Unguided Media

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Unguided Media

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1. Unguided Media Updated and revised May 6, 2005 Updated August 17, 2006Updated and revised May 6, 2005 Updated August 17, 2006

2. Data Transmission

3. Lesson Objectives By the end of this lesson, you should be able to: List the impairments found in unguided transmission media Describe the impairments found in unguided transmission media List three major design considerations for antennas and describe their relationship Describe the characteristics of terrestrial microwave, satellite, and radio wireless transmission Distinguish between LEO, MEO, and GEO satellite systems Describe what is meant by VSAT

4. Unguided Media Unguided media – no medium to control or contain signals; therefore, no boundaries Unguided media = air, atmosphere Types of unguided media systems: Microwave Satellite Radio

5. Transmission Impairments Analog signal impairments result in random modifications that degrade signal quality, and can cause errors Digital signal impairments result in bit errors Types of unguided media impairments: Free-space loss Absorption Atmospheric absorption Multipath Refraction Noise/Interference

6. Free Space Loss Free space loss is analogous to attenuation in guided mediaFree space loss is analogous to attenuation in guided media

7. Free-Space Loss Characteristics The higher the frequency, the greater the free-space loss Compensate with: Higher gain antennas Higher transmitter power Shorter spans Directional antennas Power Density – measure of an Airborne signal’s strength. Measured in Watts/meterPower Density – measure of an Airborne signal’s strength. Measured in Watts/meter

8. Absorption Waves can be absorbed by objects – buildings, trees, hills Organic materials absorb more than inorganic Pine needles especially effective in absorbing radio frequency emissions (800 MHz range) At 2.4 GHz, loss ˜ 0.35 dB/meter of loss Compensate with: Higher gain antennas Higher transmitter power levels Shorter spacing between transmitter and receiver; i.e., shorter spans Spans with fewer objects in the transmission paths When a wave is absorbed by the atmosphere, energy cannot disappear. It is converted to heat. This is the principle of a microwave. RF waves are transmitted within the oven. The frequency of the microwave used is about 2,500 MHz. This equates to a wavelength of 4.7”. This wavelength is easily absorbed by waters, fats, and sugars. The absorption of the wave is reflected in heat; hence the food becomes heated.When a wave is absorbed by the atmosphere, energy cannot disappear. It is converted to heat. This is the principle of a microwave. RF waves are transmitted within the oven. The frequency of the microwave used is about 2,500 MHz. This equates to a wavelength of 4.7”. This wavelength is easily absorbed by waters, fats, and sugars. The absorption of the wave is reflected in heat; hence the food becomes heated.

9. Atmospheric Absorption Atmospheric conditions absorb waves Water vapor, oxygen greatest contributors Peak attenuation @ 22 GHz due to water vapor; less below 15 GHz Peak attenuation @ 60 GHz due to oxygen; less below 30 GHz Rain, fog major impediments Heavy rain ˜ 0.5 dB/mile of loss (@ 5.8 GHz) Fog ˜ 0.07 dB/mile of loss (@ 5.8 GHz) Compensate with: Lower frequencies Shorter spans Referring to the previous slide, 22 GHz = 2,200 MHz. So, absorption by the water is the same as the microwave. The impact of rain is small for frequencies below 6 GHz. At 6 GHz and higher, energy is absorbed and scattered by raindropsReferring to the previous slide, 22 GHz = 2,200 MHz. So, absorption by the water is the same as the microwave. The impact of rain is small for frequencies below 6 GHz. At 6 GHz and higher, energy is absorbed and scattered by raindrops

10. Multipath Fading

11. Multipath Fading Waves reflect off of objects – buildings, vehicles, water, etc. Some reflected waves travel to intended destination One direct signal, multiple indirect signals, and… Waves arrive with different delays; result = phase differences Waves can either contribute to, or detract from, direct signal Also known as Rayleigh fading A practical application – car radio is perfectly clear. You stop at traffic light and static becomes terrible. Pull forward just a couple of feet and static is eliminated. Another example of multipath – “ghosts” or multiple images on TV screens.A practical application – car radio is perfectly clear. You stop at traffic light and static becomes terrible. Pull forward just a couple of feet and static is eliminated. Another example of multipath – “ghosts” or multiple images on TV screens.

