Nonlinear Cancellation Research Articles

Adaptive nonlinear cancellation for high-speed fiber-optic systems

The authors propose techniques for adaptive nonlinear cancellation of intersymbol interference (ISI) in the electrical signal at the receiver in Gb/s lightwave systems and describe several demonstrations of these techniques. Techniques for adjustable nonlinear cancellations are discussed and demonstrations of these techniques using commercially available integrated circuits (ICs) at data rates as high as 1.7 Gb/s are described. Techniques for automatic adjustment are discussed, and a demonstration of adaptive nonlinear cancellation at 450 Mb/s is described. The authors discuss how these techniques can be integrated onto the detector IC for operations at 2.5 Gb/s and higher data rates. These techniques allow a single IC detector with adaptive nonlinear cancellation to be used in long-haul and undersea lightwave systems to optimize the detector threshold and compensate for the ISI.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Dual mode adaptive signal processing

Adaptive signal processing using the least mean square (LMS) algorithm has been utilized in several different applications. The use of the adaptive echo canceller and the adaptive channel equalizer on many communications media has extended the rate of digital communication to near the theoretical channel capacity. Most applications of adaptive signal processing perform the task of system identification. Traditionally, this task is often performed by some predictor-corrector algorithm, with the optimal being the Kalman filter. In this respect, the LMS algorithm can be seen as an approximate limiting Kalman filter. On the other hand, another task of the traditional Kalman filter is signal smoothing. The signal smoothing task can also be performed by something close to the LMS algorithm as an approximate limiting Kalman algorithm. In this paper, the feasibility of using the LMS algorithm for signal smoothing is first explored. Then an architecture is proposed where the LMS algorithm is used for both signal smoothing and system identification. Finally, a dual mode adaptive signal processing technique using different time constants for signal smoothing and system identification is developed. An example of dual mode adaptive signal processing applied to the nonlinear echo cancellation is discussed in detail.

CMOS floating voltage-controlled negative resistor

A circuit technique for realising a floating voltage-controlled negative resistor that is suitable for fabrication in CMOS technology is presented. The negative resistor action is achieved through nonlinearity cancellation of the currents of MOS transistors in saturation.

Proposed MOSFET-C continuous time biquadratic building blocks with high-order implementation

Abstract A novel family of MOSFET-C continuous-time biquadratic building blocks is proposed in this paper. The proposed family realizes all generic transfer functions such as lowpass, highpass, bandpass, band rejection, and allpass. It achieves a complete MOS non-linearity cancellation and does not require the use of fully balanced op.-amps. The proposed blocks enjoy low passband sensitivities. A Monte Carlo analysis was done to verify the sensitivity performance. An example of bandpass filter with comparative study is given. The maximum dynamic range is adjusted using SPICE. All the design steps are matched with the 3.5 μm CMOS design rules. Then the proposed biquads are implemented to realize a transmission zeros bandpass filter of twelfth order.

Techniques for high-speed implementation of nonlinear cancellation

The authors present techniques for breaking the bottleneck caused by delays in the feedback loop, which can severely limit the maximum data rate of a detector, operating with nonlinear cancellation (NLC). The authors first show how to simplify the loop to avoid high-speed switching of analog signals by using multiple decision elements, each with a different threshold level. The authors then show how to use lookahead computation to increase the delay permissible in the feedback loop. The authors describe nonlinear cancellation, and outline some of the problems of implementing NLC at high data rates. Techniques for overcoming these problems are developed and are illustrated with the example of a simple one-tap NLC. The authors describe how to generalize the techniques to more complex NLCs and present some more examples.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Tunable operational transconductance amplifier with extremely high linearity over very large input range

A simply fully-differential transconductor is presented that achieves, based on cancellation of first and higher order non-linearities, ±0.7% linearity error over a ±6.4V differential input range for ±5 V power supplies. Common-mode input signals are cancelled at the output. The transconductance can be tuned at least by a factor 3 and f(−3dB) is 63 MHz for 10 μm channel length.

