Patent Description:
Forward Error Correction (FEC) utilizes redundancy at a transmitter, namely adding data, i.e., the redundancy, based on a particular algorithm, such that a receiver can process the received data, including information bits and the redundancy, to correct a given level of errors. FEC is used on optical and/or electrical interfaces, in order to allow operation at SNR values consistent with the high data rates (e.g., 100Gb/s and beyond) required for current technologies. FEC allows operation at lower SNRs than would be otherwise possible for current data rates.

The <CIT> discloses how to compute pre-FEC BER from SNR estimations, wherein such SNR estimations are obtained from sampled QAM modulation errors within a received signal.

The <CIT> discloses that the pre-FEC BER (BEP) can be obtained from the measured SNR. In this document, the SNR representing the scattering effect can be directly estimated from training symbols.

The paper "<NPL>et AL, discloses several telemetry metrics for evaluation and management of long-haul, coherent optical links, wherein the pre-FEC BER can be uniquely determined by the measured SNR.

The present invention is defined by the attached claims.

Again, the present disclosure relates to systems and methods for signal-to-noise ratio (SNR)-based bit error rate calculations for reporting BER in place of conventional corrected error counting techniques.

<FIG> includes two example communication links <NUM>, <NUM>, including an optical link <NUM> between a transmitter <NUM> and a receiver <NUM>, and an electrical link <NUM> between the receiver <NUM> and a backplane <NUM>. Both communication links <NUM>, <NUM> are shown in a unidirectional configuration; of course, a practical embodiment includes bidirectional communication. The optical link <NUM> is between optical modems, also referred to as transceivers, transmitters and receivers, pluggable modules, etc. The transmitter <NUM> is configured to modulate an optical signal, via a modulation format, including non-return-to-zero (NRZ), pulse amplitude modulation (PAM), or coherent modulation (e.g., quadrature amplitude modulation (QAM) and the like), as well as polarization multiplexing. The optical link <NUM> can support various baud rates through software-programmable modulation formats. The electrical link <NUM> provides communication over electrical interfaces, e.g., backplane connectors, electrical pluggable modules such as QSFP-DD, etc. In an embodiment, the electrical link <NUM> can support PAM4. The backplane <NUM> can be any electrical receiver, including one in a pluggable module, interface connectors on a printed circuit board, etc..

In an embodiment, the optical link <NUM> can utilize forward error correction including Hard Decision FEC implementations and Soft Decision FEC (SD-FEC), as another technique to trade-off complexity versus noise tolerance. In another embodiment, the electrical link <NUM> can support FEC as well. In a further embodiment, the FEC can be on both the optical link <NUM> and the electrical link <NUM>, namely both links <NUM>, <NUM> share the same FEC.

FEC is used on optical and electrical interfaces, i.e., the links <NUM>, <NUM>, in order to allow operation at SNR values consistent with the high data-rates required for current technologies, e.g., 100Gb/s and beyond. FEC allows operation at lower SNRs than would be otherwise possible for current data-rates. In general, an interface operating with FEC can operate at a high pre-FEC BER while meeting the post-FEC requirements for data transmission (typically 1x10-<NUM> to 1x10-<NUM>).

In standards implementations, a FEC threshold is typically defined, based on a pre and post FEC BER. As described herein, the FEC threshold (which can also be referred to as the pre-FEC BER threshold, FEC limit, etc.) is a value of BER below which the post FEC BER is met, and above which the FEC cannot meet the post-FEC BER requirement. That is, the FEC threshold is a maximum BER for the particular FEC to meet the link BER requirement. In many cases, the post FEC BER versus pre-FEC BER curve is very steep near the FEC threshold, transitioning rapidly from the region where the post FEC BER is < 1x10-<NUM> to the region in which all FEC codewords are errored.

Again, interfaces typically measure the pre-FEC BER by measuring the corrected bits and total bits across FEC codewords. This approach works assuming all FEC codewords are corrected. However, an uncorrected codeword does not provide information on its input BER. This causes BER estimates close to the FEC threshold to become inaccurate, and results in no BER estimate for a pre-FEC BER > the FEC threshold.

