<s>
Code-division	B-Protocol
multiple	I-Protocol
access	I-Protocol
(	O
CDMA	B-Protocol
)	O
is	O
a	O
channel	O
access	O
method	O
used	O
by	O
various	O
radio	O
communication	O
technologies	O
.	O
</s>
<s>
CDMA	B-Protocol
is	O
an	O
example	O
of	O
multiple	O
access	O
,	O
where	O
several	O
transmitters	O
can	O
send	O
information	O
simultaneously	O
over	O
a	O
single	O
communication	O
channel	O
.	O
</s>
<s>
This	O
allows	O
several	O
users	O
to	O
share	O
a	O
band	O
of	O
frequencies	O
(	O
see	O
bandwidth	B-Algorithm
)	O
.	O
</s>
<s>
To	O
permit	O
this	O
without	O
undue	O
interference	O
between	O
the	O
users	O
,	O
CDMA	B-Protocol
employs	O
spread	B-Architecture
spectrum	I-Architecture
technology	O
and	O
a	O
special	O
coding	O
scheme	O
(	O
where	O
each	O
transmitter	O
is	O
assigned	O
a	O
code	O
)	O
.	O
</s>
<s>
CDMA	B-Protocol
optimizes	O
the	O
use	O
of	O
available	O
bandwidth	B-Algorithm
as	O
it	O
transmits	O
over	O
the	O
entire	O
frequency	O
range	O
and	O
does	O
not	O
limit	O
the	O
user	O
's	O
frequency	O
range	O
.	O
</s>
<s>
IS-95	O
,	O
also	O
called	O
"	O
cdmaOne	O
"	O
,	O
and	O
its	O
3G	B-Architecture
evolution	O
CDMA2000	O
,	O
are	O
often	O
simply	O
referred	O
to	O
as	O
"	O
CDMA	B-Protocol
"	O
,	O
but	O
UMTS	B-Protocol
,	O
the	O
3G	B-Architecture
standard	O
used	O
by	O
GSM	O
carriers	O
,	O
also	O
uses	O
"	O
wideband	O
CDMA	B-Protocol
"	O
,	O
or	O
W-CDMA	O
,	O
as	O
well	O
as	O
TD-CDMA	O
and	O
TD-SCDMA	O
,	O
as	O
its	O
radio	O
technologies	O
.	O
</s>
<s>
Due	O
to	O
their	O
very	O
advantageous	O
auto	O
-	O
and	O
crosscorrelation	O
characteristics	O
,	O
these	O
spreading	O
sequences	O
have	O
also	O
been	O
used	O
for	O
radar	O
applications	O
for	O
many	O
decades	O
,	O
where	O
they	O
are	O
called	O
Barker	B-Protocol
codes	I-Protocol
(	O
with	O
a	O
very	O
short	O
sequence	O
length	O
of	O
typically	O
8	O
to	O
32	O
)	O
.	O
</s>
<s>
For	O
space-based	O
communication	O
applications	O
,	O
CDMA	B-Protocol
has	O
been	O
used	O
for	O
many	O
decades	O
due	O
to	O
the	O
large	O
path	O
loss	O
and	O
Doppler	O
shift	O
caused	O
by	O
satellite	O
motion	O
.	O
</s>
<s>
CDMA	B-Protocol
is	O
often	O
used	O
with	O
binary	O
phase-shift	O
keying	O
(	O
BPSK	O
)	O
in	O
its	O
simplest	O
form	O
,	O
but	O
can	O
be	O
combined	O
with	O
any	O
modulation	O
scheme	O
like	O
(	O
in	O
advanced	O
cases	O
)	O
quadrature	B-Device
amplitude	I-Device
modulation	I-Device
(	O
QAM	B-Device
)	O
or	O
orthogonal	B-Protocol
frequency-division	I-Protocol
multiplexing	I-Protocol
(	O
OFDM	B-Protocol
)	O
,	O
which	O
typically	O
makes	O
it	O
very	O
robust	O
and	O
efficient	O
(	O
and	O
equipping	O
them	O
with	O
accurate	O
ranging	O
capabilities	O
,	O
which	O
is	O
difficult	O
without	O
CDMA	B-Protocol
)	O
.	O
</s>
<s>
Other	O
schemes	O
use	O
