Duobinary signal encoding
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.
The following examples show how a binary input sequence is transformed into the duobinary output sequence according to the invention:. The summation circuit comprises a first transistor 35 which receives the bits of the first binary sequence a 0,1 on the base. The collector of transistor 35 is connected to a load resistor R , referred to with numeral 41, and the emitter is connected to V DD through a current source A second transistor 33 receives on its base an inverted version of the second binary sequence b 0,1 , provided by inverter The collector of transistor 33 is also connected to the load resistor 41 and the emitter to a second current source Both current sources 37 and 39 provide a current "i" through resistor R when the respective transistor conducts.
A bit y k obtained at output 5 can take three different values, iR, 2iR and 0. If the summation circuit 27 is ac coupled, the three levels become: The bias voltage V Bias is applied between electrodes 15 and 17 and the duobinary coded driving sequence is applied on electrode 19 of modulator 9, as shown in FIG.
For the case shown in FIG. The dotted line illustrates the same parameter for the duobinary scheme reported in 2! A pseudo-random bit sequence PRBS was used for this comparison.
As seen in FIG. While the invention has been described with reference to particular example embodiments, further modifications and improvements which will occur to those skilled in the art, may be made within the purview of the appended claims, without departing from the scope of the invention in its broader aspect. Year of fee payment: A first AND circuit receives the input sequence and the switch signal, and provides a first binary sequence a 0,1 , while a second AND circuit receives the input sequence and the complement of the switch signal and provides a second binary sequence b 0,1.
A method for differentially driving a M-Z modulator using a virtual ground level is also provided, which reduces the peak-to-peak drive voltage by a factor of two. Field of the Invention 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.
Background Art In the long haul, high bit rate optical fiber telecommunications, appropriate coding and modulation of the signal for transmission are essential. Coding Duobinary signaling was introduced a few decades ago and its details can be found in, for example, "Introduction To Telecommunication Systems", F. Modulation Most optical fiber transmitters use an external modulator. The following examples show how a binary input sequence is transformed into the duobinary output sequence according to the invention: It is also to be noted that the adder illustrated in FIG.
What is claimed is: A method as claimed in claim 1, wherein said first logical level "n" is logic "1", said second logical level "d" is logic "0" and said third logical level "-d" is "-0".
A method as claimed in claim 4, wherein said step of preparing comprises the sub-steps of: A method as claimed in claim 4, wherein said step of summing comprises adding the level corresponding to a bit a k of said first binary sequence a 0,1 with the level corresponding to a bit b k of said second binary sequence b 0,1.
A method as claimed in claim 7, further comprising the steps of: A method as claimed in claim 8, wherein said external modulator is a Mach-Zehnder interferometer and said step of modulating comprises: A method as claimed in claim 8, wherein said step of modulating comprises: A method as claimed in claim 11, further comprising the steps of: A method as claimed in claim 12, wherein said step of modulating comprises: A device as claimed in claim 14, wherein said means for generating is a D-type flip-flop having a clock input, an input D, an output Q and an output Q, for receiving said binary input sequence x 0,1 on the clock input, and said output Q connected to said input D.
A device as claimed in claim 14, wherein said summer comprises: A Mach-Zehnder M-Z interferometer for modulating a continuous wave CW optical carrier with a duobinary encoded driving signal, said M-Z interferometer having a first and a second travelling wave-guide, a splitter between an input port and said first and second travelling wave-guides, a combiner between said first and second travelling wave-guides and an output port, a first and a second travelling wave electrode, each associated with said respective first and second travelling wave-guide, and a control electrode, said M-Z interferometer further comprising: A Mach-Zehnder M-Z interferometer for modulating a continuous wave CW optical carrier with a duobinary encoded differential driving signal, said M-Z interferometer having a first and a second travelling wave-guide, a splitter between an input port and said first and second travelling wave-guides, a combiner between said first and second travelling wave-guides and an output port, a first and a second travelling wave electrode, each associated with said respective first and second travelling wave-guide, said M-Z interferometer further comprising: US USA en DE DED1 en EP EPB1 en DE DET2 en CA CAC en Transmission and reception of duobinary multilevel pulse-amplitude-modulated optical signals using finite-state machine-based encoder.
Transmission and reception of duobinary multilevel pulse-amplitude-modulated optical signals using subsequence-based encoder. Optical transmission system employing auto-synchronized chirped return-to-zero transmitter. Electrical domain compensation of optical dispersion in an optical communications system. Electrical domain mitigation of polarization dependent effects in an optical communications system.
Electrical domain compensation of non-linear effects in an optical communications system. Transmitter and method using half rate data streams for generating full rate modulation on an optical signal. System and method for alternate mark inversion and duobinary optical transmission.
Device for mach-zehnder modulator bias control for duobinary optical transmission and associated system and method. Method and system to provide modular parallel precoding in optical duobinary transmission systems.
T-carrier uses robbed-bit signaling: The modification of bit 7 causes a change to voice that is undetectable by the human ear, but it is an unacceptable corruption of a data stream. Data channels are required to use some other form of pulse-stuffing,  such as always setting bit 8 to '1', in order to maintain a sufficient density of ones. If the characteristics of the input data do not follow the pattern that every eighth bit is '1', the coder using alternate mark inversion adds a '1' after seven consecutive zeros to maintain synchronisation.
On the decoder side, this extra '1' added by the coder is removed, recreating the correct data. Another benefit of bipolar encoding compared to unipolar is error detection. In the T-carrier example, the bipolar signals are regenerated at regular intervals so that signals diminished by distance are not just amplified, but detected and recreated anew. Weakened signals corrupted by noise could cause errors, a mark interpreted as zero, or zero as positive or negative mark.
Every single-bit error results in a violation of the bipolar rule. Each such bipolar violation BPV is an indication of a transmission error. The location of BPV is not necessarily the location of the original error. For data channels, in order to avoid the need of always setting bit 8 to 1, as described above, other T1 encoding schemes Modified AMI codes ensure regular transitions regardless of the data being carried. A very similar encoding scheme, with the logical positions reversed, is also used and is often referred to as pseudoternary encoding.
This encoding is otherwise identical. The use of a bipolar code prevents a significant build-up of DC , as the positive and negative pulses average to zero volts. Little or no DC-component is considered an advantage because the cable may then be used for longer distances and to carry power for intermediate equipment such as line repeaters.
Bipolar encoding is preferable to non-return-to-zero whenever signal transitions are required to maintain synchronization between the transmitter and receiver. Other systems must synchronize using some form of out-of-band communication, or add frame synchronization sequences that don't carry data to the signal.
These alternative approaches require either an additional transmission medium for the clock signal or a loss of performance due to overhead, respectively. A bipolar encoding is an often good compromise: However, long sequences of zeroes remain an issue. Long sequences of zero bits result in no transitions and a loss of synchronization.
Where frequent transitions are a requirement, a self-clocking encoding such as return-to-zero or some other more complicated line code may be more appropriate, though they introduce significant overhead. The coding was used extensively in first-generation PCM networks, and is still commonly seen on older multiplexing equipment today, but successful transmission relies on no long runs of zeroes being present.
There are two popular ways to ensure that no more than 15 consecutive zeros are ever sent: T-carrier uses robbed-bit signaling: The modification of bit 7 causes a change to voice that is undetectable by the human ear, but it is an unacceptable corruption of a data stream.
Data channels are required to use some other form of pulse-stuffing,  such as always setting bit 8 to '1', in order to maintain a sufficient density of ones.