Chapter
3
3.1
Microwave Path Analysis and Design
Microwave
Path Analysis and design plays significant role in the Quality of
Service, stability and the reliability of a Microwave link A
systematic solution approach is vital in implementing this sort of
links. Even though I didn’t get a chance to work in a big
project like implementing a new Back bone transmission line, I was
fortunate to get hands on training on number of new installation
of small point to point Microwave links.
3.2
Path Analysis
Before
installing a new Microwave link, several steps have to be taken
according to the standards that are developed by the ITU. The most
important and the basic steps can be summaries as follows.
Preliminary
Path Analysis
- Based
on customer roll out plan and is generic in nature
- Does
not consider parameters like path profile, LOS clearance,
reflection, refraction and diffraction.
- Based
on the site details, network design criteria, channel plan,
and the customer input regarding the design of the network.
- Decide
equipment list
- Used
to calculate the royalty paid by the customer depending on the
number of channels used.
Steps
in preliminary path calculations
- A
connectivity plan is developed based on the roll out plan of
the customer
- Path
calculations are done for each hop using approximate data of
the site visits, coordinates and LOS clearance
- Interference
analysis for minimum frequency spots for royalty and filing
application to the regulatory body.
Final Path Analysis
- Free
Space Losses – losses due to the attenuation of the radio
signal as it travels through space.
- Atmospheric
Losses – losses due to signal attenuation resulting from
absorption of oxygen molecules and water vapor in the
atmosphere.
- Other
losses
- Jumper,
Wave guide losses are derived from manufacturer’s
specification. Loss per meter, etc, these losses represent
the total loss from the RF Unit Wave guide Flange to the
Antenna Feed point.
- Pad
losses refer to international signal attenuation caused by
resistive circuits (Pads) which are inserted between the
antenna feed point and the RF unit flange.
Rain
Attenuation
Is
the function of the rainfall (rain rate), the size of the
individual raindrops and the percentage of the total path within
the rain cell. Rain attenuation is a major concern at frequencies
above 10 GHz.
3.3 Path Design
Path
Design is a field study on Radio Link about the Line Of Sight
clearance and the feasibility. It consists of collecting all
required information and processing it for an optimum Microwave
link design
LOS
Survey (Line OF Sight Survey)
LOS survey consists of 2 steps
MAP
Survey
Maps
are the principal source of data for office study, which usually
precedes field survey, and for the field survey itself. A
preliminary study of the map is required for planning of one or
more routes which might appear to be possible between the terminal
points given, based on available data and the plotting of profiles
which are necessary preliminary, for all of the indicated paths
and alternates.
Topographic
Maps
- Show
different elevation contours
- Show
major highways, airports, and rail tracks
- Show
mountain ranges, rivers and lakes
- Available
in different scales (1:50 000,
1:25 000 etc)
Field
Survey
It
consists of the following steps
- Identification
of the site position (Using GPS)
- Calculate
link Azimuth and identify the link direction
- Check
Obstructions in the link direction
§
Near obstructions - (trees, buildings etc)
§
Far
obstructions - (hill peaks, trees over mountains etc)
Things
to be noted during field survey
- Site
near high power radars
- Link
over water paths
- Near
obstructions in LOS which may reflect/diffract energy
- Crossing
of other system routes of similar frequencies
- Building
regulations of the city
- Landmark
location
Path
Profile
Path
profile is obtained by visiting locations in short hops and using
topographical maps in long hops.
Measurements
- Geographical
Coordinates (Latitude and Longitude)
- Altitude
(AMSL - altitude above Mean Sea Level)
- Azimuth
(horizontal angle reckoned clockwise from meridian (north) to
the object used in reference to antenna alignment)
- Elevation
(the angle in the vertical plane with respect to the
horizontal ground plane, and used with the Azimuth for antenna
alignment)
Antenna Height
After
defining the working frequency, the antenna mounting height is
calculated so that, according to CCIR criteria requirements, the
minimum percentage of Fresenal radius is through obstruction free.
In calculation all present obstructions should be cleared and a
sufficient margin for future obstructions should be taken (e.g.
tree growth, high rise building under constructions, funnel zone
of a airport etc)
Various
Phenomenon’s affecting propagation
Electromagnetic
waves travel with different speed in different mediums. And they
travel at higher speed in rarer medium than in denser medium. This
property is characterized by the refractive index of the
atmosphere.
Radio
waves reflected by surfaces like the sunlight. (Reflected by the
ground, marshy land and most importantly by water sheets) may
cause constructive and destructive interference, there fore
reflecting points should be considered with much more carefully.
This
property manifests as a bending of the EM wave around any
obstacle. (Knife edge or spherical surface). And this is related
to wavelength and dimension of object. Its importance lies in the
fact that obstacles in close proximity to microwave beam can cause
additional losses during propagation.
3.4
Analyzing MTT’s Microwave Path
During my training period at MTT, I learnt about the above
importance of path design from some observations made from faulty
links and the reasons for that.For example due to a problem in the
path design one of the link near to a Air Force Base underwent
packet losses in a particular time every day.
Chapter
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