Matthew Podolsky

Limited retransmission of real-time layered multimedia

In contrast to multimedia applications that involve human-to-human communication, streaming media over the Internet enjoys relaxed delay constraints. Thus, streaming media servers are at liberty to retransmit missing packets to avoid unnecessary signal corruption. While state-of-the-art media servers employ such strategies, no work to date has proposed an optimal strategy for delay-constrained retransmissions of streaming media. In this paper, we propose a framework for streaming media retransmission based on layered media representations and explore the performance advantage of integrating layered signal structure into the retransmission strategy. In our approach, the source must choose between transmitting an older layer that expires sooner and a newer layer that expires later but is more important. To arrive at the proper mix of these two extreme strategies, we derive an optimal strategy for transmitting layered data over a binary erasure channel with instantaneous feedback. To provide a quantitative performance comparison of different transmission policies, we conduct a Markov-chain analysis, which shows that the best transmission policy is time-invariant and thus does not change as the layers approach their expiration times.

Design and verification of smart and scalable DC microgrids for emerging regions

Roughly 1.3 billion people in developing countries still live without access to reliable electricity. As expanding access using current technologies will accelerate global climate change, there is a strong need for novel solutions that displace fossil fuels and are financially viable for developing regions. A novel DC microgrid solution that is geared at maximizing efficiency and reducing system installation cost is described in this paper. Relevant simulation and experimental results, as well as a proposal for undertaking field-testing of the technical and economic viability of the microgrid system are presented.

Scalable DC Microgrids for Rural Electrification in Emerging Regions

We present the design and experimental validation of a scalable dc microgrid for rural electrification in emerging regions. A salient property of the dc microgrid architecture is the distributed control of the grid voltage, which enables both instantaneous power sharing and a metric for determining the available grid power. A droop-voltage power-sharing scheme is implemented wherein the bus voltage droops in response to low supply/high demand. In addition, the architecture of the dc microgrid aims to minimize the losses associated with stored energy by distributing storage to individual households. In this way, the number of conversion steps and line losses are reduced. We calculate that the levelized cost of electricity of the proposed dc microgrid over a 15-year time horizon is

A scalable dc microgrid architecture for rural electrification in emerging regions

0.35/kWh. We also present the experimental results from a scaled-down experimental prototype that demonstrates the steady-state behavior, the perturbation response, and the overall efficiency of the system. Moreover, we present fault mitigation strategies for various faults that can be expected to occur in a microgrid distribution system. The experimental results demonstrate the suitability of the presented dc microgrid architecture as a technically advantageous and cost-effective method for electrifying emerging regions.

Grid watch: mapping blackouts with smart phones

We present the design and experimental validation of a scalable dc microgrid architecture for rural electrification. The microgrid design has been driven by field data collected from Kenya and India. The salient features of the microgrid are distributed voltage control and distributed storage, which enable developed world grid cost parity. In this paper, we calculate that the levelized cost of electricity (LCOE) for the proposed dc microgrid system will be less than

Experience: Android Resists Liberation from Its Primary Use Case

0.40 per kW-hr. We also present experimental results from a locally installed dc microgrid prototype that demonstrate the steady state behavior, the perturbation response, and the overall efficiency of the system. The experimental results demonstrate the suitability of the presented dc microgrid architecture as a technically advantageous and cost effective method for electrifying emerging regions.

Towards Building a Community Cellular Network in the Philippines: Initial Site Survey Observations

The power grid is one of humanitys most significant engineering undertakings and it is essential in developed and developing nations alike. Currently, transparency into the power grid relies on utility companies and more fine-grained insight is provided by costly smart meter deployments. We claim that greater visibility into power grid conditions can be provided in an inexpensive and crowd-sourced manner independent of utility companies by leveraging existing smartphones. Our key insight is that an unmodified smartphone can detect power outages by monitoring changes to its own power state, locally verifying these outages using a variety of sensors that reduce the likelihood of false power outage reports, and corroborating actual reports with other phones through data aggregation in the cloud. The proposed approach enables a decentralized system that can scale, potentially providing researchers and concerned citizens with a powerful new tool to analyze the power grid and hold utility companies accountable for poor power quality. This paper demonstrates the viability of the basic idea, identifies a number of challenges that are specific to this application as well as ones that are common to many crowd-sourced applications, and highlights some improvements to smartphone operating systems that could better support such applications in the future.

GSM timeslot detection and switching for power amplifier duty cycling in community cellular networks

Network connectivity is often one of the most challenging aspects of deploying sensors. In many countries, cellular networks provide the most reliable, highest bandwidth, and greatest coverage option for Internet access. While this makes smartphones a seemingly ideal platform to serve as a gateway between sensors and the cloud, we find that a device designed for multi-tenant operation and frequent human interaction becomes unreliable when tasked to continuously run a single application with no human interaction, a seemingly counter-intuitive result. Further, we find that economy phones cannot physically withstand continuous operation, resulting in a surprisingly high rate of permanent device failures in the field. If these observations hold more broadly, they would make mobile phones poorly suited to a range of sensing applications for which they have been rumored to hold great promise.

Simulation of FEC-based error control for packet audio on the Internet

In this paper, we present preliminary site survey findings for a community cellular network project in the Philippines. We document our observations from our visits to unserved communities, notably on how they deal and adapt to the lack of cellular access. We also identify challenges in deploying the technology in the Philippine setting.

Soft ARQ for Layered Streaming Media

Despite the widespread adoption of mobile phones, some remote, rural areas still lack cellular connectivity primarily due to the high cost of traditional base station deployments. Software-defined base station implementations have recently been able to drastically reduce the cost of hardware. However, covering the operating costs remains an issue, particularly in small communities. In this paper, we present a front end system exploiting significant idle times in a rural GSM network in order to reduce the overall power consumption of the base station, thereby reducing operating costs. The front end utilizes precisely timed switching signals from the downlink transmission of a base station. The switching signals are then used for automatic, on-demand power amplifier duty cycling at the GSM timeslot level to provide virtual coverage to subscribers. An implementation using a software-defined GSM-900 base station demonstrated potential power savings of at least 80% during idle periods. The reduced power consumption is foreseen to further lower capital and operating costs, allowing more rural communities to gain access to cellular communications.