# High-Speed, High-Performance DQPSK Optical Links with Reduced Complexity VDEE Equalizers
## 4 Optical Layer Simulation
Moreover, a two-stage amplification process is used, modelled by two separate amplifiers with a 5 dB noise figure, each one of them utilized to compensate either the SMF or the DCF losses. For both of the aforementioned set up scenarios the transmitter/receiver configuration is considered identical. At the transmitter side, the DQPSK signal is generated via two different MZM (one per channel) which operate in a push-pull mode and have an extinction ratio of 35 dB each. In terms of electrical filtering two low pass third-order Bessel filters are used with a cut-off frequency of 40 GHz. For the PRBS, the modified Wichman-Hill generator [26] is used with a mark probability of 0.5. The transmitter operates at the optical frequency of 193.1 THz with 0 dBm output power. On the receiver side, an optical filter of 40 GHz bandwidth with a third-order Gaussian frequency response is utilized. In order to demodulate the DQPSK signal, two different MZDI are used. After each MZDI output port a photodiode of 1 A/W responsivity and a fourth-order Bessel frequency response electrical filter is used in order detect the optical signal. The input power is chosen to be well above the noise limit and below the nonlinear limit of the system. It is noted, however, that in a realistic system where WDM is used and the number of channels increases, the optical power and crosstalk will be dominant, and the parameters will be re-evaluated. The numerical simulations are performed by the means of Virtual Photonics Inc. software (VPI Transmission Maker) and approximately 10\({}^{6}\) bits are used for BER computation, in order to achieve higher accuracy. The results of the noise loading scenario of each equalizer (SVDFE and VDFE) are depicted in Figure 8 for both [\(M_{f}\)/\(M_{b}\)] = [5,3] and [9,5]. The specific sets of [\(M_{f}\)/\(M_{b}\)] exhibit sufficient compensation capability, and the equalizer complexity makes it feasible in terms of implementation [2]. Figure 7: Simulation generic setup for (**a**) a noise-loading technique and (**b**) multi-span scenarios with eight spans. The required OSNR with respect to accumulated dispersion is plotted for all equalizer cases discussed in Sections 2 and 3, along with the performance of the optical system when dispersion compensation is performed only by optical means (without the equalizer case). Apart from the single-ended joint version, where all four output ports are utilized (denoted in figures as 4I), each equalizer is compared with all three partially-joint input counterparts (denoted in figures as 3I). Those partially-joint single-ended configurations differentiate themselves according to the number of ports (e.g., 3I three input) and the specified disregarded input port (e.g., 3I-VDFE[5,3]-\(I_{d}\)). Depending on the output that is not utilized, every configuration exhibits slightly different performance. All of the different configurations seem to offer a significant improvement in the performance of the system under investigation, compared to the case where no electronic equalization is used (without equalizer). Assuming that the rOSNR should be approximately 18 dB, 4I equalization schemes seem to increase the amount of tolerable CD up to 1200 ps/nm (SVDFE[5,3]) and 1325 ps/nm (VDFE[5,3]), corresponding to 70 km and 80 km of uncompensated fiber, respectively. The increase of the tolerable amount of CD is even greater when equalizers with [\(M_{\prime}\),\(M_{b}\)] = [9,5] are deployed. When no equalization is used the tolerable amount of chromatic dispersion reaches up to 300 ps/nm (corresponding to 17 km of uncompensated CD), hence, utilizing equalization can extend the uncompensated distance approximately up to 60-80 km (depending on the equalizer type and the set of [\(M_{\prime}\),\(M_{b}\)]). This improvement on the CD tolerance of the system comes at the expense of increased required OSNR, since as the amount of residual dispersion increases, the value of rOSNR increase also.