Nyquist OTDM-WDM Lab Report

1 Introduction
With the continuous development of society, the optical network shows a geometrically increasing trend in scale and capacity respect. The research on high spectrum efficiency (SE) optical transmission technology has been of great urgency. Since optical network is one of the contents of the information infrastructure construction, it is necessary to reduce the cost per transported information bit. According to Nyquist’s first criterion, the maximum symbol rate is limited in a given channel with zero intersymbol interference (ISI), and therefore the highest SE is under the theoretical limit (Nyquist, 2002).
Recently, a novel orthogonal time division multiplexing (TDM) transmission scheme using optical Nyquist pulse was proposed
First of all, it uses orthogonal sinc-shaped pulses to generate the Nyquist orthogonal TDM signals, and then, multiplex these Nyquist OTDM signals with different center frequency into the Nyquist OTDM-WDM signals. Since Nyquist OTDM channel has a sharp-edged rectangular spectrum, the next wavelength channel can be adjacent to the previous without any guard band, reaching high spectral densities.

3 Simulation Setup Description
Figure 1 shows the simulation setup for the intensity modulation - direct detection (IM-DD) Nyquist OTDM-WDM signal generation and detection. One external cavity laser (ECL) with the wavelength of 1553.60 nm was used as the light source, and its linewidth was 80 kHz. In order to suppress higher sideband, a small α=0.32 was chosen, so ϵ=-0.4191, and the frequencies of two RF drive signals were 10 GHz and 30 GHz. At the output of the first MZM (Fig.1 Point A), a flat OFC with 3 tones was generated, which would be used as the seed light source of the second MZM. The driving signal of second MZM was set to synchronize with that of the first MZM, therefore, at the output of the second MZM, a flat OFC with 9 tones was generated (Fig. 1 Point
It can be found that the bandwidth of each channel was 30 GHz. The total bandwidth was 90 GHz, and the total Baud rate can reach 90 GBaud, which just exactly equaled to the total bandwidth. In other words, the SE achieved 1 baud/Hz in such a cost-effective IM-DD system. Compared to the multidimensional multiplexing system (Nyquist OTDM-WDM), although Nyquist OTDM can also achieve the same SE, it is quiet stricter in clock and timing when implementing a system with a larger capacity (xxx引用). Figure 3 shows the eye diagram of the Nyquist OTDM-WDM signal. Because of the severe ISI, the waveform is greatly damaged. It can be found clearly that, however, there is no ISI at the ideal sampling time (see the red dots in Figure 2). That means if the sampling happened at some particular time, the signal can be recovered without any distortion. 这和之前的Nyquist OTDM是一致的。（对比，） Figure 3. Eye diagrm of Nyquist OTDM signal. Red dots represent the ideal sampling time.

The BER performance of OOK Nyquist OTDM-WDM is given in Figure 4. The result indicates that the BER performance of each channel was almost the same. The received optical power required for error-free (1e-9) transmission was about -21 dBm.