Pilot symbol aided carrier phase estimation

[0001] Embodiments of the present disclosure relate to the field of optical communication systems. More particularly, the present disclosure relates to the addition of pilot symbols in modulated optical signals to aid in joint polarization phase estimation in coherent optical receivers.

[0002] In optical communication systems, various modulation schemes are used to transmit optical signals using dense wavelength division multiplexing (DWDM) where a plurality of optical channels each at a particular wavelength propagate over fiber optic cables. However, various non-linearities including, for example, self-phase modulation (SPM) and cross-phase modulation (CPM) may induce nonlinear phase shifts which effect signal reception and decoding of these transmitted signals. After propagation over long distances, the impact of these non-linearities create processing and decoding difficulties at the receiver which may compromise the integrity of the transmitted information.

[0003] Coherent decoding detects not only an optical signal's amplitude but phase and polarization as well. In other words, it detects whole characteristics of the received signal across the spectrum. This allows the receiver to compensate for the linear channel transfer function. Moreover, quadrature phase Shift Keying (QPSK) and polarization division multiplexing (PDM) schemes can be implemented and thus increase detection capability and spectral efficiency.

[0004] In coherent phase modulated optical communication systems, the carrier phase of the transmitter must be estimated and tracked at the receiver in order to decode the transmitted information signals. However, an inherent problem associated with coherent receivers in phase modulated optical communication systems is that of phase ambiguity associated with phase detection at the receiver. This is due to the presence of data modulation in the phase detection. When the phase error reaches π/4 for QPSK or π/2 for Binary Phase Shift Keying (BPSK), a “cycle slip” can occur in which symbols may be erroneously interpreted as lying in an adjacent quadrant. For systems utilizing these types of modulation techniques, this results in erroneous interpretation of the symbols following the cycle slip. Therefore, it is important to differentially pre-code data to avoid error propagation. However, differential pre-coding suffers from about 1 dB Q penalty at low optical signal to noise ratios because a single symbol error becomes a pair of consecutive symbol errors. Accordingly, it is an object of the present disclosure to overcome these problems and provide better system performance at a lower optical signal to noise ratio in optical communication systems.

[0005] Exemplary embodiments of the present disclosure are directed to a method for estimating the carrier phase of a modulated optical signal. In an exemplary method, an optical signal is received having data symbols and pilot symbols which have predefined data modulation. The carrier phase of the received pilot symbols are detected. The carrier phase of the data symbols is estimated by an Mth power scheme. A differential phase between the pilot symbol and the adjacent data symbol is determined. The differential phase is compared to a defined phase tolerance to determine if the cycle slip or phase error of the data symbols occurred before the pilot symbols. If cycle slip occurred, then the carrier phase of the data symbols are estimated by linear interpolation of the phase between a pair of pilot symbols.

[0013] The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention, however, may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, like numbers refer to like elements throughout.

[0014] Presently disclosed embodiments mitigate the impact of nonlinear phase noise through improved carrier phase estimation with increased tracking capability. Phase ambiguity associated with carrier phase estimation may be removed by inserting one or more pilot symbols, which are reference symbols, inserted at known positions in a modulated optical signal. The one or more pilot symbols represent known data information and are inserted periodically among data symbols in the modulated signal. The carrier phase of the one or more pilot symbols is estimated by determining the differential phase between the one or more pilot symbols and the carrier phase of an adjacent or previous data symbol. By utilizing pilot symbols in the modulated signal, coherent PSK without precoding may be employed which provides better system performance at a lower optical signal to noise ratio than previously realized.

[0015] FIG. 1 generally illustrates a simplified optical communication system 10 including an exemplary transmit terminal 11, receive terminal 12 and an optical transmission medium 15 disposed therebetween. The optical transmission medium may be a fiber optic cable having a plurality of fiber pairs configured to propagate communication signals between terminals 11 and 12. Terminal 11 includes a plurality of transmitters 20 which supplies a modulated optical signal 25 having a particular one of a plurality of wavelengths to multiplexer 30. Multiplexer 30 combines the modulated optical channels from the transmitters 20 and combines them in a dense wavelength division multiplexed (DWDM) signal for propagation over fiber optic cable 15. It should be understood that the exemplary transmitter 20 may also be included in terminal 12 for bidirectional transmission. Terminal 12 includes a demultiplexer 40 used to separate the received DWDM optical signal into individual wavelengths or channels 50. Once separated, each channel is supplied to a respective receiver 60 and processed to provide a demodulated optical data signal. It should be understood that the exemplary receiver 60 may also be included in terminal end 11 for bidirectional transmission.

