Andrew Thangaraj

Interference in wireless networks results in interdependent communication links between the nodes. Therefore, cross-layer design is essential and makes optimization of wireless networks complicated. In this paper, we study the problem of maximizing the information flow for a multicast session over a wireless network. Different scheduling and coding strategies to handle the interference, including the commonly used interference avoidance strategy, are compared. Results in information theory on achievable rate regions for interference networks are incorporated in the flow optimization to achieve significant improvement. Numerical results illustrate that processing interference at the physical layer results in better information flow compared to interference avoidance. © 2011 IEEE.

The performance of iterative decoding of Low Density Parity Check (LDPC) codes over Binary Erasure Channels can be completely characterized by the study of stopping sets. Therefore, the burst erasure correction capability of a given LDPC code can be readily quantified by searching for stopping sets within consecutive bit nodes. In tills work we study the optimal permutation of the bit nodes that will result in the maximum possible burst erasure correction capability for a given LDPC code. Noting that this is essentially a combinatorial optimization problem that is highly likely to be NP-hard, we adopt a simulated annealing based approach for finding the optimal permutation. We present bounds based on stopping sets that limit the burst erasure correction capability. As part of our results, we provide interleavers that greatly improve the burst erasure correction capability of protograph quasi-cyclic LDPC codes used in the WiMax standard. © 2008 IEEE.

We consider a large class of bidirectional relaying scenarios with physical layer network coding, and analytically characterize the relay's error performance in decoding the network-coded combination at high signal-to-noise ratio (SNR). Our analysis applies to scenarios with 1) binary or higher order real/complex modulation, 2) real or complex channel coefficients, and 3) linear or non-linear network maps for network coding at the relay. We consider block fading and allow the relay to choose from a set of network maps based on the channel coefficients of the source to relay links in every block. We derive expressions for pairwise error probability and approximate expected overall error probability. We also derive lower bounds for these error probabilities. We validate these expressions using simulations and show that our approximations are tight in the high SNR regime.

One of the key technologies in next generation systems for achieving high throughput and providing better coverage is relaying. Relaying has attracted a high level of recent research interest with several papers focusing on various aspects of communicating using relays with different constraints and assumptions. In this work, we are concerned with the capacity of multistage relaying from one source to one destination through an arbitrary network of half duplex relays. © 2010 IEEE.

We consider two-way relaying in a Gaussian diamond channel, where two terminal nodes wish to exchange information using two relays. A simple baseline protocol is obtained by time-sharing between two one-way protocols. To improve upon the baseline performance, we propose two compute-And-forward (CF) protocols -Compute-And-forward-Compound multiple access channel (CF-CMAC) and Compute-And-forward-Broadcast (CF-BC). These protocols mix the two flows through the two relays and achieve rates better than the simple time-sharing protocol. We derive an outer bound to the capacity region that is satisfied by any relaying protocol, and observe that the proposed protocols provide rates close to the outer bound in certain channel conditions. Both the CF-CMAC and CF-BC protocols use nested lattice codes in the compute phases. In the CF-CMAC protocol, both relays simultaneously forward to the destinations over a Compound Multiple Access Channel (CMAC). In the simpler CF-BC protocol's forward phase, one relay is selected at a time for Broadcast Channel (BC) transmission depending on the rate-pair to be achieved. We also consider the diamond channel with direct source-destination link and the diamond channel with interfering relays. Outer bounds and achievable rate regions are compared for these two channels as well. Mixing of flows using the CF-CMAC protocol is shown to be good for symmetric two-way rates. © 2013 IEEE.

