Abstract

We propose a series of modified methods covering timing and frequency synchronization in the coherent optical orthogonal frequency-division-multiplexing (CO-OFDM) system. The modified timing synchronization method eliminates the ambiguity of Park’s method, and the modified frequency synchronization method employs only one training symbol without any loss of accuracy compared to Schmidl’s method. The proposed synchronization scheme is tested in a 20Gb/s CO-OFDM simulated system, with the result that the modified synchronization scheme gives a more accurate timing estimation and a more economical frequency offset estimation with a wide estimation range in the relatively short transmission distance. Meanwhile, chromatic dispersion compensation added before the synchronization keeps the modified scheme’s performance stable over longer distances and makes better subcarrier recovery with the optical signal-to-noise ratio improved by 3 dB at the bit-error rate of 103.

© 2013 Optical Society of America

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References

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  1. W. Shieh, “Maximum-likelihood phase and channel estimation for coherent optical OFDM,” IEEE Photon. Technol. Lett., vol.  20, no. 8, pp. 605–607, Apr. 2008.
    [CrossRef]
  2. W. Shieh and C. Athaudage, “Coherent optical orthogonal frequency division multiplexing,” Electron. Lett., vol.  42, no. 10, pp. 587–589, May 2006.
    [CrossRef]
  3. T. Pollet, M. Van Bladel, and M. Moeneclaey, “BER sensitivity of OFDM systems to carrier frequency offset and Wiener phase noise,” IEEE Trans. Commun., vol.  43, no. 234, pp. 191–193, Feb./Mar./Apr. 1995.
    [CrossRef]
  4. W. Shieh, OFDM in Optical Communications. Burlington, MA: Academic, 2010, pp. 119–138.
  5. T. M. Schmidl and D. C. Cox, “Robust frequency and timing synchronization for OFDM,” IEEE Trans. Commun., vol.  45, no. 12, pp. 1613–1621, Dec. 1997.
    [CrossRef]
  6. H. Minn, V. K. Bhargava, and K. B. Letaief, “A robust timing and frequency synchronization for OFDM systems,” IEEE Trans. Wireless Commun., vol.  2, no. 4, pp. 822–839, July 2003.
    [CrossRef]
  7. B. Park, H. Cheon, and C. Kang, “A novel timing estimation method for OFDM systems,” IEEE Commun. Lett., vol.  7, no. 5, pp. 239–241, May 2003.
    [CrossRef]
  8. M. Morelli and U. Mengali, “An improved frequency offset estimator for OFDM applications,” IEEE Commun. Lett., vol.  3, no. 3, pp. 75–77, Mar. 1999.
    [CrossRef]
  9. W. Shieh, R. S. Tucker, and W. Chen, “Optical performance monitoring in coherent optical OFDM systems,” Opt. Express, vol.  15, no. 2, pp. 350–356, Jan. 2007.
    [CrossRef]
  10. L. Tombal, “On the effect of Wiener phase noise in OFDM systems,” IEEE Trans. Commun., vol.  46, no. 5, pp. 580–583, May 1998.
    [CrossRef]
  11. G. Ren, Y. Chang, and H. Zhang, “Synchronization method based on a new constant envelop preamble for OFDM systems,” IEEE Trans. Broadcast., vol.  51, no. 1, pp. 139–143, Mar. 2005.
    [CrossRef]
  12. S. J. Savory, “Digital filters for coherent optical receivers,” Opt. Express, vol.  16, no. 2, pp. 804–817, Jan. 2008.
    [CrossRef]

2008 (2)

W. Shieh, “Maximum-likelihood phase and channel estimation for coherent optical OFDM,” IEEE Photon. Technol. Lett., vol.  20, no. 8, pp. 605–607, Apr. 2008.
[CrossRef]

S. J. Savory, “Digital filters for coherent optical receivers,” Opt. Express, vol.  16, no. 2, pp. 804–817, Jan. 2008.
[CrossRef]

2007 (1)

2006 (1)

W. Shieh and C. Athaudage, “Coherent optical orthogonal frequency division multiplexing,” Electron. Lett., vol.  42, no. 10, pp. 587–589, May 2006.
[CrossRef]

2005 (1)

G. Ren, Y. Chang, and H. Zhang, “Synchronization method based on a new constant envelop preamble for OFDM systems,” IEEE Trans. Broadcast., vol.  51, no. 1, pp. 139–143, Mar. 2005.
[CrossRef]

2003 (2)

