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Chapter 4 – Transmission Media Wireless Media. 1/23. Wireless Transmission Frequencies. 1GHz to 40GHz Microwave frequency range highly directional beams are possible Point-to-point transmission Used also in satellite communications 30MHz to 1GHz Radio frequency range

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  1. Chapter 4 – Transmission Media Wireless Media 1/23

  2. Wireless Transmission Frequencies • 1GHz to 40GHz • Microwave frequency range • highly directional beams are possible • Point-to-point transmission • Used also in satellite communications • 30MHz to 1GHz • Radio frequency range • Omnidirectional (all directions) • broadcast radio and TV • 3 x 1011 to 2 x 1014 Hz • Infrared portion of the spectrum • Local point-to-point and multipoint applications(e.g., infrared remote control, wireless LAN) 2/23

  3. Antennas • Electrical conductor used to radiate or collect electromagnetic energy • Transmission antenna • radio frequency energy from transmitter converted to electromagnetic energy by antenna • radiated into surrounding environment • Reception antenna • electromagnetic energy impinging on antenna converted to radio frequency electrical energy • fed to receiver • Same antenna is often used for both purposes 3/23

  4. Antenna Radiation Pattern • Power radiated in all directions • Not same performance in all directions • as seen in a radiation pattern diagram • An isotropic antenna is a (theoretical) point in space • reference antenna • radiates in all directions equally • with a spherical radiation pattern 4/23

  5. Parabolic Reflective Antenna 5/23

  6. Antenna Gain • Measure of directionality of antenna • Defined as the output power in particular direction compared to that produced by an isotropic antenna • The antenna gain is related to the effective area of an antenna: • The isotropic antenna has a gain of 1=0dB with • Example: what is the gain of a parabolic antenna with effective area , radius r=1m and f=12GHz? 6/23

  7. Terrestrial Microwave • Used as an alternative to coaxial and fiber cables • Requires fewer repeaters but line-of-sight • Used also for short point-to-point links between buildings • Use a parabolic dish to focus a narrow beam onto a receiver antenna • 1-40GHz frequencies • Higher frequencies give higher data rates • Main source of loss is attenuation; the loss : 7/23

  8. Satellite Microwave • A satellite is a microwave relay station • Receives on one frequency band (uplink), amplifies or repeats signal and transmits on another frequency band (downlink) • eg. uplink 5.925-6.425 GHz & downlink 3.7-4.2 GHz • A satellite operate on a number of frequency bands called transponders • The satellite requires geo-stationary orbit • height of 35,784km • spaced at least 3-4° apart (to minimize interference from other satellites) • typical uses • Television distribution • long distance telephone transmission • private business networks • Global Positioning System (GPS) 8/23

  9. Satellite Point-to-Point Link 9/23

  10. Satellite Broadcast Link 10/23

  11. Broadcast Radio • Use broadcast radio, 30MHz - 1GHz, for: • FM radio • UHF and VHF television • Is omnidirectional • Still need line-of-sight • Suffers from multipath interference • reflections from land, water, other objects 11/23

  12. Infrared • modulate infrared light • are blocked by walls • no licenses required • typical uses • TV remote control • Infrared WLAN 12/23

  13. Wireless Propagation: Ground Wave • Ground wave is found in frequencies up to 2MHz • The best-known example of ground wave communication is AM radio 13/23

  14. Wireless Propagation: Sky Wave • Sky wave propagation is found in frequencies from 2MHz to 30MHz • A sky wave signal bounces back and forth between the ionosphere and the earth surface 14/23

  15. Wireless Propagation: Line-Of-Sight (LOS) • LOS propagation is found in frequencies above 30MHz • the transmitting and receiving antennas must be within a line of sight of each other 15/23

  16. Refraction • velocity of electromagnetic wave is function of material density ~3 x 108 m/s in vacuum, less in anything else • speed changes as move between media • Index of refraction (refractive index) is: • have gradual bending if medium density varies • density of atmosphere decreases with height • results in bending of radio waves towards earth • hence optical LOS horizon and radio LOS horizon are not the same 16/23

  17. Optical and Radio LOS • The optical LOS to the horizon can be expressed as: • The radio LOS to the horizon can be expressed as: • The max. distance between two antennas for LOS propagation: 17/23

  18. EXAMPLE • The max. distance between two antennas for LOS transmission if one antenna is 100m high and the other at ground level is: Now suppose that the receiving antenna is 10m high. To achieve the same distance, how high must the transmitting antenna be? The result is 18/23

  19. Free Space Loss • The free space loss for ideal isotropic antenna is: • For other antennas, we must take into account antenna gain: 19/23

  20. Free Space Path Loss • The free space path loss is a function of both the distance and the carrier frequency: 20/23

  21. Example • Determine the free space loss at 4GHz for the shortest path to a satellite from earth (35.863Km). • Now consider the antenna gain of both the satellite and earth station antennas. Typical values are 44dB and 48dB, respectively. The free space path loss: • What is the received power at the satellite antenna if the transmitted power from the earth station antenna is 250W? 21/23

  22. Impairments in wireless LOS transmission • Free space loss • loss of signal with distance • Atmospheric Absorption • from water vapour and oxygen absorption • Multipath • multiple interfering signals from reflections • Refraction • bending signal away from receiver 22/23

  23. Examples of Multipath Interference 23/23

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