Radio propagation

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Radio propagation

  1. 1. Radio Propagation CSCI 694 24 September 1999 Lewis Girod
  2. 2. 17 March 1999 Radio Propagation 2 Outline • Introduction and terminology • Propagation mechanisms • Propagation models
  3. 3. 17 March 1999 Radio Propagation 3 What is Radio? • Radio Xmitter induces E&M fields – Electrostatic field components 1/d3 – Induction field components 1/d2 – Radiation field components 1/d • Radiation field has E and B component – Field strength at distance d = E B 1/d2 – Surface area of sphere centered at transmitter
  4. 4. 17 March 1999 Radio Propagation 4 General Intuition • Two main factors affecting signal at receiver – Distance (or delay) Path attenuation – Multipath Phase differences Green signal travels 1/2 farther than Yellow to reach receiver, who sees Red. For 2.4 GHz, (wavelength) =12.5cm.
  5. 5. 17 March 1999 Radio Propagation 5 Objective • Invent models to predict what the field looks like at the receiver. – Attenuation, absorption, reflection, diffraction... – Motion of receiver and environment… – Natural and man-made radio interference... – What does the field look like at the receiver?
  6. 6. 17 March 1999 Radio Propagation 6 Models are Specialized • Different scales – Large scale (averaged over meters) – Small scale (order of wavelength) • Different environmental characteristics – Outdoor, indoor, land, sea, space, etc. • Different application areas – macrocell (2km), microcell(500m), picocell
  7. 7. 17 March 1999 Radio Propagation 7 Outline • Introduction and some terminology • Propagation Mechanisms • Propagation models
  8. 8. 17 March 1999 Radio Propagation 8 Radio Propagation Mechanisms • Free Space propagation • Refraction – Conductors & Dielectric materials (refraction) • Diffraction – Fresnel zones • Scattering – “Clutter” is small relative to wavelength
  9. 9. 17 March 1999 Radio Propagation 9 Free Space • Assumes far-field (Fraunhofer region) – d >> D and d >> , where • D is the largest linear dimension of antenna • is the carrier wavelength • No interference, no obstructions
  10. 10. 17 March 1999 Radio Propagation 10 Free Space Propagation Model • Received power at distance d is – where Pt is the transmitter power in Watts – a constant factor K depends on antenna gain, a system loss factor, and the carrier wavelength Watts)( 2 d P KdP t r
  11. 11. 17 March 1999 Radio Propagation 11 Refraction • Perfect conductors reflect with no attenuation • Dielectrics reflect a fraction of incident energy – “Grazing angles” reflect max* – Steep angles transmit max* r t • Reflection induces 180 phase shift *The exact fraction depends on the materials and frequencies involved
  12. 12. 17 March 1999 Radio Propagation 12 Diffraction • Diffraction occurs when waves hit the edge of an obstacle – “Secondary” waves propagated into the shadowed region – Excess path length results in a phase shift – Fresnel zones relate phase shifts to the positions of obstacles T R 1st Fresnel zone Obstruction
  13. 13. 17 March 1999 Radio Propagation 13 Fresnel Zones • Bounded by elliptical loci of constant delay • Alternate zones differ in phase by 180 – Line of sight (LOS) corresponds to 1st zone – If LOS is partially blocked, 2nd zone can destructively interfere (diffraction loss) Fresnel zones are ellipses with the T&R at the foci; L1 = L2+ Path 1 Path 2
  14. 14. 17 March 1999 Radio Propagation 14 Power Propagated into Shadow • How much power is propagated this way? – 1st FZ: 5 to 25 dB below free space prop. Obstruction of Fresnel Zones 1st 2nd 0 -10 -20 -30 -40 -50 -60 0o 90 180o dB Tip of Shadow Obstruction LOS Rappaport, pp. 97
  15. 15. 17 March 1999 Radio Propagation 15 Scattering • Rough surfaces – critical height for bumps is f( ,incident angle) – scattering loss factor modeled with Gaussian distribution. • Nearby metal objects (street signs, etc.) – Usually modelled statistically • Large distant objects – Analytical model: Radar Cross Section (RCS)
  16. 16. 17 March 1999 Radio Propagation 16 Outline • Introduction and some terminology • Propagation Mechanisms • Propagation models – Large scale propagation models – Small scale propagation (fading) models
  17. 17. 17 March 1999 Radio Propagation 17 Propagation Models: Large • Large scale models predict behavior averaged over distances >> – Function of distance & significant environmental features, roughly frequency independent – Breaks down as distance decreases – Useful for modeling the range of a radio system and rough capacity planning
  18. 18. 17 March 1999 Radio Propagation 18 Propagation Models: Small • Small scale (fading) models describe signal variability on a scale of – Multipath effects (phase cancellation) dominate, path attenuation considered constant – Frequency and bandwidth dependent – Focus is on modeling “Fading”: rapid change in signal over a short distance or length of time.
