Effect of frequency offset on the performance of MIMO-OFDM systems

Abstract

As the high capacity demands driven by wireless Internet and multimedia applications grow extremely, many researches on combining multi-input multi-output (MIMO) system with orthogonal frequency division multiplexing (OFDM) (MIMO- OFDM) have been studied. There are two kinds of approaches to make use of spatial diversity offered by MIMO system, which are Space-time codes (STC) and Bell Labs Layered Space-Time Architecture (BLAST). Since OFDM performs well in presence of inter-symbol interference (ISI), it is suitable for supporting wideband transmission and also well known for its spectral efficiency. However, it has some disadvantages compared to single carrier modulation. One of the major disadvantages of OFDM is its sensitivity to frequency offset. This thesis studies the effect of frequency offset on the BER performance of various MIMO-OFDM systems. Two transmit diversity schemes using Space-time block codes (STBC) for OFDM, space-time OFDM (ST-OFDM) and space-frequency OFDM (SF-OFDM) are considered and compared to maximal ratio combining scheme for OFDM (MRC-OFDM). The BER performance of Space-time block coded transmit diversity with OFDM system due to frequency offset is further degraded than that of MRC receive diversity with OFDM system. The performance degradation of Vertical-BLAST (V-BLAST) with zero-forcing criteria to frequency selective fading channel (ZF VBLAST-OFDM) caused by frequency offset is larger than that of zero-forcing linear detector with OFDM (ZF-OFDM). Also, the combining system of STBC and V-BLAST with OFDM (STBC-VBLAST-OFDM) is considered and the performance degradation due to carrier frequency offset is shown.