Abstract:
We disclose an optical transport system configured to transport an additional data stream using hierarchical modulation. In an example embodiment, copies of the additional data stream are encoded onto enhancement layers of multiple hierarchically modulated optical waves transported through the system in a multiplexed manner. An optical receiver coupled to a remote end of the optical transport link is configured to use this redundant transmission of the additional data stream to improve the bit-error rate thereof, e.g., by first extracting the signal components corresponding to the enhancement layers of the different hierarchically modulated optical waves and then combining the extracted signal components in a manner that tends to reduce, through averaging, the relative magnitude of noise in the combined signal compared to that in the individual extracted signal components. The disclosed signal-transmission format is suitable for various types of multiplexing, e.g., any combination of space-, wavelength-, and polarization-division multiplexing.

Description:
BACKGROUND 
       [0001]    1. Field 
         [0002]    The present disclosure relates to optical communication equipment and, more specifically but not exclusively, to the use of hierarchical modulation in optical transport systems. 
         [0003]    2. Description of the Related Art 
         [0004]    This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art. 
         [0005]    As used herein, the term “hierarchical modulation” refers to a modulation technique in which multiple data streams are multiplexed into a single symbol stream. In its most basic form, hierarchical modulation has two modulation layers referred to as the base layer and the enhancement layer. In this particular hierarchical-modulation variant, each constellation point of the base layer is split into multiple constellation points using the operative constellation of the enhancement layer. In more-complex hierarchical-modulation schemes, more enhancement layers can be added by further splitting each of the constellation points corresponding to the last enhancement layer using the operative constellation of the next enhancement layer. In the literature, hierarchical modulation may also be referred to as “layered modulation.” 
       SUMMARY OF SOME SPECIFIC EMBODIMENTS 
       [0006]    Disclosed herein are various embodiments of an optical transport system configured to transport an additional data stream using hierarchical modulation. In an example embodiment, copies of the additional data stream are encoded onto enhancement layers of two or more different hierarchically modulated optical waves transported through the system in a multiplexed manner. An optical receiver coupled to a remote end of the optical transport link is configured to use this redundant transmission of the additional data stream to improve the bit-error rate thereof, e.g., by first extracting the signal components corresponding to the enhancement layers of the different hierarchically modulated optical waves and then combining the extracted signal components in a manner that tends to reduce, through averaging, the relative magnitude of noise in the combined signal compared to that in the individual extracted signal components. The disclosed signal-transmission format is suitable for various types of multiplexing, e.g., any combination of space-division multiplexing, wavelength-division multiplexing, and polarization-division multiplexing. 
         [0007]    According to one embodiment, provided is an apparatus comprising: an optical de-multiplexer configured to de-multiplex an optical input signal into a plurality of hierarchically modulated optical waves; a plurality of base-layer decoders, each configured to recover a respective base data stream encoded in a base layer of a respective one of the plurality of the hierarchically modulated optical waves; a superposition module configured to generate a superposed electrical signal by superposing a plurality of electrical signals, each representing an enhancement layer of the respective one of the plurality of the hierarchically modulated optical waves; and an enhancement-layer decoder configured to decode the superposed electrical signal to recover an additional data stream, a respective copy of which is encoded in an enhancement layer of each of the plurality of the hierarchically modulated optical waves. 
         [0008]    According to another embodiment, provided is an apparatus comprising: a plurality of encoders, each configured to generate a respective electrical drive signal based on a respective one of a plurality of base data streams and a respective copy of an additional data stream; a plurality of optical modulators, each configured to be driven by the respective electrical drive signal in a manner that causes each of the optical modulators to generate a respective one of a plurality of hierarchically modulated optical waves, wherein a base layer of the respective one of the hierarchically modulated optical waves carries the respective one of the plurality of base data streams, and an enhancement layer of each of the hierarchically modulated optical waves carries the additional data stream; and an optical multiplexer configured to multiplex the plurality of the hierarchically modulated optical waves to generate an optical output signal. 
         [0009]    According to yet another embodiment, provided is a communication method comprising the steps of: (A) de-multiplexing an optical input signal into a plurality of hierarchically modulated optical waves; (B) recovering a respective base data stream encoded in a base layer of a respective one of the plurality of the hierarchically modulated optical waves; (C) generating a superposed electrical signal by superposing a plurality of electrical signals, each representing an enhancement layer of the respective one of the plurality of the hierarchically modulated optical waves; and (D) decoding the superposed electrical signal to recover an additional data stream, a respective copy of which is encoded in an enhancement layer of each of the plurality of the hierarchically modulated optical waves. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Other aspects, features, and benefits of various disclosed embodiments will become more fully apparent, by way of example, from the following detailed description and the accompanying drawings, in which: 
           [0011]      FIG. 1  shows a block diagram of an optical transmitter according to an embodiment of the disclosure; 
           [0012]      FIG. 2  shows a block diagram of an optical receiver according to an embodiment of the disclosure; 
           [0013]      FIGS. 3A-3E  graphically illustrate the signal processing in the optical receiver of  FIG. 2  according to an embodiment of the disclosure; 
           [0014]      FIG. 4  shows a block diagram of an optical transport system in which the optical transmitter of  FIG. 1  and/or the optical receiver of  FIG. 2  may be used according to an embodiment of the disclosure; 
           [0015]      FIG. 5  shows a block diagram of an optical transceiver that can be used in the optical transport system of  FIG. 4  according to an embodiment of the disclosure; and 
           [0016]      FIG. 6  shows a block diagram of an optical transceiver that can be used in the optical transport system of  FIG. 4  according to another embodiment of the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Various hierarchical-modulation schemes that may be used in some embodiments are disclosed, e.g., in U.S. Pat. Nos. 7,599,446, 8,149,752, and 8,243,601 and U.S. Patent Application Publication Nos. 2013/0272451 and 2010/0142644, all of which are incorporated herein by reference in their entirety. Some embodiments may benefit from the methods and apparatus for optical hierarchical modulation disclosed, e.g., in an article by Pan Cao, Xiaofeng Hu, Zhiming Zhuang, et al., entitled “Power Margin Improvement for OFDMA-PON Using Hierarchical Modulation,” published in OPTICS EXPRESS, 2013, Vol. 21, No. 7, pp. 8261-8268, which is incorporated herein by reference in its entirety. 
