Transcript Physical

Physical Layer
Signal
• Signal parameters of periodic signals:
–
–
–
–
period T, frequency f=1/T
amplitude A
phase shift 
sine wave as special periodic signal for a carrier:
1
s(t) = At sin(2  ft t + t)
0
t
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Why Need A Wide Spectrum:
Shannon Channel Capacity
• The maximum number of bits that can be
transmitted per second by a physical
channel is:
W log 2 (1  )
S
N
where W is the frequency range that the
media allows to pass through, S/N is the
signal noise ratio
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Signal Propagation
•
•
•
•
•
•
•
Path loss
Shadowing
Reflection at large obstacles
Refraction depending on the density of a medium
Scattering at small obstacles
Diffraction at edges
Fading (depending on the frequency)
shadowing
reflection
refraction
scattering
diffraction
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Multiplexing
• Multiplexing in 4 dimensions
– space (si)
– time (t)
– frequency (f)
– code (c)
• Goal: multiple use of a shared medium
• Important: guard spaces needed!
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Outline (Chapter 2.6 & 2.7)
Signal
Frequency allocation
Signal propagation
Antennas
Multiplexing
Modulation
Spread spectrum
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Overview of Wireless Transmissions
sender
analog
signal
bit
stream
source coding
channel coding
modulation
receiver
bit
stream
source decoding
channel decoding
demodulation
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Modulation I
• Digital modulation
– digital data is translated into an analog signal
(baseband)
– differences in spectral efficiency, power efficiency,
robustness
• Analog modulation
– shifts center frequency of baseband signal up to
the radio carrier
– Reasons
• Antenna size is on the order of signal’s wavelength
• More bandwidth available at higher carrier frequency
• Medium characteristics: path loss, shadowing, reflection,
scattering, diffraction depend on the signal’s wavelength
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Modulation and Demodulation
digital
data
101101001
digital
modulation
analog
baseband
signal
analog
modulation
radio transmitter
radio
carrier
analog
demodulation
analog
baseband
signal
synchronization
decision
digital
data
101101001
radio receiver
radio
carrier
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Modulation Schemes
• Amplitude Modulation (AM)
• Frequency Modulation (FM)
• Phase Modulation (PM)
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Digital Modulation
• Modulation of digital signals known as Shift Keying
• Amplitude Shift Keying (ASK):
– Pros: simple
– Cons: susceptible to noise
– Example: optical system, IFR
1
0
1
t
11
Digital Modulation II
• Frequency Shift Keying (FSK):
– Pros: less susceptible to noise
– Cons: requires larger bandwidth
1
0
1
t
1
0
1
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Digital Modulation III
• Phase Shift Keying (PSK):
– Pros:
• Less susceptible to noise
• Bandwidth efficient
– Cons:
• Require synchronization in frequency and phase
 complicates receivers and transmitter
t
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Phase Shift Keying
• BPSK (Binary Phase Shift
Keying):
Q
– bit value 0: sine wave
– bit value 1: inverted sine wave
– very simple PSK
– low spectral efficiency
– robust, used in satellite systems
• QPSK (Quadrature Phase Shift
Keying):
– 2 bits coded as one symbol
– needs less bandwidth compared to
BPSK
– symbol determines shift of sine wave
– Often also transmission of relative, not
absolute phase shift: DQPSK Differential QPSK
1
0
Q
10
I
11
I
00
01
A
t
11
10
00
01
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Quadrature Amplitude Modulation
• Quadrature Amplitude Modulation (QAM):
combines amplitude and phase modulation
• It is possible to code n bits using one symbol
– 2n discrete levels
• bit error rate increases with n
Q
0010
0001
0011
0000
φ
I
a
1000
• Example: 16-QAM (4 bits = 1
symbol)
• Symbols 0011 and 0001 have the
same phase φ, but different
amplitude a. 0000 and 1000 have
same amplitude but different phase
• Used in Modem
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Outline
Signal
Frequency allocation
Signal propagation
Antennas
Multiplexing
Spread spectrum
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Spread spectrum technology
• Problem of radio transmission: frequency dependent
fading can wipe out narrow band signals for duration
of the interference
• Solution: spread the narrow band signal into a broad
band signal using a special code
power
interference
spread
signal
power
detection at
receiver
• Side effects:
f
signal
spread
interference
f
– coexistence of several signals without dynamic coordination
– tap-proof
• Alternatives: Direct Sequence, Frequency Hopping
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Spreading and frequency
selective fading
channel
quality
narrowband channels
1
2
5
3
6
4
frequency
narrow band
signal
guard space
channel
quality
spread spectrum channels
1
spread
spectrum
2
2
2
2
2
frequency
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DSSS
(Direct Sequence Spread Spectrum)
• XOR of the signal
with pseudorandom number
(chipping
sequence)
– generate a signal
with a wider
range of
frequency: spread
spectrum
tb
user data
0
1
XOR
tc
chipping
sequence
01101010110101
=
resulting
signal
01101011001010
tb: bit period
tc: chip period
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DSSS System
spread
spectrum
signal
user data
X
transmit
signal
modulator
chipping
sequence
radio
carrier
transmitter
correlator
lowpass
filtered
signal
received
signal
demodulator
radio
carrier
products
sampled
sums
data
X
integrator
decision
chipping
sequence
receiver
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Effects of Spreading and
Interference
dP/df
dP/df
i)
user signal
broadband interference
narrowband interference
ii)
f
sender
dP/df
f
dP/df
dP/df
iii)
iv)
f
receiver
v)
f
f
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FHSS
(Frequency Hopping Spread Spectrum)
• Discrete changes of carrier frequency
– sequence of frequency changes determined via pseudo random
number sequence
• Two versions
– Fast Hopping:
several frequencies per user bit
– Slow Hopping:
several user bits per frequency
• Advantages
– frequency selective fading and interference limited to short period
– simple implementation
– uses only small portion of spectrum at any time
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FHSS: Example
tb
user data
0
1
f
0
1
1
t
td
f3
slow
hopping
(3 bits/hop)
f2
f1
f
t
td
f3
fast
hopping
(3 hops/bit)
f2
f1
t
tb: bit period
td: dwell time
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Comparison between Slow
Hopping and Fast Hopping
• Slow hopping
– Pros: cheaper
– Cons: less immune to narrowband
interference
• Fast hopping
– Pros: more immune to narrowband
interference
– Cons: tight synchronization  increased
complexity
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FHSS
(Frequency Hopping Spread Spectrum)
user data
modulator
modulator
frequency
synthesizer
transmitter
hopping
sequence
narrowband
signal
received
signal
data
demodulator
hopping
sequence
spread
transmit
signal
narrowband
signal
frequency
synthesizer
demodulator
receiver
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