The Myth of Spatial Reuse with Directional Antennas in Indoor Wireless Networks

Abstract. Interference among co-channel users is a fundamental prob-lem in wireless networks, which prevents nearby links from operating concurrently. Directional antennas allow the radiation patterns of wire-less transmitters to be shaped to form directed beams. Conventionally, such beams are assumed to improve the spatial reuse (i.e. concurrency) in indoor wireless networks. In this paper, we use experiments in an indoor office setting of Wifi Access points equipped with directional antennas, to study their potential for interference mitigation and spatial reuse. In contrast to conventional wisdom, we observe that the interference mit-igation benefits of directional antennas are minimal. On analyzing our experimental traces we observe that directional links do not reduce inter-ference to nearby links due to the lack of signal confinement due to indoor multipath fading. We then use the insights derived from our study to de-velop an alternative approach that provides better interference reduction in indoor networks compared to directional links. Key words: Indoor wireless networks, directional antennas, spatial reuse

Beamforming on Mobile Devices: A First Study

In this work, we report the first study of beamforming on mobile devices. We first show that beamforming is already feasible on mobile devices in terms of form factor, power efficiency and device mobility. We then investigate the optimal way of using beamforming in terms of power efficiency, by allowing a dynamic number of active antennas. We propose a simple yet effective solution, BeamAdapt, which allows each mobile client in a network to iteratively identify the optimal number of active antennas with fast convergence and close-to-optimal performance. Finally we report a WARP-based prototype of BeamAdapt and experimentally demonstrate its effectiveness in realistic environments. We also complement the prototype-based experiments with Qualnet-based simulation of a large-scale network. Our results show that BeamAdapt with four antennas can reduce the power consumption of mobile clients by more than half compared to omni directional transmission, while maintaining a required network throughput

Experimental and analytical evaluation of multi-user beamforming in wireless LANs

Adaptive beamforming is a. powerful approach to receive or transmit signals of interest in a spatially selective way in the presence of interference and noise. Recently, there has been renewed interest in adaptive beamforming driven by applications in wireless communications, where multiple-input multiple-output (MEMO) techniques have emerged as one of the key technologies to accommodate the high number of users as well as the increasing demand for new high data rate services. Beamforming techniques promise to increase the spectral efficiency of next generation wireless systems and are currently being incorporated in future industry standards. Although a significant amount of research has focused on theoretical capacity analysis, little is known about the performance of such systems in practice. In thesis, I experimentally and analytically evaluate the performance of adaptive beamforming techniques on the downlink channel of a wireless LAN. To this end. I present the design and implementation of the first multi-user beam-forming system and experimental framework for wireless LANs. Next, I evaluate the benefits of such system in two applications. First, I investigate the potential of beamforming to increase the unicast throughput through spatial multiplexing. Using extensive measurements in an indoor environment, I evaluate the impact of user separation distance, user selection, and user population size on the multiplexing gains of multi-user beamforming. I also evaluate the impact of outdated channel information due to mobility and environmental variation on the multiplexing gains of multi-user beamforming. Further, I investigate the potential of beamforming to eliminate interference at unwanted locations and thus increase spatial reuse. Second, I investigate the potential of adaptive beamforming for efficient wireless multicasting. I address the joint problem of adaptive beamformer design at the PHY layer and client scheduling at the MAC layer by proposing efficient algorithms that are amenable to practical implementation. Next, I present the implementation of the beamforming based multicast system on the WARP platform and compare its performance against that of omni-directional and switched beamforming based multicast. Finally, I evaluate the performance of multicast beamforming under client mobility and infrequent channel feedback, and propose solutions that increase its robustness to channel dynamics

Directional Antenna Diversity for Mobile Devices: Characterizations and Solutions

We report a first-of-its-kind realization of directional transmission for smartphone-like mobile devices using multiple passive directional antennas, supported by only one RF chain. The key is a multi-antenna system (MiDAS) and its antenna selection methods that judiciously select the right antenna for transmission. It is grounded by two measurementdriven studies regarding 1) how smartphones rotate during wireless usage in the field and 2) how orientation and rotation impact the performance of directional antennas under various propagation environments. We implement MiDAS using the WARP platform, and evaluate it usmg a computerized motor to rotate the prototype according to traces collected from smartphone users in the field. Our evaluation shows MiDAS achieves median of 3dB increase in link gain. Combined with rate adaptation and power control, MiDAS also improves goodput and power saving. MiDAS does not require any changes to the network infrastructure, and is therefore suitable for immediate deployment