Fundamentals of Microwave & Satellite Technologies TEL 117 Telekomunikasi Dasar
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 • 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 • Equipment, antenna, and waveguide failures • Fading and distortion from multipath reflections • Absorption from rain, fog, and other atmospheric conditions • Interference from other frequencies
Free space & atmospheric attenuation Reflections Diffractions Rain attenuation Skin affect Line of Sight (LOS) Fading Range Interference Microwave Engineering Considerations
Free Space & Atmospheric Attenuation • Free space & atmospheric attenuation is defined by theloss the signal undergoes traveling through the atmosphere. • Changes in air density and absorption by atmospheric particles.
Reflections • Reflections can occur as the microwave signal traverses a body of water or fog bank; cause multipath conditions
Diffraction • Diffraction is the result of variations in the terrain the signal crosses
Rain Attenuation • Raindrop absorption or scattering of the microwave signal can cause signal loss in transmissions.
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 SightFresnel 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 Fresnel Zone D2 D1 D1 X D2 F x D 72.2 secret formula
Microwave Fading Normal Signal Reflective Path Caused by multi-path reflections and heavy rains
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 • 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 • 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 • Digital Modem • Radio Frequency (RF) Unit • Antenna
Digital Modem • The digital modem modulates the information signal (intermediate frequency or IF).
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 • 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 Waveguidesare hollow channels of low-loss material used to direct the signal from the RF unit to the antenna.
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) • 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 • Space Diversity • Frequency Diversity • Hot Standby • PRI
Space Diversity Normal Signal Faded Signal Transmitter Receiver
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 Rx Frequency #1 Active Tx Frequency #1 Rx Frequency #2 Protect Tx Frequency #2 Transmitter Receiver
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* Active Rx #1 System Tx Primary #1 Standby Rx #2 System Tx Standby #2 failure switch Transmitter Receiver *Hot standby is designed for equipment failure only
PRI (Primary Rate Interface) PRI is an ISDN service providing user with 23,64 kbps for channel message and 1,64 kbps data channel for control & signalling. PRI is ISDN E1 interface. System Transmission Facilities System Receiver Facilities Connect to PRI interface & PSTN Connect to PRI interface & PSTN Transmitter Receiver To PSTN To PSTN
Availability Formula Percent Availability equals: 1 – (outage hours/8760 hours per year) Private microwaves have 99.99% availability
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
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 CategorizeCommunications 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 • 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 • 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 • 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 • 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 GEO : Geosynchronous equatorial orbit
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 : Low earth orbit
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 • 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 MEO : Medium Earth Orbit
GEO MEO LEO Satellite Systems GEO (22,300 mi., equatorial) high bandwidth, power, latency MEO high bandwidth, power, latency LEO (400 mi.) low power, latency more satellites small footprint V-SAT (Very Small Aperture Terminal) private WAN SATELLITE MAP SOURCE: WASHINGTON UNIV.
Geostationary Orbit SOURCE: BILL LUTHER, FCC
GPS Satellite Constellation • Global Positioning System • Operated by USAF • 28 satellites • 6 orbital planes at a height of 20,200 km • Positioned so a minimum of 5 satellites are visible at all times • Receiver measures distance to satellite SOURCE: NAVSTAR