Tapped Delay Line Equalizer Research Articles

An Si Schottky diode demultiplexer circuit for high bit-rate optical receivers

We demonstrate an Si Schottky diode sampling demultiplexer circuit for an optical receiver. A system model and the operation principle of the sampling demultiplexer are described. The Si Schottky diode sampling circuit has been realized in conductor-backed coplanar waveguide technology and fabricated in thin-film technology. The sampling circuit module was measured using a 43-Gb/s nonreturn-to-zero signal. In addition, for the purpose of reducing the intersymbol interference, a linear tapped delay-line equalizer circuit has been designed and simulated. The weight of the taps has been calculated employing a zero-forcing algorithm.

Optimization of tap spacings for the tapped delay line decision feedback equalizer

In this letter, we optimize tap spacings in a tapped delay line equalizer. We derive a set of nonlinear equations for the optimum tap spacings and tap weights for the decision feedback equalizer. A recursive numerical technique is used to obtain the optimum settings. With optimized tap spacings, the proposed solutions provide a performance upper bound for the decision feedback equalizer for a given number of taps.

Reducing nonlinear waveform distortion in IM/DD systems by optimized receiver filtering

Nonlinear waveform distortion caused by the combined effect of fiber chromatic dispersion, self-phase modulation, and amplifier noise limits the attainable performance of high bit-rate, long haul optically repeatered systems. Signal processing in the receiver is investigated and found to be effective in reducing the penalty caused by this distortion. Third order low pass filters, with and without a tapped delay line equalizer are considered. The pole locations or the tap weights are optimized with respect to a minimum bit error rate criterion which accommodates distortion, pattern effects, decision time, threshold setting and noise contributions. The combination of a third order Butterworth filter and a five-tap, fractionally spaced equalizer offers more than 4 dB benefit at 4000 km compared with conventional signal processing designs.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Non-uniformly spaced tapped-delay-line equalizers

A systematic way to design nonuniformly spaced tapped-delay-line (TDL) equalizers is described, and the performance of such equalizers is compared to that of uniformly spaced TDL equalizers with the same number of tap coefficients. It is shown that the signal-to-mean-squared-error ratio at the output of a TDL equalizer can be improved by optimally choosing the positions of the tap weights. An algorithm to find both the tap positions and the corresponding tap weights for a given delay span and a given minimum tap spacing of the equalizer is presented. Typical results are illustrated by using, as an example, the magnetic recording channel. For two target waveforms at different densities of recording, it is shown that there is a potential for saving up to seven equalizer taps. >

Performance of 10 Gb/s lightwave systems using a adjustable chirp optical modulator and linear equalization

An optical modulator with an adjustable chirp parameter and a synchronous tapped delay line equalizer can be combined to offer a low dispersion penalty for 10 Gb/s systems operating at a wavelength of 1.55 mu m with standard single-mode optical fiber. The influence of important system parameters, such as the modulator bandwidth, chirp parameter, and number of equalizer taps, on the receiver sensitivity is assessed for smart receivers which jointly optimize the decision time, decision threshold, and tap weights under a minimum bit-error-ratio criterion. The combination of an adjustable chirp modulator and equalizer offers a system reach more than twice that obtained with a chirp-free modulator. >

Precluding nonlinear ISI in direct detection long-haul fiber optic systems

Long-distance, high-rate fiber optic systems employing directly modulated 1.55- mu m single-mode lasers and conventional single-mode fiber suffer severe intersymbol interference (ISI) with a large nonlinear component. A method of reducing the nonlinearity of the ISI, thereby making linear equalization more viable, is investigated. It is shown that the degree of nonlinearity is highly dependent on the choice of laser bias current, and that in some cases the ISI nonlinearity can be significantly reduced by biasing the laser substantially above threshold. Simulation results predict that an increase in signal-to-nonlinear-distortion ratio as high as 25 dB can be achieved for synchronously spaced samples at an optimal sampling phase by increasing the bias current from 1.2 times threshold to 3.5 times threshold. The high SDR indicates that a linear tapped delay line equalizer could be used to mitigate ISI. Furthermore, the shape of the pulse response suggests that partial response precoding and digital feedback equalization would be particularly effective for this channel.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Equalization in coherent lightwave systems using a fractionally spaced equalizer

The performance of a fractionally spaced analog tapped delay line equalizer that has the advantages of being adaptive, being capable of equalizing other linear distortions such as polarization dispersion and nonideal receiver response, and eliminating chromatic dispersion over any distance if a sufficient number of taps are used is discussed. It is shown how this equalizer can be implemented at intermediate frequency and at baseband (for homodyne detection). Results show that an N-tap equalizer increases the maximum bit rate distance approximately (N-1)/twofold (e.g. a threefold increase in distance with a seven-tap equalizer).< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Application of a transversal laguerre network in adaptive equalization

Synchronous data transmission systems that transmit information over dispersive channels often employ adaptive equalization techniques in the processing of the received signals. Most equalizers presently used in communication systems are of the tapped delay line (TDL) type. In this paper, we propose a class of adaptive equalizers of which the TDL equalizer is a member. Another interesting member of this class is the Laguerre adaptive equalizer which is to be introduced in this paper. The configuration of the Laguerre equalizer is developed. It is found, as shown by the examples, that the Laguerre equalizer is more accurate, and is less sensitive to timing error than the conventional TDL equalizer.

