On frequency synchronization in OFDM-based systems
FacultiesFakultät für Ingenieurwissenschaften und Informatik
LicenseStandard (Fassung vom 03.05.2003)
Orthogonal frequency division multiplexing (OFDM) is a multi-carrier transmission method which allows a bandwidth efficient data transmission with low implementation complexity. However, it is sensitive to frequency offset. A carrier frequency offset (CFO) does not only lead to the amplitude reduction and phase rotation of the desired signal, but also introduces inter-carrier interference (ICI) between subcarriers. This drawback is in consequence inherited by other OFDM-based systems, such as MC-CDM, OFDMA, MC-CDMA, and CDM-OFDMA. In the uplink of the latter three multiple access schemes, crosstalk between subcarriers of different users may also be caused by multiple carrier frequency offsets (CFOs), i.e., multiuser interference (MUI) is introduced. Essentially, this thesis deals with frequency synchronization in OFDM-based systems. The solution proposed is to compensate for the frequency offsets in the uplink by means of vector equalization. Two conventional approaches, zero-forcing (ZF) and linear minimum mean square error (LMMSE), are applied to combat the MUI caused by CFOs. We also propose a method to reduce the computational complexity of vector equalization, which provides a good performance. To implement vector equalization, vector-valued transmission models have been derived for OFDM-based systems in the presence of CFOs. We have also investigated the influence of CFO on these systems and analyzed the signal to interference and noise ratio (SINR) in cases of OFDM, OFDMA, and MC-CDM. Furthermore, since information of CFOs are required for vector equalization, a modified estimator suited for the uplink frequency offset estimation is proposed. It improves the estimation accuracy in the uplink by approximately restoring the orthogonality among users by means of partial compensation of the CFOs.
Subject HeadingsFrequenzteiler [GND]
Orthogonal frequency division multiplexing [LCSH]