         [0018]      FIG. 1  shows a block diagram of an optical transmitter  100  according to an embodiment of the disclosure. Transmitter  100  is configured to generate a multiplexed hierarchically modulated optical signal  142  for transmission to a remote optical receiver over an optical transport link  150 . Optical signal  142  is generated to carry N+1 different (e.g., uncorrelated) data streams  110   1 - 110   N+1  using N different optical channels, where N is a positive integer greater than one. 
         [0019]    In an example embodiment, optical transport link  150  is configured to support multiple degrees of freedom, such as spatial localization, carrier frequency (wavelength), and polarization. Each of these degrees of freedom can be used for generating optical signal  142  through optical-signal multiplexing in an optical multiplexer  140 . Multiplexing techniques corresponding to these different individual degrees of freedom are referred to in the literature as space-division multiplexing, wavelength-division multiplexing (including orthogonal frequency-division multiplexing), and polarization-division multiplexing. As used herein, the term “optical channel” refers to a component of optical signal  142  configured to use a unique set of the degrees of freedom of link  150  for carrying encoded data. In a representative embodiment, two optical channels are two components of optical signal  142  that carry at least partially different sets of data and differ from one another in at least one of their degrees of freedom, e.g., in one or more of spatial localization, polarization of light, and carrier (or subcarrier) wavelength. 
         [0020]    For example, a first optical channel in link  150  may be configured to use carrier wavelength λ 1 , and a second optical channel may be configured to use carrier wavelength λ 2 . As another example, a first optical channel in link  150  may be configured to use a first propagation path of a multipath fiber or fiber-optic cable (e.g., via a first core of a multi-core fiber or a first guided mode of a multimode fiber), and a second optical channel may be configured to use a second propagation path of that multipath fiber or fiber-optic cable (e.g., via a second core of the multi-core fiber or a second guided mode of the multimode fiber). As yet another example, a first optical channel in link  150  may be configured to use a first (e.g., X) polarization, and a second optical channel may be configured to use a second (e.g., Y) polarization. Note that, in each of these examples, the first and second optical channels are described as differing from one another in the parameters of just one degree of freedom. However, different optical channels may differ from one another in the parameters of two or more degrees of freedom, such as: (i) spatial localization and wavelength; (ii) spatial localization and polarization; (iii) wavelength and polarization; or (iv) spatial localization, wavelength, and polarization. 
         [0021]    Each of the (populated) optical channels in optical link  150  is configured to carry the data received by transmitter  100  via a subset of data streams  110   1 - 110   N+1  using a respective hierarchically modulated optical wave. The hierarchical modulation is enabled in transmitter  100  by hierarchical encoders  120   1 - 120   N  and optical modulators  130   1 - 130   N . Each hierarchical encoder  120   i  (where i=1, 2, . . . , N) is configured to generate a drive signal  122 ; for optical modulator  130   i  based on data stream  110   i  and a respective copy of data stream  110   N+1  in a manner that causes the optical modulator to generate a hierarchically modulated optical wave  132   i , wherein data stream  110   i  is encoded in the base layer thereof, and data stream  110   N+1  is encoded in the enhancement layer thereof. As a result, the base layers of optical waves  132   1 - 132   N  carry different respective data streams, while the enhancement layers of optical waves  132   1 - 132   N  all carry the same data stream (i.e., data stream  110   N+1 ). Optical multiplexer  140  operates to appropriately multiplex optical waves  132   1 - 132   N , thereby generating optical signal  142  and causing it to populate a selected subset of the optical channels of link  150 . In an example embodiment, optical multiplexer  140  may operate to cause each of optical waves  132   1 - 132   N  to populate a respective one optical channel of link  150 . 
