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The subject matter of this application is related to that of U. The present invention relates to optical communication equipment and, more specifically, to equipment for processing optical duobinary signals. Duobinary signaling was introduced in the s and since then has found numerous applications in communication systems. The principle of duobinary signaling is explained, for example, in an article by A.

A signal corresponding to one of these levels i. A duobinary signal is typically generated from a corresponding binary signal using certain transformation rules. Although both signals carry the same information, the bandwidth of the duobinary signal may be reduced by a factor of 2 compared to that of the binary signal.

In addition, the duobinary signal may be constructed such that it has certain inter-symbol correlation ISC data, which can be used to implement an error-correction algorithm at the receiver. In optical communication systems, duobinary encoding is typically implemented using phase modulation of a carrier optical beam as disclosed in U. While an optical beam modulated in this manner is a three-level signal in terms of the electric field, it is a two-level signal in terms of the optical power.

A conventional binary receiver simply measures optical power. However, it would be desirable to have a specialized duobinary receiver, which, when deployed in a communication system in place of a regular binary receiver, would improve the system performance using advantages of optical duobinary coding. Problems in the prior duobinary signaling and decoding vinyl are addressed, in accordance with the principles of the present invention, by an optical receiver adapted to apply multiple-sampling processing to an optical duobinary signal received over a transmission link in an optical communication system.

In one embodiment, the receiver has an optical-to-electrical signal converter coupled to a decoder adapted to process an electrical signal generated by the converter to generate a bit sequence corresponding to the optical signal. To generate a bit value, the decoder first obtains two or more bit estimate values by sampling the electrical signal within a corresponding signaling interval two or more times. The decoder then applies a logical function to the bit estimate values, which produces the corresponding bit value for the bit sequence.

Advantageously, embodiments of the present invention improve overall back-to-back i. According to another embodiment, the present invention is a method of signal processing, comprising: According to yet another embodiment, the present invention is an optical communication system, comprising an optical receiver coupled to an optical transmitter via a transmission link, wherein the duobinary signaling and decoding vinyl receiver comprises: Other aspects, features, and benefits of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which:.

System has a duobinary transmitter coupled to a receiver via a transmission link having an optical fiber and one or more optical amplifiers Descriptions of duobinary transmitters that can be used as transmitter can be found, for example, in the following articles: Bissessur, Electronics Letters,vol. One skilled in the art will appreciate that other waveforms shown in FIG.

However, in the region between the mark and space levels, there usually exists a region of relatively low noise probability corresponding to a minimum of the noise distribution function. As already indicated above, to configure decoderone has to select the width of the sampling window and a decision threshold value.

More specifically, sampling window C duobinary signaling and decoding vinyl to a prior-art configuration of decodersampling window D corresponds to a configuration disclosed in the ' application, and sampling windows M 1 -M 2 correspond to one embodiment of the present invention.

In a typical prior-art configuration, the sampling window has a relatively large width, e. One consideration for choosing a relatively large width value is that longer integration times typically provide noise averaging, which can reduce decoding errors. It is generally believed that setting a relatively narrow sampling window will reduce the benefits of noise averaging and detrimentally affect performance of receiver However, for signals affected by dispersion, using a relatively wide sampling window increases decoding errors, e.

For similar reasons, analogous decoding errors may be caused by dispersion-free signals utilizing relatively large duty-cycle values, e. Referring again to FIG. This increases the probability of decoding errors because, even in the absence of noise, the gap between the decision threshold value and the integration result of waveform is relatively narrow.

The contribution of noise may then easily cause the integration result to overshoot the decision threshold value, thereby causing a decoding error for waveform However, an increase of the decision threshold value for sampling window C will narrow the gap between the decision threshold value and the noise-free integration result of waveform The contribution of signal spontaneous beat noise FIG. Thus, attempts to reduce said errors by simply adjusting the decision threshold value duobinary signaling and decoding vinyl sampling window C are largely ineffective.

As demonstrated in the ' application, contrary to general expectations, the number of decoding errors in decoder can be reduced for duobinary signals by decreasing the sampling window width and properly aligning said window with respect to the waveforms of the signal.

For example, when sampling window D is used for waveform FIG. This decreases the probability of decoding errors because the gap between the decision threshold value and the integration result of noise-free waveform can now be relatively large.

Consequently, it becomes more difficult for the contribution of noise to cause the integration result to overshoot the decision threshold value, which reduces the number of decoding errors. Furthermore, the gap between the decision threshold value and the noise-free integration result of waveform is relatively wide. Therefore, it becomes duobinary signaling and decoding vinyl difficult for the contribution of signal spontaneous beat noise FIG. As will be demonstrated below, the number of decoding errors in decoder can be reduced even further by utilizing two relatively short sampling windows and applying an appropriate logical function to bit estimate values corresponding to those windows.

By appropriately choosing the widths of sampling windows M 1 and M 2 and placing them in the vicinity of sampling window D, most of the above-described sampling benefits corresponding to sampling window D are retained for each sampling window. An additional improvement is then derived from further processing of the integration results obtained with sampling windows M 1 -M 2. More specifically, each of the integration results is first compared with a corresponding decision threshold value to produce a bit estimate value.

The choice of logical function applied to the bit estimate values corresponding to sampling windows M 1 and M duobinary signaling and decoding vinyl is primarily determined by the type of error-causing impediment to the optical signal.

