WO2022015305A1

WO2022015305A1 - Optical network unit (onu) initial equalization tap estimation

Info

Publication number: WO2022015305A1
Application number: PCT/US2020/042174
Authority: WO
Inventors: Frank Effenberger
Original assignee: Futurewei Technologies, Inc.
Priority date: 2020-07-15
Filing date: 2020-07-15
Publication date: 2022-01-20

Abstract

A method implemented by an OLT in a passive optical network (PON), the method comprising: receiving a signal from a new ONU in the PON; obtaining impairment information that is independent of the new ONU; calculating a channel response based on the impairment information; calculating an inverse channel response of the channel response; calculating initial equalization taps based on the inverse channel response; and performing equalization of the signal based on the initial equalization taps. A method implemented by an OLT in a PON, the method comprising: receiving an initial signal from an initial ONU in the PON; performing an initial equalization of the initial signal; and performing a curve fit based on the initial equalization and prior observations.

Description

Optical Network Unit (ONU) Initial Equalization Tap Estimation

CROSS-REFERENCE TO RELATED APPLICATIONS [0001] Not applicable.

TECHNICAL FIELD

[0002] The disclosed embodiments relate to optical networks in general and ONU initial equalization tap estimation in particular.

BACKGROUND

[0003] Optical networks are networks that use light waves, or optical signals, to carry data. Light sources such as lasers generate the optical signals; modulators modulate the optical signals with data to generate modulated optical signals; and various components transmit, propagate, amplify, receive, and process the modulated optical signals. Optical networks implement various forms of multiplexing to achieve high bandwidths. Optical networks implement data centers, metropolitan networks, PONs, longhauls, and other applications.

SUMMARY

[0004] In a first aspect, a method implemented by an OLT in a PON comprises: receiving a signal from a new ONU in the PON; obtaining impairment information that is independent of the new ONU; calculating a channel response based on the impairment information; calculating an inverse channel response of the channel response; calculating initial equalization taps based on the inverse channel response; and performing equalization of the signal based on the initial equalization taps.

[0005] An OLT equalizes a signal from a new ONU using impairment information that is independent of the new ONU. The OLT may store the impairment information before the equalization. This provides more accurate initial equalization taps, which lead to quicker equalization and reduced bandwidth consumption.

[0006] In a first implementation form of the first aspect, the method further comprises obtaining an average transmitter impairment of additional ONUs in the PON, wherein the impairment information comprises the average transmitter impairment. [0007] In a second implementation form of the first aspect or a preceding implementation form of the first aspect, the method further comprises obtaining a current channel impairment, wherein the impairment information comprises the current channel impairment.

[0008] In a third implementation form of the first aspect or a preceding implementation form of the first aspect, the current channel impairment is based on when the OLT receives the signal. [0009] In a fourth implementation form of the first aspect or a preceding implementation form of the first aspect, the current channel impairment is modeled on a fading channel characteristic depending on a level of chromatic dispersion and a total length of fibers.

[0010] In a fifth implementation form of the first aspect or a preceding implementation form of the first aspect, the current channel impairment is zero during a first time interval of a ranging process.

[0011] In a sixth implementation form of the first aspect or a preceding implementation form of the first aspect, the method further comprises storing, after performing the equalization, final equalization taps and a serial number of the new ONU.

[0012] In a seventh implementation form of the first aspect or a preceding implementation form of the first aspect, an OLT in a PON comprises: a memory configured to store instructions; and a processor coupled to the memory and configured to execute the instructions to perform any of the first aspect or a preceding implementation form of the first aspect.

[0013] In an eighth implementation form of the first aspect or a preceding implementation form of the first aspect, a computer program product comprises computer-executable instructions stored on a non-transitory medium that, when executed by a processor, cause an OLT in a PON to perform any of the first aspect or a preceding implementation form of the first aspect.

[0014] In a second aspect, a method implemented by an OLT in a PON comprises: receiving an initial signal from an initial ONU in the PON; performing an initial equalization of the initial signal; and performing a curve fit based on the initial equalization and prior observations.

