Many capacity and near-capacity achieving methods in multiple-input-multipleoutput (MIMO) wireless channels make use of the singular value decomposition (SVD) of the channel matrix. For the multi-user case, the SVD of the channel matrix for each user may result in right and left singular vectors that are similar between users. This proposes another descriptive characterization of the multi-user MIMO channel. Closely aligned singular vectors between any two users could reduce the achievable signaling rates of signal processing communication methods as one user would be more difficult to resolve in space-time from another. An examination into how this alignment can be described in realistic MIMO multipath channels using a two ring channel model is presented. The effects of correlation between singular vectors on achievable signaling rates is shown for one existing algorithm that approaches the sum capacity known as block-diagonalization. Analyzed is actual data collected in several indoor and outdoor experiments performed using newly constructed measurement hardware that extends the capabilities of an existing MIMO measurement system.

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This dissertation focuses on quantifying the effects of multi-user co-channel interference for an avionic communication system operating in a heavy-traffic aeronautical mobile environment and proposes advanced interference cancellation techniques to mitigate the interference. The dissertation consists of two parts. The first part of the work investigates the use of a visualization method to quantify and characterize the multi-user co-channel interference (multiple access interference) effects impinging on an avionic communication system. The interference is caused by complex interactions of thousands of RF signals transmitted from thousands of aircraft; each attempts to access a common communication channel, which is governed by a specific channel contention access protocol. The visualization method transforms the co-channel interference, which is specified in terms of signal-overlaps (signal collisions), from a visual representation to a matrix representation for further statistical analysis. It is found that the statistical Poisson and its cumulative distribution provide the best estimates of multi-user co-channel interference. It is shown, using Monte Carlo simulation, that the co-channel interference of a victim aircraft operating in the heavy-traffic environment could result in as high as eight signal-overlaps. This constitutes to approximately 83.4% of success rate in signal detection for the entire three thousand aircraft environment using conventional FSK receiver. One key finding shows that high-quality communications, up to 98.5% success rate, is achievable if only three overlapping signals can be decoded successfully. The interference results found in the first part set the stage for interference cancellation research in the second part. The second part of the work proposes the use of advanced interference cancellation techniques, namely sequential interference cancellation (SIC) and parallel interference cancellation (PIC), as potential solutions to mitigating the interference effects. These techniques can be implemented in radio receivers to perform multi-signal decoding functionality to remove the required interferers (three overlapping signals) so that high-quality communication, as described in the first part, can be achieved. Various performance graphs are shown for B-FSK and B-PSK for both SIC and PIC techniques. One key finding is that the system performance can be improved substantially to an additional 15% in signal reception success rate by using SIC or PIC. This means that critical information transmitted from 450 aircraft (out of approximately three thousand aircraft in the environment) is preserved and successfully decoded. Multi-signal decoding using these interference cancellation receivers comes at a small penalty of 2 - 4.5 dBs in Eb/No when sufficient signal-to-interference (SIR) ratio (7-12 dB) is provided. / Ph. D.

