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1. 1. ByManish Srivastava
2. 2. Power delay profile The power delay profile (PDP) gives the intensity of a signal received through a multipath channel as a function of time delay. The time delay is the difference in travel time between multipath arrivals. In the graph of the PDP abscissa represents units of time whereas ordinate is usually in decibels. example. It can be measured empirically and can be used to extr act certain channels parameters such as the delay spre ad.
3. 3. Power delay profile The power delay profile (PDP) gives the intensity of a signal received through a multipath channel as a function of time delay. The time delay is the difference in travel time between multipath arrivals. In the graph of the PDP abscissa represents units of time whereas ordinate is usually in decibels. example. It can be measured empirically and can be used to extr act certain channels parameters such as the delay spre ad.
4. 4. Power delay profile graph…. -90 RMS Delay Spread ( ) = 46.4 ns -90Received Signal Level (dBm) Mean Excess delay ( ) = 45 ns -95 Maximum Excess delay < 10 dB = 110 ns -100 Noise threshold -105 0 50 100 150 200 250 300 350 400 450 Excess Delay (ns)
5. 5. FADING… It is the rapid fluctuations of received signal strength over short time intervals and/or travel distances Caused by interference from multiple copies of Tx signal arriving at Rx at slightly different times. Three most important effects: 1. Rapid changes in signal strengths over small travel distances or short time periods. 2. Changes in the frequency of signals. 3. Multiple signals arriving at different times. When added together at the antenna, signals are spread out in time. This can cause a smearing of the signal and interference between bits that are received.
6. 6. Multipath Propagation - Fading a b No direct path Diffracted wave Reflected wave a a Antenna y=a+b Antenna y=0 b b a & b are in phase a & b are out of phase by Complete fading when 2 d/ = n , d is the path differenceZ. Ghassemlooy
7. 7.  Even stationary Tx/Rx wireless links can experience fading due to the motion of objects (cars, people, trees, etc.) in surrounding environment off of which come the reflections.Transmitting a short pulse over a(i) frequency-selective (delay-spread) fading channel: Transmitted Received t t Tp Tp + dt(ii) time-selective (Doppler-spread) fading channel: Transmitted Received t t Tp Tp
9. 9. Multipath time Delay Spread Figure Explaining Multi-path Fading When the waves of multi-path signals are out of phase, reduction of the signal strength at the receiver can occur. The 2 types of delay spreads are- A) FLAT FADING B) FREQUENCY SELECTIVE FADING
10. 10. DELAY SPREAD The different signal paths between Tx and a Rx corresponds to different transmission times. For an identical signal pulse from the Tx, multiple copies of the signals are received at the receiver at different moments. The signal on the shortest path (typically LOS) reaches first than those on longer paths. The direct effect of these un-simultaneous arrivals of signal causes the spread of the original signal in time domain. This spread is called the DELAY SPREAD.
11. 11.  In tele-communication, the delay spread is a measure of the MULTI-PATH RICHNESS of a communication channel. It is used most of the time in characterizing wireless channel but applies to any other multipath channel like multipath in optical fibres. Corresponding to the concept of delay spread, there is a term called COHERENCE BANDWIDTH used to measure the up-limit bandwidth that can be transmitted for a channel to be free of ISI. Defn: Defined as the 10% of the recipocal of rms delay spread. In this the channel passes all the spectral components with approx. equal gain and phase.
12. 12. Multipath Delay Spread First-arrival delay (τA) Mean excess delay e ( A ) P( )d Z. Ghassemlooy
13. 13. Effects of delay spread: It causes-INTER SYMBOL INTERFERENCE (ISI) if the bandwidth of a transmitter signal is less than the channel coherence bandwidth, the channel shows flat fading to be free of ISI. otherwise, the channel shows frequency selective fading and may suffer from ISI. •Delay spread varies with the terrain with typical values for rural, urban and suburban areas: 0 .2 s rural 3 .0 s urban 0.5 s suburban
14. 14. DOPPLER SPREAD  Defn : Time varying fading due to the motion of a scatter or the motion of the transmitter or receiver or both results in Doppler spread.  It is caused by TIME SELECTIVE FADING. i.e for a particular instance of time channel behaves as a fading channel and for rest it behaves as FLAT channel. TYPES OF FADING ON BASIS OF DOPPLER SPREAD:Fast fading – channel impulse Slow fading – channel impulseresponse changes rapidly within the response changes at a rate much slower thansymbol duration the transmitted symbol bandwidth
15. 15. EFFECT OF MOVEMENT Here is a plot of the magnitude of fading as a function of time and frequency. In this case, the channel does not change much over time. It is a slowly fading channel.
16. 16. EFFECT OF MOVEMENT Here is a plot of the magnitude of fading as a function of time and frequency. In this case, the channel does not vary with frequency, it only varies over time.
17. 17. EFFECT OF MOVEMENT Here is a plot of the magnitude of fading as a function of time and frequency. In this case, the channel varies both with frequency and time.
18. 18. Doppler Effect When the receiver or transmitter are moving, the frequency is shifted by f = v/ cos( ), v is velocity and is wave length v c is the speed of light. The maximum shift is f m fc c If the the signal is sent at fc and passed through a fading channel, the spectrum of the received signal is: Thus, not only one frequency is received, but many.
19. 19. •If a sinusoidal signal is transmitted(represented by a spectral line in thefrequency domain), after transmission over afading channel, we will receive a powerspectrum that is spread according to theimage. Figure: Measured Doppler spread at 1800 MHz. Doppler spread = 60.3 Hz•The frequency range where the powerspectrum is nonzero defines the Dopplerspread.
20. 20. The Doppler frequency V fD cos f m cos The received signal fr fc f m cos frequency• When = 0o (mobile moving away from the transmitter) fr fc fm• When = 90o (I.e. mobile circling around) fr fc• When = 180o (mobile moving towards the transmitter) fr fc fm
21. 21. Doppler spread and coherence time Doppler spread and coherence Tc 1 time (Tc) are inversely frms proportional frms is the rms value of maximum doppler shift Given as, fm frms 2 9 Tc 16 frms For 0.5 correlation
22. 22. Doppler Effect To mitigate the Doppler effect:  Use low frequencies  Transmit in bursts so the channel is constant during the burst.  Include training sequences on each frame so the channel can be re-estimated for each transmission.  Do move – indoor use only
23. 23.  THANK YOU..!!  {FOR BEING SILENT LISTENERS}