part1 - Communication Systems II Dr. Wa'il A.H. Hadi...

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Communication Systems II Dr. Wa’il A.H. Hadi 1 Digital Communication Systems The term digital communication covers a broad area of communications techniques, including digital transmission and digital radio. Digital transmission, is the transmitted of digital pulses between two or more points in a communication system. Digital radio, is the transmitted of digital modulated analog carriers between two or more points in a communication system. Why Digital There are many reasons The primary advantage is the ease with which digital signals, compared to analog signal, are regenerative. The shape of the waveform is affected by two mechanisms: (1) As all the transmission lines and circuits have some nonideal transfer function, there is a distorting effect on the ideal pulse. (2) Unwanted electrical noise or other interference further distorts the pulse waveform. Both of these mechanisms cause the pulse shape to degrade as a function of distance. During the time that the transmitted pulse can still be reliably identified, the pulse is thus regenerated. The circuit that perform this function at regular intervals along a transmission system are called regenerative repeaters.
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Communication Systems II Dr. Wa’il A.H. Hadi 2 Digital circuits are less subject to distortion and interference than analog circuits. Digital circuits are more reliable and can be produced at lower cost than analog circuits. Also, digital hardware lends itself to more flexible implementation than analog hardware. Digital techniques lend themselves naturally to signal processing functions that protect against interference and jamming. Much data communication is computer to computer, or digital instrument or terminal to computer. Such digital terminations are naturally best served by digital link. Communication System Models Generally, there are two types for communication system models, base-band model and pass-band model. In base-band model, the spectrum of signal from zero to some frequency (i.e. carrier frequency=0). For transmission of base-band signal by a digital communication system, the information is formatted so that it is represented by digital symbols. Then, pulse waveforms are assigned that represented these symbols. This step referred to as pulse modulation or base-band modulation . These waveforms can be transmitted over a cable. Base-band signal also called low-pass signal . In pass-band (or band-pass) signal, the signal has a spectral magnitude that is nonzero for frequency in some band concentrate about a frequency C f f and negligible elsewhere, where f c is the carrier frequency need to be much greater than zero. For radio
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Communication Systems II Dr. Wa’il A.H. Hadi 3 transmission the carrier is covered to an electromagnetic (EM) filed for propagation to desired destination.
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part1 - Communication Systems II Dr. Wa'il A.H. Hadi...

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