Patent Publication Number: US-2021184771-A1

Title: Receiver synchronization for higher speed passive optical networks

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. 62/947,600, filed on Dec. 13, 2019, which is herein incorporated by reference in its entirety for all purposes. 
    
    
     FIELD 
     The present disclosure relates to optical network units (ONUs) and passive optical networks (PON). The present disclosure relates also to systems and methods for receiver synchronization for an optical network receiver, or optical network unit (ONU), associated with PONs. 
     BACKGROUND 
     PON systems operating at speeds such as 10 Gbit/s or 25 Gbit/s (IEEE 802.3ca) may use synchronization state machines. Data rates in passive optical networks (PON) are increasing, e.g., to 50 Gbit/s per wavelength. With increasing transmission speed, the quality of signal transmission can be a concern. For user data, this can be mitigated by improved forward error correction (FEC) and digital equalization (DEQ). With possibly increasing bit error rates of higher speed PON running on 25 Gbit/s or 50 Gbit/s, there may be a desire for improved synchronization and detection of loss of synchronization. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  illustrates a synchronization state machine. 
         FIG. 2  illustrates an optical network unit and data signal. 
         FIG. 3  illustrates principles of error detection probabilities in synchronization patterns. 
         FIG. 4  illustrates principles of error detection probabilities in synchronization patterns. 
         FIG. 5  illustrates a synchronization state machine. 
         FIG. 6  illustrates a synchronization state machine. 
         FIG. 7  illustrates a method of operating an ONU. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a synchronization state machine  100 , according to examples described herein, which may be representative of an optical network receiver circuit (or optical network unit, ONU) for a passive optical network. The synchronization state machine  100  of  FIG. 1  shows a hunt state  120 , a pre-sync state  140 , a sync state  160 , and a re-sync state  180 . There are transitions between states shown with arrows. Transitions may be designated as forward transitions  121 ,  141 ,  181  which lead to or toward the sync state  160 . Transitions may be designated as back transitions  142 ,  162 ,  182  which lead to or toward the hunt state  120 , which may be an initial state. 
       FIG. 2  shows an incoming data signal  215  and optical network unit  200 , or receiver circuit, according to examples described herein. The incoming data signal  215  may include frames  230 , a psync pattern  210  (which can be a synchronization pattern), and/or blocks  211  of data. The frames  230  may be considered to include the psync pattern  210  and blocks  211 . The data signal  215  may have frame boundaries  212  between frames  230 , such as between consecutive frames, such as may be identified by being adjacent to psync patterns  210 .  FIG. 2  also illustrates a superframe  233  which may be a set of consecutive frames  230 . 
     An optical network receiver (ONU) circuit  200  associated with a passive optical network (PON) can include one or more processors  250 . The processor(s) may operate in a hunt state  120 , transition  121  to a pre-sync state  140 , and/or operate in the pre-sync state  140 . 
     Operating in the hunt state  120 , the one or more processors  250  may detect frame boundaries  212  associated with an incoming data signal  215 . The detection may be based on a detecting a predefined synchronization (psync) pattern  210 . The psync pattern  210  may be associated with a set of hunt frames  230  of one or more consecutive frames  230  associated with the incoming data signal. The hunt frames  230  may be received while in the hunt state  120 . 
     When the predefined psync pattern  210  is detected correctly for at least one frame  230  of the set of hunt frames, the ONU  200  may transition  121  to the presync state  140 . Compared to other methods of operation of an ONU, the forward transition  121  from hunt state  120  to pre-sync state  140  may occur more readily, particularly if the correct detection of the psync pattern  210  is a sufficient conditions for the state machine  100  to make the transition  121 . 
     In the pre-sync state  140 , the one or more processors  250  may perform forward error correction (FEC) decoding for a set of pre-sync frames  230 . The pre-sync frames  230  may be received while the processor(s)  250  are in the pre-sync state  140 . The pre-sync frames  230  may be one or more subsequent consecutive frames associated with the incoming data signal  215 . The processor(s) may determine signal statistics  293  associated with the incoming data signal  215 , such as by the FEC decoding  294  of the set of pre-sync frames. Compared to other methods of operation of an ONU, the determination of signal statistics  293  from the data, while in the pre-sync state  140 , may be allowed to stay in the pre-sync state  140  for multiple frames  230 . 
     Unlike other methods, the signal statistics  293  and/or decoding  294  may be leveraged to determine whether and how to transition, e.g. from the pre-sync state  140 . 
     The one or more processors  250  can transition  141  to a sync state  160 , such as when the FEC decoding  294  is successful for at least one frame  230  of the set of pre-sync frames. When in the sync state  160 , the one or more processors  250  can perform FEC decoding  294  for a set of sync frames. The sync frames can be one or more subsequent consecutive frames  230  which may be received while in the sync state  160 . If the FEC decoding  294  fails for at least one frame  230  of the set of sync frames, the processor(s) may transition to a re-sync state  180 . Utilizing the FEC correction to ensure sync can aid in quickly detecting a loss of sync. 
     In the re-sync state  180 , the processor(s) may perform FEC decoding  294  for a set of re-sync frames. The re-sync frames may include one or more subsequent consecutive frames  230 . The re-sync frames may be received while in the re-sync state  180 . When a number of failures associated with the consecutive FEC decoding  294  of the set of re-sync frames is less than a predefined decoding failure number  291 , the processor(s)  250  may stay in the re-sync state  180 . The processor(s)  250  may transition  181  to the sync state  160 , such as when the FEC decoding  294  is successful for at least one frame  230  of the set of re-sync frames. The signal statistics  293  and/or decoding  294  may be leveraged to determine whether and how to transition. 
