Adaptive Schemes for Multi-Carrier Based Communications Systems

基于多载波的通信系统具有更好地利用信道带宽的优点，并且因此具有简单的接收机设计。 因此，它们在有线和无线环境中得到了广泛的应用。 例如高速数字用户线（HDSL）、数字音频广播（DAD）和数字地面电视广播（DTTB）。 另外，在IMT-2000（第三代移动的通信系统）标准化过程中已经考虑了在无线PCS中使用多载波技术。 对于多用户应用，这些系统被称为多载波码分多址（MC-CDMA）。MC-CDMA的性能受到多用户干扰（MUI）的限制。 最优多用户检测通过利用所有用户的签名的知识来实现MUI消除。 由于最优多用户检测器的复杂性，各种次优检测器结构已被考虑。 在多载波调制系统中，使用紧密间隔的子载波是有利的，但并非没有对频率偏移的高性能敏感性。 后者可能是由作为移动的运动的结果的多普勒频移以及由发射机和接收机处的载波频率之间的失配引起的。 此外，在每个子载波处的期望信号的星座被旋转。 这又导致I轨和Q轨之间的轨间干扰，这进一步降低了载波干扰比（CIR）。 有人发现，相比之下，DS-CDMA，MC-CDMA的传统的全维解相关器是依赖于信道，促使使用的自适应检测器，这将在本研究中进行检查。 此外，在下行链路考虑MC-CDMA的情况下，研究人员将使用一种新颖的降低复杂度的检测器，该检测器不需要其他用户的代码的知识，而是使用所谓的复合代码来表示所有的干扰器。 这有效地将复杂度降低到2 × 2检测器，这相当于在只有两个活动用户的情况下全维检测器的复杂度。 研究人员还将开发一种自适应检测器，该检测器能够利用基于自举的自适应信号分离器来校正频率偏移的影响。

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.