EE442 Introduction An overview of modern communications EE 442 Analog & Digital Communication Systems Lecture 1

Transcription

1 EE442 Introduction An overview of modern communications EE 442 Analog & Digital Communication Systems Lecture 1 ES 442 Lecture 1 1

2 The Telegraph Revolution Near instantaneous communication Adopted worldwide Became the Victorian Internet Used by railroads, newspapers, financial organizations, businesses of all kinds, Used in the Civil War by both North and South EE 442 Lecture 1 2

3 Class Question: Name some modern communication systems that are in wide use today? What interests you most in communications systems? EE 442 Lecture 1 3

4 Selected Communication Systems In Operation Today Public Switched Telephone Network (PSTN) voice, fax, modem Radio and TV broadcasting Citizens band radio; ham short-wave radio; radio control; etc. Computer networks (LANs, MANs, WANs, and the Internet) Aviation communication bands; Emergency bands; etc. Satellite systems (Military communications) Cable television (originally CATV) for video and data Cellular networks (4 generations Most recent is LTE or 4G) Wi-Fi LANs Bluetooth GPS And of course many, many more.... EE 442 Lecture 1 4

5 Signals Carry Content in Communication Systems Data, messages, and information (i.e., useful content) sent from transmitter to receiver over a channel using electrical signals. A SIGNAL is a sequence of symbols encoding the transmitted message. Today s communication systems mostly use electrical signals that are timevarying, electrical quantities (e.g., voltages, currents, and electro-magnetic field quantities in wireless) where the time variations encodes (i.e., represents) the data, message or information. Important non-electrical signals include acoustic (voice and music). A well-defined language or code is required between sender and receiver for communication. For digital signals we use various digital codes (e.g., binary). For a signal to be considered as information we require: (1) Accurate and timely, (2) Have a specific and organized purpose or focus, and (3) Results in increased understanding or decrease in uncertainty. EE 442 Lecture 1 5

6 Advantages of Digital Over Analog 1. Digital is more robust than analog to noise and interference 2. Digital is more viable when using regenerative repeaters 3. Digital hardware is more flexible by using microprocessors and VLSI 4. Can be coded to yield extremely low error rates with error correction 5. Easier to multiplex several digital signals than analog signals 6. Digital is more efficient in trading off SNR for bandwidth 7. Digital signals are easily encrypted for security purposes VLSI = very large-scale integration SNR = signal-to-noise ratio 8. Digital signal storage is easier, cheaper and more efficient 9. Reproduction of digital data is more reliable without deterioration 10. Cost is coming down in digital systems faster than in analog systems and DSP algorithms are growing in power and flexibility DSP = digital signal processing Analog signals vary continuously and their value is affected by all levels of noise. Reading: Lathi & Ding; Section on pages 321 and 322. EE 442 Lecture 1 6

7 Electrical & Optical Signals Dominate Communication Electrical Signals found in Communication Systems EE 442 Lecture 1 7

8 Battery Voltage (V) Example: Human Speech is Analog Signal A microphone is a transducer Word: erase Resistive carbon layer Back contact Front contact voltage waveform V + Sound waves Button Diaphragm Carbon-Granular Microphone Inventor: Thomas Edison 1877 Expanded view of voltage waveform ES 442 Lecture 1 8

9 Energy Voice Bandwidth (Bell Determined 3400 Hz Was Adequate Voice Channel 0 Hz to 4,000 Hz Voice Bandwidth 300 Hz to 3,400 Hz Telephone Band Filter Shape Voice energy 0 Hz 300 Hz 3,400 Hz 4,000 Hz 7,000 Hz Frequency f (Hz) /ps379/ps8537/prod_white_paper0900aecd806fa57a.html ES 442 Lecture 1 9

10 SOUND INTENSITY LEVEL in decibels (db) Human Speech Intensity and Frequency Boundaries Acoustic signals 120 Human Hearing Chart 100 Discomfort Threshold 80 Music Speech 20 0 Hearing Threshold aging K 2K 5K 10K 20K FREQUENCY in Hertz (Hz) Presbycusis is loss of hearing with age. ES 442 Lecture 1 10

11 Discussion: Electrical-Based Communication Systems Why do electrical systems dominate modern communication systems? Electrical variables = Voltage, current, electric-field & magnetic field. This is not electrical communication. ES 442 Lecture 1 11