[0016] As described earlier, in order to transmit and receive these optical signals long distances, various modulation techniques are employed to provide a detectable optical signal at receiver 60. These modulation techniques include, for example, QPSK (quadrature phase-shift keying); DP-QPSK (dual polarization quadrature phase-shift keying), etc., in which data is identified by phases of an optical carrier. However, various known non-linear effects impact the integrity of the optical signal received by receiver 60 after propagation over cable 15. For example, the light sources used to provide the optical signal at transmitters 20 are typically external cavity lasers capable of providing linewidths in the range of 100 KHz. However, after propagation over cable 15 the received linewidth of the light may have broadened to tens of MHz due to non-linear effects such as four-wave mixing, cross-phase modulation (CPM), self-phase modulation (SPM), etc. Because of these nonlinear effects, the phase of the optical signal received at receiver 60 may have rotated such that the decoded symbols do not match those which were generated by transmitter 20. This causes symbol or data errors.

[0017] FIG. 1A is a simplified block diagram of receiver 60 shown in FIG. 1 in accordance with the present disclosure. The transmitted DWDM channels 50 are received and supplied to a polarization beam splitter (PBS) 61. The polarization split signal is provided to a pair of 90° optical hybrid interferometers 62A and 62B which extracts phase and amplitude between the received signal and local oscillator 63. A plurality of photodetectors denoted generally as 64 generate respective electrical signals proportional to the received optical signals from the hybrid interferometers 62A and 62B. A plurality of Analog to Digitial (A/D) converters 65 connected to respective photodetectors 64 receive the electrical signals and supply corresponding digital signals to digital signal processor (DSP) 70. The DSP 70 may include various modules/circuits including dispersion compensation circuit 71, clock recovery circuit 72, synchronized data resampling module 73, polarization tracking and polarization mode dispersion compensation module 74, carrier phase estimation module 75 and decision module 76. Receiver 60 is configured to decode the received modulated data symbols.

[0018] Carrier phase estimation associated with decoding QPSK modulated signals is difficult since the modulated data is present in the received optical signal. In order to decode the received signal, the modulated data must be removed from the signal to determine the carrier phase and thus, synchronize the demodulation of the data symbols based on the phase of the carrier. However, an inherent problem associated with typical coherent QPSK systems is that of phase ambiguity at the receiver. This phase ambiguity is due to the general inability of the carrier recovery circuit in an associated receiver to distinguish the reference phase from the other phase(s) of the received carrier.

[0019] In order to remove the phase ambiguity of the carrier, pilot symbols having known data information are inserted periodically within the modulated optical signal in accordance with the present disclosure. FIG. 2 illustrates an exemplary transmitted sequence of data 100 having symbols 110_N and a period length in a modulated optical signal 25 generated by transmitter 20 in an exemplary optical communication system 10. A first set of one or more pilot symbols 110-0, 110-1 is inserted within a data sequence 100 to be modulated on a carrier wave. A second set of one or more pilot symbols 110-50, 110-51 are inserted into the data sequence 100 after data symbols 110-2 . . . 110-49. The data symbols 110-2 . . . 110-49 may have, for example, data information values l, −l, i, or −i.

