Showing papers by "Xia Zhou published in 2011"

I am the antenna: accurate outdoor AP location using smartphones

Abstract: Today's WiFi access points (APs) are ubiquitous, and provide critical connectivity for a wide range of mobile networking devices. Many management tasks, e.g. optimizing AP placement and detecting rogue APs, require a user to efficiently determine the location of wireless APs. Unlike prior localization techniques that require either specialized equipment or extensive outdoor measurements, we propose a way to locate APs in real-time using commodity smartphones. Our insight is that by rotating a wireless receiver (smartphone) around a signal-blocking obstacle (the user's body), we can effectively emulate the sensitivity and functionality of a directional antenna. Our measurements show that we can detect these signal strength artifacts on multiple smartphone platforms for a variety of outdoor environments. We develop a model for detecting signal dips caused by blocking obstacles, and use it to produce a directional analysis technique that accurately predicts the direction of the AP, along with an associated confidence value. The result is Borealis, a system that provides accurate directional guidance and leads users to a desired AP after a few measurements. Detailed measurements show that Borealis is significantly more accurate than other real-time localization systems, and is nearly as accurate as offline approaches using extensive wireless measurements.

To preempt or not: Tackling bid and time-based cheating in online spectrum auctions

Abstract: Online spectrum auctions offer ample flexibility for bidders to request and obtain spectrum on-the-fly. Such flexibility, however, opens up new vulnerabilities to bidder manipulation. Aside from rigging their bids, selfish bidders can falsely report their arrival time to game the system and obtain unfair advantage over others. Such time-based cheating is easy to perform yet produces severe damage to auction performance. We propose Topaz, a truthful online spectrum auction design that distributes spectrum efficiently while discouraging bidders from misreporting their bids or time report. Topaz makes three key contributions. First, Topaz applies a 3D bin packing mechanism to distribute spectrum across time, space and frequency, exploiting spatial and time reuse to improve allocation efficiency. Second, Topaz enforces truthfulness using a novel temporal-smoothed critical value based pricing. Capturing the temporal and spatial dependency among bidders who arrive subsequently, this pricing effectively diminishes gain from bid and/or time-cheating. Finally, Topaz offers a “scalable” winner preemption to address the uncertainty of future arrivals at each decision time, which significantly boosts auction revenue. We analytically prove Topaz's truthfulness, which does not require any knowledge of bidder behavior, or an optimal spectrum allocation to enforce truthfulness. Using empirical arrival and bidding models, we perform simulations to demonstrate the efficiency of Topaz. We show that proper winner preemption improves auction revenue by 45–65% at a minimum cost of spectrum utilization.

3D beamforming for wireless data centers

Abstract: Contrary to prior assumptions, recent measurements show that data center traffic is not constrained by network bisection bandwidth, but is instead prone to congestion loss caused by short traffic bursts. Compared to the cost and complexity of modifying data center architectures, a much more attractive option is to augment wired links with flexible wireless links in the 60 GHz band. Current proposals, however, are severely constrained by two factors. First, 60 GHz wireless links are limited by line-of-sight, and can be blocked by even small obstacles between the endpoints. Second, even beamforming links leak power, and potential interference will severely limit concurrent transmissions in dense data centers. In this paper, we explore the feasibility of a new wireless primitive for data centers, 3D beamforming. We explore the design space, and show how bouncing 60 GHz wireless links off reflective ceilings can address both link blockage and link interference, thus improving link range and number of current transmissions in the data center.

On the stability and optimality of universal swarms

Abstract: Recent work on BitTorrent swarms has demonstrated that a bandwidth bottleneck at the seed can lead to the underutilization of the aggregate swarm capacity. Bandwidth underutilization also occurs naturally in mobile peer-to-peer swarms, as a mobile peer may not always be within the range of peers storing the content it desires. We argue in this paper that, in both cases, idle bandwidth can be exploited to allow content sharing across multiple swarms, thereby forming a universal swarm system. We propose a model for universal swarms that applies to a variety of peer-to-peer environments, both mobile and online. Through a fluid limit analysis, we demonstrate that universal swarms have significantly improved stability properties compared to individually autonomous swarms. In addition, by studying a swarm's stationary behavior, we identify content replication ratios across different swarms that minimize the average sojourn time in the system. We then propose a content exchange scheme between peers that leads to these optimal replication ratios, and study its convergence numerically.