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7/27/2019 MANET-Physical Layer II
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Computer Engineering and Applications
MCS 123: MANET
Physical Layer II
Manas Kumar Mishra
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Physical Layer II: Outline
Modulation
Digital modulation Analog modulation
Spread Spectrum
DSSS, FHSS
Media Access
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Digital Radio Link
MultipleAccess
Multiplex
Source
Coder
SourceCoder
ChannelCoder
ModulatorPower
Amplifier
Radio
Channel
MultipleAccess
Demultiplex
SourceDecoder
SourceDecoder
ChannelDecoder
Demodulator& Equalizer
RFFilter
Source
Destination
Carrier fc
Carrier fc
transmitted
symbol stream
received (corrupted)symbol stream
antenna
antenna
Radio Technologyand Trends
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Modulation
Digital modulation
digital data is translated into an analog signal (baseband)
ASK, FSK, PSK - main focus
differences in spectral efficiency, power efficiency, robustness
Analog modulation
shifts center frequency of baseband signal up to the radio carrier
Motivation
smaller antennas (e.g., /4)
Frequency Division Multiplexing
medium characteristics
Basic schemes Amplitude Modulation (AM)
Frequency Modulation (FM)
Phase Modulation (PM)
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synchronization
decision
digital
dataanalog
demodulation
radio
carrier
analogbaseband
signal
101101001radio receiver
digital
modulation
digitaldata
analog
modulation
radio
carrier
analog
baseband
signal
101101001radio transmitter
Modulation in action:
Modulation and demodulation
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Digital modulation
Modulation of digital signals known as Shift Keying
Amplitude Shift Keying (ASK):
very simple
low bandwidth requirements
very susceptible to interference
Frequency Shift Keying (FSK):
needs larger bandwidth
Phase Shift Keying (PSK):
more complex
robust against interference
1 0 1
t
1 0 1
t
1 0 1
t
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Advanced Frequency Shift Keying
bandwidth needed for FSK depends on the distance between the carrier frequencies
special pre-computation avoids sudden phase shiftsMSK (Minimum Shift Keying) bit separated into even and odd bits, the duration of each bit is doubled
depending on the bit values (even, odd) the higher or lower frequency, original or
inverted is chosen
the frequency of one carrier is twice the frequency of the other Equivalent to offset QPSK
even higher bandwidth efficiency using a Gaussian low-pass filter
GMSK (Gaussian MSK), used in GSM
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Example of MSK
data
even bits
odd bits
1 1 1 1 000
t
low
frequency
high
frequency
MSKsignal
bit
even 0 1 0 1
odd 0 0 1 1
signal h n n h
value - - + +
h: high frequency
n: low frequency
+: original signal
-: inverted signal
No phase shifts!
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Advanced Phase Shift Keying
BPSK (Binary Phase Shift Keying):
bit value 0: sine wave bit value 1: inverted sine wave
very simple PSK
low spectral efficiency
robust, used e.g. in satellite systems
QPSK (Quadrature Phase Shift Keying):
2 bits coded as one symbol symbol determines shift of sine wave
needs less bandwidth compared to BPSK
more complex
Often also transmission of relative, notabsolute phase shift: DQPSK -
Differential QPSK (IS-136, PHS)
11 10 00 01
Q
I
01
Q
I
11
01
10
00
A
t
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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, n=2 identical to QPSK
Bit error rate increases with n, but less errors compared to comparable PSK schemes
Example: 16-QAM (4 bits = 1 symbol)
Symbols 0011 and 0001 have
the same phase , but different
amplitude a. 0000 and 1000 have
different phase, but same amplitude.