[0020] Each of the sets of pilot symbols may comprise a pair of adjacent pilot symbols. For example, the first set of pilot symbols may include a first pilot symbol 110-0 and an adjacent pilot symbol 110-1. The second set of pilot symbols may include a first pilot symbol 110-50 and an adjacent pilot symbol 110-51. Thus, the fifty symbol period (i.e. N=50) from symbol 110-1 thru 110-50 is defined by data symbols 110-2 . . . 110-49 disposed between a first pilot symbol 110-1 and a second pilot symbol 110-50. Of course, alternative numbers of pilot symbols and sequencing with corresponding data symbols may also be employed. Data symbol 110-2 is adjacent pilot symbol 110-1 and data symbol 110-49 is adjacent pilot symbol 110-50 to form a first and second set of pilot symbols, respectively. The resultant stream 100 is transmitted to a receiver 60 via optical cable 15. The above example of the insertion of two pilot symbols (110-1, 110-50) per fifty symbol period, increases the symbol rate of the transmitted signal by 4% which causes approximately a 0.17 dB optical signal to noise ratio (OSNR) penalty. However, insertion of the pilot symbols advantageously mitigates phase tracking errors and enables the use of coherent PSK without the need of symbol precoding, and overall system performance improves by approximately 1 dB at low OSNR.

[0021] The modulated optical signal 25 is received by receiver 60 where each of the data symbols (e.g. 110-2 . . . 110-49) and pilot symbols (110-0, 110-1, 110-50, 110-51) have an associated phase consistent with the modulation format such as, for example, QPSK supplied by transmitter 20. Receiver 60 detects the received pilot symbols and estimates the carrier phase of these pilot symbols using the following equation:

[0022] $\begin{array}{cc}{\Phi }_n={\Phi }_{n+1}={\Phi }_{n-1}+\arg \left[\sum _{k=0}^1{r}_{n+k}\exp \left(-{\mathrm{i\varphi }}_{n-1}\right)\right]& \left(1\right)\end{array}$
where Φn represents the carrier phase of a first pilot symbol, Φ_n+1 represents the carrier phase of an adjacent pilot symbol, φ_n−1 represents the carrier phase of a data symbol adjacent the first pilot symbol and r_n represents the received data symbol. For example, Φn may represent the phase of pilot symbol 110-50, Φn+1 represents the carrier phase of pilot symbol 110-51 (i.e. n and n+1, respectively) and φ_n−1 represents the carrier phase of data symbol 110-49 all of which are shown in FIG. 2.

[0023] The last term of Equation (1) represents the differential phase between the pilot symbol and the previous (immediately preceding) data symbol. In the above example, this corresponds to determining the differential phase between the carrier phase of pilot symbol 110-50 and the carrier phase of data symbol 110-49 since the data symbol 110-49 immediately precedes pilot symbol 110-50. This differential phase is compared to a defined phase tolerance and if the differential phase exceeds this tolerance, then this indicates that the phase tracking of the signal at receiver 60 was lost. This phase tolerance is relatively large such as, for example, greater than π/3 radians. In the above example, if the phase differential is greater than π/3 radians, then the phase tracking of the carrier phase between the pilot symbols 110-50 and 110-1 was lost. Of course, the phase tolerance of π/3 radians is used herein as an example and other tolerances may be applicable based on the modulation scheme, length of transmission, etc.

[0024] Once the determination is made that the differential phase falls outside the phase tolerance and phase tracking has been lost between the pilot symbols, this loss of phase tracking may be corrected via linear interpolation since phase is assumed to trend linearly. The following Equation (2) is used for this linear interpolation-based correction and assumes that the period has a length of 50 symbols as described with reference to FIG. 2:

[0025] $\begin{array}{cc}{\varphi }_{n-j}={\varphi }_{n-j+1}+\frac{{\Phi }_{n-50}-{\Phi }_n}{49},j=1,2,\dots 48& \left(2\right)\end{array}$
where Φ(n) and Φ(n−50) are the estimated carrier phase of the pilot symbols 110-50 and 110-1 and again φ is the phase of an associated data symbol. Of course, the period length may be modified based on the particular application.

[0026] FIG. 3 is a notional depiction (not drawn to scale) of the phase tracking correction via linear interpolation utilizing Equation (2) above. The phase 200 is illustrated between a first pilot symbol corresponding, for example, to pilot symbol 110-1 at P1 and a second pilot symbol corresponding, for example, to pilot symbol 110-50 at P2. The differential phase (denoted graphically at 210) is determined between the pilot symbol at P2 and the phase of the data symbol preceding the pilot symbol at P2. If the differential phase is not greater than π/3 radians than phase tracking of the data symbols between the pilot symbols proceeds. Again, if the differential phase 210 is large enough, for example, greater than π/3 radians, then a correction is applied to the carrier phase of the data symbols based on linear interpolation as shown by dashed line 220, to yield a corrected phase track 230.

