Duobinary Modulation Format for Optical System- A Review

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Year of fee payment: An apparatus for modulating information bearing symbols onto an optical carrier includes a polybinary coder that operates on a binary information signal to produce an envelope compatible polybinary signal; and an optical single sideband modulator that modulates an optical carrier with the envelope compatible polybinary signal to produce a single sideband optical signal.

This provides an improvement over both polybinary signaling and optical single sideband. This action combines the chromatic dispersion advantages of SSB modulation with the DC level and bandwidth reduction of duo-binary coding. Optical fiber duobinary signal means systems are subject to distortion related to loss, noise, and nonlinearities duobinary signal means both the fiber and the modulation and amplification devices.

One of the more deleterious forms of duobinary signal means distortion is that due to chromatic dispersion. Chromatic dispersion in optical fiber is typically characterized by a linear non-flat group delay parameter.

The group refractive index of the fiber at optical frequencies near a given optical carrier frequency varies approximately linearly with wavelength or optical frequency about the carrier. This finite linear group delay imposes a quadratic phase rotation across the signal frequency band which translates to intersymbol interference in the time domain signal.

The fact that a large portion duobinary signal means the fiber in use today is dispersive duobinary signal means the desired operating wavelengths dictates that economic solutions are required. Approaches currently used to reduce the effects of chromatic dispersion include: The first is based on purely optical methods where the effects of duobinary signal means velocity dispersion are reversed while the signal duobinary signal means still in the optical domain.

Adding dispersion compensating fiber in the transmission path is one common approach. Other optical methods include compensation by differential time delay of the upper and lower sidebands of the modulated signal, see A.

Duobinary signal means second approach, in which dispersion effects are reversed in the electrical domain, is based on coherent transmission and homodyne detection followed by duobinary signal means in the electrical domain. Homodyne detection is only effective on single sideband signals. If homodyne detection were attempted with a DSB duobinary signal means, the upper and lower sidebands would overlap upon detection and the phase information duobinary signal means be lost and the higher modulation frequencies severely attenuated or distorted through frequency selective fading.

Some techniques used or proposed for post-detection equalization include the use of microstrip lines, see K. The third approach is to modify the transmission format so that duobinary signal means baseband signal spectrum is compressed. These types of approaches, which reduce the transmission bandwidth required on the fiber to transmit a given bit rate, are generally implemented by modifying the line code format in order to reduce the effective bandwidth required to transmit or receive the data, see K.

More recently it has been shown that optical single sideband transmission OSSB can combat some of the deleterious effects of chromatic dispersion.

OSSB provides a dispersion benefit directly by reducing the signal bandwidth and also by aiding in the signal restoration through post-detection dispersion compensation. The generation, transmission and detection of single sideband SSB signals has been used for both baseband and the RF and microwave regions of the electromagnetic spectrum to reduce the bandwidth of the signal by a factor of two, by sending either the upper or the lower sideband.

The structures outlined in these documents addressed the need for large added carrier in the transmitted optical signal by duobinary signal means approximations duobinary signal means time domain minimum phase signals with single sideband properties. This allowed the transmitted information to be directly modulated onto the optical electric field envelope while a special phase function was incorporated into the AM signal to cancel all or part of an information sideband.

While in digital optical SSB modulation a dispersion benefit accrues directly due to the fact that duobinary signal means transmitted signal duobinary signal means has been reduced, the more significant advantage of optical SSB transmission is that the fiber dispersion can be compensated in the electrical domain after detection. This advantage duobinary signal means similar to that for heterodyne detection of DSB signals, but with SSB transmission and detection, the signal can be homodyned directly to baseband using carrier signal added either at the source or at the receiver and thus it can be directly detected with the phase or delay information of the transmitted signal duobinary signal means.

This was shown in K. The purpose of transmitting the signals in a single sideband format is to permit these optical carrier frequencies to be spaced as closely as the maximum modulation frequency. A fundamental disadvantage of this type of dispersion compensation is found in the fact that the carrier power added to the transmitted signal must be significant thus reducing the potential signal to noise ratio at the transmitter.

To improve this situation the virtues of polybinary or duo-binary modulation may be applied. The term polybinary refers to a modification of a binary signal in which the levels have been altered so as to maintain the information while removing some of the DC content of the signal and reducing the bandwidth.

The most common implementation of this type of signal is the duobinary duobinary signal means where a two level binary signal is converted to a three level signal represented by a zero voltage level and two levels symetrically located at positive and negative voltage levels with respect to the zero voltage level.