[0015] In a first implementation form of the second aspect, the method further comprises: receiving a subsequent signal from a subsequent ONU in the PON; performing a subsequent equalization of the subsequent signal based on the curve fit; and updating the curve fit based on the subsequent equalization. [0016] In a second implementation form of the second aspect or a preceding implementation form of the second aspect, the initial ONU has been in use in the PON, and wherein the subsequent ONU is a new ONU that is new to the PON.

[0017] In a third implementation form of the second aspect or a preceding implementation form of the second aspect, the method further comprises performing additional equalizations and updating the curve fit for additional ONUs that join the PON.

[0018] In a fourth implementation form of the second aspect or a preceding implementation form of the second aspect, the prior observations comprise an average transmitter impairment of ONUs in the PON.

[0019] In a fifth implementation form of the second aspect or a preceding implementation form of the second aspect, the prior observations comprise a current channel impairment.

[0020] In a sixth implementation form of the second aspect or a preceding implementation form of the second aspect, the current channel impairment is based on when the OLT receives the initial signal.

[0021] In a seventh implementation form of the second aspect or a preceding implementation form of the second aspect, the current channel impairment is zero during a first time interval of a ranging process.

[0022] In an eighth implementation form of the second aspect or a preceding implementation form of the second aspect, an OLT in a PON comprises a memory configured to store instructions; and a processor coupled to the memory and configured to execute the instructions to perform any of second aspect or a preceding implementation form of the second aspect.

[0023] In a ninth implementation form of the second aspect or a preceding implementation form of the second aspect, a computer program product comprises computer-executable instructions stored on a non-transitory medium that, when executed by a processor, cause an OLT in a PON to perform any of second aspect or a preceding implementation form of the second aspect.

[0024] In a third aspect, a method implemented by an OLT comprises: performing a first equalization for a first PON; performing a first curve fit for the first PON based on the first equalization; and performing a second equalization for a second PON based on the first curve fit. [0025] In a first implementation form of the third aspect, the method further comprises performing a second curve fit for the second PON based on the second equalization. [0026] In a second implementation form of the third aspect or a preceding implementation form of the third aspect, an OLT comprises: a memory configured to store instructions; and a processor coupled to the memory and configured to execute the instructions to perform any of the third aspect or a preceding implementation form of the third aspect.

[0027] In a second implementation form of the third aspect or a preceding implementation form of the third aspect, a computer program product comprises computer-executable instructions stored on a non-transitory medium that, when executed by a processor, cause an OLT to perform any of the third aspect or a preceding implementation form of the third aspect. [0028] Any of the above embodiments may be combined with any of the other above embodiments to create a new embodiment. These and other features will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS [0029] For a more complete understanding of this disclosure, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts.

[0030] FIG. 1 is a schematic diagram of a PON.

[0031] FIG. 2 is a schematic diagram of an apparatus according to an embodiment.

[0032] FIG. 3 is a flowchart illustrating a method of equalization according to a first embodiment.

[0033] FIG. 4 is a flowchart illustrating a method of equalization according to a second embodiment.

[0034] FIG. 5 is a schematic diagram of a PON network.

[0035] FIG. 6 is a flowchart illustrating a method of equalization according to a third embodiment.

DETAILED DESCRIPTION

[0036] It should be understood at the outset that, although an illustrative implementation of one or more embodiments are provided below, the disclosed systems and/or methods may be implemented using any number of techniques, whether currently known or in existence. The disclosure should in no way be limited to the illustrative implementations, drawings, and techniques illustrated below, including the exemplary designs and implementations illustrated and described herein, but may be modified within the scope of the appended claims along with their full scope of equivalents.

