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S-72.227 Digital Communication Systems

S-72.227 Digital Communication Systems. Multipath Radio Channel Addendum (extracts from J-P Linnartz: Wireless Communication CDROM). Multipath radio channels. Physical backgrounds Channel modeling: statistical, semi-statistical, deterministic (empirical) Characterizing channel:

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S-72.227 Digital Communication Systems

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  1. S-72.227 Digital Communication Systems Multipath Radio Channel Addendum (extracts from J-P Linnartz: Wireless Communication CDROM)

  2. Multipath radio channels • Physical backgrounds • Channel modeling: statistical, semi-statistical, deterministic (empirical) • Characterizing channel: • channel responses • figures of merit, as RMS-delay spread and coherence bandwidth • Reception techniques to cope with multipath • diversity • antenna • power control • selection of modulation scheme: GSM, OFDM, UWB • coding: FEC / ARQ • interleaving

  3. Mapping electromagnetic fields based on physical structures Ray tracing Simplified physical environment modeled by statistical parameters Playback modeling Modeling radio channels + * increasing accuracy-> statistical semi-deterministic deterministic + * + + Radio channel modeling targets: - narrow band : averaged path loss - wide band : impulse response + Wideband modeling * Narrowband modeling

  4. Effect of mobility in communication systems

  5. Effect of mobility in communication systems (cont.)

  6. Effect of mobility in communication systems (cont.)

  7. Multipath fading characterization

  8. Multipath fading characterization (cont)

  9. Radio channel's physical backgrounds reflection scattering diffraction dispersion multipath variation as a function of time Rayleigh Rice free space loss plane earth loss Path loss shadowing

  10. Physical propagation: case A: free space B: reflection C: diffraction D: scattering

  11. Free space loss

  12. Semi-empirical channel modeling • Egli's model takes into account • loss per distance • antenna high • fits to measurements • Okimura-Hata have madea model that fits themodel into empiricalaverage path lossmeasurements

  13. Shadowing • Received signal power fluctuates due to objects obstructing the propagation path between transmitter and receiver. • These fluctuations are experienced on local-mean powers, that is, short-term averages to remove fluctuations due to multipath fading.

  14. Example of average path loss in different cities

  15. Typical delay spreads

  16. Scatter functions • Effects of path delays & Doppler spread combined in a single plot • Each path can be described by angle of arrival, complex power attenuation, Doppler shift and excess delay • Thus a plot can be made for instance to show the received wave with Doppler & Delay in x-plane and power at z plane

  17. Distribution of the angle of arrival

  18. Doppler spectra <- Theoretical spectra for an omni-directional sinusoidal

  19. Diffraction loss (single knife)

  20. Antenna diversity • A signal transmitted at a particular carrier frequency and at a particular instant of time may be received in a multipath null. • Diversity reception reduces the probability of occurrence of communication failures (outages) caused by fades by combining several copies of the same message received over different channels. • In general, the efficiency of the diversity techniques reduces if the signal fading is correlated at different branches.

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