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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 8 — Apr. 16, 2007
  • pp: 4410–4418
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Transmission simulation of coherent optical OFDM signals in WDM systems

Hongchun Bao and William Shieh  »View Author Affiliations


Optics Express, Vol. 15, Issue 8, pp. 4410-4418 (2007)
http://dx.doi.org/10.1364/OE.15.004410


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Abstract

In this letter, we first present the theoretical basis for coherent optical OFDM systems in direct up/down conversion architecture. We then demonstrate the transmission performance through simulation for WDM systems with coherent optical OFDM (CO-OFDM) including the fiber nonlinearity effect. The results show that the system Q of the WDM channels at 10 Gb/s is over 13.0 dB for a transmission up to 4800 km of standard-single-mode-fiber (SSMF) without dispersion compensation. A novel technique of partial carrier filling (PCF) for improving the non-linearity performance of the transmission is also presented. The system Q of the WDM channels with a filling factor of 50 % at 10 Gb/s is improved from 15.1 dB to 16.8 dB for a transmission up to 3200 km of SSMF without dispersion compensation.

© 2007 Optical Society of America

1. Introduction

Orthogonal frequency division multiplexing (OFDM) has been widely employed into numerous digital standards for broad-range of applications such as digital audio/video broadcasting and wireline/wireless communication systems [1

1. S. Hara and R. Prasad, Multicarrier Techniques for 4G Mobile Commmications, (Artech House, Boston, 2003).

]. Recently it has been shown that OFDM can be applied in optical long haul transmission systems and had many advantages over conventional single-carrier modulation format [2–4

2. W. Shieh and C. Athaudage, “Coherent optical orthogonal frequency division multiplexing,” Electron. Lett. 42, 587–588 (2006). [CrossRef]

]. Many key merits of the OFDM techniques have been studied and proven in the communications industry. Firstly, the frequency spectra of OFDM subcarriers are partially overlapped, resulting in high spectral efficiency. Secondly, the channel dispersion of the transmission system is easily estimated and removed, and thirdly, the signal processing in the OFDM transceiver can take advantage of the efficient algorithm of FFT/IFFT with low computation complexity. Recently, an equivalent optical-domain multi-carrier format, called coherent optical OFDM (CO-OFDM) has been proposed for long haul transmission [2

2. W. Shieh and C. Athaudage, “Coherent optical orthogonal frequency division multiplexing,” Electron. Lett. 42, 587–588 (2006). [CrossRef]

]. In the mean time, incoherent optical OFDM (IO-OFDM) has also been proposed independently, and has been shown to have similar dispersion tolerance with a much simpler detection scheme [3

3. J. Lowery, L. Du, and J. Armstrong, “Orthogonal frequency division multiplexing for adaptive dispersion compensation in long haul WDM systems,” in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, Technical Digest, (Anaheim, CA, USA, 2006), Paper PDP39.

]. However, the CO-OFDM is superior to IO-OFDM in spectral efficiency, OSNR requirement, and PMD insensitivity. It is well-known that OFDM is generally susceptible to nonlinearity and phase noise owing to high peak to average power ratio (PAPR) [1

1. S. Hara and R. Prasad, Multicarrier Techniques for 4G Mobile Commmications, (Artech House, Boston, 2003).

]. Therefore it is critical to investigate and improve the CO-OFDM system transmission performance including fiber nonlinearity, in order to ascertain its suitability for optical transmission. In this letter, we intend to answer two important questions for CO-OFDM WDM system, (i) what is the achievable system Q value ?, and (ii) what is the optimal launch power at various transmission distances ?. We first present the theoretical basis for coherent optical OFDM systems in direct up/down conversion architecture. We then demonstrate the transmission performance through simulation for WDM systems with coherent optical OFDM (CO-OFDM) including the fiber nonlinearity effect. The results show that the system Q of the WDM channels at 10 Gb/s is over 13.0 dB for a transmission up to 4800 km of standard-single-mode-fiber (SSMF) without dispersion compensation. A novel technique of partial carrier filling (PCF) for improving the non-linearity performance of the transmission is also presented. The system Q of the WDM channels with a filling factor of 50 % at 10 Gb/s is improved from 15.1 dB to 16.8 dB for a transmission up to 3200 km of SSMF without dispersion compensation.

