Digital Signal Processing Reference
In-Depth Information
Most of these multiuser receivers have focused on multiple access interference while
ignoring the ICI. In addition, important system design issues such as carrier frequency
offset recovery (CFOR) have often been neglected. In multiuser detection, the CFO of
one user not only degrades the detection of that user itself, but also makes the receiver
based on the ideal carrier frequency acquisition no longer optimal, thus degrading the
detection of the other users [41]. An alternative multiuser detection technique, denoted
interference subspace rejection (ISR), has been proposed for DS-CDMA [42]. This tech-
nique offers different modes. Each mode characterizes the interference vector in a dif-
ferent way and accordingly suppresses it. The flexibility and robustness inherent to ISR
make its exploitation in multicarrier systems of great interest.
10.3
Proposed Adaptive Multicarrier CDMA
Receiver: MC-ISR
This section is dedicated to the description, performance analysis, and implementation
of the proposed MC-ISR receiver [6]. After a short overview of MC-STAR [43], which is
our starting single-user receiver, we will describe and evaluate the adaptive interference
rejection procedure that characterizes the proposed MC-ISR receiver.
10.3.1 The General Concept of MC-STAR
The adaptive receiver MC-STAR implements joint space-time-frequency processing
over the despread data to improve the spectrum efficiency of the MC-CDMA system.
The adaptive blind channel identification and equalization as well as the acquisition
and tracking of multipaths and CFO are carried out on each subcarrier. However, their
modules are interconnected to ensure proper information exchange and joint processing
over carriers.* Mathematical details of the different adaptive procedures and their con-
nections are provided in [43]. In this section, we explain the advantages of MC-STAR by
describing the intermediate stages in our development that led to this receiver.
At the beginning, we extend original STAR, proposed for DS-CDMA [31], to a mul-
ticarrier system by placing STAR on each subcarrier. This extension requires a modi-
fication of the time-delay tracking procedure. Indeed, we introduce an intermediate
transformation of the time response to reallow estimation of the multipath delays by
simple linear regression.
Multi-carrier CDMA systems are very sensitive to the CFO. Therefore, we further
introduce joint time-delay and frequency synchronization. The effect of the CFO on
the performance of the spatiotemporal array receiver was not addressed in [31]. The
space-time separation of the channel enables us to decouple time and carrier frequency
synchronization. We can hence estimate the CFO by linear regression (LR) of the phase
variation of each fading coefficient. Once an estimate of the carrier frequency offset
* The complexity of MC-STAR, which is approximately the complexity of STAR multiplied by the
number of subcarriers, can be assessed using the results established in [44]. The latter suggests
that MC-STAR can be implemented today on a single FPGA.
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