         [0022]      FIG. 2  shows a block diagram of an optical receiver  200  according to an embodiment of the disclosure. Receiver  200  is shown in  FIG. 2  as being coupled to optical transport link  150  (also see  FIG. 1 ). When transmitter  100  applies optical signal  142  to the remote end of link  150 , the optical link causes receiver  200  to receive an optical signal  142 ′ that differs somewhat from optical signal  142 , but otherwise carries the same data. The differences between optical signals  142  and  142 ′ may be, e.g., in the amount of noise, signal distortions, and signal impairments, some of which are imposed by link  150 . The train of signal processing implemented in receiver  200  is directed at recovering data streams  110   1 - 110   N+1  from optical signal  142 ′. 
         [0023]    Receiver  200  has an optical de-multiplexer  210  that operates to separate the different optical components of optical signal  142 ′ corresponding to different populated channels of link  150 , thereby generating hierarchically modulated optical waves  212   1 - 212   N . Coherent detectors  220   1 - 220   N  perform homodyne or intradyne detection of optical waves  212   1 - 212   N , as known in the art, thereby converting each of the optical waves into a respective one of electrical digital signals  222   1 - 222   N . Digital signals  222   1 - 222   N  are then processed in the digital-signal-processor (DSP) portion of receiver  200  to recover data streams  110   1 - 110   N+1 . 
         [0024]    The DSP portion of receiver  200  comprises signal-processing modules  230   1 - 230   N , base-layer decoders  240   1 - 240   N , enhancement-layer extractors  250   1 - 250   N , an enhancement-layer superposition module  260 , and an enhancement-layer decoder  270 . 
         [0025]    Signal-processing modules  230   1 - 230   N  are configured to appropriately condition digital signals  222   1 - 222   N  for decoding in base-layer decoders  240   1 - 240   N , respectively. For example, signal-processing modules  230   1 - 230   N  may perform, inter alia, signal equalization, clock recovery, carrier recovery, and phase-error correction, as known in the art. The resulting conditioned signals are digital signals  232   1 - 232   N . 
         [0026]    In an example embodiment, each of base-layer decoders  240   1 - 240   N  is configured to decode the respective one of digital signals  232   1 - 232   N  by (i) mapping it, in each symbol period, onto the operative base-layer constellation, while treating the enhancement-layer component of the signal as noise, and (ii) recovering the respective one of data streams  110   1 - 110   N  based on the mapping. 
         [0027]    Each of enhancement-layer extractors  250   1 - 250   N  operates to extract the enhancement-layer component of the respective one of digital signals  232   1 - 232   N  based on the decoding results of base-layer decoders  240   1 - 240   N . For example, in one embodiment, enhancement-layer extractor  250   i  (where i=1, 2, . . . , N) may be configured to generate a digital signal  252   i  that represents the enhancement-layer component of digital signal  232   i  by (i) estimating, in each symbol period, the base-layer component of digital signal  232   i , e.g., using the base-layer constellation point onto which digital signal  232   i  was mapped in base-layer decoder  240   i , and (ii) subtracting the estimated base-layer component from digital signal  232   i . Despite the fact that, in each symbol period, each of digital signals  252   1 - 252   N  encodes the same constellation point defined by the corresponding segment of data stream  110   N+1 , digital signals  252   1 - 252   N  typically differ from one another, e.g., due to the different respective amounts of noise imposed during transmission onto optical signal  142  in different optical channels of link  150 . 
         [0028]    Enhancement-layer superposition module  260  is configured to superpose digital signals  252   1 - 252   N , e.g., by coherently summing them, thereby generating a superposed digital signal  262 . The superposition serves to sum the enhancement-layer components received via different optical channels of link  150  in a manner that reduces the relative magnitude of noise/distortions in signal  262  compared to that in individual ones of signals  252   1 - 252   N . The reduction occurs because the noise/distortions in individual signals  252   1 - 252   N  are generally uncorrelated and, as such, tend to average out upon summation. 
         [0029]    Enhancement-layer decoder  270  is configured to decode superposed digital signal  262  by (i) mapping it, in each symbol period, onto the operative enhancement-layer constellation and (ii) recovering data stream  110   N+1  based on the mapping. 
         [0030]    In some embodiments, receiver  200  may be configured to implement additional signal processing on digital signals  252   1 - 252   N  prior to applying these signals to enhancement-layer superposition module  260 . Such additional signal processing may include but is not limited to phase and frequency offset compensation. 
         [0031]      FIGS. 3A-3E  graphically illustrate the signal processing in receiver  200  ( FIG. 2 ) according to an embodiment of the disclosure. More specifically, each of  FIGS. 3A-3E  shows a scatter plot on the I/Q plane representing a respective modulated signal. The hierarchical constellation used in this embodiment is a 4/16-QAM hierarchical constellation. One of ordinary skill in the art will understand that, in alternative embodiments, other suitable hierarchical constellations may similarly be used. 
         [0032]    In a 4/16-QAM hierarchical constellation, two 4-QAM constellations are superposed such that each constellation point of the first (base-layer) 4-QAM constellation is split into four constellation points corresponding to the second (enhancement-layer) 4-QAM constellation, for a total of sixteen constellation points. The bits encoded by the first 4-QAM constellation are referred to as the “base bits,” and they are common for all constellation points of the 4/16-QAM hierarchical constellation located in the same quadrant of the IQ plane. The bits corresponding to the second 4-QAM constellation are referred to as the “enhancement bits.” The enhancement bits may be more vulnerable to noise than the base bits because the separating distance between the constellation points of the second 4-QAM constellation is smaller than the separating distance between the (virtual unsplit) constellation points of the first 4-QAM constellation. 
