Patent Description:
Although digital multi-band optical communication has the advantage of allowing more information to be communicated over an optical fiber (as compared to single band optical communication), there may be challenges in designing multi-band optical receivers and transmitters. For example, one challenge may be designing the multi-band optical receivers and transmitters to be backward compatible, as a large number of single carrier networks are still in use. As compared to a single carrier optical receiver, a multi-band optical receiver makes precise adjustments to the received optical signal for purposes of aligning the receiver with a center null point of the received optical signal. Misalignment of the receiver with respect to the center null point may result in all of the sub-bands being misaligned.

In accordance with example implementations that are described herein, a hybrid digital multi-band is used for the communication of data over an optical fiber using a plurality of carriers. More specifically, in accordance with example implementations, the hybrid digital multi-band spans across an allocated spectrum; and the hybrid digital multi-band includes a single master sub-band, which is located in the center of the allocated spectrum and slave sub-bands that are located on either side of the center master sub-band. In accordance with example implementations, the bandwidth of the master sub-band may be as wide as a single band in a legacy single carrier system and may be significantly wider than each of the slave sub-bands.

Due to this approach, for purposes of single carrier backward compatibility (i.e., when hybrid digital multi-band receivers and transmitters are to work with a single carrier optical system), the receiver may use the master sub-band in conjunction with a coding and modulation scheme that is used for a legacy single carrier system. When the optical system may tolerate a wider spectrum that accommodates multiple sub-bands, the slave sub-bands may be used, and the bandwidth and number of the slave sub-bands may be adjusted to optimize performance and system capacity. Unlike traditional multi-band optical communications, the master sub-band of the hybrid multi-band does not have a null point in the center of the allocated spectrum, which increases the capacity for optical communications and avoids issues with aligning the receiver to the center null point of the received optical signal. Moreover, in accordance with example implementations, the modulation and coding information for the hybrid multi-band may be carried by a control signal that is carried in the content of the master sub-band. Because the control information is carried in the master band, which is centralized in the allocated spectrum, the optical communication system may be switched hitless based on the control signal.

According to an aspect of the present disclosure, there is provided a method as defined in claim <NUM>.

According to another aspect of the present disclosure, there is provided an optical receiver apparatus as defined in claim <NUM>.

In any of the preceding aspects, controlled data of the payload data is generated to control a feature that is associated with the master sub-band and the plurality of slave sub-bands. Distributing the payload data includes designating the control data to appear in the master sub-band.

In any of the preceding aspects, generating the data to control the type of modulation includes generating first default data to set a default type of modulation that is associated with the master sub-band and the plurality of slave sub-bands and subsequently generating second data to set a different type of modulation associated with the master sub-band and the slave sub-bands.

In any of the preceding aspects, the master sub-band is wider than each of the slave sub-bands.

In any of the preceding aspects, a first number of the slave sub-bands extend in the allocated frequency spectrum above the master sub-band, a second number of the slave sub-bands extend in the allocated frequency spectrum below the master sub-band, and the first and second numbers are equal.

In any of the preceding aspects, the optical receiver apparatus further includes an analog-to-digital converter (ADC) to provide a digital signal representing the second signal corresponding to the master sub-band of the optical signal; and a voltage controlled oscillator (VCO) to provide a clock signal to the ADC. The first processor controls the VCO based on information that is contained within the master sub-band of the optical signal.

In any of the preceding aspects, the first processor controls the VCO based on a timing derived from symbols represented by the information contained within the master sub-band of the optical signal.

In any of the preceding aspects, the first processor includes a demodulation demapper to perform modulation symbol demapping associated with symbols represented by information in the master sub-band of the optical signal; and the second processor includes a plurality of demodulation demappers to demap modulation symbols associated with the slave sub-bands.

In any of the preceding aspects, the first processor includes a decoder to decode data that is associated with the master sub-band of the optical signal; and the second processor includes a plurality of decoders to decode information that is associated with the slave sub-bands of the optical signal.

In any of the preceding aspects, the decoder of the first processor is jointly coupled to the plurality of decoders of the second processor.

In any of the preceding aspects, the optical receiver apparatus includes a digital pre-processing engine to compensate for at least one of a time skew, a quadrature error, or a frequency offset associated with the master sub-band and the plurality of slave sub-bands of the optical signal.

In any of the preceding aspects, the first processor determines a type of demodulation from a plurality of types of demodulation based on information that is contained within the master sub-band of the optical signal, and applies the determined type of demodulation to the master sub-band and the plurality of slave sub-bands of the optical signal.

In any of the preceding aspects, the first processor starts the second processor in response to the master sub-band of the second signal converging with the master sub-band of the optical signal.

