1 / 15

Fading

Fading. Kevin Bolding Electrical Engineering Seattle Pacific University. Signals fade . Received Signal Strength (RSS) varies over time and space Slow Fading caused by large obstacles and terrain impairments Appears as local rises/falls in RSS around a logarithmic curve. Multipath fading.

willa
Download Presentation

Fading

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Fading Kevin BoldingElectrical EngineeringSeattle Pacific University

  2. Signals fade • Received Signal Strength (RSS) varies over time and space • Slow Fading caused by large obstacles and terrain impairments • Appears as local rises/falls in RSS around a logarithmic curve

  3. Multipath fading • Reflections and diffraction cause radio signals to follow different paths to the receiver • The received signals are summed at the receiver • The signal components will have various phases and amplitudes • This will cause positive and negative interference that varies based on the relative locations of the: • Source • Objects in the path • Receiver • As these objects move around, the interference pattern will change • The signal strength will vary significantly over small variations in the placement of these objects

  4. Fading due to reflections • Multiple signal paths will result in multiple received signals • Each reflection path is longer than the direct path – a change in the path length results in a phase difference • Reflections off of the earth usually introduce a 180 degree phase shift • In the absence of reflecting objects other than the earth, the relationship of the various reflected signal depends on • The location and height of the transmitter and receiver • The wavelength (frequency) of the signal • Multiple reflections cause the RSS to vary with frequency

  5. Fading due to multipath Doppler shift • If the receiver is moving relative to the transmitter, there will be a shift in frequency – Doppler shift • Frequency increases if moving closer • Frequency decreases if moving apart • At typical vehicle velocities, the Doppler shift is under 100Hz, which is not itself a problem • In a multipath situation, the relative velocity of the receiver varies depending on the geometry of the paths •  Different paths have different Doppler shifts, leading to slightly different frequencies • The received signal strength will vary over time as the alignment of the various signals changes • Multipath Doppler shift causes the RSS to vary over time

  6. Doppler Shift • For E/M waves, the change in frequency (Doppler shift) is computed by: • vs,r is the relative speed of the receiver w.r.t. the transmitter. vs,r is positive when the receiver is moving away from the transmitter. • f0 is the frequency and 0 is the wavelength of the E/M wave.

  7. Fading varies with time and frequency Amplitude of the channel attenuation in dB versus carrier frequency (in MHz) and time (in milliseconds).Normalized delay spread tm = 0.05 and normalized Doppler spread fD = 0.05.A time selective and frequency selective Rayleigh fading channel From: http://www.wirelesscommunication.nl/reference/chaptr03/channel.htm

  8. Multipath fading http://en.wikipedia.org/wiki/Image:Rayleigh_fading_doppler_10Hz.svg Average RSS Fades “Deep” Fades An example of multipath fading from a moving receiver.

  9. Multipath Fading Analysis • Some environments have a large number of scatterers • Ionosphere • Urban environments • If there are a large number of scattering objects, the received signal has phase and amplitude represented by a complex vector z with: • Randomly distributed Real part (Normal dist.) • Randomly distributed Imaginary part (Normal dist.) • Note: This assumes that only scattered rays are received. If scattered and the direct ray are both received, a different model applies.

  10. Im Re Complex plane • Amplitude = sqrt (x2+y2) y z = x + iy amplitude phase x See http://www.wirelesscommunication.nl/reference/chaptr03/rayjava/rayjava.htm for an animation tool

  11. Rayleigh Distribution • A random variable follows the Rayleigh distribution if it is a vector with its orthogonal components normally and independently distributed • The Real (x) and Imaginary (y) parts of the received signal are both normally distributed • The amplitude has a Rayleigh distribution PDF of a RayleighDistribution with variousVariances Mean Power < Mean Power > Mean Tail goes to infinity

  12. Rayleigh PDF RSS, 4 MPH RSS, 40 MPH Rayleigh Fading with Moving Receiver • Rayleigh fading describes the change in RSS at various locations • A Rayleigh faded channel has significant RSS variations over very small distances • A moving receiver experiences this as RSS changes over a short period of time • A moving receiver also experiences Doppler shift, which increases the rate of fading

  13. RSS, 4 MPH, Max Doppler = 10Hz Assume fade margin is setto some level. (Example: -15dB)  = ratio of margin to RMSmean (Example: -15dB =1/31.6 = 0.032)fd=Maximum Doppler shift(Example: 10Hz) Analysis using Rayleigh Model How often do fades exceed the fade margin?Analytically, compute theLevel Crossing Rate (Hz): How long is the averagefade duration for fadesbeyond the fade margin?Analytically, computethe Average Fade Duration (s):

  14. Rician Fading • If the received signal is a combination of a single line-of-sight (dominant) ray and scattered rays, the RSS follows the Rice distribution • The Rice distribution has a parameter, v, that represents the degree of dominance of the primary ray • At v=0, the Ricean distribution is the same as the Rayleigh distribution v=0, Rayleigh Note that the meanchanges with v

  15. Fading Summary • Obstructions cause fading through blockages, reflections and diffraction • Slow fading refers to deviations from free-space fading that occur over longer distances • Caused by large obstructions • Characterized by log-normal distribution • Fast fading refers to deviations that occur over very short distances • Caused by multipath • Characterized by Rayleigh or Ricean distributions

More Related