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Wireless CommunicationBy

Engr. Muhammad Ashraf Bhutta

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Antennas and Propagation

IntroductionAn antenna is a transducer that converts radio frequency electric current to electromagnetic waves that are radiated into spaceIn two-way communication, the same antenna can be used for transmission and reception

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Fundamental Antenna ConceptsReciprocityRadiation Patterns

Isotropic RadiatorGainPolarization

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Reciprocity

In general, the various properties of an antenna apply equally regardless of whether it is used for transmitting or receiving

Transmission/reception efficiencyGainCurrent and voltage distributionImpedance

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Radiation Patterns

Radiation patternGraphical representation of radiation properties of an antennaDepicted as a two-dimensional cross section

Reception patternReceiving antenna’s equivalent to radiation pattern

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Antenna GainAntenna gain

Power output, in a particular direction, compared to that produced in any direction by an isotropic antenna

Effective areaRelated to physical size and shape of the antenna

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Antenna GainRelationship between antenna gain and effective area

G antenna gainAe effective areaf carrier frequencyc speed of light ( 3 x 108 m/s) carrier wavelength

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2

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c

AfAG ee

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Polarization

Defined as the orientation of the electric field (E-plane) of an electromagnetic waveTypes of polarization

LinearHorizontalVertical

Circular

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PolarizationVertically Polarized Antenna

Electric field is perpendicular to the Earth’s surfacee.g., Broadcast tower for AM radio, “whip” antenna on an automobile

Horizontally Polarized AntennaElectric field is parallel to the Earth’s surfacee.g., Television transmission (U.S.)

Circular Polarized AntennaWave radiates energy in both the horizontal and vertical planes and all planes in between

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Types of AntennasIsotropic antenna

IdealizedRadiates power equally in all directions

OmnidirectionalDipole antennas

Half-wave dipole antennaHertz antenna

Quarter-wave vertical antennaMarconi antenna

Parabolic Reflective Antenna Smart Antenna

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RF propagation Coverable distance

The distance that a wireless link can bridge is depends on:

RF budgetgainInsertion lossReceiver sensitivity

Path lossEnvironmental Conditions (influencing the path loss)free space versus non free spaceline of sightReflections / InterferenceWeather

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RF propagationFree space versus non free space

Non-free space

Line of sight required

Objects protrude in the fresnel zone, but do not block the path

Free Space

Line of sight

No objects in the fresnel zone

Antenna height is significant

Distance relative short (due to effects of curvature of the earth)

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RF propagationFirst Fresnel Zone

Food Mart

Direct Path = L

First Fresnel Zone

Reflected path = L + /2

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RF PropagationBasic loss formula

Propagation Loss

d = distance between Tx and Rx antenna [meter]

PT = transmit power [mW]

PR = receive power [mW]G = antennae gain

R TP P Gd

( )4

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Pr ~ 1/f2 * D2 which means 2X Frequency = 1/4 Power

2 X Distance = 1/4 Power

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RF propagationRF Budget

The total amount of signal energy that is generated by the transmitter and the active/passive components in the path between the two radios, in relation to the amount of signal required by the receiver to be able to interpret the signal

Lp < Pt - Pr + Gt - It + Gr - Ir

Where:Pt = Power on transmit Pr = Power on

receiveGt = Gain of transmitting antenna It = Insertion loss

in the transmit partGr = Gain of receiving antenna Ir = Insertion loss

in the receive partLp = path loss

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RF propagation Simple Path Analysis Concept (alternative)

WP II

Satellite Town

pigtail cable

Lightning Protector

RF Cable Antenna

WP II

CTRL Bldg.

pigtail cable

Lightning Protector

RF CableAntenna

+ Transmit Power

- LOSS Cable/connectors

+ Antenna Gain + Antenna Gain

- LOSS Cable/connectors

RSL (receive signal level) > sensitivity + Fade Margin

- Path Loss over link distance

Calculate signal in one direction if Antennas and active components are equal

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RF propagation RSL and FADE MARGIN

WP II

Tx =15 dBm

1.3 dB

.7 dB

50 ft.LMR 400

3.4 dB 24 dBi

WP II

Rx = -82 dBm

1.3 dB

.7 dB

50 ft.LMR 400

3.4 dB24 dBi parabolic

For a Reliable link - the signal arriving at the receiver - RSL - should be greater than the Sensitivity of the Radio (-82dBm for 11 Mbit)

