Wireless communication channel

swimlogisticsElectronics - Devices

Nov 26, 2013 (3 years and 8 months ago)

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Wireless communication channel


Signal degradation can be classified by type :


Path Loss



happen during distance covered by the radio signal, it is called “Free
space


path loss “, it can be calculated by


LFS = 32.44 + 20 log F (MHz) +20 log d (Km)



Signal attenuation



Resulting from shadowing effects introduced by the obstacles
between transmitter and receiver



Fading of the signal




Caused by numerous effects all of which are related to the Radio


propagation phenomenon


Effects on Radio Communication

Wireless Multipath Channel

One of the most problem in communication channel is

fading

Fading Problems

1.
Shadowing (Normal fading):


The

reason

for

shadowing

is

the

presence

of

obstacles

like

large

hills

or

buildings

in

the

path

between

the

site

and

the

mobile
.




The signal strength received fluctuates around a
mean value while changing the mobile position
resulting in undesirable beats in the speech
signal.


Fading Problems

2.

Raleigh Fading (Multi
-
path Fading):


The received signal is coming from different
paths due to a series of reflection on many
obstacles. The difference in paths leads to a
difference in paths of the received components.



Parameters of multi
-
path channel

Time Domain Frequency

Domain

1
-

Max delay spread:

2
-

Coherence BW:





3
-

Coherence Time :

4
-

Doppler Shift:




𝜏



𝑇





𝐵

=
1
5
𝜏


𝑇

=

9
16
𝜋


2

𝐵


Doppler Shift


Phase change due to path length difference





Doppler shift (apparent change in freq.)



𝜑
=
2
𝜋

𝐿
𝜆
=
2
𝜋𝜈
Δ

𝜆
𝑜 𝜃



=
1
2
𝜋
.
Δ
𝜑
Δ

=
𝜈
𝜆
𝑜 𝜃






X

Y

S

𝜃

𝜃

d

Δ
𝐿

𝜈

Types of fading

𝜏


𝜏


At High Data Rate


High data rate transmission


short symbol time compared to the delay spread.





𝑇
𝑦𝑜
<

𝑇
 𝑦

𝑇
𝐷

𝑇


= Delay spread

= Symbol period

Problems


ISI

1
-

2
-

𝐵


𝐵


= signal BW

= coherence BW

Orthogonal frequency division multiplexing

(OFDM)




OFDM

was

introduced

in

1950

but

was

only

completed

in

1960
’s

Originally

grew

from

Multi
-
Carrier

Modulation

used

in

High

Frequency

military

radio
.




Patent

was

granted

in

1970
’s




Earlier

OFDM

wasn’t

popular

Large

arrays

of

sinusoidal

generators

and

coherence

demodulator

Too

expensive

and

complex
.




Later

when

DFT

and

IDFT

became

a

known

solution

to

the

arrays

of

generators

and

demodulators
.




It

was

still

not

popular

as

there

is

no

efficient

method

to

perform

the

IFFT

and

FFT

operation
.




Advances

in

VLSI

technology

allows

implementation

of

fast

and

cheap

FFT

and

IFFT

operation

drive

OFDM

popularity
.

OFDM

Orthogonal
Frequency Division Multiplexing


Frequency Division Multiplexing

-
Divide the information over several carriers

Instead of using one big truck


When
The

truck is lost…

All is lost!

Use several small trucks


When
one

truck is lost…

Only a portion of the shipment is lost!

Concept of an OFDM signal

Ch.1

Ch.2

Ch.
3

Ch.4

Ch.
5

Ch.
6

Ch.7

Ch.8

Ch.9

Ch.10

Saving of bandwidth

Ch.3

Ch.
5

Ch.
7

Ch.9

Ch.2

Ch.4

Ch.6

Ch.
8

Ch.10

Ch.
1

Conventional multicarrier techniques

Orthogonal multicarrier techniques

50% bandwidth saving

frequency

frequency


OFDM changes Frequency Selective Fading to Flat Fading
Channel

𝑇


.

.

.

