3. SPACE TIME FREQUENCY CODED ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING SYSTEM Levent AKSOY, 2004

Keywords: Space-time coding, Orthogonal frequency division multiplexing, Diversity techniques, Space diversity, Multipath fading

Nowadays, mobile communication systems require modulation and diversity techniques that ensure data transmission with high rate and low bit error in order to support internet, video and multimedia applications. In this study OFDM and space-time coding is used as a modulation technique in order to cope with mobile communication channel constraints.

Space-time coding techniques have been proposed for transmitter diversity and then employed in the OFDM system to further reduce fading and obtain the better signal quality from the diversity gain. The benefits of both spatial and frequency diversity can easily be achieved by the combination of space-time block coding (STBC) and orthogonal frequency division multiplexing (OFDM). Orthogonal Frequency Division Multiplexing is a multi-carrier modulation technique in which a single high rate data-stream is divided into multiple low rate data-streams that are modulated using subcarriers which are orthogonal to each other. OFDM has commonly been utilized to transform a frequency selective fading channel into multiple flat fading subchannels. One of the main advantages of OFDM is its multipath delay spread tolerance and efficient spectrum use by allowing overlapping in the frequency domain.

This thesis initially evaluates Space-Time Coded Orthogonal Frequency Division Multiplexing (STCOFDM) and Space-Frequency Coded Orthogonal Frequency Division Multiplexing (SFCOFDM) systems, and compares them against classical Orthogonal Frequency Division Multiplexing (OFDM). Then, a simple novel system that uses space-time-frequency coding is proposed. Space-time-frequency coded OFDM (STFCOFDM) system that is proposed in this thesis, accomplishes the coding simply across space and successive time slots as well as successive carrier frequencies.

The performance is evaluated for simple additive white Gaussian noise (AWGN) channels, as well as Rayleigh and Rician fading channels. STFCOFDM shows similar performance to STCOFDM and SFCOFDM in AWGN and Rayleigh fading channels, but performs superior in Rician fading channels.