Wireless Transceiver Design for High Velocity Scenarios

Abstract

This thesis is dedicated to transceiver designs for high data-rate wireless communication systems with rapidly moving terminals. The challenges are two-fold. On the one hand, more spectral bandwidth of the transmitted signals is required by future wireless systems to obtain higher transmission rates, which can result in the frequency selectivity of the communication channels. On the other hand, Doppler effects emerge when high mobile speeds are present, which can result in the time selectivity of the communication channels. Therefore, it is likely that future wireless communication systems operate in doubly-selective channels, which impose many difficulties on transceiver designs. In this thesis, we investigate these challenges in the following four scenarios, and propose a number of corresponding solutions. OFDM over Narrowband Channels: Orthogonal frequency-divisionmultiplexing (OFDM) is a typical multiple-carrier transmission technique. In a narrowband scenario, Doppler effects are well approximated as frequency shifts. In this manner, a narrowband doubly-selective channel for OFDM systems can be approximately characterized as a banded matrix especially when a basis expansion model (BEM) is exploited to model the channel. It thus allows to reduce the complexity of the channel equalization. However, there are various different BEM’s available. We identify a particular BEM which leads to a more efficient hardware architecture than other choices, while still maintaining a high modeling accuracy. OFDM over Wideband Channels: The Doppler effect manifests itself as a distinct phenomenon in wideband channels compared to narrowband channels. Specifically, the wideband signal waveformismeasurably dilated or compressed when Doppler is present rather than just frequency-shifted. This unique nature ofwideband time-varying channels requires new designs for wideband OFDM systems. We first quantify the amount of interference resulting from wideband doubly-selective channels which follow the multi-scale/multi-lag (MSML) model. Then we discuss an equalization method for wideband channels either in the frequency domain or in the time domain. A novel optimum resampling procedure is also introduced, which is normally unnecessary in narrowband systems. Multi-Rate Transmissions over Wideband Channels: Traditional multi-carrier transmission schemes, e.g., OFDM, use a uniformdata rate on each subcarrier, which is inherently mismatched with wideband time-varying channels. In fact, the time variation of wideband channels, i.e., the Doppler scales, imply a non-uniform sampling mechanism. To mitigate this, we propose a novel multi-rate transmission scheme by placing the information symbols at different non-overlapping sub-bands where each sub-band has a distinctive bandwidth. To combat the MSML effect of the channel, a filterbank is deployed at the receiver, where each branch of the filterbank samples the received signal at a corresponding rate. By selecting a proper transmit/receiver pulse, the effective input/output relationship can be captured by a block-diagonal channel, with each diagonal block being a banded matrix similarly as seen in narrowband OFDM systems. The benefit of this similarity is that existing low-complexity equalizers can be adopted for wideband communications. Robust Multi-band Transmissions over Wideband Channels: Accurate channel estimation for wideband doubly-selective channels is challenging and troublesome. Adaptive channel equalization is thus attractive since it does not require precise channel information and is robust to various prevailing environmental conditions. When the MSML effect emerges in wideband channels, it is not wise to adopt existing adaptive equalization designs that are previously used in other scenarios, e.g., narrowband channels. We adopt a multi-band frequency-division multiplexing (FDM) signal waveform at the transmitter to reduce the equalization complexity, while maintaining a high data rate. By carefully designing the transmit pulse, our proposed multi-layer turbo equalization, using a phase-locked loop (PLL) followed by a time-invariant finite impulse response (FIR) filter, is capable of equalizing such MSML channels.