The present disclosure notes that, when an interface is operating beyond the FEC threshold, it is useful to know the pre-FEC BER, to determine how close the interface is to the FEC threshold. Such reporting requires knowledge of the FEC limit (either theoretical or practical) for the comparison. As described herein, a link <NUM>, <NUM> may be close to the FEC threshold, and there are various options to potentially bring the link <NUM>, <NUM> below the FEC threshold.

Standards such as those from IEE, ITU, and OIF define the FEC threshold. For example, in the Optical Internetworking Forum (OIF) OIF-400ZR-<NUM> Implementation Agreement, November <NUM>, <NUM>, the contents of which are incorporated herein in their entirety, Concatenated FEC (CFEC) is used with a FEC threshold of <NUM>. 25x10-<NUM> corresponding to a post FEC BER of 1x10-<NUM>. In another example, in the International Telecommunication Union (ITU) Recommendation G. <NUM>, "OTU4 long-reach interface," (<NUM>/<NUM>), a hard-decision (HD) staircase FEC is defined with a FEC threshold of <NUM>. 5x10-<NUM>. For an example electrical link, IEEE <NUM>. 3bj-<NUM>," IEEE Standard for Ethernet Amendment <NUM>: Physical Layer Specifications and Management Parameters for <NUM> Gb/s Operation Over Backplanes and Copper Cables, describes an example backplane electrical interface using PAM4 having a FEC with a FEC threshold of about <NUM>. 2x10-<NUM>. Those skilled in the art will recognize there are various other standards specifying FEC in different protocols on associated interfaces, all of which are contemplated herewith. A key aspect of FEC and the FEC threshold is that the pre-FEC BER is a useful performance monitoring (PM) metric, one that is conventionally computed or determined while the interface is operating below the FEC threshold, and one that is reported to an operator.

In some instances FEC is applied in an external ASIC and is applied to multiple segments of a link. For example, the FEC may cover one or more segments, consisting of electrical or optical transmission. The FEC may be added and have allocations of its correction capability to multiple electrical links, or to electrical as well as optical links. In these scenarios a pre-FEC BER specification is applied to the various segments, but cannot be measured at intermediate interfaces. By using an SNR to BER conversion, it is possible to determine the operating BER of each interface.

Since standards such as those from IEEE, ITU, and OIF define specifications based on BER, the present disclosure includes expanding the reporting range beyond the FEC threshold. That is, conventionally, at or above the FEC threshold, optical or electrical transceivers do not report a pre-FEC BER value. This makes sense since it is not possible to calculate the pre-FEC BER value beyond the FEC threshold. That is, any calculation or determination would be inaccurate as the number of corrected bits is not accurate beyond the FEC threshold. In an embodiment, the present disclosure includes the ability to report an accurate pre-FEC BER beyond the FEC threshold. In another embodiment, the present disclosure provides techniques for determination of the accurate pre-FEC BER.

This reporting ability and subsequent use by a network operator, technician, etc. adds significant value; rather than simply reporting that a link has failed (FEC is overflowed) an interface can report how close it is to passing. This can give the operator a sense of link viability; if the reported pre-FEC BER is close to the threshold, then potentially with some minor adjustments the link can be made to carry traffic error-free. These minor adjustments can include, e.g., cleaning fiber connectors, changing transmit power levels, reducing lengths of fiber connections (e.g., patch cords), changing the patch cords, etc. That is, minor adjustments are typical and used in operations all the time; however, there is no indication that such adjustments may be useful without reporting of the pre-FEC BER and determination it is close to the FEC threshold. If the reported pre-FEC is much higher than the threshold, then a deeper investigation may be needed. That is, there may be a need for a different transceiver, different modulation format, etc..

The present disclosure provides real-time insight to link viability with the reporting of pre-FEC BER beyond the FEC threshold. In addition, this reporting can be extended to other performance monitoring metrics, e.g., optical link parameters or qualities such as chromatic dispersion (CD), polarization dependent loss (PDL), differential group delay (DGD), estimated SNR, etc. The determination and reporting of these optical link parameters can be used to indicate or determine why the link has exceeded the pre-FEC BER threshold, including whether it is possible to move to error-free transmission with the minor adjustments.

