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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 13, Iss. 2 — Jan. 24, 2005
  • pp: 371–376
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Performance comparison of optical 8-ary differential phase-shift keying systems with different electrical decision schemes

Hosung Yoon, Duckey Lee, and Namkyoo Park  »View Author Affiliations


Optics Express, Vol. 13, Issue 2, pp. 371-376 (2005)
http://dx.doi.org/10.1364/OPEX.13.000371


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Abstract

We examine the performance of optical 8-ary differential phase-shift keying transmission systems according to the type of receiver structure and modulation format. Compared with the approach based on a multilevel decision, we found that a bilevel receiver provides 3-dB gain in optical signal-to-noise ratio sensitivity and is more robust against chromatic dispersion for either nonreturn-to-zero or return-to-zero modulation.

© 2005 Optical Society of America

1. Introduction

Multilevel (ML) optical transmission systems have many critical advantages in the design of optical transmission links. With their smaller bandwidth, wavelength channels can be packed more closely within the limited transmission band. For the same reason, the ML optical transmission format enables much robust transmission of the signal against fiber penalties, such as chromatic dispersion or polarization mode dispersion [1

1. H. Kim and R.-J. Essiambre, “Transmission of 8x20 Gb/s DQPSK signals over 310-km SMF with 0.8-b/s/Hz spectral efficiency,” IEEE Photon. Technol. Lett. 15, 769–771 (2003). [CrossRef]

].

Among the different techniques to expand optical signals into the ML domain, phase-shift keying (PSK) has received much interest recently as the most promising candidate for the next-generation multilevel transmission format. Recent notable progress in the research of multilevel PSK includes the demonstration of optical 4-ary, differential quadrature PSK (DQPSK) [1

1. H. Kim and R.-J. Essiambre, “Transmission of 8x20 Gb/s DQPSK signals over 310-km SMF with 0.8-b/s/Hz spectral efficiency,” IEEE Photon. Technol. Lett. 15, 769–771 (2003). [CrossRef]

,2

2. R. A. Griffin, R. I. Johnstone, R. G. Walker, J. Hall, S. D. Wadsworth, K. Berry, A. C. Carter, M. J. Wale, J. Hughes, P. A. Jerram, and N. J. Parsons, “10 Gb/s optical differential quadrature phase shift key (DQPSK) transmission using GaAs/AlGaAs integration,” in Optical Fiber Communication, Vol. 70 of OSA Trends in Optics and Photonics Series (Optical Society of America, Washington, D.C., 2002), postdeadline paper FD6-1.

] and 8-ary (D8PSK) systems, by use of a combination of delay interferometers (DIs) with proper electrical processing either at the transmitter side [3

3. C. Kim and G. Li, “Direct-detection optical differential 8-level phase-shift keying (OD8PSK) for spectrally efficient transmission,” Opt. Express12, 3415–3421 (2004), http://www.opticsexpress.org. [CrossRef] [PubMed]

,5

5. M. Ohm, “Optical 8-DPSK and receiver with direct detection and multilevel electrical signals,” in IEEE/LEOS Workshop on Advanced Modulation Formats (Institute of Electrical and Electronics Engineers, New York, 2004), pp. 45–46 [CrossRef]

,6

6. Y. Han, C. Kim, and G. Li, “Simplified receiver implementation for optical differential 8-level phase-shift keying,” Electron. Lett. 40, 1372–1373 (2004). [CrossRef]

] or the receiver side [4

4. H. Yoon, D Lee, and N. Park, “Receiver sructure of generalized M-ary optical DPSK system and its semianalytical performance evaluation,” in Ninth Optoelectronics and Communications Conference and Third Conference on Optical Internet (IEEE Comunication Society, New York, 2004), paper 14C3-4.

].

2. Receiver structures

In principle, the M-ary optical DPSK signaling format has M phase codes that require M/2 phase thresholds. Figure 1(a) illustrates the direct-form implementation of such receiver hardware for an optical D8PSK system, constructed by use of as many (M/2) DIs in parallel with each DI operated at the required optical phase threshold. Even if the structure can be considered as a simple extension of an optical DQPSK receiver, the output electrical signals from the DIs now have four specific levels. Nevertheless, in the current implementation, we treat each decision variable as a BL signal and process the signal with a single threshold in its respective clock-and-data recovery (CDR) module. Note that this is possible because all the required phase thresholds are already realized in the optics of the receiver as shown in Fig. 2(a). The final step consists of recovering the original data by use of the decoding table that we propose in Table 1(a) [4

4. H. Yoon, D Lee, and N. Park, “Receiver sructure of generalized M-ary optical DPSK system and its semianalytical performance evaluation,” in Ninth Optoelectronics and Communications Conference and Third Conference on Optical Internet (IEEE Comunication Society, New York, 2004), paper 14C3-4.

