Adaptive Multicarrier CDMA Space-Time Receivers - Adaptive Signal Processing in Wireless Communications - page 280

shows that the net interference increases with the number of interferers and subcarriers,
which severely limits the capacity of the MC-CDMA system with simple MRC receivers.
Therefore, in the next section, we shall use the data decomposition of equation (10.23) to
formulate the interference suppression problem and propose a new MC-CDMA receiver
with full interference suppression capabilities.
10.2.9 Multiuser Detection Techniques for MC-CDMA Systems
Conventional multicarrier CDMA detectors—such as the matched filter, the RAKE
combiner, and the MC-STAR receiver—are optimized for detecting the signal of a sin-
gle desired user. RAKE combiners exploit the inherent multipath diversity in CDMA,
since they essentially consist of matched filters for each resolvable path of the multipath
channel. The outputs of these matched filters are then coherently combined according to
a diversity combining technique, such as maximum ratio combining, equal gain com-
bining, or selection diversity combining [13, 54]. Unlike RAKE-type receivers, which
assume perfect knowledge of the channel [3], we proposed in previous work a full space-
time receiver solution, named MC-STAR [43], that jointly implements adaptive chan-
nel identification and synchronization in both time and frequency.* These conventional
single-user detectors are inefficient, because the interference is treated as noise and there
is no utilization of the available knowledge about the mobile channel or the spreading
sequences of the interferers.
In order to mitigate the problem of MAI, [33] proposed and analyzed the optimum
multiuser detector for asynchronous Gaussian multiple access channels. This optimum
detector significantly outperforms the conventional detector, and it is near-far resistant,
but unfortunately its complexity grows exponentially with the number of interfering
users. Following this work, numerous suboptimum multiuser detectors have been pro-
posed for a variety of channels, data modulation schemes, and transmission formats.
Since MC-CDMA systems also contain a DS-CDMA component, traditional sub-
optimum multiuser detection techniques can be performed on each carrier with some
form of adaptation. A variety of linear multiuser receivers have been investigated for
MC-CDMA systems such as the minimum mean square error (MMSE) detector [34] and
the combination of MMSE and the decorrelator detector [35]. Interference cancellation
(IC) schemes constitute another variant of multiuser detection that has been applied to
MC-CDMA systems. They can be broadly divided into two categories: parallel cancel-
lation (PIC) [37, 38] and successive cancellation (SIC) [36]. At each stage in the detector,
the estimates of all the other users from the previous stage were used for reconstructing
an estimate of the MAI, and this estimate was then subtracted from the interfered signal
representing the wanted bit. A novel class of multicarrier multiuser detectors, referred
to as subspace blind detectors, was proposed by [39] and [40], where only the spreading
sequence and the delay of the desired user were known at the receiver. Based on this
knowledge, a blind subspace tracking algorithm was developed for estimating the data
of the desired user.
* MC-STAR is our starting receiver; hence, we will provide a short overview of this receiver in
section 10.3.