In any of the preceding aspects, the allocated frequency spectrum includes nulls between a first slave sub-band and a second null between a second slave sub-band and the master sub-band.

Referring to <FIG>, in accordance with example implementations that are described herein, an optical transmitter and an optical receiver are constructed to communicate over an optical medium, such as an optical fiber, using hybrid digital multi-band optical communication. More specifically, <FIG> depicts, in accordance with example implementations, a power spectral density <NUM> of an allocated spectrum <NUM> in which hybrid digital multi-band optical communication is used. In general, the power spectral density <NUM> includes a central master sub-band <NUM> (having a central wavelength disposed at a central wavelength <NUM> of the allocated spectrum <NUM>) and peripheral slave sub-bands that are disposed on either side of the master sub-band <NUM>.

Depending on the particular implementation, the optical transmitter and receiver may communicate using a single master sub-band <NUM> and a selectable number of slave sub-bands, such as zero (for legacy single carrier backward compatibility), two (as depicted in <FIG>), four, and so forth, depending on the number of carriers that are used in communications in the allocated spectrum <NUM>. For the example spectral density <NUM> that is illustrated in <FIG>, the allocated spectrum <NUM> is divided into the central master sub-band <NUM> and two adjacent slave sub-bands: a lower frequency slave sub-band <NUM> (called a "lower slave sub-band" herein); and a higher frequency slave sub-band <NUM> (called a "higher slave sub-band" herein). As also depicted in <FIG>, null points separate the sub-bands: a null point <NUM> separates the master sub-band <NUM> and the lower slave sub-band <NUM>; and a null point <NUM> separates the master sub-band <NUM> and the higher slave sub-band <NUM>.

In addition to being centered at the central wavelength <NUM> for the allocated spectrum <NUM>, the master sub-band may be further distinguished from the slave sub-bands by its relative width. In this manner, in accordance with example implementations, the bandwidth of the master sub-band <NUM> may be as wide as the bandwidth of the allocated spectrum for a single carrier system and, as illustrated in <FIG>, may be significantly wider than either slave sub-band <NUM> or <NUM> (may be two to three times wider that the slave sub-band, as an example).

Although receivers and transmitters are described herein that communicate data using multiple carriers over the master and slave sub-bands, for purpose of backward compatibility with a single carrier network, the receivers and transmitters may be designed to not use the slave sub-bands and use the master sub-band <NUM> for the single carrier communications. However, when the network allows multi-band communications, the optical receiver and transmitter may be configured with parameters for multi-band optical communication, such as parameters specifying the number and width of the slave sub-bands to optimize performance and system capacity.

In general, the master sub-band <NUM> contains control content that specifies the modulation and coding of content for the master sub-band and the slave sub-bands. More specifically, in accordance with example implementations, the master sub-band <NUM> may contain such control content as a specifically designated control string that carries information about the modulation type and coding to be used in the master sub-band and slave sub-bands. In this manner, the actual type of modulation (QPSK, QAM8, QAM16, and so forth) that is used for optical communications between the receiver and the transmitter may not be pre-programmed, or pre-configured. Rather, in accordance with example implementations, the optical receiver may accommodate a wide variety of different modulation types, so that a given optical receiver may be initialized, on start up, using a default demodulation, and based on information that is contained in a control string that is communicated over the master sub-band, the optical receiver may identify another modulation type to be used in the optical communications and adjust the demodulation that is applied by the receiver accordingly. It is noted that because the master sub-band <NUM> is centered in the allocated spectrum <NUM>, challenges pertaining to aligning the receiver to a null point of the sub-band are avoided and allow for a more robust way to communicate control information to the receiver and avoid misalignment issues.

<FIG> depicts a schematic diagram <NUM> of a hybrid digital multi-band transmitter <NUM> in accordance with example implementations. For this example implementation, the transmitter <NUM> receives client payload data <NUM> and produces a corresponding optical signal <NUM> (provided to an optical fiber <NUM>), which has a corresponding hybrid digital multi-band spectral density <NUM>, as illustrated, for example, in connection with <FIG>.

In general, the transmitter <NUM> includes a master sub-band processor <NUM> (a digital signal processor (DSP), for example); and a slave sub-band processor <NUM> (another DSP, for example). The transmitter <NUM> includes a demultiplexor <NUM>, which, according to a predefined configuration, distributes the client payload data <NUM> among the master and slave sub-bands. As an example, the demultiplexor <NUM> may be configured to route content for a master channel, including control information (content representing a coding and modulation type, as an example) to the master sub-band processor <NUM> and route content for slave channels to the slave sub-band processor <NUM>.