2. WDM transmission systems with CO-OFDM

The basic WDM CO-OFDM transmission system is shown in Fig. 1. A generic CO-OFDM system consists of an OFDM transmitter, an optical link, and an OFDM receiver. Inside the OFDM transmitter, the input data bits are mapped onto corresponding information symbols of the subcarriers within one OFDM symbol, and the digital time domain signal s(t) is obtained by using IFFT [1–2

1. S. Hara and R. Prasad, Multicarrier Techniques for 4G Mobile Commmications, (Artech House, Boston, 2003).

]:

s(t)=i=+k=1k=Nsccik(tiTs)exp(j2πfk(tiTs)
(1)
fk=k1ts
(2)
(t)={1,(ΔG<tts)0,(tΔG,t>ts)
(3)
Fig. 1. Conceptual diagram of a CO-OFDM system

s(t)=i=+k=Nsc2+1k=Nsc2cik(tiTs)exp(j2πfk(tiTs)
(4)

The digital time domain signal s(t) is then inserted with a guard interval and subsequently converted into real time waveform through digital-to-analogue converter (DAC) [1

1. S. Hara and R. Prasad, Multicarrier Techniques for 4G Mobile Commmications, (Artech House, Boston, 2003).

]. The guard interval is to eliminate the inter-symbol interference (ISI) and its interval length ΔG should satisfy the condition:

ΔGCDtNscf2ts
(5)

3. Transmission performance

A Monte Carlo simulation is conducted to identify the transmission performance of a CO-OFDM system. The OFDM parameters are OFDM symbol period of 25.6 ns, 128 subcarriers, a guard interval equal to one quarter of the observation period, QPSK encoding for each subcarrier. We apply commonly used system parameters for our simulation: WDM channel spacing of 50 GHz, 80 km span distance, fiber chromatic dispersion of 16 ps/nm/km, 0.2 dB/km loss, and a nonlinear coefficient of 2.6×10-20 m2/W. The fiber span loss is compensated by an EDFA with a gain of 16 dB and noise figure of 6 dB. The linewidth of the LD1 and LD2 are assumed to be 100 kHz, which is close to the value achieved with commercially available semiconductor lasers [6–7

6. E. Ip, J. P. Kahn, D. Anthon, and J. Hutchins, “Linewidth measurements of MEMS-based tunable lasers for phase-locking applications,” IEEE Phot. Technol. Lett. 17, 2029–2031 (2005). [CrossRef]

]. A commercial Intel laser with a similar linewidth has been recently used for coherent experiment [8

8. G. Charlet, N. Maaref, J. Renaudier, H. Mardoyan, P. Tran, and S. Bigo, “Transmission of 40Gb/s QPSK with coherent detection over ultra long haul distance improved by nonlinearity mitigation,” in Tech. Dig., ECOC’2006 (Cannes, France, 2006), post-deadline paper, Th.4.3.6.

]. In this simulation, we choose optical filters, both OBPF1 and OBPF2, as easily available second-order Gaussian filters with 40 GHz bandwidth. The simulation shows the result of transmission of 8 WDM channels with the middle (the 4th) channel detected. Simulation of larger WDM channel number (>l8) gives almost the same result. Although the traditional DFB lasers have a linewidth of about 1 MHz and will not be fit for CO-OFDM transmission, we believe that this is not a major issue because the trend is that the tunable lasers with narrow linewidth will gradually replace the traditional single-wavelength DFB laser in the market, and massive production of tunable lasers such as Intel lasers has already driven the cost down to the level of telecomm usage. In the mean time, we are also working on the algorithms to further improve the linewidth tolerance for CO-OFDM, and will report the findings in further publication.

Fig. 2. Optical Spectra for 10 Gbit/s CO-OFDM, optical duobinary and conventional IM signal with the same average power

Figure 2 shows the optical spectrum of an OFDM signal. For reference, the spectra of conventional intensity modulation (IM) signal and the optical duobinary signal are shown with the same average power. The 20 dB bandwidth of OFDM signal is around 6.8 GHz in contrast with 18 GHz for conventional IM, and 12 GH for optical duobinary signal. This signifies that CO-OFDM has potential to achieve high spectral efficiency.

