Analysis and Augmented Spatial Processing for Uplink OFDMA MU-MIMO   Receiver with Transceiver I/Q Imbalance and External Interference
  We address receiver (RX) signal processing in MIMO systems under in-phase/quadrature (I/Q) imbalance, which causes cross-talk of mirror-subcarriers in OFDM systems. We extend the typically reported single-user studies to uplink OFDMA-based multiuser MIMO, with simultaneous user multiplexing in frequency and spatial domains. We also incorporate multiple external interferers, for modeling challenging conditions in heterogeneous networks. In the signal processing developments, we exploit the augmented subcarrier processing, which processes each subcarrier jointly with its counterpart at the image subcarrier, and jointly across RX antennas. Furthermore, we derive an augmented LMMSE RX. The novel approach integrates the I/Q imbalance mitigation, interference suppression and data stream separation into a single processing stage, thus avoiding a separate transceiver calibration. Our numerical results show the signal-to-interference-plus-noise ratio and symbol-error rate of an arbitrary data stream after RX processing as a function of different system parameters. The per-subcarrier processing is shown to suffer heavily under I/Q imbalances, and being particularly sensitive to external interferers, whereas the augmented method provides efficient data stream separation and interference suppression. Finally, we extend the studies to massive MIMO framework and show that the per-subcarrier processing still suffers from performance degradation, whereas the augmented approach can fully exploit the array gain. 
Aki Hakkarainen, Janis Werner, Kapil R. Dandekar and Mikko Valkama
cs.IT math.IT