In accordance with example implementations, the master sub-band processor <NUM> includes an encoder <NUM>, which applies an encoding to the received data for the master sub-band channel, depending on the particular modulation type. For example, a <NUM>-QAM modulation type may be used in which four bits are mapped to each symbol, and the encoder <NUM> encodes the client payload data for the master sub-band channel accordingly. This encoded data, in turn, is provided to a modulation symbol mapper <NUM> of the master sub-band processor <NUM>, which maps the encoded bits to the appropriate modulation symbols. An up-sampling and pulse shaping component <NUM> of the master sub-band processor <NUM> upsamples and performs pulse shaping to produce a corresponding signal representing the content for the master sub-band channel.

As also depicted in <FIG>, the slave sub-band processor <NUM> may include multiple encoders <NUM>, where each encoder <NUM> is associated with a particular slave sub-band channel and encodes the bits, depending on the particular modulation type to be used (i.e., the same modulation type used for the master sub-band channel). An associated modulation mapper <NUM> maps the encoded bits to the modulation symbols; and up-sampling and pulse shaping components <NUM> perform the corresponding up-sampling and shaping of the pulses to produce corresponding optical signals for the corresponding slave sub-bands. A multiplexor <NUM> of the transmitter <NUM> combines the optical components of the master and slave sub-band channels to provide the composite optical signal <NUM> that is provided to the optical fiber <NUM>.

Referring to <FIG>, in accordance with example implementations, a hybrid digital multi-band optical receiver <NUM> includes front end processing components <NUM>, which include an integrated coherent receiver <NUM>, a local oscillator (LO) laser <NUM>, and an analog-to-digital converter (ADC) <NUM>, a voltage controlled oscillator <NUM> and a digital pre-processing engine <NUM>. The integrated coherent receiver <NUM> receives an optical signal <NUM> from the optical fiber <NUM>. The integrated coherent receiver <NUM> mixes the optical signal <NUM> with a laser signal that is produced by the LO laser <NUM> to produce an electrical signal at the output of the integrated coherent receiver <NUM>, which represents the mixed optical signal. The pre-processing components <NUM> further include a sample and hold circuit (not shown) that samples and holds the electrical signal that is provided by the integrated coherent receiver <NUM>; and the ADC <NUM>, which is clocked by a clock signal that is provided by the VCO <NUM>, converts the output of the sample and hold circuit into a digital signal that represent the digital version of the electrical signal that is provided by the integrated coherent receiver <NUM>.

The digital pre-processing engine <NUM>, which may be, for example, a DSP, in accordance with example implementations, pre-processes the digital signal that is provided by the ADC <NUM> for purposes of removing impairments, such as time skew, quadrature error, frequency offset, and so forth. As depicted in <FIG>, the output of the digital pre-processing engine <NUM> is provided to the input of a demultiplexor <NUM>, which provides the signal from pre-processing engine <NUM> to slave and master sub-band channels.

In accordance with example implementations, the receiver <NUM> includes a master sub-band processor <NUM> that performs processing of the content of the signal <NUM> pertaining to the master sub-band such that the processed content is provided to an input of a multiplexor <NUM> and represents the demodulated content for the master sub-band.

The receiver <NUM> further includes a slave sub-band processor <NUM>, which processes the contents for associated slave sub-band channels and provides the demodulated contents to corresponding inputs of the multiplexor <NUM>. The multiplexor <NUM> combines the contents for the slave sub-bands and master sub-bands to provide data <NUM> (at the output of the multiplexer <NUM>), which represents the content received from the optical fiber <NUM>.

In accordance with example implementations, the master sub-band processor <NUM> includes a timing recovery engine <NUM>, an equalization engine <NUM>, a phase recovery engine <NUM> and a demapping and decoding engine <NUM>. The phase recovery engine <NUM> performs a carrier phase recovery algorithm to recover the LO frequency offset (LOFO). In general, a relatively large LOFO may misalign a particular sub-band in the wrong sub-band bin, partially to completely, depending on the bin with the sub-band and the LOFO, which may significantly impact performance of the receiver. However, due to the relatively large master sub-band and the center wavelength of the master sub-band coinciding with the central wavelength of the allocated spectrum, these problems may be avoided and allow the phase recovery engine <NUM> to precisely measure the LOFO. As depicted in <FIG>, the phase recovery engine <NUM> uses the measured LOFO to provide a signal to control the LO laser <NUM>. Moreover, as also depicted in <FIG>, in accordance with example implementations, the phase recovery engine <NUM> may provide a signal, based on the LOFO, to control a frequency shift algorithm that is applied by the digital pre-processing engine <NUM>.

The timing recovery engine <NUM> of the master sub-band processor <NUM> controls the phase of the clock signal used to clock the ADC <NUM> by providing a control signal to the VCO <NUM>. The demapper and decoding engine <NUM>, as its name implies, demaps the modulation symbols, i.e., maps the symbols to encoded data and decodes the encoded data to produce the demodulated and decoded data that is provided to the input of the multiplexor <NUM> for the master sub-band.

