Methods and apparatus relating to optical transmissions

Abstract

A method is performed by a system comprising an optical transmitter and an optical receiver coupled by an optical transmission medium. The optical receiver comprises a plurality of chromatic dispersion compensation (CDC) circuitry stages. The method comprises, in the optical transmitter: determining an amount of data to be transmitted over the optical transmission medium to the optical receiver; using the amount of data to set a bit repetition factor; encoding data into a sequence of bits, in which each bit is repeated a number of times based on the bit repetition factor; and transmitting, to the optical receiver over the optical transmission medium, an optical transmission comprising the sequence of bits. The method further comprises, in the optical receiver: receiving the optical transmission; determining a number of the plurality of CDC circuitry stages to be applied to the optical transmission; and applying the determined number of CDC circuitry stages to the optical transmission. The disclosure also provides apparatus and further methods in an optical transmitter and an optical receiver.

Description

[0001] METHODS AND APPARATUS RELATING TO OPTICAL TRANSMISSIONS

[0002] Technical field

[0003] Embodiments of the disclosure relate to optical communications, and particularly to methods, apparatus and computer-readable media for controlling optical transmissions.

[0004] Background

[0005] Optical transmission over fibers is impacted by chromatic dispersion; that is, the phenomenon whereby the velocity of an electromagnetic signal propagating through the fibre will depend on the spectrum of the transmitted signal. The different spectral components of the signal propagate at slightly different speeds through the fiber. For a given modulation scheme, the impact of chromatic dispersion varies quadratically with the bit rate of the signal: the higher the bit rate, the higher the impact of chromatic dispersion. This impairs the ability for optical communication to reach longer distances, particularly at higher data rates.

[0006] For this reason, long-distance optical communication technology has developed Coherent Detection with Chromatic Dispersion Compensation (CDC) to carry 100G and above (400G, 800G, 1.6T) over distances from 40km up to thousands of kilometers. Such coherent technologies are typically oriented towards performance rather than energy efficiency, with power consumption being limited only by the heat dissipation of the transceiver cage. Other non-coherent technologies have been developed, such as Direct Detection with phase-amplitude modulation (PAM), but they can typically only operate on point-to-point (P2P) links over short distances.

[0007] The circuitry to support digital signal processing (DSP) for Chromatic Dispersion Compensation (CDC) contributes significantly to the power consumption of an optical transceiver, accounting for up to 30% additional power consumption and consequential heating. Such a power consumption might be acceptable for long-distance applications, such as in core networks, where link performances are stretched, but it might be excessive for shorter-distance applications, such as metro and access networks, where cost and energy efficiency are competitive advantages for the telecom operator. Summary

[0008] It is an object of embodiments of the present disclosure to address these and other problems.

[0009] In particular, embodiments of the disclosure provide methods, apparatus, systems and other solutions that enable chromatic dispersion compensation circuitry (and the consequential power usage of such circuitry) to be controlled intelligently so that it is used when needed and deactivated or otherwise put into a low-power state when not needed.

[0010] In a first aspect, the disclosure provides a method performed by a system comprising an optical transmitter and an optical receiver coupled by an optical transmission medium. The optical receiver comprises a plurality of chromatic dispersion compensation (CDC) circuitry stages. The method comprises, in the optical transmitter: encoding data into a sequence of bits, in which each bit is repeated a number of times based on the bit repetition factor; and transmitting, to the optical receiver over the optical transmission medium, an optical transmission comprising the sequence of bits. The method further comprises, in the optical receiver: receiving the optical transmission; determining a number of the plurality of CDC circuitry stages to be applied to the optical transmission; and applying the determined number of CDC circuitry stages to the optical transmission.

[0011] In some examples, the method comprises determining an amount of data to be transmitted over the optical transmission medium to the optical receiver; and using the amount of data to set a bit repetition factor.

[0012] The disclosure further provides a system apparatus for performing the method set out above. The system therefore comprises an optical transmitter; and an optical receiver, coupled to the optical transmitter by an optical transmission medium. The optical transmitter comprises processing circuitry configured to cause the optical transmitter to: encode data into a sequence of bits, in which each bit is repeated a number of times based on a bit repetition factor; and transmit, to the optical receiver over the optical transmission medium, an optical transmission comprising the sequence of bits. The optical receiver comprises a plurality of chromatic dispersion compensation, CDC, circuitry stages, and processing circuitry configured to cause the optical transmitter to: receive the optical transmission; determine a number of the plurality of CDC circuitry stages to be applied to the optical transmission; and apply the determined number of CDC circuitry stages to the optical transmission.

[0013] In some examples, the processing circuitry is configured to cause the optical transmitter to: determine an amount of data to be transmitted over the optical transmission medium to the optical receiver; and use the amount of data to set a bit repetition factor.

[0014] The disclosure further provides, in one aspect, a method performed by an optical transmitter. The method comprises: encoding data into a sequence of bits, in which each bit is repeated a number of times based on the bit repetition factor; and transmitting, to the optical receiver over the optical transmission medium, an optical transmission comprising the sequence of bits.

[0015] In some examples, the method comprises determining an amount of data to be transmitted over an optical transmission medium to an optical receiver; and using the amount of data to set a bit repetition factor.

[0016] Apparatus configured to perform this method is also provided. For example, an optical transmitter comprises: processing circuitry, configured to cause the optical transmitter to: encode data into a sequence of bits, in which each bit is repeated a number of times based on the bit repetition factor; and transmit, to the optical receiver over the optical transmission medium, an optical transmission comprising the sequence of bits.

[0017] In some examples, the processing circuitry is configured to determine an amount of data to be transmitted over an optical transmission medium to an optical receiver; and use the amount of data to set a bit repetition factor.

[0018] The disclosure further provides, in another aspect, a method performed by an optical receiver. The optical receiver comprises a plurality of chromatic dispersion compensation, CDC, circuitry stages. The method comprises: receiving, from an optical transmitter over an optical transmission medium, an optical transmission comprising a sequence of bits, in which each bit is repeated a number of times based on a bit repetition factor; determining a number of the plurality of CDC circuitry stages to be applied to the optical transmission; and applying the determined number of CDC circuitry stages to the optical transmission. Apparatus configured to perform this method is also provided. For example, an optical receiver comprises a plurality of chromatic dispersion compensation, CDC, circuitry stages, and processing circuitry configured to cause the optical receiver to: receive, from an optical transmitter over an optical transmission medium, an optical transmission comprising a sequence of bits, in which each bit is repeated a number of times based on a bit repetition factor; determine a number of the plurality of CDC circuitry stages to be applied to the optical transmission; and apply the determined number of CDC circuitry stages to the optical transmission.

