Automatic delay compensation for generating NRZ signals from RZ signals in communications networks

The present invention is a method of automatic delay compensation for use in data transmission, particularly in optical communication systems. The invention presents an inter-coding, real-time automated delay compensation method that minimizes the effects of polarization and inter-symbol interference without expensive dispersion compensation fiber in each data transmission channel. The invention is a simple-to-use, cost effective, single-wavelength method of delayed signal alignment with coarse and fine tuning that while conserving half of the power used to transmit wavelength information in conventional WDM (wavelength division multiplexing) systems.

FIELD OF INVENTION
 The present invention is in the field optical transmission of information.
 In particular, the invention relates to an inter-coding automatic delay
 compensation method for transmitting unipolar return-to-zero (URZ) signals
 and receiving unipolar non-return-to-zero (UNRZ) signals in optical
 communication systems such as wavelength division multiplexing (WDM).
 BACKGROUND OF INVENTION
 Data transmission in long fiber transmission paths (such as undersea or
 transcontinental cable or lightwave transmission paths) are subject to
 signal fading and accompanying degradation in the signal-to-noise ratio
 (SNR) that are caused by effects of polarization. Signal fading and
 associated SNR degradation can also result from chromatic dispersion,
 material dispersion in the fiber, and polarization mode dispersion.
 Polarization is particularly described herein to demonstrate its effects
 on SNR and signal fading.
 In a long lightwave transmission system with optical amplifiers, the SNR
 can fluctuate in a random manner due to various types of dispersion.
 Dispersion (such as the types of dispersion described above) causes delays
 in the data transmission channel, particularly in channels with long
 fiber-optic transmission paths. When the SNR of a signal in such a
 lightwave transmission system becomes unacceptably small, a signal fade
 has occurred.
 Signal fading and the associated SNR fluctuations are caused by a number of
 polarization dependent effects induced by the optical fiber itself and
 other optical components (e.g., repeaters, amplifiers, etc.) along the
 optical fiber transmission path. In systems using optical amplifiers
 between the transmitter and the receiver, the gain from an amplifier is
 dependent on the state of polarization (SOP) of the lightwave entering the
 optical amplifier. Optical amplifiers with resynchronization capabilities
 reduce the effects of signal fading and address the delay problem due to
 long fiber-optic transmission paths. For optimal signal performance, the
 SOP of the optical amplifier matches that of the incoming lightwave so
 that a maximum possible gain is achieved at the output of the lightwave.
 The SOP of the lightwave is determined by the shape of the polarization
 ellipse, i.e., the direction of the major axis and the ratio of the major
 axis to the minor axis E.sub.ox /E.sub.oy, and the phase difference
EQU Phase.sub.polarization =phase.sub.x -phase.sub.y
 Random polarization fluctuations result because of random polarization
 phase changes or individual random amplitude change of polarization
 states, or both. In particular, signal fading due to
 polarization-dependent effects over long optical fiber transmission paths
 can be attributed to polarization-dependent loss (PDL),
 polarization-dependent gain (PDG), polarization mode dispersion (PMD) and
 polarization-dependent hole-burning (PDHB). All of these effects impact
 the SOP of an optical signal being propagated along the long optical fiber
 transmission path, and thus the effectiveness of optical amplifiers in
 offsetting signal fading and delay from the transmission medium.
 A conventional solution to rectify the delay and signal fading problem in
 an optical channel is to simultaneously launch two signals of different
 wavelengths and substantially orthogonal relative polarizations into the
 same transmission path. Since the two signals are launched with equal
 power and orthogonal SOPs, the overall transmitted signal is essentially
 unpolarized. This has the advantage of reducing the deleterious effects of
 the transmission channel's non-linear signal-to-noise interactions, and
 signal delay caused by PDHB. Even though the average SNR performance
 improvement with such an arrangement can be substantial, such a system is
 still subject to significant signal fading and channel delay. Moreover,
 the two-wavelength source is still subject to signal fading. Moreover, it
 is costly and a waste of power to use two wavelengths to transmit data
 because only half the bandwidth required is needed to carry useful
 information. In addition, hardware resources such as dispersion
 compensation fiber, manual compensation tracking or variable dispersion
 compensation per channel in WDM, which may also be addressed to this
 problem, are expensive and burdensome to implement. In particular, a
 two-wavelength dispersion compensation technique for reduction of signal
 fading is practically unrealizable because information translation from
 one channel to another channel on a real-time basis is almost impossible
 to perform due to channel ranges and non-linearities in WDM.
 Therefore, there exists a need for a simple, cost-effective
 single-wavelength dispersion compensation technique for reducing effects
 of optical signal fading, without using an additional channel.
