Decorrelating Decision-feedback Detector Research Articles

An optimization approach to decision feedback detection under modulation constraints for MIMO fading channels

A new technique is proposed for designing decision feedback detectors (DFDs) wherein the per-symbol decision rules are obtained by exploiting modulation constraints. It is presented in the context of the multi-input, multi-output (MIMO) fading channel with K transmit and N receive antennas. Three cases are considered to illustrate the technique where all the transmitters employ real-valued pulse-amplitude modulation (PAM), or phaseshift keying (PSK), or complex quadrature amplitude modulation (QAM). In each case, the corresponding per-symbol decision rules are obtained by imposing the modulation constraints on a likelihood function maximization problem. When all transmitters employ PAM, it is shown that the resulting DFD (the PAM-DFD) is equivalent to the decorrelating decision feedback detector (DDFD) when the latter is used on a modified received statistic. Two DFDs are then derived (the PSK-DFDs and the QAMDFDs) for the cases when all transmitters employ PSK and QAM, respectively. To illustrate the performance benefit, we consider the Rayleigh fading channel and derive the exact joint error probability (JEP) of the PAM-DFD and show that the (possibly fractional) diversity order of the JEP is equal to N - K-1/ 2 . This greatly improves on the diversity order of N - K + 1 of the JEP obtained by the D-DFD without exploiting the real modulation constraint. Through simulations, it is shown that the PSK-DFDs and the QAM-DFDs result in significant performance improvements over the D-DFD as well. It is conjectured that one of the PSK-DFD achieves the improved diversity order of N - K-1/ 2 as well and the QAM-DFDs result in an improvement in effective SNR gain.

Decision-Feedback Multiuser Detection With Orthogonal Feedforward Transformation

Our proposed orthogonal decision feedback detector (ODFD) contains an orthogonal feedforward filter and a decision feedback structure. Compared with the decorrelating decision feedback detector (DDFD), our QR-factorization-based ODFD is computationally more efficient in dynamically updating the feedforward and feedback filter pairs. This feature is beneficial when the number of users in a system changes frequently. Using the Gaussian approximation, which was used to compute the approximate bit-error probabilities (BEPs) of multiusers for the successive interference canceller (SIC), we calculate the approximate BEPs of multiusers for the DDFD and our ODFD. The Gaussian approximation leads to good approximate BEPs for DDFD and to fine approximate BEPs for ODFD, but to poor approximate BEPs for SIC. The accuracy of these approximate BEPs is also justified by the results from Monte Carlo simulations.

Analysis of Decision Feedback Detection for MIMO Rayleigh-Fading Channels and the Optimization of Power and Rate Allocations

For an uncoded, K-transmit, N-receive antenna coherent narrow-band communication system employing a decorrelating decision feedback detector (D-DFD), the exact average (over channel realizations) joint error probability (JEP) as well as the average per-symbol error probabilities (SEPs) are derived without making any simplifying assumptions on error propagation. It is proved that the diversity orders of the JEP and the SEP (of every symbol) is limited by error propagation to N-K+1. Based on our exact error probability analysis, however, we suggest an optimization of JEP over nonnegative quadrature amplitude modulation (QAM) constellation sizes (rates) and average powers across transmitters which yield significant improvements over the usual equal power and equal rate assignment. In fact, the JEP of such an optimized design has the much improved diversity order of N (which is also the diversity order obtained through the optimum maximum-likelihood (ML) detector). Moreover, it is seen that these simple optimized designs can achieve a significant fraction of the /spl epsi/-outage capacity even without outer codes. It is also known-but only through simulations-that when the symbols are detected in certain channel realization-dependent orders it is possible to improve substantially over fixed-order detection in the case of the equal rate and equal power assignment. We provide an analysis for a recently proposed channel-dependent ordering rule and show that it does not provide an improvement of the diversity order of the JEP beyond N-K+1. Another ordering rule that was proposed earlier to maximize the worst case post-detection signal-to-noise ratio (SNR) under the perfect feedback assumption is shown to be optimal under a more compelling criterion that does not involve that simplifying assumption. While efficiently computable, this ordering rule is seen to perform almost as well as the optimal channel-dependent ordering rule that minimizes the conditional JEP (and hence the JEP). Nevertheless, a multiple-input multiple-output (MIMO) system with an optimized rate and power allocation and a fixed order of detection is not only less complex but also has a significantly lower JEP than that of the equal-power, equal-rate system, where transmitters are detected in a channel-dependent order, optimal or otherwise.

