, performs digital-to-analog conversion, and then performs single-sideband modulation based on the ONU's light source, transmitting it as an uplink signal to the OLT receiver, wherein the uplink signal occupies the other side of the spectrum of the ONU's light source. The OLT receiver performs analog-to-digital conversion and digital signal processing after coherent reception.

[0072] This involves using Digital Subcarrier Multiplexing (DSCM) technology and simplified coherent reception technology in a Passive Optical Network (PON) system to achieve bidirectional high-speed transmission between the OLT and ONU ends. This not only increases system capacity, improves sensitivity and flexibility, but also allows for the use of only one light source at the ONU end for both transmission and reception, greatly simplifying the ONU's structure and algorithms and significantly reducing system costs.

[0073] Secondly, embodiments of this application also provide a coherent PON system based on digital subcarrier multiplexing.

[0074] In one embodiment, reference is made to Figure 5 , Figure 5 This is a functional block diagram of an embodiment of a coherent PON system based on digital subcarrier multiplexing according to this application. Figure 5 As shown, a coherent PON system based on digital subcarrier multiplexing includes an OLT end and an ONU end.

[0075] The OLT end includes an OLT transmitter and an OLT receiver, and the ONU end includes multiple ONUs, each of which includes an ONU transmitter and an ONU receiver.

[0076] In the downlink transmission direction, the OLT transmitter generates an Alamouti coded signal with M digital subcarriers based on digital signal processing, performs digital-to-analog conversion on the Alamouti coded signal, and performs dual polarization modulation, and sends it to the ONU receiver as a downlink signal;

[0077] The ONU receiver is used to perform heterodyne coherent detection on the downlink signal, and to perform analog-to-digital conversion and digital signal processing. The downlink signal occupies a single-sided spectrum of the ONU light source.

[0078] In the uplink transmission direction, the ONU transmitter generates data of M digital subcarriers based on digital signal processing, performs digital-to-analog conversion, and then performs single-sideband modulation based on the ONU end light source to send as an uplink signal to the OLT receiver. The uplink signal occupies another single-sideband spectrum of the ONU end light source.

[0079] The OLT receiver performs analog-to-digital conversion and digital signal processing after coherent reception.

[0080] Furthermore, in one embodiment, the OLT transmitter generates an Alamouti-coded signal with M digital subcarriers based on digital signal processing, including:

[0081] Map the M digital subcarriers and perform Alamouti 16QAM encoding;

[0082] The encoded signal is subjected to pulse shaping, pre-equalization and digital up-conversion processing;

[0083] Carrier aggregation is performed on the processed M digital subcarriers.

[0084] Furthermore, in one embodiment, the ONU receiver includes an ONU light source, a heterodyne coherent receiver, an ONU receiver ADC, and an ONU receiver DSP, wherein the heterodyne coherent receiver includes a coupler and a balanced photodetector.

[0085] The coupler is used to couple the downlink signal and the ONU terminal light source;

[0086] The balanced photodetector is used to detect the coupled signal and convert it into an electrical signal;

[0087] The ONU receiver ADC is used to perform analog-to-digital conversion on the electrical signal;

[0088] The ONU receiver DSP is used to perform digital signal processing on the data after analog-to-digital conversion.

[0089] Correspondingly, the OLT transmitter includes an OLT transmitter DSP, an OLT transmitter DAC, and a first modulator. The OLT transmitter DSP is used to generate an Alamouti-coded signal with M digital subcarriers based on digital signal processing in the downlink transmission direction; the OLT transmitter DAC is used to perform digital-to-analog conversion on the Alamouti-coded signal; the first modulator is used to perform dual-polarization modulation on the digital-to-analog converted signal to generate a downlink signal for transmission to the ONU receiver; preferably, the first modulator is a dual-polarization IQ modulator (DP-IQM).

[0090] The ONU transmitter includes an ONU transmitter DSP, an ONU transmitter DAC, and a second modulator. The ONU transmitter DSP generates data on M digital subcarriers based on digital signal processing; the ONU transmitter DAC performs digital-to-analog conversion on the data on the M digital subcarriers; the second modulator performs single-sideband modulation based on the ONU light source to generate an uplink signal for transmission to the OLT receiver; preferably, the second modulator is a dual-polarization IQ modulator.

[0091] The OLT receiver includes an OLT coherent receiver, an OLT receiver ADC, and an OLT receiver DSP. The OLT coherent receiver is used to coherently receive the uplink signal; the OLT receiver ADC is used to perform analog-to-digital conversion on the coherently received signal; and the OLT receiver DSP is used to perform digital signal processing on the data processed by the OLT receiver coherent receiver.

[0092] Furthermore, in one embodiment,

[0093] The light source used for coherent reception at the OLT receiver has the same frequency as the light source at the ONU.