     The one or more processors  250 , when operating in the re-sync state  180 , may transition  182  to the hunt state  120 , such as when the number of failures associated with the consecutive FEC decoding  294  of the set of re-sync frames reaches the predefined decoding failure number  291 . 
     The one or more processors, when operating in the pre-sync state  140 , may detect the psync pattern  210  for the set of pre-sync frames. When a number of number of errors associated with the psync pattern  210  of one frame  230  of the set of pre-sync frames is greater than a predefined psync threshold number  295 , the processor(s) may transition back to the hunt state  120 . 
     The FEC decoding  294  may include low-density parity-check (LDPC) decoding. Success of the FEC decoding  294  may be determined based on a set of parity check equations  292 . 
     The state machine  100  can remain in a state  120 ,  140 ,  160 ,  180  while transition conditions are improved. For example, during startup, the ONU enters the hunt state  120  as an initial state. For example, the ONU remains in the hunt state  120  until the receiver is synchronized to the transmit clock and the digital equalizers are trained. 
     A PON ONU can be used to achieve frame and symbol synchronization when the ONU is connected to the PON network while the PON optical line termination (OLT) is transmitting. It can be desirable to avoid a false synchronization without a valid optical line termination (OLT) signal. When the link is active and synchronized, it can be desirable to detect a loss of sync quickly to re-synchronize the link without long service interruption. It can be desirable to avoid a false loss of sync, e.g., a re-sync being triggered when the link is stable. With the increasing bit error rate of higher speed PON running on 25 Gbit/s or 50 Gbit/s, the prior arts&#39; synchronization and loss of synchronization mechanisms may not be sufficiently reliable. Improved FEC and DEQ methods can utilize information related to the frame and symbol boundaries for training signal statistics (for FEC) and equalizer coefficients (for DEQ). 
     The ONU  200  may have at least one processor  250  and a digital equalizer  260 . The ONU may utilize or generate one or more decoding failure numbers  291  (e.g. via FEC decoding  294 ), such as a predefined decoding failure number; one or more parity check equations  292 ; one or more signal statistics  293  (e.g. associated with the incoming data signal  215 ); and any combination thereof. Herein, a low density parity check (LDPC) is a species of forward error correction (FEC). 
     For user data, transmission errors and other problems can be mitigated by improved forward error correction (FEC) and digital equalization (DEQ). During the link training, an ONU (optical network receiver), such as one represented by the state machine of  FIG. 1 , can detect the symbol and frame boundaries of the signal transmitted from the OLT (optical line termination). Improved FEC and DEQ methods can utilize information related to the frame and symbol boundaries for training signal statistics (for FEC) and equalizer coefficients (for DEQ). Herein are disclosed robust framing and symbol boundary detection methods and systems that can be used in higher speed (e.g. 25 Gbit/s or 50 Gbit/s) PON. 
     Returning to  FIG. 1 , the hunt state forward transition  121  from the hunt state  120  into the pre-sync state  140  can be performed when one or more frame boundaries  212  are detected correctly, such as by detecting the psync pattern  210 . The symbol timing and/or frame timing can be detected correctly such that the receiver, e.g. state machine  100 , is synchronized to the transmit clock. In addition, the digital equalizer(s)  260  may be converged sufficiently to detect the frame boundary  212 , such as before the forward transition  121  from the hunt state  120  into the pre-sync state  140 . 
     Transitions, especially forward transitions such as the hunt state forward transition  121  to pre-sync state  140 ; and pre sync forward transition  141  to sync state can be based at least in part on observation/identification/decoding of the psync pattern  210 . The psync pattern  210  may be identified, preferably with zero errors, or with a low number of errors. 
     In an example, synchronization detection can be based exclusively on the pysnc pattern  210 . The hunt forward transition  121  from hunt state  120  to pre-sync state  140  and/or the pre-sync forward transition  141  from pre-sync state  140  to sync state  160  can occur based exclusively on detection/identification/decoding of the psync pattern  210 . 
     For example, when a 64 bit psync pattern is detected without error in the hunt state  120 , the ONU can transition  121  to the pre-sync state  140 . Alternatively, a detection of up to 2 errors may also be adequate to trigger the hunt forward transition  121  to the pre-sync state  140 . If the forward transition is not exclusively based on the psync pattern  210 , e.g. detection of the psync pattern with 2 errors or fewer, then the superframe counter (SFC) may also be used for synchronization. The superframe counter may be protected with a 2 error correcting, 3 error detecting code, for example. 
     The state machine  100 , in the pre-sync state  140 , can reverse transition  142  into the hunt state  120  from the pre-sync state  140  when too many errors are detected in the psync pattern  210  for multiple consecutive frames  230 . 
       FIG. 1  illustrates a synchronization state machine  100  according to examples described herein. PON systems operating at lower speeds (e.g., 10 Gbit/s or 25 Gbit/s IEEE 802.3ca) can use a synchronization state machine  100  such as that illustrated in  FIG. 1 . The transitions can be based on observation of a known synchronization pattern (Psync pattern)  210 , which can be identified without errors or with a low number of errors, e.g. up to 2 errors in a 64 bit sync pattern. 
     Relevant information in the header, the SFC, and the overhead control structure (OC) can be protected with a FEC that is independent of the data FEC. It is possible that the FEC can be enabled and disabled by the overhead control structure (OC). 
