Wireless Communications Transmitter Performance Enhancement Using Advanced Signal Processing Algorithms Running in a Hybrid DSP/FPGA Platform

This paper deals with digital base band signal processing algorithms, which are seen as enabling technologies for software-enabled radios, that are intended for the correction of the analog front end. In particular, this paper focuses on the design, optimization and testability of predistortion functions suitable for the linearization of narrowband and wideband transmitters developed with a hybrid DSP/FPGA platform. To select the best algorithm for the identification of the predistortion function, singular value decomposition, recursive least squares (RLS), and QR-RLS algorithms are implemented on the same digital signal processor; and, the computation complexity, time, accuracy and the required resources are studied. The hardware implementation of the predistortion function is then carefully performed, in order to meet the real time execution requirements.     

Modified linear prediction algorithm for narrowband interference suppression in universal mobile telecommunication system

Narrow band interference (NBI) deteriorates the quality of the spectrum, leading to a poorer performance of modern universal mobile telecommunication system (UMTS) spread spectrum systems. The linear prediction algorithm is one of the most significant techniques to overcome NBI and enhance the performance of UMTS systems. In this paper, a modified linear prediction algorithm is proposed for NBI suppression in a conditionally stationary environment. This modification improves the error energy estimation in the auto‐regression model of the linear prediction. The convergence of the proposed algorithm is evaluated, and its robustness is verified using Kullback–Leibler metrics for conditionally stationary NBI signals. Computer simulations are carried out, and the obtained results demonstrate the performance of the proposed algorithm and its compliance with UMTS protocols. Copyright © 2012 John Wiley & Sons, Ltd.     

Adaptive digital/RF predistortion using a nonuniform LUT indexing function with built-in dependence on the amplifier nonlinearity

This paper reports on an integrated adaptive digital/RF predistorter using a nonuniform spaced lookup table (LUT) and in-phase/quadrature (I/Q) RF vector multiplier (VM). The LUT contents are directly deduced from the baseband input and output signals of the power amplifier (PA). In addition, a new nonlinear indexing function of the predistortion LUT with built-in dependence on the PA nonlinearity is proposed. This function is made to be robust to the input signal statistics. A comparison of this new indexation method with conventional approaches, namely, power and logarithmic power indexation functions, is carried out. The superiority of the proposed scheme is demonstrated in particular for class-AB amplifiers where the gain of the PA varies over the whole input range of the drive signal. The measured output spectrum of a linearized 90-W peak lateral double-diffused metal-oxide-semiconductor PA reveals a significant reduction of the power emission at the adjacent channels of approximately 15 dB under IS95, single-carrier, and multicarrier wide-band code-division multiple-access signals. The experimental evaluation is carried out using an RF/digital predistorter prototype that mainly includes an envelope detector, a linear I/Q RF VM, field-programmable gate array and digital signal processor, and fast analog/digital and digital/analog converters.     

Estimation of crossover DPD using orthogonal polynomials in fixed point arithmetic

This paper deals with the fixed point implementation of crossover digital predistortion (DPD). The implementation of digital predistortion linearization technique on DSPs poses major challenges, regarding cost, power consumption, speed, precision and volume. Depending on resource availability and design restrictions, fixed-point DSPs may be considered as a suitable solution. Least squares estimation of crossover DPD for multiple-input, multiple-output (MIMO) applications using conventional polynomial models face numerical instability for fixed-point processing. Orthogonal polynomials on the other hand are robust to matrix inversion. Fixed point matrix inversion was implemented using LU decomposition and triangular matrix inversion. This paper examines crossover DPD design for MIMO applications, while using fixed-point arithmetic. It also compares the linearization of 2 × 2 MIMO transmitters in the presence of radio frequency crosstalk using both orthogonal and memory polynomial models. The performance of the two crossover DPDs has been evaluated using a 3GPP standard: orthogonal crossover DPD decreased the measured adjacent channel power ratio of the signal from −42 dBc to −52 dBc.     

