CENG 531 Wireless Data Transmission Lecture 2 PDF

Lecture 2: Wireless Data Transmission Chapter 2 Dr. Abdulmalik Alwarafy CENG 531: Wireless Communications and Sensor Networks Lecture 2 Objectives • Discuss the two types of wireless transmissions • Explain the radio frequency spectrum • Describe the basic concepts & techniques related to transm...

Lecture 2: Wireless Data Transmission Chapter 2 Dr. Abdulmalik Alwarafy CENG 531: Wireless Communications and Sensor Networks Lecture 2 Objectives • Discuss the two types of wireless transmissions • Explain the radio frequency spectrum • Describe the basic concepts & techniques related to transmission of data by radio waves • Material feeds into CLO 1: – “Explain the principles of wireless transmission, reception, antennas and propagation” © Cengage Learning 2014 2 Wireless Signals • In wireless communications: – Data transmitted as electromagnetic (EM) waves – EM waves travel in space at speed of light = 3 * 10^8 m/s (300,000 km/s) • Two basic types of waves: – Infrared light – Radio waves © Cengage Learning 2014 3 Figure 2-1 Electromagnetic spectrum © Cengage Learning 2014 4 Infrared Light • It is easy to transmit information with light – Because computers & data communication equipment use binary code – Light has two properties: off & on – 1: presence of light, 0: absence of light • Infrared light: – Adjacent to visible light (although invisible) – A much better medium for data transmission – Less susceptible to interference © Cengage Learning 2014 5 Infrared Light • Infrared wireless systems require: – Emitter that transmits signal (LED) – Detector that receives signal Emitter Detector • Infrared wireless systems send data by intensity of light wave – Detector senses the higher intensity pulse of light • And produces proportional electrical current © Cengage Learning 2014 6 Infrared Light Figure 2-2 Transmitting the letter “A“ message using visible/invisible light, form © Cengagein Learning 2014of Light pulses 7 Infrared Light • Infrared wireless transmission types (or mechanisms): – Directed transmission: called line-of-sight (LoS) – Diffused transmission: relies on reflected light Figure 2-4 Directed infrared transmission © Cengage Learning 2014 9 Infrared Light Figure 2-5 Diffused infrared transmission © Cengage Learning 2014 10 Infrared Light • Illustration of how an Infrared Remote Works https://youtu.be/aoPSX3wYSXo © Cengage Learning 2014 11 Infrared Light • Advantages: – Does not interfere with other types of communication signals, except light – Infrared light does not penetrate walls • Signals are confined to a room © Cengage Learning 2014 12 Infrared Light • Limitations: – Lack of mobility – Range of coverage • Can cover range of only 50 feet (15 m) • Can only be used indoors • Light waves cannot penetrate through materials like wood or concrete – Infrared light is absorbed by most objects – Solid objects, dust & humidity can limit distance that light & infrared waves can travel © Cengage Learning 2014 13 Radio Waves • Radio waves provide the most common & effective means of wireless communications today • Energy travels through space or air in EM waves • Radio (radio telephony) waves – When electric current passes through wire, it creates magnetic field in space around wire – As this magnetic field radiates or moves out, it creates radio waves © Cengage Learning 2014 Antenna Radio Waves • Advantages of radio waves – Travel large distances – Penetrate most solid objects • With the exception of metallic ones – Invisible Figure 2-9 Radio waves can penetrate most solid objects © Cengage Learning 2014 15 How Data is Transmitted? • Radio waves are used to transmit data – Over long distances – Without needing wires • Types of data: – Analog data – Digital data © Cengage Learning 2014 16 Antennas and Transmission • Antenna – Length of copper wire, or similar material with one end free & other end connected to receiver or transmitter • Current applied to antenna generates EM waves at