Abstract:
A long transmission link for two-level information which is interrupted at spaced intervals in order that the signal&#39;s level-crossing information may be extracted and passed on. This level-crossing remodulation process discards amplitude information and retains and retransmits only the times of level crossing. By making the receiver of each remodulator sufficiently wide-banded, the overall link is protocol and bit rate independent in the sense that it can pass any of a wide variety of binary digital signals without requiring reclocking at each stage as is done with conventional regenerators. The preferred embodiment includes an application to multi-wavelength long optical transmission links, and this multi-channel approach avoids many problems associated with optical amplifiers.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a way of sending two-level information, such as binary digital data over long distances in such a way that the overall system will support almost any signal format, so long as the bandwidth of the system is not exceeded and as long as the format is of the two-level class. The effectiveness of the present invention is based on the novel use of a wide acceptance band at the receiving portion of each level-crossing remodulation function that is spaced at intervals along the transmission path, combined with the fact that at the final receiver, fairly large amounts of timing jitter can accumulate without seriously affecting the error rate, because the accumulated jitter in level crossing instant is only weakly converted into amplitude fluctuations at the point in the final receiver at which a decision is made between levels. 
     2. Discussion of the Prior Art 
     Repeaters and regenerators have been used for decades to allow transmission of electrical signals over links of great distances. Such units are interposed at spaced points on the link in such a way as to restore the attenuated and thus noisy signal back to full strength at each repeater or regenerator. Repeaters recreate the entire waveform, amplitude and phase alike, while regenerators, which are used with digital transmissions, make bit-by-bit decisions that are rendered more accurate by recovering bit timing by a process of smoothing over many bit times (often thousands), typically using a phase-locked loop with a long integration time. Repeaters are simply amplifiers, and since each must recreate the full waveform, are very vulnerable to noise. Regenerators are designed to operate at a single bit rate, and therefore the receiver, which includes the phase-locked loop, must have an acceptance bandwidth wide enough to pass the bit rate. Because of this narrow acceptance band, regenerators are much more effective against noise than repeaters and are therefore widely used for digital transmission. However, they require clocking at a known bit rate at each regenerator. The ability to build longer links with digital regenerators than is possible with analog repeaters is one of the reasons that digital transmission has almost completely replaced analog transmission in the world&#39;s telephone systems. Both repeaters and regenerators are well described in the article by E. O. Sunde, &#34;Self-Timing Regenerative Repeaters,&#34; in the Bell System Technical Journal, vol. 36, July, 1957, and in Chapters 1 and 2 of the book by P. Trischitta, Jitter in Digital Transmission Systems, Artech House, 1989. 
     In modern information systems, such as computer networks the problem of transmitting a variety of signal formats over long distances arises very commonly, and flexibility of use often demands a capability to send one format for some time period and then another with a different format over the same path. A different format usually means a different bit rate. Repeaters will provide the bit rate insensitivity desired, but are not a good solution for long distances, since the noise accumulates too rapidly along the sequence of repeaters. Regenerators are not a satisfactory solution either, if bit-rate independence is a requirement, because they involve a commitment to one chosen bit rate; moreover, they do not function with FM, where the level crossings, instead of being at regular intervals as with data, vary widely in their spacing in time. 
     Thus, repeaters have the desired format independence but poor noise performance, whereas with regenerators the converse is true. What is needed is a way of building long transmission links by inserting at appropriate points on the overall link some kind of device such that by the time the signal is observed at the final receiver, the noise will not have built up as rapidly as it would with a series of repeaters, and yet the devices must possess the bit rate independence that is not available with regenerators. 
     The present invention satisfies this requirement. It is based on simply passing on the level-crossing times at each stage, and adding a wide bandwidth capability to each receiver filter preceding the level-crossing determination at each stage, and also capitalizing on the surprisingly small conversion of timing jitter into amplitude noise at the final receiver when clock recovery by long smoothing times is used, as with a phase-locked loop. The process is called here &#34;level-crossing remodulation&#34;. The fact that the conversion of phase jitter into amplitude fluctuations is weak has been well documented in the literature, for example, in FIGS. 4.5 and 4.6 of the book by P. Trischitta, without an analysis having been made up to now of how this weak dependence can be exploited in a series of remodulators that only retain level crossing time information. The idea of passing on only the level crossing timing information while throwing away all amplitude information is mentioned in passing and rejected as unworkable in Section 1.1 of the paper by E. O. Sunde, &#34;Self-Timing Regenerative Repeaters,&#34; in the Bell System Technical Journal, vol. 36, July, 1957, but only in the context of a single predetermined bit rate. In addition, Sunde does not mention the use of a wideband filter and does not address the general problem of designing a long link that will handle arbitrary bit rates. The idea of building long links by repeated recreation of the level crossing instants, where at each such remodulation the receiver bandwidth is maintained wide enough to allow bit rate independence and of exploiting the weak conversion of timing jitter into amplitude noise at the final receiver are not known in the literature. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide the ability to transmit two-level signals of essentially any bit rate up to some maximum over long distances in spite of link attenuation which would otherwise limit the distance. Accordingly, this invention provides at spaced points along the link a level-crossing remodulation means consisting of a detector, followed by a wideband filter, followed by a level-crossing threshold element in the form of a slicing circuit, followed by electrical amplification followed by transmission of the strong two-level signal toward the destination. The final receiver at the destination consists of a detector, followed by a wideband filter, followed by a sampling device that samples at the optimum point in each bit time whether the bit was a zero or a one. The sampling device is driven by a standard clock recovery circuit, typically based on a phase-locked loop. Thus, only the transmitter at the beginning and the receiver at the end need to agree on the bit rate used between them; this information is not needed along the path, and the entire link can be used for any bit rate not exceeding the maximum allowed by the wideband filters in the remodulators. The preferred embodiment shows this technique being used in an optical wavelength division link where, at each stage the level-crossing remodulation is applied separately to the various channels, which differ in the optical wavelength at which each carries its bit stream. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, together with the written descriptions, illustrate the general principles of both single-channel and multi-channel embodiments. 