𝑁𝑇


.

.

.

N number of subcarrier

Solution to Frequency Selective Fading

When the data rate is lower

𝐵


𝐵


= Delay spread

= Symbol period

= signal BW

= coherence BW

Frequency Selective => Flat Fading



In flat fading, the amplitude varies but there is no ISI

Multicarrier Modulation


Divide broadband channel into narrowband subchannels


No ISI in

subchannels

if constant gain in every
subchannel

and if ideal
sampling


Orthogonal Frequency Division Multiplexing


Based on the fast Fourier transform


Standardized for DAB, DVB
-
T, IEEE
802.11
a,
802.16
a,
HyperLAN

II


Considered for fourth
-
generation mobile communication systems

subchannel

frequency

magnitude

subcarrier

channel

Subchannels are
312
kHz wide in
802.11
a and HyperLAN II

Carrier
1
has a maximum

where

all other
carriers
are
0

OFDM

Frequency Spectrum

s
k
T
k
f
f
1
0


8192
4096
232

Hz

5
.
4312
or
N
s
T
f
s
s





Use many carriers that are
equally

spaced:

k =
0
,
1
, … , N
-
1

T
s

= Symbol Time

1

2

3

frequency

s
T
f
1


4

5

ISI

OFDM


Many carriers with small spacing => Long symbol time

But many carriers carry a lot of information!


Long symbol time is an
advantage!


Delay Spread (Multipath)


Symbol n
-
1

Symbol n

Symbol n+
1

Direct Path

Delayed Path

ISI = Inter Symbol Interference

ISI

Symbol n
-
1

Symbol n

Symbol n+
1

OFDM


Avoid ISI and preserve Orthogonality => Guard Interval


Direct Path

Delayed Path

Integration

Period

Symbol n
-
1

Guard

Symbol n

Guard

Symbol n

Guard

Symbol n+
1

Guard

Symbol n
-
1

Guard

Symbol n

Guard

Symbol n+
1

Guard

s




ol Time
Total Symb
s
Guard
ol Time
Total Symb

Guard
μs
e
Symbol Tim


290
58
232

58
4
232







Useful Symbol length

Guard

Total Symbol length

Symbol n is added constructively
or destructively according to
phase

Avoid ICI and preserve Orthogonality
=
>

cyclic prefix

N

samples

v

samples

CP: Cyclic Prefix

CP

CP

s y m b o l

i

s y m b o l

(
i
+
1
)

copy

copy

Discrete

versus
Fast

Fourier Transform


Discrete

(DFT):



For each frequency sample ‘k’ (
0
to N
-
1
) loop ‘n’ (over
0
to N
-
1
) => N
2

complex multiplications


Fast

(FFT, Cooley
-
Tukey

algorithm):


“An efficient algorithm to calculate a DFT”


N.log(N) complex multiplications




k
n
N
j
N
n
n
k
e
x
X
.
.
2
1
0






DFT

respect to
with
0,3%

4096
12

:
FFT

152
49
12
4096
Fast
tions
multiplica

216
777
16
4096
*
4096

Discrete
4096

:
Example






.


*

.
.




N
OFDM Block Diagram




High

spectral

efficiency
.

And

high

data

rate
.




Efficient

in

multipath

environments
.




Simple

digital

realization

by

using

the

FFT

operation
.





Low

complex

receivers

due

to

avoidance

of

ISI
.



Different

modulation

schemes

can

be

used

on

individual

sub
-
carriers
.


Main advantages

x Drawbacks


Large

Peak

to

Average

Ratio

(PAR)
.



Added

sinusoid

cause

large

PAR

and

issue

of

amplifier

non
-
linearity

arises
.



Accurate

frequency

and

time

synchronization

is

required
.



More

sensitive

to

Doppler

spreads

than

single
-
carrier

schemes
.



Sensitive

to

frequency

offset

and

phase

noise

caused

by

imperfections

in

the

transmitter

and

the

receiver

oscillators
.



Guard

interval

causes

loss

in

spectral

efficiency