An Efficient Communication Abstraction for Dense Wireless Networks

In the last decade, drones became frequently used to provide eye-in-the-sky overview in the outdoor environment. Their main advantage compared to the other types of robots is that they can fly above obstacles and rough terrains and they can quickly cover large areas. These properties also open a new application; drones could provide a multi-hop, line of sight communication for groups of ground users. The aim of this thesis is to develop a drone team that will establish wireless ad-hoc network between users on the ground and distributively adapt links and spatial arrangement to the requirements and motion of the ground users. For this application, we use fixed wing drones. Such platforms can be easily and quickly deployed. Fixed wing drones have higher forward speed and higher battery life than hovering platforms. On the other hand, fixed wing drones have unicycle dynamics with constrained forward speed which makes them unable to hover or perform sharp turns. The first challenge consists in bridging unicycle dynamics of the fixed wing drones. Some control strategies have been proposed and validated in simulations using the average distance between the target and the drone as a performance metric. However, besides the distance metric, energy expenditure of the flight also plays an important role in assessing the overall performance of the flight. We propose a new methodology that introduces a new metric (energy expenditure), we compare existing methods on a large set of target motion patterns and present a comparison between the simulation and field experiments on proposed target motion patterns. The second challenge consists in developing a formation control algorithm that will allow fixed wing robots to provide a wide area coverage and to relay data in a wireless ad-hoc network. In such applications fixed wing drones have to be able to regulate an inter-drone distance. Their reduced maneuverability presents the main challenge to design a formation algorithm that will regulate an inter-drone distance. To address this challenge, we present a distributed control strategy that relies only on local information. Each drone has its own virtual agent, it follows the virtual agent by performing previously evaluated and selected target tracking strategy, and flocking interaction rules are implemented between virtual agents. It is shown in simulation and in field experiments with a team of fixed wing drones that using this distributed formation algorithm, drones can cover an area by creating an equilateral triangular lattice and regulate communication link quality between neighboring drones. The third challenge consists in allowing connectivity between independently moving ground users using fixed wing drone team. We design two distributed control algorithms that change drones' spatial arrangement and interaction topology to maintain the connectivity. We propose a potential field based strategy which adapts distance between drones to shrink and expand the fixed wing drones' formation. In second approach, market-based adaptation, drones distributively delete interaction links to expand the formation graph to a tree graph. In simulations and field experiments we show that our proposed strategies successfully maintain independently moving ground users connected. Overall, this thesis presents synthesis of distributed algorithms for fixed wing drones to establish and maintain wireless ad-hoc communication networks.

Computing in Large-Scale Dynamic Systems

While GPUs are increasingly popular for high-performance computing, optimizing the performance of GPU programs is a time-consuming and non-trivial process in general. This complexity stems from the low abstraction level of standard GPU programming models such as CUDA and OpenCL: programmers are required to orchestrate low-level operations in order to exploit the full capability of GPUs. In terms of software productivity and portability, a more attractive approach would be to facilitate GPU programming by providing high-level abstractions for expressing parallel algorithms.OpenMP is a directive-based shared memory parallel programming model and has been widely used for many years. From OpenMP 4.0 onwards, GPU platforms are supported by extending OpenMP's high-level parallel abstractions with accelerator programming. This extension allows programmers to write GPU programs in standard C/C++ or Fortran languages, without exposing too many details of GPU architectures.However, such high-level parallel programming strategies generally impose additional program optimizations on compilers, which could result in lower performance than fully hand-tuned code with low-level programming models. To study potential performance improvements by compiling and optimizing high-level GPU programs, in this paper, we 1) evaluate a set of OpenMP 4.× benchmarks on an IBM POWER8 and NVIDIA Tesla GPU platform and 2) conduct a comparable performance analysis among hand-written CUDA and automatically-generated GPU programs by the IBM XL and clang/LLVM compilers.

Wearable devices have increasingly been used in applications such as rehabilitation exercises, body fitness, physiological signal monitoring, and interactive games. In many such applications, multiple wearable devices are worn on the user, which form a wireless body sensor network to collect data of the user. The problem is that the body can easily obstruct the transmission of the radio signals from the wearable devices, thereby significantly reducing the success rate of data communication from the sensor nodes to the sinks. In this paper, we consider a wearable exercise monitoring system that places multiple sinks around the user to ensure transmission success rate. The key challenge is to determine the number of sinks required and their locations. This sink placement problem is modeled as a coverage problem in 3D space, and a heuristic algorithm is proposed to solve the problem. Experiments show that our system can ensure an almost 100% packet success rate.

