ABSTRACT
The transmission of digital information corresponds to the transmission of binary words which are encoded by the alphabet of a symbol. This chapter discusses the basic principles of three important digital modulation techniques that are not only used to transmit digital information in traditional communication systems, but are also used for underwater acoustic communication. The three digital modulation techniques are M-ary Frequency-Shift Keying (MFSK), M-ary Quadrature Amplitude Modulation (MQAM), and Orthogonal Frequency-Division Multiplexing (OFDM). For each waveform the chapter provides a time-domain description, derives its frequency spectrum, derive sbandwidth and time-average power formulas, and discusses how to demodulate the waveform. It then compares the advantages and disadvantages of each digital modulation technique. Demodulation is a procedure meant to retrieve a transmitted message at a receiver. The chapter computes the energy and time-average power of the OFDM signal.
