Frequency synchronization in multiuser OFDM-IDMA systems.
Various multiuser schemes have been proposed to efficiently utilize the available bandwidth while ensuring an acceptable service delivery and flexibility. The multicarrier CDMA became an attractive solution to the major challenges confronting the wireless communication system. However, the scheme is plagued with multiple access interference (MAI), which causes conspicuous performance deterioration at the receiver. A low-complexity multiuser scheme called the Interleave Division Multiple Access (IDMA) was proposed recently as a capable solution to the drawback in the multicarrier CDMA scheme. A combined scheme of OFDM-IDMA was later introduced to enhance the performance of the earlier proposed IDMA scheme. The multicarrier IDMA scheme therefore combats inter-symbol interference (ISI) and MAI effectively over multipath with low complexity while ensuring a better cellular performance, high diversity order, and spectral efficiency. Major studies on the OFDM-IDMA scheme emphasis only on the implementation of the scheme in a perfect scenario, where there are no synchronization errors in the system. Like other multicarrier schemes, the OFDM-IDMA scheme however suffers from carrier frequency offset (CFO) errors, which is inherent in the OFDM technique. This research work therefore examines, and analyzes the effect of synchronization errors on the performance of the new OFDM-based hybrid scheme called the OFDM-IDMA. The design of the OFDM-IDMA system developed is such that the cyclic prefix duration of the OFDM component is longer than the maximum channel delay spread of the multipath channel model used. This effectively eliminates ISI as well as timing offsets in the system. Since much work has not been done hitherto to address the deteriorating effect of synchronization errors on the OFDM-IDMA system, this research work therefore focuses on the more challenging issue of carrier frequency synchronization at the uplink. A linear MMSE-based synchronization algorithm is proposed and implemented. The proposed algorithm is a non-data aided method that focuses on the mitigation of the ICI induced by the residual CFOs due to concurrent users in the multicarrier system. However, to obtain a better and improved system performance, the Kernel Least Mean Square (KLMS) algorithm and the normalized KLMS are proposed, implemented, and effectively adapted to combat the degrading influence of carrier frequency offset errors on the OFDM-IDMA scheme. The KLMS synchronization algorithm, which involves the execution of the conventional Least Mean Square (LMS) algorithm in the kernel space, utilizes the modulated input signal in the implementation of the kernel function, thereby enhancing the efficacy of the algorithm and the overall output of the multicarrier system. The algorithms are applied in a Rayleigh fading multipath channel with varying mobile speed to verify their effectiveness and to clearly demonstrate their influence on the performance of the system in a practical scenario. Also, the implemented algorithms are compared to ascertain which of these algorithms offers a better and more efficient system performance. Computer simulations of the bit error performance of the algorithms are presented to verify their respective influence on the overall output of the multicarrier system. Simulation results of the algorithms in both slow fading and fast fading multipath scenarios are documented as well.