Wireless Coexistence: A quick primer

Wireless has quickly become an irreplaceable medium for communication, but by its very nature, wireless transmissions are vulnerable to RF (Radio Frequency) interference from various sources. This weakness is a growing problem for technologies that operate in unlicensed frequency bands, as these bands are becoming more crowded over time. The increasing density of radios, coupled with the growing number of different protocols, dictates that future wireless devices will require the ability to coexist and operate in a crowded and often unregulated electromagnetic environment.

Stanford Doctoral Dissertation Defense 2012

I successfully completed my oral university examination on October 25, 2012. Below is a recording of the open public portion of my defense. The talk aggregates the different pieces of my research at Stanford University from 2009-2012, including DOF and Picasso.

Steven Hong's Electrical Engineering Doctoral Dissertation Defense 2012

Pieces of a Puzzle: How my research fits together

The design of future wireless systems, capable of supporting high speed data protocols in such a dense and diverse spectral environment, will be very different from traditional designs. My research focuses on determining what new capabilities these systems will require, and how these systems should be architected.

1. DOF (Degrees of Freedom)   [pdf]
Predicament: To coexist in a crowded and unregulated spectral environment (e.g. ISM Band), radios will have to be aware of who their neighbors are and the nature of their operations. Coexistence mechanisms are presently designed to mediate access to the spectrum between protocols of the same type. In unlicensed spectrum, radios belonging to different protocols must share the same spectrum, and in such instances these mechanisms fail.

Solution Sketch: We've developed DOF, a wireless information plane which provides a wireless system with the knowledge of who its RF neighbors are (e.g. WiFi AP, Microwave Oven, etc.), which bands of spectrum they are operating on, and which spatial directions their signals are traversing. Devices can leverage DOF to inform them of who their neighbors are, but DOF does not dictate how the device should interact with those neighbors. DOF is simply a detector substrate on top of which smart coexistence protocols can be built.

DOF Presentation at SIGCOMM 2011

2. Picasso   [pdf]
Predicament: The available spectrum bands that DOF identifies are unlikely to be the large contiguous bands of spectrum (e.g. 802.11n uses 40MHz bands) that high throughput systems utilize today. Due to the proliferation of heterogeneous wireless devices, available unused bands are often dispersed as separate fragments. Hence, future wireless systems will need to be architectured such that they can systematically exploit fragmented spectrum.

Solution Sketch: We've developed Picasso, a novel radio design that allows simultaneous transmission and reception on separate and arbitrary spectrum fragments using a single RF frontend and antenna. Picasso leverages this capability to flexibly slice radios and fragmented spectrum into multiple slices that share the RF frontend and antenna, yet operate concurrent and independent PHY/MAC protocols.

Picasso Presentation at SIGCOMM 2012

3. PinPoint   [pdf]
Predicament: Interference is the number one cause for poor wireless performance. We've all had anecdotal experiences, where, even though the AP is quite close, we experience poor performance and more often that not, interference is to blame. Yet, in spite of these pervasive problems, we are limited in our ability to detect the nature of the interfering radio and determine where it is located. Without such localization, troubleshooting performance problems becomes hard.

Solution Sketch: PinPoint is a technique for localizing (to within a meter) rogue interfering radios that adhere to standard protocols in the inhospitable ISM band without any cooperation from the interfering radio. More generally, PinPoint is pertinent in the context of any localization application - from targeted advertising, to indoor mapping, to context detection - there are several uses for sub-meter indoor localization.

PinPoint Presentation at NSDI 2013

4. Sorting Hat   [pdf]
Predicament: Due to the heterogeneity of wireless protocols (e.g. different transmit powers, ranges, throughputs, backhauls, etc.) and unplanned/mobile nature of wireless devices, the allocation of network resources (e.g. spectrum, power, etc.) is extremely challenging, even in centrally managed cellular networks. To date, even theoretical analyses have focused only a subset of the constraints or objectives. As a result, systems will need a holistic resource allocation scheme, which takes into account all of the constraints and objectives, in order to maximize performance.

Solution Sketch: SortingHat is a tractable optimization model which leverages the advantages of the different heterogeneous components in the network to allocate resources. By focusing on the entire set of constraints and objectives, SortingHat's holistic approach enables it to outperform optimization techniques which are optimal for only a subset of the constraints and objectives. SortingHat is not meant to be an implementable systems solution, rather it is intended to be a holistic framework which can be used to benchmark more practical resource allocation algorithms.