Transceiver I/Q imbalance and widely-linear spatial processing in large antenna systems

In order to keep the total device costs low, large antenna systems require affordable radio frequency (RF) electronics. Unfortunately, this requirement results in RF impairments and may thus cause performance degradations. In this paper, we show how one of these impairments, namely in-phase/quadrature (I/Q) imbalance, distorts the received signals in an uplink multiuser multiple-input multiple-output (MU-MIMO) system where multiple users are spatially multiplexed into the same time-frequency resource. In addition, we present three receiver (RX) post-processing methods and analyze their performance with different multicarrier scenarios under transceiver I/Q imbalances. The results clearly show that the simple maximum ratio combining (MRC) based RX processing suffers heavily from the presence of multiple spatially multiplexed users, especially in case of I/Q imbalances, and cannot necessarily provide sufficient performance even with the number of RX antennas approaching infinity. In contrast, the linear minimum mean-square error (LMMSE) processing offers more flexible and efficient operation characteristics but is also shown to suffer from performance degradations due to I/Q imbalances. To overcome this problem, we formulate a widely-linear (WL) variant of the MMSE method, called WL-MMSE, which provides good performance also under I/Q imbalances in different multiple access scenarios, and is thus a good candidate for future software defined radios where flexibility is a key concern.