Digital compensation in IQ modulators using adaptive FIR filters

This paper discusses digital compensation for frequency-dependent transfer characteristics and implementation errors in digital PAM/continuous-phase frequency-shift keying (CPFSK) quadrature modulators. Recently, several methods have been proposed to digitally compensate for the shortcomings of the analog reconstruction filters in IQ modulators. While these methods have shown to be effective, they result in filters with long coefficients that are computationally demanding to implement on the DSP. Furthermore, the modulator needs to be taken offline while the precompensation filters are updated to reflect the changes in the I and Q channel characteristics. In this paper, a digital compensation method is proposed here using two adaptive finite-impulse response filters to compensate for the magnitude and phase characteristics of the analog reconstruction filters in the IQ modulator. The experimental results show that this technique is effective and lead to substantial improvement of the output envelope ripples.     

Optimum precompensation filters for IQ modulation systems

We present a technique for digitally precompensating for shortcomings in signal reconstruction filters in an IQ modulation system. Optimum digital precompensation filters are found using least squares (LS) techniques. The optimization problem is formulated in the frequency-domain, and the optimum LS solution is characterized using an eigenanalysis approach     

频变交叉耦合带通滤波器的耦合矩阵综合研究

综合代表滤波器拓扑结构和特性的耦合矩阵是交叉耦合滤波器设计的重点。提出了一种基于广义特征值的优化综合方法,通过非线性最小二次求解,将耦合矩阵的广义特征值逼近至广义切比雪夫响应多项式传输函数零极值参考点,优化求解出带有频变交叉耦合带通滤波器的耦合矩阵。通过3个数值实例演示了该方法,并验证其有效性。这是对经典带通滤波器耦合矩阵综合方法的补充,为频变交叉耦合滤波器的设计建立了基础。     

Optical intensity modulators for digital and analog applications

This tutorial describes the basic principles and performance analysis of optical intensity modulators using electrooptic and electroabsorption effects, for use in analog and digital communication systems. These include lithium niobate modulators, semiconductor electroabsorption modulators, semiconductor Mach-Zehnder modulators, and polymer modulators.     

Efficient IQ filter structure for use in adaptive equalisation

A method is presented for reducing the implementation cost of a complex digital filter structure such as that used for IQ processing in adaptive equalisation. It is shown that the number of real filter sections required can be reduced from four to three at the expense of an increase in external addition stages from two to three (or five if coefficient additions are included). The approach is applicable to both LTE and DFE structures and results in savings which approach 25% for most practical cases     

Optimal gain overdesign in analog filters

This paper presents a method for the overdesign of gain-shaping filters in an optimal way. The passband minima and the stopband maxima of a filter approximation are adjusted using an iterative method, with the objective of obtaining a circuit that satisfies exactly the specifications when the statistical deviation in the filter transfer function due to errors on its components is considered. The result is a filter that can be significantly more selective than what is obtained by a simpler overdesign method that results in an equal-ripple ideal design. Two different approaches are examined, one based in the development of a special approximation followed by synthesis, and the other by direct optimization of an initial classical design.     

Birefringence compensation for polarization-independent directionalcoupler wavelength filters

We show how polarization-independent optical wavelength filters can be achieved using asymmetric directional couplers with birefringence-compensation. Simulations demonstrate that excellent polarization behavior can be achieved with practical tolerances and without sacrificing filter bandwidth or insertion loss     

Technique for compensation of errors in analogue multipliers

A new method for significantly reducing the effect of nonlinear errors on the performance of an analogue multiplier is presented. The method is especially well suited for switched-capacitor devices where time-shared (multiplexing) techniques can be easily employed. However, the method is quite general in nature and can be applied to any type of analogue integrated multiplier. The effectiveness of the technique has been checked through extensive simulation studies.<>     

