Low complexity equalization for OFDM in doubly selective channels

Abstract

In current standards Orthogonal Frequency Division Multiplex -OFDM- is widely used for its high resistance to multi-path environments and high spectral ef- ficiency. However since the transmission duration is longer, it is affected from time variations of the channel more than single carrier systems. Orthogonality of sub-carriers are lost within an OFDM symbol and intercarrier interference(ICI) occurs as a result of time variation of the channel. Channel estimation and equalization become problematic, because the classical structures like MMSE require very complex operations. This thesis studies the channel equalization problem, as separate from the channel estimation problem. The thesis assumes that the channel coefficients are perfectly known and focuses on the estimation of data transmitted on each OFDM carrier. First, a survey of existing algorithms on channel equalization is given and simulations are provided to compare them in terms of complexity and performance under an OFDM system scenario that is consistent with the present WiMAX system parameters and operating conditions. As a novel contribution, the thesis proposes two new equalization methods by amending existing algorithms and shows that these modified algorithms improve the state-of-the-art in channel equalization in terms of complexity andperformance under certain high-mobility scenarios. Finally it is shown that the intercarrier interference cancellation problem remains a major impediment to the implementation of OFDM in high-mobility environments.