Abstract:
Optical code-division multiplexing and demultiplexing (CDM) using orthogonal codes with minimum shift keying (MSK) waveforms allows more efficient use of the spectrum and greatly reduces cross-channel interference. Receiving multiple baseband data channels, a derivation mechanism converts the data signals into series of impulses. These impulses may be split into odd and even channels, each at half the original data rate, and transmitted to a plurality of Walsh filters configured to have an impulse response corresponding to one of a plurality of orthogonal MSK waveforms. Odd and even channels, encoded with MSK waveforms, may combine into an in-phase and quadrature channel, 90° out of phase. A laser output may be divided, phase shifted, and modulated with the in-phase and quadrature channels, which divisions are then combined into a single multiplexed output. Walsh filters may decode the incoming signal in order to reproduce the original baseband channels.

Description:
BACKGROUND  
         [0001]    1. The Field of the Invention  
           [0002]    This invention relates to communications encoding, such as is used in computer and telecommunications networks and, more particularly, to novel systems and methods for optical code-division multiplexing  
           [0003]    2. Background  
           [0004]    Multiplexing is a method for transmitting multiple, distinct signals over a single physical carrier medium. Much of the protocol of computer hardware deals with the encoding and decoding of signals according to some time scheme for maintaining signal integrity and uniqueness from other signals. In conventional time-division types of multiplexing, signals are transmitted within specific time divisions or bit positions. In order to prevent individual bits from being transmitted at the same time, each is encoded into a signal and transmitted over the carrier medium at a specific time.  
           [0005]    As transmission rates increase, the individual time divisions available for each small quantity of information in a signal is reduced. However, with the advent of photonic processing, the transmission, encoding, and decoding of photonic signals taken from the electromagnetic spectrum, deserve further consideration. In conventional computer systems, as well as conventional telecommunications networks, the switching, routing, and transmission of signals throughout networks and between processors or processes are major limiting factors in performance. Typically, transmissions of a signal require encoding of the signal in a carrier medium, according to a protocol or format.  
           [0006]    Thereafter, transmission occurs as a physical phenomenon in which light, or other electromagnetic radiation, electrical signals, mechanical transmissions, or the like are transferred between a source and a destination. At the destination, a decoder must then manipulate the physical response to the incoming signals, thus reconstructing original data encoded by the sender. Communications in general may include communications between individual machines. Machines may be network-aware, hardware of any variety, individual computers, individual components within computers, and the like.  
           [0007]    Thus, the issue of sending and receiving information or message traffic is of major consequence in virtually all aspects of industrial and commercial equipment and devices in the information age. Whether communications involve sending and receiving information between machines, or telecommunications of data signals, audio signals, voice, or the like over conventional telecommunications networks, the sending and receiving requirements of rapidly encoding and decoding are present.  
           [0008]    With the advent of photonic signals and photonic signal processing, new speed limits are being approached by transmission media. Moreover, origination of signals can now be executed literally at light speeds. Accordingly, what is needed is a system for multiplexing photonic signals over photonic carrier media in such a way as to maximize throughput speed (bandwidth) and transmission speed, while maintaining the integrity of information.  
           [0009]    Traditionally, there are three ways for multiple data streams to share the same channel: time-division multiplexing (TDM), frequency division multiplexing (FDM), or code-division multiplexing (CDM).  
           [0010]    At the present time frequency (wavelength) division multiplexing and time-division multiplexing are the primary ways that signals are separated in the electromagnetic spectrum. Generally, it may be considered that both of these methods make use of very primitive codes. Both are easy to tap into or receive, but are also inherently inefficient.  
           [0011]    In optical code-division multiplexing, the multiplexing code is the temporally incoherent optical field itself. By using code-division multiplexing (CDM) the spectrum may be used more efficiently and message security is also greatly improved. In a broad bandwidth network CDM also greatly reduces the need for switching. All users transmit at the same time across the entire bandwidth and each uses a different code generator. Each user can receive any of the signals but only the signal “addressed” to the user can be transformed into usable information. Unfortunately, such statistical codes have stronger cross-correlations than one would desire, leading to significant interference between the data streams.  
