wk10 Satellite Microwave

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Published on March 24, 2008

Author: Prudenza

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Fundamentals of Microwave & Satellite Technologies:  Fundamentals of Microwave & Satellite Technologies ICS 620 Fall 2003 Week #10 Historical Perspective:  Historical Perspective Founded during WWII Used for long-haul telecommunications Displaced by fiber optic networks Still viable for right-of-way bypass and geographic obstruction avoidance Microwave Spectrum:  Microwave Spectrum Range is approximately 1 GHz to 40 GHz Total of all usable frequencies under 1 GHz gives a reference on the capacity of in the microwave range Microwave Impairments:  Microwave Impairments Equipment, antenna, and waveguide failures Fading and distortion from multipath reflections Absorption from rain, fog, and other atmospheric conditions Interference from other frequencies Microwave Engineering Considerations:  Microwave Engineering Considerations Free space & atmospheric attenuation Reflections Diffractions Rain attenuation Microwave Engineering Considerations-cont’d:  Microwave Engineering Considerations-cont’d Skin affect Line of Sight (LOS) Fading Range Interference Free Space & Atmospheric Attenuation:  Free Space & Atmospheric Attenuation Free space & atmospheric attenuation is defined by the loss the signal undergoes traveling through the atmosphere. Changes in air density and absorption by atmospheric particles. Reflections:  Reflections Reflections can occur as the microwave signal traverses a body of water or fog bank; cause multipath conditions Diffraction:  Diffraction Diffraction is the result of variations in the terrain the signal crosses Rain Attenuation:  Rain Attenuation Raindrop absorption or scattering of the microwave signal can cause signal loss in transmissions. Skin Affect:  Skin Affect Skin Affect is the concept that high frequency energy travels only on the outside skin of a conductor and does not penetrate into it any great distance. Skin Affect determines the properties of microwave signals. Line of Sight Fresnel Zone Clearance:  Line of Sight Fresnel Zone Clearance Fresnel Zone Clearance is the minimum clearance over obstacles that the signal needs to be sent over. Reflection or path bending will occur if the clearance is not sufficient. LOS & FZC-cont’d:  LOS & FZC-cont’d Fresnel Zone D1 D2 72.2 D1 X D2 F x D secret formula Microwave Fading:  Microwave Fading Normal Signal Reflective Path Caused by multi-path reflections and heavy rains Range:  Range The distance a signal travels and its increase in frequency are inversely proportional Repeaters extend range Back-to-back antennas reflectors Range-cont’d:  Range-cont’d High frequencies are repeated/received at or below one mile Lower frequencies can travel up to 100 miles but 25-30 miles is the typical placement for repeaters Interference:  Interference Adjacent Channel Interference digital not greatly affected Overreach caused by signal feeding past a repeater to the receiving antenna at the next station in the route. Eliminated by zigzag path alignment or alternate frequency use between adjacent stations Components of a Microwave System:  Components of a Microwave System Digital Modem Radio Frequency (RF) Unit Antenna Digital Modem:  Digital Modem The digital modem modulates the information signal (intermediate frequency or IF). RF Unit:  RF Unit IF is fed to the RF unit which is mounted as close physically to the antenna as possible (direct connect is optimal). Antenna:  Antenna The antenna is a passive device that radiates the modulated signal. It is fed by direct connect of the RF unit, coaxial cable, or waveguides at higher frequencies. Waveguides:  Waveguides Waveguides are hollow channels of low-loss material used to direct the signal from the RF unit to the antenna. Modulation Methods:  Modulation Methods Primarily modulated today with digital FM or AM signals Digital signal remains quiet until failure threshold bit error rate renders it unusable Bit Error Rate (BER):  Bit Error Rate (BER) The BER is a performance measure of microwave signaling throughput 10 or one error per million transmitted bits of information Data fail over is at 10 ; voice traffic can withstand this error rate -6 -3 Diversity:  Diversity Space Diversity Frequency Diversity Hot Standby PRI Space Diversity:  Space Diversity Normal Signal Faded Signal Transmitter Receiver Space Diversity-cont’d:  Space Diversity-cont’d Space Diversity protects against multi-path fading by automatic switch over to another antenna place below the primary antenna. This is done at the BER failure point or signal strength attenuation point to the secondary antenna that is receiving the transmitted signal at a stronger power rating. Frequency Diversity:  Frequency Diversity Receiver Active XTMR Frequency #1 Protect XTMR Frequency #2 RCVR Frequency #1 RCVR Frequency #2 Transmitter Frequency Diversity-cont’d:  Frequency Diversity-cont’d Frequency Diversity uses separate frequencies (dual transmit and receive systems); it monitors primary for fail over and switches to standby. Interference usually affects only one range of frequencies. Not allowed in non-carrier applications because of spectrum scarcity. Hot Standby*:  Hot Standby* Receiver System XTMR Primary #1 System XTMR Standby #2 failure switch Active RCVR #1 Standby RCVR #2 Transmitter *Hot standby is designed for equipment failure only PRI:  PRI Receiver Transmitter Connect to PRI interface & PSTN Connect to PRI interface & PSTN To PSTN To PSTN System Transmission Facilities System Receiver Facilities Availability Formula:  Availability Formula Percent Availability equals: 1 – (outage hours/8760 hours per year) Private microwaves have 99.99% availability Microwave Path Analysis:  Microwave Path Analysis Transmitter output power Antenna gain proportional to the physical characteristics of the antenna (diameter) Free space gain Antenna alignment factor Unfaded received signal level Microwave Radio Applications:  Microwave Radio Applications Satellite Communications:  Satellite Communications