Badri N. Vellambi

Rateless Codes With Unequal Error Protection Property

In this correspondence, a generalization of rateless codes is proposed. The proposed codes provide unequal error protection (UEP). The asymptotic properties of these codes under the iterative decoding are investigated. Moreover, upper and lower bounds on maximum-likelihood (ML) decoding error probabilities of finite-length LT and Raptor codes for both equal and unequal error protection schemes are derived. Further, our work is verified with simulations. Simulation results indicate that the proposed codes provide desirable UEP. We also note that the UEP property does not impose a considerable drawback on the overall performance of the codes. Moreover, we discuss that the proposed codes can provide unequal recovery time (URT). This means that given a target bit error rate, different parts of information bits can be decoded after receiving different amounts of encoded bits. This implies that the information bits can be recovered in a progressive manner. This URT property may be used for sequential data recovery in video/audio streaming

Finite-length rate-compatible LDPC codes: a novel puncturing scheme - [transactions letters]

In this paper, we study rate-compatible puncturing of finite-length low-density parity-check (LDPC) codes. We present a novel rate-compatible puncturing scheme that is easy to implement. Our scheme uses the idea that the degradation in performance is reduced by selecting a puncturing pattern wherein the punctured bits are far apart from each other in the Tanner graph of the code. Although the puncturing scheme presented is tailored to regular codes, it is also directly applicable to irregular parent ensembles. By simulations, the proposed rate-compatible puncturing scheme is shown to be superior to the existing puncturing methods for both regular and irregular LDPC codes over the binary erasure channel (BEC) and the additive white Gaussian noise (AWGN) channel.

Reliable and efficient message delivery in delay tolerant networks using rateless codes

In this paper, we consider the problem of multiple unicast message delivery in Delay Tolerant Networks (DTNs). Long delays, mobility of nodes, and lack of connectivity that are characteristics of such network make this problem very challenging. Additionally, expiry of packets in a network, considered a useful means of regulating resource consumption, reduces reliability and increases the latency of message delivery. Traditional approaches to message delivery in such networks was based on transmitting multiple copies of entire message blocks. Recently, the application of simple erasure-based codes to messages were considered. This option opened up an interesting area of research. In this paper, we effect reliable message delivery with improved latency even in the presence of packet expiry and intermittent connectivity by applying rateless codes at the source where a message is generated. We perform extensive simulations on a variety of trace data from mobility models such as the UMassDieselNet testbed, an area-based random waypoint model, and a simple campus bus model. Results reveal the superiority of our scheme in comparison to other present schemes.

An improved decoding algorithm for low-density parity-check codes over the binary erasure channel

This paper presents a new improved decoding algorithm for low-density parity-check (LDPC) codes over the binary erasure channel (BEC). The proposed algorithm combines the fact that a considerable fraction of unsatisfied check nodes are of degree two with the concept of guessing bits to perform simple graph-theoretic manipulations on the Tanner graph. The proposed decoding algorithm has a complexity similar to present improved decoding algorithms [H. Pishro-Nik et al., 2004]. Simulations of codes of very short lengths over BEC reveal the superiority of our algorithm over present improved decoding algorithms for a wide range of bit error rates.

Throughput and Latency in Finite-Buffer Line Networks

This work investigates the effect of finite buffer sizes on the throughput capacity and packet delay of line networks with packet erasure links that have perfect feedback. These performance measures are shown to be linked to the stationary distribution of an underlying irreducible Markov chain that models the system exactly. Using simple strategies, bounds on the throughput capacity are derived. The work then presents two iterative schemes to approximate the steady-state distribution of node occupancies by decoupling the chain to smaller queueing blocks. These approximate solutions are used to understand the effect of buffer sizes on throughput capacity and the distribution of packet delay. Using the exact modeling for line networks, it is shown that the throughput capacity is unaltered in the absence of hop-by-hop feedback provided packet-level network coding is allowed. Finally, using simulations, it is confirmed that the proposed framework yields accurate estimates of the throughput capacity and delay distribution and captures the vital trends and tradeoffs in these networks.

Polynomial Phase Estimation by Least Squares Phase Unwrapping

Estimating the coefficients of a noisy polynomial phase signal is important in fields including radar, biology and radio communications. One approach attempts to perform polynomial regression on the phase of the signal. This is complicated by the fact that the phase is wrapped modulo 2π and must be unwrapped before regression can be performed. In this paper, we consider an estimator that performs phase unwrapping in a least squares manner. We call this the least squares unwrapping (LSU) estimator. The LSU estimator can be computed in a reasonable amount of time for data sets of moderate size using existing general purpose algorithms from algebraic number theory. Under mild conditions on the distribution of the noise we describe the asymptotic properties of this estimator, showing that it is strongly consistent and asymptotically normally distributed. A key feature is that the LSU estimator is accurate over a far wider range of parameters than many popular existing estimators. Monte-Carlo simulations support our theoretical results and demonstrate the excellent statistical performance of the LSU estimator when compared with existing state-of-the-art estimators.

