Sunil Srinivasa

Distance Distributions in Finite Uniformly Random Networks: Theory and Applications

In wireless networks, knowledge of internode distances is essential for performance analysis and protocol design. When determining distance distributions in random networks, the underlying nodal arrangement is almost universally taken to be a stationary Poisson point process. While this may be a good approximation in some cases, there are also certain shortcomings to this model, such as the fact that, in practical networks, the number of nodes in disjoint areas is not independent. This paper considers a more-realistic network model where a known and fixed number of nodes are independently distributed in a given region and characterizes the distribution of the Euclidean internode distances. The key finding is that, when the nodes are uniformly randomly placed inside a ball of arbitrary dimensions, the probability density function (pdf) of the internode distances follows a generalized beta distribution. This result is applied to study wireless network characteristics such as energy consumption, interference, outage, and connectivity.

Superposition Coding Strategies: Design and Experimental Evaluation

We design and implement a software-radio system for Superposition Coding (SC), a multiuser transmission scheme that deliberately introduces interference among user signals at the transmitter, using a library of off-the-shelf point-to-point channel codes. We experimentally determine the set of rate-pairs achieved by this transmission scheme under a packet-error constraint. Our results suggest that SC can provide substantial gains in spectral efficiencies over those achieved by orthogonal schemes such as Time Division Multiplexing. Our findings also question the practical utility of the Gaussian approximation for the inter-user interference in Superposition-Coded systems.

Path loss exponent estimation in large wireless networks

In wireless channels, the path loss exponent (PLE) has a strong impact on the quality of the links, and hence, it needs to be accurately estimated for the efficient design and operation of wireless networks. This paper addresses the problem of PLE estimation in large wireless networks, which is relevant to several important issues in communications such as localization, energy-efficient routing, and channel access. We consider a large ad hoc network where nodes are distributed as a homogeneous planar Poisson point process and the channels are subject to Nakagami-m fading. Under these settings, we propose and study three distributed algorithms for estimating the PLE at each node, which explicitly take into account the interference in the network. Additionally, we provide simulation results to demonstrate the performance of the algorithms and quantify the estimation errors.

CREST: An Opportunistic Forwarding Protocol Based on Conditional Residual Time

Opportunistic forwarding protocols take advantage of contact opportunities to route data in intermittently connected environments. In these environments, a fully connected path between the source and destination may not always exist and the contact schedules of all the nodes are not known in advance. Hence, one of the key challenges for a node is to make effective forwarding decisions using only a limited knowledge of the contact behavior of the nodes in the network. Based on an analysis of human mobility traces that we collected from our office environment, we introduce a new link metric, conditional residual time, that accurately estimates the time remaining for a pair of nodes to meet using only the local knowledge of their past contacts. We then propose a distributed protocol called CREST, that uses the conditional residual time to opportunistically forward messages between pairs of nodes. Experimental results show that CREST has a lower end-to-end delay compared to protocols that depend on future contact schedules and global knowledge of the contact behavior across the network. Furthermore, by disseminating only a few additional copies of the message, the delivery ratio of CREST improves significantly and is comparable to that of the flooding protocol.

Implementation and Experimental Results of Superposition Coding on Software Radio

Superposition coding is a well-known capacity-achieving coding scheme for stochastically degraded broadcast channels. Although well-studied in theory, it is important to understand issues that arise when implementing this scheme in a practical setting. In this paper, we present a software-radio based design of a superposition coding system on the GNU Radio platform with the Universal Software Radio Peripheral acting as the transceiver frontend. We also study the packet error performance and discuss some issues that arise in its implementation.