     FIG. 1 is a block diagram of a typical level-crossing remodulator of the present invention. 
     FIG. 2 is a block diagram of a long link consisting of a transmitter, several link segments with a remodulator of the present invention at the end of each, except for the last segment at whose end is placed the final receiver. 
     FIG. 3 is a block diagram of the final receiver of the remodulation system of the present invention. 
     FIG. 4 is a typical eye diagram at the narrowband filter output of the final receiver of FIG. 3 and illustrates how a large amount of phase jitter is converted into a small amount of amplitude fluctuation. 
     FIG. 5 is part block diagram and part schematic diagram of the level-crossing remodulator system of the present invention used in a long multi-wavelength optical link to provide bit rate-insensitive transmission of many channels over long distances. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a single level-crossing remodulator 100 of the present invention. A weak and noisy two-level signal 1 arrives at detector 2 which produces at its output a noisy version 3 of the intended two-level transmitted information stream. Waveform 3 is passed through a wideband filter 4 whose upper frequency cutoff is high enough to pass the modulation sidebands of the highest bit rate expected to be used on the system, while allowing signals of all lower bit rates to pass through. The output of wideband filter 4 is passed to slicing circuit 5, which forwards only the observed level transitions. Because arriving signal 1 is noisy, slicing circuit 5 will make only an imperfect determination of the intended transition time, that is, it will introduce timing jitter. The output waveform 6 of the slicing circuit 5 is passed to an amplifier 7, and the amplified signal 8 is applied to a transmitter 9. Slicing circuits are well known in the art. 
     FIG. 2 shows an overall link using the level-crossing remodulation process repeatedly. The link consists of a transmitter 10, link segments 11, remodulators 12, and final receiver 13. Each of the remodulators 12, which were shown in detail in FIG. 1, adds its own contribution to the accumulated timing jitter along the chain. However, by throwing away the amplitude information at each remodulator 12, the only amplitude fluctuations that will appear at the final receiver are those accumulated on the last link segment plus any additional amplitude fluctuations that might be caused by the accumulated timing jitter. 
     The final receiver 13, well known in the art, operates as shown in FIG. 3. The predetermined bit rate has been agreed upon only by the transmitter 10 and the final receiver 13. In this case, after the noisy two-level signal 14 has been generated by a detector 15 from the arriving weak and noisy two-level signal 16, signal 14 is passed through a narrowband filter 17, whose center frequency is at the predetermined bit rate and which is wide enough in frequency only to pass most of the modulation sidebands, and then fed to a conventional narrowband phase-locked loop 18 which is operated at the predetermined bit rate, and which smooths the arriving jittered signal over many bit times, typically thousands, in order to establish the correct sampling instant, which is provided as input 19 to the sampling circuit 20. Because the smoothing time of the phase-locked loop 18 can be made so long, it can be designed to make a very clean determination of the right bit clock phase, in spite of the accumulated phase jitter at the final receiver. At the periodic sampling instants, the output of narrowband filter 17 is observed by the sampling circuit 20 to determine likely presence of a zero or one. The sequence so determined constitutes the output bit stream 21. 
     In addition to the use of wideband filters 4 at the input of each remodulation stage 12, the effectiveness of the present invention depends on the way the accumulated phase jitter maps over into mistiming of the sampling instant, which is equivalent to an additional amplitude jitter at the final receiver. FIG. 4 shows why this effect is so small. The graph of FIG. 4 shows an idealized eye diagram, well known in the art, with phase jitter only; amplitude fluctuations are not shown. Curves 22 and 23 show the averaged waveform observed at the input of sampling circuit 20. The sinusoidal shape is due to the extremely narrow width of narrowband filter 17. Dashed line 24 shows a level transition arriving with misplaced timing. It is seen that this rather large amount of timing error maps over into a small amplitude error at the sampling instant 25. In fact, the two-sided probability distribution 26 of timing jitter is seen to map over into the much narrower one-sided probability distribution 27 of amplitude fluctuation. It is this small amplitude fluctuation induced by the phase jitter that is never observed on any link segment except the last one, at which point it adds to the amplitude noise that is caused by the limited noise immunity of the last link segment only. 
