Turbo Multiuser Detection with Unknown Interferers
The turbo multiuser detection techniques developed so far
assume that the spreading waveforms of all users are known to the receiver.
Another important scenario, discussed in Chapter 3, is that the receiver has
knowledge of the spreading waveforms of some but not all of the users in a
system. Such a situation arises, for example, in a cellular system where the
base station receiver knows the spreading waveforms of the in-cell users but not
those of the out-of-cell users. In this section we discuss a turbo multiuser
detection method that can be applied in the presence of unknown interference,
which was first developed in [414].
6.4.1 Signal Model
Consider again the synchronous CDMA signal model (6.27). Here we
assume that the spreading waveforms and the received amplitudes of the first
( < K) users are known to the
receiver, whereas the rest of the users are unknown to the receiver. Since some
of the spreading waveforms are unknown, we cannot form the sufficient statistic
(6.32).
Instead, as done in Chapters
2 and 3, we sample the
received continuous-time signal r(t) at the chip
rate to convert it to discrete-time signal. The sample that corresponds to the
jth chip of the ith symbol is given by
Equation 6.72
The resulting discrete-time signal corresponding to the ith symbol is then given by
Equation 6.73
Equation 6.74
with
where
Equation 6.75
is a Gaussian random variable; n[i] ~ N(0, s2 IN);
sk is the normalized discrete-time spreading
waveform of the kth user, with cn,k 
{+1, –1}; 
; 
; and 
.
Denote by 
the matrix consisting of the first
columns of S. Denote the
remaining = K – columns of
S by 
. These
first signature sequences are unknown to the receiver. Let 
be the
-vector containing the first bits of b[i], and let
contain the remaining bits. Then we may write (6.74) as
Equation 6.76
Since we do not have knowledge of , we cannot
hope to demodulate . We therefore write (6.76) as
Equation 6.77
where 
is regarded as an interference term
that is to be estimated and removed by the multiuser detector before it computes
the a posteriori log-likelihood ratios (LLRs) for
the bits in .
Sections