0000
0001
0011
1000
Q
I
0010
a
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Hierarchical Modulation
DVB-T modulates two separate data streams onto a single DVB-T stream
High Priority (HP) embedded within a Low Priority (LP) streamMulti carrier system, about 2000 or 8000 carriers
QPSK, 16 QAM, 64QAM
Example: 64QAM
good reception: resolve the entire64QAM constellation
poor reception, mobile reception:resolve only QPSK portion
6 bit per QAM symbol, 2 mostsignificant determine QPSK
HP service coded in QPSK (2 bit),
LP uses remaining 4 bit
Q
I
00
10
000010 010101
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Spread spectrum technology
Problem of radio transmission: frequency dependent fading can wipe out narrow
band signals for duration of the interferenceSolution: spread the narrow band signal into a broad band signal using a special
code
protection against narrow band interference
Side effects:
coexistence of several signals without dynamic coordination
tap-proof
Alternatives: Direct Sequence, Frequency Hopping
detection at
receiver
interference spread signalsignal
spread
interference
f f
power power
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Effects of spreading and interference
dP/df
f
i)
dP/df
f
ii)
sender
dP/df
f
iii)
dP/df
f
iv)
receiverf
v)
user signal
broadband interference
narrowband interference
dP/df
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Spreading and frequency selective fading
frequency
channel
quality
12
3
4
5 6
narrow bandsignal
guard space
2
2
2
22
frequency
channel
quality
1
spread
spectrum
narrowband channels
spread spectrum channels
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DSSS (Direct Sequence Spread Spectrum) I
XOR of the signal with pseudo-random number (chipping sequence)
many chips per bit (e.g., 128) result in higher bandwidth of the signal
Advantages
reduces frequency selective
fading
in cellular networks
base stations can use thesame frequency range
several base stations can
detect and recover the signal
soft handover
Disadvantages precise power control necessary
user data
chipping
sequence
resulting
signal
0 1
0 1 1 0 1 0 1 01 0 0 1 11
XOR
0 1 1 0 0 1 0 11 0 1 0 01
=
tb
tc
tb: bit period
tc: chip period
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DSSS (Direct Sequence Spread Spectrum) II
X
user data
chipping
sequence
modulator
radio
carrier
spread
spectrumsignal transmit
signal
transmitter
demodulator
received
signal
radio
carrier
X
chipping
sequence
lowpass
filtered
signal
receiver
integrator
products
decision
data
sampled
sums
correlator
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Spread Spectrum
Modulation that increases signal BW
Mitigates or coherently combines ISI Mitigates narrowband interference/jamming Hides signal below noise (DSSS) or makes it hard to track (FH) Also used as a multiple access technique
Two types
Direction Sequence: Modulated signal multiplied by faster chip sequence
Frequency Hopping: Narrowband signal hopped over wide bandwidth
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Bit sequence modulated by chipsequence
Spreads bandwidth by large factor (K)Despread by multiplying by sc(t) again (sc
2(t)=1)
Mitigates ISI and narrowband interference
Direct Sequence Spread Spectrum
s(t) sc
(t)
Tb
=KTc
Tc
S(f)
Sc(f)
1/Tb
1/Tc
S(f)*S
c(f)
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DSSS: An example
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DSSS: Another Example
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Narrowband Interference Rejection (1/K)
Multipath Rejection (Autocorrelation r(t))
ISI and Interference Rejection
S(f)S(f)
I(f)
S(f)*S
c(f)
Info. Signal Receiver Input Despread Signal
I(f)*S
c(f)
S(f)aS(f)
S(f)*S
c(f)[ad(t)+b(t-t)]
Info. Signal Receiver Input Despread Signal
brS(f)
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Synchronization
Adjusts delay of sc(t-t) to hit peak value of autocorrelation. Typically synchronize to LOS component
Complicated by noise, interference, and Multipath
Synchronization offset ofDt leads to signal attenuation by r(Dt)1
-1
2n-1T
c
-T
c
Dt
r(Dt)
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Multiuser DSSS
Each user assigned a unique spreading code; transmit simultaneously oversame bandwidth
Interference between users mitigated by code cross correlation
In downlink, signal and interference have same received power
In uplink, close users drown out far users (a1>>a2: near-far problem)
a
a
a1
a2
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Main Points
DSSS rejects interference by spreading gain
DSSS rejects ISI by code autocorrelation
Maximal linear codes have good autocorrelation properties but poor crosscorrelation
Synchronization depends on autocorrelation properties of spreading code.
