Carrier frequency offset in multicarrier CDMA systems under correlated fading.

摘要

The effects of carrier frequency offset (CFO) on the performance of multicarrier code division multiple access (CDMA) systems and approaches to estimate this synchronization imperfection are studied in this thesis. We first investigate the impact of inter-carrier interference (ICI) on the performance of uplink asynchronous multicarrier direct-sequence CDMA (MC-DS-CDMA) systems with random spreading. A generalized system model is developed so that the analysis can be applied to multicarrier-CDMA (MC-CDMA) systems with straightforward modification. We adopt a more practical assumption here that all subchannels may be statistically correlated due to the system design with a large number of subcarriers or due to the channel environment shifting to a less frequency-selective condition. Then, the receiver residual CFO is incorporated in the analysis. We obtain the optimum combining filter that maximizes the signal-to-interference-plus-noise-ratio (SINR) of the combined statistics as well as exploits correlated information among subchannels. A closed-form expression for the unconditional covariance matrix of the interference-plus-noise vector, which forms the basis of our theoretical analysis of the maximum SINR and the average bit error probability (BEP) formula is also derived, which may provide insight into the choices of system parameters.; Finally, in the last part of the thesis, we propose two CFO estimation schemes using the expectation maximization (EM) algorithm; namely, block-based EM algorithm and recursive EM tracking algorithm. Block-based approach is suitable in circumstances where the accuracy of the estimate is preferable to the efficiency of it. For the block-based approach, the update for the CFO has no closed form due to the nonlinear structure of the cost function. Typically, exhaustive spectrum search is employed to locate a good estimate of the CFO. On the other hand, timely updates of system parameters would be more desirable in real time applications, which motivates the development of online algorithms to track time-varying unknowns. Moreover, we will show that the recursive update for the CFO vector can be represented in a analytically tractable manner, thanks to the quadratic structure of the cost function with respect to the target vector.