Aditya Ramamoorthy

Mobile element scheduling for efficient data collection in wireless sensor networks with dynamic deadlines

Wireless networks have historically considered support for mobile elements as an extra overhead. However, recent research has provided means by which network can take advantage of mobile elements. Particularly, in the case of wireless sensor networks, mobile elements are deliberately built into the system to improve the lifetime of the network, and act as mechanical carriers of data. The mobile element, which is controlled, visits the nodes to collect their data before their buffers are full. It may happen that the sensor nodes are sampling at different rates, in which case some nodes need to be visited more frequently than others. We present this problem of scheduling the mobile element in the network, so that there is no data loss due to buffer overflow. We prove that the problem is NP-complete and give an ILP formulation. We give some practical algorithms, and compare their performances.

Mobile Element Scheduling with Dynamic Deadlines

Wireless networks have historically considered support for mobile elementss an extra overhead. However, recent research has provided the means by which a network can take advantage of mobile elements. Particularly in the case of wireless sensor networks, mobile elements can be deliberately built into the system to improve the lifetime of the network and act as mechanical carriers of data. The mobile element, whose mobility is controlled, visits the nodes to collect their data before their buffers are full. In general, the spatio-temporal dynamics of the sensed phenomenon may require sensor nodes to collect samples at different rates, in which case, some nodes need to be visited more frequently than others. This work formulates the problem of scheduling the mobile element in the network so that there is no data loss due to buffer overflow. The problem is shown to be NP-complete and an integer-linear-programming formulation is given. Finally, some computationally practical algorithms for a single mobile and for the case of multiple mobiles are presented and their performances compared

Recognition of dynamic hand gestures

This paper is concerned with the problem of recognition of dynamic hand gestures. We have considered gestures which are sequences of distinct hand poses. In these gestures hand poses can undergo motion and discrete changes. However, continuous deformations of the hand shapes are not permitted. We have developed a recognition engine which can reliably recognize these gestures despite individual variations. The engine also has the ability to detect start and end of gesture sequences in an automated fashion. The recognition strategy uses a combination of static shape recognition (performed using contour discriminant analysis), Kalman filter based hand tracking and a HMM based temporal characterization scheme. The system is fairly robust to background clutter and uses skin color for static shape recognition and tracking. A real time implementation on standard hardware is developed. Experimental results establish the effectiveness of the approach.

On the capacity of network coding for random networks

We study the maximum flow possible between a single-source and multiple terminals in a weighted random graph (modeling a wired network) and a weighted random geometric graph (modeling an ad-hoc wireless network) using network coding. For the weighted random graph model, we show that the network coding capacity concentrates around the expected number of nearest neighbors of the source and the terminals. Specifically, for a network with a single source, l terminals, and n relay nodes such that the link capacities between any two nodes is independent and identically distributed (i.i.d.) /spl sim/X, the maximum flow between the source and the terminals is approximately nE[X] with high probability. For the weighted random geometric graph model where two nodes are connected if they are within a certain distance of each other we show that with high probability the network coding capacity is greater than or equal to the expected number of nearest neighbors of the node with the least coverage area.

Separating distributed source coding from network coding

This correspondence considers the problem of distributed source coding of multiple sources over a network with multiple receivers. Each receiver seeks to reconstruct all of the original sources. The work by Ho et al. 2004 demonstrates that random network coding can solve this problem at the potentially high cost of jointly decoding the source and the network code. Motivated by complexity considerations we consider the performance of separate source and network codes. Previous work by Effros et al. 2003 demonstrates the failure of separation between source and network codes for nonmulticast networks. We demonstrate that failure for multicast networks. We study networks with capacity constraints on edges. It is shown that the problem with two sources and two receivers is always separable. Counterexamples are presented for other cases.

Communicating the sum of sources over a network

We consider a network (that is capable of network coding) with a set of sources and terminals, where each terminal is interested in recovering the sum of the sources. Considering directed acyclic graphs with unit capacity edges and independent, unit-entropy sources, we show the rate region when (a) there are two sources and n terminals, and (b) n sources and two terminals. In these cases as long as there exists at least one path from each source to each terminal we demonstrate that there exists a valid assignment of coding vectors to the edges such that the terminals can recover the sum of the sources.

Improved lower bounds for coded caching

Caching is often used in content delivery networks as a mechanism for reducing network traffic. Recently, the technique of coded caching was introduced whereby coding in the caches and coded transmission signals from the central server were considered. Prior results in this area demonstrate that carefully designing the placement of content in the caches and designing appropriate coded delivery signals from the server allow for a system where the delivery rates can be significantly smaller than conventional schemes. However, matching upper and lower bounds on the transmission rate have not yet been obtained. In this paper, we derive tighter lower bounds on the coded caching rate than were known previously. We demonstrate that this problem can equivalently be posed as a combinatorial problem of optimally labeling the leaves of a directed tree. Our proposed labeling algorithm allows for significantly improved lower bounds on the coded caching rate. Furthermore, we study certain structural properties of our algorithm that allow us to analytically quantify improvements on the rate lower bound for general values of the problem parameters. This allows us to obtain a multiplicative gap of at most four between the achievable rate and our lower bound.

The design of efficiently-encodable rate-compatible LDPC codes - [transactions papers]

We present a new class of irregular low-density parity-check (LDPC) codes for moderate block lengths (up to a few thousand bits) that are well-suited for rate-compatible puncturing. The proposed codes show good performance under puncturing over a wide range of rates and are suitable for usage in incremental redundancy hybrid-automatic repeat request (ARQ) systems. In addition, these codes are linear-time encodable with simple shift-register circuits. For a block length of 1200 bits the codes outperform optimized irregular LDPC codes and extended irregular repeat-accumulate (eIRA) codes for all puncturing rates 0.6~0.9 (base code performance is almost the same) and are particularly good at high puncturing rates where good puncturing performance has been previously difficult to achieve.

Design of Rate-Compatible Irregular LDPC Codes for Incremental Redundancy Hybrid ARQ Systems

We present a new class of irregular low-density parity-check (LDPC) codes for finite block length (up to a few thousand symbols). The proposed codes are efficiently encodable and have a simple rate-compatible puncturing structure which is suitable for incremental redundancy hybrid automatic repeat request (IR-HARQ) systems. The codes outperform optimized irregular LDPC codes and (extended) irregular repeat-accumulate codes for rates 0.67-0.94, and are particularly good at high puncturing rates where good puncturing performance has been previously difficult to achieve. These characteristics result in good throughput performance over time-varying channels in IR-HARQ systems

Construction of short block length irregular low-density parity-check codes

We present a construction algorithm for short block length irregular low-density parity-check (LDPC) codes. Based on a novel interpretation of stopping sets in terms of the parity-check matrix, we present an approximate trellis-based search algorithm that detects many stopping sets. Growing the parity check matrix by a combination of random generation and the trellis-based search, we obtain codes that possess error floors orders of magnitude below randomly constructed codes and significantly better than other comparable constructions.