Analog quadrature modulators and demodulators have three major impairments, namely: gain imbalance, phase imbalance and dc-offset. A digital technique is presented for compensation of these modulator and demodulator impairments. Part of the RF signal is fed to an envelope detector. The detector output, along with the baseband quadrature components, is used to estimate the impairment values. The estimated impairment values are then used to compensate for the impairments. Simulation results show that spurious signals can be suppressed by more than 30 dB using this technique. The effect of modulator/demodulator impairments on RF power amplifier linearization techniques is also discussed. 1. INTRODUCTION Digital modulation techniques are increasingly being used in wireless communication systems to increase the transmission efficiency, and hence conserve the limited available frequency spectrum. The interim IS-54 standard [ 13 specifies d 4 shifted differentially encoded quadrature phase shift keying (d4-DQPSK) modulation for North American digital cellular systems. In quadrature modulation, the source bits are first encoded into in-phase (I) and quadrature-phase (Q) components. The interface between the baseband digital signals and the RF transmission channel is an analog quadrature modulator, which generates the amplitude and phase modulated RF signal. This RF signal is amplified using a RF power amplifier before transmission. At the receiver, the baseband I-and Q-signal components are recovered using an analog demodulator. Both analog modulators and demodulators have three well-known impairments [2], namely: gain imbalance, phase imbalance and dc-offset. These impairments result in spurious signals, and degrade the performance of a system. This paper presents a technique for compensating the modulator and demodulator impairments. A portion of the RF signal is fed to an envelope detector. The detector output and the I-and Q-signal component values are used to estimate the impairments, using the Newton-Baphson algorithm. The estimated values are then used to compensate for the impairments. The effects of these impairments on cartesian coordinate negative feedback-and on complex gain predistortion linearization techniques for RF power amplifiers are also discussed. QUADRATURE MODULATOR AND DEMODULATOR IMPAIRMENTSFig. 1 shows a schematic diagram of a typical quadrature modulator. The modulator consists of four components, namely: two mixers, a quadrature hybrid and an in-phase power combiner. These components are not ideal, resulting in a collective effect that can be represented by gain imbalance, phase imbalance and dc-offset. Gain imbalance represents the gain mismatch between the I-and Q-channels. Ideally, the gains in the I-and Q-channels are equal. If the phase difference between the local oscillator signals for the I-and Q-channels is not exactly 90°, phase imbalance exists. A difference in the length of the two RF paths can result in a frequency-dependent phase imbalance. Lastly, carrier feedthrough gives an un...

A b s t r a c t -An adaptive RF power amplifier between the distorted signal and the input signal is used to linearization technique is presented. In this generate an error signal. The error signal is amplified and the method, the demodulated amplifier output is output of the error amplifier is subtracted from the main compared with the baseband input signal to amplifier output to improve the linearity of the system. The estimate the amplifier's AM-AM and AM-PM LINC method [3] uses two well-matched amplifiers. The input characteristics, using cubic spline interpolation. signal is split into two constant amplitude phase-modulated The input signal i s Predistorted using these signals which are then amplified using two nonlinear RF estimated characteristics to 'Ompensate for the amplifiers. These components are passively summed in such a amplifier's nonlinearity. The proposed technique has nearly 14 dB better suppression of the way that the undesired components are in anti-phase, and hence intermodulation products than the complex gain cancel when combined. LINC requires two well-matched predistortion technique. Also, the out-of-band amplifiers, which may not be easy to obtain; also, this power emission is about 12 dB lower in the first method cannot adapt to drifts in the amplifier characteristics. two adjacent RF channels than is obtained using Drifts in the characteristics may occur because of temperature the Cartesian coordinate negative feedback variations, change in power supply voltage, frequency change, technique. etc.. The Combined Analog Locked Loop Universal Modulator (CALLUM) [4] takes feedback from the output to compensate I. INTRODUCTION for the drifts in the characteristics of the amplifiers. The Cartesian coordinate negative feedback technique [5] uses Increasing demands for RF spec"usage are forcing the synchronously demodulated signals as the feedback use of spectrally efficient linear modulation techniques to information. These signals are subtracted from the input reduce the bandwidth of transmitted signals. Linearly signals to generate loop error signals. If the loop gain is modulated RF signals have a fluctuating envelope, and sufficiently high, the feedback loop will continuously correct nonlinear amplification Of these Signals results in for the nonlinearity. This technique is simple to implement, intermodulation Products and spectral spreading. There are but the linearity and the bandwidth are critically dependent on stringent restrictions On intermodulation products and On Out-the loop time delay. Predistortion techniques operate on the of-band power emission. The interim standard IS-54 for digital basis of providing an appropriately distorted signal to the cellular radio [I] specifies power emission in adjacent and amplifier, so that the amplifier output is a simple scaled alternate channels to be 26 dB and 45 dB below the mean replica of the original input signal. Some predistortion output power, respectively. The third-order intermodulation techniques use fixed signal predistortion circu...

The performance of an adaptive digital technique for the linearization of RF power amplifiers is investigated. Cubic spline interpolation is used to estimate the amplifier's AM-AM and AM-PM characteristics. Using the computed characteristic coefficients, the baseband input signal is appropriately predistorted to compensate for the amplifier nonlinearity. This method has significantly better suppression of the intermodulation products than other predistortion techniques. The out-of-band power emission is also significantly reduced. The performance of baseband predistortion linearization techniques is adversely affected by modulator and demodulator impairments. A digital correction technique is presented to compensate for these imperfections. In this technique, part of the RF signal is fed to an envelope detector. The detector output and the baseband signal are used to estimate the impairment values, using the Newton-Raphson method. The estimated impairment values are then used to compensate for the modulator/demodulator impairments. Spurious signals can be suppressed by more than 30 dB using this technique. Cet article traite de la performance d'une technique adaptative numlrique pour la linearisation d'amplificateurs de puissance RF. Une interpolation spline cubique est utilised pour estimer les caractlristiques AM-AM et AM-PM de l'ampliflcateur. A partir des coefficients calculus, le signal d'entrle en bande de base est prfctistorsionne' en vue de compenser la non-linlarite' de l'ampliflcateur. Cette m&hode rlsulte en une suppression significativement meilleure des produits d'intermodulation que d'autres techniques de prldistorsion, et la puissance 6mise hors bande estIgalement rlduite de maniere significative. La performance des techniques de linearisation par prldistorsion en bande de base est affectle par les imperfections du modulateur et du dlmodulateur, et une technique de correction numlrique est prlsentle pour leur compensation, dans laquelle une partie du signal RF est envoyle a un dltecteur d'enveloppe. La sortie du dltecteur et le signal en bande de base sont utilises pour estimer la valeur des imperfections avec la mlthode Newton-Raphson, et pour compenser les imperfections du modulateur/dlmodulateur. Le niveau des signaux non-dlsires peut £tre r£duit de plus de 30 dB.

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