Research On Carrier Frequency Synchronization For LTE Systems Under The Time-varying Channels

The core technology of3GPP long term evolution (LTE) is orthogonal frequency division multiplexing (OFDM), The obvious defects of OFDM technology is susceptible to carrier frequency offset (CFO), carrier frequency offset will destroy the subcarriers orthogonality of OFDM system, causing the inter carrier interference (ICI), and resulting in the demodulation performance degradation and error rate increased of OFDM system. The main cause of CFO is due to carrier frequency error of the transmitter and receiver local oscillator frequency, or the Doppler shift caused by relative movement between the communication transceiver.With the development of modern communications increasingly to high carrier frequency and high-speed mobile environment, in this environment, the increased Doppler spread lead to the coherence time of channel reduced, resulting in the duration of signal greater than the coherence time of channel, communication channel gradually evolved to time-frequency doubly selective fading channel(also named time-varying channel), it included frequency selective caused by multipath delay spread and time selective caused by Doppler frequency. In time-varying channels, each channel impulse response of the transmission path is rapidly changing with time, if the carrier frequency synchronization algorithm still presumed channel is slowly varying or constant circumstances, the algorithm cannot receive the expected results.The main content of this paper is frequency synchronization techniques of LTE systems over time-varying channels, focusing on the CFO estimation algorithm of OFDM system over time-varying channels. In the time-varying channel, the channel unknown is greatly increased, it making CFO estimation becomes more complex, the existing CFO estimation algorithm under frequency-selective fading channels is no longer adapted to time-varying channel. The main trend of CFO estimation algorithms is joint CFO and channel estimation over the time-varying channel. Through construct BEM model to simplify the complex change of time-varying channel, then realized CFO estimation by pilot design, ML estimation and adaptive filtering algorithms. In-depth study on the existing CFO estimation algorithms of OFDM system over time-varying, they could divide into three types, it analyzed in chapter III, chapter IV and chapter V. Aiming at the based function choice problem for data-aided algorithm, proposed CFO estimation algorithm based on CE-BEM; aiming at the problem of weighting conjugate factor increases complexity for the algorithm based on pilot design, made simplify operations, proposed CFO estimation algorithm based on pilot-aided; Aiming at the defect of constant forgetting factor in traditional RLS algorithm cannot simultaneously satisfy convergence speed and convergence accuracy, proposed CFO estimation algorithm based on variable forgetting factor RLS. After simulation analysis, the three improved algorithms proposed enhance the CFO estimation performance certain extent.