           [0012]    What is needed is an optical code-division multiplexing technique wherein cross-channel interference is greatly reduced and the transmitted channels may be recovered at the receiver with an error rate due solely to laser power and detector sensitivity.  
           [0013]    It would also be an advance in the art to provide an efficient way of using optical CDM in order to allow multiple subchannels to be sent on a single carrier frequency.  
           [0014]    It would be a further advance in the art to provide a method wherein the baseband data may be encoded with orthogonal codes through a passive filter, thereby permitting use with higher speed photonic signals.  
           [0015]    It would be a further advance in the art to provide a modulation method used in conjunction with the orthogonal coding that would provide a constant envelope, thereby greatly reducing distortion and self-interference when passed through nonlinear elements.  
         BRIEF SUMMARY AND OBJECTS OF THE INVENTION  
         [0016]    In view of the foregoing, it is a primary object of the present invention to provide a method and apparatus for code-division multiplexing and demultiplexing that permits signals to be added together, transmitted, and recovered, while eliminating interference between the transmitted channels.  
           [0017]    It is another object to provide an apparatus and method for passively encoding the various channels input to a multiplexer functioning efficiently with higher speed photonic signals.  
           [0018]    It is another object to provide an apparatus and method for modulating the individual channels with orthogonal codes in a manner that provides a constant envelope to reduce distortion and self-interference when transmitted through nonlinear elements.  
           [0019]    Consistent with the foregoing objects, and in accordance with the invention as embodied and broadly described herein, an apparatus and method are disclosed, in suitable detail to enable one of ordinary skill in the art to make and use the invention. In certain embodiments an apparatus and method in accordance with the present invention may include a plurality of separate synchronous baseband channels, each having data bits that begin and end at the same time. The baseband data may be ones or zeros, ones and negative ones, or other signals originating from a signal generator of some kind, such as an OC3 or OC192 signal, for example.  
           [0020]    In the preferred embodiment of the present invention, the separate channels may be received by a plurality of derivation mechanisms configured to convert the baseband data signals into trains of impulses, corresponding to changes in the data from high to low. This arrangement may be used because the impulses may then be passed through a plurality of filters, each having an impulse response corresponding to one of a family of MSK orthogonal codes.  
           [0021]    A plurality of commutators may be operably connected to receive the series of impulses and divide them into two separate sub-channels comprising even and odd bits, respectively, each at half the original data rate. This method may be a way to reduce the data rate of the individual channels in the case that slower electronic components are used with the present invention.  
           [0022]    Subsequently, MSK Walsh filters having an impulse response comprising an MSK waveform, may be configured to receive the impulses of the even and odd sub-channels, therefore providing sub-channels encoded with the MSK waveforms. Likewise, the other channels may also be encoded with MSK waveforms, each being orthogonal to the others. In certain embodiments, the MSK Walsh filters may be waveguide structures. In the present invention, the MSK Walsh filters are used to encode the separate channels passively, therefore enabling the present invention to be used with higher speed photonic signals. Moreover, one further reason for making the MSK Walsh filters passive devices is that, because each data bit is encoded with an MSK Walsh code, the total bandwidth and the effective bit-rate are increased.  
           [0023]    Using Walsh codes modulated with MSK waveforms offers several advantages. The encoding of each channel with an orthogonal code, such as a Walsh code, through a filter allows for the data to be recovered through a corresponding filter in the receiver. Also, modulating the codes with MSK waveforms provides a signal with a constant envelope, thereby reducing distortion and interference caused by transmission through nonlinear elements.  
           [0024]    Eventually, all of the MSK-encoded even channels may be combined into a single consolidated output. All of the MSK-encoded odd-channels may be combined into another consolidated output.  
           [0025]    A coherent laser source may be provided and a pair of modulators may modulate the consolidated output comprising the even sub-channels into an in-phase signal, whereas the consolidated output comprising the odd sub-channels may be modulated into a quadrature signal, 90° out of phase from the in-phase signal. The in-phase and quadrature channels may subsequently be combined into a single multiplexed output for transmitting across a carrier medium. The use of quadrature amplitude modulation with the present invention provides an output with a constant envelope.  