Dirty paper coding (DPC) refers to methods for pre-subtraction of known interference at the transmitter of a multiuser communication system. There are numerous applications for DPC, including coding for broadcast channels. Recently, lattice-based coding techniques have provided several designs for DPC. In lattice-based DPC, there are two codes - a convolutional code that defines a lattice used for shaping and an error correction code used for channel coding. Several specific designs have been reported in the recent literature using convolutional and graph-based codes for capacity-approaching shaping and coding gains. In most of the reported designs, either the encoder works on a joint trellis of shaping and channel codes or the decoder requires iterations between the shaping and channel decoders. This results in high complexity of implementation. In this work, we present a lattice-based DPC scheme that provides good shaping and coding gains with moderate complexity at both the encoder and the decoder. We use a convolutional code for sign-bit shaping, and a low-density parity check (LDPC) code for channel coding. The crucial idea is the introduction of a one-codeword delay and careful parsing of the bits at the transmitter, which enables an LDPC decoder to be run first at the receiver. This provides gains without the need for iterations between the shaping and channel decoders. Simulation results confirm that at high rates the proposed DPC method performs close to capacity with moderate complexity. As an application of the proposed DPC method, we show a design for superposition coding that provides rates better than time-sharing over a Gaussian broadcast channel. © 2010 IEEE.

We study the use of Low-Density Parity-Check (LDPC) codes for two-phase, network-coded bidirectional relaying with higher-order modulation. In the multiple-access phase, the sum of transmitted symbols scaled by the channel gains is the received relay constellation, which is network-mapped (clustered) to a transmit constellation for the ensuing broadcast phase. This operation at the relay is termed Clustered-Scaled-Sum (CSS) decoding. We propose a CSS coding scheme for bidirectional relaying using a single LDPC code over a ring with higher-order PAM or QAM alphabets. We design a message-passing decoder for CSS decoding with trade-offs possible between complexity and performance. We suggest a method for completing a Constrained Partially-filled Latin Square (CPLS) to a latin square, which is used in the construction of network maps at the relay for any channel fading state. The performance of the CSS coding scheme with LDPC codes over rings is shown to be very close to information-theoretic outer bounds.

Low Density Parity Check (LDPC) codes have been proven to perform very close to the Shannon limit with low complexity encoders and decoders. The experimental proof of existence of codes that achieve a rate 0.0045 dB from the Shannon-capacity for Binary Input Additive White Gaussian Noise Channels (BIAWGN) is a big impetus for exploring similar codes and encoding-decoding schemes for other channels. In this work, our aim is to make fundamental comparisons between the performances of LDPC codes on an Inter-Symbol Interference (ISI) channel under two competing equalization methods - the time-domain BCJR algorithm and the frequency-domain Orthogonal Frequency Division Multiplexing (OFDM). Thresholds for LDPC codes with the BCJR algorithm have been derived and proved in prior work by Kavcic et al. In this paper, we study thresholds for LDPC codes over an OFDM system. We develop a rigorous density evolution method (without Gaussian approximations) to prove the existence of thresholds for LDPC codes over OFDM and evaluate the thresholds for various regular LDPC codes. We compare the OFDM thresholds with BCJR thresholds and draw some useful conclusions for code design.

In this article, we study coding strategies and code optimization for bidirectional relaying using Low-Density Parity-Check codes. We attempt to achieve extreme points in the rate region using density-evolution-based optimization and a combination of nesting and shortening of codes. The proposed method with specific choice of codes achieves rates close to capacity outer bounds. © 2014 IEEE.

We consider a three-node half-duplex Gaussian relay network where two nodes (say a, b) want to communicate with each other and the third node acts as a relay for this two-way communication. Outer bounds and achievable rate regions for the possible rate pairs (Ra, Rb) for two-way communication are investigated. The modes (transmit or receive) of the half-duplex nodes together specify the state of the network. A relaying protocol uses a specific sequence of states and a coding scheme for each state. In this paper, we first obtain an outer bound for the rate region of all achievable (Ra, Rb) based on the half-duplex cut-set bound. This outer bound can be numerically computed by solving a linear program. It is proved that at any point on the boundary of the outer bound only four of the six states of the network are used. We then compare it with achievable rate regions of various known protocols. We consider two kinds of protocols: (1) protocols in which all messages transmitted in a state are decoded with the received signal in the same state, and (2) protocols where information received in one state can also be stored and used as side information to decode messages in future states. Various conclusions are drawn on the importance of using all states, use of side information, and the choice of processing at the relay. Then, two analytical outer bounds (as opposed to an optimization problem formulation) are derived. Using an analytical outer bound, we obtain the symmetric capacity within 0.5 bits for some channel conditions where the direct link between nodes a and b is weak. © 2012 IEEE.