Decision-directed Feedback Research Articles

Maximum entropy whitening algorithm with adaptive neuron slope for blind DFE

The addressed blind decision feedback equalizer (DFE) reverses the classical order of its feed-forward and feedback filters at the beginning of adaptation to achieve the best equalization of the minimum and maximum phase components of a channel transfer function. Although very effective, this blind equalization approach deals with the feedback filter mismatch at the time of its transformation from the front-end all-pole whitener of the received signal to the decision-directed feedback equalizer placed after the feedforward filter. To eliminate this weakness, the adaptive neuron slope is introduced instead of the fixed one into the stochastic gradient whitening algorithm based on the joint entropy maximization cost. The performance of the innovated algorithm is verified by simulating m-QAM (quadrature amplitude modulation) signals transmission over multipath channels. The algorithm with the adaptive neuron slope achieves a better whitening of the received signal spectrum, and, hence, increases the equalization successfulness.

Decision-directed feedback equalisation for QAM signals

Proposed is a symbol-based decision-directed algorithm for blind equalisation of quadrature amplitude modulation (QAM) signals using a decision feedback scheme. Independently of QAM order, it presents: (i) an error equal to zero when the equaliser output coincides with the transmitted signal; (ii) simultaneous recovery of the modulus and phase of the signal; (iii) a misadjustment close to that of the normalised least-mean squares algorithm; (iv) fast convergence; and (v) the avoidance of degenerative solutions. Additionally, its stability is ensured when the step-size is properly chosen.

Joint decision-directed feedback multiuser channel gain and delay estimation for DS-CDMA uplink

This paper addresses the issue of channel parameter estimation for uplink direct-sequence code-division multiple-access (DS-CDMA) system. A frequency-selective multipath channel is considered, and a feedback (FB) structure to jointly estimate path delays and complex gains is developed. The proposed algorithms are extensions of those from , where only path-delay estimation is considered. In the current paper, a detailed mathematical analysis of the coupling between the different parallel feedback loops is carried out. From this analysis, results are derived pertaining to system dynamics and jitter variance. Furthermore, a novel operating mode, called improved decision-directed (IDD), is proposed as an improvement of the standard decision-directed (DD) mode. The resulting performance will be measured in terms of mean-square estimation errors (MSEEs) and benchmarked against the corresponding modified Crame/spl acute/r-Rao bounds (MCRBs). Finally, the impact of the accuracy of the estimated complex gains on the jitter of the path delay trackers (and vice versa) is investigated.

Performance analysis of decision-directed maximum-likelihood phase estimators for M-PSK modulated signals

We investigate performance of decision-directed maximum-likelihood carrier phase synchronizers for M-PSK signalling (M/spl les/8) on the AWGN channel. Decision-directed feedback (DD-FB) and feedforward decision feedback (FF-DFB) structures are analyzed and compared in detail. A new, accurate model for the FF-DFB synchronizer is presented. Open loop characteristics and closed loop performance, in terms of steady-state jitter and cycle slip rate, are presented in the same framework for both the DD-FB and FF-DFB synchronizers. Performance comparison between the two phase estimator architectures is finally made for each modulation scheme investigated.

A coherent spread-spectrum diversity-receiver with AFC for multipath fading channels

A direct-sequence spread-spectrum (DSSS) coherent RAKE-receiver demodulating M-ary phase-shift keyed (MPSK) signals is presented for microcellular communication systems with frequency-selective fading channels. Since the channel fading is due to multipath propagation, the received signal contains multipath or time diversity which is resolved at the receiver by a RAKE-structure. The maximum-likelihood (ML) rule is invoked to estimate the phases, and the combining factors of the receiving paths, as well as the frequency offset between the carrier of the receiver and the transmitter. The ML algorithm is simplified by using decision-directed feedback. The resulting digital algorithms for synchronization, data demodulation, and frequency error detection (FED) are well suited for digital implementation because of their fairly low numerical complexity. Analytical expressions of the bit error probability versus signal-to-noise ratio (SNR) and versus frequency offset are presented. In addition, experimental results using Monte-Carlo simulations have also been carried out. The results show a negligible influence of the decision-direction above a certain SNR, as well as a "robust" transient behavior of the phase estimation and the automatic frequency control (AFC) scheme.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Error propagation in decision feedback receivers

The problem of assessing error propagation in decision-directed feedback receivers is considered. It is shown that useful bounds on error statistics can be obtained by modelling the error mechanism using stochastic state-transition diagrams and then employing state reduction techniques to enable the complexity of the problem to be contained. This provides a more widely applicable approach to obtaining bounds on performance measures for such receivers than was previously available. The techniques are illustrated for representative error burst statistics of interest in practical applications.

Decision-Directed Detector for Overlapping PCM/NRZ Signals

A decision-directed (DD) technique for the detection of overlapping PCM/NRZ signals in the presence of white Gaussian noise is investigated. The performance of the DD detector is represented by probability of error PE versus input signal-to-noise ratio (SNR). To examine how much improvement in performance can beachieved with this technique, PE's with and without DD feedback are evaluated in parallel. Further, analytical results are compared with those found by Monte Carlo simulations. The results are shown in good agreement.

Data-Aided Carrier Tracking Loops

An innovation which improves the capability and performance of certain types of phase-coherent demodulators is introduced. This innovation has application in the fields of telemetry and tracking, e.g., relay satellite systems, deep-space communications, and military communication systems. The basic idea of the innovation centers around using the power in the composite signal sidebands to enhance the demodulator's effective signal-to-noise ratio (SNR). The vehicle through which this is accomplished employs the principle of decision-directed feedback. In its simplest form it is shown, for the first-order loop, that the maximum improvement in loop SNR relative to a standard phase-locked loop is 10 dB. The paper explores and illustrates the corresponding improvement realized in detection system efficiency. Extension to higher order loops follows directly from the analysis given here.