     In other methods of operating a state machine  100 , ONU, or the like, there can be reliance on a lower raw bit error rate of the received signal  215 , such as no errors, in order to detect the synchronization pattern  210  with low probability of mis-detection, and decode the overhead channels. Alternatively/additionally, receiver elements may require some training time, which may not be taken into account by other methods. It may not be possible, in other methods, to stay in the pre-sync state  140  for the time to train adaptive elements. Herein, a method of operation is described in which the option to enable and disable the FEC is not utilized, which may be useful for higher speed PON (e.g. at 25 or 50 Gbit/s or higher). For example, there may be no decoding of the overhead channel independent of the data FEC. 
     Synchronization detection can be based on the pysnc pattern  210 , only, which can be used for the forward transition  121  from hunt state to pre-sync state and for the forward transition  141  from pre-sync state to hunt state. The receiver  200  can remain in the pre-sync state  140  as long as the sync pattern  210  is found, while the adaptive elements are trained to reach sufficient reliability to remain in sync. It is possible to take advantage of the data FEC (e.g., an LDPC code) to protect the overhead channels and to detect a loss of synchronization. The detection of the out-of-sync condition can be derived from the data FEC. 
     There can be an allowance for an increased bit error rate, particularly when in the pre-sync state  140  and/or when determining to forward transition  121  from the hunt state to the pre-sync state. A reliable transition into the synchronized state  160  is possible. Detection of an out-of-sync condition can be done quickly and reliably. The state machine  100  can stay in the pre-sync state  140  for the time required to come to a stable operation point. After a forward transition  141  from the pre-sync state  140  into the sync state  160 , the state machine  100  can remain in the sync state  160 , such as for as long as FEC decoding (e.g. LDPC decoding) is successful. 
     Robust frame and symbol boundary detection for operation of an ONU  200  is possible, for example. The states of the sync state machine  100  of  FIG. 1  can be utilized: hunt state  120 , pre-sync state  140 , sync state  160 , and re-sync state  180 . The transition conditions, e.g. the conditions which may trigger or may be determined for transitions from one state to another, can be as in the examples described herein. For example, the ONU  200  can remain in the hunt state  120  until the receiver  200  is synchronized to the transmit clock and the digital equalizers are trained. 
     During startup, the ONU device can enter the hunt state  120  as an initial state. The forward transition  121  from the hunt state  120  into the pre-sync state  140  can be performed/triggered when the frame boundaries  212  are detected correctly by detecting the psync pattern  210 . The symbol timing can be detected correctly and/or the receiver  200  can be synchronized to the transmit clock, and the forward transition  121  from hunt state to pre-sync state can be triggered. In addition, the digital equalizers  260  may be converged sufficiently such that the frame boundary  212  is detected. 
     The initial detection of frame boundaries  212  can be based on matching of a bit pattern, such as that of a sync pattern  210 . The detection of the frame boundaries  212  can be checked for each incoming bit, which is, e.g., 25 or 50 billion times per second. The pattern matching may be designed such that the detection is successful after a few attempts, which can avoid long idle times. The probability of a mis-detect, e.g., an erroneous detection of a frame boundary  212  may be avoided. A mis-detect may delay a correct training of the link. 
       FIG. 3  shows the probability for a certain number of errors in the pattern (left) and a certain maximum number of errors in the pattern (right), according to examples described herein. The vertical axes of  FIG. 3  are each probability.  FIG. 3  can be representative of a 64-bit synchronization (psync) pattern.  FIG. 3  (left) shows the probability of a number of errors in a 64 bit sequence when the raw bit error rate is 2×10 &#39;12  (trace  302 ), 3×10 −2  (trace  303 ), and 4×10 −2  (trace  304 ).  FIG. 3  (right) shows the probability of a maximum number of errors in a 64 bit sequence when the raw bit error rate is 2×10 −2  (trace  312 ), 3&#39;10 −2  (trace  313 ), and 4×10 −2  (trace  314 ). 
     The forward transition  121  from hunt state  120  into the pre-sync state  140  can happen quickly and/or be triggered when the raw bit error rate is in an expected range, such as below a maximum value. The forward transition  121  may not happen and/or may not be triggered when the bit error rate is too high. The forward transition  121  into the pre-sync state from the hunt state can be done when the sync pattern  210  is observed with a certain maximum number of errors, e.g., with 0 errors or with up to 1 error. 
     A forward transition  121  from hunt state  120  to pre-sync state  140  may occur in which there is no check of the superframe counter. Alternatively/additionally, a forward transition  121  from hunt state  120  to pre-sync state  140  may occur in which there is no FEC decoding of the SFC codeword. The state transitions can be thereby simplified. Other synchronization patterns and/or types of synchronization patterns are contemplated, e.g. a longer synchronization pattern. With a longer synchronization pattern, the desired bit error rate for a forward transition  121  from the hunt state  120  into the pre-sync state  140  can be controlled more finely/accurately. The complexity of the pattern search operation as well as the associated transmission overhead may increase with the length of the sync pattern  210 . 
     With the introduction of soft decision input codes like LDPC codes, information on the signal statistics may be used for accurate soft information. In an example, only a raw bit error rate measurement may be required. With analog-to-digital converter-based receivers (ADC-based receivers), the signal probability density function for each possible transmit symbol can be measured. This may take some time until the measurement is sufficiently accurate to achieve the full decoder performance. 
     It may be useful, as depicted in  FIG. 4 , to compare the probability of a number of errors found in a sync pattern, for different raw bit error rates, for different error rates (as well as for a lack of sync).  FIG. 4  illustrates probabilities (vertical axis) for given numbers of errors per 64 bit sequence. When there is no sync, the probability of errors is given in trace  490 . As can be seen from trace  490  of  FIG. 4 , the most likely number of errors in a 64 bit sequence is 32.  FIG. 4  also shows the probability of the number of errors in the case where there is sync.  FIG. 4  shows the probability of a number of errors in a 64 bit sequence when the raw bit error rate is 2×10 −2  (trace  402 ), 3×10 −2  (trace  403 ), and 4×10 −2  (trace  404 ). 
