LOS Microwave

Line of Sight Microwave

SYED AHSAN RAZA

Introduction

• Widely employed means of broadband radio transmission in point to point service

• In Europe , LOS microwave is often termed as radio relay or radio links

• A link can be defined as radio connectivity from a near end transmitter to a far end receiver

• A link length is limited by Line of Sight

Introduction

• Historically, LOS microwave was the basic transport means for FDM configurations using frequency modulation

• In modern times the trend has moved from analog transmission to digital

• By digital we mean a digital baseband that modulates an analog RF carrier

• The baseband is 8 bit PCM either of DS1 hierarchy or E1 hierarchy

Introduction

Applications of LOS Microwave•Point to Point links as a backbone or tails of large

networks for common carriers linking BTS-BSC and BSC-MSC

•Point to multipoint systems for TV, telephony, data

Applications of LOS Microwave

• Transport of TV or other video signals such as Community Antenna Television (CATV) head end extension

Applications of LOS Microwave

• Optical Fiber is leading the way and replaced most of LOS microwave links because of

•▫Cost effectiveness

▫More information bandwidth

• Fiber ,however, is hampered by right of way requirements and by cables being served by construction activities

Alternatives to LOS Microwave

Criteria for LOS Microwave Systems• The signal follows a straight line or LOS path

• Signal propagation is affected by free space attenuation, precipitation and gaseous absorption

• Use of frequencies greater than 150 MHz, thereby allowing more transmission per RF carrier with wider RF bandwidths (Generally from 1 GHz to 30 GHz)

• Use of angle modulation (FM or PM), digital modulation or spread spectrum and time sharing techniques

Steps to do Link Engineering

• Selection of sites (radio equipment plus tower locations) that are in line of sight of each other

• Selection of an operational frequency band considering RF interference environment and legal constraints

Frequency Bands for LOS Microwave

•Development of path profiles to determine radio tower heights

▫If tower height exceeds a certain economical limit then sites and route should be reconfigured



• In making path profile it must be noted that Microwave energy is

▫Attenuated or absorbed by solid objects

▫Reflected from flat conductive surfaces such as water and sides of metal buildings

▫Diffracted around solid objects

▫Refracted or bent by the atmosphere


•Path Calculations are performed▫A margin is set for signal fading under all anticipated

climatic conditions

•Path survey is done to ensure accuracy of previous steps

•Establishing a frequency plan


•Equipment configuration to achieve fade margin most economically

• Installation

•Beam alignment, equipment line up, check out and acceptance by a customer


Link Design Process

Microwave Equipment Installation

Loss/ Attenuation Calculations•The loss/attenuation calculations are composed of

three main contributions

▫Propagation losses (Due to Earth’s atmosphere and terrain)

▫Branching losses (Comes from the hardware used to deliver the

transmitter/receiver output to/from the antenna)

▫Miscellaneous (other) losses (unpredictable and sporadic in character like fog,

moving objects crossing the path, poor equipment installation and less than perfect antenna alignment etc)

▫This contribution is not calculated but is considered in the planning process as an additional loss

Loss/ Attenuation Calculations

Propagation (Free Space Loss) •Consider a signal travelling between a transmitter at

A and receiver at B

Mathematical Formulation for Free Space Loss

• Let D be in Kilometers and f in Megahertz then the free space loss in decibels is calculated with the help of following formula

• If D is in statute miles then we have the following formula

Example

•Calculate free space loss for a distance separation of 40 Km and frequency of 6 GHz

Bending of Radio Wave Ray Beam above 100 MHz from Straight Line

• If radio wave above 100 MHz travelled a straight line , the engineering of LOS microwave systems would be much easier

•We could accurately predict▫Height of towers required at repeater and terminal stations▫Position of the radiating device on tower

•Our goal is to determine the height of a microwave radiator to permit reliable radio link communication from one location to the other

Bending of Radio Wave Ray Beam above 100 MHz from Straight Line

Methodology•To determine tower height

▫We establish position and height of obstacles in path between stations

▫To each obstacle height we will add earth bulge

Earth Bulge

Earth Bulge

•This is the number of feet or meters an obstacle is raised higher in elevation (into the path) owing to earth curvature or earth bulge

•The amount of earth bulge in feet at any point maybe determined by formula

Earth Bulge

•d1 is the distance from near end of the link to the point (obstacle location)

•d2 is the distance from far end of the link to the point (obstacle location)

