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  • 7/23/2019 CDMA Throughput

    1/5

    Coile

    cliuision

    multiple

    access

    CDMA)

    307

    (6.1e)

    m(t)

    m(t)p(t)

    p(t)

    Figure 5.22 Direct sequence

    D$CDMA).

    At the receiver the

    signal

    is coherently demod';lated

    by multiplying the received

    signal by

    a replica

    of the

    carrier.

    I.leglecting hernial noise, the signal

    r(f)

    at

    the

    input

    of

    the

    detector

    ov,'-pass ilter

    GPE)

    is

    given by:

    recover'ed

    data

    signal

    p(t)

    r(t)

    -

    n(t)p(t)

    ccs. ^r.t(2 os r.r.f

    :

    m(t)p(t)

    +

    m(t)p(t)

    cos

    2a,t

    (V )

    . \

    t

    'I

    {

    {

    ':

    .

    .t

    i

    i

    5

    The low-pass

    filter

    (LPF)

    eliminates

    the

    high

    frequency components at 2c..r. nd

    retainsonly the low

    frequency

    component

    u(t)

    -

    m(t)p(t).

    This component s

    then

    mu{iplied

    by

    the local code p(t)

    in phase uith the receiaed ode. n the

    product

    p(t)'-

    1.

    At the output

    of the multiplier this gives:

    2

    cosro"t

    Chp ratcR"=

    1ffs

    x(t)

    m(t)p(t)p(f) m(t)p(t)z mU)

    ry)

    (6.20)

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    Code ilivision

    maltiple access

    (CDMN

    317

    Table

    6.1

    The performance

    of

    a CDMA access

    network using

    a 36MHz

    repeater channel and

    binary

    phase shift

    keying

    (BPSK).

    Each

    carrier has the capacity of

    one 64kbit/s telephone channel

    Required

    error

    probability EuiNo

    Maximum number

    of

    accesses

    Nrro*

    Maximum total

    Efficiency

    capacity

    (%)

    82

    62

    51

    ,d

    5

    ' ,S

    . r.4

    ,d

    : . t

    s

    ' ' * i

    .j

    ,*

    g

    ',,:;

    , : i i

    . , . i

    , $

    . , 4

    .:s

    rx

    ".,.:&

    ' , , i9

    . +

    :t

    : ;

    1{

    , f

    . . I

    .,n

    ;

    .-$

    t

    x

    .il

    ,,{

    ,g

    .{

    .f

    ..*

    ',1

    '1

    ,";

    ; {

    -j

    .t'

    'j

    ,]

    .,{

    . . : l

    I

    i

    -:t

    j

    .t

    10

    {

    10

    s

    10

    6

    8.4

    dB

    9.6

    dB

    10 .5 B

    5.3Mbit/s

    4 Mbit/s

    3.3Mbit/s

    1 5

    1 1

    9

    Table 6.1 shows

    the maximum numhr

    of accesses, he maximum

    total capacity of

    the network and the

    throughput

    for a chosen

    error probability.

    The

    throughput, of

    the

    order af

    10Vo,

    s low

    compard, for

    example, with

    TDMA

    (Section

    6.5.5).

    The

    values in the table

    are optimistic; thermal noise is

    neglected,user codes

    are

    assumed

    to be

    orthogonai

    and no

    account

    is

    taken

    of degradation

    due to

    the demodulator.

    6.6.6

    Conclusion

    Code division

    multiple

    access perateson the

    principle of spread spectrurn

    transmission,

    ecalledn

    Figure

    6.29.

    hecode

    equence hich

    serves

    o

    spread

    he

    Figure 6.29

    Spread spectrum transmission

    in

    a code division

    multipl'e access

    system.

    Four

    spectral

    clcnsity

    (psd)

    @

    +

    t m

    lffi

    ffi

    ffi

    mmm

    ffi

    Bffiffi

    0 W

    trequency

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    308

    Multiple

    access

    This signal

    is

    then integrated

    over

    one

    bit

    period to filter

    the noise. The

    transmitted

    message m(t) is recovered

    at the

    integrator

    output.

    4.6.1.2

    Spectral ccupation

    The

    spectrum

    of

    the

    carrierc(t), of power

    C and frequency

    Fr, s

    given

    by:

    f |

    (\

    _

    \ - \ /

    -

    " ( f

    -

    F.) /R.

    (w

    lHz)

    (6.21)

    It is represented in

    Figure

    6.23.

    For

    comparison purposes,

    this

    spectrum

    is

    superimposed

    on that which

    the

    carrier would

    have if modulated

    by

    the message

    m(t)

    alone. t

    can

    be

    seen

    hat, with

    CDMA, c(f) has

    a spectrum

    which is

    broadened

    by the

    spreading

    ratio

    R./R6. This

    is the

    result

    of combining

    the message

    with

    the chip sequence.

    It will now be

    shown that this

    combination

    permits multiple

    ACCESS.

