Linear Decorrelating Detector Research Articles

Near-far resistant channel estimation for CDMA systems using the linear decorrelating detector

The well-known linear decorrelating detector (LDD) for direct-sequence code-division multiple-access (CDMA) systems provides near-far resistant performance when the timing of each user is accurately known. Traditional CDMA acquisition techniques suffer from high differences in power levels. The estimation accuracy for a user overwhelmed by stronger ones is likely to be unsatisfactory; at the same time, the interference from a user undergoing acquisition or tracking is not removed by the standard LDD. In this paper, a fully near-far resistant technique for acquisition and tracking for asynchronous CDMA systems applying the LDD is proposed, considering realistic band-limited signals. This technique is based on the adoption of a pair of special sequences equivalent to a dedicated access channel and is shown to provide a relatively fast and robust means to perform channel estimation both in case of single- and multipath channels.

Frame-extended linear decorrelating detector for asynchronous CDMA

The standard linear decorrelating detector (LDD) for asynchronous code-division multiple access involves complicated computations and long detection delay. The one-shot LDD with a frame of 1-bit period length leads to a simpler architecture with less than 1-bit detection delay but significant noise enhancement compared with the standard LDD. In this paper, extending the frame to more than one bit period, a frame-extended LDD (FELDD) is proposed. With a slight increase of complexity and detection delay, the noise enhancement of the FELDD is greatly reduced compared with the one-shot LDD and approaches that of the standard DD. The total number of users supported is also increased compared with that of the one-shot LDD. Theoretical analysis and computer simulations are used to demonstrate the feasibility of the proposed scheme.

Blind adaptive energy estimation for decorrelating decision-feedback CDMA multiuser detection using learning-type stochastic approximations

This paper investigates the application of linear reinforcement learning stochastic approximation to the blind adaptive energy estimation for a decorrelating decision-feedback (DDF) multiuser detector over synchronous code-division multiple-access (CDMA) radio channels in the presence of multiple-access interference (MAI) and additive Gaussian noise. The decision-feedback incorporated into the structure of a linear decorrelating detector is able to significantly improve the weaker users' performance by cancelling the MAI from the stronger users. However, the DDF receiver requires the knowledge of the received energies. In this paper, a new novel blind estimation mechanism is proposed to estimate all the users' energies using a stochastic approximation algorithm without training data. In order to increase the convergence speed of the energy estimation, a linear reinforcement learning technique is conducted to accelerate the stochastic approximation algorithms. Results show that our blind adaptation mechanism is able to accurately estimate all the users' energies even if the users of the DDF detector are not ranked properly. After performing the blind energy estimation and then reordering the users in a nonincreasing order, numerical simulations show that the DDF detector for the weakest user performs closely to the maximum likelihood detector, whose complexity grows exponentially with the number of users.

Recursive structures and finite impulse response implementations of linear multiuser detectors for an asynchronous CDMA system

We present order-recursive structures for implementing the linear decorrelating and linear minimum mean square error (MMSE) detectors in a computationally efficient manner. These infinite memory length, linear time invariant, noncausal systems can be approximated by implementing them as K-input K-output finite impulse response (FIR) filters. We developed a linear multiuser receiver architecture called a recursive linear multiuser detector which has lower computational and memory requirements then an equivalent tap delay line FIR filter implementation. We establish the tradeoff between window length and bit error rate and find that relatively small window size yields performance indistinguishable from the ideal decorrelating detector and the ideal MMSE detector.

Orthogonal on-off BPSK: a convenient signaling scheme for near-far resistant detection in DS/CDMA

This paper proposes a convenient signaling scheme-orthogonal on-off BPSK (O/sup 3/BPSK)-for near-far (NF) resistant detection in asynchronous direct-sequence code-division multiple-access (DS/CDMA) systems (uplink). The temporally adjacent bits from different users in the received signals are decoupled by using the on-off signaling, and the original data rate is maintained with no increase in transmission rate by adopting an orthogonal structure. The detector at the receiver is a one-shot linear decorrelating detector, which depends upon neither hard decision nor specific channel coding. The application of O/sup 3/ strategy to the differentially encoded BPSK (D-BPSK) sequences is also presented. Finally, some computer simulations are shown to confirm the theoretical analysis.

Capacity of the linear decorrelating detector for QS-CDMA

We study the performance and capacity of a quasi-synchronous code-division multiple-access (QS-CDMA) cellular system employing the decorrelating detector. It is assumed that each of N users has a GPS generated local clock, and attempts to transmit in synchrony with the other users in its cell. The detector makes use of the small timing uncertainties to build the interference signal space and perfectly cancel the interference. Results are compared with those of an adaptive MSE receiver. It is shown that when the LMS adaptive algorithm is employed, the mean-squared-error misadjustment significantly affects the receiver performance. Finally, system capacities in terms of users/cell are evaluated.

