MQAM-SSB system for higher spectral efficien-cy aiming for elastic optical network

Abstract

With the growing of global internet traffic, increasing of transmission speed is inevitable. Although wavelength division multiplexed (WDM) optical transmission system provides multiple Tb/s transmission capacity, we need to develop higher capacity transmission technology. Since the bandwidth of an optical amplifier is finite, the transmission capacity can be increased 1) by increasing the transmission bits per unit frequency, defined as the spectral efficiency (SE) and 2) by decreasing guard bands between WDM channels. On one hand, the SE can be increased by using advanced modulation format such as M-quadrature phase shift key (MQAM). Then, further increase of the SE can be realized with a single side bad (SSB) transmission. On the other hand, the concept of Elastic Optical Network (EON) came out to have a flexible optical transmission. By dividing bandwidth flexibly, instead of using fixed ITU, the guard band can be minimized. Although these methods enhance the SE and flexibility in principle, there exist many technical problems to be solved. Comparing with Double Side Band (DSB) modulation, SSB signal saves half of the bandwidth at the price of more complex hardware implementation. Usually SSB is done with amplitude modulated (AM) signal. However, higher modulation format like MQAM-SSB signal has inherited Inter Symbol Interference (ISI) making MQAM hard to achieve SSB modulation. Several researchers tried to achieve QPSK-SSB signal, but they all have problems such as, bandwidth is not half reduced or the BER vs OSNR performance is almost same with 16QAM-DSB signal. In this work, we are going to make a deeper analyzation about G. Ohta’s work. His work has one drawback that it needs two lasers and two coherent receivers, which will be quite expensive for coherent communication system because the lasers used for coherent communication should have very small linewidth and are quite expensive. However, this work will apply G. Ohta’s scheme in EON, so that one laser can be shared by two adjacent channels with the help of one MZM as a complementary frequency shifter. In this thesis, we investigate the effect of ASE on MQAM-SSB (M=4,16,64), since G. Ohta only investi-gated the QPSK-SSB signal’s performance. The results show that MQAM-SSB performs better than MQAM-DSB signal when bandwidth is limited. When bandwidth is unlimited, MQAM-SSB performs slightly worse than MQAM-DSB signal. Meanwhile chromatic dispersion (CD) effect is analyzed. Because SSB signals occupy only half of the bandwidth of DSB signal, SSB signal always performs better than same bit rate DSB signal. Then system imperfections are investigated, too. Hilbert Filter (HF) can’t be realized ideally. Our analyzations show that HF should be designed into Type III FIR filter. In real system because of the imperfect HF, signal’s spectrum is broadened. So that Gaussian band pass filer (BPF) should be used. Different order of Gaussian Filters (GF) have different effect on system performance. Meanwhile the ratio between 3dB bandwidth of BPF and signal symbol rate will also effect system’s performance. So the joint effect of length of HF, order of GF, and the ratio between 3dB bandwidth of BPF and signal symbol rate is investigated. Users can choose the desired hardware implementation based on their requirement.