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Radio Wave Propagation

Radio Wave Propagation

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Radio Wave Propagation

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  1. Radio Wave Propagation Property of R. Struzak

  2. Radio Wave Components Property of R. Struzak

  3. Absorption • = the conversion of the transmitted EM energy into another form, usually thermal. • The conversion takes place as a result of interaction between the incident energy and the material medium, at the molecular or atomic level. • One cause of signal attenuation due to precipitations (rain, snow, sand) and atmospheric gases Property of R. Struzak

  4. Diffraction • = the mechanism the waves spread as they pass barriers in obstructed radio path (through openings or around barriers) • Each point on a wave front acts as a source of secondary spherical wavelets. When the wave front approaches an opening or barrier, only the wavelets approaching the unobstructed section can get past. They emit new wavelets in all directions, creating a new wave front, which creates new wavelets and new wave front, etc. - the process self-perpetuates. • [Huygens, 1629-1695]. Property of R. Struzak

  5. Reflection • = the abrupt change in direction of a wave front at an interface between two dissimilar media so that the wave front returns into the medium from which it originated. Reflecting object is large compared to wavelength. • Reflection may be specular (i.e., mirror-like) or diffuse (i.e., not retaining the image, only the energy) according to the nature of the interface. • The phase of the reflected wave may change depending on the nature of the media and interface and wave polarization. Property of R. Struzak

  6. Refraction • = redirection of a wavefront passing through a medium having a refractive index that is a continuous function of position (e.g., a graded-index optical fiber, or earth atmosphere) or through a boundary between two dissimilar media or • For two media of different refractive indices, the angle of refraction is closely approximated by Snell's Law. Property of R. Struzak

  7. Scattering • - of a wave propagating in a material medium, a phenomenon in which the direction or polarization of the wave is changed when the wave encounters discontinuities in the medium. • Involves objects smaller than the wavelength (e.g. foliage, street signs, …) • Scattering results in a disordered or random change in the incident energy distribution. Property of R. Struzak

  8. Fading • In a received signal, the variation (with time) of the amplitude or relative phase, or both, of one or more of the frequency components of the signal. • Fading is caused by changes in the characteristics of the propagation path with time. Property of R. Struzak

  9. Outdoor Propagation Min. acceptable level (wanted signal) Coverage (useful, service) range Max. tolerable level (unwanted signal) Denied (occupied, sterile, excluded) range Power density Distance n ~ 4, dominates Diffraction & Rayleigh statistics n ~ 2, dominatesLOS & Rice statistics Property of R. Struzak

  10. Propagation Models • Different dominating propagation mechanism • For various frequencies • For various applications • For various environments • For the wanted or interfering signals • Variability due to randomly changing factors • Probabilistic approach Property of R. Struzak

  11. Some Popular Models • Longley-Rice Model (ITS Irregular Terrain Model) • Point-to-Point and Point-to-Area modes, 40MHz-100GHz • Okumura Model • 150MHz-3GHz, urban areas, 1-100km • Hata Model • Based on Okumura model • ITU Model • Atlas of curves Property of R. Struzak

  12. ITU Propagation Models Property of R. Struzak

  13. Received power PR = Kd-n n = 2 in free space Typically 3 n 4 Free space Signal strength (log) Open area (LOS) Urban Suburban Distance (log) Outdoor Propagation Property of R. Struzak

  14. LOS - Fresnel Zone • Fresnel zones are loci of points of constant path-length difference of /2 (=constant phase difference of 1800) • The 1st Fresnel zone corresponds to /2. The n-th zone is the region enclosed between the 2 ellipsoides giving path-length differences n(/2) and (n-1)(/2) R T d1 d2 Property of R. Struzak

  15. Fresnel Zone 2 • Energy transmission from T to R concentrates in the 1st Fresnel zone. If this zone is not obstructed, the energy transmitted approximates energy transmitted in free-space. • An obstruction may lie to the side, above, or below the path. Ridges, bridges, cliffs, buildings, and trees are examples of obstructions. • It means, path obstructions that do not obstruct the 1st Fresnel zone can be ignored. Sometimes one ignores obstructions up to ½ of the 1st Fresnel zone. Property of R. Struzak

  16. Free-Space Model Property of R. Struzak

  17. Troposphere • Troposphere - the lower layer of atmosphere, between the earth surface and the stratosphere, in which the change of temperature with height is relatively large. It is the region where convection is active and clouds form. • The thickness of the troposphere varies with season and latitude. It is usually 16 km to 18 km thick over tropical regions, and less than 10 km thick over the poles. • This layer contains ~80% of the total air mass. Property of R. Struzak

  18. LOS – Radio Horizon • Radio waves go behind the geometrical horizon Radio horizon Geometrical horizon Property of R. Struzak

  19. Refraction in Troposphere • The EM waves travel in atmosphere with slightly lower velocity (v) than in a vacuum (c). • Refractive index: n = c/v – (~1) • Modified refractive index: m = n + h/a • Refractivity N = (n-1)x106 Atmospheric pressure, mbar Vapor pressure, mbar Temperature of the atmosphere, Kelvins Property of R. Struzak