transmitting side • EM Waves hitting antenna induce current in antenna at receiving end © Cengage Learning 2014 17 Antennas and Transmission • How Antennas Work: https://youtu.be/ZaXm6wau-jc © Cengage Learning 2014 18 Radio Frequency Spectrum – Frequency Bands • Radio frequency spectrum: – Entire range of all radio frequencies that exist – Range extends from 3 KHz to over 30 GHz – Spectrum is divided into 450 different sections (bands) © Cengage Learning 2014 Radio Frequency Spectrum – Frequency Bands 3 Table 3-3 Radio frequency bands © Cengage Learning 2014 20 Radio Frequency Spectrum – Frequency Bands • Radio frequencies of common devices include: Garage door openers, alarm systems: 40 MH Radio-controlled cars: 75 MHz Radio-controlled airplanes: 72 MHz Wildlife tracking collars: © Cengage Learning 2014 215 MHz–220 MHz Baby monitors: 49 MHz Global positioning system (GPS): 1.227 GHz & 1.575 GHz Radio Frequency Spectrum – Frequency Bands https://www.ntia.gov/sites/default/files/publications/januar © Cengage Learning 2014 y_2016_spectrum_wall_chart_0.pdf Radio Frequency Spectrum – Licensed & Unlicensed • International spectrum allocations are established by International Telecommunications Union (ITU) • License exempt or Unregulated spectrum: – Radio spectra available without charge to any users without license – Devices from different vendors may use same frequency (disadvantage) © Cengage Learning 2014 23 Radio Frequency Spectrum – Licensed & Unlicensed Table 3-4 Unregulated bands © Cengage Learning 2014 24 (example of analog modulation scheme) 25 © Cengage Learning 2014 © Cengage Learning 2014 26 © Cengage Learning 2014 27 Transmitted Received Multipath fading at shorter timescales © Cengage Learning 2014 28 0 1 0 1 0 ? ? ? ? ? © Cengage Learning 2014 29 Digital Modulation © Cengage Learning 2014 Digital Modulation 1. Amplitude Shift Keying (ASK) – Binary modulation technique that changes height (e.g., amplitude) of carrier signal to represent 1 or 0 bit Figure 2-27 Amplitude shift keying (ASK) © Cengage Learning 2014 31 Digital Modulation 2. Frequency Shift Keying (FSK) – Binary modulation technique that changes frequency of carrier signal to represent 1 or 0 bit Figure 2-28 Frequency shift keying (FSK) © Cengage Learning 2014 32 Digital Modulation 3. Phase Shift Keying (PSK) – Less susceptible to noise – Transmitter varies the starting point of wave • E.g. phase 0 to send 0 & phase 180 to send 1 – Quaternary Phase Shift Keying (QPSK) • 4 different phases to send bits 00, 01, 10,11 110 111 101 Figure 2-29 Phase shift keying (PSK) © Cengage Learning 2014 Figure 2-30 Phase modulation angles Digital Modulation 4. Quadrature amplitude modulation (QAM) – Modulate amplitude in addition to phase of carrier – Further increases speed of transmission by sending more bits of data • Two carriers shifted by 90 degree phase – One in-phase & other out of phase – Vary amplitude of each carrier to convey bit information © Cengage Learning 2014 35 © Cengage Learning 2014 Single vs Multi-Carrier Modulation • ASK, PSK, QAM are all single carrier modulation schemes – Only one carrier signal is modulated – Exact amplitude & phase of carrier should be known to decode correctly – Complex Receiver! • Multi-carrier modulation to the rescue! – Split bit stream into multiple parallel streams – Modulate each stream with multiple carriers – Different carrier frequencies for each stream in same band (64 subcarriers for WiFi 802.11) – This technique called orthogonal frequency division Multiplexing (OFDM). • Standard in all high speed systems today! © Cengage Learning 2014 Content adapted from: https://www.cse.iitb.ac.in/~mythili/teaching/cs653_spring2014/slides/lecture03-phy-modulation.pdf 36 Single vs Multi-Carrier Modulation Carrier_1 © Cengage Learning 2014 … Carrier_n 37 Spread Spectrum Principle • Narrow-band transmissions: – Radio signal transmissions are narrow-band – Each signal transmits on: • one radio frequency or • very narrow range of frequencies – Vulnerable