The non-zero voltage level represent logical zeros and the zero level represents logical ones. These signals are part of a class duobinary signal means signals duobinary signal means to as correlative coding or partial response signals, so named because they allow a controlled amount of inter-symbol interference to achieve another advantage such as reduced bandwidth and reduced dc content.

The resulting duobinary signal means level sequence is called a duo-binary sequence and the data sequence b k may be recovered from c k by the following rule. The immediate advantage of this type of signal is a reduced bandwidth requirement for a duobinary signal means information rate.

The disadvantage is related to the fact that a single symbol error will propagate through the data sequence due to correlation between the symbols. To overcome this a precoding scheme may be used. In this case the binary information is unipolar in that a mark is represented as a 1 and a space is represented as a duobinary signal means. An intermediate pre-coded sequence is generated duobinary signal means in. A sample data sequence with precoding and duobinary encoding is shown in Table 1.

The information sequence b k is mapped into c k such that the individual symbols duobinary signal means independent and the bandwidth requirement for duobinary signal means given information rate is halved. Other types of correlative coding are possible if duobinary signal means coding rule is allowed to be more complicated. Some of these more complicated schemes are known as modified duobinary and alternate mark inversion. Additionally a binary signal may be mapped into a multilevel or polybinary signal where there are 5, 7 or more amplitude levels.

From the optical direct detection perspective, the reduction of the dc component compared to the binary case and the bandwidth reduction inherent in duobinary coding are the most favourable benefits of duobinary signaling along with the fact that the signal is recoverable by envelope detection.

Complete removal of the dc component may be achieved for a different coding rule known as modified duo-binary. In this case the precoder rule is given by. Table 2 shows the modified duobinary coding method. Thus for a trade-off in coder length, an SNR advantage is gained in the sense that none of the finite power of the optical signal is wasted on the non-information carrying Duobinary signal means or carrier component of the signal.

The coding rule in this case is given by. The precoded sequence is then converted to the j level signal duobinary signal means. The demodulation rule is slightly more difficult than the duo-binary case in that marks are taken from the odd levels duobinary signal means the polybinary sequence and the spaces are taken from the even levels.

Nonetheless the information is still recoverable using envelope detection techniques. This invention provides an improvement over both polybinary signaling and optical single sideband by implementing polybinary modulation on a single sideband modulator.

Therefore there is provided an apparatus for modulating information bearing symbols onto an optical carrier, the apparatus comprising a polybinary coder that operates on a binary information signal to produce an envelope compatible polybinary signal; and an optical single sideband modulator that modulates an optical carrier with the envelope compatible polybinary signal to produce a single sideband optical signal.

In a further aspect of the invention, the polybinary coder produces an envelope compatible polybinary signal by dividing a polybinary signal into at least a pair of unipolar signals. In a further aspect of the invention, the polybinary coder produces an envelope compatible polybinary signal by separately operating on a polybinary signal in a first stream to convert all negative symbols in the polybinary stream to zero symbols and in a second stream to convert duobinary signal means positive symbols in the polybinary signal to duobinary signal means symbols.

In a further aspect of the invention, the optical single sideband modulator operates on the optical carrier using the duobinary signal means compatible polybinary signal to produce a single sideband optical signal whose phase is determined by operating on the output of the polybinary coder with a Hilbert transform and whose magnitude is determined by the magnitude of the output of the polybinary coder.

The output of the polybinary coder may be converted by a logarithmic operator prior to input to the Hilbert transform. The output of the polybinary coder may be adjusted to duobinary signal means singularities in the operation of the logarithmic operator.

There will now be described a preferred embodiment of the invention with reference to the drawings, in which like reference characters denote like elements, by way of illustration only, and in which:. An improvement over both polybinary signaling duobinary signal means optical single sideband may be obtained when polybinary modulation is implemented on a single sideband modulator.

To duobinary signal means this, we consider the basic theory of envelope compatible signal sideband modulation. Generation of single sideband is based on the analytic signal. Let A t be a bandlimited analytic signal defined by. The signal duobinary signal means in 11 will have no negative frequency content in its Fourier transform.

Let q t be a bandpass signal defined by. Clearly, if A t is analytic with no negative frequencies then q t is a single sideband signal. For direct detection optical systems the modulating signal in 15 using the information signal defined as in 11 is unsuitable on direct detection optical systems since coherent detection is required to extract the information signal.

This situation is partially rectified by adding duobinary signal means to the modulated signal which, through square duobinary signal means detection, will allow recovery of a distorted information signal s t. The deficiency may be eliminated however by ensuring that the duobinary signal means signal is single sideband but also minimum phase in the time domain. The minimum phase modulator is shown in FIG. The term minimum phase is used due to the fact that the modulated signal has the characteristic relationship between the magnitude and phase that is observed in minimum phase signals.