This dissertation addresses multi-layer optimization aspects of multiple input multiple output (MIMO) and deep learning-based communication systems. The initial focus is on the rate optimization for multi-user MIMO (MU-MIMO) configurations; specifically, multiple access channel (MAC) and interference channel (IC). First, the ergodic sum rates of MIMO MAC and IC configurations are determined by jointly integrating the error and overhead effects due to channel estimation (training) and feedback into the rate optimization. Then, we investigated methods that will increase the achievable rate for parallel Gaussian IC (PGIC) which is a special case of MIMO IC where there is no interference between multiple antenna elements. We derive a generalized iterative waterfilling algorithm for power allocation that maximizes the ergodic achievable rate. We verified the sum rate improvement with our proposed scheme through extensive simulation tests. Next, we introduce a novel physical layer scheme for single user MIMO spatial multiplexing systems based on unsupervised deep learning using an autoencoder. Both transmitter and receiver are designed as feedforward neural networks (FNN) and constellation diagrams are optimized to minimize the symbol error rate (SER) based on the channel characteristics. We first evaluate the SER in the presence of a constant Rayleigh-fading channel as a performance upper bound. Then, we quantize the Gaussian distribution and train the autoencoder with multiple quantized channel matrices. The channel is provided as an input to both the transmitter and the receiver. The performance exceeds that of conventional communication systems both when the autoencoder is trained and tested with single and multiple channels and the performance gain is sustained after accounting for the channel estimation error. Moreover, we evaluate the performance with increasing number of quantization points and when there is a difference between training and test channels. We show that the performance loss is minimal when training is performed with sufficiently large number of quantization points and number of channels. Finally, we develop a distributed and decentralized MU-MIMO link selection and activation protocol that enables MU-MIMO operation in wireless networks. We verified the performance gains with the proposed protocol in terms of average network throughput. / Doctor of Philosophy / Multiple Input Multiple Output (MIMO) wireless systems include multiple antennas both at the transmitter and receiver and they are widely used today in cellular and wireless local area network systems to increase robustness, reliability and data rate. Multi-user MIMO (MU-MIMO) configurations include multiple access channel (MAC) where multiple transmitters communicate simultaneously with a single receiver; interference channel (IC) where multiple transmitters communicate simultaneously with their intended receivers; and broadcast channel (BC) where a single transmitter communicates simultaneously with multiple receivers. Channel state information (CSI) is required at the transmitter to precode the signal and mitigate interference effects. This requires CSI to be estimated at the receiver and transmitted back to the transmitter in a feedback loop. Errors occur during both channel estimation and feedback processes. We initially analyze the achievable rate of MAC and IC configurations when both channel estimation and feedback errors are taken into account in the capacity formulations. We treat the errors associated with channel estimation and feedback as additional noise. Next, we develop methods to maximize the achievable rate for IC by using interference cancellation techniques at the receivers when the interference is very strong. We consider parallel Gaussian IC (PGIC) which is a special case of MIMO IC where there is no interference between multiple antenna elements. We develop a power allocation scheme which maximizes the ergodic achievable rate of the communication systems. We verify the performance improvement with our proposed scheme through simulation tests. Standard optimization techniques are used to determine the fundamental limits of MIMO communications systems. However, there is still a gap between current operational systems and these limits due to complexity of these solutions and limitations in their assumptions. Next, we introduce a novel physical layer scheme for MIMO systems based on machine learning; specifically, unsupervised deep learning using an autoencoder. An autoencoder consists of an encoder and a decoder that compresses and decompresses data, respectively. We designed both the encoder and the decoder as feedforward neural networks (FNNs). In our case, encoder performs transmitter functionalities such as modulation and error correction coding and decoder performs receiver functionalities such as demodulation and decoding as part of the communication system. Channel is included as an additional layer between the encoder and decoder. By incorporating the channel effects in the design process of the autoencoder and jointly optimizing the transmitter and receiver, we demonstrate the performance gains over conventional MIMO communication schemes. Finally, we develop a distributed and decentralized MU-MIMO link selection and activation protocol that enables MU-MIMO operation in wireless networks. We verified the performance gains with the proposed protocol in terms of average network throughput.

