Transmit Diversity for Down-Link MC-CDMA Based on Energy Spreading Transform

In this paper, we investigate transmit diversity for MC-CDMA down-link mobile communications. We apply recently developed energy spreading transform (EST) in MC-CDMA systems to pick up diversity in frequency-selective wireless channels and to improve signal detection performance. It is shown by computer simulation that the required E_b/N_o´s for a 10^-2 bit-error-rate (BER) are improved by about 5 dB for both an indoor channel and an urban channel compared with a MC-CDMA system without an EST. For a system with two transmit antennas and Alamouti´s code, the required E_b/N_o for an EST based MC-CDMA system is improved by 2.7 dB for an indoor channel and 3.3 dB for an outdoor channel compared with a MC-CDMA system without an EST. For a system with multiple transmit antennas, symbol shuffling scheme, together with an EST, has been proposed for transmit diversity. Even though symbol shuffling scheme alone is not as good as Alamouti´s code, it can be combined with Alamouti´s code to reach matched filter bound (MFB) for systems with over four transmit antennas. Comparison of our transmit-diversity approach with the widely used MMSE detection for MC-CDMA systems is also provided.