subcarriers	O
based	O
on	O
binary	B-Device
offset	I-Device
carrier	I-Device
modulation	I-Device
(	O
BOC	O
modulation	O
)	O
,	O
which	O
is	O
inspired	O
by	O
Manchester	B-Protocol
codes	I-Protocol
and	O
enable	O
a	O
larger	O
gap	O
between	O
the	O
virtual	O
center	O
frequency	O
and	O
the	O
subcarriers	O
,	O
which	O
is	O
not	O
the	O
case	O
for	O
OFDM	B-Protocol
subcarriers	O
.	O
</s>
<s>
Many	O
carriers	O
(	O
such	O
as	O
AT&T	O
and	O
Verizon	B-Application
)	O
shut	O
down	O
CDMA	B-Protocol
networks	O
in	O
2022	O
,	O
rendering	O
CDMA	B-Protocol
handsets	O
unusable	O
for	O
calls	O
,	O
even	O
to	O
911	O
.	O
</s>
<s>
The	O
technology	O
of	O
code-division	B-Protocol
multiple	I-Protocol
access	I-Protocol
channels	O
has	O
long	O
been	O
known	O
.	O
</s>
<s>
In	O
the	O
US	O
,	O
one	O
of	O
the	O
earliest	O
descriptions	O
of	O
CDMA	B-Protocol
can	O
be	O
found	O
in	O
the	O
summary	O
report	O
of	O
Project	O
Hartwell	O
on	O
"	O
The	O
Security	O
of	O
Overseas	O
Transport	O
"	O
,	O
which	O
was	O
a	O
summer	O
research	O
project	O
carried	O
out	O
at	O
the	O
Massachusetts	O
Institute	O
of	O
Technology	O
from	O
June	O
to	O
August	O
1950	O
.	O
</s>
<s>
The	O
technology	O
of	O
CDMA	B-Protocol
was	O
used	O
in	O
1957	O
,	O
when	O
the	O
young	O
military	O
radio	O
engineer	O
Leonid	O
Kupriyanovich	O
in	O
Moscow	O
made	O
an	O
experimental	O
model	O
of	O
a	O
wearable	O
automatic	O
mobile	O
phone	O
,	O
called	O
LK-1	O
by	O
him	O
,	O
with	O
a	O
base	O
station	O
.	O
</s>
<s>
Synchronous	O
CDM	O
(	O
code-division	B-Protocol
'	O
multiplexing	O
 '	O
,	O
an	O
early	O
generation	O
of	O
CDMA	B-Protocol
)	O
was	O
implemented	O
in	O
the	O
Global	O
Positioning	O
System	O
(	O
GPS	O
)	O
.	O
</s>
<s>
The	O
UMTS	B-Protocol
3G	B-Architecture
mobile	I-Architecture
phone	O
standard	O
,	O
which	O
uses	O
W-CDMA	O
.	O
</s>
<s>
CDMA	B-Protocol
has	O
been	O
used	O
in	O
the	O
OmniTRACS	O
satellite	O
system	O
for	O
transportation	O
logistics	O
.	O
</s>
<s>
CDMA	B-Protocol
is	O
a	O
spread-spectrum	B-Architecture
multiple-access	O
technique	O
.	O
</s>
<s>
A	O
spread-spectrum	B-Architecture
technique	O
spreads	O
the	O
bandwidth	B-Algorithm
of	O
the	O
data	O
uniformly	O
for	O
the	O
same	O
transmitted	O
power	O
.	O
</s>
<s>
A	O
spreading	O
code	O
is	O
a	O
pseudo-random	O
code	O
that	O
has	O
a	O
narrow	O
ambiguity	B-Algorithm
function	I-Algorithm
,	O
unlike	O
other	O
narrow	O
pulse	O
codes	O
.	O
</s>
<s>
In	O
CDMA	B-Protocol
a	O
locally	O
generated	O
code	O
runs	O
at	O
a	O
much	O
higher	O
rate	O
than	O
the	O
data	O
to	O
be	O
transmitted	O
.	O
</s>
<s>
The	O
figure	O
shows	O
how	O
a	O
spread-spectrum	B-Architecture
signal	O
is	O
generated	O
.	O
</s>
<s>
(	O
Note	O
:	O
bandwidth	B-Algorithm
is	O
proportional	O
to	O
,	O
where	O
=	O
bit	O