12 Communication Systems ES 442 Lecture 1 12

13 Electromagnetic Spectrum (There is only one in the universe) The gateway to WIRELESS. EE 442 Lecture

14 3 khz Frequency Allocation by FCC 30 khz 300 khz AM radio 3 MHz 30 MHz FM radio 300 MHz 3 GHz 30 GHz 300 GHz ES 442 Lecture 1 14 National Telecommunications & Information Administration (NTIA) is an agency of the United States Department of Commerce

15 Opacity Radio and Optical Windows in Atmosphere Just as sight depends upon the Visible Window, wireless communication depends upon the existence of the Radio Window in the EM spectrum. 100 % Charged Particles Microwave Windows Radio Window Partial IR Windows Water & Carbon Dioxide Visible Window Ozone & Molecular Oxygen 0 % MHz GHz Frequency THz Increasing frequency PHz EE 442 Lecture 1 15

16 Zenith Attenuation (db) Total, Dry Air and Water-vapor Zenith Attenuation from Sea Level 1000 W-band & V-band used in satellite communications V-band is 50 to 75 GHz W-band is 75 to 100 GHz 1 Why W/V band for satellite communications? Radio Window Total Water vapor Frequency (Hz) Dry air W & V bands have no crowding in frequency, hence, this provides reduced interference, large bandwidth availability, reduced antenna and electronic components size, and more security in point-to-point links due to smaller beamwidths. EE 442 Lecture 1 16

17 Example: Unlicensed Spectrum ISM and & UHII RF Bands ISM: Industrial, Scientific & Medical & UNII: Unlicensed National Information Infrastructure ISM I ISM II UNI I UNI II ISM III UNI III Frequency (GHz) Band ISM I ISM II ISM III UNII I UNII II UNII III Applications Cordless phones; 1G Wireless Cellular Wi-Fi; Bluetooth; ZigBee; Microwave ovens Cordless phones; Wireless PBX Wi-Fi a/n Short-range indoor; Campus applications Long-range outdoor; Point-to-Point links Frequency Range MHz GHz GHz GHz GHz GHz EE 442 Lecture 1 17

18 Wireless Communication: Radiation from Dipole Antenna dipole Single Direction Shown Here antenna Electric & Magnetic Fields EE 442 Lecture 1 18

19 Channel Limitations and Challenges Propagation loss The greater the distance, the greater the loss (All channels are lossy unless they have gain built into them) Frequency selectivity Most media are transmitted over selective frequency bands (FCC assigns these bands) Time variation Many channels have natural varying conditions which change transmission properties (e.g., temperature changes and moisture content) Nonlinearity Ideally a channel is linear; however, exceptions exist such as satellite communication through the ionosphere Shared usage Most channels are not dedicated to a single user so they must contend with multiple users Noise All channels contribute noise to the signal as it travels through the medium Interference Channels can pick up adjacent communication signals and noise which interfere with the intended signals All of these influence and/or limit the choice of modulation schemes & transmitter/receiver (transceiver) design. EE 442 Lecture 1 19

20 Challenges in Wireless: Fading in Cellular Telephony Radio Waves Base Transceiver Station Also, Moisture in atmosphere causes variations in radio signal strength. Multipath Reception Mobile Station: MS EE 442 Lecture 1 20

21 Shannon-Weaver Model for Communication From ES 101A Wireline, EM waves or Fiber Message being sent Signal Transmitted Signal Received Message received Information Source Transmitter Channel Receiver Information Destination Message put into a format appropriate for transmitting over channel Noise Noise distorts signal with random additions Signal retrieved from channel and converted into a format appropriate for the destination Transmitter will... Encode message data Add a carrier signal (modulation) Set signal parameters for channel transmission and transmit Receiver will... Receive signal Remove the carrier signal (demodulation) Decode the data to put it into format for destination EE 442 Lecture 1 21

22 Amplitude Radio Superheterodyne Receiver Antenna Receiver RF Amplifier & Tuner Mixer IF Amplifier Filter Demodulator AF Amplifier Local Oscillator Audio Speaker LO IF RF frequency f EE 442 Lecture 1 22

23 Why do we cover Analog if Digital is so dominant? 1. The world is fundamentally an analog world (People respond primarily to analog symbols, images & sounds) 2. Digital signals are actually analog signals just encoding digital data (bits still must be converted to physical waveforms) 3. Digital communication systems make use of components leveraged from analog communication systems (e.g., ADC & DAC converters, mixers, amplifiers and antennas) 4. Analog communication systems illustrate high-level issues and principles (especially true as we push data rate limits) 5. Analog communication systems are still in use (e.g., AM and FM radio) IMPACT: We must convert analog data to digital data and vice versa. EE 442 Lecture 1 23

