Patent ID: 12244348

DETAILED DESCRIPTION

The description herein may be embodied in other specific apparatus and/or methods. The described embodiments are to be considered in all respects as only illustrative and not restrictive. In particular, the scope of the invention is indicated by the appended claims rather than by the description and figures herein. Embodiments within the meaning and range of equivalency of the claims are to be embraced within their scope.

FIG.1is a schematic diagram illustrating selected components of an exemplary PON100in which some embodiments may be implemented. Note that PON100may, and in many implementations will, include additional components, and the configuration shown inFIG.1is intended to be exemplary rather than limiting. The PON100includes at least one OLT110, an optical distribution network (ODN)120including at least one splitter/combiner140, and a plurality of ONUs130. In an embodiment, the PON network100is based on a point to multi-point (p2mp) architecture wherein the OLT110broadcasts a time division multiplexed (TDM) downstream signal to the multiple ONUs130over a first downstream wavelength. In the upstream direction, the communication from the plurality of ONUs130to the OLT110is achieved via burst mode time-division multiple access signal on a second upstream wavelength, which is different from the downstream wavelength.

The ONUs130are located at different subscriber premises and are connected or connectable to a subscriber network and/or one or more devices of the subscriber (not shown). The OLT120is typically located at a service provider location referred to as a central office. The central office may house multiple OLTs110, each managing their own respective plurality of ONUs130. The OLT110communicates directly or indirectly with various sources of content and network-accessible services (not shown) that are or may be made available to the subscribers associated with the PON100.

In an embodiment, the ODN120may be a data distribution system, which may comprise optical fiber cables, couplers, splitters, distributors, and/or other equipment. In an embodiment, the optical fiber cables, couplers, splitters, distributors, and/or other equipment may be passive optical components. Specifically, the optical fiber cables, couplers, splitters, distributors, and/or other equipment may be components that do not require power to distribute data signals between the OLT110and the ONUs130. Alternatively, the ODN120may comprise one or a plurality of processing equipment, such as optical amplifiers. The ODN120may typically extend from the OLT110to the ONUs130in a branching configuration but may be alternatively configured in any other point-to-multi-point configuration.

At least one splitter/combiner140in the ODN120splits and distributes the downstream signal from the OLT110to the plurality of ONUs130. As such, the ONUs130receive the same downstream signal from the OLT110. The splitter/combiner140also serves as a combiner for combining upstream traffic from the ONUs130to the OLT110. The splitter/combiner140may be located, for example, in a street-side cabinet near the subscribers' premises. This cabinet or similar structure may be referred to as the outside plant. Note, however, that no particular network device or configuration is a requirement of the present embodiments.

The ITU-T recently approved a time-division multiplexing (TDM) PON standard that employs 50 Gb/s per wavelength (λ) non-return to zero (NRZ) modulation in the downstream (DS) signal and either 12.5 Gb/s or 25 Gb/s per wavelength NRZ modulation in the upstream (US) signal. See, Recommendation ITU-T G.9804.3, “50-Gigabit-capable passive optical networks (50G-PON): Physical media dependent (PMD) layer specification” (September 2021). In addition to ITU-T G.9804.3, the embodiments described herein may also be implemented in PON systems compliant in whole or in part to ITU-T Recommendations G.987 dated June 2012 (XG-PON) or to the ITU-T Recommendation G.9807 dated June 2016 (XGS-PON) or to ITU-T Recommendation G.989 (NG-PON2) or to IEEE 802.3ca standard dated June 2020 (25G-EPON, 50G-EPON). Though these specific standards are cited, the embodiments described herein may be implemented in other standards, current or future. In addition, the embodiments described herein may be implemented in other types of networks.

The ITU-T G.9804.3 standard still employs NRZ modulation for a 50 Gb/s rate per wavelength (λ) in the downstream signal. To obtain higher bit rates, different modulation formats may be necessary. For example, 4 level pulse amplitude modulation (PAM4) has been proposed for modulation of the downstream signal. While NRZ includes two amplitude levels, PAM4 includes four amplitude levels. In one example, to upgrade to PAM4 modulation, a new downstream wavelength may be implemented with PAM4 modulated signals. In this example, the OLT110needs to be upgraded as well as the ONUs130to receive and transmit the new downstream wavelength. Since the OLT110is located in the central office and accessible to a network operator, it may be more easily upgraded within a predetermined time period. However, the ONUs130in the field are more difficult to upgrade. For example, the upgrades of the ONUs may only occur upon a subscriber request. Since some subscribers may never request to upgrade, the legacy ONUs may remain with equipment that is configured for NRZ or other legacy modulations. Thus, in an embodiment, rather than upgrading to PAM4 modulation on a new downstream wavelength, a flexible PON is proposed that includes two or more modulation formats on the same downstream wavelength.

FIG.2illustrates a schematic diagram of an embodiment of a frame format230for a multimodulation downstream signal200. One mechanism to address backward compatibility with legacy ONUs130is by employing multimodulation in the downstream signal200, e.g. including two or more modulation formats on the same wavelength in the downstream signal.FIG.2illustrates an exemplary PON downstream frame format230that spans a duration of 125 microseconds for 50 Gb/s downstream frame with NRZ transmission. The frame230includes 360 forward error correction (FEC) codewords each of length 17280 bits (6220800 bits in all).

The frame230includes a Physical Synchronization Block downstream (PSBd)240, a Framing Sublayer (FS) header250and a FS payload260. The PSBd240includes three separate structures, e.g., a Physical Synchronization sequence (Psync)242, a Superframe Counter (SFC) Structure244and an Operation Control (OC) structure246. The start of the downstream PHY frame is bound to transmission or receipt of the first bit of the PSync sequence242. The ONU uses this sequence to achieve alignment at the downstream PHY frame boundary. The SFC structure244includes a superframe counter (SFC) and a hybrid error control (HEC). The SFC value in each downstream PHY frame is incremented by one with respect to the previous PHY frame. Whenever the SFC reaches its maximum value (all ones), it is set to 0 on the following downstream PHY frame. The OC structure246includes the OLT identification and optical parameters of the transceiver.

The FS Header250includes a (Header Length downstream) HLend structure252, a bandwidth map (BWmap)254structure and Physical Layer Operations, Administration and Maintenance (PLOAM) structure256. HLend is a structure that controls the size of the variable length partitions within the downstream FS header250. The BWmap254is used to allocate upstream transmissions. The PLOAM256includes one or more PLOAM messages. The FS payload comprises a series of XGEM frames addressed to specific XGEM port-IDs.

To ensure backward compatibility, information that is intended for both legacy and advanced ONUs130is modulated with a legacy modulation MOD1. For example, the start of the frame270, including the PSBd240and FS Header250, are encoded with a legacy modulation MOD1 (such as NRZ). The legacy modulation may extend beyond the end of the FS header portion in order to align with FEC codeword boundaries. For example, the start of the frame270may correspond to a predetermined number of FEC codewords, such as 10 FEC codewords. The legacy modulation MOD1 in the start of the frame270may be decoded by both the legacy and advanced ONUs130receiving the frame230.

The FS payload260comprises a series of XGEM frames262addressed to specific XGEM port-IDs. The FS payload260may be encoded using a plurality of modulation formats280. For example, it may include a first legacy modulation MOD1 (such as, e.g. NRZ) that is interleaved in one or more first portions210with a second modulation MOD2 (e.g., such as PAM4) in one or more second portions220. The first legacy modulation may have a lower bit rate (such as 50 Gb/s for NRZ) than the second modulation (such as 100 Gb/s for PAM4). These portions210of the payload modulated with MOD1 may be decoded by all the ONUs (legacy and advanced). But portions220of the payload modulated with MOD2 may only be decoded by advanced ONUs130and not legacy ONUs130. The portions210,220may have varying length depending on the balance of usage between the legacy and advanced ONUs130.

The frame format230in PON100described herein is exemplary only. Other types of frame formats may be implemented in a flexible, multimodulation PON100or implemented in other types of networks may be employed herein.

FIG.3illustrates a schematic diagram of an embodiment of a PON300with a multimodulation system. The PON300includes a multimodulation OLT110(i.e., an OLT capable of transmitting a downstream signal with two or more modulations on a same wavelength) and a mixture of legacy ONUs130aand advanced ONUs130b. In this exemplary embodiment, a first modulation includes a non-return to zero (NRZ) modulation and a higher order modulation includes 4-Level Pulse Amplitude Modulation with four amplitude levels (PAM4). In such an exemplary network, the hybrid modulation OLT110transmits a downstream signal200including both modulations to the legacy ONUs130aand the advanced ONUs130b.

