Contents I. Introduction
Increasing network capacity is required by service operators to cope with year over year traffic growth. Current commercial 100G and advance 200G systems with dual-polarization quadrature phase shift keying (DP-QPSK) and digital coherent detection at 50-GHz spacing, allowing network operators to attain a capacity up to 9.6 Tb/s [1]–[6] per fiber using the -band alone. To upgrade system capacity, the following techniques can be applied: 1) reducing channel spacing [7]; 2) employing higher order modulation formats [9]–[13]; 3) transmit more optical channels over wider transmission window [12], [13]. Due to the increased optical signal- to-noise ratio (OSNR) requirement of higher order modulation formats such as dual-polarization 16 quadrature amplitude modulation (DP-16QAM), the reported high-capacity experiments utilized newly developed fiber with advantageous properties, e.g., large core area and ultralow loss. This fiber can help maintain the transmission OSNR by reducing fiber attenuation and raising signal launch power to further obtain high fiber capacity. However, specialty fibers are not feasible for a network deployment because of their high manufacturing cost at the current moment. In the industry, it is unlikely that any network service providers will perform large scale network fiber infrastructure upgrade any time soon. Thus, it is important to investigate how much capacity that network service providers can achieve with their existing fiber infrastructure.