[0019] Brief description of the drawings

[0020] For a better understanding of the embodiments of the present disclosure, and to show how it may be put into effect, reference will now be made, by way of example only, to the accompanying drawings, in which:

[0021] Figure 1 is a schematic diagram showing an optical system according to embodiments of the disclosure;

[0022] Figure 2 is a flowchart of a method in a first optical transceiver according to embodiments of the disclosure;

[0023] Figure 3 is a schematic diagram showing the variation of spectrum width of an optical transmission according to embodiments of the disclosure;

[0024] Figure 4 is a flowchart of a method in a second optical transceiver according to embodiments of the disclosure;

[0025] Figure 5 shows first and second optical transceivers according to embodiments of the disclosure;

[0026] Figure 6 shows first and second optical transceivers according to further embodiments of the disclosure;

[0027] Figure 7 shows receiver circuitry of an optical transceiver according to embodiments of the disclosure; Figure 8 shows an example of optical transmissions transmitted according to methods of the disclosure;

[0028] Figure 9 is a schematic diagram of a second optical transceiver according to embodiments of the disclosure; and

[0029] Figure 10 is a schematic diagram of a first optical transceiver according to embodiments of the disclosure.

[0030] Detailed description

[0031] Some of the embodiments contemplated herein will now be described more fully with reference to the accompanying drawings. Other embodiments, however, are contained within the scope of the subject matter disclosed herein. The disclosed subject matter should not be construed as limited to only the embodiments set forth herein; rather, these embodiments are provided by way of example to convey the scope of the subject matter to those skilled in the art.

[0032] The embodiments set forth below represent information to enable those skilled in the art to practice the embodiments. Upon reading the following description in light of the accompanying figures, those skilled in the art will understand the concepts of the description and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the description.

[0033] In the following description, numerous specific details are set forth. However, it is understood that embodiments may be practiced without these specific details. In other instances, well-known circuits, structures, and techniques have not been shown in detail in order not to obscure the understanding of the description. Those of ordinary skill in the art, with the included description, will be able to implement appropriate functionality without undue experimentation.

[0034] References in the specification to “one embodiment,” “an embodiment,” “an example embodiment,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to implement such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

[0035] As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” and / or “including” when used herein, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0036] Embodiments of the present disclosure relate to methods, systems, apparatus and computer-readable media for optical transmissions. A method in a second (receiving) optical transceiver controls the amount of chromatic dispersion compensation (CDC) applied when receiving an optical transmission from a first (transmitting) optical transceiver, based on a spectrum of the optical transmission. For example, some or all of the CDC circuitry within the second optical transceiver may be disabled if not required, therefore reducing power consumption in the second optical transceiver. The disclosure also provides a method in the first (transmitting) optical transceiver, providing an intelligent way to control the spectrum used in an optical transmission to the second optical transceiver, e.g., based on an amount of data that is to be transmitted. Those skilled in the art will appreciate that use of the method in the second optical transceiver, for receiving an optical transmission, does not depend on or require use of the method in the first optical transceiver, for transmitting the optical transmission. However, use of both methods, in a system comprising both the first and second optical transceivers, is particularly advantageous. The disclosure therefore also provides a system and corresponding method combining both methods, as will be set out in greater detail below.

[0037] Figure 1 is a schematic diagram showing an optical system 100 according to embodiments of the disclosure.

[0038] The system 100 comprises a first optical network element 102 and a second optical network element 104, connected by an optical link 106. Each optical network element is generally capable of both transmitting and receiving optical transmissions over the optical link, and therefore the terms “optical network element” and “optical transceiver” are used interchangeably herein. For ease of discussion, the first optical transceiver 102 is described in the context of a device which transmits an optical transmission (and thus the first optical transceiver 102 may also be termed “an optical transmitter”), and the second optical transceiver 104 is described in the context of a device which receives that optical transmission (and thus the second optical transceiver 104 may also be termed “an optical receiver”). However, the disclosure specifically contemplates a system in which both the first and second optical transceivers 102, 104 transmit optical transmissions (e.g., using the methods described herein) and receive those optical transmissions (e.g., using the methods described herein).

[0039] The optical link 106 may be any optical transmission medium suitable for the transportation of optical signals between the transceivers, such as an optical fiber (e.g., single, double or multiple fiber strands). Signals may be transmitted between the first and second optical transceivers using the same optical strand (duplex operation) or different strands (simplex operation). The signals may use the same frequency or different frequencies. The optical link 106 may also be referred to as an “optical line” or simply “line” herein.

[0040] Figure 2 illustrates a method according to embodiments of the present disclosure. The method may be performed by a first optical network element (e.g., a first optical transceiver), such as the first optical transceiver 102 described above, and / or any of the first optical transceivers 500, 600 and 900 described below.

[0041] The method begins at step 202, in which the first optical transceiver determines an amount of data to be transmitted. That is, the first optical transceiver receives data from a client which is local to or remote from the first optical transceiver, for transmission over an optical link to a second optical transceiver. The data may be input to a buffer at the first optical transceiver, and output from the buffer to be encoded into optical transmissions. The amount of data in the buffer may therefore be used as an indication of the amount of data to be transmitted. Note that the amount of data may be expressed in absolute values (e.g., in terms of a number of bytes or other units), or relative values (e.g., expressed as a fill amount of the buffer). Additionally or alternatively, data rates may be determined in step 202. For example, a rate of data being received by the first optical transceiver from the client, or being input to the buffer, may be used. Alternatively, a rate at which the amount of data in the buffer is changing may be used. Additionally or alternatively, the data to be transmitted may include a prediction of an amount of data to be transmitted. For example, a machine-learning model may be trained on historical data to recognize patterns in the amount of data transmitted by the first optical transceiver, or by optical transceivers in the system more generally. The machine-learning model may take any suitable parameters as input, including the time of day, the services being used by clients connected to the optical transceivers, etc. However, there is no requirement to use machine-learning algorithms, and the amount of data to be transmitted may instead be predicted using non-machine-learning statistics. For example, the time of day may have an impact on the amount of data to be transmitted. Less data may be expected to be transmitted across the network at night, for example.

[0042] In step 204, the first optical transceiver uses the amount of data determined in step 202 to set a bit repetition factor. In step 206, the bit repetition factor is used to encode data into a sequence of bits without changing the line rate of the transmitted signal. In particular, the data is encoded such that each data symbol is transmitted as a sequence of n identical bits, where n is the bit repetition factor (an integer). Where n is equal to 1 , there is no bit repetition. The data is encoded at a maximum symbol rate, where there is a 1-to-1 correspondence between the number of bits and the number of data symbols. Where n is equal to 2, for example, each data symbol is transmitted over two identical bits in the sequence of bits, one immediately following the other. This halves the symbol rate from its maximum value, when n is equal to 1 . The bit repetition factor may be set to any suitable integer, with higher values (more repetitions) reducing the symbol rate of the outgoing encoded optical transmissions.