In this example ofFIG.3, the OLT110interleaves PAM4 modulated portions320a-bwith NRZ modulated portions310a-bin the downstream signal200. The portions310a-band320a-bmay have different durations depending on usage by the various ONUs130. As described with respect toFIG.2, a start of the frame270is modulated with NRZ so that both the legacy and advanced ONUs may decode the information. The legacy ONUs130aare operable to decode the portions310a-bwith NRZ modulation, and the advanced or upgraded ONUs130bare operable to receive and decode the portions310of the payload with NRZ modulation and the portions320with PAM4 modulation.

The PAM4 modulation is a multilevel modulation format with four amplitude levels. Conventional PAM4 modulation encodes two bits into each symbol. This effectively doubles a network's data rate from NRZ modulation, enabling for example 50 Gb/s to 100 Gb/s or higher bit rates for transmission in a PON. The advanced or upgraded ONUs130bcapable of successfully detecting and decoding the higher bit rate PAM4 modulated signal320may thus have services with a higher throughput as compared with the legacy ONUs130athat can only detect and decode the NRZ modulated signal310. Thus, by using a plurality of modulation formats on a same wavelength, different modulations and line rates may be assigned to ONUs, e.g. based on one or more factors such as ONU capability and link budgets.

In an embodiment, the different modulations are applied to forward error correction (FEC) codewords as a minimum unit rather than symbol-by-symbol modulations. Thus, each portion310and320may include one or more FEC codewords. Such multimodulation signals may thus be encoded per codeword or over a group of codewords. In another embodiment, the portions may correspond to one or more PON frames, e.g. as defined in one or more PON standards described herein and/or within one or more variable or fixed time windows. Other partitions between the portions of the two or more types of modulations in the downstream signal200may also be implemented herein.

One of the challenges of multimodulation in the point to multi-point network is that the legacy ONUs130a still need to maintain synchronization with the transmitter clock in the downstream signal200even during the PAM4 modulated signal320. For example, clock recovery needs to continue working when the ONU130receives a PAM4 signal at the NRZ sensitivity limit. Clock recovery includes detecting embedded clock information in the data stream, allowing the clock timing of the OLT110to be determined by the ONU130. The embedded clock information is used in a phase-locked loop or similar adjustable oscillator to produce a local clock signal that can be used to time the downstream signal in the periods between the clock signals. Maintaining synchronization to the transmitter clock is also critical for upstream performance since the downstream clock recovered at the ONU130is re-used to clock its upstream transmission. The upstream transmission performance might therefore be impacted by any performance issues in synchronization as well.

In addition to problems with legacy ONUs130a, some upgraded or advanced ONUs130may have difficulties in detecting and decoding a PAM4 modulated signal due to signal quality of the downstream channel. Although PAM4 enables optical network operators to pursue short haul 100 Gb/s transmission, it does create a penalty on signal to noise ratio (SNR). The distance for high signal quality becomes shorter, e.g. in the realm of distances up to 10 km. Since the ONUs130have varying distances from the OLT110, the ONUs130receive varying signal qualities of the downstream signal200. In addition, receivers configured for PAM4 modulated signals are more sensitive to jitter and inter-symbol interference (ISI) as compared to NRZ receivers, and this sensitivity is exacerbated when bandwidth limited components and receiver side equalization are employed. Thus, some advanced ONUs130bcapable of receiving PAM4 modulation may still not be able to decode the PAM4 downstream signal due to jitter, ISI, or other signal quality problems.

In general, the PON standards have been designed to ensure that an optical network unit (ONU) operating under the maximum path loss and dispersion can still meet the performance requirements. Until now, design for the channel conditions of individual ONUs130has not been specifically considered. In an embodiment, the PON system determines the channel and operating conditions of individual ONUs130. The PON system then assigns a first portion of the downstream signal with a first lower bit rate modulation to legacy ONUs130aand/or advanced ONUs130bthat receive a low quality downstream signal. The PON system may then assign advanced ONUs130bthat operate under more benign channel conditions with a second portion of the downstream signal having a second higher bit rate modulation with a higher throughput capability.

In an embodiment, the second modulation having a higher throughput capability includes a multilevel amplitude modulation with adaptations to one or more amplitude transitions. For example, in one or more embodiments described herein, a modified 4-level pulse amplitude modulation (PAM4) is generated and employed to modulate the downstream signal200. The modified PAM4 encodes the data bits in the downstream signal such that a probability of one or more predetermined transitions between one or more amplitude levels (e.g., transitions between one or more of the four amplitude levels) are modified. This adaptation of the probability of predetermined transitions is configured to assist legacy ONUs130a(e.g., configured for NRZ modulated signals) in clock recovery of the downstream signal. In addition, the modified PAM4 helps advanced ONUs130bachieve a stable clock and data recovery (CDR) and equalizer performance by enhancing jitter and ISI tolerance. The modified PAM4 may increase a probability of one or more predetermined transitions and/or decrease the probability of one or more transitions. This adaptation of the probability of one or more transitions between levels may be termed “probabilistic transition shaping.” By selecting between the lower bit rate modulation (such as NRZ) and the modified PAM4 format, the PON may adapt the information rate transmitted to the ONUs. For example, the ONUs130may be assigned frames in the downstream signal that are modulated with NRZ or modulated with modified PAM4. The selection may be based on one or more of: ONU capability, signal quality of the received downstream signal, link budget, subscriber status or other factors. The modulation format and information rate may thus be selected per ONU.

FIG.4Aillustrates an embodiment of a signal400with a conventional PAM4 format400. The conventional PAM4 modulation is a four-level pulse amplitude modulation that combines two bits into a single symbol. The information rate for the conventional PAM4 is thus 2 bits per symbol. As seen inFIG.4A, the four amplitude levels in the conventional PAM4 modulation have the following example associated data bits (assuming Gray mapping is employed). Level 0 is associated with data bits 00, level 1 is associated with data bits 01, level 2 is associated with data bits 11, and level 3 is associated with data bits 10. The symbols in conventional PAM4 modulation are each associated with two data bits. Thus, conventional PAM4 has a rate of 2 bits per symbol.

FIG.4Billustrates an embodiment of signal410with a modified PAM4 format. In an embodiment, the modified PAM4 format encodes three data bits over two symbols. The information rate for this modified PAM4 is reduced to 1.5 bits per symbol. This reduction in bits/symbol rate means that not all the symbol patterns with a length L=2 are needed. The three data bits include 23=8 unique bit sequences, thus requiring only 8 symbol patterns to encode the 8 unique bit sequences. A PAM4 symbol pattern of length L=2 has a total of 42=16 symbol patterns. Thus, more symbol patterns are available than are needed for mapping the 8 unique sequences of data bits. This disparity enables a deletion of one or more of the 16 symbol patterns.

In an embodiment, symbol patterns including undesirable transitions between amplitude levels may be deleted, e.g. not used in the mapping of the 8 unique bit sequences. This selection of the valid symbol patterns allows a modification of a probability of predetermined transitions between amplitude levels, as described in more detail herein. The modified PAM4 modulation thus modifies or adapts the probability of predetermined transitions between amplitude levels in the downstream signal.

FIG.4Cillustrates an embodiment of a downstream signal200with multimodulation in a PON. In this embodiment, the OLT110interleaves NRZ modulated portions410a-cand modified PAM4 portions (e.g., PAM4 with probabilistic transition shaping)420a-bon the same wavelength in the downstream signal200. The modulated portions410a-b,420a-cmay have different durations depending on scheduling and usage by the various legacy and advanced ONUs. Thus, adaptation of the information rate may be achieved by assigning NRZ or modified PAM4 formatted signal to an ONU. Also, as described above, the modified PAM4 portions420a-bmay still include headers modulated with NRZ so that legacy ONUs may determine the destination of the frames.

FIG.4Dillustrates another embodiment of a downstream signal200with multimodulation in a PON. In this embodiment, the OLT110interleaves NRZ portions410a,410band modified PAM4 modulated portions420a,420band conventional PAM4 portions430a,430bon the same wavelength in the downstream signal200. The downstream signal200may thus include three or more modulation formats which allows for further adaptation of the information rate by assigning NRZ or modified PAM4 or conventional PAM4 formatted signal to an ONU.

FIG.4Eillustrates another embodiment of a downstream signal200with modified PAM4 in a PON. In this embodiment, the OLT110transmits the downstream signal200using only modified PAM4 portions420a-d. Thus, the downstream signal200may only include a modified PAM4 format.