  19. 19. 17 March 1999 Radio Propagation 19 Large Scale Models • Path loss models • Outdoor models • Indoor models
  20. 20. 17 March 1999 Radio Propagation 20 Free Space Path Loss • Path Loss is a measure of attenuation based only on the distance to the transmitter • Free space model only valid in far-field; – Path loss models typically define a “close-in” point d0 and reference other points from there: 2 0 0 )()( d d dPdP rr dB dBr d d dPLdPdPL 0 0 2)()]([)( What is dB?
  21. 21. 17 March 1999 Radio Propagation 21 Log-Distance Path Loss Model • Log-distance generalizes path loss to account for other environmental factors • Choose a d0 in the far field. • Measure PL(d0) or calculate Free Space Path Loss. • Take measurements and derive empirically. dB d d dPLdPL 0 0 )()(
  22. 22. 17 March 1999 Radio Propagation 22 Log-Distance 2 • Value of characterizes different environments Environm ent Exponent Free Space 2 Urban area 2.7-3.5 Shadowed urban area 3-5 Indoor LOS 1.6-1.8 Indoor no LOS 4-6 Rappaport, Table 3.2, pp. 104
  23. 23. 17 March 1999 Radio Propagation 23 Log-Normal Shadowing Model • Shadowing occurs when objects block LOS between transmitter and receiver • A simple statistical model can account for unpredictable “shadowing” – Add a 0-mean Gaussian RV to Log-Distance PL – Markov model can be used for spatial correlation
  24. 24. 17 March 1999 Radio Propagation 24 Outdoor Models • “2-Ray” Ground Reflection model • Diffraction model for hilly terrain
  25. 25. 17 March 1999 Radio Propagation 25 2-Ray Ground Reflection • For d >> hrht, – low angle of incidence allows the earth to act as a reflector – the reflected signal is 180 out of phase – Pr 1/d4 ( =4) RT ht hr Phase shift!
  26. 26. 17 March 1999 Radio Propagation 26 Ground Reflection 2 • Intuition: ground blocks 1st Fresnel zone – Reflection causes an instantaneous 180 phase shift – Additional phase offset due to excess path length – If the resulting phase is still close to 180 , the gound ray will destructively interfere with the LOS ray. RT ht hr p1 p0 180
  27. 27. 17 March 1999 Radio Propagation 27 Hilly Terrain • Propagation can be LOS or result of diffraction over one or more ridges • LOS propagation modelled with ground reflection: diffraction loss • But if there is no LOS, diffraction can actually help!
  28. 28. 17 March 1999 Radio Propagation 28 Indoor Path Loss Models • Indoor models are less generalized – Environment comparatively more dynamic • Significant features are physically smaller – Shorter distances are closer to near-field – More clutter, scattering, less LOS
  29. 29. 17 March 1999 Radio Propagation 29 Indoor Modeling Techniques • Modeling techniques and approaches: – Log-Normal, <2 for LOS down corridor – Log-Normal shadowing model if no LOS – Partition and floor attenuation factors – Computationally intensive “ray-tracing” based on 3-D model of building and attenuation factors for materials
  30. 30. 17 March 1999 Radio Propagation 30 Outline • Introduction and some terminology • Propagation Mechanisms • Propagation models – Large scale propagation models – Small scale propagation (fading) models
  31. 31. 17 March 1999 Radio Propagation 31 Recall: Fading Models • Small scale (fading) models describe signal variability on a scale of – Multipath effects (phase cancellation) dominate, path attenuation considered constant – Frequency and bandwidth dependent – Focus is on modeling “Fading”: rapid change in signal over a short distance or length of time.