time	O
.	O
)	O
</s>
<s>
Therefore	O
,	O
the	O
bandwidth	B-Algorithm
of	O
the	O
data	O
signal	O
is	O
and	O
the	O
bandwidth	B-Algorithm
of	O
the	O
spread	B-Architecture
spectrum	I-Architecture
signal	O
is	O
.	O
</s>
<s>
Since	O
is	O
much	O
smaller	O
than	O
,	O
the	O
bandwidth	B-Algorithm
of	O
the	O
spread-spectrum	B-Architecture
signal	O
is	O
much	O
larger	O
than	O
the	O
bandwidth	B-Algorithm
of	O
the	O
original	O
signal	O
.	O
</s>
<s>
Each	O
user	O
in	O
a	O
CDMA	B-Protocol
system	O
uses	O
a	O
different	O
code	O
to	O
modulate	O
their	O
signal	O
.	O
</s>
<s>
Choosing	O
the	O
codes	O
used	O
to	O
modulate	O
the	O
signal	O
is	O
very	O
important	O
in	O
the	O
performance	O
of	O
CDMA	B-Protocol
systems	O
.	O
</s>
<s>
To	O
avoid	O
confusion	O
,	O
people	O
could	O
take	O
turns	O
speaking	O
(	O
time	O
division	O
)	O
,	O
speak	O
at	O
different	O
pitches	O
(	O
frequency	O
division	O
)	O
,	O
or	O
speak	O
in	O
different	O
languages	O
(	O
code	B-Protocol
division	I-Protocol
)	O
.	O
</s>
<s>
CDMA	B-Protocol
is	O
analogous	O
to	O
the	O
last	O
example	O
where	O
people	O
speaking	O
the	O
same	O
language	O
can	O
understand	O
each	O
other	O
,	O
but	O
other	O
languages	O
are	O
perceived	O
as	O
noise	O
and	O
rejected	O
.	O
</s>
<s>
Similarly	O
,	O
in	O
radio	O
CDMA	B-Protocol
,	O
each	O
group	O
of	O
users	O
is	O
given	O
a	O
shared	O
code	O
.	O
</s>
<s>
In	O
general	O
,	O
CDMA	B-Protocol
belongs	O
to	O
two	O
basic	O
categories	O
:	O
synchronous	O
(	O
orthogonal	B-Application
codes	O
)	O
and	O
asynchronous	O
(	O
pseudorandom	O
codes	O
)	O
.	O
</s>
<s>
Synchronous	O
CDMA	B-Protocol
exploits	O
mathematical	O
properties	O
of	O
orthogonality	B-Application
between	O
vectors	O
representing	O
the	O
data	O
strings	O
.	O
</s>
<s>
If	O
the	O
dot	O
product	O
is	O
zero	O
,	O
the	O
two	O
vectors	O
are	O
said	O
to	O
be	O
orthogonal	B-Application
to	O
each	O
other	O
.	O
</s>
<s>
Some	O
properties	O
of	O
the	O
dot	O
product	O
aid	O
understanding	O
of	O
how	O
W-CDMA	O
works	O
.	O
</s>
<s>
If	O
vectors	O
a	O
and	O
b	O
are	O
orthogonal	B-Application
,	O
then	O
and	O
:	O
</s>
<s>
Each	O
user	O
in	O
synchronous	O
CDMA	B-Protocol
uses	O
a	O
code	O
orthogonal	B-Application
to	O
the	O
others	O
 '	O
codes	O
to	O
modulate	O
their	O
signal	O
.	O
</s>
<s>
An	O
example	O
of	O
4	O
mutually	O
orthogonal	B-Application
digital	O
signals	O
is	O
shown	O
in	O
the	O
figure	O
below	O
.	O
</s>
<s>
Orthogonal	B-Application
codes	O
have	O
a	O
cross-correlation	O
equal	O
to	O
zero	O
;	O
in	O
other	O
words	O
,	O
they	O
do	O
not	O
interfere	O
with	O
each	O
other	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
IS-95	O
,	O
64-bit	O
Walsh	B-Error_Name
codes	I-Error_Name
are	O
used	O
to	O
encode	O
the	O
signal	O
to	O
separate	O
different	O