In another embodiment, modified PAM4 formats having different information rates may be implemented in the downstream signal200. For example, the information rate for the modified PAM4 in portions420a,420cmay be 1.5 bits per symbol (e.g., three bits encoded over two symbols). While the information rate for the modified PAM4 in portions420b,420dmay be 1.75 bits per symbol (e.g., 7 bits encoded over 4 symbols). In another example, the information rate for one or more of portions410may be 1.25 (e.g., 5 bits encoded over 4 symbols). So adaptation of the information rate may be achieved by encoding different number of N data bits over L symbols in a modified PAM4 format. The examples inFIG.4describe a modified PAM4 modulation with probabilistic transition shaping. However, probabilistic transition shaping as described herein may be applied to other PAMx formats, wherein x is the number of amplitude levels and is greater than 2. In addition, probabilistic transition shaping may be applied to other types of modulation, as described in more detail below. The application of probabilistic transition shaping to the PAM4 format is used herein only as an example for illustrative purposes.

FIG.5illustrates an embodiment of an encoder500to generate a modified PAM4 signal with probabilistic transition shaping. In a first embodiment, the encoder500includes at least one state machine having a plurality of states510a,510b,510cthat encodes data bits to adapt the transitions in the modified PAM4 signal and encode the data to adapt the information rate. In an embodiment, the encoder500encodes the data bits over symbols such that certain transitions between amplitude levels are either decreased or increased. The modified PAM4 modulation thus modifies the probability of predetermined transitions between amplitude levels in the downstream signal.

In this specific example, the encoder500performs transition tuning encoding based on a 3-state machine (state S1, S2, and S3). In specific, the modified PAM4 encodes N=3 data bits over L=2 symbols. As such, the information rate in the modified PAM4 modulation is reduced to 1.5 bits per symbol (e.g. less than 2 bits per symbol in conventional PAM4 modulation). The encoding of three bits over two PAM4 symbols allows the probability of certain transitions between amplitude levels to be decreased or increased. For example, the probability of the transition from level 0 to level 3 is decreased in this modified PAM4 format. Similarly, the probability of the transition from level 3 to level 0 is lowered in this modified PAM4 format. In addition, the probability of transition between level 0 and level 0 is increased as well as the probability between level 3 and level 3. In other embodiments, the probabilities of transitions between additional or alternate amplitude levels may be adapted.

FIG.6illustrates an embodiment of a decoder600to decode a modified PAM4 signal. The first column610lists an amplitude level of a first symbol and the first row620lists an amplitude level of a second symbol in the modified PAM4 signal. Some symbol patterns are valid and included in the mapping. For example, the valid symbol patterns are mapped to three data bits as seen inFIG.6. However, some symbol patterns are not included in the mapping in the modified PAM4 format. The x's indicate these symbol patterns that are not valid and not transmitted.

For example, when a first amplitude level is 0 and the second amplitude level is 0, then the associated or mapped data bits are 000. When a first amplitude level is 1 and the second amplitude level is 0, then the mapped data bits are 001. When a first amplitude level is 2 and the second amplitude level is 0, then the mapped data bits are 011.

However, note that no mapping is provided when a first amplitude level is 3 and the second amplitude level is 0. So this symbol pattern (e.g., amplitude level 3 to amplitude level 0) is not selected for mapping and is not transmitted as one of the mapped symbol patterns. This mapping thus reduces the probability of a level 3 to level 0 transition in a modified PAM4 signal.

Note that similarly, no mapping is provided when a first amplitude level is 0 and the second amplitude level is 3. So this symbol pattern (e.g., amplitude level 0 to amplitude level 3) is not selected in a modified PAM4 mapping and is not transmitted as one of the mapped symbol patterns. This encoding thus reduces the probability of a level 0 to level 3 transition in a modified PAM4 signal. The encoding in this exemplary modified PAM4 format thus reduces the probability of the following transitions between amplitude levels: between level 3 and level 0 and between level 0 and level 3.

In an embodiment, the modified PAM4 format also increases the probability of the following transitions between amplitude levels: between level 0 and level 0 and between level 3 and level 3. In this embodiment, the state machine with the plurality of states510determines the terminating symbol (ST) of the preceding symbol pattern. When the terminating symbol is a level 0, the state machine tries to encode a symbol pattern beginning with a level 0. For example, fromFIG.6, the data bits 000 are mapped to a symbol pattern of level 0, level 0. The terminating symbol of the symbol pattern is thus level 0. Suppose the next data bits for mapping are 001. These data bits 001 are mapped to two valid symbol patterns inFIG.6, e.g. the first symbol pattern is level 0, level 1 and the second pattern is level 1, level 0. The encoder500must decide between these two symbol patterns. Since the previous terminating symbol is a level 0, then the encoder500selects the first symbol pattern of level 0, level 1. The encoder500thus selects the symbol pattern having an initial symbol SIthat is the same amplitude level of the terminating symbol STof the preceding symbol pattern. This mapping of data bits based on the terminating symbol STand initial symbol SIof consecutive symbol patterns increases the probability of transitions between level 0 to level 0.

The encoder500performs a similar process when the previous terminating symbol is a level 3. The state machine tries to encode a next symbol pattern beginning with a level 3. For example, fromFIG.6, the data bits 110 are mapped to a symbol pattern of level 2, level 3. The terminating symbol STof the symbol pattern is thus level 3. Suppose the next data bits for mapping are 110. These data bits 110 are mapped to two valid symbol patterns inFIG.6, e.g. the first symbol pattern is level 2, level 3 and the second pattern is level 3, level 2. Since the previous terminating symbol ST is a level 3, then the encoder500selects the second symbol pattern of level 3, level 2. This mapping and encoding increases the probability of transitions between level 3 to level 3.

FIG.7illustrates a graphical representation700of probability of transitions between levels in this example of the modified PAM4710and in conventional PAM4720. The vertical axis represents the probability730of level transitions740shown on the horizontal axis. As seen in the graph, the level transitions in conventional PAM4720have the same probability. In theory, the level transitions in conventional PAM4720are exactly equal when random equiprobable data is assumed, however there may be some variance in simulations or in the field, especially if considered over a short duration. However, in general over time, the transitions between amplitude levels in conventional PAM4720should have approximately the same probability and theoretically have the same probability.

In contrast, in this example of modified PAM4710, the probability730of level transitions740between the outermost levels 0 and 3 (e.g. 0 to 3 or 3 to 0), in this example of modified PAM4710is theoretically lowered from 12.5% to 7.5%. The lowered theoretical probability is less than the theoretical probability of other level transitions in modified PAM4710. For example, the probability730of level transitions740between the outermost levels 0 and 3 in modified PAM4710(e.g., 7.5%) is less than the probability of the transition between 0 and 2 (e.g., 12%) or between 0 and 0 (e.g., 20%) in modified PAM4710. In addition, the decreased probability730of level transitions740between the outermost levels 0 and 3 in modified PAM4 (e.g., 7.5%) is less than the probability of the same level transition between the outermost levels in 0 and 3 in conventional PAM4 (e.g., 12.5%).

The “probability of a level transition” as used herein means the theoretical probability and the probability in practice or in operation, e.g. when random equiprobable data is assumed of a level transition. The “decreased probability of a level transition” in modified PAM4 means that the probability of a level transition is less than the probability of the same level transition in a conventional PAM4720. The “decreased probability of a level transition” in modified PAM4 also means that it has a lower probability of one or more other level transitions in modified PAM4.

In another example, the probability730of level transitions740between level 3 and level 3 (e.g., 20%) is greater in modified PAM4710than the probability of other level transitions in modified PAM4710. In specific, the probability730of level transitions740between the level 3 and level 3 in modified PAM4 is greater than the probability of transitions between 0 and 2 or between 0 and 3 in modified PAM4. In addition, the probability in modified PAM4 of the level transition between level 3 and level 3 (e.g., 20%) is greater than the probability of the same level transition between level 3 and level 3 in conventional PAM4720(e.g., 12.5%).

In a still further example, the probability730of level transitions740between level 0 and level 0 (e.g., 20%) is greater than the probability of other level transitions in modified PAM4710. In specific, the probability730of level transitions740between the outermost levels 0 and 0 (e.g., 20%) is greater than the probability730of level transitions740between 0 and 2 or between 0 and 3. In addition, the probability in modified PAM4 of the level transition between level 0 and level 0 (e.g., 20%) is greater than the probability of the same level transition between level 0 and level 0 in conventional PAM4720(e.g., 12.5%). The “increased probability of a level transition” in modified PAM4 means that the probability of the level transition is greater than the probability of the same level transition in a conventional PAM4720. The “increased probability of a level transition” in modified PAM4 also means that it has a greater probability of one or more other level transitions in modified PAM4.

The probabilistic transition shaping of PAM4 modulation thus adapts (e.g., increases or decreases) the probability of at least one predetermined level transition between one or more amplitude levels. For example, the probability of at least one level transition in modified PAM4 is greater than the probability of other level transitions in modified PAM4 and/or greater than the probability of the same at least one level transition in conventional PAM4. In addition or alternatively, in modified PAM4, the probability of at least one level transition is less than the probability of one or more other level transitions in PAM4, and less than the probability of the same at least one level transitions in conventional PAM4. This adaptation of the probabilities of level transitions may assist to stabilize CDR and equalizer performance in the ONUs130and to enhance jitter and ISI tolerance.

