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Presentation on theme: "WIRELESS COMMUNICATIONS Assist.Prof.Dr. Nuray At."— Presentation transcript:


2 The mobile radio channel places fundamental limitations on the performance of wireless communication systems.  Radio channels are extremely random and do not offer easy analysis  The speed of motion impacts how rapidly the signal level fades as a mobile terminal moves.  Modeling issues Radio Wave Propagation The mechanisms behind electromagnetic wave propagation are diverse:  Reflection at large obstacles  Diffraction at edges  Scattering at small objects  … 2

3 Reflection Diffraction Scattering Due to multiple reflections from various objects in urban areas, the electromagnetic waves travel along different paths of varying lengths.  The interaction between these waves causes multipath fading at a specific location  The strengths of the waves decrease as the distance between the transmitter and receiver increases, path loss 3

4 Small-scale or fading models characterize the rapid fluctuations of the received signal strength over very short travel distances or short time durations As a mobile moves over very small distances, the instantaneous received signal strength may fluctuate rapidly giving rise to small-scale fading. The reason for this is that the received signal is a sum of many contributions coming from different directions. Since the phases are random, the sum of the contributions varies widely. In small-scale fading, the received signal power may vary by as much as three or four orders of magnitude (30 or 40dB) when the receiver is moved by only a fraction of a wavelength. 4

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6 Small-scale and Large-scale Fading 6

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8  The Friis free space equation shows that the received power falls off as the square of the T-R separation distance.  The gain of an antenna is related to its effective aperture, A e, by The effective aperture is related to the physical size of the antenna.  An isotropic radiator is an ideal antenna which radiates power with unit gain uniformly in all directions. The effective isotropic radiated power (EIRP): represents the maximum radiated power available from a transmitter in the direction of max. antenna gain, as compared to an isotropic radiator 8

9 In practice, effective radiated power (ERP) is used instead of EIRP to denote the maximum radiated power as compared to a half-wave dipole antenna. Since a dipole antenna has a gain of 1.64 (2.15 dB above an isotropic), the ERP will be 2.15 dB smaller than the EIRP for the same transmission system. The Path Loss represents signal attenuation and is defined as the difference (in dB) between the effective transmitted power and the received power. The path loss for the free space model When the antennas are assumed to have unity gain, 9

10  The Friis free space model is valid for values of d which are in the far-field of the transmitting antenna.  The far-field, or Fraunhofer region, of a transmitting antenna is defined as the region beyond the far-field distance d f. The Fraunhofer distance d f is given by where D is the largest physical linear dimension of the antenna.  Additionally, to be in the far-field region, d f must satisfy  Furthermore, large scale propagation models use a close-in distance d 0, as a known received power reference point. Hence, 10

11 Reflection Occurs when a propagating EM wave impinges upon an object which has very large dimensions when compared to the wavelength of the propagating wave. Reflections occur from the surface of the earth and from buildings and walls. When a radio wave propagating in one medium impinges upon another medium having different electrical properties, the wave is partially reflected and partially transmitted.  If the plane wave is incident on a perfect dielectric, part of the energy is transmitted into the second medium and part of the energy is reflected back into the first medium, and there is no loss of energy in absorption.  If the plane wave is incident on a perfect conductor, then all incident energy is reflected back into the first medium without loss of energy. The Three Basic Propagation Mechanisms 11

12 The Three Basic Propagation Mechanisms 12

13 The Three Basic Propagation Mechanisms 13

14 The Three Basic Propagation Mechanisms 14

15 Example: Some typical values The Three Basic Propagation Mechanisms SurfaceRelative dielectric constant Dry ground4-7 Average ground15 Wet ground25-30 Sea water81 Fresh water81 15

16 The Three Basic Propagation Mechanisms 16

17 The Three Basic Propagation Mechanisms 17

18 Ground Reflection (Two-Ray) Model (1/6) Useful propagation model that is based on geometric optics, and considers both the direct path and a ground reflected propagation path between transmitter and receiver. In most mobile communication systems, the max. T-R separation distance is at most only a few tens of kilometers, and the earth may be assumed to be flat. The Three Basic Propagation Mechanisms 18

19 The Three Basic Propagation Mechanisms 19

20 The Three Basic Propagation Mechanisms 20

21 Ground Reflection (Two-Ray) Model (4/6) When the T-R separation distance d is very large compared to h t + h r, The phase difference between the two E-field components: The time delay between the arrival of the two components: If the received E-field is evaluated at some time, say at t = d’’/c The Three Basic Propagation Mechanisms 21

22 Ground Reflection (Two-Ray) Model (5/6) Note that as d becomes large, the difference between the distances d’ and d’’ becomes very small, and the amplitudes of E LOS and E g are virtually identical and differ only in phase. E TOT (d) decays in an oscillatory fashion. When As long as The Three Basic Propagation Mechanisms 22

23 Ground Reflection (Two-Ray) Model (6/6) The received power at a distance d from the transmitter: This is a much more rapid path loss than is experienced in free space. The Three Basic Propagation Mechanisms 23

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