Block Transmission Techniques for Wireless Communications

In order to meet the market demand for high datarates, most digital wireless communication systems rely on broadband channels and therefore suffer from Inter Symbol Interference (ISI a phenomenon that needs to be combatted at the receiver by appropriate equalization techniques in order to restore the transmitted information. In this context, block transmission techniques based on the use of a Cyclic-Prefix (CP) have attracted a lot of attention in the last years for they allow an efficient and computationally cheap ISI cancellation procedure. Historically, OFDM (Orthogonal Frequency Division Multiplexing) was the first proposed block transmission scheme and has been adopted in numerous standards for high-speed data transmission in both wired and wireless applications. In the wireless context however, OFDM suffers of several problems, both on an implementational point of view and from a performance perspective. Some recently proposed block transmission techniques, namely Single-Carrier Cyclic Prefix (SC-CP) and Known Symbol Padding (KSP) partly solve these implementation problems whilst preserving the possibility of using efficient equalization schemes that have the same low complexity as the OFDM equalizers. These techniques also enable the use of specific equalization schemes that offer higher performance at the cost of an increased computational complexity. In this thesis, we analyze these alternative block transmission techniques in the specific context of wireless communications with a special focus on KSP. We first detail the equalization schemes that are traditionally used with the different considered block transmission techniques. We then carry out a detailed analysis of the achievable BER performance of these block transmission techniques in the light of existing Wireless Local Area Networks (WLAN) standards under the working assumption that the receiver has perfect channel knowledge. We then analyze the possibilities offered by the different block transmission techniques towards the identification of the transmission channel. KSP appears to be the best suited scheme since the padded sequences can be seen as short training sequences inserted in the stream of data symbols. We propose a new semi-blind Gaussian Maximum Likelihood (ML) method for the identification of the transmission channel which is specifically suited to this KSP context. This method asymptotically achieves the Cramer-Rao Bound and significantly outperforms existing semi-blind methods and has the advantage of a low computational complexity. In the light of this discussion on channel identification, we propose a new KSP transmission scheme that simultaneously allows low-complexity equalization and accurate estimation of the channel, namely, Shifted KSP (S-KSP). A detailed analysis of the achievable BER performance of the considered block transmission techniques where realistic channel estimates are used for the design of the equalizers highlights the promising performance of the proposed S-KSP scheme. Besides this work on channel identification and equalization, we also discuss several approaches for Direct Sequence Estimation that are suited to the considered block transmission techniques. We specifically propose a new iterative Maximum Likelihood Sequence Estimation (MLSE) technique that is suited for both KSP and SC-CP transmission. Finally, we consider the situation where the mobility of the users is increased. The channel then becomes time-varying and changes significantly during the transmission of a packet of data symbols. Relying on a newly proposed technique for the modeling of time-varying multipath channels, namely the Basis Expansion Model (BEM), we propose novel methods that allow to estimate the channel relying on the padded sequences of the KSP scheme. This method outperforms existing ones in the considered context with a limited computational complexity. A detailed analysis of the overall syste performance shows that KSP is also a suitable candidate for data transmission in the context of time-varying channels.