1 CS 6910 – Pervasive Computing Spring 2007 Section 3 (Ch.3): Mobile Radio Propagation Prof. Leszek Lilien Department of Computer Science Western Michigan.

1

CS 6910 – Pervasive ComputingSpring 2007

Section 3 (Ch.3):

Mobile Radio Propagation

Prof. Leszek LilienDepartment of Computer Science

Western Michigan University

Slides based on publisher’s slides for 1st and 2nd edition of: Introduction to Wireless and Mobile Systems by Agrawal & Zeng

© 2003, 2006, Dharma P. Agrawal and Qing-An Zeng. All rights reserved.

Some original slides were modified by L. Lilien, who strived to make such modifications clearly visible. Some slides were added by L. Lilien, and are © 2006-2007 by Leszek T. Lilien.

Requests to use L. Lilien’s slides for non-profit purposes will be gladly granted upon a written request.

Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 2

Chapter 3

Mobile Radio Propagation


Outline of Chapter 3We skip a lot of detailed information from this Chapter

Introduction Types of Waves

Speed, Wavelength, Frequency Radio Frequency Bands Propagation Mechanisms Radio Propagation Effects Free-Space Propagation Land Propagation Path Loss Fading: Slow Fading / Fast Fading Delay Spread Doppler Shift Co-Channel Interference The Near-Far Problem Digital Wireless Communication System Analog and Digital Signals Modulation Techniques


3.1. Introduction

This section covers the distinguishing features of mobile radio propagation

Model of wireless mobile channel Time-varying communication path between 2

terminals Fixed BS and mobile MS

Mobility introduces new challenges into radio propagation

© 2007 by Leszek T. Lilien


Speed, Wavelength, Frequency

System Frequency Wavelength

AC current 60 Hz 5,000 km

FM radio 100 MHz (88-99, 100-108)

3 m

Cellular (original) 800 MHz 37.5 cm

Ka band satellite 20 GHz 15 mm

Ultraviolet light 1015 Hz 10-7 m

Light speed = Wavelength x Frequency = 3 x 108 m/s = 300,000 km/s


Speed, Wavelength, Frequency – cont.


** OPTIONAL ** Broadcast Frequencies (cf.

http://en.wikipedia.org/wiki/Radio_frequencies): AM radio bands:

long waves (now 153–279 kHz, historically up to 413 kHz) medium waves (520–1,710 kHz in the Americas) short waves (2,300–26,100 kHz)

TV Band I (Channels 2 - 6) = 54MHz - 88MHz (VHF) FM Radio Band II = 88MHz - 108MHz (VHF) TV Band III (Channels 7 - 13) = 174MHz - 216MHz

(VHF) TV Bands IV & V (Channels 14 - 69) = 470MHz -

806MHz (UHF) Most common cellular bands now in uses worldwide:

850/900/1800/1900 MHz E.g., China uses yet another


3.2. Types of Radio Waves

Transmitter Receiver

Earth

Sky wave

Space wave

Ground waveTroposphere

(0 - 12 km)

Stratosphere (12 - 50 km)

Mesosphere (50 - 80 km)

Ionosphere (80 - 720 km)

Ground , space, and sky waves Cellular systems use ground & space

waves


Radio Frequency BandsClassification Band

Initials Frequency Range Characteristics

Extremely low ELF < 300 Hz

Ground waveInfra low ILF 300 Hz - 3 kHz

Very low VLF 3 kHz - 30 kHz

Low LF 30 kHz - 300 kHz

Medium MF 300 kHz - 3 MHz Ground/Sky wave

High HF 3 MHz - 30 MHz Sky wave

Very high VHF 30 MHz - 300 MHz

Space waveUltra high UHF 300 MHz - 3 GHz

Super high SHF 3 GHz - 30 GHz

Extremely high EHF 30 GHz - 300 GHz

Tremendously high

THF 300 GHz - 3000 GHz

AM long wave

AM med. wave

AM short wave

FM, TV

TV, cellphonesWLAN


3.3. Propagation Mechanisms Ideal propagation in free space (no obstacles)

Radio signals can penetrate simple walls to some extentLarge structure, a hill – difficult to pass through

Propagation effects due to obstacles Reflection

Propagation wave impinges on an object which is large as compared to wavelength

E.g., the surface of the Earth, buildings, walls, etc. Diffraction

Radio path between transmitter and receiver obstructed by surface with sharp irregular edges