     To support the signal statistics measurement, it is possible to stay in the pre-sync state  140  for multiple frames  230 , which may not be the case for other methods. 
     In case that the receive bit error rate is higher than expected and/or indicates that there is a loss of sync or an erroneous transition into the pre-sync state  140 , the ONU  200  can back transition  142  from the pre-sync state  140  to the hunt state  120 .  FIG. 4  shows the probability to detect a 64 bit pattern with up to Nmax errors for different raw bit error rates. This can be compared to the probability of detecting the pattern  210  with up to Nmax errors in a random bit sequence, which can correspond to a loss of sync. 
     Observing  FIG. 4 , the ONU  200  can decide to stay in Pre-Sync state  140  when the number of errors is less than a threshold, e.g. a threshold defined by the intersection of the no sync curve  490  and the maximum bit error rate curve ( 402 ,  403 ,  404 , which may depend on the tolerance for error), such as that illustrated in  FIG. 2 . For example, up to N esync =10 errors can be the threshold for a system designed for up to 2e−2 raw bit error rate. In case that more than 10 errors are observed, a back transition  142  back into the hunt state  120  may be triggered. The value of N esync  can be selected for the maximum raw bit error rate that is supported by the receiver  200  and/or not adjusted during synchronization. 
     In one example, a binary (hard decision) input signal û t  ∈ {0,1} is available at the receiver  200 , which can be used to derive the soft information for the LDPC decoder. In the pre-sync state  140 , the error probabilities are measured. The transmitted bit sequence on the psync pattern u t  is known to the receiver, which receives a sequence û t . In a channel without memory, there are the probabilities for 0 transmitted and a 1 received p u,û (u=0,û=1) and the probability for 1 transmitted and 0 received p u,û (u=1,û=0), which are derived by counting the corresponding events. For a channel with memory, the probabilities are for û=0 or 1, for a given sequence of received bits û post =[u t−N  . . . u t−1 ] and û pre =[u t+1  . . . u t+K ]. p u,û , û post , û pre  can be measured by counting the corresponding events. In another example, an oversampled binary signal can be available at the receiver  200 , e.g., a twice oversampled signal, the binary signal vectors may contain the intermediate samples, e.g., û post =[u t−N , u t−N+1/2  . . . u t−1 , u t−1/2 ] and û post =[u t+1/2 , u t+1  . . . u t+k−1/2 , u t+K ]. 
     In another example, an analog-to-digital converter (ADC) with b bits is available at the receiver  200  and the receiver signal y t  ∈ {0, . . . 2 b −1} gives b bits per sample. In this case, the receiver  200  can measure the probability p u,y (u=0, y=0, . . . , 2b−1) and p u,y (u=1, y=0, . . . , 2b−1) for all possible values of y, using the Psync sequence  210 . This can be practical if the resolution b of the analog-to-digital converter is low. With higher resolutions, it can be more practical to derive statistical parameters describing the receive signal, e.g., the mean of the receive signal for 0 transmitted μ(u=0) and for 1 transmitted μ(u=1) and the variances σ 2 (u=0) and σ 2 (u=1). When the statistics measurement is sufficiently stable, the statistics information can be used to derive the soft information for the LDPC decoder. 
     When the LDPC decoding is successful for multiple consecutive frames  230 , e.g. at least 2 consecutive frames, and the superframe counter is valid, the ONU  200  can forward transition  141  into the sync state  160  from the pre-sync state  140 . 
     In case of a loss of sync, particularly when in the sync state  160 , a quick detection of the failure state is important to keep the service interruption short. The out-of-sync detection can be based on the LDPC decoder status. In one example, only one LDPC codeword per frame  230 , e.g., the first codeword of the frame, is used to check the out-of-sync condition. A successful FEC decoding can be checked by a set of parity check equations  292 . When all parity check equations  292  are satisfied for the hard decision output of the FEC decoder, the codeword is valid. 
     In case that one or more of the parity check equations  292  are un-satisfied, particularly while in the sync state  160 , the decoding has failed. If this happens for the first codeword of one frame, the ONU can back transition  162  into the re-sync state  180  from the sync state  160 . When, in the re-sync state  180 , the decoding fails for M−1 more consecutive frames  230  for the first codeword of the frame  230 , a loss of synchronization is detected and the ONU can back transition  182  to the hunt state 120  from the re-sync state  180 . 
     In another example, each FEC codeword can be considered to detect the out-of-sync condition. When in the sync state  160 , when decoding of any LDPC codeword fails and one or more of the parity check equations  292  are unsatisfied, the ONU  200  back transitions  162  into the re-sync state  180 ; and if, in the re-sync state  180 , the decoding fails on another M−1 consecutive FEC codewords, a loss of synchronization is detected and the ONU back transitions  182  into the hunt state  120 . With that, the out-of sync can be detected more quickly. The receiver  200  may thereby be more susceptible to back-transitions from the sync state  160  and/or re-sync state  180 , particularly to error events which affect multiple consecutive codewords. 