The field of application for distributed algorithms is growing with the ongoing demand for new network applications. Wherever a global goal must be achieved by a number of peers, a distributed algorithm consisting of local computations and communication protocols must be developed. In asynchronous environments the design of a distributed algorithm must respect the fact that processes may vary in speed and that messages might be delayed for an arbitrarily long time. Moreover, it is often required that the algorithm still works in the presence of crash failures, i.e., in scenarios, where some processes stop working at some time. The complex nature of concurrent executions in distributed systems often hinder the application of common methods for modeling and verification. Although many different formal approaches for modeling fault tolerant distributed algorithms, like e.g. I/O automata and process calculi have been proposed, many works in the field of distributed systems use informal pseudo-code representations to introduce new algorithms. On most cases this leads to informal and crude arguments for correctness, which usually become incomprehensible if all race conditions must be taken into account. This work proposes new formal strategies to model and verify distributed algorithms, whereas we focus on two main goals. Firstly, we introduce a means to satisfy a designer’s demands by using methods that are easy to understand and to work with and which are very similar to common applied methods as e.g. TLA and Abstract State Machines. Secondly, our strategies are suitable for the formal use in a theorem proving environment. This enables mechanical verification of our algorithms and therefore, we are able to produce machinechecked proofs for correctness. Our model adopts abstractions from the work of Fuzzati, which examines two similar distributed algorithms in a logical framework. We lifted these abstractions to a more general level to make them suitable and reusable for as many distributed algorithms as possible. Furthermore, we present a new method to place local computation steps in the respective global context. The advantage of our method is that, on the on hand, like with pseudo code, computations can be described as local steps, but, on the other hand, it is possible to formally reason over global system states. Our model for communication mechanisms comprises modules for different kinds of message passing, broadcasts, and shared memory access. We are convinced that our model can easily be extended to support almost all kinds of communication infrastructures that can formally be described. We show how properties that have to be verified can be expressed in terms of our model to enable formal reasoning. Furthermore, we present appropriate strategies to verify different classes of properties. For the theorem proving environment Isabelle/HOL we provide a library which incorporates all introduced abstractions and can, therefore, be reused and extended for further applications, new algorithms, and further verification projects. Finally, we present case studies for the verification of different distributed algorithms to demonstrate the applicability of our strategies. Four known algorithms are verified in our model using our framework for Isabelle/HOL. The algorithms vary in complexity, the used communication infrastructure, the failure model, and other constraints, so that many different modeling and verification aspects are exemplified.

This paper concerns randomized leader election in synchronous distributed networks. A distributed leader election algorithm is presented for complete n-node networks that runs in O(1) rounds and (with high probability) uses only O(nlog3/2⁡n) messages to elect a unique leader (with high probability). When considering the “explicit” variant of leader election where eventually every node knows the identity of the leader, our algorithm yields the asymptotically optimal bounds of O(1) rounds and O(n) messages. This algorithm is then extended to one solving leader election on any connected non-bipartite n-node graph G in O(τ(G)) time and O(τ(G)nlog3/2⁡n) messages, where τ(G) is the mixing time of a random walk on G. The above result implies highly efficient (sublinear running time and messages) leader election algorithms for networks with small mixing times, such as expanders and hypercubes. In contrast, previous leader election algorithms had at least linear message complexity even in complete graphs. Moreover, super-linear message lower bounds are known for time-efficient deterministic leader election algorithms. Finally, we present an almost matching lower bound for randomized leader election, showing that Ω(n) messages are needed for any leader election algorithm that succeeds with probability at least 1/e+ε, for any small constant ε>0. We view our results as a step towards understanding the randomized complexity of leader election in distributed networks.

