Sairam Goguri

A distributed, real-time and non-parametric approach to demand response in the smart grid

This paper considers a novel approach to demand response in a smart grid, wherein a set of flexible loads with given energy requirements and deadlines for completion adjust their instantaneous power consumption in such a way as to make the aggregate total power consumption on the grid as smooth as possible. Each load periodically adjusts its own power level using only a minimal amount of feedback i.e. the aggregate power consumption observed in the previous period, and knowledge of its own energy requirement and deadline. Our approach allows for a fully distributed, real-time implementation and does not require any knowledge of arrival process statistics or forecasts of future load. We illustrate the working of our algorithm with a detailed simulation of generation and load for a fictional small city modeled after the city of Ames, IA using data from a variety of sources. In this simulation, charging electric vehicles serve as flexible loads. We also present an approximate analysis of the bahaviour of flexible loads in our scheme based on a continuous time version of the proposed algorithm to gain insight into its properties and limitations.

Experimental demonstration of nullforming from a fully wireless distributed array

We consider distributed nullforming using an array of wireless transmitters that coordinate their transmissions to achieve destructive interference at a designated receiver. We describe the first experimental demonstration of distributed nullforming to a target receiver from an array of three distributed transmitters using (mostly) off-the-shelf hardware and simple and standard signal processing techniques. We are motivated by the goal of using distributed arrays to achieve increased spectrum reuse through interference cancellation. Our results show interference suppression in excess of 25dB over uncoordinated transmission. We build on our recent experimental demonstration of beamforming from a distributed antenna array after estimating and compensate for the combined effects of unknown propagation channels, hardware mismatches and clock drifts between the array nodes. The transmitters do not share clocks or have any wired back channels. They coordinate achieve nullforming entirely using in-band wireless message exchanges. Thus these methods can be implemented on portable mobile devices rather than being limited to Base Stations.

Experimental demonstration of retrodirective beamforming from a fully wireless distributed array

We report on recent results from our ongoing work on demonstrating retrodirective transmission from distributed arrays. Specifically, we describe a successful experimental demonstration of retrodirective beamforming to a non-cooperating receiver from an array of three distributed transceivers using (mostly) off-the-shelf hardware and simple and standard signal processing techniques. We build on our recently reported work that describes a synchronization procedure that allows the array nodes to estimate and compensate for the combined effects of frequency offsets and drifts in the oscillators as well as nonreciprocal elements in the transceiver hardware. We show how the array nodes that have performed the above synchronization process can then use an opportunistic incoming transmission from an external target to perform retrodirective beamforming back to that target without any coordination with that target. Our experimental results show beamforming gains greater than 90%. A key distinguishing feature of our work is that our procedure requires no wired links between the array nodes, no GPS, nor any other shared signal. To the best of our knowledge, this is the first ever demonstration of retrodirective beamforming from a fully wireless distributed array.

Optimizing wireless power transfer with multiple transmitters

We present a simple theoretical model and supporting experimental evidence for a new approach to maximizing the efficiency of wireless power transfer (WPT) to a receiver from multiple transmitters. Specifically, we consider a multiple-input single-output (MISO) WPT system using near-field inductive coupling to transfer power from multiple transmitting coils to a single receiver; the use of multiple transmitters can potentially allow the system to efficiently focus the transmitted power in the direction of the receiver similar to beamforming from a phased array. This idea is not new and such systems have been extensively studied in previous work using lumped RLMC circuit models to analyze their behavior. However, the difficulty of constructing tractable and realistic circuit models has limited our ability to accurately predicting and optimizing the performance of these systems. Our key novelty is to take the more abstract approach of modeling the WPT system as a linear circuit whose input-output relationship is expressed in terms of a small number of unknown parameters that can be thought of equivalent impedances and transconductances. The crucial advantage of this approach is the economy of the representation: the number of unknown parameters can be much smaller than the number of lumped circuit elements required for a complete and accurate RLMC circuit representation. We present a simple derivation of the optimal voltage excitations to be applied at the transmitters to maximize power transfer efficiency, and also some general properties of the optimal solution. This optimal excitation is, of course, a function of the unknown parameters in our abstract circuit model. We describe a simple procedure for estimating these parameters using a small set of direct measurements. We describe a simple experimental setup with two transmitter coils and a receiver designed to illustrate our approach and present results to demonstrate the efficiency increase achieved using the calculated optimal solution from our model.

A class of scalable feedback algorithms for beam and null-forming from distributed arrays

We describe a class of scalable algorithms for shaping the transmission pattern of a distributed antenna array to steer beams and nulls to cooperating receivers. The key distinguishing feature of these algorithms is that no explicit channel state feedback from the receiver to individual transmitters, no channel reciprocity and minimal coordination between the transmitters is required. Instead, these algorithms are iterative, whereby the transmitters periodically make small adjustments to their transmit array weights, and the intended beam or null target responds by sending a common feedback message every period to all the transmitters. This message consists of a single complex number that represents the aggregate signal amplitude and phase of the array nodes combined transmission at the receiver. By repeatedly probing the receiver in this way and using the resulting feedback messages, we show that the transmitters can converge to the correct array weights necessary to steer beams and nulls at the receiver in a completely decentralized fashion. We describe the intuition behind this technique as well as numerical comparisons with previous work.

Maximizing wireless power transfer using distributed beamforming

We consider the problem of maximizing the total wireless signal power delivered by a distributed antenna array to a receiver where the transmitting nodes each have known frequency-selective channel responses to the receiver and are subject to individual total transmit power constraints. This optimization problem is mathematically quite different from the power maximization problems involving single transmitters or for narrowband systems. We show that the power maximizing solution involves the array nodes performing distributed beam-forming while concentrating their power in a small, finite set of frequencies resulting in an overall received signal consisting of a small number of sinusoidal tones. We derive some properties of the power maximizing solution and describe an iterative algorithm that efficiently computes the solution. We show using numerical simulations that the power maximization problem can yield substantially larger received power compared to alternatives such as a matched filter for frequency-selective channels.

An approach to Kalman filtering for oscillator tracking

Distributed MIMO communications involve multiple transmitters and receivers organizing themselves into virtual antenna arrays. As these carry individual clocks and oscillators that drift, maintaining sychronization requires that the frequency and the unwrapped phase of each oscillator be tracked using Kalman filters. Kalman filters in turn are sensitive to how well the process and measurement noise variances are known. Existing methods for estimating unknown system variances do not work well for oscillators as the process noise variances are very small (orders of 10-21). In this paper we modify the most advanced technique for estimating the noise variances to develop a scheme that leads to faster and more accurate estimation of the noise variances, using fewer observations.