    6.6.1.3 Realisationf multipleaccess

    The

    earthstation eceivesrom

    the

    channel he

    wanted

    carrier

    c(f)

    superimposed

    n

    the carr iers

    i ( f ) ( i

    7 ,

    2, . . . , .^y ' -

    1)

    of the / -

    1

    otherusers ransmi t ted

    n

    the

    same requency;hence:

    (;)

    s in fzr (

    -F, . ) lR,- )

    ) '

    with:

    Figure

    6.23

    carrier

    would

    r ( t ) : c ( t )+ f r ' ( t )

    (V )

    c( f )

    :

    m( t )p( t )

    coscu. f

    f

    r,(t)

    :

    t

    m

    ft)p(f

    cos

    ,,'.f

    (6.22)

    - R c

    - R b

    F c

    R b

    The

    spectrum

    of the carrier in DS-CDMA

    have if

    modulated

    by the message

    r(/).

    R c

    frequency

    _+

    together with the

    spectrum

    modulation

    by

    m(t)p(t)

    which

    the

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    308 Multiple

    access

    This

    signal

    is then integrated

    over one

    bit

    period

    to

    filter the

    noise.

    The

    transmitted

    messagem(t)

    is rec overed at the

    integrator

    output.

    4 .6 .1 .2

    The spectrum

    of

    the

    carrier

    c(t),

    of

    Spectral ccupation

    power

    C

    and

    l " ( f

    -

    F.) /R.

    F., is

    given by:

    ( - (

    f\

    -

    (w

    lHz)

    requency

    , l

    fr

    l ' )

    ,I

    ln

    (fr)

    (6.21)

    r ( f

    -

    F . ) / R .

    It is represented in Figure

    6.23.

    For

    comparison purposes, this spectrum is

    superimposed

    on

    that which

    the carrier would have if

    modulated by the message

    rn

    f

    )

    alone. It

    can

    be seen hat, with

    CDMA,

    c(f) has

    a spectrum which is broadened

    by the spreading ratio

    R./Ru. This is the result

    of combining the message with

    the chip sequence. It will now

    be shown that this combination permits multiple

    access.

    6.6.1.3

    Realisation f multiple access

    The

    earth station receives rom the

    channel the wanted

    carrier c(f)

    superimposed

    on

    the carr iers ci ( f ) ( i

    :7,

    2, . . . , lJ

    -

    1)

    of the N

    -

    1

    other users

    transmit ted

    on

    the

    same

    frequency;

    hence:

    r ( t1 c( f )

    I r ' ( t )

    (V)

    c( t)

    :

    m(t)p( t ) cos

    u,r . /

    L r,G) L*,ft)p i(t)

    cos

    ,t

    (6.22)

    z-

    - R b

    F c

    R b

    Figure 6.23

    The spectrum

    of the carrier

    in DS-CDMA

    carrier

    would have

    if

    moduiated

    by

    the message

    (f).

    R s

    frequencY

    +

    together

    with

    the

    spectrum

    which

    -

    F l c

    the

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    Conclusion

    327

    time for

    protocols of

    the DAMA type

    with explicit reservation

    as

    a

    functior"r of

    transmission

    throughput.

    6.9 CONCLUSION

    There is a

    large variety of

    solutions

    to

    the

    problem of multiple

    access

    o

    a satellite

    by

    a group of

    network

    stations. The choice of access type depends

    above

    all

    on

    economic considerations;

    these are the

    global

    cost in

    terms of

    invesinent

    and

    operating

    costs

    ancl

    the

    benefits in terms of revenues.

    General

    indications

    can be given according

    to

    the

    type

    of

    traffic:

    -For

    traffic characterised by long messages, rnplying

    continuous or quasicontin-

    uous transmission

    of a carrier,

    FDMA,

    TDMA, and CDMA access

    echniques

    are

    the most appropriate. This involves, for example, telephone traffic, television

    transmission

    and

    videoconferencing.

    If the

    volume

    of

    traffic

    per

    carrier is large

    and

    the

    number of accesses s

    small

    (trunking),

    FDMA has the

    advantage

    of

    operational

    simplicity. When the traffic

    per

    carrier

    is

    small and the number of

    accesses

    arge, FDMA loses much in

    efficiency of usage of the

    space

    segment and

    TDMA

    and CDMA

    are the

    best

    candidates. However"

    TDMA

    requires

    relatively

    costly earth

    station

    equipment.

    For

    small

    stations exposed to inter-svstern

    interference"

    CDMA may be preferred despite its low throughput.

    THROTJGHPUT

    oh

    1

    Figure 6.34

    Comparison

    throughput

    corresponds

    to

    repeater

    channel).

    N Q

    NI,IIGEROF

    rcCESSES

    of throughput

    for different

    multiple

    access

    techniques. A 1,00%

    the capacity

    considedng

    one access

    nly

    (one

    carrier

    within

    a single