H. Minn, V. K. Bhargava, and K. B. Letaief, “A robust timing and frequency synchronization for OFDM systems,” IEEE Trans. Wireless Commun., vol.  2, no. 4, pp. 822–839, July 2003.
[CrossRef]

B. Park, H. Cheon, and C. Kang, “A novel timing estimation method for OFDM systems,” IEEE Commun. Lett., vol.  7, no. 5, pp. 239–241, May 2003.
[CrossRef]

1999 (1)

M. Morelli and U. Mengali, “An improved frequency offset estimator for OFDM applications,” IEEE Commun. Lett., vol.  3, no. 3, pp. 75–77, Mar. 1999.
[CrossRef]

1998 (1)

L. Tombal, “On the effect of Wiener phase noise in OFDM systems,” IEEE Trans. Commun., vol.  46, no. 5, pp. 580–583, May 1998.
[CrossRef]

1997 (1)

T. M. Schmidl and D. C. Cox, “Robust frequency and timing synchronization for OFDM,” IEEE Trans. Commun., vol.  45, no. 12, pp. 1613–1621, Dec. 1997.
[CrossRef]

1995 (1)

T. Pollet, M. Van Bladel, and M. Moeneclaey, “BER sensitivity of OFDM systems to carrier frequency offset and Wiener phase noise,” IEEE Trans. Commun., vol.  43, no. 234, pp. 191–193, Feb./Mar./Apr. 1995.
[CrossRef]

Athaudage, C.

W. Shieh and C. Athaudage, “Coherent optical orthogonal frequency division multiplexing,” Electron. Lett., vol.  42, no. 10, pp. 587–589, May 2006.
[CrossRef]

Bhargava, V. K.

H. Minn, V. K. Bhargava, and K. B. Letaief, “A robust timing and frequency synchronization for OFDM systems,” IEEE Trans. Wireless Commun., vol.  2, no. 4, pp. 822–839, July 2003.
[CrossRef]

Chang, Y.

G. Ren, Y. Chang, and H. Zhang, “Synchronization method based on a new constant envelop preamble for OFDM systems,” IEEE Trans. Broadcast., vol.  51, no. 1, pp. 139–143, Mar. 2005.
[CrossRef]

Chen, W.

Cheon, H.

B. Park, H. Cheon, and C. Kang, “A novel timing estimation method for OFDM systems,” IEEE Commun. Lett., vol.  7, no. 5, pp. 239–241, May 2003.
[CrossRef]

Cox, D. C.

T. M. Schmidl and D. C. Cox, “Robust frequency and timing synchronization for OFDM,” IEEE Trans. Commun., vol.  45, no. 12, pp. 1613–1621, Dec. 1997.
[CrossRef]

Kang, C.

B. Park, H. Cheon, and C. Kang, “A novel timing estimation method for OFDM systems,” IEEE Commun. Lett., vol.  7, no. 5, pp. 239–241, May 2003.
[CrossRef]

Letaief, K. B.

H. Minn, V. K. Bhargava, and K. B. Letaief, “A robust timing and frequency synchronization for OFDM systems,” IEEE Trans. Wireless Commun., vol.  2, no. 4, pp. 822–839, July 2003.
[CrossRef]

Mengali, U.

M. Morelli and U. Mengali, “An improved frequency offset estimator for OFDM applications,” IEEE Commun. Lett., vol.  3, no. 3, pp. 75–77, Mar. 1999.
[CrossRef]

Minn, H.

H. Minn, V. K. Bhargava, and K. B. Letaief, “A robust timing and frequency synchronization for OFDM systems,” IEEE Trans. Wireless Commun., vol.  2, no. 4, pp. 822–839, July 2003.
[CrossRef]

Moeneclaey, M.

T. Pollet, M. Van Bladel, and M. Moeneclaey, “BER sensitivity of OFDM systems to carrier frequency offset and Wiener phase noise,” IEEE Trans. Commun., vol.  43, no. 234, pp. 191–193, Feb./Mar./Apr. 1995.
[CrossRef]

Morelli, M.

M. Morelli and U. Mengali, “An improved frequency offset estimator for OFDM applications,” IEEE Commun. Lett., vol.  3, no. 3, pp. 75–77, Mar. 1999.
[CrossRef]

Park, B.

B. Park, H. Cheon, and C. Kang, “A novel timing estimation method for OFDM systems,” IEEE Commun. Lett., vol.  7, no. 5, pp. 239–241, May 2003.
[CrossRef]

Pollet, T.

