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DSP-Based 40 GB/s Lane Rate Next-Generation Access Networks^{ †}

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## Abstract

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## 1. Introduction

## 2. System Architecture and Simulation Parameters

^{L+1}branches per each symbol, where K is the alphabet size of the modulation scheme, and L is the memory length. The MLSE processor chooses the path with the smallest metric, and produces the most likely sequence by tracing back [13]. The complexity of MLSE is exponentially proportional to memory length. Therefore, we limit the memory length to 2 for electrical/optical Duobinary and 1 for PAM-4, so that all three schemes have four states of MLSE. Electrical/optical Duobinary can also incorporate 2-state MLSE with a memory length of 1.

^{−3}is assumed, which has been adopted in 10G Ethernet PON standard. The transmitter output optical power is set to be 10 dBm, and the signal carrier wavelength is 1550 nm. A split-length SMF model is adopted with a fiber loss of 0.2 dB/km, a chromatic dispersion of 17 ps/km/nm, and a nonlinear coefficient γ = 1.3 W

^{−1}/km. A second-order optical band-pass Gaussian filter is considered to model the MUX/De-MUX with a 40 GHz 3 dB bandwidth, which corresponds to a 50G WDM grid. A responsivity of 3.2 A/W is used for the optical receiver. Note that the optimization of the thresholds is performed in the receiver decoders for all cases.

## 3. Simulation Results

#### 3.1. Optimization of Transceiver Bandwidth

^{−3}on chromatic dispersion subject to different transceiver bandwidths is shown in the figure. It clearly indicates that there exists an optimum transmitter/receiver bandwidth, corresponding to which PAM-4 has the best dispersion tolerance, and/or the maximum receiver optical power sensitivity. The optimum transmitter bandwidth is identified to be 20 GHz, which gives rise to the best trade-off between dispersion tolerance and receiver sensitivity. This is attributed to the fact that a larger transmitter bandwidth allows less ISI but experiences a stronger impact of chromatic dispersion. The optimum receiver bandwidth is 12.5 GHz, which is mainly determined by two factors: the receiver noise and the receiver-caused ISI. The optimization of transceiver bandwidths for 40-Gb/s electrical Duobinary and optical Duobinary is similar, which has been detailed in [18] and [11], respectively. The optimum transmitter/receiver bandwidth for electrical Duobinary (optical Duobinary) is 25 GHz/12.5 GHz (17 GHz/17.5 GHz) [11,18]. Note that the 50 G-ITU grid is adopted for all cases. It is worth mentioning that the system performance not only depends on transceiver 3-dB bandwidth, but can also be affected by the spectral profile beyond the 3 dB bandwidth frequency [5].

#### 3.2. Transmission Performance

^{−3}. The performance comparison considers four receiver signal processing cases: without equalization, which adopts only simple threshold devices in the receiver side [11], linear equalization only, linear and nonlinear equalizations, and MLSE together with (non-)linear equalizations. Figure 2a shows the optical Duobinary system where the optical sensitivity shows little variation with different SMF lengths within 10 km, beyond which the power penalty increases with increasing fiber length. Without equalization, the system shows the worst power sensitivity at all considered fiber lengths. Linear equalization brings about a slightly improvement in power sensitivity, compared to the no equalization case, which agrees with published results in [19]. The optical Duobinary performance is dominated by the nonlinear interaction of MZM modulation response with fiber dispersion. Such a nonlinear effect is further enhanced by the three-level signal to two-level signal transformation upon the square-law direct detection [11]. Not surprisingly, nonlinear equalization enables an approximately 1dB improvement in sensitivity at different fiber lengths compared to the case without equalization. The MLSE can further improve the power sensitivities for fiber lengths beyond 10 km. Significant improvement is achievable when the number of MLSE states increases from 2 to 4. Only nonlinear equalization and MLSE enable an SMF transmission distance of 20 km, and a 4-state MLSE can further achieve 25 km of SMF transmission.

#### 3.3. Link Power Budget

^{−3}, obtained in Figure 4 and for a 10 dBm of optical launch power. The vertical length of each bar in Figure 5 represents the corresponding link power budget, and it is explicitly marked with a value in dB. Comparatively, for fiber lengths beyond 10 km, there are some blank area with bars missing, which means that the link under such a receiver signal processing configuration fails, because the receiver BER cannot achieve 1 × 10

^{−3}.

