Phase Equalization Research Articles

Channel Equalization for Single Carrier MIMO Underwater Acoustic Communications

Multiple-input multiple-output (MIMO) underwater acoustic (UWA) channels introduce both space-time interference (STI) and time-varying phase distortion for transmitted signals. In such cases, the equalized symbols produced by conventional equalizer aiming for STI cancelation suffer phase rotation and thus cannot be reliably detected. In this paper, we propose a new equalization scheme for high data rate single carrier MIMO UWA channels. Different from existing methods employing joint equalization and symbolwise phase tracking technology, the proposed scheme decouples the interference cancelation (IC) operation and the phase compensation operation, leading to a generalized equalizer structure combining an IC equalizer with a phase compensator. The decoupling of the two functionalities leads to robust signal detection, which is most desirable in practical UWA applications. MIMO linear equalizer (LE) is adopted to remove space-time interference, and a groupwise phase estimation and correction method is used to compensate the phase rotation. In addition, the layered space-time processing technology is adopted to enhance the equalization performance. The proposed equalization scheme is tested to be very robust with extensive experimental data collected at Kauai, Hawaii, in September 2005, and Saint Margaret's Bay, Nova Scotia, Canada, in May 2006.

Equalization and Carrier Phase Recovery of CMA and MMA in Blind Adaptive Receivers

This paper derives general formulae for normalized error surface curvatures by using the constant modulus algorithm (CMA), the multimodulus algorithm(MMA), and two well known blind carrier phase recovery algorithms (CPRA) at the stationary points with arbitrary two-dimensional symmetric signal constellations. Based on the derived error surface curvatures, both the performance of the CMA followed by a blind CPRA and the performance of the MMA are forecasted and evaluated in terms of blind equalizer convergence and carrier phase recovery.

MICROCONTROLLER-BASED SYNCHRONOSCOPE IMPLEMENTED

In this study a Synchronoscope based on microcontroller has been developed to show phase equality, synchronization time and frequency equality while connecting an alternator and grid in parallel. Voltage signals belongs to alternator and grid are converted to certain levels using a measurement circuit developed and then transformed to square waveform by zero crossing detecting circuit. These signals are analyzed in PIC16F877 microcontroller using the program written in C language. Later on phase differences between signals are defined and related LED on the front panel turns on depending on phase difference. This process continuous till synchronizing time and then the related LED indicating the synchronization become on continuously. After that, synchronization relay is energized. So that, Synchronoscope equipment based on a microcontroller has been developed and a new automatic parallel connection system for alternators has been introduced.

Improved Adaptive Noise Cancellation Using Allpass IIR Filter Banks

In this paper, a subband noise canceller that is based on using low complexity infinite impulse response IIR filter banks at the analysis stage is proposed. Aliasing distortion is greatly reduced this way, while amplitude distortion is minimised by optimising a finite impulse response FIR filter bank at the synthesis stage. The motivation behind this work is the fact that, the least mean square LMS adaptive noise cancellers suffer from slow convergence due to coloured interfering signals, as well as high computational burden, due to long signal paths in acoustic environments. Efficiency of the proposed canceller was evaluated experimentally for white and coloured interferences. The contribution of this work is brought about by offering a design that is capable of improving convergence behaviour of the adaptive noise canceller with low implementation cost. Compared to similar literature solutions, the proposed scheme avoids the use of notch filtering or phase equalisers. The new scheme offers an improved p...

Meet the challenge of high-pass filter and ST-segment requirements with a DC-coupled digital electrocardiogram amplifier

