Telecom lect 8
- 1. WIRELESS/ RADIATED TRNANSMISION (UNGUIDED MEDIA) MAINLY THREE TYPES: 1- RADIO TRANSMISSION 2- MICROWAVE TRANSMISSION a- Terrestrial Microwave b- Satellite Microwave 3- INFRARED TRANSMISSION
- 2. A Basic Wireless Communications System Works by: β’ Modulating the input signal with a suitable carrier. β’ Amplifying the modulated carrier wave, β’ Radiating it in free space with the use of an antenna. β’ A receiver tuned to the right carrier frequency can recover the transmitted information. β’ An antenna can be used to both transmit and receive E-M waves.
- 3. More on Antennas β’ Antennas are essential components of both the transmitter and receiver of a radio communications system. β’ When transmitting, the antenna radiates energy from a transmission line into free space in the desired direction. β’ When receiving, the antenna picks up the signals transmitted towards it. β’ The signals received are then guided by a transmission line (coaxial cable) to the receiver circuit for demodulation.
- 4. β’ Transmission and reception ANTENNAS CAN BE (Two Types): A- Directional: 2GHz to 300GHz (Microwave) βFocused beam βCareful alignment required βPoint to point/ Point to multipoint βSatellite B- Omni-directional (RADIO WAVES) βSignal spreads in all directions βCan be received by many antenna
- 5. β’ Transmitted using radio waves β’ Energy travels through the air rather than copper or glass β’ Conceptually similar to radio, TV, cellular phones β’ Can travel through walls and through an entire building β’ Can be long distance or short distance βLong distance with satellite relay
- 6. Terrestrial Microwave β’ Typically parabolic dish, focused beam, line of sight β’ Max distance between T and R antenna: d=7.14 * Sqrt(hK) ; K=4/3, ; h=antenna height in meters ; d=distance in km so two 1 meter antenna can be 7.14*Sqrt(4/3)=8.2 km apart β’ Applications β Long haul telecommunications, television. β May need repeaters β Short range (for closed-circuit TV)
- 7. Terrestrial Microwave β’ Attenuation βLoss varies with the square of the distance βTP/Coax: loss varies with log of distance (linear) βTherefore, we donβt need as many repeaters with microwave β’ Interference and Raindrop Attenuation βFrequency bands strictly regulated βUse lower frequency to avoid raindrop problem
- 8. Modes of Radio Wave Propagation β’ Modes describe the way that E-M waves propagate in free space after radiation from an antenna. β’ Various physical characteristics affect the E-M waves propagation in free space. β’ Radio waves may be: Reflected: From the Earth's surface. Refracted: Using upper layers of the atmosphere, to increase range beyond optical horizon. β’ Diffracted: This enables long to medium wave bands to propagate around obstacles.
- 9. Mainly three types of waves propagate: 1- DIRECT WAVE or SPACE WAVE 2- Ground wave or Surface wave 3- Sky waves
- 10. Space Wave or Direct Wave It travels in a straight line directly from transmitter antenna to receiving antenna. It is called Line of Sight (LoS) communication. These are not refracted nor they follow the curvature of earth. Because they travel in straight line they can be blocked by curvature of earth. Therefore as the distance increases height of both antennas should also increase
- 11. If D= distance b/w transmitter & receiver ht= height of transmitter antenna hr = height of receiver antenna Distance traveled by the space wave is: d = sqrt(2ht + 2 hr), h in feet, d in miles If ht=150 ft, hr= 40 ft, d= 26.22 miles
- 12. Direct Wave communication is Important above 30Mhz (VHF, UHF, MICROWAVES). Signals at these frequencies cross the ionosphere because these neither refracted nor reflected. Transmission distances at these frequencies are limited. That is the reason FM and TV broadcasts cover only limited distances. Direct wave is the transmission mechanism for microwave radio and satellite communications. For long distance communication REPEATERS are required. See next slide.
- 14. Ground Wave/ Surface Wave β’ Transmission close to the earth's surface. β’ Main mechanism at VLF and MF for up to 1500 km. β’ Surface waves follow the earth's curvature as it guides (diffracts) them. See next slide
- 15. Ground waves leave the antenna and remains close to the earth and will follow the curvature of earth and can therefore travel distances beyond the horizon. GW is strongest at low and medium frequencies, 30Khz-3-Mhz. Can propagate several hundred miles. AM signals mainly propagate by ground waves. After 3Mhz, earth begins to attenuate the radio signals
- 16. β’ Sky Waves is one that is radiated by the antenna into upper atmosphere where it is reflected back to earth by IONOSPHERE, 30 miles to 250 miles from earth (UV rays from SUN ionize upper atmosphere). Radio Signals are reflected to earth with minimum loss and travel extremely long distances at 3Mhz-30Mhz β’ e.g. AM short wave radio: SW1, SW2, SW3,β¦β¦ β’ VHF and beyond go through the ionosphere (not reflected back).
- 17. Sky Wave & Scatter
- 19. β’ A satellite is a physical object that revolves around some body. The earth and other planets are satellites which revolve around the sun. β’ The Moon is satellite of earth. β’ A BALANCE BETWEEN THE INERTIA OF THE REVOLVING SATELLITE AND THE GRAVITATIONAL PULL OF THE ORBITED BODY KEEPS THE SATELLITE IN ORBIT β’ Artificial satellites are launched into orbit for communication purpose. β’ Space Craft needs speed of 25000 mph to break the gravitational pull of earth. β’ Satellites are taken into their orbits by ROCKETS which shoot them into space.
