Study On Synchronization And Channel Estimation Issues In OFDM Systems

OFDM has been implied in mobile telecommunication, DAB/DVB and LAN because of itshigh frequency spectrum efficiency and robustness against multipath fading. Carriersynchronization, symbol timing synchronization, as well as channel estimation are three criticaltechnologies of OFDM. Generally, data aided synchronization and pilot aided channel estimationalgorithms can achieve better performance and are easily implemented. However, they’ll costmore resource in transmitting training data. Blind algorithms can achieve higher transmissionefficiency, while their performances are poorer and cost more computing amount. We still cannot figure out the conflict between transmission efficiency and performance of synchronizationand channel estimation.This thesis hammers at researching into synchronization and channel estimation issues ofOFDM systems, with focus on carrier synchronization, symbol timing synchronization andchannel estimation issues in fading channel. The main work and innovative contributionsobtained in this thesis can be summarized as follows.1. Research on carrier frequency synchronization (CFS) technology in OFDM systems.Study deficiency of CFS methods in existence through analysis and simulation. A blind carriersynchronization algorithm taking advantage of OFDM signal structure is proposed. In theproposed algorithm, carrier synchronization is implemented in two steps: First, construct a costfunction utilizing the orthogonality between signal subspace and noise subspace, estimatedecimal carrier frequency offset (CFO) based on the cost function; after decimal CFO iscompensated, because integral CFO will make non-zero elements of diagonal in auto-correlationmatrix moving along diagonal line, we can estimate integral CFO based on this principle. Thesimulation results show that the proposed algorithm can estimate frequency offset in large scale.Furthermore, it can achieve a high resolution. During simulation, it is showed that the algorithmis robust to modulation mode of subcarriers. Besides, the algorithm only needs little data toconverge. Also error source of simulation is analyzed.2. Research on symbol timing synchronization (STS) technology in OFDM systems. Studydeficiency of STS synchronization methods in existence. ML timing algorithm based on cyclicprefix and improved method, iterative ML algorithm are inefficient in multipath and Rayleighchannel. So, we proposed a blind timing synchronization algorithm based on cyclic prefix. Thealgorithm is implemented as follow: get initial symbol timing offset (STO) estimation usingtraditional ML algorithm. Update STO estimation during iterative process till the maximum iterative times. Finally utilize several continuous segments of received data to overcomemultipath fading and Rayleigh fading. The simulation results show that the proposed algorithm isefficient in AWGN channel, multipath channel as well as Rayleigh channel.3. Research on channel estimation technology in OFDM systems. Because subspace basedblind channel estimation method converges slowly, it is not suitable for time-varying. Aim at thisproblem, this thesis proposed a blind channel estimation method based on particle filtering (PF).We get initial particle set by traditional subspace based method, and then track the change ofchannel response. The simulation results show that the proposed algorithm can estimateinvariable and time-varying channel efficiently.