12. Fresnel Zones

13. Fresnel Zones – So What? F1 = 1st Fresnel Zone. Every point where distance is exactly ½ wavelength longer than direct path F2 = 2nd Fresnel Zone. Every point where distance is exactly 1 wavelength longer than direct path Reflections from: Odd Fresnel Zone – reduces signal level at receiver Even Fresnel Zone – increases signal level at receiver

14. Refraction Waves are bent as they pass through atmosphere Signal speed increases with altitude Somewhat predictable, but weather conditions can cause aberrations in tendencies

15. Noise Noise – unwanted electromagnetic energy inserted in the signals somewhere between transmission and reception Types of Noise: Thermal Noise Cochannel interference Intermodulation noise

16. Thermal Noise As with guided media, thermal noise is unavoidable Arise from the thermal activity of devices and media Impact increases as signal strength decreases Also known as white noise. Has greatest impact on satellite systems, because signal is very weak due to distance it has to travel.Also known as white noise. Has greatest impact on satellite systems, because signal is very weak due to distance it has to travel.

17. Spectrum Reuse Wireless spectrum is limited – a major limitation to wireless systems Two fundamental solution sets: Space division – carve up geography into smaller coverage areas

18. Cochannel Interference Occurs when more than 1 transmitter in wireless system is on same frequency Caused by frequency assignments with too little geographic dispersion By-product of basic tenet of cellular systems – frequency reuse Managed or reduced by: Reducing power levels Maintaining geographic dispersion Types of antennas Management of cochannel interference is the number 1 limiting factor in maximizing capacity of a wireless system

19. Cochannel Interference

20. Intermodulation Interference Occurs whenever signals of different frequencies share the same medium When two frequencies share the same medium, supplemental frequencies are produced (harmonics) a + ß, a – ß Could interfere with deliberate signals at these resultant frequencies Degree of noise is a function of power output Occurs when there is some nonlinearity in system Can be managed through compensating circuits

21. Unguided Transmission Key is the antenna Role of antenna – conversion between electrical signals and airborne signals Transmission – antenna gets electrical signals, and radiates airborne energy into the medium; i.e., air Reception – antenna receives airborne waves from the surrounding medium and converts them to electrical signals Every wireless system MUST have antennas Antenna design is related to three major considerations: Frequency to be transmitted Direction of transmission Power needed for transmission

22. Antenna Relationship to Frequency Wavelength (?) = An ideal antenna size is half the wavelength. However, other proportions also work well – ¼, full size. Speed of light = 186,000 miles/secondAn ideal antenna size is half the wavelength. However, other proportions also work well – ¼, full size. Speed of light = 186,000 miles/second

23. Impact of Direction on Antenna Design The higher the frequency, the easier to focus in a directional beam. AnimationThe higher the frequency, the easier to focus in a directional beam. Animation

24. Microwave First used by military in WWII Successful application led to civilian use – substitute for coaxial cable in late 1940s Generally operates at 1 GHz – 50 GHz Vulnerable to reflections, absorption, frequency reuse Highly directional beam Affected by weather Requires line-of-sight; free of obstructions Distance between Systems also dependent upon frequencies 2, 4, 6 GHz system towers could range 45 miles; with LOS restrictions, closer to 35 miles 18, 23, 45 GHz systems range 1 – 5 miles In 1960s, Jack Goeken built a microwave system between Chicago and St. Louis to carry long distance telephony. This led to MCI being the first real competition to AT&T, which in turn ultimately led to Divestiture of the Bell System in 1984.In 1960s, Jack Goeken built a microwave system between Chicago and St. Louis to carry long distance telephony. This led to MCI being the first real competition to AT&T, which in turn ultimately led to Divestiture of the Bell System in 1984.