[0043] The bit repetition factor may be set based on a comparison of the amount of data to one or more thresholds. For example, the amount of data may be set to a first value responsive to a determination that the amount of data is less than a threshold, and to a second value responsive to a determination that the amount of data is greater than the threshold. The first value is greater than the second value, such that more repetition is applied when there is relatively less data to transmit. This idea can be extended by applying multiple thresholds for multiple values of the bit repetition factor.

[0044] The bit repetition factor may be determined as an absolute value based on the comparison of the amount of data to one or more thresholds (e.g., as described above), or may be changed based on the comparison of the amount of data to one or more thresholds. In the latter case, the bit repetition factor may be incremented by 1 (or some other increment) responsive to a determination that the amount of data has fallen below a threshold value; conversely, the bit repetition factor may be decremented by 1 (or some other decrement) responsive to a determination that the amount of data has risen above a threshold value.

[0045] The thresholds may be the same for increasing or decreasing amounts of data for transmission; that is, the bit repetition factor is changed when the amount of data increases above a threshold or reduces below the same threshold. In other embodiments, hysteresis, may be introduced such that the thresholds for increasing amounts of data are different from the thresholds used for decreasing amounts of data. In other words, the bit repetition factor may be changed from a first value to a second value responsive to a determination that the amount of data is greater than a first threshold, and changed from the second value to the first value responsive to a determination that the amount of data is less than a second threshold (which is different to the first threshold). Specifically, the second threshold may be lower than the first threshold, such that the bit repetition factor does not change rapidly when the amount of data remains at or near to a threshold value. Other mechanisms may be introduced to address this issue, such as permitting changes to the bit repetition factor only after a certain time duration.

[0046] As noted above, the bit repetition factor may be set in step 204 based on the amount of data to be transmitted. For example, if the first optical transceiver has a relatively large amount of data to be transmitted (e.g., the data level in the buffer is high, the data level in the buffer is increasing, the input data rate is high, etc), the bit repetition factor may be set to a relatively low value. This increases the symbol rate of optical transmissions, such that data does not suffer excessive delay while waiting for transmission. Conversely, if the first optical transceiver has a relatively small amount of data to be transmitted (e.g., the data level in the buffer is low, the data level in the buffer is decreasing, the input data rate is low, etc), the bit repetition factor may be set to a relatively high value.

[0047] Increasing the bit repetition factor decreases the symbol rate of optical transmissions, but this may be acceptable if there is relatively little data to transmit. Repeating bits also has a consequential effect on the spectrum width used by the optical transmission, as shown in Figure 3. Figure 3 is a schematic diagram showing the variation of the spectrum of the transmitted signal with different bit repetition factors. The horizontal axis corresponds to frequency; the vertical axis corresponds to received energy (or equivalent metrics). The dashed line illustrates the line rate spectrum, i.e., the spectrum of the underlying clock signal. The spectrum is relatively wide, covering a relatively large range of frequency components. If the bit repetition factor is set to 1 , the transmitted optical spectrum has the bandwidth expected for the given bit rate, If the bit repetition factor is increased, such that each bit is repeated one or more times, the spectrum keeps the same central frequency, but narrows in width (bandwidth) so reducing the impact of chromatic dispersion due to the fibre length. This spectrum is shown with a solid line in Figure 3.

[0048] In step 208, the first optical transceiver transmits an optical transmission comprising the sequence of bits to the second optical transceiver. This transmission may be received and processed, for example, according to the method set out below with respect to Figure 4.

[0049] In one embodiment, the optical transmission utilizes a constant line-out rate and / or a constant modulation scheme. For example, the line-out rate may be set to a maximum value that can be output by the first optical transceiver, or a different nominal value. Further, those skilled in the art will appreciate that the bit modulation factor may vary as the amount of data to be transmitted varies. Thus the optical transmission may comprise a plurality of sequences of bits, where each sequence of bits utilizes a respective bit repetition factor. Despite the changing bit repetition factor, the line-out rate and / or the modulation scheme may remain constant between the different sequences. Further detail is set out below with respect to Figure 8.

[0050] Optionally, in step 210, the first optical transceiver transmits to the second optical transceiver an indication of the spectrum width used by the optical transmission, or a related parameter such as the bit repetition factor, the symbol rate, etc. The information may be transmitted over the optical link (e.g., the same link as the optical transmission of step 208) or a different link or mechanism.

[0051] As noted above, the disclosure specifically contemplates optical transceivers that both transmit optical transmissions (e.g., as set out with respect to Figure 2), and receive optical transmissions (e.g., as set out with respect to Figure 4). Further detail regarding the structure of transmitting and receiving optical transceivers is set out below with respect to Figures 5 and 6. It will therefore be understood that the transmitting components (e.g., as illustrated in the first optical transceivers) may also be provided in the second optical transceivers; similarly, the receiving components (e.g., as illustrated in the second optical transceivers) may also be provided in the first optical transceivers.

[0052] Figure 4 illustrates a method according to embodiments of the present disclosure. The method may be performed by a second optical network element (e.g., a second optical transceiver), such as the second optical transceiver 104 described above, and / or any of the second optical transceivers 550, 650 and 1000 described below.

[0053] The method begins at step 402, in which the second optical transceiver receives an optical transmission from a first optical transceiver over an optical transmission medium. See Figure 2 above for description of a method by which the optical transmission may be transmitted.

[0054] The optical transmission may therefore comprise a first sequence of bits, with each bit repeated a number of times based on a bit repetition factor. In one embodiment, the optical transmission utilizes a constant line-out rate and / or a constant modulation scheme. For example, the line-out rate may be set to a maximum value that can be output by the first optical transceiver, or a different nominal value. Further, those skilled in the art will appreciate that the bit modulation factor may vary as the amount of data to be transmitted varies. Thus the optical transmission may comprise a plurality of sequences of bits, where each sequence of bits utilizes a respective bit repetition factor. Despite the changing bit repetition factor, the line-out rate and / or the modulation scheme may remain constant between the different sequences. Further detail is set out below with respect to Figure 8.

[0055] In step 404, the second optical transceiver determines an amount of chromatic dispersion compensation to be applied to the optical transmission. In particular, chromatic dispersion compensation implemented by the second optical transceiver may comprise a plurality of chromatic dispersion compensation stages. See Figure 7 for more detail. In such embodiments, the amount of chromatic dispersion compensation may be controlled by determining the number of CDC stages to be applied to the optical transmission, e.g., selectively activating or deactivating one or more stages of the plurality of stages. The plurality of chromatic dispersion compensation stages may have different granularity, and / or provide an amount of chromatic dispersion compensation that varies at different rates. For example, in one embodiment the plurality of stages comprises a first chromatic dispersion compensation stage and a second chromatic dispersion compensation stage. The first chromatic dispersion compensation stage provides a relatively large (e.g., bulk) amount of chromatic dispersion compensation; the second chromatic dispersion compensation stage provides a relatively small (e.g., fine) amount of chromatic dispersion compensation.