Convergence behaviour of a jointly adaptive transversal and memory-based echo canceller

A convergence analysis of a jointly adaptive transversal and memory-based or nonlinear echo canceller is presented. The coefficients of the transversal echo canceller (TEC) and the memory echo canceller (MEC) are updated simultaneously using the well-known least mean square (LMS) algorithm with separate adaptation constants and a common cancellation error. The amount of linear echo cancellation load-sharing between the TEC and MEC is shown to be governed by a fixed relationship that exists between the MEC coefficients and those TEC coefficients updated using the shared portion of the input symbol span. The average mean square error is derived for the case of binary input symbols and compared to that of an identical canceller adopting a sequential adaptation method where the TEC coefficients are adapted first and permitted to converge before MEC coefficient adaptation is initiated for jointly adaptive operation. Bounds for both adaptation constants are found for MSE convergence for the jointly adaptive case. The adaptation constants that yield the fastest jointly adaptive convergence are also derived. The sequential method is shown to have an overall convergence rate advantage that decreases as the ratio of the number of TEC coefficients to MEC coefficients becomes large. The theoretical results are supported by computer simulations.

High-gain filters for non-linear stochastic systems

The use of high-gains in the filtering problem for non-linear stochastic systems is studied. It is shown that a high-gain filter has the same structure as a Luenberger observer. The use of high-gains is studied in the cancellation of non-linearities in order to simplify the filter design for non-linear systems. Singular perturbation techniques are used to show how the error dynamics reaches stable equilibrium in a very fast transient time, ensuring that the slow dynamics of the filter are just those of the non-linear stochastic system. © 1990 Taylor & Francis Group, LLC.

Transistor-only frequency-selective circuits

The possibility of implementing frequency-selective circuits using only MOS transistors is considered. The advantages of such circuits are small chip area, good matching properties for individual elements, a potential for very high frequency of operation, and suitability for fabrication using standard digital VLSI processes. Disadvantages include the implementation of automatic tuning schemes that reliably eliminate unpredictabilities and variations, device modeling difficulties of common simulators, and nonlinearity cancellation. A procedure for the topology development of filters in which transistors are used as URC elements is presented together with a technique which uses transistors as capacitors, along with first-order correction for the nonidealities of such 'capacitors' and of the active elements. Measured results from three experimental chips are reported in the 1-100-MHz range. >

Cancellation of nonlinearities in bandpass QAM systems

An orthogonalized input-output characterization of nonlinear bandpass quadrature amplitude modulation systems with memory is developed. The orthogonal expansion is used with a nonlinear canceller structure to counteract the effect of nonlinear distortion introduced by high-power amplifiers at the transmitter. It is shown through simulations that when the transmitted symbols are known at the canceller, relatively simple structures can eliminate the effect of the nonlinearity. Even when the transmitted symbols are not known at the canceller, the improvement in performance with the nonlinear canceller, with respect to a linear receiver, is significant. >

Electrical signal processing techniques in long-haul fiber-optic systems

The potential for electrical signal processing to mitigate the effect of intersymbol interference in long-haul fiber-optic systems is discussed. Intersymbol interference can severely degrade performance and consequently limit both the maximum distance and data rate of the system. Several techniques for reducing intersymbol interference in single-mode fiber systems with single-frequency lasers are presented, and those techniques which are appropriate at high data rates in direct coherent detection systems are identified. The performances of linear equalization (tapped delay lines), nonlinear cancellation (variable threshold detection), maximum-likelihood detection, coding, and multilevel signaling are analyzed. The results for a simulated binary 8-Gb/s system show that simple techniques can be used to reduce intersymbol interference substantially, thereby increasing the system margin by several decibels. A six-tap linear equalizer increases the dispersion-limited distance (due to chromatic or polarization dispersion) by 20% (or reduces the optical power penalty by as much as a factor of two) in direct detection systems, even when the distortion is nonlinear. A nonlinear cancellation technique (adjusting the decision threshold in the detector based on previously detected bits) can more than double the dispersion-limited distance and/or data rate.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

High-gain observers in the state and parameter estimation of robots having elastic joints

The paper deals with the state and parameter estimation of robots having elastic joints. The estimation of the unknown parameters of the robot is reduced to the estimation of its state variables by a state space immersion. The Luenberger observer is used in the state estimation of the extended robot model. The use of high gains is studied in the cancellation of nonlinearities in order to simplify the observer design. The high gain induces a time scale separation between the robot model and the observer and therefore the singular perturbation theory can be used in the stability analysis of the estimate error dynamics. In particular, it is shown that the error dynamics reaches the stable equilibrium in a very fast transient ensuring that the slow dynamics of the observer is just that of the given robot.

