The current interest in linear modulation and multilevel signals has resulted in an emphasis on DSP implementations to achieve precision signal manipulation. However, most transceivers, and direct conversion designs in particular, rely on analog implementations of the quadrature modulator and demodulator, thereby sacrificing much of the precision gained through DSP. The present paper focuses on the three principal impairments of analog quadrature modulators and demodulators: gain imbalance, phase imbalance, and DC offset. The paper contains three main contributions. First is an analysis and quantitative assessment of the losses-primarily a degraded BER and out-of-band power in the transmitted signal-due to imbalances and offsets. The second contribution is an adaptive compensation technique for the quadrature modulator at the transmitter, and the third is a compensation technique for the quadrature demodulator at the receiver. Both compensation methods converge quickly and present only a modest computational load. >

Adaptive compensation for imbalance and offset losses in direct conversion transceivers

https://www.dtc.umn.edu/s/resources/gg-se.pdf

A bibliography on nonlinear system identification

This paper describes an adaptive predistortion system to linearize class B and class C radio frequency high power amplifiers. The system, which can handle a 30 dB power control range, uses a digital signal processing microprocessor and memory look-up tables to predistort the baseband drive signals. Low data rate services such as are found in mobile satellite applications ( >

Adaptive linearization using predistortion-experimental results

The current trend in building low-cost yet flexible radio transceivers is to use the so-called direct-conversion principle, which is based on complex (I/Q) up- and down conversions. Such transceivers are, however, sensitive to mismatches between the I and Q branches. These mismatches are unavoidable in any practical implementation, and result in finite attenuation of the mirror frequencies. In addition to the mirror-frequency interference problem, I/Q mismatches can severely compromise the performance of power amplifier linearization techniques based on pre-distortion. The effects of these impairments are becoming more pronounced as higher order modulated waveforms and/or more wideband multichannel signals are used. This brief focuses on digital-signal-processor-based I/Q mismatch calibration in wideband direct-conversion transmitters, assuming the challenging case of frequency-dependent I/Q mismatch. First, a novel widely linear (WL) calibration structure is introduced, suitable for frequency-dependent calibration. Then, two alternative principles for calibration parameter estimation are proposed. The first estimation approach stems from second-order statistics of complex communication signals, while the second technique is based on WL least-squares model fitting. Both estimators are shown by simulations to yield very good calibration performance. The obtainable performance is further assessed using laboratory RF signal measurements.

Frequency-Selective I/Q Mismatch Calibration of Wideband Direct-Conversion Transmitters

Adaptive Linearization Using Predis tortion-Experiment a1 Results

A technique for the automatic correction of errors in RF quadrature modulators is described. The method uses a diode detector to provide amplitude feedback to guide the adaption of baseband circuits that correct for carrier leak, differential gain and phase mismatch errors.

Automatic adjustment of quadrature modulators

One method of linearisation for class C RF power amplifiers is to pre-distort the drive signal. It is necessary to adjust the pre-distort waveform so that it can track changes in amplifier characteristics. An adaptive predistortion technique which uses a digital signal-processing microprocessor and memory look-up tables is described. The method differs from a previously described adaptive predistortion technique by using much smaller table sizes, with the result that conversion speeds are much higher. The disadvantage is that the processing overhead is higher, which puts a limit on the maximum data rate of the system. Low data rate services such as are found in mobile satellite applications ( >

W. Yates

Papers

Automatic adjustment of quadrature modulators

Adaptive linearisation using pre-distortion