 Furthermore, an automated dispersion compensation method is needed for
 reduction of signal fading on a real-time basis without optical signal
 power penalty and without prohibitive, additional retransmission
 adjustment methods. A dispersion compensation method is needed to rectify
 signal fading and SNR degradation due to various factors in optical
 transmission such as polarization, chromatic dispersion, material
 dispersion in the fiber and polarization mode dispersion.
 SUMMARY OF THE INVENTION
 The problems related to signal fading in long optical fiber transmission
 paths is overcome by using a cost-effective, easy-to-use automatic delay
 compensation method in accordance with the invention. The method of the
 invention provides an inter-coding automatic synchronization technique
 that transmits unipolar return-to-zero (URZ) data and receives unipolar
 non-return-to-zero (UNRZ) data, incorporating a synchronization reference
 signal transmitted using the low energy regions in the transmitter power
 spectrum without additional channel requirements. The invention presents a
 single-wavelength, real-time automated delay compensation method that
 minimizes the effects of polarization and inter-symbol interference
 without expensive dispersion compensation fiber in each data transmission
 channel. The invention can be used in delay compensation for transmitting
 information over communication systems such as wireless, optical, modem,
 cable television, free space media, or data networks.

DETAILED DESCRIPTION
 The method of the invention is an inter-coding dispersion compensation
 technique in which data are transmitted in URZ format and received in UNRZ
 format, while conserving hardware and software resources without memory
 and bandwidth penalty. The invention uses a dual-stage synchronization
 approach of coarse and fine tuning while using only a single optical
 wavelength transmission path. In order to achieve such an improvement in
 power penalty while using only one wavelength channel, the present
 invention takes advantage of a unique hybrid coding scheme that enables
 two occurrences of identical URZ data to be transmitted (one occurrence
 being delayed by a given amount) where the two transmissions are later
 combined to produce UNRZ data. As is explained more fully in co-pending
 U.S. patent application Ser. No. 09/197,223 entitled "SYSTEM AND METHOD
 FOR GENERATING NRZ SIGNALS FROM RZ SIGNALS IN COMMUNICATION NETWORKS"
 (GEHLOT-7), which is incorporated herein by reference, this hybrid coding
 method has significant advantages over prior art in that it is an
 efficient way to receive UNRZ data by transmitting URZ coded data. This is
 significant, particularly for optical systems, in that URZ coded data has
 properties that favor optical transmission and it is therefore desirable
 to be able to transmit URZ data instead of UNRZ data and still be able to
 realize the benefits of receiving UNRZ data.
 Preferred embodiments of the invention are hereafter described. It is noted
 that, while the method of the invention is illustrated based on URZ and
 UNRZ coding, it will readily be seen by those ordinarily skilled in the
 art that such method can be modified for other coding or modulation
 techniques applied in an all-optical or a combination of optical and
 electrical communication systems. Although effects of polarization on
 delay and signal fading are described herein, the dispersion compensation
 method in the invention can also be used to rectify optical signal fading
 and delay in the data channel due to various factors such as chromatic
 dispersion, material dispersion in the fiber and polarization mode
 dispersion. The invention can be used in conjunction with single-path,
 two-path or multi-path transmission channels.
 The method of the invention uses a dual-stage synchronization approach that
 uses coarse and fine tuning in order to ascertain proper alignment of URZ
 and URZ.sub.d data transmitted in an optical communications system. In
 accordance with the invention, coarse tuning monitors and compares a
 2-byte sample of data in both the URZ and URZ.sub.d signals. Fine tuning,
 on the other hand, monitors and compares a 1-bit (or 1-pulse) sample of
 data in both the URZ and URZ.sub.d signals. Prior to transmission to the
 data channel, the transmitter modulates both the 2-byte and 1-bit data
 samples with two offset frequencies, f.sub.N and f.sub.M, that provide a
 reference for the coarse and fine tuning. At the receiver, the two
 reference frequencies (f.sub.N and f.sub.M) are tapped and the coarse and
 fine tuning sample data recovered. After performing the coarse and fine
 tuning operations at the receiver, the results of that coarse and fine
 tuning are fed back to the main data channel in the receiver and applied
 for the URZ and URZ.sub.d signals to achieve proper alignment for those
 signals. Once synchronization of the two signals is accomplished, the
 receiver combines the URZ and URZ.sub.d signals to obtain a UNRZ signal
 that is representative of the original data at the data source.
 FIG. 1 illustrates a transmitter according to the method of the invention.
