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Radio Propagation

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Radio Propagation. 5. Objective ... Natural and man-made radio interference... What does the field look like at the receiver? ... – PowerPoint PPT presentation

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Title: Radio Propagation


1
Radio Propagation
  • CSCI 694
  • 24 September 1999
  • Lewis Girod

2
Outline
  • Introduction and terminology
  • Propagation mechanisms
  • Propagation models

3
What is Radio?
  • Radio Xmitter induces EM 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

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
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
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
Outline
  • Introduction and some terminology
  • Propagation Mechanisms
  • Propagation models

8
Radio Propagation Mechanisms
  • Free Space propagation
  • Refraction
  • Conductors Dielectric materials (refraction)
  • Diffraction
  • Fresnel zones
  • Scattering
  • Clutter is small relative to wavelength

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
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
Refraction
  • Perfect conductors reflect with no attenuation
  • Dielectrics reflect a fraction of
    incident energy
  • Grazing angles reflect max
  • Steep angles transmit max

q
qr
qt
  • Reflection induces 180? phase shift

The exact fraction depends on the materials and
frequencies involved
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
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)

Path 1
Path 2
Fresnel zones are ellipses with the TR at the
foci L1 L2l
14
Power Propagated into Shadow
  • How much power is propagated this way?
  • 1st FZ 5 to 25 dB below free space prop.

LOS
0 -10 -20 -30 -40 -50 -60
0o
90
180o
dB
Obstruction
Rappaport, pp. 97
Tip of Shadow
1st 2nd
Obstruction of Fresnel Zones ?
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
Outline
  • Introduction and some terminology
  • Propagation Mechanisms
  • Propagation models
  • Large scale propagation models
  • Small scale propagation (fading) models

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
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
Large Scale Models
  • Path loss models
  • Outdoor models
  • Indoor models

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

What is dB?
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
Log-Distance 2
  • Value of ? characterizes different environments

Rappaport, Table 3.2, pp. 104
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
Outdoor Models
  • 2-Ray Ground Reflection model
  • Diffraction model for hilly terrain

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
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.

180?
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
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
Indoor Modeling Techniques
  • Modeling techniques and approaches
  • Log-Normal, ?
  • 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
Outline
  • Introduction and some terminology
  • Propagation Mechanisms
  • Propagation models
  • Large scale propagation models
  • Small scale propagation (fading) models

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
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
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
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)

t
35
Channel Sounding
SKIP
  • 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 cant account for mobility or environmental
    changes

h(t,?)
?
?
36
Characterizing Fading
Adapted from EE535 Slides, Chugg 99
  • 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

In time domain, roughly corresponds to the
fidelity of the response sharper pulse
requires wider band
37
Effect of Delay Spread
  • Does the channel distort the signal?
  • if W
  • Amplitude and phase distortion only
  • if W Bc Frequency Selective Fading
  • If T
  • 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
Qualitative Delay Spread
Typical values for ? Indoor 10-100 ns Outdoor
0.1-10 ?s
Noise threshold
Power(dB)?
Delay?
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
Effect of Coherence Time
  • Is the channel constant over many uses?
  • if T
  • 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
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
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
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
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

Rappaport, Fig. 4.24, pp. 185.
45
Saleh Valenzuela (1987)
Rappaport, pp. 188
  • 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
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
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
The End
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
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, receivers antenna
    catches some of this flux

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
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
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
Ground Reflection 1.5
  • The power at the receiver in this model is
  • derivation calculates E field
  • Pr E2Ae 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
Convolution Integral
  • Convolution is defined by this integral

Indexes relevant portion of impulse response
Scales past input signal
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
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
Materials
  • Attenuation values for different materials

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

60
dB 2
  • Ex Attenuation from transmitter to receiver.
  • PT100, PR10
  • attenuation is ratio of PT to PR
  • PT/PRdB 10 log(PT/PR) 10 log(10) 10 dB
  • Useful numbers
  • 1/2dB ? -3 dB
  • 1/1000dB -30 dB

61
dB 3
  • dB can express ratios, but what about absolute
    quantities?
  • Similar units reference an absolute quantity
    against a defined reference.
  • n mWdBm n/mWdB
  • n WdBW n/WdB
  • Ex 1 mWdBW -30 dBW

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
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
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
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
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
Convolutions 2
  • y(t) is the sum of scaled, time-delayed responses

x(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
Convolutions 3
  • Graphical method Flip Slide

x(t)

?
Pairwise multiply xh and integrate over ?
x(?)
y(t)
and Store y(t)
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
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
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
Flat Fading
  • T ?d and W

r(t)
s(t)
h(t,?)
Delay spread

Time domain (convolve)
t
t
t
0
?
0
Ts
0
Ts?
Coherence BW
Freq domain (filter)

f
f
f
fc
fc
fc
73
Frequency Selective Fading
  • T BC ? ISI

r(t)
s(t)
h(t,?)
Delay spread

Time domain (convolve)
t
t
0
?
0
Ts
0
Ts
Ts?
Coherence BW
Freq domain (filter)

f
f
f
fc
fc
fc
74
Review
  • Object of radio propagation models
  • predict signal quality at receiver
  • Radio propagation mechanisms
  • Free space (1/d2)
  • Diffraction
  • Refraction
  • Scattering

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
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
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
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
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
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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