         [0033]      FIG. 3A  shows an example scatter plot corresponding to one of optical waves  132   1 - 132   N  (see  FIG. 1 ) before it is coupled into link  150  for transmission to receiver  200  ( FIG. 2 ). For illustration purposes and without undue limitation, let us assume that this scatter plot represents optical wave  132   1 . One of ordinary skill in the art will appreciate that scatter plots representing the other ones of optical waves  132   1 - 132   N  may be similar to the scatter plot shown in  FIG. 3A . 
         [0034]      FIG. 3B  shows an example scatter plot representing electrical digital signal  222   1 , which corresponds to optical wave  132   1  at receiver  200  ( FIG. 2 ). Comparison of  FIGS. 3A and 3B  reveals the effects of noise and signal distortions imposed by link  150  and the front end of receiver  200 . One of ordinary skill in the art will appreciate that scatter plots representing the other ones of digital signals  222   1 - 222   N  may be similar to the scatter plot shown in  FIG. 3B . 
         [0035]      FIG. 3C  shows an example scatter plot representing electrical digital signal  232   1 , which is generated by processing electrical digital signal  222   1  in signal-processing module  230   1  ( FIG. 2 ). Comparison of  FIGS. 3B and 3C  reveals the benefits of, e.g., signal equalization and phase-error correction carried out in signal-processing module  230   1 . The four clusters of received symbols corresponding to the four constellation points of the first (base-layer) 4-QAM constellation of the 4/16-QAM hierarchical constellation are now well-separated from one another for reliable base-layer decoding in decoder  240   1 . The splitting of the base-layer constellation points caused by the presence of the enhancement layer is also evident in  FIG. 3C . 
         [0036]      FIG. 3D  shows an example scatter plot representing electrical digital signal  252   1 , which is generated by enhancement-layer extractor  250   1  based on electrical digital signal  232   1  and the estimated base-layer component thereof determined in decoder  240   1  (see  FIG. 2 ). If the signal illustrated in  FIG. 3D  were to be decoded directly by an enhancement-layer decoder similar to decoder  270  ( FIG. 2 ), then the resulting bit-error rate (BER) would have been about 2.1×10 −2 . One of ordinary skill in the art will appreciate that scatter plots representing the other ones of digital signals  252   1 - 252   N  may be similar to the scatter plot shown in  FIG. 3D . 
         [0037]      FIG. 3E  shows an example scatter plot representing superposed digital signal  262 , which is generated by enhancement-layer superposition module  260  by superposing digital signals  252   1 - 252   N . Visual comparison of  FIGS. 3D and 3E  reveals the improved separation between the four clusters of received symbols corresponding to the four constellation points of the second (enhancement-layer) 4-QAM constellation of the 4/16-QAM hierarchical constellation. When the signal illustrated in  FIG. 3E  is decoded by enhancement-layer decoder  270  ( FIG. 2 ), the resulting BER is about 1.4×10 −3 , which is a significant improvement compared to the BER corresponding to the signal shown in  FIG. 3D . 
         [0038]      FIG. 4  shows a block diagram of an optical transport system  400  in which optical transmitter  100  ( FIG. 1 ) and/or optical receiver  200  ( FIG. 2 ) may be used according to an embodiment of the disclosure. System  400  is illustratively shown as having two optical transceivers (TxRx), labeled  410   1  and  410   2 , respectively. One of ordinary skill in the art will understand that, in an alternative embodiment, system  400  may have a different number of optical transceivers  410 . 
         [0039]    In an example embodiment, each optical transceiver  410  includes a respective instance (physical copy) of optical transmitter  100  and a respective instance of optical receiver  200 , which enables bidirectional data transport in system  400 . For example, optical transceiver  410   1  may be configured to transmit data to and receive data from optical transceiver  410   2 . Similarly, optical transceiver  410   2  may be configured to transmit data to and receive data from optical transceiver  410   1 . Optical transceivers  410   1  and  410   2  may also be configured to transmit data to and receive data from other optical transceivers  410  (if any) in system  400 . Example embodiments of optical transceiver  410  are described in more detail below in reference to  FIGS. 5 and 6 . 
         [0040]    System  400  includes an optical path  420  that connects optical transceivers  410   1  and  410   2  as indicated in  FIG. 4 . Optical path  420  comprises reconfigurable optical add-drop multiplexers (ROADMs)  422   1  and  422   2  that may be optically connected to one another by one or more stretches of optical fiber or fiber-optic cable. 
         [0041]    The configurations of optical transceivers  410   1  and  410   2  and ROADMs  422   1  and  422   2  are controlled by a controller  430 , e.g., via control paths  408   1 ,  408   2 ,  418   1 , and  418   2 . For example, each of control paths  408   1  and  408   2  may be used to: (i) supply data streams, such as data streams  110   1 - 110   N+1  (see  FIG. 1 ), from the data plane of the system to the corresponding transceiver  410  for transmission over optical path  420  and (ii) retrieve the received data streams, such as data streams  110   1 - 110   N+1  (see  FIG. 2 ), from the corresponding transceiver  410  for distribution over the data plane to respective recipients. Each of control paths  418   1  and  418   2  may be used to appropriately configure the corresponding one of ROADMs  422   1  and  422   2  to add/drop optical signals to/from optical path  420 . In one embodiment, controller  430  can be a software-defined networking (SDN) controller. 
