Study Of Synchronization Algorithm In OFDM-based Cooperative Communication Systems

Orthogonal Frequency Division Multiplexing (OFDM) has recently attracted increased attention for achieving faster data speed and higher spectral efficiency in the next generation communication. The relay technology can exploit space diversity to combat channel fading and improve system performance in multi-path environment. The combination of OFDM and relay is a promising technique for the future broadband mobile communication systems. The synchronization problems are focused on in OFDM-based cooperative communication systems. The main contribution of this paper as follows:Firstly, the principles of OFDM and OFDM based cooperative communication systems are explained in detail. Another emphasis is devoted to analyze how the symbol timing offsets and multiple carrier frequency offsets affects the OFDM systems performance, respectively. At the same time, we investigate the influence of timing and carrier frequency offsets on amplify and forward cooperative systems and decode and forward cooperative systems.Secondly, based on several typical time synchronization algorithm of the OFDM cooperative communication system, a new algorithm is proposed by using Constant Amplitude Zero auto-Correlation (CAZAC) sequences and the channel characteristic of multipath fading. With the knowledge of channel length, multiple timing offsets car. be estimated by our proposed algorithm within one OFDM block. At the same time, the proposed algorithm can achieve channel estimation which doesn’t need to spend extra overhead. Simulation results show that our proposed algorithm can achieve better system accuracy than unequal period synchronization patterns (UPSPs) approach under any signal-to-noise ratio condition.Finally, For the OFDM-based cooperative communication system, the timing offset and carrier frequency offset (CFO) problems are investigated. New training sequence structure with adjustable parameters, which consists of one OFDM block, is proposed for both the multiple timing offsets and CFO estimation. Timing offsets estimation can be obtained through using a cross-correlation approach and at the same time, FFT (Fast Fourier Transform) and IFFT (Inverse Fast Fourier Transform) is used to reduce the complexity of the sliding correlation operation. CFO estimation can be also achieved by utilizing subspace-based algorithm. The parameters of the proposed training sequences are adjusted properly to acquire better system performance.