WIRELESS OVERVIEW, CHANNEL

8/7/2019 WIRELESS OVERVIEW, CHANNEL

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Wireless Communication

Lecture 1: Overview

Sqn Ldr Sohail Ahmed


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Transmitting voice and data using electromagneticwaves in open space

Electromagnetic waves

Travel at speed of light (c = 3x108

m/s) Has a frequency f and wavelength P

c = f P

Higher frequency means higher energy photons

The higher the energy photon the more penetrating is the

radiation


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Wireless Communication: Advantages

Ubiquity: Can travel long distances and acrossoceans

Mobility

Easy to esta

blish a wireless link: no hardwarebased medium

Can penetrate buildings

Suitable both for indoor and outdoorcommunication

Omni-directional: can travel in all directions Can be narrowly focused at high frequencies

(greater than 100MHz) using parabolicantennas (like satellite dishes)


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Wireless Communication: Challenges

Mobility leads to Doppler effect

Multipath effect

Wireless channel is time varying: difficult to estimate Interferences from other users and sources

Limited bandwidth

Limited power (especially in mobile communication)


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Wireless Communication: Applications

Satellite Links

LOS Microwave

Cellular communication

Wireless LAN

Wireless Local Loop (WLL)

Space Communication

Remote controls

Sensor networks

Mobile computing

Cordless phones

Pagers

Radars


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History

1831: Faraday had first started experimenting withelectromagnetic waves.

Electromagnetic wave:

one of the waves that are propagated by simultaneous

periodic variations of electric and magnetic field intensityand that include

radio waves

infrared

visible light

ultraviolet,

X rays

Gamma rays

Started with Marconis invention of radio.


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First Wireless Telegraphy

The first use of wireless telegraphy

in military occurred during the

Anglo-Boer War (1899-1902). The

British Army experimented with

Marconi's system and the BritishNavy successfully used it for

communication among naval

vessels in Delagoa Bay.


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Wireless History

First Mobile Radio Telephone 1924


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History Early Achievements

1901: Marconi successfully transmits radio signal across AtlanticOcean from Cornwall to Newfoundland

1902: First bidirectional communication across Atlantic

1909: Marconi awarded Nobel prize for physics

1914: First voice over radio transmission

1920s: Mobile receivers installed in police cars in Detroit

1930s: Mobile transmitters developed; radio equipment occupied

most of police car trunk

by 1934: Amplitude Modulation (AM) systems used by police carsand stations

1935: Edwin Armstrong demonstrated frequency modulation (FM)for the first time. Majority of police systems converted to FM


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History Mobile Telephony

1946: First public mobile telephone service was introduced.First interconnection of mobile users to public switched

telephone network (PSTN)

1949: FCC (Federal Communications Commission) ofUS

recognizes mobile radio as new class of service

1950-1960: AT&T Bell Labs developed theory andtechniques for cellular telephony


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History Pagers, Cordless Phones and

Cellular Telephony 1959: The term "pager" was first used, referring to a Motorola radio

communications product

1968: AT&T proposed cellular telephony to FCC ofUS.

1974: The first pager was introduced byMotorola.

1977: Public cell phone testing began. 1979:Worlds first cellular system was implementedby NTT Japan.

1980: 3.2 million pagers used wordwide. They had limited range.

1980: Cordless phones started to emerge.

Early 1980s: Wireless modems emerged.

1981:

European Nordic

Mo

bile

Telephone (N

MT) System wasdeveloped

1983: FCC allocated wireless spectrum for mobile telephony.

1983: AMPS, first USA analog cellular telephony standard wasdeveloped


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History Wireless LANs, GSM

1987: IEEE 802.11 Wireless LAN working group founded.

1989: In Europe, GSM was defined.

1990: In Europe, GSM deployed.

by 1990: Wide-area paging had been invented and over 22

million pagers were in use

1990: FCC allocated spectrum in 900 Mhz for cordlessphones.

1990: Announcement ofWireless LAN products


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Recent History

1991: First US digital cellular hardware was installed. IS-54 and IS-136 emerged.

