## Offset-QAM based coherent WDM for spectral efficiency enhancement |

Optics Express, Vol. 19, Issue 15, pp. 14617-14631 (2011)

http://dx.doi.org/10.1364/OE.19.014617

Acrobat PDF (1304 KB)

### Abstract

Optically multiplexed multi-carrier systems with channel spacing reduced to the symbol rate per carrier are highly susceptible to inter-channel crosstalk, which places stringent requirements for the specifications of system components and hinders the use of high-level formats. In this paper, we investigate the performance benefits of using offset 4-, 16-, and 64-quadrature amplitude modulation (QAM) in coherent wavelength division multiplexing (CoWDM). We compare this system with recently reported Nyquist WDM and no-guard-interval optical coherent orthogonal frequency division multiplexing, and show that the presented system greatly relaxes the requirements for device specifications and enhances the spectral efficiency by enabling the use of high-level QAM. The achieved performance can approach the theoretical limits using practical components.

© 2011 OSA

## 1. Introduction

1. X. Zhou, J. Yu, M. F. Huang, Y. Shao, T. Wang, L. Nelson, P. Magill, M. Birk, P. I. Borel, D. W. Peckham, and R. Lingle, “64Tb/s (640×107Gb/s) PDM-36QAM transmission over 320km using both pre- and post-transmission digital equalization,” *Optical Fiber Communication Conference* (2010), paper PDPB9.

2. A. Sano, E. Yamada, H. Masuda, E. Yamazaki, T. Kobayashi, E. Yoshida, Y. Miyamoto, R. Kudo, K. Ishihara, and Y. Takatori, “No-guard-interval coherent optical OFDM for 100Gb/s long-haul WDM transmission,” J. Lightwave Technol. **27**(16), 3705–3713 (2009). [CrossRef]

5. G. Bosco, A. Carena, V. Curri, P. Poggiolini, and F. Forghieri, “Performance limits of Nyquist-WDM and Co-OFDM in high-speed PM-QPSK systems,” IEEE Photon. Technol. Lett. **22**(15), 1129–1131 (2010). [CrossRef]

6. A. D. Ellis and F. C. G. Gunning, “Spectral density enhancement using coherent WDM,” IEEE Photon. Technol. Lett. **17**(2), 504–506 (2005). [CrossRef]

*J*is small (e.g.

*J*= 2) and the occupied bandwidth is ~(

*J*+ 1)/

*T*, where

*T*is the symbol period, resulting in redundancy in the spectral usage [8

8. S. Yamamoto, K. Yonenaga, A. Sahara, F. Inuzuka, and A. Takada, “Achievement of sub-channel frequency spacing less than symbol rate and improvement of dispersion tolerance in optical OFDM transmission,” J. Lightwave Technol. **28**(1), 157–163 (2010). [CrossRef]

9. Y. Cai, J. X. Cai, C. R. Davidson, D. Foursa, A. Lucero, O. Sinkin, A. Pilipetskii, G. Mohs, and S. N. Bergono, “High spectral efficiency long-haul transmission with pre-filtering and maximum a posteriori probability detection,” Proc. *European Conference on Optical Communication* (2010), paper We.7.C.4.

12. S. B. Weinstein and P. M. Ebert, “Data transmission by frequency division multiplexing using the discrete Fourier transform,” IEEE Trans. Commun. Technol. Com. **19**(5), 628–634 (1971). [CrossRef]

3. S. Chandrasekhar and X. Liu, “Experimental investigation on the performance of closely spaced multi-carrier PDM-QPSK with digital coherent detection,” Opt. Express **17**(24), 21350–21361 (2009). [CrossRef] [PubMed]

5. G. Bosco, A. Carena, V. Curri, P. Poggiolini, and F. Forghieri, “Performance limits of Nyquist-WDM and Co-OFDM in high-speed PM-QPSK systems,” IEEE Photon. Technol. Lett. **22**(15), 1129–1131 (2010). [CrossRef]