  32. 32. 17 March 1999 Radio Propagation 32 Factors Influencing Fading • Motion of the receiver: Doppler shift • Transmission bandwidth of signal – Compare to BW of channel • Multipath propagation – Receiver sees multiple instances of signal when waves follow different paths – Very sensitive to configuration of environment
  33. 33. 17 March 1999 Radio Propagation 33 Effects of Multipath Signals • Rapid change in signal strength due to phase cancellation • Frequency modulation due to Doppler shifts from movement of receiver/environment • Echoes caused by multipath propagation delay
  34. 34. 17 March 1999 Radio Propagation 34 The Multipath Channel • One approach to small-scale models is to model the “Multipath Channel” – Linear time-varying function h(t, ) • Basic idea: define a filter that encapsulates the effects of multipath interference – Measure or calculate the channel impulse response (response to a short pulse at fc): h(t, ) t
  35. 35. 17 March 1999 Radio Propagation 35 Channel Sounding • “Channel sounding” is a way to measure the channel response – transmit impulse, and measure the response to find h( ). – h( ) can then be used to model the channel response to an arbitrary signal: y(t) = x(t) h( ). – Problem: models the channel at single point in time; can‟t account for mobility or environmental changes h(t, ) SKIP
  36. 36. 17 March 1999 Radio Propagation 36 Characterizing Fading* • From the impulse response we can characterize the channel: • Characterizing distortion – Delay spread ( d): how long does the channel ring from an impulse? – Coherence bandwidth (Bc): over what frequency range is the channel gain flat? – d 1/Bc *Adapted from EE535 Slides, Chugg „99 In time domain, roughly corresponds to the “fidelity” of the response; sharper pulse requires wider band
  37. 37. 17 March 1999 Radio Propagation 37 Effect of Delay Spread* • Does the channel distort the signal? – if W << Bc: “Flat Fading” • Amplitude and phase distortion only – if W > Bc: “Frequency Selective Fading” • If T < d, inter-symbol interference (ISI) occurs • For narrowband systems (W 1/T), FSF ISI. • Not so for wideband systems (W >> 1/T) For a system with bw W and symbol time T...
  38. 38. 17 March 1999 Radio Propagation 38 Qualitative Delay Spread RMS Delay spread ( ) Mean excess delay Noise threshold Delay Power(dB) Typical values for : Indoor: 10-100 ns Outdoor: 0.1-10 s
  39. 39. 17 March 1999 Radio Propagation 39 Characterizing Fading 2* • Characterizing Time-variation: How does the impulse response change with time? – Coherence time (tc): for what value of are responses at t and t+ uncorrelated? (How quickly is the channel changing) – Doppler Spread (fd): How much will the spectrum of the input be spread in frequency? – fd 1/tc
  40. 40. 17 March 1999 Radio Propagation 40 Effect of Coherence Time* • Is the channel constant over many uses? – if T << tc: “Slow fading” • Slow adaptation required – if T > tc: “Fast fading” • Frequent adaptation required • For typical systems, symbol rate is high compared to channel evolution For a system with bw W and symbol time T...
  41. 41. 17 March 1999 Radio Propagation 41 Statistical Fading Models • Fading models model the probability of a fade occurring at a particular location – Used to generate an impulse response – In fixed receivers, channel is slowly time-varying; the fading model is reevaluated at a rate related to motion • Simplest models are based on the WSSUS principle
  42. 42. 17 March 1999 Radio Propagation 42 WSSUS* • Wide Sense Stationary (WSS) – Statistics are independent of small perturbations in time and position – I.e. fixed statistical parameters for stationary nodes • Uncorrelated Scatter (US) – Separate paths are not correlated in phase or attenuation – I.e. multipath components can be independent RVs • Statistics modeled as Gaussian RVs
  43. 43. 17 March 1999 Radio Propagation 43 Common Distributions • Rayleigh fading distribution – Models a flat fading signal – Used for individual multipath components • Ricean fading distribution – Used when there is a dominant signal component, e.g. LOS + weaker multipaths – parameter K (dB) defines strength of dominant component; for K=- , equivalent to Rayleigh
  44. 44. 17 March 1999 Radio Propagation 44 Application of WSSUS • Multi-ray Rayleigh fading: – The Rayleigh distribution does not model multipath time delay (frequency selective) – Multi-ray model is the sum of two or more independent time-delayed Rayleigh variables s(t) R1 R2 r(t) Rappaport, Fig. 4.24, pp. 185.
