MANET-Physical Layer II

7/27/2019 MANET-Physical Layer II

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Computer Engineering and Applications

MCS 123: MANET

Physical Layer II

Manas Kumar Mishra

[email protected]


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Mobile Ad Hoc Networks 18.01.2012- 2

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


7/32Mobile Ad Hoc Networks 18.01.2012- 7

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



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!



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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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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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

Documents

MANET-Physical Layer II