FIG.8illustrates a graphical representation800of probability of levels in modified PAM4810and in conventional PAM4820. The vertical axis corresponds to the probability830of a level while the horizontal axis corresponds to a PAM4 level840. The graph illustrates that in this embodiment with a state machine, the probabilities of the levels (e.g., 25%) are uniform in both modified PAM4810and in conventional PAM4820. In this embodiment of modified PAM4, the probabilities of level transitions (shown inFIG.7) are adapted but the probabilities of the levels (shown inFIG.8) are not adapted, e.g. probabilistic level shaping is not implemented.

In another embodiment, probabilistic level shaping may also be implemented as well as probabilistic transition shaping to generate a modified PAM4 signal. This implementation of both probabilistic level shaping and transition shaping may further optimize the performance of the ONUs130.

The modified PAM4 format in this embodiment maps 3 bits over two PAM4 symbols to generate a 1.5 bit per symbol rate. In another embodiment of modified PAM4, 7 bits may be mapped over four PAM4 symbols to provide a 1.75 bit per symbol rate. In yet another embodiment of modified PAM4, 5 bits may be mapped over four PAM4 symbols to provide a 1.25 bit per symbol rate. When a lower symbol rate, or more states, bits, and/or more PAM4 symbols are used in the encoding, transition shaping and rate-adaptation may be extended further for optimizing reception of the downstream signal in a PON.

In another embodiment, the modified PAM4 modulation is encoded in the downstream signal using one or more codebooks or code mappings. The one or more codebooks include one or more databases that map the data bits to PAM4 symbol patterns in accordance with desired level transition probabilities. The encoding of the PAM4 signal with codebooks may control the level transitions in the downstream signal with more flexibility.

In one example, a modified PAM4 modulated signal has four amplitude levels {0, 1, 2, 3} with N bits mapped over symbol patterns with a length L. The code book is configured to exclude symbol patterns with transitions between the outermost levels, i.e., 0→3 and 3→0. In this embodiment, in order to lower complexity and to limit error propagation, symbol pattern lengths are chosen to span a small number, L, of PAM4 symbols, and the last symbol (called the terminating symbol ST) in the span is constrained to transmit only two levels, for example, the outermost levels {0, 3}. Thus, valid symbol patterns include a terminating symbol STof level 0 or level 3. In view of these constraints, the number of valid symbol patterns with a length L, and the number of data bits that can be transmitted over these valid symbol patterns with a length L are shown inFIG.9.

FIG.9illustrates a graphical representation900of a number of valid symbol patterns with a length L and a number N of data bits that can be transmitted over these valid symbol patterns. The N data bits are mapped to symbol patterns with a length L, wherein N/L is less than 2. The graph900compares the number N of data bits910on the vertical axis that can be mapped to valid patterns of PAM4 (or other 4-ary) symbols of length (L)920on the horizontal axis. As seen in the graph900, N=5 bits may be encoded over symbol patterns of length L=4 or a number N=13 bits may be encoded over symbol patterns of length L=8.

In the example of N=5 bits encoded over symbol patterns of length L=4 in a modified PAM4 format, the valid patterns of PAM4 symbols920exclude transitions between the outermost levels (e.g., 0→3 and 3→0 for PAM4 and terminate at the outermost levels (e.g. the terminating symbol is either 3 or 0 for PAM4). Though these guidelines are implemented to generate valid symbol patterns in this embodiment, other guidelines may be implemented alternatively or in addition to these guidelines.

For example, in an embodiment using PAM4, when the symbol patterns have a length L=4, then there 256 patterns (Lx, wherein L=4 and x=4 amplitude levels). Of these 256 patterns, there are 61 valid symbol patterns that do not contain 0→3 or 3→0 transitions, and that terminate either at the 0 or 3 level. The 61 valid patterns are illustrated in Table 1 below:

TABLE 1VALID PATTERNS for L = 40000001000200100011001200200021002201000101010201100111011201200121012201310132020002010202021002110212022002210222023102320001300230113012301330213022302331013102311131123113312131223123313131323133320132023211321232133221322232233231323232333

The first 31 patterns (shown in the first four rows in Table 1 above) terminate at the 0 level, while the remaining 30 patterns terminate at the 3 level. Rounding the log 2(N)=VP, wherein VP is the number of valid patterns (61) to the nearest smaller integer, then N=4 bits of information may be encoded. A subset of 2{circumflex over ( )}4=16 patterns from the set of 61 valid patterns are needed for mapping, and thus the (61) patterns in Table 1 may be further refined using one or more guidelines.

In an embodiment, the following guidelines are used to select the valid symbol patterns and perform the mapping of the valid symbol patterns to data bits in the codebook database.1. The number of symbol patterns terminating in the level 0 are selected to be equal to those terminating in the level 3. This guideline ensures that one of the bits (e.g., the MSB) can be used to select the terminating level with equal probability. This is also beneficial in determining a ‘complementary codebook’ for the case where the previously transmitted symbol is a level 3.2. For a given terminating level, the patterns may be partitioned into two equal sized groups; one where the first symbol is 0 and another where the first symbol is not zero. In this case, since at least 8 patterns starting with 0 exist for both terminating levels, one bit can be used to make this selection.3. Alternatively, symbol patterns in the codebook may be selected to maximize Euclidean distance. Note that most patterns are at an Euclidean distance of at least 2 with respect to others, but there are still a few patterns at Euclidean distance 1 with respect to other patterns.4. Construct a complementary codebook that is used when the previously transmitted symbol is a 3 (e.g. instead of a 0). The symbol patterns may be constructed using a mirrored mapping scheme, i.e., where the level i from the codebook is replaced with the value 3—i in the complementary codebook.

Steps 1 and 4 ensure a symmetric distribution of levels. In an embodiment, a codebook database stores the mapping of the valid symbol patterns to the data bits. An exemplary codebook and complementary codebook for symbol patterns of length L=4 encoding a number of N=5 data bits for a modulated PAM4 format is shown inFIG.10.

FIG.10illustrates an embodiment of a codebook database for encoding and decoding a modified PAM4 signal, wherein N=5 data bits are encoded per symbol pattern of length L=4. A codebook1020illustrates encoding for the N data bits1010when the last previously transmitted PAM4 symbol is a level 0. The complementary codebook1030illustrates the encoding for the N data bits1010when the previously transmitted PAM4 symbol is a level 3. This encoding by the codebook1020and the complementary codebook1030restricts the transitions between the outermost symbol levels, e.g. between levels 0 to 3 and between levels 3 to 0 in a modified PAM4 signal. Note that the adaptation of the probability of these transitions between amplitude levels is exemplary, and other transitions between other amplitude levels may be implemented alternatively or in addition thereto.

FIG.11illustrates a graphical representation1100of the resulting probability of the symbol levels1120with the codebook and complementary codebook shown inFIG.10. The horizontal axis illustrates the symbol levels {0, 1, 2, 3}1130of the PAM4 modulation, and the vertical axis shows the probability of the symbol level1120in this embodiment of the codebook and complementary codebook. Almost or approximately equal probabilities (e.g., approximately 25%) are achieved across the four levels. However, there is a minimal difference in the probabilities of the levels in this embodiment of the codebooks. This difference results in an insignificant or minimal amount of probabilistic level shaping. In other embodiments of the codebooks, a different probability distribution of the levels may be achieved by a different selection of valid patterns. For example, an equal distribution of probabilities of the levels may be achieved through a different selection of the valid patterns in the codebooks from the example inFIG.11. In another embodiment, a statistically significant or meaningful probabilistic level shaping may be achieved as well as probabilistic transition shaping through selection of the valid patterns in the codebooks. This implementation of both probabilistic level shaping and transition shaping may further optimize the performance of the ONUs130.

Though the symbol pattern length L=4 and number of bits=5 in this example, other lengths L of symbol patterns may be encoded. For example, as shown inFIG.9, a number N=13 data bits may be encoded across PAM4 symbol patterns of length L=8. As shown inFIG.6, when the symbol pattern length L=2, then N=3 data bits may be encoded. The information rate may thus be adapted in the modified PAM4 signal by implementing a plurality of different mappings, wherein the mappings include different symbol rates N/L.

The application of probabilistic transition shaping to the PAM4 format is used herein only as an example for illustrative purposes. The probabilistic transition shaping may be applied to other PAMx formats, wherein x is the number of amplitude levels and x is greater than 2. For example, probabilistic transition shaping may be applied to PAM8 or to PAM3.

