Cryptography Reference
In-Depth Information
11.2
Multi-user turbo detection and its applica-
tion to CDMA systems
11.2.1
Introduction and some notations
In a Code Division Multiple Access (CDMA) system,
such as the one shown
in Figure 11.18, user
k
(
1
k
K
) transmits a sequence of binary elements
{
1
with an amplitude
A
k
. For each of the users, a channel encoder (
CC
k
)
is used, followed by an external interleaver (
π
k
) before the spreading operation
(multiplication by size
N
, normalized spreading code
s
k
,) which provides binary
symbols called
chips
. This code can vary at each symbol time.
d
k
}
=
±
Figure 11.18 - CDMA transmitter.
The received signal,
r
, can be written in matrix form by:
r
=
SAb
+
n
(11.61)
where:
•
S
is the
N
K
matrix formed by the normalized codes of each user (the
k
-th column represents the
k
-th code
s
k
whose norm is equal to unity),
×
•
A
is a diagonal
K
×
K
matrix made up of the amplitudes
A
k
of each user.
•
b
is the vector of dimension
K
made up of the elements transmitted after
coding channel by the
K
users.
•
n
is the
N
-dimensional centred Gaussian vector with covariance matrix
σ
2
I
N
.
The source data rates of the different users can be different. The size of the
spreading code is such that the chip data rate (after spreading) is the same for all
