radio propagation l.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Radio Propagation PowerPoint Presentation
Download Presentation
Radio Propagation

Loading in 2 Seconds...

play fullscreen
1 / 80

Radio Propagation - PowerPoint PPT Presentation


  • 921 Views
  • Uploaded on

Radio Propagation CSCI 694 24 September 1999 Lewis Girod Outline Introduction and terminology Propagation mechanisms Propagation models What is Radio? Radio Xmitter induces E&M fields Electrostatic field components µ 1/d 3 Induction field components µ 1/d 2

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Radio Propagation' - albert


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
radio propagation

Radio Propagation

CSCI 694

24 September 1999

Lewis Girod

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

Radio Propagation

what is radio
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

Radio Propagation

general intuition
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.

Radio Propagation

objective
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

models are specialized
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

Radio Propagation

outline7
Outline
  • Introduction and some terminology
  • Propagation Mechanisms
  • Propagation models

Radio Propagation

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

Radio Propagation

free space
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

Radio Propagation

free space propagation model
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

Radio Propagation

refraction
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

Radio Propagation

*The exact fraction depends on the materials and frequencies involved

diffraction

T

R

1st Fresnel zone

Obstruction

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

Radio Propagation

fresnel zones
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 T&R at the foci; L1 = L2+l

Radio Propagation

power propagated into shadow
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 

Radio Propagation

scattering
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)

Radio Propagation

outline16
Outline
  • Introduction and some terminology
  • Propagation Mechanisms
  • Propagation models
    • Large scale propagation models
    • Small scale propagation (fading) models

Radio Propagation

propagation models large
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

Radio Propagation

propagation models small
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.

Radio Propagation

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

Radio Propagation

free space path loss
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?

Radio Propagation

log distance path loss model
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.

Radio Propagation

log distance 2
Log-Distance 2
  • Value of  characterizes different environments

Rappaport, Table 3.2, pp. 104

Radio Propagation

log normal shadowing model
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

Radio Propagation

outdoor models
Outdoor Models
  • “2-Ray” Ground Reflection model
  • Diffraction model for hilly terrain

Radio Propagation

2 ray ground reflection

T

R

ht

hr

Phase shift!

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)

Radio Propagation

ground reflection 2

T

R

p0

ht

hr

p1

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

Radio Propagation

hilly terrain
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!

Radio Propagation

indoor path loss models
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

Radio Propagation

indoor modeling techniques
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

Radio Propagation

outline30
Outline
  • Introduction and some terminology
  • Propagation Mechanisms
  • Propagation models
    • Large scale propagation models
    • Small scale propagation (fading) models

Radio Propagation

recall fading models
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.

Radio Propagation

factors influencing fading
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

Radio Propagation

effects of multipath signals
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

Radio Propagation

the multipath channel

h(t,)

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

Radio Propagation

channel sounding

SKIP

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,)

Radio Propagation

characterizing fading
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

Radio Propagation

effect of delay spread
Effect of Delay Spread*

For a system with bw W and symbol time T...

  • 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)

Radio Propagation

qualitative delay spread

RMS Delay spread ()

Mean excess delay

Qualitative Delay Spread

Typical values for  :

Indoor: 10-100 ns

Outdoor: 0.1-10 s

Noise threshold

Power(dB)

Delay

Radio Propagation

characterizing fading 2
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

Radio Propagation

effect of coherence time
Effect of Coherence Time*

For a system with bw W and symbol time T...

  • 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

Radio Propagation

statistical fading models
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

Radio Propagation

wssus
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

Radio Propagation

common distributions
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

Radio Propagation

application of wssus

s(t)

R1

r(t)

R2

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.

Radio Propagation

saleh valenzuela 1987
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.

Radio Propagation

saleh valenzuela 2
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

Radio Propagation

references
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.

Radio Propagation

the end
The End

Radio Propagation

scattering 2
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:

Radio Propagation

free space 2a
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

Radio Propagation

free space 2b
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

Radio Propagation

free space 2c
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

Radio Propagation

lnsm 2
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

Radio Propagation

ground reflection 1 5
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

Radio Propagation

convolution integral
Convolution Integral
  • Convolution is defined by this integral:

Indexes relevant portion of impulse response

Scales past input signal

Radio Propagation

partition losses
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

Radio Propagation

partition losses 2
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

Radio Propagation

materials
Materials
  • Attenuation values for different materials

Radio Propagation

what does db mean
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

Radio Propagation

slide60
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

Radio Propagation

slide61
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

Radio Propagation

channel sounding 2
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

Radio Propagation

channel sounding 3
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

Radio Propagation

channel sounding 4
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

Radio Propagation

channel sounding 5
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)

Radio Propagation

digression convolutions
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

Radio Propagation

convolutions 2

y(t)

h(t)

y(t)

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.

+

Radio Propagation

convolutions 3

y(t)

h(t-)

h(t-)

h(t-)

h(t-)

h(t-)

h(t)

Flip & Slide: h(t-)

Flip & Slide: h(t-)

Flip & Slide: h(t-)

Flip & Slide: h(t-)

Flip & Slide: h(t-)

Convolutions 3
  • Graphical method: “Flip & Slide”

x(t)

=

Pairwise multiply x*h

and integrate over 

x()

y(t)

and Store y(t)

Radio Propagation

frequency and time domains
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.

Radio Propagation

frequency domain
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)

Radio Propagation

frequency domain 2
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”

Radio Propagation

flat fading
Flat Fading
  • T >> d and W<< BC  minimal ISI

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

Radio Propagation

frequency selective fading
Frequency Selective Fading
  • T << d and W>> 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

Radio Propagation

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

Radio Propagation

review 2
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

Radio Propagation

review 3
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

Radio Propagation

relevance to micronets
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?

Radio Propagation

relevance 2
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

Radio Propagation

relevance 3
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.

Radio Propagation

relevance 4
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?

Radio Propagation