FIG.12Aillustrates a schematic diagram of an embodiment of the OLT110configured to generate a modified PAMx signal using a code book database. The OLT includes a processing device1200with one or more processors and a memory device1210. The memory device1210includes a non-transitory computer readable storage medium that stores one or more sets of executable instructions that when executed by the one or more processors causes the OLT110to perform one or more aspects of the embodiments described herein. The memory device may include, any storage medium, or combination of storage media, accessible by the OLT110during use to provide instructions and/or data to the OLT110. Such storage media can include, but is not limited to, optical media (e.g., compact disc (CD), digital versatile disc (DVD), Blu-Ray disc), magnetic media (e.g., floppy disc, magnetic tape, or magnetic hard drive), volatile memory (e.g., random access memory (RAM) or cache), non-volatile memory (e.g., read-only memory (ROM) or Flash memory), or microelectromechanical systems (MEMS)-based storage media. The computer readable storage medium may be embedded in the OLT110(e.g., system RAM or ROM), fixedly attached to the OLT110(e.g., a magnetic hard drive), removably attached to the OLT110(e.g., an optical disc or Universal Serial Bus (USB)-based Flash memory), or coupled to the OLT110via a wired or wireless network (e.g., network accessible storage (NAS)).

The OLT110further includes a transmitter/receiver (transceiver)1230including at least one signal encoder1240and modulator1250. The signal encoder1240generates a digital encoding signal1242, wherein the data bits are mapped to an amplitude modulated signal with probabilistic transition shaping as described herein. In an embodiment, the signal encoder1240encodes two or more modulations formats, such as a modified PAMx with probabilistic transition shaping, another AM format with probabilistic transition shaping, conventional PAMx, and/or NRZ. The one or more different modulations are interleaved in one or more portions on a same wavelength. The portions may have fixed or variable durations and be based on one or more frames or on fixed or variable time windows.

The modulator1250uses the digital encoding signal1242to modulate an optical carrier signal1262from a laser light source1260. The transceiver1230may thus generate a downstream signal with multimodulation, e.g., wherein at least a first portion of the downstream signal is modulated using a first modulation format and at least a second portion is modulated using a modified amplitude modulation format with probabilistic transition shaping. In another embodiment, the transceiver1230may generate a downstream signal including only an amplitude modulation format with probabilistic transition shaping.

The modulator1250and laser light source1250may include a directly modulated laser diode (DML) or an integrated laser diode with an electro-absorption modulator (EAM) in a single integrated circuit (EML). The modulated optical carrier signal is transmitted from the OLT110as the downstream signal200to the optical distribution network (ODN)140.

In an embodiment, the OLT110stores a code book database1270in the memory device1210. The codebook database1270includes one or more codebooks and complementary codebooks. Each of the codebook and complementary codebook includes a mapping of a predetermined number N of data bits to symbol patterns with a predetermined length L. The codebook provides the mapping when the previously transmitted PAMx symbol is a first outermost level (such as level 0 for PAM4 or PAM8) and the complementary codebook stores the mapping when the previously transmitted PAMx symbol is a second outermost level (such as level 3 for PAM4 or level 7 for PAM8).

The signal encoder1240uses the code book database1270to map the data bits to a valid pattern of symbols as described herein. The mapped symbols and the pre-computed sequences are output as the digital encoded signal1242and are uploaded to the modulator1250. The modulator1250may include a memory and CMOS digital to analog converter (DAC) for high speed modulation of the optical carrier signal1262.

Another objective of using multiple modulation formats on the downstream signal200is the ability to assign different line rates per ONU130. The assignment may be performed in response to one or more parameters, such as link budget, ONU capability, signal quality at the ONU, subscriber status, etc. The OLT110transmits an assignment of a modulation format to the ONU130.

FIG.12Billustrates a schematic diagram of an embodiment of the OLT110configured to generate a modified PAMx signal using a state machine with one or more states. The OLT110includes a state machine circuit1280including at least one state machine with a plurality of states510, e.g. as described with respect toFIG.5. The signal encoder1240employs the state machine circuit1280to map N data bits to a valid pattern of symbols of length L.

In another embodiment, the OLT110may implement both a code book database1270and state machine circuit1280, wherein the devices work independently. Alternatively, the OLT100may use both in combination to map the data bits to a valid pattern of PAMx symbols.

FIG.13Aillustrates a schematic diagram of an embodiment of the ONU130using a code book database to decode a modified PAMx downstream signal200. The ONU130includes a processing device1300with one or more processors and a memory device1310. The memory device1310includes a non-transitory computer readable storage medium that stores one or more sets of executable instructions that when executed by the one or more processors causes the ONU130to perform one or more aspects of the embodiments described herein. The memory device1310may include, any storage medium, or combination of storage media, accessible by the ONU130during use to provide instructions and/or data to the ONU130. Such storage media may include, but is not limited to, optical media (e.g., compact disc (CD), digital versatile disc (DVD), Blu-Ray disc), magnetic media (e.g., floppy disc, magnetic tape, or magnetic hard drive), volatile memory (e.g., random access memory (RAM) or cache), non-volatile memory (e.g., read-only memory (ROM) or Flash memory), or microelectromechanical systems (MEMS)-based storage media. The computer readable storage medium may be embedded in the ONU130(e.g., system RAM or ROM), fixedly attached to the ONU130(e.g., a magnetic hard drive), removably attached to the ONU130(e.g., an optical disc or Universal Serial Bus (USB)-based Flash memory), or coupled to the ONU130via a wired or wireless network (e.g., network accessible storage (NAS)).

The ONU130further includes a transmitter/receiver (transceiver)1320including an optical signal detector1390including at least one photodiode and an analog to digital convertor. For next generation high speed PON of 50 Gb/s and above line rates, the ONU transceiver1320may include a digital signal processing (DSP)1340. The DSP1340includes an equalizer1350(such as a feedforward and decision feedback equalizer (FFE/DFE)) and clock and data recovery circuit (CDR)1360. In high speed PON, there is substantial inter symbol interference (ISI) due to components' limited bandwidth, fiber dispersion and transmitter chirp. The DSP1340helps to combat these problems to detect the modified PAMx symbols, especially when the downstream signal200has data rates of 50 Gb/s and above and has a multimodulation, e.g., of a modified PAMx with probabilistic transition shaping, another AM format with probabilistic transition shaping, conventional PAMx, and/or NRZ.

The clock and data recovery circuit1360is configured to perform clock recovery during periods when the downstream signal is modulated with a PAMx format and during periods when modulated with the NRZ format. When assigned frames with PAMx modulation, the CDR1360and/or signal decoder1330decodes its designated data from detected PAMx symbols. When assigned frames with NRZ modulation, the CDR1360and/or signal decoder1330decodes its designated data from detected NRZ symbols.

In an embodiment, the signal decoder1330is configured to decode data from the detected modified PAMx symbols using the code book database1370. The codebook database1370includes the same codebook and complementary codebook used by the OLT110to encode the PAMx symbols. The codebook and complementary codebook may be predefined (e.g., in a PON standard) or stored during configuration of the OLT110and ONU130or exchanged between the OLT110and ONU130using an established management channel.

At the ONU130, when PAMx is implemented, standard PAMx decisioning may be used to detect the L-1 non-terminating symbols. When the terminating symbol is limited to the outermost levels (e.g., 0 or 3 for PAM4 and 0 or 7 for PAM8), then standard NRZ decisioning may be used to detect the terminating symbol.

A valid pattern for the detected block of L symbols is then determined. The level of the previous terminating symbol (e.g., the last symbol of the previous block of L symbols) is used to determine whether the codebook (previous symbol is level 0) or complementary codebook (previous symbol is level 3) should be selected for pattern-to-bit mapping.

In case the detected block of L symbols matches a valid pattern in the selected book (i.e., codebook or complementary codebook), an estimate of the transmitted bits may be determined directly from the selected codebook. In case the detected block of L symbols does not match a valid pattern in the selected book (codebook or complementary codebook), a pattern mapper unit1380may determine a valid pattern from the detected block of L symbols, e.g., determine a valid pattern that is nearest to the detected pattern in terms of Euclidean distance. Then, the transmitted bits may be determined directly from the selected codebook. When multiple valid patterns exist at the nearest Euclidean distance, the bits wherein the nearest neighbor patterns differ may be marked as erasures for further decoding using the forward error correction (FEC) data in the downstream signal200.

FIG.13Billustrates a schematic diagram of another embodiment of the ONU130that implements at least one state machine with a plurality of states510to decode a modified PAMx downstream signal200. In this embodiment, the ONU130includes a state machine circuit1385including at least one state machine with a plurality of states510. The signal decoder1330is configured to decode data from the detected PAMx symbols using the state machine circuit1380.

In another embodiment, the ONU130may implement both a code book database1370and a state machine circuit1385, wherein the devices work independently. Alternatively, the ONU130may use both in combination to decode the modified PAMx downstream signal.

