D.R. Brown

Distributed transmit beamforming: challenges and recent progress

Distributed transmit beamforming is a form of cooperative communication in which two or more information sources simultaneously transmit a common message and control the phase of their transmissions so that the signals constructively combine at an intended destination. Depending on the design objectives and constraints, the power gains of distributed beamforming can be translated into dramatic increases in range, rate, or energy efficiency. Distributed beamforming may also provide benefits in terms of security and interference reduction since less transmit power is scattered in unintended directions. Key challenges in realizing these benefits, however, include coordinating the sources for information sharing and timing synchronization and, most crucially, distributed carrier synchronization so that the transmissions combine constructively at the destination. This article reviews promising recent results in architectures, algorithms, and working prototypes which indicate that these challenges can be surmounted. Directions for future research needed to translate the potential of distributed beamforming into practice are also discussed.

On throughput efficiency of geographic opportunistic routing in multihop wireless networks

Geographic opportunistic routing (GOR) has shown throughput efficiency in coping with unreliable transmissions in multihop wireless networks. The basic idea behind opportunistic routing is to take advantage of the broadcast nature and spacial diversity of the wireless medium by involving multiple neighbors of the sender into the local forwarding, thus improve transmission reliability. The existing GOR schemes typically involve as many as available next-hop neighbors into the local forwarding, and give the nodes closer to the destination higher relay priorities. In this paper, we show that it is not always the optimal way to achieve the best throughput. We introduce a framework to analyze the one-hop throughput of GOR, provide a deeper insight into the trade-off between the benefit (packet advancement and transmission reliability) and cost (medium time delay) associated with the node collaboration, and propose a local metric named expected one-hop throughput (EOT) to balance the benefit and cost. We also identify an upper bound of EOT and its concavity, which indicates that even if the candidate coordination delay were negligible, the throughput gain would become marginal when the number of forwarding candidates increases. Based on the EOT, we also propose a local candidate selection and prioritization algorithm. Simulation results validate our analysis and show that the EOT metric leads to both better one-hop and path throughput than the corresponding pure GOR and geographic routing.

On Geographic Collaborative Forwarding in Wireless Ad Hoc and Sensor Networks

In this paper, we study the geographic collaborative forwarding (GCF) scheme, a variant of opportunistic routing, which exploits the broadcast nature and spatial diversity of the wireless medium to improve the packet delivery efficiency. Our goal is to fully understand the principles, the gains, and the tradeoffs of the node collaboration and its associated cost, thus provide insightful analysis and guidance to the design of more efficient routing/forwarding protocols. We first identify the upper bound of the expected packet advancement (EPA) that GCF can achieve and prove the concavity of the maximum EPA. With energy efficiency as a major concern, we propose a new metric, EPA per unit energy consumption, which balances the packet advancement, reliability and energy consumption. By leveraging the proved properties, we then propose an efficient algorithm which selects a feasible candidate set that maximizes this local metric. We validate our analysis results by simulations, and justify the effectiveness of the new metric by comparing the performance of GCF with those of the existing geographic and opportunistic routing schemes.

Natural cooperation in wireless networks

This tutorial provides an introduction to game theoretic analysis of selfish behavior in wireless ad hoc networks with a focus on the packet forwarding and relaying scenarios.

Secret communication with feedback

Secure communication with feedback is studied. An achievability scheme in which the backward channel is used to generate a shared secret key is proposed. The scenario of binary symmetric forward and backward channels is considered, and a combination of the proposed scheme and Maurers coding scheme is shown to achieve improved secrecy rates. The scenario of a Gaussian channel with perfect output feedback is also analyzed and the Schalkwijk-Kailath coding scheme is shown to achieve the secrecy capacity for this channel.

RAKE reception for UWB communication systems with intersymbol interference

Recently, ultra wideband (UWB) technology has been proposed for use in wireless personal area networks (WPAN). Under the conditions where such transceivers are expected to operate, intersymbol interference (ISI) will become a significant performance limitation, and improvements to conventional RAKE reception will be necessary. We propose a modified RAKE receiver that finds an optimal balance between the goal of gathering multipath signal energy avoiding ISI, and suppressing narrowband interference. For fixed RAKE finger delays, we develop a closed-form expression for the minimum mean squared error (MMSE) combining weights that account for ISI. We then examine the optimal choice of RAKE finger delays, and show that significant performance gains can be achieved, particularly in an undermodeled situation when there are more channel paths than RAKE fingers. Several numerical examples are presented which compare our proposed scheme to a conventional RAKE with maximal ratio combining (MRC).

Opportunistic collaborative beamforming with one-bit feedback

An energy-efficient opportunistic collaborative beamformer with one-bit feedback is proposed for ad hoc sensor networks over Rayleigh fading channels. In contrast to conventional collaborative beamforming schemes in which each source node uses channel state information to correct its local carrier offset and channel phase, the proposed beamforming scheme opportunistically selects a subset of source nodes whose received signals combine in a quasi-coherent manner at the intended receiver. No local phase-precompensation is performed by the nodes in the opportunistic collaborative beamformer. As a result, each node requires only one bit of feedback from the destination in order to determine if it should or should not participate in the collaborative beamformer. Analytical results show that the received signal power obtained with the proposed beamforming scheme scales linearly with the number of available source nodes. Since the optimal node selection rule requires an exhaustive search over all possible subsets of source nodes, two low-complexity selection algorithms are developed. Simulation results confirm the effectiveness of opportunistic collaborative beamforming with the low-complexity selection algorithms.

Distributed massive MIMO: Algorithms, architectures and concept systems

Making MIMO truly “massive” involves liberating it from the shackles of form factor constraints, by allowing arbitrarily large groups of neighboring nodes to opportunistically form virtual antenna arrays for both transmission and reception. Moving such distributed MIMO (DMIMO) systems from the realm of information theory to practice requires synchronization of the cooperating nodes at multiple levels. We are interested in all-wireless systems which are severely constrained in the amount of information that can be exchanged among the cooperating nodes, in contrast to recent proposals in massive MIMO (co-located antennas) or base station cooperation (which relies on a high-speed wired backhaul). The goal of this paper is to point out some of the research issues unique to scaling up such DMIMO systems. We briefly review the significant technical progress in design and demonstration over the past few years, and describe a research agenda for the next few years based on fundamental questions in attaining the “distributed coherence” required to realize concept systems such as DMIMO communication at large carrier wavelengths (e.g., white space frequencies for which standard antenna arrays are too bulky) and distributed 911 for emergency and rescue scenarios.

Adaptive whitening in electromyogram amplitude estimation for epoch-based applications

Epoch-based electromyogram (EMG) amplitude estimates have not incorporated signal whitening, even though whitening has demonstrated significant improvements for stream-based estimates. This work presents new epoch-based algorithms, for both single- and multiple-channel EMG, which include a whitening stage. The best multiple-channel whitening processor provided a 21.4%-22.5% improvement over single-channel unwhitened estimation in an EMG-to-torque application.

Fundamental limits on phase and frequency tracking and estimation in drifting oscillators

Distributed beamforming requires phase and frequency synchronization. As oscillators drift, through Brownian motion induced phase noise, their instantaneous phases must be tracked and compensated. Several papers and IEEE 1588 have proposed Kalman Filter (KF) based tracking algorithms using the unwrapped phase measurements. This paper quantifies the effect of Brownian Motion induced drift at two levels. First we derive Cramer-Rao Lower Bounds (CRLB) manifest in one shot estimation of frequency and phase from unwrapped phase observations, and reveal fundamental and illuminating differences with the existing frequency and phase estimation CRLBs in the literature derived in the absence of Brownian motion. Second, we consider a KF that tracks the instantaneous phase in intervals where there is no beamforming, and is switched off during beamforming. Bounds are derived relating the error growth as a function of the underlying duty cycle.