Studies On Key Techniques Of MIMO-OFDM Systems

Future wideband wireless communications are supposed to provide data rate of over 100Mbps to satisfy audio and various multimedia services. To achieve this goal, there are two main challenges, i.e., multi-path fading channel and bandwidth efficiency. OFDM technology is able to convert the frequency selective fading channel into parallel flat fading sub-channels by serial to parallel conversion, which can eliminate the inter-symbol interference and significantly simplify the equalizers complexity at the receiver. Hence, OFDM has become another key wireless access technology after CDMA. MIMO is an emerging technology using multiple antennas at both transmitter and receiver. It takes advantages of space diversity by spatially separated antennas in a multipath environment to boost the system capacity. MIMO-OFDM, with combined advantages of MIMO and OFDM technology, is regarded as a most promising wireless access scheme for the fourth generation mobile communications and has been adopted by some standardization organizations.This thesis does extensive studies on key techniques concerning MIMO-OFDM systems and mainly focuses on its coding techniques. Firstly, the fundamentals of OFDM technology are introduced, and the key techniques concerning MIMO-OFDM systems are presented as well as their research status. Coding techniques of MIMO-OFDM systems are studied in detail, including the space-time coding and space-frequency coding, as well as their implement methods, advantages and disadvantages. The same as SISO-OFDM systems, MIMO-OFDM systems are very sensitive to frequency offset as well. It breaks the orthogonality between sub-carriers and induces the inter-carrier interference, which degrades the system performance greatly. To solve this problem, in this thesis, deep studies are performed on the ICI self-cancellation space-frequency coding to suppress the ICI induced by frequency offset. Simulation result shows, MIM-OFDM systems are very sensitive to frequency offset, and an error floor occurs. When adopting the ICI self-cancellation space-frequency coding, system performance is also degraded as the frequency offset increases, but the error floor is eliminated. This coding scheme has very small implementation complexity, and hence has very important application and research values.