FIG.14Aillustrates an eye diagram1400of an optical signal modulated using the modified PAM4 format. In this embodiment, PAM4 symbols with a pattern length L=8 are mapped to N=13 bits in the codebook and complementary codebook. As seen inFIG.9, there are a total of 9805 valid patterns (log 2(9805)=13.26). The modified PAM4 codebook may thus map N=13 bits to a selection of 8192 patterns, e.g. that are a subset of the 9805 valid patterns.

The horizontal axis of the eye diagram1400illustrates the time1410over which the amplitude1420of the PAM4 symbols1420on the vertical axis are measured. The eye diagram is measured at 2 samples/symbol. The eye diagram1400illustrates that the absence of transitions between the outermost levels, e.g. between 0 to 3 and between 3 to 0 in the modified PAM4 modulation, improves the horizontal eye opening of the outer eyes. During experimentation, an eye quality measurement of at least TDECQ=1.36 dB was obtained. The eye diagram1400also shows the terminating NRZ symbol.

FIG.14Billustrates a graph1450of power spectral density (PSD)1460of a modified PAM4 format. In this embodiment, the PAM4 symbols with a pattern length L=8 are also mapped to N=13 bits as inFIG.14A. The power spectral density (PSD)1460is represented on the vertical axis for a range of frequencies1470shown in the horizontal axis. As seen inFIG.14B, the power spectral density1460of the modified PAM4 signal1480with probabilistic transition shaping is more constrained than for the conventional PAM4 signal1490.

FIG.15illustrates a block diagram of an embodiment of a method1500for generating a modified PAMx modulated signal, wherein x is greater than 2. A modified PAM4 format is described in many examples herein. However, probabilistic transition shaping may be applied to other PAMx formats having an x number of amplitude levels (PAMx), wherein x is greater than 2. The method1500inFIG.15describes a generalized embodiment of encoding a modified PAMx signal having an adapted probability of a transition between at least one predetermined level. It is to be noted that this method may also be applied to other modulation formats as described further herein.

Referring toFIG.15, a length L for the symbol patterns is determined at1502, and a number N of data bits for mapping to the symbol patterns of length L is determined at1504. The number of sequences of the N data bits (e.g., equal to 2N) is then determined at1506.

Next, valid symbol patterns with a length L are selected at1508. The valid patterns are selected to adapt (increase or decrease) a probability of one or more predetermined level transitions. For example, the valid patterns may be selected to exclude any patterns with transitions between the outermost levels (e.g., 0→3 and 3→0 for PAM4 or 0→7 and 7→0 for PAM8). The valid patterns may be selected such that the last symbol (called the terminating symbol) is constrained to the outermost levels (e.g., {0,3} for PAM4 or {0,7} for PAM8). In another embodiment, patterns may be selected to reduce the occurrence of transitions between asymmetrical levels, such as between 1 and 3 or between 0 and 2. in PAM4. Additionally or alternatively, patterns having two or more consecutive amplitude levels that are the same value may be selected or preferred. For example, a symbol pattern of levels 0, 2, 2, 2, 3 may be selected over a symbol pattern of levels 0, 1, 2, 1, 3. These examples are not inclusive and a probability of other level transitions may be adapted using similar methods. Other considerations in selecting the valid patterns are discussed further with respect toFIG.16.

One or more valid patterns are then mapped to a sequence of the N data bits to generate the modified PAMx encoding at1510. A first valid pattern may be mapped to a sequence of N data bits that begins with a 3. A second valid pattern may be mapped to the same sequence of N data bits that begins with a 0, e.g. by subtracting 3-i from the first pattern.

This mapping is used to encode a sequence of N data bits for transmission on the downstream signal. The encoding may be performed using a state machine with one or more states510as illustrated inFIG.5or using a codebook database1000as illustrated inFIG.10. The encoding may also include determining the terminating symbol of the preceding symbol pattern. When the terminating symbol STis a 0, then for the next consecutive pattern, a valid pattern with an initial symbol SIwith a 0 is selected for encoding of the sequence of N data bits, e.g. such that ST=SI. When the terminating symbol is a 3, then a valid pattern that begins with a level 3 is used for encoding the next sequence of N data bits. Though PAM4 is used as an example, the same method may be implemented using other PAMx formats or other multilevel modulations wherein the amplitude levels are greater than 2.

A signal encoder1240generates a digital encoding signal1242with the modified PAMx format for the sequence of N data bits. The digital encoding signal1242is then used by a modulator1250to modulate an optical carrier signal at1512.

The signal encoder1240may further encode data bits with another modulation format, such as an NRZ format. The transceiver1230may then generate a downstream signal200with multimodulation, wherein at least a first portion of the downstream signal is modulated using an NRZ format and at least a second portion is modulated using a modified PAMx format.

FIG.16illustrates a block diagram of an embodiment of a method1600for selecting valid symbol patterns with a length (L). First, the set of patterns having the number (L) of symbols is determined. For example, this set will include 4Lpatterns for PAM4 or 8Lpatterns for PAM8. From the total set of patterns of the PAMx symbols, a subset of valid patterns are selected based on one or more factors, wherein the subset of patterns includes less patterns than the set of patterns. The subset of valid patterns may be selected to decrease transitions between one or more predetermined levels. The subset of valid patterns may also be selected to increase transitions between one or more predetermined levels.

For example, at1604, patterns may be selected for the subset that adapts transitions between one or more predetermined levels. As described herein, for one embodiment of modified PAMx, this selection reduces or eliminates patterns including transitions between outermost levels, such as levels 0 and 3 for PAM4. For PAM 8, this selection may eliminate patterns including transitions between levels 0 and 7 and vice versa. Alternatively or additionally, the patterns may be selected that reduces transitions between asymmetrical levels, such as between 1 and 3 or between 0 and 2 for PAM4. In another example, the patterns may be selected to reduce transitions between inner levels, such as between 1 and 2 for PAM4. In yet another example, patterns may be selected that include transitions from level 1 to level 3 but patterns including transitions between level 3 to level 1 are excluded. The selection of patterns may be selected to increase the probability of a transition between a same level, such as the transition between level 0 and level 0 or the transition between level 3 and level 3. The adapted probability of a predetermined level transition thus includes one or more of: (i) an increase or decrease in probability of a predetermined transition between two different levels; or (ii) an increase or decrease in probability of a predetermined transition between the same levels.

In an embodiment, from the subset of patterns, a same number of patterns terminating in a first predetermined level is selected as terminating in a second predetermined level at1606. For PAM4, a same number of valid patterns ending in a first outermost level 0 may be selected as ending in a second outermost level 3. For PAM8, a same number of valid patterns ending in 0 may be selected as ending in 7. However, the patterns do not have to end in the outermost levels. The valid patterns may be selected to end in only innermost levels (e.g., 1 or 2 in PAM4 and 3 or 4 in PAM8) or other predetermined levels.

At this point, there may still be unnecessary valid patterns. For example, in PAM4 with L=4, then there are 61 valid patterns that do not contain 0→3 or 3→0 transitions, and that terminate either at the 0 or 3 level. When PAM4 is mapped to 5 bits, then only 25 or 32 valid patterns are needed. Thus, the number of valid patterns needs to be further reduced. In this example, the valid patterns may be further selected to maximize Euclidean distance between levels at1608. The valid patterns may be further selected to reduce a probability of transitions between asymmetrical levels or to increase a probability of transitions between a same level.

A codebook may then be generated using the subset of valid patterns having a length (L) of symbols at1610. This subset of valid patterns is mapped to the N data bits in the codebook. In addition, a complementary codebook is generated at1612. For PAM4, the level i is replaced with 3-i in the complementary codebook. In another embodiment, for PAM8, the level i is replaced with 7-I in the complementary codebook.

FIG.17illustrates a block diagram of an embodiment of a method1700for encoding data bits in a modified PAMx signal. The transceiver1230of the OLT110may perform one or more of these steps using the code book database1270and/or at least one state machine with one or more states510and/or using software and/or other devices. At1702, a set of N data bits is obtained for mapping and transmitting.

At1704, the terminating symbol ST of the preceding symbol pattern is determined. When the terminating symbol ST is a first predetermined level (e.g. level 0 for PAM4 and for PAM8), then the N data bits are mapped using a first codebook at1706. The first codebook includes the valid patterns of symbols that begin with the same first predetermined level. When the terminating symbol ST is a second predetermined level (e.g. level 3 for PAM4 and level 7 for PAM 8), then the N data bits are mapped using a second, complementary codebook at1708. The second, complementary codebook includes the valid patterns of symbols that begin with the second predetermined level. Though the outermost levels are used as examples as the first and second predetermined symbol, other symbols may be encoded as the first and second predetermined symbols. The N data bits are then encoded using either the first codebook or the second, complementary codebook. As such, the terminating symbol ST of the preceding symbol pattern is at the same amplitude level as the initial symbol SI of the consecutive symbol pattern. Though codebooks are described herein, a similar process may be implemented with one or more state machines or other processor devices.