           [0026]    On the receiving end, a splitter may receive the multiplexed output from the carrier medium and split the output into a number of daughter signals, corresponding to the number of channels multiplexed at the transmitter. Each of these daughter signals may be subsequently split into a pair of granddaughter signals, with a 90° phase shift imposed on one of them with respect to the other.  
           [0027]    The granddaughter signals may be received by MSK Walsh decoders identical to the filters of the transmitter, wherein a data value is only received for baseband signal encoded with the corresponding MSK waveform. Each decoder receives the waveforms from all of the channels, but will only have an output for the corresponding MSK waveform encoded in the transmitter. All other MSK waveforms will provide a zero, since they are orthogonal to the desired MSK waveform.  
           [0028]    The data values may be sent to a plurality of integrating mechanisms, which integrate over a time corresponding to the length of one MSK waveform, and output the corresponding even and odd baseband data signals for each channel of the transmitter. In certain embodiments, the odd and even subchannels may be left as two separate channels or may be combined to reproduce the original baseband data signals at the transmitter. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]    The foregoing and other objects and features of the present invention will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. Understanding that these drawings depict only typical embodiments of the invention and are, therefore, not to be considered limiting of its scope, the invention will be described with additional specificity and detail through use of the accompanying drawings in which:  
         [0030]    [0030]FIG. 1 is a schematic block diagram of an optical code-division multiplexer using minimum shift keying waveforms.  
         [0031]    [0031]FIG. 2 is a schematic block diagram of various waveforms and coding used in the multiplexer and demultiplexer of FIGS. 1 and 3.  
         [0032]    [0032]FIG. 3 is a schematic block diagram of a demultiplexer used in accordance with the invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]    It will be readily understood that the components of the present invention, as generally described and illustrated in the Figures herein, may be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the system and method of the present invention, as represented in FIGS. 1 through 3, is not intended to limit the scope of the invention. The scope of the invention is as broad as claimed herein. The illustrations are merely representative of certain, presently preferred embodiments of the invention. Those presently preferred embodiments of the invention will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout.  
         [0034]    Those of ordinary skill in the art will, of course, appreciate that various modifications to the details of the Figures may easily be made without departing from the essential characteristics of the invention. Thus, the following description of the Figures is intended only by way of example, and simply illustrates certain presently preferred embodiments consistent with the invention as claimed.  
         [0035]    Referring to FIG. 1, while referring generally to FIG. 2, an optical multiplexer  10  using code-division minimum shift keying may be configured to receive a plurality of baseband data signals  12   a - c . The baseband data signals  12   a - c  may represent digital data signals devoid of a carrier frequency, such as is represented by signals  84   a - d  or waveforms  84   a - d  of FIG. 2. The baseband data signals may be comprised of ones and zeros or ones and negative ones, for example, such as might be supplied by a signal generator outputting OC3 or OC192.  
         [0036]    A plurality of derivation devices  16   a - c  may be configured to receive the baseband data signals  12   a - c  from lines  14   a - c,  taking the derivative of transitions in the data from one digital signal to the next, producing a train of impulses, such as the signal  26 . Positive impulses would be representative of a transition from low to high of the signal  12 , whereas a negative impulse would be representative of a transition from high to low of the signal  12 . The advantage of converting the data pulses to impulses is that the resulting impulses may be passed through a device having an impulse response corresponding to a desired code, such as a Walsh code.  
         [0037]    In other embodiments, the present invention may not convert the baseband data  12  into impulses, but may actually convert the high and low data signals of signal  12  directly from baseband data  12  into a coded signal by way of a filter or other encoding device.  
         [0038]    A plurality of commutators may be operably connected to receive the impulses through lines  18   a - c , and divide the signal  18   a - c  into two signals  22   a - c ,  24   a - c , each at half the data rate. All even bits may go one direction  22   a - c , while all odd bits may go another direction  24   a - c . One advantage of this may be that the two bit streams, now at half the original data rate, may make use of slower electronic devices, if needed. Another advantage of this method may be that by dividing the signal  18   a - c  into a pair of signals  22   a - c ,  24   a - c , the signal  22   a - c  may be used as an in-phase component and the signal  24   a - c  may be used as a quadrature component, 90 degrees out of phase from the in-phase component. This method may allow for the use of certain types of modulation techniques, such as quadrature amplitude modulation, which may provide an output envelope that is less easily distorted.  
