Over the last few years, internet traffic exponentially increased globally. Due to the huge-capacity of optical fiber, it becomes the most promising solution to provide a fast internet access to the end users, utilizing the advanced multiplexing techniques and modulation formats. This book, therefore, provides a study on a new multiplexing technique entitled Duty-Cycle Division Multiplexing (DCDM) for optical communication systems. The working principles of multiplexer and demultiplexer of DCDM and their implementations are discussed. Power Spectral Density and Bit Error Rate estimation of DCDM is derived theoretically. Performance of DCDM is evaluated in terms of receiver sensitivity, Optical Signal-to-Noise Ratio (OSNR), dispersion tolerance, Self-Phase Modulation, number of optical amplifiers, spectral bandwidth, and transmission distance. Meanwhile, the main impairment factors are discussed. The working principles and structures of transmitter and receiver of different modulation and multiplexing techniques including TDM, WDM, OFDM, PDM, ASK, DPSK, DQPSK, QAM, and Duobinary, are reviewed in this book and their performance is compared against DCDM.