R-3 Repository :: Browsing by Author "Chakrabarti, Arnab"

Browsing by Author "Chakrabarti, Arnab"

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Code design for the relay channel
(2007) Chakrabarti, Arnab; Aazhang, Behnaam
The design of wireless communication networks is based on the premise that networks are collections of reliable point-to-point links between communicating nodes. Such a communication model, although simple, is inefficient because wireless propagation is not point-to-point in nature. It is more efficient for nodes to share their resources by cooperating at the symbol level to forward each other's physical layer packets collectively to the destination. The improved paradigm is known as cooperative communication. My work develops practical coding techniques that approach the theoretical limits of cooperation predicted by information theory. Cooperative strategies, often called relay protocols, describe the processing performed by the information forwarding (relaying) nodes. My research proposes implementable schemes for two relay protocols - decode-and-forward and estimate-and-forward. In each case, the starting point is an optimal but non-constructive information theoretic random coding scheme, which motivates a practical code construction. Novel code design princi ples and some surprising insights emerge from this work of research. The performance of the each scheme developed here is found to approach theoretical limits. For decode-and-forward relaying, we propose dual-rate low-density parity-check (LDPC) codes. In our designs, the source transmission is decoded with the help of side information in the form of additional parity bits from the relay. The key challenge is to discover codes that simultaneously perform well for the source-relay and the source-destination links. 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. In estimate-and-forward relaying, the key challenge is to form a quantized estimate of the source transmission from the received signal at the relay. We illustrate with an example that the existing approach of distortion minimization at the relay is suboptimal. We derive an improved quantizer design criterion based on rate-constrained mutual information maximization between the source transmission and the quantizer output, using which, we obtain performance less than 0.9 dB from the achievable rate at a BER of 10-4.
COOPERATIVE COMMUNICATIONS: Fundamental Limits and Practical Implementation
(2006-01-01) Chakrabarti, Arnab; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
This chapter summarizes theoretically achievable gains and the construction of practical codes for user-cooperation. Most of these results relate to the relay channel, which is a three-terminal channel that captures the essence of usercooperation and serves as one of the primary building blocks for cooperation on a larger scale. In investigating the fundamental limits of relaying, we present information-theoretic results on the achievable throughput of relay channel in mutual-information terms. We also include results on Gaussian channels, and for the practically important case of half-duplex relaying. In the domain of relay coding, we specifically discuss pragmatic code constructions for half as well as full-duplex relaying, using LDPC codes as components.
Data Collection by a Mobile Observer in a Single-hop Sensor Network
(2005-01-15) Chakrabarti, Arnab; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
In this paper, we demonstrate significant reduction in communication power by using a mobile observer to collect data from a wireless single-hop network of static sensors. The key challenge in using a mobile observer is that it remains within range of any sensor for a limited period, and inability to transfer data in this period causes data loss. This prompts us to choose the fraction of nodes successful in communicating data as our performance constraint as we try to minimize network power. First, we establish that the process of data collection by a mobile observer is analogous to a queue. The queuing-based data collection model is then used as a design tool to identify the combination of system parameters that satisfies the performance metric with minimum power. Second, we derive a sufficient condition on node placement for zero data loss in a mobile observer network. As our third contribution, we demonstrate that in a dense network, the data collection analysis is much simpler, which allows us to obtain closed expressions for the network power. Power comparisons with static observer networks show that power reduction by multiple orders of magnitude is typical. The scenarios chosen for power comparisons also provide guidelines on the choice of path, if such a choice is available. As an example, a disc shaped network is analyzed and among the set of concentric circular paths, the best path is identified.
Exploiting Predictable Observer Mobility for Power Efficient Sensor Network Communication
(4/20/2003) Chakrabarti, Arnab; Center for Multimedia Communications
This thesis introduces techniques that exploit predictable mobility of the observer (data sink) to reduce communication range, thereby saving power in wireless sensor networks (WSNs). The movement of public transport vehicles is predictable, and such vehicles can act as mobile observers in wide area WSNs for applications such as pollution monitoring. To understand the gains due to mobility in single hop WSNs, data collection is modelled as a queuing process, with random arrivals representing randomness in the spatial distribution of sensors. This queuing model aids in ana- lyzing the success in data collection and in quantifying network power consumption. A communication protocol is proposed which achieves the predicted power savings. In multihop WSNs, mobility reduces the expected number of hops from sensors to the observer, leading to significant power savings. In addition, having dedicated high data-rate relays near the path of the observer reduces power by orders of magnitude.
Exploiting predictable observer mobility for power-efficient sensor network communication
(2003) Chakrabarti, Arnab; Aazhang, Behnaam
This thesis introduces techniques that exploit predictable mobility of the observer (data sink) to reduce communication range, thereby saving power in wireless sensor networks (WSNs). The movement of public transport vehicles is predictable, and such vehicles can act as mobile observers in wide area WSNs for applications such as pollution monitoring. To understand the gains due to mobility in single hop WSNs, data collection is modelled as a queuing process, with random arrivals representing randomness in the spatial distribution of sensors. This queuing model aids in analyzing the success in data collection and in quantifying network power consumption. A communication protocol is proposed which achieves the predicted power savings. In multihop WSNs, mobility reduces the expected number of hops from sensors to the observer, leading to significant power savings. In addition, having dedicated high data-rate relays near the path of the observer reduces power by orders of magnitude.