         [0042]      FIG. 5  shows a block diagram of an optical transceiver  500  that can be used as transceiver  410  ( FIG. 4 ) according to an embodiment of the disclosure. Transceiver  500  is configured to use wavelength-division multiplexing (WDM) to transmit and receive hierarchically modulated optical signals. For illustration purposes and without loss of generality, the operation of transceiver  500  is described below in reference to two carrier wavelengths, labeled λ 1  and λ 2 . From the provided description, one of ordinary skill in the art will understand how to modify transceiver  500  for three or more carrier wavelengths. 
         [0043]    The transmitter portion of transceiver  500  includes light sources  508   1  and  508   2  configured to generate carrier waves at wavelengths λ 1  and λ 2 , respectively. These carrier waves are modulated with data in optical modulators  530   1  and  530   2  to generate hierarchically modulated optical waves  532   1  and  532   2 . Drive signals for optical modulators  530   1  and  530   2  are generated by encoders  520   1  and  520   2 , respectively, based on data streams  510   1 - 510   3  received from a data plane  504 . More specifically, encoder  520   1  operates to cause hierarchically modulated optical wave  532   1  to carry data stream  510   1  in the base layer thereof, and data stream  510   3  in the enhancement layer thereof. Encoder  520   2  similarly operates to cause hierarchically modulated optical wave  532   2  to carry data stream  510   2  in the base layer thereof, and data stream  510   3  in the enhancement layer thereof. Hierarchically modulated optical waves  532   1  and  532   2  are multiplexed in an optical multiplexer (MUX)  540  to generate a hierarchically modulated WDM signal  542 , which is then directed by a circulator  546  to a corresponding ROADM, such as ROADM  422  ( FIG. 4 ). 
         [0044]    Encoders  520   1  and  520   2  are embodiments of encoders  120  ( FIG. 1 ). Optical modulators  530   1  and  530   2  are embodiments of optical modulators  130  ( FIG. 1 ). Optical multiplexer  540  is an embodiment of optical multiplexer  140  ( FIG. 1 ). 
         [0045]    The receiver portion of transceiver  500  includes optical receivers (Rx&#39;s)  556   1  and  556   2  configured to receive hierarchically modulated optical waves  552   1  and  552   2 , respectively. Optical waves  552   1  and  552   2  are generated by an optical de-multiplexer (DMUX)  550  by demultiplexing a hierarchically modulated WDM signal  548  received from a remote transmitter via the corresponding ROADM and circulator  546 . Hierarchically modulated WDM signal  548  may be generally analogous to hierarchically modulated WDM signal  542  and generated by the remote transmitter in a similar manner. In an example embodiment, optical waves  552   1  and  552   2  have carrier wavelengths λ 1  and λ 2 , respectively. 
         [0046]    Optical receivers  556   1  and  556   2  operate to recover the data streams encoded in the base layers of optical waves  552   1  and  552   2 , respectively. These data streams, labeled  510   4  and  510   5 , are applied directly to data plane  504 . Optical receivers  556   1  and  556   2  further operate to extract enhancement-layer components  558   1  and  558   2  of optical waves  552   1  and  552   2 , respectively, for coherent summation in a an enhancement-layer superposition module (SUP)  560 . A resulting superposed digital signal  562  is then decoded in an enhancement-layer decoder  570  to recover the data stream encoded in the enhancement layers of optical waves  552   1  and  552   2 . The recovered data stream, labeled  510   6 , is then applied by decoder  570  to data plane  504 . 
         [0047]    In an example embodiment, each of optical receivers  556   1  and  556   2  may comprise a respective copy of coherent detector  220  ( FIG. 2 ), a respective copy of signal-processing module  230  ( FIG. 2 ), a respective copy of base-layer decoder  240  ( FIG. 2 ), and a respective copy of enhancement-layer extractor  250  ( FIG. 2 ). Enhancement-layer superposition module  560  is an embodiment of enhancement-layer superposition module  260  ( FIG. 2 ). Enhancement-layer decoder  570  is an embodiment of enhancement-layer decoder  270  ( FIG. 2 ). 
         [0048]      FIG. 6  shows a block diagram of an optical transceiver  600  that can be used as transceiver  410  ( FIG. 4 ) according to an embodiment of the disclosure. Transceiver  600  is functionally analogous to transceiver  500  ( FIG. 5 ) and employs many of the same circuit elements. The description of these elements is not repeated here. Instead, the following description of transceiver  600  primarily focuses on the differences between the two transceivers. 
