A hybrid of space time coding and orthogonal zero padding for multirate CDMA

This work presents a simple, yet effective method, referred to as orthogonal zero padding (OZP), for multirate CDMA systems. The OZP is an ingenuous zero padding scheme which pads the spreading codes of different spreading gains with appropriate number of zeros at adequate time slots. A new set of orthogonal spreading codes can then be constituted to separate data with different transmission rates, thus alleviating the multiple access interference (MAI) effect. In addition, to further enhance the performance in downlink transmission, a transmit diversity, space time coding (STC), is also incorporated. Such a hybrid of STC and OZP not only possesses the diversity gain in dispatching symbols for multirate CDMA systems but it also requires little redesign of the original transceivers. Simulations are also furnished to verify the proposed approaches.