[0056] For the avoidance of doubt, the amount of chromatic dispersion compensation applied may be zero; that is, all chromatic dispersion compensation circuitry (whether multiple stages or a single stage) may be deactivated.

[0057] In one embodiment, the number of CDC circuitry stages is determined such that a fewest number of CDC circuitry stages is applied to the optical transmission to achieve a given signal quality (e.g., bit error rate, signal-to-noise ratio, etc).

[0058] For example, a trial-and-error process may be used, whereby the second optical transceiver selects a first number of CDC circuitry stages, measures the signal quality of the resulting signal, and then varies the number of selected CDC circuitry stages based on the measured signal quality. If the signal quality is too low (e.g., below some acceptable threshold), the number of CDC circuitry stages may be increased; if the signal quality is high (e.g., above the acceptable threshold), the number of CDC circuitry stages may be decreased, to test whether the signal quality remains acceptable (above the threshold) while using fewer CDC circuitry stages and therefore consuming less power. The first number of stages may be selected so as to provide the best-possible signal quality initially (e.g., all stages are activated); alternatively, the first number of stages may be selected so as to provide maximum power saving (e.g., no stages are activated); or intelligently based on statistical (e.g., historical) data.

[0059] In another embodiment, step 404 comprises the second optical transceiver receiving, from the first optical transceiver, an indication of the number of CDC circuitry stages to be applied to the optical transmission (see step 210 above). The indication of the number of CDC circuitry stages comprises an indication of a parameter from which the number of CDC circuitry stages can be determined, such as one or more of: the number of CDC circuitry stages; an indication of a spectrum width used by the optical transmission; an indication of a symbol rate used by the optical transmission; and an indication of the bit repetition factor.

[0060] Alternatively, the second optical transceiver may measure a parameter from which the number of CDC circuitry stages can be determined, such as one or more of: a spectrum width used by the optical transmission; a symbol rate used by the optical transmission; and the bit repetition factor.

[0061] There are many options for determining the spectrum width and / or a related parameter. For example, square-law timing recovery using either a phase-locked loop or a Digital Fourier Transform to extract the current value of the symbol rate. In some examples, the receiver determines the spectrum width of the received signal within the receiver, i.e. without receiving signaling from the transmitter indicating the spectrum width. In some examples, from the spectrum width the receiver determines the bit repetition factor. In some examples, the receiver determines the amount of chromatic dispersion compensation (e.g. the number of stages of chromatic dispersion compensation) based on the determined bit repetition factor or spectrum width.

[0062] It may also be possible to determine that the symbol rate has changed (e.g., between different sequences of bits in the optical transmission), and use this as an input to determine whether to change the amount of chromatic dispersion compensation to be applied. For example, when changing from a higher to a lower symbol rate, the change may be identified by noticing that each of the received bits is repeated at least once, and assuming that the bit repetition factor is the same as the shortest sequence of consecutive ones or zeroes in a data frame. As an example, if the shortest sequence is two bits, it implies the bit repetition factor is 2, etc.

[0063] On the other hand, if the symbol rate is changed by increasing the symbol rate (reducing or removing bit repetition), this approach is not possible. One approach may be to deduce the appropriate amount chromatic dispersion compensation from an estimation of the quality of the demodulated signal, e.g., considering the Bit Error Rate (BER). If the quality is low (e.g., BER is high), or decreases (BER increases), it may be inferred that the symbol rate has increased, and the amount of chromatic dispersion compensation may also be increased. Conversely, if the quality is sufficiently high that there is room to reduce the compensation whilst still meeting acceptable performance, finding the bit repetition factor in the same way as above, the amount of chromatic dispersion compensation may be reduced.

[0064] Alternatively, multiple physical coding sublayer (PCS) blocks may be applied to the received optical transmission, each assuming a different bit repetition factor. Those PCS blocks using an incorrect bit repetition factor will issue a loss-of-frame (LOF) alarm.

[0065] In an alternative embodiment, wherein the second optical transceiver uses adaptable digital bandpass-filtering, a change from lower to higher symbol rate may be determined by comparing the signal power before and after application of the digital bandpass-filter. If the symbol rate is increased for a fixed filter bandwidth, it will result in a noticeable additional power loss due to the filter (i.e. the power of the signal prior to the filter is higher than the power of the signal after the filter); the filter bandwidth and chromatic dispersion compensation may then both be increased.

[0066] In step 406, the second optical transceiver applies the determined number of CDC circuitry stages to the optical transmission to compensate for chromatic dispersion.

[0067] Figure 5 shows a system comprising a first optical transceiver 500 and a second optical transceiver 550 according to embodiments of the disclosure. The first optical transceiver 500 may correspond to the first optical transceiver 102 described above, and the second optical transceiver 550 may correspond to the second optical transceiver 104 described above. The first and second optical transceivers 500, 550 may be configured to perform the methods described with respect to Figures 2 and 4 respectively.

[0068] The first optical transceiver 500 comprises a throughput monitor buffer (TMB) 502, more generally referred to as a “buffer” herein. The buffer 502 receives and buffers an input client signal comprising data for transmission to the second optical transceiver. Data is taken from the buffer 502 for transmission to the second optical transceiver over the optical line. An increasing amount of data in the buffer 502 indicates that more data is being received from the client than is being output in transmissions to the second optical transceiver. The amount of data in the buffer, or more correctly the rate of change of the amount of data in the buffer, may be reduced by increasing the symbol rate of transmissions to the second optical transceiver. One method of affecting the symbol rate, as described above, is to repeat each bit in the transmission. If the bit repetition factor is 1 (i.e., there is no bit repetition), the symbol rate is at its maximum value as a different symbol is transmitted with every transition of the underlying clock signal. The symbol rate can be reduced by increasing the bit repetition factor, such that the symbol is only changed every n transitions of the underlying clock signal (where n is an integer). Accordingly, the first optical transceiver further comprises a bit repetition module (BRM) 504, which applies a given bit repetition factor to data taken from the buffer 502. In this way, each bit in an output sequence of bits is repeated a certain number of times before the next bit.

[0069] The sequence of bits is output from the BRM 504 (e.g., as non-return-to-zero bits) to digital signal processing (DSP) circuitry 506, which processes the signal, and outputs it to the transmitter optical sub-assembly (TOSA) 508 for onward transmission to the second optical transceiver over an optical line. The TOSA 508 performs various functions as known to those skilled in the art, including at least converting the electrical signal output from the DSP circuitry 506 to an optical signal.

