The recent interest in I/Q signal processing based transceivers has resulted in a new domain of research in flexible, low-power, and low-cost radio architectures. The main advantage of complex or I/Q up- and downconversion is that it does not produce any image signal and eliminates the need of expensive RF filters. This greatly simplifies the transceiver front-end and permits single-chip radio transceiver solutions. The analog quadrature modulators and demodulators are, however, sensitive to two kinds of implementation impairments: gain imbalance, and phase imbalance. These impairments originate due to the non-ideal behavior of the electronic components in the I- and Q- channels of the modulators/demodulators. As a result, they compromise the infinite image signal attenuation and adversely affect the performance of a wireless system. Furthermore, new higher order modulated waveforms and wideband signals are especially susceptible to these impairments and achieving sufficient image signal attenuation is a fundamental requirement for future wireless systems. Therefore, digital techniques which enhance the dynamic range of front-end with minimum amount of additional analog hardware are becoming more popular, being also motivated by the constantly increasing number crunching power of digital circuitry.
In this thesis, some recently developed algorithms for I/Q imbalance estimation and compensation are studied on the transmitter side. The calibration algorithms use a baseband test signal combined with a feedback loop from I/Q modulator output back to transmitter digital parts to efficiently estimate the modulator I/Q mismatch. In the feedback loop, the RF signal is demodulated and compared with the original test signal to estimate the I/Q imbalance and the needed pre-distortion parameters. The actual digital transmit signal is then properly pre-distorted with the obtained I/Q imbalance knowledge, in order to cancel the effects of modulator I/Q imbalance at the data transmission phase. The performance of the compensation algorithms is first evaluated with computer simulations. A prototype system using laboratory instruments is also developed to illustrate the effects of I/Q imbalance in direct conversion and low-IF transmitters and is used to prove the usability of algorithms in real life front-ends. The results of computer simulations and laboratory measurements prove that the compensation algorithms yield a good calibration performance by suppressing the image signal interference close to or even below the noise floor. /Kir1
