Adaptive interference cancelation techniques for multicarrier modulated systems

Multicarrier code division multiple access (MC-CDMA), a hybrid of orthogonal frequency division multiplexing (OFDM) and CDMA, is envisioned for future wireless communications systems. The performance of MC-CDMA is limited by the presence of multiple access interference (MAI). Downlink communications also suffers from MAI as a result of the multipath channel effect, even if it implements orthogonal code multiplexing. Additionally, downlink power-control may be used to increase the system capacity, essentially creating a near-far problem for some users.; To combat the MAI, a near-far resistant detector may be used. Due to the MC-CDMA signal structure the conventional decorrelator is dependent on the channel coefficients, suggesting the use of an adaptive multiuser detector. The performance of a blind adaptive multiuser detector for MC-CDMA is investigated and compared to the performance of the conventional decorrelator.; The downlink signal structure is less complex due to assumed synchronized transmission. Moreover, the mobile receiver only needs to detect the desired user. Reduced complexity detectors are proposed that do not require knowledge of the active users nor their respective codes, but rather use a combined code to represent all the interfering users at once. The performance of an adaptive reduced complexity detector is compared to the performance of the reduced complexity conventional decorrelator.; For spectral-efficiency, multicarrier modulated system use closely spaced subcarriers. As a result, the performance is highly sensitivity to a frequency offset, resulting from a Doppler shift and/or carrier frequency synchronization error at the receiver. To mitigate this problem an adaptive decorrelator based frequency offset correction scheme is developed for OFDM and its performance is investigated. Additionally, a blind frequency offset estimation and correction structure is proposed based on a stochastic gradient method. The convergence and statistical properties of this estimator are investigated.; A blind adaptive joint multiuser detection and frequency offset correction structure for downlink MC-CDMA is developed. This detector is a combination of the structures for multiuser detection for MC-CDMA and frequency offset correction for OFDM. Moreover, the performance of this detector is investigated and compared to a joint detector based on a minimum mean square error (MMSE) criterion.