Microwave communication link Basic Design Considerations

Chap-13 (TB)Wyne Tomasi

Link Design

• Path Clearance • Earth Bulging• K Factor • Fresnel Zone • Link Budget

Path Clearance

• LOS Signals• Curvature of the earth surface• H= D1 D2 /1.5 K

• K is constant • Depends upon • Actual propagation • Of wave • energy

Effects of atmospheric Density

• reflection refraction diffraction• higher above earth surface , atmospheric

density decreases • Lower part of the signal wave front travel

little slower than upper part • A k value of 4/3 can be used to describe such

a path

Earth Bulge• Humid coastal areas• Atmospheric inversion condition• Atmospheric density increase with increase in

height• This causes microwave signals to bend opposite

to the curvature of the earth• The bending may be severe enough to cause the

microwave signal diverted back to the earth blocking the propagation of the signal

• K = 2/3

Ducting• The change in refractive index is normally linear

and gradual,• but under certain atmospheric conditions a layer,

of warm air may be trapped above cooler air, • often over the surface of water. • The result is that the refractive index will decrease

far more rapidly with height than is usual.• This happens near the ground, often within 30 m

of it.

• Rapid variation in reduction of refractive index may be caused by atmospheric condition where there is a layer of warm air trapped over a relatively cooler air “ Temperature Inversion Region”

• In this region , temperature increases with height @ 6.5C/km

• Duct propagation is used at microwaves to cover the distance beyond LOS

• M=(N-1) x 106

Conditions

• Critical Angle : when angle of incidence is less than critical angle

• Cut off frequency:• (max) = 0.084 d3/2

• d = duct height in meter

Effect of Fresnel zone

• number of concentric ellipsoids which define volumes in the radiation pattern of a (usually) circular aperture.

• Fresnel zones result from diffraction by the circular aperture

• The area around the visual line-of-sight that radio waves spread out into after they leave the antenna. This area must be clear or else signal strength will weaken.

• If the propagation path is so chosen that it does not block the area represented by the first Fresnel Zone , the path is said to have first Fresnel zone clearance.

• Partial /significant cancellation of the desired signal if the clearance equals an even fresnel zone number

• The Fresnel zone clearance can be computed from

• F1 = 72.2 √D1 D2 /D.f

• D=Total Path length in Miles• F = frequency in GHz• D1D2 = Distance in Miles from the point in

question to each end of the path • F1 = Distance in feet from the path line to the

edge of the first Fresnel Zone• FN =F1√N

Link Budget

Transmitter Receiver

Transmit Output Power

Wave guide losses

Antenna Gain

Antenna Gain

Wave guide losses

Receiver Threshold

Free Space Loss

Transmitter Receiver

Transmit Output Power

Wave guide losses

Antenna Gain

Antenna Gain

Wave guide losses

Receiver Threshold

Free Space Loss

Transmitter Receiver

Transmit Output Power

Wave guide losses

Antenna Gain

Antenna Gain

Wave guide losses

Receiver Threshold

Free Space Loss

• The received signal needs to be stronger than the minimum detectable signal by the receiver also known as receiver sensitivity.

• Fading :• Atmospheric condition• Geometry of path• Fade Margin:• “Fudge Factor” included in system gain =n• Fm = 30logD + 10 log (6ABf)- 10 log (1-R)-70

• A=roughness factor• 4 for very smooth terrain, including over

water. • 1 for average terrain, with some roughness. • .25 for mountainous, very rough, or very dry

areas.

• B=factor to convert a worst month probability to an annual probability

• 1 to convert an annual availability to a worse month basis

• .5 for gulf coast or similar hot, humid areas. • .25 for normal interior temperate or northern

areas. • .125 for mountainous or very dry areas.

free space loss

• The free space loss is computed based on the path length and frequency using the equation:

• Loss (indB)= 96.6 + 20 LOG(F) + 20 LOG(D)• where:• L is the attenuation in dB. • F is the frequency in GHz. • D is the distance in miles.

Reliability

• Path availability is expressed in terms of outage time per year.

• The reliability of a system based on the computed fade margin is calculated based on the following equation

• U = a x b x 2.5 x 10-6 x f x D3 x 10-F/10

• where:• U is the non-diversity outage probability. • a is the terrain factor. • b is the climate factor. • f is the frequency in GHz. • D is the path length in miles. • F is the fade margin in dB.

Diversity

• When there is more than one transmission path or method of transmission available

Types of diversity

• Frequency Diversity• Space Diversity• Polarization Diversity• Hybrid Diversity• Quad Diversity

Space Diversity• 30-60 feet• The space diversity improvement factor for vertically

separated receive antennas is computed as:• I = (7 x 10-5 x f x s2 x 10F/10 ) / D• where:• I is the space diversity improvement factor • f is the frequency in GHz. • s is the vertical antenna spacing in feet. • D is the path length in miles. • F is the lower fade margin in dB

• Usd = a x b x (3.6 x 10-2 ) (D4 )(10 -2F/10 )/S2