Experimental Demonstration of Flexible Hybrid Modulation Formats for Future Adaptive Optical Transmission Systems

The exponentially growing demand for high-speed data transmission is one of the main reasons for the ongoing evolution from fixed and static to link-adaptive systems in fiber-optic communications. The need to maximize the flexibility as well as the capacity of the system, while keeping the optical network infrastructure unchanged, leads to modifications of the system terminals, resulting in so-called flexible transceivers. A highly desired property of such a transceiver is a fine data rate granularity, which allows us to match the transmission capacity to the link budget. In this paper, we experimentally analyze flexible quadrature-division hybrid modulation formats, namely Flex-PAM, as a promising candidate for flexible transceivers. This method works at every integer number of bits-per-symbol, and, compared with other candidates proposed in the literature, requires a very simple digital signal processing scheme. We focus in particular on the effects of the effective number of bits in state-of-the-art digital/analog and analog/digital converters and fiber nonlinearities on the performance of Flex-PAM transmission. We analyze the capabilities of flexible modulation in a 5 × 32 GBd wavelength division multiplexing coherent transmission on a dense 37.5 GHz grid and support the results by thorough numerical simulations. We successfully demonstrate the capability of Flex-PAM modulation to handle a wide range of bit-rates (5 × 128 to 5 × 320 Gbit/s), a maximum spectral efficiency of 6.8 (b/s)/Hz, and propose a simple and adaptive digital signal processing scheme to compensate for the major linear and nonlinear impairments of the system.