548 mw intro
TRANSCRIPT
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.
Wireless Transmission
Transmission and reception are achieved by means of an antenna .
Directional : Transmitt ing antenna puts out focused
beam . Transmitter and receiver must be
aligned . Omnidirectional “Isotropically” :
Signal spreads out in all directions . Can be received by many antennas .
Wireless Examples Terrestrial microwave transmission . Satellite transmission . Broadcast radio . Infrared .
Terrestrial Microwave Used for long-distance telephone service . Uses radio frequency spectrum, from 2 to 40
GHz . Parabolic dish transmitter, mounted high . Used by common carriers as well as private
networks . Requires unobstructed line of sight between
source and receiver . Curvature of the earth requires stations
(repeaters) ~30 miles apart .
Microwave Applications
Television distribution . Long-distance telephone transmission . Private business networks .
Microwave Advantages :
No cabling needed between sites . Wide bandwidth . Mult ichannel transmissions .
Disadvantages : Line of sight requirement . Expensive towers and repeaters . Subject to interference -e.g. passing airplanes,
rain .
Satellite Microwave Transmission
A microwave relay station in space . Can relay signals over long distances .
Wireless TechnologiesMicrowave Microwave systems transmit voice and data through the atmosphere as
super-high-frequency radio waves
One particular characteristic of the microwave system is that it cannot bend around corners; therefore microwave antennas must be in "line of sight" of each other
The following are some of the characteristics of the microwave system:
1. High Volume 2. Long distance transmission 3. Point to point transmission 4. High frequency radio signals are transmitted from one terrestrial transmitter
to another 5. Satellites serve as a relay station for transmitting microwave signals over
very long distances. See image next slide
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.
Microwave Engineering Considerations
Free space & atmospheric attenuation. Reflections. Diffractions. Rain 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 can occur as the microwave
signal traverses a body of water or fog
bank; cause multipath conditions
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.
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.
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
Waveguides are 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.
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
Receiver
Active XTMRFrequency #1
Protect XTMRFrequency #2
RCVRFrequency #1
RCVRFrequency #2
Transmitter
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*
Receiver
System XTMRPrimary #1
System XTMRStandby #2
failure switch
Active RCVR#1
Standby RCVR#2
Transmitter
*Hot standby is designed for equipment failure only
PRI
ReceiverTransmitter
Connect to PRI interface
& PSTN
Connect to PRI interface
& PSTN
To PSTN To PSTN
System Transmission
Facilities
System Receiver Facilities
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