Wireless communication systems

We developed a cross layer design which combines retransmission diversity and
multi-user diversity for wireless communication. To this end, a joint design of
adaptive modulation and coding with retransmission-based automatic repeat request
protocol is outlined. This design is applied to devise multi-user scheduling schemes,
which can optimally capture the available multi-user and retransmission diversities. In
addition, the proposed on-line scheduling algorithms can operate even when the
underl ying fading channel distribution is unknown, while asymptotically converging
to the offline benchmark with guarantees on prescribed fairness and rate requirements.
Numerical results are provided to verify the merits of our novel schemes for
multi-user transmissions over Nakagami block fading channels.

In mobile ad hoc networks, it is challenging to solve the standard problems
encountered in fixed network because of the unpredictable motion of mobile nodes.
Due to the lack of a fixed infrastructure to serve as the backbone of the network, it
is difficult to manage nodes' locations and ensure the stable node performance. The
virtual mobile node (VMN) abstraction that has been applied implements an virtual
mobile node that consists of a set of real nodes traveling on one predetermined virtual
path to collect messages and deliver them to the destinations when they meet. It
conquers the unpredictable motion with virtual nodes' predictable motion. But it
encounters unavoidable failure when all the nodes leave the VMN region and stop
emulating the VMN. We extend the idea of the VMN abstraction to the Multi-path
Intelligent Virtual Mobile Node (MIVMN) abstraction, which allows the messages
to switch between multiple Hamiltonian paths to increase the message delivery ratio
and decrease the failure rate of the virtual nodes. Through simulation results we
show that the MIVMN abstraction successfully meets our goals.

The underwater channel poses numerous challenges for acoustic communication.
Acoustic waves suffer long propagation delay, multipath, fading, and potentially
high spatial and temporal variability. In addition, there is no typical underwater
acoustic channel; every body of water exhibits quantifiably different properties. Underwater
acoustic modems are traditionally operated at low frequencies. However, the
use of broadband, high frequency communication is a good alternative because of the
lower background noise compared to low-frequencies, considerably larger bandwidth
and better source transducer efficiency. One of the biggest problems in the underwater
acoustic communications at high frequencies is time-selective fading, resulting
in the Doppler spread. While many Doppler detection, estimation and compensation
techniques can be found in literature, the applications are limited to systems operating
at low frequencies contained within frequencies ranging from a few hundred Hertz
to around 30 kHz.

Channel assignment in multi-radio networks is a topic of great importance because
the use of multiple channels and multiple radios reduces interference and increases the
network throughput. The goal of our research is to design algorithms that maximize the
use of available resources while providing robustness to primary users that could reclaim
one or more channels. Our algorithms could be used in ad hoc networks, mesh networks,
and sensor networks where nodes are equipped with multiple radios. We design
algorithms for channel assignment which provide robustness to primary users without
assuming an accurate primary user behavior model. We also compute bounds for capacity
in grid networks and discuss how the capacity of a network changes when multiple
channels are available. Since preserving energy is very important in wireless networks,
we focus on algorithms that do not require powerful resources and which use a reduced
number of messages.

High data rate acoustic communications become feasible with the use of communication systems that operate at high frequency. The high frequency acoustic transmission in shallow water endures severe distortion as a result of the extensive intersymbol interference and Doppler shift, caused by the time variable multipath nature of the channel. In this research a Single Input Multiple Output (SIMO) acoustic communication system is developed to improve the reliability of the high data rate communications at short range in the shallow water acoustic channel. The proposed SIMO communication system operates at very high frequency and combines spatial diversity and decision feedback equalizer in a multilevel adaptive configuration. The first configuration performs selective combining on the equalized signals from multiple receivers and generates quality feedback parameter for the next level of combining.