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FHSS (Frequency Hopping Spread
Spectrum) I
Discrete changes of carrier frequency
sequence of frequency changes determined via pseudo random numbersequence
Two versions
Fast Hopping:several frequencies per user bit
Slow Hopping:
several user bits per frequencyAdvantages
frequency selective fading and interference limited to short period
simple implementation
uses only small portion of spectrum at any time
Disadvantages not as robust as DSSS
simpler to detect
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FHSS (Frequency Hopping Spread
Spectrum) II
user data
slow
hopping
(3 bits/hop)
fast
hopping
(3 hops/bit)
0 1
tb
0 1 1 t
f
f1
f2
f3
t
td
f
f1
f2
f3
t
td
tb: bit period t
d: dwell time
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FHSS (Frequency Hopping Spread
Spectrum) III
modulator
user data
hopping
sequence
modulator
narrowbandsignal spread
transmit
signal
transmitter
received
signal
receiver
demodulator
data
frequency
synthesizer
hopping
sequence
demodulator
frequency
synthesizer
narrowband
signal
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DSSS vs FHSS
Collocation / Aggregate Rate
DSSS biggest advantage over FHSS is its capability to provide rates of up to 11 Mbps. When covering the whole 2.4 GHz
band, three systems may be installed, providing an aggregate rate of 33 Mbps. (Overall efficiency: 33Mbps/83.5MHz = 0.39
bits/Hz). Additional systems, if installed, will share the spectrum with the already installed systems, lowering the overall
aggregate rate / throughput because of collision occurrences.
In a 2.4 GHz FHSS synchronized environment, up to 12 systems can be collocated, providing an overall aggregate rate of 36
Mbps (Overall efficiency: 36Mbps/83.5MHz= 0.43 bits/Hz).
In a licensed FDD FHSS synchronized environment, up to 6 systems may be collocated in a 12 MHz band, providing an
aggregate rate of 18 Mbps (Overall efficiency: 18Mbps/12MHz = 1.5 bits/Hz).
Contiguous band
IEEE 802.11 DSSS needs 22MHz, contiguous. If such a band is not available, the system can not be operated. FHSS does not
require contiguous band for correct operation. If some frequencies are not available (administrative reasons), FHSS system
could be set to use sequences that do not include the unavailable frequencies.
Coverage
11 Mbps DSSS and 3 Mbps FHSS, cover more or less the same distances.
Near / far problem
Present in DSSS, not critical in FHSS.
Multipath sensitivity
DSSS is extremely sensitive, especially when operated at 11Mbps. To minimize multipath effects, directional antennas
should be used, limiting the DSSS technology mainly to point to point applications.
Bluetooth interference
FHSS are significantly less sensitive to Bluetooth interference.
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Cell structure
Implements space division multiplex
base station covers a certain transmission area (cell)
Mobile stations communicate only via the base station
Advantages of cell structures
higher capacity, higher number of users
less transmission power needed
more robust, decentralized
base station deals with interference, transmission area etc. locally
Problems
fixed network needed for the base stations handover (changing from one cell to another) necessary
interference with other cells
Cell sizes from some 100 m in cities to, e.g., 35 km on the country side
(GSM) - even less for higher frequencies
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Frequency planning I
Frequency reuse only with a certain distance between the base stations
Standard model using 7 frequencies:
Fixed frequency assignment:
certain frequencies are assigned to a certain cell
problem: different traffic load in different cells
Dynamic frequency assignment:
base station chooses frequencies depending on the frequencies already used inneighbor cells
more capacity in cells with more traffic assignment can also be based on interference measurements
f4
f5
f1
f3
f2
f6
f7
f3
f2
f4
f5
f1
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Frequency planning II
f1
f2
f3
f2
f1
f1
f2
f3
f2
f3
f1
f2
f1
f3f
3
f3f
3
f3
f4
f5
f1
f3
f2
f6
f7
f3
f2
f4
f5
f1
f3
f
5
f
6
f7f
2
f
2
f1f1 f1
f2
f3
f2
f3
f2
f3
h1
h2
h3
g1
g2
g3
h1
h2
h3
g1
g2
g3
g1
g2
g3
3 cell cluster
7 cell cluster
3 cell cluster
with 3 sector antennas
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Computer Engineering and Applications
Thank you!
Mobile Ad Hoc Networks
Manas Kumar [email protected]
18.01.2012