users	O
.	O
</s>
<s>
Since	O
each	O
of	O
the	O
64	O
Walsh	B-Error_Name
codes	I-Error_Name
is	O
orthogonal	B-Application
to	O
all	O
other	O
,	O
the	O
signals	O
are	O
channelized	O
into	O
64	O
orthogonal	B-Application
signals	O
.	O
</s>
<s>
Start	O
with	O
a	O
set	O
of	O
vectors	O
that	O
are	O
mutually	O
orthogonal	B-Application
.	O
</s>
<s>
(	O
Although	O
mutual	O
orthogonality	B-Application
is	O
the	O
only	O
condition	O
,	O
these	O
vectors	O
are	O
usually	O
constructed	O
for	O
ease	O
of	O
decoding	O
,	O
for	O
example	O
columns	O
or	O
rows	O
from	O
Walsh	B-Algorithm
matrices	I-Algorithm
.	O
)	O
</s>
<s>
An	O
example	O
of	O
orthogonal	B-Application
functions	O
is	O
shown	O
in	O
the	O
adjacent	O
picture	O
.	O
</s>
<s>
Since	O
it	O
is	O
not	O
mathematically	O
possible	O
to	O
create	O
signature	O
sequences	O
that	O
are	O
both	O
orthogonal	B-Application
for	O
arbitrarily	O
random	O
starting	O
points	O
and	O
which	O
make	O
full	O
use	O
of	O
the	O
code	O
space	O
,	O
unique	O
"	O
pseudo-random	O
"	O
or	O
"	O
pseudo-noise	O
"	O
sequences	O
called	O
spreading	O
sequences	O
are	O
used	O
in	O
asynchronous	O
CDMA	B-Protocol
systems	O
.	O
</s>
<s>
These	O
spreading	O
sequences	O
are	O
used	O
to	O
encode	O
and	O
decode	O
a	O
user	O
's	O
signal	O
in	O
asynchronous	O
CDMA	B-Protocol
in	O
the	O
same	O
manner	O
as	O
the	O
orthogonal	B-Application
codes	O
in	O
synchronous	O
CDMA	B-Protocol
(	O
shown	O
in	O
the	O
example	O
above	O
)	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
unlike	O
synchronous	O
CDMA	B-Protocol
,	O
the	O
signals	O
of	O
other	O
users	O
will	O
appear	O
as	O
noise	O
to	O
the	O
signal	O
of	O
interest	O
and	O
interfere	O
slightly	O
with	O
the	O
desired	O
signal	O
in	O
proportion	O
to	O
number	O
of	O
users	O
.	O
</s>
<s>
All	O
forms	O
of	O
CDMA	B-Protocol
use	O
the	O
spread-spectrum	B-Architecture
spreading	O
factor	O
to	O
allow	O
receivers	O
to	O
partially	O
discriminate	O
against	O
unwanted	O
signals	O
.	O
</s>
<s>
Since	O
each	O
user	O
generates	O
MAI	O
,	O
controlling	O
the	O
signal	O
strength	O
is	O
an	O
important	O
issue	O
with	O
CDMA	B-Protocol
transmitters	O
.	O
</s>
<s>
A	O
CDM	O
(	O
synchronous	O
CDMA	B-Protocol
)	O
,	O
TDMA	O
,	O
or	O
FDMA	O
receiver	O
can	O
in	O
theory	O
completely	O
reject	O
arbitrarily	O
strong	O
signals	O
using	O
different	O
codes	O
,	O
time	O
slots	O
or	O
frequency	O
channels	O
due	O
to	O
the	O
orthogonality	B-Application
of	O
these	O
systems	O
.	O
</s>
<s>
This	O
is	O
not	O
true	O
for	O
asynchronous	O
CDMA	B-Protocol
;	O
rejection	O
of	O
unwanted	O
signals	O
is	O
only	O
partial	O
.	O
</s>
<s>
This	O
leads	O
to	O
a	O
general	O
requirement	O
in	O
any	O
asynchronous	O
CDMA	B-Protocol
system	O
to	O
approximately	O
match	O
the	O
various	O