•Atmospheric refraction may cause ray beam to be bent towards the earth or away from the earth

Earth Bulge

• If it is bent towards the earth , it is as if we shrank the earth bulge or lowered it from its true location

• If the beam is bent away from the earth it is as if we expanded the earth bulge or raised it up towards the beam above its true value

•This lowering or raising is handled mathematically by adding a factor K to the earth bulge equation

Earth Bulge

•The K factor can be calculated from the formula

Earth Bulge

•The value commonly used for r0 is 6370 km

• The effective earth radius is calculated from the formula

• Where exp means e the natural number

• Ns is surface refractivity

Earth Bulge

•Taking surface refractivity value of 301 and calculating r

•K-factor comes out to be 4/3 or 1.33

Earth Bulge

Surface Refractivity

•This is the refractivity at the altitude of LOS microwave site that we selected or the average refractivity of the path

•The sea level refractivity can be obtained for the area from the nearby weather bureau or from a chart



•To calculate Ns when we are given N0 the mean sea level refractivity we have

•Where hs is the altitude above mean sea level in km of the LOS radio site


K-factor and Tower Height

•For k>1 , ray beam is bent towards earth which essentially allows us to shorten radio link towers

•For k<1, earth bulge is effectively increased and tower height has to be increased as range is reduced

Fresnel Zone Clearance

• Direct radio waves will obviously travel in a straight line from Tx to Rx

• There are other waves inside the signal cone or circle if antenna is Omni-directional

• If there is no obstacle in the way these waves will go all the way

• In case of an obstacle these waves will be deflected and might reach the Rx with straight line signals causing phase cancelling effect resulting in fading

Fresnel Zone Clearance

•Phase cancelling effect is a function of the signal strength and extent of phase variation

•Fresnel provided a means to calculate how out of phase the deflections between Tx and Rx will be

•Each Fresnel zone is an ellipsoidal shape

Fresnel Zone Clearance – Phase Cancellation


•Deflections from obstacles occurring in zone 1 will create signals that will be 0 to 90 degree out of phase

• In zone 2 they will be 90 to 270o out of phase

• In zone 3 they will be 270 to 450o out of phase and so on


•Even numbered zones have the maximum phase cancelling effect

•Odd numbered zones may actually add to the signal power

The rule of thumb is that 60% of the 1st Fresnel zone must be clear of obstacles



Fresnel Zone Clearance – Earth Curvature

Fresnel Zone Radius Calculation

Example Fresnel Zone Clearance

•Calculate the clearance value where an obstacle is 5 mi from the transmit antenna, total path length

is 20 mi and operational frequency is 6 GHz

Path Profiling- Steps to Perform• On topological map draw a straight line connecting the

two adjacent radio link sites

• Carefully mark all obstacles or obstructions and possible points of reflections such as bodies of water, marshes or desert areas

• Assign consecutive letters to each obstacle

• Plot horizontal location of each point on the graph paper

• Mark the path midpoint which is the point of maximum earth bulge and should be marked as an obstacle

• Determine K factor by any of the following methods

• Method 1 :-

▫Refer to a sea level refractivity profile chart

▫Select appropriate N0 for the area of interest

▫Calculate K factor using equation

Path Profiling- Steps to Perform

•Method 2 :-

▫Lacking refractivity information, plot path profile using 3 values of K factor 1.33, 1.0 and 0.5

▫Later path survey will help in deciding appropriate K factor

▫Table can be helpful in choosing K



• For each obstacle point compute d1, distance to one repeater site and d2, the distance to other

• Compute the equivalent earth curvature for each point using

• Compute Fresnel zone clearance by equation 5.8

• For obstacles other than midpoint of path , find the percentage of the total path from transmitter to obstacle



•For example an obstacle 5 mile on a 30 mile path is 25 mile from another end

•Alternatively, this point is 16.6% or 83% of path length

•From graph it is 76% of midpath clearance

• If midpath Fresnel clearance were 40 ft, only 30.4 ft would be required at the 5 mi point


•Set up a table at the bottom of profile chart


Path Profile Exercise

Path Profile Exercise

Path Calculations (Link Budget)• Path calculation (link budget) provides parameters for dimensioning the

radio equipment

• Parameters include

• Antenna size or aperture

• Transmitter Power Output

• Receiver Noise Figure

• Required Bandwidth

•Required Diversity and type

•Coding gain for digital systems

•Performance measured in Signal to Noise ratio

•Noise in derived voice channel for FDM/FM systems

Path Calculations (Link Budget)

•Far end receiver is the starting point on a path calculation or link budget

•Main question to be asked is

What signal level entering the receiver will give the desired link performance?