<FIG> is a flowchart of a process <NUM> for determining BER from a SNR measurement, which is used by the present invention to provide an accurate determination of pre-FEC BER beyond the FEC threshold. The process <NUM> can be implemented as a method having steps, via a receiver (either an optical or electrical receiver) configured to implement the steps, and via circuitry configured to implement the steps.

The process <NUM> includes measuring signal-to-noise ratio (SNR) of a first signal (step <NUM>), extrapolating the measured SNR of the first signal to determine SNR of a second signal (step <NUM>), and determining a bit error ratio (BER) from the SNR of the second signal (step <NUM>). As noted above, above the FEC threshold, it is not accurate to determine or calculate the pre-FEC BER. As such, the present disclosure and the process <NUM> contemplates a measurement of SNR and extrapolation of that SNR to BER.

In a coherent optical signal, the first signal can be a pilot or training signal, and the second signal can be a data signal, that is operating above the FEC threshold. Details are described herein related to measuring the SNR of the pilot or training signal. Of note, the first signal, such as the pilot or training signal, supports a lower SNR than the second signal, i.e., the data signal. In the measuring step <NUM>, pilot symbols are extracted from the received symbols and pilot SNR is measured from the received pilot symbols. So, once the SNR is measured of pilot or training signal, the present disclosure can provide an accurate pre-FEC BER of the data signal, from the measured SNR of the pilot or training signal and from an extrapolated SNR for the data signal. The extrapolating step <NUM> takes into account any difference in the transmitted power between pilots and data symbols and yields a corresponding data SNR from the measured pilot SNR.

The determining step <NUM> is a theoretical SNR to BER conversion for the modulation format in use (e.g., <NUM>-QAM). Those skilled in the art will recognize there are known techniques to take a measured SNR and provide a BER based thereon. The technique is based on calculations that are based on a type of modulation format. This calculation can be based on analytical formulas, look up tables, or other methods.

Also, it is possible to measure other aspects of the pilot or training signal, namely CD, DGD, PDL, etc..

In an electrical signal, such as a PAM signal, the first signal and the second signal can be the same and the measurement can measure the SNR of the received electrical signal. For example, in PAM4, the SNR can be measured from a histogram of the received signal. Once the SNR is determined, the pre-FEC BER can be determined based on the measured SNR.

<FIG> is a graph of a <NUM>-quadrature amplitude modulation (QAM) constellation. <FIG> is a graph of a <NUM>-QAM constellation. As is known in the art, coherent modulation includes transmission of symbols, namely <NUM> symbols in <NUM>-QAM, each representing four bits, and <NUM> symbols in <NUM>-QAM, each representing six bits. Each symbol is represented by a dot in the graphs of <FIG>. Of note, due to noise, non-linear effects, etc., in the optical link <NUM>, the symbols can spread out, leading to bit errors.

The pilot or training signal uses a subset of the symbols in the constellation. The pilot or training signal is used to establish a frame lock, to train a digital signal processor (DSP) on the received signal, etc. That is, the pilot ortraining signal is a normal aspect of a coherent optical link. Importantly, since the pilot or training signal for a coherent constellation uses a subset of the constellation points (such as using only the outer <NUM> points of a <NUM>-QAM constellation), they can be acquired at low SNR and will have a lower BER than the full data constellation. For example, the subset of the constellation points can be outer points <NUM> in the <NUM>-QAM constellation and the <NUM>-QAM constellation. That is, it is possible to measure SNR of the optical link above the FEC threshold. In order to calculate the BER of the full data, the SNR of the pilot or training signal is scaled to the effective SNR of the full data constellation. This data SNR is then translated into BER using knowledge of the SNR to BER conversion for the operating mode. Note that this will be different for different operating modes.

The process <NUM> includes an SNR measurement to get a BER estimate beyond the FEC threshold, since it is not possible to calculate the BER beyond the FEC threshold. For coherent optical interfaces, pilot and training symbols for the pilot or training signal are inserted periodically to allow robust framing. In order to frame at low SNR values, these pilot and training symbols are defined as a subset of symbols which can tolerate a low SNR.