A Minimum-Error Probability Tapped Delay Line Equalizer

This paper considers the design of a tapped delay line (TDL) equalizer from an error probability point of view. We define a new class of TDL equalizers, which includes the zero-forcing equalizer (ZFE) as a special case. Some optimal properties of the new equalizers are presented and their relationships to mean-square equalizers (MSE's) and ZFE are discussed. It is shown that, for a certain class of additive noise, some of the new equalizers result in a probability of error lower than that of the MSE.

Self-Orthogonalizing Adaptive Equalization Algorithms

A comparison is made of several self-orthogonalizing adjustment algorithms for linear tapped delay line equalizers. These adaptive algorithms accelerate the rate of convergence of the equalizer tap weights to those which minimize the output mean-squared error of a data transmission system. Accelerated convergence of the estimated gradient algorithm is effected by premultiplying the correction term in the algorithm by a matrix which is an estimate of the inverse of the channel correlation matrix. The various algorithms differ in the manner in which this estimate is sequentially computed. Depending on the degree of complexity available, the equalizer convergence time may be reduced more than an order of magnitude from that required by the simple gradient algorithm.

An Equalizer Structure with Reduced Sampling Time Reference Sensitivity

In the last decade the work on linear tapped delay line equalizers has proved successful in reducing intersymbol interferenceone of the largest problems in data communication systems. An adjoining problem is the selection of a sampling time reference. An equalizer structure less sensitive to sampling time reference is studied for adaptive equalization. The equalization is simply done by means of tap coefficient adjustment. The equalizer consists of two parallel branches, each containing a transversal filter with adjustable tap coefficients. The branches are connected by a fixed filter, the transfer function of which is selected such that the equalizer can perform a sampling time reference displacement and a cancellation of intersymbol interference simultaneously. The criterion of goodness is the conventional meansquare error (including noise) between the actual output and a desired output. The equalizer is applied in two examples showing-in contrast to a conventional equalizer with the same total number of tap coefficients-the insensitivity of the minimum mean-square error (MMSE) to how the the sampling time reference is selected. Thus no extra circuits are needed for this purpose, while a mean-square error that is as low as the best that can be obtained with a conventional equalizer is adaptively maintained.

Recursive Equalization in Data Transmission--A Design Procedure and Performance Evaluation

A design procedure for the receiver of a pulse amplitude modulation system is developed for data transmission in the presence of intersymbol interference and noise. In the receiver, an estimate of each source symbol is made based on the WienerKolmogorov theory of minimum variance estimation for stationary time series. The resulting structure is called a recursive equalizer and its taps are obtained by taking the canonical factorization of the spectral density function followed by the operations of polynomial multiplication and division. The performance of the system is evaluated by calculating the probability of error versus signal-signal-to-noise ratio and Comparing it to the performance of a conventional tapped delay line equalizer. The specific example presented uses raised-cosine signaling through a channel with slope-attenuation distortion and a zero phase characteristic. For this example, the recursive equalizer is shown to exhibit superior-performance.

On the use of Channel Introduced Redundancy for Error Correction

The problem considered is that of reducing intersymbol interference caused by time dispersive channels. Similarity between the response of a time dispersive channel and that of a cyclic algebraic encoder has led to a method of treating the dispersive channel as a linear encoder. A tapped delay line equalizer is used to complete the encoding process so that the result can be decoded and errors corrected. The tap gain vector for the equalizer is developed, and an upper bound on the probability of error is derived. The analysis and computer simulation results show that under certain conditions the error rate can be reduced by orders of magnitude.

Adaptive Equalizer for Pulse Transmission

Results are reported which are of interest in the automatic adaptive equalization of telephone lines or other narrowbandwidth channels where data transmission is limited primarily by intersymbol interference. We describe an algorithm for adjusting the tap gains of a tapped delay line (TDL) equalizer which converges for all channels to the setting, which minimizes the mean-square error (rose) at the filter output due to both noise and intersymbol interference. This algorithm has been implemented using digital microcircuits and metal-oxide semiconductor (MOS) shift registers to construct a 63 tap all-digital TDL filter covering 19.7 ms. The modulation system used is vestigal sideband with bandwidth 1.6 kHz. The data rate is 3200 pulses/s, or 9600 bit/s with 8 data levels. A description of the hardware realization and performance data over various telephone lines is given.

Automatic channel equalization algorithm

Results are reported which are of interest in the automatic equalization of telephone lines or other narrow bandwidth channels where data transmission is limited primarily by intersymbol interference. We describe an algorithm for adjusting the tap gains of a tapped delay line equalizer which converges for all channels. The algorithm also adjusts the tap gains to match a changing channel.