], which is implemented in the logic circuitry following the CDR modules.

As an alternative approach that uses the minimum number of DIs, Fig. 1(b) shows a receiver structure that uses a ML electrical decision technique for the optical D8PSK system, similar to the quadrature receiver scheme commonly used in an electrical PSK system [7

7. B. Sklar, Digital Communications: Fundamentals and Applications (Prentice-Hall, Englewood Cliffs, N.J., 1988).

]. In this structure, the missing phase information from an insufficient number of DIs can be fulfilled in the electrical domain by increasing the number of electrical decision thresholds. Also shown in Fig. 2(b) and Table 1(b) are the associated constellation graph with detection

Table 1. Phase decoding tables of optical D8PSK for (a) bilevel (BL) and (b) multilevel (ML) receiver; a + or - mark represents the polarity of the decision variable, and the number of marks indicates the expected signal level

table-icon
View This Table

(optical and electrical) thresholds and a decoding table for the ML receiver, respectively.

Fig 1. Implementations of optical D8PSK receiver incorporating (a) bilevel electrical decision (BL D8PSK) and (b) multilevel electrical decision (ML D8PSK)
Fig 2. Gray-coded constellations of optical D8PSK for (a) bilevel (BL) and (b) multilevel (ML) receiver. Dotted lines are optical thresholds achieved by DI; dashed ones electrical thresholds by multilevel decision circuits of ML-receiver.

It should be mentioned that inevitable surplus sets exist that are not matched to the constellations of the optical D8PSK signals. Note that the number of all the possible decision sets is 16 (24 for a BL receiver and 42 for a ML receiver). Even though the redundant sets also represent symbol errors, they can be corrected to the nearest symbols in a maximum-likelihood manner [4

4. H. Yoon, D Lee, and N. Park, “Receiver sructure of generalized M-ary optical DPSK system and its semianalytical performance evaluation,” in Ninth Optoelectronics and Communications Conference and Third Conference on Optical Internet (IEEE Comunication Society, New York, 2004), paper 14C3-4.

,7

7. B. Sklar, Digital Communications: Fundamentals and Applications (Prentice-Hall, Englewood Cliffs, N.J., 1988).

]. Such a bit-error-rate (BER) enhancement technique is not included in the current analysis.

3. Analysis methodology

To estimate the performance of an optically preamplified M-ary optical DPSK receiver exactly, we utilized the well-known Karhunen–Loève series expansion technique formulated in the frequency domain [8

8. J. -S. Lee and C. -S. Shim, “Bit-error-rate analysis of optically preamplified receivers using an eigenfunction expansion method in optical frequency domain,” J. Lightwave Technol. 12, 1224–1229 (1994). [CrossRef]

]. To analyze the effect of intersymbol interference (ISI), we also used a semianalytical BER calculation [9

9. P. J. Winzer, S. Chandrasekhar, and H. Kim, “Impact of filtering on RZ-DPSK reception,” IEEE Photon. Technol. Lett. 15, 840–842 (2003). [CrossRef]

,10

10. J. Wang and J. M. Kahn, “Impact of chromatic and polarization-mode dispersions on DPSK systems using interferometric demodulation and direct detection,” J. Lightwave Technol. 22, 362–371 (2004). [CrossRef]

], including a pseudorandom bit sequence (PRBS) with a sufficient code length to take account of ISI caused by the symbol patterning effect.

The decision error rate of each DI obtained with the above step is utilized to calculate the system’s BER as follows:

BER[1r=1Nr(1ERr)]log2M
(1)

where ERr is the decision error rate of the rth DI, and Nr is the number of DIs in each receiver structure (M/2 for the BL receiver and 2 for the ML receiver).