[0037] The following abbreviations apply:

ASIC: application-specific integrated circuit

CO: central office

CPU: central processing unit

DSP: digital signal processor

EO: electrical -to-optical

FPGA: field-programmable gate array

ISI: inter-symbol interference km: kilometer(s)

MAC: media access control ODN: optical distribution network OE: optical -to-electrical OLT : optical line terminal ONT : optical network terminal ONU: optical network unit PON: passive optical network P2MP: point-to-multipoint RAM: random-access memory RF : radio frequency ROM: read-only memory RX: receiver unit SRAM: static RAM

TCAM: ternary content-addressable memory TDMA: time-division multiple access TX: transmitter unit ps: microsecond(s). [0038] FIG. 1 is a schematic diagram of a PON 100. The PON 100 comprises an OLT 110, ONUs 120, and an ODN 130 that couples the OLT 110 to the ONUs 120. The PON 100 is a communications network that may not require active components to distribute data between the OLT 110 and the ONUs 120. Instead, the PON 100 may use passive optical components in the ODN 130 to distribute data between the OLT 110 and the ONUs 120.

[0039] The OLT 110 communicates with another network and with the ONUs 120. Specifically, the OLT 110 is an intermediary between the other network and the ONUs 120. For instance, the OLT 110 forwards data received from the other network to the ONUs 120 and forwards data received from the ONUs 120 to the other network. The OLT 110 comprises a transmitter and a receiver. When the other network uses a network protocol that is different from the protocol used in the PON 100, the OLT 110 comprises a converter that converts the network protocol to the PON protocol and vice versa. The OLT 110 is typically located at a central location such as a CO, but it may also be located at other suitable locations.

[0040] The ODN 130 is a data distribution network that comprises optical fiber cables, couplers, splitters, distributors, and other suitable components. The components include passive optical components that do not require power to distribute signals between the OLT 110 and the ONUs 120. The ODN 130 extends from the OLT 110 to the ONUs 120 in a branching configuration as shown, but the ODN 130 may be configured in any other suitable P2MP configuration.

[0041] The ONUs 120 communicate with the OLT 110 and customers and act as intermediaries between the OLT 110 and the customers. For instance, the ONUs 120 forward data from the OLT 110 to the customers and forward data from the customers to the OLT 110. The ONUs 120 comprise optical transceivers that receive optical signals from the OLT 110, convert the optical signals into electrical signals, and provide the electrical signals to the customers. The transceivers also receive electrical signals from the customers, convert the electrical signals into optical signals, and transmit the optical signals to the OLT 110. ONUs 120 and ONTs are similar, and the terms may be used interchangeably. The ONUs 120 are typically located at distributed locations such as customer premises, but they may also be located at other suitable locations.

[0042] The PON 100 is developing to operate at higher speeds, while having a limited bandwidth. The limited bandwidth causes ISI, which in turn reduces communication performance absent compensation. To reduce the ISI, the OLT 110 may implement a DSP-based equalizer. Because the components of the PON 100 act as low-pass filters, the equalizer boosts higher frequencies to provide flatter signals. The equalizer does so in an adaptive manner by adapting equalization taps, or coefficients.

[0043] Each ONU 120 may transmit with different impairments because the ONUs 120 may have manufacturing variations and different fiber lengths. The OLT 110 therefore stores equalization taps for each ONU 120 and pre-loads those equalization taps before receiving signals from respective ONUs 120. The OLT 110 knows when the ONUs 120 are to transmit signals, and thus when the OLT 110 is to receive those signals, because the PON 100 is a TDMA PON in which the ONUs 120 transmit bursts at allotted times.

[0044] However, when new ONUs 120 join the PON 100, the OLT 110 does not have accurate initial equalization taps for those ONUs 120. In some approaches, the OLT 110 begins from arbitrary initial equalization taps, but those initial equalization taps may be poor estimates of final equalization taps and cause significant delay and bandwidth consumption in obtaining final equalization taps and completing equalization. There is therefore a desire to obtain accurate initial equalization taps for the ONUs 120.