 Referring to the figure, the data clock is operated to drive a frequency
 synthesis means such as one using direct digital synthesis (DDS). That
 frequency synthesis means operates to generate two offset frequencies from
 the clock frequency (f.sub.M and f.sub.N) which are applied at 103 and
 104. Preferrably, the offset frequencies will be chosen in a region in the
 transmitting spectrum characterized by low signal energy. An exemplary DDS
 chip for use as the frequency synthesizer is the AD7008 DDS Modulator
 found in the ANALOG DEVICES REFERENCE MANUAL 1994, REV. A, pages 21-35
 through 21-41, which is incorporated by reference herein.
 Referring again to the choice of the offset frequencies, f.sub.N and
 f.sub.M, in the low energy portion of the transmission spectrum, those
 frequencies may be selected from either a higher frequency region or a
 lower frequency region of the data spectrum. For transmission of URZ data,
 the low energy regions preferrably start beyond 1.5 times the clock rate.
 The clock rate multiplication factors for the frequency synthesizer are
 indicated as N (103) and M (104) in the figure. Although N and M may be
 equal, for the illustrated embodiment, it is preferred that N not be equal
 to M.
 Assuming, for example, the data clock is operating a 5 Ghz N and M could be
 set at 1.09 and 1.08, respectively. After multiplication by the clock
 rate, the offset frequencies of f.sub.N and f.sub.M become f.sub.N
 =5.times.1.09=5.45 Ghz, and f.sub.m =5.times.1.08=5.40 Ghz, respectively.
 The frequency translation based on the indicated values of N and M results
 in the offset frequency signal being in the frequency range of (5
 {character pullout} 0.5) Ghz, which represents a low-energy region in the
 data spectrum.
 Two-byte coarse alignment clock signal (2-Byte Start/Stop at 105) and 1-bit
 fine alignment signal (1-BIT at 106) are then selected in the transmitter.
 Coarse tuning at 105 and fine tuning at 106 insert into the data stream
 two reference signals that monitor a 2-byte interval and a 1-bit interval,
 respectively, in the data stream. For example, in coarse tuning the system
 injects a reference signal with a pulse that starts and stops in two
 bytes, e.g., 1111, to monitor data in the interval of those two bytes.
 Similarly, in fine tuning the system injects a reference signal with a
 single pulse that starts and stops in one bit to monitor data in the
 single-bit interval. To avoid overlapping alignment tracking, the 2-byte
 interval of coarse tuning and the 1-bit interval of fine tuning
 preferrably are established at least two bytes apart.
 The coarse and fine tuning signals are summed together by Adder 107. The
 summed signal is delayed by a half-bit period (T/2) at Delay 108 for
 superposition with URZ.sub.d encoded data, from URZ.sub.d Data source 102,
 at Adder 116. Prior to that superposition operation, however, the summed
 coarse and fine tuning signals are applied to modulate the offset
 frequency f.sub.M at Multiplier 110.
 In a similar fashion, the summed coarse and fine tuning signals are used to
 modulate offset frequency f.sub.N (at Multiplier 112) and that modulated
 signal is then added to the URZ encoded data (from URZ Data source 101) at
 Adder 115. Band pass filters (BPF) are used to restrict noise bandwidth
 where appropriate (e.g., 109, 111, 113 and 114).
 Both URZ and URZ.sub.d signals now have coarse and fine reference signals
 incorporated in their data streams. After modulation of an optical carrier
 from Laser 119 by external optical modulators EOM1 and EOM2, the URZ and
 URZ.sub.d modulated signals are combined and transmitted in an optical
 transmission channel to a receiving location.
 For implementation in a WDM system, a skilled artisan can readily see that
 the transmitter in FIG. 1 is advantageously transmitting data on a single
 wavelength. According to the method of the invention, dispersion
 compensation is advantageously performed on a dual-stage synchronization
 basis of modulating URZ and URZ.sub.d data with coarse (2-byte alignment)
 and fine (1-bit alignment) tuning, without requiring an extra channel for
 transmitting additional wavelength information. System resources are
 optimally conserved since additional compensation fiber and variable
 compensation mechanisms for unnecessary channel requirements are
 eliminated.
 FIG. 2 illustrates a receiver according to the method of the invention,
 which will be operated in conjunction with the transmitter in FIG. 1.
 Referring to FIG. 2, after incoming data are received from the
 transmission channel, the data are sent to a main data channel (which is
 the combination of OD1, OD2, Rx1, Rx2, 219 and 220). At the same time, a
 portion of the received optical signal is split off by an Optical Tap (as
 shown) and provided to a separate synchronization channel where the coarse
 and fine tuning signals are recovered and applied. The task of the main
 data channel is to split the incoming data into two signals, apply
 appropriate delay for those two signals to obtain URZ and URZ.sub.d
 signals in synchronization, and combine the two signals to obtain UNRZ
 data. Optical delays are performed at OD1 and OD2. Electrical delays are
 performed at ED1 and ED2, which are integrated in receivers Rx1 and Rx2,
 respectively. The URZ and URZ.sub.d signals are combined by adder 219 to
 obtain UNRZ data at 220.