Fast Optimal and Suboptimal Any-Time Algorithms for CDMA Multiuser Detection Based on Branch and Bound

A fast optimal algorithm based on the branch-and-bound (BBD) method is proposed for the joint detection of binary symbols of K users in a synchronous code-division multiple-access channel with Gaussian noise. Relationships between the proposed algorithms (depth-first BBD and fast BBD) and both the decorrelating decision-feedback (DF) detector and sphere-decoding algorithm are clearly drawn. It turns out that decorrelating DF detector corresponds to a "one-pass" depth-first BBD; sphere decoding is, in fact, a type of depth-first BBD, but one that can be improved considerably via tight upper bounds and user ordering, as in the fast BBD. A fast "any-time" suboptimal algorithm is also available by simply picking the "current-best" solution in the BBD method. Theoretical results are given on the computational complexity and the performance of the "current-best" suboptimal solution.

Multiple Model Adaptive Estimation for Multiuser Detection in CDMA Communication

This paper introduces a blind adaptive multiuser detection scheme for code division multiple acces (CDMA) communication, based on the interacting multiple model algorithm. The system estimates the received power and input data sequence which are corrupted by multiple access interference (MAI) and noise. A hybrid system is used to model the synchronous code division multiple access, and multiuser detection is implemented using the interacting multiple model (IMM) technique. The performance of the IMM based multiuser detector is studied and simulations are used to compare with the decorrelating decision feedback detector (DDFD) and decision-feedback blind adaptive multiuser detector (DFBD).

The Cholesky-iterative detector: a new multiuser detector for synchronous CDMA systems

The decorrelating decision-feedback (DDF) multiuser detector based on Cholesky factorization has been shown to alleviate the performance degradation of the users in the detection process, especially for relatively low cross-correlation values between user signals. A new detection concept for multiple users described in this paper employs two triangular matrices (upper and lower) and soft output information to improve the data estimates. Simulation results show that significant performance gains can be achieved over the DDF. Also, the users tend to have their bit error probabilities clustered. Thus, the performance of a given user is less dependent on its position in the detection process than for the DDF detector.

Novel synchronous CDMA multiuser detection scheme: orthogonal decision-feedback detection and its performance study

The authors propose a novel multiuser detection scheme for synchronous DS/CDMA systems, the orthogonal decision-feedback detector (ODFD), which performs equally well as the decorrelating decision-feedback detector (DDFD) but with a much reduced complexity. The ODFD employs a match filter bank matching to a set of orthonormal sequences which are generated by the Gram–Schmidt orthogonalisation procedure based on spreading codes. The ODFD algorithm involves only the orthonormal coefficient matrix which requires no frequent recalculations even when system parameters change. Successive decision-feedback detection is performed immediately at the output of the ODFD match filter bank without the transform matrix which is required in the DDFD.

Synchronous DS-SSMA system with improved decorrelating decision-feedback multiuser detection

An improved decorrelating decision-feedback multiuser detector (IDDFD) is proposed, whose complexity is linear in the number of users, but performs closely to the maximum likelihood detector (MLD), whose complexity grows exponentially with the number of users. The IDDFD also relaxes the requirement of correct power ordering which is essential to successfully cancel multiuser interference (MUI) for the standard decorrelating decision-feedback detector (DDFD). Simulation results show that in a direct-sequence spread-spectrum multiple-access (DS-SSMA) system using Gold codes of length 31 an IDDFD with only twice the complexity of the DDFD can simultaneously service 31 users with an error rate of each user close to the interference-free case. The IDDFD is then extended and studied to user numbers larger than the processing gain, complex signature waveforms, and MPSK signaling.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>