A Statistical Mechanics-Based Framework to Analyze Ad Hoc Networks with Random Access

Characterizing the performance of ad hoc networks is one of the most intricate open challenges; conventional ideas based on information-theoretic techniques and inequalities have not yet been able to successfully tackle this problem in its generality. Motivated thus, we promote the totally asymmetric simple exclusion process (TASEP), a particle flow model in statistical mechanics, as a useful analytical tool to study ad hoc networks with random access. Employing the TASEP framework, we first investigate the average end-to-end delay and throughput performance of a linear multihop flow of packets. Additionally, we analytically derive the distribution of delays incurred by packets at each node, as well as the joint distributions of the delays across adjacent hops along the flow. We then consider more complex wireless network models comprising intersecting flows, and propose the partial mean-field approximation (PMFA), a method that helps tightly approximate the throughput performance of the system. We finally demonstrate via a simple example that the PMFA procedure is quite general in that it may be used to accurately evaluate the performance of ad hoc networks with arbitrary topologies.

Throughput-delay-reliability tradeoffs in multihop networks with random access

In ad hoc networks, performance objectives are often in contention with each other. Indeed, due to the transmission errors incurred over wireless channels, it is difficult to achieve a high rate of transmission in conjunction with reliable delivery of data and low latency. In order to obtain favorable throughput and delay performances, the system may choose to compromise on its reliability and have nodes forcibly dropping a small fraction of packets. The focus of this paper is on the characterization of tradeoffs between the achievable throughput, end-to-end delay and reliability in wireless networks with random access. We consider a multihop ad hoc network comprising several source-destination pairs communicating wirelessly via the slotted ALOHA channel access scheme. Employing ideas from statistical mechanics, we present an analytical framework for evaluating the throughput, end-to-end delay and reliability performances of the system. The main findings of this paper are (a) when the system is noise-limited, dropping a small fraction of packets in the network leads to a smaller end-to-end delay though the throughput suffers as well, and (b) when the system is interference-limited, however, there exist regimes where dropping a few packets in the network may actually reduce the end-to-end delay as well as increase the system throughput. We also present some empirical results which corroborate the results obtained analytically.

Combining stochastic geometry and statistical mechanics for the analysis and design of mesh networks

We consider a two-dimensional mesh network comprising several source-destination pairs, each communicating wirelessly in a multihop fashion. First, we introduce a novel transmission policy for multihop networks according to which all the buffering in the network is performed at source nodes while relays just have unit-sized buffers. We demonstrate that incorporating this buffering scheme in conjunction with minor amendments to the medium access control (MAC) layer yields several benefits such as keeping packet delays small and helping regulate the traffic flow in a completely distributed fashion. Second, we employ a novel combination of tools from stochastic geometry and statistical mechanics to characterize the throughput and end-to-end delay performances of multihop wireless networks for two different channel access mechanisms, Carrier Sense Multiple Access (CSMA) and ALOHA. Our study also offers valuable insights from a system design stand-point such as determining the optimum density of transmitters or the optimal number of hops along a flow that maximizes the systems throughput performance. We corroborate our theoretical analyses via simulations.

Scheduling using Superposition Coding: Design and Software Radio implementation

Cellular base stations typically orthogonalize downlink transmissions, although this approach is not always throughput-optimal. Indeed, it can be shown that removing the orthogonality constraint (as in Superposition Coding) can provide significant benefits in some scenarios. Based on this principle, we propose a scheduling algorithm for a two-user downlink that leverages the disparity in their respective channel qualities. By a judicious reallocation of the transmit power and bandwidth, this algorithm improves the throughput to both users vis-à-vis an orthogonal scheme. We design a software-defined radio platform to implement the scheduler and experimentally verify the gains promised by theory.

A practical approach to strengthen vulnerable downlinks using superposition coding

We propose and experimentally demonstrate a novel approach to improve the packet delivery efficiency on a vulnerable downlink (e.g., from a transmitter to a far-away receiver) using superposition coding, a multiuser transmission scheme that forgoes orthogonal transmission and deliberately introduces interference among signals at the transmitter. On a software radio platform that uses off-the-shelf point-to-point channel codes, we show that a transmitter serving multiple links can use simple two-user superposition codes to dramatically improve (compared to time division multiplexing) the packet delivery efficiency on its most vulnerable links. Interestingly, our results suggest that superposing signals of far-away users on to those of high-traffic users yields the maximum benefits - implying that the degrees-of-freedom gain in doing so can more than compensate for the increased interference from signal superposition.