Half-Duplex Estimate-and-Forward Relaying: Bounds and Code Design
(2006-07-01) Chakrabarti, Arnab; de Baynast, Alexandre; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
We propose a coding scheme for half-duplex estimate-and-forward relaying. Our first contribution is to present an achievable rate for the estimate-and-forward protocol. We also derive the achievable rate for the special case of a Gaussian half-duplex relay channel. Guided by the information theoretic coding scheme for the aforementioned achievable rate, we propose a practical coding scheme. The proposed coding scheme incorporates several design choices to reduce receiver complexity without compromising performance. Binary LDPC codes are used in the source broadcast phase. Estimation is performed by 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 symbol level source-relay synchronization. The codes outperform direct and two-hop channel capacities, as well as decode-and-forward relaying when the relay-destination link is strong.
Half-Duplex Estimate-and-Forward Relaying:Bounds and Code Design
(2006-01-01) Chakrabarti, Arnab; de Baynast, Alexandre; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
We propose a coding scheme for half-duplex estimate-and-forward relaying. Our first contribution is to present an achievable rate for the estimate-and-forward protocol. We also derive the achievable rate for the special case of a Gaussian half-duplex relay channel. Guided by the information theoretic coding scheme for the aforementioned achievable rate, we propose a practical coding scheme. The proposed coding scheme incorporates several design choices to reduce receiver complexity without compromising performance. Binary LDPC codes are used in the source broadcast phase. Estimation is performed by 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 symbol level source-relay synchronization. The codes outperform direct and two-hop channel capacities, as well as decode-and-forward relaying when the relay-destination link is strong.
LDPC Code Design for Half-Duplex Decode-and-Forward Relaying
(2005-09-01) Chakrabarti, Arnab; de Baynast, Alexandre; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
We propose LDPC code designs for the half-duplex relay channel. Our designs mimic the information theoretic random coding scheme for decode-and-forward relaying. An important advantage of our scheme is that it is built entirely of single-user codes that can be decoded by belief propagation. The optimization of relay LDPC code profiles presents unique challenges, which are met by using the density evolution algorithm with additional constraints for relaying. To speed up our optimization, we use a Gaussian approximation of density evolution that converts the infinite dimensional code profile optimization into a simple linear programming problem. The thresholds of the discovered relay code profiles are within 0.4 dB of the achievable lower bound for decode-and-forward relaying.
LDPC Code Design for Relay Channel in Time-Division mode
(2006-06-01) de Baynast, Alexandre; Chakrabarti, Arnab; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
We address the problem of 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.
LDPC Code-Design for the Relay Channel
(2006-06-01) Chakrabarti, Arnab; de Baynast, Alexandre; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
We propose LDPC code designs for the half-duplex relay channel. Our designs are derived from the information theoretic random coding scheme for decode-and-forward relaying. An important advantage of our scheme is that it is built entirely of single-user codes. The optimization of relay LDPC code profiles presents unique challenges, which are met by modifying the density evolution algorithm by introducing additional constraints for relaying. To speed up optimization, we use a Gaussian approximation of density evolution that converts the infinite dimensional code profile optimization into a simple linear programming problem. The thresholds of the discovered relay code profiles are a fraction of a dB from the achievable lower bound for decode-and-forward relaying.
LDPC Code-Design for the Relay Channel
(2006-02-01) Chakrabarti, Arnab; de Baynast, Alexandre; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
We propose LDPC code designs for the half-duplex relay channel. Our designs are derived from the information theoretic random coding scheme for decode-and-forward relaying. An important advantage of our scheme is that it is built entirely of single-user codes. The optimization of relay LDPC code profiles presents unique challenges, which are met by modifying the density evolution algorithm by introducing additional constraints for relaying. To speed up optimization, we use a Gaussian approximation of density evolution that converts the infinite dimensional code profile optimization into a simple linear programming problem. The thresholds of the discovered relay code profiles are a fraction of a dB from the achievable lower bound for decode-and-forward relaying.
MultiHop Communication is Order Optimal for Homogeneous Sensor Networks
(2004-04-01) Chakrabarti, Arnab; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
The main goal of this paper is to show that multi-hop single-user communication achieves the per node transport capacity of O(lnN/N) in homogeneous sensor networks, making it order-optimal. Our contributions in this paper are threefold. 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 O(lnN/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.
Sensitivity of Achievable Rates for Half-Duplex Relay Channel
(2005-06-01) Chakrabarti, Arnab; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
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.
Using Predictable Observer Mobility for Power Efficient Design of Sensor Networks
(2003-04-20) Chakrabarti, Arnab; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
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 ve- hicles (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 mod- eling is performed for a network which uses only single hop communi- cation, 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.
Wireless @ Wired Speeds
(2004-12-01) Steger, Christopher; Chakrabarti, Arnab; Center for Multimedia Communications (http://cmc.rice.edu/)
This is a poster from the first 100x100 project meeting. It outlines some basic concepts in MIMO algorithm and system design at a relatively high, accessible level.

Browsing by Author "Chakrabarti, Arnab"

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