         [0049]    The transmitter portion of transceiver  600  differs from the transmitter portion of transceiver  500  in that hierarchically modulated WDM signal  542  is generated by combining three conventionally modulated (single-layer) optical signals  632   1 - 632   3  in an optical combiner  640 . More specifically, optical signal  632   1  is generated in a conventional manner by an optical modulator  630   1 , which receives carrier wavelength λ 1  from light source  508   1  and is driven by a drive signal generated by an encoder  620   1  based on data stream  510   1 . Optical signal  632   2  is similarly generated by an optical modulator  630   2 , which receives carrier wavelength λ 2  from light source  508   2  and is driven by a drive signal generated by an encoder  620   2  based on data stream  510   2 . Optical signal  632   3  has both carrier wavelengths λ 1  and λ 2  and is generated by an optical modulator  630   3 , which is driven by a drive signal generated by an encoder  620   3  based on data stream  510   3 . 
         [0050]    Carrier wavelengths λ 1  and λ 2  are applied to optical modulator  630   3  by an optical combiner  628  that is coupled to the outputs of light sources  508   1  and  508   2  via optical splitters  608   1  and  608   2  as indicated in  FIG. 6 . The splitting ratio of each of optical splitters  608   1  and  608   2  is selected such as to provide an appropriate power ratio between the base and enhancement layers in the corresponding WDM (λ 1  and λ 2 ) components of hierarchically modulated WDM signal  542 , wherein: (i) optical signal  632   1  provides the base layer for the λ 1  component of WDM signal  542 ; (ii) optical signal  632   2  provides the base layer for the λ 2  component of WDM signal  542 ; and (iii) optical signal  632   3  provides the enhancement layers for both of the λ 1  and λ 2  components of WDM signal  542 . 
         [0051]    The receiver portion of transceiver  600  is substantially the same as the receiver portion of transceiver  500  ( FIG. 5 ). 
         [0052]    According to an example embodiment disclosed above in reference to  FIGS. 1-6 , provided is an apparatus (e.g.,  200 ,  FIG. 2 ;  400 ,  FIG. 4 ;  500 ,  FIG. 5 ;  600 ,  FIG. 6 ) comprising: an optical de-multiplexer (e.g.,  210 ,  FIG. 2 ) configured to de-multiplex an optical input signal (e.g.,  142 ′,  FIG. 2 ) into a plurality of hierarchically modulated optical waves (e.g.,  212   1 - 212   N ,  FIG. 2 ); a plurality of base-layer decoders (e.g.,  240   1 - 240   N ,  FIG. 2 ), each configured to recover a respective base data stream (e.g.,  110   1 - 110   N ,  FIGS. 1-2 ) encoded in a base layer of a respective one of the plurality of the hierarchically modulated optical waves; a superposition module (e.g.,  260 ,  FIG. 2 ) configured to generate a superposed electrical signal (e.g.,  262 ,  FIG. 2 ) by superposing a plurality of electrical signals (e.g.,  252   1 - 252   N ,  FIG. 2 ), each representing an enhancement layer of the respective one of the plurality of the hierarchically modulated optical waves; and an enhancement-layer decoder (e.g.,  270 ,  FIG. 2 ) configured to decode the superposed electrical signal to recover an additional data stream (e.g.,  110   N+1 ,  FIGS. 1-2 ), a respective copy of which is encoded in an enhancement layer of each of the plurality of the hierarchically modulated optical waves. 
         [0053]    In some embodiments of the above apparatus, a base layer of a first hierarchically modulated optical wave (e.g.,  212   1 ,  FIG. 2 ) of the plurality of the hierarchically modulated optical waves is configured to carry a first data stream (e.g.,  110   1 ,  FIGS. 1-2 ); a base layer of a second hierarchically modulated optical wave (e.g.,  212   2 ,  FIG. 2 ) of the plurality of the hierarchically modulated optical waves is configured to carry a second data stream (e.g.,  110   2 ,  FIGS. 1-2 ) different from the first data stream; an enhancement layer of the first hierarchically modulated optical wave is configured to carry the additional data stream different from each of the first and second data streams; and an enhancement layer of the second hierarchically modulated optical wave is configured to carry the additional data stream. The plurality of base-layer decoders comprises: a first decoder (e.g.,  240   1 ,  FIG. 2 ) configured to recover the first data stream by decoding a first electrical signal (e.g.,  232   1 ,  FIG. 2 ) generated using the first hierarchically modulated optical wave; and a second decoder configured to recover the second data stream by decoding a second electrical signal (e.g.,  232   2 ,  FIG. 2 ) generated using the second hierarchically modulated optical wave. The plurality of electrical signals comprises (i) a first electrical signal (e.g.,  252   1 ,  FIG. 2 ) representing the enhancement layer of the first hierarchically modulated optical wave and (ii) a second electrical signal (e.g.,  252   2 ,  FIG. 2 ) representing the enhancement layer of the second hierarchically modulated optical wave. 
         [0054]    In some embodiments of any of the above apparatus, a base layer of a third hierarchically modulated optical wave (e.g.,  212   N ,  FIG. 2 ) of the plurality of the hierarchically modulated optical waves is configured to carry a third data stream (e.g.,  110   N ,  FIGS. 1-2 ) different from each of the first, second, and additional data streams; an enhancement layer of the third hierarchically modulated optical wave is configured to carry the additional data stream; the plurality of base-layer decoders comprises a third decoder (e.g.,  240   N ,  FIG. 2 ) configured to recover the third data stream by decoding a third electrical signal (e.g.,  232   N ,  FIG. 2 ) generated using the third hierarchically modulated optical wave; and the plurality of electrical signals comprises a third electrical signal (e.g.,  252   N ,  FIG. 2 ) representing the enhancement layer of the third hierarchically modulated optical wave. 
