cdma throughput
TRANSCRIPT
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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