Waves bend around the obstacle, even when LOS (line of sight) does not exist

Scattering Objects smaller than the wavelength of the propagation wave

E.g. foliage, street signs, lamp posts


Radio Propagation Effectshb – heights of BS antenna, hm - heights of MS antenna


Quality of signal reaching receiver via different types of radio waves Direct propagation is the best Reflection is 2nd best Diffraction is 3rd best Scattering is 4th best

If no direct-path waves (LOS waves) can reach receiver mainly by reflection or diffraction


3.4. Free-space Propagation

The received signal power Pr at distance d:

Ae is effective area covered by the transmitter (on the receiver’s side - e.g., receiver’s antenna)

Gt is the transmitting antenna gain (a better antenna has a higher gain)

Pt is transmitting power(Assuming that the radiated power is uniformly distributed over the surface of the sphere.)

Transmitter Distance dReceiver

hb

hm

2r

4P

d

PGA tte


** SKIP ** Antenna Gain For a circular reflector antenna Gain G = ( D / )2

= net efficiency (depends on the electric field distribution over the antenna aperture, losses, ohmic heating , typically 0.55)

D = diameter thus, G = ( D f /c )2, c = f (c is speed of light)

Example: Antenna with diameter = 2 m, frequency = 6 GHz, wavelength = 0.05 m G = 39.4 dB Frequency = 14 GHz, same diameter, wavelength = 0.021 m G = 46.9 dB * The higher the frequency, the higher the gain for the same-size antenna


Free-space Path Loss

Definition of path loss LP :

Path loss in free space (no obstacles):

where fc is the carrier frequency

The greater the fc , the higher is the loss (see next

slide)

,r

tP P

PL

),(log20)(log2045.32)( 1010 kmdMHzfdBL cPF

Pt is transmitted signal power Pr is received signal power


Example of Path Loss (Free-space)

Path Loss in Free-space

70

80

90

100

110

120

130

0 5 10 15 20 25 30

Distance d (km)

Path

Los

s Lf

(dB)

fc=150MHz

fc=200MHz

fc=400MHz

fc=800MHz

fc=1000MHz

fc=1500MHz


3.5. Land Propagation

The received signal power:

where: Gt is the transmitter antenna gain,

Gr is the receiver antenna gain,

L is the propagation loss in the channel,

i.e., L = LP LS LF

L

PGGP trt

r

Fast fading (long-term f.)

Slow fading (short-term f.)

Path loss


Fast and Slow Fading and Path Loss

Fast fading (short-term fading) – Microscopic aspect of a channel for mobile comm. For a moving MS, it represents fading over its every

step Due to scattering of signal by objects near transmitter

=> Changing wave diffractions

Slow fading (long-term fading) – Variation of propagation loss in a local area (k * 10m) Due to obstacles (e.g., buildings) For an MS, it represents overall average fading over

short distance (e.g., a couple of blocks) traveled by MS Path loss

Propagation loss over long distances (more below)



Schematic Diagram of Propagation Loss (and Fading)

Signal Strength

(dB)

Distance

Path Loss

Slow Fading (Long-term fading)

Fast Fading (Short-term fading)

Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 19© 2007 by Leszek T. Lilien


3.6. Path Loss (for Land Propagation)

Path loss as a fcn of distance Lp = A dα

where: A and α: propagation constantsd : distance between transmitter and receiver

Value of : - 2-4 - normally - below 2 in some waveguides - 2 in free space, - 3-4 in typical urban areas - 4 is for relatively lossy environments. - 4-6 in some environments, such as buildings, stadiums & other indoor environments


Path Loss

Path loss in decreasing order: Urban area (large city) Urban area (medium and small city) Suburban area Open area

Path

loss


Path Loss (Urban, Suburban and Open areas)

Urban area:

where

Suburban area:

Open area:

)(log)(log55.69.44

)()(log82.13)(log16.2655.69)(

1010

1010

kmdmh

mhmhMHzfdBL

b

mbcPU

citymediumsmallfor

MHzfformh

MHzfformh

cityelforMHzfmhMHzf

mh

cm

cm

cmc

m &,400,97.4)(75.11log2.3

200,1.1)(54.1log29.8

arg,8.0)(log56.1)(7.0)(log1.1

)(2

10

210

1010

4.528

)(log2)()(

2

10

MHzfdBLdBL c

PUPS

94.40)(log33.18)(log78.4)()( 102

10 MHzfMHzfdBLdBL ccPUPO

Notice how LPU is subtracted from in LPS and LPO

-

hb, hm – see slide 20

(Units: MHz, m, etc., in parentheses)


