# A Pulse Shaping Based Optical Transmission System of 128 QAM for DWDM with N \(\times\) 904 Gbps
## 3 System Architecture and Transceiver Units  
The ultra-high-speed DWDM transmission system with a large capacity based on 128 QAM and Gaussian optical filtering is shown in Figure 2.  
The multi-carrier signals with the adjacent wavelength space of 0.7 nm produced by multiple coherent photonic transmitters with the same modulation format of 128 QAM are multiplexed into an SSMF via the DWDM-Mux; the multiplexed multiple carriers are amplified by EDFA 1; the amplified optical signals are transmitted over 80 km with the SSMF, amplified again via EDFA 2, de-multiplexed via DWDM-Demux, mixed with the Gaussian distributed optical white noise (GDOWN), and filtered by GOFs successively; The filtered photonic signals are then successively processed via the photonic heterodyne coherent receivers, ultra-fast digital signal processors (UDSP), normalized threshold setting for 128 QAM (NTS-128 QAM), 128 QAM sequence decoders (128 QAM-SD), and parallel-to-serial converters (PTSC). Finally, the useful information is demodulated and the ultra-fast digital sequence signals are obtained. The main parameters of the critical components and key modules are shown in Table 1.  
The functional block diagram of the coherent photonic transmitter is shown in Figure 3. The ultra-fast digital sequence signal is divided into two signals by the serial-to-parallel converter (STPC); the two signals are identically processed, and the up one is used to illustrate the modulation process of the ultra-fast signal. It is divided into two signals again via the 128 QAM sequence generator 1, \begin{table}
\begin{tabular}{c l} \hline \hline
**DWDM-Mux** & **Number of input ports: N; Bandwidth: 70 GHz; Additional loss: 0 dB; Filter Depth: 100 dB; Filter** \\ \multicolumn{2}{c}{**Type: Gaussian; Filter order: 3; Channel spacing: 0.7 nm; Noise dynamic: 3 dB.**} \\ \hline DWDM-Demux & Number of output ports: N; Bandwidth: 70 GHz; Additional loss: 0 dB; Filter Depth: 100 dB; Filter Type: Gaussian; Filter order: 3; Channel spacing: 0.7 nm; Noise dynamic: 3 dB. \\ \hline GOF \(\pm\) N & Frequency: (1550 \(\pm\) 0.7 \(\times\) N) nm; Bandwidth: 70 GHz; Additional loss: 0 dB; Depth: 100 dB; Order: 3. \\ \hline EDFA 1 & Operation mode: Power control; Noise figure: 4 dB; Power: 0 dBm. \\ \hline SSMF & Length: 80 km; Attenuation: 0.2 km; Reference wavelength: 1550 nm; Dispersion: 16.75 ps/nm/km. \\ \hline EDFA 2 & Operation mode: Gain control; Noise figure: 4 dB; Gain: 16 dB. \\ \hline UDSP & Polarization type: Dual; Modulation type: 128 QAM; Type of filter: Bessel; Insertion loss: 0 dB; Cutoff frequency: 0.75 \(\times\) Bit rate/14; Depth: 100 dB; Number of symbols: Sequence length/14; Order: 3; Roll off factor: 1; Symbol rate: Bit rate/14; Samples per block: 2048; Interpolation type: Cubic: RF interpolation method: Cubic; Average window size: 512; AE number of taps: 3; Delay X: 0; Delay Y: 0; AE number of iterations: 10; Dispersion order: 2; Initial taps index: 1; Step CMA: 10\({}^{-6}\); Step RD: 1.125 \(\times\) 10\({}^{-7}\); Number of test phases: 256; Power order of symbols: 4; Power order: 4; CPE symbols per block: 20. \\ \hline NTS-128 QAM & Reference bit rate: Bit rate of transmitting; Modulation format: 128 QAM; Polarization type: Dual; Symbol rate: Bit rate/14; Star and circular level radii: 1-2-3-4. \\ \hline
128 QAM-SD & Bits per symbol: 7; Star and circular level radii: 1-2-3-4; Gray code: False. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Configuration of the parameters in the DWDM transmission system. Figure 2: Setup used for ultra-high-speed transmission (DWDM: Dense Wavelength Division Multiplexing; GOF: Gaussian Optical Filter; QAM: Quadrature Amplitude Modulation; SSMF: Standard Single Mode Fiber; EDFA: Erbium Doped Fiber Amplifier; GDOWN: Gaussian Distributed Optical White Noise). and these two signals are processed by M-ary pulse generator 1 and M-ary pulse generator 2, thus obtaining the two M-ary pulse signals. The two M-ary pulse signals are processed identically, we use the up one to illustrate the process.