T. Pollet, M. Van Bladel, and M. Moeneclaey, “BER sensitivity of OFDM systems to carrier frequency offset and Wiener phase noise,” IEEE Trans. Commun., vol.  43, no. 234, pp. 191–193, Feb./Mar./Apr. 1995.
[CrossRef]

Ren, G.

G. Ren, Y. Chang, and H. Zhang, “Synchronization method based on a new constant envelop preamble for OFDM systems,” IEEE Trans. Broadcast., vol.  51, no. 1, pp. 139–143, Mar. 2005.
[CrossRef]

Savory, S. J.

Schmidl, T. M.

T. M. Schmidl and D. C. Cox, “Robust frequency and timing synchronization for OFDM,” IEEE Trans. Commun., vol.  45, no. 12, pp. 1613–1621, Dec. 1997.
[CrossRef]

Shieh, W.

W. Shieh, “Maximum-likelihood phase and channel estimation for coherent optical OFDM,” IEEE Photon. Technol. Lett., vol.  20, no. 8, pp. 605–607, Apr. 2008.
[CrossRef]

W. Shieh, R. S. Tucker, and W. Chen, “Optical performance monitoring in coherent optical OFDM systems,” Opt. Express, vol.  15, no. 2, pp. 350–356, Jan. 2007.
[CrossRef]

W. Shieh and C. Athaudage, “Coherent optical orthogonal frequency division multiplexing,” Electron. Lett., vol.  42, no. 10, pp. 587–589, May 2006.
[CrossRef]

W. Shieh, OFDM in Optical Communications. Burlington, MA: Academic, 2010, pp. 119–138.

Tombal, L.

L. Tombal, “On the effect of Wiener phase noise in OFDM systems,” IEEE Trans. Commun., vol.  46, no. 5, pp. 580–583, May 1998.
[CrossRef]

Tucker, R. S.

Van Bladel, M.

T. Pollet, M. Van Bladel, and M. Moeneclaey, “BER sensitivity of OFDM systems to carrier frequency offset and Wiener phase noise,” IEEE Trans. Commun., vol.  43, no. 234, pp. 191–193, Feb./Mar./Apr. 1995.
[CrossRef]

Zhang, H.

G. Ren, Y. Chang, and H. Zhang, “Synchronization method based on a new constant envelop preamble for OFDM systems,” IEEE Trans. Broadcast., vol.  51, no. 1, pp. 139–143, Mar. 2005.
[CrossRef]

Electron. Lett. (1)

W. Shieh and C. Athaudage, “Coherent optical orthogonal frequency division multiplexing,” Electron. Lett., vol.  42, no. 10, pp. 587–589, May 2006.
[CrossRef]

IEEE Commun. Lett. (2)

B. Park, H. Cheon, and C. Kang, “A novel timing estimation method for OFDM systems,” IEEE Commun. Lett., vol.  7, no. 5, pp. 239–241, May 2003.
[CrossRef]

M. Morelli and U. Mengali, “An improved frequency offset estimator for OFDM applications,” IEEE Commun. Lett., vol.  3, no. 3, pp. 75–77, Mar. 1999.
[CrossRef]

IEEE Photon. Technol. Lett. (1)

W. Shieh, “Maximum-likelihood phase and channel estimation for coherent optical OFDM,” IEEE Photon. Technol. Lett., vol.  20, no. 8, pp. 605–607, Apr. 2008.
[CrossRef]

IEEE Trans. Broadcast. (1)

G. Ren, Y. Chang, and H. Zhang, “Synchronization method based on a new constant envelop preamble for OFDM systems,” IEEE Trans. Broadcast., vol.  51, no. 1, pp. 139–143, Mar. 2005.
[CrossRef]

IEEE Trans. Commun. (3)

L. Tombal, “On the effect of Wiener phase noise in OFDM systems,” IEEE Trans. Commun., vol.  46, no. 5, pp. 580–583, May 1998.
[CrossRef]

T. Pollet, M. Van Bladel, and M. Moeneclaey, “BER sensitivity of OFDM systems to carrier frequency offset and Wiener phase noise,” IEEE Trans. Commun., vol.  43, no. 234, pp. 191–193, Feb./Mar./Apr. 1995.
[CrossRef]

T. M. Schmidl and D. C. Cox, “Robust frequency and timing synchronization for OFDM,” IEEE Trans. Commun., vol.  45, no. 12, pp. 1613–1621, Dec. 1997.
[CrossRef]

IEEE Trans. Wireless Commun. (1)

H. Minn, V. K. Bhargava, and K. B. Letaief, “A robust timing and frequency synchronization for OFDM systems,” IEEE Trans. Wireless Commun., vol.  2, no. 4, pp. 822–839, July 2003.
[CrossRef]

Opt. Express (2)

Other (1)

W. Shieh, OFDM in Optical Communications. Burlington, MA: Academic, 2010, pp. 119–138.

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Figures (9)

Fig. 1.
Fig. 1.