         [0039]    A filter  28   a,  having an impulse response such as waveform  32 , may be configured to receive the impulses from the line  22   a,  producing the waveform  32  for each positive impulse, or the negative of the waveform  32  for each negative impulse. The result may be a train of successive waveforms (either negative or positive).  
         [0040]    The filters  28   a,    30   a  may encode the signals  22   a,    22   b  with the same waveform  32 , while the filter pair  28   b,    30   b,  and the pair  28   c,    30   c  may encode the signals  22   b,    24   b  and the signals  22   c,    24   c,  respectively, with waveforms orthogonal to the waveform  32 . This topic will be discussed in more depth in the description for FIG. 2.  
         [0041]    The signals  22   a - c ,  24   a - c  may be encoded by the filters  28   a - c ,  30   a - c , in part, because the filters  28   a - c ,  30   a - c  may be implemented to function passively, which may allow the filters  28 ,  30  to keep pace with higher speed photonic. In reality, the signals  22   a - c ,  24   a - c  may be modulated electronically with orthogonal codes at the inputs  14   a - c . However, modulating the signals electronically may be too slow to work in higher speed photonic systems. Moreover, one further reason for making the filters passive devices may be that, because each data bit is encoded with an MSK Walsh code, the bit-rate may be effectively increased and thus the total effective informational bandwidth may also increase. Thus, it may be difficult to provide non-passive electronic components that are able to keep pace with this higher effective bit-rate.  
         [0042]    The even signals  34   a - c , each encoded with a different orthogonal code, may then be combined in a combiner  40  to form a consolidated signal  44 . Likewise, the odd signals  36   a - c , each encoded with a different orthogonal code (equivalent to their even counterpart) may then be combined in another combiner  42  to form a second consolidated signal  46 . The consolidated signals  44 ,  46  may be thought of as an in-phase  44  and a quadrature signal  46 .  
         [0043]    A coherent laser source  50  may be provided, in accordance with the invention, to produce a coherent laser  58  on line  58 , which may be split into a pair of daughter signals  60   a,    60   b.  A phase shifter  62  may also be provided to shift the daughter signal  60   b  by 90° to produce a phase-shifted signal  64 . Subsequently, a pair of amplitude modulators  56   a,    56   b  may be configured to modulate the laser signals  60   a,    64  with the consolidated signals  44 ,  46 , respectively, producing an in-phase signal  66  and a quadrature signal  67 , 90° out of phase with the in-phase signal  66 .  
         [0044]    In other embodiments, the modulators  56   a,    56   b  may use other modulation techniques, such as amplitude, phase, frequency, and spatial modulation. Generally, the object of the modulators  56   a ,  56   b  is to encode the laser signals  60   a,    64  in some way with the information contained in the signals  44 ,  46 , thereby using the laser signals  60   a,    64  as carriers of the information.  
         [0045]    The signals  66 ,  67  may then be added together in a combiner  69  to form a multiplexed output  72  for transmitting across a carrier medium, such as an optical fiber  72 . The use of quadrature amplitude modulation, using an in-phase and quadrature component as described above, may have the advantage of providing an output  72  with a constant envelope. As a result, a constant envelope may reduce distortion in the output  72  if the output  72  is amplified by a nonlinear amplifier, for example.  
         [0046]    Referring to FIG. 2, while referring generally to FIG. 1, the orthogonal codes provided by the filters  28   a - c ,  30   a - c  (referring back to FIG. 1) may be illustrated by a matrix  88 , such as the Walsh-code matrix  88 . Each Walsh code may be represented by a row  90  of ones or negative ones, each orthogonal from the other. This means that when the elements of each individual code  90  are squared and added together the result is nonzero, but when the individual elements of each row  90  are multiplied with the corresponding elements of another row  90  (either above or below), the sum of the products is equal to zero.  
