In this paper, three multiple access schemes, single carrier (SC)-CDMA, multicarrier (MC)-CDMA, and frequency & time spread (FTS)-CDMA, with space time transmit diversity (STTD) are presented for a broadband mobile communication system. In general, broadband communication systems are easily affected by severe frequency selective fading which results in the degradation of the system performance due to the multipath. In order to improve the system performance in such a fading environment, STTD is applied to these CDMA systems. Computer simulation is carried out to evaluate the system performance, and the BER results are compared with each other. From the simulation results, it is found that the performance of MC-CDMA with STTD is better than that of the other systems, and SC-CDMA cannot improve the system performance by using STTD.

This paper presents a novel transmission diversity scheme for code division multiple access system. Conventional diversity receivers in mobile stations require space and complicated circuits, however, the proposed diversity schemes present significant diversity effect without any diversity equipment at the mobile station. It is possible to use the transmitter diversity at the base station by using the feature of time division duplex (TDD) which has strongly correlated fading patterns in both forward and reverse link. Computer simulation is performed to evaluate the performance of the proposed systems for single user environment. The performance of the system 1, which select best situated antenna, is analyzed and the BER performance for multiple access is presented.

Multi-carrier (MC) signal has a large peak to mean envelope power ratio, so that the MC signal suffers from a high level of inter-modulation distortion due to the nonlinearity of the power amplifier stage. For portable terminals, it is undesirable to use linear amplifiers because high power efficiency is needed. To solve this problem, we propose a time division duplex (TDD)-code division multiple access (CDMA) communication system which uses an asymmetric modulation scheme between the forward and reverse links. The system consists of multicarrier modulation for the forward link and single carrier modulation for the reverse link. A pre-equalization method for the forward link transmission is also presented in this paper. In frequency selective fading, the system achieves a path diversity effect without any channel estimation unit at the mobile station by using the pre-phase equalizer. From the simulation results, it it found that the proposed system achieves better BER performance than the conventional MC-CDMA system and the single carrier RAKE system equipped at the mobile unit since the proposed system has the ability to suppress other user interfering signals.