5. G. Bosco, A. Carena, V. Curri, P. Poggiolini, and F. Forghieri, “Performance limits of Nyquist-WDM and Co-OFDM in high-speed PM-QPSK systems,” IEEE Photon. Technol. Lett. **22**(15), 1129–1131 (2010). [CrossRef]

13. J. Zhao and A. D. Ellis, “Electronic impairment mitigation in optically multiplexed multi-carrier systems,” J. Lightwave Technol. **29**(3), 278–290 (2011). [CrossRef]

14. G. Gavioli, E. Torrengo, G. Bosco, A. Carena, V. Curri, V. Miot, P. Poggiolini, M. Belmonte, F. Forghieri, C. Muzio, S. Piciaccia, A. Brinciotti, A. L. Porta, C. Lezzi, S. Savory, and S. Abrate, “Investigation of the impact of ultra-narrow carrier spacing on the transmission of a 10-carrier 1Tb/s superchannel,” *Optical Fiber Communication Conference* (2010), paper OThD3.

15. D. Hillerkuss, T. Schellinger, R. Schmogrow, M. Winter, T. Vallaitis, R. Bonk, A. Marculescu, J. Li, M. Dreschmann, J. Meyer, S. Ben Ezra, N. Narkiss, B. Nebendahl, F. Parmigiani, P. Petropoulos, B. Resan, K. Weingarten, T. Ellermeyer, J. Lutz, M. Moller, M. Huebner, J. Becher, C. Koos, W. Freude, and J. Leuthold, “Single source optical OFDM transmitter and optical FFT receiver demonstrated at line rates of 5.4 and 10.8 Tbit/s,” *Optical Fiber Communication Conference* (2010), paper PDPC1.

6. A. D. Ellis and F. C. G. Gunning, “Spectral density enhancement using coherent WDM,” IEEE Photon. Technol. Lett. **17**(2), 504–506 (2005). [CrossRef]

6. A. D. Ellis and F. C. G. Gunning, “Spectral density enhancement using coherent WDM,” IEEE Photon. Technol. Lett. **17**(2), 504–506 (2005). [CrossRef]

16. S. K. Ibrahim, J. Zhao, F. C. Garcia Gunning, P. Frascella, F. H. Peters, and A. D. Ellis, “Coherent WDM: analytical, numerical, and experimental studies,” IEEE Photon. J. **2**(5), 833–847 (2010). [CrossRef]

13. J. Zhao and A. D. Ellis, “Electronic impairment mitigation in optically multiplexed multi-carrier systems,” J. Lightwave Technol. **29**(3), 278–290 (2011). [CrossRef]

18. B. Hirosaki, S. Hasegawa, and A. Sabato, “Advanced groupband data modem using orthogonally multiplexed QAM technique,” IEEE Trans. Commun. **34**(6), 587–592 (1986). [CrossRef]

## 2. Principle

*j*,

*H*

_{D,j}(

*ω*+

*ω*

_{j}), may allow through parts of the signals from other channels, e.g. channels (

*j*-1) and (

*j*+ 1), in addition to the targeted channel, such that

*I*

_{k,j}(

*t*) and

*Q*

_{k,j}(

*t*) (

*k*≠

*j*) are not zero for all

*t*. However, the symbol decisions are made based only on the final samples of the signal, where the final decision samples are those obtained after all over- or non-over-sampling based digital processing in coherent detection. Consequently, we only require that the crosstalk and ISI levels at the final sampling points are zero. By setting

*t*=

*mT*and (

*m*+ 0.5)

*T*in Eq. (1) for the in-phase and quadrature tributaries respectively, we have the decoded logical data

*a*

^{’}

_{j,m}and

*b*

^{’}

_{j,m}:

*ω*

_{k}-

*ω*

_{j}) = 2π(

*k*-

*j*)/

*T*. The first, second, and third terms on the right-hand of Eqs. (5.1) and (5.2) represent the signal, ISI, and crosstalk, respectively. Note that in Eq. (5), the phase of the targeted

*j*

^{th}channel,

*ϕ*

_{j}, is assumed to be compensated. In practice, similar to other coherent-detection systems,

*ϕ*

_{j}varies with time and an adaptive algorithm is required to mitigate the impact of phase noise, unless the laser linewidth is sufficiently narrow and

*ϕ*

_{j}is approximately constant for the acquired data window.

