Analysis and mitigation of i/q imbalances in multi-antenna transmission systems

The implementation challenges in building compact and low-cost radios for future wireless systems are continuously growing. This is partially due to the introduction of multi-antenna transmission techniques as well as the use of wideband communication waveforms and high-order symbol alphabets. In general, implementations of several parallel radios with wide bandwidth and high performance are required in single devices. Then, to keep the overall implementation costs and size feasible, simplified radio architectures and lower-cost electronics are typically used. This in turn implies that various nonidealities in the used analog radio frequency (RF) modules, stemming from the unavoidable physical limitations of the used electronics, are expected to play a critical role in future multi-antenna radio systems. In this thesis, one example of such nonidealities, called in-phase/quadrature (I/Q) imbalance related to the amplitude and phase matching of transceiver I/Q branches, is studied in a multi-antenna communication system context. Assuming the individual analog front-ends are based on the direct-conversion radio architecture, the essence of the thesis concentrates on the analysis and digital compensation of the I/Q imbalance effects in multi-antenna transmission systems. Both transmitter and receiver sides are taken into account. In most of studies carried out in this thesis, the I/Q imbalances are assumed to be frequency-dependent and both single-carrier and multi-carrier waveforms are considered. More specifically, analytical signal models for depicting the imbalanced analog front-end processing are derived for three types of multi-antenna transmission systems, namely the space-time coded (STC) single-carrier (SC) transmission system, the space-time coded (STC)-orthogonal frequency division multiplexing (OFDM) transmission system and the spatial multiplexing (SM)-multiple-input multiple-output (MIMO)-OFDM transmission system. The resulting waveform distortion and link performance degradation are then analyzed in terms of the achievable signal-to-interference ratio (SIR) at detector input in the receiver. This analysis offers a valuable analytical tool for assessing the I/Q imbalance effects in typical multi-antenna systems, without lengthy system simulations. The analysis results also indicate that in general the I/Q imbalance effects are fundamentally different and more challenging in the multi-antenna context compared to traditional single-antenna systems. Two types of digital compensation methods are then also proposed for combating the I/Q imbalance effects on the receiver side. The first approach is based on algebraic properties of the derived signal models combined with proper pilot data and is applicable in both single-carrier and multi-carrier multi-antenna transmission systems. The second one is based on blind signal separation principles and is mainly targeted for the single-carrier transmission case. The compensation performance of both methods is verified using extensive computer simulations. The results indicate that the proposed techniques can efficiently mitigate the signal distortion and performance degradation due to I/Q imbalance. Some practical problems such as the effects of channel estimation errors, residual carrier offsets and pilot interpolation are also considered in the thesis. Finally, pilot-based compensation techniques for combating the I/Q imbalance effects in individual OFDM transmitters and receivers are also developed in the thesis. Generally, this approach offers an alternative way to cope with the I/Q imbalance effects in the multi-antenna scenario by calibrating the individual radios in an efficient manner on both sides of a wireless link.