to interference from other signals Figure 2-33 Spread spectrum vs. narrow-band transmission © Cengage Learning 2014 38 Spread Spectrum Principle • Spread spectrum transmission: – Takes narrow-band signal & spreads it over broader portion of frequency band – Results in less interference & fewer errors – Two common methods • Frequency hopping & direct sequence Figure 2-33 Spread spectrum vs. narrow-band transmission © Cengage Learning 2014 39 Frequency Hopping Spread Spectrum (FHSS) • Uses a range of frequencies – Changes frequencies several times during transmission • Hopping code: – Defines sequence of changing frequencies – Receiver must also know hopping code – Both transmitter & receiver generate exact same sequence because: • Both use same pseudo-random generator algorithm & seed – Multiple radios can use different sequence of frequencies within same area • And never interfere with each other 40 © Cengage Learning 2014 Frequency Hopping Spread Spectrum (FHSS) Figure 2-34 Frequency hopping spread spectrum (FHSS) transmission © Cengage Learning 2014 41 FHSS Cycle • Pattern is repeated after every eight hops • Hopping pattern known to both transmitter & receiver © Cengage Learning 2014 42 How does FHSS differ from FDM? © Cengage Learning 2014 43 Advantages of FHSS • If interference is encountered on frequency – Only small part of message is lost • FHSS reduces impact of interference from other signals • An interfering signal will affect FHSS signal only when multiple stations transmit at same frequency & at same time – Because FHSS transmits short bursts over wide range of frequencies, the extent of interference is small • The error can be detected through error checking • Message can be easily retransmitted © Cengage Learning 2014 44 Direct Sequence Spread Spectrum (DSSS) • Uses expanded redundant code to transmit each data bit – then a modulation technique such as QPSK – DSSS signal is effectively modulated twice • Barker (or chipping or pseudo-random) code – particular sequence of 1s & 0s – As in FHSS, both transmitter & receiver generate exact same sequence because: • Both use same pseudo-random generator algorithm & seed • Before transmission, embed original data bit to chipping code © Cengage Learning 2014 45 Direct Sequence Spread Spectrum (DSSS) Figure 2-36 Encoding before modulation in a DSSS transmission • In this example, XOR data bit to be sent with barker code creates bit pattern to be transmitted © Cengage Learning 2014 46 Direct Sequence Spread Spectrum (DSSS) • DSSS system transmits combinations of multiple chips – 11 chips are transmitted at rate 11 times faster than data rate • Characteristics: – Frequency of DSSS signal is much higher than that of original data (chip rate) – A plot of frequency spectrum of DSSS signal looks similar to random noise – All information contained in original signal (0 & 1) is still there! © Cengage Learning 2014 47 Direct Sequence Spread Spectrum (DSSS) Figure 2-37 Spreading the signal over a wider range of frequencies © Cengage Learning 2014 48 Direct Sequence Spread Spectrum (DSSS) • Advantages: – DSSS signal appears to unintended receivers as noise – Noise can cause some of chips to change value • Receiver can recover original data bit – Using statistical techniques & mathematical algorithms – Thus avoiding need for retransmission • Disadvantages: – DSSS devices are more expensive to manufacture than FHSS systems © Cengage Learning 2014 49 Summary • Infrared & radio wave communication – Different characteristics, different application • Different spectrum bands used for radio communication • Principles of radio communication – Channel propagation effects – Motivation for digital modulation – Different modulation techniques • Single carrier ASK, FSK, PSK, QAM • Multi carrier modulation OFDM – Spread spectrum techniques FHSS & DSSS © Cengage Learning 2014 50 Extra Resources • Single and Multi-carrier modulation – https://www.youtube.com/watch?v=rKy5dOl3Et4 © Cengage Learning 2014 51