Sharing a single, large touchscreen between several collaborating users is the research area of multi-user touchscreen interfaces, also named collaborative groupware. Collaborative groupware concerns how to best facilitate teamwork over such shared digital spaces. The technology is used in a number of high-technological and expert-dominated domains including emergency coordination, urban planning, military strategy, etc. This thesis investigates the problem of how to facilitate such mixed-focus work on a multi-user touchscreen in a two-dimensional geospatial context - a digital map - while avoiding the problematic use of widgets and click interfaces. Through a background study of previous work, a new solution is suggested. It is a handchord-input-based interface that uses hand-pattern recognition to interface with multiple users simultaneously and provides graphical tools to enable and enhance geospatial collaborative work. This solution is implemented and evaluated in two user studies. The first investigates the solution in the aspects of practical interfacing, tool impact, and hybrid collaboration on small teams of two. A second smaller expert user study evaluates the technique in the aspects of responsiveness, effectiveness, and representation; with more formalized procedures. The results show that a strong majority of users in the first study could successfully and quickly learn chord-input interfacing, and use it to access a variety of functionality with effectivity and enhancement of collaboration. The results of the second survey received moderately positive feedback in all aspects, with reserved opinions for its usage potential. These results indicate that chord-based interfacing, successfully implemented, could contribute to improved teamwork and task-solving in relevant collaborative contexts. / Kollaborativa gruppprogramet är området som faciliterar samarbete på delade digitala utrymmen. Den här tesen utreder problemet om hur man faciliterar kollaborativ, mix-fokuserad arbete på en fleranvändarpekskärm i en geospatial konext utan att använda problematiska knappar och klickbara gränssnitt. Kollaborativa geospatiala gränsnittanvänds i ett flertal av högteknologiska expertdominerade domäner som inkluderar nödsituationskoordinering, , militär strategi, etc. Den föreslagna lösningen är ett handackordsbaserat gränsnitt som använder fingeringenkänning för att interagera flera användare samtidigt och förse verktyg som möjliggör och förbättrar geospatialt samarbete. Denna lösning implementeras och är möjligtvis den mest omfattande implementationen av hand-ackord, men med signifikanta buggar. Denna utvärderas i två användarstudier. Den första undersöker lösningen i aspekterna av handackordsinteraktion, verktygens påverkan och hybridsamarbete i små lag om två. En andra, mindre studie utvärderar tekniken i termer av gensvar, effektivet och representation. Resultatet visar att ackord-baserade gränssnitt kan framgångsrikt förse en mångfald av funktionalitet med effektivtet och förbättring av samarbete i en geospatial kontext. Resultatet av den första studien visar att handackordbaserade kan bidra till förbättrat samarbete och problemlösning i geospatiala kontexter på en fleranvändarpekskärm, där vissa användare finner det mer praktiskt och enklare att bemästra än andra. Resultatet av den andra studien påvisade tämligen positiv respons i alla aspekter, med reservation för dess tillämpningspotential.

Η παρούσα διατριβή αναδεικνύει τα πλεονεκτήματα της διδακτικής στρατηγικής της Μάθησης Βασισμένης σε Προβλήματα (ΜΒΠ), αλλά και των περιορισμών που υπάρχουν στην εφαρμογή της στην εργαστηριακή εκπαίδευση. Τα Εικονικά Περιβάλλοντα Πολλών Χρηστών (Multi-User Virtual Environments - MUVEs) όπως το Second Life (SL) είναι τρισδιάστατα συνεργατικά εικονικά περιβάλλοντα τα οποία θα μπορούσαν να αποτελέσουν συμπληρωματικούς ή και εναλλακτικούς κόσμους για την υλοποίηση εργαστηριακών δραστηριοτήτων ΜΒΠ με χαμηλό κόστος, υψηλή διαθεσιμότητα και ασφάλεια. Ο στόχος της παρούσας διατριβής ήταν ο σχεδιασμός και η εμπειρική αξιολόγηση μιας απλής δραστηριότητας ΜΒΠ τόσο στον εικονικό όσο και στον πραγματικό κόσμο. Η εμπειρική αξιολόγηση περιλάμβανε μια περιγραφική (Ν=30) και μια ημι-πειραματική έρευνα (Ν=150) με φοιτητές Πανεπιστημίου. Τα αποτελέσματα υποδεικνύουν ότι δεν υπάρχουν στατιστικά σημαντικές διαφορές μεταξύ των περιβαλλόντων στο μαθησιακό αποτέλεσμα, την ευχρηστία, την ικανοποίηση από τη συνεργασία και την ευχαρίστηση από την εμπειρία. Ωστόσο υπάρχουν ενδείξεις ότι το SL εκλαμβάνεται ως πιο ευχάριστο και άτυπο περιβάλλον μάθησης. Η ικανοποίηση συσχετίζεται θετικά με την αίσθηση παρουσίας. Η δραστηριότητα διαρκεί περισσότερο στο SL λόγω των διαδικασιών χειρισμού και της μειωμένης μη λεκτικής επικοινωνίας, παράγοντες που φαίνεται να «επιβάλλουν» ένα πιο κατακερματισμένο μοτίβο ομιλίας. / This dissertation identifies the advantages of the Problem-Based Learning (PBL) instructional strategy and acknowledges the limitations of classic laboratory education to support it. Multi-User Virtual Environments (MUVEs) like Second Life (SL) are 3D collaborative virtual environments that could act as complementary or alternative worlds for the implementation of PBL activities offering low cost, safe and always available environments. The aim of this dissertation was to design and empirically evaluate a simple PBL activity in both the real and virtual worlds, by carrying out a descriptive study (N=30) and a quasi-experimental study (N=150) with University students. The results showed that the MUVE provided similar learning outcomes, engagement, usability and satisfaction to the real world condition. SL is perceived as more pleasurable and informal learning environment than reality. Satisfaction is positively correlated with presence. The PBL activity tends to last longer in SL that in real world, mainly due to control procedures and reduced nonverbal communication, that also “impose” a more fragmented pattern of verbal communication.

