Sheetno.(15) 1

Microwave Communications

Principles

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Radio

Radio is the wireless transmission of signals, by modulation of electromagnetic waves with frequencies below those of visible light.

When radio waves pass an electrical conductor, the oscillating fields induce an alternating current in the conductor. This can be detected and transformed into sound or other signals that carry information.

The word 'radio' is used to describe this phenomenon, and radio transmissions are classed as radio frequency emissions.

Today, the term 'radio' often refers to the actual transceiver device or chip, whereas 'wireless' refers to the system and/or method used for radio communication

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Uses of radio

Radar (RAdio Detection And Ranging). WLAN Mobile communications Radio broadcasting (AM or FM) Aircraft comm using VHF. Marine voice Government, police, fire and commercial voice services Civil and military TETRA (Terrestrial Trunked Radio) is a digital cell phone system for

military, police and ambulances TV satellite navigation GPS

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Radio Frequencies

Ultra-low frequency (ULF): 0-3 Hz Extremely low frequency (ELF): 3 Hz - 3 kHz Very low frequency (VLF): 3kHz - 30 kHz Low frequency (LF): 30 kHz - 300 kHz Medium frequency (MF): 300 kHz - 3 MHz High frequency (HF): 3MHz - 30 MHz Very high frequency (VHF): 30 MHz - 300 MHz Ultra-high frequency (UHF): 300MHz - 3 GHz Super high frequency (SHF): 3GHz - 30 GHz Extremely high frequency (EHF): 30GHz - 300 GHz

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Microwaves The term microwave generally refers to "alternating current signals

with frequencies between 300 MHz and 300 GHz (UHF, SHF, EHF). However, both IEC and IEEE define "microwave" frequencies as

starting at 1 GHz (30 cm wavelength). Hence, it is usually defined as 1 GHz to 1000 GHz range

Waves longer than microwaves are called "radio waves“ Electromagnetic radiation with shorter wavelengths may be called

"millimeter waves", terahertz radiation Most common applications are within the 1 to 40 GHz range Microwave radio is used in broadcasting and telecommunication

transmissions because, due to their short wavelength, highly directive antennas are smaller and therefore more practical than they would be at longer wavelengths

There is also more bandwidth in the microwave spectrum than in the rest of the radio spectrum; the usable bandwidth below 300 MHz is less than 300 MHz while many GHz can be used above 300 MHz.

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Microwave: Frequency Bands L band

1 to 2 GHz S band

2 to 4 GHz C band

4 to 8 GHz X band

8 to 12 GHz Ku band

12 to 18 GHz K band

18 to 26.5 GHz Ka band

26.5 to 40 GHz

Q band 30 to 50 GHz

U band 40 to 60 GHz

V band 50 to 75 GHz

E band 60 to 90 GHz

W band 75 to 110 GHz

F band 90 to 140 GHz

D band 110 to 170 GHz

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Radio path

1. Fresnelzone

Obstruction loss

Reflections

Free space lossRain loss

Reflection

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Basic System Structure Spectrum forming

Spectrum formingEqualization

MOD TX

DEM RX

TX MOD

RX DEMPOL

Branching

Branching Branching

Branching

ModulationCCoding

Adaptive EqualizationBit error correctionCorrectionektur

XPD

Radiation

1

n

1

n

1

n

1

n

POL

System loss

Free space loss

Fading

Interference

Split mounting System (Indoor-outdoor systems)where parts of the active components are housed outside near the antenna in order to save the installation of waveguides.

Fully indoor systemsAll active components are housed inside and only the antenna is mounted outside, connected via a coaxial cable (< 3 GHz) or a waveguide (> 3GHz)

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Electromagnetic Radiation

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The alternating electrical field generates an alternating magnetic field, and reverse.

Optimum of electromagnetic energy is radiated (or received) ifthe electrical length of the antenna is tuned to the wavelength

(/4-, /2- Dipole), andAntenna and transmitter (receiver) have the same impedance.

The real antenna of the parabolic antenna is a short dipole just mounted at the transmitter output (receiver input); the parabolic dish is used as reflector only in order to get the directivity

Antenna Radiation

electrical field

magnetical field

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Polarization According to the direction of the electrical field with respect to the ground

polarization is vertical if the electrical field is vertical

polarization is horizontal if the electrical field is horizontal

At the receiver site the antenna has to have the same polarization as at the transmitter site in order to transmit the maximum of energy.

