Introduction

1. Introduction Analog and Digital Communications Autumn 2005-2006 CS477: Analog and Digital Communications

2. 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 CS477: Analog and Digital Communications

3. 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 CS477: Analog and Digital Communications

4. 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 CS477: Analog and Digital Communications

5. 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 CS477: Analog and Digital Communications

6. t t 1 1 1 0 0 0 0 t 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 CS477: Analog and Digital Communications

7. Elements of Communication Systems • Transmitter • Modulation • Coding • Channel • Attenuation • Noise • Distortion • Interference • Receiver • Detection (Demodulation+Decoding) • Filtering (Equalization) CS477: Analog and Digital Communications

8. Elements of Communication Systems Text Images Video Audio Transmitter Source Decoder Receiver Source Encoder Channel • 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 CS477: Analog and Digital Communications

9. What do we cover in CS477? What do we cover in CS477? n(t) Analog or Digital “Modulator” Analog or Digital “Modulator” Analog or Digital “Demodulator” h(t) + Transmitter Transmitter Channel Receiver • 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 CS477: Analog and Digital Communications

10. 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 CS477: Analog and Digital Communications

11. Block Diagram (Modulator) Analog or Digital Demodulator Channel Transmitter Receiver CS477: Analog and Digital Communications

12. 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 CS477: Analog and Digital Communications

13. 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: CS477: Analog and Digital Communications