[0070] In the illustrated embodiment, the first optical transceiver 500 further comprises chromatic dispersion compensation control (CDCC) circuitry 510. This optional component may monitor the TMB 502 to determine the bit repetition factor in the manner discussed above with respect to step 204. Thus the CDCC circuitry 510 may output a control signal to the BRM 504 comprising an indication of the determined bit repetition factor.

[0071] The second optical transceiver 550 comprises a receiver optical sub-assembly (ROSA) 552, which receives an optical transmission from the first optical transceiver (over the optical line). The ROSA 552 performs various functions as known to those skilled in the art, including at least converting the received optical transmission to an electrical signal. For example, the ROSA 552 may comprise a photodiode for this purpose.

[0072] The electrical signal is output from the ROSA 552 to DSP circuitry 554, which processes the signal including at least applying an amount of chromatic dispersion compensation. Further detail regarding the mechanism for applying different amounts of chromatic dispersion compensation may be found in Figure 7, for example. The compensated signal is provided to a spectrum monitoring control (SMC) module 556 and a symbol rate detection (SRD) module 558. These modules respectively determine the spectrum width of the optical transmission received by the second optical transceiver and the symbol rate used by the optical transmission received by the second optical transceiver.

[0073] There are many options for determining the spectrum width and / or a related parameter (see step 204 described above), and thus for implementation of the SMC and SRD blocks.

[0074] The values of the spectrum width and the symbol rate are provided as input to CDCC circuitry 562, which determines an appropriate amount of chromatic dispersion compensation to be applied and provides this as a control signal to the DSP circuitry 554.

[0075] The CDCC circuitry 562 further determines the amount of bit repetition used in the optical transmission and provides that as a control signal to a BRM module 560. The BRM module 560 receives the compensated signals output from the DSP circuitry 554, and de-duplicates the bits (removes the repeated symbols) to recover the data to be provided to the client.

[0076] Figure 5 therefore shows a system in which the second optical transceiver 550 itself determines the appropriate amount of chromatic dispersion compensation to be applied to the received optical transmission, e.g., through measuring the spectrum width of the optical transmission, recovering the symbol rate, etc. In other embodiments, the first optical transceiver may provide this information directly to the second optical transceiver. Figure 6 is a schematic diagram showing an optical system according to such embodiments.

[0077] The system comprises a first optical transceiver 600 and a second optical transceiver 650. The components of the two transceivers are similar to those of the transceivers 500, 550 described above. The first optical transceiver 600 in particular is structurally identical to the first optical transceiver 500 described with respect to Figure 5. However, the CDCC circuitry 610 of the first optical transceiver 600 is configured to provide information to the CDCC circuitry 662 of the second optical transceiver 650. The information provided may include, for example, one or more of: an indication of the spectrum width of an optical transmission; an indication of the symbol rate of the optical transmission; an indication of a bit repetition factor used by the optical transmission. The information may be provided over the optical link or a different channel.

[0078] By providing this information directly to the CDCC circuitry 662, the second optical transceiver 650 no longer requires the ability to determine the spectrum width or the symbol rate of the optical transmission itself. Therefore SMC and SRD modules may be omitted, simplifying processing at the second optical transceiver 650.

[0079] Figure 7 is a schematic illustration of processing circuitry 700, such as may be employed within an optical transceiver, for receiving and processing a coherent optical transmission. Figure 7 shows in particular the application of chromatic dispersion compensation circuitry to a received optical transmission (e.g., implemented within digital signal processing circuitry), and may be employed in any of the optical transceivers described herein.

[0080] The processing circuitry 700 comprises an electro-optical front-end 702, an analogue-to- digital converter (ADC) 704, a first stage equalizer 706 and a second-stage equalizer 708. A received optical signal Eft) is mixed with a local oscillator signal ELO and converted to an electronic signal within the electro-optical front-end 702. The resulting signal components are converted to the digital domain in the ADC 704, producing Q digital x-components of the signal (where Q is an integer) and Q digital y-components of the signal.

[0081] These components are (or may be) provided to the first-stage equalizer 706, which applies a first amount of chromatic dispersion compensation and outputs intermediate chromatic dispersion compensated signal components. These intermediate signal components are (or may be) then provided to the second-stage equalizer 708, which in the illustrated embodiment comprises a butterfly equalizer.

[0082] The first-stage equalizer 706 may apply a first amount of chromatic dispersion compensation. The first amount may be relatively large (e.g., bulk), and / or vary at a rate which is relatively low; that is, the first-stage equalizer 706 may provide a relatively consistent amount of chromatic dispersion compensation.

[0083] The second-stage equalizer 708 may apply a second amount of chromatic dispersion compensation. The second amount may be relatively small (e.g., fine), and / or vary at a rate which is relatively high; that is, the second-stage equalizer 708 may provide an amount of chromatic dispersion compensation which is small (compared to that provided by the first-stage equalizer 706), and / or varies at a rate which is faster than the compensation applied by the first-stage equalizer 706.

[0084] According to embodiments of the disclosure, the amount of chromatic dispersion compensation applied to an optical transmission may be varied (e.g., as described above with respect to step 406) by selectively activating or deactivating one or more of the firstand second-stage equalizers 706, 708. For example, where the demodulated optical signal has an adequate quality (e.g., error rate below a given threshold) without chromatic dispersion compensation, both first- and second-stage equalizers 706, 708 may be deactivated. This of course achieves the maximum power saving. Where a certain amount of chromatic dispersion compensation is required to achieve an adequate quality, one or other of the first- and second-stage equalizers 706, 708 may be activated in order to apply an amount of chromatic dispersion compensation to the received optical transmission. If the quality of the optical transmission is still inadequate (e.g., error rate above the threshold), both or all stages of chromatic dispersion compensation may be activated.

[0085] Those skilled in the art will appreciate that alternative architectures than that shown in Figure 7 are possible, without departing from the scope of the claims appended hereto. For example, the chromatic dispersion compensation circuitry may be implemented in different numbers of stages, which can be selectively activated or deactivated. In one example, the chromatic dispersion compensation may be implemented within a single ‘stage’, which is either activated or deactivated. In other examples, the chromatic dispersion compensation may be implemented in more than two stages. The stages may apply the same or different amounts of compensation (e.g., bulk compensation, fine compensation, finer compensation, etc).

[0086] Figure 8 shows an example of optical transmissions transmitted according to methods of the disclosure, e.g., as following the method of Figure 2. The figure includes fourtime- aligned traces, showing the variation of: in the uppermost trace a), the amount of data available for transmission by the first optical transceiver; in the second trace b), the bit repetition factor used for transmissions by the first optical transceiver; in the third trace c), the symbol rate of transmissions by the first optical transceiver; and in the lowermost trace d), the spectrum width of transmissions by the first optical transceiver. Those skilled in the art will appreciate that the third and fourth traces follow as a consequence of the changing bit repetition factor shown in trace b). In the illustrated embodiment, bit repetition factors of 1 , 2, and 4 are used. Those skilled in the art will appreciate that different bit repetition factors may be used.