signal	O
power	O
levels	O
as	O
seen	O
at	O
the	O
receiver	O
.	O
</s>
<s>
In	O
CDMA	B-Protocol
cellular	O
,	O
the	O
base	O
station	O
uses	O
a	O
fast	O
closed-loop	O
power-control	O
scheme	O
to	O
tightly	O
control	O
each	O
mobile	O
's	O
transmit	O
power	O
.	O
</s>
<s>
In	O
theory	O
CDMA	B-Protocol
,	O
TDMA	O
and	O
FDMA	O
have	O
exactly	O
the	O
same	O
spectral	O
efficiency	O
,	O
but	O
,	O
in	O
practice	O
,	O
each	O
has	O
its	O
own	O
challenges	O
–	O
power	O
control	O
in	O
the	O
case	O
of	O
CDMA	B-Protocol
,	O
timing	O
in	O
the	O
case	O
of	O
TDMA	O
,	O
and	O
frequency	O
generation/filtering	O
in	O
the	O
case	O
of	O
FDMA	O
.	O
</s>
<s>
Asynchronous	O
CDMA	B-Protocol
offers	O
a	O
key	O
advantage	O
in	O
the	O
flexible	O
allocation	O
of	O
resources	O
i.e.	O
</s>
<s>
In	O
the	O
case	O
of	O
CDM	O
(	O
synchronous	O
CDMA	B-Protocol
)	O
,	O
TDMA	O
,	O
and	O
FDMA	O
the	O
number	O
of	O
simultaneous	O
orthogonal	B-Application
codes	O
,	O
time	O
slots	O
,	O
and	O
frequency	O
slots	O
respectively	O
are	O
fixed	O
,	O
hence	O
the	O
capacity	O
in	O
terms	O
of	O
the	O
number	O
of	O
simultaneous	O
users	O
is	O
limited	O
.	O
</s>
<s>
There	O
are	O
a	O
fixed	O
number	O
of	O
orthogonal	B-Application
codes	O
,	O
time	O
slots	O
or	O
frequency	O
bands	O
that	O
can	O
be	O
allocated	O
for	O
CDM	O
,	O
TDMA	O
,	O
and	O
FDMA	O
systems	O
,	O
which	O
remain	O
underutilized	O
due	O
to	O
the	O
bursty	O
nature	O
of	O
telephony	O
and	O
packetized	O
data	O
transmissions	O
.	O
</s>
<s>
There	O
is	O
no	O
strict	O
limit	O
to	O
the	O
number	O
of	O
users	O
that	O
can	O
be	O
supported	O
in	O
an	O
asynchronous	O
CDMA	B-Protocol
system	O
,	O
only	O
a	O
practical	O
limit	O
governed	O
by	O
the	O
desired	O
bit	O
error	O
probability	O
since	O
the	O
SIR	O
(	O
signal-to-interference	O
ratio	O
)	O
varies	O
inversely	O
with	O
the	O
number	O
of	O
users	O
.	O
</s>
<s>
In	O
a	O
bursty	O
traffic	O
environment	O
like	O
mobile	O
telephony	O
,	O
the	O
advantage	O
afforded	O
by	O
asynchronous	O
CDMA	B-Protocol
is	O
that	O
the	O
performance	O
(	O
bit	O
error	O
rate	O
)	O
is	O
allowed	O
to	O
fluctuate	O
randomly	O
,	O
with	O
an	O
average	O
value	O
determined	O
by	O
the	O
number	O
of	O
users	O
times	O
the	O
percentage	O
of	O
utilization	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
asynchronous	O
CDMA	B-Protocol
is	O
ideally	O
suited	O
to	O
a	O
mobile	O
network	O
where	O
large	O
numbers	O
of	O
transmitters	O
each	O
generate	O
a	O
relatively	O
small	O
amount	O
of	O
traffic	O
at	O
irregular	O
intervals	O
.	O
</s>
<s>
CDM	O
(	O
synchronous	O
CDMA	B-Protocol
)	O
,	O
TDMA	O
,	O
and	O
FDMA	O
systems	O
cannot	O
recover	O
the	O
underutilized	O
resources	O
inherent	O
to	O
bursty	O
traffic	O
due	O
to	O
the	O
fixed	O
number	O
of	O