- 1. Matched filter to minimize the noise impact, which places restrictions on the selection of the receiver filter (
*H*_{D,j}(*ω*+*ω*_{j}) =*H*_{s}*(*ω*)) but not on the transmitted signal pulse shape; - 2. Nyquist ISI criterion for ISI free operation in generic communication systems, which is satisfied by only particular set of signal pulse shapes with associated matched receiver filters. Fortunately, the selection of signal pulse shape under this restriction is not stringent and a signal generated by a practical transmitter in the conventional WDM or single-channel case can achieve ISI free operation, unless the system is bandwidth-limited [1,9
1. X. Zhou, J. Yu, M. F. Huang, Y. Shao, T. Wang, L. Nelson, P. Magill, M. Birk, P. I. Borel, D. W. Peckham, and R. Lingle, “64Tb/s (640×107Gb/s) PDM-36QAM transmission over 320km using both pre- and post-transmission digital equalization,”

*Optical Fiber Communication Conference*(2010), paper PDPB9.];9. Y. Cai, J. X. Cai, C. R. Davidson, D. Foursa, A. Lucero, O. Sinkin, A. Pilipetskii, G. Mohs, and S. N. Bergono, “High spectral efficiency long-haul transmission with pre-filtering and maximum a posteriori probability detection,” Proc.

*European Conference on Optical Communication*(2010), paper We.7.C.4. - 3. Channel orthogonality specific to optically multiplexed multi-carrier systems for crosstalk free operation. This condition strictly limits the freedom of selecting the spectral profiles of the signal before demultiplexing (
*H*_{s}(*ω*)) and the associated matched filter (*H*_{D,j}(*ω*+*ω*_{j})).

*j*+ 1)

^{th}channel to the targeted

*j*

^{th}channel at the sampling point, equal to the integration of the sine and cosine wave over the time period of

*T*, is zero. However, precluded by the limitations of device fabrication, these signal pulses cannot be practically realized. On the other hand, commonly used practical signal pulses do not satisfy the condition for crosstalk free operation or channel orthogonality, as depicted by the second and third rows of Fig. 2(b) where the signal pulse is a raised cosine with roll-off coefficient of 0.4. The figure clearly shows that crosstalk exists. It is also observed that when the carrier phase difference between channels is π/2, the crosstalk is only from the other quadrature of the (

*j*+ 1)

^{th}channel. This is the principle of conventional CoWDM [13

13. J. Zhao and A. D. Ellis, “Electronic impairment mitigation in optically multiplexed multi-carrier systems,” J. Lightwave Technol. **29**(3), 278–290 (2011). [CrossRef]

### 2.1 Relaxed Condition for Crosstalk Free Operation

*j*+ 1)

^{th}channel is offset by

*T*/2 in time (the bottom row). It can be seen that in this specific example, the crosstalk from both the in-phase and quadrature tributaries of the (

*j*+ 1)

^{th}channel becomes zero even for the practical raised-cosine pulse shape. This implies that potential performance benefits could be obtained by offset-QAM CoWDM. Note that in Fig. 2(b), as will be shown later, crosstalk and ISI free operation is still not ideally achieved because the spectrum of a raised-cosine shaped pulse remains infinite such that the crosstalk from channels (

*j*-2) and (

*j*+ 2) are not eliminated. In this subsection, we will firstly identify the condition to obtain the optimum operation of offset-QAM CoWDM. For a simple illustration, we firstly study the crosstalk levels to the in-phase tributary of the targeted

*j*

^{th}channel (i.e. (5.1)), with

*I*

_{k,j}((

*m*-

*n*)

*T*) and

*Q*

_{k,j}((

*m*-

*n*)

*T*) obtained from Eq. (2): Here, we have used (

*ω*

_{k}-

*ω*

_{j}) = 2π(

*k*-

*j*)/

*T*and the condition of a matched filter with

*H*

_{D,j}(

*ω*+

*ω*

_{j}) =

*H*

_{s}*(

*ω*). Without giving detailed mathematical manipulations, we simplify Eq. (6) as: We place the first requirement on the signal pulse to achieve channel orthogonality:

