Contents 1. INTRODUCTION
As an inevitable trend, current optical networks evolve towards enhanced flexibility and elasticity with variable modulation formats, optical power, and channel configurations. By adaptively allocating the modulation format and spectrum according to the distance and capacity requirements, network performance significantly improved compared to traditional wavelength switched optical networks (WSON) with fixed modulation and wavelength grid [1]. By exploiting this feature, accurate estimation of the quality of transmission (QoT) for light paths has become an essential requirement for increasing the network efficiency. For evaluating the QoT of light path for coherent optical communication systems, Gaussian noise (GN) model, Enhanced GN (EGN) model have been proposed [2-4], assuming the Gaussian distribution of the nonlinear noise. Currently, many QoT estimations are based on the homogenous channel configurations, where the nonlinear noise of a group of adjacent wavelength channels with the same modulation format, launch powers are evaluated to be the same, assuming a worst-case assumption. However, to improve the QoT evaluation accuracy, the nonlinear noise should be calculated by a channel-by-channel case, where the inter-channel nonlinearity with various modulation formats, launch power, channel spacing should be taken into consideration. In this paper, we investigate the influence of inter-channel nonlinear interference (ICNI) on the QoT estimation with various modulation format, launch power and channel spacing. The numerical results show that the ICNI power at a channel spacing of 50 GHz is 3.6 dB higher than the ICNI power at 100 GHz. When the interfering channel power increases, the effect of the modulation format becomes more obvious. When the power is 3 dBm, the Q factor in the case of QPSK, 16QAM and 64QAM is 3.6 dB, 2.1 dB and 1.3dB larger than that estimated by GN model respectively, which should not be neglected.
