By Shinji Tsuji, Yuji Hamasaki
64QAM is recognized as a standard for the digital television transmission method on cable plants. This is more robust than the traditional VSB-AM analog method in white noise and nonlinear distortion environment that is typical in the CATV environment. As it is a digital method, it can be regenerated as necessary. Therefore, it will be a better format for long distance transmission using optical fiber. In this paper, we propose an economical CATV backbone solution using a 64QAM transmission system. The system has one digital headend and several analog headends connected by optical fiber cable. The digital headend facility is shared by other headends. We conducted experiments to verify the concept of the digital 64QAM backbone. The experimental system has 250km fiber with 5 cascaded erbium doped optical fiber amplifiers (EDFA) and 11 channels of 64QAM signal or 60CW signals were transmitted as test signals. Such parameters as input/output of EDFAs and modulation depth of laser diode were intentionally changed to find the optimized value. We conclude that it is feasible to transmit 30 channels of 64QAM signals through 250km of optical fiber with EDFAs without regeneration.