As shown in Duobinary signal means. This yields the signal. Next, the Hilbert transform of the natural log ln of p t is duobinary signal means as in. Clearly there is a fixed relationship between the amplitude and phase of Additionally A mp t is still analytic and as such has no negative frequencies.

A complex signal composed in the above manner may be modulated onto the optical carrier to create a single sideband signal in a variety of optical modulator structures such as: The critical characteristic from the direct detection optical aspect is that the information is contained in the envelope of the signal while the phase which is lost in the square-law detection process is present only to cancel a sideband of the optical signal.

Duobinary signal means it has been shown that approximations may be made in the signal synthesis that maintain significant spectral sideband cancellation while reducing the complexity of the signal conditioner.

One of these approximations is to combine the signal as in the development above but to eliminate the step where the logarithm is taken ie. The modulator structure is reduced in complexity with the trade-off that the SSB signal emitted therefrom is duobinary signal means. The above modulators provide a number of approaches to envelope compatible SSB modulation however they all require a unipolar signal to synthesize the required phase function.

Considering the combination of duo-binary and envelope compatible OSSB signaling, the raw duobinary signal duobinary signal means not unipolar and will not directly yield the correct phase function envelope compatible SSB. However modifications may be applied that allow duobinary envelope compatible single sideband.

A duobinary signal may be represented as the superposition of two unipolar signals: As an example consider the duobinary signal:. The above embodiment apples to specifically to duobinary signals where the original binary stream is converted to a three level signal where the levels consist of positive, negative and zero symbols.

Other types of coding may be applied where there are more than three levels in the coded data stream. This is referred to as polybinary and may be implemented in a similar manner as duobinary. At 1 the binary information is applied to the duobinary coder and converted to the required precoded doubinary sequence. The information symbols may also be pulse shaped in some manner to bandlimit the signals as required by the modulating equipment.

A small DC offset is added at 3 to prevent the formation of a singularity in the following log operation. This is only required where a log operation is implemented. If the approximate phase operation, where only the Hilbert transform of the binary data is duobinary signal means or an approximate log operation is used the input signal is immune to the formation of a singularity and the offset is not required. At 5 the nonlinear operation required to produce phase signals to convert the double sideband envelope signals to single sideband is implemented.

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This invention is directed to an encoding and modulation technique for communication systems, and more particularly to a duobinary coding and modulation technique for optical transmission systems. In the long haul, high bit rate optical fiber telecommunications, appropriate coding and modulation of the signal for transmission are essential. One of these limitations is the chromatic dispersion, which can be relaxed if modulation bandwidth of the optical signal is reduced.

Duobinary signaling was introduced a few decades ago and its details can be found in, for example, "Introduction To Telecommunication Systems", F. Stremler, Addison-Wesley Publishing Company, 2nd edition, In binary transmission systems, only two symbols "d" and "n" are used, and more particularly, the bits of information take on two values, logic "1" and logic "0".

One of these two possible signals is transmitted during each T-second signaling interval. Duobinary signaling uses two levels for the non-zero signals, for example, "-1" and "1", resulting in three symbols: Thus, the duobinary signal has one half the transmission bandwidth of the binary signal for encoding the same information. Therefore, this type of signaling can be used to reduce the effect of dispersion, which in turn reduces the high inter-symbol interference at long transmission distances.

There are a number of solutions for constructing a duobinary sequence from a binary one. In general, any duobinary encoding scheme is based on introducing inter-symbol interference ISI , controlled in such a way that it comes only from the immediately preceding symbol. Solving this equation implies providing additional circuitry at the receiver. In addition, decoding errors tend to propagate in the system according to this solution. The XOR-coded binary sequence p k is then used to form a time varying binary signal.

This simplifies the decoding rule, in that the receiver makes each binary decision based only on the current received sample, the ISI still being controlled. The duobinary encoding is followed by an appropriate filtering of the encoded signal. When the input binary "1"s are separated by an odd number of "0"s, the "1"s are encoded as pulses of opposite polarity in the duobinary sequence.

When the input binary "1"s are separated by an even number of "0"s, the "1"s are encoded as pulses of the same polarity in the duobinary sequence.

