mobile computing and wireless networking lec 02 03/03/2010 ecom 6320
TRANSCRIPT
Mobile Computing and Wireless Networking
Lec 02
03/03/2010
ECOM 6320
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Outline
Characteristic of wireless channels Antennas and Signal Propagation Multiplexing
3
)2cos()2sin(2
1)(
11
nftbnftactgn
nn
n
1
0
1
0
t t
ideal periodical digital signal
decomposition
Fourier Transform: Every Signal Can be Decomposed as a Collection of Harmonics
- Two representations: - time domain; frequency domain
- Knowing one can recover the other
Time domain Frequency domain
Examples
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Try spectrum1.m and spectrum2.m
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Objective encode digital data into analog signals at the
right frequency range
Recap: Modulation
Basic schemes Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM)
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Modulation of digital signals known as Shift Keying Amplitude Shift Keying (ASK):
Frequency Shift Keying (FSK):
Phase Shift Keying (PSK):
1 0 1
t
1 0 1
t
1 0 1
t
Modulation
Example
Suppose fc = 1 GHz(fc1 = 1 GHz, fc0 = 900 MHzfor FSK)
Bit rate is 1 Mbps Encode one bit at a time Bit seq: 1 0 0 1 0
Q: How does the wave look like for each scheme?
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t
1 0 1
t
1 0 1
t
1 0 1
t
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BPSK (Binary Phase Shift Keying): bit value 0: sine wave bit value 1: inverted sine wave very simple PSK
Properties robust, used e.g. in satellite systems
Q
I01
Phase Shift Keying: BPSK
Q: What is the spectrum usage of BPSK?
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Spectral Density of BPSK
b
Rb =Bb = 1/Tbb
fc : freq. of carrier
fc
Spectral Density =
bit rate-------------------
width of spectrum used
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Phase Shift Keying: QPSK
11 10 00 01
Q
I
11
01
10
00
A
t
QPSK (Quadrature Phase Shift Keying): 2 bits coded as one symbol symbol determines shift of sine
wave often also transmission of relative,
not absolute phase shift: DQPSK - Differential QPSK
Antennas and Signal Propagation
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Isotropic radiator: a single point equal radiation in all directions (three dimensional) only a theoretical reference antenna
Radiation pattern: measurement of radiation around an antenna
zy
x
z
y x idealisotropicradiator
Antennas: Isotropic Radiator
Q: how does power level decrease as a function of d, the distancefrom the transmitter to the receiver?
Antennas: directed and sectorized
side view (xy-plane)
x
y
side view (yz-plane)
z
y
top view (xz-plane)
x
z
top view, 3 sector
x
z
top view, 6 sector
x
z
directedantenna
sectorizedantenna
Often used for microwave connections or base stations for mobile phones (e.g., radio coverage of a valley)
Signal propagation ranges
Transmission range communication possible low error rate
Detection range detection of the signal
possible no communication
possible
Interference range signal may not be
detected signal adds to the
background noise
distance
sender
transmission
detection
interference
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Free-Space Isotropic Signal Propagation
In free space, receiving power proportional to 1/d² (d = distance between transmitter and receiver)
Suppose transmitted signal is x,received signal y = h x, where h is proportional to 1/d²
2
4
dGG
P
Ptr
t
r
Pr: received power
Pt: transmitted power
Gr, Gt: receiver and transmitter antenna gain
(=c/f): wave length
Sometime we write path loss in log scale: Lp = 10 log(Pt) – 10log(Pr)
Free Space Signal Propagation
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1 0 1
t
1 0 1
t
1 0 1
t
at distance d
?
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Real Antennas
Real antennas are not isotropic radiators Some simple antennas: quarter wave /4 on car roofs or
half wave dipole /2 size of antenna proportional to wavelength for better transmission/receiving
/4/2
Q: Assume frequency 1 Ghz, = ?
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Dipole: Radiation Pattern of a Dipole
http://www.tpub.com/content/neets/14182/index.htmhttp://en.wikipedia.org/wiki/Dipole_antenna
Why Not Digital Signal (revisited) Not good for spectrum usage/sharing The wavelength can be extremely large
to build portal devices e.g., T = 1 us -> f=1/T = 1MHz ->
wavelength = 3x108/106 = 300m
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Receiving power additionally influenced by shadowing (e.g. through a wall or a door) refraction depending on the density of a medium reflection at large obstacles scattering at small obstacles diffraction at edges
reflection
scattering
diffraction
shadow fadingrefraction
Signal Propagation
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Signal Propagation: Scenarios
Details of signal propagation are very complicated
We want to understand the key characteristics that are important to our objective
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Shadowing
Signal strength loss after passing through obstacles
Some sample numbers
i.e. reduces to ¼ of signal10 log(1/4) = -6.02
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Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction
Multipath
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Example: reflection from the ground: received power decreases proportional to 1/d4 instead of 1/d² due to the destructive interference between the direct signal and the signal reflected from the ground
Multipath Can Reduce Signal Strength
ground
For detail, see page 9: http://www.eecs.berkeley.edu/~dtse/Chapters_PDF/Fundamentals_Wireless_Communication_chapter2.pdf
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signal at sender
Multipath Can Spread Delay
signal at receiver
LOS pulsemultipathpulses
LOS: Line Of Sight
Time dispersion: signal is dispersed over time
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signal at sender
Multipath Can Cause ISI
signal at receiver
LOS pulsemultipathpulses
LOS: Line Of Sight
dispersed signal can cause interference between “neighbor” symbols, Inter Symbol Interference (ISI)
Assume 300 meters delay spread, the arrival time difference is 300/3x108 = 1 msif symbol rate > 1 Ms/sec, we will have serious ISI
In practice, fractional ISI can already substantially increase loss rate
Multiplexing in 4 dimensions space (si) time (t) frequency (f) code (c)
Goal: multiple use of a shared medium
Important: guard spaces needed!
s2
s3
s1
Multiplexing
f
t
c
k2 k3 k4 k5 k6k1
f
t
c
f
t
c
channels ki
Frequency multiplex
Separation of the whole spectrum into smaller frequency bands
A channel gets a certain band of the spectrum for the whole time
Advantages no dynamic coordination
necessary works also for analog signals
Disadvantages waste of bandwidth
if the traffic is distributed unevenly
inflexible
k2 k3 k4 k5 k6k1
f
c
f
c
k2 k3 k4 k5 k6k1
Time multiplex
A channel gets the whole spectrum for a certain amount of time
Advantages only one carrier in the
medium at any time throughput high even
for many users
Disadvantages precise
synchronization necessary
f
Time and frequency multiplex Combination of both methods A channel gets a certain frequency band for a certain
amount of time Example: GSM
Advantages better protection against
tapping protection against frequency
selective interference but: precise coordination
required
t
c
k2 k3 k4 k5 k6k1
Code multiplex
Each channel has a unique code
All channels use the same spectrum at the same time
Advantages bandwidth efficient no coordination and synchronization
necessary good protection against interference
and tapping Disadvantages
varying user data rates more complex signal regeneration
Implemented using spread spectrum technology
k2 k3 k4 k5 k6k1
f
t
c