Arnab Chakrabarti

Using predictable observer mobility for power efficient design of sensor networks

In this paper, we explore a novel avenue of saving power in sensor networks based on predictable mobility of the observer (or data sink). Predictable mobility is a good model for public transportation vehicles (buses, shuttles and trains), which can act as mobile observers in wide area sensor networks. To understand the gains due to predictable mobility, we model the data collection process as a queuing system, where random arrivals model randomness in the spatial distribution of sensors. Using the queuing model, we analyze the success in data collection, and quantify the power consumption of the network. Even though the modeling is performed for a network which uses only single hop communication, we show that the power savings over a static sensor network are significant. Finally, we present a simple observer-driven communication protocol, which follows naturally from the problem formulation and can be used to achieve the predicted power savings.

Low density parity check codes for the relay channel

We propose Low Density Parity Check (LDPC) code designs for the half-duplex relay channel. Our designs are based on the information theoretic random coding scheme for decode-and-forward relaying. The source transmission is decoded with the help of side information in the form of additional parity bits from the relay. We derive the exact relationships that the component LDPC code profiles in the relay coding scheme must satisfy. These relationships act as constraints for the density evolution algorithm which is used to search for good relay code profiles. To speed up optimization, we outline a Gaussian approximation of density evolution for the relay channel. The asymptotic noise thresholds of the discovered relay code profiles are a fraction of a decibel away from the achievable lower bound for decode-and-forward relaying. With random component LDPC codes, the overall relay coding scheme performs within 1.2 dB of the theoretical limit.

Communication power optimization in a sensor network with a path-constrained mobile observer

We present a procedure for communication power optimization in a network of randomly distributed sensors with an observer (data collector) moving on a fixed path. The key challenge in using a mobile observer is that it remains within communication range of any sensor for a brief duration, and inability to transfer data in this duration leads to data loss. We establish that the process of data collection can be modeled by a queue with deadlines, where arrivals correspond to the observer entering the range of a sensor and a missed deadline means data loss. The queuing model is then used to identify the combination of system parameters that ensures adequate data collection with minimum power. The results obtained from the queuing analogy take a simple form in the asymptotic regime of dense sensor networks. Additionally, for sensor networks that cannot tolerate data loss, we derive a tight bound on minimum sensor separation that ensures that no data will be lost on account of mobility. We present two examples to illustrate our results, from which it is seen that power reduction by two orders of magnitude or more is typical relative to a static sensor network. The scenarios chosen for power comparisons also provide guidelines on the choice of path, if such a choice is available.

Half-Duplex Estimate-and-Forward Relaying: Bounds and Code Design

We propose a practical coding scheme for half-duplex estimate-and-forward relaying. The proposed construction is guided by the information theoretic coding scheme for the estimate-and-forward relay protocol. Our construction incorporates several design features to reduce receiver complexity without compromising performance. Observing that the relaying gain is significant only at low SNRs, we use binary LDPC codes in the source broadcast phase of relaying. Estimation is performed by entropy constrained scalar quantization of the received signal at the relay. Finally, a procedure similar to maximal ratio combining is used to aggregate direct and relayed signals at the destination. An important practical advantage of our scheme is that it does not require source-relay symbol synchronization. The codes outperform direct and two-hop channel capacities, as well as decode-and-forward relaying when the relay-destination link is strong

Sensitivity of achievable rates for half-duplex relay channel

This paper investigates the sensitivity of achievable rates for the half-duplex AWGN relay channel to the choice of modulation, receiving time fraction, and source-relay correlation. Our study is motivated by practical considerations that limit the choice of parameters in a real relay network. We show that BPSK modulation suffices to achieve much of the relaying throughput in the SNR range where relaying is most advantageous. We also show that smartly choosing either completely correlated or uncorrelated signals incurs negligible loss in rate compared to optimally correlated signals. Furthermore, the achievable rate is not diminished significantly if we use equal time slots for relay transmission and reception, instead of using the optimum time ratio. Finally, we demonstrate that even when time and correlation are both restricted, relaying with appropriate power control still carries substantial gains over direct and two-hop channels.

Multi-hop communication is order-optimal for homogeneous sensor networks

The main goal of this paper is to show that multi-hop single-user communication achieves the per node transport capacity of /spl Theta/((ln N)/N) in homogeneous sensor networks, making it order-optimal. Our contributions in this paper are three-fold. First, we construct a route-discovery and scheduling scheme based on spatial TDMA for sensor networks. Second, we show that our schedule achieves a per node transport capacity of /spl Theta/ ((ln N)/N), the same as that achievable by beamforming. Third, we compare multi-hop communication and beamforming based methods in terms of the network power consumption required to attain a fixed throughput. Based on our power calculations, we conclude that if the channel attenuation is above a certain threshold (which we calculate), then multi-hop communication performs better, whereas below the threshold, beamforming is preferable.

A Systematic Construction of LDPC Codes for Relay Channel in Time-Division mode

We address the problem of low density parity-check (LDPC) code design for relay channel in time-division mode based on a distributed strategy: In the first time slot, the source transmits part of the codeword. The relay and the destination receive it but only the relay can decode it. In the second time slot, the relay transmits the additional redundant bits to the destination. The destination is able to decode the transmitted codeword based on the data received in both time slots. The asymptotic performance of the LDPC codes that we designed are as close as 0.6 decibel from the theoretical limit.

Codes for Half Duplex Relay Channels

In this paper, we study LDPC code constructions for the half-duplex relay channels. Our work is motivated by the observation that half-duplex relaying where the relay cannot simultaneously transmit and receive in the same band yields substantially higher rate than both direct and two-hop communication with the same average power constraint. The profile optimization technique and the resulting LDPC codes prove to be very effective in fading environments. The performance thresholds determined using Gaussian approximation are extremely close to theoretical limits