  45. 45. 17 March 1999 Radio Propagation 45 Saleh & Valenzuela (1987) • Measured same-floor indoor characteristics – Found that, with a fixed receiver, indoor channel is very slowly time-varying – RMS delay spread: mean 25ns, max 50ns – With no LOS, path loss varied over 60dB range and obeyed log distance power law, 3 > n > 4 • Model assumes a structure and models correlated multipath components. Rappaport, pp. 188
  46. 46. 17 March 1999 Radio Propagation 46 Saleh & Valenzuela 2 • Multipath model – Multipath components arrive in clusters, follow Poisson distribution. Clusters relate to building structures. – Within cluster, individual components also follow Poisson distribution. Cluster components relate to reflecting objects near the TX or RX. – Amplitudes of components are independent Rayleigh variables, decay exponentially with cluster delay and with intra-cluster delay
  47. 47. 17 March 1999 Radio Propagation 47 References • Wireless Communications: Principles and Practice, Chapters 3 and 4, T. Rappaport, Prentice Hall, 1996. • Principles of Mobile Communication, Chapter 2, G. Stüber, Kluwer Academic Publishers, 1996. • Slides for EE535, K. Chugg, 1999. • Spread Spectrum Systems, Chapter 7, R. Dixon, Wiley, 1985 (there is a newer edition). • Wideband CDMA for Third Generation Mobile Communications, Chapter 4, T. Ojanpera, R. Prasad, Artech, House 1998. • Propagation Measurements and Models for Wireless Communications Channels, Andersen, Rappaport, Yoshida, IEEE Communications, January 1995.
  48. 48. 17 March 1999 Radio Propagation 48 The End
  49. 49. 17 March 1999 Radio Propagation 49 Scattering 2 • hc is the critical height of a protrusion to result in scattering. • RCS: ratio of power density scattered to receiver to power density incident on the scattering object – Wave radiated through free space to scatterer and reradiated: )sin( θ8 λ i c h )log(20)log(20)π4log(30 ]dB[)λlog(20)dBi()dBm()dBm( 2 RT TTR dd mRCSGPP
  50. 50. 17 March 1999 Radio Propagation 50 Free Space 2a • Free space power flux density (W/m2) – power radiated over surface area of sphere – where Gt is transmitter antenna gain • By covering some of this area, receiver‟s antenna “catches” some of this flux 2 π4 d GP P tt d
  51. 51. 17 March 1999 Radio Propagation 51 Free Space 2b • Fraunhofer distance: d > 2D2/ • Antenna gain and antenna aperture – Ae is the antenna aperture, intuitively the area of the antenna perpendicular to the flux – Gr is the antenna gain for a receiver. It is related to Ae. – Received power (Pr) = Power flux density (Pd) * Ae 2 λ π4 e A G π4 λ 2 G Ae
  52. 52. 17 March 1999 Radio Propagation 52 Free Space 2c – where L is a system loss factor – Pt is the transmitter power – Gt and Gr are antenna gains – is the carrier wavelength Watts )π(4 λ1 )( 2 2 2 L GGP d dP rtt r
  53. 53. 17 March 1999 Radio Propagation 53 LNSM 2 • PL(d)[dB] = PL(d0) +10nlog(d/d0)+ X – where X is a zero-mean Gaussian RV (dB) • and n computed from measured data, based on linear regression
  54. 54. 17 March 1999 Radio Propagation 54 Ground Reflection 1.5 • The power at the receiver in this model is – derivation calculates E field; – Pr = |E|2Ae; Ae is ant. aperture • The “breakpoint” at which the model changes from 1/d2 to 1/d4 is 2 hthr/ – where hr and ht are the receiver and transmitter antenna heights 4 22 d hh GGPP rt rttr
  55. 55. 17 March 1999 Radio Propagation 55 Convolution Integral • Convolution is defined by this integral: τ)τ()τ()( )()()( dthxty thtxty Indexes relevant portion of impulse response Scales past input signal
