WLC1 92-93-1 Introduction

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12 Δεκ 2013 (πριν από 3 χρόνια και 8 μήνες)

64 εμφανίσεις

لاسمین
ل
ّ
وا
93
-
92

ت
ّ
مه نیشفا

رای

رتویپماک یسدنهم هدکشناد

را
ّ
یس تارباخم
(
626
-
40
)

یفرعم

References



A Goldsmith, Wireless Communications,


Cambridge University Press, 2005.




D. Tse and D. Vaswanth, Fundamentals of


Wireless Communications, Cambridge


University Press, 2005.




T. Rappaport, Wireless Communications,


Principles and Practice, 2
nd

Edition, Prentice


Hall.



J. Fayyaz, Radio Design of Cellular Networks,


Naghoos Press, 2011.

2

Contents


Background and Preview


Wireless Propagation Channel


Multiple Access Methods


Cellular Systems


Diversity Issues


Information Transmission Capacity


Multiple Antenna Technologies


Cooperative Communications

3

Wired Vs. Wireless Communications

Wired

Wireless

Each cable is a different channel

One media (cable) shared by all

Signal attenuation is low

High

signal attenuation

No interference

High interference

noise; co
-
channel interference; adjacent
channel interference

4



Advantages


Sometimes it is impractical to lay cables


User mobility


Cost




Limitations



Bandwidth



Fidelity



Power



Security

Why Wireless?

5

Wireless ≈ Waves


Electromagnetic

radiation


Emitted

by

sinusoidal

current

running

through

a

wire

(transmitting

antenna)


Creates

propagating

sinusoidal

magnetic

and

electric

fields

according

to

Maxwell’s

equations
:




Fields induce current in receiving antenna.

6

Propagation Principle

electric

field

magnetic

field

propagation direction

7

Propagation Mechanisms

Non Line
-
of
-
Sight

Reflection

λ << D

Diffraction

λ



D

Scattering

λ
>>

D

S

D

Line
-
of
-
Sight

8

Propagation in the “Real World”

a wave


can

be absorbed

reflect

penetrate

bend

9

The Cluttered World of Radio Waves

walls

hallways

windows

trees

vehicles

rain

hills

girders

10

Electromagnetic Spectrum



Propagation characteristics are different in each frequency band.


UV

1
MHz

1
kHz

1
GHz

1
THz

1
PHz

1
EHz

infrared

visible

X rays

Gamma rays

LF

HF

VHF

UHF

SHF

EHF

MF



902


928
Mhz

2.4


2.4835
Ghz

5.725


5.785
Ghz

ISM band



30
kHz

300
kHz

3
MHz

30
MHz

300
MHz

30
GHz

300
GHz

10
km

1
km

100
m

10
m

1
m

10
cm

1
cm

100
mm

3
GHz

11

Evaluating Frequencies


50
MHz
-
250
MHz
: Good for outdoor range, large
antenna size, bending and penetrating. No foliage
problems. “Sees” metallic building structures,
doesn’t pass through windows or down corridors.



450
MHz to
2
GHz
:
-

Good compromise for cellular
-
type systems. Small antenna, but big enough for
outdoor range. Minor foliage effects. OK for windows
walls and corridors.



5
-
20
GHz
: Antenna too small for outdoor range.
Foliage and rain effects. Indoor microcells, Point
-
to
-
point, and Satellites to ground stations.

12

Unlicensed Radio Spectrum

(ISM: Industrial, Science, Medicine)

902
Mhz

928
Mhz

26
Mhz

83.5
Mhz

125
Mhz

2.4
Ghz

2.4835
Ghz

5.725
Ghz

5.850
Ghz

cordless phones

baby monitors

WaveLan

802.11
b

Bluetooth

Microwave oven

802.11
a



33
cm

12
cm

5
cm

13

Free
-
space Path
-
loss


Power

of

wireless

transmission

reduces

with

square

of

distance

(due

to

surface

area

increase

of

sphere)
.


Reduction

also

depends

on

wavelength
:


Long wave length (low frequency) has less loss


Short wave length (high frequency) has more loss

2
4
.
.









d
L
P
14

Path
-
loss Models


Path
-
Loss Exponent Depends on environment:


L(d) = L(d
0
)(d/d
0
)
n







Free space




n =
2

Urban area cellular


n =
2.7
to
3.5

Shadowed urban cell


n =
3
to
5

In building LOS



n =
1.6
to
1.8

Obstructed in building


n =
4
to
6

Obstructed in factories


n =
2
to
3


15

Multi
-
path Propagation


Electromagnetic

waves

bounce

off

of

conductive

(metal)

objects
.


