Anand Prabhu Subramanian

Minimum Interference Channel Assignment in Multi-Radio Wireless Mesh Networks

In this paper, we consider multi-hop wireless mesh networks, where each router node is equipped with multiple radio interfaces and multiple channels are available for communication. We address the problem of assigning channels to communication links in the network with the objective of minimizing overall network interference. Since the number of radios on any node can be less than the number of available channels, the channel assignment must obey the constraint that the number of different channels assigned to the links incident on any node is atmost the number of radio interfaces on that node. The above optimization problem is known to be NP-hard. We design centralized and distributed algorithms for the above channel assignment problem. To evaluate the quality of the solutions obtained by our algorithms, we develop a semidefinite program formulation of our optimization problem to obtain a lower bound on overall network interference. Empirical evaluations on randomly generated network graphs show that our algorithms perform close to the above established lower bound, with the difference diminishing rapidly with increase in number of radios. Also, detailed ns-2 simulation studies demonstrate the performance potential of our channel assignment algorithms in 802.11-based multi-radio mesh networks.

Minimum Interference Channel Assignment in Multiradio Wireless Mesh Networks

In this paper, we consider multihop wireless mesh networks, where each router node is equipped with multiple radio interfaces, and multiple channels are available for communication. We address the problem of assigning channels to communication links in the network with the objective of minimizing the overall network interference. Since the number of radios on any node can be less than the number of available channels, the channel assignment must obey the constraint that the number of different channels assigned to the links incident on any node is at most the number of radio interfaces on that node. The above optimization problem is known to be NP-hard. We design centralized and distributed algorithms for the above channel assignment problem. To evaluate the quality of the solutions obtained by our algorithms, we develop a semidefinite program and a linear program formulation of our optimization problem to obtain lower bounds on overall network interference. Empirical evaluations on randomly generated network graphs show that our algorithms perform close to the above established lower bounds, with the difference diminishing rapidly with increase in number of radios. Also, ns-2 simulations, as well as experimental studies on testbed, demonstrate the performance potential of our channel assignment algorithms in 802.11-based multiradio mesh networks.

Understanding traffic dynamics in cellular data networks

We conduct the first detailed measurement analysis of network resource usage and subscriber behavior using a large-scale data set collected inside a nationwide 3G cellular data network. The data set tracks close to a million subscribers over thousands of base stations. We analyze individual subscriber behaviors and observe a significant variation in network usage among subscribers. We characterize subscriber mobility and temporal activity patterns and identify their relation to traffic volume. We then investigate how efficiently radio resources are used by different subscribers as well as by different applications. We also analyze the network traffic from the point of view of the base stations and find significant temporal and spatial variations in different parts of the network, while the aggregated behavior appears predictable. Broadly, our observations deliver important insights into network-wide resource usage. We describe implications in pricing, protocol design and resource and spectrum management.

Drive-By Localization of Roadside WiFi Networks

We use a steerable beam directional antenna mounted on a moving vehicle to localize roadside WiFi access points (APs), located outdoors or inside buildings. Localizing APs is an important step towards understanding the topologies and network characteristics of large scale WiFi networks that are deployed in a chaotic fashion in urban areas. The idea is to estimate the angle of arrival of frames transmitted from the AP using signal strength information on different directional beams of the antenna - as the beam continuously rotates while the vehicle is moving. This information together with the GPS locations of the vehicle are used in a triangulation approach to localize the APs. We show how this method must be extended using a clustering approach to account for multi-path reflections in cluttered environments. Our technique is completely passive requiring minimum effort beyond driving the vehicle around in the neighborhood where the APs need to be localized, and is able to improve the localization accuracy by an order of magnitude compared with trilateration approaches using omnidirectional antennas, and by a factor of two relative to other known techniques using directional antennas.

Near-Optimal Dynamic Spectrum Allocation in Cellular Networks

In this paper, we address the spectrum allocation problem in cellular networks under the coordinated dynamic spectrum access (CDSA) model. In this model, a centralized spectrum broker owns a part of the spectrum and issues dynamic spectrum leases to competing base stations in the region it controls. We consider a dynamic auction based approach where the base stations bid for channels depending on their demands. The broker allocates channels to them with an objective to maximize the overall revenue generated subject to wireless interference in the network. This problem is known to be NP-hard and has been addressed before in limited context. We address this problem in a very generic context where (i) interference in the network is modeled using pairwise and physical interference models and (ii) base stations can bid for heterogeneous channels of different width using generic bidding functions. We propose efficient approximation algorithms that give near optimal solutions with provable analytical bounds. Detailed simulation studies using randomly generated and real base station networks show that our algorithms scale very well for large network sizes.