FIG.18illustrates a block diagram of an embodiment of a method1800for decoding data bits in a modified PAMx signal. A receiver, e.g. in the ONU130, detects a span of symbols with a length L. For the first L-1 symbols in the span, a PAMx decision process of determining one of the x levels is implemented at1802. In an embodiment, the terminating symbol ST in the span is limited to one of two predetermined levels (e.g., outermost levels 0 or 3 for PAM4 and 0 or 7 for PAM8). So then standard NRZ or bilevel modulation decision process may be used to detect the terminating symbol ST at1804.

Next, the predetermined level of the terminating symbol ST is determined at1806. When the preceding terminating symbol is a first predetermined level (e.g. outermost level 0 for PAM4 and for PAM8), then a first codebook is used to decode the span of L detected symbols at1808. The first codebook includes the valid patterns of L symbols that begin with the same first predetermined level. When the preceding terminating symbol is a second outermost level (e.g. level 3 for PAM4 or level 7 for PAM8), then a complementary codebook is used to decode the span of L detected symbols at1810. The second, complementary codebook includes the valid patterns of L symbols that begin with the second outermost level.

When the span of L detected symbols matches a valid pattern in the selected book (i.e., codebook or complementary codebook), an estimate of the transmitted bits may be determined directly from the mapping in the selected codebook. In case the detected block of L symbols does not match a valid pattern in the selected book (codebook or complementary codebook), a valid pattern may be determined based on Euclidean distance. For example, a valid pattern in the selected codebook is determined that is nearest to the span of L detected symbols in terms of Euclidean distance. Then, the transmitted bits may be determined from the mapping in the selected codebook. When multiple valid patterns exist at the same nearest Euclidean distance, the bits wherein the nearest neighbor patterns differ may be marked as erasures for further decoding using the forward error correction (FEC) in the downstream signal200. The transmitted data bits are then generated at1812.

FIG.19illustrates a block diagram of an embodiment of a method1900to determine a modulation type for an ONU130in a PON100. In an embodiment, the ONUs130may be assigned different modulation types to achieve different bit rates. The assignment may be determined in response to one or more parameters. A first parameter may include a capability of the ONU130at1902. For example, the capability may include configuration of the ONU130and/or equipment of the ONU130, such as a DSP1340with an equalizer1350and CDR1360. For example, in a flexible PON implementing NRZ format and a modified PAM4 format with probabilistic transition shaping, when an ONU130has not been configured for PAM4 format or does not have a DSP, then it may not be able to successfully detect and decode the modified PAM4 signal. So, such an ONU130may be assigned to the NRZ modulation with a lower bit rate.

In another example, the signal quality of the received downstream signal at the ONU130is determined at1904. The ONU130may measure the signal quality and provide the information to the OLT110. The signal quality may be measured using a signal to noise (S/N) ratio, bit error rate (BER), received power level or other parameters. In addition or alternatively, the OLT110may determine the link budget of the fiber to the ONU130at1906. Due to differing distances from the OLT110, the ONUs130may experience differing signal quality and link budget. For example, when the link budget and/or signal quality fails to meet a threshold, e.g. such that the ONU130may not detect a modified PAM4 signal due to ISI, the ONU130may be assigned to detect NRZ modulation in the downstream signal.

Another parameter that may be obtained is subscriber status information associated with the ONU130at1908. The PON may have different subscriber options, e.g. with higher line rates costing more than lower line rates. Thus, even if the ONU130is capable of receiving and decoding a PAM4 modulation, the subscriber status may only include access to lower line rates and so the ONU is assigned to detect NRZ modulation in the downstream signal. Using one or more of these parameters, the OLT110determines a modulation format (e.g., NRZ or modified PAM4 or modified PAM8) for the ONU130at1910. The OLT110may also determine an information rate for the selected modulation format. For example, depending on one or more of the above factors, the OLT110may determine a modified PAM4 format with a 1.5 symbol rate or with a 1.75 symbol rate. The OLT110may thus select the modulation format and information rate for ONUs130. The OLT110then transmits data in the downstream signal200to the ONU130using the assigned modulation format and information rate.

In an embodiment, the OLT110may assign a different modulation format to the ONU130for receiving data on the downstream signal130than for transmitting data on the upstream signal. For example, the ONU130may have the capability to receive and transmit data using PAM4 but the received signal quality of the downstream signal200may be below a threshold at the ONU130. The OLT110may then assign to the ONU130an NRZ modulation format for receiving the downstream signal200but assign a modified PAM4 or conventional PAM4 format for transmitting data on the upstream wavelength.

FIG.20illustrates a block diagram of an embodiment of a method2000for decoding a downstream signal with a plurality of modulation formats by an ONU130. The ONU130receives the downstream signal including at least a first portion with a first modulation format, such NRZ modulation, and a second portion with a second modulation format, such as modified PAMx with probabilistic transition shaping. The ONU130is configured to perform clock recovery during periods when the downstream signal is modulated with a modified PAMx format and during periods when modulated with the NRZ format at2002. The ONU130determines its assigned modulation format, e.g. modified PAMx or NRZ or other assigned formats at2004. The ONU130monitors the downstream signal for frames of data addressed to destinations associated with the ONU130, and/or a timeslot designated for data for the ONU130. When not receiving frames addressed to a destination associated with the ONU130, the ONU130continues to perform clock recovery at2010. When the frames are addressed for the ONU, the CDR1360and/or signal decoder1330decodes the data using its assigned modulation format at2008. For example, when assigned modified PAMx modulation, at least the payload of the frames addressed to the ONU130are modulated in modified PAMx format. The ONU130decodes the payloads of the fames using one or more codebooks and/or one or more state machines as described herein.

In one or more embodiments described herein, a PON100transmits a downstream signal200with a plurality of modulation formats, wherein at least one of the modulation formats is a modified amplitude modulation (AM) format with x amplitude levels, wherein x is greater than 2. The modified AM modulation encodes the data bits in the downstream signal such that the probability of one or more predetermined transitions between amplitude levels is modified. This probabilistic transition shaping assists legacy ONUs130(e.g., configured for NRZ modulated signals) in clock recovery of the downstream signal. In addition, the modified AM modulation helps advanced ONUs130bachieve a stable clock and data recovery (CDR) and equalizer performance by enhancing jitter and ISI tolerance.

The assignment of one of the plurality of modulation formats to an ONU130in the PON100may be based on one or more parameters. These parameters include one or more of: ONU capability, signal quality of the received downstream signal, link budget, subscriber status or other factors. The modulation format assigned to the ONU130in the downstream signal may be different than the modulation format assigned to the ONU130in the upstream signal. Thus, the modulation format, and so the line data rate, may be selected and assigned per ONU130in the PON100.

The probabilistic transition shaping has been described with respect to a PAMx format. However, probabilistic transition shaping may be implemented with other complex modulations, e.g. any N-ary modulation, wherein N is greater than 2. Probabilistic transition shaping may be generalized as adapting transitions between constellation points of a constellation map for any N-ary modulation, wherein N is greater than 2.

FIG.21Aillustrates a constellation map2100for conventional PAM4 modulation. In a constellation map, the distance of a constellation point from the origin represents a measure of the amplitude or power of the signal. The angle of the constellation point from the horizontal axis represents a phase shift of the carrier wave. Since PAM4 is a one-dimensional amplitude modulation with no phase shifts, the constellation points are positioned on the horizontal axis. There are NP=4 constellation points that represent the four different amplitude levels in PAM4. So, constellation point A represents amplitude level 0, constellation point B represents level 1, constellation point C represents amplitude level 2 and constellation point D represents amplitude level 3.

In conventional PAM4, a symbol pattern has a length L=1. Each symbol, e.g. each of the constellation points on the diagram, are mapped to a sequence of N=2 data bits. One example of this encoding is shown in the constellation map2100.

FIG.21Billustrates a constellation map2110for a modified PAM4 format, wherein N=3 data bits are mapped to constellation patterns having a length L=2. Two symbols, e.g., two of the constellation points on the constellation map, are mapped to N=3 data bits. By having constellation patterns with a length L greater than 1, the transitions between the constellation points may be adapted.

The constellation map2110illustrates exemplary constellation patterns and mapped data bits but is not inclusive, e.g. multiple patterns may be mapped to a same sequence of N=3 data bits. The dotted lines and arrows indicate the pattern of the two constellation points from the first symbol to the second symbol. In this example, the pattern of constellation points A, B is encoded to data bits 001. The pattern of constellation points A, A is encoded to data bits 000. The pattern of constellation points A, C is encoded to data bits 010. The pattern of constellation points D, D is encoded to 111. The full mapping for this example is shown inFIG.21B.