Path Calculations (Link Budget) for Receiver

•Receiver will have a noise floor or noise threshold

•Thermal noise power is given by

•For a microwave receiver with 10 MHz bandwidth and 3 dB noise figure, thermal noise threshold is given by


• RSL (Received Signal Level) must be above this noise level or floor by some value in dB

• Factors on which this level depends

▫Desired link reliability (Time availability)▫BER or S/N in voice channel▫Modulation type▫Fading and fade margin▫Type of coding▫ Interference environment



Bandwidth for FM Systems

FM Systems•Deviation is a measure of instantaneous frequency

variation

•Direct function of the level of the modulating signal

•Higher the level of the modulating signal , the more deviation we can expect

Pre emphasis / De emphasis in FM

•Pre emphasis increases the peak deviation during the FM modulation process for higher baseband frequencies

•This increase of peak frequency deviation is done in accordance with a compensation curve



• If a transmitter is over deviated ▫More band width is required▫Increased thermal noise▫More intermodulation distortion

•From the relation discussed before for noise power threshold , doubling IF bandwidth increases thermal noise by 3 dB


FM Improvement Threshold

•At FM receiver, FM improvement will be achieved if Carrier to Noise ratio is at least 10 dB

•At that point , derived FDM voice channel experiences an increase of SNR by 20 dB



Received Signal Level• Received level at the input to a receiver’s first active stage

• On most LOS microwave receivers , the first active stage is the mixer

• In other cases it would be the input of the low noise amplifier (LNA)

• RSL is commonly measured in dBm or dBW

• RSL is synonymous with Carrier level C

• C/N can be written as RSL/N

Steps required in Link Budget Calculations

•Calculate EIRP of transmitter

•From that value subtract Free Space Loss (FSL)

•Resulting value is Isotropic Received Level (IRL)

•Add receiver antenna gain and subtract transmission line losses

•Resulting value is unfaded RSL



Examples of Link Budget

Link Budget Scenario

Link Budget Example

Link Budget Example

Mechanisms of Fading

•Multipath Fading

•Power Fading

Multipath Fading• Stems from interference between direct wave and reflected wave

• The reflection maybe from the ground or from atmospheric sheets or layers

• May display fades in excess of 30 dB for periods of seconds or minutes

• This form of fading will be observed during quiet, windless and foggy nights when temperature inversion occurs near ground

Multipath Fading

Multipath Fading

Multipath Fading

•Deviation from normal change of an atmospheric property with altitude

•Normally refers to temperature inversion

•Refers to increase in temperature with height

Multipath Fading- Temperature Inversion

• Usually within the lower atmosphere , air near surface of earth is warmer than the air above it

• Atmosphere is heated from below as solar radiations warm the earth surface

• Earth surface in return warm the atmosphere directly above it

• This rising of heat from earth surface is responsible for clouds formation through convection


•An inversion layer is when the normal temperature (warm air below, cold air above) profile is reversed

•Stable configuration of dense, cold air sitting below lighter, warm air is formed

•An elevated inversion layer is thus a region of warm air above a region of cold air, but higher in the atmosphere (generally not touching the surface)


•Cloud formation from the lower layer is "capped" by the inversion layer

• If the capping inversion layer or "cap" is too strong (too close to the surface), it will prevent thunderstorms from developing

•A strong cap can result in foggy conditions


Multipath Fading- Temperature Inversion--Scotland

Multipath Fading- Temperature Inversion-- Bratislava

Multipath Fading- Temperature Inversion– Shanghai ,China

Propagation Losses Calculation-Case Study

Propagation Losses Calculation-Case Study

Estimating Fade Margin for Analog Microwave Links

•Experience in the design o many radio link systems shows that multipath fading depends on

▫Path length

▫Frequency

▫Climate

▫Terrain Conditions


• It has been found that hops over following climatic and terrain conditions produce lowest amount of fading

▫Dry

▫Windy

▫Mountainous


• Worst amount of fading usually occur in coastal areas that are hot and humid

• Flat terrain along a radio path increases the probability of fading

• Irregular terrain like hilly areas with vegetation reduce the effect of fading



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LOS Microwave