# Low Complexity DSP for High Speed Optical Access Networking +
## 3 Experimental Results at 20 km Transmission  
The eye diagrams of the modulator output shown in Figure 2 exhibit strong eye closure and asymmetry with smaller lower eyes. The degradation of the signal quality caused by the MZM is obvious by comparing the eye diagram with the DAC output eye diagram. The latter is much more open and the apertures are more uniform, although a slight eye closure is observed due to the DAC bandwidth limitation. The eye diagrams of the detected signals are also shown in Figure 2, in which they are completely closed, regardless of the fiber transmission distance. This is a direct result of the joint effect of limited transceiver bandwidth and fiber dispersion. Figure 3a identifies an optimum operating point that is far below the linear regime, which is verified by the BER versus modulator bias voltage curve presented in Figure 3b. Considering a small differential driving signal swing of about 500 mV, which is about one ninth of the V\({}_{\pi}\) of the MZM, a bias closer to the null point brings about a lower CSPR (namely, a higher signal to noise ratio (SNR) for a fixed optical power by suppressing optical carrier) but stronger signal-signal beating interference upon square-law detection at the same time. The optimum bias gives rise to the best trade-off between CSPR and beating interference. This leads to a low optical output power, but it can be amplified by a booster EDFA. This configuration is critical to reduce the transceiver cost and power due to the avoidance of an RF driver in each channel since the EDFA cost can be shared by all channels. Figure 4a presents the BER versus the received optical power performance under various receiver DSP configurations for an SMF length of 20 km. With only a linear FFE with an identified optimum tap count of \(M=8\), the BER improves slowly with increasing optical power but an error floor appears at a BER level of \(5\times 10^{-5}\). The use of DFE with an optimum tap count \(N=4\), in addition to FFE, shows a performance enhancement, especially in lowering the error floor. Based on the optimum FFE and DFE tap counts, the 2 nd-order nonlinear terms given in Equation (1) are additionally introduced, requiring a total number of 44/10 FFE/DFE taps according to Equation (1). It is obvious that nonlinear equalization brings a tremendous improvement in both the power sensitivity and error floor. Further improvement is achieved by using an NS filter, which is more significant at lower BERs. Overall, a total optical power sensitivity improvement of approximately 2 dB (4 dB) is obtained at the 10-G EPON FEC (KP-4 FEC) threshold BER of \(1\times 10^{-3}\) (\(2\times 10^{-4}\)), respectively. Figure 2: The eye diagrams (10 ps/div time resolution) obtained at the output of the DAC (marker A), the modulator (marker B), and the photodiode (marker C), respectively. Figure 4: (**a**) BER vs. optical power performance using different equalization approaches for transmission of 20 km single-mode fiber (SMF) and (**b**) power spectral densities. (**c**) Histograms of signals corresponding to DSP configurations marked in BER curves with \(-25\) dBm optical power. (**d**) Received simulated histograms for avalanche photodiode (APD) case (w/o receiver EDFA) with and without DSP at \(-25\) dBm optical power. Figure 3: (**a**) The Mach–Zehnder intensity modulator (MZM) transfer function and the identified optimum modulation regime in the inset block and (**b**) bit-error-rate (BER) vs. modulator bias voltage for optical back to back case. The improvement enabled by the receiver adaptive DSP is firstly attributed to--as indicated in Figure 4b--the nonlinear equalization, that effectively mitigates the signal-signal beating interference due to fiber dispersion and square-law detection, leading to approximately a 3 dB reduction in noise power. Secondly, the nonlinear response of the MZM near to the null bias point can also be significantly compensated by nonlinear equalization.