         [0055]    In some embodiments of any of the above apparatus, the superposition module is configured to generate the superposed electrical signal by summing the plurality of electrical signals in a manner that causes the superposed electrical signal to have a lower level of noise compared to a level of noise in any one of the plurality of the electrical signals taken individually (e.g., as indicated in  FIGS. 3D and 3E ). 
         [0056]    In some embodiments of any of the above apparatus, the optical de-multiplexer is configured to perform one or more of space-division demultiplexing, wavelength-division demultiplexing, and polarization-division demultiplexing to generate the plurality of the hierarchically modulated optical waves. 
         [0057]    In some embodiments of any of the above apparatus, the apparatus further comprises a plurality of enhancement-layer extractors (e.g.,  250   1 - 250   N ,  FIG. 2 ), each coupled between a respective one of the plurality of the base-layer decoders and the superposition module and configured to generate a respective one of the plurality of the electrical signals by (i) estimating, in each symbol period, a base-layer component of the respective one of the plurality of the hierarchically modulated optical waves and (ii) subtracting, in each symbol period, the estimated base-layer component from an electrical signal generated by optically detecting (e.g., using one of 220 1 - 220   N ,  FIG. 2 ) the respective one of the plurality of the hierarchically modulated optical waves. 
         [0058]    In some embodiments of any of the above apparatus, each of the plurality of the hierarchically modulated optical waves has a same (e.g., common, nominally identical) symbol rate. 
         [0059]    In some embodiments of any of the above apparatus, each of the base data streams has a first (e.g., common, nominally identical) bit rate. 
         [0060]    In some embodiments of any of the above apparatus, the additional data stream has a second bit rate different from the first bit rate. 
         [0061]    In some embodiments of any of the above apparatus, the additional data stream has the first bit rate. 
         [0062]    In some embodiments of any of the above apparatus, the apparatus further comprises an optical transmitter (e.g.,  100 ,  FIG. 1 ) configured to cause the optical de-multiplexer to receive the optical input signal. 
         [0063]    In some embodiments of any of the above apparatus, the optical transmitter comprises: a plurality of encoders (e.g.,  120   1 - 120   N ,  FIG. 1 ), each configured to generate a respective electrical drive signal (e.g.,  122   1 - 122   N ,  FIG. 1 ) based on a respective one of the plurality of the base data streams (e.g.,  110   1 - 110   N ,  FIG. 1 ) and a respective copy of the additional data stream; a plurality of optical modulators (e.g.,  130   1 - 130   N ,  FIG. 1 ), each configured to be driven by the respective electrical drive signal in a manner that causes each of the optical modulators to generate the respective one of the plurality of the hierarchically modulated optical waves, wherein the base layer of the respective one of the hierarchically modulated optical waves carries the respective one of the plurality of the base data streams, and the enhancement layer of each of the hierarchically modulated optical waves carries the additional data stream; and an optical multiplexer (e.g.,  140 ,  FIG. 1 ) configured to multiplex the plurality of the hierarchically modulated optical waves in a manner that causes the optical de-multiplexer to receive the optical input signal. 
         [0064]    In some embodiments of any of the above apparatus, the optical multiplexer is configured to perform one or more of space-division multiplexing, wavelength-division multiplexing, and polarization-division multiplexing to multiplex the plurality of the hierarchically modulated optical waves. 
         [0065]    In some embodiments of any of the above apparatus, the apparatus further comprises an optical link (e.g.,  150 ,  FIGS. 1-2 ) configured to optically couple the optical transmitter and the optical de-multiplexer. 
         [0066]    In some embodiments of any of the above apparatus, the optical link comprises at least one reconfigurable optical add-drop multiplexer (e.g.,  422 ,  FIG. 4 ) located between the optical transmitter and the optical de-multiplexer. 
         [0067]    According to another example embodiment disclosed above in reference to  FIGS. 1-6 , provided is an apparatus (e.g.,  100 ,  FIG. 1 ;  400 ,  FIG. 4 ;  500 ,  FIG. 5 ;  600 ,  FIG. 6 ) comprising: a plurality of encoders (e.g.,  120   1 - 120   N ,  FIG. 1 ), each configured to generate a respective electrical drive signal (e.g.,  122   1 - 122   N ,  FIG. 1 ) based on a respective one of a plurality of base data streams (e.g.,  110   1 - 110   N ,  FIG. 1 ) and a respective copy of an additional data stream (e.g.,  110   N+1 ,  FIG. 1 ); a plurality of optical modulators (e.g.,  130   1 - 130   N ,  FIG. 1 ), each configured to be driven by the respective electrical drive signal in a manner that causes each of the optical modulators to generate a respective one of a plurality of hierarchically modulated optical waves (e.g.,  132   1 - 132   N ,  FIG. 1 ), wherein a base layer of the respective one of the hierarchically modulated optical waves carries the respective one of the plurality of base data streams, and an enhancement layer of each of the hierarchically modulated optical waves carries the additional data stream; and an optical multiplexer (e.g.,  140 ,  FIG. 1 ) configured to multiplex the plurality of the hierarchically modulated optical waves to generate an optical output signal (e.g.,  142 ,  FIG. 1 ). 
