Performance improvement in decision feedback equalisers by using`soft decision'

One method to combat intersymbol interference (ISI) in digital transmission is to use a decision feedback equaliser (DFE). It is well known that a major part of the errors made by a DFE is due to the fact that the equaliser makes instantaneous decisions without taking into account data received later. It is therefore obvious that the performance of a DFE could be improved by allowing some type of `soft decision'. A method to introduce `soft decisions' in a decision feedback equaliser is presented. The method gives a performance improvement in SNR of about 1.5 dB with almost no increase in complexity compared to an ordinary DFE     

Approach to blind decision feedback equalisation

Based on the fact that a decision feedback equaliser (DFE) can be divided into a linear equaliser and a prediction error filter with feedback, a new blind adaptation method for the DFE is proposed. The proposed method provides reliable convergence and the resulting symbol error rate is very close to that of the DFE using a training sequence     

Adaptive feedforward/feedback architectures for multiuser detectionin high data rate wireless CDMA networks

We consider the design and performance of nonlinear minimum mean-square-error multiuser detectors for direct sequence code-division multiple-access (CDMA) networks. With multiple users transmitting asynchronously at high data rates over multipath fading channels, the detectors contend with both multiple-access interference (MAI) and intersymbol interference (ISI). The cyclostationarity of the MAI and ISI is exploited through a feedforward filter (FFF), which processes samples at the output of parallel chip-matched filters, and a feedback filter (FBF), which processes detected symbols. By altering the connectivity of the FFF and FBF, we define four architectures based on fully connected (FC) and nonconnected (NC) filters. Increased connectivity of the FFF gives each user access to more samples of the received signal, while increased connectivity of the FBF provides each user access to previous decisions of other users. We consider three methods for specifying the FFF sampling and propose a nonuniform FFF sampling scheme based on multipath ray tracking that can offer improved performance relative to uniform FFF sampling. For the FC architecture, we capitalize on the sharing of filter contents among users by deriving a multiuser recursive least squares (RLS) algorithm and direct matrix inversion approach, which determine the coefficients more efficiently than single-user algorithms. We estimate the uncoded bit-error rate (BER) of the feedforward/feedback detectors for CDMA systems with varying levels of power control and timing control for multipath channels with quasi-static Rayleigh fading. Simulations of packet-based QPSK transmission validate the theoretical BER analysis and demonstrate that the multiuser RLS adapted detectors train in several hundred symbols and avoid severe error propagation during data transmission mode     

Simplified parallel decision feedback equalisation for CCITT V.32/V.32 bis coded modems

A simplified parallel decision feedback equaliser (SPDFE) with the 32/64/128 AMPM CCITT trellis code is considered. The SPDFE detector consists of a whitened matched filter (WMF) and a reduced parallel decision feedback equaliser incorporated in the Viterbi decoder. The bit error rate simulation results show substantial improvement over the conventional detector with a linear equaliser and a separate Viterbi algorithm, although the implementation complexity remains the same.<>     

Iterative decision feedback equalisation of linearly precoded OFDM systems

An iterative extension of the decision feedback equaliser (DFE) receiver for linearly precoded OFDM is proposed, which improves the performance significantly, but requires a Cholesky factorisation for each iteration every time the channel changes. Next, a lower complexity form for Hadamard precoded systems is proposed, which requires just one Cholesky factorisation per OFDM symbol, with no loss in performance     

On the equivalence of the simultaneous and separate MMSEoptimizations of a DFE FFF and FBF

There is great interest in the use of decision feedback equalization (DFE) to mitigate the effects of intersymbol interference (ISI) on wireless multipath fading channels. The coefficients of a DFE feedforward filter (FFF) and feedback filter (FBF) are usually adjusted based on the minimum mean square error (MMSE) criterion. The equalizer coefficients can be calculated by recursive adaptation or by direct computation based on a channel estimate. The equivalence of the simultaneous and separate MMSE optimization of the FFF and FBF of a finite-length DFE is established     