As shown in the constellation map2110, there is no transition between constellation point A (e.g. amplitude level 0) and constellation point D (e.g., amplitude level 3). Bits are not mapped to this pattern of constellation points A, D. This mapping decreases the probability of the transition between constellation points A and D in a modulated signal.

In addition, when mapping data bits for transmission, consecutive constellation patterns may be selected such that the terminating constellation point Pt of the first pattern and the initial constellation point Pi of the second pattern are the same (e.g., both are Point A or both are Point D). Additional or alternate mapping may be performed as described herein with respect to PAMx to adapt the probability of transitions between the constellation points.

Thus, the probabilistic transition shaping may be generalized to adapting a transition between one or more constellation points of the modulation, where a number NPof constellation points is greater than 2. This principle of adapting a probability of a transition between one or more constellation points may apply to other types of modulation formats, such as phase shift keying (PSK) or quadrature amplitude modulation (QAM).

FIG.22Aillustrates an embodiment of a constellation map2200for a conventional phase shift keying (PSK) modulation with 4 phases (4-PSK). The 4-PSK modulation is also known as quadrature PSK (QPSK). The angle of a constellation point (A, B, C, D), measured counterclockwise from the horizontal axis, represents the phase of the carrier wave. So constellation point A represents a 0° phase, point B represents a 90° phase, point C represents a 180° phase, and point D represents a 270° phase. The distance of a constellation point from the origin represents a measure of the amplitude or power of the signal. Since there is no amplitude modulation in 4-PSK, the constellation points are equidistant to the origin. In conventional 4-PSK modulation, each constellation point is encoded with N=2 data bits. An example of the mapping of the N=2 data bits to each of the constellation points is shown in the constellation map2200.

FIG.22Billustrates an embodiment of a constellation map2210for a modified 4-PSK modulation with probabilistic transition shaping. This example has N=3 data bits mapped to constellation patterns having a length L=2. As such, the constellation patterns include two constellation points. An exemplary mapping is illustrated for some constellation patterns and is not inclusive for all possible constellation patterns, e.g. multiple patterns may be mapped to a same sequence of N=3 data bits.

In this example, an adapted probability of at least one transition between one or more constellation points is implemented for the modified 4-PSK modulation. For example, bits are not mapped for a transition between constellation point A (representing 0° phase) and point D (representing a 270° phase). The example mapping may thus eliminate transitions between constellation points A and D. So these constellation patterns (e.g., phase of 0° to 270° and phase of 270° to 0°) are not selected for mapping and are not transmitted as one of the mapped constellation patterns. This mapping thus adapts the probability of the 0° to 270° phase transition and the 270° to 0° phase transition in a modified 4-PSK signal.

In addition, when mapping bits for transmission, consecutive constellation patterns may be selected such that the terminating constellation point Pt of the first pattern and the initial constellation point Pi of the second pattern are the same, e.g., both may be predetermined to be Point A. This mapping increases the probability of the transitions between Point A and Point A. Additional or alternate mapping may be performed as described herein with respect to PAMx to adapt the probability of transitions between the constellation points in 4-PSK. Thus, the probabilistic transition shaping may be generalized to PSK modulation format by adapting a probability of a transition between one or more constellation points during mapping of the N bits.

FIG.23Aillustrates an embodiment of a constellation map2300for a conventional QAM with 8 constellation points (8 QAM). Higher order QAM formats may also be implemented herein, such as 16 QAM, 32 QAM and 64 QAM, but for simplicity, 8 QAM is used as an example. QAM includes both phase and amplitude modulation using two carrier waves of the same frequency that are out of phase with each other by 90°. So the constellation points for QAM vary by both phase and amplitude. As such, as seen inFIG.23A, the constellation points have varying distances from the origin and from the horizontal axis. In a conventional 8 QAM format, each of the number NP=8 of constellation points are encoded with N=3 bits for a 3 bit/symbol rate. An example of one possible mapping of the N=3 bits to each of the constellation points is shown inFIG.23A.

FIG.23Billustrates an embodiment of a constellation map2310for a modified 8 QAM modulation with probabilistic transition shaping. For simplicity, this example has N=3 data bits mapped to constellation patterns having a length L=2. As such, the constellation patterns include two constellation points. An exemplary mapping is illustrated but additional and/or alternate bit mappings may be implemented as well.

In this example, data bits are not mapped for a transition between constellation points A and D. The example mapping may thus eliminate transitions between constellation points A and D. So these symbol patterns (e.g., phase shift of 0° to 270° and phase shift of 270° to 0° at a same amplitude) is not selected for mapping and is not transmitted as one of the mapped symbol patterns. This mapping thus adapts the probability of the 0° to 270° phase transition and the 270° to 0° phase transition in a modified QAM signal. In another example, there is no mapping for a transition between constellation points B and D. This mapping thus adapts the probability of the 90° to 270° phase transition and the 270° to 90° phase transition in a modified QAM signal. Thus, the probabilistic transition shaping may be generalized to QAM modulation format by adapting a probability of a transition between one or more constellation points.

The number N of data bits and the length L of constellation patterns may be varied to generate a modified 8-QAM format. For example, N=6 data bits may be mapped over symbol patterns with a length L=4. In another example, N=13 data bits may be mapped to symbol patterns having a length L=8. In addition, the probability of alternate or additional transitions may be increased or decreased.

FIG.24illustrates a flow diagram of an embodiment of a method2400for generating a modulated signal with probabilistic transition shaping. In general, modulation formats with a number NP of constellation points greater than two may be modified to include probabilistic transition shaping. The number NP of constellation points for a modulation format is determined at2402. The length L of constellation patterns and number N of data bits is selected at2404. The calculations shown inFIG.9may be used in this determination. For example, the number of valid patterns of length L is obtained along with the number of bits that can be mapped to the number of valid patterns. The desired symbol rate and bit rate may also be considered.

The set of constellation patterns with the length L is determined at2406, and the subset of valid patterns is selected at2408. One or more parameters as described herein may be used in the determination. For example, constellation patterns including one or more transitions between constellation points may be eliminated. Or constellation patterns only terminating and/or beginning with one or more constellation points may be selected. This selection of valid patterns adapts the probability of one or more transitions between constellation points.

The N data bits are then mapped to one or more of the valid patterns of constellation points at2410. For example, a sequence of N data bits may be mapped to a first valid pattern with a predetermined initial constellation point PIand a second valid pattern with another predetermined initial constellation point PI. In another example, a sequence of N data bits may be mapped to a first valid pattern with a predetermined terminating constellation point PTand a second valid pattern with another predetermined terminating constellation point PT. The mapped sequences of data bits and associated constellation patterns are used to generate one or more codebooks. Alternatively or in addition thereto, at least one state machine with a plurality of states is generated using the mapping.

During use, a digital encoding signal is generated with an adapted probability of at least one transition between one or more constellation points at2414. In an embodiment, as described herein, probabilistic transition shaping may also be performed during selection of consecutive constellation patterns in the digital encoding signal. For example, a first constellation pattern is selected with a terminating symbol or constellation point PT. The next consecutive constellation pattern having an initial constellation point PIis selected such that the constellation points of PTand PIare the same. In another embodiment, the consecutive constellation patterns may be selected such that the constellation points of PTand PIare one of a plurality of predetermined patterns. Some patterns of constellation points may be more advantageous. So the two consecutive patterns are selected such that constellation points PTand PIare one of the plurality of predetermined patterns. The digital encoding signal may be generated using one or more codebooks and/or using one or more state machines.

In step2416, the digital encoding signal is then used to generate a modulated signal with an adapted probability of at least one transition between one or more constellation points. The modulated signal may be an optical signal, a wireless electromagnetic signal or electronic signal.

The specific devices and portions or elements of devices described herein are not required and may be substituted for one or more different devices or elements. Additional devices and elements of devices may also be included though not described or illustrated herein. One or more methods or steps of a method described herein may not be performed, or steps or methods may be performed in addition to those described. Still further, the sequence in which methods or steps of methods are listed are not necessarily the order in which they are performed. Also, the concepts have been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the claims as set forth below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the claims.

Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any feature(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature of any or all the claims. Moreover, the particular embodiments disclosed above are illustrative only, as the disclosed subject matter may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. No limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope of the disclosed subject matter. Accordingly, the protection sought herein is as set forth in the claims below.

Although multiple embodiments of the present invention have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it should be understood that the present invention is not limited to the disclosed embodiments, but is capable of numerous rearrangements, modifications, and substitutions without departing from the invention as set forth and defined by the following claims.