  56. 56. 17 March 1999 Radio Propagation 56 Partition Losses • Partition losses: same floor – Walls, furniture, equipment – Highly dependent on type of material, frequency • Hard partitions vs soft partitions – hard partitions are structural – soft partitions do not reach ceiling • “open plan” buildings
  57. 57. 17 March 1999 Radio Propagation 57 Partition Losses 2 • Partition losses: between floors – Depends on building construction, frequency – “Floor attenuation factor” diminishes with successive floors – typical values: • 15 dB for 1st floor • 6-10 dB per floor for floors 2-5 • 1-2 dB per floor beyond 5 floors
  58. 58. 17 March 1999 Radio Propagation 58 Materials • Attenuation values for different materials M aterial 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 Alum inum siding 20.4 815 M Hz Sheetrock (3/4”) 2 9.6 GHz Sheetrock (3/4”) 5 57.6 GHz Turn corner in corridor 10-15 1.3 GHz
  59. 59. 17 March 1999 Radio Propagation 59 What does “dB” mean? • dB stands for deciBel or 1/10 of a Bel • The Bel is a dimensionless unit for expressing ratios and gains on a log scale • Gains add rather than multiply • Easier to handle large dynamic ranges ))log()(log(10log10 P P 12 1 2 10 dB1 2 PP P P
  60. 60. 17 March 1999 Radio Propagation 60 dB 2 • Ex: Attenuation from transmitter to receiver. – PT=100, PR=10 – attenuation is ratio of PT to PR – [PT/PR]dB = 10 log(PT/PR) = 10 log(10) = 10 dB • Useful numbers: – [1/2]dB -3 dB – [1/1000]dB = -30 dB
  61. 61. 17 March 1999 Radio Propagation 61 dB 3 • dB can express ratios, but what about absolute quantities? • Similar units reference an absolute quantity against a defined reference. – [n mW]dBm = [n/mW]dB – [n W]dBW = [n/W]dB • Ex: [1 mW]dBW = -30 dBW
  62. 62. 17 March 1999 Radio Propagation 62 Channel Sounding 2 • Several “Channel Sounding” techniques can measure the channel response directly: – Direct RF pulse (we hinted at this approach) – Sliding correlator – Frequency domain sounding
  63. 63. 17 March 1999 Radio Propagation 63 Channel Sounding 3 • Direct RF Pulse – Xmit pulse, scope displays response at receiver – Can be done with off-the-shelf hardware – Problems: hard to reject noise in the channel – If no LOS • must trigger scope on weaker multipath component • may fail to trigger • lose delay and phase information
  64. 64. 17 March 1999 Radio Propagation 64 Channel Sounding 4 • Sliding correlator – Xmit PseudoNoise sequence – Rcvr correlates signal with its PN generator – Rcvr clock slightly slower; PN sequences slide – Delayed components cause delayed correlations – Good resolution, good noise rejection
  65. 65. 17 March 1999 Radio Propagation 65 Channel Sounding 5 • Frequency domain sounding – Sweep frequency range – Compute inverse Fourier transform of response – Problems • not instantaneous measurement • Tradeoff between resolution (number of frequency steps) and real-time measurement (i.e. duration as short as possible)
  66. 66. 17 March 1999 Radio Propagation 66 Digression: Convolutions • The impulse response “box” notation implies the convolution operator, – Convolution operates on a signal and an impulse response to produce a new signal. – The new signal is the superposition of the response to past values of the signal. – Commutative, associative
  67. 67. 17 March 1999 Radio Propagation 67 y(t) y(t) Convolutions 2 • y(t) is the sum of scaled, time-delayed responses x(t) h(t) = + h(t) Each component of the sum is scaled by the x(t)dt at that point; in this example, the response is scaled to 0 where x(t) = 0.