Fuzzy adaptive decision feedback equaliser

As a new approach to the equalisation of nonlinear channels, a fuzzy adaptive decision feedback equaliser based on a fuzzy adaptive filter is proposed. With perfect knowledge of the channel, the proposed method not only improves equalisation performance but also reduces the computational complexity compared with the conventional fuzzy adaptive filter     

Analysis of quantized feedback low-frequency restoration in digitalregenerators

Quantized feedback, a DC restoration technique utilizing decision feedback equalization, can be used to compensate for unavoidable low-frequency cutoff in digital regenerators. An analysis is provided of the error probability and sensitivity to timing offsets of quantized feedback for a single-pole high-pass filter regenerator characteristic. Error probability is expressed in terms of the moments of the intersymbol interference (ISI), and these moments are calculated using the Markov chain properties of the ISI. To keep degradations small, the low-frequency cutoff should be no greater than 15% of the bit rate     

Decision feedback demodulation-based adaptive linear equalisers fordifferentially coherent DPSK systems

The authors propose to combine linear feedback equalisation and decision feedback demodulation for the equalisation of differentially coherent PSK signalling. By modifying the equaliser output based on the decision feedback demodulation before feeding back, the proposed equaliser can be made to behave like one with a decision feedback structure. Indeed, computer simulation results demonstrate that this equaliser performs much better than existing equalisers, such as linear equalisers for differentially coherent detection. Furthermore its performance is even comparable to that of a decision feedback equaliser with coherent detection     

Nonlinear feedback equaliser employing a soft limiter

An adaptive feedback equaliser using a soft limiter in the feedback path is briefly described. Curves and figures describing its adaptive behaviour are presented, and a brief comparison with the decision-feedback equaliser, which uses a hard limiter in the feedback path, is made.     

Blind decision feedback equaliser for multichannels with commonzeros

Blind equalisation of a fractionally spaced channel (FSC) is generally difficult if all subchannels have common zeros. A new blind equalisation structure is proposed so that the uncommon part of the FSC is equalised with a fractionally spaced equaliser and the common part of the FSC is equalised with a decision feedback equaliser which operates at the baud rate     

Turbo-equalisation for symbol-spread block transmission system

The authors consider coded block transmission over a memoryless Rayleigh fading channel based on an orthonormal transform. A new block-turbo equaliser is designed and its performance investigated and compared to an improved iterative block decision feedback equaliser     

Performance of adaptive multisensor decision feedback equaliser fortime-varying frequency selective radio channels

A multisensor decision feedback equaliser based on the minimum mean squared error (MMSE) criterion is studied. The superiority of the performance of the multisensor equaliser is shown by simulation of a whole communication system in which the adaptive equaliser is incorporated. The recursive least squares (RLS) algorithm is used to update the coefficients. From the results obtained for a time-varying urban terrain channel model, the extremely interesting tracking capability of the multisensor equaliser is shown     

Joint coding and decision feedback equalization for broadbandwireless channels

This paper introduces a new approach for joint convolutional coding and decision feedback equalization (DPE). To minimize error propagation, the DFE uses a combination of soft decisions and delayed tentative decisions to cancel intersymbol interference (ISI). Soft decisions are obtained by passing the DFE output through a (soft) nonlinear device. This simple method is shown to perform almost as well as an optimum soft feedback approach on wireless channels with diversity. Tentative decisions from the Viterbi decoder are used to cancel ISI due to multipath with large delays, thus remedying the increasing effect of error propagation in channels with large delay spreads. We consider the use of this soft/delayed feedback DFE (S/D-DFE) technique in broadband wireless channels (with delay spreads up to several tens of the symbol period) typical in high-bitrate mobile data applications. Simulation results indicate that the proposed joint coding and S/D-DFE technique performs to within 1-2 dB [in required signal-to-noise ratio (SNR)] of an ideal coded DFE without error propagation. When combined with antenna diversity and a reduced-complexity DFE concept with adaptive feedforward tap assignment, it provides high packet throughput against Rayleigh fading, severe delay spreads, and high Doppler rates     

