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Small scale fading

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Small scale fading

  1. 1. Small Scale Fading: By AJAL JOSE
  2. 2. Radio Block Diagram  In today's class:  How does the signal propagate? What are the prominent effects? --- Small scale fading Coding Modulation Antenna DemodulationDecoding Antenna
  3. 3. Main story  Communication over a flat fading channel has poor performance due to significant probability that channel is in deep fading.  Reliability is increased by provide more signal paths that fade independently.  Diversity can be provided across time, frequency and space.  Name of the game is how to expoited the added diversity in an efficient manner.
  4. 4. Antenna Diversity Receive Transmit Both
  5. 5. Current strategy : Transmit More when Channel is Good
  6. 6. Small-scale Multipath Propagation The mobile radio channel as a function of time and space. Illustration of Doppler effect. d X Y Δl d υ
  7. 7. Channel issues
  8. 8. Accuracy of Models • The accuracy of the models depends on their purpose: – detailed models are needed for detailed coverage and capacity analysis – for rough capacity and range calculation needed in radio interface design, simple and easy-to-use models are sufficient • It is important to analyze the sensitivity of the result with respect to the propagation model used
  9. 9. Accuracy of Modeling
  10. 10. Small-Scale Multipath Propagation • The three most important effects – Rapid changes in signal strength over a small travel distance or time interval – Random frequency modulation due to varying Doppler shifts on different multipath signals – Time dispersion caused by multipath propagation delays • Factors influencing small-scale fading – Multipath propagation: reflection objects and scatters – Speed of the mobile: Doppler shifts – Speed of surrounding objects – Transmission bandwidth of the signal • The received signal will be distorted if the transmission bandwidth is greater than the bandwidth of the multipath channel. • Coherent bandwidth: bandwidth of the multipath channel.
  11. 11. Fading Channel Large-scale: Path loss Medium-scale: Shadowing Small-scale: Multipath fading d Pr/Pt
  12. 12. 12 From Signals to Packets Analog Signal “Digital” Signal Bit Stream 0 0 1 0 1 1 1 0 0 0 1 Packets 0100010101011100101010101011101110000001111010101110101010101101011010111001 Header/Body Header/Body Header/Body ReceiverSender Packet Transmission
  13. 13. Materials • Attenuation values for different materials Material Loss (dB) Frequency Concrete block 13-20 1.3 GHz Plywood (3/4”) 2 9.6 GHz Plywood (2 sheets) 4 9.6 GHz Plywood (2 sheets) 6 28.8 GHz Aluminum siding 20.4 815 MHz Sheetrock (3/4”) 2 9.6 GHz Sheetrock (3/4”) 5 57.6 GHz Turn corner in corridor 10-15 1.3 GHz
  14. 14. Can I see Whats there taking place actually in the channel
  15. 15. NORMAL SHADOWING Tx Rx0 Rx d0 d
  16. 16. Small Scale Fading • Causes of small scale fading – Multipaths • Coherence distance, time, and Doppler spread – Coherence distance: the signal strength experiences change from largest to smallest traveling the distance – Coherence time: the time a moving Rx travels the coherence distance – Doppler spread: coherence time expressed in time domain • Coherence bandwidth and delay spread – Coherence bandwidth: the frequency band causing signal changes from largest to smallest – Delay spread: coherence bandwidth expressed in time domain
  17. 17. Small scale fading  Rapid fluctuations of the signal over short period of time  Invalidates Large-scale path loss  Occurs due to multi-path waves  Two or more waves (e.g: reflected/diffracted/scattered waves)  Such waves differ in amplitude and phase  Can combine constructively or destructively resulting in rapid signal strength fluctuation over small distances Example of Multipath Phase difference between original and reflected wave
  18. 18. Small Scale Fading • The signal variation over a short period of time or a short distance
  19. 19. Large Scale Fading • Large scale fading is due to the shadowing effect of large size objects (buildings, mountains) • It is the mean signal strength (or power) vs. (large) distance between Tx and Rx • Path loss and path gain – The ratio of total transmitted power over the received power – The path gain is 1/path loss
  20. 20. ECE6331 Spring 2009 Multipathradio propagation in urban areasMultipathradio propagation in urban areas
  21. 21. Small Scale Fading: Different types of transmitted signals undergo different types of fading depending upon the relation between the Signal Parameters: Bandwidth, Symbol Period & Channel Parameters: RMS Delay Spread, Doppler Spread  In any mobile radio channel a wave can be dispersed either in Time or in Frequency.  These time and frequency dispersion mechanisms lead to four possible distinct effects which depend on the nature of transmitted signal, the channel and the velocity.
