Vikram Srinivasan

Extending the lifetime of wireless sensor networks through mobile relays

We investigate the benefits of a heterogeneous architecture for wireless sensor networks (WSNs) composed of a few resource rich mobile relay nodes and a large number of simple static nodes. The mobile relays have more energy than the static sensors. They can dynamically move around the network and help relieve sensors that are heavily burdened by high network traffic, thus extending the latters lifetime. We first study the performance of a large dense network with one mobile relay and show that network lifetime improves over that of a purely static network by up to a factor of four. Also, the mobile relay needs to stay only within a two-hop radius of the sink. We then construct a joint mobility and routing algorithm which can yield a network lifetime close to the upper bound. The advantage of this algorithm is that it only requires a limited number of nodes in the network to be aware of the location of the mobile relay. Our simulation results show that one mobile relay can at least double the network lifetime in a randomly deployed WSN. By comparing the mobile relay approach with various static energy-provisioning methods, we demonstrate the importance of node mobility for resource provisioning in a WSN.

CloudIQ: a framework for processing base stations in a data center

The cellular industry is evaluating architectures to distribute the signal processing in radio access networks. One of the options is to process the signals of all base stations on a shared pool of compute resources in a central location. In this centralized architecture, the existing base stations will be replaced with just the antennas and a few other active RF components, and the remainder of the digital processing including the physical layer will be carried out in a central location. This model has potential benefits that include a reduction in the cost of operating the network due to fewer site visits, easy upgrades, and lower site lease costs, and an improvement in the network performance with joint signal processing techniques that span multiple base stations. Further there is a potential to exploit variations in the processing load across base stations, to pool the base stations into fewer compute resources, thereby allowing the operator to either reduce energy consumption by turning the remaining processors off or reducing costs by provisioning fewer compute resources. We focus on this aspect in this paper.n Specifically, we make the following contributions in the paper. Based on real-world data, we characterise the potential savings if shared homogeneous compute resources are used to process the signals from multiple base stations in the centralized architecture. We show that the centralized architecture can potentially result in savings of at least 22 % in compute resources by exploiting the variations in the processing load across base stations. These savings are achievable with statistical guarantees on successfully processing the base stations signals. We also design a framework that has two objectives: (i) partitioning the set of base stations into groups that are simultaneously processed on a shared homogeneous compute platform for a given statistical guarantee, and (ii) scheduling the set of base stations allocated to a platform in order to meet their real-time processing requirements. This partitioning and scheduling framework saves up to 19 % of the compute resources for a probability of failure of one in 100 million. We refer to this solution as CloudIQ. Finally we implement and extensively evaluate the CloudIQ framework with a 3GPP compliant implementation of 5 MHz LTE.

Cooperative Asynchronous Multichannel MAC: Design, Analysis, and Implementation

Medium access control (MAC) protocols have been studied under different contexts for decades. In decentralized contexts, transmitter-receiver pairs make independent decisions, which are often suboptimal due to insufficient knowledge about the communication environment. In this paper, we introduce distributed information sharing (DISH), which is a distributed flavor of control-plane cooperation, as a new approach to wireless protocol design. The basic idea is to allow nodes to share control information with each other such that nodes can make more informed decisions in communication. This notion of control-plane cooperation augments the conventional understanding of cooperation, which sits at the data plane as a data relaying mechanism. In a multichannel network, DISH allows neighboring nodes to notify transmitter-receiver pairs of channel conflicts and deaf terminals to prevent collisions and retransmissions. Based on this, we design a single-radio cooperative asynchronous multichannel MAC protocol called CAM-MAC. For illustration and evaluation purposes, we choose a specific set of parameters for CAM-MAC First, our analysis shows that its throughput upper bound is 91 percent of the system bandwidth and our simulations show that it actually achieves a throughput of 96 percent of the upper bound. Second, our analysis shows that CAM-MAC can saturate 15 channels at maximum and our simulations show that it saturates 14.2 channels on average, which indicates that, although CAM-MAC uses a control channel, it does not realistically suffer from control channel bottleneck. Third, we compare CAM-MAC with its noncooperative version called UNCOOP, and observe a throughput ratio of 2.81 and 1.70 in single-hop and multihop networks, respectively. This demonstrates the value of cooperation. Fourth, we compare CAM-MAC with three recent multichannel MAC protocols, MMAC, SSCH, and AMCP, and find that CAM-MAC significantly outperforms all of them. Finally, we implement CAM-MAC and UNCOOP on commercial off-the-shelf hardware and share lessons learned in the implementation. The experimental results confirm the viability of CAM-MAC and the idea of DISH.