[0094] Furthermore, in one embodiment, the ONU end light source is a distributed feedback laser or an external cavity laser.

[0095] The above are merely preferred embodiments of this application and do not limit the patent scope of this application. Any equivalent structural or procedural transformations made using the content of this application's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of this application.

Claims

1. A method of coherent transmission based on digital subcarrier multiplexing, characterized in that, The coherent transmission method based on digital subcarrier multiplexing includes the following steps: In the downlink transmission direction, the OLT transmitter generates an Alamouti coded signal with M digital subcarriers based on digital signal processing, performs digital-to-analog conversion on the Alamouti coded signal, and performs dual polarization modulation, and sends it to the ONU receiver as a downlink signal; The ONU receiver performs heterodyne coherent detection on the downlink signal, and performs analog-to-digital conversion and digital signal processing. The downlink signal occupies a single-sided spectrum of the ONU's light source. In the uplink transmission direction, the ONU transmitter generates data of M digital subcarriers based on digital signal processing, performs digital-to-analog conversion, and then performs single-sideband modulation based on the ONU end light source, which is then sent to the OLT receiver as an uplink signal. The uplink signal occupies another single-sideband spectrum of the ONU end light source. The OLT receiver performs analog-to-digital conversion and digital signal processing after coherent reception. The OLT transmitter generates an Alamouti-coded signal with M digital subcarriers based on digital signal processing, including: Map the M digital subcarriers and perform Alamouti 16QAM encoding; The encoded signal is subjected to pulse shaping, pre-equalization and digital up-conversion processing; Carrier aggregation is performed on the processed M digital subcarriers; The ONU receiver performs heterodyne coherent detection on the downlink signal, and performs analog-to-digital conversion and digital signal processing, including: A coupler is used to couple the downlink signal to the light source at the ONU terminal; The coupled signal is detected using a balanced photodetector and converted into an electrical signal; The electrical signal is converted from analog to digital using the ADC at the ONU receiver. The ONU receiver's DSP performs digital signal processing on the data after analog-to-digital conversion.

2. The coherent transmission method based on digital subcarrier multiplexing as described in claim 1, characterized in that, The OLT receiver uses a light source with the same frequency as the ONU light source for coherent reception.

3. The coherent transmission method based on digital subcarrier multiplexing as described in claim 1, characterized in that, The ONU terminal light source is a distributed feedback laser or an external cavity laser.

4. A coherent PON system based on digital subcarrier multiplexing, characterized in that, The coherent PON system based on digital subcarrier multiplexing includes an OLT end and an ONU end; The OLT end includes an OLT transmitter and an OLT receiver, and the ONU end includes multiple ONUs, each of which includes an ONU transmitter and an ONU receiver; In the downlink transmission direction, the OLT transmitter generates an Alamouti coded signal with M digital subcarriers based on digital signal processing, performs digital-to-analog conversion on the Alamouti coded signal, and performs dual polarization modulation, and sends it to the ONU receiver as a downlink signal; The ONU receiver is used to perform heterodyne coherent detection on the downlink signal, and to perform analog-to-digital conversion and digital signal processing. The downlink signal occupies a single-sided spectrum of the ONU light source. In the uplink transmission direction, the ONU transmitter generates data of M digital subcarriers based on digital signal processing, performs digital-to-analog conversion, and then performs single-sideband modulation based on the ONU end light source to send as an uplink signal to the OLT receiver. The uplink signal occupies another single-sideband spectrum of the ONU end light source. The OLT receiver performs analog-to-digital conversion and digital signal processing after coherent reception. The OLT transmitter generates an Alamouti-coded signal with M digital subcarriers based on digital signal processing, including: Map the M digital subcarriers and perform Alamouti 16QAM encoding; The encoded signal is subjected to pulse shaping, pre-equalization and digital up-conversion processing; Carrier aggregation is performed on the processed M digital subcarriers; The ONU receiver includes an ONU light source, a heterodyne coherent receiver, an ONU receiver ADC, and an ONU receiver DSP. The heterodyne coherent receiver includes a coupler and a balanced photodetector. The coupler is used to couple the downlink signal and the ONU terminal light source; The balanced photodetector is used to detect the coupled signal and convert it into an electrical signal; The ONU receiver ADC is used to perform analog-to-digital conversion on the electrical signal; The ONU receiver DSP is used to perform digital signal processing on the data after analog-to-digital conversion.

5. The coherent PON system based on digital subcarrier multiplexing as described in claim 4, characterized in that: The light source used for coherent reception at the OLT receiver has the same frequency as the light source at the ONU.

6. The coherent PON system based on digital subcarrier multiplexing as described in claim 4, characterized in that, The ONU terminal light source is a distributed feedback laser or an external cavity laser.

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