The massive growth of data consumption and the variety of wireless technology emergence has made the handover (HO) an attractive research topic nowadays, mainly due to the popularity of Wireless Local Area Networks (WLANs), which allow users to reach high-speed data communication while they are in movement. Moreover, mobile devices such as tablets and smartphones have also become increasingly popular due to their low cost and ease of use, and an increase in mobile device use is expected to accelerate in the coming years, along with the availability and use of applications such as real-time services and online gaming. The traditional HO methods will likely not meet the requirements of mobile devices for modern applications due to the lack of intelligence, lack of awareness Quality of Service (QoS) and Quality of Experience (QoE) requirements of mobile users. We, therefore, introduce a novel architecture that supports horizontal HO in homogenous networks. This architecture is based on the Software-Defined Wireless Networking (SDWN) concept, where the wireless network is controlled centrally and the wireless Access Points (APs) are programmable. In this architecture, HO algorithms will assist wireless users to find the network that could best support the application requirements through Quality of Service (QoS) and Quality of Experience (QoE) management policies. The first HO algorithm proposed in this thesis is called Quality of Experience Oriented Handover Algorithm. This algorithm will guarantee the best possible connectivity to the users in terms of their QoE and QoS requirements and outperforms the traditional methods in a sparse network environment. The second contribution is called Optimised Handover Algorithm for Dense WLAN Environments. This algorithm has been designed to address dense network environments via taking into consideration the Adaptive Hysteresis Value (AHV). The AHV will help the Optimised Handover Algorithm via reducing the so-called ping-pong effect. This contribution shows promising performance results by selecting the best candidate AP, decreasing the number of redundant HO and avoiding the ping-pong effect. The final contribution is called Priority Based Handover Algorithm. We extended our proposed SDWN architecture in order to include the concept of prioritising users and make a smart decision during the process of HO. This algorithm will prioritise a certain class of users to avoid the effect of the over-congestion. The results show that the approach based on priority outperforms the state of the art and provides better QoE to the high priority users despite the over-congestion situation.

This thesis is devoted to the investigation and realization of an 868.35 MHz two-way digital RF transceiving system, including design of the 2-way communication firmware encoding/decoding algorithms associated, performance optimization of the RF transceiver module used, and design of a planar antenna of reduced length. The format of the wireless data packet basically includes the following various fields: training preamble, sync header, payload data, frame check sequence, and stop symbol. In any open wireless communication channel, nature-generated and human-made noises are ubiquitous. In order to enhance the performance reliability and system range of the digital wireless communication system under study, the 2-way encoding/decoding communication firmware has to be embedded with effective real-time noise-robust digital signal processing algorithms. In this thesis, The Cyclic Redundancy Check sequence is adopted as the error-detecting mechanism, Hamming coding and the error syndromes/error patterns associated are used to provide the necessary error-correcting capability. In view of the intermittent burst noise created by other radio users, interleaving/de-interleaving techniques are utilized to effectively spread out the burst errors possible.Moreover, a 2-way digital wireless communication protocol is designed so as to demonstrate the 2-way communication capability and facilitate the subsequent 2-way ping-pong range test. Since all the foregoing encoding, decoding, and anti-noise tasks for the wireless data packets to be communicated are realized with the aid of a microcontroller and the assembly language, the microcontroller, RF transceiver module, and antenna are finally integrated and put to 1-way and 2-way range tests in open site or space.

