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

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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