Patent Description:
A passive optical network (PON) typically has a point-to-multipoint (P2MP) architecture in which passive optical splitters are used to enable a single optical transmitter to broadcast data transmissions to multiple subscribers. An exemplary PON includes an optical line terminal (OLT) at a service provider's network location and a plurality of optical network units (ONUs) near or at the end-user locations. The ONUs are connected to the OLT by way of an optical distribution network (ODN) that comprises one or more passive optical splitters. In a time-division-multiplexed (TDM) PON, downstream signals are typically broadcast to all ONUs, with upstream signals from the individual ONUs controlled using some type of multiple-access protocol.

The recently promulgated international standard ITU-T G. <NUM> for TDM PON provides a fixed <NUM> Gb/s ("<NUM>") transmission rate in the downstream direction by employing non-return-to-zero (NRZ) modulation, independent of the particular channel conditions within the network. However, the fixed rate <NUM> standard may be problematic for use by a portion of the already installed base, with older generation receiver equipment within certain ONUs not able to operate well with <NUM> NRZ data for a variety of reasons. First, the front-end receiver components (both optical and electronic) in some ONUs lose performance capabilities as they age (such as lower signal-to-noise ratio in the O/E conversion), introducing an unacceptable bit error rate (BER) in the recovered data stream. Additionally, various ONUs may have been originally designed to operate near margin conditions (link loss, for example) associated with prior standard transmission rates. These are just two examples, and other factors related to the performance of the existing base of ONU modules may be found to hamper the acceptance of a <NUM> fixed rate NRZ for downstream transmission.

The needs remaining in the art are addressed by the present invention, which relates to a time-division multiplexed (TDM) passive optical network (PON) that is capable of supporting not only the standard-defined fixed rate <NUM> NRZ data, but also a lower line-rate modulation scheme that utilizes the same front-end receiver configuration as used for <NUM> NRZ, thus allowing for the installed base of ONUs to support a <NUM> Gb/s ("<NUM>") data stream in additional to the <NUM> data stream.

In accordance with the principles of the present invention, an apparatus and method are disclosed that utilize a particular delay modulation technique (referred to hereinafter at times as "Miller encoding") to encode <NUM> data for inclusion with the <NUM> NRZ traffic in the downstream broadcast transmission from an optical line terminal (OLT) to a plurality of optical network units (ONUs) through an optical distribution network (ODN). More generally, the delay modulation technique allows for a secondary data stream, operating at half the rate of the NRZ data) to also be sent, since both signals are recovered using the same clocking circuitry at the ONU.

Advantageously, <NUM> Miller-encoded data requires a clock running at twice that rate to recover the transmitted data stream. Inasmuch as the recently-published PON system standard is based upon the use of <NUM> NRZ modulation, the clock and data recovery (CDR) circuitry within the ONU's receiver needs to operate at a <NUM> Gbaud rate to be in compliance with the standard. Thus, this same <NUM> Gbaud rate (being twice the rate of the Miller-encoded <NUM> data) is precisely the clocking speed required to recover the encoded <NUM> Miller data. The installed ONU base is thus able to easily recover both <NUM> NRZ data and <NUM> Miller data.

In addition to providing extended reach and/or higher loss capability, the inclusion of data transmission operating at a <NUM> rate offers a "safe mode" option for operating the PON system in the event of certain ODN fault scenarios that may limit <NUM> (or higher) transmission capabilities.

An exemplary embodiment takes the form of apparatus comprising transmission circuitry and encoding circuitry. The transmission circuitry is configured to create a downstream transmission in a point-to-multipoint (P2MP) passive optical network (PON) from at least two separate input signals. The two signals including a first input signal comprising NRZ modulated data operating at a first data rate and a second input signal operating at a second data rate that is one-half of the first data rate. The encoding circuitry is configured to be responsive to an input data stream operating at the second data rate, to generate therefrom a delay-modulation encoded output signal and to thereafter apply it as the second input signal to the transmission circuitry.