Example of Path Loss (Urban Area: Large City)

Path Loss in Urban Area in Large City

100

110

120

130

140

150

160

170

180

0 10 20 30

Distance d (km)

Pat

h Lo

ss L

pu (

dB)

fc=200MHz

fc=400MHz

fc=800MHz

fc=1000MHz

fc=1500MHz

fc=150MHz Should be on top of this fc list


Example of Path Loss (Urban Area: Medium and Small Cities)

Path Loss in Urban Area for Small & Medium Cities

100

110

120

130

140

150

160

170

180

0 10 20 30

Distance d (km)

Pat

h Lo

ss L

pu (

dB) fc=150MHz

fc=200MHz

fc=400MHz

fc=800MHz

fc=1000MHz

fc=1500MHz


Example of Path Loss (Suburban Area)

Path Loss in Suburban Area

90

100

110

120

130

140

150

160

170

0 5 10 15 20 25 30

Distance d (km)

Path

Los

s Lp

s (d

B)

fc=150MHz

fc=200MHz

fc=400MHz

fc=800MHz

fc=1000MHz

fc=1500MHz

Compare loss for, e.g., 30 km & 1500 MHz with Large/Small Urban


Example of Path Loss (Open Area)[repeated for completeness]

Path Loss in Open Area

80

90

100

110

120

130

140

150

0 5 10 15 20 25 30

Distance d (km)

Path

Los

s Lp

o (d

B)

fc=150MHz

fc=200MHz

fc=400MHz

fc=800MHz

fc=1000MHz

fc=1500MHz

Compare loss for, e.g., 30 km & 1500 MHz with Large/Small Urban & Suburb.


3.7. Slow Fading Slow fading (shadowing or log-normal fading) of a signal = long-

term spatial and temporal variations in signal strength over “large enough “ distances Distance is “large enough “ if it produces gross variation in signal strength

for the overall path between the transmitter and receiver

Slow fading obeys log-normal distribution (hence one of its names)

- The pdf of the received signal level is given in decibels (dB) by

where

M is the true received signal level m in dB (i.e., 10log10m) M is the area average signal level (i.e., the mean of M) is the standard deviation in dB

,2

1 2

2

2

MM

eMp


** SKIP ** Log-normal Distribution

MM

2

p(M)

The pdf of the received signal level(log-normal distribution)


3.8. Fast Fading Fast fading (short-term fading) of a signal = rapid fluctuations in

the spatial and temporal signal characteristics caused by local mulipath Multipath – since scattered, diffracted, and reflected signals gets

to receiver’s antenna over many paths

Fast fading occurs over distances of about half a wavelength Example:

VHF and UHF (30 MHz - 300 MHz & 300 MHz - 3 GHz) Vehicle traveling at 30 mph It passes through several fades in a second

Mobile radio signal consists of: Fast fading signal Superimposed on a local mean value

Local mean value: constant over a small area / varies slowly as the receiver moves



Modeling fast fading: When receiver is far from transmitter

No direct radio waves between transmitter & receiver

Reflected waves stronger

Modeled by the Rayleigh distribution (below) Useful for modeling when no LOS

When receiver is close to transmitter Direct radio wave between transmitter &

receiver is stronger than other waves Stronger than reflected, diffracted, or scattered

Modeled by the Rician distribution (below)


Fast Fading - cont. 1


Rayleigh Distribution

The pdf of the Rayleigh Distribution

r – envelope of the fading signal; - standard deviation

r2 4 6 8 10

P(r)

0

0.2

0.4

0.6

0.8

1.0

=1

=2

=3

rm = 1.777 rm is middle value of envelope signal (i.e. such value that: P(r ≤ rm) = 0.5)


Rician Distribution

r

Pdf

p(r

)

r86420

0.6

0.5

0.4

0.3

0.2

0.1

0

= 2 = 1

= 0

= 1

= 3

The pdf of the Rician Distribution

r – envelope of the fading signal - amplitude of the direct signal ( = 0 means no direct signal)

= 0 means no direct signal => Rayleigh distr. - see the same shape as for Rayleigh distr. with = 1

For large (= very strong signal), Rician distr. can be approximated by Gaussian distribution


Modeling fast fading – cont.:

When receiver is far from transmitter Modeled by the Rayleigh distribution

When receiver is close to transmitter Modeled by the Rician distribution

Generalized model for any distance – Nakagami distrib.