[0087] Four events labelled A, B, C and D trigger symbol rate changes over the time period covered by the traces shown. Dashed lines illustrate the times at which events A, B, C and D occur.

[0088] Initially, the buffer fill level is small enough to allow use of a bit repetition factor equal to four, which in turn reduces both the symbol rate and the spectrum width with a factor of four with respect to their maximum values that would be used by default without the invention. This also implies that selected parts of the CDC circuitry in the receiver can be switched off. After some time, the buffer fill level exceeds 25% (event A) and the symbol rate needs to be increased to satisfy capacity demands. The bit repetition rate is therefore reduced to two, which doubles the symbol rate and spectrum width, and some of the CDC circuitry may need to be activated.

[0089] The buffer fill level eventually reaches 50% (event B), which calls for an additional increase of the symbol rate and spectrum width, which is achieved by disabling the bit duplication such that the symbol rate and spectrum width are set to their respective maximum values. Later, when the capacity demand reduces, the system can return to using lower symbol rates (events C and D).

[0090] Note that while in this example, switches of symbol rate are made at buffer fill levels of 50%, 25%, etc, in other embodiments different thresholds (buffer fill levels) may be used to introduce some margin to make the system more robust. For example, with a 5% margin, the symbol rate switches may instead be made at buffer fill levels of 20% and 45%. In this way, the capacity of the optical transmission to carry data may be increased (by reducing the bit repetition factor) slightly before it becomes absolutely necessary.

[0091] In further embodiments, the symbol rate may be changed at different thresholds (different buffer fill levels) when increasing the rate as compared to when decreasing the rate. That is, some hysteresis may be introduced so that the bit repetition factor is not changed rapidly when the buffer fill level remains at or close to a threshold. It is also worth mentioning that the spectrum width, in this example, is assumed to depend linearly on the symbol rate. In practice, the dependency of the spectrum width on the symbol rate might be nonlinear and therefore a conservative approach may be taken on spectrum width compensation. This does not have a significant impact, however, as the spectrum is a deterministic function of the symbol rate which allows accurate prediction of the amount of CDC needed in the receiver.

[0092] Figure 9 is a schematic diagram of an optical network element or optical transceiver 900 according to embodiments of the disclosure. The optical network element comprises processing circuitry 902, a non-transitory computer readable medium 904 and one or more interfaces 906.

[0093] The processing circuitry 902 may comprise a combination of one or more of a microprocessor, controller, microcontroller, central processing unit, digital signal processor, application-specific integrated circuit, field programmable gate array, or any other suitable computing device, resource, or combination of hardware, software and / or encoded logic operable to provide, either alone or in conjunction with other components of the optical network element 900, such as the memory 904, functionality of the optical network element 900. For example, the processing circuitry 902 may be configured to cause the optical network element to perform the method as described with reference to Figure 2.

[0094] Thus, in one embodiment, the processing circuitry 902 is configured to cause the optical network element 900 to: determine an amount of data to be transmitted over an optical transmission medium to an optical receiver; use the amount of data to set a bit repetition factor; encode data into a sequence of bits, in which each bit is repeated a number of times based on the bit repetition factor; and transmit, to the optical receiver over the optical transmission medium, an optical transmission comprising the sequence of bits.

[0095] The optical transmitter may be further caused to vary the bit repetition factor as a function of the amount of data to be transmitted to the optical receiver. The optical transmission may comprise a plurality of sequences of bits, with each sequence of bits being associated with a respective value of the bit repetition factor, and each bit in a sequence of bits being repeated a number of times based on the respective value of the bit repetition factor. The optical transmission may have a constant line rate and / or use a constant modulation scheme, e.g., as the bit repetition factor varies.

[0096] A spectrum width of the optical transmission may vary as a function of the bit repetition factor.

[0097] The bit repetition factor may be set based on a comparison of the amount of data to one or more thresholds. For example, the bit repetition factor may be set to a first value responsive to a determination that the amount of data is less than a threshold, and to a second (smaller) value responsive to a determination that the amount of data is greater than the threshold. In a further example, the bit repetition factor may be changed from the first value to the second value responsive to a determination that the amount of data is greater than a first threshold, and changed from the second value to the first value responsive to a determination that the amount of data is less than a second (different) threshold.

[0098] In addition, the bit repetition factor may be set using a prediction of an amount of data that is to be transmitted by the optical transmitter.

[0099] In some embodiments, the processing circuitry 902 includes a system on a chip (SOC). Such a device integrates the logic for implementing the functions described herein and the memory (e.g., RAM) required to process incoming optical signals. Typically highspeed memory such as DDR6 or HBM (high bandwidth memory) may be integrated in the same SOC. In some embodiments, the processing circuitry 902 includes optical frequency transceiver circuitry. In some embodiments, the optical transceiver circuitry may be on separate chips (or sets of chips), boards, or units, such as optical units. In alternative embodiments, part or all of optical transceiver circuitry may be on the same chip or set of chips, boards, or units.

[0100] The memory 904 may comprise any form of volatile or non-volatile computer-readable memory including, without limitation, persistent storage, solid-state memory, remotely mounted memory, magnetic media, optical media, random access memory (RAM), readonly memory (ROM), mass storage media (for example, a hard disk), removable storage media (for example, a flash drive, a Compact Disk (CD) or a Digital Video Disk (DVD)), and / or any other volatile or non-volatile, non-transitory device-readable and / or computerexecutable memory devices that store information, data, and / or instructions that may be used by the processing circuitry 902. The memory 904 may store any suitable instructions, data, or information, including a computer program, software, an application including one or more of logic, rules, code, tables, and / or other instructions capable of being executed by the processing circuitry 902 and utilized by the optical network element 900 (e.g., for performing the method of Figure 2). The memory 904 may be used to store any calculations made by the processing circuitry 902 and / or any data received via the communication interface 906. In some embodiments, the processing circuitry 902 and memory 904 are integrated.

[0101] The interface(s) 906 comprise at least one optical interface (e.g., an optical link, as described above), for bidirectional optical communication with a remote optical network element. The optical transceiver circuitry described above may be coupled to such an interface for controlling the transmission of optical signals over the interface 906 and for the detection and / or measurement of optical signals received over the interface 906.

[0102] Figure 10 is a schematic diagram of an optical network element or optical transceiver 1000 according to embodiments of the disclosure. The optical network element comprises processing circuitry 1002, a non-transitory computer readable medium 1004 and one or more interfaces 1006.

