Wireless Sensor Networks

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21 Νοε 2013 (πριν από 3 χρόνια και 6 μήνες)

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1

Wireless Sensor Networks

Physical Layer

Mario
Č
agalj

mario.cagalj@fesb.hr


FESB

University of Split


2013.

Based on “Protocols and Architectures for Wireless Sensor Networks”,
Holger Karl
, 2005.

2

Goal of this lecture

o
Get an understanding of the peculiarities of wireless
communication

>
“Wireless channel” as abstraction of these properties


e.g.,
bit error patterns

>
Focus is on radio communication

o
Impact of different factors on communication
performance

>
Frequency band, transmission power, modulation scheme, etc.

>
Transciever design

o
Understanding of energy consumption for radio
communication

3

o
Which part of the electromagnetic spectrum is used for
communication


>
Not all frequencies are equally suitable for all tasks


e.g., wall
penetration, different atmospheric attenuation


o
VLF = Very Low Frequency


UHF = Ultra High Frequency

o
LF = Low Frequency


SHF = Super High Frequency

o
MF = Medium Frequency


EHF = Extra High Frequency


o
HF = High Frequency


UV = Ultraviolet Light

o
VHF = Very High Frequency

1 Mm

300
Hz

10 km

30 kHz

100 m

3 MHz

1 m

300 MHz

10 mm

30 GHz

100

m

3 THz

1

m

300 THz

visible
light

VLF

LF

MF

HF

VHF

UHF

SHF

EHF

infrared

UV

optical transmission

coax cable

twisted
pair

Radio spectrum for communication

4

o
Some frequencies are
allocated to specific uses

>
Cellular phones, analog
television/radio
broadcasting, DVB
-
T, radar,
emergency services, radio
astronomy, …

o
Particularly interesting:
ISM bands

(“Industrial,
scientific, medicine”)


license
-
free operation


Some typical ISM bands

Frequency

Comment

13,553
-
13,567 MHz

26,957


27,283 MHz

40,66


40,70 MHz

433


464 MHz

Europe

900


928 MHz

Americas

2,4


2,5 GHz

WLAN/WPAN

5,725


5,875 GHz

WLAN

24


24,25 GHz

Frequency allocation

5

http://www.ntia.doc.gov/osmhome/allochrt.pdf


Electromagnetic Spectrum

6

Transmitting data using radio waves

o
Produced by a resonating circuit (e.g., LC)

o
Transmitted through and antenna

o
Basics: Transmit
er

can send a radio wave, receive
r

can
detect whether such a wave is present and also its
parameters

o
Parameters of a wave
(e.g, a
sine function
)





s(t)=A(t) sin( 2
π
f(t)t +

(t)

)


>
Parameters: amplitude A(t), frequency f(t), phase

(t)


o
Manipulating these three parameters allows the sender to
express data; receiver reconstructs data from signal

o
Simplification: Receiver

sees” the same signal that the
sender generated


not true, see later!

7

Time and Frequency Domains


Different representations of the same signal.


Spectral representation obtained using
F
FT

(Fast Fourier Transform)

frequency

amplitude

A

f

A/3

3f

frequency

amplitude

A

f

A/3

3f

8

Signal Modulation (I)

o
How to manipulate a given signal parameter?

>
Set the parameter to an arbitrary value:
analog modulation

>
Choose parameter values from a finite set of legal values:
digital keying


o
Modulation?


>
Data to be transmitted is used
to
select transmission
parameters as a function of time

>
These parameters modify a basic sine wave, which serves as a
starting point for modulating

the signal onto it

>
This basic sine wave has a
center frequency f
c

>
The resulting signal

requires a certain
bandwidth

to be
transmitted (centered around center frequency)

9

Signal Modulation (I
I
)

o
Use data to modify the
amplitude

of a carrier
frequency
-

Amplitude Shift
Keying

(ASK)





o
Use data to modify the
frequency

of a carrier
frequency
-

Frequency Shift
Keying
(FSK)




o
Use data to modify the
phase

of a carrier frequency
-

Phase
Shift Keying

(PSK)


© Tanenbaum, Computer Networks

10

Signal Modulation (II
I
)

o
Quadrature PSK (QPSK): Two bits

>
00


A
sin
(2πft

+

7
π
/4
)

>
01


A
sin
(2πft

+

5
π
/4
)

>
10


A
sin
(2πft

+

3
π
/4
)

>
11


A
sin
(2πft

+

π
/
4)



o
Quadrature Amplitude and Phase Modulation (QAM)