This EXTRA signal strength is FADE MARGIN

FADE MARGIN can be equated to UPTIME

Minimum Fade Margin = 10 dB

Links subject to interference (city) = 15dB

Links with Adverse Weather = 20dB

Calculate RSL > -82 + 10 = -72dBm

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RF propagation Sample Calculation

WP II

Tx =15 dBm

1.3 dB

.7 dB

50 ft.LMR 400

3.4 dB 24 dBi

WP II

Rx = -82 dBm

1.3 dB

.7 dB

50 ft.LMR 400

3.4 dB24 dBi parabolic

RSL > PTx - Cable Loss + Antenna Gain - Path loss + Antenna Gain - Cable Loss

16 Km = - 124 dB

+ 15 dBm

- 2 dB

- 3.4 dB

+ 24 dBi

- 124 dB

+ 24 dBi

- 3.4 dB

- 2 dB

- 71.8 dB > -72

This lets us know that if the Fresnel zone is clear, the Link should work. If RSL < than -72 then MORE GAIN is needed, using Higher Gain Antennas or Lower loss Cables or Amplifiers (not a Agere Systems provided option)

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RF PropagationAntenna Height requirements

Fresnel Zone Clearance = 0.6 first Fresnel distance (Clear Path for Signal at mid point)

•57 feet for 40 Km path

• 30 feet for 10 Km path

Clearance for Earth’s Curvature

•13 feet for 10 Km path

•200 feet for 40 Km path

Midpoint clearance = 0.6F + Earth curvature + 10' when K=1

First Fresnel Distance (meters) F1= 17.3 [(d1*d2)/(f*D)]1/2 where D=path length Km, f=frequency (GHz) , d1= distance from Antenna1(Km) , d2 = distance from Antenna 2 (Km)

Earth Curvature h = (d1*d2) /2 where h = change in vertical distance from Horizontal line (meters), d1&d2 distance from antennas 1&2 respectively

Earth Curvature

Obstacle Clearance

Fresnel Zone Clearance Antenna

HeightAntenna Height

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RF Propagation Reflections

Signals arrive 180° out of phase ( 1/2 ) from reflective surface

Cancel at antenna - Try moving Antenna to change geometry of link - 6cm is the difference in-phase to out of phase

Path 6cm ( 1/2 ) longer

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RF propagationEnvironmental conditions

WeatherSnow

Ice and snow when attached to the antenna has negative impact

heavy rain on flat panelsWhen rain creates a “water film” it will negatively impact performanceRainfall in the path has little impact

StormCan lead to misalignment

LightningSurge protector will protect the equipment against static discharges that result of lightning. It cannot protect the system against a direct hit by lightning, but will protect the building from fire in such a case

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Propagation Characteristics of mobile radio channels

In an ideal radio channel, the received signal would consist of

only a single direct path signal, which would be a perfect

reconstruction of the transmitted signal.

In real the received signal consists of a combination of

attenuated, reflected, refracted, and diffracted replicas of the

transmitted signal

.It can cause a shift in the carrier frequency if the transmitter, or

receiver is moving (Doppler effect).

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Attenuation

•Attenuation is the drop in the signal power when transmitting from one point to another.

• It can be caused by the transmission path length, obstructions in the signal path, and multipath effects.

•Figure on next slide shows some of the radio propagation effects that cause attenuation.

•Any objects that obstruct the line of sight signal from the transmitter to the receiver can cause attenuation. 

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•Shadowing of the signal can occur whenever there is an obstruction between the transmitter and receiver.

• It is generally caused by buildings and hills, and is the most important environmental attenuation factor.

•Shadowing is most severe in heavily built up areas, due to the shadowing from buildings.

• Radio signals diffract off the boundaries of obstructions, thus preventing total shadowing of the signals behind hills and buildings.

• However, the amount of diffraction is dependent on the radio frequency used, with low frequencies diffracting more then high frequency signals.

•Thus high frequency signals, especially, Ultra High Frequencies (UHF), and microwave signals require line of sight for adequate signal strength.

•To over come the problem of shadowing, transmitters are usually elevated as high as possible to minimise the number of

obstructions

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Multipath Effects

Rayleigh fading

•In a radio link, the RF signal from the transmitter may be reflected from objects such as hills, buildings, or vehicles.