In this thesis, we cope with the fundamental mathematical and physical problems of the space-time-Doppler UWB system design for fading channels: multi-input multi-output (MIMO) wireless communication with adaptive filtering. The estimation algorithm from equalizer providing an accurate parameters estimate is investigated. UWB radio is a fast emerging technology with uniquely attractively features inviting major advances in wireless communications, networking, radar, imaging, and positioning systems. By its rule-making proposal in 2002, the Federal Communications Commission (FCC) in the United States essentially unleashed huge new bandwidth (3.6-10.6 GHz) at the noise floor, where UWB radios overlaying coexistence RF systems cerate using low-power ultra-short information bearing pulses. Right now, there are three basic types of UWB: multiband OFDM UWB (MB-OFDM UWB), impulse-UWB (I-UWB), and direct sequence UWB (DS-UWB). We focus on I-UWB and MB-OFDM-UWB, and compare their performance. A simple means for spreading the spectrum of low duty cycle pulse trains is time hopping (TH), with pulse position modulation (PPM) for data modulation at the rate of many pulses per data bit. The main contribution of this thesis for I-UWB is the design of leading edge detectors, coherent correlation detector (CCD), and pilot performance enhancement compared with different Rayleigh and Ricean fading conditions. For MB-OFDM UWB, we have considered the performance analysis and design criteria of MIMO OFDM UWB systems for high data-rate wireless transmission. The signal processing of frame detection, time synchronization, frequency synchronization, channel estimation, synchronization tracking using pilot subcarriers and 2-D MIMO detection algorithms is discussed. The main contributions of this thesis are 1) Compared with receiver employing the Maximum Likelihood (ML) detector, the receiver employing a low-complexity linear Minimum Mean Square Error (MMSE) demodulator can perform a limited performance loss in spatially uncorrelated channels. 2) Due to the operations of spreading and dispreading combined with MMSE-based frequency domain equalization, the adverse effects of the low-SNR subcarriers on the average BER performance are potentially improved. This is a direct consequence of spreading, because even if the signal corresponding to a specific chip is obliterated by a deep frequency domain channel fade, after dispreading these effects are spread over the Walsh-Hadamard Transform (WHT) length. Hence there is a high chance of still recovering all the partially affected subcarrier symbols without errors. 3) As a test case, the OFDM-based 200 Mb/s MB-UWB network (IEEE 802.15.3a) are considered, but the simulation results are shown above section. 4) The simulation channel can be easily applied to NOS and NLOS situation with WHT transformation from Simulink model. The fading channels of UWB with low/high signal to noise ratio (SNR), multipath effects, and multiuser interference may introduce large errors in location. The location engine performs the following tasks: █ Data fusion techniques: Different data fusion techniques are implemented using Time Of Arrival (TOA), Angle Of Arrival (AOA), or a combination of both. █ Channel modeling: A multipath, multiuser channel environment is created that models path loss, shadowing, Rayleigh fading, and Doppler frequency effects. █ Parameter estimation: TOA and AOA estimation algorithms are implemented as part of the location finding engine. Different variations of algorithms are implemented for performance and comparison purposes. █ Configuring the physical layer for different wireless networks. Among the programmable parameters are spreading factor, packet size, training length, constellation type, modulation technique, carrier frequency, level of transmitted signal power, and the number of antennas at the transmitter for both sides. █ Configuring the mobile user conditions. The wireless channel models depend on the Doppler frequencies present in the environment. The Doppler frequency depends on the mobile speed and carrier frequencies as input and generates a Rayleigh fading channel with the U-shape Doppler spectrum. █ Configuring the environmental parameters and network geographical structure. One of the factors affecting the performance of the wireless location system is the environment type (e.g., bad urban, urban, suburban, or rural area). For example, in a bad urban area with many blocking objects and buildings, non-line of sight (NLOS) effects plays an important role in the estimation accuracy.

