Adaptive compensation for imbalance and offset losses in direct conversion transceivers

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