Rayleigh distribution is its special case Rician distribution can be closely approximated

by it “Can often be better” than Rician


Fast Fading - cont. 2


Characteristics of Instantaneous Amplitude Instantaneous amplitude => potentially fast changing

signal

Characteristics of instantaneous amplitude Level crossing rate at a specified threshold (signal level)

Average number of times per second that the signal envelope crosses the threshold in positive-going direction

Fading rate Number of times that the signal envelope crosses the middle

value rm in positive-going direction per unit time Depth of fading

Ratio of mean square value of fading signal and minimum value of fading signal

Average depth of fading often used instead Fading duration

Duration for which signal is below a given threshold


3.9. Doppler Effect (Doppler Shift) Doppler effect - occurs when a wave source and a receiver are

moving towards or away from each other When they are moving toward each other, the frequency of the

received signal is higher than the source frequency When they are moving away from each other, the frequency is

lower than the source frequency

Received signal frequency is

where: fC - source frequency ,

fD - the Doppler shift in frequency

Doppler shift is

where: v - the moving speed - the wavelength of the carrier of the source signal

cosv

fD

DCR fff

MS

Signal from source

Moving direction of the receiver (MS)


*** SKIP *** Moving Speed Effect[LTL:] This textbook figure is not consistent with text referring to it. It is confusing in the context of the Doppler effect (which has to do with signal frequency not signal strength, as the figure suggests).


3.10. Delay Spread When a signal propagates from a transmitter to a receiver, it can

suffer one or more reflections [Agrawal & Zeng]

Now, instead of single-path direct signal we have a multipath reflected signal reaching the receiver

More descriptively: Each reflection of the original signal produces a new (fainter) “child” signal that follows its own path

The path followed by the “child” signal is determined by reflection parameters

When all “children” signals are put together, we have multiple reflected signals (all originating from the original signal) following a multipath towards the receiver

Some of the “children” signals may be too faint to reach the receiver

Some of the “children” signals may be reflected in wrong direction(s) and never reach the receiver



Delay Spread – cont.

Different paths in the multipath have different lengths, so the time of arrival for each of many reflected “child” signals is different

This spreads out the received signal

=> is called “delay spread” If we had direct signal only, there would be one signal

path, no children signals, and no delay spread

See Figure (next)


Delay Spread

Delay

Sig

nal S

tren

gth

The signals reflected from nearby reflectors

The signals reflected from intermediate reflectors

The signals reflected from distant reflectors

Q1: A signal reflected by a nearby reflector arrives sooner than a signal reflected by a more distant reflector. Why?


Delay Spread – cont. 1


Q1: A signal reflected by a nearby reflector arrives sooner than a signal reflected by a more distant reflector. Why?

A: First note that the direct path is the shortest path from the xmitter to the receiver.

A signal reflected by a nearby reflector already traveled most of the distance d as a direct signal. In other words, such signal already traveled most of the distance d along the shortest path to the receiver.

A signal reflected further away from the receiver was reflected earlier, so it traveled more of the distance d as a multipath reflected signal, i.e., its components (children signals) traveled more of the distance d along a longer (not the shortest) path to the receiver.


Delay Spread (again)

Delay

Sig

nal S

tren

gth

The signals reflected from nearby reflectors

The signals reflected from intermediate reflectors

The signals reflected from distant reflectors

Q2: A signal reflected by a nearby reflector is stronger than a signal reflected by a more distant reflector. Why?




Q2: A signal reflected by a nearby reflector is stronger than a signal reflected by a more distant reflector. Why?

A: A signal reflected by a nearby reflector already traveled most of the distance d as the strongest (single-path & direct) signal.

A signal reflected further away from the receiver was reflected earlier, and traveled more of the distance d as a weaker (multipath & reflected) signal. To be precise, it traveled as a multipath collection of reflected children signals, collectively weaker than the original single-path & direct signal would be.




Typical delay spread: 3 microsec. in a city area 10 microsec. in a hilly terrain

Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 44© 2007 by Leszek T. Lilien


3.11. Intersymbol Interference (ISI) Intersymbol Interference (ISI) - caused by delay spread

Example of ISI: 101… digital signal sent from xmitter to rcver

4 multipaths due to reflections Single-path direct signal 4 multipath reflected children

signals Each child generated by another reflection of the

original signal

See figures in the next 2 slides

Observation – case of ISI: In Slide +2 multipath components for symbol “1” arrive at

the time when only multipath components for symbol “0” should be arriving => symbol “1” interferes with symbol “0”


[Agrawal&Zeng]