Satellite-Related Terms:  Satellite-Related Terms Earth Stations – antenna systems on or near earth Uplink – transmission from an earth station to a satellite Downlink – transmission from a satellite to an earth station Transponder – electronics in the satellite that convert uplink signals to downlink signals Ways to Categorize Communications Satellites:  Ways to Categorize Communications Satellites Coverage area Global, regional, national Service type Fixed service satellite (FSS) Broadcast service satellite (BSS) Mobile service satellite (MSS) General usage Commercial, military, amateur, experimental Classification of Satellite Orbits:  Classification of Satellite Orbits Circular or elliptical orbit Circular with center at earth’s center Elliptical with one foci at earth’s center Orbit around earth in different planes Equatorial orbit above earth’s equator Polar orbit passes over both poles Other orbits referred to as inclined orbits Altitude of satellites Geostationary orbit (GEO) Medium earth orbit (MEO) Low earth orbit (LEO) Geometry Terms:  Geometry Terms Elevation angle - the angle from the horizontal to the point on the center of the main beam of the antenna when the antenna is pointed directly at the satellite Minimum elevation angle Coverage angle - the measure of the portion of the earth's surface visible to the satellite Minimum Elevation Angle:  Minimum Elevation Angle Reasons affecting minimum elevation angle of earth station’s antenna (>0o) Buildings, trees, and other terrestrial objects block the line of sight Atmospheric attenuation is greater at low elevation angles Electrical noise generated by the earth's heat near its surface adversely affects reception GEO Orbit:  GEO Orbit Advantages of the the GEO orbit No problem with frequency changes Tracking of the satellite is simplified High coverage area Disadvantages of the GEO orbit Weak signal after traveling over 35,000 km Polar regions are poorly served Signal sending delay is substantial LEO Satellite Characteristics:  LEO Satellite Characteristics Circular/slightly elliptical orbit under 2000 km Orbit period ranges from 1.5 to 2 hours Diameter of coverage is about 8000 km Round-trip signal propagation delay less than 20 ms Maximum satellite visible time up to 20 min System must cope with large Doppler shifts Atmospheric drag results in orbital deterioration LEO Categories:  LEO Categories Little LEOs Frequencies below 1 GHz 5MHz of bandwidth Data rates up to 10 kbps Aimed at paging, tracking, and low-rate messaging Big LEOs Frequencies above 1 GHz Support data rates up to a few megabits per sec Offer same services as little LEOs in addition to voice and positioning services MEO Satellite Characteristics:  MEO Satellite Characteristics Circular orbit at an altitude in the range of 5000 to 12,000 km Orbit period of 6 hours Diameter of coverage is 10,000 to 15,000 km Round trip signal propagation delay less than 50 ms Maximum satellite visible time is a few hours Frequency Bands Available for Satellite Communications:  Frequency Bands Available for Satellite Communications Satellite Link Performance Factors:  Satellite Link Performance Factors Distance between earth station antenna and satellite antenna For downlink, terrestrial distance between earth station antenna and “aim point” of satellite Displayed as a satellite footprint (Figure 9.6) Atmospheric attenuation Affected by oxygen, water, angle of elevation, and higher frequencies Satellite Footprint:  Satellite Footprint Satellite Network Configurations:  Satellite Network Configurations Capacity Allocation Strategies:  Capacity Allocation Strategies Frequency division multiple access (FDMA) Time division multiple access (TDMA) Code division multiple access (CDMA) Frequency-Division Multiplexing:  Frequency-Division Multiplexing Alternative uses of channels in point-to-point configuration 1200 voice-frequency (VF) voice channels One 50-Mbps data stream 16 channels of 1.544 Mbps each 400 channels of 64 kbps each 600 channels of 40 kbps each One analog video signal Six to nine digital video signals Frequency-Division Multiple Access:  Frequency-Division Multiple Access Factors which limit the number of subchannels provided within a satellite channel via FDMA Thermal noise Intermodulation noise Crosstalk Forms of FDMA:  Forms of FDMA Fixed-assignment multiple access (FAMA) The assignment of capacity is distributed in a fixed manner among multiple stations Demand may fluctuate Results in the significant underuse of capacity Demand-assignment multiple access (DAMA) Capacity assignment is changed as needed to respond optimally to demand changes among the multiple stations FAMA-FDMA:  FAMA-FDMA FAMA – logical links between stations are preassigned FAMA – multiple stations access the satellite by using different frequency bands Uses considerable bandwidth DAMA-FDMA:  DAMA-FDMA Single channel per carrier (SCPC) – bandwidth divided into individual VF channels Attractive for remote areas with few user stations near each site Suffers from inefficiency of fixed assignment DAMA – set of subchannels in a channel is treated as a pool of available links For full-duplex between two earth stations, a pair of subchannels is dynamically assigned on demand Demand assignment performed in a distributed fashion by earth station using CSC Reasons for Increasing Use of TDM Techniques:  Reasons for Increasing Use of TDM Techniques Cost of digital components continues to drop Advantages of digital components Use of error correction Increased efficiency of TDM Lack of intermodulation noise FAMA-TDMA Operation:  FAMA-TDMA Operation Transmission in the form of repetitive sequence of frames Each frame is divided into a number of time slots Each slot is dedicated to a particular transmitter Earth stations take turns using uplink channel Sends data in assigned time slot Satellite repeats incoming transmissions Broadcast to all stations Stations must know which slot to use for transmission and which to use for reception FAMA-TDMA Uplink:  FAMA-TDMA Uplink FAMA-TDMA Downlink:  FAMA-TDMA Downlink

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