1 / 14

Fading Techniques

Fading Techniques . Rayleigh Fading Channel. We starts from additive white Gaussian noise (AWGN), with statistically independent Guassian noise sample corrupting data samples free of intersymbol interference (ISI). Causes to degradation: Thermal noise at the receiver

demetrius
Download Presentation

Fading Techniques

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 Techniques

  2. Rayleigh Fading Channel • We starts from additive white Gaussian noise (AWGN), with statistically independent Guassian noise sample corrupting data samples free of intersymbol interference (ISI). • Causes to degradation: • Thermal noise at the receiver • External interference received by the antenna • Signal attenuation vs. distance

  3. Rayleigh Fading Channel • Path loss or free space loss: Ls(d) = (4d)2 where Ls(d): path loss d: distance between transmitter and receiver : wavelength of the propagation signal • For this case of idealized propagation, received signal power is very predictable. 

  4. Rayleigh Fading Channel • In wireless sys, a signal can travels over multiple reflective paths, which is referred to as multipath propagation. It causes to multipath fadingor scintillation. • Scintillation described the multipath fading caused by physical changes in the propagation medium.

  5. a b No direct path Diffracted wave Reflected wave a a Antenna Antenna y = a + b y = 0 b b Complete fading when 2d/ = n, d is the path difference Multipath Propagation - Fading a & b are in phase a & b are out of phase by 

  6. Mobile Radio Propagation: Large-Scale and Small-Scale Fading • Large Scale Fading at Fig. 1: • Large scale fading due to motion over large areas • Mean signal attenuation vs. distance • Variation about the mean • These above fadings are affected by prominent terrain contours. It is as being “shadowing”. • It described in terms of a mean-path loss (nth-power law) & log-normally distribution variation.

  7. Mobile Radio Propagation: Large-Scale and Small-Scale Fading • Small Scale Fading at Fig. 1: • Small-scale fading due to small changes in position • Time spreading of the signal • Time variance of the channel • Small scale fading manifests itself in: • Time-spreading of the signal (or signal dispersion) • Time-variant bahavior of the channel • Small scale fading is also called Rayleigh fading

  8. Mobile Radio Propagation: Large-Scale and Small-Scale Fading • Generally: r(t) = s(t) * hc(t) , where: * : convolution r(t): receive signal s(t): transmitted signal hc(t): the impulse response of the channel

  9. Mobile Radio Propagation: Large-Scale and Small-Scale Fading • In case of mobile radios: r(t) = m(t) x r0(t), where: m(t): large-scale-fading component or local mean or log-normal fading. r0(t): Small-scale-fading component or multipath or Rayleigh fading.

  10. Challenges of Communicating Over Fading Channels • Sources of noise degrade the system performance • AWGN (ex. Thermal noise) • Man-made and natural noise • Interferences • Band-limiting filter induces the ISI effect • Radio channel results in propagation loss • Signal attenuation versus distance over free space. For example, • Multi-path fading  cause fluctuations in the received amplitude, phase, angle of arrival

  11. Characterizing Mobile-radio Propagation • Large-scale fading • Signal power attenuation due to motion over large area • Is caused by the prominent terrain (ex. hills, forest, billboard…) between the transmitter and the receiver • Statistics of path loss over the large-scale fading • Mean-path loss (nth-power law) • Log-normal distributed variation about the mean • Is evaluated by averaging the received signal over 10 to 30 wavelengths

  12. Characterizing Mobile-radio Propagation • Small-scale fading • Time-spreading of the signal • Time delays of multi-path arrival • Time-variant behavior of the channel • Motion between the transmitter and the receiver results in propagation path changes • Statistics of envelop over the small-scale fading • Rayleigh fading if there are large number of reflective paths, and if there is no line-of –sight signal components • Rician pdf while a line-of-sight propagation path is added to the multiple reflective paths

  13. Basic Mechanisms for Signal Propagation • Reflection • Electromagnetic wave impinges on a smooth surface with very large dimensions relative to the RF wavelength • Diffraction • Propagation path between the transmitter and the receiver is obstructed by a dense body, causing secondary waves to be formed behind the obstructing body • Scattering • A radio wave impinges on either a large, rough surface or any surface whose dimensions are on the order of l or less, causing the energy to be spread out

  14. Thank you

More Related