Cognitive Radio Transmission Based on Chip-level Space Time Block Coded MC-DS-CDMA over Fast-Fading Channel

In this paper, a transmit diversity technique using chip level space time block coding (chip-STBC) is investigated for the downlink transmission of multicarrier direct sequence code division multiple access (MC-DS-CDMA) based cognitive radio base station. Synchronous transmission of multiple users’ signals over fast fading frequency selective Rayleigh channel is considered. Conventional symbol level space time block coding techniques cannot be applied in such channel conditions, since the coherence time of the channel will be less than the symbol period. Therefore, chip level STBC with chip level linear minimum mean square error frequency domain equalization (LMMSE-FDE) is utilized to achieve space-time diversity. The proposed cognitive radio system involves three adaptive techniques namely, variable-length-code spreading, adaptive modulation, and adaptive subcarrier deactivation, to achieve flexibility and improved performance in a dynamic wireless environment. Orthogonal Variable Spreading Factor (OVSF) codes are used to support users of different data bit rates. Adaptive modulation technique is used to improve the data throughput of the system. Adaptive subcarrier deactivation method is proposed for deactivating a set of subcarriers to avoid causing interference to the primary user. The simulation results show significant bit error ratio (BER) performance gain over conventional single antenna systems.