Frequency-Domain Equalization and Channel Estimation for Broadband Wireless Communications

New services such as high-speed data transmission and multimedia applications have tremendously increased the bandwidth requirements for communications in the last two decades. On the other hand, the emerging need for access to information everywhere at every time (ubiquity) has focused attention over wireless communications. At the physical layer, wireless broadband transmission is characterized by dispersion of the channel and its possibly time-varying nature. When more transmitters share the same radio bandwidth, co-channel interference arises. This thesis addresses three disturbances affecting the received broadband signal: the distortion introduced by the channel dispersion, the distorsion due to the time-variations of the channel and the interference due to the simultaneous transmission of various devices. In order to face these phenomena we consider both the equalization of the channels, and the use of multiple antennae. We also devise new methods to estimate the channel parameters needed for the equalization. In all the proposed techniques, operations on the signals are performed in the frequency domain, in order to obtain efficient solutions. For a static channel, a new transmission format is proposed for a single carrier transmission, which allows a reduced complexity implementation of the decision feedback equalizer, due to the frequency-domain operations of the feedforward filter. For a time-varying channel scenario, a new iterative interference cancellation scheme is considered for an orthogonal frequency division multiplexing modulation. Another contribution of the thesis is the derivation of the optimum adaptation of a multiple antennae transmitter in order to minimize the impact of co-channel interference in a co-channel interference scenario. For both the time-invariant and the time-varying cases, new reduced complexity channel parameter estimators are derived. Lastly, the thesis proposes also new channel estimators. For the static case, two estimators are presented: one is based on the efficient realization of the inverse Fourier transform, while the latter performs a polynomial interpolation of the frequency response of the channel. For the time-varying scenario, a multistage estimator is proposed, where the estimate is performed according to a model based on the Taylor expansion of the time-varying channel taps.