Background The high-pass filter (HPF) in an electrocardiogram (ECG) amplifier can distort the ST segment required for ischemia interpretation. Therefore, the current standards and guidelines require −3 dB for monitoring and −0.9 dB for diagnostic purposes at 0.67 Hz. In addition, a minimal reaction to a rectangular pulse of 300 μV has to be proven. We raise the question of why the design of a DC-coupled digital ECG amplifier is reasonable when today the AC-coupled digital ECG amplifier including a 0.05-Hz HPF works so well, meets all required standards, and is already safe. We make the hypothesis that a digital DC-coupled ECG amplifier can as well meet the requirements and guarantee the same safety levels at the same time provide a higher degree of freedom for future improvements of the ECG signal quality. Methods Firstly, a historical research of the origin of the 0.05-Hz requirement has been made. Secondly, triangular pulses simulating unipolar QRS complexes have been passed through a digital filter to get qualitative results of the HPF response. And finally, to quantitatively describe the filter response, corresponding test requirement signals have been passed through a digital filter to simulate the HPF behavior, therefore understanding the reasons for the required tests. Results The oldest reference found to the 0.05-Hz filter dates from 1937. At that time, DC-coupled analogue ECG amplifiers were used. The simulation of the AC-coupled ECG amplifier with a first-order analogue HPF shows that the rectangular 300- μV pulse is a phase requirement and more restrictive than the frequency requirements. The phase requirement in fact corresponds to the requirement of a 0.05-Hz first-order analogue HPF (−3 dB) even if −0.9 dB at 0.67 Hz is required. The DC-coupled ECG amplifier (without an analogue HPF and during online and off-line acquisition) fulfils the phase and frequency requirements, just as the digital AC-coupled ECG amplifier does. Conclusions An AC-coupled ECG amplifier based on a first-order analogue HPF must have a maximum cutoff frequency of 0.05 Hz or requires a phase equalizer causing a delay of the acquired ECG. Because the desired delay during online acquisition should be short, the solution is practical but could be improved. Not the frequency cutoff of the HPF but the phase distortion of such a filter should be discussed. The DC-coupled ECG amplifier is as safe as the AC-coupled ECG amplifier; but it provides a higher degree of freedom for future filter designs certainly improving the ECG signal quality, while the safety can be guaranteed. Furthermore, the DC-coupled ECG amplifier allows investigation of the HPF, which is not easily possible when an AC-coupled ECG amplifier including the HPF is to be investigated.

Blind Equalization and Carrier Phase Recovery in a 16-QAM Optical Coherent System

Blind equalization and carrier phase recovery in a simulated 14 Gbaud 16-QAM optical coherent system are investigated. Equalization techniques to compensate for linear transmission impairments are presented using the constant modulus algorithm (CMA), the recursive least-squares (RLS)-CMA, and the radius directed equalization (RDE). With 7 T/2-spaced taps, the RDE and the RLS-CMA can compensate up to 1000 ps/nm of CD in the 16-QAM coherent system with performances comparable to the decision-directed (DD) equalizer. We show that the RDE is a promising technique for blind equalization in a 16-QAM coherent system with lower complexity than the RLS-CMA. Blind carrier phase recovery is investigated in a decision-directed-mode. We show that the blind carrier phase recovery algorithm can recover the Square-16-QAM constellation for laser beat linewidths of DeltanuTs ~ 10-4 in a polarization-multiplexed (POLMUX) 16-QAM coherent system with the RDE algorithm giving better overall performance than the CMA when compensating for CD and differential group delay (DGD). Finally, the dynamical characteristics of the equalizers to track endless polarization rotations are discussed. With the adaptation parameters optimized, the equalizers can track angular rate of rotation ~ 105 rad/s.

A FULLY COMPLEX-VALUED RADIAL BASIS FUNCTION NETWORK AND ITS LEARNING ALGORITHM

In this paper, a fully complex-valued radial basis function (FC-RBF) network with a fully complex-valued activation function has been proposed, and its complex-valued gradient descent learning algorithm has been developed. The fully complex activation function, sech(.) of the proposed network, satisfies all the properties needed for a complex-valued activation function and has Gaussian-like characteristics. It maps C(n) --> C, unlike the existing activation functions of complex-valued RBF network that maps C(n) --> R. Since the performance of the complex-RBF network depends on the number of neurons and initialization of network parameters, we propose a K-means clustering based neuron selection and center initialization scheme. First, we present a study on convergence using complex XOR problem. Next, we present a synthetic function approximation problem and the two-spiral classification problem. Finally, we present the results for two practical applications, viz., a non-minimum phase equalization and an adaptive beam-forming problem. The performance of the network was compared with other well-known complex-valued RBF networks available in literature, viz., split-complex CRBF, CMRAN and the CELM. The results indicate that the proposed fully complex-valued network has better convergence, approximation and classification ability.

