wireless network

WIRELESS NETWORK - DDPP 3573

CHAPTER 1

FADING AND ITS CHALLENGES

Effect of EnvironmentOnce radio signal has been radiated, it travels or propagates through space and ultimately reaches the receiver.

The energy level of the signal decreases rapidly with the distance from the transmitter.

The EM wave are also affected by objects that it encounters along the way such as trees, buildings, and other large objects.

The path of EM signal taken to the receiver depends upon many factors, including the frequency of the signal, atmospheric conditions and time of day.

All these factors has to be taken into account to predict the propagation of radio waves from transmitter to receiver. DDPP3573 SECTION 11

1.1 Optical properties of Radio waves

Waves will be reflected, refracted and diffracted as it travels through the atmosphere.

These three properties are shared by light and radio waves.

Radio waves are identical to light waves except for the frequency.

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1.1.1 Reflection

Radio waves are reflected by any medium such as metal surfaces or the earth’s surface.Reflection of waves from a smooth surface results in the angle of reflection being equal to the angle of incidence

There is a change in the phase of the incident and reflected waves as seen by the difference in the direction of polarization.If the electric field is parallel to the reflecting surface, the electric field will be shorted out and all of the electromagnetic energy is dissipated in the form of the generated surface currents in the conductor.

Reflection of wavefront

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Corner reflector

Parabolic reflector

Same angle of incident and reflection but differentorientation

Diffuse reflection

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1.1.2 Refraction• Refraction occurs when

waves pass from a medium of one density to another medium of a different density

• Results in the bending of radio waves

• Snell’s Law governs the behaviour of electromagnetic waves being refracted:

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1.1.3 Diffraction

Phenomenon whereby waves travelling in straight paths bend around an obstacle

This effect is the result of Hugyens’ principle, an advance by Dutch astronomer Christian Hugyens in 1690

The principle states that each point on a spherical wavefront may be considered as the source of a secondary spherical wavefront

Areas beyond the shadow zone will be able to receive signal and the lower the frequency, the quicker the process of diffraction (i.e the shadow zone is smaller)

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Diffraction around an object

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1.2 Radio wave propagation through space

Terrestrial propagation modes include:

Ground waves

Sky waves

Space-wave (Line-of-sight )propagation

Tropospheric Scatter

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1.2.1 Ground Wave propagationA radio wave that travels along the earth’s surface and sometimes refers to as surface wave.

Follow the curvature of the earth and sometimes beyond the horizon

Must be vertically polarized because the earth will short out the electric field if it is horizontally polarized

Above 2 MHz – the ground waves attenuated very quickly.

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Increasing tilt

Earth surface

Direction of travel

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Ground wave propagation

as the distance of wave propagation increases from the TX, there is a tendency for the waves to “tilt” towards the horizontal, increasing losses and eventually the ground waves will die down

Wave tilting horizontally depends on the radio frequency of the wave; the higher the frequency- the quicker the waves tilt and will only propagate shorter distance.

Operate at frequencies 3 kHz to 2 MHz (VLF, LF, MF).

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Ground wave propagationAt LOW frequency with the sufficient power, the ground waves are able to propagate

around the world more than once before it’ loses its energy .

The conductivity of the earth also determineshow well ground waves are

propagated. The better the conductivity, the less the attenuation and the greater the

distance the waves can travel.

• It much better to propagate over water (especially salt water – excellent conductor)

than very dry (poor conductivity) desert terrain.

Applications Example:

- Military TX = 76 Hz,

- International navigation (LORAN-C)= 100 kHz.

- Standard AM broadcast

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1.2.2 Space wave Propagation

Used for signals transmission in the VHF and higher range.

Two types of space waves:

Direct wave

Reflected wave

The direct wave is most widely used mode.

uses direct radiation from the transmitter to the receiver

But this direct space wave does have severe limitation – limited to line-of-sight transmission distances.

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Space Wave Propagation

Line-of-sight PropagationDirect or space waves are not refracted, nor do they follow the curvature of the earth.

Because of their straight-line nature, direct wave signal travel horizontally from the transmitting antenna until they reach the horizon, at which point they will be blocked.

If a direct wave signal is to be received beyond the horizon, the receiver must be high enough to intercept it.

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LOS

Line-of-sight communication is a characteristic of most radio signals with frequencies above 30 MHz, particularly VHF, UHF and microwave signals.