  68. 68. 17 March 1999 Radio Propagation 68 Flip & Slide: h(t- )h(t- ) Flip & Slide: h(t- )h(t- ) Flip & Slide: h(t- )h(t- ) Convolutions 3 • Graphical method: “Flip & Slide” x(t) x( ) h(t) = Pairwise multiply x*h and integrate over and Store y(t) y(t) y(t) Flip & Slide: h(t- )h(t- ) Flip & Slide: h(t- )h(t- )
  69. 69. 17 March 1999 Radio Propagation 69 Frequency and Time Domains • The channel impulse response is f(time) – It describes the channel in the “time domain” • Functions of frequency are often very useful; – Space of such functions is “frequency domain” • Often a particular characteristic is easier to handle in one domain or the other.
  70. 70. 17 March 1999 Radio Propagation 70 Frequency Domain • Functions of frequency – usually capitalized and take the parameter “f” – where f is the frequency in radians/sec – and the value of the function is the amplitude of the component of frequency f. • Convolution in time domain translates into multiplication in the frequency domain: – y(t) = x(t) h(t) Y(f) = X(f)H(f)
  71. 71. 17 March 1999 Radio Propagation 71 Frequency Domain 2 • Based on Fourier theorem: – any periodic signal can be decomposed into a sum of (possibly infinite number of) cosines • The Fourier Transform and inverse FT – Convert between time and frequency domains. – The frequency and time representations of the same signal are “duals”
  72. 72. 17 March 1999 Radio Propagation 72 Flat Fading • T >> d and W << BC minimal ISI 0 Ts 0 0 Ts+ fc fcfc t t t f f f s(t) r(t)h(t, ) Time domain (convolve) Freq domain (filter) = = Delay spread Coherence BW
  73. 73. 17 March 1999 Radio Propagation 73 Frequency Selective Fading • T << d and W >> BC ISI 0 Ts 0 0 Ts+ fc fcfc t t f f f s(t) r(t)h(t, ) Time domain (convolve) Freq domain (filter) = = Delay spread Coherence BW Ts
  74. 74. 17 March 1999 Radio Propagation 74 Review • Object of radio propagation models: – predict signal quality at receiver • Radio propagation mechanisms – Free space (1/d2) – Diffraction – Refraction – Scattering
  75. 75. 17 March 1999 Radio Propagation 75 Review 2 • Factors influencing received signal – Path loss: distance, obstructions – Multipath interference: phase cancellation due to excess path length and other sources of phase distortion – Doppler shift – Other radio interference
  76. 76. 17 March 1999 Radio Propagation 76 Review 3 • Approaches to Modelling – Models valid for far-field, apply to a range of distances – large scale models: concerned with gross behavior as a function of distance – small scale (fading) models: concerned with behavior during perturbations around a particular distance
  77. 77. 17 March 1999 Radio Propagation 77 Relevance to Micronets • Micronets may require different models than most of the work featured here – Smaller transmit range – Likely to be near reflectors: on desk or floor. • On the other hand, at smaller scales things are less smooth: “ground reflection” may turn into scattering – Outdoors, throwing sensors on ground may not work. Deployable tripods?
  78. 78. 17 March 1999 Radio Propagation 78 Relevance 2 • Consequences of “Fading” – You can be in a place that has no signal, but where a signal can be picked up a short distance away in any direction • Ability to move? Switch frequencies/antennas? Call for help moving or for more nodes to be added? • If stuck, may not be worth transmitting at all – Reachability topology may be completely irrelevant to location relationships
  79. 79. 17 March 1999 Radio Propagation 79 Relevance 3 • Relevant modelling tools: – Statistical models (Rice/Rayleigh/Log Normal) • Statistical fading assumes particular dynamics, this depends on mobility of receivers and environment – CAD modelling of physical environment and ray tracing approaches. • For nodes in fixed positions this is only done once.
  80. 80. 17 March 1999 Radio Propagation 80 Relevance 4 • An approach to modelling? – Characterize wireless system interactions with different materials, compare to published data – Assess the effect of mobility in environment on fixed topologies, relate to statistical models – Try to determine what environmental structures and parameters are most important: • Scattering vs. ground reflection? • can a simple CAD model help?

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