Research On Multiuser Frequency Synchronization For OFDMA Systems

As a new multiple-access scheme based on orthogonal frequency-division multiplexing (OFDM),orthogonal frequency division multiple access (OFDMA) has several advantages in its immunity to multipath fading, its flexibility in resource allocation and its high spectral efficiency. OFDMA is a promising candidate for future broad wireless communication systems, but similar to OFDM, it is extremely sensitive to timing errors and carrier frequency offsets (CFOs). The CFOs disrupt the orthogonality between subcarriers and lead to intercarrier interference (ICI). Furthermore,in an OFDMA system, the ICI between different users is generally referred to as multiple-access interference (MAI). Aiming at resolving this problem,the dissertation focuses on estimation and compensation of multiuser carrier frequency offsets in OFDMA system. The main work of the dissertation is listed as follows:Firstly, the basic principle and realization scheme of OFDMA system is elaborated and the influences of frequency synchronization errors on demodulation performance are quantified by theoretical deduction. Mainstream methods to achieve multiuser frequency synchronization in OFDMA are summarized and classified.Secondly, the multiple signal classification (MUSIC) method based on searching polynomial roots was applied for estimating multiuser CFOs in the uplink of OFDMA system with interleaved subcarrier assignment (ICAS). Comparing with the general MUSIC algorithm, the proposed method avoids spectral peak searching and resolves the problem of peak ambiguity, with the computational complexity reduced.Thirdly,we derive the linear signal structure in OFDMA uplink with ICAS, which is similar to the linear signal structure in the code division multiple access (CDMA) system, on condition that multiuser CFOs have been estimated and known. A compensation scheme of multiuser CFOs based on linear minimum mean square error (MMSE) detection is analysed, and another scheme based on matched filtering by multistage parallel interference cancellation is proposed to improve bit-error-rate performance.Finally, according to the OFDMA physical layer standard prescribed in IEEE 802.16e, the dissertation designs a training symbol without constraint on subchannel assignment for OFDMA uplink in WiMAX system. A new iterative algorithm for multiuser CFOs estimation is proposed base on the training symbol. Comparing with the the maximum likelihood estimation methods, the algorithm we propose shows lower complexity, and comsumes less system resource without sacrificing system performance.