Performance Of Decision Feedback Equalizer Research Articles

PERFORMANCE EVALUATION OF DECISION FEEDBACK EQUALIZATION (DFE) BASED DISPERSION COMPENSATOR BY VARYING FORWARD TAP SPACE & ADAPTIVE STEP SIZE IN THE SCENARIO OF 4X10-GBIT/S WDM SYSTEM

With the advancement of technology, the demand for bandwidth increases exponentially with time. Hence, optical fiber communication becomes the optimum choice for backbone networks. As the efficiency of the optical system is limited by congenital parameters such as chromatic dispersion. So, in this work, we have evaluated the ability of electronic equalization scheme reduce the CD (chromatic dispersion for an optical communication system capable of transporting 4 channels at 10 Gbps line rate over a distance of 30km. Further, we compare the performance of decision feedback equalizer with two different configurations.

Study of Linear and Decision Feedback Equalizers on Fading Channels

For evaluating the performance of wireless digital communication system the most important part is the simulation of wireless channel, as whole performance of the system is depends upon it. The Band limited communication channel, due to its dispersive time response, may restrict the high data rate, due to Inter symbol Interference (ISI). If the ISI is not taken care off and compensated may cause high Bit Error Rate (BER). Equalizers are used for this purpose. In this paper the performance of Linear and Decision Feedback Equalizers (DFE) are evaluated on three wireless channels i.e. two ray path, Rayleigh and Rician. A model of communication system having Quadrature Amplitude Modulation (QAM) with fading channels is implemented using MATLAB. The performance of Linear and DFE is evaluated on the basis of Bit Error Rate (BER) with varying Signal to Noise Ratio (SNR). The simulation results showed the performance of two equalizers on different channels. The Rayleigh channel model is the most suitable model for urban areas as it is the model which simulates a channel with no Line of Sight (LOS) path and is the most difficult environment. General Terms ISI (Inter-symbol Interference), BER (Bit Error Rate), SNR (Signal to Noise Ratio)

A 0.41 pJ/Bit 10 Gb/s Hybrid 2 IIR and 1 Discrete-Time DFE Tap in 28 nm-LP CMOS

An ideal infinite impulse response (IIR) decision feedback equalizer (DFE) can have an effect on wireline received waveforms similar to a continuous-time equalizer, but without the associated amplification of noise and crosstalk. However, an IIR DFE's performance degrades significantly as the feedback loop delay increases. Fortunately, adding a single discrete-time tap can eliminate the degradation. The implementation of a half-rate DFE with two IIR taps and one discrete-time tap is presented here. The two IIR filters have different time constants to accommodate a variety of channel pulse responses having a long tail. The discrete-time tap cancels the first post-cursor inter-symbol interference (ISI) term and alleviates feedback loop timing issues. The DFE can receive data transmitted with a low swing of 150 mVpp-diff through 24 dB of channel loss at half the bitrate while consuming 4.1 mW at 10 Gb/s. Digital foreground calibration of clock phase shifters and offset cancellation is described. The receiver, including the DFE, clock buffers and clock phase adjustment, occupies an area of 8760 µm 2 in an ST 28 nm LP CMOS process.

Convergence and Performance Analysis of Multi-Split Decision Feedback Equalizer

In this paper, the idea of multi-split adaptive filter is applied to a Decision Feedback Equalizer (DFE). The feed-forward and feedback sections in the DFE are divided into parallel sub-filters by imposing separately the symmetry and antisymmetry conditions on the impulse responses of filters with the use of appropriate and distinct sets of linear constraints. The objective of this work is to obtain a new structure that gives better performance than the conventional structure and to study its convergence behavior. The performance of multi-split DFE is tested with different algorithms (LMS, RLS, and DCT-LMS) and compared with the performance of multi-split linear transversal Equalizer. We show also by simulation that the feedback section of DFE need not be split.

On the Decision-Feedback Equalizer in Optically Amplified Direct-Detection Systems

This paper investigates the combined feed-forward and decision-feedback equalizer (DFE) in a lightwave system with optical amplifiers and a direct-detection receiver. Based on a nonlinear channel model, the paper provides a modification of the classical minimum mean square error theory of the DFE. Furthermore, an analytical method that is capable of accurate bit error rate (BER) evaluation is used to optimize the DFE for minimum BER. The paper evaluates the DFE performance for both optical ON-OFF keying and duobinary modulation formats in the presence of chromatic dispersion as well as the DFE performance for the mitigation of higher order polarization mode dispersion in first-order compensated systems. The paper shows that the DFE can compensate for the BER degradation due to narrow-band receiver-side optical filtering and can significantly improve the spectral efficiency of dense wavelength-division multiplexed systems.