In accordance with example implementations, the slave sub-band processor <NUM> includes equalization engines <NUM> for each of the slave sub-band channels as well as phase recovery engines <NUM> for each of the slave sub-bands and demapping and decoding engines <NUM> for each of the slave sub-band channels. The demapping and decoding engines <NUM> provide respective outputs to inputs of the multiplexor <NUM> representing the demodulated content, which the multiplexor <NUM> provides as part of the data <NUM>.

In accordance with example implementations, a controller <NUM> controls the start up of the receiver <NUM>. More specifically, referring to <FIG> in conjunction with <FIG>, in accordance with example implementations, the controller <NUM> performs a technique <NUM> during the initialization, or startup, of the receiver <NUM>. Pursuant to the technique <NUM>, the controller <NUM> initially starts up the master sub-band processor <NUM>, pursuant to block <NUM>, which allows the equalizer engine <NUM> to converge and the LOFO to be determined by the carrier phase recovery algorithm that is performed by the phase recovery engine <NUM>. In response to determining that the equalizer engine <NUM> has converged and the LOFO has been determined (decision block <NUM>), the master sub-band processor <NUM> adjusts (block <NUM>) the LO laser <NUM>. Next, the controller <NUM> starts (block <NUM>) the phase recovery by the slave sub-band processor <NUM> (i.e., starts the phase recovery engines <NUM>), pursuant to block <NUM>. The technique <NUM> next includes the controller <NUM> starting (block <NUM>) the master and slave band demapping and decoding engines <NUM> and <NUM>.

Referring to <FIG>, in accordance with example implementations, the control signal that is transmitted in the master sub-band may include a pre-defined modulation identification (MID) string <NUM>, which represents a particular modulation format, or type, and a coding scheme to be used for both the master and slave sub-bands. As depicted in <FIG>, for an example data sequence <NUM>, the MID string <NUM> may be adjacent in time to a training sequence <NUM>, which may be the same for all combinations of modulation format and coding schemes. The MID string <NUM> may be either before or after (as depicted in <FIG>) the training sequence <NUM>, which, in general, improves the robustness of detecting the string MID <NUM>.

Moreover, as depicted in <FIG>, in accordance with some implementations, the MID string <NUM> may repeat in a couple of frames to further improve its robustness. The repetition, as well as the frame length, may depend on the payload structure, such as FEC encoding and the client data.

The MDI string <NUM> allows different modulation formats and coding schemes to be communicated to the receiver <NUM>. In this manner, as an example, in accordance with some implementations, the receiver <NUM> may be preset with a default modulation type (a QPSK or BPSK modulation type, for example) as a default such that upon the initialization of the receiver <NUM>, the MID string <NUM> may be communicated to the receiver <NUM> to change the modulation format of the receiver <NUM> to another format. As such, in accordance with example implementations, it may unnecessary to preset the particular modulation format for the receiver <NUM>.

Referring to <FIG> in conjunction with <FIG>, in accordance with some implementations, the receiver <NUM> may perform a technique <NUM> for purposes of initializing the modulation format and coding scheme for the receiver <NUM>. Pursuant to the technique <NUM>, the master sub-band processor <NUM> is initialized (pursuant to block <NUM>) a particular demodulation, such as here, QPSK modulation; and as such, the initial demodulation is applied using QPSK demodulation, as depicted in block <NUM>. In response to the master sub-band processor <NUM> determining (decision block <NUM>) that the equalizer <NUM> has converged, then the master sub-band processor <NUM> may, from the demodulated and decoded data, detect the string <NUM>, as depicted in block <NUM> and set the demodulation and encoding for the master and slave sub-bands according to the demodulation format and encoding indicated by the string, pursuant to block <NUM>.

Claim 1:
A method comprising:
distributing payload data among a master sub-band and a plurality of slave sub-bands, wherein the master sub-band and the plurality of slave sub-bands collectively extend over an allocated frequency spectrum, the master sub-band and the plurality of slave sub-bands being associated with different carrier frequencies, and the master sub-band having a center frequency corresponding to a center frequency of the allocated frequency spectrum;
generating modulated data for the master sub-band and the plurality of slave sub-bands based on the distributed payload data; and
transmitting an optical signal representing the modulated data to an optical medium, characterized in that the method further comprises generating control data of the payload data to control a feature associated with the master sub-band and the plurality of slave frequency bands, wherein distributing the payload data comprises designating the control data to appear in the master sub-band and wherein the control data comprises data to set a type of modulation associated with the master sub-band and the plurality of slave sub-bands.