     An Optical Networks Unit (ONU) or optical network receiver (ONU) circuit  200  for Passive Optical Networks (PON), which synchronizes to the OLT transmission is disclosed. Herein is described:
         a hunt state  120 , in which frame boundaries  212  are detected; the clock recovery can be established and adaptive digital equalizers  260  are optionally trained.   a pre-sync state  140  in which signal statistics for the soft input FEC can be determined.   a forward transition  121  from the initial (hunt) state  120  to the pre-sync state  140  which can be based on thresholds of the number of errors observed in a known (Psync) pattern  210 , e.g. a psync pattern  210  that is sent with every frame  230 . If the synchronization pattern  210  is found with a certain minimum number of errors (e.g. less than a maximum number of errors), the ONU  200  can forward transition  121  to the pre-sync state  140 .   a back transition  142  from the pre-sync state  140  to the hunt state  120  can be triggered, for example, if, in the pre-sync state  140 , the number of errors observed in the Psync pattern  210  exceeds a certain threshold.   a forward transition  141  to the synchronized state  160  from the pre-sync state  140  can be performed/triggered when the soft input FEC is correcting all codewords for multiple successive codewords, e.g. at least two consecutive codewords.       

     Herein, an Optical Networks Unit (ONU), or optical network receiver circuit  200 , for Passive Optical Networks (PON) is described. The ONU  200  can detect a loss of synchronization based on multiple consecutive failures of the data FEC, e.g. based on un-satisfied parity check equations  292 . 
       FIG. 5  illustrates a synchronization state machine  500  according to examples described herein. The example illustrated in  FIG. 5  may be compared to that of  FIG. 1 . The synchronization state machine of  FIG. 5  may be representative of an optical network receiver circuit (or optical network unit, ONU) for a passive optical network. The synchronization state machine  500  of  FIG. 5 , like that of  FIG. 1 , shows a hunt state, a pre-sync state, a sync state, and a re-sync state. 
     As illustrated in  FIG. 5 , a transition from hunt state to pre-sync state may be triggered when there is a detection of the psyc pattern with no errors, and the superframe counter (SFC) is usable. A back transition may occur when the psync and/or SFC fails. A forward transition from the pre-sync state to the sync state may be triggered and/or occur when both the psync and SFC are verified. In the sync state, when both the psync pattern and the SFC are verified, the ONU can remain in the sync state. When at least one of the psync pattern and the SFC fails, there can be (triggered) a back transition to the re-sync state. A forward transition to the sync state from the re-sync state can occur if bot the psync pattern and the SFC are verified. Otherwise, if there are Pscyn and/or SFC failures, the ONU can remain in the re-sync state up to a certain number of consecutive PSync and/or SFC failures, e.g. M−1, at which there can be triggered a back transition from the re-sync state to the hunt state. 
       FIG. 6  illustrates a synchronization state machine  600  according to examples described herein. The example illustrated in  FIG. 6  may be compared to that of  FIG. 1 . The synchronization state machine  600  of  FIG. 6  may be representative of an optical network receiver circuit (or optical network unit, ONU) for a passive optical network. The synchronization state machine  600  of  FIG. 6 , like that of  FIG. 1 , shows a hunt state, a pre-sync state, a sync state, and a re-sync state. 
     As illustrated in  FIG. 6 , a transition from hunt state to pre-sync state may be triggered when there is a detection of the psyc pattern with no errors. The ONU may stay in the pre-sync state when there is up to Nesync errors detected, and/or LDPC decoding fails. When there are more than Nesync errors, a back transition from the pre-sync state to the hunt state may be triggered. It is possible that the SFC is not utilized in the determinations to remain or transition between the pre-sync state and hunt state. 
     A forward transition from the pre-sync state to the sync state may be triggered and/or occur when LDPC decoding is successful and/or SFC is incrementing. The ONU may remain in the sync state while the LDPC decoding is successful; LDPC decoding success may be the only requirement to remain in the sync state. If LDPC decoding fails, there can be triggered a transition from sync state to re-sync state; LDPC decoding failure may be the only requirement to trigger the transition. When, in the re-sync state, and the LDPC decoding is successful, there can be a transition to the sync state; LDPC decoding success may be the only requirement to transition from the re-sync state to the sync state. The ONU can remain in the re-sync state when there are LDPC decoding failures, however, when there are a threshold number of consecutive LDPC decoding failures, e.g. M−1, a transition from re-sync state to hunt state can be triggered. The ONU remains in the hunt state while no psync pattern is detected. 
       FIG. 6  illustrates an optical network receiver (ONU) circuit associated with a passive optical network (PON). The ONU includes one or more processors. The processors can operate in a hunt state. When frame boundaries associated with an incoming data signal are detected, based on a detecting a predefined synchronization (psync) pattern associated with a set of frames (e.g. hunt frames), the ONU can transition to a pre-sync state. The hunt frames may be one or more consecutive frames associated with the incoming data signal, which are detected while the ONU is in the hunt state. When the predefined psync pattern is detected correctly for at least one frame of the set of hunt frames, the transition to the pre-sync state can occur. 
     In the pre-sync state, the one or more processors performs forward error correction (FEC) decoding for a set of pre-sync frames, e.g. one or more subsequent consecutive frames associated with the incoming data signal which are received while in the pre-sync state. The processor(s) can determine signal statistics associated with the incoming data signal. The statistics can be based at least in part on FEC decoding. 
     As illustrated in  FIG. 6 , the one or more processors can transition to a sync state, when the FEC decoding is successful for at least one frame of a set of pre-sync frames, e.g. frames received while in the pre-sync state. 
     As illustrated in  FIG. 6 , the one or more processors in the sync state, can perform FEC decoding for a set of sync frames, which may be received while in the sync state. The sync frames may include one or more subsequent consecutive frames. When the FEC decoding fails for at least one frame of the set of sync frames, the ONU may transition to the re-sync state. 