• This gives rise to multiple transmission paths at the receiver. Figure in next slide show some of the possible ways in which multipath signals can occur.

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The relative phase of multiple reflected signals can cause constructive or destructive interference at the receiver.

This is experienced over very short distances (typically at half wavelength distances), thus is given the term fast fading. These variations can vary from 10-30dB over a short distance. Figure 4 shows the level of attenuation that can occur due to the fading

31Figure Typical Rayleigh fading while the Mobile Unit is moving (for at 900 MHz)

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The Rayleigh distribution is commonly used to describe the statistical time varying nature of the received signal power. It describes the probability of the signal level being received due to fading.

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Frequency Selective Fading

In any radio transmission, the channel spectral response is not flat.

It has dips or fades in the response due to reflections causing cancellation of certain frequencies at the receiver.

Reflections off near-by objects (e.g. ground, buildings, trees, etc) can lead to multipath signals of similar signal power as the direct signal. This can result in deep nulls in the received signal power due to destructive interference.

For narrow bandwidth transmissions if the null in the frequency response occurs at the transmission frequency then the entire signal can be lost.

This can be partly overcome in two ways. 

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. This can be partly overcome in two ways. 

By transmitting a wide bandwidth signal or spread spectrum as CDMA, any dips in the spectrum only result in a small loss of signal power, rather than a complete loss. Another method is to split the transmission up into many small bandwidth carriers, as is done in a COFDM/OFDM transmission. The original signal is spread over a wide bandwidth and so nulls in the spectrum are likely to only affect a small number of carriers rather than the entire signal. The information in the lost carriers can be recovered by using forward error correction techniques

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Delay Spread

•The received radio signal from a transmitter consists of typically a direct signal, plus reflections off objects such as buildings, mountings, and other structures.

•The reflected signals arrive at a later time then the direct signal because of the extra path length, giving rise to a slightly different arrival times, spreading the received energy in time. Delay spread is the time spread between the arrival of the first and last significant multipath signal seen by the receiver.

•In a digital system, the delay spread can lead to inter-symbol interference. This is due to the delayed multipath signal overlapping with the following symbols. This can cause significant errors in high bit rate systems, especially when using time division multiplexing (TDMA). Figure 5 shows the effect of inter-symbol interference due to delay spread on the received signal. As the transmitted bit rate is increased the amount of inter-symbol interference also increases. The effect starts to become very significant when the delay spread is greater then ~50% of the bit time

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Table shows the typical delay spread for various environments. The maximum delay spread in an outdoor environment is approximately 20 us, thus significant inter-symbol interference can occur at bit rates as low as 25 kbps.

Delay Spread Maximum Path Length Difference

Indoor (room) 40 nsec - 200 12 m - 60 m

Outdoor 1 sec - 20 sec 300 m - 6 km

Environment or cause

Inter-symbol interference can be minimized in several ways. One method is to reduce the symbol rate by reducing the data rate for each channel (i.e. split the bandwidth into more channels using frequency division multiplexing, or OFDM). Another is to use a coding scheme that is tolerant to inter-symbol interference such as CDMA. 

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Doppler Shift

When a wave source and a receiver are moving relative to one another the frequency of the received signal will not be the same as the source. When they are moving toward each other the frequency of the received signal is higher then the source, and when they are approaching each other the frequency decreases. This is called the Doppler effect. An example of this is the change of pitch in a car’s horn as it approaches then passes by. This effect becomes important when developing mobile radio systems. 

The amount the frequency changes due to the Doppler effect depends on the relative motion between the source and receiver and on the speed of propagation of the wave. The Doppler shift in frequency can be written:

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(from [12])

fd=fo v/c

Where fd is the change in frequency of the source seen at the receiver , fo is the frequency of the source, v is the speed difference

between the source and transmitter, and c is the speed of light.

For example: Let fo= 1GHz, and v = 60km/hr (16.7m/s) then the

Doppler shift will be:

This shift of 55Hz in the carrier will generally not effect the transmission. However, Doppler shift can cause significant problems if the transmission technique is sensitive to carrier frequency offsets (for example OFDM) or the relative speed is higher (for example in low earth orbiting satellites).

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What is function of SMH?

What sort of processing is done with SU in outgoing processor at MTP level 2 ?

What is the function of Sevice indicator (SI)In SIO?