Another embodiment comprises a method of forming a downstream transmission in a passive optical network (PON), where the method includes the steps of: accepting a first input data signal in an NRZ modulation format and operating at a first data rate; accepting a second input data signal operating at a second data rate that is one-half of the first data rate; applying a delay-modulation encoding to the second input data signal; and forming the downstream transmission including both the NRZ-modulated first input data signal and the delay-modulation encoded second input data signal.

Other and further embodiments and aspects of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.

A PON system is proposed that provides a degree of flexibility to the <NUM> fixed rate standard of ITU-T G. <NUM> in a broadcasted downstream transmission. Based on the presumption that the installed base of ONUs include receiver components that perform <NUM> Gbaud clock and data recovery (CDR), as necessary for the <NUM> NRZ data, a flexible PON system is formed in accordance with the present invention that is also able to support transmission and recovery of lower line-rate data within the same ONU receiver design. In particular, it has been found that a <NUM> data stream encoded by a particular delay modulation scheme (also referred to as "Miller encoding" as a tribute to its inventor Armin Miller, as documented in <CIT>), allows for transmission at the lower rate of <NUM> (extending the reach of the PON system and providing additional loss margin) while using the same in-place <NUM> Gbaud CDR at the ONUs to provide recovery of the original data. Miller-encoded data requires the use of a clock running at twice its data rate to properly recover the original data. Therefore, in a PON system using a <NUM> Gbaud CDR capability, a <NUM> Miller-encoded transmission may also be supported. As discussed below, an inventive flexible PON system is able to continue to support legacy ONUs by including this <NUM> Miller-encoded signalling option.

<FIG> is a schematic block diagram of a TDM PON system <NUM> useful in understanding the implementation of a flexible <NUM>/<NUM> downstream modulation transmission system in accordance with the principles of the present invention. <FIG> illustrates a conventional topology including an optical line terminal (OLT) <NUM> that communicates with a plurality of optical network units (ONUs) <NUM> via an optical distribution network (ODN) <NUM>. The specific number of ONUs associated with a given OLT may be a function of the particular application (for example, a last-mile application of FTTx may serve a relatively small number of ONUs, while a mid-span link in a radio network may need to support hundreds of ONUs).

ODN <NUM> can be thought of as a data distribution system that comprises optical fiber cables, couplers, splitters, and other suitable passive components. ODN <NUM> typically extends from OLT <NUM> to ONUs <NUM> in a branching configuration, simply represented in this diagram as comprising a single 1xN splitter <NUM>. In general, ODN <NUM> may be configured in any other suitable point-to-multipoint (P2MP) manner for TDM PON systems.

In operation of flexible rate PON <NUM>, OLT <NUM> is configured to broadcast a single downstream transmission (illustrated as <NUM>/<NUM> in <FIG>) to all of the ONUs <NUM>, where each individual ONU <NUM>. x is configured to extract its unique "user block" of data from each received PON frame, discarding the remainder of the blocks. In this flexible environment, each downstream PON frame may include both <NUM> Miller-encoded data and <NUM> NRZ data.

An exemplary OLT <NUM> is shown in <FIG> as including a transmission circuitry module <NUM> and a receiver circuitry module <NUM>. Transmission circuitry <NUM> is configured to construct the PON frames for downstream transmission in a manner well-known in the art, where in this embodiment of the present invention, two input data streams (<NUM> Miller and <NUM> NRZ) are provided as inputs to transmission circuitry module <NUM>. Receiver circuitry <NUM> functions in a conventional manner that is not germane to the subject matter of the present invention, which is specifically directed to the provisioning of a downstream transmission that accommodates different modulation formats.