Transmission distances at those frequencies are extremely limited, and it is obvious why very high transmitting antennas must be used for FM and TV broadcast.

The antennas for transmitters and receivers operating at VHF are typically located on top of the tall buildings or on mountains, to increases the range of transmission and reception.

To extend the communication distance, special technique named repeater stations have been adopted.

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LOS Repeater

A repeater is a combination of a receiver and transmitter operating at different frequencies.

The receiver picks up a signal from a remote transmitter, amplifies it, and retransmits it (on another frequency) to a remote receiver.

Usually, the repeater is located between the transmitting and receiving stations, therefore it extends the communication distance.

Repeaters have extremely sensitive receivers and high power transmitters, their antennas are located at high points.

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Ghosting in TV Reception

Reflected wave can cause reception problems

This occurs when the signal is reflected by large objects like hills or buildings.

There are not only phase cancellation but also time differences between the direct and reflected waves.

This condition results ghosts that appear in TV reception.

Ghosts: When the same signal arrives at the TV receiver at two different times; the reflected signal has further distance to travel and is weaker than the direct signal, resulting in a double image.

For fixed receivers this problem is reduce d by using directional receiving antenna.

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Ghosting interference

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1.2.3 Sky-wave ( Ionospheric) propagationMost frequently used method for long distance transmission in the high-frequency (HF) band.

The concept of transmission is radiating signal towards the ionosphere (at point A) and are refracted back by the ionosphere ( at point B) to the ground. Then it is reflected by the earth surface ( at point C) back to the ionosphere and again refracted from the ionosphere ( at point D) and arrives at the receiving antenna (at point E).

Both refracting and reflecting action can be called as skipping.

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Sky Wave Propagation

1.4 Fading

Time variation in signal amplitude (power) at the receiver caused by characteristics of signal path and changes in it over short time intervals or distance

Static environment: slowly varying properties that depend primarily on the distance between Tx and Rx.

Free space path loss

Power decay with respect to a reference point

The two-ray model

General characterization of systems using the path loss exponent.

Diffraction

Scattering

Fading in mobile environment

Rapidly changing signal characteristics primarily caused by movement and multipath.

Fading is caused by 3 factors:

1. Rapid changes in signal strengths over small travel distances or short time periods.

2. Changes in the frequency of signals.

3. Multiple signals arriving a different times. When added together at the antenna, signals are spread out in time. This can cause a smearing of the signal and interference between bits that are received.

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1.4.1 Multipath Propagation

Reflection – EM signal encounters a surface that is large relative to the wavelength o the signal. In cases where mobile antenna is lower than most human-made structures multipath interference occurs.

Diffraction – occurs at the edge of an impenetrable body that is large compared to the wavelength of the radio wave.

Scattering – occurs when the size of the obstacle in the order of the wavelength of the signal or less

CAR

R

S

D

Lamp Post

R

3 important Propagation Mechanisms: Reflection (R) , Scattering (S) , Diffraction (D).

Effects of Multipath Propagation

Multiple copies of a signal may arrive at different phases. If add destructively, signal level relative to noise will decline.

Intersymbol inteference (ISI) occurs in digital communication. Signal arrives at different times and with different amplitude.

ISI

1.4.2 Types of Fading

Fast fading – 20 to 30 dB variation in amplitude over a short distance, affect not only mobile phones in automobiles but even a mobile phone user walking down an urban street.

Slow fading – fluctuations in amplitude but at longer distances

Fading graphs

1.4.3 Error Compensation Mechanisms

Three general categories

Forward Error Correction

Adaptive equalization

Diversity techniques

Forward Error Correction

Information contained in the incoming digital transmission corrects bit errors in data.

Coding algorithm where transmitter adds a number of additional values, redundant bits to each transmission blocks of data – error correcting codes

Receiver then calculates a new error correcting code for the incoming data

If errors is below a threshold, receiver will determine and correct the error

Adaptive Equalization

Applied to analog transmission eg voice or video or digitized information eg digital data, voice or video

Diversity Techniques

Space diversity –multiple nearby antenna used to receive message and signals from these antennas are combined to reconstruct the most likely transmitted signal

Frequency diversity – signal is spread out over a larger frequency bandwidth or carried on multiple frequency carriers

Time diversity – spread data over time so that a noise burst affects fewer bits. Quite effective in slow fading environment