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Lecture 2A- April 7 wireless channel +802-11-v2

Lecture 2A- April 7 wireless channel +802-11-v2 - CS 117...

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CS 117 Spring 09 Lecture 2A, April 7, 2009 The wireless channel The 802.11 MAC
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Lecture Outline 1. The wireless channel Propagation 2. The 802.11 MAC and Wireless LANs Ref: Tanenbaum Chpt 2, pg 100-107; pg 162-166 (CDMA); pg 292- 301
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3 Spectrum and Propagation
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EM Spectrum ν Propagation characteristics are different in each frequency band LF HF VHF UHF SHF EHF MF AM radio UV S/W radio FM radio TV TV cellular ν 1 MHz 1 kHz 1 GHz 1 THz 1 PHz 1 EHz infrared visible X rays Gamma rays 902 – 928 Mhz 2.4 – 2.4835 Ghz 5.725 – 5.785 Ghz ISM band λ 30kHz 300kHz 3MHz 30MHz 300MHz 30GHz 300GHz 10km 1km 100m 10m 1m 10cm 1cm 100mm 3GHz
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Unlicensed Radio Spectrum 902 Mhz 928 Mhz 26 Mhz 83.5 Mhz 125 Mhz 2.4 Ghz 2.4835 Ghz 5.725 Ghz 5.785 Ghz cordless phones baby monitors Wireless LANs (first generation) 802.11 Bluetooth Microwave oven 802.11a λ 33cm 12cm 5cm This spectrum is free (unlicensed) However, users must implement “Spread Spectrum” Techniques in order to minimize mutual interference DSSS or Frequency Hopping
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Mobile Radio Propagation Free space loss Two ray model Slow fading Fast fading Delay spread
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Radio propagation: path loss P t P r P r near field path loss = 10 log (4 π r 2 / λ ) r 8m = 58.3 + 10 log ( r 3.3 /8) r > 8m r path loss in 2.4 Ghz band near field far field r 2 r 8m r > 8m r 3.3 Two ray (direct + reflected) model
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Multipath Fading Tx Rx Fading: rapid fluctuation of the amplitude of a radio signal over a short period of time or travel distance Fading Varying doppler shifts on different multipath signals Time dispersion (causing inter symbol interference) Effects of multipath:
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Reflection,diffraction, scattering of radio signals Scatterin g
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The delay spread of a signal
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Propagation mechanisms Reflection: propagation wave reflected by smooth surface larger than wavelength Diffraction (shadowing): wave obstructed by surface with sharp, irregular edges. Scattering: wave hits loose objects smaller than wavelength; signal scattered in bunch of outgoing weaker signals (wavelets)
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Fading FAST FADING - observed over traveled distance of about half a . For VHF and UHF, a vehicle traveling 30 mph passes through several fast fades in a sec. SLOW FADING - caused by large obstructions Therefore, the mobile radio signal consists of a Short-Term Fast-Fading signal + Long-Term Slow Fading signal, a local mean value that remains constant over a small area and varies slowly as the receiver moves. λ
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Schematic diagram of propagation loss and fading L f = fast fading: rapid fluctuations due to multipath fading L s = slow fading (shadow fading): path loss “variation” caused by changes in landscape, i.e., building. VHF, 30 mph In City 2 λ
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Fast fading: Raleigh distribution ( 29 0 , 2 2 2 2 > = - r e r r p r s s P robability D ensity F unction of Raleigh distribution when sigma=1,2,and 3 Where r is the envelope of faded signal,    is the  standard deviation of signal level in decibels.
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