Some concepts of Wireless Communication

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Nov 21, 2013 (3 years and 8 months ago)

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1: The Wireless Channel
SomeConceptsofWireless Communication
Someconceptsof
Wireless Communication
Juan Cousseau
DepartmentofElectricalandComputerEngineering
Universidad Nacional del Sur
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1: The Wireless Channel
SomeConceptsofWireless Communication
LecturesObjectives
•Past decade has seen a surge of research activities in
the field of wireless communication.
•Emerging from this research thrust are new points of
view on how to communicate effectively over wireless
channels.
•The goal of theselecturesis to study in a unified way the
fundamentals andsomeconceptsthatleadtonew
research developments.
•The concepts are illustrated using examples from
several modern wireless systems (GSM, IS-95, CDMA
2000 1x EV-DO, Flarion'sFlash OFDM, ArrayComm
systems.)
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1: The Wireless Channel
SomeConceptsofWireless Communication
System Implementation
Capacity limits and
communication techniques
Channel modeling
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1: The Wireless Channel
SomeConceptsofWireless Communication
Outline
1.The Wireless Channel
2. Diversity
3.Multiple Access and Interference Management
4.Capacity of Wireless Channels
1: The Wireless Channel
SomeConceptsofWireless Communication
1. The Wireless Channel
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1: The Wireless Channel
SomeConceptsofWireless Communication
Wireless MultipathChannel
Channel varies at two spatial scales:
large scale fading
small scale fading
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1: The Wireless Channel
SomeConceptsofWireless Communication
Large-scale fading
•In free space, received power attenuates like 1/r
2.
•With reflections and obstructions, can attenuate even
more rapidly with distance. Detailed modeling
complicated.
•Time constants associated with variations are very long
as the mobile moves, many seconds or minutes.
•More important for cell site planning, less for
communication system design.
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1: The Wireless Channel
SomeConceptsofWireless Communication
Small-scale multipathfading
•Wireless communication typically happens at very high
carrier frequency. (eg. fc
= 900 MHz or 1.9 GHz for
cellular)
•Multipathfading due to constructiveand destructive
interference of the transmitted waves.
•Channel varies when mobile moves a distance of the
order of the carrier wavelength. This is about 0.3 m for
900 MHz cellular.
•For vehicular speeds, this translates to channel variation
of the order of 100 Hz.
•Primary driver behind wireless communication system
design.
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1: The Wireless Channel
SomeConceptsofWireless Communication
Physical Models
•Wireless channels can be modeled as linear time-
varying systems:
where ai(t) and τi(t) are the gain and delay of path i.
•The time-varying impulse response is:
•Consider first the special case when the channel is time-
invariant:
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1: The Wireless Channel
SomeConceptsofWireless Communication
Passbandto BasebandConversion
•Communication takes place at
•Processing takes place at baseband
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1: The Wireless Channel
SomeConceptsofWireless Communication
Complex BasebandEquivalent Channel
•The frequency response of the system is shifted
from the passbandto the baseband.
•Each path is associated with a delayand a
complex gain.
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1: The Wireless Channel
SomeConceptsofWireless Communication
Modulation and Sampling
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1: The Wireless Channel
SomeConceptsofWireless Communication
MultipathResolution
Sampled baseband-equivalent channel model:
where hl
is the l thcomplex channel tap.
and the sum is over all paths that fall in the delay bin
System resolves the multipathsup to delays of 1/W .
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1: The Wireless Channel
SomeConceptsofWireless Communication
Sampling Interpretation
•h
l
is the l thsample of the
low-pass version of the
channel response hb(∙).
•Contribution of the i th
path is the projection of
aib
δ(τ-τi) onto sinc(Wτ-l).
1
W
Main contribution l = 0
Main contribution l = 0
Main contribution l = 1
Main contribution l = 2
Main contribution l = 2
i = 0
i = 1
i = 2
i = 3
i = 4
012
l
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1: The Wireless Channel
SomeConceptsofWireless Communication
Flat and Frequency-Selective Fading
•Fading occurs when there is destructive interference of
the multipathsthat contribute to a tap.
Delay spread
Coherence bandwidth
single tap, flat fading
multiple taps, frequency selective
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1: The Wireless Channel
SomeConceptsofWireless Communication
Effective channel depends on both physical environment and bandwidth!
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1: The Wireless Channel
SomeConceptsofWireless Communication
Time Variations
Doppler shiftof the i thpath
Doppler spread
Coherence time
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1: The Wireless Channel
SomeConceptsofWireless Communication
Two-path Example
v= 60 km/h, f
c
= 900 MHz:
direct path has Doppler shift of -50 Hz
reflected path has shift of +50 Hz
Doppler spread = 100 Hz
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1: The Wireless Channel
SomeConceptsofWireless Communication
Doppler Spread
Doppler spread is proportional to:
•the carrier frequency f
c;
•the angular spread of arriving paths.
where θi
is the angle the direction of motion
makes with the i thpath.
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SomeConceptsofWireless Communication
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1: The Wireless Channel
SomeConceptsofWireless Communication
Types of Channels
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1: The Wireless Channel
SomeConceptsofWireless Communication
Typical Channels are Underspread
•Coherence time T
c
depends on carrier frequency
and vehicular speed, of the order of milliseconds
or more.
•Delay spread T
d
depends on distance to
scatterers, of the order of nanoseconds(indoor)
to microseconds (outdoor).
•Channel can be considered as time-invariant
over a long time scale.
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1: The Wireless Channel
SomeConceptsofWireless Communication
Statistical Models
•Design and performance analysis based on statistical
ensemble of channels rather than specific physical
channel.
•Rayleighflat fading model: many small scattered paths
Complex circular symmetric Gaussian .
Squared magnitude is exponentially distributed.
•Ricianmodel: 1 line-of-sight plus scattered paths
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1: The Wireless Channel
SomeConceptsofWireless Communication
Correlation over Time
•Specified by autocorrelation
function and power spectral
density of fading process.
•Example: Clarke’s (or Jake’s)
model.
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1: The Wireless Channel
SomeConceptsofWireless Communication
Additive Gaussian Noise
•Complete baseband-equivalent channel model:
•Special case: flat fading:
•Will use this throughout these lectures.
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1: The Wireless Channel
SomeConceptsofWireless Communication
Summary
•We have understood how time and frequency
selectivity of wireless channels depend on key
physical parameters.
•We have come up with statistical channel
models that are useful for analysis and design.