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 measurement-driven 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 and its antenna selection methods using the WARP platform. We evaluate the implementation using a computerized motor to rotate the prototype according to traces collected from smartphone users in the field. Our evaluation shows that MiDAS achieves a median of 3dB increase in link gain. We demonstrate that rate adaptation and power control can be combined with MiDAS to further improve goodput and power saving. Real-time experiments with the prototype show that the link gain translates to 85% goodput improvement for a low SNR scenario. The same gain translates to 51% transmit power reduction for a high SNR scenario. Compared to other methods in realizing directional communication, MiDAS does not require any changes to the network infrastructure, and is therefore suitable for immediate or near-future deployment.