Reflected

waves

received

along

with

direct

wave
.


16

Multi
-
Path Effect


Multi
-
path

components

are

delayed

depending

on

path

length,

causes

delay

spread
.



Phase

shift

causes

frequency

dependent

constructive

/

destructive

interference
.

17

Multi
-
transmitter Interference


Similar to multi
-
path



Two

transmitting

stations

will

constructively/destructively

interfere

with

each

other

at

the

receiver
.



Receiver

will

“hear”

the

sum

of

the

two

signals,

which

usually

means

garbage
.

18

Modulation


Modulation

allows

the

wave

to

carry

information

by

adjusting

its

properties

(
Amplitude
,

Frequency
,

and

Phase
)

in

a

time

varying

way
.


Digital

modulation

using

discrete

“steps”

so

that

information

can

be

recovered

well

despite

noise/interference
.


8
VSB
-

US HDTV


BFSK
-

Mote Sensor Networks


QPSK
-

2
Mbps
802.11
& CMDA(IS
-
95
)

19

Symbol Rate & Bandwidth


Modulation

allows

transmission

of

one

of

several

possible

symbols

(two

or

more)
.



Data

stream

is

encoded

by

transmitting

several

symbols

in

succession
.



Symbol rate ≈ Bandwidth


Symbol Rate or Baud Rate (symbols/sec)


Bit Rate or Throughput (bits/sec)


Spectrum Usage or Bandwidth (Hz)



Inter
-
symbol

interference

(ISI)

occurs

unless

delay

spread

<<

symbol

time
.

20

Thermal Noise


Ever
-
present thermal noise in wireless
medium.



Sums with any wireless transmission.



Potentially causes errors in reception
(digital) or degradation of quality (analog).



Effectively

limits

transmission

range

when


transmitting

signal

strength

falls

below

a

threshold
.

21

Thermal Noise Calculation


Noise power depends on channel bandwidth:



Noise Power =
-
174
dBm/Hz +
10
log(BW in Hz)



So for
802.11


BW =
25
MHz for
802.11
b or
802.11
a channel



Thus noise power is about
-
100
dBm



-
100
dBm

=
10
-
10

mW

22

Physical Channel Properties Review


Received signal power
depends on:


Transmit power


Loss over distance (falls off by
d
n
)


Shadowing (e.g. absorption by walls)


Multi
-
path (e.g. bouncing off of metal objects)



Noise power
depends on:


Thermal noise


Environmental noise (e.g. microwave ovens)



Channel Quality


Related to
Signal to Noise Ratio

23

Current Wireless Technologies



Cellular Telephony (GSM, CDMA
2000
)




Fixed Wireless Access (WiMax)





Wireless Local Area Networks




Local Networks (Bluetooth, UWB)




Satellite Communications


24

Cellular Telephony (
1
)

25

Cellular Telephony (
2
)

26

Cellular Telephony (
3
)



Data is bursty, whereas voice is continuous.




3
G widens the data pipe

o

384
Kbps to few Mbps

o

Standard based of WCDMA

o

Packet
-
based switching for both voice and data




New generations

o

HSDPA, HSPA+, LTE

o

WiMAx added to
3
G




4
G systems start to come up

o

Mostly based on OFDM

27

Fixed Wireless Networks (WiMax)



Provide broadband wireless access to


homes/offices in a few Km range.




A potential replacement of ADSL




Provide high speed Internet and VOD

28

Wireless Local Area Networks (
1
)



WLANs connect local computers
(
100
m to a few Km range)




Breaks data into packets




Channel is shared (random access)




Backbone internet provides best
-
effort service




Poor performance in some applications (e.g. video)

29



802.11
b (oldest)

o

Standard for
2.4
GHz ISM band (
80
MHz BW)

o

Frequency hopped spread spectrum

o

1.6
-
10
Mbps, few hundred meter range



802.11
a (old)

o

Standard for
5.7
GHz NII band (
300
MHz BW)

o

OFDM with time division

o

20
-
70
Mbps, variable range



802.11
g (newer)

o

Standard for
2.4
GHz and
5.7
GHz bands

o

OFDM

o

Up to
54
Mbps, few hundred meter range



802.11
n (current)

o

U
p to
140
Mbps

o

Uses smart antenna technology

Wireless Local Area Networks (
2
)

30

Ultra Wide
-
Band



Also known as “Impulse Radio”.




Use very high bandwidth to decrease power level.




Hard synchronization




No license problem, but seems to interfere with GPS




Now mainly targeted to small distance applications.


(home networks to replace Bluetooth)




Smaller delay and longer range than Bluetooth




Emerging as competitor for
802.11

31

Satellite Communications



Cover very large areas.




