Abstract-Direct conversion orthogonal frequency division multiplexing (OFDM) systems suffer from transmit and receive analog processing impairments such as in-phase/quadrature (I/Q) imbalance causing inter-carrier interference (ICI) among sub-carriers. Another source of performance-limiting ICI in OFDM systems is Doppler spread due to mobility. However, the nature of ICI due to each of them is quite different. Unlike previous work which considered these two impairments separately, we develop a unified mathematical framework to characterize, estimate, and jointly mitigate ICI due to I/Q imbalance and high mobility. Based on our general model, we derive a closed-form expression for the degradation in signal-to-interference-plus-noise ratio (SINR) due to the impairments. Moreover, we exploit the special ICI structure to design efficient OFDM channel estimation and digital baseband compensation schemes for joint transmit/receive frequency-independent (FI) and frequency-dependent (FD) I/Q imbalances under highmobility conditions. Index Terms-Channel estimation, in-phase/quadrature (I/Q) imbalance, inter-carrier interference (ICI), minimum mean square error (MMSE) equalization, mobility, orthogonal frequency division multiplexing (OFDM), signal-to-interference-plus-noise ratio (SINR).

Abstract-Direct-conversion multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) transceivers enjoy high data rates and reliability at practical implementation complexity. However, analog front-end impairments such as I/Q imbalance and high mobility requirements of nextgeneration broadband wireless standards result in performancelimiting inter-carrier interference (ICI). In this paper, we study the effects of ICI due to these impairments for OFDM with space frequency block codes and spatial multiplexing, derive a generalized linear model and propose a non-iterative reduced-complexity digital baseband joint compensation scheme. Furthermore, we present a pilot scheme for joint estimation of the channel and the I/Q imbalance parameters and evaluate its performance through simulations. Our proposed scheme is effective in estimating and compensating for frequency-independent and frequencydependent transmit and receive I/Q imbalances even in the presence of a residual frequency offset.

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In our previous work [1], digital baseband compensation of joint Tx/Rx I/Q imbalance in mobile SC-FDE transceivers was investigated. In this sequel, the training sequence design for joint channel and I/Q imbalance parameter estimation in mobile SC-FDE transceivers is discussed in greater detail.

Abstract-I/Q imbalance and high mobility in OFDM systems result in performance-limiting intercarrier interference (ICI). However, the nature of ICI due to each of them is quite different. Unlike previous works which considered these two impairments separately, we develop a unified mathematical framework to characterize and mitigate ICI when both impairments are present. In addition, we exploit the special ICI structure to design efficient OFDM channel estimation and digital baseband compensation schemes for I/Q imbalance under high-mobility conditions.