Fig 3. BER vs. received OSNR in back-to-back transmission at a fixed bit rate of 40Gb/s. Lines are from KLSE with 26-1 PRBS for DQPSK and 29-1 PRBS for D8PSK; symbols from Monte-Carlo error counting method with 215-1 PRBS.

For approximation (1), it is obvious that approximations exist for mutual independence among the DIs in the receiver and Gray coding [11

11. G. Santella and F. Mazzenga, “A hybrid analytical-simulation procedure for performance evaluation in M-QAM-OFDM schemes in presence of nonlinear distortions,” IEEE Trans. Vehicular Technol. 47, 142–151 (1998). [CrossRef]

]. It can easily be shown that the approximations hold for all the practically meaningful BER levels (e.g., less than 10-3). In fact, the BER results obtained from the above method showed excellent agreement with the Monte Carlo error counting method to within 0.5 dB of the estimation error (Fig. 3).

4. Numerical results and discussion

Optical amplified spontaneous emission (ASE) from the preamplifier was modeled as additive white Gaussian noise. To characterize the optical noise, the OSNR was calculated with unpolarized ASE power within a reference bandwidth of 0.1 nm. To simulate the frequency response of an arrayed waveguide grating, we assumed a first-order Gaussian optical bandpass filter. Electrical noise from photodiodes was ignored; we focused instead on the ASE noise-limited systems. After we used balanced photodetectors, fifth-order Bessel electrical low-pass filters were used. For all the numerical analyses, electrical decision thresholds (in CDR modules) were optimized to minimize the BER.

To determine the optimum receiver design and to study the effects of filtering in the D8PSK receivers [9

9. P. J. Winzer, S. Chandrasekhar, and H. Kim, “Impact of filtering on RZ-DPSK reception,” IEEE Photon. Technol. Lett. 15, 840–842 (2003). [CrossRef]

] at the same time, we evaluated the OSNR penalty in a back-to-back condition as a function of optical (Bo) and electrical (Be) bandwidths for both ML and BL D8PSKs, with NRZ and RZ modulation formats (Fig. 4). The results are referenced to the minimum OSNR to achieve a 10 -12 BER for each format: 22.4, 25.4, 23.5, and 26.6 dB for RZ BL, RZ ML, NRZ BL, and NRZ ML D8PSK, respectively.

Figure 4 also shows that the NRZ D8PSK format suffers from suboptimum filtering more severely than RZ modulation. A comparison of the BL and ML receivers showed that the BL receiver was more resistant to filtering-induced ISI for both modulation formats.

It is important to note the existence of a 3-dB gain in OSNR sensitivity for BL D8PSK (compared with that for ML D8PSK). We attribute this observation to the difference in the distances between error-dominant levels. Note that, in Fig. 5, fp and fn govern the BER for ML D8PSK, and fg governs the BER for BL D8PSK. As the distance for ML D8PSK is 2 1/2 times smaller than that for BL D8PSK, the ML D8PSK signals require 3 dB more OSNR than the BL D8PSK signals to achieve the same BER. Note that OSNR ∝ Q2 ∝ |ded|2 [12

12. E. Desurvire, Erbium-Doped Fiber Amplifiers: Principles and Applications (Wiley, New York, 1994).

], where Q is the Q factor and ded is the distance between error-dominant levels. Here, we assume an ASE noise-limited condition with a fixed received optical power, ignoring the data dependency of the beat noise power that can be justified for M-ary DPSK with ordinary optical filtering [13

13. X. Zhang, G. Zhang, C. Xie, and L. Wang, “Noise statistics in optically preamplified differential phase-shift keying receivers with Mach-Zehnder interferometer demodulation,” Opt. Lett. 29, 337–339 (2004). [CrossRef] [PubMed]

]. Expressed in mathematical terms, we obtain

ΔOSNRdB=10log10{2sin(π8)[sin(3π8)sin(π8)]2}=3(dB)
(2)

in agreement with the observed OSNR gain.

Fig 4. OSNR penalty due to optical and electrical filtering for (a) NRZ ML D8PSK, (b) RZ ML D8PSK, (c) NRZ BL D8PSK, and (d) RZ BL D8PSK.

By comparing BL and ML receivers we observed that the BL receiver exhibited greater dispersion tolerance than the ML receiver: 1.05 and 1.3 times larger for RZ and NRZ D8PSK, respectively, with a 1-dB OSNR penalty. As inferred from Fig. 5, we attribute this to the relatively larger margin of the central eye around the electrical ground, which is the dominant factor on the performance of a BL receiver.