For example, the OIF 400ZR IA uses a <NUM>-QAM constellation to encode <NUM> bits of data per polarization, but a quadrature phase shift keying (QPSK) constellation to allow the pilot and training symbols to acquire lock at a lower SNR than the FEC. A QPSK constellation is just four symbols, one in each quadrant.

<FIG> is a graph of pre-FEC BER and optical signal-to-noise ratio (OSNR) for a <NUM> ZR optical interface. The graph includes delivered OSNR (in dB) on the x-axis. This means the actual OSNR over the optical link <NUM>. The graph includes a line <NUM> of pre-FEC BER which shows pre-FEC BER (on the left side y-axis) vs. the delivered OSNR. The graph further includes a line <NUM> of reported OSNR (on the right side y-axis) vs. the delivered OSNR. In this example, the FEC threshold is at <NUM>. 3dB OSNR, noted by a line <NUM>.

Note, on the right side of the line <NUM>, the FEC corrects all errors and the pre-FEC BER can be calculated/determined normally. On the left side of the line <NUM>, the FEC is overflowed, unable to correct all of the errors. The process <NUM> can provide the pre-FEC BER based on the measurement of SNR. That is, the graph shows an example measurement of estimated Pre-FEC BER and OSNR, before and after FEC overflow. Before FEC overflow, the Pre-FEC BER and OSNR is estimated using the FEC decoder. The pre-FEC BER and OSNR after FEC overflow is the focus of the process <NUM>.

Note, the pilot or training signal can be measured up until loss of lock (LOL). LOL is where the pilot or training signal is no longer received error free.

As known in the art, coherent modulation on the optical link <NUM> typically includes polarization multiplexing where an optical signal is modulated on both a horizontal (X) and vertical (Y) polarizations. Also, the FEC is distributed across both the polarizations. Because the FEC is distributed across the polarizations, typically only an aggregate BER is calculated. Using SNR to calculate BER, in the process <NUM>, allows the two polarizations to have their BER independently calculated. Since the standards specifications and typical use cases are for the aggregate BER, the SNR's of the two polarizations can be combined to determine the overall BER.

<FIG> is a graph of effective SNR vs. SNR delta for illustrating the SNR between the two polarizations in an example. Assume in this example:<MAT><MAT>.

A line <NUM> is the effective SNR which is the result of combining the BER from both polarization - it is the true answer. A line <NUM> is the SNR avg mean squared error which is the result of adding the noise power from both polarizations - a good approximation to the line <NUM>. A line <NUM> is the SNR min which is the results of taking the min SNR over both polarizations, a poor estimate.

Electrical interfaces also use FEC to improve their performance. SNR for an electrical interface can be estimated based on histograms of the levels. These histograms can be measured on either side of the FEC threshold. <FIG> and <FIG> are a graph of an eye diagram (<FIG>) and a histogram (<FIG>) of an electrical pulse amplitude modulation with <NUM> levels (PAM4). It is possible to determine the SNR based on the histograms and to use the process <NUM> to then determine the pre-FEC BER. In some embodiments, the same FEC is shared between the optical link <NUM> and the electrical link <NUM>. Here, only a portion of the available FEC coding gain is applied to the electrical interface, and since the FEC may not be terminated after the electrical interface, the BER from this interface is not directly measured. Using SNR as a BER estimate can provide an approach to determine if the BER from the electrical interface is within the allocation.

Standards defined in OIF and IEEE <NUM> are moving to concatenated FEC architectures in which an end-to-end FEC is enhanced by an inner FEC to improve performance. In these implementations a portion of the overall FEC is allocated to electrical interfaces, with the remainder allocated as an outer code for the optical link. In these scenarios having knowledge of the electrical FEC for each segment to enhance fault isolation will become increasingly important. Additionally, standards such as IEEE <NUM> specify a BER allocation to electrical interfaces. Using an SNR to BER conversion provides a means to verify if the electrical link is meeting the defined BER allocated to it. This can be applied in any link having interfaces at which FEC is unterminated.