*h*

_{s}(

*t*) is a even function (real and symmetric). This applies to the majority of practically generated signals. In systems with transmission impairments such as chromatic dispersion, compensation using optical or digital devices is assumed. By using this kind of signal pulse, it can be proved that in (7),

*h*

_{s}((

*m*-

*n*)

*T*/2 +

*τ*’)⋅

*h*

_{s}*(

*τ*’-(

*m*-

*n*)

*T*/2) and

*h*

_{s}((

*m*-

*n*-0.5)

*T*/2 +

*τ*’)⋅

*h*

_{s}*(

*τ*’-(

*m*-

*n*-0.5)

*T*/2) are also even functions. Therefore, (7) can be re-written as: Physically, Eq. (8) implies that

*I*

_{k,j}((

*m*-

*n*)

*T*) is always real, while

*Q*

_{k,j}((

*m*-

*n*)

*T*) is imaginary for odd (

*k*-

*j*) and real for even (

*k*-

*j*). We then place the second requirement: CoWDM with the phase difference between channels of π/2. Without loss of generality, we define

*ϕ*

_{k}= (

*k*-1)⋅π/2, and can obtain from Eq. (5.1):With the same mathematical manipulations, we can also obtain that

*Q*

_{k,j}((

*m*-

*n*+ 0.5)

*T*) is always real, while

*I*

_{k,j}((

*m*-

*n*+ 0.5)

*T*) is imaginary for odd (

*k*-

*j*) and real for even (

*k*-

*j*). Consequently, the detected logical data for the quadrature tributary of the

*j*

^{th}channel,

*b*

^{’}

_{j,m}, is:It is clear from Eqs. (9.1) and (9.2) that the crosstalk to a particular quadrature of a particular channel

*j*has only contributions from the same quadrature of channels more than one channel distant from the targeted channel (i.e. channels (

*j*-2) and (

*j*+ 2) and beyond). Therefore, we place the third requirement for crosstalk free operation: the spectral profiles of the signal and its associated matched receiver filter are designed to avoid the spectral overlap between the targeted channel

*j*and channels (

*j*-2) and (

*j*+ 2).

- a). The spectral profile of the demultiplexing filter is matched to that of the signal.
- b). The design of
*h*_{s}(*t*) satisfies Nyquist ISI criterion for ISI free operation. - c).
*h*_{s}(*t*) is a even function. - d). The transmitter is coherent with optimal phase difference between channels of π/2.
- e).
*h*_{s}(*t*) is designed to avoid the spectral overlaps between the targeted channel (e.g. the*j*^{th}channel) and channels more than one channel distant (e.g. the (*j*-2)^{th}and (*j*+ 2)^{th}channels).

*j*-1) and (

*j*+ 1) can only come from the other tributary, which however experiences a zero crossing at the sampling point provided that the signal pulse of the other tributary of the adjacent channels (

*j*-1) and (

*j*+ 1) is the image of the impulse response of the receiver filter about the time point

*T*/4.

### 2.2 Crosstalk Analysis

*T*/2 time offset for the quadrature tributary. From Eq. (5.1), it is clear that the essential step for the semi-analytical crosstalk analysis is to obtain

*I*

_{k,j}((

*m*-

*n*)

*T*) and

*Q*

_{k,j}((

*m*-

*n*)

*T*) given the signal pulse shape and the associated matched receiver filter. By using similar mathematical manipulation to [13

**29**(3), 278–290 (2011). [CrossRef]

*ω*∈[-

*π*/

*T π*/

*T*] and

*k*≠

*j*. The folded spectra of

*I*

_{k,j}(

*t*) and

*Q*

_{k,j}(

*t*),

*FH*

_{in-phase,k,j}(

*ω*) and

*FH*

_{quadrature, k,j}(

*ω*), are defined as: Therefore,

*I*

_{k,j}((

*m*-

*n*)

*T*) and

*Q*

_{k,j}((

*m*-

*n*)

