Rice University logoGeorge R. Brown School of Engineering
 
Electrical and Computer Engineering
 
 

Alacheswar Sahai and Samantha Summerson tied for best ECE PhD Defense. The award is decided on by the ECE Graduate Awards Committee each year. Sahai and Summerson were selected out of many outstanding dissertations.

 

sahai  Sahahi's defense was titled, "Wireless Full-Duplex: From Practice to Theory." 

His abstract: "Full-duplex is the ability of a node to transmit and receive simultaneously in the same band. Ideal wireless full-duplex communication can double the spectral efficiency compared to the traditional half-duplex communication. In this dissertation, we study the challenges in realizing full-duplex communication. We tackle the challenges from two different perspectives: node and network. Node perspective: Simultaneous transmission and reception results in a large self-interference due to the proximity of transmit and receive antennas at the full-duplex node. To establish the feasibility of wireless full-duplex, we develop a wideband real-time physical layer and evaluate its performance on the WARP testbed. Self-interference reduction in our proposed physical layer is achieved through passive suppression and active cancellation. Based on the constraints of the physical layer, we propose a MAC layer protocol which is designed specifically to discover and enhance opportunities to communicate in the full-duplex mode. Further, based on the physical layer, we propose a MAC layer protocol which is designed to discover full-duplex opportunities in a network. Experimental evaluation of our physical layer design, as well as several other full-duplex designs proposed in literature, reveal that active cancellation does not push self-interference all the way up to the thermal noise floor. In this dissertation, we explore the bottlenecks limiting active cancellation in full-duplex systems. We show that the amount of active cancellation is limited by transmitter side noise, particularly by the phase-noise in the local oscillator at the transmitter of the full-duplex node. Thus, unlike conventional half-duplex systems where receiver thermal noise is a limiting factor, full-duplex systems are limited by transmitter side noise. As a key by-product of our analysis, we propose a signal model for a wideband MIMO full-duplex system. We use our proposed signal model to study the performance limits of a system where the start of transmission and the start of reception at a full-duplex node are not synchronized. Interestingly, we discover that the bit-error-rate of the communication mode where the start of transmission precedes the start of reception is better than the mode where start of transmission follows reception of a packet at the full-duplex node. Network perspective: In order to extract gains in capacity from full-duplex operation in a multi-user network, we propose to use full-duplex capable nodes to simultaneously operate uplink and downlink in a network. Such operation results in a new type of interference in the network -- internode interference, i.e., the uplink transmission from each mobile user starts interfering with the downlink receptions at all the other mobile users. We show a physical layer coding strategy that aligns interference over time and extracts gains in degrees-of-freedom of the network. Finally, we recognize that larger gains from full-duplex operation are possible by leveraging the fact that the strength of the internode interference channel is often different from the uplink/downlink channel. By analyzing the uplink/downlink capacity of a network composed of one base-station and two mobile users, we show that full-duplex not only out-performs half-duplex, but also recovers some of the degrees-of-freedom lost due to lack/delay of channel state information of the network." Sahai defended this past January. 
 summerson2  Summerson's defense was titled, "Engineering Deep Brain Stimulation as a Treatment for Parkinson's Disease: from Models to Materials." She analyzed deep brain stimulation (DBS) as a treatment for the motor symptom's of Parkinson's Diseas at multiple levels.

Her abstract: "Although this treatment is currently used on human patients, little is understood about the mechanism of action which allows patients to experience therapeutic benefits. The work here investigates efficacy of DBS in computational and experimental manners in order to enhance the understanding of the effects on neural activity and behavior. First, I examine computational models of the nuclei within the motor circuit of the brain and used these models to test novel electrical stimulation signal designs. I show that irregular spacing of stimulation pulses allows for increased variability in neuronal firing rate responses within the basal ganglia. Also, I develop a model of the stimulation-frequency-dependent nature of antidromic spiking induced in the motor cortex as a result of DBS. Second, I use the hemi-Parkinsonian rat model to demonstrate motor and cognitive behavioral effects of DBS in the globus pallidus internus (GPi). The work validates this animal model for translational research on DBS of the GPi and demonstrates results consistent with reports for DBS of the subthalamic nucleus (STN) in the same model. Additionally I study recorded neural activity in the motor cortex while stimulating the STN in order to characterize the corresponding changes in neural activity. I found that regular and irregular stimulation patterns both decrease Parkinsonian entropic noise in the output layer of the motor cortex, with irregular stimulation having the greatest benefit towards reducing this noise. Third, I consider a new material for its biocompatibility and applicability as a material for stimulating electrodes. In the rat model that I previously validated, I verify that behavioral results using a stimulating electrode made from carbon nanotube fibers (CNTf) match results from previous experiments using standard platinum iridium (PtIr) electrodes. Additionally, it is shown that CNTf electrodes produce lower inflammation, gliosis and damage to the blood brain barrier. Together, all three aspects of the work demonstrate significant contributions to the functionality and engineering of DBS as a neuromodulation therapy for PD." Summerson defended in April.