Link-Adaptive Antenna Systems

With the proliferation of rich data and multimedia services in cellular and wireless local area networks (WLANs), mobile operators and service providers are looking for solutions to make more efficient use of their spectrum to keep up with consumer demand. A key challenge in wireless systems research is to identify air interface techniques that increase spectral efficiency and reduce outage probability of a communications network. Currently, cellular operators are deploying small cells to improve the spectral efficiency per unit area, but this approach requires large capital investments for installation and precise network planning to manage inter-cell interference. Wireless standards makers and baseband device vendors have proposed higher order QAM modulation schemes. However, the capacity improvements scale logarithmically with the number of IQ constellation points, and these techniques only work in regions close to the base-station where the quality of the communication channel is already good. Other approaches like transmit beamforming and multiuser-MIMO are power-limited and require a large number of spatially separate antenna array elements./nFundamentally, in order to improve capacity, a wireless system must maximize the link quality and suppress noise and interference between the transmitters and receivers in the network. This dissertation explores the concept and experimental validation of link-adaptive or modal antenna systems, consisting of a single fed antenna structure, RF switch, and algorithm. The antenna system uses active RF switching to adjust its radiation state to the multipath propagation environment and provide a peak signal at the transceiver. We investigate the impact of modal antennas from a wireless communication systems perspective: RF front end and baseband integration, multipath fading, and network performance. The analysis is extended from SISO to MIMO systems with one or more active elements. We compare active modal antennas with standard passive antennas in the context of mobile cellular handsets and WLAN access points and client devices. Depending on the application, we have tailored algorithms based on the device usage and propagation environment./nThis research contributes to the state of the art in wireless communication systems by integrating several cross-disciplinary areas, such as antenna design, signal processing, electromagnetic wave propagation and fading, and software systems. We begin with a theoretical and practical discussion of novel algorithms designed to integrate modal antennas using real-time channel state information feedback from the device baseband communications processor. We discuss how to optimize these algorithms to achieve good performance in both slow-fading and fast-fading environments. We then present cellular field test results with thousands of hours of data collected using a handset equipped with a modal antenna. To the best of the author’s knowledge, this constitutes the first comprehensive measured results of radiation pattern switching antennas in mobile terminals on a live cellular network./nWe extend the antenna system architecture and analysis to wireless LAN systems and indoor fading environments. The novel modal antenna algorithms are further generalized to cover MIMO point-to-multipoint systems, such as access points equipped with one or more modal antennas serving multiple users in a network. We believe that the systems described in this dissertation make a strong argument for embedding adaptive modal antennas in mobile devices as a practical and effective method to increase both spectral efficiency and link reliability of a wireless network.

Advisor: Yablonovitch, Eli