QAM
-
4, QAM
-
16, QAM
-
64, QAM
-
256

>
s(t) = I(t) cos( 2
π
f
c
t
)
-

Q(t) sin( 2
π
f
c
t
)

I(t) and Q(t) are the modulating signals (analog modulation)

>
I(t) > “in
-
phase” componenet, Q(t) > “quadrature” component

>
s(t) is a linear combination of two orthogonal signal waveforms

>
Received signal (ideal case) I(t) component is demodulated as

r(t) = s(t) cos( 2
π
f
c
t
) = ½ I(t) + ½ [I(t) cos(
4
π
f
c
t
) + Q(t) sin(
4
π
f
c
t
)]

>
By filtering a low
-
pass filter we can recover the I(t) and the Q(t) terms

11

Signal Modulation (
IV
)

o
Quadrature PSK (QPSK): Two bits

>
00


A
sin
(2πft

+

7
π
/4
)

>
01


A
sin
(2πft

+

5
π
/4
)

>
10


A
sin
(2πft

+

3
π
/4
)

>
11


A
sin
(2πft

+

π
/
4)



o
Quadrature Amplitude and Phase Modulation (QAM)

QAM
-
4, QAM
-
16, QAM
-
64, QAM
-
256

Q

I

01

11

00

10

Q

I

QAM
-
4

QAM
-
16

Q

I

0

1

Binary

12

Signal Modulation (
examples
)

Carrier

Modulating

data

Modulating

data

11

10

01

00

101

110

011

000

Resulting

signal QPSK

Resulting

signal

QAM
-
8

13

Bit rate vs
.

Baud rate

o
Bit rate = bits/second

o
Baud (
S
ymbol) rate = Symbols/second

o
Binary

PSK,

1 symbol encodes 1 bit

o
QAM
-
4, 1 symbol encodes 2 bits

o
QAM
-
16
,

1 symbol encodes 4 bits

Q

I

01

11

00

10

Q

QAM
-
4

QAM
-
16

Q

I

0

1

Binary

14

Receiver: Demodulation

o
The receiver looks at the received wave form and matches it
with the data bit that caused the transmitter to generate
this wave form

>
Necessary: one
-
to
-
one mapping between data and wave form

>
Because of channel imperfections, this is at best possible for digital
signals, but not for analog signals


o
Problems caused by

>
Carrier synchronization: frequency can vary between sender and
receiver (drift, temperature changes, aging, …)

>
Bit synchronization (actually: symbol synchronization): When does
symbol representing a certain bit start/end?

>
Frame synchronization: When does a packet start/end?

>
Biggest problem: Received signal is
not

the transmitted signal!

15

Antenna (I)

o
A resonating circuit (e.g., LC) connected to an antenna causes an
antenna to emit EM

(electromagnetic)

waves

o
A receiving antenna converts the EM waves into electrical current

o
Many types of antennas with different
gains

(G)

Gain: 10
-
55dB

Isotropic

D
irectional

O
mnidirectional

Gain: 2dB

16

dB, dBm, dBi, ...

dBm


=

dB

value

of

Power

/

1

m
W
att



Used

to

describe

signal

strength
.

dBW

=

dB

value

of

Power

/

1

W
att


Used

to

describe

signal

strength
.

dBi


=

dB

value

of

antenna

gain

relative

to

0
dBi

is

by

default

the

gain

of

an




the

gain

of

an

isotropic

antenna

isotropic

antenna







A

linear

number

is

converted

into

dB,

using

the

following

formula
:


X
(dB)

=

10
log
10
(
X
)


X
(dBm)

=

10
log
10
(
X
/
1
mW)


E
.
g
.

1
W

=

0
dBW

=

+
30
dBm

17

Antenna (II): Gain vs
.

Beamwidth

©
Constantine A. Balanis
,

Antenna Theory: Analysis and Design, 3rd Edition

o
Antenna radiation pattern

>
Beamwidth

of a pattern is the angular separation between two identical points on
opposite side of the pattern maximum












o
FNBW >
First Null BeamWidth

o
HPBW >
Half
-
Power BeamWidth


>
The power reduced by half or 3dB of its maximum
-
> 3dB beamwidth

18

Antenna (II
I
): Gain vs
.