[0103] The processing circuitry 1002 may comprise a combination of one or more of a microprocessor, controller, microcontroller, central processing unit, digital signal processor, application-specific integrated circuit, field programmable gate array, or any other suitable computing device, resource, or combination of hardware, software and / or encoded logic operable to provide, either alone or in conjunction with other components of the optical network element 1000, such as the memory 1004, functionality of the optical network element 1000. For example, the processing circuitry 1002 may be configured to cause the optical network element to perform the method as described with reference to Figure 4.

[0104] According to embodiments of the disclosure, the optical network element further comprises a plurality of chromatic dispersion compensation (CDC) circuitry stages. For example, the plurality of CDC circuitry stages may be implemented as shown in Figure 7 and as described with respect to that Figure. In one embodiment, the plurality of chromatic dispersion compensation stages comprises a first CDC circuitry stage and a second CDC circuitry stage, with the first CDC circuitry stage providing a first amount of chromatic dispersion compensation, and the second CDC circuitry stage providing a second (smaller) amount of chromatic dispersion compensation. The first amount of chromatic dispersion compensation may vary at a first frequency, and the second amount of chromatic dispersion compensation may vary at a second (higher) frequency. Those skilled in the art will appreciate that the plurality of CDC circuitry stages may be implemented within processing circuitry 1002 or different (e.g., dedicated) circuitry.

[0105] Thus, in one embodiment, the processing circuitry 1002 is configured to cause the optical network element 1000 to: receive an optical transmission; determine a number of the plurality of CDC circuitry stages to be applied to the optical transmission; and apply (e.g., by selectively activating and deactivating CDC circuitry stages in accordance with the determination) the determined number of CDC circuitry stages to the optical transmission.

[0106] The optical network element 1000 may be caused to determine the number of CDC circuitry stages by determining the number of CDC circuitry stages such that a fewest number of CDC circuitry stages is applied to the optical transmission to achieve a given signal quality (e.g., a signal quality meeting a threshold value). The signal quality may be measured in any suitable metric, such as a bit error rate; a signal-to-noise ratio, etc.

[0107] In one embodiment, the optical network element 1000 is caused to determine the number of CDC circuitry stages by measuring the signal quality after applying a first number of CDC circuitry stages, and varying the number of CDC circuitry stages based on the measured signal quality. Such a method may comprise a trial-and-error process.

[0108] Alternatively, the optical network element 1000 may be caused to determine the number of CDC circuitry stages by receiving, from the optical transmitter, an indication of the number of CDC circuitry stages to be applied to the optical transmission. Such an indication may comprise an indication of a parameter from which the number of CDC circuitry stages can be determined, such as one or more of: the number of CDC circuitry stages; an indication of a spectrum width used by the optical transmission; an indication of a symbol rate used by the optical transmission; and an indication of the bit repetition factor.

[0109] In a further embodiment, the optical network element 1000 may be caused to determine the number of CDC circuitry stages by measuring a parameter from which the number of CDC circuitry stages can be determined, such as one or more of: a spectrum width used by the optical transmission; a symbol rate used by the optical transmission; and the bit repetition factor.

[0110] The number of CDC circuitry stages is determined from a range from zero CDC circuitry stages (i.e., not CDC circuitry stages are activated or applied to the optical transmission) to all the plurality of CDC circuitry stages (i.e., all of the CDC circuitry stages are activated or applied to the optical transmission).

[0111] In some embodiments, the processing circuitry 1002 includes a SOC. Such a device integrates the logic for implementing the functions described herein and the memory (e.g., RAM) required to process incoming optical signals. Typically high-speed memory such as DDR6 or HBM (high bandwidth memory) may be integrated in the same SOC. In some embodiments, the processing circuitry 1002 includes optical frequency transceiver circuitry. In some embodiments, the optical transceiver circuitry may be on separate chips (or sets of chips), boards, or units, such as optical units. In alternative embodiments, part or all of optical transceiver circuitry may be on the same chip or set of chips, boards, or units.

[0112] The memory 1004 may comprise any form of volatile or non-volatile computer-readable memory including, without limitation, persistent storage, solid-state memory, remotely mounted memory, magnetic media, optical media, random access memory (RAM), readonly memory (ROM), mass storage media (for example, a hard disk), removable storage media (for example, a flash drive, a Compact Disk (CD) or a Digital Video Disk (DVD)), and / or any other volatile or non-volatile, non-transitory device-readable and / or computerexecutable memory devices that store information, data, and / or instructions that may be used by the processing circuitry 1002. The memory 1004 may store any suitable instructions, data, or information, including a computer program, software, an application including one or more of logic, rules, code, tables, and / or other instructions capable of being executed by the processing circuitry 1002 and utilized by the optical network element 9001000 (e.g., for performing the method of Figure 2). The memory 1004 may be used to store any calculations made by the processing circuitry 1002 and / or any data received via the communication interface 1006. In some embodiments, the processing circuitry 1002 and memory 1004 are integrated. The interface(s) 1006 comprise at least one optical interface (e.g., an optical link, as described above), for bidirectional optical communication with a remote optical network element. The optical transceiver circuitry described above may be coupled to such an interface for controlling the transmission of optical signals over the interface 1006 and for the detection and / or measurement of optical signals received over the interface 1006.

[0113] The methods of the present disclosure may be implemented in hardware, or as software modules running on one or more processors. The methods may also be carried out according to the instructions of a computer program, and the present disclosure also provides a computer readable medium having stored thereon a program for carrying out any of the methods described herein. A computer program embodying the disclosure may be stored on a computer readable medium, or it could, for example, be in the form of a signal such as a downloadable data signal provided from an Internet website, or it could be in any other form.

[0114] It should be noted that the above-mentioned examples illustrate rather than limit the disclosure, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended embodiments. The word “comprising” does not exclude the presence of elements or steps other than those listed in an embodiment or claim, “a” or “an” does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the embodiments. Any reference signs in the claims shall not be construed so as to limit their scope.

Claims

26CLAIMS1 . A method performed by a system comprising an optical transmitter (102) and an optical receiver (104) coupled by an optical transmission medium (106), the optical receiver comprising a plurality of chromatic dispersion compensation, CDC, circuitry stages (706, 708), the method comprising: in the optical transmitter: encoding (206) data into a sequence of bits, in which each bit is repeated a number of times based on a bit repetition factor; and transmitting (208), to the optical receiver over the optical transmission medium, an optical transmission comprising the sequence of bits, and in the optical receiver: receiving (402) the optical transmission; determining (404) a number of the plurality of CDC circuitry stages to be applied to the optical transmission; and applying (406) the determined number of CDC circuitry stages to the optical transmission.

2. The method of claim 1 , wherein the optical transmitter (102) varies the bit repetition factor as a function of the amount of data to be transmitted to the optical receiver, comprising: determining (202) an amount of data to be transmitted over the optical transmission medium to the optical receiver; using (204) the amount of data to set the bit repetition factor.