Turbo equalisation in non-Gaussian impulsive noise

Turbo equalisation is a state-of-the-art receiving scheme for coded data transmission over channels introducing intersymbol interference (ISI). The author investigates turbo equalisation performance in the presence of ISI and impulsive noise. The design imperfections contributing to the non-robustness of the standard turbo equaliser to outliers are identified, and a novel turbo equaliser, at almost no additional increase in complexity, is proposed for joint mitigation of ISI and impulsive noise. The proposed turbo equaliser incorporates a Talwar penalty function into the maximum a posteriori (MAP) component equaliser to serve two purposes. First, it improves the estimation of the transition probabilities for all transitions through the trellis and for subsequent determination of the a posteriori log-likelihood ratio. Secondly, it absorbs the outliers and prevents them from spreading into the MAP constituent decoder. Simulation results based on Proakis's channel models show that the proposed turbo equaliser achieves a dramatic improvement over the standard turbo equaliser in impulsive noise. At a bit error rate (BER) of 10/sup -2/, the performance gain is as large as 3.5 to 5 dB, and as large as 7 to 8 dB at a BER of 10/sup -3/.     

Error-rate evaluation of linear equalization and decision feedbackequalization with error propagation

We propose applying an approximate Fourier series to evaluate efficiently the bit-error-rate (BER) performance of finite-length linear equalization (LE) and decision feedback equalization (DFE). By extending the Fourier series, we enable BER calculations for quadrature phase-shift keying (QPSK) transmission on complex channels with in-phase and crosstalk intersymbol interference (ISI). The BER calculation is based on determining the residual ISI samples and background Gaussian noise variance at the equalizer output for static channels or for realizations of quasi-static fading channels. A simple bound on the series error magnitude in terms of the Fourier series parameters ensures the required accuracy and precision. Improved state transition probability estimates are derived and verified by simulation for an approximate Markov model of the DFE error propagation for the case in which residual ISI exists even when the previous decisions stored in the feedback filter (FBF) are correct. We demonstrate the ease and widespread applicability of our approach by producing results which elucidate a variety of equalization tradeoffs. Our analysis includes symbol-spaced and fractionally spaced minimum mean-square error (MMSE)-LE, zero-forcing (ZF)-LE, and MMSE-DFE (with and without error propagation) on static ISI channels and multipath channels with quasi-static Rayleigh fading; a comparison between suboptimum and optimum receiver filtering in conjunction with equalization; and an assessment of the accuracy of some widely used equalization BER approximations and bounds     

Wavelet based decision feedback equaliser

A decision feedback equaliser (DFE) structure based on wavelets (WBDFE) is presented. The WBDFE shows faster convergence than the conventional DFE, with a very small increase in computation. In addition, the WBDFE has good time-frequency localisation, and is suitable for time-varing channels     

New gradient decision feedback equaliser algorithm

An efficient algorithm for a least-squares gradient lattice decision feedback equaliser is described. Simulation results for telephone channel and HF radio channel models are presented. Performance is compared with other decision feedback algorithms.     

Chromatic dispersion compensation with FFE-DFE utilising MZ filter for diverse VSB filtering

Chromatic dispersion compensation of non-return-to-zero on?off keying (NRZ-OOK) signals with a diverse-vestigial-sideband-filtering feed forward equaliser and a decision feedback equaliser (DVSB-FFEDFE) is presented, in which diverse vestigial sideband (VSB) filtering is realised with a Mach-Zehnder (MZ) filter. In a 10.709 Gbit/s NRZOOK system, experiment results show that the standard singlemode fibre transmission distance at 3 dB optical signal-to-noise ratio penalty with DVSB-FFE-DFE is doubled, compared to that with conventional FFE-DFE.     

Filtre correcteur de phase pour égaliseurs sous-optimaux

We propose a receiver structure intended for high bit rate transmissions. In order to ensure a good compromise between performance and complexity, the receiver is composed of a transversal phase- corrective filter followed by an equaliser, which takes only partially the channel impulse response into account. We propose a non-iterative technique to adjust the filter tap coefficients, based on the cepstral properties of a minimum phase response. The performance of the filter is evaluated and shown by simulating a transmission system including a multipath mobile radio channel, the filter and a dfse equaliser (decision feedback sequence estimator).