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

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

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Unformatted text preview: CS 117 Spring 09 Lecture 2A, April 7, 2009 The wireless channel The 802.11 MAC 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 3 Spectrum and Propagation EM Spectrum Propagation characteristics are different in each frequency band LF HF VHF UHF SHF EHF MF A M r a d i o UV S / W r a d i o F M r a d i o T V T V c e l l u l a r 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 30kHz 300kHz 3MHz 30MHz 300MHz 30GHz 300GHz 10km 1km 100m 10m 1m 10cm 1cm 100mm 3GHz 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 33cm 12cm 5cm This spectr um is fr ee (unlicensed) H owever , user s must implement Spr ead Spectr um Techniques in or der to minimize mutual inter fer ence DSSS or Fr equency H opping Mobile Radio Pr opagation Free space loss Two ray model Slow fading Fast fading Delay spread Radio propagation: path loss P t P r P r near field path loss = 10 log (4 r 2 / ) 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 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: Reflection,diffr action, scatter ing of r adio signals Scatter in g The delay spr ead of a signal Pr opagation 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) 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. Schematic diagr am of pr opagation 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....
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Lecture 2A- April 7 wireless channel +802-11-v2 - CS 117...

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