Addressing deafness and hidden terminal problem in directional antenna based wireless multi-hop networks

We address deafness and directional hidden terminal problem that occur when MAC protocols are designed for directional antenna based wireless multi-hop networks. Deafness occurs when the transmitter fails to communicate to its intended receiver, because the receiver’s antenna is oriented in a different direction. The directional hidden terminal problem occurs when the transmitter fails to hear a prior RTS/CTS exchange between another pair of nodes and cause collision by initiating a transmission to the receiver of the ongoing communication. Though directional antennas offer better spatial reuse, these problems can have a serious impact on network performance. In this paper, we study various scenarios in which these problems can occur and design a MAC protocol that solves them comprehensively using only a single channel and single radio interface. Current solutions in literature either do not address these issues comprehensively or use more than one radio/channel to solve them. We evaluate our protocol using detailed simulation studies. Simulation results indicate that our protocol can effectively address deafness and directional hidden terminal problem and increase network performance.

Experimental characterization of sectorized antennas in dense 802.11 wireless mesh networks

Sectorized antennas can increase wireless network capacity through greater spatial reuse. Despite their increasing popularity, their real-world performance characteristics in dense wireless mesh networks are not well understood. This paper conducts a systematic experimental study on a mesh network testbed using commodity 802.11 hardware and multi-sector antennas. Our study results in the following main observations. (i) Sector selection should be based on explicit measurement in all sectors, though the measurement overhead can be significantly reduced by exploiting spatio-temporal characteristics of the best sector. (ii) Multi-sector activation typically reduces the signal strength of a link compared to single sector activations due to antenna design constraints. (iii) Spatial reuse is constrained by characteristics of antenna radiation pattern in different sectors (iv) Physical layer capture reduces the effect of directional hidden terminal problem. Finally, we discuss the implications of these observations on the design of practical directional MAC and topology control protocols.

DAL: A Distributed Localization in Sensor Networks Using Local Angle Measurement

We study the localization problem in sensor net- works by using local angle measurement. Localization using local angle information was recently proposed as an effective localization technique, which can be used for geographical rout- ing with guaranteed delivery. However, the existing approach is based on linear programming (LP) and can not be implemented distributedly. We propose, design, and evaluate DAL: a purely distributed localization protocol in sensor networks using local angle measurement. Localization with local angle poses unique challenge in sensor networks due to information uncertain- ties identified in this paper. DAL specifically addresses these challenges. Via extensive simulations using ns2 and our own simulator, we show that the performance of DAL is comparable with that of the centralized LP approach in most cases. Our preliminary results with noisy angle measurement show that DAL keeps the global geometry of the sensor network fairly well. its neighbors, thus the localization problem could be made easier. Local angles between adjacent edges can be measured by using multiple ultrasound receivers (17) or by using directional antennas (9, 20). In our work, we assume that each node is equipped with such antenna array so that it can measure the local angle between two edges of itself and two neighbor nodes. Localization using local angle information was recently proposed as an effective localization technique, which can be used for geographical routing with guaranteed delivery (5). However, the existing approach is based on linear pro- gramming and can not be easily implemented in a distributed manner. In this paper, we propose, design and evaluate DAL: aD istributed, local Angle-based Localization protocol in sensor networks. Since the nodes only measure local angles, our approach does not rely on any pre-defined orientation. Anchor-free local angle-based localization does not yield ground truth coordinates. It is subject to not only scaling, but also global translation and rotation. We show that with only one anchor node, DAL can overcome these limitations and give absolute coordinates for the sensor nodes. Besides, in DAL, we identify some other information uncertainties, which pose unique challenge to localization in sensor net- works, and show how DAL addresses these issues. DAL consists of several stages and thus some synchro- nization is needed. In each stage, however, all the nodes communicate with each other simultaneously (using node IDs and other simple information), therefore preventing the error estimation from accumulating and propagating into a global scope. For the noisy angle measurement, we propose modified mass string optimization technique in DAL. Via extensive simulations using both ns2 simulator and our own simulator, we show DAL performance is comparable with the existing linear programming approach in most cases.

Multi-sector antenna performance in dense wireless networks

Sectorized antennas provide an attractive solution to increase wireless network capacity through higher spatial reuse. Despite their increasing popularity, the real-world performance characteristics of such antennas in dense wireless mesh networks are not well understood. In this demo, we demonstrate our multi-sector antenna prototypes and their performance through video streaming over an indoor wireless network in the presence of interfering nodes. We use our graphical tool to vary the sender, receiver, and interferer antenna configurations and the resulting performance is directly visible in the video quality displayed at the receiver.

Interference mitigation in WiFi networks using multi-sector antennas

Sectorized antennas provide an attractive solution to increase wireless network capacity through interference mitigation. Despite their increasing popularity, the real-world performance characteristics of such antennas in dense wireless mesh networks are not well understood. We demonstrate our multi-sector antenna prototypes and their performance through video streaming over an indoor wireless network in the presence of interfering nodes. We use our graphical tool to vary the sender, receiver, and interferer antenna configurations and the resulting performance is directly visible in the video quality displayed at the receiver.