1991: RAM mobile (mobitex) data service 1992: HyperLAN in Europe

1992:World Radio Conference in Malaga (WRC-92) allocated

frequencies for futureUMT

S use. Frequencies 1885 - 2025 and 2110 - 2200 MHz were identified for IMT2000 use

1993: First GSM 1800 system in commercial operation in UK

1993: IS-95 code-division multiple-access (CDMA) spread-spectrum digital cellular system deployed in US

1993: CDPD (Cellular Digital Packet Data) over AMPS was realized 1994: GSM system deployed in US

1994: there were over 61 million pagers in use and pagers becamepopular for personal use.


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History Personal Area Networks

1998: Bluetooth was born. SIG for Bluetooth has beenestablishedby the leadership of 5 companies: Ericsson, IBM,Intel, Toshiba, Nokia

1998: HomeRFWorking Group was formed.

1998: FCC gave 2.5 GHz spectrum for cordless phones

1998 ETSI SMG meeting in Paris both W-CDMA and TD-CDMA proposals were combined to UMTS air interfacespecification.


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History 3G Trials and Progress

1998: The first call using a Nokia W-CDMA terminal inDoCoMo's trial network was completed at Nokia's R&D unitnear Tokyo in Japan.

Jun 1998: CDMA2000 submitted to ITU for IMT-2000

Dec 1998: The first meetings of the 3GPP TechnicalSpecification Groups in France.

1999: IEEE 802.11b approved (11 Mbps)

1999: The first open Bluetooth specification 1.0 is released.


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History 3G Progress

Jul 1999: Phase 1 CDMA2000 standard complete andapproved for publication

Jul 1999: Korea Telecom Freetel launches world's first IS-95B network in Korea

1999: Nokia claimed that it has completed the firstWCDMAcall through the public switched telephone network in theworld

Nov 1999: ITU-RTask Group 8/1 endorses CDMA2000standards (three modes) for IMT-2000


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History 3G Progress

1999: ETSI Standardization finished for UMTS Release 1999specificationsboth for FDD and TDD in Nice, France.

Mar 1999: March 1999 ITU approves radio interfaces for thirdgeneration mobile systems

1999:W

orld Radio Conference (W

RC-99) handled spectrum andregulatory issues for advanced mobile communicationsapplications in the context ofIMT-2000

June 2000: Telstra and Nortel complete first 3G CDMA2000 1Xdata transmission

2001 Ericsson and Vodafone UKclaim to have made the world's

first WCDMA voice call over commercial network. Jun 2001: NTT DoCoMo launched a trial 3G service

June 2001: CDMA2000 1xEV-DO recognized as part of the 3GIMT-2000 standard


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History 3G Commercial Services

Aug 2001: 1 million commercial CDMA2000 1X subscribers

Oct 2001 NTT DoCoMo launched the first commercial WCDMA 3G mobilenetwork

Nov 2001: Nokia and AT&TWireless complete first live 3G EDGE call.

Dec 2001: Telenor launched in Norway the first commercial UMTS network

Jan 2002: Verizon Wireless (US) launches commercial CDMA2000 1Xservice

Jan 2002: Verizon Wireless (US) launches commercial CDMA2000 1Xservice

Feb 2002: Nokia and Omnitel Vodafone claims to have made the first richcall in an end-to-end All-IP mobile network at the 3GSMWorld Congress inCannes, France.

May 2002: 10 million commercial CDMA2000 1X subscribers

Jun 2003: Target date for UMTS Release 6

2005: UMTS service will be world-wide


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History The Future

4G:

Must support data traffic much more cost-effectively than 3G

Large peak data rates

Over 2 billion voice users worldwide

Preferably, global convergence to a single standard

May not be based on CDMA; multi-carrier transmissionbeingconsidered.

WiMAX Ultrawideband (UWB) systems

Software defined radio


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What is Mobility?

