DIGITAL MODULATION

Digital Modulation Basics l The bit rate defines the rate at which information is passed. l The baud (or signaling) rate defines the number of symbols per second. Each symbol represents n bits, and has M signal states, where M = 2n. This is called M- ary signaling. l The maximum rate of information transfer through a baseband channel is given by: Capacity fb = 2 W log2M bits per second where W = bandwidth of modulating baseband signal

Digital Modulation • Any form of digital modulation necessarily uses a finite number of distinct signals to represent digital data. • In the case of PSK, a finite number of phases are used. • In the case of FSK, a finite number of frequencies are used. • In the case of ASK, a finite number of amplitudes are used.

Digital Modulation

Digital Modulation Formats ASK PSK FSK

Amplitude Shift Keying (ASK) The most basic (binary) form of ASK involves the process of switching the carrier either on or off, in correspondence to a sequence of digital pulses that constitute the information signal. One binary digit is represented by the presence of a carrier, the other binary digit is represented by the absence of a carrier. Frequency remains fixed

Amplitude Shift Keying (ASK) Amplitude-shift keying (ASK) is a form of modulation that represents digital data as variations in the amplitude of a carrier wave. The amplitude of an analog carrier signal varies in accordance with the bit stream (modulating signal), keeping frequency and phase constant. The level of amplitude can be used to represent binary logic 0s and 1s. We can think of a carrier signal as an ON or OFF switch. In the modulated signal, logic 0 is represented by the absence of a carrier, thus giving OFF/ON keying operation and hence the name given.

Amplitude Shift Keying (ASK)

Frequency Shift Keying (FSK) The most basic (binary) form of FSK involves the process of varying the frequency of a carrier wave by choosing one of two frequencies (binary FSK) in correspondence to a sequence of digital pulses that constitute the information signal. Two binary digits are represented by two frequencies around the carrier frequency. Amplitude remains fixed

Frequency Shift Keying (FSK) A form of frequency modulation used especially in telegraph, data, and facsimile transmission, in which the modulating wave shifts the output frequency between predetermined values corresponding to the frequencies of correlated sources. Abbreviated FSK. Also known as frequency-shift modulation; frequency-shift transmission.

Frequency Shift Keying (FSK)

Phase Shift Keying (PSK) Varying the angle of a wave in a carrier in order to transmit analog or digital data. For digital signals, phase modulation (PM) is widely used in conjunction with amplitude modulation (AM). For example, quadrature amplitude modulation (QAM) uses both phase and amplitude modulation to create different binary states for transmission

Phase Shift Keying (PSK)

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