#### 3.4. DSP Complexity

## 4. Discussion

## 5. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 2.**Schematic architectures of 40 Gb/s lane rate NG-PON downstream links with (

**a**) electrical Duobinary; (

**b**) optical Duobinary; and (

**c**) PAM-4 formats; (

**d**) the optical Duobinary modulator transfer function where the Mach-Zehnder modulator (MZM) is biased at the null point, and the eye-diagrams correspond to the electrical driving signal, and the output optical intensity signal.

**Figure 3.**Receiver sensitivity versus fiber dispersion for PAM-4, subject to various (

**a**) transmitter bandwidths and (

**b**) avalanche photo-detector trans-impedance amplifier (APD-TIA) receiver bandwidths. An Rx bandwidth of 12.5 GHz is used in (

**a**), and a Tx bandwidth of 20 GHz is used in (

**b**). (De-)MUX filters as shown in Figure 2 are included. Only seven tap feedforward equalizer (FFE) and five tap decision feedback equalizer (DFE) with first-order terms are considered.

**Figure 4.**Optical power sensitivity at a bit error rate (BER) of 10-3 versus fiber lengths for (

**a**) optical Duobinary; (

**b**) electrical Duobinary; and (

**c**) PAM-4 subject to different post-equalization and detection conditions. For receiver digital signal processing (DSP), the following configurations are considered: without equalization (w/o EQ) where no DSP considered; linear EQ, which uses only 7-tap/5-tap linear FFE/DFE; nonlinear EQ includes both linear FFE/DFE terms and second-order nonlinear FFE/DFE terms; maximum likelihood sequence estimation (MLSE) contains linear and nonlinear EQ, as well as 2- or 4-state MLSE. Unless indicated explicitly, the four DSP configurations apply throughout this work.

**Figure 5.**Link power budget for each modulation format under various DSP configurations for transmission distances of (

**a**) 10 km, (

**b**) 20 km, (

**c**) 30 km, and (

**d**) 40 km. The optical power sensitivity shown in Figure 4 is considered to calculate the link power budget.

DSP Algorithm | Modulation | Multiplications | Sums |
---|---|---|---|

Linear FFE–DFE (M = 7, N = 5) | Opt./Electr. Duobinary, PAM-4 | M + N 12 | M + N − 1 11 |

(Non)linear FFE–DFE (M = 7, N = 5) | Opt./Electr. Duobinary, PAM-4 | $\frac{{M}^{2}+3M+{N}^{2}+N-2}{2}+12$ 61 | $\frac{{M}^{2}+M+{N}^{2}-N}{2}+11$ 49 |

(Non)linear FFE–DFE–MLSE (K = 2, L = 2) | Opt./Electr. Duobinary | $\left(L+1\right)\xb7{K}^{L+1}+61$ 85 | $\left(L+2\right)\xb7{K}^{L+1}$ + 49 81 |

(Non)linear FFE–DFE–MLSE (K = 4, L = 1) | PAM-4 | $\left(L+1\right)\xb7{K}^{L+1}+61$ 93 | $\left(L+2\right)\xb7{K}^{L+1}$ + 49 97 |

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**MDPI and ACS Style**

Wei, J.; Zhou, J.; Giacoumidis, E.; Haigh, P.A.; Tang, J.
DSP-Based 40 GB/s Lane Rate Next-Generation Access Networks. *Future Internet* **2018**, *10*, 118.
https://doi.org/10.3390/fi10120118

**AMA Style**

Wei J, Zhou J, Giacoumidis E, Haigh PA, Tang J.
DSP-Based 40 GB/s Lane Rate Next-Generation Access Networks. *Future Internet*. 2018; 10(12):118.
https://doi.org/10.3390/fi10120118

**Chicago/Turabian Style**

Wei, Jinlong, Ji Zhou, Elias Giacoumidis, Paul A. Haigh, and Jianming Tang.
2018. "DSP-Based 40 GB/s Lane Rate Next-Generation Access Networks" *Future Internet* 10, no. 12: 118.
https://doi.org/10.3390/fi10120118