Different orbit heights

o

GEO (
36000
Km), MEO and LEO (<
2000
Km)




Optimized for one
-
way transmission

o

Radio (DAB) and TV (DVB
-
S) broadcast




Two
-
way systems

o

Expensive alternative to terrestrial systems

32

Evolution of Current Technologies



Link:


Modulation, Coding, Adaptivity, Smart Antennas




Network:


Dynamic resource allocation, Mobility support




Hardware:


Better batteries, Circuits/Processors




Applications:


Soft and adaptive QoS (main issue in real
-
time


multi
-
media services)

33

Moving toward Real Multimedia

Voice

Data

Video

Delay

< 100
mSec

-

< 100
mSec

Packet loss

< 1%

0

<1%

BER

10
-
3

10
-
6

10
-
6

Data Rate

8
-
32Kbps

1
-
100 Mbps

1
-
20 Mbps

Traffic

Continuous

Bursty

Continuous

34

Design Challenges



Wireless channels are capacity
-
limited


broadcast communication medium.

o

Two main problems in wireless media:



Fading



Interference




Traffic patterns, user locations, and network


conditions are constantly changing.




Energy and delay constraints change design


principles across all layers of the protocol


stack.

35

Emerging Systems



Ad
-
hoc Wireless Networks




Wireless Sensor Networks




Distributed Control Networks




Cooperative Networks




Cognitive Radio Networks

36

Ad
-
hoc Wireless Networks (
1
)

37

Ad
-
hoc Wireless Networks (
2
)



Peer
-
to
-
peer communications



No backbone infrastructure



Multi
-
hop routing



Dynamic topology



Fully connected with different link SNRs

38

Ad
-
hoc Wireless Networks (
3
)



Ad
-
hoc Wireless Networks provide a flexible


network infrastructure for any emerging


application.




The capacity of such a network is generally


unknown.




Transmission, access, and routing strategies are


generally ad
-
hoc




Energy constraints impose interesting design


tradeoffs for communication and networking.

39

Wireless Sensor Networks (
1
)

40

Wireless Sensor Networks (
2
)



Energy is the driving constraint.



Nodes powered by non
-
rechargeable batteries.



Data flows to central location.



Low per
-
node rates but up to
100000
nodes.



Data highly correlated in transmission.



Nodes can cooperate in transmission, reception,


compression, and signal processing.

41

Wireless Sensor Networks (
3
)

(Energy
-
constrained nodes)




Short
-
range networks must consider


transmit, circuit, and processing energy.

o

Sleep modes save energy but complicate


networking.





Changes everything about the network


design:

o

Optimization of bit allocation across all protocols

o

Delay vs. throughput vs. node/network lifetime


tradeoffs

o

Optimization of nodes cooperation

o

Efficient MAC layer communication and Scheduling

42

Distributed Control (
1
)



Packet loss and/or delay impacts controller


performance.



Controller design should be robust to network faults.



Joint application and communication network delay.



Interesting ideas in packet
-
based communication


and file transfer.

43



There

is

no

methodology

to

incorporate

random



delays

or

packet

losses

into

control

system

design
.



The

best

rate/delay

tradeoff

for

a

communication



system

in

distributed

control

can

not

be

determined
.



Current

autonomous

vehicle

platoon

controllers

are



not

string

stable

with

any

communication

delay
.



What

are

the

best

routing

technologies?

Distributed Control (
2
)

44

Cooperative Networks (
1
)

45



Increase coverage area




Reduce number of “blind spots”




Reduce transmit power per node




Increase in transceiver complexity





More complex synchronization problems




More interference to be handled properly




Higher end
-
to
-
end delays




Additional delay to be handled in real
-
time


applications

Cooperative Networks (
2
)

46

Cognitive Radio (
1
)



Available spectrum looks scarce.




Measurements show the allocated spectrum is vastly


underutilized.

47

Cognitive Radio (
2
)

48

Cognitive Radio (
3
)



Sense, learn, and exploit the environment



One “simple” option:

o

Use “un
-
used” spectrum

o

Agile Radios

o

Give priority to ”primary” users



But not receiving a signal in wireless environment


does not mean that no signal is actually transmitted


at that frequency!



Even in simplest form is very challenging.



Many ideas such as:

o

Game Theory

o

Near
-
Noise Level Signal detection

o

BW transactions

o

Trust theories (how to identify users with “bad” intensions?)

49

Summary



The

wireless

vision

encompasses

many



exciting

systems

and

applications
.




Technical challenges go across all layers


of the system design.




Wireless systems today still have


limited performance and interoperability.

50