Research On Synchonization And Channel Estimation Algorithm For Ofdm Based Relay System

Relay transmission technique are considered as an important candidate for LTE-Advanced, which has the ability to extend the coverage, improve the channel quality for the cell-edge users, overcome multipath channel fading and increase system capacity. Meanwhile, the frequency selective fading will cause inter symbol interference in the high speed wireless communication system, thus orthogonal frequency division multiplexing (OFDM) technology is widely used because of its high spectral efficiency and robustness against frequency selective fading. The combination of cooperative relay technology and OFDM is a promising way of future wireless communication systems. However, the spatial separation of relay nodes give rise to multiple timing offsets (MTOs) and different oscillator of each node with Doppler shift results in multiple carrier frequency offsets (MCFOs), which makes the synchronization problem more challenging and complex compared with the traditional communication systems. Moreover, channel estimation is a necessary foundation to realize coherent detection, space-time decoding and ensure the system bit error rate. To solve the problems above, this paper focuses on synchronization and channel estimation technology for OFDM-based relay system. The primary work of this paper is as follows:First, this paper will introduce the basic principles of relay transmission system and OFDM, and then the effect of synchronization errors, including timing errors and carrier frequency offsets, in OFDM system performance is analyzed theoretically. Some classical synchronization method for OFDM systems is provided. Finally, we discuss the specificity of the synchronization problems in relay transmission system.Considering the particularity of relay system synchronization problems, an efficient scheme for timing and frequency estimation in multi-relay cooperative networks operating with both large and small carrier frequency offset (CFO) over frequency selective channels is proposed. A novel preamble based on constant amplitude zero auto-correlation (CAZAC) sequence is proposed, and a corresponding practical multistage scheme is presented. Joint timing and integral frequency synchronization is involved to resist multi-relay interference (MRI). Then, fractional frequency estimation is carried out, and fine timing estimation completes the synchronization scheme.Finally, we addresses channel estimation for amplify-and-forward relay networks using training sequences in the presence of both time and frequency offsets. A practical training scheme is provided to reduce the complexity at the relay nodes and to combat the effects of time and frequency offsets at the destination. By exploiting the training scheme, a piecewise channel estimator with respect to signal-to-noise ratio (SNR) is proposed to reduce the computational complexity of the minimum mean square error (MMSE) method.