1 Communication System 1 September 2007 Prepared and Lectured by Assoc Prof. Thuong Le-Tien Slides with references from HUT Finland, La Hore uni., Mc. Graw Hill Co., and A.B. Carlson’s Communication Systems book

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

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

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

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

6 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

7 Elements of Communication Systems Transmitter Modulation Coding Channel Attenuation Noise Distortion Interference Receiver Detection (Demodulation+Decoding) Filtering (Equalization)

8 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

9 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) Analog or Digital “Modulator” Transmitter

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

11 Block Diagram (Modulator) Analog or Digital Demodulator Transmitter Channel Receiver

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

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:

14 Example: PSTN Public Switched Telephone Network Components Phone set (analog signal is generated) Local exchange (A/D conversion) Long-haul exchange Characteristics Circuit-switched network Designed for voice communications (analog???) Faxes and modems use PSTN for transmission of digital data in analog form

15 Example:PSTN Local exchange International exchange Long distance exchange Local exchange Local line Long distance line International line

16 Example: Cellular Islamabad MTSO PSTN MTSO Lahore MTSO: Mobile Telephone Switching Office

17 Example: Cellular Cellular Communication System A cell is assigned some number of channels Typically one channel is allocated to a user Users communicate with a base station Base station is connected to MTSO/PSTN AMPS is an analog system Uses FM and frequency-division multiple access Digital systems use digital modulation

18 Example: Radio broadcast Two modes are used AM Amplitude modulation kHz 10kHz channels FM Frequency modulation MHz Channels centered at 200kHz intervals starting at 88.1MHz

19 Example: Wireless LANs Various standards IEEE a/b/g popular IEEE b 11Mb/s data rate GHz band Modulation: Direct sequence spread spectrum (DSSS) IEEE a 55Mb/s data rate GHz band (in U.S.) Uses orthogonal frequency division multiplexing (OFDM)

20 Example: LANs and WANs Local Area Networks (LANs) Connect “closely” located computers Data bits are transmitted in chunks (packets) for efficiency/feasibility reasons Various LAN protocols are used in practice Wide Area Networks (WANs) A wide area backbone network connects different LANs A standard protocol is needed for such communication (TCP/IP)

21 Example: Ad Hoc Networks Various devices connected to each other without using an infrastructure Sensor Networks Similar to ad hoc Networks (may be considered a special case of ad hoc networks) Have power constraints (Use non-rechargeable battery) Mesh Networks Another example of ad hoc networks Used for provide communications to remote areas

22 A Generic Communication System (Modulator) Analog or Digital Demodulator Transmitter Channel Receiver

23 Elements of Communication Systems Transmitter Modulation Coding Channel Attenuation Noise Distortion Interference Receiver Detection (Demodulation+Decoding) Filtering (Equalization)

24 Transmitter What does modulation do? Encodes messages (analog) or bits (digital) into amplitude, frequency, or phase of a carrier signal Also makes transmitted signal robust against channel impairments Coding Source coding – remove redundancy Channel coding – add redundancy

25 Channel Channel introduces impairments Noise Thermal noise is the most significant Additive white Gaussian noise (AWGN) Distortion Inter-symbol interference Attenuation and fading Constant attenuation Variable attenuation Interference Crosstalk

26 Receiver What does demodulator do? Extracts messages or bits from the received signal Mitigates channel impairments by making use of equalizers Decodes the signal, especially if channel coding was performed at the transmitter

27 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

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