# A Pulse Shaping Based Optical Transmission System of 128 QAM for DWDM with N \(\times\) 904 Gbps
## 4 Simulations and Performance Analysis
Consequently, we can improve OSNR to ameliorate the transmission performance of the communication system. Furthermore, when the parameters of transmission rates in the single channel are configured as 900 Gbps, 901 Gbps, 902 Gbps, 903 Gbps, 904 Gbps, and 905 Gbps, we can then obtain the constellation-diagrams and diagram of curves between the bit rates and BER, and the bit rates and EVM, which are shown in Figures 11 and 12, respectively. Figure 6:4 The relation curve between BER and transmission rates with constellation-diagrams. Figure 6:5 The relation curve between OSNR and EVM with constellation-diagrams. BER is a very important index to measure the performance of a transmission system. According to the conventions of the communication industry, the BER can be acceptable when it falls below the forward error correction threshold of 3.8 \(\times\) 10\({}^{-3}\) (that is too say, \(-\)lg (BER) \(\geq\) 2.42) for a high-speed digital coherent optical communication system. From Figure 11, it can be seen that when the transmission rate of a single wavelength for the DWDM system configures 904 Gbps, the BER is about 3.8 \(\times\) 10\({}^{-3}\); that is too say, when it is configured as N \(\times\) 904 Gbps for the DWDM system, the transmission performance can be acceptable; meanwhile, the constellation-diagrams are very clear, that is too say, at the receiving terminal, the signals recover well and the distortion is not obvious. Consequently, from N \(\times\) 900 Gbps to N \(\times\) 904 Gbps, the transmission rate configuration for the DWDM system is reasonable, and it is also a good choice. EVM is an index to measure the quality of modulated signal. It indicates the proximity between I/Q components and the ideal components when the receiver demodulates the signals; and the quality of EVM is related to the quality of the signal. In a high-speed digital optical communication, the EVM can be acceptable when it falls below 5%; and this indicator is commonly used in high speed coherent optical communication systems. From Figure 12, it can be obtained that when the bit rate of a single wavelength for the DWDM system is 904 Gbps, the EVM is about 5%; that is too say, when it is configured as N \(\times\) 904 Gbps for the DWDM system, the transmission performance can be acceptable; Moreover, the constellation-diagrams are very clear, that is too say, at the receiving terminal, the signals recover well and the distortion is not obvious. Consequently, from N \(\times\) 900 Gbps to N \(\times\) 904 Gbps, the transmission rate configuration for the DWDM system is reasonable, and it is also a good choice. From Figures 11 and 12, it can be seen that BER has a specific relationship with EVM in terms of the acceptability of transmission performance corresponding to the rate configuration. This further verifies the rationality of equation (20). From the configurations and results of the simulations, we calculate that the optical spectral utilization efficiency of the transmission channels is up to 10.33 bps/Hz (904 Gbps/87.5 GHz \(\approx\) 10.33 bps/Hz). According to all simulations and demonstrations, we determine that the performances of the transmission system are acceptable when the bit rates of the transmission are configured from 900 Gbps to 904 Gbps.