microwave communication link basic design considerations
TRANSCRIPT
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