     In the re-sync state, FEC decoding may occur for a set of re-sync frames, which may be frames received while in the re-sync state. The re-sync frames may be one or more subsequent consecutive frames. The ONU may stay in the re-sync state while a number of failures associated with the consecutive FEC decoding of the set of re-sync frames is less than a predefined decoding failure number, which may be a type of threshold. When the FEC decoding is successful for at least one frame of the set of re-sync frames, there may be a transition back to the sync state. 
     From the re-sync state, it is also possible to transition back to the hunt state, such as when the number of failures associated with the consecutive FEC decoding of the set of re-sync frames reaches a predefined decoding failure number. 
     The FEC decoding can include a low-density parity-check (LDPC) decoding. 
     Herein, success of the FEC decoding can be determined based on a set of parity check equations. 
       FIG. 7  illustrates a method  700  of operating an optical network receiver (ONU), such as in a PON. The network receiver may be operated, as illustrated in the method  700  depicted in  FIG. 7 , in accordance with the descriptions to the state machines  100 ,  500 ,  600 , described herein; the state machines  100 ,  500 ,  600  described herein may operate in accordance with the method  700  described herein. 
     As an initial state, the ONU may operate in a hunt state  720 , which may transition to a pre-sync state  740 . A back transition from the pre-sync state  740  to the hunt state is possible. A forward transition from the pre-sync state  740  into a sync state  760  is also possible. From the sync state  760 , there may be a back transition to a re-sync state  780 . A transition from the sync state  760  back to the pre-sync state  740  may not be allowed, according to an example. From the re-sync state  780 , there may be either a back transition or a forward transition, to the hunt state  720  and sync state  760 , respectively. From the hunt state  720 , it may be that the only transition allowed is to the pre-sync state  740 . 
       FIG. 7  may illustrate a method  700  for an optical network receiver (ONU) circuit  200  which includes one or more processors  250 . The method  700  includes operating the one or more processors in a hunt state  720 . The one or more processors  250  can detect frame boundaries associated with an incoming data signal based on a detecting a predefined synchronization (psync) pattern associated with a set of hunt frames. The hunt frames may include one or more consecutive frames  230  associated with the incoming data signal  215 . The processor(s)  250  may transition to a pre-sync state  740 , such as when the predefined psync pattern  210  is detected correctly for at least one frame  230  of the set of hunt frames. 
     The one or more processors  250 , in the pre-sync state  740 , may perform forward error correction (FEC) decoding  294  for a set of pre-sync frames, which may be one or more subsequent consecutive frames  230  associated with the incoming data signal  215 . The pre-sync frames may be received while in the pre-sync state  740 . The pre-sync frames and the FEC decoding may be used to determine signal statistics  293  associated with the incoming data signal  215 . 
     As illustrated in  FIG. 7 , there may be a transition, e.g. by the one or more processors  250 , to a sync state  760 , such as when the FEC decoding is successful for at least one frame of the set of pre-sync frames. 
     The method  700  illustrated in  FIG. 7  may also include operating the one or more processors, in the sync state  760 , in which FEC decoding is performed for a set of sync frames of one or more subsequent consecutive frames. The sync frames may be received while in the sync state  760 . The method may allow for a transition to a re-sync state  780 , such as when the FEC decoding fails for at least one frame of the set of sync frames. 
     As illustrated in  FIG. 7 , the method  700  may include operating the one or more processors in the re-sync state  780 . The one or more processors can perform FEC decoding for a set of re-sync frames of one or more subsequent consecutive frames, which may be received while in the re-sync state. From the re-sync state  780 , there may be a transition back (or forward) to the sync state  760 , such as when the FEC decoding is successful for at least one frame of the set of re-sync frames. 
     The one or more processors may transition from the re-sync state  780  back to the hunt state  720 , such as when the number of failures associated with the consecutive FEC decoding of the set of re-sync frames reaches a predefined decoding failure number. 
     The one or more processors may detect the psync pattern for the set of pre-sync frames, such as when the one or more processors is operating in the pre-sync state  740 . 
     The processor(s) may transition back to the hunt state  720 , for example when a number of errors associated with the psync pattern of one frame of the set of pre-sync frames is greater than a predefined psync threshold number. 
     With reference to  FIGS. 1, 6, and 7 , an optical network receiver circuit (ONU) for a passive optical network (PON) can have the one or more processors configured to operate in each of a hunt state, a pre-sync state, a sync state, and a re-sync state. The ONU may transition from the hunt state to the pre-sync state, transition from the pre-sync state to the sync state, transition from the sync state to the re-sync state, and transition from the re-sync state to the hunt state. 
     A first sufficient and/or necessary condition for a transition from the hunt state to the pre-sync state can be that a sync pattern is detected with no errors while in the hunt state. A second sufficient and/or necessary condition for a second transition from the pre-sync state to the hunt state can be a detection of errors of the sync pattern above a threshold while in the pre-sync state. A third sufficient and/or necessary condition for a third transition from the sync state to the re-sync state can be forward error check decoding failure of at least one frame received while in the sync state. A fourth sufficient and/or necessary condition for a fourth transition from the re-sync state to the sync state can be a successful forward error check decoding of at least one frame received while in the re-sync state. 
     The transition from the hunt state to the pre-sync state can be directly reversible; the ONU can transition back and forth directly between hunt and pre-sync state depending on conditions. The transition from the pre-sync state to the sync state can be directly irreversible; going from pre-sync state to the sync state may be possible, while the direct transition from the sync state directly back to the pre-sync state may not be allowed. As seen in the relevant figures (e.g.  FIGS. 1, 6, and 7 ), the transition from the sync state to the re-sync state can be directly reversible. The transition from the re-sync state to the hunt state can directly irreversible. 
