Research On Estimation And Compensation Algorithms For IQ Imbalance And Channel In Coherent Optical OFDM Systems

The high-speed, reliable, low-cost, high-capacity optical transmission system have been the inevitable trend of future development, since people’s sustained increasing network bandwidth requirements. Coherent optical orthogonal frequency-division multiplexing(CO-OFDM) combined OFDM and coherent optical communication technology. The OFDM technology is realized by using the transmission mode which allows the information is loaded on a number of mutually orthogonal sub-carriers, it can effectively reduce the date rate and improve the spectrum utilization. The coherent optical communication technology introduce the in phase/orthogonal(IQ) modulater and coherent receiver, aiming to achieve the radio to optical coherent modulation and heterodyne or homodyne detection technology, which bring the advantages of high receiver sensitivity, long distance relay. The CO-OFDM system has been the most promising candidate for the next generation high speed and reliable communication technology for the full integration of these two technologies.However, when the optical signal generate the amplitude loss and phase deviation due to the presence of Tx IQ modulater and Rx mixer, the signal will appear the mismatch between the I and Q branch which is called Tx and Rx IQ imbalance respectively. This nonlinearity destroy the orthogonality between subcarriers, which bring mirror interference, consequently degrade the BER performance of CO-OFDM systems. Besides, the IQ imbalance would indirectly result in noise enhancement in the channel estimation. Therefore, in the thesis, we focus on algorithms for IQ imbalance and channel estimation. Several theoretical and simulation studies are implemented as follows:Firstly, a CO-OFDM system based on OFDM and coherent optical communication technology is build using Optisystem and Matlab. Then we give the Tx and Rx IQ imbalance models respectively.Secondly, a decision feedback(DF) estimation algorithm for Tx IQ imbalance and channel is proposed. The algorithm realize the dependent estimation between IQ imbalance factor and channel function through insert two special training sequences at the beginning of every OFDM frame. Moreover, the DF produre achieve dynamic tracking for time-varying IQ imbalance and channel.At last, since the conventional time-domain method, Gram-Schmidt orthogonal procedure(GSOP), has been demonstrated not efficiently to compensate Tx IQ imbalance. So a novel time-domain compensation scheme for IQ imbalance is studied. This scheme which can remove the mirror interference with a low OSNR penalty has a wide application. The simulation results shows it not only has a larger amplitude and phase tolerance than GSOP, but also improves the BER performance of CO-OFDM systems.