Transmission over Time- and Frequency-Selective Mobile Wireless Channels

The wireless communication industry has experienced rapid growth in recent years, and digital cellular systems are currently designed to provide high data rates at high terminal speeds. High data rates give rise to intersymbol interference (ISI) due to so-called multipath fading. Such an ISI channel is called frequency selective. On the other hand, due to terminal mobility and/or receiver frequency offset the received signal is subject to frequency shifts (Doppler shifts). Doppler shift induces time-selectivity characteristics. The Doppler effect in conjunction with ISI gives rise to a so-called doubly selective channel (frequency- and time-selective). In addition to the channel effects, the analog front-end may suffer from an imbalance between the I and Q branch amplitudes and phases as well as from carrier frequency offset. These analog front-end imperfections then result in an additional and significant degradation in system performance, especially in multi-carrier based transmission techniques. In this thesis, novel channel estimation and equalization techniques are devised to combat the doubly selective channel effects. Single carrier (SC) transmission techniques and orthogonal frequency division multiplexing (OFDM) transmission techniques are considered. In the context of SC transmission, a set of linear and decision feedback timevarying finite impulse response (FIR) equalizers are proposed to overcome the doubly selective channel effects. The basis expansion model (BEM) is used to approximate the doubly selective channel and to model the time-varying FIR equalizers. By doing so, a complicated time-varying 1-D deconvolution problem is turned into a simpler time-invariant (TI) 2-D deconvolution problem in the TI coefficients of the channel BEM and the time-varying FIR equalizer BEM coefficients. The design criteria considered in this context are the Zero-Forcing (ZF) and the Minimum Mean-Square Error (MMSE) criterion. It is shown that the ZF solution exists when the system has a number of receive antennas equal to the number of transmit antennas plus at least one. Using the MMSE criterion, on the other hand, the time-varying FIR equalizer always exists for any number of receive antennas. This approach is shown to unify and extend the previously proposed equalization techniques for TI channels. So far, in this approach the doubly selective channel is assumed to be known at the receiver, which is far from practical. To facilitate practical scenarios channel estimation and direct equalization techniques are also investigated. The proposed techniques range from pilot symbol assisted modulation based techniques to blind and semi-blind techniques. For OFDM transmission, a set of time-domain and frequency domain equalization techniques are proposed. In addition to assuming the channel is rapidly time-varying, the so-called cyclic prefix (CP) length is assumed to be shorter than the channel impulse response length. This assumption introduces interblock interference (IBI which in conjunction with the Doppler shift induces severe intercarrier interference (ICI). Time-domain equalizers (TEQs) are then needed to shorten the channel to fit within the CP length and to eliminate the channel time-variation. Doing so the TEQ is capable of restoring orthogonality between subcarriers. While the TEQ optimizes the performance on all subcarriers in a joint fashion, an optimal frequency-domain per-tone equalizer (PTEQ), that optimizes the performance on each subcarrier separately, can also be obtained by transferring the TEQ operation to the frequency domain. Finally, analog front-end impairments may have a big impact on system?s performance. In this context, joint channel equalization and compensation techniques are proposed to equalize the channel and compensate for the analog front-end impairments for OFDM transmission over TI channels. The analog front-end impairments treated in this thesis are the in-phase and quadraturephase (IQ) imbalances and the carrier frequency-offset (CFO). While IQ imbalance causes a mirroring effect, CFO induces ICI. A frequency-domain PTEQ is then proposed to overcome the problems associated with the analog front-end and to equalize the TI channel. The PTEQ is obtained here by transferring two TEQ operations to the frequency domain. One TEQ is applied to the received sequence, and the other one is applied to a conjugated version of the received sequence. Each TEQ is implemented as a time-varying FIR and modeled using the BEM. Once again, the BEM modeling here shows to be efficient to combat the problem of ICI induced due to CFO. Other analog front-end impairments like phase noise, nonlinear power amplifiers, etc. are out of the scope of this thesis.