In particular, OLT <NUM> is shown as further comprising a delay modulation encoder <NUM> (also referred to at times as a "Miller encoder" or "encoding circuitry"), which is responsive to an input digital data stream operating at <NUM> and generates therefrom Miller-encoded data for transmission to ONUs <NUM>. Miller encoding, as described in detail below in association with <FIG>, is represented in the diagram of <FIG> by its finite state machine form within encoder <NUM>. The symbolic representation in this form is fashioned into logic circuitry that creates the specific encoding explained in detail below. In operation, the delay modulation (Miller) encoding scheme uses the information of two adjacent input information bits to generate the encoded output symbol (hence, a first bit is "delayed" in being coded until the second bit is present). For ease of understanding, "delay modulation" encoding will be described hereinafter as "Miller" encoding.

In particular, Miller encoding applies the following rules: (<NUM>) each logic <NUM> is represented by a mid-bit transition; (<NUM>) a logic <NUM> that is followed by and preceded by a logic <NUM> is ignored; and (<NUM>) a pair of adjacent logic <NUM>'s is represented by a transition at the mid-point of the pair. In the context of the present invention, the Miller encoded data stream output from encoder circuitry <NUM> is referred to as "<NUM> Miller", which is transmitted along with <NUM> NRZ, as shown in flexible PON system <NUM> of <FIG>.

<FIG> illustrates a data stream encoded using the Miller coding scheme embodied by encoder circuitry <NUM>. As shown, each logic <NUM> is coded as a transition in the middle of the bit period (the transition itself may occur in either direction, depending on the values of the previous bits, with both directions shown in <FIG>). Isolated logic <NUM>'s are ignored (as per rule (<NUM>) mentioned above), and a transition is inserted at the beginning of a second bit period for a pair of adjacent logic O's. As mentioned above and discussed in more detail below, decoding the Miller-encoded data requires a clock running at twice its data rate and, therefore, the <NUM> Gbaud CDR circuitry used for decoding <NUM> NRZ data is appropriate for handling the <NUM> Miller-encoded data as well.

Since Miller encoding is a run-length limited code (here, the longest time span possible without a transition is two bits in length), it has very good timing/ clock recovery properties and is immune to polarity inversion. That is, Miller encoded data is self-clocking, with the clock signal necessary for decoding being directly obtained from the encoded data itself. As a result, a Miller encoded <NUM> signal looks very much like a <NUM> NRZ signal in terms of bit transitions while still maintaining many of the benefits of a lower baud-rate signal.

<FIG> is a block diagram of an exemplary ONU <NUM> that is able to recover both the <NUM> NRZ data and <NUM> Miller data in accordance with the principles of the present invention. Here, ONU <NUM> includes an optical coupling component <NUM> (e.g., a wavelength division multiplexer) that is used to direct the broadcasted downstream <NUM>/<NUM> transmission (operating at a known wavelength λD) to an optical receiver component <NUM>. An optical transmitter component <NUM> is included within ONU <NUM> and directs upstream data traffic (on a separate wavelength λU) through WDM <NUM> and back into ODN <NUM> in a well-known manner.

For the purposes of the present invention, optical receiver component <NUM> is considered as comprising an O/E element <NUM> and a CDR circuitry module <NUM>, where in order to recover <NUM> NRZ data, CDR circuitry module <NUM> functions to extract a <NUM> clock from the stream and use it to re-time the stream and recover the transmitted data. The recovered <NUM> clock is also depicted in <FIG> and illustrates how the use of a clock operating at twice the Miller encoding rate is able to recover the original Miller-encoded data stream.

In particular, since the original Miller encoding relies on the value of adjacent bits to perform the coding (i.e., a "delayed modulation" scheme), the double-sampling of each transmitted <NUM> Miller data symbol essentially reverses the delay. Conceptually and as shown in <FIG>, two samples of each received <NUM> Miller symbol are created by using the <NUM> clock. The logic values of the two samples are passed through an XOR logic gate to produce the original Miller-encoded bit. That is, in accordance with standard XOR logic circuitry, a (<NUM>,<NUM>) or (<NUM>,<NUM>) pair of inputs to an XOR gate generates a logic <NUM> output; a (<NUM>,<NUM>) or (<NUM>,<NUM>) pair of inputs to an XOR gate generates a logic <NUM> output. The application of the XOR function is shown in <FIG> as applied to the pair of samples from each received <NUM> Miller data symbol, with the original data stream shown as being recovered by this function.