- 20. TRANSPOPNDER: The Transmitter- Receiver combination in the satellite is called Transponder. Its main function is amplification and frequency translation. Frequency translation because transponder cannot transmit and receive on the same frequency, Strong transmitter signals would overload the receiver. Typical uplink frequency = 6Ghz Typical downlink frequency = 4Ghz
- 21. FERQUENCY REUSE It is a technique by which uplink and downlink signals can be at the same frequency. This DOUBLES the BANDWIDTH and information carrying capacity of the satellite. The two signals operating on the same ferquencies, are isolated from one an other by special ANTENNA TECHNIQUE based on polarization of radiated signal.
- 22. If the electric field of radiating signal is horizontal to earth the signal is called HORIZATALLY POLARISED. If the electric field of radiated signal is vertically polarised with respect to earth, the signal is called βVERTICALLY POLARISEDβ Polarization of radiated signal can be CIRCULAR as well. It can be LEFT HAND or RIGHT HAND CIRCULAR POLARIZATION.
- 23. Thus by using TRNASMITTING and RECEIVING ANTENNAS that are VERTICALLY or HORIZANTALLY POLARISED or if they use RIGHT or LEFT HAND POLARIZATION, two completely separate sets of transponders operating at same frequency can be used simultaneously without any interference. Such a technique provides FREQUENCU REUSE.
- 24. TYPES OF SATELLITES: Mainly three: 1- Geosynchronous- Earth Orbit satellites or simply GEO sat 2- Medium-Earth Orbit satellites or MEO sat 3- Low-Earth Orbit satellites or LEO sat GEO Sats: β’ Speed of Sat = Speed of earth β’ Three sats at the altitude of 22000 miles or 35786 kms from the surface of earth β’ Each 120 degrees from another in geosynchronous orbit around the EQUATOR. β’ Complete one revolution in 24 hours
- 25. MEO sats: β’ May be positioned b/w 5000 kms to 1500 kms from the surface of earth. β’ A sat may take about 6 hours to circle the earth β’ GPS Global Positioning system is an example. β’ 24 sats in GPS at the altitude of 18000 kms provide time & location of ships and vehicles on earth β’ At any time FOUR sats are visible at any time from any point on earth. β’ A receiver in your vehicle can send a signal to four sats and measure how long it takes to for the signal to return and then calculates position on earth
- 26. LEO sats: β’ Can be positioned b/w 500 kms to 2000 kms from the surface of earth β’ Rotation period around earth may be 90 to 120 minutes β’ A sat may have speed of 20,000 -25000 kmph β’ Footprint of sat may have dia of 8000 km β’ Round trip delay time of 20ms β’ A LEO system is a constellation of satellites that work together as a network β’ Examples: Iridium System 66 sats ( by Motorolla), Globalstar system 48 sats , Teledesic system 288 sats
- 27. Satellite Frequency Bands BAND DOWNLINK GHz UPLINK GHz BW MHz L 1.5 1.6 15 S 1.9 2.2 70 C 4 6 500 Ku 11 14 500 Ka 20 30 3500
- 28. More about satellites β’ Satellite is relay station β’ Satellite receives on one frequency, amplifies or repeats signal and transmits on another frequency/frequencies (transponder channels) β’ Applications β TV, telephone β Private business networks β VSAT (Very Small Aperture Terminal) β’ Large corp. with distributed sites β’ Small receiver to Ku-band satellite to Big earth hub β’ Cellular Phones β’ Weather information gathering β’ GPS
- 29. Satellite Transmission Characteristics β’ Optimum Frequency Range 1-10Ghz β Below 1Ghz, natural noise. Above 10Ghz, attenuation from the atmosphere β Most applications use the 5.925-6.425 Ghz range uplink, 4.2-4.7Ghz range downlink (4/6 Ghz Band) β’ Propagation delay β 35784000m / 3.0 * 108 m/s ο 0.12 seconds (one way) β About quarter second propagation delay round trip, (e.g. Listen News bulletin on PTV Network & PTV world) β noticeable for phone conversations, β problem for two-way communications β’ Low orbit satellites a solution? (LEO Sats)
- 30. INFRARED β’Infrared light transmission through air. β’Similar to technology used in TV remote control β’Can propagate throughout a room β’(bouncing off surfaces), β’will not penetrate walls/ Blocked by walls. β’Line of sight (or reflection) β’Low speed, 1-4 Mbps β’Short range, 50-75 feet maximum β’Remote control, IRD port (PC), networking Hubs etc
- 31. LASER βUnidirectional, like microwave βHigher speed than microwave βUses laser transmitter and photo- sensitive receiver at each end β’Point-to-point, typically between buildings β’Can be adversely affected by weather
- 32. Choosing a medium βCopper wire is mature technology, rugged and inexpensive; maximum transmission speed is limited βGlass fiber: β’ Higher speed β’ Resistant to electro-magnetic interference β’ Spans longer distances β’ More expensive
- 33. Choosing a medium β’Radio, microwave, infrared don't require physical connection β’Radio/ infrared may be used for mobile connections β’Laser does not need physical connection/ supports higher speeds
- 34. Network Transmission Error Media Type Cost Distance Security Rates Speed Twisted Pair LAN Low Short Good Low Low-high Coaxial Cable LAN Mod. Short-Mod Good Low Low-high Fiber Optics any High Mod.-long V. Good V-Low High-V-High Network Transmission Error Media Type Cost Distance Security Rates Speed Radio (FM) LAN Low Short Poor Mod Low Infrared LAN Low Short Poor Mod Low Microwave WAN Mod Long Poor Low-Mod Mod Satellite WAN Mod Long Poor Low-Mod Mod Guided Media Radiated Media