*T*) are proportional to the Fourier series coefficients of

*FH*

_{in-phase,k,j}(

*ω*) and

*FH*

_{quadrature,k,j}(

*ω*) respectively, which can be determined by

*H*

_{s}(

*ω*) and

*H*

_{D,j}(

*ω*+

*ω*

_{j}) with Eqs. (3) and (11).Tables 1 and 2 compare the calculated signal level, ISI and crosstalk on the received

*m*

^{th}sample of the in-phase tributary of the

*j*

^{th}channel (

*a*’

_{j,m}in Eq. (5.1)) for the conventional system and offset-QAM CoWDM respectively. The signal pulse shape before demultiplexing and the impulse response of the demultiplexing filter in both tables are raised cosine with the roll-off coefficient of 0.4. The phase difference between channels is π/2. Note that in the conventional system, the crosstalk level is only weakly dependent on this phase difference [13

**29**(3), 278–290 (2011). [CrossRef]

*j*-1) and (

*j*+ 1), so would improve the performance when compared to the conventional system. It is also observed that in offset-QAM CoWDM, the crosstalk from channels (

*j*-2) and (

*j*+ 2) is still not fully eliminated due to the infinite spectral tails of raised-cosine signal pulse.

*H*

_{s}(

*ω*) and

*H*

_{D,j}(

*ω*+

*ω*

_{j}) both being the square root of a raised-cosine function with the roll-off coefficient of 0.4. The phase difference between channels is π/2. In the conventional system (Table 3), the bandwidth-limited signal spectrum results in the crosstalk only arising from the adjacent channels (

*j*-1) and (

*j*+ 1). However, the crosstalk levels from not only the

*m*

^{th}symbol but also the (

*m*-1)

^{th}and (

*m*+ 1)

^{th}symbols of channels (

*j*-1) and (

*j*+ 1) are increased when compared to Table 1 because of the long pulse tails in the time domain. Therefore, the total crosstalk levels might not be reduced. In contrast, crosstalk and ISI free operation can be achieved when using offset-QAM CoWDM. It can be proved that in this case, the requirements (a)–(e) in subsection 2.1 are satisfied. In practice, this function can be readily achieved by using commercial components.

## 3. Simulation Setup

*T*/2 time offset for the quadrature tributary. In the latter case, pre-filtering was used such that the output of the pre-filter had a raised-cosine shaped spectrum with the roll-off coefficient of 0.1 [5

**22**(15), 1129–1131 (2010). [CrossRef]

**29**(3), 278–290 (2011). [CrossRef]

^{−4}for the central channel by direct error counting, where

## 4. Results

### 4.1 Comparison of Fundamental Performance Limit

^{−4}for the 4-QAM format and could not support the 16-QAM format. On the other hand, Nyquist WDM used rectangular spectral profile and improved the performance by using optical pre-filtering. However, residual crosstalk still existed, which resulted in ~5dB penalty for 16-QAM at BER of 5 × 10

^{−4}. In contrast, the presented system showed very clear constellation diagrams even for offset 64-QAM and the performance could approach the fundamental limits using practical devices with optimized bandwidths. This clearly illustrates the performance benefits of the proposed system, with the potential to achieve crosstalk and ISI free operation.

### 4.2 Relaxed Transmitter Specifications

### 4.3 Relaxed Receiver Specifications

### 4.4 Performance Sensitivity to Phase Difference between Channels

## 5. Conclusions

## Acknowledgments

## References and links

1. | X. Zhou, J. Yu, M. F. Huang, Y. Shao, T. Wang, L. Nelson, P. Magill, M. Birk, P. I. Borel, D. W. Peckham, and R. Lingle, “64Tb/s (640×107Gb/s) PDM-36QAM transmission over 320km using both pre- and post-transmission digital equalization,” |

2. | A. Sano, E. Yamada, H. Masuda, E. Yamazaki, T. Kobayashi, E. Yoshida, Y. Miyamoto, R. Kudo, K. Ishihara, and Y. Takatori, “No-guard-interval coherent optical OFDM for 100Gb/s long-haul WDM transmission,” J. Lightwave Technol. |

