  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 efficiently mitigate intersymbol interference effects. However,
low-cost implementation of STBC-OFDM with direct-conversion transceivers is
sensitive to In-phase/Quadrature-phase imbalance (IQI). In this paper, we
quantify the performance impact of IQI at both the transmitter and receiver
radio frequency front-ends on differential STBC-OFDM systems which has not been
investigated before in the literature. In addition, we propose a widely-linear
compensation algorithm at the receiver to mitigate the performance degradation
caused by the IQI at the transmitter and receiver ends. Moreover, a
parameter-based generalized algorithm is proposed to extract the IQI parameters
and improve the performance under high-mobility. The adaptive compensation
algorithms are blind and work in a decision-directed manner without using known
pilots or training sequences. Numerical results show that our proposed
compensation algorithms can effectively mitigate IQI in differential STBC-OFDM.
