Chapter One: Communication Processes PDF

Chapter One Background and Preview  The Communication Process  In the most fundamental sense, communication involves the transmission of information from one point to another through a succession of processes.  There are three basic elements to every communication syst...

Chapter One Background and Preview  The Communication Process  In the most fundamental sense, communication involves the transmission of information from one point to another through a succession of processes.  There are three basic elements to every communication system: (Transmitter, Channel, and Receiver)  Communication System  The transmitter is located at one point in space and its purpose is to transform the message signal produced by the source of information into a form suitable for transmission over the channel.  The receiver has the task of operating on the received signal so as to reconstruct a recognizable form of the original message signal and to deliver it to the user destination.  The channel is the physical medium that connects the transmitter and the receiver. The transmission medium may be a cable, an optical fiber, or free space if using radio or infrared communication. - The source of information: voice, music, pictures, videos, or data files.  Communication Channels  The communication channel is the physical medium that do the transmission from transmitter to receiver.  The channels can be classified in different ways: – A channel may be linear or nonlinear. Examples: Telephone channel is linear Satellite channel is nonlinear – A channel may be time-invariant or time-varying. Examples: Optical fiber is time-invariant Mobile radio channel is time-varying – A channel may be bandwidth limited or power limited. Examples: Telephone channel is bandwidth limited Optical fiber and satellite channel are power limited  Example: Communication System Transmitter (Tx) Source Source Channel Modulation D/A input Coder Coder Channel Distortion and noise + Reconstructed Signal Source Channel demodulation A/D output decoder decoder Receiver (Rx)  The Modulation Process  The purpose of a communication system is to deliver a message signal from an information source to a user destination.  To do this, the transmitter modifies the message signal into a form suitable for transmission over the channel.  This modification is achieved by means of a process known as modulation, which involves varying some parameters of a carrier wave in accordance with the message signal.  The receiver recreates the original message from the transmitted signal after propagation through the channel using a process known as demodulation.  Purpose of Modulation For a signal (like the electric signals coming out of a microphone) to be transmitted by an antenna, signal wavelength has to be comparable to the length of the antenna (signal wavelength is equal to 0.1 of the antenna length or more). If the wavelength is extremely long, modulation must be used to reduce the wavelength of the signal to make the length of the required antenna practical. To receive transmitted signals from multiple sources without interference between them, they must be transmitted at different frequencies (frequency multiplexing) by modulating carriers that have different frequencies with the different information signals.  Modulation Baseband Passband m(t) s(t) (modulated) message Modulator signal signal c(t) Carrier  Primary Communication Resources  In a communication system, there are two primary resources to be employed: 1. Transmitted Power: The transmitted power refers to the average power of the transmitted signal. 2. Channel Bandwidth: The channel bandwidth is defined as the band of frequencies allocated for the transmission of the message signal.  A general system design objective is to use these two resources as efficiently as possible.  The Decibel (dB)  Measure of power transfer: 1 dB = 10 log10 (Pout / Pin) 1 dBm = 10 log10 (P / 10-3) where P is in Watts 1 dBmV = 20 log10 (V / 10-3) where V is in Volts