Wireless Sensor Networks Physical Layer Based on “Protocols and Architectures for Wireless Sensor...
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Wireless Sensor Networks Physical Layer Based on “Protocols and Architectures for Wireless Sensor Networks”, Holger Karl, 2005. Mario Čagalj [email protected]
Wireless Sensor Networks Physical Layer Based on Protocols and
Architectures for Wireless Sensor Networks, Holger Karl, 2005.
Mario agalj [email protected] FESB, 9/4/2014.
Slide 2
2 Introduction: OSI model oThe Open Systems Interconnection
model (OSI) is a conceptual model that characterizes and
standardizes the internal functions of a communication system by
partitioning it into abstraction layers. - Wikipedia
Slide 3
3 Goal of this lecture oGet an understanding of wireless
communication >Wireless channel as abstraction of these
properties >Focus is on radio communication oImpact of different
factors on communication >Frequency band, transmission power,
modulation scheme, etc. >Transciever design oUnderstanding of
energy consumption for radio communication
Slide 4
VLF = Very Low FrequencyUHF = Ultra High Frequency LF = Low
Frequency SHF = Super High Frequency MF = Medium Frequency EHF =
Extra High Frequency HF = High Frequency UV = Ultraviolet Light 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 VLFLFMFHF
VHFUHFSHFEHFinfraredUV optical transmission coax cable twisted pair
Radio spectrum 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
4
Slide 5
Frequency allocation 5 Some frequencies are allocated to
specific uses Cellular phones, analog television/radio
broadcasting, DVB-T, radar, emergency services, radio astronomy,
Particularly interesting: ISM (Industrial, Scientific, Medical)
frequency bands License-free operation Overcrowding leads to
cognitive radio systems Some typical ISM bands FrequencyComment
13,553-13,567 MHzRFID smart cards 26,957 27,283 MHz 40,66 40,70 MHz
433,05 434,79 MHzEurope 902 928 MHzAmericas 2,4 2,5 GHzWLAN/WPAN
microwave owen 5,725 5,875 GHzWLAN 24 24,25 GHz
Binary PSK (BPSK): 1 bit per symbol T b bit duration, f c
carrier frequency (f c >> 1/ T b ) E b transmitted signal
energy per bit (i.e., ) Signal space representation This implies
that the in-phase component is given as and therefore and Digital
modulation - example 19 binary 0 represented by binary 1
represented by Q (quadrature) I (in-phase) bit 0 bit 1 symbol
amplitude
Slide 20
Binary PSK (BPSK) Esentially an amplitude modulation with a
square wave Digital modulation - example 20 Digital Phase
Modulation: A Review of Basic Concepts by James E. Gilley, 2003
10110100
Slide 21
Binary PSK (BPSK) spectrum and pulse shaping Digital modulation
- example 21 Digital Phase Modulation: A Review of Basic Concepts
by James E. Gilley, 2003
Slide 22
Demodulation of BPSK signal 22 Let r(t) be the received signal
in a noise-free scenario The demodulation process Guess signal s 1
(t) (or binary 1) was transmitted if Guess signal s 0 (t) (or
binary 0) was transmitted if http://www.gaussianwaves.com
Slide 23
The simplest channel model - Additive White Gaussian Noise
(AWGN) channel Transmission corrupted by noise 23 databaseband
digital modulator passband channel Noise detector digital
demodulator bandpass filter
Slide 24
Let r(t) be the received signal in a AWGN scenario The signal s
i (t) is corrupted by zero-mean Gaussian noise n(t) with variance N
0 /2 (the noise spectral power density), i.e., r(t) = s i (t) +
n(t) The output of the correlation receiver Here n I is a
projection of n(t) onto the in-phase axis (also Gaussian and zero
mean with with variance N 0 /2) Demodulation of BPSK signal in AWGN
24 http://www.gaussianwaves.com
Slide 25
Demodulation of BPSK signal in AWGN 25 We assume that bits 0
and 1 are equally likely http://www.gaussianwaves.com
Slide 26
Demodulation of BPSK signal in AWGN 26 We assume that bits 0
and 1 are equally likely Finally, the BPSK bit error rate (BER) is
given by http://www.gaussianwaves.com
Slide 27
27 BER (bit-error rate) Bit error rate (BER) E b /N 0 [dB] bit
energy to noise density ratio [SNR/bit]
Slide 28
Quadrature PSK (QPSK): 2 bits per symbol Digital multi-level
modulation - example 28 binary 00 represented by binary 01
represented by binary 10 represented by binary 11 represented
by
Slide 29
Quadrature PSK (QPSK): 2 bits per symbol Using the identity
cos(a+b)=cos(a)cos(b)-sin(a)sin(b), we can rewrite the QPSK symbols
as follows where Digital multi-level modulation - example 29 Q
(quadrature) I (in-phase) 01 11 00 10
Slide 30
The same bit error rate as BPSK But more bits per symbol QPSK Q
(quadrature) I (in-phase) 01 11 0010 30 Quadrature In-phase
01101001 0 1 01 10 10 01
Slide 31
Quadrature Amplitude and Phase Modulation (QAM) QAM-4, QAM-16,
QAM-64, QAM-256 On one hand, we increase the the data rate On the
other hand, denser constellations imply higher bit error rates Q I
01 11 00 10 Q I QAM-4 (QPSK) QAM-16 Q I 0 1 BPSK Digital
multi-level modulations 31
Slide 32
Bit rate = bits/second Baud (symbol) rate = symbols/second
BPSK, 1 symbol encodes 1 bit QPSK (QAM-4), 1 symbol encodes 2 bits
QAM-16, 1 symbol encodes 4 bits Bit rate vs. baud rate 32 Q I 01 11
00 10 Q I QAM-4 (QPSK) QAM-16 Q I 0 1 BPSK
Slide 33
