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Propagation Characteristics

Propagation Characteristics. Lecture 2. Frequencies for communication. Frequencies and regulations. Signal propagation. Propagation in free space always like light (straight line) Receiving power proportional to 1/d² (d = distance between sender and receiver)

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Propagation Characteristics

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  1. Propagation Characteristics Lecture 2

  2. Frequencies for communication

  3. Frequencies and regulations

  4. Signal propagation Propagation in free space always like light (straight line) Receiving power proportional to 1/d² (d = distance between sender and receiver) Receiving power additionally influenced by • Fading (frequency dependent) • Shadowing • Reflectionat large obstacles • Refraction depending on the density of a medium • Scattering at small obstacles • Diffraction at edges

  5. Signal propagation

  6. Real world example

  7. Free space loss, ideal isotropic antenna Pt= signal power at transmitting antenna Pr= signal power at receiving antenna λ= carrier wavelength d = propagation distance between antennas c= speed of light (3x108 m/s) where d and λ are in the same units (e.g., meters)

  8. Free Space Loss Free space loss equation can be rewritten:

  9. Free Space Loss Free space loss accounting for gain of other antennas Isotropic ant : Gt=1, Gr=1 Gt= gain of transmitting antenna Gr= gain of receiving antenna At= effective area of transmitting antenna Ar= effective area of receiving antenna

  10. Free Space Loss Free space loss accounting for gain of other antennas can be recast as

  11. Propagation model Path loss: function of distance between TX and RX d0 : close-in distance, received power reference point, commonly 1Km used d:T-R separation: distance n: path loss exponent Log-normal shadowing: amplitude has a log-normal PDF Addition of random variable Xσ: zero-mean Gaussian distributed random variable (in dB) with standard deviation σ

  12. Path loss model parameter

  13. Path loss model parameter Two values are computed from measured data, using linear regression method

  14. Empirical Models • Okumura model • Empirically based (site/freq specific) • Awkward (uses graphs) • Hata model • Analytical approximation to Okumura model • Cost 231 Model: • Extends Hata model to higher frequency (2 GHz) • Walfish/Bertoni: • Cost 231 extension to include diffraction from rooftops Commonly used in cellular system simulations

  15. (Okumura) model

  16. Hata model

  17. Slow Fast Very slow Propagation Characteristics • Path Loss (includes average shadowing) • Shadowing (due to obstructions): reflection, refraction, diffraction • Multipath Fading Pr/Pt Pt Pr v d=vt d=vt

  18. Channel characteristics • Channel characteristics change over time and location • signal paths change • different delay variations of different signal parts • different phases of signal parts • quick changes in the power received • (short term fading) • Additional changes in • distance to sender • obstacles further away • slow changes in the average power • slow changes in the average term fading received (long term fading)

  19. Combined Path Loss & Fading

  20. Path Loss Modeling • Maxwell’s equations • Complex and impractical • Free space path loss model • Too simple • Ray tracing models • Requires site-specific information • Simplified power falloff models • Main characteristics: good for high-level analysis • Empirical Models • Don’t always generalize to other environments

  21. Small Scale fading • Variations due to shadowing occur over relatively large distances– often many meters • Signals in multipath environments also undergo small scale fading – variations that occur over the wavelength of the signal • This is due to the different multipath components combining either constructively or destructively

  22. Small-scale fading (2) Offset of only a fraction of a wavelength can lead to large change in signal level:

  23. Multipath propagation Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction • Time dispersion: signal is dispersed over time • interference with “neighbor” symbols Inter Symbol Interference (ISI) • The signal reaches a receiver directly and phase shifted • distorted signal depending on the phases of the different parts

  24. The Effects of Multipath Propagation • Due to the different paths taken by the multipath components, they may arrive at different times • If the symbol period TS is smaller than the delay spread, i.e. TS < Tm, Inter-Symbol Interference (ISI) will occur • The receiver cannot determine which symbol each multipath component belongs to:

  25. The Effects of Multipath Propagation

  26. Delay Spread The Delay Spread Tm is defined as the difference between times-of arrival of the first and last multipath components Typical values are as follows:

  27. (Doppler shift)

  28. Fading

  29. Coherence Bandwidth • The Coherence Bandwidth Bcis a statistical measure of the range of frequencies over which the attenuation of the channel is approximately constant • Two frequency components f1 and f2 will experience similar attenuation if (f1 – f2) << Bc • Coherence Bandwidth is approximately related to the Delay Spread by: • Bc(Hz) = 1/Tm • e.g. in a particular factory environment, • Tm= 120ns, Bc= 1/(120 x 10-9) = 8.33 MHz

  30. Coherence Bandwidth (2) • If the transmitted signal has a bandwidth (Bu) much smaller than the Coherence Bandwidth(Bc), i.e. Bu<< Bc, all frequency components will be attenuated similarly. • This is called Flat Fading • Else, it will undergo Frequency-selective fading, with different components attenuated differently. This causes distortion of the signal

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