Techniques to reduce multiple-access interference in multi-carrier CDMA systems

For future high data rate wireless applications, multi-carrier code division multiple access (CDMA) has emerged as the most promising multiple access scheme due to its numerous advantages over single-carrier CDMA (SC-CDMA). However, the performance of multi-carrier CDMA systems similar to the conventional SC-CDMA systems is limited by the multiple access interference (MAI). Therefore, it is important to suppress the MAI in multi-carrier CDMA systems. This dissertation focuses mainly on techniques to reduce the MAI. In literature, two multi-carrier CDMA systems namely MC-CDMA and multi-carrier direct sequence CDMA (MC-DS-CDMA) have been proposed. In this dissertation both schemes are considered.; In the first part of this dissertation, assuming the signature sequences are random sequences, the impact of the carrier spacing and the chip waveforms on the performance of asynchronous MC-DS-CDMA systems is analyzed, for a given system bandwidth. The family of band-limited chip waveforms is considered. The average MAI at the output of the receiver is taken as the performance measurement. The analysis demonstrates that there exists an optimal carrier spacing for a given chip waveform which gives the minimum MAI. Motivated by this observation, a method to jointly design the chip waveform and the carrier spacing to achieve the minimum MAI is presented.; In the second part, an adaptive carrier interferometry scheme is proposed for synchronous MC-CDMA systems. By exploiting the additional degree of freedom in selecting the amplitudes of the subcarriers in accordance with the channel condition, the proposed scheme attains a significant performance gain over the conventional carrier interferometry MC-CDMA systems in which a constant amplitude is set for all the carriers. Two adaptation strategies, namely local adaptation and global adaptation, are proposed for estimating the appropriate subcarrier amplitudes at the receiver in the proposed systems. A further advantage of the proposed scheme is that it reduces the peak-to-average power ratio (PAPR) problem present in the conventional carrier interferometry MC-CDMA systems.; Finally, in the last part of this dissertation, a synchronous MC-DS-CDMA system especially for downlink, called phase offset assisted MC-DS-CDMA is proposed. Two phase offset assignment schemes are introduced: (i) the Carrier-Based phase offset, in which a carefully chosen phase offset is introduced in each carrier to minimize the MAI and (ii) the User-Based phase offset, in which each user is assigned a different phase offset. However, in contrast to the Carrier-Based phase offset, the same phase offset is used in all the carriers. It is shown both schemes achieve approximately a 50% reduction in MAI level.