Initial access and beam-steering mechanisms for mmWave wireless systems

Abstract

Future millimeter-wave networks will support very high densities of devices and access points. This vastly increases the overhead required for access point selection and beam training. Due to unfavorable radio propagation, mmWave systems will exploit largescale MIMO and adaptive antenna arrays at both the transmitter and receiver to realize sufficient link margin. Beamforming is vital to overcome the high attenuation in wireless millimeter-wave networks. It enables nodes to steer their antennas in the direction of communication. Fortunately, the quasi-optical properties of millimeter-wave channels make location-based network optimization a highly promising technique to reduce control overhead in such millimeter-wave WLANs. In this thesis we present tools to improve mmWave systems. We start by designing an effective lightweight sector beam-pattern design for using as a baseline for hybrid analog-digital structures. We deal with practical constraints of mmWave transceivers and propose a novel, geometric approach to synthesize multi-beamwidth beam patterns that can be leveraged for simultaneous multi-direction scanning. Then we make use of this multi-direction scanning to create a beam training protocol which effectively accelerates the link establishment by exploiting the ability of mobile users to simultaneously receive from multiple directions. We propose smart beam training and tracking strategies for fast mm-wave link establishment and maintenance under node mobility. We leverage the ability of hybrid analog-digital transceivers to collect channel information from multiple spatial directions simultaneously and formulate a probabilistic optimization problem to model the temporal evolution of the mm-wave channel under mobility. We propose a mechanism to extract full channel state information (CSI) regarding phase and magnitude from coarse signal strength readings on off-the-shelf IEEE 802.11ad devices. Using this CSI, transmitters dynamically compute a transmit beam pattern that maximizes the signal strength at the receiver. Channel properties and antenna design at 60GHz are ideal for path angular information extraction, following an almost ideal geometric channel model. Due to this, we present some localization method specifically designed for the 60GHz band. We merge the ideas presented in this thesis and by extracting channel state information from off-the-shelf routers we estimate the user location to manage a location aware beam-training and device handling method. The resulting scheme can predict blockage, optimize access point association, and select the most suitable antenna beam patterns while significantly reducing the beam training overhead.