Saurabha Rangrao Tavildar

FlashLinQ: a synchronous distributed scheduler for peer-to-peer ad hoc networks

This paper proposes FlashLinQ - a synchronous peer-to-peer wireless PHY/MAC network architecture for distributed channel allocation. By leveraging the fine-grained parallel channel access of OFDM, FlashLinQ develops an analog energy-level based signaling scheme that enables SIR (Signal to Interference Ratio) based distributed scheduling. This new signaling mechanism and the corresponding allocation algorithms permit efficient channel-aware spatial resource allocation, leading to significant gains over a CSMA/CA system with RTS/CTS. FlashLinQ is a complete system architecture including (i) timing and frequency synchronization derived from cellular spectrum, (ii) peer discovery, (iii) link management, and (iv) channelaware distributed power, data-rate and link scheduling. We implement FlashLinQ over licensed spectrum on a DSP/FPGA platform. In this paper, we present performance results for FlashLinQ using both implementation and simulations.

Approximately universal codes over slow-fading channels

Performance of reliable communication over a coherent slow-fading multiple-input multiple-output (MIMO) channel at high signal-to-noise ratio (SNR) is succinctly captured as a fundamental tradeoff between diversity and multiplexing gains. This paper studies the problem of designing codes that optimally tradeoff the diversity and multiplexing gains. The main contribution is a precise characterization of codes that are universally tradeoff-optimal, i.e., they optimally tradeoff the diversity and multiplexing gains for every statistical characterization of the fading channel. This characterization is referred to as approximate universality; the approximation is in the connection between error probability and outage capacity with diversity and multiplexing gains, respectively. The characterization of approximate universality is then used to construct new coding schemes as well as to show optimality of several schemes proposed in the space-time coding literature.

Rate Region of the Quadratic Gaussian Two-Encoder Source-Coding Problem

We determine the rate region of the quadratic Gaussian two-encoder source-coding problem with separate distortion constraints. This region is achieved by a simple architecture that separates the analog and digital aspects of the compression. Furthermore, this architecture requires higher rates to send a Gaussian source than it does to send any other source with the same covariance. The proof technique can be used to partially solve problems with more than two encoders or more general distortion constraints.

On the design of device-to-device autonomous discovery

This paper proposes a synchronous device discovery solution for ad-hoc networks based on the observations that time synchronization, along with an FDM based channel resource allocation, can lead to gains in terms of energy consumption, discovery range, and the number of devices discovered. These attributes are important for the success of proximity-aware networking, where devices autonomously find peer-groups over human mobility scales. In this paper, we develop the PHY and MAC protocols to enable autonomous device discovery. Using both simulations and stochastic-geometry based analysis, we validate our design, and argue that there can be significant gains over a conventional Wi-Fi based solution.

On the Sum-rate of the Vector Gaussian CEO Problem

We study the vector version of the Gaussian CEO problem. We present a lower bound on the sum-rate of any class of distributed quantizers that meet the quadratic distortion constraint, and show that it is tight in various situations including the symmetric one

Permutation codes: achieving the diversity-multiplexing tradeoff

This paper considers reliable communication over a parallel (correlated) fading channel for short periods of time. We derive a code design criterion by taking a compound channel viewpoint of the outage capacity of the channel. Motivated by the criterion, we show existence of simple codes that achieve the optimal diversity-multiplexing tradeoff curve, introduced recently in (Zheng, L et al., 2003), simultaneously for every correlated parallel channel. We demonstrate a code with simple encoding and decoding for a parallel channel with two diversity branches. The codes for the parallel channel can be used on a correlated MIMO channel by using the DBLAST architecture to simultaneously achieve the diversity-multiplexing tradeoff curve for arbitrary fading channels.

The Gaussian Many-Help-One Distributed Source Coding Problem

Jointly Gaussian memoryless sources are observed at N distinct terminals. The goal is to efficiently encode the observations in a distributed fashion so as to enable reconstruction of any one of the observations, say the first one, at the decoder subject to a quadratic fidelity criterion. Our main result is a precise characterization of the rate-distortion region when the covariance matrix of the sources satisfies a ¿tree-structure¿ condition. In this situation, a natural analog-digital separation scheme optimally trades off the distributed quantization rate tuples and the distortion in the reconstruction: each encoder consists of a point-to-point Gaussian vector quantizer followed by a Slepian-Wolf binning encoder. We also provide a partial converse that suggests that the tree-structure condition is fundamental.

Distributed Synchronization for Device-to-Device Communications in an LTE Network

We study the problem of distributed time and frequency synchronization for device-to-device communication in LTE [1]. LTE is an OFDMA system that crucially uses tight time and frequency synchronization between UEs and the base station for intracell and intercell interference coordination. For consistency with the cyclic prefix length and tone spacing used in LTE, we target an accuracy of a few microseconds for time synchronization and 150 Hz for frequency synchronization. We examine the problem both from a link and a system-level perspective. Link-level challenges involve designing a synchronization signal for computationally and energy efficient receiver algorithms, and system-level challenges involve achieving consistent timing using a distributed algorithm while leveraging the multiuser aspect of the problem to improve performance.

Bit-Permuted Coded Modulation for Polar Codes

We consider the problem of using polar codes with higher order modulation over AWGN channels. Unlike prior work, we focus on using modulation independent polar codes. That is, the polar codes are not re-designed based on the modulation used. Instead, we propose bit-permuted coded modulation (BPCM): a technique for using the multilevel coding (MLC) approach for an arbitrary polar code. The BPCM technique exploits a natural connection between MLC and polar codes. It involves applying bit permutations prior to mapping the polar code to a higher order modulation. The bit permutations are designed, via density evolution, to match the rates provided by various bit levels of the higher order modulation to that of the polar code. We demonstrate performance of the BPCM technique using link simulations and density evolution for the AWGN channel. We compare the BPCM technique with the bit-interleaved coded modulation (BICM) technique. When using polar codes designed for BPSK modulation, we show gains for BPCM over BICM with random interleaver of up to 0.2 dB, 0.7 dB and 1.4 dB for 4-ASK, 8-ASK, and 16-ASK respectively.

Tradeoff-optimality of D-BLAST

We present coding schemes that achieve the diversity-multiplexing tradeoff for Rayleigh fading channels with one or two receive antennas and arbitrary number of transmit antennas. Codes designed for a parallel channel are the key constituents along with the D-BLAST architecture. We show that the joint ML receiver for D-BLAST outperforms the traditional MMSE with successive cancellation receiver.