24 amplitude amplitude Analog Signals versus Digital Signals Analog Signals represent the values of physical parameters which vary in time. Amplitude can be any value within a range of values, and Amplitude is time-varying Digital Signals represent a sequence of numbers. The values restricted to a set of discrete values Example: Binary signal with only two values (1 and 0). Amplitude is time-varying but magnitude is not important time time All signal waveforms are analog the difference is what they represent! ES 442 Lecture 1 24

25 amplitude amplitude amplitude ADC Process: Sampling, Quantizing & Encoding Analog to Digital Data Conversion (ADC) Process time time time Analog Signal Analog signal is continuous in time & amplitude Sample Quantize Encode Sampling selects the data points we use to create the digital data Captured Sampled Data Values Discrete time values: few amplitudes from analog signal Quantizing chooses the amplitude values used to encode Quantized Sampled Data Now have discrete Values in both time & amplitude Encoding assigns binary numbers to those amplitude values Digital Signal Now have the digital data which is the final result Note: Discrete time corresponds to the timing of the sampling. EE 442 Lecture 1 25

26 Pulse Code Modulation To communicate sampled values, we send a sequence of bits that represents the quantized value. For 16 quantization levels, 4 bits are required. PCM can use a binary representation of value. The PSTN (1976) uses PCM Figure 1.5 on page 8 of Lathi & Ding EE 442 Lecture 1 26

27 Examples of Digital Encoding Format NRZ Symbols per Bit 1 Selfclocking? No Duty Factor (%) % RZ 2 No 0-50 % NRZI 1 No % Manchester (Biphase L) 2 Yes 50 % Miller 1 Yes % Biphase M (Bifrequency) 2 Yes 50 % And many, many more are possible.... EE 442 Lecture 1 27

28 Data Rate Limits (Shannon Capacity) Noise Dependent Data rate R is limited by channel bandwidth, signal power, noise power and distortion Without distortion or noise, we could transmit without limit in the data rate. However, this is never reality. The Shannon capacity C is the maximum possible data rate for a system with noise and distortion Maximum rate approached with bit error probability close to 0. For additive white Gaussian noise (AWGN) channels, signal power C B log 2 1 in bits per second noise power Shannon obtained C = 32 kbps for telephone channels Nowhere near capacity in wireless systems READ: Lathi & Ding, section on pages EE 442 Lecture 1 28

29 Additive White Gaussian noise Corrupts Signals White means noise power is uniform over all frequencies Digital signal corrupted by White Gaussian noise EE 442 Lecture 1 29

30 Digital Signal Errors From Noise and Interference EE 442 Lecture 1 30

31 Analog Signal Corrupted by Noise Question: Is it possible to recover the analog signal from noise after it has been corrupted (i.e., signal + noise waveform is shown below)? Signal + Noise EE 442 Lecture 1 31

32 The Four Primary Enablers of the Communication Age (from ES 101A Communications in the Information Age ) Alessandra Volta Battery (1800) 1. Harnessing of Electricity Electric Power Generation (1880s) Telegraph & Telephone Radio Waves Guglielmo Marconi Radio Waves ( began in 1896 with wireless telegraphy ) 3. Digitization 4. Transistors & Integrated Circuits Started in 1940s (but accelerated in the 1970s) Transistor 1948 IC invented 1958 (Jack Kilby & Robert Noyce) Moore s Law EE 442 Lecture 1 32

33 Selective History of Communication Technologies 1794 Claude Chappe develops an armature signal telegraph 1837 First commercial electrical telegraph was Cooke-Wheatstone telegraph 1837 Samuel Morse independently develops and patents an electrical telegraph (leads to Morse Code) 1876 Alexander Graham Bell demonstrates voice-based telephone 1896 Wireless telegraphy (radio telegraphy) by Guglielmo Marconi 1901 First transatlantic radio telegraph transmission (Marconi) 1906 First AM radio broadcast by Reginald Fessenden 1920 First commercial radio stations 1921 First mobile radio service (Detroit Police Department) 1928 First television station in United States (W3XK) 1935 Edwin Armstrong demonstrates FM radio 1947 Bell Telephone laboratories proposed cellular concept 1947 BTL invents and demonstrates solid-state transistor 1958 Integrated circuit is invented (Jack Kilby at Texas Instruments) 1984 AMPS cellular mobile service by Motorola 1991 GSM cellular service (digital) service begins 1997 IEEE (b) wireless LAN standard EE 442 Lecture 1 33