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Wireless Networks

Wireless Networks. Lecture 3 Introduction to Wireless Communication Dr. Ghalib A. Shah. Outlines. Review of previous lecture #2 Multiplexing Transmission Mediums Propagation modes Multi-path propagation Fading Summary of today’s lecture. Last Lecture Review. Wireless Transmission

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Wireless Networks

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  1. Wireless Networks Lecture 3 Introduction to Wireless Communication Dr. Ghalib A. Shah

  2. Outlines • Review of previous lecture #2 • Multiplexing • Transmission Mediums • Propagation modes • Multi-path propagation • Fading • Summary of today’s lecture

  3. Last Lecture Review • Wireless Transmission • Digital data analog signal • Baseband/bandpass signal • Encoding techniques/Modulation • Receiver synchronization / Demodulation • Noises • Thermal noise • Intermodulation noise • Crosstalk • Impulse Noise • Manmade noise / Natural noise • Losses / Gain

  4. Multiplexing • Capacity of transmission medium usually exceeds capacity required for transmission of a single signal • Multiplexing - carrying multiple signals on a single medium • More efficient use of transmission medium

  5. Multiplexing

  6. Reasons for Widespread Use of Multiplexing • Cost per kbps of transmission facility declines with an increase in the data rate • Cost of transmission and receiving equipment declines with increased data rate • Most individual data communicating devices require relatively modest data rate support

  7. Multiplexing Techniques • Frequency-division multiplexing (FDM) • Takes advantage of the fact that the useful bandwidth of the medium exceeds the required bandwidth of a given signal • Time-division multiplexing (TDM) • Takes advantage of the fact that the achievable bit rate of the medium exceeds the required data rate of a digital signal

  8. Frequency-division Multiplexing

  9. Time-division Multiplexing

  10. Classifications of Transmission Media • Transmission Medium • Physical path between transmitter and receiver • Guided Media • Waves are guided along a solid medium • E.g., copper twisted pair, copper coaxial cable, optical fiber • Unguided Media • Provides means of transmission but does not guide electromagnetic signals • Usually referred to as wireless transmission • E.g., atmosphere, outer space

  11. Unguided Media • Transmission and reception are achieved by means of an antenna • Configurations for wireless transmission • Directional • Omnidirectional

  12. General Frequency Ranges • Microwave frequency range • 1 GHz to 40 GHz • Directional beams possible • Suitable for point-to-point transmission • Used for satellite communications • Radio frequency range • 30 MHz to 1 GHz • Suitable for omnidirectional applications • Infrared frequency range • Roughly, 3x1011 to 2x1014 Hz • Useful in local point-to-point multipoint applications within confined areas

  13. Terrestrial Microwave • Description of common microwave antenna • Parabolic "dish", 3 m in diameter • Fixed rigidly and focuses a narrow beam • Achieves line-of-sight transmission to receiving antenna • Located at substantial heights above ground level • Due to attenuation particularly rainfall, requires repeaters/amplifiers placed farther apart 10-100 km. • Applications • Long haul telecommunications service • 4 – 6 GHz band is common • But due to increased congestion 11 GHz is coming into use now • Microwave links provide TV signals to local CATV and then distributed to subscribers via coaxial cable. • Short point-to-point links between buildings • Enterprise offices, university campuses

  14. Satellite Microwave • Description of communication satellite • Communication satellite is Microwave relay station • Used to link two or more ground-based microwave transmitter/receivers • Receives transmissions on one frequency band (uplink), amplifies or repeats the signal, and transmits it on another frequency (downlink) • Broadcast in nature • Applications • Television distribution • Long-distance telephone transmission • Used for point-to-point trunks between telephone exchange offices. • Private business networks

  15. Satellite microwave • Transmission characteristics • Optimum range is 1-10 GHz • Below 1 GHz, significant natural noise (solar, galactic, atmospheric) and manmade • Above 10 GHz, higher attenuation due to atmospheric absorption • Mostly use 5.925-6.425 for uplink and 3.7-4.2 GHz for downlink referred as 4/6 GHz band • Due to saturation, 12/14 GHz band has been developed. Uplink: 14-14.5, downlink: 11.7-12.2 GHz • In Future, 20/30GHz. Uplink: 27.5-30.0, downlink: 17.7-20.2 GHz • Long propagation delay of about 250 ms, which is noticeabe in telephone conversation. • Broadcast in nature and suitable for TV broadcast service.