Adaptive Feedback Linearization and Tracking for Induction Motors

We discuss an approach to the control of voltage fed induction motors, which takes explicitly into account the nonlinear multivariable nature of the model and the presence of uncertainties in the motor inductances. Exact input-output linearization techniques are used to design a controller for motor torque and flux. The parameter adaptation is then proposed to overcome their chief drawback, the fact that they rely on an exact cancellation of the system nonlinearities in order to get linear input-output behaviour. The linearization objective is perturbed by the presence of model uncertainties which appear linearly. A direct adaptation scheme which depends on the output’s strong relative degree, yields asymptotically bounded tracking i.e. provides bounded state evolutions, and outputs which track asymptotically the desired trajectories. The method has been tested in simulation, on a high power induction motor model.

A new floating resistor for CMOS technology

A new floating resistor consisting entirely of MOS devices in saturation has been developed. Linearity within +or-0.04% is achieved through nonlinearity cancellation via current mirrors over an applied voltage range of +or-1 V. The frequency response exceeds 10 MHz, and the resistors are expected to be useful in implementing integrated circuit active RC filters.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A digital signal processor for an ANSI standard ISDN transceiver

The digital signal processing chip of a two-chip ISDN (integrated services digital network) basic access transceiver based on the ANSI standard 2B1Q code is described. Nonlinear echo cancellation is used to improve the loop coverage. The chip features a multiprocessor architecture, where each processor is optimized for the algorithm used. Full observability of internal signals and adaptive filter coefficients is supported. The device is fabricated in a 1.25- mu m double-level-metal CMOS process with an active area of 47 mm/sup 2/. >

Improvement in adaptive noise canceling using a nonlinear filter based on the pontryagin minimum principle

This paper introduces a new nonlinear filter that is used for adaptive noise canceling. The derivation and convergence properties of the filter are presented. The performance, as measured by the root mean square error between the signal and its estimate, is compared with that of the commonly used least mean square (LMS) algorithm. It is shown, through simulation, that the proposed nonlinear noise canceler has, on the average, better performance than the LMS canceler. The proposed adaptive noise canceler is based on the Pontryagin minimum principle and the method of invariant imbedding. The computational time for the proposed method is about 10% of that of the LMS, in the studied cases, which is a substantial improvement.

A new MOSFET-C universal filter structure for VLSI

A continuous-time all-MOS universal filter structure is proposed. The structure is based on the MOSFET-C design approach. It achieves complete MOS nonlinearity cancellation and does not require the use of fully balanced op-amps. General topological requirements are established that are necessary for the conversion of active-RC prototypes to MOSFET-C counterparts, such that MOS nonlinearity cancellation is achieved. Accordingly, a universal active-RC prototype filter structure, which meets the necessary requirements, is presented and its MOSFET-C version is developed. Nonideal effects that may degrade the performance at high frequency are discussed and ways for improvement are proposed. Results obtained from a test chip have verified the viability of the proposed structures. The chip is an implementation of a MOSFET-C universal filter in a 3.5- mu m CMOS process. The filter is successfully tuned over a wide range of pole frequencies (0 to 100 kHz) using op-amps with a measured gain bandwidth of only 1.2 MHz. >

Four-transistor continuous-time MOS transconductor

A general condition for nonlinearity cancellation in the recently reported four-transistor continuous-time MOS transconductor is presented. It is shown that the circuit possesses a high degree of versatility and can operate with different sets of signal excitations using enhancement or depletion transistors.

Continuous-time MOSFET-C filters in VLSI

The desirable features of fully integrated, VLSI-compatible continuous-time filters are discussed. A recently proposed integrated continuous-time filtering technique is reviewed, in which MOS transistors are used in place of resistors along with nonlinearity cancellation and on-chip automatic tuning. The filters obtained using this technique are compared to switched-capacitor (SC) filters, digital filters, and continuous-time filters using different techniques. Representative experimental results are given, demonstrating the high performance that can be achieved.

Continuous-time MOSFET-C filters in VLSI

The desirable features of fully integrated, VLSI-compatible continuous-time filters are discussed. A recently proposed integrated continuous-time filter technique is reviewed, in which MOS transistors are used in place of resistors along with the nonlinearity cancellation and on-chip automatic tuning. The filters obtained using this technique are compared to switched-capacitor filters, digital filters, and continuous-time filters using different techniques. Representative experimental results are given, demonstrating the high performance that can be achieved.