basics of transmission

8/22/2019 Basics of Transmission

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Training on Microwave Communication

ADA Cell works

Sachin Bhushan

Basic concepts


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Contents:

Possible media for communication

Introduction to Communication Media

Introduction to Microwave communication

Manufacturers of Microwave

Why Microwave?

Characteristics of microwave

Types of Microwave communication

Types of Microwave Links

Requirements for the microwave communication

What is LOS?

Wave Propagation in the atmosphere

Multi path Propagation

LOS Purpose & requirements

Limitations of Line of Sight Systems Design of Line of Sight Microwave Links

K- factor

Variations of the ray curvature as a function of k

Fresnel zoneFresnel zone


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Contents:

Obstacles & Loses

Knife Edge Obstacles

Smooth Spherical Earth Obstacles

Path Loss

Other losses

Why vertical polarization favorable at high freq

Antenna type & Gain

RECEIVER SENSITIVITY, FADE MARGIN AND SIGNAL TO NOISE RATIO

Fading MarginFading Margin

Reliability


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Possible Media for communication

Copper media Microwave media Optical fiber Media Satellite Media


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What is Microwave Communication

Microwave communication system is used to transfer data from onenode to the other node using the frequency ranging from 2GHz to60GHz.

Small capacity systems generally employ the frequencies less than

3 GHz while medium and large capacity systems utilize frequenciesranging from 3 to 15 GHz. Frequencies > 15 GHz are essentiallyused for short-haul transmission.


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Manufacturers of Microwave equipments

Few well known Radio ManufacturersNokiaNera

NECSiemensDigital Microwave CorporationFujitsu

EricssonAlcatelHariss


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Why Microwave?

Fast Deployment

Flexibility Low implementation Cost Link across Mountains and Rivers are economical & feasible Quick maintenance of the system

GHz band has very low noise LOW MTTR

Drawback of Microwave

Needs frequency license Environment dependant link quality (e.g. rainfall) LOS not always available


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Characteristics of microwave

Microwave are used for point to point and point to multipointcommunication

Microwaves are the electromagnetic waves comprises of electricaland magnetic field at angle of 90 degree to each other.

Normally communication on microwave is done between 3GHz to30 GHz frequency


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Types of Microwave communication

Point to point communication



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Types of Microwave Links

Long Haul Radios: ~ 30 - 80 km2 GHz, 7 GHz

Medium Haul Radios: ~ 25 - 45 km10 GHz, 13 GHz, 15 GHz

Short Haul Radios: ~ 5 - 30 km18 GHz, 23 GHz, 26 GHz, 38 GHz, Nokia Metro hopper: < 1 km

57 GHz(uses oxygen absorption in air to limit range)


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Requirements for the microwave communication

Microwave communication requires a clear line of sight between twonodes

A Fresnel ellipsoids and their clearance criteria concept is used tocalculate the radio Line of sight

Antenna height Calculation for clear LOS Parameters design like Power ,Frequency , Rx level and many more


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What is LOS?

Radio signals, like all electromagnetic radiation, usually travel instraight lines. However, at low frequencies (below approximately2 MHz or so) diffraction effects cause significant ray bending,allowing ray bundles to partially follow the Earth's curvature, thusenabling AM radio signals in low-noise environments to be recievedwell after the transmitting antenna has dropped below the horizon.

Additionally, frequencies between approximately 1 and 30 MHz, canbe reflected by the ionosphere, thus giving radio transmissions inthis range a potentially global reach (see shortwave radio).

However, at higher frequencies, neither of these effects apply, andso any obstruction between the transmitting and receiving antennawill block the signal, just like the light that the eye senses.

Therefore, as the ability to visually sight a transmitting antenna (withregards to the limitations of the eye's resolution) roughlycorresponds with the ability to receive a signal from it, thepropagation characteristic of high-frequency radio is called "line-of-sight" as per radiowave propagation is called as "radio horizon".


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Wave Propagation in the atmosphere


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Multipath Propagation

When rays reaches to the receiver from different paths then twopossibilities are there

A) If they reaches in the same phase then the signal strength increases.

B) If they reaches in opposite phase then its cause fading called multipathfading


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LOS Purpose & requirements

1. Purpose For the establishment of short / long haul LOS links

Feasibility studies Submission of tenders Up gradation of existing links

2. Requirements of LOS links site locations planned antenna height direction to the other end of link

restrictions to cherry-picker, etc.

Output LOS/NLOS minimum antenna height exact antenna location (rooftop) panorama picture with

landmarks and their directions extra observations

(forests,building sites etc.)


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(Limitations of Line of Sight Systems)

How far we can go: The range of LOS microwave systems is limited by:- Curvature of earth-Actual Technical radio characteristics (K-factor)-Modified Earth Curvature Actual Obstructions en-route in each hop RF effect of fresnel zone

Path loss Transmitter power Antenna gains Transmission line looses Frequency of operation Received power

Receiver threshold Signal to noise ratio Fade margin required Desired reliability of link


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Design of Line of Sight Microwave Links

Link Design: The design of microwave links, involves three sets ofcalculations.