<FIG> is a somewhat more detailed illustration of an exemplary receiver <NUM> included in ONU <NUM> as described above in association with <FIG>. Here, O/E element <NUM> is shown as comprising a photodiode <NUM> and a transimpedance amplifier circuitry (TIA) <NUM>. The electrical output from TIA <NUM> is thereafter applied as an input to the <NUM> Gbaud CDR circuitry module <NUM>. In this embodiment, CDR module <NUM> includes a decision-directed, symbol-based Mueller-Muller timing error detector (TED) circuit <NUM> with an integrated <NUM>-tap fast forward error correction (FFE) equalizer circuit <NUM>. The use of a <NUM>-tap FFE equalizer is very similar to the specified reference receiver as specified in G. In the case of recovering the <NUM> NRZ data, the output of FFE equalizer <NUM> is the data signal itself.

In accordance with the principles of the present invention, the same CDR circuitry (i.e., providing the <NUM> Gbaud rate) is appropriate for recovering the lower line-rate Miller-encoded data. In particular for the <NUM> Miller signal, decoding of the recovered bit stream is accomplished by XORing two consecutive <NUM> bits, as shown in the diagram of <FIG>, to convert the individual recovered bits into the <NUM> Miller symbols as originally encoded. In the diagram of <FIG>, this additional step of recovering the Miller-encoded data is shown by XOR gate <NUM>, which receives a pair of samples in the manner described above, providing as the output the recovered <NUM> Miller data.

<FIG> includes measured eye diagrams for both <NUM> Miller data (shown in diagrams (a)) and <NUM> NRZ data (shown in diagrams (b)). The top diagram in each case is experimental data measured in a "back-to-back" (b2b) arrangement of OLT-ONU, with the bottom diagram showing measured results when including <NUM> of optical fiber between the OLT and the ONU. In particular, the illustrated data was measured at the output of FFE equalizer <NUM>. The outer modulation amplitude (OMA) of the signals was optimized for the <NUM>/<NUM> "mixed signal" operation and set to the same value so as to maintain a similar extinction ratio (ER) for either case. Quite evident from the data is that the <NUM> Miller eye diagram in the b2b configuration resembles the <NUM> NRZ data in the same condition. In both cases, a <NUM>-tap T-spaced FFE equalizer was used. The more open eye result of transmitted <NUM> Miller data through <NUM> of fiber can be attributed to, at least in part, a higher chromatic dispersion tolerance for the reduced transmission rate.

Thus, it appears that the inclusion of a <NUM> Miller encoding option in the <NUM> PON system may allow for extended reach transmissions well beyond the exemplary <NUM> value. Of course, the extended reach benefit of the <NUM> Miller option comes at the cost of a lower line rate. It is contemplated that certain applications may be well-suited to prefer the extended reach over the higher transmission rate. The ability of the flexible PON system of the present invention to provide both alternatives is considered to be an advantageous, important feature.

While the above-described embodiments have focused on supplementing the <NUM> NRZ standard transmission rate (associated with ITU-T G. <NUM>), it is to be understood that a flexible PON system of the present invention may also be enhanced to support the transmission of other modulation schemes that use similar CDR/ equalizer components. Indeed, it is well-known that a <NUM> Gb/s ("<NUM>") PAM4 modulated signal also uses a 50Gbaud clock to perform data recovery and, therefore, an alternative embodiment of the inventive flexible PON system may be configured to support all three schemes: <NUM> Miller, <NUM> NRZ, and <NUM> PAM <NUM>. <FIG> shows a flexible PON system 10A that utilizes these three rates, depicted as downstream transmission <NUM>/<NUM>/<NUM>. In this example, ONU <NUM> is shown as operating with the included <NUM> PON4 data, ONUs <NUM> and <NUM> operating with the <NUM> NRZ data, and ONU <NUM>. N including an XOR gate <NUM> for recovering the <NUM> Miller-encoded data.