Beamwidth

o
Gain

of an antenna:

>
The ratio of the intensity, in a given direction, to the radiation intensity that
would be obtained if the power accepted by antenna were radiated isotropically.












source)

isotropic

the

of

intensity

(radiation

4
P
U
source)

isotropic

(lossless
P
U
4

power
(accepted)
input

total
intensity

radiation
4
Gain
input
0
input
π
π
π



©D. Adamy, A First Course on Electronic Warfare

19

Transmitted signal <> received signal!

o
Wireless transmission
distorts

any transmitted signal

>
Received <> transmitted signal; results in uncertainty at receiver

about which bit sequence originally caused the transmitted signal

>
Abstraction:
Wireless channel

describes these distortion effects

o
Sources of distortion

>
Attenuation


energy is distributed to larger areas with increasing
distance

>
Reflection/refraction


bounce of a surface; enter material

>
Diffraction


start “new wave” from a sharp edge

>
Scattering


multiple reflections at rough surfaces

>
Doppler fading


shift in frequencies (loss of center)


20

Signal Propagation: Diffraction, Reflection,
Scattering

o
Reflection:

W
hen the surface is large relative to the wavelength of signal (λ =
c/f), c

=

speed of light

>
May cause phase shift from original / cancel out original or increase it

o
Diffraction
:

W
hen

the

signal

hits

the

edge

of

an

impenetrable

body

that

is

large

relative

to

the

wavelength

λ

>
Enables the reception of the signal even if Non
-
Line
-
of
-
Sight (NLOS)

o
Scattering:

obstacle size is in the order of λ. (e.g., a lamp post)









o
In LOS

(Line
-
of
-
Sight)

diffracted and scattered signals not significant
compared to the direct signal, but reflected signals can be (multipath effects)

o
In NLOS, diffraction and scattering are primary means of reception

Reflection

Scattering

Diffraction

21

Doppler
s
hift


o
If the transmitter and/or receiver are mobile, the frequency of
the received signal changes

>
When they are moving closer, the frequency increases

>
When they are moving away, the frequency decreases

Frequency difference = velocity/
w
avelength

o
Example:

λ
(
2.4 GHz) = 3x10
8
/2.4x10
9

= 0.125m

o
120km/hr = 33.3 m/s

o
Freq
.

diff = 33.3/.125 = 267 Hz

22

Signal Propagation (
attenuation and path loss
)

o
Effect of attenuation: received signal strength is a function
of the distance
R

between sender and
receiver

o
Captured by
Friis equation

(a simplified form)





>
Gr and Gt are antenna gains for the receiver and transmiter


>

λ

is the wavelength and
α

is a
path
-
loss exponent

(2
-

5)

>
Attenuation depends on frequencies, for free
-
space
α
=2

o
Path loss

(PL)










R
4
G
G
P
P
r
t
tx
rx
π
λ












R
4
G
G
dB
P
dB
P
P
P
PL(dB)
r
t
rx
tx
rx
tx
π
λ
log
10
)
(
)
(
log
10
23

Suitability of different frequencies


Attenuation

o
Attenuation depends on the used frequency


o
Can result in a
frequency
-
selective channel

>
If bandwidth spans frequency ranges with different attenuation
properties


© http://141.84.50.121/iggf/Multimedia/Klimatologie/physik_arbeit.htm

24

Signal Propagation (Strength)

©D. Adamy, A First Course on Electronic Warfare

XMTR

RCVR

Path through link

Signal Strength (dBm)

Transmitted
Power

Antenna Gain

Antenna Gain

Received
Power

LINK LOSSES

Spreading and
Atmospheric
Loss

To calculate the received signal level (in dBm), add the transmitting antenna

gain (in dB), subtract the link losses (in dB), and add the receiving antenna

gain (dB) to the transmitter power (in dBm).

25

Receiver sensitivity

o
The smallest signal (the lowest signal strength) that a receiver can
receive and still provide the proper specified output.


o
Example:

>
Transmitter Power (1W) = +30dBm

>
Transmitting Antenna Gain = +10dB

>
Spreading Loss = 100dB

>
Atmospheric Loss = 2dB

>
Receiving Antenna Gain = +3dB


Receiver Power

(dBm)

= +30dBm
+

10dB


100dB


2dB + 3dB =
-
59dBm


R
eceiver

1

sensitivity is
-
6
2
dBm

and the receiver 2 is
-
65dBm >
receiver

1 and 2

will
receive the signal as
if
there is still
3
dBm
and 6dBm
of

margin on the link
,
respectively.


Recv 2 is 3dB (a factor of two) better than recv 1; recv 2 can hear signals that are
half the strength of those heard by recv1.