         [0047]    For example, when the individual elements of a row  90   a  are multiplied with the individual elements of another row  90   b  and added together, the result is zero. The same rule holds true for any pair of rows  90  selected from the matrix  88 . Additionally, the Walsh-code matrix  88  need not be limited to the rows  90  comprising four elements as illustrated, but each row may comprise 2 n  elements for any whole number n. The number n may be determined by engineering according to the number of codes needed to encode the data signals  12  (Referring back to FIG. 1) input to the multiplexer  10 .  
         [0048]    To represent the Walsh codes, square pulse Walsh functions may be replaced with windowed pulses such as half-sine, Gaussian, or some other smoothly shaped function such as Blackman, Hanning, or Hamming windows, as illustrated by the waveforms  80   a - d.  Walsh functions of selectable length (L 2 n , where n=2, 3, 4 . . . ) may be used to produce a new higher rate bit stream (because each data bit may now be represented by a code having multiples “bits”) which is transmitted using MSK modulation. Walsh codes, which may be square waves, are represented using MSK modulation because of some advantageous properties of MSK modulation. MSK modulation may provide a constant envelope, which decreases distortion and self-interference when passed through nonlinear devices, such as amplifiers. Because of the properties of the coding, multiple MSK modulated signals may be added together and transmitted. These signals may be subsequently separated and recovered at the receiver by passing them through filters like those in the transmitter.  
         [0049]    For example, referring back to FIG. 1, each pair of MSK Walsh filters, such as a pair  28   a ,  30   a  may encode the odd and even signals  22   a,    24   a  with an MSK modulated Walsh code, such as is illustrated by waveform  80   a.  Likewise, other pairs  28   b - d,    30   b - c  of filters may be configured to encode the signals  22   b - c,    24   b - c  with other waveforms  80   b - d,  orthogonal to the first waveform  80   a.  As previously stated, the waveform  80   a - d  may represent a positive data bit, whereas a negative data bit may be represented by the waveform  80   a - d  inverted across the horizontal axis, as illustrated by the waveforms  82   a - d.  Finally, in the demultiplexer  100  or receiver  100 , the original baseband data signals  12   a - c  may be extracted, as illustrated by the waveforms  84   a - d.    
         [0050]    Referring FIG. 3, the demultiplexer  102  in accordance with the present invention may include a splitter configured to receive the multiplexed signal  101  from a carrier medium, such as an optical fiber  72 . The splitter  102  may subsequently split the multiplexed signal  101  into identical daughter signals  104   a - c . A second set of splitters  106   a - c  may be configured to split the signals  104   a - c  into granddaughter signals  108   a - c ,  110   a - c,  the signals  110   a - c  being phase shifted by 90°. MSK Walsh decoders  112   a - c  may be configured to extract the even signals from the signals  108   a - c , while decoders  114   a - c  may be configured to extract the odd signals from the signals  110   a - c.  The word “decoder” is used herein to describe a device or method that provides an output for a selected waveform, such as is produced by the filters  28   a,    30   a  in the multiplexer  10  in accordance with the present invention, but does not provide an output for other orthogonal waveforms.  
         [0051]    Each decoder  112 ,  114  may actually receive all of the MSK encoded signals from all the channels, but will only have an output for the corresponding MSK waveform encoded in the transmitter. All other MSK waveforms may each provide a zero to the decoder  112 ,  114 , since they are orthogonal to the desired MSK waveform. One embodiment in accordance with the present invention may also comprise integrating mechanisms  120   a - c,    122   a - c  which may be configured to integrate over a time corresponding to the length of one MSK waveform, and output the corresponding even and odd baseband data signals  124   a - c ,  126   a - c  for each channel of the transmitter. In certain embodiments, the signals  124 ,  126  may be combined to reproduce the original baseband signals  12   a - c  of FIG. 1, or they may be kept separate to provide two distinct signals at half the data rate.  
         [0052]    The present invention may be embodied in other specific forms without departing from its structures, methods, or other essential characteristics as broadly described herein and claimed hereinafter. The described embodiments are to be considered in all respects only as illustrative, and not restrictive. The scope of the invention is, therefore, indicated by the appended claims, rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.