Since the first communication systems were developed, the scientific community has been witnessing attempts to increase the amount of information that can be transmitted. In the last 10--15 years there has been a tremendous amount of research towards developing multi-antenna systems which would hopefully provide high-data-rate transmissions. However, increasing the overall amount of transmitted information increases the complexity of the necessary signal processing. A large portion of this thesis deals with several important issues in signal processing of multi-antenna systems. In almost every particular case the goal is to develop a technique/algorithm so that the overall complexity of the signal processing is significantly decreased. In the first part of the thesis a very important problem of signal detection in MIMO (multiple-input multiple-output) systems is considered. The problem is analyzed in two different scenarios: when the transmission medium (channel) 1) is known and 2) is unknown at the receiver. The former case is often called coherent and the later non-coherent MIMO detection. Both cases usually amount to solving highly complex NP-hard combinatorial optimization problems. For the coherent case we develop a significant improvement of the traditional sphere decoder algorithm commonly used for this type of detection. An interesting connection between the new improved algorithm and the H-infinity estimation theory is established, and the performance improvement over the standard sphere decoder is demonstrated. For the non-coherent case we develop a counterpart to the standard sphere decoder, the so-called out-sphere decoder. The complexity of the algorithm is viewed as a random variable; its expected value is analyzed and shown to be significantly smaller than the one of the overall exhaustive search. In the non-coherent case, in addition to the complexity analysis of the exact out-sphere decoder, we analyze the performance loss of a suboptimal technique. We show that only a moderate loss of a few dbs in power required at the transmitter will occur if a polynomial algorithm based on the semi-definite relaxation is used in place of any exact technique (which of course is not known to be polynomial). In the second part of the thesis we consider a few problems that arise in wireless broadcast channels. Namely, we consider the problem of the information symbol vector design at the transmitter. A polynomial linear precoding technique is constructed. It enables achieving data rates very close to the ones achieved with DPC (dirty paper coding) technique. Additionally, for another suboptimal polynomial scheme (the so-called nulling and cancelling), we show that it asymptotically achieves the same data rate as the optimal, exponentially complex, DPC. In the last part of the thesis we consider a quantum counterpart of the signal detection from classical communication. In quantum systems the signals are quantum states and the quantum detection problem amounts to designing measurement operators which have to satisfy certain quantum mechanics laws. A specific type of quantum detection called unambiguous detection, which has numerous applications including quantum filtering, has recently attracted a lot of attention in the research community. We develop a general framework for numerically solving this problem using the tools from the convex optimization theory. Furthermore, in the special case where the two quantum states are of rank 2, we construct an explicit analytical solution for the measurement operators. At the end we would like to emphasize that the contribution of this thesis goes beyond the specific problems mentioned here. Most algorithmic optimization techniques developed in this paper are generally applicable. While it is a fact that our results were originally motivated by wireless and quantum communications applications, we believe that the developed techniques will find applications in many different areas where similar optimization problems appear.

We address the multicast issue in a wide area WDM network in which a source broadcasts its message to some sites in the network. This problem can be formalized as the following optimal multicast tree problem. Given a directed graph G = (V, E) with a source s and a set S of nodes, assume that every node in S (⊂ V) is reachable from s. Associated with every link e ∈ E, there is a set Λ(e) of available wavelengths on it, |V| = n and |E| = m, find a multicast tree rooted at s including all the nodes in S with minimizing the cost of the tree, in terms of the sum of the cost of wavelength conversion at nodes and the cost of using wavelengths on links in the tree. That is, not only do we need to find a directed tree rooted at s, but also do we need to assign a specific wavelength Λ ∈ Λ(e) to every tree edge e and to set the switches at every node in the tree. Since this problem is NP-complete, it is unlikely that there is a polynomial algorithm to find an exact solution for it. Instead, we first present an approximation algorithm for the problem which runs in time O(k2n + km +kn log(kn) + (kn)1/e|S|), and delivers a solution within O(|S|e) of the optimal, for any fixed 0 > e ≤ 1. We then present a distributed version of the proposed algorithm. The communication and time complexities of the distributed algorithm are O(km) and O(kn) respectively, and the solution delivered is |S| times of the optimal, where k is the number of wavelengths in the network. Finally, we conduct some experiments to verify the applicability of the proposed algorithms. The experimental results matches our algorithmic claims.

IEEE 802.16 is a new communication standard for supporting the Wireless Metropolitan Area Network. IEEE 802.16 replaces the last one mile by the way of wireless connects and fetches. Because it used high-efficiency wireless signal transmitting and treatment technology, it is farthest to 48km to transmit the distance, wide 300Kbps- 2Mbps bandwidth of end. Compared with the tradition wired network, IEEE 802.16 possesses the characteristics of the fast laying of hardware, being free from the effect of topography, and low cost. Multimedia applications over mobile wireless network are becoming popular in recent years. High video quality depends on the wide bandwidth but the wide bandwidth restricts the number of users in the network system. Effective bandwidth utilization is the major problem in wireless network because the bandwidth resource in wireless environment is limited and precious. For this reason, we propose an active dropping mechanism to deal with the effective bandwidth utilization problem. In the proposed mechanism, if the network loading exceeds the threshold, the dropping mechanism starts to drop the enhancement layer data for low level user and the dropping probability is varying with the different network loading. For the multimedia application, we use the characteristic of the Scalable Video Coding (SVC) extension of H.264/AVC standard to provide different video quality for different level user. By the dropping mechanism, base station increases the system capability and users can obtain better quality of service when the system is under heavy loading. In this research, we study the network of the 802.16e standard and add the QoS-based Active Dropping mechanism to the MAC layer. The dropping mechanism works when the base station loading is exceeding the bandwidth threshold. We adjust the threshold value dynamically to balance between the bandwidth utilization and video quality. By the dropping mechanism, network system can release bandwidth which is used to transmit the dropped data before and base station can support more users to access this network system. In the simulation results, we improve the bandwidth utilization and the network capability and achieve the QoS for each user.