  22. 22. Fading Channel Large-scale Fading Small-scale Fading Path Loss Shadowing Effect Multipath Delay Spread Doppler Spread Flat Fading Frequency Selective Fading Fast Fading Slow Fading Signal BW << Channel BW Symbol period >> Delay spread Signal BW > Channel BW Symbol period < Delay spread High Doppler spread Symbol period > Coherence Time Signal variation < Channel variation Low Doppler spread Symbol period << Coherence Time Signal variation >>Channel variation Mobile SpeedPropagation Environment
  23. 23. 2424 Types of Small Scale FadingTypes of Small Scale Fading Multipath time delayMultipath time delay Doppler SpreadDoppler SpreadDoppler SpreadDoppler Spread Flat fadingFlat fading Frequency Selective Fading Frequency Selective Fading FastFast FadingFading FastFast FadingFading Slow fadingSlow fadingSlow fadingSlow fading Two types of fading are independent of each other.Two types of fading are independent of each other.
  24. 24. Types of Small-scale Fading Small-scale Fading (Based on Multipath Tİme Delay Spread) Flat Fading 1. BW Signal < BW of Channel 2. Delay Spread < Symbol Period Frequency Selective Fading 1. BW Signal > Bw of Channel 2. Delay Spread > Symbol Period Small-scale Fading (Based on Doppler Spread) Fast Fading 1. High Doppler Spread 2. Coherence Time < Symbol Period 3. Channel variations faster than baseband signal variations Slow Fading 1. Low Doppler Spread 2. Coherence Time > Symbol Period 3. Channel variations smaller than baseband signal variations
  25. 25. Small scale fading Multi path time delay Doppler spread Flat fading BC BS Frequency selective fading BC BS TC TSSlow fading Fast fading TC TS Small scale fading
  26. 26. Simulating Doppler/Small-scale fadingSimulating Doppler/Small-scale fading
  27. 27. Doppler Shift Geomerty
  28. 28. Doppler ShiftDoppler Shift
  29. 29. • Doppler Shift – A mobile moves at a constant velocity v, along a path segment having length d between points X and Y. – Path length difference – Phase change – Doppler shift θθ coscos tvdl ∆==∆ θ λ π λ π φ cos 22 tvl ∆ = ∆ =∆ θ λ φ π cos 2 1 v t fd = ∆ ∆ ⋅=
  30. 30. Simulating Doppler fadingSimulating Doppler fading
  31. 31. Doppler spectrumDoppler spectrum
  32. 32. Doppler Spectrum If one transmits a sinusoid, … what are the frequency components in the received signal? • Power density spectrum versus received frequency • Probability density of Doppler shift versus received frequency • The Doppler spectrum has a characteristic U-shape. • Note the similarity with sampling a randomly-phased sinusoid • No components fall outside interval [fc- fD, fc+ fD] • Components of + fD or -fD appear relatively often • Fades are not entirely “memory-less”
  33. 33. How do systems handle Doppler Spreads?•Analog •Carrier frequency is low enough to avoid problems •GSM • Channel bit rate well above Doppler spread • TDMA during each bit / burst transmission the channel is fairly constant. • Receiver training/updating during each transmission burst • Feedback frequency correction •DECT •Intended to pedestrian use: •only small Doppler spreads are to be anticipated for •Original DECT concept did not standardize an equalizer •IS95 •Downlink: Pilot signal for synchronization and channel estimation •Uplink: Continuous tracking of each signal
  34. 34. How to handle fast multipath fading? Analog •User must speak slowly GSM •Error correction and interleaving to avoid burst errors •Error detection and speech decoding •Fade margins in cell planning DECT •Diversity reception at base station IS95 •Wideband transmission averages channel behaviour This avoids burst errors and deep fades
