Xi Liu

Maximizing Spatial Reuse in Indoor Environments Degree Type: Ph.D. in Computer Science
Advisor(s): Srinivasan Seshan, Peter Steenkiste
Graduated: May 2011

Abstract:

Wireless technologies have gained tremendous popularity over the last couple of years. Such popularity causes dense and usually chaotic, i.e., unplanned and unmanaged, deployments of wireless devices in indoor environments such as homes and offices. Due to the broadcast nature of wireless communication, wireless interference is becoming the most serious issue in these dense and usually chaotic wireless deployments. Existing techniques, however, cannot solve the interference problem at this scale and meet this growing demand for wireless capacity.

In this dissertation, we propose to tackle the interference problem by optimizing spatial reuse (increase the number of simultaneous transmissions in an area without using additional frequencies) using directional transmission and power control. We propose two directional systems: DIRC for enterprise wireless networks with directional APs and omni-directional clients, and Speed for future wireless networks with directional APs and directional clients. While directional antennas can be very useful, there are certain cases where deploying directional antennas can be very difficult, e.g., due to cost or size issues. Thus, in order to extend the optimization of spatial reuse to existing omni-directional antenna networks, we also propose Opera that uses the technique of power control to achieve spatial reuse for these networks. Both techniques of directional transmission and power control can allow multiple transmissions to happen simultaneously even in the same channel and even when the nodes are closely located in an area.

While there are many systems use directional transmission and/or power control in outdoor application scenarios, achieving spatial reuse in our application scenario, i.e., chaotic deployment of wireless devices in indoor environments, is decidedly more complicated. This is primarily due to the key characteristics of these scenarios: Due to both rich scattering and unplanned node locations, the signal and interference patterns from the senders to the receivers are rather unpredictable. The problem is further complicated by the fact that today's carrier sensing based medium access control (MAC) protocols interact poorly with both techniques.

In this dissertation, we first show that contrary to conventional wisdom, directional transmission and power control can be very effective even in rich-scattered indoor environments. Then, we show how to build the three systems in a practical and lightweight fashion to exploit the capabilities of directional transmission and power control. Specifically, we use the SINR model to facilitate choosing the appropriate antenna orientations and power levels, and we also use a timeslot and timeslot reservation based MAC protocol (both centralized and distributed versions) to coordinate the APs and the clients. Finally, we evaluate our systems in several indoor testbeds to illustrate their effectiveness in practice.

Our contributions demonstrate that there exist practical and lightweight solutions to maximize indoor wireless spatial reuse.

Thesis Committee:
Srinivasan Seshan (Co-Chair)
Peter Steenkiste (Co-Chair)
David Andersen
Konstantina Papagiannaki

Jeannette Wing, Head, Computer Science Department
Randy Bryant, Dean, School of Computer Science

Keywords:
Wireless networks, directional antennas, power control, spatial reuse, wireless capacity, wireless performance, 802.11