Abstract:
In an over-sampled maximum-likelihood sequence estimation (MLSE) receiver system, the optimal sample spacing is determined for a variety of conditions. In an illustrative implementation, the system includes an optical filter for tightly filtering an incoming optical data signal with an on-off-keying (OOK) non-return-to-zero (NRZ) format, followed by an optical-to-electrical converter, an electrical filter, a sampler, and a MLSE receiver. The sampler samples the filtered electrical data signal twice each bit period with unequal sample spacings. For wide optical filtering bandwidths, the optimal sample spacing occurs at less than 50% of a bit period. For narrow bandwidths, the optimal sample instances occur closer to the maximum eye opening.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to the field of high-speed optical data communications, and in particular, to the detection of signals using maximum likelihood sequence estimation.  
       BACKGROUND INFORMATION  
       [0002]     Maximum likelihood sequence estimation (MLSE) receivers have been used in fiber optic communication systems operating at data rates up to 10 Gb/s to counteract signal distortions due to chromatic and polarization-mode dispersion. (See, e.g., H. F. Haunstein et al., “Principles for Electronic Equalization of Polarization-Mode Dispersion,” J. Lightwave Technol., vol. 22, pp. 1169-1182, 2004; F. Buchali et al., “Viterbi equalizer for mitigation of distortions from chromatic dispersion and PMD at 10 Gb/s,” in Proc. Opt. Fiber Commun. Conf. (OFC), MF85, 2004; A. Farbert et al., “Performance of a 10.7-Gb/s receiver with digital equalizer using maximum likelihood sequence estimation,” Proc. European Conf. on Opt. Commun. (ECOC), p. Th4.1.5, 2004; and J. J. Lepley et al., “Excess penalty impairments of polarization shift keying transmission format in presence of polarization mode dispersion,” IEEElectron. Lett., vol. 36, no. 8, pp. 736-737, 2000.)  
         [0003]     MLSE has also been used to mitigate distortions due to narrow-band electrical filtering such as is typically used in receivers. (See, e.g., F. Buchali et al., “Correlation sensitive Viterbi equalization of 10 Gb/s signals in bandwidth limited receivers,” Proc. Opt. Fiber Commun. Conf. (OFC), OFO2, 2005; and H. F. Haunstein et al., “Optimized Filtering for Electronic Equalizers in the Presence of Chromatic Dispersion and PMD,” Proc. Opt. Fiber Commun. Conf. (OFC), MF63, 2003.)  
         [0004]     The performance of receiver systems employing MLSE may be improved under certain conditions by over-sampling the received data signal and applying the MLSE algorithm to the increased number of samples.  
       SUMMARY OF THE INVENTION  
       [0005]     In an exemplary embodiment, the present invention provides an optical communication receiver in which maximum likelihood sequence estimation (MLSE) is performed on an over-sampled received signal. In an aspect of the present invention, the optimal sample spacing is determined for various conditions. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  is a schematic representation of a model of an optical communication system with an exemplary embodiment of a receiver in accordance with the present invention.  
         [0007]      FIGS. 2A and 2B  are eye diagrams illustrating exemplary sampling instants for one and two samples per bit, respectively.  
         [0008]      FIGS. 3A and 3B  illustrate the performance of an exemplary receiver under different conditions for narrow and wide bandwidth electrical filtering, respectively. 
     
    
     DETAILED DESCRIPTION  
       [0009]      FIG. 1  is a schematic representation of a model of an exemplary optical communication system  100  with an exemplary embodiment of a receiver  120  in accordance with the present invention. The bit sequence to be transmitted, denoted â1, â2, . . . , ân, with âiε{ 0 ,  1 } and with time index i ε{ 1 , . . . , n}, is modulated by a modulator  110 . The modulator, may be, for example, a chirp-free Mach-Zehnder modulator producing a non-return-to-zero (NRZ) on-off-keying (OOK) signal. Other signal formats may also be used in conjunction with the present invention, including RZ and phase-modulated signals, among others.  