Dynamic spectrum access in DTV whitespaces: design rules, architecture and algorithms

In November 2008, the FCC ruled that the digital TV whitespaces be used for unlicensed access. This is an exciting development because DTV whitespaces are in the low frequency range (50-698 MHz) compared to typical cellular and ISM bands, thus resulting in much better propagation characteristics and much higher spectral efficiencies. The FCC has also mandated certain guidelines for short range unlicensed access, so as to avoid any interference to DTV receivers. We consider the problem of WiFi like access (popularly referred to as WiFi 2.0) for enterprizes. We assume that the access points and client devices are equipped with cognitive radios, i.e., they can adaptively choose the center frequency, bandwidth and ower of operation. The access points can be equipped with one or more radios. Our goal is to design a complete system, which (i) does not violate the FCC mandate, (ii) dynamically assigns center frequency and bandwidth to each access point based on their demands and (iii) squeezes the maximum efficiency from the available spectrum. This problem is far more general than prior work that investigated dynamic spectrum allocation in cellular and ISM bands, due to the non-homogenous nature of the whitespaces, i.e., different whitespace widths in different parts of the spectrum and the large range of frequency bands with different propagation characteristics. This calls for a more holistic approach to system design that also accounts for frequency dependent propagation characteristics and radio frontend characteristics. In this paper, we first propose design rules for holistic system design. We then describe an architecture derived from our design rules. Finally we propose demand based dynamic spectrum allocation algorithms with provable worst case guarantees. We provide extensive simulation results showing that (i) the performance of our algorithm is within 94% of the optimal in typical settings and (ii) and the DTV whitespaces can provide significantly higher data rates compared to the 2.4GHz ISM band. Our approach is general enough for designing any system with access to a wide range of spectrum.

Dependent link padding algorithms for low latency anonymity systems

Low latency anonymity systems are susceptive to traffic analysis attacks. In this paper, we propose a dependent link padding scheme to protect anonymity systems from traffic analysis attacks while providing a strict delay bound. The covering traffic generated by our scheme uses the minimum sending rate to provide full anonymity for a given set of flows. The relationship between user anonymity and the minimum covering traffic rate is then studied via analysis and simulation. When user flows are Poisson processes with the same sending rate, the minimum covering traffic rate to provide full anonymity to m users is O(log m). For Pareto traffic, we show that the rate of the covering traffic converges to a constant when the number of flows goes to infinity. Finally, we use real Internet trace files to study the behavior of our algorithm when user flows have different rates.

Opportunistic energy-efficient contact probing in delay-tolerant applications

In many delay-tolerant applications, information is opportunistically exchanged between mobile devices that encounter each other. In order to affect such information exchange, mobile devices must have knowledge of other devices in their vicinity. We consider scenarios in which there is no infrastructure and devices must probe their environment to discover other devices. This can be an extremely energy-consuming process and highlights the need for energy-conscious contact-probing mechanisms. If devices probe very infrequently, they might miss many of their contacts. On the other hand, frequent contact probing might be energy inefficient. In this paper, we investigate the tradeoff between the probability of missing a contact and the contact-probing frequency. First, via theoretical analysis, we characterize the tradeoff between the probability of a missed contact and the contact-probing interval for stationary processes. Next, for time-varying contact arrival rates, we provide an optimization framework to compute the optimal contact-probing interval as a function of the arrival rate. We characterize real-world contact patterns via Bluetooth phone contact-logging experiments and show that the contact arrival process is self-similar. We design STAR, a contact-probing algorithm that adapts to the contact arrival process. Instead of using constant probing intervals, STAR dynamically chooses the probing interval using both the short-term contact history and the long-term history based on time of day information. Via trace-driven simulations on our experimental data, we demonstrate that STAR requires three to five times less energy for device discovery than a constant contact-probing interval scheme.

