Radio Propagation

Radio Propagation PowerPoint PPT Presentation


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17 March 1999. Radio Propagation. 2. Outline. Introduction and terminologyPropagation mechanismsPropagation models. 17 March 1999. Radio Propagation. 3. What is Radio?. Radio Xmitter induces E

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Radio Propagation

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1. Radio Propagation CSCI 694 24 September 1999 Lewis Girod

2. 17 March 1999 Radio Propagation 2 Outline Introduction and terminology Propagation mechanisms Propagation models

3. 17 March 1999 Radio Propagation 3 What is Radio? Radio Xmitter induces E&M fields Electrostatic field components µ 1/d3 Induction field components µ 1/d2 Radiation field components µ 1/d Radiation field has E and B component Field strength at distance d = E?B µ 1/d2 Surface area of sphere centered at transmitter Start with some simplified background. The electrostatic and induction fields decay rapidly and can be ignored. Start with some simplified background. The electrostatic and induction fields decay rapidly and can be ignored.

4. 17 March 1999 Radio Propagation 4 General Intuition Two main factors affecting signal at receiver Distance (or delay) ? Path attenuation Multipath ? Phase differences

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? Radio propagation is complicated and highly dependent on details of the environment. Most models are statistically based or take into account a general case. Examples: probability distributions to simulate noise, 4-ray configuration to simulate propagation down a streetRadio propagation is complicated and highly dependent on details of the environment. Most models are statistically based or take into account a general case. Examples: probability distributions to simulate noise, 4-ray configuration to simulate propagation down a street

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 Scale: The terminology may be confusing; both large and small scale models assume “far-field” region and may be applied to the same set of distances. The difference lies in the granularity of prediction provided by the model. Applications: Models are usually developed with a specific purpose in mind and are not always generally applicable because they may make assumptions that are not generally valid. For example, some models are good for outdoor systems while others are more accurate for indoor systems. Macrocell - large outdoor cells Microcell - smaller outdoor cells or large indoor systems Picocell - room or desk sized systemsScale: The terminology may be confusing; both large and small scale models assume “far-field” region and may be applied to the same set of distances. The difference lies in the granularity of prediction provided by the model. Applications: Models are usually developed with a specific purpose in mind and are not always generally applicable because they may make assumptions that are not generally valid. For example, some models are good for outdoor systems while others are more accurate for indoor systems. Macrocell - large outdoor cells Microcell - smaller outdoor cells or large indoor systems Picocell - room or desk sized systems

7. 17 March 1999 Radio Propagation 7 Outline Introduction and some terminology Propagation Mechanisms Propagation models

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 The building blocks of propagation models are these three basic ways that radio energy can interact with the environment. Most materials are neither perfect conductors nor perfect insulators but it is useful to describe the models separately. Note that conductivity and permittivity (a measure of resistance to electric fields) are frequency dependent.The building blocks of propagation models are these three basic ways that radio energy can interact with the environment. Most materials are neither perfect conductors nor perfect insulators but it is useful to describe the models separately. Note that conductivity and permittivity (a measure of resistance to electric fields) are frequency dependent.

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 If there are no obstructions or interference, free space model applies. Think of power dissipated over surface of radius d sphere. Function of distance; proportional to |E|^2, 1/d^2 (ignore high order terms) If there are no obstructions or interference, free space model applies. Think of power dissipated over surface of radius d sphere. Function of distance; proportional to |E|^2, 1/d^2 (ignore high order terms)

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

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* This is actually quite simplified; the actual characteristics depend on the polarization of the incident wave. Depending on polarization there can also be a 180 degree phase shift in the reflected component. “grazing” angles often result in “ground reflection” for large T-R distances This is actually quite simplified; the actual characteristics depend on the polarization of the incident wave. Depending on polarization there can also be a 180 degree phase shift in the reflected component. “grazing” angles often result in “ground reflection” for large T-R distances

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

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)

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.

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) Scattering is generally difficult to model because the environmental conditions that cause it are complex (e.g. modelling position of every street sign is not feasible). Large distant objects such as mountains are more reasonable: they don’t move, and they are far enough away that their angular position can be independent of motion of the receiver. RCS is the ratio of scattered power density at the receiver to the power density incident to the scattering object. Scattering is generally difficult to model because the environmental conditions that cause it are complex (e.g. modelling position of every street sign is not feasible). Large distant objects such as mountains are more reasonable: they don’t move, and they are far enough away that their angular position can be independent of motion of the receiver. RCS is the ratio of scattered power density at the receiver to the power density incident to the scattering object.

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 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. 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. 17 March 1999 Radio Propagation 19 Large Scale Models Path loss models Outdoor models Indoor models

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: Path loss.. Ratio of transmit power to receive power in decibels. “close-in” points are especially useful for gathering empirical data: measurements are made at the d_0 point and compared with measurements from farther distances.Path loss.. Ratio of transmit power to receive power in decibels. “close-in” points are especially useful for gathering empirical data: measurements are made at the d_0 point and compared with measurements from farther distances.

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.

22. 17 March 1999 Radio Propagation 22 Log-Distance 2 Value of ? characterizes different environments

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. 17 March 1999 Radio Propagation 24 Outdoor Models “2-Ray” Ground Reflection model Diffraction model for hilly terrain

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)

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.

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. 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. 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. 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. 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. 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 If the tx b/w is greater than channel b/w, frequency selective fading can occur. The signal will be distorted but it may be possible to recover from fading because it will occur on a scale smaller than individual symbols.If the tx b/w is greater than channel b/w, frequency selective fading can occur. The signal will be distorted but it may be possible to recover from fading because it will occur on a scale smaller than individual symbols.

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. 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):

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

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

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)

38. 17 March 1999 Radio Propagation 38 Qualitative Delay Spread

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. 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

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. 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. 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. 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

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.

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. 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. 17 March 1999 Radio Propagation 48 The End

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:

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

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

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

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. 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

55. 17 March 1999 Radio Propagation 55 Convolution Integral Convolution is defined by this integral:

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. 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. 17 March 1999 Radio Propagation 58 Materials Attenuation values for different materials

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 The Bel is named after Bell of telephone fame Its common use stemmed from the need to conveniently express and manipulate amplifier gains. Decibels are a convenient way to describe ratios when the range of the values will span many orders of magnitude. The Bel is named after Bell of telephone fame Its common use stemmed from the need to conveniently express and manipulate amplifier gains. Decibels are a convenient way to describe ratios when the range of the values will span many orders of magnitude.

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. 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. 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. 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. 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. 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. 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. 17 March 1999 Radio Propagation 67 Convolutions 2 y(t) is the sum of scaled, time-delayed responses

68. 17 March 1999 Radio Propagation 68 Convolutions 3 Graphical method: “Flip & Slide”

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. 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. 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. 17 March 1999 Radio Propagation 72 Flat Fading T >> ?d and W << BC ? minimal ISI

73. 17 March 1999 Radio Propagation 73 Frequency Selective Fading T << ?d and W >> BC ? ISI

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. 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. 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. 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. 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. 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. 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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