[0045] Disclosed herein are embodiments for ONU initial equalization tap estimation. In a first embodiment, an OLT equalizes a signal from a new ONU using impairment information that is independent of the new ONU. The OLT may store the impairment information before the equalization. In a second embodiment, the OLT performs an initial equalization on an initial signal from an initial ONU, performs a curve fit based on the initial equalization, performs a subsequent equalization of a subsequent signal from a subsequent ONU based on the curve fit, and updates the curve fit based on the subsequent equalization. The OLT may perform future equalizations using initial equalization taps from the curve fit. In a third embodiment, the OLT performs a first curve fit based on equalization for a first PON, then uses that curve fit for equalization for a second PON. Thus, the OLT shares curve fits, and thus initial equalization taps, among PONs. The embodiments provide more accurate initial equalization taps, which lead to quicker equalization and reduced bandwidth consumption.

[0046] FIG. 2 is a schematic diagram of an apparatus 200 according to an embodiment. The apparatus 200 may implement the disclosed embodiments. For instance, the apparatus 200 implements the OLT 110 in some embodiments. Further, the apparatus 200 implements the ONU 120 in some embodiments (including both the OLT 110 and the ONU 120 in some embodiments). The apparatus 200 comprises ingress ports 210 and an RX 220 or receiving means to receive data; a processor 230 or logic unit, baseband unit, CPU, or processing means to process the data; a TX 240 or transmitting means and egress ports 250 to transmit the data; and a memory 260 or data storing means to store the data. The apparatus 200 may also comprise OE components, EO components, or RF components coupled to the ingress ports 210, the RX 220, the TX 240, and the egress ports 250 to provide ingress or egress of optical signals, electrical signals, or RF signals.

[0047] The processor 230 is any combination of hardware, middleware, firmware, or software. The processor 230 comprises any combination of one or more CPU chips, cores, FPGAs, ASICs, or DSPs. The processor 230 communicates with the ingress ports 210, the RX 220, the TX 240, the egress ports 20, and the memory 260. The processor 230 comprises an equalization component 270, which implements the disclosed embodiments. The equalization component 270 in some embodiments comprises software or instructions to be executed by the processor 230. The inclusion of the equalization component 270 therefore provides a substantial improvement to the functionality of the apparatus 200 and effects a transformation of the apparatus 200 to a different state. Alternatively, the memory 260 stores the equalization component 270 as instructions, and the processor 230 executes those instructions. The equalization component 270 may implement MAC and DSP functions in order perform equalization.

[0048] The memory 260 comprises any combination of disks, tape drives, or solid-state drives. The apparatus 200 may use the memory 260 as an over-flow data storage device to store programs when the apparatus 200 selects those programs for execution and to store instructions and data that the apparatus 200 reads during execution of those programs, for instance as a computer program product. The memory 260 may be volatile or non-volatile and may be any combination of ROM, RAM, TCAM, or SRAM.

[0049] A computer program product may comprise computer-executable instructions stored on a non-transitory medium, for instance the memory 260, that when executed by a processor, for instance the processor 230, cause an apparatus to perform any of the embodiments.

[0050] FIG. 3 is a flowchart illustrating a method 300 of equalization according to a first embodiment. The OLT 110 performs the method 300. [0051] At step 310, a signal is received from a new ONU. For instance, the ingress ports 210 and the RX 220 receive the signal from a new ONU 120. The new ONU 120 may have just activated itself and connected to the PON 100.

[0052] At step 320, an average transmitter impairment is obtained. For instance, manufacturers of the ONUs 120 calculate transmitter impairments of the ONUs 120 in a laboratory and provide those impairments. A manufacturer or an operator of the OLT 110 calculates an average of those impairments and stores that average as the average transmitter impairment in the memory 260. The average transmitter impairment may be an average of transmitter impairments of all ONUs 120 in the PON 100, a subset of those ONUs 120, a representative sample of those ONUs 120, or all ONUs 120 that have completed ranging or another process. The equalization component 270 then obtains the average transmitter impairment from the memory 260.