In wireless orthogonal frequency division multiple-access (OFDMA) standards,
subcarriers are grouped into chunks and a chunk of subcarriers is made as the minimum allocation unit for subcarrier allocation. We investigate the chunk-based resource allocation for OFDMA downlink, where data streams contain packets with diverse bit-errorrate (BER) requirements. Supposing that adaptive transmissions are based on a number of discrete modulation and coding modes, we derive the optimal resource allocation scheme that maximizes the weighted sum of average user rates under the multiple BER and total power constraints. With proper formulation, the relevant optimization problem is cast as an integer linear program (ILP). We can rigorously prove that the zero duality gap holds for the formulated ILP and its dual problem. Furthermore, it is shown that the optimal strategy for this problem can be obtained through Lagrange dual-based gradient iterations with fast convergence and low computational complexity per iteration. Relying on the stochastic optimization tools, we further develop a novel on-line algorithm capable of dynamically learning the underlying channel distribution and asymptotically approaching the optimal strategy without knowledge of intended wireless channels a priori. In addition, we extend the proposed approach to maximizing the a-fair utility functions of average user rates, and show that such a utility maximization can nicely balance the trade-off between the total throughput and fairness among users.

The present research is a targeted endeavor to study the underlying characteristics and novel applications of millimeter (mm) wave through terahertz (THz) spectrum of electromagnetic (EM) energy. Focused thereof are the following specific tasks broadly considered pertinent to the said EM spectral range: (i) To elucidate the material characteristics vis-à-vis the interaction with EM energy at the test frequencies; (ii) to identify biomedical applications based on the material characteristics studied and applied to biomedia; and (iii) to model the wireless communication channels supporting EM waves at the test frequency bands of interest. Commensurate with the scope as above, the objectives of the research are as follows:

Nowadays the widespread availability of wireless networks has created an interest
in using them for other purposes, such as localization of mobile devices in indoor
environments because of the lack of GPS signal reception indoors. Indoor localization
has received great interest recently for the many context-aware applications it could make possible. We designed and implemented an indoor localization platform for Wi-Fi nodes (such as smartphones and laptops) that identifies the building name, floor number, and room number where the user is located based on a Wi-Fi access point signal fingerprint pattern matching. We designed and evaluated a new machine learning algorithm, KRedpin, and developed a new web-services architecture for indoor localization based on J2EE technology with the Apache Tomcat web server for managing Wi-Fi signal data from the FAU WLAN. The prototype localization client application runs on Android cellphones and operates in the East Engineering building at FAU. More sophisticated classifiers have also been used to improve the localization accuracy using the Weka data mining tool.

The scalability of QUIC-TCP was examined by expanding previous
developmental 11-node, 4-flow topology to over 30 nodes with 11 flows to validate
QUIC-TCP for larger networks. The topology was simulated using ns-2 network
simulator with the same ns-2 module of FAST-TCP modified to produce QUIC-TCP
agent that the original development used. A symmetrical topology and a random
topology were examined. Fairness, aggregate throughput and the object of the utility
function were used as validation criteria. It was shown through simulation that QUICTCP
optimized the utility function and demonstrated a good balance between aggregate
throughput and fairness; therefore QUIC-TCP is indeed scalable to larger networks.

In the present computer age, cellular technology and portable computers are becoming an integral part of the life. Each computer user wants to access the computing resources, irrespective of the location. Because of this need the computing paradigm "Mobile Computing" has assumed a primary role in modern computer communication technology. While dimensioning the network resources, it is very important to know how the users move around the geographical area covered by the cellular network. This knowledge allows us to plan the system resources in order to achieve the QoS required. The major factors that affect the performance, along with the mobility pattern of the mobile user, are the speed at which the user is moving and the load on the network. In this research, we study different types of mobility patterns the user can follow and it's impact on the network services. We have proposed and evaluated a reservation scheme to improve the QoS in the cellular network. The reservation scheme reserves some part of the bandwidth for handoff connections. We have developed simulation programs and have studied three mobility patterns namely leading movement type, random motion, and square-street mobility pattern for measuring the QoS for cellular network. It has been observed from the results that at an average speed of 50 miles per hour with the average loading of the network, a significant improvement in QoS has been achieved for all the mobility patterns by using the reservation scheme.