SPACE TIME BLOCK CODE FOR MIMO OFDM SYSTEMS

Abstract

Multiple-input-multiple-output (MIMO) antenna architecture has the ability to increase capacity and reliability of a wireless communication system. Orthogonal frequency division multiplexing (OFDM) is another popular technique in wireless communication which is famous for the efficient high speed transmission and robustness to frequency selective channels. Therefore, the integration of the two technologies probably has the potential to meet the ever growing demands of future communication systems. Because of aforementioned merits of these two techniques, this thesis was about MIMO and MIMO-OFDM system, and it addressed three different issues. Firstly it has investigated the performance of MIMO-STBC systems, Secondly, it has focused on performance of MIMO-OFDM system. In last part, BER comparison of space-time block coded OFDM system (STBC) was calculated for different frequency offset using simulation software. The results proved that the reliability of the wireless link increases as the number of transmits and received antenna increase. OFDM is a special form of multicarrier modulation (MCM), where a single data stream is transmitted over a number of lower rate subcarriers. The basic principle of OFDM is to split a high-data-rate sequence into a number of low-rate sequences that are transmitted simultaneously over a number of subcarriers OFDM is relatively higher sensitive to carrier frequency offset (CFO) errors, compared to a single carrier system. The frequency offset error is caused by the misalignment in carrier frequencies at the receiver due to fluctuations in receiver RF oscillators or a channel's Doppler frequency. This frequency offset can destroy the subcarrier orthogonality of the OFDM signal, introducing inter-carrier interference (ICI). The ICI results in severe degradation of the bit-error-rate (BER) performance of OFDM systems. iii Since OFDM subcarriers are closely packed compared to the system bandwidth the amount of tolerable frequency offset is a small fraction of the OFDM bandwidth. CFO introduces both attenuation/rotation of the useful signal and ICI which destroys the orthogonality of demodulated subcarriers. ICI and attenuation/phase rotation effects if not compensated, increase the system error rate and reduce the overall throughput.