[0053] At step 330, a current channel impairment is obtained. A channel impairment of the new ONU 120 may be modeled on a fading channel characteristic that depends on a level of chromatic dispersion and a total length of fibers connecting the new ONU 120 to the OLT 110. Because the new ONU 120 is new, the equalization component 270 does not yet know the total length of those fibers. However, during a ranging process, the ONUs 120 that are closer to the OLT 110 than the new ONU 120 will respond to the OLT 110 before the new ONU 120 responds, due to the shorter amount of time it takes signals to travel across the shorter fibers in the ODN 130. Similarly, the ONUs 120 that are farther from the OLT 110 than the new ONU 120 will respond to the OLT 110 after the new ONU 120 responds. The distance between the ONUs 120 and the OLT 110, and thus lengths of corresponding fibers in the ODN 130, are linearly dependent on a ranging delay.

[0054] Thus, at the beginning of the ranging process, or during a first time interval of the ranging process when the ONUs 120 closest to the OLT 110 respond, the equalization component 270 assumes zero channel impairment because the ONUs 120 closest to the OLT 110 may be essentially co-located with the OLT 110. As the ranging process proceeds and time elapses, the equalization component 270 increases the channel impairment. The equalization component 270 may increase the channel impairment at specified intervals, for instance every 10 ps of elapsed time. A manufacturer or an operator of the OLT 110 may set a length of the intervals based on the channel response. For instance, the manufacturer or the operator may set the length to be short enough that the channel response changes less than a desired amount during the interval. At each interval, the channel impairment may be an average of calculated channel impairments from the prior interval, a greatest calculated channel impairment from the prior interval, or a predetermined channel impairment. At some point, the new ONU 120 responds, and at that point the equalization component 270 obtains the current channel impairment, which is the last available channel impairment.

[0055] Together, the average transmitter impairment and the current channel impairment may be referred to as impairment information. The impairment information is independent of the new ONU 120. Specifically, the equalization component 270 knows values of the impairment information without knowing anything about the new ONU 120 other than when the OLT 110 receives a response from the new ONU 120.

[0056] At step 340, a channel response is calculated based on the average transmitter impairment and the current channel impairment. The equalization component 270 may do so by adding the average transmitter impairment and the channel impairment.

[0057] At step 350, an inverse channel response of the channel response is calculated. The inverse channel response is a function of frequency. For instance, the inverse channel response H ¹ is as follows:

H ¹ = 1 + 2TtjF (1) where j is a unit imaginary number such that j = V— Ϊ, and F is a frequency.

[0058] At step 360, initial equalization taps are calculated based on the inverse channel response. For instance, the OLT 110 calculates a Fourier transform of the channel response, calculates an impulse response corresponding to the Fourier transform, performs sampling of the impulse response, and calculates the initial equalization taps based on the sampling.

[0059] Finally, at step 370, equalization of the signal is performed based on the initial equalization taps. For instance, during a first iteration of the equalization, the equalization component 270 uses the initial equalization taps. The equalization component 270 updates the initial equalization taps to current equalization taps based on the first iteration. The equalization component 270 iterates the updating until an equalized value of the signal converges and the equalization component 270 obtains a serial number of the new ONU 120. The equalization may employ any known method. After completing the iterating and thus the equalization, the equalization component 270 stores final equalization taps and the serial number in the memory 260.

[0060] FIG. 4 is a flowchart illustrating a method 400 of equalization according to a second embodiment. The OLT 110 performs the method 400.

[0061] At step 410, an initial signal is received from an initial ONU. For instance, the ingress ports 210 and the RX 220 receive the initial signal from an ONU 120 that has been in use in the PON 100. Thus, the initial ONU 120 may have communicated signals with the OLT 110 before the initial signal.

[0062] At step 420, an initial equalization of the initial signal is performed. For instance, the equalization component 270 performs an initial equalization like in step 370 in the method 300. [0063] At step 430, a curve fit is performed based on the initial equalization and prior observations. For instance, the equalization component 270 performs a curve fit of the final equalization taps from step 420 in the method 400 onto prior observations obtained from prior activity. For instance, the prior observations may be the average transmitter impairment in step 320 in the method 300, the current channel impairment in step 330 in the method 300, or both the average transmitter impairment in step 320 and the current channel impairment in step 330. The curve fit then provides estimates for initial equalization taps for future equalizations. The curve fit may also be referred to as interpolation or extrapolation.