A new unequal three-tone signal method for AM-AM and AM-PMdistortion measurements suitable for characterization of satellitecommunication transmitters/transponders

A new method for characterization of AM-AM and AM-PM distortion of power amplifiers (PAs) using only power measurements is proposed in this paper This technique was found to be suitable for the characterization of satellite communication transmitters/transponders. It consists of using an unequal three-tone signal (UTTS) to drive either the base-station PA's transmitter or the satellite transponder's PA and to measure the level of this UTTS at the output of the transmitter/transponder. By comparison of the three tone levels at the input and output of the transmitter/transponder, one can calculate the AM-AM compression factor and AM-PM conversion coefficient of the transmitter/transponder PA using closed-form expressions     

Direct parameter-extraction method for HBT small-signal model

An accurate and broadband method for the direct extraction of heterojunction bipolar transistor (HBT) small-signal model parameters is presented in this paper. This method differs from previous ones by extracting the equivalent-circuit parameters without using special test structures or global numerical optimization techniques. The main advantage of this method is that a unique and physically meaningful set of intrinsic parameters is extracted from the measured S-parameters for the whole frequency range of operation. The extraction procedure uses a set of closed-form expressions derived without any approximation. An equivalent circuit for the HBT under a forward-bias condition is proposed for extraction of access resistances and parasitic inductances. An experimental validation on a GaInP/GaAs HBT device with a 2×25 μm emitter was carried out, and excellent results were obtained up to 30 GHz. The calculated data-fitting residual error for three different bias points over 1-30 GHz was less then 2%     

A simple large signal model for III–V HBT devices exceeding VBIC performances

This paper presents a large signal compact circuit simulation model for III–V heterojunction bipolar transistors (HBTs). The model is implemented as a user compiled model (UCM), in the agilent ADS circuit simulator, using C code. Though its simplicity, the model includes all physical effects taking place in power III–V-based HBT devices. It is verified by comparing its simulations to measurements in DC, small-signal and large-signal operation modes. The proposed model is further tested by comparing it to the established VBIC model and the newly introduced Agilent HBT model. It is found that despite its reduced complexity, the proposed model gives better agreement with measurements than the VBIC model in all modes of operation.     

Lipolysis of butter oil by immobilized lamb pregastric esterase: I. Uniresponse kinetics-pH and temperature effects

Lamb pregastric esterase, immobilized by physical adsorption on microporous polypropylene in a hollow fiber reactor, has been employed to effect the continuous hydrolysis of the triglycerides in butter oil. Experimental data were obtained at temperatures from 35 to 45 degrees C and pH values from 5.5 to 6.5. The overall rate of hydrolysis was fastest at 40 degrees C and a pH of 6.0. Nonlinear regression methods were employed to determine the kinetic parameters of rate expressions based on a generic Ping-Pong Bi Bi mechanism. The best nonlinear fit of the data was consistent with a mechanism that assumes that acylation of the enzyme is the rate-limiting step in the hydrolysis reaction.     

An experimental study of the effects of harmonic loading onmicrowave MESFET oscillators and amplifiers

This paper reports an extensive experimental investigation of the effects of second harmonic loading on the performance of microwave GaAs MESFET oscillators; and strongly driven amplifiers. The measurement system used is an active load system based on six-port techniques. Harmonic load pull measurements were obtained for the NE72084 MESFET; the measurements show how the second harmonic load can influence the power gain and the power added efficiency in strongly driven amplifiers. The device line characterization technique was combined with the harmonic load pull technique; the measurement results illustrate how the output power and the DC to RF conversion efficiency of an oscillator depend on the choice of the second harmonic load. Amplifier and oscillator circuits have been designed using these measurements; the circuits have been constructed and measured. The results validate the experimental approach used and clearly illustrate the importance of properly selecting the second harmonic load in amplifier and oscillator circuits. Significant improvements in gain, output power and efficiency have been achieved by properly selecting the second harmonic load