In Concatenated FEC implementations, the inner FEC may be terminated at a different interface than the outer FEC. In these scenarios the BER of the inner FEC can not be measured using conventional error-counting techniques. The SNR to BER approach allows a means of measuring the BER of the link covered by the inner FEC.

<FIG> is process <NUM> of signal-to-noise ratio (SNR)-based bit error rate calculations for reporting beyond a forward error correction threshold. The process <NUM> can be implemented as a method having steps, via a receiver (either an optical or electrical receiver) configured to implement the steps, and via circuitry configured to implement the steps. For example, the process <NUM> can be implemented at the receiver <NUM> or at a receiver associated with the backplane <NUM>, and associated circuitry therein.

The process <NUM> includes measuring signal-to-noise ratio (SNR) of a first signal (step <NUM>); extrapolating the SNR of a first signal to determine an SNR of a second signal (step <NUM>); and determining a pre-FEC bit error rate (BER) for the second signal based on the determined SNR of the second signal (step <NUM>). The process <NUM> can further include reporting the determined pre-FEC BER (step <NUM>). Note, the process <NUM> can be used when the second signal is beyond a forward error correction (FEC) threshold.

The process <NUM> can further include determining an estimate of optical link parameters including any of chromatic dispersion (CD), polarization dependent loss (PDL), and differential group delay (DGD) of the first signal; and reporting the estimate of optical link parameters.

The process <NUM> can further include, responsive to the determined pre-FEC BER being at or near the FEC threshold, reporting the determined pre-FEC BER and one or more adjustments to perform to attempt to reach or operate below the FEC threshold.

The received signal is a coherent optical signal, the first signal is a pilot or training signal associated with the coherent optical signal, and the second signal is a data signal for the coherent optical signal. The pilot or training signal has a reduced number of symbols from the data signal. The coherent optical signal can include two polarizations including an X polarization and a Y polarization, and the process <NUM> can further include determine the pre-FEC BER for each of the two polarizations, and one or more (<NUM>) combine the pre-FEC BER for each of the two polarizations and (<NUM>) report each of the pre-FEC BER for each of the two polarizations.

The received signal can be an electrical signal utilizing pulse amplitude modulation (PAM), and the first signal and the second signal are both the electrical signal. The received signal can be an electrical signal, and the measured SNR is from a histogram of a received electrical signal. The process <NUM> can further include, responsive to a received signal being below the FEC threshold, determine the pre-FEC BER based on total received bits and total corrected errors.

In addition to using the SNR of a first signal to measure the BER of a second signal, the SNR of the first signal can also be used to calculate other SNR metrics of the second signal, such as eSNR, MER, EVM, and other standards-defined metrics.

<FIG> is a diagram of another communication link <NUM> with concatenated FEC. Specifically, the process <NUM> can be used to estimate the BER of an Inner code in a concatenated FEC scheme, as in <FIG>. To illustrate a concatenated FEC scheme, the communication link <NUM> includes two 800LR modules <NUM>, <NUM> connected to one another over a fiber link. Two host devices <NUM>, <NUM> are connected to one another via the 800LR modules <NUM>, <NUM>. For example, the 800LR modules <NUM>, <NUM> can be pluggable modules in the host devices <NUM>, <NUM>, which can be routers, switches, network devices, etc. The 800LR modules <NUM>, <NUM> can utilize a BCH inner code, and the host devices <NUM>, <NUM> can use a RS(<NUM>, <NUM>) outer code.

In these schemes, the inner code may not fully correct all errors and cannot always calculate BER based on bit-error counts. Again, specs for the Inner code are based on BER, so monitoring is useful. These concatenated codes are being implemented both for direct detect and for coherent in IEEE and OIF.

Those skilled in the art will understand there are various equations and approaches for determining BER from SNR, all of which are contemplated herewith. There are simplified expressions for QPSK, 16QAM, etc. to convert an SNR value to BER. For amplitude modulation, such as PAM-N interfaces, one such example is described in <NPL>, the contents of which are incorporated by reference.