3. | S. Chandrasekhar and X. Liu, “Experimental investigation on the performance of closely spaced multi-carrier PDM-QPSK with digital coherent detection,” Opt. Express |

4. | J. Yu, Z. Dong, X. Xiao, Y. Xia, S. Shi, C. Ge, W. Zhou, N. Chi, and Y. Shao, “Generation, transmission and coherent detection of 11.2 Tb/s (112×100Gb/s) single source optical OFDM superchannel,” |

5. | G. Bosco, A. Carena, V. Curri, P. Poggiolini, and F. Forghieri, “Performance limits of Nyquist-WDM and Co-OFDM in high-speed PM-QPSK systems,” IEEE Photon. Technol. Lett. |

6. | A. D. Ellis and F. C. G. Gunning, “Spectral density enhancement using coherent WDM,” IEEE Photon. Technol. Lett. |

7. | J. Zhao and A. D. Ellis, “A novel optical fast OFDM with reduced channel spacing equal to half of the symbol rate per carrier,” |

8. | S. Yamamoto, K. Yonenaga, A. Sahara, F. Inuzuka, and A. Takada, “Achievement of sub-channel frequency spacing less than symbol rate and improvement of dispersion tolerance in optical OFDM transmission,” J. Lightwave Technol. |

9. | Y. Cai, J. X. Cai, C. R. Davidson, D. Foursa, A. Lucero, O. Sinkin, A. Pilipetskii, G. Mohs, and S. N. Bergono, “High spectral efficiency long-haul transmission with pre-filtering and maximum a posteriori probability detection,” Proc. |

10. | R. R. Mosier and R. G. Clabaugh, “Kineplex, a bandwidth-efficient binary transmission system,” AIEE Trans. Commun. |

11. | R. W. Chang, “Synthesis of band-limited orthogonal signals fro multi-channel data transmission,” Bell Syst. Tech. J. |

12. | S. B. Weinstein and P. M. Ebert, “Data transmission by frequency division multiplexing using the discrete Fourier transform,” IEEE Trans. Commun. Technol. Com. |

13. | J. Zhao and A. D. Ellis, “Electronic impairment mitigation in optically multiplexed multi-carrier systems,” J. Lightwave Technol. |

14. | G. Gavioli, E. Torrengo, G. Bosco, A. Carena, V. Curri, V. Miot, P. Poggiolini, M. Belmonte, F. Forghieri, C. Muzio, S. Piciaccia, A. Brinciotti, A. L. Porta, C. Lezzi, S. Savory, and S. Abrate, “Investigation of the impact of ultra-narrow carrier spacing on the transmission of a 10-carrier 1Tb/s superchannel,” |

15. | D. Hillerkuss, T. Schellinger, R. Schmogrow, M. Winter, T. Vallaitis, R. Bonk, A. Marculescu, J. Li, M. Dreschmann, J. Meyer, S. Ben Ezra, N. Narkiss, B. Nebendahl, F. Parmigiani, P. Petropoulos, B. Resan, K. Weingarten, T. Ellermeyer, J. Lutz, M. Moller, M. Huebner, J. Becher, C. Koos, W. Freude, and J. Leuthold, “Single source optical OFDM transmitter and optical FFT receiver demonstrated at line rates of 5.4 and 10.8 Tbit/s,” |

16. | S. K. Ibrahim, J. Zhao, F. C. Garcia Gunning, P. Frascella, F. H. Peters, and A. D. Ellis, “Coherent WDM: analytical, numerical, and experimental studies,” IEEE Photon. J. |

17. | J. G. Proakis, |

18. | B. Hirosaki, S. Hasegawa, and A. Sabato, “Advanced groupband data modem using orthogonally multiplexed QAM technique,” IEEE Trans. Commun. |

**OCIS Codes**

(060.2330) Fiber optics and optical communications : Fiber optics communications

(060.4080) Fiber optics and optical communications : Modulation

**ToC Category:**

Fiber Optics and Optical Communications

**History**

Original Manuscript: May 25, 2011

Revised Manuscript: July 7, 2011

Manuscript Accepted: July 8, 2011

Published: July 14, 2011

**Citation**

J. Zhao and A. D. Ellis, "Offset-QAM based coherent WDM for spectral efficiency enhancement," Opt. Express **19**, 14617-14631 (2011)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-15-14617