25. Microwave Antennas Highly Directional Most common form is a parabolic reflector Dish 6 – 10’ in diameter Radome loss ˜ 0.5 – 1 dB Radome – any protective covering over an antenna to protect it from the environment, while still allowing RF energy to pass through.Radome – any protective covering over an antenna to protect it from the environment, while still allowing RF energy to pass through.

26. Microwave – Pros and Cons Cost savings Portability Reconfiguration flexibility Bandwidth Requires line-of-sight Susceptible to natural environmental conditions Regulatory licensing requirements Potential community environmental restrictions

27. Satellite 1947 – Arthur Clarke (2001: A Space Odyssey) presented a paper suggesting the use of satellites for communications 1963 NASA launched 1st experimental satellite 1965 – 1st commercial satellite 2003 – space clutter: >250 communications satellites, total satellites exceed 700; plus 250,000 pieces of debris Satellite = microwave repeater/relay station Receives transmissions on uplink, retransmits them on downlink

28. Satellite Effectiveness

29. Satellite Characteristics Key component: transponder Accepts signal from earth Shifts signal to another frequency Amplifies signal and… Rebroadcasts signal to earth Distance has impact on system: Requires significant power Amount of delay is measurable and significant factor Uplink always at a higher frequency than downlink

30. Classes of Satellites

31. GEO Satellites Geosynchronous earth orbit 22,300 miles above earth Requires the most power Adds greatest delay: 0.25 sec/leg Position is constant relative to earth – same rotational speed as the earth Provides largest footprint of all satellites Three satellites can cover earth Applications: One way broadcasts, international TV

32. MEO Satellites Middle earth orbit Orbit 6,200 – 9,400 miles above earth Delay reduced to 0.05 per leg Smaller footprint; requires 10-15 to cover earth Applications: regional use due to footprint and speed, such as mobile voice, low-speed data Most rapidly growing application: GPS Why does GPS use MEO instead of GEO – needs 3 – 4 satellite signals in order to determine exact location.Why does GPS use MEO instead of GEO – needs 3 – 4 satellite signals in order to determine exact location.

33. LEO Satellites Low earth orbit Closest to earth: 400 – 1,000 miles above earth Least amount of delay: 0.025 seconds/leg Least amount of power required; can be directed into user’s handheld device Smallest footprint: requires approximately 60 to cover earth Functionality is new due to speed and small footprint – switching capability was needed and the system is very complex Jitter is a significant issue Applications: mobile voice, low-speed data, high-speed data

34. VSAT Very Small Aperture Terminal Characterized by very small antenna (0.6 meters or less) Low cost, easy and quick installation Applications: Vehicle tracking systems Broadband Internet access (Hughes DirecPC™ provides downlinks @ 2 Mbps) Business video

35. Satellites – Pros and Cons Access to remote areas Covers large geographies Insensitive to topology Insensitive to distance-related costs High bandwidth Economic value increases with number of locations High initial cost Propagation delay Vulnerable to environmental interference Licensing requirements Vulnerable to space clutter Low security – requires encryption

36. Radio Microwave Antennas are less directional, ranging to full omnidirectional Common frequency range 3 KHz – 300 GHz Most significant application – mobile telephony

37. Radio – Pros and Cons Less sensitive to environmental attenuation Cost savings Portability Reconfiguration flexibility Bandwidth Requires line-of-sight Regulatory licensing requirements Potential community environmental restrictions Vulnerable to multipath interference

38. What We’ve Covered List the impairments found in unguided transmission media Describe the impairments found in unguided transmission media List three major design considerations for antennas and describe their relationship Describe the characteristics of terrestrial microwave, satellite, and radio wireless transmission Distinguish between LEO, MEO, and GEO satellite systems Describe what is meant by VSAT

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