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1. 1. Presented By :Presented By :Er. Rajesh KumarEr. Rajesh KumarINDIAINDIA
6. 6. EXAMPLEEXAMPLE For example, consider the common experience ofFor example, consider the common experience ofstopping at traffic lights and hearing a lot of staticstopping at traffic lights and hearing a lot of staticon your FM broadcast radio, which is immediatelyon your FM broadcast radio, which is immediatelycorrected if you move less than a meter. Cellularcorrected if you move less than a meter. Cellularphones also exhibit similar momentary fades. Thephones also exhibit similar momentary fades. Thereason for these losses of signal is the destructivereason for these losses of signal is the destructiveinterference that multiple reflected copies of theinterference that multiple reflected copies of thesignal make with itself. To understand how asignal make with itself. To understand how asignal can destructively interfere with itself,signal can destructively interfere with itself,consider the sum of twoconsider the sum of two sinusoidalsinusoidal waveformswaveforms(which are similar to modulated carrier signals)(which are similar to modulated carrier signals)with different phases.with different phases.
8. 8. SMALL SCALE MULTIPATH PROPAGATIONSMALL SCALE MULTIPATH PROPAGATION Multipath in the radio channel creates smallMultipath in the radio channel creates smallscale fading effects. The three mostscale fading effects. The three mostimportant effects are :important effects are : Rapid changes in signal strength over aRapid changes in signal strength over asmall travel distance or time interval.small travel distance or time interval. Random frequency distribution due toRandom frequency distribution due tovarying Doppler shifts on differentvarying Doppler shifts on differentmultipath signals.multipath signals. Time dispersion (Echoes) caused byTime dispersion (Echoes) caused bymultipath propagation delays.multipath propagation delays.
10. 10. (3) Speed of surrounding objects :If objects in the radio channel are in motion, they induced a timevarying Doppler Shift on multipath components. If the surroundingobjects move at a greater rate than the mobile, then this effectdominates the Small scale Fading. Otherwise, motion of surroundingobjects may be ignored and only the speed of the mobile need beconsidered. The coherence time defines the “static ness” of thechannel, and is directly impacted by the Doppler shift.(4) The transmission bandwidth of the signal :If the transmitted radio signal bandwidth is greater than the“bandwidth” of the multipath channel, the received signal will bedistorted, but the received signal strength will not fade much over alocal area (i.e., the small scale signal fading will not be significant). Aswill be shown, the bandwidth of the channel can be quantified by thecoherence bandwidth which is related to the specific multipath structureof the channel. The coherence bandwidth is a measure of the maximumfrequency difference for which signals are still strongly correlated inamplitude. If the transmitted signal has a narrow bandwidth ascompared to the channel, the amplitude of the signal change rapidly,but the signal will not be distorted in time.
11. 11. DOPPLER SHIFTDOPPLER SHIFT
12. 12.  Consider a mobile moving at aConsider a mobile moving at aconstant velocityconstant velocity vv, along a path, along a pathsegment having lengthsegment having length dd betweenbetweenpoints X and Y, while it receivespoints X and Y, while it receivessignals from a remote source assignals from a remote source asillustrated in Fig (1) in previous slide :illustrated in Fig (1) in previous slide : The difference in path length traveledThe difference in path length traveledby the wave from source S to theby the wave from source S to themobile at points X and Y ismobile at points X and Y is∆∆l = dcosθ = v∆tcosθl = dcosθ = v∆tcosθ
13. 13. The phase change in received signalThe phase change in received signaldue to difference in path lengths isdue to difference in path lengths istherefore,therefore,∆∆ Φ =Φ = 2 π ∆2 π ∆ ll == 2 π v ∆2 π v ∆ tt cos θcos θλ λλ λand hence the apparent change inand hence the apparent change infrequency or Doppler shift, is givenfrequency or Doppler shift, is givenbyby fdfd, where, wherefdfd == 11 .. ∆Φ∆Φ == vv . cosθ. cosθ2π ∆ t λ2π ∆ t λ
14. 14. TYPES OF SMALL SCALE FADINGTYPES OF SMALL SCALE FADINGFig. (2) : Types of small scale fading
15. 15. FLAT FADINGFLAT FADING Flat fadingFlat fading, where the bandwidth of, where the bandwidth ofthe signal is less than thethe signal is less than thecoherence bandwidthcoherence bandwidth of the channelof the channelor theor the delay spreaddelay spread is less than theis less than thesymbol periodsymbol period..
16. 16. Fig. (3):Fig. (3): Flat Fading channel characteristicsFlat Fading channel characteristics
17. 17. FREQUENCY SELECTIVE FADINGFREQUENCY SELECTIVE FADING Frequency selective fadingFrequency selective fading, where, wherethe bandwidth of the signal is greaterthe bandwidth of the signal is greaterthan the coherence bandwidth of thethan the coherence bandwidth of thechannel or the delay spread ischannel or the delay spread isgreater than the symbol period.greater than the symbol period.
18. 18. Fig. (4) :Fig. (4) : Frequency selective fading channel characteristicsFrequency selective fading channel characteristics