 With respect to the synchronization portion of the received optical signal
 which is split off at the Optical Tap, this signal is first split into two
 portions with one portion initially applied to Optical Delay (OD) element
 201 and the other portion applied to OD 211. After application of
 appropriate delay by OD 201 and OD 211, the data signals are sent to
 Receivers (Rx) 203 and 213, which operate, respectively, to detect and
 recover the synchronization reference information associated with offset
 frequencies f.sub.N and f.sub.M, along with a sampled portion of the
 transmitted data. Band pass filters (BPF) are used to restrict noise
 bandwidth at 203 and 213, respectively. A synchronization reference signal
 and a data signal are multiplied by Multiplier 210 with the resultant
 being operated on by either Coarse Align 204 or Fine Align 214. Note that
 Coarse Align 204 and Fine Align 214 are operated alternatively and each
 functions independently with Multiplier 210. The specific operation of
 Coarse Align 204 and Fine Align 214 is described below in conjunction with
 FIG. 3.
 The setting of switch (SW1) determines which alignment (coarse 204) or
 (fine 214) is being run. For the case illustrated in the figure, SW1 is
 set to coarse alignment 204. After coarse alignment is performed and
 synchronization is determined for a 2-byte interval, then fine alignment
 begins to operate in order to determine synchronization for a 1-bit
 interval. During the operation of coarse and fine alignment at 204 and
 214, respectively, the system feeds back alignment data to optical delays
 at 201, 211, at the input to the synchronization stage, and, through
 switch SW1, to optical delays OD1, OD2, and electrical delays ED1, ED2 of
 the main data channel, for delay adjustment in order to achieve signal
 synchronization. After synchronization is achieved, URZ and URZ.sub.d
 signals from Rx1 and Rx2, respectively, are combined by adder 219 to
 obtain UNRZ data (220) which are representative of the original data at
 the data source of the transmitter in FIG. 1.
 FIG. 3 is a flow diagram of coarse and fine signal adjustment according to
 the method of the invention. Assuming SW1 at FIG. 2 initially sets the
 system for coarse alignment, the first step (301 of FIG. 3) sets a 2-byte
 interval in which data alignment is monitored. According to the method of
 the invention, a data signal and a reference signal are multiplied by a
 multiplier (such as 210 in FIG. 2) to obtain a multiplication result. Then
 the result is tested to see if it matches the maximum voltage at 302. The
 multiplication can result in a minimum or a maximum voltage, depending on
 the current alignment of the two signals. A minimum voltage (e.g., 0
 volts) result indicates that the two signals are completely out of sync. A
 maximum voltage result indicates that the two signals are perfectly
 aligned. If the multiplication produces a result that is falls between the
 minimum and the maximum voltages, the signals are re-aligned and
 multiplied again for the maximum voltage test. The process is repeated
 until the multiplication result matches the maximum result.
 Referring again to FIG. 3, step 302 performs the maximum voltage test
 (described hereinabove) on the alignment of the two signals. If the
 multiplication result does not reach maximum voltage, the two signals are
 re-aligned at 305. The process is repeated until the multiplication result
 at 302 matches the maximum voltage, which indicates synchronization of the
 two signals for the 2-byte interval.
 Once coarse alignment is complete, step 303 begins the process of fine
 alignment. Step 304 performs the maximum voltage test on the alignment of
 the two signals in a 1-bit interval. If the multiplication result does not
 reach maximum voltage, the data signal and the reference (fine) signal are
 re-aligned at 305 and are sent to 301 where the process of coarse
 alignment starts again. Once coarse alignment is ascertained at 302, the
 process of fine alignment is re-started at 303. The process is repeated
 until the multiplication result at 304 matches the maximum voltage, which
 indicates synchronization of the two signals for the 2-byte and the 1-bit
 intervals.
 CONCLUSION
 The invention relates to a method of automatic delay compensation for use
 in data transmission, particularly in optical communication systems. The
 present invention is a single-wavelength dispersion compensation method
 that reduces signal fading due to effects of polarization in long optical
 fiber transmission paths. In particular, the invention presents a
 simple-to-use, cost effective method of delayed signal alignment using
 coarse (2-byte) and fine (1-bit) tuning while conserving half of the power
 used to transmit wavelength information in conventional WDM systems.
 Changes and modifications in the specifically described embodiments can be
 carried out without departing from the scope of the invention. Although
 preferred embodiments are disclosed herein, they do not limit the scope of
 the present invention.