InitiallyInternet and Telephone Networks isdesigned assuming the user terminals are static

No change of location during a call/connection

A user terminals accesses the network always from a fixed

location Mobility and portability Portability means changing point of attachment to the

network offline

Mobility means changing point of attachment to the networkonline


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Degrees of Mobility

Walking Users Low speed

Small roaming area

Usually uses high-bandwith/low-latency access

Vehicles High speeds

Large roaming area

Usually uses low-bandwidth/high-latency access

Uses sophisticated terminal equipment (cell phones)


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Major Mobile Radio Standards

USAStandard Type Year

Intro

MultipleAccess

Frequency Band

(MHz)

Modulation Channel

BW

(KHz)

AMPS Cellular 1983 FDMA 824-894 FM 30

USDC Cellular 1991T

DM

A 824-894 DQPSK

30

CDPD Cellular 1993 FH/Packet 824-894 GMSK 30

IS-95 Cellular/PCS 1993 CDMA 824-8941800-2000

QPSK/BPSK 1250

FLEX Paging 1993 Simplex Several 4-FSK 15

DCS-1900(GSM)

PCS 1994 TDMA 1850-1990 GMSK 200

PACS Cordless/PCS 1994 TDMA/FDMA 1850-1990 DQPSK 300


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Major Mobile Radio Standards - Europe

Standard Type Year

Intro

MultipleAccess

Frequency Band

(MHz)

Modulation Channel

BW

(KHz)

ETACS Cellular 1985 FDMA 900 FM 25

NMT-900Cellular 1986 FDMA 890-960 FM 12.5

GSM Cellular/PCS 1990 TDMA 890-960 GMSK 200KHz

C-450 Cellular 1985 FDMA 450-465 FM 20-10

ERMES Paging 1993 FDMA4 Several 4-FSK 25

CT2 Cordless 1989 FDMA 864-868 GFSK 100

DECT Cordless 1993 TDMA 1880-1900 GFSK 1728

DCS-1800 Cordless/PCS 1993 TDMA 1710-1880 GMSK 200


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Lecture 2: Wireless Channel

Sqn Ldr Sohail Ahmed


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Basics - Propagation

Radio waves are Easy to generate

Can travel long distances

Can penetrate buildings

They are

both used for indoor and outdoor communication

They are omni-directional

They can be narrowly focused at high frequencies (greater than100MHz) using parabolic antennas (like satellite dishes)

Waves behave more like light at higher frequencies Difficulty in passing obstacles

More direct paths

Theybehave more like radio at lower frequencies Can pass obstacles


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AtVLF, LF, and MF b

ands, radiowaves follow the ground. AM radio

broadcasting uses MF band

At HF bands, the ground

waves tend to be absorbed by the

earth. The waves that reach ionosphere

(100-500km above earth surface),are refracted and sent back to

earth.

absorption

reflection

Ionosphere


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LOS path

Reflected Wave

-Directional antennas are used

-Waves follow more direct paths

- LOS: Line-of-Sight Communication- Reflected waves interfere with the

original signal

VHF Transmission


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Radio Propagation Mechanisms

The physical mechanisms that govern radio propagation are complexand diverse, but generally attributed to the following three factors

1. Reflection

2. Diffraction

3. Scattering Reflection

Occurs when waves impinges upon an obstruction that is muchlarger in size compared to the wavelength of the signal

Example: reflections from earth and buildings

These reflections may interfere with the original signalconstructively or destructively


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Diffraction Occurs when the radio path between sender and receiver is

obstructed by an impenetrable body and by a surface with sharpirregularities (edges)

Explains how radio signals can travel urban and ruralenvironments without a line-of-sight path

Scattering Occurs when the radio channel contains objects whose sizes are

on the order of the wavelength or less of the propagating waveand also when the number of obstacles are quite large.

They are produced by small objects, rough surfaces and otherirregularities on the channel

Follows same principles with diffraction Causes the transmitter energy to be radiated in many directions Lamp posts, trees and street signs may cause scattering


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Building Blocks

D

R

S

R: ReflectionD: Diffraction

S: Scattering

transmitter

receiver

D

Street


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As a mobile moves through a coverage area, these 3mechanisms have an impact on the instantaneousreceived signal strength.

If a mobile does have a clear line of sight path to the base-

station, than diffraction and scattering will not dominate thepropagation.

If a mobile is at a street level without LOS, then diffraction andscattering will probably dominate the propagation.


