MIMO structures for multicarrier CDMA systems

This thesis focuses on a particular multi-carrier system, known as Multi-Carrier Code Division Multiple Access (MC-CDMA), which simultaneously accommodates several users on the same frequency range. This technique, being a combination of Direct-Sequence CDMA (DS-CDMA) and Orthogonal Frequency Division Multiplexing (OFDM), takes advantage of the frequency diversity gains provided by DS-CDMA systems and the simple FFT architecture of transmit/receive structures in OFDM systems. In addition, it is widely known that Multi-Input Multi-Output (MIMO) structures have the potential to increase the capacity of communication systems. Furthermore, the potential of such structures to provide diversity without exhausting any further time or frequency resources and hence to increase the reliability of the communication has made MIMO structures a very popular area of research.;In this thesis, MIMO structures for MC-CDMA systems in frequency selective channels are investigated. Several known architectures are analyzed and an architecture, which combines spatial multiplexing with space-time coding in order to achieve higher data rates with high quality of service, is proposed. The bit error rate (BER) performance of the proposed system is evaluated and compared with other MIMO systems and it is shown that the proposed system, achieves better performance, is very robust to channel estimation errors and performs well in most of spatially correlated MIMO channels. In addition, a study on the complexity of the proposed system and the other MIMO architectures is carried out and the results show that our system is considerably less complex. A frequency multiplexing scheme is introduced which under certain conditions can significantly decrease the complexity of the system for a required bit error rate. A thorough BER analysis of MC-CDMA systems in a frequency selective channel is performed and BER expressions for MC-CDMA multiuser system and space-time coded structure are derived, which perfectly match the simulation results. Furthermore, the impact of the variations of the delay spread of the channel on the bit error performance of MC-CDMA system is studied.