A new learning algorithm with logarithmic performance index for complex-valued neural networks

In a fully complex-valued feed-forward network, the convergence of the Complex-valued Back Propagation (CBP) learning algorithm depends on the choice of the activation function, learning sample distribution, minimization criterion, initial weights and the learning rate. The minimization criteria used in the existing versions of CBP learning algorithm in the literature do not approximate the phase of complex-valued output well in function approximation problems. The phase of a complex-valued output is critical in telecommunication and reconstruction and source localization problems in medical imaging applications. In this paper, the issues related to the convergence of complex-valued neural networks are clearly enumerated using a systematic sensitivity study on existing complex-valued neural networks. In addition, we also compare the performance of different types of split complex-valued neural networks. From the observations in the sensitivity analysis, we propose a new CBP learning algorithm with logarithmic performance index for a complex-valued neural network with exponential activation function. The proposed CBP learning algorithm directly minimizes both the magnitude and phase errors and also provides better convergence characteristics. Performance of the proposed scheme is evaluated using two synthetic complex-valued function approximation problems, the complex XOR problem, and a non-minimum phase equalization problem. Also, a comparative analysis on the convergence of the existing fully complex and split complex networks is presented.

Joint Multilevel Turbo Equalization and Continuous Phase Frequency Shift Keying

A novel type of turbo coded modulation scheme, called multilevel turbo coded-continuous phase frequency shift keying (MLTC-CPFSK), is designed to improve the overall bit error rate (BER) and bandwidth efficiency. Then, this scheme is combined with a new double decision feedback equalizer (DDFE) to remove the interference and to enhance BER performance for the intersymbol interference (ISI) channels. The entire communication scheme is called multilevel turbo equalization-continuous phase frequency shift keying (MLTEQ-CPFSK). In these schemes, parallel input data sequences are encoded using the multilevel scheme and mapped to CPFSK signals to obtain a powerful code with phase continuity over the air. The performances of both MLTC-CPFSK and MLTEQ-CPFSK systems were simulated over nonfrequency and frequency-selective channels, respectively. The superiority of the two level turbo codes with 4CPFSK modulation is shown against the trellis-coded 4CPFSK, multilevel convolutional coded 4CPFSK, and TTCM schemes. Finally, the bit error rate curve of MLTEQ-CPFSK system over Proakis B channel is depicted and ISI cancellation performance of DDFE equalizer is shown against linear and decision feedback equalizers.

Stable-phase microwave-signal transmission channels and simulation synthesis of wave beams performed on the basis of beam waveguides

The prominent contribution of Academician V.A. Kotel’nikov to creation of an underground testing ground formed from a periodic sequence of phase equalizers (lenses and reflectors) is reported. A series of experiments that were performed on this unique facility and that involved laser-beam transmission with low loss over a line of a highly stable electric length are described. It is shown that the experience in investigations dealing with beam waveguides initiated research aimed at formation of wave beams with unique properties.

The design of a linear phase superconducting filter with quasi-elliptic response

This paper presents the design of a linear phase superconducting filter with quasi-ellipticresponse. The coupling structure of the filter contains two trisections and one quadruplet.The two trisections are applied to generate two independent transmission zeros for highselectivity and are realized by meandered open-loop microstrip resonators; the quadrupletis applied for phase equalization and realized by four novel L-shaped microstrip resonators.The filter is designed at 1950 MHz with a bandwidth of 20 MHz. It is fabricated on aLaAlO3 wafer withdouble-coated Tl2Ba2CaCu2O8 films. The filter shows good selectivity together with flat group delay over 80% of thepassband both in the simulation and measurements.

Using fractional delay to control the magnitudes and phases of integrators and differentiators

The use of fractional delay to control the magnitudes and phases of integrators and differentiators has been addressed. Integrators and differentiators are the basic building blocks of many systems. Often applications in controls, wave-shaping, oscillators and communications require a constant 90° phase for differentiators and −90° phase for integrators. When the design neglects the phase, a phase equaliser is often needed to compensate for the phase error or a phase lock loop should be added. Applications to the first-order, Al-Alaoui integrator and differentiator are presented. A fractional delay is added to the integrator leading to an almost constant phase response of −90°. Doubling the sampling rate improves the magnitude response. Combining the two actions improves both the magnitude and phase responses. The same approach is applied to the differentiator, with a fractional sample advance leading to an almost constant phase response of 90°. The advance is, in fact, realised as the ratio of two delays. Filters approximating the fractional delay, the finite impulse response (FIR) Lagrange interpolator filters and the Thiran allpass infinite impulse response (IIR) filters are employed. Additionally, a new hybrid filter, a combination of the FIR Lagrange interpolator filter and the Thiran allpass IIR filter, is proposed. Methods to reduce the approximation error are discussed.