[0064] As an example of a curve fit, the OLT 110 has equalized a first ONU 120 that is 10 km away from the OLT 110 and a second ONU 120 that is 20 km away from the OLT 110. Thus, the OLT 110 knows a first channel response of the first ONU 120 and a second channel response of the second ONU 120. The OLT 110 may then assume that a third channel response of a third ONU 120 that is 15 km away from the OLT 110 is an average of the first channel response and the second channel response. Though the example is linear, the curve fit may also be non-linear.

[0065] At step 440, a subsequent signal is received from a subsequent ONU. For instance, the ingress ports 210 and the RX 220 receive the subsequent signal from a new ONU 120 that is new to the PON 100. Thus, the new ONU 120 may have just activated itself and connected to the PON 100, and the subsequent signal may be the first signal from the new ONU 120 to the OLT 110. [0066] In step 450, a subsequent equalization of the subsequent signal is performed based on the curve fit. For instance, starting with initial equalization taps from the curve fit, the equalization component 270 performs a subsequent equalization like in step 370 in the method 300. The subsequent equalization will complete sooner than it otherwise would without the curve fit because the initial equalization taps will be more accurate.

[0067] Finally, at step 460, the curve fit is updated based on the subsequent equalization. For instance, the equalization component updates the curve fit using the final equalization taps from step 450.

[0068] Steps 440-460 may repeat as long as the OLT 110 is in operation. As more ONUs 120 join the PON 100 and as the OLT 110 performs additional equalizations of signals from those ONUs 120 and updates the curve fit, the curve fit will become more accurate. As a result, each additional equalization of each subsequent ONU 120 will start with more accurate initial equalization taps and will complete sooner.

[0069] FIG. 5 is a schematic diagram of a PON network 500. The PON network 500 comprises three PONs 505, 525, 540, which are similar to the PON 100. The PONs 505, 525, 540 share an OLT 520. The PON 505 comprises ONUs 510 and an ODN 515, the PON 525 comprises an ODN 530 and ONUs 535, and the PON 540 comprises an ODN 545 and ONUs 550. Though 3 PONs 505, 525, 540 and 12 ONUs 510, 535, 550 are shown, the OLT 520 may support about 128 PONs and about 4,000 ONUs.

[0070] FIG. 6 is a flowchart illustrating a method 600 of equalization according to a third embodiment. The OLT 520 performs the method 600.

[0071] At step 610, a first equalization is performed for a first PON. For instance, the OLT 520 performs equalization of one or more of the ONUs 510 in the PON 505. The OLT 520 may do so in a manner similar to step 420 in the method 400.

[0072] At step 620, a first curve fit is performed for the first PON based on the first equalization. For instance, the OLT 520 performs the first curve fit in a manner similar to step 430 in the method 400.

[0073] At step 630, equalization is performed for a second PON based on the first curve fit. For instance, the OLT 520 performs a second equalization of one or more of the ONUs 535 in the PON 525. The OLT 520 may do so in a manner similar to step 450 in the method 400. [0074] Finally, at step 640, a second curve fit is performed for the second PON based on the second equalization. For instance, the OLT 520 performs the second curve fit in a manner similar to step 460 in the method 400.

[0075] Though the OLT 520 uses the first curve fit from the PON 505 to equalize ONUs 535 in the PON 525, the OLT 520 may also use the first curve fit from the PON 505 to equalize ONUs 550 in the PON 540. Thus, the OLT 520 may use data, including curve fittings, from one PON 505, 525, 540 to perform equalization of another PON 505, 525, 540. Because the OLT 520 may support 128 PONs and about 4,000 ONUs, the OLT 520 may obtain and use statistically-significant data for equalization. In addition, the OLT 520 may share that data with other OLTs or other networks in a central office or even another city.

[0076] The terms “about,” “approximately,” and their derivatives mean within ±10% of a subsequent modifier. While several embodiments have been provided in the present disclosure, it may be understood that the disclosed systems and methods might be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted, or not implemented.

