MIMO OFDM systems with digital RF impairment compensation
Signal Processing
Pilot designs for channel estimation of MIMO OFDM systems with frequency-dependent I/Q imbalances
IEEE Transactions on Communications
Time domain IQ imbalance compensation for wideband wireless systems
IEEE Communications Letters
Efficient compensation of transmitter and receiver IQ imbalance in OFDM systems
EURASIP Journal on Advances in Signal Processing
Receiver IQ Imbalance Estimation via Pure Noise for 60 GHz Systems
Wireless Personal Communications: An International Journal
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New air interfaces are currently being developed to meet the high spectral efficiency requirements of the emerging wireless communication systems. In this context, OFDM is considered as a promising air interface candidate for both indoor and outdoor communications. Besides spectral efficiency and power consumption, the production cost of the transceiver should also be optimized. Direct-conversion radio frequency receivers are appealing because they avoid costly intermediate frequency hardware. However, they imply analog IQ separation, introducing a phase and amplitude mismatch between the I and Q branches. A communication system based on OFDM is sensitive to synchronization errors, such as CFO, and to front- end non-idealities, such as IQ imbalance. The goal of this paper is to use the iterative EM algorithm to acquire jointly the CFO and the IQ imbalance. The solution relies on a standard compliant repetitive preamble and does not require the knowledge of the propagation channel. Based on a second order approximation of the likelihood function, the complexity of the EM algorithm is significantly reduced. The algorithm is shown to perform extremely well: the estimates of the CFO and of the IQ imbalance converge to their ML estimate after less than 3 iterations. It outperforms state-of-the-art solutions significantly and suffers from a lower computational complexity. While the CFO estimate is robust against variations of the SNR, the IQ imbalance estimate accuracy is reduced at values of the SNR below 10 dB and above 35 dB.