Intersymbol Interference (ISI) Example

Modified by LTL

Figure: Xmitter and receiver are synchronized

Red broken-line arrows show synchronization “Short delay”: all 4 delayed multipath signals showing

NULL1 arrive within duration of their “time slot “

Time

Time

Transmission signal

Received signal (Short delay)

1

0

1

Propagation time

Delayed multipathsignals

NULL


Intersymbol Interference (ISI) Example

Modified by LTL

Figure Xmitter and receiver are synchronized

Red broken-line arrows show synchronization “Long delay”: only 1 delayed multipath signal showing

NULL1 arrives within duration of its “time slot” 3 delayed multipath signals arrive too late (within the next “time slot”)

Time

Time

Transmission signal

Received signal (Long delay)

1

0

1

Delayed multipathsignalsPropagation time

NULL


Burst error– a contiguous sequence of symbols, received over a data transmission channel, such that:(a) the first symbol and the last symbol are in error,

and (b) there exists no contiguous subsequence of m

correctly received symbols within the error burst The last symbol in a burst and the first symbol in

the following burst are separated by m correct bits or more

The length of a burst of errors in a frame is defined as the number of bits from the first error to the last, inclusive

Error burst – occurrence of burst errors Burst error rate – measures the rate of burst errors

ISI can cause burst errors=> ISI has impact on the burst error rate of a channel

3.11. Intersymbol Interference (ISI) – cont. 1



3.11. Intersymbol Interference (ISI) – cont. 2

To assure low bit-error rate (BER), digital transmission rate R must be limited by delay spread d as follows:

dR

21


3.12. Coherence Bandwidth Gain [cf. http://en.wikipedia.org/wiki/Gain]

= the mean ratio of the signal output of a system to the signal input of the system

Gain has wide use to characterize amplifiers

Linear phase channel (or filter) [cf. http://en.wikipedia.org/wiki/Linear_phase]

= channel with no distortion due to the frequency-selective delays

All frequency components have equal delay times

In contrast, in a filter/channel with non-linear phase, delay varies with frequency, resulting in phase distortion

Flat channel Passes all frequencies (“spectral components”) with

approxi-mately equal gain and linear phase



3.12. Coherence Bandwidth – cont.1

Coherence bandwidth Bc

= channel frequency range over which the channel can be considered “flat”

In other words: the frequency interval over which two different frequencies f1 and f2 of a signal are likely to experience correlated (amplitude) fading [cf. http://en.wikipedia.org/wiki/Coherence_bandwidth]

*** SKIP this bullet *** - NOT EXPLAINED WELL – SEEMS INCONSISTENT WITH THE REST Bc represents the correlation between 2 fading signal envelopes at

frequencies f1 and f2 Correlation => a statistical measure

Bc is a function of delay spread Bigger delay spread => lower correlation

Two frequencies that are larger than their coherence bandwidth Bc fade independently

Concept of coherence bandwidth useful in diversity reception Diversity reception = multiple copies of same message are sent using

different frequencies


3.12. Coherence Bandwidth – cont.2

Let S = transmitted signalB = bandwidth of SBc = channels’ coherence bandwidth

If B < Bc => linear transmission of S Only gain and phase of S are changed

=> S not distorted Intuitively, whole B for S is flat (whole B within Bc)

If B > Bc => non-linear transmission of S Part of S truncated

=> S severely distorted Intuitively, only part of B for S is flat (part of B outside of Bc)



3.13. Cochannel Interference

Cells reuse frequencies Same freq assigned to different cells

=> cells using the same frequency interfere with each other

Let: rd - the desired signal

ru - the interfering undesired signal

- the protection ratio for which rd ru (so that interference of ru is limited by )

(Probability of) cochannel interference between cells reusing a frequency

Pco = P(rd ru )

where P(rd ru ) = probability that rd ru


3.13. Cochannel Interference

Cells having the same frequency interfere with each other rd is the desired signal

ru is the interfering undesired signal is the protection ratio for which rd ru (so that interference of ru

is limited by )

Cochannel interference between cells using the same frequency

Pco = P(rd ru )

where P(rd ru ) = probability that rd ru


The End of Section 3

1 CS 6910 – Pervasive Computing Spring 2007 Section 3 (Ch.3): Mobile Radio Propagation Prof. Leszek Lilien Department of Computer Science Western Michigan.

Documents

agrawal zeng

mobile radio propagation

radio bands

lilien slide

ground space waves slide

vhf fm radio band

kms slide

mobile systems