Cryptography Reference
In-Depth Information
Figure 11.26 - Interference cancellation unit for the turbo SIC decoder in CDMA for
the
k
-th user and at iteration
m
.
thus obtained
Δ
e
m,k
is subtracted from residual signal
e
m,k
to obtain the new
residual signal
e
m,k
+1
of the following user (if
k<K
)
or to obtain the new
residual signal
e
m
+1
,
1
for the first user at the following iteration (
e
m,K
+1
=
e
m
+1
,
1
). Here,
y
m,k
is written in the form
y
m,k
=
A
k
b
k
+
ν
m,k
where
ν
m,k
(residual multiple access interference plus the additive noise) is approximated
by a centred Gaussian random variable whose variance is given by:
var
(
ν
m,k
)=
i<k
A
i
ρ
i,k
1
b
2
m,i
+
i>k
A
i
ρ
i,k
1
b
2
m−
1
,i
+
σ
2
−
−
(11.73)
We show that the extrinsic information of user
k
at iteration
m
is given by:
λ
m
(
b
k
)=log
P
[
y
m,k
/b
k
=+1]
P
[
y
m,k
/b
k
=
=
2
y
m,k
A
k
var
(
ν
m,k
)
(11.74)
−
1]
This extrinsic information serves as the input at the decoder associated with the
k
-th user.
Some simulations
To give an idea of the performance of the turbo SIC decoder, Gold sequences
of size 31 are generated. The channel turbo encoder (rate
R
=1
/
3
) normalized
for UMTS [11.1] is used. We consider frames of 640 bits per user. The external
interleavers of the different users are produced randomly. The BER and PERs
are averaged over all the users. For the channel turbo encoder, the Max-Log-
MAP algorithm is used, 8 being the number of iterations internal to the turbo
decoder. Figure 11.27(a) gives the performance of the turbo SIC decoder for one,
two and three iterations with
K
=31
users (that is, 100% load rate) having the
same power. The performance of the single-user detector and of the conventional
detector are also indicated. Figure 11.27(b) shows performance in terms of PER.
Turbo SIC/RAKE detector
In the case where the propagation channel of the
k
-th user has an impulse
response with multiple paths
c
k
(
t
)
, it suces to replace the despreading function