     The ONU can remain in the pre-sync state when the errors detected of the sync pattern are equal to or less than the threshold and the forward error check decoding fails. The threshold can be more than zero. 
     The apparatus and methods described herein may aid in reliable, rapid, and/or robust synchronization of optical network receivers associated with passive optical networks, particularly those operating at higher speeds such as 25 Gbit/s, 50 Gbit/s. Alternatively/additionally, the apparatus and methods may allow for rapid detection of loss of synchronization, and robust and/or rapid means of returning to synchronization. 
     Examples can include subject matter such as a method, means for performing acts or blocks of the method, and at least one machine-readable medium (e.g. a computer readable medium, which may be a nontransitory medium) including instructions that, when performed by a machine cause the machine to perform acts of the method or of an apparatus or system for concurrent communication using multiple communication technologies according to examples described herein. 
     For convenience and/or further description, an enumerated list of examples are provided hereinbelow. 
     Example 1 is an optical network receiver (ONU) circuit associated with a passive optical network (PON) comprising one or more processors configured to operate in a hunt state, wherein the one or more processors is configured to detect frame boundaries associated with an incoming data signal based on a detecting a predefined synchronization (psync) pattern associated with a set of hunt frames comprising one or more consecutive frames associated with the incoming data signal; transition to a pre-sync state, when the predefined psync pattern is detected correctly for at least one frame of the set of hunt frames; and operate in the pre-sync state, wherein the one or more processors is configured to perform forward error correction (FEC) decoding for a set of pre-sync frames comprising one or more subsequent consecutive frames associated with the incoming data signal. Optionally, the FEC decoding, particularly while in the pre-sync state, may be used to determine signal statistics associated with the incoming data signal. 
     Example 2 is an ONU circuit, including the subject matter of example 1, wherein the one or more processors is further configured to transition to a sync state, when the FEC decoding is successful for at least one frame of the set of pre-sync frames. 
     Example 3 is an ONU circuit, including the subject matter of any of examples 1-2, including or omitting elements, wherein the one or more processors, when operating in the sync state, is configured to perform FEC decoding for a set of sync frames comprising one or more subsequent consecutive frames; and transition to re-sync state, when the FEC decoding fails for at least one frame of the set of sync frames. 
     Example 4 is an ONU circuit, including the subject matter of any of examples 1-3, including or omitting elements, wherein the one or more processors, when operating in the re-sync state, is further configured to perform FEC decoding for a set of re-sync frames comprising one or more subsequent consecutive frames; stay in the re-sync state, when a number of failures associated with the consecutive FEC decoding of the set of re-sync frames is less than a predefined decoding failure number; and transition back to the sync state, when the FEC decoding is successful for at least one frame of the set of re-sync frames. 
     Example 5 is an ONU circuit, including the subject matter of any of examples 1-4, including or omitting elements, wherein the one or more processors, when operating in the re-sync state, is further configured to transition back to the hunt state, when the number of failures associated with the consecutive FEC decoding of the set of re-sync frames reaches the predefined decoding failure number. 
     Example 6 is an ONU circuit, including the subject matter of any of examples 1-5, including or omitting elements, wherein the one or more processors, when operating in the pre-sync state, is further configured to detect the psync pattern for the set of pre-sync frames; and transition back to the hunt state, when a number of number of errors associated with the psync pattern of one frame of the set of pre-sync frames is greater than a predefined psync threshold number. 
     Example 7 is an ONU circuit, including the subject matter of any of examples 1-6, including or omitting elements, wherein the FEC decoding comprises low-density parity-check (LDPC) decoding. 
     Example 8 is an ONU circuit, including the subject matter of any of examples 1-7, including or omitting elements, wherein a success of the FEC decoding is determined based on a set of parity check equations. 
     Example 9 is a method for an optical network receiver (ONU) circuit comprising one or more processors, the method comprising operating, the one or more processors, in a hunt state, wherein the one or more processors is configured to detect frame boundaries associated with an incoming data signal based on a detecting a predefined synchronization (psync) pattern associated with a set of hunt frames comprising one or more consecutive frames associated with the incoming data signal; transitioning, the one or more processors, to a pre-sync state, when the predefined psync pattern is detected correctly for at least one frame of the set of hunt frames at the one or more processors; and operating, the one or more processors, in the pre-sync state, wherein the one or more processors is configured to perform forward error correction (FEC) decoding for a set of pre-sync frames comprising one or more subsequent consecutive frames associated with the incoming data signal. Optionally, the FEC decoding, particularly while in the pre-sync state, may be used to determine signal statistics associated with the incoming data signal. 
     Example 10 is a method, including the subject matter of example 9, further comprising transitioning, the one or more processors, to a sync state, when the FEC decoding is successful for at least one frame of the set of pre-sync frames. 
     Example 11 is a method, including the subject matter of any of examples 9-10, including or omitting elements, further comprising operating, the one or more processors, in the sync state, wherein the one or more processors is configured to perform FEC decoding for a set of sync frames comprising one or more subsequent consecutive frames; and transitioning, the one or more processors, to a re-sync state, when the FEC decoding fails for at least one frame of the set of sync frames. 
     Example 12 is a method, including the subject matter of any of examples 9-11, including or omitting elements, further comprising operating, the one or more processors, in the re-sync state, wherein the one or more processors is configured to perform FEC decoding for a set of re-sync frames comprising one or more subsequent consecutive frames; and transitioning, the one or more processors, back to the sync state, when the FEC decoding is successful for at least one frame of the set of re-sync frames. 