Indoor Position Tracking Using Multiple Optical Receivers

An indoor positioning system is a key component in enabling location-based services in future wireless networks. The need for a highly accurate indoor positioning system is rapidly increasing. In the past couple of years, several positioning systems based on visible light communications that achieve good positioning accuracy have been proposed. Some of these systems are based on assumptions such as complete knowledge of the height of the receiver, and exact alignment of the transmitter and receiver normals to the normal of the ceiling. Some other systems do not support user mobility because they require a user to vary the receiver orientation at a fixed location. Another common assumption is that the transmitters are at the same height from the ground. In order to support user mobility, this paper proposes a novel positioning system using multiple optical receivers that provides coordinates and an orientation of the mobile receiver. The remarkable features of the system are as follows: 1) the receiver can be mobile; 2) the positioning is done within 2.5 ms in our experimental setup; 3) the heights of the transmitters need not be the same; 4) the receivers height need not be known; and 5) the receivers normal need not be aligned with those of the transmitters. Experimental results show that mean position errors of less than 0.06 m is achievable even when the average receiver speed is 1.3 m/s.

Results on the Improved Decoding Algorithm for Low-Density Parity-Check Codes Over the Binary Erasure Channel

In this correspondence, we first investigate some analytical aspects of the recently proposed improved decoding algorithm for low-density parity-check (LDPC) codes over the binary erasure channel (BEC). We derive a necessary and sufficient condition for the improved decoding algorithm to successfully complete decoding when the decoder is initialized to guess a predetermined number of guesses after the standard message-passing terminates at a stopping set. Furthermore, we present improved bounds on the number of bits to be guessed for successful completion of the decoding process when a stopping set is encountered. Under suitable conditions, we derive a lower bound on the number of iterations to be performed for complete decoding of the stopping set. We then present a superior, novel improved decoding algorithm for LDPC codes over the binary erasure channel (BEC). The proposed algorithm combines the observation that a considerable fraction of unsatisfied check nodes in the neighborhood of a stopping set are of degree two, and the concept of guessing bits to perform simple and intuitive graph-theoretic manipulations on the Tanner graph. The proposed decoding algorithm has a complexity similar to previous improved decoding algorithms. Finally, we present simulation results of short-length codes over BEC that demonstrate the superiority of our algorithm over previous improved decoding algorithms for a wide range of bit error rates

Indoor localization using visible light and accelerometer

Indoor positioning has attracted a lot of attention in the literature. Positioning systems using the existing wireless network have low deployment cost but the position error can be up to several meters. Some proposed systems have low position error; however, they require extra hardware, thereby resulting in high deployment costs. In this work, we propose a positioning system that offers low position error at a low deployment cost. The proposed system uses visible light communications (VLC) together with the accelerometers in mobile devices. In contrast to existing works on VLC for positioning, our system neither requires the knowledge of the height of the receiver from the ground, nor does it require the receiver to be oriented such that the angle of irradiance of a transmitter equals the incidence angle at the receiver. The proposed system has low complexity, and simulation results show that it achieves position errors of less than 0.5 meter.

CRBcast: a reliable and energy-efficient broadcast scheme for wireless sensor networks using rateless codes

This paper introduces a novel two-phase broadcast scheme referred to as collaborative rateless broadcast (CRBcast). CRBcast is a scalable approach for reliable and energy-efficient broadcasting in a multihop wireless sensor networks that also addresses load balancing, while requiring no knowledge of network topology. CRBcast combines the energy-efficiency offered by probabilistic broadcasting (PBcast) with the reliability features offered by application-layer rateless coding. In the first phase of CRBcast, packets encoded using a rateless code are dispersed into the network based on PBcast. In the second phase, simple collaboration of neighboring nodes ensures that all nodes recover original data with a very high probability of success. Since the performance of CRBcast rests heavily on that of PBcast, first part of this paper analyzes both analytically and via simulations the probabilistic broadcasting scheme. We then study the effectiveness of CRBcast. We show that CRBcast provides both reliability and energy efficiency simultaneously. Simulation results indicate that CRBcast provides an energy savings of at least 72% and 60% in comparison with flooding and PBcast, respectively.