WIRELESS COMMUNICATIONS Assist.Prof.Dr. Nuray At

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 d 1 from the transmitter and d 2 from the receiver h: effective height The Three Basic Propagation Mechanisms 2

Knife-edge diffraction geometry Equivalent knife-edge geometry Assume that and The excess path length, the difference between the direct path and the diffracted path: The corresponding phase difference is The Three Basic Propagation Mechanisms 3

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The electric field strength, E d, 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 5

Knife-edge diffraction gain The Three Basic Propagation Mechanisms 6

Example: Given the following geometry, determine a.The loss due to knife-edge diffraction b.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 7

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Most radio propagation models are derived using a combination of analytical and emprical methods. Log-distance Path Loss Model Both 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, d 0 is the close-in reference distance. Link Budget Design Using Path Loss Models 10

 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 11 EnvironmentPath Loss Exponent, n Free space2 Urban area cellular radio2.7 to 3.5 Shadowed urban cellular radio3 to 5 In building line-of-sight1.6 to 1.8 Obstructed in building4 to 6 Obstructed in factories2 to 3

Link Budget Design Using Path Loss Models 12

Link Budget Design Using Path Loss Models 13

Link Budget Design Using Path Loss Models 14

By choosing the signal level such that where Link Budget Design Using Path Loss Models 15

Example: Four received power measurements were taken at distances of 100m, 200m, 1km, and 3km from a transmitter. Let d 0 = 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 16 Distance from TransmitterReceived Power 100m0 dBm 200m-20 dBm 1000m-35 dBm 3000m-70 dBm

 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 17

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 18

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 19

The phase change in the received signal due to the difference in path lengths The apparent change in frequency, or Doppler shift f d : 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 20