Performance analysis of error propagation effects in the dfe for atsc dtv receivers

The paper analyzes the error propagation phenomenon in the decision feedback equalizer (DFE) for the receivers of Advanced Television Systems Committee (ATSC) digital television (DTV) and presents the performance upper-limits of the DFE by comparing various error propagation cases and the no-error propagation case. As one approach to the performance limit, we consider a blind DFE, adopting a trellis decoder with a trace-back depth of 1 as a decision device. Through simulation, we show how much the DFE performance in ATSC DTV receivers is affected by error propagation. We found that while blind equalization is preferable to decision-directed (DD) equalization at signal-to-noise ratio (SNR) values less than 18 dB, DD equalization is superior to blind equalization at SNR values greater than 18 dB. In addition, symbol error rate curves quantitatively show that the performance difference in the DFE caused by error propagation becomes clearer at the trellis decoder following the DFE. The analysis results presented are very informative for developing equalization algorithms for ATSC DTV receivers.

A comparison of GSM receivers for fading multipath channels with adjacent and co-channel interference

We evaluate and compare several data detection schemes used in GSM systems. In particular, we compare the performance of decision feedback equalization (DFE) and nonlinear data directed estimation (NDDE) to that of maximum likelihood sequence estimation (MLSE). Establishing the performance of the basic NDDE detector is a first step in investigating the applicability of block transmission techniques to GSM systems. Our simulation results, obtained both for fading multipath channels and adjacent- and co-channel interference scenarios, suggest that the NDDE offers certain performance advantages over the DFE, and the performance of both detectors is comparable to that of the MLSE for the SNR region of interest in practical systems. Thus, they both represent viable alternatives to the MLSE detector.

Valuation of the effects of intersymbol interference in decision-feedback equalizers

For channels which suffer predominantly from additive noise and intersymbol interference, the decision-feedback equalizer has provided a relatively simple solution for reducing the effects of interfering symbols at the input to the decision device. A technique is developed that enables fast, accurate calculation of the error performance of decision-feedback equalization for a number of channel models. The method is to calculate the n-step transition probability for an associated Markov process and then use this transition probability as an approximation to the stationary probability distribution. For systems with finite memory, it is proved that the method converges. If the signal-to-noise ratio (SNR) is high and the signal amplitude is more than twice the worst-case interference, it is shown that the convergence is rapid. Numerical results indicate that the convergence is rapid enough to make this an efficient method of calculation, even for channels for which the interference does not fully satisfy this condition. Two examples are given here, but the technique has been tested on most of the examples that have been presented in the literature. The method yields results in closer agreement with simulation results than previous results obtained using bounding techniques, especially at low to moderate SNRs, and requires less computation.

A comparison of detection methods in the presence of nonlinearities

In this paper we evaluate the detectability of nonlinear signals characteristic of high-density, magnetic recording channels read by a magneto-resistive (MR) head. We first introduce a signal model which performs nonlinear superposition of experimentally-collected MR-head transition responses. Feeding signals synthesized from this model through a detection simulator, we compare the performance of Decision Feedback Equalization (DFE), RAM DFE, Partial Response Maximum-likelihood detection with PR4, EPR4, and E/sup 2/PR4 targets, and Fixed Delay Tree Search with Decision Feedback (FDTS/DF) in the presence of time and amplitude nonlinearities. Signals are generated at a variety of signal-to-noise ratios (SNRs) and densities with runlength constraints of (0,3) and (1,7). We found that the most detrimental nonlinearities come from amplitude asymmetry characteristic of poorly biased MR heads, and such nonlinearities cause a loss of between 0.5 and 1 orders of magnitude in bit error rate. We found that of the schemes, FDTS/DF provided the great robustness against all nonlinearities in the presence of RLL(1,7) coding, and EPR4ML and E/sup 2/PR4ML preformed the best with RLL(0,3) coding.

MSK-type signals in the presence of adjacent channel interference and white noise

In this letter, the problem of adjacent channel interference (ACI) caused by the close packing of constant envelope MSK-type users in a given frequency band is considered. An optimal receiver filter, based on the theory of matched filtering, is found, and it serves as an upper bound on the signal-to-interference ratio. The intersymbol interference (ISI) caused by the time response of the matched filter is eliminated by a decision feedback equalizer (DFE) which, however, degrades performance. It was found that the matched-filter upper bound allows about 3-15 dB more ACI than the performance of a classical correlation detector (for additive white Gaussian noise only), depending on the frequency separation between channels. The DFE performance is only a little bit worse than that of the matched filter.