The Effect of a Selective Receiver on CDMA

The quest for higher spectral efficiency in wireless communications is creating a new impetus for selective and sensitive base-station receivers. One consequence of high selectivity is phase dispersion, which critically affects the operation of a phase-modulated communications system. If phase equalization is not used, increasing selectivity will eventually compromise the performance of a receiver. These performance limitations are examined in a coherent and noncoherent direct-sequence spread-spectrum system

Design of 2-D FIR digital filters with specified magnitude and group delay responses by GA approach

An effective genetic algorithm (GA) approach is proposed for designing two-dimensional (2-D) real finite-duration impulse response (FIR) digital filters with complex-valued frequency responses. The method is extended from the one-dimensional (1-D) filter designs. By minimizing a quadratic measure of the error in the frequency band, the real-valued chromosomes of a population are evolved to get the filter coefficients. It is used to design two major classes of 2-D FIR digital filters including multi-band filters and all-pass phase equalizers. Two examples are presented to demonstrate the efficiency and effectiveness of this proposed approach.

Self-phase equalised bidirectional distributed amplifier

A novel bidirectional distributed amplifier is configured by integrating a bidirectional amplifier (BDA) into a distributed amplifier. The BDA is formed by connecting two identical FETs in ‘reverse’ parallel. Experimental results show that symmetric S-parameters are obtained in the range 0.8–2.2 GHz without additional phase equalisation.

Channel Equalization in Filter Bank Based Multicarrier Modulation for Wireless Communications

Channel equalization in filter bank based multicarrier (FBMC) modulation is addressed. We utilize an efficient oversampled filter bank concept with 2x-oversampled subcarrier signals that can be equalized independently of each other. Due to Nyquist pulse shaping, consecutive symbol waveforms overlap in time, which calls for special means for equalization. Two alternative linear low-complexity subcarrier equalizer structures are developed together with straightforward channel estimation-based methods to calculate the equalizer coefficients using pointwise equalization within each subband (in a frequency-sampled manner). A novel structure, consisting of a linear-phase FIR amplitude equalizer and an allpass filter as phase equalizer, is found to provide enhanced robustness to timing estimation errors. This allows the receiver to be operated without time synchronization before the filter bank. The coded error-rate performance of FBMC with the studied equalization scheme is compared to a cyclic prefix OFDM reference in wireless mobile channel conditions, taking into account issues like spectral regrowth with practical nonlinear transmitters and sensitivity to frequency offsets. It is further emphasized that FBMC provides flexible means for high-quality frequency selective filtering in the receiver to suppress strong interfering spectral components within or close to the used frequency band.

Linear-phase fiber lattice equalizer for Gaussian-like spectra

We present an all-fiber linear-phase equalizer for equalizing Gaussian-like spectra. The equalizer uses an optical lattice filter in a symmetric folded structure. Compared to existing solutions to linear phase equalization, the proposed equalizer has guaranteed linear phase responses and lower power attenuation. As an illustrative example, the effectiveness of the proposed equalizer is demonstrated with the flattening of the output spectrum of a superluminescent light emitting diode.

A novel algorithm for the design of selective FIR filters with arbitrary amplitude and phase characteristics

In this paper, a novel analytic technique is presented for the design of finite impulse response digital filters with simultaneous amplitude and phase requirements. The design process relies on the interpolation concept and is based on the generation of arbitrary amplitude and phase polynomials. The new class of FIR filters lays in between the exact linear phase filters at all frequencies and the minimum phase FIR filters based on the amplitude approximation only. It interpolates arbitrary (e.g., linear) phase response in the passband and arbitrary amplitude characteristics in pass- and stopbands. The introduced class of filters can be used on the amplitude and phase equalization of both FIR and IIR digital filters. Illustrative examples are included in order to investigate the main properties of these filters and to demonstrate that the proposed design method leads to a significant saving in the arithmetic computations.

Fully dispersion-compensated ?500?fs pulse transmission over 50?km single-mode fiber

We demonstrate essentially distortionless 50 km fiber transmission for approximately 500 fs pulses, using dispersion-compensating fiber and a programmable pulse shaper as a spectral phase equalizer. This distance is approximately five times longer than previously achieved at similar pulse widths.

Novel allpass filter configuration employing single OTRA

A new circuit configuration uses a single operational transresistance amplifier (OTRA) and four passive elements to realize both first- and second-order allpass filters. Owing to internal grounding of the OTRA input terminals, effects of input parasitics are significantly reduced. Applications of the circuit include the realization of the phase equalizer, the quadrature oscillator, and the high-Q bandpass filter for analog signal processing. The performance of the allpass filter is illustrated by circuit-level simulation of a quadrature oscillator with CMOS realization of the OTRA.