Fig 5. Eye diagram and decision error probability between adjacent levels as a function of decision thresholds. OSNR values are 23.5dB for NRZ D8PSK and 22.4dB for RZ D8PSK
Fig 6. OSNR penalty to acquire 10 -12 BER according to the accumulated chromatic dispersion

Summarizing the above analysis, we conclude that the BL receiver with RZ modulation is the best design choice over other D8PSK schemes in terms of its strong dispersion tolerance as well as its low OSNR sensitivity.

5. Conclusion

We investigated the performance of an 8-ary optical DPSK transmission system by comparing different receiver structures (bilevel versus multilevel) and signal modulation (RZ versus NRZ) formats. We proved the existence of a 3-dB gain in OSNR sensitivity of the BL D8PSK structure both numerically and analytically. In the future, we expect the RZ BL D8PSK system to be used in spectrally efficient high-capacity transmission systems.

References and links

1.

H. Kim and R.-J. Essiambre, “Transmission of 8x20 Gb/s DQPSK signals over 310-km SMF with 0.8-b/s/Hz spectral efficiency,” IEEE Photon. Technol. Lett. 15, 769–771 (2003). [CrossRef]

2.

R. A. Griffin, R. I. Johnstone, R. G. Walker, J. Hall, S. D. Wadsworth, K. Berry, A. C. Carter, M. J. Wale, J. Hughes, P. A. Jerram, and N. J. Parsons, “10 Gb/s optical differential quadrature phase shift key (DQPSK) transmission using GaAs/AlGaAs integration,” in Optical Fiber Communication, Vol. 70 of OSA Trends in Optics and Photonics Series (Optical Society of America, Washington, D.C., 2002), postdeadline paper FD6-1.

3.

C. Kim and G. Li, “Direct-detection optical differential 8-level phase-shift keying (OD8PSK) for spectrally efficient transmission,” Opt. Express12, 3415–3421 (2004), http://www.opticsexpress.org. [CrossRef] [PubMed]

4.

H. Yoon, D Lee, and N. Park, “Receiver sructure of generalized M-ary optical DPSK system and its semianalytical performance evaluation,” in Ninth Optoelectronics and Communications Conference and Third Conference on Optical Internet (IEEE Comunication Society, New York, 2004), paper 14C3-4.

5.

M. Ohm, “Optical 8-DPSK and receiver with direct detection and multilevel electrical signals,” in IEEE/LEOS Workshop on Advanced Modulation Formats (Institute of Electrical and Electronics Engineers, New York, 2004), pp. 45–46 [CrossRef]

6.

Y. Han, C. Kim, and G. Li, “Simplified receiver implementation for optical differential 8-level phase-shift keying,” Electron. Lett. 40, 1372–1373 (2004). [CrossRef]

7.

B. Sklar, Digital Communications: Fundamentals and Applications (Prentice-Hall, Englewood Cliffs, N.J., 1988).

8.

J. -S. Lee and C. -S. Shim, “Bit-error-rate analysis of optically preamplified receivers using an eigenfunction expansion method in optical frequency domain,” J. Lightwave Technol. 12, 1224–1229 (1994). [CrossRef]

9.

P. J. Winzer, S. Chandrasekhar, and H. Kim, “Impact of filtering on RZ-DPSK reception,” IEEE Photon. Technol. Lett. 15, 840–842 (2003). [CrossRef]

10.

J. Wang and J. M. Kahn, “Impact of chromatic and polarization-mode dispersions on DPSK systems using interferometric demodulation and direct detection,” J. Lightwave Technol. 22, 362–371 (2004). [CrossRef]

11.

G. Santella and F. Mazzenga, “A hybrid analytical-simulation procedure for performance evaluation in M-QAM-OFDM schemes in presence of nonlinear distortions,” IEEE Trans. Vehicular Technol. 47, 142–151 (1998). [CrossRef]

12.

E. Desurvire, Erbium-Doped Fiber Amplifiers: Principles and Applications (Wiley, New York, 1994).

13.