For example, as duobinary signaling and decoding vinyl indicated above, for the eye diagram of FIG. In a different situation, e. In contrast, waveforms a and c are displaced with respect to waveform b by negative and positive timing jitter, respectively. The arrows in FIG. For illustration purposes, it is assumed that there is a common decision threshold value for all three sampling windows indicated by the horizontal dashed line.

To graphically compare an integration result duobinary signaling and decoding vinyl to a particular window with the decision threshold value, one can simply project the arrow onto a waveform and note the position of the crossing point of the projection with respect to the dashed decision threshold line. For example, when the crossing point is above the dashed line, the integration result exceeds the decision threshold value.

Similarly, when the crossing point is below the dashed duobinary signaling and decoding vinyl, the integration result is smaller than the decision threshold value. As duobinary signaling and decoding vinyl be seen in FIG. This yields a correct decoding result, i. However, in the presence of jitter, the configuration corresponding to sampling window D produces a decoding error, while the duobinary signaling and decoding vinyl corresponding to duobinary signaling and decoding vinyl windows M 1 and M 2 is able to decode the signal correctly.

This error is avoided with the configuration corresponding to sampling windows M 1 and M 2 as follows. For waveform athe integration result for sampling window M 1 is below the decision threshold value, while the integration result for sampling window M 2 is above that value. Analogously, for waveform duobinary signaling and decoding vinylthe integration result for sampling window M duobinary signaling and decoding vinyl is below the decision threshold value, while the integration result for sampling window M 1 is above that value.

Similar to the decoding of waveform bdecoding of waveform d yields a correct result, i. However, for waveform ethe configuration corresponding to sampling window D produces a duobinary signaling and decoding vinyl error, while the configuration corresponding to sampling windows M 1 and M 2 avoids that error.

However, the integration results for sampling window M 1 and M 2 are above and below the decision threshold value, respectively. Similar to receiver of FIG. However, receiver implements dual-sampling processing in accordance with an embodiment of the invention. To provide an appropriate time reference for decision circuitreceiver has a clock recovery circuit and a clock multiplier Clock multiplier then multiplies the frequency of the first clock signal and generates a second clock signal applied to decision circuit In a representative implementation of receiverthe second clock signal has a frequency value four times that of the first clock signal.

Decision circuit uses clock pulses of the second clock signal to sample the received signal. More specifically, for each clock pulse, decision circuit generates a signal sample integration resultcompares it with a decision threshold value, and outputs a binary value corresponding to the comparison result. Sampling windows of decision circuit are aligned with respect to the signal such that, for each signaling interval, two sampling windows correspond to sampling windows M 1 and M 2 shown in FIG.

As a duobinary signaling and decoding vinyl, the bit stream at the output of decision circuit has bits carrying bit estimate values corresponding to sampling windows M 1 -M 2 and described above in the context of FIGS. These bits are however interleaved with other bits not related to sampling windows M 1 -M 2.

To separate the bits carrying bit estimate values from the rest of the bit stream output by decision circuitreceiver has a de-multiplexer having a number of output ports equal to the number of signal samples taken by decision circuit per signaling interval. Therefore, each output port of de-multiplexer receives one bit from the bit stream during each signaling interval, with two particular output ports of de-multiplexer receiving the bits carrying bit estimate values for sampling windows M 1 -M 2.

However, instead of the serial sample processing technique realized in receiverreceiver implements a parallel sample processing duobinary signaling and decoding vinyl. However, unlike decision circuitdecision circuits a - b operate directly at the clock frequency generated by clock recovery circuit without any frequency multiplication. Each decision circuit generates a signal sample integration result using a relatively short sample window, compares the sample with a corresponding decision threshold value, and outputs a binary value corresponding to the comparison result.

Note that decision circuits a - b may or may not have the same decision threshold value. This serves to introduce a desired time lag between the sampling windows of decision circuits a and b.

These sampling windows are then aligned with respect to the signal to correspond to sampling windows M 1 and M 2 shown in FIG. Consequently, bit streams at the outputs of decision circuits a - b carry bit estimate values corresponding to sampling windows M 2 and M 1respectively.

Duobinary signaling and decoding vinyl configuration I, receiver is receiver FIG. The data shown in FIGS. Data points in FIGS. As can be seen from the comparison of FIG.

In summary, embodiments of the present invention improve dispersion tolerance and reduce optical power corresponding to a duobinary signaling and decoding vinyl BER value in duobinary transmission systems, thereby improving overall back-to-back i. While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Although the present invention is described with reference to duobinary signals, it can also be used for processing other types of signals, tradingsystemforex elite section. Various modifications of the described embodiments, as well as other embodiments of the invention, which are apparent to persons skilled in the art to which the invention pertains are deemed to lie within the principle and scope of the invention as expressed in the following claims.

Although the steps 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 steps, those steps are not necessarily intended to be limited to being implemented in that particular sequence. Year of fee payment: An optical receiver adapted to apply multiple-sampling processing to an optical duobinary signal received over a transmission link in an optical communication system.

Field of the Invention The present invention relates to optical communication equipment and, more specifically, to equipment for processing optical duobinary signals. Description of the Related Art Duobinary signaling was introduced in the s and since then has found duobinary signaling and decoding vinyl applications in communication systems.

A method of signal processing, comprising:

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