Adaptive Schemes for Multi-Carrier Based Communications Systems

Multi-carrier based communication systems have the advantage of better exploitation of the channel bandwidth, and hence simple receivers design. As a result they find broad applications in wireline as well as wireless environments. Examples are the high-rate digital subscriber line (HDSL), digital audio broadcasting (DAD) and digital terrestrial television broadcasting (DTTB). Additionally, the use of multi-carrier techniques in wireless PCS has been considered in the IMT-2000 (third generation mobile communication systems) standardization process. For multi-user application, these systems are referred to as Multi-Carrier Code Division Multiple Access (MC-CDMA). The performance of MC-CDMA is limited by multiusers interference (MUI). Optimal multi-user detection accomplishes MUI cancellation by exploiting the knowledge of all users' signatures. Due to complexity of the optimum multi-user detector, various sub-optimum detectors structures have been considered. In a multicarrier modulated system it is advantageous to use closely spaced subcarriers, but not without high sensitivity of performance to frequency offset. The latter might be caused by Doppler shift as result of mobile movement, as well as from a mismatch between the carrier frequencies at the transmitter and receiver. In addition, the constellation of the desired signal at each subcarrier is rotated. This in turn results in inter-rail interference between the I- and Q-rails, which further reduces the carrier to interference ration (CIR). It was found that, in contrast to DS-CDMA, for MC-CDMA the conventional full dimensional decorrelator is dependent on the channel, prompting the use of an adaptive detector, which will be examined in this research. Additionally, where MC-CDMA is considered for downlink, the investigators will use, a novel reduced complexity detector which does not require the knowledge of the other users' codes, but instead, uses a so-called compound code to represent all the interferers. This effectively reduces the complexity to a 2 x 2 detector, which is equivalent to the complexity of a full-dimensional detector in the case of only two active users. The investigators will also develop an adaptive detector that has he capability to correct the effects of a frequency offset utilizing a bootstrap based adaptive signal separator.