The de-coupling between vertical and horizontal polarization is called Cross Polarization Discrimination (XPD)

For Radio Relay Systems this affect is applied in order to re-use channels on the same path, that means, on the same path two channels are using the same frequency but different polarization.

In case, only one polarization is needed, usually the vertical polarization is selected due to improved transmission characteristics relating to reflection and rain loss

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In case the impedance of the transmission components are different, only parts of the maximum possible energy is transmitted.

The strength of the reflected power depends on the differences between the impedance of the connected devices

Return loss is the ratio between the reflected power and the maximum possible power related to a reference impedance.

A return loss of 30 dB implies, that only 1/1000 of the output power is reflected; against that with a return loss of 10 dB 1/10 of the output power is reflected.

In general transmitter, feeder, receiver and antenna of microwave systems have an impedance of 50 .

Return Lossoutput power

Z1 Z2

reflected power

Transmitter

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An isotropic antenna generates a uniform spherical radiation of the energy.

This antenna has no dimensions, and is therefore of theoretical nature only.

The spherical radiated power PT generates a power flux density of :

S = PT / ( 4 * d2 ) W/m2

Isotropic Antenna 1m2

d

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Antenna Types

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Power CalculationCalculation of power levels within a transmission path

P5 = P1 * V1 * V2 / (A1 * A2 )Using logarithmic values:

P5 = P1 + V1 + V2 - A1 - A2

P1 P2 P3 P4 P5

amplificationV1

attenuationA1

attenuationA2

amplificationV2

p = 10 * log P W / 1W dBW

p = 10 * log P mW / 1mW dBm

p = 10 * log P pW / 1pW dBpW

P mW = 10 p dBm / 10

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Line-of-Sight Considerations Microwave radio communication requires a clear line-of-

sight (LOS) condition Radio LOS takes into account the concept of Fresnel

ellipsoids and their clearance criteria Radius of the first Fresnel zone

R=17.32(x(d-x)/fd)1/2

where d = distance between antennas (in Km)

R= first Fresnel zone radius in meters

f= frequency in GHz

x

y

d=x+yR

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Link Budget (1)

The link budget is a calculation involving the gain and loss factors associated with the antennas, transmitters, transmission lines and propagation environment, to determine the maximum distance at which a transmitter and receiver can successfully operate

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Link BudgetReceive Signal Level (RSL)

RSL = Po – Lctx + Gatx – Lcrx + Gatx – FSL

Link feasibility formulaRSL Rx (receiver sensitivity threshold)

Po = output power of the transmitter (dBm)Lctx, Lcrx = Loss (cable,connectors, branching unit) between transmitter/receiver and

antenna(dB)Gatx = gain of transmitter/receiver antenna (dBi)FSL = free space loss (dB) Free-space loss - when the transmitter and receiver have a

clear, unobstructed line-of-sightLfsl= 92.45 + 20log(f) + 20log(d) [dB]

where f = frequency (GHz), d = LOS range between antennas (km)

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Radio path link budget

Received Power (Rx)

Transmitter 1

Receiver 1

Splitter Splitter

Transmitter 2

Receiver 2

OutputPower (Tx)

Branching Losses

waveguide

Prop

agat

ion

Los

ses

Ant

enn a

G

ain

Ant

enna

G

ain

Branching Losses

Receiver threshold Value

Fade Margin

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Microwave Applications

Before the advent of fiber optic transmission, most long distance telephone calls were carried via microwave point-to-point links

Starting in the early 1950's, FDM was used to send up to 5,400 telephone channels on each microwave radio channel, with as many as ten radio channels combined into one antenna for the hop to the next site, up to 70 km away.

WLAN protocols, such as Bluetooth and the IEEE 802.11 MAN protocols, such as WiMAX Wide Area Mobile Broadband Wireless Access (MBWA) protocols

based on standards specifications such as IEEE 802.20 Cable TV and Internet access on coax cable as well as broadcast

television use some of the lower microwave frequencies Some mobile phone networks, like GSM Most radio astronomy uses microwaves.

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Applications: Cellular backhaul

Mobile Switching Center

Base StationController

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Applications: Customers wireless access

BusinessDistricts

Central Office

High trafficCustomers

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Applications: long distance connecting regional networks

LocalExchange

RegionalNetwork

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Applications: private voice/data networks

Headquarters

BranchOfficeBranch

Office

Hospitals

CorporateNetworks

Universities