However, the pre-coding operation necessary according to the above technique results in a somewhat complex structure of the transmitter. Most optical fiber transmitters use an external modulator. In many cases, the transmitter's light source is a semiconductor laser operating in continuous wave CW mode and the external modulator changes the phase of the CW signal at the appropriate bit rate. One such modulator is a Mach-Zehnder M-Z interferometer. A M-Z interferometer comprises a pair of wave-guide channels, or arms, connected between an optical wave-guide splitter and a wave-guide combiner.

The light source is optically coupled to the wave-guide splitter, which serves as a Y-branch splitter or directional coupler. The two light beams from the splitter travel through the wave-guide arms and are reunited by the wave-guide combiner. The recombined light exits the output port of the wave-guide combiner and is then optically coupled to an optical fiber for transmission.

The optical M-Z interferometer operates on the principle of interference between the two optical waves, which have been separated from a common wave at the modulator's input port, at the point of their recombination near the modulator's output port. The interference condition is controlled by the difference between distances travelled by these two waves between the point of separation and the point of recombination.

These distances are controlled by varying the optical indices of the two wave-guides which define the optical paths between separation and recombination.

It is common practice to ensure a particular condition of interference, despite wave-guide variations in modulator manufacture, by combining the varying drive voltages which are used to modulate the condition of interference with a substantially constant bias voltage.

This is often done using a bias tee. In the following is assumed that the modulator has been adequately biased. In M-Z interferometric modulators with a three-electrode configuration, a first and second electrode is each associated with an optical wave-guide arm. These are also called travelling wave electrodes. A third electrode is generally disposed between the arms.

Disadvantageously, the available drive voltage according to this method of modulation effects a phase shift in only the arm associated with the first electrode, thereby limiting the achievable modulation depth for a given voltage in comparison with other methods described herein.

This phase shifting modulation method is known as push-pull. In both above modulation techniques, the two beams arrive at the wave-guide combiner in phase in the absence of a modulating voltage, giving an intensity maximum or an "on" condition.

Conversely, a modulating voltage supplied to one or both arms results in a differential phase change, giving rise to an intensity minimum or "off" condition. As such, the push-pull configuration utilizes the drive voltage more efficiently than the one arm modulation in that, for a given voltage, twice the net phase shift is effected. The reported literature on experiments using the above-identified duobinary pre-coding and modulation techniques can be classified according to the choice of the bias voltage to the external modulator and the intensity levels in the drive signal.

This latter approach simplifies the detection scheme, however, neither technique can always reduce the signal bandwidth by a factor of two. The transmitter includes an encoder comprising an XOR gate with a delayed feedback path for determining each symbol of the duobinary sequence from the current and the previous symbol. The receiver comprises two decision circuits, one having a low threshold to distinguish a "0" level from a "1" level and the other having a high threshold to distinguish a "1" level from a "2" level.

The tests concluded that the dispersion has less effect on the duobinary receiver than on the binary receiver over the distance range tested. However, additional hardware has to be installed at the receiver for decoding the incoming signal, with the resulting penalty in receiver sensitivity.

The optical signal has a central level resulting in maximum extinction, the "0" optical level, and two outer levels resulting in equal intensities, the "1" optical level. The optical signal exhibits characteristics that meet the requirements of existing SDH and SONET interface standards, and therefore a conventional receiver is required for reception.

However, the examples discussed in these publications use a differentially encoded data stream with a bias voltage for the Mach-Zehnder modulator about the point of maximum extinction for nullifying the optical carrier, with the inherent control circuitry. As these references disclose standard duobinary coding techniques which involve pre-coding of the signal and push-pull or single arm modulation methods, they all suffer from the drawbacks described above.

There is a need for a duobinary encoding technique that is simple, does not require additional circuitry at the receiver, uses an easy-to-implement encoding circuit, and provides an encoded signal with a low dc component and reduced transmission bandwidth. There is also a need for a modulation method which may be used efficiently in high speed operation, is voltage efficient and suitable for use with a duobinary coding technique to obtain a reduced bandwidth of the transmitted signal and prolongs the lifespan of the external modulator.

It is an object of this invention to provide a duobinary coding and modulation technique for optical communication systems which reduces the drawbacks inherent with the prior art techniques.

It is another object of the invention to provide a duobinary coding circuit that is used to drive an external modulator, no pre-coding circuit being necessary at the transmitter site, and no additional decoding circuitry being necessary at the receiver site. The coding circuit according to this invention also behaves as a band limiting element. It is another object of the invention to provide a duobinary coding technique that is simple, provides a modulation signal with substantially no dc component and provides a bandwidth reduction factor of substantially two for a given symbol rate.