在多人虛擬環境系統的設計上，讓使用者能自行設計化身的行為，並即時以動畫元件的形式安裝到虛擬世界中，是3D內容能否達到共享的關鍵。本論文主要研究的部分包含三個部分：第一個部分是提供虛擬環境系統動態載入動畫元件的機制，使得虛擬環境系統可以動態加入客製化之化身行為；第二個部分是在現有的虛擬環境系統中加入語意的描述，並增加互動訊息傳送時的彈性；第三個部分是實現可客製化的動畫元件，分別以化身和環境間以及化身彼此間的互動，來說明上述機制的可行性。在動態的載入動畫元件的部分，客製化的動畫元件在系統上得以即時安裝並執行。動畫元件在安裝上可使用XML片段，並交由OSGi Framework中的服務來處理此XML標籤。另外，在加入物件的語意描述後，使得這些動畫元件可以取得世界的資訊，並進一步產生符合當時環境限制或應用需求的動畫。我們以Ontology來描述環境和化身的資訊，並實際製作路徑規劃器元件和化身間互動元件兩個範例。我們利用動態安裝及語意資訊兩個機制，以實例說明如何達到實現化身可客製化行為之目的。 / In the design of multi-user virtual environments, in order to share 3D contents designed by users, it is crucial to allow the behaviors of an avatar to be designed as animation components and loaded at the run time. In this thesis, we attempt to address the problem of designing a semantic virtual environment by considering the following three parts. First, we have designed a mechanism for user-designed animation procedures to be installed and loaded at run time. Second, we have augmented our virtual environment system with semantic descriptions and enhanced the flexibility of message interchange. Third, we have used two types of interaction scenarios, avatar-environment and avatar-avatar, to illustrate how customized animation components can be designed to enhance the functionality of a virtual environment. In our system, we allow users to design their own XML tags and the corresponding animation components managed in the OSGi framework. These components can acquire world information and generate appropriate animations according to application requirements and environment constraints. We have used ontology to describe the semantics of environments and avatars. Two example components: the motion planner and the avatar-avatar interaction have been designed to illustrate the dynamic installation process and the retrieval of semantic information for the realization of customizing avatar behaviors.

Online language learning and teaching is a field that has received a significant amount of research attention. What factors could affect student participation in simpler online learning environments has been investigated by researchers, but there has been limited study of factors affecting participation in complex Multi-User Virtual Environments. By using the typical Multi-User Virtual Environment Second Life, three English courses offered by Swedish universities were examined in this thesis. The courses were video-recorded, and selected parts of the recordings were transcribed. The transcribed recordings were complemented by author(s)’ observation, participants’ reflection, an online questionnaire and an online interview. Participation from the courses was measured both quantitatively and qualitatively. Quantitative methods were used to measure, for example, floor space, number of utterances, turn length, number of turns; the qualitative analysis centered on, for instance, utterance functions, discourse analysis, and Conversational Analysis. The results were published in five papers that focused on different central factors affecting participation in Second Life. In this thesis, the findings from those articles are synthesized. Furthermore, on the basis of the findings, a general model of factors affecting participation is presented and discussed to highlight that different factors interrelate and that some factors are particularly important in terms of affecting participation in Multi-User Virtual Environments. These are students’ technical skills, task design, course design, technical support, and Second Life technology. The complex technology also places critical demands on teachers’ technical skills, teaching strategies, and roles that teachers should play. Finally, this thesis argues that it is important to choose a suitable technology for an English course.