orthogonal	B-Application
codes	O
,	O
time	O
slots	O
or	O
frequency	O
channels	O
that	O
can	O
be	O
assigned	O
to	O
individual	O
transmitters	O
.	O
</s>
<s>
Furthermore	O
,	O
it	O
would	O
require	O
significant	O
overhead	O
to	O
continually	O
allocate	O
and	O
deallocate	O
the	O
orthogonal-code	O
,	O
time-slot	O
or	O
frequency-channel	O
resources	O
.	O
</s>
<s>
By	O
comparison	O
,	O
asynchronous	O
CDMA	B-Protocol
transmitters	O
simply	O
send	O
when	O
they	O
have	O
something	O
to	O
say	O
and	O
go	O
off	O
the	O
air	O
when	O
they	O
do	O
not	O
,	O
keeping	O
the	O
same	O
signature	O
sequence	O
as	O
long	O
as	O
they	O
are	O
connected	O
to	O
the	O
system	O
.	O
</s>
<s>
Most	O
modulation	O
schemes	O
try	O
to	O
minimize	O
the	O
bandwidth	B-Algorithm
of	O
this	O
signal	O
since	O
bandwidth	B-Algorithm
is	O
a	O
limited	O
resource	O
.	O
</s>
<s>
However	O
,	O
spread-spectrum	B-Architecture
techniques	O
use	O
a	O
transmission	O
bandwidth	B-Algorithm
that	O
is	O
several	O
orders	O
of	O
magnitude	O
greater	O
than	O
the	O
minimum	O
required	O
signal	B-Algorithm
bandwidth	I-Algorithm
.	O
</s>
<s>
These	O
systems	O
were	O
designed	O
using	O
spread	B-Architecture
spectrum	I-Architecture
because	O
of	O
its	O
security	O
and	O
resistance	O
to	O
jamming	O
.	O
</s>
<s>
Asynchronous	O
CDMA	B-Protocol
has	O
some	O
level	O
of	O
privacy	O
built	O
in	O
because	O
the	O
signal	O
is	O
spread	O
using	O
a	O
pseudo-random	O
code	O
;	O
this	O
code	O
makes	O
the	O
spread-spectrum	B-Architecture
signals	O
appear	O
random	O
or	O
have	O
noise-like	O
properties	O
.	O
</s>
<s>
CDMA	B-Protocol
is	O
also	O
resistant	O
to	O
jamming	O
.	O
</s>
<s>
The	O
jammer	O
can	O
either	O
spread	O
its	O
energy	O
over	O
the	O
entire	O
bandwidth	B-Algorithm
of	O
the	O
signal	O
or	O
jam	O
only	O
part	O
of	O
the	O
entire	O
signal	O
.	O
</s>
<s>
CDMA	B-Protocol
can	O
also	O
effectively	O
reject	O
narrow-band	O
interference	O
.	O
</s>
<s>
Since	O
narrow-band	O
interference	O
affects	O
only	O
a	O
small	O
portion	O
of	O
the	O
spread-spectrum	B-Architecture
signal	O
,	O
it	O
can	O
easily	O
be	O
removed	O
through	O
notch	O
filtering	O
without	O
much	O
loss	O
of	O
information	O
.	O
</s>
<s>
Convolution	B-Error_Name
encoding	I-Error_Name
and	O
interleaving	O
can	O
be	O
used	O
to	O
assist	O
in	O
recovering	O
this	O
lost	O
data	O
.	O
</s>
<s>
CDMA	B-Protocol
signals	O
are	O
also	O
resistant	O
to	O
multipath	O
fading	O
.	O
</s>
<s>
Since	O
the	O
spread-spectrum	B-Architecture
signal	O
occupies	O
a	O
large	O
bandwidth	B-Algorithm
,	O
only	O
a	O
small	O
portion	O
of	O
this	O
will	O
undergo	O
fading	O
due	O
to	O
multipath	O
at	O
any	O
given	O
time	O
.	O
</s>
<s>
Another	O
reason	O
CDMA	B-Protocol
is	O
resistant	O
to	O
multipath	O
interference	O
is	O
because	O