Recent research has shown that the cooperative diversity can efficiently combat the fading effects caused by the hostile wireless channels just as the multiple-input multiple-output (MIMO) antenna systems do. Besides, due to the inherent cost and size constraints by many small area wireless networks, e.g. wireless ad hoc networks and wireless sensor networks, in such environments, it would be almost impossible to deploy the antenna array in a mobile node. However, cooperative diversity can provide an economical and viable solution by sharing each mobile node’s single-antenna with others to form a virtual antenna array, while it still can maintain significant performance gains similar to MIMO systems. Meanwhile, the adaptive modulation is a proven technique which can significantly increase the spectral efficiency when perfect channel estimation can be achieved, and many wireless standards have adopted it as an implementation option. It also has been shown that combining these two techniques can greatly raise the system efficiency. Nevertheless, in the multiple nodes cooperative communication networks, the randomness of demodulation process at each relay node makes the task of correctly selecting the forwarding relay nodes very complicated. In view of this problem, a novel adaptive node selection algorithm is proposed for the destination node to effectively select the forwarding relay nodes to help the source node transfer the signals. In this paper, the decode-and-forward (DF) protocol is employed as the cooperative diversity signaling, and the constant-power variable-rate M-ary phase shift keying (M-PSK) is adopted as the adaptive modulation scheme. At the destination node, the maximal-ratio-combining (MRC) method is used to combine all of the signals from diversity branches with the main path transmitted directly from the source node. Because of the random phenomenon of the demodulation process, the outage requirement is used as the baseline in the first stage of the proposed node selection algorithm to choose the maximal transmission rate. Hence the maximal overall system throughput in terms of rate can be achieved. In the meantime, the first candidate forwarding sets of the relay nodes which collectively meet the required outage probability are also been chosen. Then in the second stage, we can further refine the number of the relay nodes among the first candidate forwarding sets by using the following criterion — maximum average rate per node. It will screen out the first candidate sets with the minimum number of the relay nodes. If only one candidate set is left at the end of the second stage, it will be the end of the proposed algorithm. However, if the result of the screening still has more than one set, then the final election among the second candidate sets would be the one which can give the minimum outage probability. In the simulation results, our proposed node selection algorithm can even further improve the spectral efficiency while the deleteriously fading effect induced by the wireless channels can also be effectively removed.

Secure compilation is an emerging field that puts together advances in security, programming languages, verification, systems, and hardware architectures in order to devise secure compilation chains that eliminate many of today's vulnerabilities. Secure compilation aims to protect a source language's abstractions in compiled code, even against low-level attacks. For a concrete example, all modern languages provide a notion of structured control flow and an invoked procedure is expected to return to the right place. However, today's compilation chains (compilers, linkers, loaders, runtime systems, hardware) cannot efficiently enforce this abstraction: linked low-level code can call and return to arbitrary instructions or smash the stack, blatantly violating the high-level abstraction. The emerging secure compilation community aims to address such problems by devising formal security criteria, efficient enforcement mechanisms, and effective proof techniques. This seminar strived to take a broad and inclusive view of secure compilation and to provide a forum for discussion on the topic. The goal was to identify interesting research directions and open challenges by bringing together people working on building secure compilation chains, on developing proof techniques and verification tools, and on designing security mechanisms.

A wireless multiple-input multiple-output (MIMO) communications transceiver is a promising candidate for next generation wireless systems. It can enhance the capacity and reliability of the communication link. MIMO is enabled by the presence of multiple transmitting antennas and multiple receiving antennas in the communication link. The benefits of MIMO communication are obtained through a combination of antenna arrays, signal processing, and coding algorithms, which adapt the link to the changing channel. In this critical review paper, a propagation-oriented viewpoint is adopted to discuss wireless MIMO. We give an overview of current research areas and discuss some open problems.