1. 01_Intro-A-DCom

Introduction to Analog and Digital Communications August 2013 Lectured by Assoc Prof Dr Thuong Le-Tien Slides with references from HUT Finland, La Hore uni., Mc Graw Hill Co., and A.B Carlson’s Communication Systems book Textbook: A B Carlson, et al "Communication Systems", third ed., McGraw-Hill Inc., New York, 1986, 2002 ISBN: 0-07-100560-9 Communications   Communications = Information transfer This course is about communications in electrical engineering  Limited to information in electrical form   not be considered delivering newspapers Primarily cover information transfer at systems levels  little deal with circuits, chips, signal processing, microprocessors, protocols, and networks What exactly is information?  Information is a word that is too generic for our purposes  use the word “message”   A physical manifestation of information What communication systems have to with messages?  Communication systems are responsible for producing an “acceptable” replica of message at the destination 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  Signal source and message interchangeably 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 Examples for basic definitions  Analog Signals   Values are taken from an infinite set t Digital Signals  Values are taken from a discrete set t  Binary Signals  Digital signals with just two discrete values 1 0 t n( t ) m(t ) (Modulator) Analog or Digital s( t ) Transmitter  Transmitter    Demodulator Channel Receiver Elements of Communication Systems Channel      Modulation Coding h( t )  m(t ) Attenuation Noise Distortion Interference Receiver   Detection (Demodulation+Decoding) Filtering (Equalization) 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 in digital communication systems    Source coding – remove redundancy Channel coding – add redundancy Encryption Coding – hide information Channels  Channel introduces impairments  Noise    Distortion   Inter-symbol interference (ISI) Attenuation and fading    Thermal noise is the most significant Additive white Gaussian noise (AWGN) Constant attenuation Variable attenuation Interference  Crosstalk Receiver  What does demodulator do?    Extracts messages (analog systems) or bits (digital systems) from the received signal Mitigates channel impairments by making use of equalizers Decodes the signal, especially if channel coding was performed at the transmitter 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     Noise     Channel must be able to allow signal to pass through Channels usually have limited bandwidth Can we reduce signal bandwidth? Do “something” at source Can we reduce it? Can we reduce its effects? Do something at the transmitter and receiver Signal to Noise Ratio (SNR) 11 Performance Criterion    How a “good” communication system can be differentiated from a “sloppy” one? For analog communications   How close is m (t ) to m ( t ) ? Fidelity!  SNR is typically used as a performance metric For digital communications  Data rate and probability of error (BER)  No channel impairments, no error  With noise, error probability depends upon data rate, signal and noise powers, modulation scheme 12 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: C = B log(1 + SNR) Bits/second 13 Example: PSTN, ADSL   Public Switched Telephone Network, Asymmetrical Digital Subscriber Line Components     Phone set (analog signal is generated), MODEM Local exchange (A/D conversion) Long-haul exchange Characteristics    Circuit-switched network Designed for voice communications and Internet Faxes and modems use PSTN for transmission of digital data in analog form 14 Example: PSTN Long distance line Local exchange Local line International exchange International line Long distance exchange Long distance line Long distance users Local exchange 15 ADSL Asymmetric Digital Subscriber Line Customer Premises DSL Modem Local Carrier End Office Main Dist ribution Frame Line Splitter Voice Telepho ne Network Local Loop Hub Telepho ne Computer Computer Customer Premises ATM Switch ISP POP DSL Access Multiplexer ISP POP ISP POP ISP POP Customer Premises 25 Example: Cellular PSTN MTSO MTSO 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 (Advanced Mobile Phone System, 1st generation) is an analog system, uses FM and frequency-division multiple access Digital systems use digital modulations, D-AMPS (2nd generation); GSM (2nd genration up); CDMA (3rd generation up) 18 Example: Radio broadcast  Two popular modes are used in analog communication systems  AM     Amplitude modulation 600-1600kHz (MW), 1600kHz-22MHz (SW) 10kHz channels FM    Frequency modulation 88-108MHz Channels centered at 200kHz intervals starting at 88.1MHz 19 Example: Wireless LANs    Various standards IEEE 802.11a/b/g popular IEEE 802.11b     11Mb/s data rate 2.4-2.4835GHz band Modulation: Direct sequence spread spectrum (DSSS), Frequency hopping spread spectrum (FHSS) IEEE 802.11a    55Mb/s data rate 5.725-5.825GHz 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 23