     FIG. 5 shows the use of level-crossing remodulation to produce a bit rate independent long multi-wavelength optical link. Each transmitter 10 of FIG. 2 takes the form of a laser diode 28, each of these operating at a different wavelength from one another. A conventional wavelength division multiplexor 29, typically a diffraction grating, combines the outputs of laser diodes 28 onto the transmission fiber 30, which plays the role of the link segments 11 of FIG. 2. At intervals along the overall link, there is a wavelength division demultiplexor 31, a set of photodetectors 32, each playing the role of detector 2 of FIG. 1, wideband filters 4, slicing circuits 5, amplifiers 7, and another set of laser diodes 28, each at its different wavelength and each playing the role of transmitter 9 of FIG. 1. The laser diode outputs are remultiplexed onto the next link segment by means of another wavelength division multiplexor 29. This process is repeated at the end of each link segment until the final receiver is reached where a demultiplexor 31 feeds the usual set of photodetectors 32, each of which now plays the role of detector 15 of FIG. 3, followed by narrowband filters 17, phase-locked loops 18 and sampling circuits 20 that were shown earlier in FIG. 3. 
     From the system point of view, the fact that signals are converted to electronic form at the end of each segment allows one to isolate a given channel at a given segment end, tap off that bitstream and reinsert a new one. This is the &#34;add-drop&#34; capability, important in many applications, and unavailable with optical amplifier chains. It is also possible to change at a segment end the assignment of which bitstream will be forwarded on which wavelength channel, either on a fixed, hard-wired basis, or on a dynamic basis by inserting a crossbar switch after the set of amplifiers 7 and before the laser diodes 28. 
     The multi-channel realization of this invention solves many problems faced in building long wavelength-division optical links using chains of optical amplifiers. Erbium doped fiber amplifiers do not have enough bandwidth to access the entire low-attenuation bandwidth of optical fiber, whereas the level-crossing remodulation technique does not restrict the laser diodes 28 and 32 to operate at wavelengths close to each other. If one substitutes laser diode amplifiers for erbium doped fiber optical amplifiers, problems of crosstalk and polarization dependence result. All these amplifier problems are described in Chapter 6 and Sections 8.11 and 8.14 of the book Fiber Optic Networks, by P. E. Green, Jr., Prentice Hall, 1992, in which it is also pointed out that erbium amplifiers also suffer from irregularities of gain as a function of wavelength. The multi-channel level-crossing remodulation invention does not have this disadvantage; in fact, wavelength-by-wavelength gain variations between the various amplifiers 7 within one multi-channel remodulator can be deliberately introduced to compensate, for example, for the wavelength dependence of the attenuation of the fiber constituting the link segment. Also, the level-crossing remodulation technique avoids the buildup of so-called amplified spontaneous emission introduced by each optical amplifier and also described in Chapter 6 of the book by Green, and the buildup of nonlinearity effects described in Section 3.14 of the same book. 
     The present invention avoids numerous additional problems that occur with prior art transmission systems. The following is a list of some but not all of the problems avoided: 
     1. Avoids the need for bit clock recovery at the end of any segment prior to the last one on a long single channel link and for each channel of a long multi-channel link. 
     2. Avoids the use of chains of linear amplifiers while still preserving bit rate independence of the link. 
     3. Avoids the bandwidth limitations of chains of wideband amplifiers in a multi-wavelength or multi-frequency multi-channel link. 
     4. Avoids the crosstalk disadvantages of chains of wideband laser diode amplifiers in a multi-wavelength optical link. 
     5. Avoids the polarization dependence disadvantages of chains of laser diode amplifiers in an optical link. 
     6. Avoids irregularities of gain with wavelength or frequency in chains of wideband amplifiers in a multi-wavelength or multi-frequency multi-channel link, where the gains of the various amplifiers corresponding to the various wavelengths or frequencies are adjusted to have appropriately different values so as to compensate for the irregularities. 
     7. Compensates for differences of link segment attenuation as a function of wavelength or frequency in long links, where the gains of the various amplifiers corresponding to the various wavelengths or frequencies are adjusted to have appropriately different values so as to compensate for the irregularities. 
     8. Avoids the buildup of amplified spontaneous emission noise and its intermodulation products in chains of wideband optical amplifiers in a multi-wavelength multi-channel optical link. 
     9. Avoids the buildup of the effects of nonlinearities in the fiber or other medium in chains of wideband amplifiers in a multi-wavelength or multi-frequency multi-channel optical link. 
     10. Avoids the buildup of the effects of dispersion in the fiber or other medium in chains of wideband amplifiers in a multi-wavelength or multi-frequency multi-channel optical link. 
     While the invention has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.