26

Distortion effects: Non
-
line
-
of
-
sight paths

o
Because of reflection, scattering, …, radio communication is
not limited to direct line of sight communication

>
Effects depend strongly on frequency, thus different behavior at
higher frequencies








o
Different paths have different lengths = propagation time

>
Results in
delay spread

of the wireless channel

>
Closely related to frequency
-
selective fading properties of the
channel

>
With movement:
fast fading


Line
-
of
-

sight path

Non
-
line
-
of
-
sight path

S
ignal at receiver

LOS pulses

M
ultipath

pulses

© Jochen Schiller, FU Berlin

27

Wireless signal strength in a multi
-
path
environment

o
Brighter color = stronger signal

o
Obviously, simple (quadratic)
free space attenuation formula
is not sufficient to capture
these effects

© Jochen Schiller, FU Berlin

28

Generalizing the attenuation formula

o
To take into account stronger attenuation than only caused by
distance (e.g., walls, …), use a larger
path
-
loss
exponent
α

> 2





>
Rewrite in logarithmic form (in dB):




o
Take obstacles into account by a random variation

>
Add a Gaussian random variable with 0 mean, variance

2

to dB representation

>
Equivalent to multiplying with a lognormal distributed
random variable

in metric
units

>

lognormal fading


α








R
R
)
(R
P
(R)
P
0
0
recv
recv










0
0
R
R
)[dB]
PL(R
PL(R)[dB]
log
α
10
[dB]
X
R
R
)[dB]
PL(R
PL(R)[dB]
0
0












log
α
10
(R
0
is a referent distance)

29

From waves to bits: symbols and bit errors

o
Extracting symbols out of a distorted/corrupted wave form
is
filled

with errors

>
Depends essentially on strength of the received signal compared to
the corruption

>
Captured by
signal to noise and interference ratio (SINR)




o
SINR allows to compute
bit error rate
(
BER
)

for a given
modulation

>
Also depends on data rate
R
(# bits/symbol) of modulation

>
E.g., for simple DPSK















K
i
1
i
0
recv
I
N
P
10log
SINR
R
1
SINR
N
E

where

,
e
2
1
BER(SINR)
0
b
N
E
0
b




30

Examples for SINR

to

BER mappings

1e-07
1e-06
1e-05
0.0001
0.001
0.01
0.1
1
-10
-5
0
5
10
15
Coherently Detected BPSK
Coherently Detected BFSK
BER

SINR

31

WSN
-
specific channel models

o
Typical WSN properties

>
Small transmission range

>
Implies small delay spread (nanoseconds, compared to
micro/milliseconds for symbol duration)

>
Frequency
-
non
-
selective fading, low to negligible inter
-
symbol
interference


o
Some example

measurements

>

α

-

path loss exponent

>
Shadowing variance

2


>
Reference path

loss at 1 m


Average

α

32

Transceiver design

o
Strive for good power efficiency at low transmission power

>
Some amplifiers are optimized for efficiency at high output
power

>
To radiate 1 mW, typical designs need 30
-
100 mW to operate
the transmitter


WSN nodes: 20 mW (mica motes)

>
Receiver can use as much or more power as transmitter at
these power levels


Sleep state is important

o
Startup energy/time penalty can be high

>
Examples take 0.5 ms and ¼ 60 mW to wake up

o
Exploit communication/computation tradeoffs

>
Might payoff to invest in rather complicated
coding/compression schemes

33

Transceiver

design

o
One exemplary design point: which modulation to use?

>
Consider: required data rate, available symbol rate, implementation
complexity, required BER, channel characteristics, …

>
Tradeoffs: the faster one sends, the longer one can sleep


Power consumption can depend on modulation scheme

>
Tradeoffs: symbol rate (high?) versus data rate (low)


Use m
-
ary transmission to get a transmission over with ASAP


But: startup costs can easily void any time saving effects


o
Adapt modulation choice to operation conditions

>
Similar

to dynamic voltage scaling

(DVS) introduced in the last lecture
,
introduce
Dynamic Modulation Scaling

>
When there are no packets present, a small value for
m

(bits per
symbol)

can be used, having low

energy consumption. As backlog
increases,
m
is increased as well to reduce the backlog quickly

and
switch back to lower values of
m
.

34

Summary

o
Wireless radio communication introduces many
uncertainties into a communication system


o
Handling the unavoidable errors will be a major
challenge for the communication protocols


o
Dealing with limited bandwidth in an energy
-
efficient
manner is the main challenge