Fig. 3. Constellation of received data
Fig. 4. Constellation of received data after removing chromatic dispersion. Stars show the average of each OFDM symbol

Figure 3 shows the received constellations of 51 OFDM symbols after transmitting 3200 km fiber. The constellation is rotated with respect to each other owing to a phase shift from fiber chromatic dispersion [2–3

2. W. Shieh and C. Athaudage, “Coherent optical orthogonal frequency division multiplexing,” Electron. Lett. 42, 587–588 (2006). [CrossRef]

]. The phase shift due to chromatic dispersion is

ϕ=12β2ω2L
(6)
β2=λ22πcD
(7)

Where β2 is group velocity dispersion parameter, D is fiber dispersion parameter, L is the fiber length, ω is the optical frequency at each subcarriers. The phase shift can be estimated by using training sequences and compensated. The constellations of 51 OFDM symbols after removing chromatic dispersion is shown in Fig. 4, where the stars show the average of each OFDM symbol. The constellation of received OFDM information symbol drifts between each OFDM symbol showing smearing of constellation, and this is due to the phase noise of the transmitter and receiver lasers.

Fig. 5. OFDM symbol phase evolution
Fig. 6. Constellations of received data after removing chromatic dispersion and average phase noise of one OFDM symbol

Figure 5 shows the 51 OFDM symbol phase after transmitting 3200 km fiber. The OFDM symbol phase ϕi is estimated by averaging over the phases of 128 subcarriers given by

ϕi=0.25mod(4arg(Cik),2π)
(8)

q=(1NNSCi=1Nk=1NSCCikCi,AVG2Ci,AVG2)1
(9)
Ci,AVG=Cikk
(10)

where Cik' is the received information symbol for the kth subcarrier in the ith OFDM symbol, Ci,AVG' is average of received information symbol, N and NSC are the number of OFDM symbols and subcarriers in each OFDM symbol respectively. The bit error rate of 0.5×erc(q/2) matches with our numerical simulation results. 510 OFDM symbols are simulated equivalent to 510×128 bits for each Q computation. We find that the simulated Q value is very stable for different pseudorandom data and different random phase noise of laser.

Fig. 7. System Q versus the optical power of each WDM channel

Figure 7 shows the system Q of the received data versus the optical launch power of each WDM for different fiber lengths. It can be seen from Fig. 7 that the optimal optical launch power of each WDM channel is from -10 dBm to – 8 dBm.

Fig. 8 Optimal optical power of each WDM channel and maximum system Q versus fiber transmission distance

4. Partial carrier filling (PCF) for improvement of fiber nonlinearity performance

Fig. 9 (a) Original OFDM symbol. (b) OFDM symbol after filling zeros

The impact of system nonlinearity on the OFDM symbol could be moderated by partially filling the OFDM spectrum, or in practice, assigning redundant zero values to certain OFDM subcarrier as shown in Fig. 9. We define a filling factor (FF) as the number of effective subcarriers (with data) divided by the total number of the subcarriers in each OFDM symbol. Fig. 9 shows the case of a filling factor of ½. As shown in Fig. 9, there are zero subcarriers between every two data subcarriers. A large proportion of the spurious components generated by fiber nonlinearity ‒ four wave mixing will be located in the unfilled (zero) subcarriers, which has no impact on the filled (data) subcarriers. Therefore inter-subcarrier and inter-channel cross talk due to fiber nonlinearity are reduced by partial carrier filling. This scheme takes advantage of the powerful signal processing capability of the OFDM and would be difficult to implement in single-carrier system, where a large number of very fine electrical filters have to be employed for the same purpose.

Fig. 10. System Q versus the optical power of each WDM channel with and without partial filling

Fig. 11. Maximum system Q for different filling factors

Figure 11 shows the maximum system Q at different filling factors. As the filling factor increases, the maximum system Q increases. Increase of filling factor also leads the increase of electrical and optical bandwidth of transmitting data. Figure 12 shows the optical spectrum of an OFDM signal with a filling factor of ½. For reference, the spectra of an OFDM signal without filling, conventional intensity modulation (IM) signal and the optical duobinary signal are shown with the same average power. Although partial carrier filling broadens the bandwidth of an OFDM signal, due to its high spectral efficiency, the 20 dB bandwidth of OFDM signal with a filling factor of ½ is around 13 GHz, which is still comparable with conventional IM of 18 GHz, and optical duobinary signal of 12 GHz.

Fig. 12. Optical Spectra for 10 Gbit/s CO-OFDM of filling zero factor ½, CO-OFDM without filling zeros, optical duobinary and conventional IM signal with the same average power

5. Conclusions

References and links

1.

S. Hara and R. Prasad, Multicarrier Techniques for 4G Mobile Commmications, (Artech House, Boston, 2003).

2.