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### References

- X. Zhou, J. Yu, M. F. Huang, Y. Shao, T. Wang, L. Nelson, P. Magill, M. Birk, P. I. Borel, D. W. Peckham, and R. Lingle, “64Tb/s (640×107Gb/s) PDM-36QAM transmission over 320km using both pre- and post-transmission digital equalization,” Optical Fiber Communication Conference (2010), paper PDPB9.
- A. Sano, E. Yamada, H. Masuda, E. Yamazaki, T. Kobayashi, E. Yoshida, Y. Miyamoto, R. Kudo, K. Ishihara, and Y. Takatori, “No-guard-interval coherent optical OFDM for 100Gb/s long-haul WDM transmission,” J. Lightwave Technol. 27(16), 3705–3713 (2009). [CrossRef]
- S. Chandrasekhar and X. Liu, “Experimental investigation on the performance of closely spaced multi-carrier PDM-QPSK with digital coherent detection,” Opt. Express 17(24), 21350–21361 (2009). [CrossRef] [PubMed]
- J. Yu, Z. Dong, X. Xiao, Y. Xia, S. Shi, C. Ge, W. Zhou, N. Chi, and Y. Shao, “Generation, transmission and coherent detection of 11.2 Tb/s (112×100Gb/s) single source optical OFDM superchannel,” Optical Fiber Communication Conference (2011), paper PDPA6.
- G. Bosco, A. Carena, V. Curri, P. Poggiolini, and F. Forghieri, “Performance limits of Nyquist-WDM and Co-OFDM in high-speed PM-QPSK systems,” IEEE Photon. Technol. Lett. 22(15), 1129–1131 (2010). [CrossRef]
- A. D. Ellis and F. C. G. Gunning, “Spectral density enhancement using coherent WDM,” IEEE Photon. Technol. Lett. 17(2), 504–506 (2005). [CrossRef]
- J. Zhao and A. D. Ellis, “A novel optical fast OFDM with reduced channel spacing equal to half of the symbol rate per carrier,” Optical Fiber Communication Conference (2010), paper OMR1.
- S. Yamamoto, K. Yonenaga, A. Sahara, F. Inuzuka, and A. Takada, “Achievement of sub-channel frequency spacing less than symbol rate and improvement of dispersion tolerance in optical OFDM transmission,” J. Lightwave Technol. 28(1), 157–163 (2010). [CrossRef]
- Y. Cai, J. X. Cai, C. R. Davidson, D. Foursa, A. Lucero, O. Sinkin, A. Pilipetskii, G. Mohs, and S. N. Bergono, “High spectral efficiency long-haul transmission with pre-filtering and maximum a posteriori probability detection,” Proc. European Conference on Optical Communication (2010), paper We.7.C.4.
- R. R. Mosier and R. G. Clabaugh, “Kineplex, a bandwidth-efficient binary transmission system,” AIEE Trans. Commun. 76, 723–728 (1958).
- R. W. Chang, “Synthesis of band-limited orthogonal signals fro multi-channel data transmission,” Bell Syst. Tech. J. 45, 1775–1796 (1966).
- S. B. Weinstein and P. M. Ebert, “Data transmission by frequency division multiplexing using the discrete Fourier transform,” IEEE Trans. Commun. Technol. Com. 19(5), 628–634 (1971). [CrossRef]
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- G. Gavioli, E. Torrengo, G. Bosco, A. Carena, V. Curri, V. Miot, P. Poggiolini, M. Belmonte, F. Forghieri, C. Muzio, S. Piciaccia, A. Brinciotti, A. L. Porta, C. Lezzi, S. Savory, and S. Abrate, “Investigation of the impact of ultra-narrow carrier spacing on the transmission of a 10-carrier 1Tb/s superchannel,” Optical Fiber Communication Conference (2010), paper OThD3.
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