Fundamentals of Microwave

Technologies

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 :

Transmitting 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 . Multichannel 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

Wireless TechnologiesLow-Orbit Satellite and Microwave

Transmission

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.

Skin affect Line of Sight (LOS) Fading Range Interference

Microwave Engineering Considerations

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

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’dFresnel Zone

D1

D2

72.2D1 X D2

F x D

secret formula

Microwave FadingNormal 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

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.

Diversity Space Diversity Frequency Diversity Hot Standby PRI

Space DiversityNormal 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

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

Microwave Radio Applications