Conference: The 2007 International Conference on Wireless Networks (ICWN’07). Part of the 2007 World Congress in Computer Science, Computer Engineering, & Applied Computing (Worldcomp’07),
In this paper, a new scheme for reducing the computational complexity of multiuser receivers is presented. It utilizes the transformation matrix (TM) algorithm to improve the performance of multiuser receivers by effectively reducing the bit error rate (BER). In addition, a deterministic formalization of the processing gain (PG) for a multiuser DS CDMA system is presented. The proposed formalization of the PG demonstrates that how the reduced BER could be used to achieve reasonable values of PG by which unwanted signals or interference can be suppressed relative to the desired signal at the receiving end. The proposed algorithms not only are shown to substantially improve the performance of the multiuser detectors by means of reduced BER but also have a much lower multi-access interference. The performance measure adopted in this paper is the achievable bit rate for a fixed probability of error (10-7) and consistent values of the PG
In this letter, we derive a recursive, additive metric for
complexity-constrained maximum likelihood detection for multiuser CDMA
using breadth-first detection algorithms. The metric requires linear
filtering of the matched-filtered received signal vector. It is shown
that a class of filters fulfilling certain requirements lead to
identical performance
In this paper, we compare several optimization methods for solving the optimal multiuser detection problem exactly or approximately. The purpose of using these algorithms is to provide complexity constraint alternatives to solving this nondeterministic polynomial-time (NP)-hard problem. An approximate solution is found either by relaxation or by heuristic search methods, while the branch and bound algorithm is used to provide an exact solution. Simulations show that these approaches can have bit-error rate (BER) performance which is indistinguishable from the maximum likelihood performance. A tabu search method is shown to be an effective (in terms of BER performance) and efficient (in terms of computational complexity) heuristic when compared to other heuristics like local search and iterative local search algorithms. When the number of users increases, the tabu search method is more effective and efficient than the semidefinite relaxation approach.
A maximum likelihood optimum detector with an asymptotic
complexity per user of 0(1.5<sup>K</sup>) has been derived for the
synchronous DS/CDMA channels. The detector employs a local descent
algorithm through the Voronoi regions for the equivalent hypothesis
detector
Simple reduced tree-search detection schemes of the breadth-first
type are applied to suboptimal joint multiuser detection in
bit-synchronous code-division multiple access (CDMA) systems over both
Gaussian and two-path Rayleigh-fading channels. It is pointed out that
in contrast to the case of the optimal multiuser detector, the choice of
the receiver filter severely influences the performance of suboptimal
multiuser detectors. Simulation results supported by analysis show that
breadth-first tree-search algorithms using a decorrelating noise
whitening receiver filter perform better than similar receivers, which
solely use a matched filter (MF) for virtually all nonsingular spreading
code sets studied. Most of the code sets are randomly generated. The M-
and T-algorithm detectors based on decorrelating noise whitening filter
(WF) outputs can achieve near optimum performance at a very low
complexity compared to the optimal detector, although the proposed
detectors are more complex than some known suboptimum detectors.
Furthermore, the use of combining techniques is considered for a
two-path Rayleigh-fading channel, and a semi-synchronous CDMA structure
is proposed. It is shown that if maximum ratio combining (MRC) is
employed, the decorrelating noise WF still exists. The corresponding
suboptimal combining detector with a decorrelating noise WF outperforms
a similar noncombining detector
Consider a Gaussian multiple-access channel shared by K users who transmit asynchronously independent data streams by modulating a set of assigned signal waveforms. The uncoded probability of error achievable by optimum multiuser detectors is investigated. It is shown that the K -user maximum-likelihood sequence detector consists of a bank of single-user matched filters followed by a Viterbi algorithm whose complexity per binary decision is O(2^{K}) . The upper bound analysis of this detector follows an approach based on the decomposition of error sequences. The issues of convergence and tightness of the bounds are examined, and it is shown that the minimum multiuser error probability is equivalent in the Iow-noise region to that of a single-user system with reduced power. These results show that the proposed multiuser detectors afford important performance gains over conventional single-user systems, in which the signal constellation carries the entire burden of complexity required to achieve a given performance level.
A review of adaptive detection techniques for direct-sequence code
division multiple access (CDMA) signals is given. The goal is to improve
CDMA system performance and capacity by reducing interference between
users. The techniques considered are implementations of multiuser
receivers, for which background material is given. Adaptive algorithms
improve the feasibility of such receivers. Three main forms of receivers
are considered. The minimum mean square error (MMSE) receiver is
described and its performance illustrated. Numerous adaptive algorithms
can be used to implement the MMSE receiver, including blind techniques,
which eliminate the need for training sequences. The adaptive
decorrelator can be used to eliminate interference from known
interferers, though it is prone to noise enhancement. Multistage and
successive interference cancellation techniques reduce interference by
cancellation of one detected signal from another. Practical problems and
some open research topics are mentioned. These typically relate to the
convergence rate and tracking performance of the adaptive
algorithm
