  Differential space time block coding (STBC) achieves full spatial diversity
and avoids channel estimation overhead. Over highly frequency-selective
channels, STBC is integrated with orthogonal frequency division multiplexing
(OFDM) to achieve high performance. However, low-cost implementation of
differential STBC-OFDM using direct-conversion transceivers is sensitive to
In-phase/Quadrature-phase imbalance (IQI). In this paper, we quantify the
performance impact of IQI at the receiver front-end on differential STBC-OFDM
systems and propose a compensation algorithm to mitigate its effect. The
proposed receiver IQI compensation works in an adaptive decision-directed
manner without using known pilots or training sequences, which reduces the rate
loss due to training overhead. Our numerical results show that our proposed
compensation algorithm can effectively mitigate receive IQI in differential
STBC-OFDM.