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Radio Propagation Models: small-scalefading

As the mobile moves over small distances, theinstantaneous received signal will fluctuate rapidlygiving rise to small-scale fading

The reason is that the signal is the sum of many contributors

coming from different directions and since the phases ofthese signals are random, the sum behave like a noise (e.g.Rayleigh fading).

In small scale fading, the received signal power may changeas much as 3 or 4 orders of magnitude (30dB or 40dB), whenthe receiver is only moved a fraction of the wavelength.


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Radio Propagation Models: Large-scalePath Loss

As the mobile moves away from the transmitter over largerdistances, the local average received signal will graduallydecrease. This is called large-scale path loss.

Typically the local average received power is computed by averagingsignal measurements over a measurement track of 5P to 40PFor PCS,

this means 1m-10m track)

The models that predict the mean signal strength for anarbitrary-receiver transmitter (T-R) separation distance arecalled large-scale propagation models

Useful for estimating the coverage area of transmitters


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Small-Scale and Large-Scale Fading

14 16 18 20 22 24 26 28

T-R Separation (meters)

-70

-60

-50

-40

-30

Received Power (dBm)

This figure is just an illustration

to show the concept. It is not based on readdata.


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Large-Scale Fading: Free-Space

Propagation Model Free space power receivedby a receiver antenna separated froma radiating transmitter antenna by a distance d is given by Friisfree space equation:

Pr(d) = (PtGtGrP2) / ((4T)2d2L) [Equation 1]

Pt is transmitted power Pr(d) is the received power Gt is the transmitter antenna gain (dimensionless quantity) Gr is the receiver antenna gain (dimensionless quantity) d is T-R separation distance in meters L is system loss factor not related to propagation (L >= 1) L = 1 indicates no loss in system hardware (for our purposes we

will take L = 1, so we will ignore it in our calculations). P is wavelength in meters.


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Free-Space Propagation Model

The gain of an antenna G is related to its affective apertureAeby:

G = 4TAe / P2 [Equation 2]

The effective aperture of Ae is related to the physical size of

the antenna, P is related to the carrier frequencyby:

P = c/f = 2Tc / [c [Equation 3]

f is carrier frequency in Hertz

[c is carrier frequency in radians per second.

c is speed of light in meters/sec


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Free-Space Propagation Model: Path

Loss Path loss, which represents signal attenuation as positive

quantity measured in dB, is defined as the difference (indB) between the effective transmitted power and thereceived power.

PL(dB) = 10 log (Pt/Pr) = -10log[(GtGrP2)/(4T)2d2] [Equation 4]

If antennas have unity gains

PL(dB) = 10 log (Pt/Pr) = -10log[P2/(4T)2d2] [Equation 5]


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Reference Distance d0

Eq. 1 does not hold for d=0. So we define reference distance d0 Pr(d) = Pr(d0)(d0/d)

2

Reference distance d0 for practical systems:

For frequncies in the range 1-2 GHz

1 m in indoor environments

100m-1km in outdoor environments


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Long distance Path loss model

Extension of the free-spacemodel to other channels.

The average large-scale pathloss for an arbitraryT-Rseparation is expressed as afunction of distanceby using apath loss exponent n.

The value ofn depends on thepropagation environment: forfree space it is 2; when

obstructions are present it hasa larger value.

)log(10)()(

)()(

0

0

0

d

dndPLdBPL

d

ddPL

n

!

w


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Path Loss Exponent for Different

EnvironmentsEnvironment Path Loss Exponent, n

Free space 2

Urban area cellular radio 2.7 to 3.5

Shadowed urban cellular radio 3 to 5

In building line-of-sight 1.6 to 1.8

Obstructed in building 4 to 6

Obstructed in factories 2 to 3


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Log-normal Shadowing

Path loss equation given above does not consider thefact the surrounding environment maybe vastlydifferent at two locations having the same T-Rseparation

This leads to measurements that are different fromthe predicted values obtained using the aboveequation.

Measurements show that for any value d, the pathloss PL(d) in dBm at a particular location is random

and distributed normally.


