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Unformatted text preview: ECE 342 Communication Theory Fall 2005, Class Notes Prof. Tiffany J Li www: http://www.eecs.lehigh.edu/ jingli/teach email: email@example.com Analog Communications Modulation and Communication Systems Modulation is a process that causes a shift in the range of frequencies in a signal. In effect, modulation converts the message signal from lowpass to bandpass . Two types of communication systems. Baseband communication: does not use modulation. * The term baseband is used to designate the band of frequen- cies of the signal delivered by the source or the input transducer. In telephony, the baseband is the audio band (band of voice signals) of 0 to 3.5kHz. In television, the baseband is the video band occupying 0 to 4.3MHz. For digital data or pulse-code modulation (PCM) using bipo- lar signaling at a rate R b pulses per second, the baseband is 0 to R b Hz. * Baseband signals cannot be transmitted over a radio link but are suitable for transmission over a pair of wires, coaxial cables, or optical fibers. Examples include: local telephony communication short-haul PCM between two exchanges long-distance PCM over optical fibers Carrier communication: uses modulation. * Eg: amplitude modulation, frequency modulation, phase mod- ulation. 1 * A comment about pulse-modulated signals (including pulse amplitude modulation (PAM), pulse width modulation (PWM), pulse position modulation (PPM), pulse code modulation (PCM) and delta modulation (DM)): despite the term modulation, these signals are baseband signals. Pulse-modulation schemes are really baseband coding schemes, and they yield baseband signals. These signal must still modulate a carrier in order to shift their spectra. Objectives of Modulation To translate the frequency of the lowpass signal to the passband of the channel so that the spectrum of the transmitted bandpass signal will match the passband characteristics of the channel. (Eg. in transmission of speech over microwave links, the transmission frequencies must be increased to the gigahertz range.) To reduce the size of the antennas. (To obtain efficient radiation of electromagnetic energy, the antenna must be longer than 1/10 of the wavelength.) To accommodate for the simultaneous transmission of signals from sev- eral message sources (e.g. frequency-division multiplexing). To expand the bandwidth of the transmitted signal in order to increase its noise and interference immunity in transmission over a noisy channel (i.e. use modulation to exchange transmission bandwidth for the signal- to-noise ratio). Energy Spectral Density and Power Spectral Density Three ways of computing the energy of a signal g ( t ): E g = Z - | g ( t ) | 2 dt = R g (0) = Z - | G ( f ) | 2 df where R g ( ) = g ( ) * g * (- ) is the (time) auto correlation function of g ( t )....
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This note was uploaded on 08/06/2008 for the course ECE 342 taught by Professor Li during the Fall '05 term at Lehigh University .
- Fall '05