3. The method of claim 1 or 2, wherein the optical transmission comprises a plurality of sequences of bits, each sequence of bits being associated with a respective value of the bit repetition factor, and wherein each bit in a sequence of bits is repeated a number of times based on the respective value of the bit repetition factor.

4. The method of any one of the preceding claims, wherein the optical transmission has a constant line rate and uses a constant modulation scheme.

5. The method of any one of the preceding claims, wherein a spectrum width of the optical transmission varies as a function of the bit repetition factor.

6. The method of any one of the preceding claims, wherein the bit repetition factor is set based on a comparison of the amount of data to one or more thresholds.

7. The method of claim 6, wherein the bit repetition factor is set to a first value responsive to a determination that the amount of data is less than a threshold, and to a second value responsive to a determination that the amount of data is greater than the threshold, and wherein the first value is greater than the second value.

8. The method of claim 6 or 7, wherein the bit repetition factor is changed from the first value to the second value responsive to a determination that the amount of data is greater than a first threshold, and changed from the second value to the first value responsive to a determination that the amount of data is less than a second threshold, and wherein the first and second thresholds are different.

9. The method of any one of claims 6 to 8, wherein the bit repetition factor is further set using a prediction of an amount of data that is to be transmitted by the optical transmitter.

10. The method of any one of the preceding claims, wherein the optical receiver (104) determining (404) the number of CDC circuitry stages comprises determining the number of CDC circuitry stages such that a fewest number of CDC circuitry stages is applied to the optical transmission to achieve a given signal quality.

11. The method of claim 10, wherein the signal quality comprises one or more of: a bit error rate; and a signal-to-noise ratio.

12. The method of claim 10 or 11 , wherein the optical receiver (104) determining (404) the number of CDC circuitry stages comprises measuring the signal quality after applying a first number of CDC circuitry stages, and varying the number of CDC circuitry stages based on the measured signal quality.

13. The method of any one of claims 1 to 11 , wherein the optical receiver (104) determining (404) the number of CDC circuitry stages comprises receiving, from the optical transmitter (102), an indication of the number of CDC circuitry stages to be applied to the optical transmission.

14. The method of claim 13, wherein the indication of the number of CDC circuitry stages comprises an indication of a parameter from which the number of CDC circuitry stages can be determined.

15. The method of claim 14, wherein the parameter comprises one or more of: the number of CDC circuitry stages; an indication of a spectrum width used by the optical transmission; an indication of a symbol rate used by the optical transmission; and an indication of the bit repetition factor.

16. The method of any one of claims 1 to 11 , wherein the optical receiver (104) determining (404) the number of CDC circuitry stages comprises measuring a parameter from which the number of CDC circuitry stages can be determined.

17. The method of claim 16, wherein the parameter comprises one or more of: a spectrum width used by the optical transmission; a symbol rate used by the optical transmission; and the bit repetition factor.

18. The method of any one of the preceding claims, wherein the number of CDC circuitry stages is determined from a range from zero CDC circuitry stages to all the plurality of CDC circuitry stages.

19. The method of any one of the preceding claims, wherein applying (406) a CDC circuitry stage to the optical transmission comprises activating the CDC circuitry stage, and wherein not applying a CDC circuitry stage to the optical transmission comprises deactivating the CDC circuitry stage.

20. The method of any one of the preceding claims, wherein the plurality of chromatic dispersion compensation stages comprises a first CDC circuitry stage (706) and a second CDC circuitry stage (708), wherein the first CDC circuitry stage provides a first amount of chromatic dispersion compensation, and wherein the second CDC circuitry stage provides a second amount of chromatic dispersion compensation, wherein the second amount of chromatic dispersion compensation is smaller than the first amount of chromatic dispersion compensation.

21. The method of claim 20, wherein the first amount of chromatic dispersion compensation varies at a first frequency, which is lower than a second frequency at which the second amount of chromatic dispersion compensation varies.2922. A method performed by an optical transmitter (102), the method comprising: encoding (206) data into a sequence of bits, in which each bit is repeated a number of times based on a bit repetition factor; and transmitting (208), to the optical receiver over the optical transmission medium, an optical transmission comprising the sequence of bits.

23. The method of claim 22 comprising: determining (202) an amount of data to be transmitted over an optical transmission medium (106) to an optical receiver (104); using (204) the amount of data to set the bit repetition factor.

24. A method performed by an optical receiver (104), the optical receiver comprising a plurality of chromatic dispersion compensation, CDC, circuitry stages (706, 708), the method comprising: receiving (402), from an optical transmitter (102) over an optical transmission medium (106), an optical transmission comprising a sequence of bits, in which each bit is repeated a number of times based on a bit repetition factor; determining (404) a number of the plurality of CDC circuitry stages to be applied to the optical transmission; and applying (406) the determined number of CDC circuitry stages to the optical transmission.

25. A system (100) comprising: an optical transmitter (102, 900); and an optical receiver (104, 1000), coupled to the optical transmitter by an optical transmission medium (106); wherein the optical transmitter comprises processing circuitry (902) configured to cause the optical transmitter to: encode data into a sequence of bits, in which each bit is repeated a number of times based on a bit repetition factor; and transmit, to the optical receiver over the optical transmission medium, an optical transmission comprising the sequence of bits, wherein the optical receiver (104, 1000) comprises a plurality of chromatic dispersion compensation, CDC, circuitry stages (706, 708), and processing circuitry (1002) configured to cause the optical transmitter to:30 receive the optical transmission; determine a number of the plurality of CDC circuitry stages to be applied to the optical transmission; and apply the determined number of CDC circuitry stages to the optical transmission.

26. The system of claim 25, wherein the processing circuitry (902) of the optical transmitter (102, 900) further causes the optical transmitter to perform the steps according to any one of claims 2 to 9.

27. The system of claim 25 or 26, wherein the processing circuitry (1002) of the optical receiver (104, 1000) further causes the optical receiver to perform the steps according to any one of claims 10 to 21.

28. An optical transmitter (102, 900), comprising: processing circuitry (902), configured to cause the optical transmitter to: encode data into a sequence of bits, in which each bit is repeated a number of times based on a bit repetition factor; and transmit, to the optical receiver over the optical transmission medium, an optical transmission comprising the sequence of bits.

29. An optical receiver (104, 1000), the optical receiver comprising a plurality of chromatic dispersion compensation, CDC, circuitry stages (706, 708), and processing circuitry (1000) configured to cause the optical receiver to: receive, from an optical transmitter over an optical transmission medium, an optical transmission comprising a sequence of bits, in which each bit is repeated a number of times based on a bit repetition factor; determine a number of the plurality of CDC circuitry stages to be applied to the optical transmission; and apply the determined number of CDC circuitry stages to the optical transmission.

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