# DSP-Based 40 GB/s Lane Rate Next-Generation Access Networks +
## 2 System Architecture and Simulation Parameters  
The WDM-PON architecture with downstream transceivers is depicted in Figure 2a, based on electrical Duobinary modulation. Figure 2b,c is for optical Duobinary and PAM-4, respectively, which adopts the same WDM-PON architecture that is shown in Figure 2a. For the electrical Duobinary case, the optical line terminal (OLT) transmitter simply consists of a digital pre-coder conventional and an external electro-absorption modulator (EAM). The transmitter is like a non-return-to-zero (NRZ) transmitter. The pre-coder helps to overcome the error propagation in decoding. The EAM output signal passes through a fiber link, which is comprised of a WDM multiplexer (MUX), a standard single-mode fiber (SMF), and a WDM de-multiplexer (De-MUX), which is in the remote node, to select the signal of a specific wavelength to a targeted optical network unit (ONU). At the ONU receiver, the transmitted signal is detected by an avalanche photo-detector (APD), and then recovered by DSP. The DSP includes a nonlinear equalizer, a low complex MLSE, and a Duobinary-to-NRZ convertor that which is based on two-threshold slicers, followed by an exclusive-or (XOR) gate [3]. The equalizer performs as a duobinary filter targeting a three-level Duobinary signal. 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. For optical Duobinary, the transmitter consists of a pre-coder, a shaping filter and a Mach-Zehnder modulator (MZM). The shaping filter is designed to generate a three-level electrical Duobinary waveform, as indicated in Figure 2d. A fully driven MZM with a push-pull configuration is used for both amplitude and phase modulations for optical Duobinary signals. The MZM is biased at the null point; thus the output signal has only two intensity levels, as indicated in Figure 2d. The receiver is a simple NRZ receiver. The receiver DSP also consists of a nonlinear equalizer and a MLSE. For PAM-4, the transmitter has a bits-to-symbol encoder and an EAM. The receiver also incorporates an APD, a nonlinear equalizer followed by a MLSE, and a symbol-to-bits decoder. It should be noted that MLSE is also considered for nonlinear equalization operation. MLSE is especially efficient for compensating channel memory-caused inter-symbol interference (ISI). The MLSE process involves Trellis, as defined by states and branches, which analyze the K\({}^{L+1}\) 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. The MZM used for optical Duobinary is configured in a push-pull mode, and it has zero chirp. Comparatively, both electrical Duobinary and PAM-4 uses an EAM, which considers the chirp model based on experimental measurements [14] and behaves a negative chirp. The detailed modeling of EAM can be found in [11]. The model reveals an important relationship relating the carrier frequency deviations with the power variations through the time-dependent chirp-parameter for electro-absorption modulated lasers (EMLs). It is essentially similar to, but slightly different from the frequency chirp model that is used for the directly modulated laser [15]. The receiver equalizer for each scheme is similar. The equalizer is based on an FFE) and DFE structure, and includes both linear and nonlinear terms.