相干光OFDM系统中IQ不平衡的时域补偿算法

通过研究相干光正交频分复用(CO-OFDM)系统中正交/同相(IQ)不平衡对系统性能的影响,提出一种新型IQ不平衡时域补偿TS(基于训练序列)算法,并与传统的时域格拉姆-施密特正交化(GSOP)算法进行了对比。     

高性能的MIMO-OFDM接收机IQ不平衡补偿新算法

在分析MIMO-OFDM接收机IQ不平衡信号模型基础上,以OFDM物理层为背景提出了一种新的时频结合的MIMO-OFDM接收机IQ不平衡补偿算法,即先在时域对IQ不平衡预补偿,然后在频域对残余的IQ不平衡进行校正.仿真结果表明:本文所提算法性能优于传统的频域补偿算法,该算法在AWGN信道下能达到理想性能,在多径衰落信道下当误比特率等于10-3时性能损失可以减小到0.5dB左右.     

Efficient fully 3-D iterative SPECT reconstruction with Monte Carlo-based scatter compensation

Quantitative accuracy of single photon emission computed tomography (SPECT) images is highly dependent on the photon scatter model used for image reconstruction. Monte Carlo simulation (MCS) is the most general method for detailed modeling of scatter, but to date, fully three-dimensional (3-D) MCS-based statistical SPECT reconstruction approaches have not been realized, due to prohibitively long computation times and excessive computer memory requirements. MCS-based reconstruction has previously been restricted to two-dimensional approaches that are vastly inferior to fully 3-D reconstruction. Instead of MCS, scatter calculations based on simplified but less accurate models are sometimes incorporated in fully 3-D SPECT reconstruction algorithms. We developed a computationally efficient fully 3-D MCS-based reconstruction architecture by combining the following methods: 1) a dual matrix ordered subset (DM-OS) reconstruction algorithm to accelerate the reconstruction and avoid massive transition matrix precalculation and storage; 2) a stochastic photon transport calculation in MCS is combined with an analytic detector modeling step to reduce noise in the Monte Carlo (MC)-based reprojection after only a small number of photon histories have been tracked; and 3) the number of photon histories simulated is reduced by an order of magnitude in early iterations, or photon histories calculated in an early iteration are reused. For a 64 x 64 x 64 image array, the reconstruction time required for ten DM-OS iterations is approximately 30 min on a dual processor (AMD 1.4 GHz) PC, in which case the stochastic nature of MCS modeling is found to have a negligible effect on noise in reconstructions. Since MCS can calculate photon transport for any clinically used photon energy and patient attenuation distribution, the proposed methodology is expected to be useful for obtaining highly accurate quantitative SPECT images within clinically acceptable computation times.     

An adaptive approach for designing phase compensation filters

The objective of this communication is to introduce an adaptive method for designing filters to compensate for nonlinear phase shift introduced into a data stream by fixed (i.e., nonadaptive) recursive digital filters, e.g., those obtained via the bilinear transform.     

Waveguide grating filters for dispersion compensation and pulsecompression

Dispersion compensation and pulse compression are theoretically demonstrated using aperiodic waveguide gratings. The gratings are designed to have both a flat amplitude and a quadratic phase response over the pulse bandwidth. This results in nearly transform-limited compressed pulses. The appropriate waveguide grating parameters are obtained by applying the Gel'fand-Levitan-Marchenko inverse scattering method to the coupled mode equations which describe propagation. The technique is illustrated by designing an aperiodic grating which compresses a 60-ps pulse by a factor of three. Limitations and possible extensions of the general method are discussed     

A novel active compensation scheme for active-RC filters

A novel scheme using two internally compensated operational amplifiers with unmatched gain-bandwidth products is suggested as a replacement for the conventional single-amplifier integrators, inverters or summers. The scheme, while providing exact phase compensation, contributes to magnitude compensation as well. Filters realized using such a scheme have an extended range of frequency of operation with reduced error in pole frequency and in Q.     

A compensation technique for positioning errors in planarnear-field measurements

Probe positioning errors are among the major sources of inaccuracy in the planar near-field-far-field transformation technique, as their presence destroys the Fourier-transform relationship between the tangential components of the near field and those of the plane-wave spectrum of the test antenna. An optical systematic correction technique is developed which requires only the knowledge of the true probe positions. The `corrected' values are computed by an iterative procedure which turns out to be rapidly convergent and fairly accurate even in the presence of rather high displacement errors. It is concluded that the developed correction technique either includes or compares favorably with alternative methods. It is an optimal generalization of the Z -correction method, being exact in principle and allowing simple control of the attainable precision     

Decoupled compensation of IQ imbalance in MIMO OFDM systems

The direct-conversion architecture is an attractive front-end design for multi-input multi-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. These systems are typically small in size and provide a good flexibility to support growing number of wireless standards. However, direct-conversion based OFDM systems are generally very sensitive to front-end component imperfections. These imperfections are unavoidable especially when cheaper components are used in the manufacturing process and can lead to radio frequency (RF) impairments such as in-phase/quadrature-phase (IQ) imbalance. These RF impairments can result in a severe performance degradation. In this paper, we propose training based efficient compensation schemes for MIMO OFDM systems impaired with transmitter and receiver frequency selective IQ imbalance. The proposed schemes can decouple the compensation of the transmitter and receiver IQ imbalance from the compensation of the channel distortion. It is shown that the proposed schemes result in an overall lower training overhead and a lower computational requirement as compared to a joint estimation/compensation of IQ imbalance and the channel distortion.     

Nonuniformly coupled microstrip transversal filters for analog signal processing

The Fourier transform relationship between frequency response and impedance profile for single nonuniform transmission lines is used to derive the time-domain step response of single and coupled nonuniform lines. The expression for the step response of a characteristically terminated nonuniformly coupled transmission line structure is shown to correspond to the characteristic impedance profile. By using this relationship, any arbitrary step response can be realizing by utilizing nonuniformly coupled strip or microstrip lines for possible applications as waveform-shaping networks and chirp filters. A numerical procedure to compute the step response of the nonuniform coupled line four-port is also formulated in terms of frequency-domain parameters of an equivalent cascaded uniform coupled line model with a large number of sections. Sinusoidal and chirp responses are presented as examples that are readily implemented using coupling microstrip structures. The step response of an experimental nonuniformly coupled microstrip structure is presented to validate the theoretical results.<>     

Enhancing design-for-test for active analog filters by using CLP

We describe a computer-aided approach to automatic fault isolation in active analog filters which enhances the design-for-test (DFT) methodology proposed by Soma (1990). His primary concern was in increased controllability and observability while the fault isolation procedure was sketched only in general terms. We operationalize tionalize and extend the DFT methodology by using CLP() to model analog circuits and by a model-based diagnosis approach to implement a diagnostic algorithm. CLP() is a logic programming language which combines symbolic and numeric computation. The diagnostic algorithm uses different DFT test modes and results of voltage measurements for different frequencies and computes a set of suspected components. Ranking of suspected components is based on a measure of (normalized) standard deviations from predicted mean values of component parameters. The diagnosis is performed incrementally, in each step reducing the set of potential candidates for the detected fault. Case studies show encouraging results in isolation of soft faults of a given low-pass biquad filter.     

Switched capacitor analog modulo integrator for application in open loop Sigma-Delta modulators

We introduce the switched capacitor analog modulo integrator, which to our knowledge is a new circuit. We introduce the amplitude modulated open loop ΣΔ modulator (OLSDM), which is an analog modulo integrator followed by a quantizer and a modulo differentiator. The mathematical equivalence between low pass ΣΔ modulators and OLSDM is explained. Behavioral simulations confirm the equivalence. The necessary circuit, a switched capacitor analog modulo integrator, is explained in detail. Behavioral level simulations in SPICE of the analog modulo integrator verify the function, and prove the concept of amplitude modulated OLSDM.