  20. K- Factor • M = N + (h/a)x106 – Refractive modulus • Optics: Snell’s law • In standard conditions the radio wave • travels approximately along an arc bent slightly downward. K-factor is a scaling factor of the ray path curvature. K=1 means a straight line. For the standard atmosphere K=4/3 • Departure from the standard conditions • may led to subrefraction, superrefraction or duct phenomena. • Strong dependence on meteorological phenomena. Superrefraction Hight, h Subrefraction Duct M 0.12 (M x 10-6)/m – Standard atmosphere Property of R. Struzak

  21. K=4/3 K=2 K=1 Examples Long LOS paths over water or desert may show ducting phenomena, - surface ductsor elevated ducts. Property of R. Struzak

  22. Atmospheric Absorption • At frequencies above 10 GHz the atmosphere introduces attenuation due to interaction of radio wave at molecular/ atomic level 10 Specific Attenuation dB/km 10 H2O O2 0.1 10 100 GHz Property of R. Struzak

  23. Multipath Propagation • Reflection coefficient Property of R. Struzak

  24. Reflected signal • The reflected and direct signals received differ due to • Reflection process: it changes the magnitude and phase of the reflected signal • Path-lengths difference of the reflected and direct rays: it introduces phase delay Property of R. Struzak

  25. Reflected signal 2 • The reflected and direct signals received also differ due to • Directive transmitting antenna: the magnitudes and phases of the signals radiated in the receiver direction and the reflection point direction are different • Directive receiving antenna: the magnitudes and phases of the signals received from the transmitter direction and the reflection point direction are different Property of R. Struzak

  26. Ray Tracing • SISP – Site Specific propagation models based on deterministic analysis of all possible rays between the transmitter and receiver to account for reflection, diffraction & scattering • Requires exact data on the environment • Indoor: detailed 3D data on building, room, equipment • Outdoor: 3D data on terrain infrastructure, streets, buildings, etc. • Large databases • Satellite/ aerial photographs or radar images, Property of R. Struzak

  27. 2 Rays: Path-length Difference h1 h2 D h1 Property of R. Struzak

  28. 2 Ray Propagation Model Edir  E R Erefl Property of R. Struzak

  29. Distance Dependence, 2 Rays 6 dB 0 dB relative to free-space Slope: 40 dB/decade Field-strength ~d-2 Power ~d-4 Amplitude, relative to Free-space Distance (h1h2)/ Property of R. Struzak

  30. Simulated Experiments • Distance dependence • Height dependence • Frequency dependence Property of R. Struzak

  31. Time – Frequency Characteristics • Radio channel can be treated as a linear two-terminal-pair transmission channel (input port: transmitting antenna; output port: receiving antenna). Property of R. Struzak

  32. Direct ray y(t) Amplitude x(t) a1 + Amplitude Reflected ray a2 Time   Time  = c(dref – ddir) Path-length difference Light velocity Time Response, 2 Rays Transmitted signal Received signal Property of R. Struzak

  33. Direct RF Pulse Sounding Detector Propagation Channel Key BPF Digital Storage Oscilloscope Pulse Generator Property of R. Struzak

  34. Frequency Domain Sounding Vector Network Analyzer & Swept Frequency Osillator X() Y() Port 1 S-Parameter Test Set Port 2 S21()  H() = [X()] /[X()] Inverse DFT Processor h(t) h(t) = Inverse Fourier Transform of H() Property of R. Struzak

  35. Power Delay Profile • The dispersion of the channel is normally characterized using the RMS Delay Spread, or standard deviation of the power delay profile Relative Power Time Property of R. Struzak

  36. Delay Spread • If an impulse is sent from transmitter in a multiple-reflection environment, the received signal will consist of a number of impulse responses whose delays and amplitudes depend on the reflecting environment of the radio link. The time span they occupy is known as delay spread. Property of R. Struzak

  37. Inter-symbol Interference • The delay spread limits the maximum data rate: no new impulse should reach the receiver before the last replica of the previous impulse has perished. • Otherwise the symbol spreads into its adjacent symbol slot, the two symbols mix, the receiver decision-logic circuitry cannot decide which of the symbols has arrived, and inter-symbol interference occurs. Property of R. Struzak

  38. Inter-symbol Interference Symbols Received Symbols Sent Property of R. Struzak

  39. Microcell vs Macrocell Item Microcell Macrocell Cell radius 0.1-1 km 1-20 km Tx power 0.1-1 W 1-10 W Fading Ricean Rayleigh RMS delay spread 10-100 ns 0.1-10us Max. Bit Rate 1 Mbps 0.3 Mbps After R.H.Katz CS294-7/1996 Property of R. Struzak

  40. Error Bursts • When the delay spread becomes a substantial fraction of the bit period, error bursts may happen. • These error bursts are known as irreductible since it is not possible to reduce their value by increasing the transmitter power. Property of R. Struzak

  41. Error Reduction • Antenna diversity (~10 dB) • Dual antennas placed at /2 separation • Automatic Repeat Request (ARQ) • Retransmission protocol for blocks in error • Error- resistant • modulation, • code, • protocol Property of R. Struzak

  42. Summary • Propagation presents a number of problems we do not control • Dependence on environment, including meteorological phenomena, difficult to predict Property of R. Struzak

  43. References • Many good books, e.g. • Freeman RL: Radio System Design for Telecommunications, J Wiley • Coreira LM: Wireless Flexible Persdonalised Communications, J Wiley • Shigekazu Shibuya, A Basic Atlas of Radio-Wave Propagation, J Wiley • ITU-R Recommendations, SG 3 Property of R. Struzak