  35. 35. How do systems handle delay spreads? Analog • Narrowband transmission GSM • Adaptive channel equalization • Channel estimation training sequence DECT • Use the handset only in small cells with small delay spreads • Diversity and channel selection can help a little bit “pick a channel where late reflections are in a fade” IS95 • Rake receiver separately recovers signals over paths with excessive delays Digital Audio Broacasting • OFDM multi-carrier modulation The radio channel is split into many narrowband (ISI-free) subchannels
  36. 36. Flat Fading  Occurs when the amplitude of the received signal changes with time  For example according to Rayleigh Distribution  Occurs when symbol period of the transmitted signal is much larger than the Delay Spread of the channel  Bandwidth of the applied signal is narrow.  May cause deep fades.  Increase the transmit power to combat this situation.
  37. 37. Flat Fading h(t,τ) s(t) r(t) 0 TS 0 τ 0 TS+τ τ << TS Occurs when: BS << BC and TS >> στ BC: Coherence bandwidth BS: Signal bandwidth TS: Symbol period στ: Delay Spread
  38. 38. Frequency Selective Fading  Occurs when channel multipath delay spread is greater than the symbol period.  Symbols face time dispersion  Channel induces Intersymbol Interference (ISI)  Bandwidth of the signal s(t) is wider than the channel impulse response.
  39. 39. Frequency Selective Fading h(t,τ) s(t) r(t) 0 TS 0 τ 0 TS+τ τ >> TS TS Causes distortion of the received baseband signal Causes Inter-Symbol Interference (ISI) Occurs when: BS > BC and TS < στ As a rule of thumb: TS < στ
  40. 40. Frequency Selective Fading • If the channel possesses a constant-gain and linear phase response over a bandwidth that is smaller than the bandwidth of transmitted signal, then the channel creates frequency selective fading. signal spectrum channel response received signal spectrum f f f )( fS CB
  41. 41. Fast Fading  Due to Doppler Spread  Rate of change of the channel characteristics is larger than the Rate of change of the transmitted signal  The channel changes during a symbol period.  The channel changes because of receiver motion.  Coherence time of the channel is smaller than the symbol period of the transmitter signal Occurs when: BS < BD and TS > TC BS: Bandwidth of the signal BD: Doppler Spread TS: Symbol Period TC: Coherence Bandwidth
  42. 42. Slow Fading  Due to Doppler Spread  Rate of change of the channel characteristics is much smaller than the Rate of change of the transmitted signal Occurs when: BS >> BD and TS << TC BS: Bandwidth of the signal BD: Doppler Spread TS: Symbol Period TC: Coherence Bandwidth
  43. 43. 4545 Multipath terms associated with fadingMultipath terms associated with fading TTss = Symbol period or reciprocal bandwidth= Symbol period or reciprocal bandwidth BBss = Bandwidth of transmitted signal= Bandwidth of transmitted signal BBcc = coherence bandwidth of channel= coherence bandwidth of channel Note :Note : BBcc= 1/50= 1/50σσττ wherewhere σσττ is rms delay spreadis rms delay spread
  44. 44. Different Types of Fading Transmitted Symbol Period Symbol Period of Transmitting Signal TS TS TC στ Flat Slow Fading Flat Fast Fading Frequency Selective Slow Fading Frequency Selective Fast Fading With Respect To SYMBOL PERIOD
  45. 45. Different Types of Fading Transmitted Baseband Signal Bandwidth BS BD Flat Fast Fading Frequency Selective Slow Fading Frequency Selective Fast Fading BS Transmitted Baseband Signal Bandwidth Flat Slow Fading BC With Respect To BASEBAND SIGNAL BANDWIDTH
  46. 46. Small-scale Multipath Measurements
  47. 47. Small-scale Multipath Measurements • Three methods of wideband channel sounding techniques 1.Direct RF Pulse System 2.Spread Spectrum Sliding Correlator Channel Sounding 3.Frequency Domain Channel Sounding
  48. 48. Small-scale Multipath Measurements) • Direct RF Pulse System ★Determine the power delay profile of any channel by using pulse signal with pulse width τbb . The main problem with this system is that it is subject to interference and noise. • Another disadvantage is that the phases of the individual multipath components are not received.