W. Shieh and C. Athaudage, “Coherent optical orthogonal frequency division multiplexing,” Electron. Lett. 42, 587–588 (2006). [CrossRef]

3.

J. Lowery, L. Du, and J. Armstrong, “Orthogonal frequency division multiplexing for adaptive dispersion compensation in long haul WDM systems,” in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, Technical Digest, (Anaheim, CA, USA, 2006), Paper PDP39.

4.

I. B. Djordjevic and B. Vasic, “Orthogonal frequency division multiplexing for high-speed optical transmission,” Opt. Express 4, 3767–3775 (2006). [CrossRef]

5.

“Supplement to IEEE standard for information technology telecommunications and information exchange between systems - local and metropolitan area networks - specific requirements. Part 11: wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: high-speed physical layer in the 5 GHz band,” in IEEE Std 802.11a-1999, (1999)

6.

E. Ip, J. P. Kahn, D. Anthon, and J. Hutchins, “Linewidth measurements of MEMS-based tunable lasers for phase-locking applications,” IEEE Phot. Technol. Lett. 17, 2029–2031 (2005). [CrossRef]

7.

J. D. Berger, D. Anthon, S. Dutta, F. Ilkov, and I. -F. Wu, “Tunable MEMS Devices for Reconfigurable Optical Networks,” in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, Technical Digest (CD) (Optical Society of America, 2005), paper OThD1. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2005-OThD1.

8.

G. Charlet, N. Maaref, J. Renaudier, H. Mardoyan, P. Tran, and S. Bigo, “Transmission of 40Gb/s QPSK with coherent detection over ultra long haul distance improved by nonlinearity mitigation,” in Tech. Dig., ECOC’2006 (Cannes, France, 2006), post-deadline paper, Th.4.3.6.

9.

N. S. Bergano, “Wavelength division multiplexing in long-haul transoceanic transmission systems,” J. Lightwave Technol. 23, 4125–4139 (2005). [CrossRef]

OCIS Codes
(060.1660) Fiber optics and optical communications : Coherent communications
(060.4080) Fiber optics and optical communications : Modulation

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: January 17, 2007
Revised Manuscript: March 26, 2007
Manuscript Accepted: March 26, 2007
Published: April 3, 2007

Citation
Hongchun Bao and William Shieh, "Transmission simulation of coherent optical OFDM signals in WDM systems," Opt. Express 15, 4410-4418 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-8-4410


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References

  1. S. Hara and R. Prasad, Multicarrier Techniques for 4G Mobile Commmications, (Artech House, Boston, 2003).
  2. W. Shieh and C. Athaudage, "Coherent optical orthogonal frequency division multiplexing," Electron. Lett. 42,587 - 588 (2006). [CrossRef]
  3. J. Lowery, L. Du, and J. Armstrong, "Orthogonal frequency division multiplexing for adaptive dispersion compensation in long haul WDM systems," in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, Technical Digest, (Anaheim, CA, USA, 2006), Paper PDP39.
  4. I. B. Djordjevic and B. Vasic, "Orthogonal frequency division multiplexing for high-speed optical transmission," Opt. Express 4,3767-3775 (2006). [CrossRef]
  5. "Supplement to IEEE standard for information technology telecommunications and information exchange between systems - local and metropolitan area networks - specific requirements. Part 11: wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: high-speed physical layer in the 5 GHz band," in IEEE Std 802.11a-1999, (1999)
  6. E. Ip, J. P. Kahn, D. Anthon and J. Hutchins, "Linewidth measurements of MEMS-based tunable lasers for phase-locking applications," IEEE Phot. Technol. Lett. 17,2029 - 2031 (2005). [CrossRef]
  7. J. D. Berger, D. Anthon, S. Dutta, F. Ilkov, and I. -F. Wu, "Tunable MEMS Devices for Reconfigurable Optical Networks," in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, Technical Digest (CD) (Optical Society of America, 2005), paper OThD1. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2005-OThD1.
  8. G. Charlet, N. Maaref, J. Renaudier, H. Mardoyan, P. Tran, S. Bigo, "Transmission of 40Gb/s QPSK with coherent detection over ultra long haul distance improved by nonlinearity mitigation," in Tech. Dig., ECOC’2006 (Cannes, France, 2006), post-deadline paper, Th.4.3.6.
  9. N. S. Bergano, "Wavelength division multiplexing in long-haul transoceanic transmission systems," J. Lightwave Technol. 23,4125-4139 (2005). [CrossRef]

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