1. Working out antenna heights for the link. K-factor is major dominant variable. Earth bulge.

Fresnel zone radius. Actual obstructions on the route Path Loss Operating frequency. Path profile: it indicates the distance from one of the transmitter

site where obstructions to the line of sight radio link may occur.

The objective of this calculation is to arrange tower heights alongthe entire route of the link, so that an obstruction in the path doesnot enter into the fresnel zone by a specified amount for a specifiedK-factor used.


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(Design of Line of Sight Microwave Links)

2. To determine equipment and other parameters for each hop. Transmit power. Antenna type and gain. Transmission type. Other losses. (Absorption, Diffraction, Reflection or Scattering etc.) Maximum received power.

Receiver threshold.This will decide the thermal fade margin, which we will be able to get for each hop.3. To determine the reliability of each hop and overall reliability of the link. Climatic factor. Terrain roughness. Average annual temperature Annual rain.

This will decide, what is total expected outage time per annum for each hop as well as forthe entire link.


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K- factor

Earth bulge and K-factor: The propagation of radio beam is affected by atmospheric conditions and the

obstructions on the way. It can be subjected to:DiffractionReflectionRefraction

Most important is refraction, which is caused by changes in the density ofatmospheric layers confronted by the radio beam front. The curvature of earth and slight bending of waves as it is refracted downwards by

the earths atmosphere are two factors, that, must be considered while makingpath profiles.

The earths curvature and microwave beam refraction are combined to formfictitious earth curvature or earth bulge.

EARTH CURVATURE (M) = 0.078 x d1 x d2 / K WHERE K = EFFECTIVE EARTH RADIUS/TRUE EARTH RADIUS

EARTH BULGE = d1 x d2 / 12.75 x KEARTH BULGE FOR K=4/3 = d1 x d2 / 17EARTH BULGE FOR K=2/3 = d1 x d2 / 8.5


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Variations of the ray curvature as a function of k

Different K values

K=

True Earths curvature

= 6,371 Km

K=1K=0.5

K=0.33


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Fresnel zoneFresnel zone

Fresnel zone:Fresnel zone: The radio beam energy travels in an ellipsoidal wave front, the

different components of which maintains different path lengths. The distance from microwave beams center is commonly

measured in fresnel zones to take into account both frequencyand distance.

The first fresnel zone (FFZ) is the surface of the point along whichthe distance to the ends of the path is exactly wave lengthlarger than the direct end to end path.

FFZ radius in meters=17.32d1*d2/fDWhere d1 & d2 are in kms, f is the frequency in GHz and D is the

hop distance in Kms. In order to achieve a free space propagation condition for a radiobeam at least 60 % of FFZ should be cleared under the standardatmospheric condition of K=4/3.


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More on fresenal zone

Keep 1st Fresnel zone clear ofobstacles

nth Fresnel zone: Ellipse around directpath, where path difference to directline is n*/2.

d

b

1st Fresnel zone2nd3rd

Radius for n-th zone = b * sqrt(n)b

d km

f MHzm= 274

[ ]

[ ][ ]


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More on Fresnal Zone

1. If f=2.5 GHz and D=30 Km, then FFZ=32.99 M



Conclusion :FFZ radius decreases with increase in frequency.




Conclusion: FFZ radius increases with increase in distance


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(FIRST FRESNEL ZONE AND EARTH BULGE)

With 100% fresenal zone

NATURAL EARTH FEATURES

EARTH BULGE

FIRSTFRESNELZ

ONERADIUS

A B

T

BUILDING

d1 d2

D

f


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Obstacles & Loses

Types of Loss

Obstacle loss

Knife edgeobstacle loss

Smoothspherical

earth obstacleloss


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Knife Edge Obstacles


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Knife Edge Loss


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Knife Edge Loss


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Smooth Spherical Earth Obstacles


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Smooth Spherical Earth Loss


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Path Loss

Free Space LossFree space loss: consider a signal is traveling betweentransmitter at A to a receiver at B. There is for a givenfrequency and distance, a characteristic loss. This loss increaseswith both distance and frequency. It is known as free space loss.