X. Zhang, G. Zhang, C. Xie, and L. Wang, “Noise statistics in optically preamplified differential phase-shift keying receivers with Mach-Zehnder interferometer demodulation,” Opt. Lett. 29, 337–339 (2004). [CrossRef] [PubMed]

OCIS Codes
(060.2330) Fiber optics and optical communications : Fiber optics communications
(060.2360) Fiber optics and optical communications : Fiber optics links and subsystems
(060.4080) Fiber optics and optical communications : Modulation
(060.4510) Fiber optics and optical communications : Optical communications
(060.5060) Fiber optics and optical communications : Phase modulation

ToC Category:
Research Papers

History
Original Manuscript: November 8, 2004
Revised Manuscript: December 30, 2004
Published: January 24, 2005

Citation
Hosung Yoon, Duckey Lee, and Namkyoo Park, "Performance comparison of optical 8-ary differential phase-shift keying systems with different electrical decision schemes," Opt. Express 13, 371-376 (2005)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-13-2-371


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References

  1. H. Kim and R.-J. Essiambre, �??Transmission of 8x20 Gb/s DQPSK signals over 310-km SMF with 0.8-b/s/Hz spectral efficiency,�?? IEEE Photon. Technol. Lett. 15, 769�??771 (2003). [CrossRef]
  2. R. A. Griffin, R. I. Johnstone, R. G. Walker, J. Hall, S. D. Wadsworth, K. Berry, A. C. Carter, M. J. Wale, J. Hughes, P. A. Jerram, and N. J. Parsons, �??10 Gb/s optical differential quadrature phase shift key (DQPSK) transmission using GaAs/AlGaAs integration,�?? in Optical Fiber Communication, Vol. 70 of OSA Trends in Optics and Photonics Series (Optical Society of America, Washington, D.C., 2002), postdeadline paper FD6-1.
  3. C. Kim and G. Li, �??Direct-detection optical differential 8-level phase-shift keying (OD8PSK) for spectrally efficient transmission,�?? Opt. Express 12, 3415�??3421 (2004), <a href="http://www.opticsexpress.org.">http://www.opticsexpress.org.</a> [CrossRef] [PubMed]
  4. H. Yoon, D Lee, and N. Park, �??Receiver structure of generalized M-ary optical DPSK system and its semi-analytical performance evaluation,�?? in Ninth Optoelectronics and Communications Conference and Third Conference on Optical Internet (IEEE Comunication Society, New York, 2004), paper 14C3-4.
  5. M. Ohm, �??Optical 8-DPSK and receiver with direct detection and multilevel electrical signals,�?? in IEEE/LEOS Workshop on Advanced Modulation Formats (Institute of Electrical and Electronics Engineers, New York, 2004), pp. 45�??46 [CrossRef]
  6. Y. Han, C. Kim, and G. Li, �??Simplified receiver implementation for optical differential 8-level phase-shift keying,�?? Electron. Lett. 40, 1372�??1373 (2004). [CrossRef]
  7. B. Sklar, Digital Communications: Fundamentals and Applications (Prentice-Hall, Englewood Cliffs, N.J., 1988).
  8. J. -S. Lee and C. -S. Shim, �??Bit-error-rate analysis of optically preamplified receivers using an eigenfunction expansion method in optical frequency domain,�?? J. Lightwave Technol. 12, 1224�??1229 (1994). [CrossRef]
  9. P. J. Winzer, S. Chandrasekhar, and H. Kim, �??Impact of filtering on RZ-DPSK reception,�?? IEEE Photon. Technol. Lett. 15, 840�??842 (2003). [CrossRef]
  10. J. Wang and J. M. Kahn, �??Impact of chromatic and polarization-mode dispersions on DPSK systems using interferometric demodulation and direct detection,�?? J. Lightwave Technol. 22, 362�??371 (2004). [CrossRef]
  11. G. Santella and F. Mazzenga, �??A hybrid analytical-simulation procedure for performance evaluation in M-QAM-OFDM schemes in presence of nonlinear distortions,�?? IEEE Trans. Vehicular Technol. 47, 142�??151 (1998). [CrossRef]
  12. E. Desurvire, Erbium-Doped Fiber Amplifiers: Principles and Applications (Wiley, New York, 1994).
  13. X. Zhang, G. Zhang, C. Xie, and L. Wang, �??Noise statistics in optically preamplified differential phase-shift keying receivers with Mach�??Zehnder interferometer demodulation,�?? Opt. Lett. 29, 337�??339 (2004). [CrossRef] [PubMed]

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