<FIG> contains plots of the magnitude of the frequency response of the taps which are exemplary of an FFE equalizer portion used at the ONUs for equalization of the signal, as illustrated in <FIG>. For comparison purposes, the frequency response of the tap weights for a <NUM> NRZ are also shown. It is observed that the filter response for <NUM> NRZ data is quite different from the set of three encodings (<NUM> Miller, <NUM> NRZ, and <NUM> PAM4) used in flexible PON system 10A of the present invention. This difference in filter response can be attributed to the fact that for a conventional <NUM> NRZ signal, the <NUM> Gbaud T-spaced equalizer will generate two samples per symbol. However, for the Miller-encoded <NUM> signal, the FFE filter response is shown to closely match those of the <NUM> NRZ and <NUM> PAM4 modulations (since it is also based on a 50Gbaud clocking rate). This data clearly supports the supposition that it is feasible, even when FFE equalization is performed at the receiver, to include <NUM> Miller-encoded data with existing <NUM>/ <NUM> PON systems, providing a <NUM>/<NUM>/ <NUM> flexible PON system that provides the ability to use a lower line-rate in certain circumstances.

Summarizing, it is proposed to extend the capability of a <NUM> flexible PON by also providing a <NUM> modulation option (and, similarly, providing a <NUM>/<NUM>/ <NUM> operation as well). The lower modulation rate allows for provided an extended reach to remote ONU locations, as well as higher loss capability. Another benefit provided by including a <NUM> option is as a "safe mode" option when certain fault scenarios occur at an ONU so that the standard <NUM> signals cannot be recovered. Inasmuch as the receiver front-end within a significant portion of the installed ONU equipment is already based upon <NUM> compatible elements, this reality of the equipment "limitation" can be exploited by the inventive technique of incorporating <NUM> Miller encoded data within the downstream signal.

While contemplated as a preferred embodiment, it is to be understood that the use of Miller encoding at any given baud rate R may be used in downstream PON communications with NRZ-encoded data utilizing a CDR at a 2R baud rate (and, similarly, supporting transmission of PAM4 signals using the same 2R baud rate for data recovery ). As long as the CDR is configured to operate at a 2R baud rate, a pre-existing 2R flexible PON system (or even simply at a 2R PON system) may easily accommodate additional transmissions based on Miller encoding at the basic data rate of R.

As used in this application, the term "circuitry" may refer to one or ore or all of the following: (a) hardware-only circuit implementations (such as implementations in only analog and/or digital circuitry) and (b) combinations of hardware circuits and software, such as (as applicable): (i) a combination of analog and/or digital hardware circuit(s) with software/firmware and (ii) any portions of hardware processor(s) with software (including digital signal processor(s)), software, and memory(ies) that work together to cause an apparatus, such as a mobile phone or server, to perform various functions) and (c) hardware circuit(s) and or processor(s), such as a microprocessor(s) or a portion of a microprocessor(s) that requires software (e.g., firmware) for operation, but the software may not be present when it is not needed for operation.

Claim 1:
Apparatus (<NUM>) comprising:
transmission circuitry (<NUM>) configured to create a downstream transmission in a point-to-multipoint, P2MP, passive optical network, PON, from at least two separate input signals, a first input signal comprising NRZ modulated data operating at a first data rate and a second input signal operating at a second data rate that is one-half of the first data rate; and
encoding circuitry (<NUM>) configured to be responsive to an input data stream operating at the second data rate, to generate therefrom a delay-modulation encoded output signal and to thereafter apply it as the second input signal to the transmission circuitry.