# Chapter 2: Digital Modulation

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Chapter 2: Digital Modulation
Introduction Electronic communications  is the transmission, reception and the processing of the information with the use of electronic circuits Information knowledge or intelligence that is communicated ( transmitted or received ) Digital modulation the transmittal of digitally modulated analog signals (carrier) between two points or more Digital radio  Relatively high-frequency analog carriers modulated by relatively low-frequency digital signals (can be carried by free space)

If the information signal is digital and the amplitude (V) is varied proportional to the information signal amplitude shift keying (ASK) If the information signal is digital and the phase of the carrier is varied proportional to the information signal phase shift keying (PSK) If the information signal is digital and the frequency (f) is varied proportional to the information signal frequency shift keying (FSK)

Digital modulation is good for the use in both cable and wireless system such as
Low-speed voice band data communication modem High-speed data transmission system such as DSL (digital subscriber line) Digital microwave and satellite communication systems Cellular telephone Personal communications systems (PCS)

Information theory  study the efficient use of bandwidth to propagate information through electronic communication systems Information capacity  represent the number of independent symbols (usually binary digit or bit) that can be carried through a system in a given unit of time (bit rate  bps) Hartley’s Law Shannon limit for information capacity

 the binary information signal directly modulates the amplitude of an analog carrier  there are only two output amplitudes possible  mathematically written as

Frequency-shift keying (FSK)
 mathematically written as  when the binary input change logic from 1 to logic 0, the output frequency change from a mark (fm) to a space frequency (fs) as shown in picture

Continuous-Phase Frequency-Shift Keying (CP-FSK)
is normal binary FSK except the mark and space frequencies are synchronized (synchronous does not have to be equal, but there is a precise time relationship between the two frequency )  with CP-FSK, the mark and space frequencies are selected such that they are separated from the center frequency by an exact multiple of one-half the bit rate (fm and fs =n*(fb/2)) in discontinuity FSK, when the input changes, there is an abrupt phase discontinuity in analog signal. So, the demodulator has trouble following the shift frequency causing error CP-FSK has better bit-error performance but expensive to implement

Binary Phase-Shift Keying
two phases are for carrier. One phase represents logic 1 and the other represents logic 0  the phase of the output carrier shifts between two angles that are separated by 180 degree Quadrature –Amplitude Modulation (QAM) is a form of digital modulation similar to PSK but digital information is contained in both the amplitude and phase of the transmitted carrier