         [0010]     The model used to illustrate the present embodiment assumes, for the sake of simplicity, a linear channel, in which case amplified spontaneous emission (ASE) noise is added at  115  from in-line optical amplification in both polarization modes carried by a single-mode optical fiber. The present invention applies equally to other optical channel models, including non-linear optical channels. For each polarization independently, the ASE noise can be modeled as additive, white, circularly symmetric complex Gaussian noise (AWGN).  
         [0011]     At the receiver  120 , the noisy signal is filtered by an optical bandpass filter  125  of bandwidth B o , which can be implemented, for example, as a first or third-order Gaussian filter.  
         [0012]     As a measure of the performance of the receiver  120 , the optical-signal-to-noise ratio (OSNR) at the receiver  120  input that is required by the receiver for operation at a predefined bit error ratio (e.g., 10 −3 ) can be used. The OSNR is defined as P s /(2N ASE B ref ), where P s  is the optical signal power entering the receiver, N ASE  is the ASE power spectral density per polarization, B ref  is the reference bandwidth (e.g., 12.5 GHz), and the factor of 2 takes into account both ASE polarizations.  
         [0013]     After the filter  125 , the optical signal is provided to an optical-to-electrical converter  130 , such as, for example, a square-law photodetector or a coherent receiver. The resultant electrical signal is filtered by a low-pass filter  140  of bandwidth B e . The filter  140  can be implemented, for example, as a fifth-order Bessel filter.  
         [0014]     The filtered electrical signal is then sampled by a sampler  150  at the bit rate or at multiples thereof.  FIGS. 2A and 2B  show eye diagrams over one bit period T bit  of the electrical signal at the input to the sampler  150 . The maximum eye opening is designated by the line  250 . Although shown at approximately the middle of the bit period, the maximum eye opening  250  can occur at other instances within the bit period.  
         [0015]      FIG. 2A  shows the case of a single sample, designated r i , in which the sampling instant preferably substantially corresponds to the maximum eye opening  250  of each bit period.  
         [0016]      FIG. 2B  shows the two-sample case, in which the two sampling instants (yielding samples r i,a  and r i,b ) are displaced about the maximum eye opening  250  of each bit period. The first sampling instance is displaced by a time period  251  before the maximum eye opening  250  and the second sampling instance is displaced by a time period  252  after the maximum eye opening  250 . The time periods  251  and  252  may or may not be equal. In an exemplary embodiment, the maximum eye opening  250  is substantially at the middle of the bit period and the time periods  251  and  252  are substantially equal, each being 0-25% of the bit period T bit . Note that where the time periods  251 ,  252  are between 0% and 25% of the bit period T bit , this leads to an over-sampled MLSE receiver with unequal sample spacing. Note that in the presence of severe signal distortions, the eye may be fully closed. In that case, the samples are characterized by two not necessarily equal time periods denoting the spacing between two successive samples. Note also that the present invention may be generalized to more than two samples in a straightforward way.  
         [0017]     The samples are then processed by a receiver  160 . Where the optical-to-electrical converter  130  is a square law photodetector, the samples would represent the absolute magnitude of the optical field and would be real quantities. (As can be appreciated by one of ordinary skill in the art, where a coherent receiver is used to perform the optical to electrical conversion, a first detector yielding the in-phase (real) part, and a second detector yielding the quadrature (imaginary) part of the optical field could be used, in which case, the receiver  160  would process complex samples.)  
         [0018]     The receiver  160  comprises a maximum likelihood sequence estimation (MLSE) receiver, preferably a correlation-sensitive MLSE receiver. The detected data sequence, denoted ã 1 , ã 2 , . . . , ãn, should match the original transmitted bit sequence â 1 , â 2 , . . . , ân.  
         [0019]      FIGS. 3A and 3B  show the resulting OSNR performance of the correlation-sensitive MLSE receiver  160  with one and two samples per bit as a function of the bandwidth B o  of the optical filter  125  for electrical filter  140  bandwidths B e  of 0.75R bit  and 2R bit , respectively. Curves  300 ,  305 ,  310 ,  315 ,  320  and  325  are indicative of the performance of the correlation-sensitive MLSE receiver  160 . For comparison, the curves  330  represent the OSNR performance of a threshold receiver using a de Brujin bit sequence (DBBS) and the curves  340  represent the OSNR performance of such a threshold receiver for a single pulse.  