         [0068]    In some embodiments of the above apparatus, the optical multiplexer is configured to perform one or more of space-division multiplexing, wavelength-division multiplexing, and polarization-division multiplexing to generate the optical output signal. 
         [0069]    In some embodiments of any of the above apparatus, a base layer of a first hierarchically modulated optical wave (e.g.,  132   1 ,  FIG. 1 ) of the plurality of the hierarchically modulated optical waves is configured to carry a first data stream (e.g.,  110   1 ,  FIG. 1 ); a base layer of a second hierarchically modulated optical wave (e.g.,  132   2 ,  FIG. 1 ) of the plurality of the hierarchically modulated optical waves is configured to carry a second data stream (e.g.,  110   2 ,  FIG. 1 ) different from the first data stream; an enhancement layer of the first hierarchically modulated optical wave is configured to carry the additional data stream different from each of the first and second data streams; and an enhancement layer of the second hierarchically modulated optical wave is configured to carry the additional data stream. 
         [0070]    In some embodiments of any of the above apparatus, a base layer of a third hierarchically modulated optical wave (e.g.,  132   N ,  FIG. 1 ) of the plurality of the hierarchically modulated optical waves is configured to carry a third data stream (e.g.,  110   N ,  FIG. 1 ) different from each of the first, second, and additional data streams; and an enhancement layer of the third hierarchically modulated optical wave is configured to carry the additional data stream. 
         [0071]    According to yet another example embodiment disclosed above in reference to  FIGS. 1-6 , provided is a communication method comprising the steps of: de-multiplexing an optical input signal (e.g.,  142 ′,  FIG. 2 ) into a plurality of hierarchically modulated optical waves (e.g.,  212   1 - 212   N ,  FIG. 2 ); recovering a respective base data stream (e.g.,  110   1 - 110   N ,  FIGS. 1-2 ) encoded in a base layer of a respective one of the plurality of the hierarchically modulated optical waves; generating a superposed electrical signal (e.g.,  262 ,  FIG. 2 ) by superposing a plurality of electrical signals (e.g.,  252   1 - 252   N ,  FIG. 2 ), each representing an enhancement layer of the respective one of the plurality of the hierarchically modulated optical waves; and decoding the superposed electrical signal to recover an additional data stream (e.g.,  110   N+1 ,  FIGS. 1-2 ), a respective copy of which is encoded in an enhancement layer of each of the plurality of the hierarchically modulated optical waves. 
         [0072]    While this disclosure includes references to illustrative embodiments, this specification is not intended to be construed in a limiting sense. Various modifications of the described embodiments, as well as other embodiments within the scope of the disclosure, which are apparent to persons skilled in the art to which the disclosure pertains are deemed to lie within the principle and scope of the disclosure, e.g., as expressed in the following claims. 
         [0073]    Unless explicitly stated otherwise, each numerical value and range should be interpreted as being approximate as if the word “about” or “approximately” preceded the value of the value or range. 
         [0074]    It will be further understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain the nature of this disclosure may be made by those skilled in the art without departing from the scope of the disclosure, e.g., as expressed in the following claims. 
         [0075]    Although the elements in the following method claims, if any, are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those elements, those elements are not necessarily intended to be limited to being implemented in that particular sequence. 
         [0076]    Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the disclosure. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments. The same applies to the term “implementation.” 
         [0077]    Also for purposes of this description, the terms “couple,” “coupling,” “coupled,” “connect,” “connecting,” or “connected” refer to any manner known in the art or later developed in which energy is allowed to be transferred between two or more elements, and the interposition of one or more additional elements is contemplated, although not required. Conversely, the terms “directly coupled,” “directly connected,” etc., imply the absence of such additional elements. 
         [0078]    The description and drawings merely illustrate the principles of the disclosure. It will thus be appreciated that those of ordinary skill in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the disclosure and are included within its spirit and scope. Furthermore, all examples recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the disclosure and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the disclosure, as well as specific examples thereof, are intended to encompass equivalents thereof. 
         [0079]    The functions of the various elements shown in the figures, including any functional blocks labeled as “processors,” may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software. When provided by a processor, the functions may be provided by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared. Moreover, explicit use of the term “processor” or “controller” should not be construed to refer exclusively to hardware capable of executing software, and may implicitly include, without limitation, digital signal processor (DSP) hardware, network processor, application specific integrated circuit (ASIC), field programmable gate array (FPGA), read only memory (ROM) for storing software, random access memory (RAM), and non volatile storage. Other hardware, conventional and/or custom, may also be included. Similarly, any switches shown in the figures are conceptual only. Their function may be carried out through the operation of program logic, through dedicated logic, through the interaction of program control and dedicated logic, or even manually, the particular technique being selectable by the implementer as more specifically understood from the context.