the	O
delayed	O
versions	O
of	O
the	O
transmitted	O
pseudo-random	O
codes	O
will	O
have	O
poor	O
correlation	O
with	O
the	O
original	O
pseudo-random	O
code	O
,	O
and	O
will	O
thus	O
appear	O
as	O
another	O
user	O
,	O
which	O
is	O
ignored	O
at	O
the	O
receiver	O
.	O
</s>
<s>
Some	O
CDMA	B-Protocol
devices	O
use	O
a	O
rake	O
receiver	O
,	O
which	O
exploits	O
multipath	O
delay	O
components	O
to	O
improve	O
the	O
performance	O
of	O
the	O
system	O
.	O
</s>
<s>
In	O
a	O
CDMA	B-Protocol
system	O
,	O
the	O
same	O
frequency	O
can	O
be	O
used	O
in	O
every	O
cell	O
,	O
because	O
channelization	O
is	O
done	O
using	O
the	O
pseudo-random	O
codes	O
.	O
</s>
<s>
Reusing	O
the	O
same	O
frequency	O
in	O
every	O
cell	O
eliminates	O
the	O
need	O
for	O
frequency	O
planning	O
in	O
a	O
CDMA	B-Protocol
system	O
;	O
however	O
,	O
planning	O
of	O
the	O
different	O
pseudo-random	O
sequences	O
must	O
be	O
done	O
to	O
ensure	O
that	O
the	O
received	O
signal	O
from	O
one	O
cell	O
does	O
not	O
correlate	O
with	O
the	O
signal	O
from	O
a	O
nearby	O
cell	O
.	O
</s>
<s>
Since	O
adjacent	O
cells	O
use	O
the	O
same	O
frequencies	O
,	O
CDMA	B-Protocol
systems	O
have	O
the	O
ability	O
to	O
perform	O
soft	O
hand-offs	O
.	O
</s>
<s>
In	O
contrast	O
,	O
CDMA	B-Protocol
systems	O
use	O
the	O
soft	O
hand-off	O
,	O
which	O
is	O
undetectable	O
and	O
provides	O
a	O
more	O
reliable	O
and	O
higher-quality	O
signal	O
.	O
</s>
<s>
A	O
novel	O
collaborative	O
multi-user	O
transmission	O
and	O
detection	O
scheme	O
called	O
collaborative	O
CDMA	B-Protocol
has	O
been	O
investigated	O
for	O
the	O
uplink	O
that	O
exploits	O
the	O
differences	O
between	O
users	O
 '	O
fading	O
channel	O
signatures	O
to	O
increase	O
the	O
user	O
capacity	O
well	O
beyond	O
the	O
spreading	O
length	O
in	O
the	O
MAI-limited	O
environment	O
.	O
</s>
<s>
The	O
authors	O
show	O
that	O
it	O
is	O
possible	O
to	O
achieve	O
this	O
increase	O
at	O
a	O
low	O
complexity	O
and	O
high	O
bit	O
error	O
rate	O
performance	O
in	O
flat	O
fading	O
channels	O
,	O
which	O
is	O
a	O
major	O
research	O
challenge	O
for	O
overloaded	O
CDMA	B-Protocol
systems	O
.	O
</s>
<s>
In	O
this	O
approach	O
,	O
instead	O
of	O
using	O
one	O
sequence	O
per	O
user	O
as	O
in	O
conventional	O
CDMA	B-Protocol
,	O
the	O
authors	O
group	O
a	O
small	O
number	O
of	O
users	O
to	O
share	O
the	O
same	O
spreading	O
sequence	O
and	O
enable	O
group	O
spreading	O
and	O
despreading	O
operations	O
.	O
</s>
<s>
An	O
enhanced	O
CDMA	B-Protocol
version	O
known	O
as	O
interleave-division	O
multiple	O
access	O
(	O
IDMA	O
)	O
uses	O
the	O
orthogonal	B-Application
interleaving	O
as	O
the	O
only	O
means	O
of	O
user	O
separation	O
in	O
place	O
of	O
signature	O
sequence	O
used	O
in	O
CDMA	B-Protocol
system	O
.	O
</s>