     Example 13 is a method, including the subject matter of any of examples 9-12, including or omitting elements, further comprising transitioning the one or more processors from the re-syn state back to the hunt state, when the number of failures associated with the consecutive FEC decoding of the set of re-sync frames reaches the predefined decoding failure number. 
     Example 14 is a method, including the subject matter of any of examples 9-13, including or omitting elements, further comprising detecting, using the one or more processors, the psync pattern for the set of pre-sync frames, when the one or more processors is operating in the pre-sync state; and transitioning, the one or more processors, back to the hunt state, when a number of number of errors associated with the psync pattern of one frame of the set of pre-sync frames is greater than a predefined psync threshold number. 
     Example 15 is a method, including the subject matter of any of examples 9-14, including or omitting elements, wherein the FEC decoding comprises low-density parity-check (LDPC) decoding. 
     Example 16 is an optical network receiver circuit for a passive optical network (PON), including one or more processors. The processor(s) operate in each of a hunt state, a pre-sync state, a sync state, and a re-sync state. The processor(s) can transition from the hunt state to the pre-sync state, transition from the pre-sync state to the sync state, transition from the sync state to the re-sync state, and transition from the re-sync state to the hunt state. A first sufficient condition for a transition from the hunt state to the pre-sync state is that a sync pattern is detected with no errors while in the hunt state. A second sufficient condition for a second transition from the pre-sync state to the hunt state is a detection of errors of the sync pattern above a threshold while in the pre-sync state. 
     In example 17, which includes the optical network receiver circuit of example 16, the transition from the hunt state to the pre-sync state is directly reversible. The transition from the pre-sync state to the sync state is directly irreversible. The transition from the sync state to the re-sync state is directly reversible. The transition from the re-sync state to the hunt state is directly irreversible. 
     Example 18, which includes the optical network receiver circuit of either examples 16 or 17, has the processors configured such that a third sufficient condition for a third transition from the sync state to the re-sync state is forward error check decoding failure of at least one frame received while in the sync state. 
     Example 19, which includes the optical network receiver circuit of any of examples 16-18. The optical network receiver circuit of claim  18 , wherein the one or more processors is configured such that a sufficient condition for a transition from the re-sync state to the sync state is a successful forward error check decoding of at least one frame received while in the re-sync state. 
     Example 20 is the optical network receiver circuit of any or claim  17 - 19 , in which the one or more processors can remain in the pre-sync state when the errors detected of the sync pattern are equal to or less than the threshold, and the forward error check decoding fails. The threshold can be more than zero. 
     Herein, a transition from one state to another state may be described to be a back transition or forward transition; “forward” and “back” are used to aid in understanding. A back transition does not require that the device, ONU, state machine, or the like to have previously been in the state to which the “back transition” occurs; nevertheless a back transition to a previous state is possible. “Back transition” may be used to indicate a transition that brings the machine from an initial state into the hunt state or closer to the hunt state. Similarly, “forward” transitions may not necessarily require that the state machine is advancing to a state in which the machine has not already been. “Forward transition” may be used to indicate a transition that brings the device, ONU, state machine, or the like from an initial state into the sync state or closer to the sync state. 
     Herein a set of frames is not necessarily a superframe. For example, a set of consecutive frames is not necessarily a superfame. A set of frames, e.g. a set of consecutive frames, may be fewer in number than that of a superframe. 
     Herein, adaptive elements may refer to digital equalizers. 
     Herein, parity check equation(s) may determine the success of FEC decoding. 
     Herein, the meeting of various conditions, described herein according to various examples, may be considered to trigger a transition from one state to another of an ONU, state machine, and/or processor(s). To “trigger” or “be triggered” may be used in the sense of to allow or to have met condition(s), e.g. necessary and/or sufficient condition(s), for the transition of state. 
     The transition may be considered to be a transition from one state to another of a processor or plurality of processors. Conditions may be as described herein and/or described in the figures, such as  FIGS. 5 and/or 6 . For example, the text of  FIG. 6  associated with arrows between states shows conditions for transitions between states. For example, the conditions described in  FIG. 6  may be the only condition(s) which are met to cause/trigger a particular transition; e.g. a psync detection with no errors is necessary and sufficient for the transition from hunt state to pre-sync state (see  FIG. 6 ); e.g. a successful LDPC decoding is necessary and sufficient for the transition from re-sync state to sync state occurs (see  FIG. 6 ). 
     Herein a receiver such as an optical network receiver may be regarded as a type of optical network unit and/or circuit. The abbreviation ONU may refer to an optical network unit, or, more specifically, to an optical network receiver, optical network receiver circuit, and/or optical network receiver circuitry. Optical network receiver circuit, and optical network receiver circuitry may be used interchangeably. Herein ONU may refer to a circuit, receiver, an optical receiver, and/or an optical receiver unit; herein a state machine may refer to an ONU. Herein frame boundary detection may refer to detection of multiple frame boundaries. 
     Herein “state machine,” “synchroniziation state machine,” “sync state machine,” and “system” may be used interchangeably. 
     Herein the term N esync  may refer to a type of threshold. 
     Herein a trailing(s) indicates one or more items; for example, processor(s) means one or more processors. 
     While examples of apparatuses have been illustrated and described with respect to one or more implementations, alterations and/or modifications may be made to the illustrated examples without departing from the spirit and scope of the appended claims. In particular regard to the various functions performed by the above described components or structures (assemblies, devices, circuits, systems, etc.), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component or structure which performs the specified function of the described component (e.g., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary implementations. 
     In particular regard to the various functions performed by the above described components (assemblies, devices, circuits, systems, etc.), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component or structure which performs the specified function of the described component (e.g., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary implementations of the disclosure. In addition, while a particular feature may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application.