<FIG> is a diagram of a PAM4 amplitude histogram and approach for determining SNR therefrom. <FIG> is Figure <NUM> from the OIF Implementation Agreement, Common Management Interface Specification (CMIS), Revision <NUM>, OIF-CMIS-<NUM>, April <NUM>, <NUM>, the contents of which are incorporated by reference. The histogram represents the distribution from the eye diagram of <FIG>. This describes how to estimate/determine SNR from a PAM histogram. Once SNR is determined, the above approaches can be used to determine BER based thereon.

In an embodiment, the present disclosure includes a transceiver module with circuitry configured to implement the various techniques described herein. The transceiver module can be a coherent optical modem, a pluggable optical module, a coherent pluggable optical module, a line card, an electrical transceiver, a pluggable electrical transceiver, and the like. That is, the present disclosure contemplates any physical embodiment of a transceiver, electrical or optical. As described herein, the present disclosure enables reporting of BER, SNR, and other metrics even after a signal is beyond the FEC threshold.

The transceiver can include circuitry configured to receive and process a signal with forward error correction (FEC), measure or determine the pre-FEC bit error ratio (BER), wherein the pre-FEC BER is measured in a first regime above a FEC threshold and determined in a second regime below the FEC threshold, and provide the pre-FEC BER for reporting. The transceiver can utilize coherent modulation where the pre-FEC BER is determined in the second regime based on measuring a constellation BER where the constellation is smaller than a constellation of the signal and extrapolating the pre-FEC BER therefrom.

It will be appreciated that some embodiments described herein may include one or more generic or specialized processors ("one or more processors") such as microprocessors; central processing units (CPUs); digital signal processors (DSPs): customized processors such as network processors (NPs) or network processing units (NPUs), graphics processing units (GPUs), or the like; field programmable gate arrays (FPGAs); and the like along with unique stored program instructions (including both software and firmware) for control thereof to implement, in conjunction with certain non-processor circuits, some, most, or all of the functions of the methods and/or systems described herein. Alternatively, some or all functions may be implemented by a state machine that has no stored program instructions, or in one or more application-specific integrated circuits (ASICs), in which each function or some combinations of certain of the functions are implemented as custom logic or circuitry. Of course, a combination of the aforementioned approaches may be used. For some of the embodiments described herein, a corresponding device in hardware and optionally with software, firmware, and a combination thereof can be referred to as "circuitry configured or adapted to," "logic configured or adapted to," etc. perform a set of operations, steps, methods, processes, algorithms, functions, techniques, etc. on digital and/or analog signals as described herein for the various embodiments.

Moreover, some embodiments may include a non-transitory computer-readable storage medium having computer-readable code stored thereon for programming a computer, server, appliance, device, processor, circuit, etc. each of which may include a processor to perform functions as described and claimed herein. Examples of such computer-readable storage mediums include, but are not limited to, a hard disk, an optical storage device, a magnetic storage device, a read-only memory (ROM), a programmable read-only memory (PROM), an erasable programmable read-only memory (EPROM), an electrically erasable programmable read-only memory (EEPROM), Flash memory, and the like. When stored in the non-transitory computer-readable medium, software can include instructions executable by a processor or device (e.g., any type of programmable circuitry or logic) that, in response to such execution, cause a processor or the device to perform a set of operations, steps, methods, processes, algorithms, functions, techniques, etc. as described herein for the various embodiments.

Claim 1:
A method (<NUM>, <NUM>) performed by a receiver comprising steps of:
measuring (<NUM>, <NUM>) signal-to-noise ratio, SNR, of a first signal;
extrapolating (<NUM>, <NUM>) the SNR of the first signal to determine an SNR of a second signal; and
determining (<NUM>, <NUM>) a pre-forward error correction, FEC, bit error ratio, BER, for the second signal based on the determined SNR of the second signal, characterised in that: the SNR is measured of the first signal and extrapolated to the second signal responsive to the second signal being beyond a FEC threshold, to determine the pre-FEC BER for the second signal, the FEC threshold being the value of BER below which the post-FEC BER requirement is met.