dc.description.abstract | OFDM has been widely used in communication systems and is the key technology of 4G in the field of mobile communication, because of its excellent performance in overcoming multi-path channel and narrow band interfering. However the principle weakness of OFDM is sensitivity to synchronization. So synchronization is very important to OFDM systems. By researching various of the synchronization factors, according to simulation we introduce a method of improving the synchronization system for broad band data communication based on 802. 11a.
OFDM is the most famous modulating technology of the fourth mobile communication system nowadays.Recently years, OFDM technology has widely used in DAB、DVB、IEEE 802.11aandHIPERLAN-2. OFDM system is the multi-carrier transmission system, it can be divided every high speed transmission data into several low speed data packet style. It can use wideband efficiently, reducing noise, improving cryptogram and reducing multi-path fading etc. has the disparity in communication the signal in OFDM receiver will appear frequency offset, the frequency offset will affect interference in every sub carriers. Because in every sub carrier frequency offset will destruct the orthogonality, producing phase noise, so that synchronization in timing and frequency is the main processing procedure in OFDM receiver to demodulate signal correctly.
Due to wideband efficiency and deal with frequency selecting fading has the well efficiency, recently years had widely used in various wireless transmission technology, especially in WLAN, DAB, DVB and DRM etc. OFDM is very sensitive in frequency offset effect, this is because they have frequency error between TX and RX, so that it produces ISI (Inter-Symbol-Interference), destruct orthogonality in OFDM symbols. Others, frequency sampling offset, multi path effect, will also reduce system efficiency. So that, in RX needs a well estimator to reconstruct original signal.
In the thesis, we focus on the foundation of WLAN 802.11a, it will appear several estimating algorithm, and proposing a valid synchronization structure in OFMD system. This structure including the estimation in the start point of signal frame in time domain, estimating the edge of signal frame and estimating frequency offset, and the completion of reminding timing sequence and phase offset in frequency domain. To deal with the need in real environment and the reality in practice, we adopt the dynamical simulation to help us to understand the reality in every situation. So that, we add the limitation of wideband, amplitude modulation and demodulation, low pass filter, sampling in continuous time, simulating in AWGN and Rayleigh fading channel. According to the simulation, our proposed method in the IEEE channel model of 802.11a get a well done effect with synchronization. We conduct extensive experiments to examine several synchronization timing methods in depth exploration into wireless communications systems. | en_US |