Currently, the only available wideband (multiple MHz), accurate, amplifier linearization method is feedforward. Feedforward, though, requires automatic adaptation of key parameters for reliable distortion cancellation as operational and environmental conditions vary. In this thesis, previous analysis on adaptive feedforward linearization is extended to include an _alternative placement of the adaptive signal cancellation coefficient.In contrast to previous analysis, this placement results in a non-quadratic error surface. Consequently, two available criteria for the optimization of the signal cancellation coefficient result in different optimal values. This result can have practical implementation consequences under certain operating conditions. A new analysis is presented that shows that various inaccuracies & the implementation of baseband correlation, such as frequency and phase oeets, filter mismatches, and incomplete image suppression, do not affect the final converged coefficient values. With a novel and appropriate use of DSP, a feedforward linearizer has been implemented with adaptation driven by easily computed gradient signals. This overcomes the dficulties, such as DC -. offsets at the output of analog mixers and masking of weak signals by stronger ones, that slow and/or cause incorrect convergence of many previously reported implementations. The result is 40 dB reduction of intermodulation spectra over a bandwidth of 7 MHz. Coefficient convergence occurs within 50 msec of start-up, and following a 6 dB change in input power, reconvergence occurs \in 3 msec with no loss in distortion suppression. ACKNOWLEDGEMENTSNothing is ever done alone; thus, I would like to thank certain people who helped me along the way with this project. Thanks to my Senior Supervisors, Jim Cavers and Paul Goud, who, I feel, were responsible for advancing my knowledge and confidence in the area of wireless communications to a new and hopehlly much higher level than before.Deserving of thanks as well, is my wife, Irma, for her constant support the whole way through the project, fiom start to finish.
Goud, P. A. (1991). A Spread spectrum radiolocation technique and its application to cellular radio (Unpublished master's thesis).
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...
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