Graduation Year

2008

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Electrical Engineering

Major Professor

Wilfrido A. Moreno, Ph.D.

Co-Major Professor

Paris H. Wiley, Ph.D.

Keywords

Wireless systems, Wlans, Wavelets, Space time coding, Multiplexing

Abstract

The current popularity of WLANs is a testament primarily to their convenience, cost efficiency and ease of integration. Even now the demand for high data rate wireless communications has increased fourfold as consumers demand better multimedia communications over the wireless medium. The next generation of high speed WLANs is expected to meet this increased demand for capacity coupled with high performance and spectral efficiency. The current generation of WLANs utilizes Orthogonal Frequency Division Multiplexing (OFDM) modulation. The next generation of WLAN standards can be made possible either by developing a different modulation technique or combining legacy OFDM with Multiple Input Multiple Output (MIMO) systems to create MIMO-OFDM systems. This dissertation presents two different basis technologies for the next generation of high speed WLANs: OWSS and MIMO-STC-OFDM.

OWSS, or Orthogonal Wavelet Division Multiplexed - Spread Spectrum is a new class of wavelet pulses and a corresponding signaling system which has significant advantages over current signaling schemes like OFDM. In this dissertation, CSMA/CA is proposed as the protocol for full data rate multiplexing at the MAC layer for OWSS. The excellent spectral characteristics of the OWSS signal is also studied and simulations show that passband spectrum enjoys a 30-40% bandwidth advantage over OFDM. A novel pre-distortion scheme was developed to compensate for the passband PA non-linearity. Finally for OWSS, the fundamental limits of its system performance were also explored using a multi-level matrix formulation. Simulation results on a 108 Mbps OWSS WLAN system demonstrate the excellent effectiveness of this theory and prove that OWSS is capable of excellent performance and high spectral efficiency in multipath channels.

This dissertation also presents a novel MIMO-STC-OFDM system which targets data rates in excess of 100 Mbps and at the same time achieve both high spectral efficiency and high performance. Simulation results validate the superior performance of the new system over multipath channels. Finally as channel equalization is critical in MIMO systems, a highly efficient time domain channel estimation formulation for this new system is also presented. In summary, both OWSS and MIMO-STC-OFDM appear to be excellent candidate technologies for next generation of high speed WLANs.

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