Sep 06, 2005CS477: Analog and Digital Communications1 Introduction Analog and Digital Communications Autumn

Sep 06, 2005CS477: Analog and Digital Communications2 Communications Communications = Information transfer This course is about communications Limited to information in electrical form We will not consider delivering newspapers We will primarily cover information transfer at systems level We will not deal [too much] with circuits, chips, signal processing, microprocessors, protocols, and networks

Sep 06, 2005CS477: Analog and Digital Communications3 What exactly is information? Information is a word that is too generic for our purposes We will use the word message A physical manifestation of information What do communication systems have to do with messages? Communication systems are responsible for producing an “acceptable” replica of message at the destination

Sep 06, 2005CS477: Analog and Digital Communications4 Is Signal = Message? Just like information, signal is also a generic word Derived directly from information Scientists and Engineers use signal to denote information in electrical form We will use signal and message interchangeably

Sep 06, 2005CS477: Analog and Digital Communications5 Can we classify signals? Messages or signals can be classified: Analog A physical quantity that varies with “time”, usually in a smooth or continuous fashion Fidelity describes how close is the received signal to the original signal. Fidelity defines acceptability Digital An ordered sequence of symbols selected from a finite set of discrete elements When digital signals are sent through a communication system, degree of accuracy within a given time defines the acceptability

Sep 06, 2005CS477: Analog and Digital Communications6 Examples Analog Signals Values are taken from an infinite set Digital Signals Values are taken from a discrete set Binary Signals Digital signals with just two discrete values tt t

Sep 06, 2005CS477: Analog and Digital Communications7 Elements of Communication Systems Transmitter Modulation Coding Channel Attenuation Noise Distortion Interference Receiver Detection (Demodulation+Decoding) Filtering (Equalization)

Sep 06, 2005CS477: Analog and Digital Communications8 Elements of Communication Systems Encoder: Message  Message Signal or bits Transmitter: Message signal  Transmitted signal Channel: Introduces noise, distortion, interference Receiver: Received Signal  Message Signal Decoder: Message Signal  Original Message Example: Microphone > Speaker Text Images Video Audio Source Encoder Source Decoder Channel Receiver Transmitter

Sep 06, 2005CS477: Analog and Digital Communications9 What do we cover in CS477? Modulation converts message signal or bits into amplitude, phase, or frequency of a sinusoidal carrier (Am, FM, QPSK) Modulation may make the transmitted signal robust to channel impairments Channel introduces noise, distortion, and interference Demodulator tries to mitigate the channel impairments Analog or Digital “Modulator” TransmitterChannel h(t) + Analog or Digital “Demodulator” Receiver n(t) What do we cover in CS477? Analog or Digital “Modulator” Transmitter

Sep 06, 2005CS477: Analog and Digital Communications10 Fundamental Limitations If practical implementation is not a concern and we don’t worry about feasibility, is there something else that limits acceptable communications? Bandwidth Channel must be able to allow signal to pass through Channels usually have limited bandwidth Can we reduce signal bandwidth? Do “something” at source Noise Can we reduce it? Can we reduce its effects? Do something at the transmitter and receiver Signal to Noise Ratio

Sep 06, 2005CS477: Analog and Digital Communications11 Block Diagram (Modulator) Analog or Digital Demodulator Transmitter Channel Receiver

Sep 06, 2005CS477: Analog and Digital Communications12 Performance Criterion How a “good” communication system can be differentiated from a “sloppy” one? For analog communications How close is to ? Fidelity! SNR is typically used as a performance metric For digital communications Data rate and probability of error No channel impairments, no error With noise, error probability depends upon data rate, signal and noise powers, modulation scheme

Sep 06, 2005CS477: Analog and Digital Communications13 Limits on data rates Shannon obtained formulas that provide fundamental limits on data rates (1948) Without channel impairments, an infinite data rate is achievable with probability of error approaching zero For bandlimited AWGN channels, the “capacity” of a channel is: