Nonlinear distortions in an anti-series varactor pair (ASVP) are analyzed by a perturbation method. To the authors' knowledge, this paper presents the first derivation of an analytical expression that explicitly shows intermodulation and harmonic distortions of the ASVP. In addition to canceling the expected even-order distortion, the third-order distortion can be suppressed simultaneously when a certain condition is satisfied. We also find that the second- and third-order distortions can be greatly suppressed without dependence on dc bias voltage if the varactors in the ASVP have an ideal abrupt p-n junction. These theoretical predictions are verified by measuring the second- and third-order harmonic distortions of an ASVP. The experimental results show that the second-order harmonic distortion is suppressed by approximately 20 dB. The third-order harmonic distortion is suppressed to the same extent in the theoretically predicted dc bias voltage range.

While a multicarrier approach of achieving frequency diversity performs well in the presence of partial-band interference, it suffers from the effects of intermodulation distortion (IMD) due to power amplifier (PA) nonlinearity. On the other hand, transmit diversity using multiple transmit antennas has the benefit of no IMD effects, but can suffer from a larger performance degradation due to partial-band interference (e.g., jamming or narrowband signals in a overlay system) compared to the multicarrier approach. Hence, hybrid diversity schemes which use both multicarrier as well as multiple transmit antennas are of interest. Techniques to suppress IMD effects in such hybrid diversity schemes are important. In this paper, we propose and evaluate the performance of a minimum mean square error (MMSE) receiver to suppress the intermodulation distortion in a coded multicarrier multiple transmit antenna (P transmit antennas) DS-CDMA system with M subcarriers on each transmit antenna, for both BPSK and QPSK modulation. The system uses rate-1/M convolutional coding, interleaving and space-time coding. We compare the performance of a (M = 4,P = 2) scheme and a (M = 2,P = 4) scheme, both having the same diversity order. We show that the proposed MMSE receiver effectively suppresses the IMD effects, thus enabling to retain better antijamming capability without much loss in performance due to IMD effects.

A flexible and robust rate-distortion optimization algorithm is presented to select macroblock coding mode for H.264/AVC transmission over wireless channels subject to burst errors. A two-state Markov model is used to describe the burst errors on the packet level. With the feedback information from the receiver and the estimation of the channel errors, the algorithm analyzes the distortion of the reconstructed macroblock at the decoder due to the channel errors and spatial and temporal error propagation. The optimal coding mode is chosen for each macroblock in rate-distortion (R-D)-based framework. Experimental results using the H.264/AVC test model show a significant performance of resilience to the burst errors.

Modern spaceborne precision radar altimeters transmit radio signals of a spectrum bandwidth up to 300 MHz, but the bandwidth should be still increased for precise estimation of the roughness of the sea surface. In this research, the influence of the ionosphere on wideband radar signals is investigated and then it is shown that the signals are strongly influenced by the dispersive distortions in the atmosphere of the Earth even in Ku-band. Finally, the allowable bandwidth of a space borne precision radar altimeter signals is estimated, at which we could ignore the presence of these distortions.

This paper presents a compensating method based on Self-Organizing Map (SOM) for nonlinear distortion, which is caused by high-power amplifier (HPA) in 16-QAM-OFDM system. OFDM signals are sensitive to nonlinear distortions and different methods are studied to solve them. In the proposed scheme, the correction is done at the receiver by a SOM algorithm. Simulations are carried out considering an additive white Gaussian (AWG) transmission channel. Simulation results show that the SOM algorithm brings perceptible gains in a complete 16-QAM-OFDM system.

A simple method has been proposed for the measurement of the output power and phase characteristics of the 3rd-order inter-modulation distortion (IM3) components appearing in multistage power amplifiers. By adopting a unique definition of the phase for the IM3 components that is independent of the delay time caused by transmission lines and other instrument devices, it is possible to measure the phase, merely by using a vector signal analyzer. It is demonstrated that an accurate estimation of the IM3 characteristics of two-stage cascaded power amplifiers for cellular radio handheld terminals can be made by using the IM3 characteristics of the 1st and 2nd-stage amplifiers as measured by the proposed method. The results indicate that it is possible to reduce the dissipation power by 18% at 28 dBm RF output power with respect to conventional measurement methods. Further studies show that the error in the resultant vector of the estimated IM3 is less than 1 dB, when the asymmetry characteristics of the IM3 sidebands in the 2nd-stage amplifier are less than 7.3%.

A DFT-Based method (DBM) has been proposed to compensate for the performance degradation caused by clipping distortion at the expense of bandwidth expansion. On the other hand, in any communication systems, conventional channel coding methods can be employed to improve performance. In this letter, the performance of the DBM and the channel coding methods (CCM) are compared. Furthermore, we introduce a hybrid system which outperforms both the DBM and the CCM.

We propose Optimal Temporal Decomposition (OTD) of speech for voice morphing preserving Δ cepstrum. OTD is an optimal modification of the original Temporal Decomposition (TD) by B. Atal. It is theoretically shown that OTD can achieve minimal spectral distortion for the TD-based approximation of time-varying LPC parameters. Moreover, by applying OTD to preserving Δ cepstrum, it is also theoretically shown that Δ cepstrum of a target speaker can be reflected to that of a source speaker. In frequency domain interpolation, the Laplacian Spectral Distortion (LSD) measure is introduced to improve the Inverse Function of Integrated Spectrum (IFIS) based non-uniform frequency warping. Experimental results indicate that Δ cepstrum of the OTD-based morphing spectra of a source speaker is mostly equal to that of a target speaker except for a piecewise constant factor and subjective listening tests show that the speech intelligibility of the proposed morphing method is superior to the conventional method.

In recent years, digital predistortion linearizers have been used in power amplifiers for mobile communications because they are simpler and provide higher power efficiency than conventional feedforward systems. However, in systems that cover a wider frequency band, it is impossible to disregard the frequency characteristics of their various parameters since the degradation that can result causes a decline in output power efficiency which is the most important property of a power amplifier. To date, no detailed studies have been carried out on predistortion compensation systems. Thus, we focused our research on these systems and in this paper we report the simulation and experimental results we obtained for clarifying these effects. In our experiments, we used a W-CDMA power amplifier to determine how much the distortion compensation effect is degraded by the frequency characteristics of analog RF circuits. The results of experiments to determine the relationship between the ACLR (Adjacent Channel Leakage power Ratio) and power efficiency are also reported.

This work presents a new sigma-delta modulator (SDM) architecture for a wide bandwidth receiver. This architecture contains dual-bandwidth for W-CDMA and GSM system applications. Low-distortion swing-suppressing SDM and interpolative SDM cascaded units are used together. Using the low-distortion swing-suppressing technique, the resolution can be improved even under non-linearity effects. The interpolative SDM extends the signal bandwidth and represses the high-band noise. The SDM used in the W-CDMA and GSM applications was designed and simulated using 0.25-µm 1P5M CMOS technology. The simulated peak SNDR of W-CDMA and GSM are 72/70 dB and 82/84 dB in Low-IF/Zero-IF standards.

A novel geometrically invariant watermarking scheme based on gravity center is presented which treating the geometrically invariant gravity centers of host image and its supplement image as reference points. Thus watermark synchronization is obtained. Simulation results show the effectiveness of our scheme to the geometrical distortion including rotation and/or scaling.

In this paper a second order Volterra series model of an operational amplifier (opamp) circuit is presented. Such a model is suitable to the investigation of the rectification and demodulation effects of radio frequency (RF) interference superimposed on the nominal input signals and on the power supply voltage of an opamp. On the basis of the new model, some design criteria to improve the immunity of opamps to RF interference are proposed. Model predictions are verified by comparison with experimental test results.

We present ultra low noise- and wide dynamic range performances of an AlGaN/GaN heterostructure FET (HFET). An HFET fabricated on a high quality epitaxial layers grown on a semi-insulating SiC substrate exhibited impressively low minimum noise figure (NF min ) of 0.4 dB with 16 dB associated gain at 2 GHz. The low NF (near NF min ) operation was possible in a wide drain bias voltage range, i.e. from 3 V to 15 V. At the same time, the device showed low distortion character as indicated by the high third order input intercept point (IIP3), +13 dBm. The excellent characteristics are attributed to three major factors: (1) high quality epitaxial layers that realized a high transconductance and very low buffer leakage current; (2) excellent device isolation made by selective thermal oxidation; (3) ultra low gate leakage current realized by Pd based gate. The results demonstrate that the AlGaN/GaN HFET is a strong candidate for front-end LNAs in various mobile communication systems where both the low noise and the wide dynamic range are required.

An 18 GHz-band Microwave Monolithic Integrated Circuit (MMIC) diode linearizer using a parallel capacitor with a bias feed resistance is presented. The newly employed parallel capacitor is able to control gain and phase deviations of the linearizer. This implies that the gain deviation of the linearizer can be controlled without changing the phase deviation. The presented linearizer can compensate the distortion of an amplifier sufficiently. The operation principle of the linearizer with the parallel capacitor is investigated. It is clarified that the gain deviation can be adjusted without changing the phase deviation by using the parallel capacitor. Two variable gain buffer amplifiers and the core part of the linearizer which consists of a diode, a bias feed resistor, and a capacitor are fabricated on the MMIC chip. The amplifiers cancel the frequency dependence of the core part of the linearizer to improve bandwidth of the MMIC. Further, the amplifiers contribute to earn low reflection and compensate insertion loss of the linearizer. The MMIC chip is size of 2.5 mm 1 mm. The linearizer has demonstrated improvement of 3rd Inter-Modulation Distortion (IMD3) of 12 dB at 18 GHz and improvement of more than 6 dB between 17.8 GHz and 18.6 GHz.

An optical FM system using an optical FM LD (laser diode) and an optical frequency discriminator (OFD), in which a nonlinear compensation scheme based on the interaction between its nonlinearities can minimize intermodulation distortion. This paper theoretically investigates the minimization influence for 3rd plus 5th order intermodulation distortion power for an optical FM radio-on-fiber system. The carrier to noise-plus-distortion power ratio (CNDR) is theoretically analyzed in employing the OFD whose transmission characteristic is controlled by a phase shifter. The results show that the designed receiver can achieve higher CNDR in the application of multicarrier transmission.

The equidistortion principle[1] has recently been proposed as a basic principle for design of an optimal vector quantization (VQ) codebook. The equidistortion principle adjusts all codebook vectors such that they have the same contribution to quantization error. This paper introduces a novel VQ codebook design algorithm based on the equidistortion principle. The proposed algorithm is a variant of the law-of-the-jungle algorithm (LOJ), which duplicates useful codebook vectors and removes useless vectors. Due to the LOJ mechanism, the proposed algorithm can establish the equidistortion condition without wasting learning steps. This is significantly effective in preventing performance degradation caused when initial states of codebook vectors are improper to find an optimal codebook. Therefore, even in the case of improper initialization, the proposed algorithm can achieve minimization of quantization error based on the equidistortion principle. Performance of the proposed algorithm is discussed through experimental results.

A linear voltage-to-current convertor without current mirror circuit is proposed for low distortion applications employing short channel MOSFET's. Twin current sources and current sinks pair of MOSFET's having the same drain-source voltage are employed for a substitute of the current mirror circuits, in order to eliminate the channel length modulation factor of the short channel MOSFET's. HSPICE simulation is shown in order to evaluate the proposed circuits. As an application, a low distortion OTA is realized by employing the proposed linear voltage-to-current convertor with short channel MOSFET's.

The paper is concerned with an identification-based predistortion scheme for compensating nonlinearity of high power amplifiers (HPA). The identification algorithms for the Wiener-Hammerstein nonlinear model are developed in the frequency domain. By approximately modeling the nonlinear distortion part in HPA by polynomial or spline functions, and introducing linear distortion parts in the input and output of the nonlinear element, the iterative identification schemes are proposed to estimate all the uncertain parameters and to construct an inverse system for the predistortion.

This paper investigates a new method for creating robust speaker models to cope with inter-session variation of a speaker in a continuous HMM-based speaker verification system. The new method estimates session-independent parameters by decomposing inter-session variations into two distinct parts: session-dependent and -independent. The parameters of the speaker models are estimated using the speaker adaptive training algorithm in conjunction with the equalization of session-dependent variation. The resultant models capture the session-independent speaker characteristics more reliably than the conventional models and their discriminative power improves accordingly. Moreover we have made our models more invariant to handset variations in a public switched telephone network (PSTN) by focusing on session-dependent variation and handset-dependent distortion separately. Text-independent speech data recorded by 20 speakers in seven sessions over 16 months was used to evaluate the new approach. The proposed method reduces the error rate by 15% relatively. When compared with the popular cepstral mean normalization, the error rate is reduced by 24% relatively when the speaker models were recreated using speech data recorded in four or more sessions.

This paper presents a practical implementation scheme of the variable gain amplifier (VGA) using a Cds photo coupler (Cds PC) as a variable resister at the feedback loop. The fundamental design policies of IF amplifier stage in superheterodyne receiver were described. We demonstrated the VGA's experimental results. The results indicated the excellent IIP3 of +25 dBm achieved by a gain of 15 dB, and the reasonable thermal stability and variable gain range. Third-order intermodulation distortion (IMD3) comparison between the proposed VGA and conventional PIN diode attenuation type VGA was evaluated and the result indicated that the proposed VGA surpassed the PIN VGA. The proposed VGA was practically fabricated in 455 kHz IF amplifier stage for an airborne VHF communication receiver in order to improve the large signal handling capability to eliminate numerous interferences resulting from the collocated airborne VHF communication systems on the aircraft.