[0027] The carrier phase of the data symbols (φ_n) between the pilot symbols (110-1 and 110-50) is estimated. This may be done using an M^th power scheme with or without joint polarization carrier phase estimation. The equation for M^th power scheme where M=4 (i.e. 4^th power) is as follows:

[0028] $\begin{array}{cc}{\varphi }_n={\varphi }_{n-1}+\frac{\mu }{4}\arg \left[\sum _{k=0}^N{r}_{n+k}^4\exp \left(-i4{\varphi }_{n-1}\right)\right]& \left(3\right)\end{array}$
where μ is a Kalman filter coefficient. It is important to note that the drawbacks associated with M^th power scheme (i.e. phase estimation error) referenced above is associated with removing data modulation from the signal without the use of pilot symbols to estimate the carrier phase. The use of the M^th power scheme of Equation (3) is with respect to the carrier phase of the data symbols between the pairs of pilot symbols.

[0029] Alternatively, the carrier phase of the data symbols between the pairs of pilot symbols may be estimated by using Mth power scheme with joint polarization carrier phase estimation as described in ECOC '2008, paper No. 4, D.2, M. Kuschnerov, et al., Brussels, September 2008, the contents of which are incorporated herein by reference. The phase of the X and Y polarization tributaries associated with the modulated data symbols is calculated separately using the following equations:

[0030] $\begin{array}{cc}{\varphi }_n\left(X\right)={\varphi }_{n-1}\left(X\right)+\frac{\mu }{4}\arg \left[\sum _{k=0}^N{r}_{n+k}^4\left(X\right)e^{-{\mathrm{i4\varphi }}_{n-1}\left(X\right)}+C\sum _{k=0}^N{r}_{n+k}^4\left(Y\right)e^{-{\mathrm{i4\varphi }}_{n-1}\left(Y\right)}\right]\mathrm{and}{\varphi }_n\left(Y\right)={\varphi }_{n-1}\left(Y\right)+\frac{\mu }{4}\arg \left[\sum _{k=0}^N{r}_{n+k}^4\left(Y\right)e^{-{\mathrm{i4\varphi }}_{n-1}\left(Y\right)}+C\sum _{k=0}^N{r}_{n+k}^4\left(X\right)e^{-{\mathrm{i4\varphi }}_{n-1}\left(X\right)}\right]& \left(4\right)\end{array}$
where C is the coupling coefficient between X-polarization and Y-polarization tributaries. Joint polarization phase estimation improves the performance of carrier phase estimation by effectively increasing the averaging window size. The optimal coupling coefficient Cε [0, 1] may vary depending on nonlinear XPM effect which leads to reduced correlation between the carrier phase of QPSK polarization tributaries. However, the use of pilot symbols significantly helps to tolerate a non-optimized C value. This is because the phase of the pilot symbols can be estimated with high accuracy, but without ambiguity. The value of the coupling coefficient can be selected by default (e.g., 0.7) and the accumulated phase error of data symbols due to non-optimal C values are corrected periodically with the pilot symbols. In this manner the carrier phase of the data symbols between the pilot symbols in a received symbol sequence may be estimated and coherent decoding using QPSK may be utilized to realize better system performance.

[0031] FIG. 4 is a flow chart graphically illustrating the preceding method of pilot symbol aided carrier phase estimation 400. In particular, at step 401 pilot symbols having known data information are inserted periodically within a sequence of data symbols of a modulated optical signal at an optical transmitter. The data symbols and the pilot symbols each have an associated phase consistent with a predefined data modulation for optical signal transmission. The phase of each of the pilot symbols is detected at step 402 at a receiver. The carrier phase of the data symbols between the pilot symbols is estimated. This is done using M^th power scheme with joint polarization carrier phase estimation at step 403. The phase of the data symbol (e.g. data symbol 110-49 of FIG. 2) immediately preceding or adjacent the pilot symbol (e.g. pilot symbol 110-50 of FIG. 2) is detected at step 404. The differential phase between the carrier phase of the pilot symbol and the carrier phase of the data symbol which immediately precedes the pilot symbol is determined at step 405. This differential phase is compared to a phase tolerance (e.g. π/3 radians) at step 406 to determine if cycle slip or phase error of the data symbols occurred before the pilot symbol (e.g. pilot symbol 110-50).