Conceptual diagram for a complete CO-OFDM system. S/P, serial-to-parallel; GI, guard interval insertion; D/A, digital-to-analogue; (I)DFT, (inverse) discrete Fourier transform; LPF, low-pass filter; BPF, bandpass filter; OBPF, optical bandpass filter; CD, chromatic dispersion.

Fig. 2.
Fig. 2.

(a) Timing metrics of the four methods in different CD conditions (CD=0). (b) Timing metrics of the four methods in different CD conditions (CD=3400ps/nm). (c) Timing metrics of the four methods in different CD conditions (CD=6800ps/nm).

Fig. 3.
Fig. 3.

(a) MSE of timing offset versus CD for the four methods (Gcp=1/8). (b) MSE of timing offset versus CD for the four methods (Gcp=1/4).

Fig. 4.
Fig. 4.

Average estimated value versus normalized frequency offset for the three methods (all operated with one training symbol).

Fig. 5.
Fig. 5.

MSE of estimated frequency offset versus both frequency offset and CD for the modified method.

Fig. 6.
Fig. 6.

(a) MSE of timing metric versus OSNR for the four methods (Gcp=1/8). (b) MSE of timing metric versus OSNR for the four methods (Gcp=1/4).

Fig. 7.
Fig. 7.

MSE of frequency offset estimation for the three methods.

Fig. 8.
Fig. 8.

(a) PA+MLDF phase estimation without CD compensation (OSNR=25dB). (b) PA+MLDF phase estimation with CD compensation (OSNR=25dB).

Fig. 9.
Fig. 9.

BER sensitivity versus OSNR for PA+MLDF phase and channel estimation with and without CD compensation and back to back.

Tables (3)

Tables Icon

TABLE I Comparison of Four Timing Methods

Tables Icon

TABLE II Comparison of Three Frequency Offset Estimation Methods

Tables Icon

TABLE III Comparison of Two Subcarrier Recovery Schemes

Equations (26)

Equations on this page are rendered with MathJax. Learn more.

sm=k=0N1ckej2πkm/N,
s(t)=i=+k=1Nckisk(tiTs),
r(t)=ej(2πfoff·t+ϕ(t))s(t)h(t)+N(t)=ej(2πfoff·t+ϕ(t))r0(t)+N(t),
foff=frft.
ϕ(t)=ϕr(t)ϕt(t),
cki=ejϕiHkicki+ςki,
ςki=εki+nki.
Hki=ejϕkiDϕkiD=π·c·Dt·fk2fL2,
Spa=[ApaBpaApa*Bpa*],
Smodi(k)=Spa(k)p(k),k=1,2,,N1,N.
r(k)=r(k·ts/N)=ej(2πfoffk·ts/N+ϕ(k))r0(k)+N(k).
Mmodi(d)=|Pmodi(d)|2(Rmodi(d))2,
Pmodi(d)=k=0N/21r(N/2+dk)·r(N/2+d+k)·p(N/2+1k)·p(N/2+1+k)
Rmodi(d)=k=1N/2|r(d+k)|2.
MSE_timing=E[(d^d)2]=1NN(d^Nd)2.
ϕ=πfoffΔf,
ϕ^=angle[Pfreq(d^)],
Pfreq(d^)=k=0N/21r*(d^+k)·r(d^+k+N/2)=k=0N/21|r(d^+k)|2ejπfoffΔf+o(n).
f^off=ϕ^πΔf.
ϕ^πΔf+2m·Δf,
B(m)=|kXxk+2m·ck*|2(kX|ck|2)2,X={0,2,,N4,N2}.
f^off=ϕ^πΔf+2m^·Δf.
MSE_freq_norm=E[(f^offfoffΔf)2]=1NN(f^off_NfoffΔf)2.
cki=ejϕiHckik+ςki.
ϕi=arg(kckiHk*cki*δk2),
Hk=icki·cki*·ejϕii|cki|2.