Adaptive equalization for co-channel interference in a multipath fading environment

The paper concerns the feasibility and achievable performance of adaptive filtering in an interference-limited multipath fading environment as encountered in indoor wireless communications. In a typical cellular radio application, the performance-limiting impairment is interference due to synchronous data streams from other co-channel and adjacent channel users (CCI and ACI). The receiver under consideration employs an adaptive fractionally spaced decision feedback equalizer (DFE) Which exploits the correlation of the cyclostationary interference to achieve superior performance relative to the worst case when the interference is stationary noise. The paper presents ideal calculations which confirm that significant DFE performance gains are potentially achievable by explicitly accounting for the cyclostationary CCI. Two adaptive DFE strategies are considered. One approach is to adapt the DFE directly using iterative algorithms such as least mean square (LMS) or recursive least squares (RLS). Another approach is to compute the minimum mean square error DFE using an RLS channel impulse response (CIR) estimate and a sample estimate of the CCI autocorrelation obtained from the CIR estimation error during training. The best approach for adaptive equalization, in terms of adaptation speed and system performance, is to employ an RLS DFE which does not explicitly estimate the CIR or the CCI autocorrelation. >

Performance analysis of decision-feedback equalization with maximum-likelihood detector in high-density recording channels

The performance of decision-feedback equalization combined with maximum-likelihood detection (DFE/ML) using the FDTS/DF algorithm was estimated analytically in terms of the length of the feedback-filter and the depth of the ML-detector. Performance degradation due to error propagation in the feed-back loop and in the ML-detector was taken into account. It was quantitatively shown that signal-to-noise-ratio performance in high-density magnetic recording channels can be improved by combining an ML-detector and a feedback-filter. In particular, it was found that near-optimum performance can be achieved over the channel density range from 2 to 3 by increasing the total of the feedback-filter length and the ML-detector depth to six bits.

The effect of adjacent channel interference on the capacity of FDMA cellular systems

In FDMA cellular radio systems, the adjacent channel interference (ACI) is suppressed by the IF filter and also by a distance factor. Channels immediately adjacent to each other in frequency are assigned to different cells and, as such, the distance plays a role in reducing their mutual interference. The authors study the feasibility of optimizing the frequency spacing between carriers such that the combined effect of distance and IF filter will be enough to control the interference while allowing the maximum number of channels in a given bandwidth. They also study the decision feedback equalizer (DFE) as a means to reduce any excess ACI that results from reducing the channel spacing. The paper starts with a model that estimates the cumulative probability density of the signal to adjacent channel interference ratio (SIR) as a function of the channel spacing. They then present the results of a computer simulation study to estimate the performance of DFE in mitigating the effect of ACI. The results indicate that, under the conditions of slow flat fading and selection diversity, the DFE is very effective in reducing the effect of excess ACI interference which allows for a considerable reduction in the frequency spacing. >

Performance of decision feedback equaliser on measured indoor radio channels

The measured multipath profiles from five different factories are used for the performance analysis of a binary phase shift keying (BPSK) modem, with and without a decision feedback equaliser (DFE). Both average bit error rate and outage probability are calculated as the performance criteria.

Recovery Times of Decision Feedback Equalizers on Noiseless Channels

The performance of decision feedback equalizers (DFE's) working on a noiseless channel with correct tap weights is studied. We show that the space of channel parameters can accordingly be partitioned into a finite number of sets. The error recovery performance of a DFE is the same for all DFE's within one such set, and is determinable. We also discuss some tight bounds for recovery time statistics, which are particularly important when the number of equalizer taps is not small. We argue that minimum phase or near minimum phase character for the channel does not necessarily guarantee short recovery time.

Tight Bounds on the Error Probabilities of Decision Feedback Equalizers

A decision feedback equalizer (DFE) with correct tap weights operating on a noisy channel is considered. We show how the results concerning a noiseless channel can be extended to yield tight bounds on the stationary error probability performance for the noisy case. The effect of noise on DFE performance is classified according to the noise distribution and the channel parameters.

Adaptive Equalization Techniques for HF Channels

Data transmission at rates of 1.2 kbits/s or higher through voiceband ionospheric channels is subject to impairment from severe linear distortion, fast channel time variations, and severe fading. In this paper, we have focused on the performance of DFE (decision feedback equalization) receivers for communication over 3 kHz bandwidth HF channels. We describe the results of simulations for a wide range of fading rates on simulated and real recorded HF channels, using fractionally spaced DFE receivers. Both LMS (least mean square) and FRLS (fast recursive least squares) adaptation algorithms with periodic restart were evaluated, and both ideal-reference and decision-directed operation was observed. The results indicate that FRLS adaptation yields superior performance to LMS in rapid fading conditions, but that this performance advantage diminishes at low signal-to-noise ratios. Also, fade rates greater than about 1 Hz produced relatively high error rates, irrespective of which adaptation method was employed. Finally, a novel modification of the simple LMS algorithm which improves its tracking ability was evaluated. This involved preceding the LMS DFE receiver with an adaptive lattice whitening filter.