WIRELESS COMMUNICATIONS

Assist.Prof.Dr. Nuray At

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Diffraction Occurs when the radio path between the T-R is obstructed by a surface that has sharp irregularities (edges). Diffraction allows radio signals to propagate around the curved surface of the

earth, beyond the horizon, and to propagate behind obstructions. Diffraction field has often sufficient strength to produce a useful signal. Fresnel Zone Geometry Obstructing screen is placed between T-R at a distance d1 from the transmitter and d2 from the receiver h: effective height

The Three Basic Propagation Mechanisms

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Knife-edge diffraction geometry Equivalent knife-edge geometry

Assume that andThe excess path length, the difference between the direct path and the diffracted

path:

The corresponding phase difference is

The Three Basic Propagation Mechanisms

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The Fresnel-Kirchoff diffraction parameter v is given by

then can be expressed as

Generally, it is impossible to make very precise estimates of the diffraction losses.

When shadowing is caused by a single object such as a hill or mountain, the attenuation caused by diffraction can be estimated by treating the obstruction as a diffracting knife edge.

The corresponding phase difference is

The Three Basic Propagation Mechanisms

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The electric field strength, Ed, of a knife edge diffracted wave

The diffraction gain:An approximate solution provided by Lee is

Hwk: Plot both the diffraction gain and its approximate solution by Lee.

The Three Basic Propagation Mechanisms

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Knife-edge diffraction gain

The Three Basic Propagation Mechanisms

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Example: Given the following geometry, determine a. The loss due to knife-edge diffractionb. The height of the obstacle required to induce 6dB diffraction loss.Assume f = 900 MHz.

Knife edge

T 100m 50m R 25m 10km 2km

The Three Basic Propagation Mechanisms

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Scattering Occurs when the medium through which the wave travels consists of objects with dimensions that are small compared to the wavelength, and where the number of obstacles per unit volume is large. When a radio wave impinges on a rough surface, the reflected energy is spraed

out (diffused) in all directions due to scattering. Surface roughness is tested using the Rayleigh criterion, which defines a critical

height (hc) of surface protuberances for a given angle of incidence

A surface is considered smooth if its minimum to maximum protuberance h < hc, and is considered rough if the protuberance h > hc

The Three Basic Propagation Mechanisms

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For rough surfaces, the flat surface reflection coefficient needs to be multiplied by a scattering loss factor, .

Ament found to be given by

where is the standard deviation of the surface height about the mean surface height. Modified scattering loss factor by Boithias:

The Three Basic Propagation Mechanisms

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Most radio propagation models are derived using a combination of analytical and emprical methods.Log-distance Path Loss ModelBoth theoretical and measurement-based propagation models indicate that average received signal power decreases logarithmically with distance, whether in outdoor or indoor radio channels. The average large-scale path loss for an arbitrary T-R separation

or

where n is the path loss exponent, d0 is the close-in reference distance.

Link Budget Design Using Path Loss Models

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The value of a path loss exponent n depends on the specific propagation environment . Typical path loss exponents

Link Budget Design Using Path Loss Models

Environment Path Loss Exponent, n

Free space 2

Urban area cellular radio 2.7 to 3.5

Shadowed urban cellular radio 3 to 5

In building line-of-sight 1.6 to 1.8

Obstructed in building 4 to 6

Obstructed in factories 2 to 3

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Log-normal ShadowingThe previous model (log-distance path loss model) does not consider the fact that the surrounding environmental clutter may be vastly different at two different locations having the same T-R separation. Measurements have shown that at any value of d, the path loss PL(d) at a

particular location is random and distributed log-normally (normal in dB) about the mean distance-dependent value.

is a zero-mean Gaussian distributed random variable (in dB) with standart deviation (also in dB)

Link Budget Design Using Path Loss Models

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Log-normal shadowing implies that measured signal levels at a specific T-R separation have a Gaussian distribution about the distance dependent mean. In practice, the values of n and are computed from measured data. Since PL(d) is a random variable with a normal distribution in dB about the distance dependent mean, so is Pr(d)

Thus, the probability that the received signal level (in dB power units) will

exceed a particular level can be given by

where the Q-function is defined as

Link Budget Design Using Path Loss Models

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Determination of Percentage of Coverage Area Due to random effects of shadowing, some locations within a coverage area

will be below a particular desired received signal threshold. For a circular coverage area having radius R from a base station, let there be

some desired received signal threshold . Let U() be the percentage of useful service area, the percentage of area with a

received signal that is equal or greater than , given a known likelihood of coverage at the cell boundary.

Letting d = r represent the radial distance from the transmitter. Then,

Link Budget Design Using Path Loss Models

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By choosing the signal level such that

where

Link Budget Design Using Path Loss Models

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Example: Four received power measurements were taken at distances of 100m, 200m, 1km, and 3km from a transmitter. Let d0 = 100m. Assuming the log-normal shadowing path loss model,a. Find the minimum mean square error (MMSE) estimate for the path loss

exponent, n.b. Calculate the standard deviation about the mean value. c. Estimate the received power at d = 2km using the resulting model.

Link Budget Design Using Path Loss Models

Distance from Transmitter Received Power

100m 0 dBm

200m -20 dBm

1000m -35 dBm

3000m -70 dBm

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In built-up urban areas, fading occurs because the height of the mobile antennas are well below the height of surrounding structures.

The incoming radio waves arrive from different directions with different propagation delays.

The signal received by the mobile at any point in space may consist of a large number of plane waves having randomly distributed amplitudes, phases and angles of arrival.

These multipath components combine vectorially at the receiver antenna, and can cause the signal received by the mobile to distort or fade.

Even when a mobile receiver is stationary, the received signal may fade due to movement of surrounding objects in the radio channel.

Small-Scale Multipath Propagation

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The three most important small-scale fading effects: Rapid changes in signal strength over a small travel distance or time interval Random frequency modulation due to varying Doppler shifts on different

multipath signals Time dispersion (echoes) caused by multipath propagation delays.

Small-Scale Multipath Propagation

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The Doppler Shift Due to the relative motion between the mobile and the base station, each

multipath wave experiences an apparent shift in frequency. The shift in received signal frequency due to motion is called the Doppler shift.

Consider a mobile moving at a constant velocity v, along a path segment having length d between points X and Y, while it receives signals from a remote source S. The difference in path lengths

Small-Scale Multipath Propagation

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The phase change in the received signal due to the difference in path lengths

The apparent change in frequency, or Doppler shift fd:

If the mobile is moving toward the direction of arrival of the wave, the Doppler shift is positive (i.e., the apparent received frequency is increased)

If the mobile is moving away from the direction of arrival of the wave, the Doppler shift is negative (i.e., the apparent received frequency is decreased)

Small-Scale Multipath Propagation