         [0020]     The curves  300 - 325  indicate the performance of the receiver  160  for various spacings of the samples from the maximum eye opening. Curve  300  corresponds to the case in which the samples are spaced 0% of the bit period from the maximum eye opening; in other words, the single-sample case illustrated in  FIG. 2A . Curve  305  represents the case in which the samples are spaced 5% of the bit period from the maximum eye-opening; i.e., with reference to  FIG. 3B , the time periods  251  and  252  are substantially equal to 5% of the bit period T bit , leading to 10%/90% sampling intervals. Analogously, curves  310 ,  315 ,  320  and  325  represent the 10%, 15%, 20% and 25% spacing cases, respectively. The 25% case represented by curve  325  corresponds to uniform sampling at twice the bit rate, i.e. 50%/50% sampling intervals.  
         [0021]     As shown in  FIGS. 3A and 3B , for large optical filter bandwidths B o , curves  305 - 325  are below curve  300  indicating that using two samples per bit, as opposed to one, improves the performance of the MLSE receiver (i.e., the receiver can provide the requisite bit error rate performance at a lower OSNR). As indicated by curves  305 - 325 , moving the two samples away from the maximum eye opening gradually improves performance, up to a point, as the second sample increasingly contains more new (uncorrelated) information. In other words, increasing the spacing between the two samples within a bit initially causes the samples to become less correlated, which improves MLSE performance.  
         [0022]     As shown by  FIGS. 3A and 3B , however, the optimum sampling spacing is not necessarily half the bit period (curves  325 ), where the samples are spaced apart the furthest (±25% from the maximum eye opening), and their correlation is minimum. This is because the “quality” of the individual samples degrades when the displacement from the optimum sampling instant at the maximum eye opening is increased beyond a certain point. This effect can be understood by considering the extreme case of a narrow return-to-zero (RZ) signal: sampling at ±25% of the bit period off the maximum eye opening would yield no useful data information at all, since all samples, even though largely uncorrelated, would be taken in between information-bearing signal pulses. This loss of useful information leads to a performance degradation of the MLSE receiver. The optimum spacing of the two sampling instants within a bit is thus given by trading off the gain due to reduced noise correlation between the samples with the loss due to reduced information content of the samples.  
         [0023]     As can be seen from the wide optical bandwidth regions of  FIGS. 3A and 3B , the performance of the MLSE receiver improves up to a sample spacing of approximately ±20% of the bit duration (curves  320 ), but degrades slightly for larger sample spacings of, e.g., ±25% (curves  325 ). Furthermore, as can be seen from  FIG. 3B , the MLSE receiver with over-sampling can perform better ( 310 - 325 ) than a conventional threshold receiver with a single pulse ( 340 ).  
         [0024]     With respect to the narrow optical bandwidth regions of  FIGS. 3A and 3B , the smaller the optical and/or electrical filter bandwidths, the higher the correlation will be between adjacent samples. Thus, the MLSE receiver gains less new information by over-sampling in this regime, and the second effect, pertaining to the reduced information quality of the two samples, becomes more prevalent. Therefore, for optical filter bandwidths below approximately 0.8 Rbit, the MLSE receiver with one sample/bit has a better OSNR performance ( 300 ) than the MLSE receiver with two samples/bit ( 305 - 325 ), and the performance of the MLSE receiver with two samples/bit degrades with increasing displacement of the two samples from the maximum eye opening.  
         [0025]     Note that while the above-described embodiments address degradations through narrow-band optical filtering, the present invention also applies to other signal degradations, such as chromatic dispersion, polarization mode dispersion, and others.  
         [0026]     It is understood that the above-described embodiments are illustrative of only a few of the possible specific embodiments which can represent applications of the present invention. Numerous and varied other arrangements can be made by those skilled in the art without departing from the spirit and scope of the invention.