Digital Compensation of Frequency-Dependent Joint Tx/Rx I/Q Imbalance in OFDM Systems Under High Mobility

Abstract

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).

Novel Neural Network Model of Power Amplifier Plus Iq Imbalances

“…Due to its peculiarity of a high Peak-to-Average Power Ratio (PAPR) [6], OFDM-based signals (WLAN and LTE signals) are very sensitive to the PA nonlinear distortions. Another important distortion is TX IQ imbalances [7,8], represented by the gain imbalance between I and Q paths, and the shift from the ideal phase difference between the two local oscillators (LOs). TX IQ imbalances are also frequency-dependent with wideband [7] signals.…”

“…TX IQ imbalances are also frequency-dependent with wideband [7] signals. The TX IQ imbalances distortion may introduce Mirror Frequency Interference (MFI) [8].…”

Single carrier frequency domain equalization and energy efficiency optimization for MIMO cognitive radio.

“…In general, it is difficult to compensate the IQ imbalance in analog domain due to its cost and inefficiency [34]. Consequently, many related works have focused on mitigating the effect of IQ imbalance in digital baseband.…”

“…These results show that the effect of IQ imbalance produces so called mirror carrier interference (MCI) causing the loss of orthogonality among the subcarriers. Various IQ imbalance estimation and compensation schemes have been proposed for the compensation of frequency-independent [38] and frequency-dependent IQ imbalance [34]. Initially, most solutions use linear compensation methods, such as Least Square (LS) [39] and Minimum Mean Square Error (MMSE) [40].…”

“…The input-output relation given in (69) is a simplified model of [34], since it only considers the time invariant channel and ignores the effect of ideal low-pass filter at receiver side (The ideal low-pass filter in [34] does not contribute to the system I/Q imbalance and can be jointly considered with the wireless channel).…”