  16. Broadcast Radio • Description of broadcast radio antennas • Omnidirectional • Antennas not required to be dish-shaped • Antennas need not be rigidly mounted to a precise alignment • Applications • Broadcast radio • VHF and part of the UHF band; 30 MHZ to 1GHz • Covers FM radio and UHF and VHF television • Characteristics • Because of longer wavelength, radio waves relatively suffer less attenuation. • Prime source of impairments is multi-path interference. Reflection from land water and human made objects can create multiple paths. • Less sensitive to rainfall

  17. Propagation Modes • Ground-wave propagation • Sky-wave propagation • Line-of-sight propagation

  18. Ground Wave Propagation

  19. Ground Wave Propagation • Follows contour of the earth • Can Propagate considerable distances • Frequencies up to 2 MHz, which are low frequencies and have tendency to tilt downwards • EM waves of low frequency are scattered by the atmosphere such that they do not penetrate the upper atmosphere. • Example • AM radio

  20. Sky Wave Propagation

  21. Sky Wave Propagation • Signal reflected from ionized layer of atmosphere back down to earth • Signal can travel a number of hops, back and forth between ionosphere and earth’s surface • Reflection effect caused by refraction • Examples • Amateur radio • CB radio

  22. Line-of-Sight Propagation

  23. Line-of-Sight Propagation • Transmitting and receiving antennas must be within line of sight • Satellite communication – signal above 30 MHz not reflected by ionosphere • Ground communication – antennas within effective line of site due to refraction • Refraction – bending of microwaves by the atmosphere • Velocity of electromagnetic wave is a function of the density of the medium • When wave changes medium, speed changes • Wave bends at the boundary between mediums

  24. Line-of-Sight Equations • Optical line of sight • Effective, or radio, line of sight • d = distance between antenna and horizon (km) • h = antenna height (m) • K = adjustment factor to account for refraction, rule of thumb K = 4/3

  25. Line-of-Sight Equations • Maximum distance between two antennas for LOS propagation: • h1 = height of antenna one • h2 = height of antenna two

  26. Example Let h1 = 100 m, h2 = 0 or the second antenna is at ground level. D = 3.57 ( 4/3x100)^1/2 + 0 = 41 km. Now suppose that h2 = 10m. To achieve same distance, what must be h1? 41 = 3.57(Kh1)^1/2+(13.3)^1/2 h1 = 46.2m

  27. Propagation Factors 27 The transmitter’s power output The frequency being transmitted The effect of the Earth’s shape in between the points The conductivity of the Earth along the transmission path The microclimate through which the signal passes

  28. Multipath Propagation • Reflection - occurs when signal encounters a surface that is large relative to the wavelength of the signal • Diffraction - occurs at the edge of an impenetrable body that is large compared to wavelength of radio wave • Scattering – occurs when incoming signal hits an object whose size in the order of the wavelength of the signal or less

  29. Multipath Propagation

  30. The Effects of Multipath Propagation • Multiple copies of a signal may arrive at different phases • If phases add destructively, the signal level relative to noise declines, making detection more difficult • Intersymbol interference (ISI) • One or more delayed copies of a pulse may arrive at the same time as the primary pulse for a subsequent bit

  31. Types of Fading • Fast fading • Rapid variation in signal strength occurs over distance about one-half of wavelength. • At 900 Mhz cellular badn, lambda is 0.33 m. • Slow fading • Users cover distance well in excess of a wavelength as it passes buildings of different heights, vacant lots and so on • A slow variation in signal strength. • Flat fading • Selective fading

  32. Fading channel • Additive white Gaussian noise (AWGN) channel • Signal is degraded only by thermal noise • Accurate for space communication and some wire communication such as coaxial cable. • Rayleigh fading • Fading occurs when there are multiple indirect paths but no direct LOS path • Suitable for Outdoor environment • Rician fading • When there exist a direct LOS path in addition to multiple paths. • Suitable for smaller cells and indoor environment

  33. Summary of today’s lecture • Multiplexing • FDM, TDM • Transmission Mediums • Guided media • Unguided media • Microwave • Radio waves • Infra red • Propagation modes • Ground wave propagation • Sky-wave propagation • LOS propagation • Multi-path propagation • Fading • Next lecture • Error detecting and correcting techniques

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