Free space loss LdB=92.44+20 log10F+20 log10DWhere F is in GHz and D is in km's.If D is 40 Km and F is 6 GHz, then free space in dBL

dB=92.44+20 log 40+20 log 6=92.44+20*1.6021+20*0.7782=92.44+32.042+15.564=140.046 dB


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Variation in free Space Loss

Example:- Free space loss if F=2.5 GHz and D=30 KmFSL (dB) = 92.44 + 20 log 2.5 + 20 log 30

=92.44 + 20*0.398 + 20*1.478=92.44 + 7.96 + 29.56 = 129.96 dB

Now, ifF=7.5 GHz (changed) and D=30 Km (unchanged)

FSL (dB) = 92.44 + 20 log 7.5 + 20 log 30=92.44 + 20*0.875 + 20*1.478

=92.44 + 17.5 + 29.56 = 139.5 dBNow, if F=2.5 GHz (unchanged) and D=40 Km (changed)FSL (dB) = 92.44 + 20 log 2.5 + 20 log 40

=92.44 + 20*0.398 + 20*1.602=92.44 + 7.96 + 32.04 = 132.44 dBIt can be seen, that, free space loss increases both withdistance and frequency


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Other losses

Precipitation

Transmission of microwave signal above 10 GHz is vulnerable toprecipitation The energy is attenuated due to radiation(scattering) and absorption (heating) Scattering

Radio waves are a time varying electromagnetic field, the incident

field willinduce a dipole moment in the raindrop. The rain drop will also

have the same time Variation as the radio waves and will act as anantenna and reradiate the energy. As rain drop-antenna have lowdirectivity it will radiate energy arbitrary direction and add to loss.

AbsorptionWhen the wavelength becomes small (High freq. < 18GHz) relativeto the raindrop size more energy is absorbed by heating of theraindrop.


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Why vertical polarization favorable at high freq?

As the rain-drop increases in size they depart spherical shape andextended in the

the horizontal direction.For freq. Higher than 18 GHz thewavelength is generally

in mm. So these rain-drops attenuate horizontally polarized wavesthan the vertical Polarized. Raindrop shapes

1mm 1.5mm 2mm 2.5mm


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To determine equipment and other parameters for each hop

Transmit power : Transmit power is the power in dB that is required for thesignal to travel from one node to other.

The max and minimum transmission power for the equipment is vendorspecific and changes with the capacity of the E1 carried by the radio.


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Microwave Radio - Modules

Digital Modem : To interface with customer equipment and toconvert customer traffic to a modulated signal

RF Unit : To Up and Down Convert signal in RF Range

Passive Parabolic Antenna : For Transmitting and Receiving RF

Signal


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Antenna type & Gain

Antenna type : There are different types of antenna used for the Microwavecommunication. Mostly parabolic antennas are used.

Antenna can be put in vertical or horizontal mode. Also cross polarizedantennas are available that can carry both horizontal and vertical beamswith a very low interference.

Gain of Antenna: Gain of the antenna is calculated by the approximateformula

Gain = 17.8 + 20 log (D.f) dBi

Where

D = Antenna diameter [m]F = Frequency in GHz


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(RECEIVER SENSITIVITY, FADE MARGIN AND SIGNAL TO NOISE RATIO)

Receiver Sensitivity: Sensitivity or Threshold Power of receiver is the level of signalwhich would produce a 30 dB signal to noise ratio out of the base band of ananalogue receiver, or a bit error ratio (BER)=10-4 out of the base band of a digitalreceiver. Typically it is -70 to -90 dB.

Fading: Received Signal vary with time due to multipath fading and rain etc. Refractiveindex of atmosphere varies with Temp. humidity and pressure which in turn cause theelectromagnetic waves to change direction. Another cause for Multipath fading isground reflection. So a fade margin is built in Link Designing.

Fade Margin: The fade margin is the power level, that, the unfaded received signal canfall to until it reaches the receiver threshold. This margin will vary depending ongeographic and climatic conditions of different geographic areas and desiredreliability of the system. Typically it is 20-40 dB.Fade Margin dB=P

rx-P

thresh

Signal to Noise Ratio: Its the minimum power difference between the wanted receivedsignal and received noise.Signal/Noise Ratio (dB)=10 log

10(Signal Power/Noise Power)

Typically it is > 50 dB, logically it should be more than the Fade Margin, so that it isalways below the threshold level.


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Received Power

Fading Margin

Receiverthreshold

Fading margin:

Safety margin. Should be

large enough to guarantee

that quality and availability

objectives are met duringfading conditions.

Typical value ~ 40 dB

Fading MarginFading Margin


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Reliability

Reliability of the link: Outage time for each hop and for the complete link is to beworked out, which in turn will give the over all reliability of the link in terms ofpercentage.Single hop reliability (%) Fade Margin

99.9 28 dB99.99 38 dB99.999 48 dB

CCIR defines its availability objective for radio relay systems over a hypotheticalreference circuit as 99.7 %. Resulting unavailability 0.3 % is of three components.Outage due to power failureOutage due to equipment failureOutage due to propagationIt is reasonable to allot 50 % of the outage time to power and equipment failures and50 % for propagation. Considering propagation alone, system should have an

availability (reliability) of 99.85 % apportioned across the 2500 Km route. This provideguide to establish a per hop propagation reliability for a particular system.Planner rather first set the limit for the reliability and for wide band links it is betterthan 99.99 %.

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