  49. 49. Small-scale Multipath Measurements Direct RF channel impulse response measurement system Pulse Generator fc Tx RF link τbb τREP τ x(τ) bbτ 2 BPF Digital storage Oscilloscope Resolution = Pulse Width BW = Rx detector
  50. 50. Small-scale Multipath Measurements • Spread Spectrum Sliding Correlator Channel Sounding ★The advantage of a spread spectrum system is that, while the probing signal may be wideband, it is possible to detect the transmitted signal using a narrow band receiver, thus improving the dynamic range of the system as compared to the direct RF pulse system. ★The transmitter chip clock is run at a slightly faster rate than the receiver chip clock. This implementation is called a sliding correlator. ★A disadvantage of the spread spectrum system is that measurements are not made in real time, but they are compiled as the PN codes slide past one another.
  51. 51. Small-scale Multipath Measurements Spread spectrum channel impulse response measurement system. Pulse Sequence Generator fc Tx chip clock Rc = α[Hz] = 1/Tc Tx BPF detector Digital storage Oscilloscope BW = 2Rc Rc = β[Hz] Pulse Sequence Generator Correlation Bandwidth BW 2(α-β)≒ Wideband Filter Narrowband Filter Rx @ fc BPF
  52. 52. Small-scale Multipath Measurements • Frequency Domain Channel Sounding ★Measure the frequency response of the channel first then convert it to time response. ★It is useful only for very close measurements (indoor channel sounding). ★It is a non-real time measurement.
  53. 53. Small-scale Multipath Measurements Frequency domain channel impulse response measurement system. )( )( )()(21 wX wY wHwS =∝ Vector Network Analyzer Swept Frequency Oscillator S-parameter test set Inverse DFT Processor h(t)=FT-1 [H(w)] Tx Rx X(w) Y(w) Port 1 Port 2
  54. 54. Extra slides
  55. 55. Multipath Components Component 2 Component 1 Component N Radio Signals Arriving from different directions to receiver Receiver may be stationary or mobile.
  56. 56. Trace Collection Setup Receiver Transmitter Receiver movement trajectory Origin 2o feet If you measure signal at the receiver, what do you expect to see?
  57. 57. Measured Signal Signal Strength -80 -70 -60 -50 -40 -30 -20 -10 0 Time dBm Channel 4 Avg. Signal Strength +20 feet +20 feet +20 feet +20 feet +20 feet +20 feet +20 feetOrigin Long range path lossSmall scale fading
  58. 58. Fundamental Design considerations Data signal x(t) Recovered data signal power spectrum Noise, interference ratio must be above some threshold for correct reception I C Channel Attenuation Distortion
  59. 59. Radio Propagation: Fading and multipath 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
  60. 60. Review of basic concepts  Fourier Transform  Channel Impulse response  Power delay profile  Inter Symbol Interference  Coherence bandwidth  Coherence time
  61. 61. Understanding the effect of attenuation Parabolic Yagi Patch Omni 24 dBi 14 dBi 8 dBi 0 dBi 20 dBm cable loss
  62. 62. Questions?Questions?

Hinweis der Redaktion

  • Fig. 3.2
  • Fig. 2.16