33 A resonating circuit (e.g., LC) connected to an antenna
causes an antenna to emit EM waves (modulated signals) A receiving
antenna converts the EM waves into electrical current Many types of
antennas with different gains (G) Gain: 10-55dB Isotropic
Directional Omnidirectional Gain: 2dB Antenna 33
Slide 34
dBm = dB value of Power / 1 mWatt Used to describe signal
strength. dBW = dB value of Power / 1 Watt dBi = dB value of
antenna gain relative to (0dBi is by default the gain of an the
gain of an isotropic antenna isotropic antenna) The ratio of a
quantity Q 1 to another comparable quantity Q 0 : Thus: and For
example: 1W = +30dBm, 100mW = +20dBm Power and gain quantities
34
Slide 35
Antenna: Gain vs. Beamwidth (1/2) Antenna radiation pattern
Reciprocity theorem: the transmitting and receiving patterns of an
antenna are identical at a given wavelength Gain is a measure of
how much of the input power is concentrated (radiated) in a
particular direction (relative to the isotropic antenna with the
same input power, e.g., 20dBi means 100 times more) Beamwidth of a
pattern is the angular separation between two identical points on
opposite side of the pattern maximum 35
Slide 36
Antenna: Gain vs. Beamwidth (2/2) 36 Power density P D = P in
/4R 2, where P in is the input/radiated power (no losses)
http://www.kyes.com/antenna/navy/basics/antennas.htm When the angle
in which the radiation is constrained is reduced, the gain goes up
in that direction.
Slide 37
Signal propagation Wireless transmission distorts a transmitted
signal Results in uncertainty at receiver about which bit sequence
originally caused the transmitted signal Abstraction: Wireless
channel describes these distortion effects 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) 37
Slide 38
Effect of attenuation: received signal strength is a function
of the distance R between sender and receiver Captured by Friis
equation (a simplified form) G r and G t are antenna gains for the
receiver and transmiter is the wavelength and is a path-loss
exponent (2 - 5) Attenuation depends on the enviroment, for
free-space =2 Path loss (PL) Attenuation and path loss 38
Slide 39
39 WSN-specific channel models oTypical WSN properties
>Small transmission range oSome example measurements > - path
loss exponent >Shadowing variance 2 >Reference path loss at 1
m Average
Slide 40
Signal Propagation (Strength) 40 D. Adamy, A First Course on
Electronic Warfare XMTR RCVR Path through link Signal Strength
(dBm) Transmitted Power 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).
Slide 41
Receiver sensitivity The smallest signal (the lowest signal
strength) that a receiver can receive and still provide the proper
specified output. 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 41 Receiver 1
sensitivity is -62dBm and the receiver 2 is -65dBm: receiver 1 and
2 will receive the signal as if there is still 3dBm 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.
Slide 42
Wireless signal in a real environments Brighter color =
stronger signal Obviously, simple (quadratic) free space
attenuation formula is not sufficient to capture these effects 42
Jochen Schiller, FU Berlin
Slide 43
Generalizing the attenuation formula To take into account
stronger attenuation than only caused by distance (e.g., walls, ),
use a larger path-loss exponent > 2 Rewrite in logarithmic form
(in dB): Take obstacles into account by a random variation Add a
Gaussian random variable with 0 mean and variance 2 to dB
representation Equivalent to multiplying with a lognormal random
variable in metric units: lognormal fading 43 (R 0 is a referent
distance)
Reflection, diffraction and scattering Reflection: when the
surface is large relative to the wavelength of signal May cause
phase shift from original / cancel out original or increase it
Diffraction: when the signal hits the edge of an impenetrable body
that is large relative to the wavelength Enables the reception of
the signal even if Non-Line-of-Sight (NLOS) Scattering: obstacle
size is in the order of Doppler shift Signal propagation In LoS
(Line-of-Sight) diffracted and scattered signals not significant
compared to the direct signal, but reflected signals can be
(multipath effects) In NLoS, diffraction and scattering are primary
means of reception 45 Reflection Scattering Diffraction
Slide 46
Reflections and multipath fading Multiple copies of a radio
signal take different paths to the receiver The effects of
multipath include constructive and destructive interference, and
phase shifting of the signal at the receiver Destructive
interference causes signal fading 46 Reflection
Slide 47
Signal-to-Noise ratio (SNR) per bit (E b /N 0 ) E b - energy
per bit, E s - energy per symbol N 0 - noise power spectral density
S (i.e., P rx ) - received signal power N - received noise power B
- receivers bandwidth Ts - symbol duration R s - baud rate, R b -
bit rate, r=R b /R s 47 BER (bit-error rate) E b /N 0 [dB]
Slide 48
48 Summary oWireless radio communication introduces many
uncertainties into a communication system oHandling the unavoidable
errors will be a major challenge for the communication protocols
oDealing with limited bandwidth in an energy-efficient manner is
the main challenge
