Limits of Spectral Efficiency and Transmission Reach in DWDM Networks

The continuously increasing demand of capacity requires a highly efficient use of the available optical bandwidth of the fiber. Additional needs for optical networks are more flexibility and higher transparency. Optical orthogonal frequency division multiplexing (O-OFDM) is a very flexible approach on the physical layer, which distributes data on a high number of low data rate subcarriers. The digital signal processing in both the transmitter and receiver allows one to very easily change the signal properties (e.g. modulation format, data rate, number of subcarriers) by software in contrast to fixed hardware implementations. We analyzed the limits of spectral efficiency and transmission reach over SSMF by an O-OFDM DWDM system with bit-rates ranging from 10 - 600 Gb/s with QPSK and n-QAM modulation reaching a spectral efficiency up to 12 bit/s/Hz. The transmission reach was determined allowing for available OSNR, required OSNR, fiber nonlinearities (including SPM, XPM, and FWM), and ~3 dB system reserve. Besides the classical DWDM scenario using the ITU 50 GHz grid we also looked at transmission of OFDM superchannels with no need of a fixed frequency grid supporting channels with different data rates and/or modulation on neighbouring channels. We could show, that dependent on the bit rate the use of a guard band can be useful to increase the achievable transmission reach, especially at lower bitrates 60 Gb/s. Transmission reach ranging from ~300 km to 3000 km can be achieved with spectral efficiency up to 11.8 bit/s/Hz which is only about 15 % below the absolute theoretical limit.