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Log-normal Shadowing, n and W

The log-normal shadowing model indicates thereceived power in dBs at a distance d is normallydistributed with a distance dependent mean and witha standard deviation ofW

In practice the values of n and Ware computed frommeasured data using linear regression so that thedifference between the measured data and estimatedpath losses are minimized in a mean square errorsense.


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Small Scale Fading: Multipath

Propagation

Many echoes are received due to reflections

As data rate goes up, number ofbits sent between echoesgoes up

È0 ` È1 ` È2 `

(1 (2

E0

E1

E2


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Signal Transmissions in MultipathEnvironments


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Mathematical Model of Multipath

Channel Channel Impulse Response and Transfer

Function

? Atfetstx 02)()( T!Transmitted Signal

Received Signal after time-varying channel

? A !n

nn ntxtty )()()( XE

Attenuation Delay


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Channel Impulse Response

-

!

tfj

n

n

tj

n ettsettyn 02

EnvelopeComplexChanged

)()]([)()(

TU XE

)(2)( 0 tft nn XTJ |Where

g

g

!

!

XX

XXHX

XT

J

detctfH

tettc

fj

n

n

tj

nn

2

)(

),(),(

FunctionTransfertheAnd

)]([)(),(


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Time-Varying Impulse Response Model


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Multipath Channel: Received Signal

Received signal consists of many multipath

components

Amplitudes change slowly

Phases change rapidly Constructive and destructive addition of signal

components

Amplitude fading of received signal

The channel has time-varying statistics


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Effect ofFading on QPSK Signal


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Fading Ruins Performance

Unmitigated, flatfading kills thesystem

About an extra30dB needed!(1000 times moresignal power )

Clearly, wireless

systems need away around thiscatastrophe

30 dB


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Fading Channels: Doppler Effect

cd

idi

f

c

vf

ff

!

! Ucos

DirectionOf Movement

Buildings

iU


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Fading Channel: Doppler Spread

The Average Received Power

g

g!

!

iiAb

2

0 2

1

2

0

1

)(

!

d

dfff

bfS

T

Power Spectrum of c(t)

df

df


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Fading Channel: Coherence Time

Doppler spread Bd is the range of frequencies over whichthe Doppler spectrum is non-zero.

Bd fd Coherence time Tc is the statistical measure of the time

during which the channel impulse response remains moreor less invariant

Tc 1/ BdTwo pulses arriving with a time separation ofgreater than Tc will be affected differently by the

channel.


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Multipath Spread and the MultipathIntensity Profile

Multipath or delay spread Tm is the time between thefirst and last received component. Also referred to asmaximum excess delay.

Multipath intensity profile (MIP) gives the average

power output of the channel impulse response (CIR) as afunction of time delay.

There is measurements-based evidence that the MIP can beapproximated by an exponentially decaying function

where is the standard deviation of the delay , i.e.,


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Coherence Bandwidth


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Fading Channel: Coherence Bandwidth


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Frequency Selective Fading


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Frequency Selective Fading

Large Differential Path Delays

Signal BW> Coherence Bandwidth

Phase and Amplitude Distortion

Introduces ISI

Urban/Sub-Urban Cellular Channel

Wideband signal


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Flat and Frequency Selective Fading


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Various Fading Distributions

Used to model the statistical nature ofthe envelope (amplitude) of thereceived signal when the transmittedsignal suffers from small-scale fading.

Rayleigh

Rician

Nakagami-m


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FadingDistributions:Rayleigh fading


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Rayleigh fading: Received Signal


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FadingDistributions:Rician Fading

I0 is the modified Bessel function of zeroth order

K=s2/22, ratio of the power of the direct signal component to the

diffuse signal power


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FadingDistributions: Nakagami-m

distribution


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Outage Probability


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Level Crossing Rate (LCR) and AverageD

uration ofF

ades (ADF)


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LCRand ADF (Outdoor Channel)


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Multiuser Interference

Wireless frequencyspectrum is limited

All wireless users

inherently interfere witheach other (Power fallsoff rapidly with distancethough, thankfully)

How to divide the

resources and be robustto interference?

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