Still another object of this invention is to provide an optical modulation technique based on a single ended push-pull driven modulator or a differentially driven modulator with virtual ground. The invention also provides a modulation technique using a Mach-Zehnder M-Z interferometer for modulating a continuous wave CW optical carrier with a duobinary encoded differential driving signal, the M-Z interferometer having a first and a second travelling wave-guide, a splitter between an input port and the first and second travelling wave-guides, a combiner between the first and second travelling wave-guides and an output port, a first and a second travelling wave electrode, each associated with the respective first and second travelling wave-guide, the M-Z interferometer further comprises: Advantageously, the technique according to this invention converts a binary input to a duobinary output regardless of the incoming data rate.

Hence, a factor of two bandwidth reduction is obtained, which is not the case with the standard duobinary scheme. The coded signal generated according to the invention also has a smaller dc component.

The smaller dc component results in better suppression of the carrier frequency. This, in turn, shifts the onset of stimulated Brillouin scattering threshold to higher launch powers. Hence, a higher optical power may be launched onto the fibers. In addition, the modulation technique using a duobinary encoded differential driving signal according to the invention allows for a reduced amplitude required of individual drivers to approximately half that required for the conventional push-pull drive configurations.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of the preferred embodiments, as illustrated in the appended drawings, where:. As a result, for any odd number of "1"s in a row, modulation bandwidth of the drive signal is reduced by a factor of two. As well, the E-field has no component at the carrier frequency. However, this is not the case with an input sequence having an even number of successive "1"s, as shown in the examples of Table 1.

In the above examples, the bandwidth of the driving signal is identical to the bandwidth of the binary input signal, and the dc component is the same as that of the binary input signal.

Therefore, this scheme does not eliminate the dc component in general, rather, it reduces the dc for certain patterns. The duobinary encoding according to the present invention affects only non-zero input bits.

An input bit "0" results in a "0" output bit. The input bits "1" are replaced by output bits "1"s or "-1"s. This is done in such a way that the only allowed transitions at the output are from "1", to any number of "0"s and to "-1", or vice versa. In this way, the number of "1"s in the driving signal is substantially equal to the number of "-1"s.

Hence, a maximum reduction of the dc component is obtained. A coding circuit 1 receives the binary input sequence x 0,1 at the input 3 and provides the duobinary output sequence y 0,1,-1 at output 5. The output sequence y 0,1,-1 is input to driver 7 which provides the driving signal 12 on electrode 19 of modulator 9. Modulator 9 illustrated in the embodiment of FIG. A bias voltage V Bias is applied between travelling wave electrodes 15 and A laser 11 provides a CW optical carrier signal 14 to the input port 8 of the optical wave-guide splitter of modulator 9 in the known manner.

A modulated optical signal 16 is obtained at the output port 10 of the wave-guide combiner of the modulator and coupled into optical fiber 13 for transmission.

Coding circuit 1 comprises a D-type flip-flop 21 connected with the inverting output Q to the D input for obtaining a delay with a period T, which is needed for simultaneously obtaining the bits x k and x k The binary stream x 0,1 is applied to the clock input of the flip-flop Whenever a logic "1" x k bit is followed by a logic "0", flip-flop 21 changes its state. This is shown in rows 2 and 3 of Tables 2 and 3 below. This is shown in row 4 of Tables 2 and 3 below.

This is illustrated in row 5 of Tables 1 and 2 below. The output of both AND gates 23 and 25 is applied to a summation circuit 27 to provide the coded stream y 0,1,-1 at output 5. The summation circuit 27 effects an algebraic summation of the signals, rather than the logic "AND" effected by gates 23 and Another advantageous implementation of the optical modulation operation according to this invention is based on a M-Z modulator configuration that is driven differentially, as shown in FIG.

In this case, the driving signal is a differential signal provided on lines 12 and 12', respectively, these being generated with a differential pair of drivers 19 from duobinary sequence y 1,0, Travelling wave electrodes 15, 17 receive each on a first end, close to input 8, the active electrically modulated signal from the respective line 12 or 12'. A first matching impedance Z 1 connects the second end of electrode 15, close to the output port 10 of the modulator to ground, while a second matching impedance Z 2 connects the second end of electrode 17 to ground.

In this way, the impedance of each travelling wave electrode is substantially twice the impedance to ground of the individual active lines, creating a virtual ground line.

This virtual ground line is not electrically connected to a physical ground, but is located somewhere between the travelling wave drive electrode, substantially parallel to the direction of propagation of the drive RF wave. The effect of using this drive implementation is to reduce the drive amplitude required of individual drive circuits to approximately half that required for the push-pull drive configuration described earlier.