Performance Evaluation of Practical OFDM Systems with Imperfect Synchronization

This work aims to expose the potential performance loss due to synchronization errors in the downlink of the two major cellular standards of OFDM systems, i.e., the WiMAX OFDM physical layer and the LTE. Different to most results in literature, the physical layer coded throughput is utilized as the major performance measure. The influence of an imperfect carrier frequency synchronization or symbol timing is evaluated via analytical modeling and standard compliant link level simulations. In the frequency aspect, a modified differential estimator for the residual frequency offset in WiMAX is proposed. It is shown that the theoretical performance of such an estimator approaches the Cramer-Rao lower bound and provides a significant gain in terms of the mean squared error. However, such an improvement becomes negligible in terms of the coded throughput. Therefore, a throughput loss prediction model is proposed for the LTE downlink. This model takes into account the entire physical layer of the LTE downlink and is able to indicate the throughput loss given an arbitrary signal-to-noise ratio and carrier frequency offset. In the timing aspect, a quantitative evaluation shows that a late timing mainly causes the inter-carrier and inter-symbol interference, whereas an early timing degrades the channel estimation performance. Therefore, an extremely accurate symbol timing is required in a coherent OFDM transmission system. Although it is generally believed that the carrier frequency offset and symbol timing offset cause merely inter-carrier interference and inter-symbol interference, other effects, i.e., the common phase error and the additional phase variation induces negative impacts as well. Due to the overall system design constraint, these impacts may interfere the performance of other signal processing units and further degrades the system performance. These side-effects to some extent raise the requirement on the synchronization accuracy in practical OFDM systems.