[0077] In addition, techniques, systems, subsystems, and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, components, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as coupled may be directly coupled or may be indirectly coupled or communicating through some interface, device, or intermediate component whether electrically, mechanically, or otherwise. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and may be made without departing from the spirit and scope disclosed herein.

Claims

CLAIMS What is claimed is:

1. A method implemented by an optical line terminal (OLT) in a passive optical network (PON), the method comprising: receiving a signal from a new optical network unit (ONU) in the PON; obtaining impairment information that is independent of the new ONU; calculating a channel response based on the impairment information; calculating an inverse channel response of the channel response; calculating initial equalization taps based on the inverse channel response; and performing equalization of the signal based on the initial equalization taps.

2. The method of claim 1, further comprising obtaining an average transmitter impairment of additional ONUs in the PON, wherein the impairment information comprises the average transmitter impairment.

3. The method of claim 1, further comprising obtaining a current channel impairment, wherein the impairment information comprises the current channel impairment.

4. The method of claim 3, wherein the current channel impairment is based on when the OLT receives the signal.

5. The method of claim 3, wherein the current channel impairment is modeled on a fading channel characteristic depending on a level of chromatic dispersion and a total length of fibers.

6. The method of claim 3, wherein the current channel impairment is zero during a first time interval of a ranging process.

7. The method of claim 1, further comprising storing, after performing the equalization, final equalization taps and a serial number of the new ONU.

8. An optical line terminal (OLT) in a passive optical network (PON), the OLT comprising: a memory configured to store instructions; and a processor coupled to the memory and configured to execute the instructions to perform any of claims 1-7.

9. A computer program product comprising computer-executable instructions stored on a non-transitory medium that, when executed by a processor, cause an optical line terminal (OLT) in a passive optical network (PON) to perform any of claims 1-7.

10. A method implemented by an optical line terminal (OLT) in a passive optical network (PON), the method comprising: receiving an initial signal from an initial optical network unit (ONU) in the PON; performing an initial equalization of the initial signal; and performing a curve fit based on the initial equalization and prior observations.

11. The method of claim 10, further comprising: receiving a subsequent signal from a subsequent ONU in the PON; performing a subsequent equalization of the subsequent signal based on the curve fit; and updating the curve fit based on the subsequent equalization.

12. The method of claim 11, wherein the initial ONU has been in use in the PON, and wherein the subsequent ONU is a new ONU that is new to the PON.

13. The method of claim 11, further comprising performing additional equalizations and updating the curve fit for additional ONUs that join the PON.

14. The method of claim 10, wherein the prior observations comprise an average transmitter impairment of ONUs in the PON.

15. The method of claim 14, wherein the prior observations comprise a current channel impairment.

16. The method of claim 15, wherein the current channel impairment is based on when the OLT receives the initial signal.

17. The method of claim 16, wherein the current channel impairment is zero during a first time interval of a ranging process.

18. An optical line terminal (OLT) in a passive optical network (PON), the OLT comprising: a memory configured to store instructions; and a processor coupled to the memory and configured to execute the instructions to perform any of claims 10-17.

19. A computer program product comprising computer-executable instructions stored on a non-transitory medium that, when executed by a processor, cause an optical line terminal (OLT) in a passive optical network (PON) to perform any of claims 10-17.

20. A method implemented by an optical line terminal (OLT), the method comprising: performing a first equalization for a first passive optical network (PON); performing a first curve fit for the first PON based on the first equalization; and performing a second equalization for a second PON based on the first curve fit.

21. The method of claim 20, further comprising performing a second curve fit for the second PON based on the second equalization.

22. An optical line terminal (OLT) comprising: a memory configured to store instructions; and a processor coupled to the memory and configured to execute the instructions to perform any of claims 20-21.

23. A computer program product comprising computer-executable instructions stored on a non-transitory medium that, when executed by a processor, cause an optical line terminal (OLT) to perform any of claims 20-21.