Research On Joint Estimation Of CFO And Transmitter/Receiver I/Q Imbalances In OFDM Systems

Orthogonal frequency division multiplexing (OFDM) is an attractive modulation scheme used in wideband communications, and the fourth generation (4G) air interface technology is based on OFDM. Because it can increase the frequency utilization efficiency, the rate of data transmission and it essentially transforms the frequency selective fading channel into a flat fading channel to reduce multipath interference. In addition, OFDM combined with direct-conversion transceiver has been widely used in modern communication systems because of low cost, low power consumption, simple implementation structure and easy integration. However, in the multipath fading channels, the major problem of OFDM systems is carrier frequency offset (CFO), and direct-conversion transceiver introduce transmitter/receiver in-phase/quadrature-phase (I/Q) imbalances and direct current offset (DCO) due to the analogy processing uncontrollability and imperfections. Such impairments may have a tremendous impact on the performance and cannot be efficiently eliminated in the analog devices or analogy processing. Therefore, an efficient joint estimation of CFO and transmitter/receiver I/Q imbalances in the digital baseband signal processing would be highly desirable for the OFDM systems.However, the research on joint estimation of CFO and receiver I/Q imbalance for OFDM systems without knowing channels is more widely, which rarely considers the transmitter I/Q imbalance. And the computational complexity and performance of the existing algorithms still remain to improve. Therefore, according to the current research on joint estimation of CFO and transmitter/receiver I/Q imbalances for OFDM systems, we intensive study and analyze some classic joint algorithms for CFO and transmitter/receiver I/Q imbalances. Then we proposed two new joint estimations, including the joint estimation of CFO and receiver I/Q imbalance and the joint estimation of CFO and transmitter/receiver I/Q imbalances.Firstly, based on IEEE802.11a/g preamble structure, we intensive study and analyze the joint estimation of CFO and receiver I/Q imbalance, which include LS-based joint estimation of CFO and receiver I/Q imbalance in OFDM systems and a closed form solution for joint estimation of CFO and receiver I/Q imbalance. Based on the analysis of these algorithms, an improved joint estimation of CFO, DCO and receiver I/Q imbalance in OFDM systems is proposed.Secondly, the joint estimation of CFO and transmitter/receiver I/Q imbalances for OFDM systems is emphasis researched and analyzed, which include channel residual energy (CRE) based joint estimation of CFO and transmitter/receiver I/Q imbalances for OFDM systems and its improved algorithm. Based on the analysis of these algorithms, a novel joint estimation of CFO and transmitter/receiver I/Q imbalances with training sequence for OFDM systems under unknown channels is proposed.Finally, through the simulation comparison of all algorithms, simulation results show that the effectiveness and accuracy of the proposed method are superior to the existing algorithms, and the proposed has a less computational complexity.