Flexible Orthogonal Code for CDMA Based MIMO-OFDM Systems with Space-Time-Frequency Spreading

We propose a novel spreading code, called flexible-orthogonal code (FOC), for space-time-frequency spreading (STFS) scheme in code division multiple access (CDMA) based multiple input multiple output (MIMO) orthogonal frequency division multiplexing (OFDM). The goal of our design is to obtain an improved bit-error rate (BER) performance without loss of bandwidth efficiency. We make use of multiple antennas to achieve space diversity and balanced signal to interference and noise ratio (SINR) among users on each subscriber device. The proposed code exhibits length flexibility, which breaks up the "power of two" constraint in spreading factor. Our results indicate that the proposed code improves overall performance of the system, by exploiting both space diversity and SINR balancing. The flexibility in code length also results in high spectral efficiency.