Cryptography Reference
In-Depth Information
users. The received signal
r
is given by the contribution of all the
K
users plus
a centred AWGN with variance
σ
2
.Fromobservation
r
, we wish to recover the
information bits
d
k
of each user. Figure 11.19 gives the diagram of the receiver
using a turbo CDMA type technique to jointly process the multi-detection and
the channel decoding:
Figure 11.19 - Turbo CDMA receiver.
11.2.2 Multi-user detection
This section presents the main multi-user detection methods. In order to simplify
the description of these methods, only the case of synchronous transmissions over
Gaussian channels is considered.
Standard receiver
The simplest (conventional or standard) detector is the one which operates as
if each user was alone on the channel. The receiver is quite simply made up of
the filter adapted to the signature of the user concerned (this operation is also
called despreading), see Figure 11.20.
At the output of the adapted filter bank the signal can be written in the
form:
y
=
S
T
r
=
RAb
+
S
T
n
(11.62)
We note that the vector of the additive noise at the output of the adapted
filter bank, is made up of correlated components. Its covariance matrix depends
directly on the intercorrelation matrix of the spreading sequences used,
R
=
S
T
S
.Wehave
S
T
n
∼
N
(0
,σ
2
R
)
.