MOTA: engineering an operator agnostic mobile service

There are two emerging trends in the mobile data world. First, mobile data is exploding at a rapid rate with analysts predicting 25-50X growth by the year 2015. The second trend is that users are demanding greater degree of flexibility in selecting their operators at fine timescales. Across Asia, dual-SIM phones have become popular, while Apple is rumored to be designing a Universal SIM that will allow iPhone users to toggle between different operators. This latter trend points towards an impending disruption in wireless service models which could also be the need of the hour from the spectrum shortage perspective.n This points towards a new service model where users can choose an operator based on application needs. However, if users make this choice greedily without network assistance, it can exacerbate spectrum scarcity and degrade user experience. In this work, we consider user devices with multiple network interfaces (3G, LTE etc.) that can be simultaneously active and each running multiple applications. We propose the MOTA service model to enable users to associate each interface with the operator of choice at fine time scales. Under the MOTA service model, through concise signalling information, operators provide information about their own network, so that each user can (i) choose a suitable operator for each interface, and (ii) choose an interface for each active application. We make the following contributions in this paper. First, we propose concise network signalling that assists users to make informed choices even under mobility. Second, we develop user-choice algorithms that maximize a suitable notion of user satisfaction while using spectrum resources efficiently. Third, we perform extensive evaluation over actual base station deployment in a city coupled with real signal propagation maps. Our results with two operators show that, MOTA service model provides capacity gain in the range 2.5-4X over the current existing service model. Finally, we argue that our solution is practically implementable by combining appropriate IEEE standards and IETF proposals.

Power Control for Distributed MAC Protocols in Wireless Ad Hoc Networks

In centralized wireless networks, reducing the transmission power normally leads to higher network transport throughput. In this paper, we investigate power control in a different scenario, where the network adopts distributed MAC layer coordination mechanisms. We first consider widely adopted RTS/CTS based MAC protocols. We show that an optimal power control protocol should use higher transmission power than the just enough power in order to improve spatial utilization. The optimal protocol has a minimal transmission floor area of Theta(dijdmax), where dmax is the maximal transmission range and dij is the link length. This surprisingly implies that if a long link is broken into several short links, then the sum of the transmission floors reserved by the short links is still comparable to that reserved by the long link. Thus, using short links does not necessarily lead to higher throughput. Another consequence of this is that, with the optimal RTS/CTS based MAC, rate control can at best provide a factor of 2 improvement in transport throughput. We then extend our results to other distributed MAC protocols which uses physical carrier sensing or busy-tone as the control signal. Our simulation results show that the optimal power controlled scheme outperforms other popular MAC layer power control protocols.

MAX: Wide area human-centric search of the physical world

We propose MAX, a system that facilitates human-centric search of the physical world. Instead of organizing objects a priori, it allows humans to search for and locate them as needed. Designed for the following objectives: (i) human-centric operation, (ii) privacy, and (iii) efficient searching of any tagged object, MAX provides location information in a form natural to humans, that is, with reference to identifiable landmarks (such as, “on the dining table”) rather than precise coordinates. In the system, all physical objects—from documents to clothing—can be tagged, users then locate objects using an intuitive search interface. To make searching efficient, MAX adopts a hierarchical architecture consisting of tags (bound to objects), substations (bound to landmarks), and base-stations (bound to localities). Tags can be marked as either public or private, with private tags searchable only by the owner. MAX also provides for privacy of physical spaces. It requires minimal initial configuration, and is robust to reconfiguration of the physical space. We also present a methodology to design energy-optimal and delay-optimal query protocols for a variety of device choices, this optimizes system performance, and affords insight into the appropriate actions for various scenarios. We have implemented a simple prototype of MAX, demonstrating the feasibility of the system for human-centric search over several locations across a wide area. We contend that a MAX-like search system will enable sharing (e.g., books on a college campus) and trading (e.g., buying and selling used books) of physical resources, and will be the engine for a host of new applications.

Energy-Efficient Strategies for Cooperative Multichannel MAC Protocols

Distributed Information SHaring (DISH) is a new cooperative approach to designing multichannel MAC protocols. It aids nodes in their decision making processes by compensating for their missing information via information sharing through neighboring nodes. This approach was recently shown to significantly boost the throughput of multichannel MAC protocols. However, a critical issue for ad hoc communication devices, viz. energy efficiency, has yet to be addressed. In this paper, we address this issue by developing simple solutions that reduce the energy consumption without compromising the throughput performance and meanwhile maximize cost efficiency. We propose two energy-efficient strategies: in-situ energy conscious DISH, which uses existing nodes only, and altruistic DISH, which requires additional nodes called altruists. We compare five protocols with respect to these strategies and identify altruistic DISH to be the right choice in general: it 1) conserves 40-80 percent of energy, 2) maintains the throughput advantage, and 3) more than doubles the cost efficiency compared to protocols without this strategy. On the other hand, our study also shows that in-situ energy conscious DISH is suitable only in certain limited scenarios.