Multiuser detection of quasisynchronous CDMA signals using linear decorrelators

A linear decorrelator detector is proposed for a quasisynchronous code-division multiple-access (QS-CDMA) cellular system. It is assumed that each of N users has a Global Positioning System (GPS) generated local clock and attempts to transmit synchronously with the other users in its cell. The base station thus receives N signature waveforms which are approximately synchronized, but with a small timing error due to local oscillator drift, Doppler shifts, and variable distances from the mobiles to the base. The residual timing offset is discretized, and a decorrelator is constructed which attempts to reject the multiuser interference vectors corresponding to the set of possible offsets. A noncoherent version of the decorrelator detector is presented, along with a maximum-likelihood scheme for synchronizing to the desired signal. Bounds and analytical results for the bit-error rate (BER) are obtained for various interferer conditions.

Near-far resistant detection of CDMA signals via isolation bit insertion

This paper presents a novel scheme for near-far resistant CDMA detection: isolation bit insertion (IBI). At the transmitter, isolation bits are inserted into the information bit sequence before modulation, and a practical linear decorrelating detector (LDD) is obtained at the receiver. All the advantages that an LDD theoretically offers are retained and realised in practice. >

A family of multiuser decision-feedback detectors for asynchronous code-division multiple-access channels

It is important to identify simple and reliable interference rejection methods for code-division multiple-access (CDMA) channels, since the conventional matched filter receiver has high error rates, and the optimal detector is too complex. We introduce decision-feedback and partial feedback detectors for asynchronous CDMA channels. Two-stage detectors with decision-feedback in the second stage are also studied. The derivation of the decision-feedback detector is based on spectral factorization which leads to a white-noise channel model. We also describe two implementations of the maximum-likelihood detector for this model. Comparisons among the proposed detectors, the conventional detector, and the linear decorrelating detector are undertaken for several asynchronous CDMA channels. In these examples, high bandwidth efficiency systems are explored, and user energies are varied form being similar to being very different. We find that decision-feedback detectors compare favorably with more complex two-stage methods and maintain good performance under diverse channel conditions. >

On the performance of near-far resistant CDMA detectors in the presence of synchronization errors

Little has so far been reported on the performance of the near-far resistant CDMA detectors in the presence of the synchronization errors. Starting with the general mathematical model of matched filters, this paper examines the effects of three classes of synchronization errors (i.e. time-delay errors, carrier phase errors, and carrier frequency errors) on the performance (bit error rate and near-far resistance) of an emerging type of near-far resistant coherent DS/SSMA detectors, i.e. the linear decorrelating detector (LDD). For comparison, the corresponding results for the conventional detector are also presented. It is shown that the LDD can still maintain a considerable performance advantage over the conventional detector even when some synchronization errors exist. Finally, several computer simulations are carried out to verify the theoretical conclusions.

Multiuser detectors for synchronous CDMA communication over non-selective Rician fading channels

Synchronous code division multiple access (CDMA) data transmission over a non-selective Rician fading channel is considered, where the faded signal components arrive at the receiver in synchronism with the specular signal component. Under the assumption that the fading parameters are uncorrelated, this fading CDMA channel is shown to be equivalent to a Gaussian CDMA channel over which a modified signal set is employed. A fading counterpart of the multiuser asymptotic efficiency performance measure is defined. The asymptotic efficiencies of the optimum, decorrelating, and conventional detectors designed for the fading channel are derived. The conventional detector, which has low complexity, is shown to degrade rapidly in near-far environments. The optimal detector for the fading channel exhibits a significant improvement in performance but at the price of a computational complexity that increases exponentially with the number of users. The linear decorrelating detector for the fading case is optimally near-far resistant. The asymptotic efficiencies of multiuser detectors designed for the Gaussian channel when employed over the Rician fading channel are also obtained, thereby quantifying the loss in performance incurred by these mismatched detectors.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Decorrelating decision-feedback multiuser detector for synchronous code-division multiple-access channel

A decorrelating decision-feedback detector (DF) for synchronous code-division multiple-access (CDMA) that uses decisions of the stronger users when forming decisions for the weaker ones is described. The complexity of the DF is linear in the number of users, and it requires only one decision per user. It is shown that performance gains with respect to the linear decorrelating detector are more significant for relatively weak users and that the error probability of the weakest user approaches the single-user bound as interferers grow stronger. The error rate of the DF is compared to those of the decorrelator and the two-stage detector. >