[0032] If the differential phase is not greater than the phase tolerance, the process proceeds to step 408 where no loss of phase tracking was detected and the data is decoded. If the differential phase is greater than the phase tolerance, then the process proceeds to step 407 where correction is applied to phase tracking of the data symbols between the pilot symbols using linear interpolation. The process proceeds to step 408 where there is no loss of phase tracking and the data may be decoded.

[0033] The foregoing carrier phase estimation method was tested using orthogonally launched independent QPSK modulated signals with pilot symbols and decoded at a receiver. The receiver utilized carrier frequency estimation (CFE) method without the aid of pilot symbols and was compared to the carrier phase estimation (CPE) method with pilot symbols. In order to compare these two methods along with ideal performance, the bit error rate (BER) was plotted in FIG. 5. It is important to note that since the pilot symbols were added in the transmitted signal in accordance with the present disclosure, coherent QPSK could be used to decode the symbol information rather than using conventional M^th power CPE which could only employ differential decoding (DC-QPSK) to decode the symbols. The decoding results illustrate a 1 dB Q-factor improvement at low SNR and 0.6 dB improvement at, high SNR. As is known to one of ordinary skill in the art, the term “Q-factor” suggests the minimum SNR required to obtain a specific BER for a given received signal.

[0034] FIGS. 6A and 6B graph the phase tracking results without the use of pilot symbols and the phase tracking results using the pilot symbols respectively. In particular, FIG. 6A shows plot 600 of the estimated carrier phase assuming the data phase is known and plot 601 is the estimated carrier phase obtained by the conventional 4^th power estimation method (i.e., not using pilot symbol aided joint polarization estimation as in a presently disclosed embodiment). As can be seen from the divergence of plots 600 and 601, track loss was not corrected. In contrast, FIG. 6B shows plot 602 of the estimated carrier phase assuming the data phase is known and plot 603 of the estimated carrier phase obtained using pilot symbol aided joint polarization CPE together with coherent QPSK. The pilot symbol are indicated at points 605A and 605B with tracking of the data symbols therebetween. As indicated by plots 602 and 603, phase tracking is advantageously maintained, thereby enabling the use of coherent PSK for symbol decoding.

[0035] With pilot symbol aided joint polarization CPE, phase tracking is guaranteed because there is no phase ambiguity. Although loss of tracking may still occur between pilot symbols, such track loss is advantageously detected and corrected which is an improvement over the prior art.

[0036] Embodiments of the present disclosure may be implemented at a transmitter and receiver of an optical communication system. A processor may be used to effectuate operations associated with a communication system, as is known to one of ordinary skill in the art. A processor as used herein is a device for executing stored machine-readable instructions for performing tasks and may comprise any one or combination of hardware, software, and firmware. A processor may also comprise memory storing machine-readable instructions executable for performing tasks. A processor acts upon information by manipulating, analyzing, modifying, converting, or transmitting information for use by an executable procedure or an information device, and/or by routing the information to an output device. A processor may use or comprise the capabilities of, e.g., a controller or microprocessor. A processor may be electrically coupled with any other processor, enabling interaction and/or communication therebetween. A processor comprising executable instructions may be electrically coupled by being within stored executable instructions enabling interaction and/or communication with executable instructions comprising another processor. A user interface processor or generator is a known element comprising electronic circuitry or software, or a combination of both, for generating display images or portions thereof.

[0037] An executable application comprises code or machine readable instructions for conditioning the processor to implement predetermined functions, such as those of an operating system, a context data acquisition system, or other information processing system, e.g., in response to user command or input. An executable procedure is a segment of code or machine readable instruction, sub-routine, or other distinct section of code or portion of an executable application for performing one or more particular processes. These processes may include receiving input data and/or parameters, performing operations on received input data and/or performing functions in response to received input parameters, and providing resulting output data and/or parameters.

[0038] The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Further, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes.