Distributed Cooperative Communications and Wireless Power Transfer

In telecommunications, distributed cooperative communications refer to techniques which allow different users in a wireless network to share or combine their information in order to increase diversity gain or power gain. Unlike conventional pointto-point communications maximizing the performance of the individual link, distributed cooperative communications enable multiple users to collaborate with each other to achieve an overall improvement in performance, e.g., improved range and data rates. The first part of this dissertation focuses the problem of jointly decoding binary messages from a single distant transmitter to a cooperative receive cluster. The outage probability of distributed reception with binary hard decision exchanges is compared with the outage probability of ideal receive beamforming with unquantized observation exchanges. Low-dimensional analysis and numerical results show, via two simple but surprisingly good approximations, that the outage probability performance of distributed reception with hard decision exchanges is well-predicted by the SNR of ideal receive beamforming after subtracting a hard decision penalty of slightly less than 2 dB. These results, developed in non-asymptotic regimes, are consistent with prior asymptotic results (for a large number of nodes and low per-node SNR) on hard decisions in binary communication systems. We next consider the problem of estimating and tracking channels in a distributed transmission system with multiple transmitters and multiple receivers. In order to track and predict the effective channel between each transmit node and each receive node to facilitate coherent transmission, a linear time-invariant statespace model is developed and is shown to be observable but nonstabilizable. To quantify the steady-state performance of a Kalman filter channel tracker, two methods are developed to efficiently compute the steady-state prediction covariance. An asymptotic analysis is also presented for the homogenous oscillator case for systems with a large number of transmit and receive nodes with closed-form results for all of the elements in the asymptotic prediction covariance as a function of the carrier frequency, oscillator parameters, and channel measurement period. Numeric results confirm the analysis and demonstrate the effect of the oscillator parameters on the ability of the distributed transmission system to achieve coherent transmission. In recent years, the development of efficient radio frequency (RF) radiation wireless power transfer (WPT) systems has become an active research area, motivated by the widespread use of low-power devices that can be charged wirelessly. In this dissertation, we next consider a time division multiple access scenario where a wireless access point transmits to a group of users which harvest the energy and then use this energy to transmit back to the access point. Past approaches have found the optimal time allocation to maximize sum throughput under the assumption that the users must use all of their harvested power in each block of the “harvest-then-transmit” protocol. This dissertation considers optimal time and energy allocation to maximize the sum throughput for the case when the nodes can save energy for later blocks. To maximize the sum throughput over a finite horizon, the initial optimization problem is separated into two sub-problems and finally can be formulated into a standard box-constrained optimization problem, which can be solved efficiently. A tight upper bound is derived by relaxing the energy harvesting causality. A disadvantage of RF-radiation based WPT is that path loss effects can significantly reduce the amount of power received by energy harvesting devices. To overcome this problem, recent investigations have considered the use of distributed transmit beamforming (DTB) in wireless communication systems where two or more individual transmit nodes pool their antenna resources to emulate a virtual antenna array. In order to take the advantages of the DTB in the WPT, in this dissertation, we study the optimization of the feedback rate to maximize the energy efficiency in the WPT system. Since periodic feedback improves the beamforming gain but requires the receivers to expend energy, there is a fundamental tradeoff between the feedback period and the efficiency of the WPT system. We develop a new model to combine WPT and DTB and explicitly account for independent oscillator dynamics and the cost of feedback energy from the receive nodes. We then formulate a “Normalized Weighted Mean Energy Harvesting Rate” (NWMEHR) maximization problem to select the feedback period to maximize the weighted averaged amount of net energy harvested by the receive nodes per unit of time as a function of the oscillator parameters. We develop an explicit method to numerically calculate the globally optimal feedback period. ACKNOWLEDGEMENTS There are so many people I want to thank, for without their help and accompany, I would have never made it this far. My words here are insufficient to capture all of my appreciation. First, and most importantly, I want to thank Mom and Dad unconditional support and love. They are the people who give me life and are my personal heroes in the world. They have been my guides in this world from my first, tentative, unsure steps, both as an infant and as a young man attending university aboard. No matter where I am or how much space, or how many zones, separate us, they always give me power and encourage me to struggle in front of difficulties. I am forever indebted to them. I feel incredibly fortunate to have the most awesome Professor Rick Brown as my advisor. I am very appreciated that he promoted me to a Ph.D. student after one year entering WPI when I was a master at that time. I have been extremely lucky to have a supervisor who cared so much about my work, and who responded to my questions and queries so promptly. I am very thankful to him for his efforts on helping me to understand problems, revise each paper and bring new ideas to my research. His advice on both research as well as on my career have been priceless. Besides my advisor, I am thankful to the rest of my dissertation committee members, Professor Lai and Professor Andy Klein, for reviewing my dissertation, participating in my dissertation defense, and for asking me challenging questions and giving me professional comments on my dissertation. I also thank them for always being kind enough to give me valuable comments and ideas on my research. I would also like to thank everyone who I interacted with over the weekly teleconference calls. I want to thank Professor Dasgupta for letting me cooperate with him and Professor Madhow for his insightful feedback and comments on my first i paper in life. Thanks also go to my current/previous labmates at WPI who all make my experience here special. I would like to thank Min who gave me lots of unselfish helps both in research and daily life when I entered the laboratory. I would also like to thank my other labmates who have made the lab a better workplace: Radu, Sabah, Shahab, Kirty, Kim. Finally, I wish to thank my friends Chenyun and Bingwen, who were also my roommates during the second year of my Ph.D. career. I am grateful to them not only for their insights and discussions on many problems and also lots of unselfish helps in my daily life.